Principles and Applications of Miniaturized Near‐Infrared (NIR) Spectrometers

Abstract This review article focuses on the principles and applications of miniaturized near‐infrared (NIR) spectrometers. This technology and its applicability has advanced considerably over the last few years and revolutionized several fields of application. What is particularly remarkable is that the applications have a distinctly diverse nature, ranging from agriculture and the food sector, through to materials science, industry and environmental studies. Unlike a rather uniform design of a mature benchtop FTNIR spectrometer, miniaturized instruments employ diverse technological solutions, which have an impact on their operational characteristics. Continuous progress leads to new instruments appearing on the market. The current focus in analytical NIR spectroscopy is on the evaluation of the devices and associated methods, and to systematic characterization of their performance profiles.


Introduction
Near-infrared (NIR)s pectroscopy has gained remarkable value as an ondestructive analytical technique and it has become the tool of choice in several fields of application. [1,2] Its primary advantages in practical roles, 1) applicability to aw ide variety of samples;a nd 2) rapid, noninvasive analysis, form ag ood synergyw ith the autonomous, portable spectrometers that are capable of on-sitea nalysis. Such NIR spectrometers have emergedi nt he past decadea nd led to as ignificant leap in the evolution of the practical applications of this technique. [3,4] Nevertheless, severali ssues connected with the peculiarity of miniaturized spectrometers have become apparent. In contrast to the maturedesign of aF TNIR benchtop spectrometer,h andheld devices are much less uniform and implement diverse and novel technological solutions. Thisr esults in differing performance profiles of miniaturized spectrometers from that of laboratory instrumentsa nd between each model as well. The most apparent distinctiveness is the narrower spectral regions and/or lower spectral resolution with which the compact devices operate. For these reasons, current research focus is directed to at horough systematic evaluationo ft he applicability limits and analytical performance of such devicesi navariety of applications. The scope of this review is to provide comprehensivei nformation on miniaturized NIR spectrometers, including the principles of the technology,c urrent applications, and the potential for future advances.

Practicalimportance and remaining challenges in the application of miniaturizedNIR spectroscopy
The value of NIRs pectroscopyi na nalytical chemistry results from combined physicochemical and instrumental reasons. These are briefly discussed in Sections 2.1 and2 .2. Here, attention should be given to the primary driverb ehind the adoption of NIR spectroscopy in av ariety of practical roles. Often, it is af easible alternativet ot ime-inefficient and resource-intensive conventional methods of analysis, such as HPLC. Within the established framework of NIR spectrala nalysis, these demandingm ethods are required only once, and their role is to provide reference data for subsequent calibration. Once ar eliable calibration model that links the measured NIR spectra with ag iven property of the sample (e.g.,c oncentration of as elected compound or ag roup of compoundsa vailablef rom reference analysis) is established, rapid and efficient spectral measurements can substitute the less efficient analytical method in furtherroutines. [5] Consequently,the greatest gain from the application of NIR spectroscopy is in the analyses, in which a large amount of samples of relatively uniform properties are used. NIR spectroscopy is therefore widely adopted in highthroughput analysis in agriculture and variousi ndustries. In this scheme of an efficient, short-time-to-result method, the bottleneck limitation originally was the still unavoidable laboratory setup for spectral measurements. Therefore, the appearance of autonomous, portable NIR spectrometers could be seen as am ajor breakthrough in severale stablished roles.F urthermore, this leap offered the development of entirely new, previously unattainable applications. Scenarios in which spectral measurements are necessary directly on-site becamep ossible, af actor that meets keen interest from,f or example, agrifood or natural medicine industry.T he potential of portable NIR spectroscopy in such applicationsh as repeatedly been demonstrated.
Notably,t he miniaturization of the instrumentation is not solely specific to NIRs pectroscopy.R ather,i ti satrend observedt hroughout widely understood spectroscopy and spectrometry. [3] In certain applications,a ttenuated total reflection infrared (ATR-IR) or Raman techniques are possible alternatives to NIR spectroscopy. [6,7] In both cases, the portable devices cannotm atch neither the affordability nor the compact factor of miniaturized NIR spectrometers. On the otherh and, certain other techniques for which instruments with such features are available, for example, fluorescence, are inferior to NIR spec-troscopyi nf undamental capabilities, such as the chemical specificity of the methoda nd applicability to aw ide selection of samples. [4] The number of reports in the literatureo nm iniaturized NIR spectrometers is rapidlyi ncreasing nowadays, reflecting the sharp edge in applicabilityo ft his technology to conventional spectroscopy that is limited to laboratory use. However,t he revolutionary step into miniaturization has required implementing new technological solutions, which unequivocally affected the performance of the portable NIR spectrometers. Severald istinct designp rinciples have been implemented in the instrumentsi ntroduced into the market over the past decade. Furthermore, in several applications, the cost-per-unit of portable spectrometers is criticalf or wide adoption, and there is an economic stimulus for offering highly affordablei nstruments. Given these two reasons, operating characteristics largely differ between the availablem iniaturized NIR spectrometers. Systematic feasibility studies are necessary to evaluate the accuracyand robustness, in an analyticals ense, of these instruments in various applications. Currently,t his is an active area of research in analytical chemistry.

Essential Background
Unique strengths andl imitations of miniaturized NIR spectroscopy result from the underlying factors of both physical and instrumental nature.P rior to discussing these features, necessary key information is provided, and the interested reader will find more exhaustive information in the referenced literature.

Physical principles of NIR spectroscopy
NIR spectroscopy extracts information from the sample through molecular vibrational excitations, similart oI Ra nd Raman techniques. However,t he principle difference between NIR spectroscopy andt he last two techniques is that, in the NIR spectralr egion (typically defined as 12 500-4000 cm À1 or 800-2500 nm), only overtones andc ombination transitions can be observed.
These are "forbidden" transitions, with meaningful consequences for the scopeoft he presentr eview.T he probability of such transitions occurring is significantly lower than that of fundamental transitions (i.e.,t he most relevant in IR and Raman spectroscopy), which is directly observeda sam uch lower absorption index of as ample in the NIR region. [8] This results in ad eeper penetration of NIR radiation beneath the sample surface (from af ew mm to af ew cm), giving the possibility of investigating al arger sample volumeb ym eans of NIR spectroscopy.F urthermore, the band intensities decrease towards higher NIR wavenumbers, whereas the local-modee ffect makes the spectra relatively simpler. [9] In contrastt oI Rand Raman spectra, numerouse xtensively overlapping bands lead to broad line shapes being observedi nN IR spectra.H ence, a high spectralr esolution of as pectrometer becomes relatively less importanti nN IR spectroscopy.F urthermore, this peculiarity of NIR absorption bands makes direct interpretationo ft he spectra more difficult. [9] Ones hould mention the short-wave NIR (SW-NIR) region, typically defined as the region at about 14 285-9090 cm À1 (700-1100 nm), although the exact boundaries are rather arbitrary in this case. [10] Available technological solutionsm akei tp ossible to construct very compacta nd affordable spectrometers operating in this region. SW-NIR spectroscopys hows great potential and is often used in food analysis. [11] Typically,avery low absorption index( givingt he possibility to sense deepb eneath the sample surface), suitabilityo f examining moist samples, and good performance in analyzing highly scattering samples should be notedi nt he context of SW-NIR spectroscopy in such applications. [10] Krzysztof B. Beć

Basics of instrumentation in NIR spectroscopy
The design of ab enchtop NIR spectrometer is in keepingw ith ag eneral scheme of any instrumentu sed in optical absorption spectroscopy.T he main buildingb locks include al ight source, aw avelength selector,a nd ad etector.T here are two major classes of such instruments, differing by the principle of how wavelength selection is achieved. Dispersive instrumentsl et only selected wavelengths (narrow waveband) reach the detector at the same time;t hese wavelengths are selected by,f or example, ad iffraction grating and opticals lit system.T he benchtop instrumentation based on this schemeh as largely been marginalized by Fourier-transform (FT) devices with either the most popular Michelson or less common polarization interferometer.F Tspectrometers let the entire wavelength region reach the detector in the form of an interferogram, a frequency-dependent quantity.I np rinciple, this gives a straightforward gain in optical throughputofthe spectrometer, resultingi nabetter signal-to-noise (S/N) ratio (SNR). It should be noted, however,t hat mostp ortable NIR spectrometers use distinctively different technologies and spectroscopic elements to acquire spectra.

