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 RenatoZenobi 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 IndianaUniversity. 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 electrosprayionisation, 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 electrosprayionisation.
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 metolcarbto 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 µg/kg (ppb) for pesticides in fruit juice and 0.01-5 µg/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.
