Determination of aldehydes and ketones using derivatization with 2,4-dinitrophenylhydrazine and liquid chromatography–atmospheric pressure photoionization-mass spectrometry

doi:10.1016/j.chroma.2004.08.149

Journal of Chromatography A

Volume 1058, Issues 1–2, 26 November 2004, Pages 107-112

https://doi.org/10.1016/j.chroma.2004.08.149 Get rights and content

Abstract

Atmospheric pressure photoionization-mass spectrometry (APPI-MS) is used for the analysis of aldehydes and ketones after derivatization with 2,4-dinitrophenylhydrazine (DNPH) and liquid chromatographic separation. In the negative ion mode, the [M − H]⁻ pseudomolecular ions are most abundant for the carbonyls. Compared with the established atmospheric pressure chemical ionization (APCI)-MS, limits of detection are typically lower using similar conditions. Automobile exhaust and cigarette exhaust samples were analyzed with APPI-MS and APCI-MS in combination with an ion trap mass analyzer. Due to improved limits of detection, more of the less abundant long-chain carbonyls are detected with APPI-MS in real samples. While 2,4-dinitrophenylazide, a known reaction product of DNPH with nitrogen dioxide, is detected in APCI-MS due to dissociative electron capture, it is not observed at all in APPI-MS.

Introduction

The analysis of aldehydes and ketones in air samples is an important task in the fields of occupational medicine and atmospheric chemistry. Due to their reactivity, a stabilization of the carbonyls prior to analysis is advantageous. Therefore, a large number of derivatization reagents for aldehydes has been introduced in the last decades. Many of these use an aromatic hydrazine group, which reacts with aldehydes and ketones in acidic media under formation of the respective hydrazones [1]. 2,4-Dinitrophenylhydrazine (DNPH) is known for this purpose since more than 20 years [2], [3], [4] and has become the most popular reagent for the analysis of aldehydes. After derivatization (see Scheme 1), the hydrazones are separated by reversed-phase liquid chromatography and detection is performed by UV–vis absorption spectroscopy. Due to its good performance for the analysis of liquid and gas phase samples, the DNPH method has been introduced as national and international standard method by several standardization bodies [5], [6], [7]. As the resolving power of liquid chromatography is limited for the DNPH derivatives and as the number of carbonyl compounds is strongly increasing with increasing alkyl chainlength, UV–vis detection is not sufficient for the analysis of DNPH derivatives of higher aldehydes and ketones with four or more carbon atoms [8]. Furthermore, problems are described for the analysis of formaldehyde in the presence of ozone [9], [10] or nitrogen dioxide [11], as potentially coeluting compounds are formed. Recently, it was found out that the analysis of unsaturated aldehydes in the gas phase may be accompanied by interferences, when a large excess of reagent is still present after sampling and when strongly acidic pH is used [12].

Mass spectrometric detection of the hydrazones by using atmospheric pressure chemical ionization (APCI) in the negative ion mode was introduced in 1998 by Oehme and coworkers [13]. They used an ion trap mass spectrometer and investigated the fragmentation pathways of reference compounds. Soon thereafter, other groups adapted this method to investigate various types of air samples [14], [15], [16]. Oehme and coworkers later refined their method with respect to fragmentation pathways [17] and quantitative aspects [18]. Van den Bergh et al. [19], [20] applied the method to study oxidation products formed in the reaction between alpha- and beta-pinene and OH radicals. Manini and coworkers [21] determined patterns of biologically relevant aldehydes, e.g., acrolein or 4-hydroxynonenal, in exhaled breath using DNPH derivatization and LC with tandem mass spectrometric detection. Richardson et al. [22] and Zwiener et al. [23] determined aldehydes by LC–MS in ozonated drinking waters and outdoor swimming pools after chlorination, respectively.

Four years ago, Bruins and coworkers introduced atmospheric pressure photoionization (APPI), a new method for the analysis of non-polar analytes by LC–MS [24]. A vacuum ultraviolet (VUV) lamp is used as source of photons with an energy of approximately 10 eV. A dopant is added to obtain a great abundance of dopant photoions, which then react with the analytes. The ion source is similar to an APCI source, with the major difference that the corona discharge needle is replaced by a VUV lamp. The method has rapidly become commercially available for the state-of-the-art instruments of most major LC–MS manufacturers, and has already been covered in recent reviews [25], [26]. The number of publications in this field is therefore increasing rapidly, with some papers being devoted to fundamental investigations, e.g., about negative ion-APPI-MS [27], solvent [28] or dopant [29], [30] effects. Most papers in this field, however, focus on novel applications for the analysis of analytes with low polarity, e.g., flavonoids [31], anabolic steroids [32], idoxifene and its metabolites [33], hydrophobic peptides [34] and even polycyclic aromatic hydrocarbons [35]. However, nitroaromatics have not been studied by APPI-MS yet, and the DNPH derivatives are particularly interesting because of their broad application and the established use of APCI-MS for their analysis. For this reason, a method for the determination of aldehydes based on DNPH derivatization, LC separation and APPI-MS detection has been developed. The results are compared with those obtained with APCI-MS detection.

Section snippets

Chemicals

DNP hydrazone standards (DNPH derivatives of formaldehde, acetaldehyde, acrolein, 2-butanone, p-tolualdehyde and 1-hexanal) were synthesized according to [8]. Solvents for LC were acetonitrile and water, both LC–MS grade, purchased from Biosolve Ltd. (Valkenswaard, The Netherlands). DNPH coated sampling cartridges were purchased from Supelco (Bellefonte, PA, USA).

Instrumentation

For the LC–MS setup, an Agilent Technologies (Waldbronn, Germany) HP1100 liquid chromatograph consisting of binary gradient pump

Results and discussion

First investigations showed already that the DNPH derivatives can be detected well using LC–APPI-MS without dopant. As in case of APCI-MS, the most abundant ion for the derivatives is the [M − H]⁻ pseudomolecular ion in the negative ion mode. An APPI(−) mass spectrum of the acetaldehyde DNPH derivative is presented in Fig. 1. The [M − H]⁻ peak with an m/z = 223 is observed with highest intensity, and without consideration of quantitative aspects, the mass spectrum is identical to that obtained

Conclusions

Dopant-free APPI-MS has shown to be an attractive alternative to APCI-MS, as the limits of detection typically are slightly lower and more different carbonyls can be detected at low levels in real samples from automobile exhaust and cigarette smoke. As no dopant is required, the technical effort for both methods is identical and routine analysis with APPI-MS in well possible. As could be expected, DNPA, which is ionized by dissociative electron capture in APCI-MS, is not detected at all in

Acknowledgement

Financial support by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO, Den Haag, The Netherlands) is gratefully acknowledged.

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Determination of aldehydes and ketones using derivatization with 2,4-dinitrophenylhydrazine and liquid chromatography–atmospheric pressure photoionization-mass spectrometry

Abstract

Introduction

Section snippets

Chemicals

Instrumentation

Results and discussion

Conclusions

Acknowledgement

Chemosphere

J. Chromatogr.

J. Chromatogr. A

J. Chromatogr. A

J. Chromatogr. A

J. Am. Soc. Mass Spectrom.

Int. J. Mass Spectrom.

J. Chromatogr. A

J. Chromatogr. A

J. Chromatogr. A

Anal. Chim. Acta

Fresenius J. Anal. Chem.

J. Chromatogr. Sci.

Anal. Chem.

Anal. Chem.

Environ. Sci. Technol.