Light sources
In principle, two different types of NIR radiation sources are used in commercially available miniaturized spectrometers. The first one, at ungsten halogen light bulb, is aw ell-known standard used in benchtop instruments. It is at hermal radiation source,i nw hich af ilamentu ndergoes resistive heatingb ya n electric current passing through it. Through ah alogen cycle, tungstenc irculates between the filamenta nd halogen gas filling the volume of the bulb. It yields light of high brightness with an intensity and spectral emission profile that depends on the temperature of both the filament and the inner wall of the lamp. Following Planck's law,t os timulate the emission with ap eak maximum located in the NIR region, relatively higher temperatures need to be reached than those in the case of IR radiation. At hermale mission source is reliable, inexpensive,a nd gives as table output once thermale quilibrium is reached. However, for adoption in miniaturizedd evices,a number of additional challenges and prerequisites need to be addressed. In addition to the elevatedr equirement for the source's powere fficiency andi ts physicald imensions,t he thermal stability may become an issue. Handheld spectrometers are particularly prone to temperature variationsf or severalr easons. In-field operation exposes the device to external conditions. Furthermore, compactd imensions reduce the thermal capacityo ft he device, easing temperature buildup over operation time. The simplest solution recommended by somev endors is to perform frequentr eference scans, to keept he background signalm ost recent. Frequent reference scans are not problematic, in many cases, because these devices often feature rapid scanning. However, in certaina pplications, such a solution maynot be feasible.I th as been shown that an insuffi-cient thermal stabilityn egatively influences the analytical performance (MicroNIR 2200). [12] Solutions in the form of thermoelectric cooling have been proposed. On the other hand, some newerd evices offer at emperature-correction functiont oa ccount for the drift of the emission profile of the source (Micro-NIR ES 1700). At ungsten halogen radiation source is employed in, for example, MicroNIR series or microPHAZIRh andheld spectrometers.
The second solution, suitable for miniaturized spectrometers, are light-emittingd iodes (LEDs). In principle, aL ED is as emiconductor element, in which,u pon current flow,r ecombination of electrons and electron holes occurs and excess energy is emitted as photons. [13] LEDs have severalc onsiderable advantages for application in highly miniaturized spectrometers. They feature very compactd imensions, low power consumption, require low voltages for their operation, and are robust and inexpensive. However,t here are significant limitations of LEDs as light sources, in general, in spectroscopy.I ti sp rimarily an arrow emission bandwidth, for example, aG aAs LED has a maximum emission at 870 nm and ab andwidth of only 50 nm. [14] Furthermore, the availability of LEDs emitting in the NIR region remains very limited. Sources covering the Vis/SW-NIR region are, however,a vailable and commerciallyu sed in miniaturized spectrometers, for which the compact dimensions and affordability are emphasized (e.g.,S CiO).

Wavelength selection techniques
The most essential elementf or as pectrometer is the wavelength selector. Unlike benchtop NIR spectrometers, which are dominated by FT instrumentsequipped with aMichelsoninterferometer,p ortable devices show much diversity here. Instrumentsb ased on the principles of aF abry-PØroti nterferometer, Hadamard mask,l inear variable filter (LVF), or digital micromirror array are availableo nt he market. Furthermore, some miniaturized designs implement aM ichelson interferometer on a microscale. [4] The wavelengths election mechanisms dictate whether ac ost-effective single-pixel detector (i.e.,s ingle spectral resolution element) or ac omplex array detector needs to be used in the spectrometer.S everal of the wavelength selectors could be miniaturized through micro-electromechanical systems( MEMS;o rm icro-opto-electromechanical systems (MOEMS)i fm icro-optics is also included). [15] These optomechanical devicesa re capable of digitall ight processing (DLP). MEMS are manufactured in at echnology similar to that used for integrated circuitry (microfabrication in silicon).
Despite becoming the standard in benchtop spectrometers, aM ichelsoni nterferometer in am iniaturized spectrometer initially faced limitations in implementation. Nonetheless, commerciallys uccessful instrumentsb ased on this elementa re available( e.g.,N eoSpectra sensors). Additionally,M EMS-based FTNIR spectrometers face the problem of limitedlight-throughput efficiency;h owever,m easures are takent oi mprovet his parameter.AHadamard-transform (HT) spectrometer seems more feasiblef or miniaturization. The design is analogous to that of ag rating instrument. [16] In its simplestf orm of as ingly encoded HT spectrometer,t he light beam is focusedo nto as lit and, after passing through ag rating and associated optics, it is encoded by aH adamard mask (a multiaperture mask) andd irected to as ingle-pixel detector.T his optical configuration leads to aH adamard-encoded signal reaching the detector, and the spectrum is recovered through HT.T he advantages of Hadamard spectrometers were demonstrated in theory relatively early, sharing the opticala dvantages (multiplex (Felgett), frequency-precision( Connes), and throughput (Jacquinot)) of FT spectrometry,w ithout the need to use moving parts. [16,17] Practicalb enefitso fHadamard NIRs pectrometers werew ell explained by Fateley and associates. [18,19] AF abry-PØrot interferometer is equally suitable as am iniaturized wavelength selector.I tu ses aF abry-PØrot filter with two parallel mirrors separated by ad istance d,c reating an optical cavity.E ither fixed or variable d can be used. The optical cavity controls the interference condition throught he effect of as tanding electric field wave between the two mirrors. Only wavelengths that are in resonance with the opticalc avity can pass through the filter.T he configurationo ft he Fabry-PØrot interferometer enablest he incident polychromatic light to be divided into several narrower wavelength bands. Af ully programmable Fabry-PØrot interferometer can be microfabricated by using MEMS technology.I mportantly,s uch as olution enables the spectrometer to be easily reconfigured to operate over awidespectral region (e.g.,NIRONE sensor series).
AM EMS-based array of mirrorsf orms ad igital micromirror device (DMD). This element is being used to obtain am icroscale dispersive scanning spectrometer with no macroscale movingp arts. DMD takes the functionalr ole of am ovingd ispersion grating. In addition to gains in the cost and size of the wavelength selectore lement itself, such an opticalc onfiguration allows the use of an inexpensive single-pixel detector.
Wavelength selectors suitable for miniaturized NIR spectrometers can be constructed by using technology other than that of MEMS. AL VF is an otable example. It operates as an optical bandpass filter by using an optical coatingo fv arying thickness across its wedged geometry.T his yields al inear change in the transparence of the filter against different wavelengths.A wavelength selector based on aL VF is highly cost-effective and, in contrast to MEMS, does not involve av ery high initial investment cost that is characteristic for semiconductor manufacturing. It is extremely compact and allows spectrometers with very short opticalp ath to be built. Furthermore, no macro-or micromoving parts are used, improvingt he ruggedness of the spectrometer.H owever,t he LVFr equires the use of ac omplicated array detector placed behindt he filter;e ach element of the array createsaseparater esolution channel. Notwithstanding, such am ultichannel optical configuration provides ag ood optical throughput and the capacity to achieve a very short time of spectra acquisition. [3]

Detectors
Twoc lasses of detector are typically used in miniaturizeds pectrometers. [3] Photovoltaic Si diodes maintain as uitable sensitivity over the 14 285-9100 cm À1 (700-1100 nm) region, and thus, are only suitable for compact, inexpensive spectrometers oper-ating in the Vis and SW-NIR regions. Furthermore, this solution yields ar ather inferiorl evel of S/N. Photodiodes used in portable spectrometers require the use of wavelength cutoff filters to eliminate the risk of the detector responding to sunlight. If mid-and long-wave (LW) sections of the NIR region (i.e.,9 500-400 cm À1 ;1 050-2500 nm) are considered, other detectors are indispensable. Because of the need to maintain an adequate S/N in am iniaturized spectrometer,h igh-performing InGaAs photodetectors dominate in these devices. [3] The typical range of sensitivity is about 11 100-5882 cm À1 (900-1700 nm). Compared with other detectors, InGaAs has ar apid response time, good quantume fficiency,a nd low dark current at ag iven sensora rea, enabling short scanning times with good S/N. There exists an extended InGaAs variant, suitable for detecting wavelengths longert han 1700 nm;h owever,i tf eaturesl ower S/N and requires cooling. [3]

