Selective multiresidue methods

Selective multiresidue methods, can identify and measure a small number of structurally similar pesticides and are used when MRMs cannot determine potential pesticide residues of interest Recently registered pesticides, as well as many pesticides that have been registered for some years, frequently have chemical and physical properties that are radically different from those for which the MRMs were developed. A selective MRM usudly can determine fewer than a dozen residues. These methods are also frequently more complicated than MRMs and often require more expertise, or they may require recently developed, sophisticated instrumentation. 

When a first column of a very short length (and therefore a low selectivity) is used (this is especially suitable for multiresidue methods), we talk about an on-line precolumn (PC) switching technique coupled to LC (PC-LC or solid-phase extraction (SPE)-LC). This is particulary useful for the enrichment of analytes, and enables a higher sample volume to be injected into the analytical column and a higher sensitivity to be reached. The sample is passed through the precolumn and analytes are retained, while water is eliminated then, by switching the valve, the analytes retained in the precolumn are transferred to the analytical column by the mobile phase, and with not just a fraction, as in the previous cases. 

Chlorophenoxy acids are relatively polar pesticides which are usually determined by LC because volatile derivatives have to be prepared for GC analysis. This group of herbicides can be detected by multiresidue methods combined with automated procedures for sample clean-up, although selectivity and sensitivity can be enhanced by coupled-column chromatographic techniques (52). The experimental conditions for Such analyses are shown in Table 13.1. 

Although both polyclonal and monoclonal antibodies have been effectively used in immunochemical assays, only the latter can provide the high specificity required in some applications. Antibody specificity, on the other hand, is both a major advantage and disadvantage for immunochemical methods. It allows for highly selective detection of analytes but at the same time may complicate the development of multiresidue methods. Moreover, production of monoclonal antibodies requires special expertise and it is much more expensive than polyclonal antibodies. Thus, in cases where a range of analytes similar in molecular structure are required to be determined, a polyclonal may be more suitable than a monoclonal antibody. 

OP compounds and carbamate are widely used as insecticides, pesticides, and warfare agents [20,21], Detection of pesticides is usually carried out by multiresidue methods (MRMs) of analysis, which are able to detect simultaneously more than one residue and have been developed mainly based on chromatographic techniques. Two groups of MRMs are used (i) multiclass MRMs that involve coverage of residues of various classes of pesticides, and (ii) selective MRMs, which concern multiple residues of chemically related pesticides (e.g., IV-methyl carbamate pesticides (NMCs), carboxylic acids, phenols, etc.). As foods are usually complex matrices all of the pre-analytical steps (matrix modification, extraction, and clean-up) are often necessary. 

Jans son et al. [52] described a multiresidue method for PCA analysis in biological samples by acetone-hexane and diethylether-hexane extraction, oxidation with sulfuric acid to remove lipid, and isolation of PCAs on SX-3 Biobeads GPC. A combination of silica and activated charcoal column chromatography was then used to isolate other OCs. With diethylhexylphthalate (DEHP) as a retention indicator and (1 1) DCM/hexane as the mobile phase for GPC, PCAs were selectively removed from other persistent OCs by collecting an initial fraction that corresponded to a factor times the retention time of DEHP. All other... 

A.O. Olsson, S.E. Baker, J.V. Nguyen, L.C. Romanoff, S.O. Udunka, R.D. Walker, K.L. Flemmen, D.B. Barr, A LC-MS-MS multiresidue method for quantification of specific metabolites of OPP, synthetic pyrethroids, selected herbicides, and DEEP in human urine. Anal. Chem., 76 (2004) 2453. 

Increasing demand for screening our environment for pesticide residues has generated a need for more efficient residue methods. Multiresidue methods, or those which measure several compounds at once, are generally more efficient than single residue methods in satisfying these needs. A mass spectrometer can be used as a universal selective detector for multiresidue analysis in different sample matrices, since it is generally blind to interferences present in the sample. In addition to selectivity, use of a mass spectrometer offers structure confirmation, and in many cases, can eliminate sample cleanup steps. 

Thermospray LC/MS has been extensively used for the study of sulfonylurea herbicides (1-2). These compounds are thermally labile and can not be successfully analyzed by conventional GC/MS. Early applications of thermospray LC/MS included metabolite identification and product chemistry studies. We have recently evaluated the use of thermospray LC/MS for multi-sulfonylurea residue analysis in crops and have found the technique to meet the criteria for multiresidue methods. LC/MS offers both chromatographic separation and universal mass selectivity. Our study included optimization of the thermospray ionization and LC conditions to eliminate interferences and maximize sensitivity for trace level analysis. The target detection levels were SO ppb in crops. Selectivity of the LC/MS technique simplified sample extraction and minimized sample clean up, which saved time and optimized recovery. Average recovery for these compounds in crop was above 85%. 

