During the last 15 years or so there have been substantial improvements in the ability to screen pesticide residues in food. Typically 10 times more analytes, at 10 times lower concentrations, with more assured identification, even in complex matrices. These advancements have been driven by the introduction of generic extraction methods in parallel with new instrument technologies. Using a combination of LC-MS and GC-MS it is common for laboratories to analyse 400-500 pesticides and metabolites, ideally each at the 10 ng/g concentration in a single extract. Still, difficult challenges remain, not least the analysis of the polar ionic pesticides.
Historically, pesticides such as glyphosate, glufosinate, fosetyl, and alike were analysed individually using specialist methods involving derivatisation or ion pairing to overcome unwanted interactions during extraction and chromatographic separation. Consequently these frequently-used pesticides were infrequently tested because of the additional costs involved.
To help bridge the gap, a method based on extraction with acidified methanol without partition or clean-up, the so called Quick Polar Pesticides (QuPPe) method, has been developed by the European Reference Laboratory responsible for single residue methods EURL-SRM. The QuPPe extracts often contain high amounts of co-extractives which can impact on the robustness of the method, thus isotopically labelled standards are recommended to improve the precision of analysis. Despite these imperfections, the method is used by laboratories in Europe and has resulted in recent findings of residues of chlorate, perchlorate, phosphonic acid and glyphosate in food and beverages.
Implications and regulatory response to some these findings
In Europe and in 2014, the EURL-SRM reported approximately 26% of 1,661 samples tested contained residues of chlorate above the Maximum Residue Level (MRL) of 0.01mg/kg, a surprise at the time. In 2015 the European Food Safety Authority (EFSA) Panel on Contaminants in the Food Chain (CONTAM Panel) published a scientific opinion on the presence of chlorate in food. The chronic dietary exposure to chlorate can affect the thyroid gland and inhibition of iodine uptake especially in infants and vulnerable groups. The CONTAM Panel assumed that chlorate in food results mainly from the use of chlorinated water for food processing (e.g. washing) and from the disinfection of surfaces and food processing equipment coming into contact with food. The EFSA panel also concluded that more occurrence data were needed for foods for which there are currently no data (e.g. animal derived foods, tea, coffee, beer). More data was also needed on chlorate in foods where there are currently indications of high chlorate levels such as infant/follow-on formula and yoghurt.
Another surprise was the fact that 36% of samples containing chlorate also contained perchlorate, but without any obvious correlation. As a follow-up a Commission Regulation (EU) 2015/682 on the monitoring of the presence of perchlorate in food was issued on of 29 April 2015 and a scientific opinion on the risk to human health of the presence of perchlorate in food issued by EFSA on 26 May 2015. Because of tension in the market a European Commission statement updated on 23 June 2015 revised levels perchlorate to those considered as low as reasonably achievable applying good practices. These documents state that the use of certain fertilizers containing high levels of perchlorate is an important contributor to the presence of perchlorate in fruits and vegetables.
However, other sources may also contribute to the presence and further investigations are needed to have a better view of the different sources of contamination of food, in particular fruits and vegetables, with perchlorate.
Another controversial polar pesticide is glyphosate, the world’s most widely used herbicide. This compound has come under intense scrutiny after the International Agency for Research on Cancer (IARC) that informs the World Health Organization (WHO) on cancer risk factors, classified glyphosate as a ‘probable carcinogen’ last March 2015. By contrast, EFSA concluded glyphosate unlikely to cause cancer. What is not in doubt is the fact that the use of glyphosate as a weed killer on genetically modified crops (soy, wheat) tolerant to glyphosate, and as a desiccant on cereal crops, results in a high frequency of residues in cereal-based products such as bread and breakfast cereals. The permitted levels are typically higher than for other pesticides. Recently in Switzerland, some retail chains have removed glyphosate products from the shelves because of public pressure, and in Germany there have been media reports of glyphosate in beer although health concerns were allayed in an assessment by the BfR. In the US, a report by the Government Audit Office criticised the responsible government agencies (EPA, FDA and UDSA) for the lack of testing for glyphosate residues in food. Reuters reported: “Given increased public interest in glyphosate in the US, the EPA may recommend increased sampling for glyphosate in the future.”
Information relating to only 3 polar pesticides have been mentioned briefly yet there are more than 20 such pesticides and metabolites identified in the scope of the QuPPE method. Despite the controversy and uncertainties surrounding much of the information available, one obvious need is for more sensitive and robust methods to enable more cost effective monitoring to provide more data underpin a more valid assessment of the frequency and levels of residues in the food supply.
Is ion chromatography-mass spectrometry the way forward?
The use of high pressure, high resolution ion-exchange chromatography (IEC) coupled with a triple quadrupole mass spectrometer could prove to be the answer for the determination of polar ionic pesticides in QuPPe extracts. The charged nature of the polar analytes which is problematic for more conventional separations can be turned into a huge advantage using IEC. Not least because electrolytically reagent-free suppression of the mobile phase, post column, allows the use of high capacity analytical columns which are tolerant to the matrix-heavy QuPPe extracts. Also, the use of reagent free eluent generation only requires addition of water to generate reproducible elution gradients. To achieve reproducible results at low ng/g concentrations compliant with regulatory requirements and the current SANTE Analytical Quality Control Guidelines (SANTE/11945/2015). The development of new robust methods is in progress so watch this space.
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