Optical materials
Optical materials not absorbing in the Vis region are most often transparent throughout al argep ortion of the NIR region as well. Importantly,t his makesg lass optics suitable, whereas, for best operation in the LW-NIR region,h igh-quality (i.e.,w ithout impurities containingO ÀHg roups)f used silica (fused quartz)o ptics may be required. This forms as ignificant advantage for portable NIR spectrometers, making them cheaper and suitable for operation under humid conditions because no alkali halides are used as optical materials.These are critical advantages in on-site analysiso rp rocessm onitoring. For rugged operation, as cratch-resistant optical windowa tt he sample interfacei sf avored for feasible contact-mode operation. Sapphire is am echanically durable material used in such ar ole (e.g.,i nM icroNIR spectrometer). However,i th as the disadvantage of ah igh refractive index (above1 .7 in the Vis and NIR regions), which increases optical loss due to reflection;t his makes the materialm ore suitable for instruments with good opticalt hroughput (e.g.,m ultiplexed or multichannels pectrometers).
Notably,o nly some miniaturized spectrometers require reflective elements (mirrors) in their design. In this role, goldplated surfaces offer superior effectiveness, with ar eflectivity of at least 96 %t hroughout 700 to 2500 nm, that is, the entire NIR region.

Overview of the designofselected handheld NIR spectrometers
In this ands ubsequent sections, primary focus is given to the instruments intended for laboratory and on-site analysis, whereas the designsi ntended for online applications in industry are mentioned briefly.A lthough the principal advantages of Hadamard spectrometers were demonstrated early, [16][17][18][19] these devices became practicallym eaningful once ap rogrammable microscale Hadamard mask becamei mplementable through MEMS.T his solution was used by the Polychromix company in aN IR spectrometer designed for the exploration of the moon by NASA. Such an application requires ar ugged, reliable, and compacti nstrument.C losely connected to that designw as the first handheld NIR spectrometer officially launchedi n2 006, mi-croPHAZIR (presently,t he intellectual property of Thermo Fisher Scientific Inc.;T able 1a nd Figure 1a). [20] This instrument uses ap rogrammableH adamard mask in the form of aM EMS chip with electronically actuated bars akin to apiano keyboard. It is combined with al ow-power tungstenb ulb source and a single-pixel InGaAs detector.T he device offers ar apid scanning capability with af ulls pectrum recorded under 10 s, good S/N level, and ar easonably good opticalr esolution of 11 nm. However,t he device operates in ar ather narrow-wavelength region of 1596-2396 nm (6267-4173 cm À1 ). MicroPHAZIR is equipped with its own power source( Li-ion battery,w hich can be exchanged, and thus as pare battery can be carriedf or prolonged operation), displays creen, and user interface for entirely autonomous operation. Therefore, this device is perfectly suited for on-site measurements.
The initial successo ft he microPHAZIRd esign allowed the anticipation that similar constructions( aH adamard mask implemented through aM EMS-based programmablea ctuator combined with diffractiong rating) would rapidly dominate the market of miniaturized NIR instruments. [3] However,f urther developmentsp rogressed differently with the appearance of numerous, distinct designs. Ad ifferent solution was implemented in the Digital Light Processor (a trademark owned by Te xas instruments) NIRscan module from Texas Instruments( Ta ble 1 and Figure1b). In this design, aM EMS DMD enables the use of as tationary diffraction grating in as cannings pectrometer. This allows an expensive microarray detector to be replaced by al arge single-pixel detector.T hus, av ery simple, cost-effective construction of am iniature dispersive instrumentw as achieved. The low powerc onsumption of such ad esign should  also be noted. These spectrometers offer spectralr esolution and sensitivity that is adequate for certain applications. The developed technologyi sa vailable commercially as two EVMs: HP EVM featuring aD LP NIRscans ensora nd aM SE VM with a DLP NIRscan Nano (Table 1). [21] Although the larger DLP NIRscan sensori sa dvertised as being suitable for integration into spectrometers intended for both in-fielda nd industrial online use, the compact NIRscan Nano module is suited for portable spectrometers. The latter solution is implemented in aN IR-S-G1 instrument offered by InnoSpectra [22] and availablea sacustomized product from SphereOptics, [23] Sagitto, [24] Allied Scientific, [25] and Tellspec. [26] The NIR-S-G1spectrometer has extremely compactd imensions (82 63 43 mm 3 ; < 145 gw eight) and uses at ungsten halogen source and InGaAs detector.T he workings pectralr egion of the device (11111-5082 cm À1 ;9 00-1700 nm) covers aw ide part of the NIR wavelengths. The spectrometer is controlled through am obile applicationa nd communicates with as martphone over al ow-power wireless (Bluetooth) interface. Miniaturized FTNIR spectrometers with aM ichelson interferometer were commercialized by Si-Ware Systems in their Neo-Spectra device (Table 1a nd Figure 1c). [27] As explained in Section 2.3.2, this particular principle of operation is difficult for miniaturization withoutp erformance penalties. However,c ontinuousd evelopment has focusedo ni mprovingt he operation of the interferometer and the opticalt hroughputo fs uch devices should be noted. For example, the nanoFTIR NIR spectrometer was announced last year by Hefei SouthNest Technology (Table 1a nd Figure1d). [28] The device uses aM EMS Michelson interferometer with al arge mirror (relative to the area of the MEMS chip) to enhance its opticalt hroughput. Notably,t he spectrometer operates over the entire NIR region( 12 50-846 cm À1 ;8 00-2600 nm) with ar elatively high spectralr esolution (6 nm at 1600 nm). The instrumenth as compact dimensions (14.3 4.9 2.8 cm 3 )a nd low weight (220 g). Rugged build and fiber-probec onnectivity of the instrument make it particularly suitable foro nline analysis. An ew MEMS-based FTNIR spectrometer wasr ecently introducedb yH amamatsu as well. [29] To overcomeo pticalt hroughput limitations, the device features a3mm in diameter movingm irror,r esulting in ah igh S/N over the wide spectral region of 1100-2500 nm (9090-4000 cm À1 ). The device is offered as ac ompact spectrometer module (ca. 300 go fw eight), with USB connectivity and a fiber-probe interface.
At unable Fabry-PØrot interferometer miniaturized as a MEMS is used in the NIRONES ensorS instrument (Table 1a nd Figure 1e)a vailable since 2016. [30] Despite very compactd imensions( 25 25 17.5 mm 3 ;1 5g weight), the sensor is equipped with two tungsten halogenl amps and as ingle-element detector.S everal variants of the sensor are available that differ by the operational wavenumber region, resolution, and S/N parameter ( Table 1). Implementation of the Fabry-PØrot interferometer enabled an opticalc onfiguration of the sensor suitable for ar elativelyl arge area, with either an InGaAs or an extendedI nGaAs detector;t he former type yields S/N up to 15 000:1, grantingareasonable level of sensitivity and specificity of the instrument. Sensor Xi sascaled-down variant, opti-mized for cost-effectiveness and ease of manufacturability.I mportantly, currenta dvances in the technology of Fabry-PØrot interferometers are promising with regard to ultraminiaturization. Hamamatsur ecently introduced an ultracompact NIR sensor that integrated aF abry-PØrot interferometer through MEMS technology and operated in the 1550-1850 nm (6452-5405 cm À1 )r egion. [31] The sensori sh ermetically sealed for high reliability under high humidity,w hile remaining extremely compact(< 1gof weight).
