Optimization of a modified QuEChERS method by means of experimental design for multiresidue determination of pesticides in milk and dairy products by GC–MS
Introduction
Pesticide residues are organic contaminants which are able to damage to endocrine, nervous and immune system as well as causing to cancer, by accumulating in the fatty tissue in human body. They can easily reach the food chain and concentrate in human and animal tissues. For food quality assurance, maximum residue limits (MRLs) of pesticides have been established by European Community for food and animal products (EC No 396/2005) [1].
Due to low-level MRLs set by regulatory agencies, it is necessary to develop efficient, accurate and sensitive analytical methods. Multi-residue pesticides determination is primarily based on gas chromatography–mass spectrometry (GC–MS) [2,3], GC-electron capture detector [[4], [5], [6]], GC-flame photometric detector [7,8], GC–MS/MS [9,10] and liquid chromatography tandem MS (LC-MS/MS) [11,12]. GC–MS is a powerful technique to separate, identify and quantify trace components in a wide variety of complex sample mixtures. In particular, GC–MS is a practical analysis method for multi-residue pesticides having a high degree of volatility, less polar characteristic and thermal stability [11].
Milk is known as an essential nutrient source especially for infant, school-age children and elders and is consumed as both raw and dairy products. Milk-producing animals, such as sheep, goat and cow may accumulate pesticide residues by means of carryover processes from contaminated aliments including feed, grass and corn silage or various sources such as water, top-layer soil, and air [13]. Thereafter, these accumulated pesticide residues are excreted through milk fat and by the consumption of contaminated milk or dairy products by human beings, serious hazard to human health can easily appear. Hence, monitoring programs of milk and dairy products with regard to pesticide residues are carried out in many countries.
One of the major drawbacks with respect to analysis of these contaminants in milk and dairy products is the high protein and fat content frequently be able to interfering with the analytical determination. Accordingly, sample pretreatment and/or extraction process including several clean-up steps to remove interference from the matrix can be required, which can take a long time and be tedious. Many sample preparation methods have been reported in the literature for the determination of pesticide residues such as solid-phase microextraction (SPME) [14,15], dispersive liquid–liquid microextraction (DLLME) [16], gel permeation chromatography (GPC) [17] and QuEChERS (quick, easy, cheap, effective, robust and safe) [18]. The QuEChERS method consists of an extraction step with acetonitrile followed by a clean-up step involving a dispersive solid-phase extraction (dSPE) [19]. Instead of the original QuEChERS method which was introduced in 2003 for determining multi-residue pesticides in plant foods such as fruits and vegetables, currently there are a wide variety of modified QuEChERS method for different sample matrices including animal orgin foods such as fish [20], chicken [21], egg [22], dry samples such as wheat, white flour and bran [23], honey [24], milk [25,26] and soil-sediment [27] in the literature. While primary secondary amine (PSA) and C18 are frequently employed as conventional dSPE sorbents for the purpose of increasing the clean-up efficiency in samples with ≥2% of fat, novel dSPE sorbents including graphene [28], multi-walled carbon nanotubes [29], graphitic carbon nitride [30], zirconium oxide [31], chitosan [32,33] and diatomaceous earth [24,[32], [33], [34], [35]] have been also used to improve efficiency of sample pretreatment for different kinds of pesticides and matrices. Among them, diatomaceous earth which is cheap, having high surface area hence very high absorption capacity and with low specific weight was used as dispersant in the extraction of pesticides mainly cereals [[32], [33], [34], [35]], vegetables and fruits [24]. To the best of our knowledge, diatomaceous earth has not yet been employed in milk and dairy products for the purpose of obtaining high recoveries in QuEChERS.
Selection of extraction solvents and sorbents, pH, amount of salts and buffers and so on in dSPE methods is so important that they should be optimized in order to have high efficiency. Statistical optimization tehcniques or Response Surface Methodology (RSM) are able to predict and determine optimum conditions of the desired process by statistical calculations [25,36,37]. This methodology is more useful and efficient compared to a univariate optimization also called One-Factor-at-a-Time (OFAT) and moreover, has less number of experimental run [38].
In this stuy, firstly, indepented variables such as acetonitrile volume, amounts of sample, C18, diatomaceous earth, magnesium sulphate, sodium acetate and sodium chloride, percent of formic acid and centrifugation time in the sample pretreatment method were investigated by Plackett-Burman design which is a two-level fractional factorial screening design. Then, Box-Behnken design being a type of RSM was used in order to optimize significant variables of the modified QuEChERS method for simultaneous GC–MS determination of 25 pesticides in milk and dairy products such as cheese, yogurt and cream.
Section snippets
Reagents and chemicals
Pesticides and triphenyl phosphate (TPP) internal standard were purchased from Dr. Ehrenstorfer GmbH (Augsburg, Germany). Acetonitrile (HPLC grade), methanol (HPLC grade), sodium chloride (NaCl), sodium acetate (NaAc), 98% formic acid (FA), magnesium sulphate anhydrous (MgSO4) and sodium sulphate anhydrous (Na2SO4) were purchased from Merck (Darmsdart, Germany) in analytical grade. C18 was supplied from Sigma-Aldrich Chemie GmbH (Sternheim, Germany) and diatomaceous earth was supplied from
Experimental design approach and optimization process screening design-Plackett-Burman design (PBD)
For effective sample preparation and hence acquiring high recoveries for all analytes, preliminary studies were determinative in the selection of extraction parameters. The extraction efficiency is closely associated with the organic solvents used, due to the nature of chemical properties of the pesticide residues. In this respect, the effects of acetonitrile, THF and their various mixture containing different ratio of formic acid on the extraction efficiency were investigated. It was observed
Conclusions
A reliable modified QuEChERS method was developed for the simultaneous GC–MS determination of 25 pesticide residues in milk and dairy products. The optimal experimental conditions for the proposed extraction method were obtained using Box-Behken experimental design followed by Plackett-Burman design on the purpose of optimization and screening. Respectively. The most significant variable in the optimization study was found as the amount of sodium acetate. The usage of diatomaceous earth
Conflicts of interest
None.
Acknowledgments
This study was supported by Yildiz Technical University Scientific Research Projects Coordination Department. Project Number: 2014-01-02-DOP02.
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