Multiresidue methods are the most powerful procedures for the analysis of pesticides in environmental, food, and/or feed samples. The maximum residue limits of the pesticides prescribed by health authorities include not only the residues of the parent compounds but their toxic metabolites as well. There is also a trend toward inclusion of residues of highly polar and conjugated metabolites (126). The ever-increasing demands on the quality of food, feed, and environmental samples, and the lowering of the maximum residue limits, require the development of new and more sensitive methods. To distinguish the pesticide signals from the interfering coextractives of samples demands more selective detection. TLC methods alone are not sufficient and should be combined with GLC, HPLC, mass spectrometry, etc. 

SFC has received increasing attention in recent years in the environmental field. The main advantages of this technique include shorter retention times in the analysis of moderately polar and thermally labile pesticides, and compatibility with most LC and GC detectors (UV, FID, NPD, MS, and BCD) [162-165]. Packed-column SFC is nowadays competitive with LC and GC as it combines the speed and efficiency of GC and the wide selectivity adjustment capabilities of LC, thereby facilitating the determination of polar and thermobile analytes [166-168]. A multiresidue method for the analysis of 35 contaminants including pesticides in river waters by use of SPE coupled with SFC with silica-packed columns has provided good sensitivity (detection limits between 0.4 and 2.5 xg/L) [169]. 

A single multiresidue method was developed to determine 109 priority compoimds listed in the 76/464/EEC Coimcil Directive on Pollution of the EU [49]. For trapping analytes, automated off-line SPE with a polymeric sorbent "Oasis" 60 mg cartridge, was optimized. A multianalyte method for the confirmation and quantitation of 16 selected sulfonylurea, imidazolinone, and sulfonamide herbicides in surface water has been proposed [50]. This method is based on analyte extraction with a polymeric material (RP-102) and extract cleanup with a strong anion exchanger (SAX) cartridge stacked on... 

ID target residues have been measured in wastewater samples with fully validated, highly selective multiresidue assays. Briefly, water samples were generally sohd-phase extracted, purified, and subsequerrtly analyzed by hqtrid chromatograplty— tandem mass spectrometry or alternative methods. These methods are reviewed elsewhere in this book (see Chapter 3). 

Whereas most multiresidue methods employ SPE for sample enrichment, Grulick and Alder [80] utilized direct sample injection for the determination of 300 pesticides in water samples. A conventional reversed-phase LC separation was developed with gradient elution using a Cl 8 column with a run time of approximately 23 min. The LC was coupled to a triple quadrupole mass spectrometer with electrospray ionization in the positive ion mode. Two selected reaction monitoring transitions were collected for each analyte by means of repeated analyses (i.e., two sample injections). Because interfering signals were noted for many of these transitions, both the LC separation and confirmatory transitions were essential for correct analyte identification. 

Sample preparation consists of homogenization, extraction, and cleanup steps. In the case of multiresidue pesticide analysis, different approaches can have substantially different sample preparation procedures but may employ the same determinative steps. For example, in the case of soil analysis, the imidazolinone herbicides require extraction of the soil in 0.5 M NaQH solution, whereas for the sulfonylurea herbicides, 0.5M NaOH solution would completely decompose the compounds. However, these two classes of compounds have the same determinative procedure. Some detection methods may permit fewer sample preparation steps, but in some cases the quality of the results or ruggedness of the method suffers when short cuts are attempted. For example, when MS is used, one pitfall is that one may automatically assume that all matrix effects are eliminated because of the specificity and selectivity of MS. 

Another characteristic example of analytical strategy is that followed in the United Kingdom for the analysis of tranquilizers and -blockers (78). A total of 180 samples distributed over a whole year (15 samples per month) should be analyzed within a turnaround time of 28 days from receipt of sample. In that case, the expense of developing a two-tier analytical strategy was not justified by the sample throughput. Thus, the selection was direct application of a multiresidue LC/PDA confirmatory method (80). 

While the selection of an isotopically-labelled or analogue internal standard is relatively easy in quantitative bioanalysis, the situation is more complicated in multiresidue analysis. It is difficult to select appropriate analogue standards for a wide variety of target compounds, while isotopically-labelled standards are often not available for all target compounds. In addition, if one would introduce one standard per target compound, this would seriously limit the sensitivity of the method as it doubles the number of SRM transitions that have to be monitored. Another problem in multiresidue analysis is the selection of appropriate blanks for the production of the matrix-matched standards and the number of matrices that might have to be studied. When no adequate blank matrix is available, the standard addition method is the only way to achieve sufficiently accurate and precise results [123]. This method is time-consuming and labourious. 

The LC/MS separation and detection shown in this method illustrate the multiresidue capabilities of LC/MS. The combination of the LC separation for six sulfonylurea herbicides within 25 minutes and the selectivity and structure confirmation of the mass selective detector offer the basis for efficient residue analysis. LC/MS residue methodology can be a cost effective alternative to conventional residue methodology. The mass selectivity will simplify the extraction procedure and minimize sample clean up. A simplified sample preparation saves time and reduces analyte loss during sample preparation which will maximizes recovery. 

---