Ar adically differentd esign approach wase mployed in the MicroNIR series of instrumentsf rom VIAVI (Table 1a nd Figure 1f). This compacts pectrometer uses ac omplex and more expensive array detector (InGaAs)c ombined with aL VF element. This yields an extremelys mall and robustd evice, with no moving parts andr eliable operation under difficult conditions;s imilart echnology is used in sensors aimed at process control.N ewer variantso faMicroNIR instrument improve the stabilityo ft he operation time by implementingatemperature-correction function. The opticalc onfigurationo ft his multichannel device enables rapid measurement and good quality of the spectra. The MicroNIR 1700 device is the variant powered andc ontrolled by aU SB interface, andt herefore, during on-site use, it requires continuous connection to am aster device (e.g.,n otebook PC). Am ore convenient version of the sensor,M icroNIRO nSite-W,i se quipped with its own battery power source and water-and dust-resistant case, making it even more suited for on-siteu se. [32] VIAVI offers as imilar design in their line of spectrometers intended for online and inline monitoring of processes in the chemical, pharmaceutical, and food industries. MicroNIR PATa nalyzers are offered in three variants (MicroNIR PAT-U, PAT-W, and PAT-Ux), in which different features required in the applicationsa re accommodated, for example, an increased ruggedness, resistance to environment( e.g.,w ater,d ust, chemical agents), fiber-probe connectivity or sanitary (hygienic) flange mounts, and control software designed for such uses. [33] An umber of dedicated portable NIR sensors intendedf or online use are available, for example, Axsun IntegraSpecX LN IR from Excelitas Te chnologies, [34] or am obile unit (mounted on am ovable cart and equipped with its own powers ource)M B3600-PH FTNIR spectrometer from ABB. [35] Interestingly,t he Axsun instrumenti ncorporates MEMS tunable laser technology,y ielding fast measurement speed, high spectralr esolution, and good S/N, which are helpful in process monitoring applications. [34] Some of the extremelyc ompact and cost-effectiveN IR spectrometers have been engineered by accepting as omewhat limitedg eneral applicabilitya nd performance in general use, yet adequate suitability for certaina pplications.Agood example of such an instrument is the SCiO NIRm icrospectrometer from Consumer Physics, which has been available since about 2015. [36] Advertised as the first pocket-sized spectrometer,t he deviceh as dimensions of 67.7 40.2 18.8 mm 3 andaweight of 35 g, and is aimed at the consumer market. Unparalleled affordability was achieved through the use of aL ED light source and as imple Si photodiode array detector (4 3c onfiguration), with opticalf ilterso ver the individual pixels creating a1 2channeld evice. However,n oticeable penalties to the per- formance of the spectrometer have been unavoidable in such ad esign. As ubpar S/N, very narrow Vis/SW-NIR operating wavelength region (13 514-9346 cm À1 ;7 40-1070 nm), and poor spectral resolution of about 28 nm because of al ow number of resolution elements shouldb en oted. The product is addressed for consumer use, and therefore, ac ompromise in the performance in exchange fora ffordability seems justified.
It should be noted that, in research laboratories and in many applications, the calibration is generated by the operator,o ften by using software different from that providedb y the vendoro ft he device. Althought he vast majority of miniaturized instrumentsm ay operate as such general-purposeN IR spectrometers, an umber of them are also sold preconfigured with calibrationsp repared for intended analyses. Their operations do not requirei n-depth knowledge about spectroscopy or data-analytical methods, and satisfactory analyses may be performed by untrained personnel. Such dedicated analyzers are becoming populari nr egularu se, particularly in agriculture. For instance, NIR4 Farm, availablef rom AB Vista, is ap ortable spectrometer intended for forage analysis. [37] Unlike the commont ime-consuming way of shipping samples to ac ommercial laboratory,adirect on-farm analysise nables quick optimizationo ft he ration and maximizes milk yield. Other conceptually similarp roducts shouldb em entioned as well, for example, the AURA handheld NIR [38] or X-NIR analyzer, [39] which are both preconfigured for the assessment of grain, or slightly larger,p ortable in as uitcase format, StellarCASE Portable NIR analyzer,i ntendedf or material analysis. [40] Comparable concepts exist for sensors intended for online use, for instance, a nondrift NIR analyzer from MoistTech Corp. [41] Notably,s ome of the instruments on the market are available either as ag eneral-purpose spectrometer or in ap reconfigured setup. For instance, microPHAZIRh as been sold as an analyzer for animal feed and ingredients, with several calibrations prepared for such as cope. [42] Offered as the microPHAZIR AG handheld analyzer,t he device is intendedf or easeo fu se, with the ability to estimate the key components in feed (e.g., moisture, protein, fiber,s tarch). Other turnkeyp reconfigurations of the same hardware should be mentioned as well, for example, ap lastics recycling analyzer (microPHAZIR PC), [43] a pharmaceutical analyzer (microPHAZIR RX), [44] and an asbestos analyzer (microPHAZIRAS). [45] The current development trend aims for furtheru ltraminiaturization to the extent of enabling implementing aN IR spectrometerd irectly in smartphone devices. [46] Such designs have already been announced, although not yet available as the final product, indicating that certainc hallenges have not yet been fully overcome. It should be briefly mentioned that there are spectrometers mountable in airborne drones( unmanned aerial vehicles (UAVs)), which can be considered ah ighly specialized type of portable NIR spectrometer.T he development of such technology faces similar challenges ands haress imilar goals with the topic reviewed herein. Notably,U AV-borne spectral sensors are currently mostly used for multispectral imaging. [47] Quantitative analysis of severalp ropertieso fc rops was shown to be feasible based upon acquisition of narrow spectral bands from UAV-borne spectrometers, additionally com-pared in performance with ground-based instrument. [48] In some cases, higherr esolution spectrali nformation covering the NIR region is processed. [49] The advances in sensor technology andd ata-analytical tools, as well as studies aimed at resolving fundamentali ssues, such as reliable direct measurement of reflectance, [50] form important steps in fully suited airborne NIR spectroscopy.
2.3.6. Principles, treatment, and consequences of the instrumental differences in the contextofportable spectrometers Instrumental difference is aw ell-known phenomenon, in which even very subtle dissimilarities are observed between the spectral line shapes measured on variouss pectrometers for the same sample and under identical external conditions. It is apparentf or benchtop FTNIR instruments and leads to nonstraightforward transferability of calibration models. In other words, calibrations developed on ag iven instrument, most of the time, cannot be used for the analysis of unknown samples on any other spectrometers. Because calibrationi satime-and resource-consuming process, calibration transfer procedures were intensively researched. [51] Compared with conventional instruments, diversity in the technical principles underlying microspectrometers goes much further,a sp resented in Section 2.3.5. Nonidentical wavelengthr esolutions, spectral sensitivity,a nd S/N levels strongly contribute to instrumental differences.A sm ay be concludedf rom the information provided in Ta ble 1, miniaturized spectrometers strongly vary in these regards.T herefore, their distinct operational characteristics may lead to highly elevated instrumental dissimilarities. Furthermore,d ecisive differencesb etween the wavelength regions that these devices can measure may directly determine the applicability to certain types of samples. For instance, in food analysis, the SW-NIR regioni sc onsidered to be suitable for macronutrient determinationi nm oist samples or water-content analysis. However,o ne may not be able to measure weak absorption of less abundant chemicals in the SW-NIR region with adequate accuracy.T his enables the suitability of SW-NIR spectrometers for certain purposes to be roughly estimated. On the other hand, differences between instruments, in their capacityt oa nalyze the LW-NIR region, are more difficult to predicta nd requiremore systematic approaches. [52] The currents tate of technology only requires am iniaturized NIR spectrometer to be ab alance between the level of miniaturization, performance, and economic cost. Furthermore,t his type of instrumentation continues its rapid development, with new spectrometers pushing the envelope in operational characteristics, for example, mini-FTNIRd evices capable of operating over the entire NIR wavelength region (Table 1). Therefore, the applicabilityo ft hese devices in variouss cenarios, as well as their analytical performance, is an intensively explored area. [4] 3. Miniaturized NIR Spectroscopy in Practical Applications 3.1. Major fields of application

