A glowing reference for food-borne pesticides
[October 15, 2008]

 

Source of Article: http://www.spectroscopynow.com/coi/cda/detail.cda?id=19634&type=Feature&chId=4&page=1


The control of pesticides on foods bound for sale is an ever-present problem for the food industry with dire consequences for consumers if the protection systems fail. A recent publication by the British Crop Protection Council estimated that there are about 860 active compounds in current pesticides, belonging to more than 100 classes. Many of these are toxic, so regulatory authorities have set maximum residue limits (MRLs) for safety purposes, those of the EC ranging from several ppb to ppm.

Traditional methods for measuring the levels of pesticides in all types of foods involve extraction, purification and, in some cases, enrichment steps. These are generally followed by analysis using one of the hyphenated techniques such as GC/MS or LC/MS, often in tandem mass spectrometry mode. Although these methods are accurate with relatively low limits of quantification and detection, the often lengthy sample preparation and resultant extended analysis times are undesirable in quality control and testing laboratories.

These limitations have been tackled by Renato Zenobi and colleagues from the Department of Chemistry and Applied Biosciences at ETH Zurich, who turned to a novel ambient mass spectrometry technique. Earlier in 2008, the development of the flowing afterglow atmospheric pressure glow discharge (APGD) source was announced by the group run by Gary Hieftje at Indiana University. Zenobi built a version of the source in his own lab and attached it to a quadrupole-time-of-flight mass spectrometer.

A glow discharge was generated in a helium flow at atmospheric pressure using a tungsten rod as cathode and a stainless steel plate as anode. Within the discharge, the metastable species and other excited species that were formed reacted with ambient air to produce protonated water clusters and radical cations such as NO+.. The clusters, in turn, ionised the analyte molecules in a chemical ionisation process.

In this instance, the system was tested for the analysis of 10 pesticides in food products. Apple, orange, cranberry and grape juices were diluted in water, spiked with the pesticides and droplets were added to filter paper. Similarly, salad leaves and small pieces of apple skin were washed before the pesticides were spotted onto small areas. There was no further treatment before the samples were placed between the source and the mass spectrometer inlet for analysis.

The pesticides, covering the carbamate, triazine, urea and organo-chlorine classes, were carbendazim, carbofuran, metolcarb, propoxur, alachlor, metolachlor, dinoseb, atrazine, simazine and isoproturon.

The mass spectra obtained with the APGD source were very similar to those obtained by electrospray ionisation, with strong protonated molecular ion peaks (except for alachlor which had a [M-CH3OH+H]+ peak). The tandem mass spectra recorded under the same collision-induced dissociation conditions were also similar for both ionisation processes. However, the fragment ion intensities tended to be higher with APGD, pointing to APGD being a more energetic ionisation than electrospray ionisation. The probable thermal loss of the CH3OH group from the alachlor molecule appears to support this deduction.

For identification, 9 of the 10 pesticides fulfilled the minimum requirement of three identification points laid down in the EC guidelines for quality control in pesticide analysis. They matched the protonated molecular ion plus two of the product ions, while the tenth pesticide, metolachlor, matched the protonated molecular ion and one product ion.

The limits of determination in the fruit juices ranged from 1 ng/mL (ppb) for metolcarb to 500 ng/mL for alachlor. The researchers were encouraged to find that the more complex nature of orange juice, due to the presence of fruit pulp, did not have any noticeable effects on these limits. For spiked apple skin, the limits of determination ranged from 0.01-5 ng, corresponding to 0.009-5.0 ppb. These values for juices and skin are well above the EU MRLs of 1-500 g/kg (ppb) for pesticides in fruit juice and 0.01-5 g/kg in apple skin.

Reproducibilities of APGD MS were in the region of 20% r.s.d., which is acceptable for screening purposes. They could be improved by adopting a more precise procedure for positioning samples in front of the source. First attempts at quantification in fruit juice did not produce linear calibration curves, with larger errors for higher pesticide concentrations. However, the team noted that it was still easy to determine the order of magnitude of the pesticide concentrations.

This relatively poor performance achieved in the early stages of development suggest that the technique could be used for initial screening to determine which batches of products would require more accurate examination, perhaps by LC/MS. This outcome is essential "for the control of the labelling of bio-/organic foods."

A revised source geometry might make it possible to analyse pesticides on whole fruits, rather than small strips of skin, with the potential for several sampling sites on a single fruit. Miniaturisation and mounting on a portable mass spectrometer would allow for pesticide control in the field.

However, the main advantages of the APGD MS method are the lack of sample preparation which speeds up the whole analysis process, and the good detection limits.

Article by Steve Down 

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

 

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