Full scan analysis limitations

The majority of stndies on TSP LC-MS analysis of pesticides, herbicides, and insecticides concerns the evalnation of interface performance, detection limits, and information content. Typical full-scan detection limits of various classes of pesticides are summarized in Table 4.2 [73]. 

Pesticides. There are numerous references to the use of HPLC/MS for the analysis of pesticides and herbicides [14, 16-20]. Some major classes of pesticides and herbicides including carbamate, triazines, organophosphorus, and phenolic acid have been analyzed by HPLC/MS using Cl or ion evaporation ionization. While these ionization techniques often resulted in excellent sensitivity (thermospray/MS full scan detection limits of 1-10 ng), usually only [M+H] and/or [M+NH4] ions were formed. This limitation can be overcome using tandem4MS [20], moving belt [17], and most recently through the use of particle beam HPLC/MS. 

MS has always been seen as one of the most conclusive techniques for positive identification of organic compounds. The availability, since the beginning of the 1980s, of benchtop GC-MS systems based on quadrupole mass analyzers (GC-Q-MS) made such an analytical tool extremely popular also for routine applications. However, when GC-Q-MS is operated in the full scan mode, limits of detection (LODs) are too high, especially in trace analysis, and its use is seldom restricted to a confirmation technique.When the selected ion monitoring mode (SIM) is employed, the sensitivity is dramatically enhanced. On the other hand, SIM implies the detection of specific analytes with the consequent loss of all other information. 

Two general modes of data acquisition are available in MS full-scan acquisition and selective ion monitoring (SIM). In full-scan analysis, a continuous series of mass spectra is acquired during the chromatographic run. For high-efficiency open capillary GC columns, sufficiently fast scanning is required in order to acquire a sufficient number of data points (typically 10 to 20) to adequately describe the chromatographic peak profile. However, in routine quantitative analysis of a limited number of components, better results in terms of lower detection limits are achieved by the use of SIM, in which the intensity of a (number of) ion(s) is monitored. The choice between full-scan and SIM acquisition in a particular application depends on the required detection limit and information content. 

To improve sensitivity, selected ion monitoring (SIM) mode may be used for the detection of routine YOCs and SVOCs and in low resolution dioxin/furan analysis (EPA Method 8280). In the SIM mode, only specific ions from the analyte s spectrum are scanned for the detector s dwelling time on each ion is increased resulting in higher sensitivity. A mass spectrometer operated in the SIM mode is approximately ten times as sensitive as one in the full scan mode. The SIM mode has limitations, such as the capacity to monitor only a limited number of ions and the need to monitor multiple ions for each compound to improve the degree of confidence in compound identification. That is why typically no more than 20 compounds can be analyzed simultaneously in the SIM mode. 

Pleasance et al. [23] described residue analysis of erythromycin A and its metabolites in salmon tissue using LC-ESl-MS. Detection limits of erythromycin A in salmon tissue were below 10 pg/kg in SIM and below 50 pg/kg in SRM, while confirmatory full-scan LC-MS or LC-MS-MS was achieved at the 0.5- and 1-mg/kg level, respectively. Next to erythromycin A, a variety of metabolites were detected, e.g., anhydroerythromycin and N-desmethylerythromycin. 

Regarding MS detection, authors described methods performed in positive ion mode using full scan and selected ion monitoring for PSP analysis [42,43], The pioneering PSP research in the area of electrospray ionization-mass spectrometry (ESl-MS) detection analysis has been conducted in the group of Quilliam et al. [46, 47]. In their studies, the authors used LC separation before pneumatically assisted ESI. Detection limits as low as 30 pg were obtained for positive ions of several STX analogs [46]. 

The use of different eluents in LC-RP separations was examined to optimize LC separation and determination of carbamates besides other pesticides by TSP-LC-MS [244]. The effects of additives on LC separation and of the vaporizer temperature on ion formation in TSP-FIA-MS analysis were studied for N-methylcarba-mate [252], carbamates [242] and (thio)carbamates [242] pesticides. A strong reduction in abundance of the characteristic ions [M-tH-CHjNCOj and [M-H-CH3NC0] for methiocarb and its sulone were found because of thermal degradation at 90 °C which made quantitation difficult [252]. The addition of trialkylammo-nium formates increased selectivity and sensitivity, detection limits being < 20 ng in full scan mode [242]. 

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