Pharmaceuticals
Pharmaceutical analysis is ap articularly demanding scenario for analytical techniques, in which accuracy andr eliability are foremost important. Nonetheless, flexibility for on-site measurements is ac onsiderable gain for the pharmaceutical industry as well, and therefore, miniaturized NIR sensors have attracted keen attention. The current state of industrial applications of NIR spectroscopy have been thoroughly described by Chavan et al. [53] It was pointed out relatively early that the applicability and performance of handheldN IR instruments for qualitative andq uantitative analyses of pharmaceutical formulations require systematic feasibility studies. Subsequent examinations revealed thatc ertainm iniaturized devices could offer competitive levels of accuracy and reliability,f or example, as demonstrated by Alcalà et al. [54] for MicroNIR spectrometers. Twov ersions of this device were used:t he standardo ne operating in the 10 526-6060 cm À1 (950-1650 nm) range and variant with an extended InGaAs detectort hat covers the 8695-4651 cm À1 (1150-2150 nm) range. Qualitative applications,t hat is, classification, have wide practical importance because of drug counterfeiting,i llegal drug imports, and online drug trading. [54][55][56][57] Portable NIR spectrometerso ffer great potential for rapid analyses performed directly by border control or postal staff, largely improving the throughput of the control procedures. Qualitative analysisi s, most of the time, less sensitivet o the instrumental factorsa nd could be accurately performed with aM icroNIRd evice. [54] First, the identification of raw materials commonly used in pharmaceutical formulations could be performed by meanso fr outinely used methods (e.g.,p rincipal components analysis( PCA)), as demonstrated on the basis of 22 compounds. False-positive and near-false-positive identifications occurred only among chemically similarm aterials. Furthermore, successful discriminationb etween authentic and counterfeit drugs with 95 %c onfidence was accomplished for severalp harmaceutical products (Alli,V iagra,C ialis)a nd their illegal counterfeits,w hich were analyzed as tablets and capsuleb lends. Furthermore,q uantitative determination of active pharmaceutical ingredients (APIs),i nt his case, acetylsalicylic acid (ASA), ascorbic acid (ASC), and caffeine (CAF), in pharmaceutical formulations in powdered form was performed. Accurate determination of the concentrations of active principal ingredients in unknown test samples could be performed through partial least squares regression (PLS-R) models.
Direct comparisons of the applicability and performanceo f differentm iniaturized NIRs pectrometersi na nalyzing pharmaceuticalf ormulations were performed. For example, recently, Yana nd Siesler examined four instruments that used distinct design principles:N eoSpectra, NIRONE (SpectralE ngines NR-2.0 W), DLP NIRscan, and MicroNIR. [58] Consequently,t he captured spectral information differs notably between these instruments, as presented in Figure 2. The subjects for quantita-tive analysisw eret wo excipients (cellulose and starch) and three APIs (ASA, ASC, CAF). The study indicated that the predictionp erformance of these four instrumentv aried for the studied analytes, with the exception of CAF,i nw hichc ase all devicesp erformed comparably well. The conclusions drawn indicatedt hat the LVF/InGaAs array spectrometer (MicroNIR) yielded the most balanced performance, with the DMD-based device (NIRscan) following closely behind. The remaining two spectrometers performed notably worse, althoughs till offered prediction capabilities acceptable for quantitative analysisi n the investigated scenario. This provides evidence for the performance penaltieso fm iniaturized interferometer-based spectrometers (int hat study,N eoSpectra with aM ichelson interferometera nd NIRONE with aF abry-PØrot interferometer) clearly translating into their practical applicability.
In ar ecent study by Guillemain et al., [59] in addition to the routinelyu sed unsupervised methodP CA, an umber of supervised classification techniques (linear discriminant analysis (LDA), quadratic discriminant analysis (QDA), support vector machine, SVM; k-nearest neighbors (KNN)) were evaluated in tablet authentication from the spectral measurements performed with two different NIR miniaturized spectrometers (Mi-croNIR and SCiO). The compared instruments differ notably in their design and operational characteristics, foremost, in the measured spectral region (Table 1). Interestingly,d espite being extremelya ffordable, the SCiO instrument performed better in this application.F or each of these two instruments, different classification methods were concluded to be the best performing ones:S VM for SCiO and LDA for MicroNIR. The classification performance for 29 pharmaceutical products was reported to be 100/96 (% of correct identification in calibration/in validation)i nt he former and 99.9/91.1 in the latter.T his suggests that spectral informationr elevant for the authentication of the pharmaceutical formulations investigated in that study was relatively more accessible in the SW-NIR region (over which the SCiO instrument operates;T able 1).
The development of applicationso fm iniaturized NIR spectrometers in the quantitative determination of APIs continues. Af ocus on improving their performance in more demanding scenarios can be observed. For example, ar ecent study aimed at the simultaneous quantification in at ablet of two APIs (paracetamol andt ramadol), which are spectrally similar and present in the sample in greatlyd ifferent mass proportions. [60] Despite these difficulties, ah andheld NIR spectrometer (MicroNIR) performed comparably well to that of ab enchtop spectrometer.H owever,i ts hould be noted that preselection of the spectral region for subsequent analysisw as neededi nt his case. [60] The need fora pplying different pretreatments to the spectra acquired by various spectrometers, for example, as the result of poorer spectralr esolution of S/N, should be mentioned, but such as tep does not significantly complicate the flow of the analysis. [54] Often,a dditional reasonsf or extended supervision result from the pronounced differencesb etween the spectral regions measured by variousm iniaturized instruments. For instance, ac omparatives tudy reported that different classification methodsm aximized the accuracy of counterfeit detection in the case of pharmaceutical tablets performed by two porta-ble instruments. [59] In the case of microPHAZIR, which operates in the conventionalN IR region, the application of LDA was deemedt ob em ore successful. In contrast, SVM classification yielded the best resultsf or datac ollected with the SW-NIR SCiO spectrometer. [59] These studies indicatedt hat, in the case of miniaturized NIR spectrometers, more attention should be given to the careful selection of spectralp retreatments and chemometric methods for subsequent analysis. [54,59,60] The importance of properp reprocessing confirms that certain limitations of compact instruments, for example, narrow observed spectral regions, SNR, or spectralr esolution, need to be taken into account more carefully than in the case of benchtop spectrometers. In some cases, ah igherr equirement of sample preparation for successful analysis by miniaturizeds pectrometers was reported as well. For instance, milling of plant materialm ay significantly improvethe performance of the analysis. [52] Notably,h andheld NIRs pectrometers face competition from Ramani nstruments in applications, in which cost-effectiveness does not play ac rucial role. The strengthsa nd limitations of both techniques in the analysis of pharmaceutical products were directly compared, for example, in ar ecent study by Ciza et al. [61] The general conclusion was that miniaturized NIR spectroscopyw as more reliable in specific product identification; however,t he choice of ab etter technique might depend on the particular application. It was suggested that the future appearance of cost-effective instrumentsc ombining both NIR and Ramans pectrometers would result in substantial progress. [61] Aw ider perspective of how portable NIR spectrometers competew ith Raman instruments in the analysis of pharmaceuticals has been presented in arecent reviewarticle. [62]

