Pesticide Residue Analysis in Foods

ABSTRACT

The common methods that are employed for analyses of pesticide residues in food are LC-MS, LC-MS/MS, GC-MS and GC-MS/MS. As there are no analyses that would separate, identify and measure the exact content of pesticides usually due to high sample matrix effect it is crucial to define what methods are most effective for different commodities as the amount of pesticides used in the food production is on the rise. Advantages and drawbacks of these methods are also reviewed, especially, efficiency, cost and simplicity. Moreover, stages that significantly influence the final analysis such as extraction techniques and sample preparation are as well briefly discussed; mostly QuEChERS technique as it is considered as the most efficient one. Finally, the review focuses on new developments to achieve efficiency, automation, miniaturization and environmentally friendly practice.

LIST OF ABBREVIATIONS

APCI            atmospheric-pressure chemical ionization 

dSPE            dispersive solid phase extraction

ESI            electrospray ionization  

EI            electron impact

GC            gas chromatography

GC-ECD        electron capture detection

GC-MS          gas chromatography-mass spectrometry 

GC-MS/MS    gas chromatography-tandem-mass spectrometry

GC-NPD        gas chromatography-nitrogen phosphorus detection

GC-TSD gas chromatography-thermionic sensitive detection

GPC            gel permeation chromatography 

HRMS            high resolution mass spectrometry 

LC-MS liquid chromatography-mass spectrometry  

LC-MS/MS    liquid chromatography-tandem-mass spectrometry

MAE              microwave assisted extraction

         mLC            micro liquid chromatography or micro-flow liquid chromatography

MRLs            maximum residue limits

MRM             multiresidue method/multiple reaction monitoring

PLE            pressurised liquid extraction

SFE            supercritical fluid extraction

SPE            solid-phase extraction

SPME            solid-phase microextraction

TOF            time-of-flight

QuEChERS Quick, Easy, Cheap, Effective, Rugged and Safe method

INTRODUCTION

Pesticides are commonly used in agriculture not only to control pests but also to control weeds, insects, fungi, and bacteria. If not controlled the use of pesticides might be increased as weeds, pests, etc. may become resistant and hence higher amount or stronger concentration of specific chemical might be required. This, in turn, can cause long-term health risks and allergy. For instance, in 2005, glyphosate (the most common herbicide) and two insecticides such as malathion and diazinon, were classified by the International Agency for Research on Cancer as probable carcinogens (IARC, 2015). This gradually raised public awareness and created significant discussion on the safety of pesticides on human health. For this reason, Regulation No 396/2005 on the allowed and statutory maximum residue limits (MRLs) of pesticides in food has been implemented by the European Commission (EC) from 2008 onwards. Since then pesticide analyses have become an important part of food safety as it has become necessary to control the level of used pesticides in agriculture; for food crops and for animal feed, which should be kept at the lowest possible level to ensure that food is safe for consumption. Another important institution that controls the maximum level and toxicity of pesticide residues in food is the European Food Safety Authority (EFSA) (European Communities, 2008).

As many different pesticides are used during the entire crop production the analysis should be able to identify and quantify various pesticides’ residues from one sample during one analytical run. Such methods are known as multiresidue method (MRMs) and are a combination of various stages such as different extractions, sample cleaning and final analysis (FDA, 2018). Moreover, MRMs should be time efficient, relatively easy and should guarantee good reproducibility. Most common MRMs are GC-ECD, GC-NPD, GC-TSD, GS-MS, GC-MS/MS, LC-MS, LC-MS/MS (Alder et al. 2006; González-Curbelo at al., 2012). However, this review will only focus on LC-MS, LC-MS/MS, GC-MSand GC-MS/MS as these methods are most commonly used for detection of pesticides.

METHODS

To conduct a proper analysis it is crucial to choose a method that would be appropriate for polarity, volatility and chemical composition of targeted compounds. In addition, to enhance the detection of pesticides the methods that are discussed below are coupled with mass spectroscopy that is used to measure and identify the characteristics of individual molecules.

In any method, the first important step is to choose a technique for a sample extraction which should provide a high level of selectivity as in this way the need for sample clean-up will be reduced or even not necessary what would reduce cost, time and labour requirements of the analysis (Wilkowska and Biziuk, 2011). There are many techniques used for extraction such as SPE, SFE, GPC, MAE, SPME or PLE but mostly used and the most promising method that also cleans up the sample is called QuEChERS. This method is combined with dSPE what makes it highly efficient as the time is reduced and amount of solvent used is smaller when compared to other methods (González-Curbelo et al., 2012; Zhang et al., 2019). Furthermore, the manual two-step QuEChERS technique is considered as a base of further improvement of pesticide analyses and for that reason one-step automatic QuEChERS was developed and tested in a study by Wang et al. (2017). It was proved that the method was as good as the previous one but the time and labour requirements were decreased; hence, the simplicity was achieved.

