High-order calibration for the spectrofluorimetric determination of pesticides based on photochemical derivatization. A solution of the problems of inner-filter effects and matrix interferences in complex environmental water

https://doi.org/10.1016/j.chemolab.2016.05.004Get rights and content

Highlights

  • Emission-excitation-irradiation time-volume fourth-order data array was measured.

  • An alternating fitting weighted residue quadrilinear decomposition algorithm was developed.

  • It is based on the incompletely unfolded matrices along two different modes.

  • It has been further extended to address the matrix effect from environmental water.

  • Quantifying pesticides with no or weak native fluorescence was realized.

Abstract

The paper reports a simple and effective spectrofluorimetric method for the quantitative analysis of the pesticide with no or weak fluorescence in complex environmental water using photochemical derivatization and high-order calibration. Through constructing four objective functions according to the pseudo-fully unfolded matrices of quadrilinear model along two different modes, a new third-order calibration algorithm named as alternating fitting weighted residue quadrilinear decomposition (AFWRQLD) has been proposed with excellent performance. Most importantly, since the matrix effect from environmental water induces partial fluorescence quenching of the fluorescent photochemical derivative of imidacloprid, AFWRQLD has been further extended for the analysis of fourth-order emission–excitation–UV irradiation time–volume data array. Satisfactory results has been obtained for the quantitative analysis of artificial river samples spiked with imidacloprid by extracting the extra mode to extract the relationship between the target analyte and the volume of environmental water. The spectral and UV irradiation time profiles yielded by the methodology are in good agreement with experimental observations. The average recovery is 93.9 ± 5.2% with a root-mean-square error of prediction of 7.4 ng mL 1 and the detection limit of 16.0 ng mL 1. Due to inner-filter effects and matrix interferences widely existing in increasingly severe aquatic environment, the strategy can be expected to open a new avenue for further insight into the health of aquatic ecosystem.

Introduction

The increasing worldwide contamination of surface and ground waters with pesticides, as a result of the widespread use, becomes one of the severe environmental problems facing humanity [1], [2], [3], [4]. Although most of pesticides are present at low concentrations, they are toxic by design and the potential hazard for aquatic life and human health should not be neglected, such as chronic health effects including carcinogenesis, neurotoxicity as well as reproductive and developmental effects caused by long-term exposure to pesticides [5], [6], [7]. Therefore, it is of very great significance to explore highly sensitive and accurate quantification methods for analytically monitoring the transport, fate and effect of trace-level pesticides in aquatic environment.

To date, there are a growing number of analytical methods for the determination of pesticide residues in various environmental samples including water, sediments, air, and soil [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Among them, liquid chromatography coupled to mass spectrometry (LC–MS) as a high-performance technique has gained wide acceptance for trace analysis [9], [10], [11], [12], [13], [14]. However, it is well-known that the LC–MS method requires skilled technicians and the use of expensive instrumentation both of which are not in the access of many laboratories in developing countries. The spectrofluorimetry methods as an excellent alternative have proved capable of providing high sensitivity and clean spectrum acquisition, and its cost is so much cheaper than MS detection that it can be easily available in the general case. In addition, the spectrofluorimetric equipment is relatively easier to run and especially appropriate for inaccessible regions in the area of that the trained technical personnel are not enough to be distributed to the analytical systems. The fluorescence method opens another opportunity for trace analysis of pesticides. Although the number of pesticides presenting an intrinsic fluorescence is limited, there are various kinds of derivatization techniques such as photochemical, chemical and electrochemical methods, through which the analyte with no or weak native fluorescence can converse into strongly fluorescent species and then be measured via the fluorescence detection (FD) [4], [19], [20], [21], [22]. Compared with chemical/electrochemical derivatization, the photochemical derivatization is a better technique to yield high the fluorescent photoproduct due to these two advantages: (1) UV radiation for relevant analysis is sufficiently simple and it can make the analyte(s) exhibit strong fluorescence with the use of the direct irradiation of the liquid solution containing the analyte(s), and (2) their chemical structure(s) of the fluorescent product(s) formed after UV irradiation is/are not necessary to be identified, provided that photo-chemically induced fluorescence signals are reproducibly yielded [23]. However, if without thorough previous separation procedures, when using the spectrofluorimetric method to directly analyze the pesticides in complex environmental samples, it will face the challenges such as serious interferences or the matrix effect (fluorescence quenching/enhancement which probably exists in systems).