Natural medicines
Despite apparent similarities to the field discussed in Section 3.1.1, the purpose and characteristics of the analyses performed on natural medicines (i.e.,p lant extracts or fresh plant materials) differ in several aspects. Natural products are incomparably more complex in their chemical composition (i.e.,l eading to considerable matrix effects), often with the active compoundb eing in ar elatively low concentration, the chemical and physical properties of the sample vary from batch to batch and may be affected by the sample preparation procedures.A lthough these factorsi ncreaset he difficulty of the analysis,t hey also make miniaturized NIR spectroscopy particularly important for its capacity to perform rapid and highthroughput quality controlo fs uch products.
Miniaturized NIR spectrometers can be successfully used for quantitative determination of the active compounds in natural medicines, for example, as shown by Kirchler et al. [63] The performance of two handheldN IR spectrometers (MicroNIR and microPHAZIR) in analyzing 60 samples of extracts from rosemary leaves (Rosmarini folium)w as compared with the results obtained from ab enchtop instrument( B üchi NIRFlex N-500). The analysiswas directed at the quantitative determination of polyphenolic content, mostly contributed to by rosmarinic acid (RA), in the medicine. Instrumental differencew as examined, in Figure 2. NIR spectraoft he pure ingredients of the pharmaceutical formulation investigated by Yanand Siesler [58] recorded with the miniaturized spectrometers: a) NeoSpectra, b) NIRONE, c) DLP NIRScan, and d) MicroNIR.R eproduced with permission from ref. [58].Copyright Elsevier,2 018. detail, in the study,i ncluding the clear disparity in the covered spectralregions ( Figure 3). The study included ad etailed analysis of the vibrational transitions of RA, yielding ad etailed overview of which vibrations thesep ortable instruments can observe and how relevant the correspondingw avenumbers are in PLS-R modelso fN IR spectral features associatedw ith RA content in the studied samples. This enabled ad eeper insight into the instrumental differences to be obtained. Further insights into the assessment of the relative sensitivity of each spectrometer towards the absorption regions associated with specific molecular vibrations werep rovidedb yq uantum mechanical simulations of the NIR spectrum of RA andt wo-dimensional correlation analysis( 2D-COS). The quantitative analytical performance of the three evaluated spectrometers indicated that, although the best results were providedb yt he benchtop NIRFlex N-500 instrument, among the miniaturized devices,t he efficacy of MicroNIRw as also satisfactory and superior to microPHAZIR in that particular application.T he study revealed that the wider observed spectral region, presenting more relevant vibrations of RA to the regression procedure, led to better results of the former device. At the same time, the benchtop spectrometer outperformed both portable devices, although all three yielded performance levels that were satisfactory for quantitative analysis. Notwithstanding, the study confirmed that the physical and chemical properties of plant extracts might pose increased difficultiesf or the successful application of miniaturized NIR spectrometers.
It is of particular note that portable NIR spectrometers can be used with great success for highly specificp urposes in phytopharmaceutical applications. Through direct on-site measurements on pharmaceutically relevant fresh plant material, a rapid determination of the ideal harvest time of am edicinal plant may be performed. Optimization of the conditions of cultivation towards the best quality (e.g.,h ighest concentration of the activet herapeutic ingredient in plant tissues)b ecomes possible, withs ignificant economicg ain for the industry.A n example of af easibility study that demonstrated the emerging potentialf or such applications may be served by Pezzei et al. [64] Their case study involved the evaluation of the applica-bility of ah andheldN IR spectrometer (microPHAZIR) in predicting an optimal harvest time of am edicinal plant, Verbena officinalis,w ith reference analysisc arried out through HPLC with ad iode-array detector based on mass spectrometry (HPLC-DAD-MS) and reference NIR spectral analysis performed on ab enchtop FTNIR instrument.T he optimal harvest time of the plant was relatedt ot he concentrationo ft he main therapeutic ingredients, verbenalin and verbascoside. Despite predictions,t he powerd emonstrated by the handheld spectrometer in absolute terms was inferior compared with that of the benchtop instrument. The study confirmed that such ap erformance level was adequate for this particulara pplication. Hence, miniaturized NIR spectrometers deployed on-sitef or direct measurements of fresh plant material offer great potential for guaranteeing ac onsistently high phytopharmaceutical quality of herbal raw materials.

Agri-foodsector
Agriculture-and food-relateda nalysis is ap articularly rich field of application of miniaturized NIR spectrometers. PortableN IR spectroscopy is ap erfect tool for monitoring crop quality to determine optimal cultivation conditions. The importanceo f controlling food quality,a sar esult of the generally high vulnerability of foods to content variation, maintaining freshness to prevent the risk of quality loss, and possibility of illegal adulteration, and so forth, have led to the wide adoption of NIR spectroscopy as an ondestructive, rapid, and high-throughput analytical method. Furthermore, the complex nature of food production and delivery chain, as well as the necessity to reduce the analysist ime to am inimum, have contributed to emergentp ortable spectrometers being ar evolutionarys tep forwardi nt his field. The particular exposition of food products to safety risks of various kinds and the key role of conventional NIR spectroscopy as atool for controlling food safety and quality have been discussed in detail by Qu et al. [65] Portable instruments weres uccessfully validated in food analyses relatively early,a s, for example, summarized in 2013 in review articles by Te ixeira dos Santos et al. [66] and Alander et al. [67] Ap erspective view on the particular potential offered by portable NIR instruments and the feasibility of their use throughout the food supply chains has been presented by Ellis and co-workers. [68] The majority of studies focus on establishing successful methods of determiningt he quality parameters of final food products. The great variety of chemical (e.g.,c omplex matrix, high moisture content)a nd physical( e.g.,s urface texture) properties of such products and often the need to perform analyses in an entirely nonintrusive manner( i.e., through the originalpackaging)appears. These factorsincrease the difficulty of such analyses,a nd the applicability of certain instruments greatly varies, depending on ap articularc ase. Nevertheless, portable NIR spectroscopy has been adopted with great success in the food sector anda pplication development continues to be an active research direction, with numerous reports appearing in the recent literature. Ta ble 2s ummarizes current progress being made in this direction. An overview of current progress in food analytical applicationsofm ini-  microPHAZIRP CA, PLS-R high-levelm eat adulterations (> 10 %): fully feasible with benchtop spectroscopy, improvementsr equired for miniaturized instrument (e.g.,l arger sample set);low-level meat adulterations( < 10 %): improvements are needed for both typesofi nstrumentation [131] feasibility study of miniaturized NIR spectrometer in determining nutritional parameterso f pasta/sauce blends commercial products:5pasta products, 5s auce products; for each,5differentp asta/ sauce-type blend combinations( 0-100 %( w/w) sauce addition) MicroNIR PLS-Rsatisfactory prediction accuracy for quantifying energy, carbohydrate,f at, fiber,protein, and sugar in pasta/sauce meal through miniaturized NIR spectroscopy in ar ealistic analytical scenario [132]f easibility study for miniaturized NIR spectroscopyina nalyzing matcha tea quality index am odel strategyb ased on portable NIR spectroscopy was successfully developed,w ith promising potential for predicting and classifying the content of polyphenols and aminoa cids in matcha tea [133] Trek (ASD Inc.,Boulder, CO, USA), NIRscan Nano MSC and PLS-R;V IP analysisf or determiningt he important spectral features (relevant peak related to intramuscular fat) captured by the NIRscanN ano device prediction performance not affectedbys ample temperature equilibrationt ime;frozen samples:good performance of Lab-Spec5000,L abSpec4, and Trek instruments;bias (measurement time-wise) observedf or NIRscan Nano( instrumental variations); freshmeat:N IRscan Nano performed well and is ag ood alternative to handheld spectrophotometers for rapid and real-time classification of freshl amb aturized NIR spectrometersr eveals that, while compactd evices can be successfully adopted for numeroust ypes of analyses, the applicabilityp otential and relative performance between devices largely varies. [126][127][128][129][130][131][132][133][134][135][136][137] The instrumental differences seem to be particularly evident in such applications, for example, highly affordable SW-NIR spectrometers may yield good performance in analyzing somem acronutrients, yet their applicability is largely limited in other scenarios. This makes it difficult to anticipate the performance of ag iven spectrometer without prior systematic feasibilitys tudies. Recently,asuccessful attempt to obtain more general insights into instrumental differences, expressedt hrough the ability to analyze chemical contents with significant differencesi nt heir NIR absorption spectra, was conducted by Mayr et al. [52] Differences in the prediction of CAF and l-theanine contents (considered the quality parameters) in black tea by microPHAZIR and MicroNIRi nstruments could be associated with their sensitivity towards characteristic absorption bandso ft hese two constituents. Furthermore, spectrals imulations yielded ad etailed correlation between the absorption bands and structuralf eatures of these molecules. Hence, the wavenumber regions meaningful for NIR analysisp erformed with both devices,w hich, in this case, largely differed in their operational spectralr egions, couldb e established. These accomplishments enabled the performance of each of theset wo spectrometers to be predicted in analyzing compounds similar to either CAF or l-theanine. [52] Importantly,f ood analysist owards provingi ts quality,o rigin, authenticity,o rn utritional values attracts great interestf rom consumers. In this direction, the barrier between professional and causalu se of NIR spectroscopy has already been broken. Inexpensive SW-NIR spectrometers intended to be operated througha"black-box" application installed on as martphone are already availableand advertised on the market. [4] Agricultural investigationsf ocus on analyzing raw products, with an extensive literaturer ecord corresponding to the qualitative and quantitative analysis of crops, fruits,a nd vegetables. An extensive amount of literature reports on applicationso f portableN IR spectroscopy aimed in this direction are available because miniaturized NIR spectrometers were widely adopted in agriculturala pplications relativelye arly. [66] Often, the methods and generalf eatures of such analyses resemble those described in the paragraph above.C onsiderablea ttention has been given to fruit analyses by portable NIR spectroscopy. [69][70][71][72][73] Amongo thers, the capacity of compact spectrometers to successfully determining quality parameters, [69] monitoring moisture contenta nd controlling drying processes, [70,73] determining and authenticating fruit varieties, [71] as well as quantitatively determining various properties of fruits [70][71][72] were demonstrated in the recent literature. Similarly high attention was given to the advantages of portable NIR spectroscopyi nt he analysis of grains. Recent examples of such applicationsi nclude, for example,t he quantitative determination of the total antioxidant capacityi nv arious gluten-free grains. [74] The study compared the performance of three different portable NIR instruments (microPHAZIR, MicroNIRa nd SCiO) versus ab enchtop spectrometer.T he best results were achievedu pon measuring nonmilled samples with the MicroNIR instrument,a lthough all evaluated spectrometers performed satisfactorily in this application. The SCiO device performed consistently slightly poorer; however,i ts high affordability should be stressed. On the other hand, protein analysisi ng rains provedt ob emore challenging for miniaturizeds pectrometers,p articularly for nonmilled sam-  ples. [75] The suggested reasons for poorer performance may include highers usceptibility to detrimental effectso fs cattering at the sample surface. Thisc oincides with the observations made in an earlier study,i nw hich sample milling led to amore pronounced performance improvement of portable spectrometers. [74] Those conclusions seem to indicate that, in certain applications, portables pectrometers may requirem ore sophisticated sample preparations. The potentialf or nonintrusive examinationso fe ntire seeds by NIR spectroscopy seems to be well-recognizedi na griculture-related studies. Miniaturized spectrometers werer ecently used for analyzing whole canola seeds, [76] coffee beans, [77] barley,c hickpeas, and sorghum. [78] An umber of other ongoing studies have demonstrated portable NIR spectroscopy to be a suitable tool for the quantitative and qualitative examination of variouso ther agricultural products, for example, cherry tomatoes, [79] mulberry leaves, [80] grapes, and peaches. [81] It is noteworthy that NIR spectroscopy is as uitable tool for the analysiso fs oil and water,a nd portablei nstruments markedly improve the ability to perform such monitoringo nalarge scale. Such as cope also fits the topic of environmental studies, which are discussed jointly in Section 3.1.4. Currently,r evolutionary concepts and novel technology lead to the era of precision agriculture. [82] Am ajor role in precision agriculture is played by spectralm ethods of analysisa nd, in particular, remote sensing. This technique is based on miniaturized spectrometers mounted on UAVs (that is,a irborned rones), enabling remote sensing of large areas of agricultural crops. [47,48] The solutions employed are based on specialized technology. However,p rogress made therein in both instrumentation and data-analytical tools contributes to the general advances of portable NIR spectroscopy.

