Tandem mass spectrometry background

More recently, liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS) have been evaluated as possible alternative methods for carfentrazone-ethyl compounds in crop matrices. The LC/MS methods allow the chemical derivatization step for the acid metabolites to be avoided, reducing the analysis time. These new methods provide excellent sensitivity and method recovery for carfentrazone-ethyl. However, the final sample extracts, after being cleaned up extensively using three SPE cartridges, still exhibited ionization suppression due to the matrix background for the acid metabolites. Acceptable method recoveries (70-120%) of carfentrazone-ethyl metabolites have not yet been obtained. 

Background. As an analytical technique, tandem mass spectrometry is just entering its second decade of development. The variety of reported applications belies its relative youth. Tandem mass spectrometry (MS/MS) grew out of early work which used metastable ion transitions in order to establish ion structures and interrelationships. After extensive applications to ion structural studies, its usefulness in direct catplex mixture analysis became apparent with the early work of Cooks (1-3). Its successes in problem solving are summarized in a recent book edited by McLafferty (4). New, with several ccnmercial instruments available, MS/MS is being evaluated for application in several new areas, including biochemical analysis, forensic chemistry, and food and flavor analyses. The principles of MS/MS will be surmarized in the first part of this chapter. The second part of the chapter will deal with the reported applications of MS/MS to flavor analysis. 

In addition, due to lack of MS/MS capability, SIM has been more commonly performed on single quadrupole MS, while SRM has been broadly adapted on triple quadrupole (Figure 13-4) and ion-trap mass spectrometers. The increase in sensitivity and selectivity of SRM stem from the ion-chromatogram (i.e., LC-MS/MS) obtained by specific precursor-to-product ion transition for an analyte of interest (Figure 13-4). Conversely, in an SIM mode, the relative background noise due to the presence of other isobaric species (i.e., ions with a same m/z as the analyte of interest) can result in a lower signal-to-noise ratio for the analyte. Due to the widespread acceptance of SRM in quantitative analysis, the remaining part of this section focuses on a description of tandem-mass spectrometry (MS/MS), which is utilized in SRM (or MRM) experiments. 

An example of the application of the wide isolation window in tandem mass spectrometry is the detection and quantitative imaging of cocaine in postmortem human brain tissue (Reich et al., 2008 Reich et al., 2010). In this instance, it was determined necessary to develop an MS wide isolation window method because of interfering background ions. Cocaine detected in brain tissue was confirmed by matching six MS product ions with those from a cocaine standard. For the quantitation of cocaine analyzed from the tissue, the trideuterated ( Hs) internal standard was spiked beneath the tissue at known concentrations before matrix application to develop a calibration curve. The MS wide isolation method was then employed for the analysis of cocaine and cocaine-fi 3. Cocaine was analyzed successfully and quantified using this approach. 

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Coupling of LC with tandem MS may be a solution for improving detection limits by reducing the background noise, but this combination is two or three times more expensive than its LC-MS analogue. Mass spectrometry has become a standard tool in every modern laboratory, but there will be a growing need for even more sophisticated couplings. 

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