To address the above-mentioned challenges, it could be a feasible and valid strategy by introducing more extra factors (i.e., analytical time, pH, sample volume, etc.) as new analytical dimensions to construct three-way or higher-way data which exploits more information inherent in sample in conjunction with chemometrics tools [16], [17], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]. In the context of chemometrics, second-order calibration as a famous method, which allows the accurate quantification of analyte(s) in samples with unexpected components that are potential interferences, has been obtained wide application [24], [25], [26]. However, since third- or higher-order data introduce more information, third- or higher calibration methods can allow for better performance than second-order calibration, such as a theoretical increase in the predictive ability of the models, a high tolerance for heavily collinear data, the growing sensitivity and selectivity, and so on. In recent years, a series of new third-order calibration methods have been developed to resolve third-order data of complex samples [16], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]. Some of them have also been extended for five-way data arrays in particular cases [17], [18]. Generally, they can be classified into two types. One is built on the direct least-squares procedure, for example, trilinear least-squares algorithm coupled with residual trilinearization (TLLS/RTL) [19], unfolded and N-way partial least-squares (N-PLS) and unfolded principal component analysis (U-PCA) combined with RTL (U-PLS/RTL [46], N-PLS/RTL [47] and U-PCA/RTL [48], respectively). The other is based on the iterative least squares, including 4-way parallel factor analysis (FPARAFAC) [5], four-way self-weighted alternating normalized residue fitting (4-way SWANRF) [14], alternating penalty quadrilinear decomposition (APQLD) [15], alternating weighted residue constraint quadrilinear decomposition (AWRCQLD) [17], alternating quadrilinear decomposition (AQLD) [16] as well as regularized self-weighted alternating quadrilinear decomposition (RSWAQLD) [8]. These algorithms have been used to successfully extract the qualitative and quantitative information of single component or multi-components in complex practical systems. However, the algorithms belonging to the second type whose objection functions are constructed according to the fully stretched matrices along single mode present relatively slow convergence rate, such as FPARAFAC and 4-way SWANRF. In addition, they are generally sensitive to the over-estimated number of components used in calculation. But this problem can be addressed to some extent when the exact number of components is provided by some kinds of diagnostics, such as core consistency [49], split half analysis [50], the ADD-ONE-UP truncating and fitting method (ADD-ONE-UP) [51] and so on. By contrast, the approaches, involving APQLD, AWRCQLD, AQLD and RSWAQLD, which are based on the pseudo-fully unfolded matrices along single mode have been considered as great alternatives to overcome the above-mentioned drawbacks. However, the penalty factors of chosen in both of APQLD and AWRCQLD need to be very large (more than 106 as the authors suggested), sometimes the convergence of them may be slow and meaningless, especially when the collinearity is extremely serious. And the application of AQLD is restricted when the noise level is too high. It follows that no best alternative with all the advantages can successfully resolve different third-order data problems in any cases, up to now. In other words, the further development of third-order calibration algorithm is urgently needed and full of significance.

In this paper, we propose a new third-order calibration method, namely alternating fitting weighted residue quadrilinear decomposition (AFWRQLD), whose four objective functions are built according to the pseudo-fully unfolded matrices along two different directions. To the best of our knowledge, this is the first attempt to develop a third-order algorithm based on the incompletely unfolded matrix forms of the quinquelinear model which are stretched along different modes. A detailed study comparing AFWRQLD with FPARAFAC and AWRCQLD by using a set of simulated data and an already published experimental data array was carried out to reveal the merits of the proposed algorithm. In addition, combined with photochemical derivatization, the newly developed method has been employed to explore the spectrofluorimetric analysis of quantifying the level of a model pesticide, imidacloprid, with no native fluorescence in environmental water. Since the matrix effect from environmental water leads to the partial fluorescence quenching of the fluorescent photochemical derivative of imidacloprid, AFWRQLD has been further extended for the resolution of the emission–excitation–UV irradiation time–volume of environmental water fourth-order data array. The satisfactory results demonstrate that the combination of the new algorithm and photochemical derivatization can achieve highly sensitive and accurate quantitative detection of the pesticide with no native fluorescence in complex environmental water.

Section snippets

Different forms of the quadrilinear component model

An emission–excitation fluorescence matrix (EEFM) with size of I × J for the lth sample at the kth UV irradiation time point (UVITP) can be experimentally obtained with I emission wavelengths and J excitation wavelengths (Fig. 1A, left). If the EEFMs for the samples with the same concentration level at K different UVITPs are collected along the K mode, a three-way data array (I × J × K) can be produced (Fig. 1A, middle). If L samples, consisting of L0 calibration samples and LP prediction samples,

Simulated data

A simulated four-way data array produced by a two-dimensional liquid chromatography system with diode array detection on ten samples with four species which has been given in Ref. 42 was employed to evaluate the performance of the newly proposed algorithm. The first chromatographic profiles a1a4 were produced by:a1,i=gs2412i,a2,i=gs2413i,a3,i=gs2414i,a4,i=gs2415i,where i = 1, 2,. .., 30; gs (x, a, b, i) refers to the value at x of a Gaussian function with standard deviation a and centre b, i.e.gs

Choice of constraint coefficients of pa, pb, pc and pd

The constraint coefficients of pa, pb, pc and pd were firstly determined according to the results yielded by using the AFWRQLD algorithm to decompose the simulated four-way data arrays when running 100 times. The results indicate that the algorithm is insensitive to the excess factors used for calculations and exhibits fast convergence rate no matter how the parameters vary. However, when the parameters pa = pb = pc = pd < 104, the AFWRQLD algorithm sometimes suffered from “swamps” in the convergence

Conclusion

In summary, a quantitative spectrofluorimetric method by photochemical derivatization and high-order calibration has been developed for the determination of imidacloprid as a model pesticide in river water. A new third-order calibration algorithm AFWRQLD with excellent performance has been firstly proposed by constructing four objective functions according to the pseudo-fully unfolded matrices of quadrilinear model along two different directions. In addition, the fourth-order AFWRQQLD algorithm

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21565012, 21205021 and 21365009), the Guangxi Natural Science Foundation (No. 2014GXNSFAA118033), the Scientific Research Fund of Guangxi Education Department (No. 201203YB090), the Open Foundation of State Key Laboratory of Chemo/biosensing and Chemometrics in Hunan University (No. 2014005) as well as the Project of High Level Innovation Team/Outstanding Scholar and Key Laboratory of Food Safety and Detection in

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