Soil
Analysis of soil has prime importance for environmental research and agriculture. All conventional methods used for this purpose previously,i np ractice, required time-consuminga cquisitiona nd transportation of contaminated soil samples to the laboratory.Therefore, portable NIR spectrometers have rapidly attracted much attentionf or this application.T he analytical feasibility of miniaturized NIR spectrometers for such tasks has been exhaustively studied in the literature. [83,84] For example, O'Brien et al. developed am ethod for analyzing hydrocarbon contamination in soil using aM icroNIR spectrometer. [83] Highly accuratea nd reliable quantitative analysis of hydrocarbon contaminant in soil was possible, in spite of the heterogeneity of this type of sample. Soon after,asimilars tudy by Altinpinar et al. demonstrated that microPHAZIR, aM EMS-Hadamard spectrometer,w as able to rapidly analyze hydrocarbon contaminants (gasoline, diesel, and oil) in soil, while maintaining standard errors of cross validation between 0.3 and 0.5% (w/w), depending on the type of soil. [84] These early studies demonstrated the high potential of miniaturized NIR spectrometers to be employed for on-site, real-time evaluation of contaminateds oil for remediation applications.K een interesti nt he use of NIR spectroscopy for analysiso fs oil quality and contam-inationc an be deducted from an umber of recently published articles (e.g.,r efs. [85][86][87]).
Importantly,s oil characterization itselfr emains of keen interest in agriculture. The classification of soil types and the predictiono ft he physical, biological, and chemical properties of a soil and its quality for cultivation, typicallyr equired complex, resource-intensive methods, often combining field observations and laboratory analysis. [88] Therefore, the application of miniaturized NIR spectrometers for such at ask offers decisive gains.Astudy by Lopo et al. demonstrated the feasibility of using ap ortable dispersive NIR spectrometer (Model NIR-512, Ocean Optics) to classify vineyard soils. [88] Samples of topologically similar origin were identified with 75 to 100 %a ccuracy, depending on the soil type. The performance of ap ortable instrumentw as compared with ab enchtop one;i nterestingly, both delivered similarly good results. Tang et al. compared the performance of four different compact/transportable Vis/NIR instruments, including aN eoSpectra miniaturized NIR spectrometer,i nt he rapid analysis of soil towards predictingv arious properties:c lay,s and, and total carbon( TC) content; cation-exchange capacity (CEC);p H; and exchangeable Mg and Ca content. [89] Despite an arrow operating spectral window,t he NeoSpectra devicep erformed admirably,w ith slightly lower accuracyo nly in predictingT C, sand, and clay contents. [89] In another study,am iniaturized NIR spectrometer (NeoSpectra) was compared with ah igh-performing compactb enchtop instrumento perating over the Vis/SW-NIR/NIR spectral regions (350-2500 nm). The analysisa imed to determine the organic carbon (OC) and TC content in soil, whicha re important parameters of land suitability for crop cultivation. Althought he benchtopi nstrument offered better performance, the Neo-Spectra device yieldeds tatistically acceptable predictions, which werec oncluded to be satisfactory for the intended purpose. However,i nt his case, the spectral analysisw as preceded by in-laboratory sample preparation procedures (drying, grinding, and sieving). Further studies validating miniaturized NIR spectroscopy as af ully in situ method are necessary.S imilar commentsm ay be made about the recent analyses of soil carbon and nitrogen contentsb yu sing the MicroNIR spectrometer. [90] 3.1. 5