GC-MS and GC-MS/MS

It was reported by many publications that GC-MS has very good selectivity and sensitivity especially when it is used with triple quadrupole mass analyser (GC-MS/MS) (see table 1) (Alder et al., 2006; Hernández et al., 2013). According to Masiá et al. (2016), the data analysis cannot be targeted in GC-MS but might identify a great degree of analytes even the unexpected ones. What is more, Dömötörová and Matisová (2008) reported that there is no need for sample clean up when the low-pressure GC-MS/MS is used as a final analyser.

However, as stated by Wang et al. (2017) matrix enhancement occurred, for the majority of the tested pesticides, what made the results less efficient. Furthermore, Alder et al. (2006) stated that another limitation of GC-MS/MS is that it lacks soft ionization  that could be used for good reproducibility of many different pesticides. Hard EI ionization is mostly used with GC-MS and it is also not sufficient as it produces low-intensity ions. Nevertheless, many years ago GC-MS/MS was still not well developed and Hernández et al. (2013) claim that tandem MS with new ionization such as APCI or more sophisticated analysers, for example, triple quadrupole mass or Qtrap have improved selectivity and sensitivity of GC-MS/MS.

LC-MS and LC-MS/MS

Masiá et al. (2016) reported that LC-MS and LC-MS/MS are typically used to analyse targeted pesticides. However, new advances in technology such as HRMS analysers (for example TOF, Orbitrap) enabled LC to identify non-targeted compounds.

As stated in Alder et al. (2006) the LC-MS is not used very often as produces strong signals that interfere in the detection of pesticides. Also, Masiá et al. (2016) claim that the biggest challenge is the high matrix effect in LC. The LC coupled with tandem MS/MS is more efficient and, hence, is mostly used as there is no need for sample

cleanup (Alder et al., 2006). Therefore, the cost and time of analysis are reduced and it is also labour efficient. However, it might not be the case if the sample matrix is too complex and might require to be purified what in turn would increase the cost and time of the analysis. Moreover, ESI or APCI soft ionization and triple quadrupole mass or Qtrap analysers combined with LC-MS/MS are the best choices for the detection of pesticides by LC (Alder et al., 2006; Hakme et al., 2018).

However, ESI might cause ion suppression or ion enhancement and as a result the detector will provide inaccurate data (Hakme et al., 2018). Wang et al. (2017) further confirm that matrix suppression occurred during LC-MS/MS for the majority of the tested pesticide.

Table 1 Methods used for determination of pesticide residues invarious food 

             matrixes and their sensitivity.

COMPARISON

Table 2 LC-MS versus GC-MS

FUTURE WORK

There are studies that seek miniaturization of the equipment what in turn would decrease the time and cost of analyses. According to Masiá et al. (2016) using smaller columns in LC (mLC) would provide more efficient data as the amount of the required sample and the retention time would be decreased. Also, coupling with MS would be less complex. It would also decrease the amount of material used during the mobile and stationary phases which is believed to have an adverse effect on the environment. This creates a necessity for analytical chemistry to be more environmentally friendly.

As reported by Armenta and de la Guardia (2016) there are many aspects that should be taken under consideration to achieve green chromatography and, among others,

that would involve reduction or elimination of the sample cleanup, using a lower number of solvents/reagents, decrease waste products and leaning towards automation and miniaturization.

Magnetic nanoparticles have a promising future in extraction and separation of pesticides. According to Smith (2018) this method is quick, does not involve expensive sample preparation and the same nanoparticle may be utilized up to 30 times.

Until now this method was used on vegetables to detect pyrethroids and the result showed that it can detect low concentrations (0.02 ng/g). Furthermore, Yaneva et al. (2018) reported that Magnetic-nanoparticles-based fluorescent immunoassay was tested in the detection of two insecticides (dichlorvos, paraoxon) in milk and the quantification of residues was acceptable. It is believed that this technique might be further developed and used for a wider range of foods and might also get fully automated.

Conclusion

As the demand for food is increasing with an increasing population the amount of used pesticides to ensure high yield is also on the rise. Many methods for detection of pesticides are expensive and time-consuming and are also not environmentally friendly. The sample extraction is the first step to be improved to make the methods more effective. Therefore, many studies aim to develop extraction methods which would be quick, simple, cost-effective, automated and efficient in the determination of pesticides. Until now, chromatographic methods are mostly used in the detection of pesticide residues. As they are frequently coupled with detectors the direct focus is based on examination of the efficiency of different detectors in relation to different sample matrices. To increase the efficiency of analyses even more different ionization and different analysers are also tested for their performance in relation to specific sample matrices.

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