Residue analysis MRMs

Most modern methods of analysis to determine pesticide residues in food commodities, whether a multi-residue method (MRM) or a single-residue method (SRM), can be broken down into three or four basic steps sample processing, sample extraction, extract cleanup (optional) and instrumental determination. 

In addition to MRM, the other scan modes available on a QqQ have occasionally been used for residue analysis as well. A precursor ion scan can be used to identify precursor ions from a product ion, and therefore to identify analytes and metabolites or impurities, which generate the same product ion, in complex matrices. For example, erythromycin B was identified in yogurt using this function. In this application, Q3 was held constant to measure a fragment ion at m/z 158, which is a typical product ion of compounds or impurities related to erythromycin A with a desosamine residue. Q1 was then scanned over an appropriate range, from which a precursor ion at m/z 718 was detected. The latter was identified as erythromycin B, which was an impurity in the erythromycin fermentation product. Constant neutral loss scan, which has rare applications for antibiotic analysis, records spectra that show all the precursor ions that have fragmented by the loss of a specific neutral mass. In this instance, both Q1 and Q3 scan together with a constant mass offset between the two quadrupoles. Both precursor ion and constant neutral loss scans can be performed only with ion beam tandem in-space mass spectrometers. 

An early article by Gimther and Blinn (72) outlined the basic principles of pesticide residue analysis, including the possibilities for systematization and standardization of analytical methods. In many respects this thinking led to the evolution of today s methods, both single residue (SRM) and particularly integrated multi-residue methods (MRMs) that can include several hundred individual anal es, both parent compounds and significant breakdown products (13). 

This chapter focuses on LC—MS/MS applied to pesticide residue analysis, as this technique is the most attractive and efficient nowadays for developing MRMs [11], including both parent pesticides and metabolites. Sample treatment (mainly extraction and cleanup) are briefly commented on, with emphasis on those commonly applied in MRMs. A brief mention is made of problematic pesticides that do not fit in MRMs and consequently need to be determined with individual-specific LC—MS/MS methods. The use of HR MS in combination with LC also is briefly treated, either for the investigation of parent pesticides or for metabolite research, as this is a field of major interest at present. 

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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. 

A dramatic selective enrichment in nitrotyrosine content was observed within apoA-I recovered from serum and human atherosclerotic lesions. The analysis of serum from sequential subjects demonstrates that the nitrotyrosine contents of apoA-I were markedly higher in individuals with cardiovascular disease. An increase in nitrotyrosine was observed in the atherosclerotic lesions and plasma of apolipoprotein A-I-deficient mice (Parastatidis et al., 2007). LC MS/MS studies confirm that nitration of apoA-I occurs specifically on tyrosine-192 (Shao et al., 2005). Similarly, LC—MS/MS methods were used to identify tyrosine residues 292 and 422 at the carboxyl terminus of the 3 chain as the principal sites of fibrinogen nitration in vivo (Parastatidis et al., 2008). Stable isotope dilution LC MRM/MS methodology will likely be employed extensively in the future to assess the role of protein tyrosine nitration in the oxidative stress associated with cardiovascular disease. 

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