. Forensics and security
Portable NIR spectroscopy offers significant potential for detectingi llegal drugs and euphoriants (see Section 3.1.1). Alongside the successes reported in al aboratory setting, studies on establishing am ethodd irectly applicable on-site are currently being performed. [91][92][93] Recent feasibility studies presenting the capacityo fp ortable NIR spectrometers to perform accurate on-sited etermination of heroin [92] and cocaine [93] should be noted. In two other recent investigations,aMicroNIR spectrometer was successfully used to detect amphetamine [94] and cocaine [95] in human oral liquids. The performance of existing instrumentation and data-analyticalt ools seem sufficient for such tasks and may be expected to be furtherr efined with advancements in technology.H owever,f urther systematic investi-gationsa re necessary to ascertain the reliability of the analysis in real-life applications. This seems an indispensable step into adoptingN IR spectroscopy into existings ecurity protocols.
Recently,i tw as demonstrated that portable NIR spectroscopy could successfully be used to authenticateb anknotes. [96] A case study showed1 00 %a ccuracyi nd iscriminating between authentic and counterfeit banknotes, based on aP LS-DA model developed for NIR spectra measured with aM icroNIR spectrometer.
Interestingly,i tw as shown recently that miniaturized NIR spectroscopy was capable of analyzing blood stains, [97,98] and that the sensitivity of the method did not seem to present a major issue. This shows the potentialf or NIR spectroscopy to competew ith IR or Raman techniques, which are already in use for such purposes. [99][100][101][102] NIR spectroscopy has been demonstratedt ob es uitable for the detection of various explosive materials. [103] An on-site NIR analytical approach based on am icroPHAZIR spectrometer was successfully developed and used in analyzing the chemical composition of explosives. [104] Accurate determination of the oxidizer( RDX), stabilizer (Methylene di-tertiary butyl phenol), and plasticizer (isodecyl pelargonate( IDP)) contentsi se ssential to maintain the condition and stability of military warheads that use plasticized explosives. The applicationo faportable NIR spectrometer decreases safety hazards and largely improvest he cost-effectiveness of safety protocols. As tudy indicates that as imilar method based on aN IR mini-spectrometer could be used to control the condition of solid propellant in missiles. [104] More recently,afully on-site screening methodf or the detection of explosives on human hands was developed on the basis of aM icroNIR spectrometer. [105] It was demonstrated to be feasible to detect andc lassify,b ym eanso fP LS-DA, trace amounts of explosives (< 10 mg) in ac omplex matrix of the human skin surface. This accomplishment is an essential contribution along the path of establishing miniaturized NIR technology as at ool for detecting trace explosives.
NIR spectrometers demonstrated their usefulness in various successful applications in forensics and security.T he particular potentialo fp erforming different types of analyses with a single, compact NIR instruments eems to be particularly promising for practical use in this field.

Wood analysis
NIR spectroscopy has been established as ap owerful analytical technique for the assessment of wood properties and quality. [106][107][108][109] However,i ts full potential in this field was uncovered after the appearance of portables pectrometers. This essential breakthrough enabled effectiveN IR on-site analysisi nf orestry. [110][111][112][113] Similar to the previously overviewed fields of application, attention hasb een given to systematic evaluation studies on the applicability and performance, in aq ualitative and quantitative sense, of portable NIRs pectrometers in relation to laboratory equipment.R ecently published examples include the evaluation of miniaturized instrumentation well established in other areas, for example, the popular MicroNIR spectrometer.T he performance and applicabilityo ft his instrument, in comparison with ab enchtop spectrometer,w ere determined for discriminating between wood sawdust samples collected from two Eucalyptus species, E. pellita and E. benthamii. [114] PCA, LDA, and PLS-DA chemometrict ools were used to predict the origin of unknown samples;o nly the last two yieldedt he intended results. While the MicroNIR instrument was concluded to be inferior in this application to that of the benchtop instrument, it still offered ap erformance level that was satisfactory for the intended use. Interestingly,t he authors concluded that the observed differencei np erformance was likely to result from ah ighers catterp rofile observed in spectra measured by the miniaturized spectrometer.T his factor led to a lesser robustnessa nd am ore critical importance of performing spectrala cquisition with care if using am iniaturized spectrometer. [114] Recent literaturei nt his field notes that ap articular requirement for ruggedness and capacity to operate in ah arsh environment appears for the portable instrument. [115] This creates the need for the development of specialized,y et cost-effective, spectrometers by utilizing modules existing on the market. A prototypei nstrument developed by Sandak et al. in 2020 used ar ugged metal case, while maintaining low-powerc onsumption, high cost-effectiveness, and performance levelss uitable for detecting woodd efects. [115] Importantly,t he specialized instrumenti mplements adjustable focusing optics to improvei ts applicability to collecting NIR spectra of chained wood logs that feature profound surface roughness and irregularities. Such surfaces decrease the quality of spectra measured by standards pectrometers, and the surface itself is not suitable for on-site preparation.

Polymers and textiles
Polymer analysis by portable NIR spectrometers may find critical importance in various applications. Recent literature indicates that attention is being given to applying handheld NIR spectrometers for the analysis of variousp olymers. For example, as tudy by Yana nd Siesler comprehensively evaluated four different miniaturized NIR spectrometers (NeoSpectra,N IRONE Sensor, NIRscan Nano, and MicroNIR) in the qualitative and quantitative analyses of several polymers of major importance for industry and the environment:p olyethylene, polypropylene, polyethylene terephthalate (PET), polyvinyl chloride, and polystyrene. [116] Samples in different morphologies (pellets, films, plates, fibers, and powders) were used. The study provided ad etailed analysis of the classification performance of these four spectrometers. In conclusion, however,a ll evaluated instruments successfully identified test samples of all considered polymers. [116] An oteworthy potential for the particularly important adoption of miniaturized spectrometers stems from environmental pollution and growing public concerns about the omnipresence of microplastics in the environment. NIR spectroscopy is ap romising tool for monitoring the threat arising from these pollutants. [117] Portable NIR spectrometersw ere also assessed for their performance and applicability to textile analysis. [118,119] Rodgers et al. demonstrated that three evaluated portable NIR spec-trometers offered good performances in analyzing cotton fiber micronaire. [118] The key quality parameters of this textile, maturity and fineness, were successfully determinedb yu sing miniaturized NIR instruments. Conventional methods of controlling the properties of micronaire require resource-intensive in-lab procedures with tightly controlled environmental conditions. Therefore, the ability to rapidly assess fiber micronaire, maturity,a nd finenesso n-siteu nder ambient conditions by portable NIR spectroscopy is am ajor step forward in textile analysis. Yana nd Siesler emphasized the potential of using handheld NIR spectrometers for textile authentication. [119] They provided evidencet hat miniaturized instrumentsp erformed adequately in identifying most common textile types (poly(acrylonitrile)/ acrylic, wool/cashmere, cotton,e lastane, Kevlar,N omex, PA6/ PA66, PET,a nd silk).

Fuel
The potentialo fm iniaturized NIR spectrometers for fuela nalysis was demonstrated, for example, by Lutz et al. [120] The developed method was able to successfully predict the content of bioethanol in gasoline. Importantly,t his study highlighted the importance of maintaining stable conditions for measurement in the case of portable spectrometers. First, ac ustom transflectance sampling cell was designed for the highly reproducible measurement of liquid samples by using af used silica cuvette with af ixed optical pathlength. The sampling cell was equipped with as pherical goldm irror,f or directly focusing the incident beam onto the detector.T his solution was deemed mandatory for the quantitative analysis of liquid samples with the miniaturized spectrometer being operated by hand. Second,i tw as determined that the temperature drift of the miniaturized spectrometer mustb ec orrected.T his was accomplished throughapurpose-built thermoelectric cooling system for the MicroNIR spectrometer,l argely improving its thermal stability, regardless of theh eat built up in the device itself over the measurement time or external conditions. These custom solutions were concludedt ob ea dvantageous fori ncreasing the accuracy of the collected spectral data sets and for highly reproducible quantification of ethanol in fuel by the MicroNIR spectrometer. [120] The MicroNIR spectrometer,a longside ab enchtop instrument, was also used by da Silva et al. for determiningt he quality parameters for gasoline and diesel/biodiesel blends. [121] This study also aimed to developa ne fficient calibration transfer methodf or use in the practical analysis of fuels, which involved an exhaustive number of chemicals( ethanol, pentane, 1-pentene, hexane, 1-hexene, heptane, toluene, isooctane, xylene,e thylbenzene, n-hexadecane,a nd biodiesel B100 standard;F igure 4) that served as "virtual standards." By proposing av irtuals tandards approach, there was no need for the measurement of standard samples and transfer between distant laboratoriesw as achieved;t his bringsaconsiderable advantage in terms of security regulations in the fuel industry.A more efficient and less-resource-intensive operation of am iniaturized spectrometer on-sitem ay use this approach based on ac alibration model developed under betterc ontrolled condi-