Diagnostic ion

Analysis. The infrared (ii), ultraviolet M, and nuclear magnetic resonance (nmr) spectra are distinct and characteristic for benzene and are widely used in analysis (78—80). Benzene also produces diagnostic ions in the mass spectmm (81,82) (see Analytical methods). 

It has been shown [18] that when high cone-voltages were used in conjunction with negative-ion electrospray, phosphopeptides produce diagnostic ions at m/z 63 (P02 ) and m/z 79 (P03 ). LC-MS analysis of a trypsin digest of bovine... 

Ions present on the fragment ion spectrum can either be sequence specific (they can provide information about the sequence) or non-seqence-specific (they cannot provide direct information on the sequence). For example, all ions belonging to six basic ion series (a, b, c, x, y, z) are sequence specific, whereas immonium ions are not (but they still are valuable diagnostic ions). 

Commercially available GC-MS systems present major differences in their detection and recording system. Many quadrupole instruments use SIM for the determination of analytes at trace levels. With this type of instrumentation, more than 1-10 ng of the analyte is required to record a full-scan mass spectrum. In contrast, instruments based on ion-trap technology can record a full-scan mass spectrum on an analyte at pg level. With SIM, a limited number of ions are monitored during a selected time interval of the chromatogram. The presence of the analyte is determined by the presence of these diagnostic ions at the correct retention time and in the correct abundance ratio (33). 

Unlike with GC-MS, quality criteria for identification of drug residues by LC-MS have not been yet defined within the European Union, but this is currently under review. Criteria for GC-MS stipulate the measurement of preferably at least four diagnostic ions. However, this is not always possible with LC-MS because most compounds will only produce an M ion in positive mode or a M ion in negative mode, with little fragmentation when using thermospray (TSP), electrospray (ESP), or atmospheric pressure chemical ionization (APCI). Even where the ions and ratios are in agreement, there will be still possibility of misidentification. For this reason, mass spectra data are often interpreted with additional supporting data such as the LC retention times, as, for example, in the LC-MS analysis of sulfadimethoxine and sulfadoxine that present identical mass spectra (24). 

Low-Resolution-M ss Spectrometry At least two, preferably four, diagnostic ions should be scanned Relative intensities as % of base peak should be within 20% in Cl mode and 10% in El mode with respect to standard analyte... 

Detection of additional but non-diagnostic ion High-resolution mass spectrometry Peak match of molecular ion... 

A method for the determination of 11 OPPs in a water sample involved SPE followed by LC-ISP-MS and was developed with a limit of detection between 0.01 and 0.2 tg/L (88). The authors observed that ISP spectra display diagnostic ions of OPPs, which could be used for the identification of unknowns. Comparison of LC-ISP-MS with TSP-MS analysis of the same compounds showed that ISP is to be preferred It has about 100-fold better sensitivity, and the thermal degradation of trichlorfon observed with TSP was not found with ISP. Liquid chromatography-ISP-MS is sometimes better when it comes to distinguishing isomers like trans- and m-mevinphos, which is difficult to do under current LC-DAD conditions owing to the poor chro-mophore of cw-mevinphos. 

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