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Determination of Pesticide Minimum Residue Limits in Essential Oils -
Report No 3By Professor R. C. Menary & Ms S. M. Garland
June 2004
RIRDC Publication No 04/023 RIRDC Project No UT-23A
The capacity of disposable C18 cartridges to separate components of boronia oil was found to be limited with the majority of boronia components being eluted on the solvent front, with little to no separation achieved. The cartridges were useful, however, in establishing the likely interaction of reverse phases (RP) C18 columns with components of essential oils, using polar mobile phases . The loading of large amounts of oil onto RP HPLC columns presents the risk of permanently contaminating the bonded phases. The lack of retention of components on disposable SPE C18 cartridges, despite the highly polar mobile phase, presented a good indication that essential oils would not accumulate on HPLC RP columns.
The removal of non-polar essential oil components by solvent partitioning of distilled oils was minimal, with the recovery of pesticides equivalent to that recorded for the essential oil components.
However application of this technique was of advantage in the analysis of solvent extracted essential oils such as those produced from boronia and blackcurrant.
ECD was found to be successful in the detection of terbacil, bromacil, haloxyfop ester, propiconazole, tebuconazole and difenaconzole. However, analysis of pesticide residues in essential oils by application of GC ECD is not sufficiently sensitive to allow for a definitive identification of any contaminant. As a screen, ECD will only be effective in establishing that, in the absence of a peak eluting with the correct retention time, no gross contamination of pesticide residues in an essential oil has occurred . In the situation where a peak is recorded with the correct elution characteristics, and which is enhanced when the sample is fortified with the target analyte, a second means of contaminant identification would be required. ECD, then, can only be used to rule out significant contamination and could not in itself be adequate for a positive identification of pesticide contamination.
Benchtop GC daughter, daughter mass spectrometry (MSMS) was assessed and was not considered practical for the detection of pesticide residues within the matrix of essential oils without comprehensive clean-up methodologies. The elution of all components into the mass spectrometer would quickly lead to detector contamination.
Method validation for the detection of 6 common pesticides in boronia oil using GC high resolution mass spectrometry was completed. An analytical technique for the detection of monocrotophos in essential oils was developed using LC with detection by MSMS. The methodology included an aqueous extraction step which removed many essential oil components from the sample.
Further method development of LC MSMS included the assessment of electrospray ionisation (ESI) and atmospheric pressure chemical ionisation (APCI. For the chemicals trialed, ESI has limited application. No response was recorded for some of the most commonly used pesticides in the essential oil industry, such as linuron, oxyflurofen, and bromacil. Overall, there was very little difference between the sensitivity for ESI and APCI. However, APCI was slightly more sensitive for the commonly used pesticides, tebuconazole and propiconazole, and showed a response, though poor, to linuron and oxyflurofen. In addition, APCI was the preferred ionisation method for the following reasons, ¦ APCI uses less nitrogen gas compared to ESI, making overnight runs less costly; ¦ APCI does not have the high back pressure associated with ionisation by ESI such that APCI can be run in conjunction with UV-VIS without risk of fracturing the cell, which is pressure sensitive.
Analytes that ionised in the negative APCI mode were incorporated into a separate screen which included bromacil, terbacil, and the esters of the fluazifop and haloxyfop acids. Further work using APCI in the positive mode formed the basis for the inclusion of monocrotophos, pirimicarb, propazine and difenaconazole into the standard screen already established. Acephate, carbaryl, dimethoate, ethofumesate and pendimethalin all required further work for enhanced ionisation and / or improved elution profiles. Negative ionisation mode for APCI gave improved characteristics for dicamba, procymidone, MCPA and mecoprop.
The thirteen pesticides included in this general screen were monocrotophos, simazine, cyanazine, pirimicarb, propazine, sethoxydim, prometryb, tebuconazole, propiconazole, , difenoconazole and the esters of fluroxypyr, fluazifop and haloxyfop.. Bromacil and terbacil were not included as both require negative ionisation and elute within the same time window as simazine, which requires positive ionisation. Cycling the MS between the two modes was not practical.
The method validation was tested against three oils, peppermint, parsley and fennel.
Detection limits ranged from 0.1 to 0.5 mgkg-1 within the matrix of the essential oils, with a linear relationship established between pesticide concentration and peak height (r2 greater than 0.997) and repeatabilities, as described by the relative standard deviation (r.s.d), ranging from 3 to 19%. The type of oil analysed had minimal effect on the response function as expressed by slope of the standard curve.
The pesticides which have an carboxylic acid moiety such as fluazifop, haloxyfop and fluroxypyr, present several complications in any analytical method development. The commercial preparations usually have the carboxylic acid in the ester form, which is hydrolysed to the active acidic form on contact with soil and vegetation. In addition, the esters may be present in several forms, such as the ethoxy ethyl or butyl esters. Detection using ESI was tested. Preliminary results indicate that ESI is unsuitable for haloxyfop and fluroxypyr ester. Fluazifop possessed good ionisation characteristics using ESI, with responses approximately thirty times that recorded for haxloyfop. Poor chromatography and response necessitated improved mobile phase and the effect of pH on elution characteristics was considered the most critical parameter. The inclusion of acetic acid improved peak resolution.
The LC MSMS method for the detection of dicamba, fluroxypyr, MCPA, mecoprop and haloxyfop in peppermint and fennel distilled oils underwent the validation process. Detection limits ranged from 0.01 to 0.1 mgkg-1 Extraction protocols and LC MSMS methods for the detection of paraquat and diquat were developed.
ESI produced excellent responses for both paraquat and diquat, after some modifications of the mobile phase. Extraction methodology using aqueous phases were developed. Extraction with carbonate buffer proved to be the most effective in terms of recovery and robustness. A total ion chromatogram of the LC run of an aqueous extract of essential oil was recorded and detection using a photodiode array detector confirmed that very little essential oil matrix was co-extracted. The low background noise indicated that samples could be introduced directly into the MS. This presented a most efficient and rapid way for analysis of paraquat and diquat, avoiding the need for specialised columns or modifiers to be included in the mobile phase to instigate ion exchange.
The adsorbtion of paraquat and diquat onto glass and other surfaces was reduced by the inclusion of diethylenetriamine (DETA). DETA preferentially accumulates on the surfaces of sample containers, competitively binding to the adsorption sites. All glassware used in the paraquat diquat analysis were washed in a 5% solution of 0.1M DETA, DETA was included in all standard curve preparations, oils were extracted with aqueous DETA and the mobile phase was changed to 50:50 DETA / methanol.
The stainless steel tubing on the switching valve was replaced with teflon, further improving reproducibility. Method validation was undertaken of the analysis of paraquat and diquat using the protocols established. The relationship between analyte concentration and peak area was not linear at low concentrations, with adsorption more pronounced for paraquat, such that the response for this analyte was half that seen for diquat and the 0.1 mgkg-1 level.
The development of a method for the detection of the dithiocarbamate, mancozeb was commenced.
Disodium N, N'-ethylenebis(dithiocarbamate) was synthesised as a standard for the derivatised final analytical product. An LC method, with detection using MSMS, was successfully completed. The inclusion of a phase transfer reagent, tetrabutylammonium hyrdrogen sulfate, required in the derivatisation step, contaminated the LC MSMS system, such that any signal from the target analyte was masked. Alternatives to the phase transfer reagent are now being investigated.
Monitoring of harvests were undertaken for the years spanning 1998 to 2001. Screens were conducted covering a range of solvent extracted and distilled oils. Residues tested for included tebuconazole, simazine, terbacil, bromacil, sethoxydim, prometryn, oxyflurofen, pirimicarb, difenaconazole, the herbicides with acidic moieties and paraquat and diquat. Problems continued for residues of propiconazole in boronia in the 1998 / 1999 year with levels to 1 mgkg-1 still being detected.
Prometryn residues were detected in a large number of samples of parsley oil.
Finally the information gleaned over years of research was collated into a manual designed to allow intending analysts to determine methodologies and equipment most suited to the type of the pesticide of interest and the applicability of analytical equipment generally available.
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