Full scan

Important to environmental analysis is the ability to automate the injection, as weU as the identification and quantitation of large numbers of samples. Gc/ms systems having automatic injectors and computerized controllers have this capabiUty, even producing a final report in an unattended manner. Confirmation and quantitation are accompHshed by extracting a specific ion for each of the target compounds. Further confirmation can be obtained by examining the full scan mass spectmm. 

A full-scan mass spectrum can easily be obtained from this amount of material and it should be clear, therefore, that even high-pnrity (and nsually expensive ) solvents can give rise to a significant mass spectral backgronnd, hence rendering the interpretation of both qnalitative and qnantitative data difficult. 

Like the UV detector, the mass spectrometer may be employed as either a general detector, when full-scan mass spectra are acquired, or as a specific detector, when selected-ion monitoring (see Section 3.5.2.1) or tandem mass spectrometry (MS-MS) (see Section 3.4.2) are being used. 

Another advantage of mass spectrometry is its sensitivity - a full-scan spectrum, and potentially an identification, can be obtained from picogram (pg) amounts of analyte. In addition, it may be used to provide quantitative information, usually to low levels, with high accuracy and precision. 

In the ion-trap, ionization of the sample is carried out as in conventional operation and ions of all m/z ratios take up stable trajectories within the trap. In the production of a conventional full-scan mass spectrum, ions of different m/z values are then sequentially made unstable and ejected from the trap to the detector. In MS-MS operation, ions of all m/z ratios, except that required for further study, are made unstable and ejected from the trap. The ions remaining in the trap, only those of the selected m/z ratio, are now excited to bring about their dissociation. The resulting product ions are then sequentially made unstable and sent to the detector to generate the product-ion spectrum. 

The data considered above have been derived from a TIC trace, i.e. have been acquired from full scanning, but the same methodology is used for analysing... 

To answer this question, we must consider the ways in which the data are acquired. An RIC is generated, post-acquisition, from consecutive full scans in which a small amount of time is spent monitoring each ion, as discussed above. The data produced in a SIM experiment are generated by monitoring only a small number of ions, thus taking advantage of the increased time spent monitoring each ion. 

Figure 5.27 Selective detection of lactolated peptides from a tryptic digest of / -lacto-globulins by LC-electrospray-MS-MS, showing (a) the total-ion-cnrrent trace in full-scan mode, and (b) the total-ion-current trace in neutral-loss-scanning mode. Figure from Selective detection of lactolated peptides in hydrolysates by liquid chromatography/ electrospray tandem mass spectrometry , by Molle, D., Morgan, F., BouhaUab, S. and Leonil, J., in Analytical Biochemistry, Volume 259, 152-161, Copyright 1998, Elsevier Science (USA), reproduced with permission from the publisher.

Figure 5.41 The total-ion-current (TIC) trace and reconstructed ion chromatograms from the predicted pseudomolecular ions of Indinavir m/z 614) and its mono- (m/z 630) and dihydroxy metabolites (m/z 646), generated from full-scan LC-MS analysis of an incubation of Indinavir with rat liver S9. Reprinted by permission of Elsevier Science from Identification of in vitro metabolites of Indinavir by Intelligent Automated LC-MS/MS (INTAMS) utilizing triple-quadrupole tandem mass spectrometry , by Yu, X., Cui, D. and Davis, M. R., Journal of the American Society for Mass Spectrometry, Vol. 10, pp. 175-183, Copyright 1999 by the American Society for Mass Spectrometry.

Figure 18. Gas chromatogram from the full-scan GC/MS analysis of a high-temperature distillation fraction (343 °C) containing diamondoids. Taken from Ref. [11] with permission.

Specificity is unsurpassed. Traditionally, MS was performed on very large and expensive high-resolution sector instruments operated by experienced specialists. The introduction of low-resolution (1 amu), low-cost, bench-top mass spectrometers in the early 1980s provided analysts with a robust analytical tool with a more universal range of application. Two types of bench-top mass spectrometers have predominated the quadrupole or mass-selective detector (MSD) and the ion-trap detector (ITD). These instruments do not have to be operated by specialists and can be utilized routinely by residue analysts after limited training. The MSD is normally operated in the SIM mode to increase detection sensitivity, whereas the ITD is more suited to operate in the full-scan mode, as little or no increase in sensitivity is gained by using SIM. Both MSDs and ITDs are widely used in many laboratories for pesticide residue analyses, and the preferred choice of instrument can only be made after assessment of the performance for a particular application. 

The full-scan mode is needed to achieve completely the full potential of fast GC/MS. Software programs, such as the automated mass deconvolution and identification system (AMDIS), have been developed to utilize the orthogonal nature of GC and MS separations to provide automatically chromatographic peaks with background-subtracted mass spectra despite an incomplete separation of a complex mixture. Such programs in combination with fast MS data acquisition rates have led to very fast GC/MS analyses. 

An advantage of the microbore gas chromatrography/time-of-flight mass spectrometry (GC/TOFMS) method over the other two approaches is that separation efficiency need not be compromised for speed of analysis. The rapid deconvolution of spectra ( scan rate ) with TOFMS makes it the only MS approach to achieve several data points across a narrow peak in full-scan operation. However, the injection of complex extracts deteriorates performance of microbore columns quickly, and an increased LOD and decreased ruggedness result. Microbore columns may be used in water analysis if the LOD is sufficiently low, but they can rarely be used in real-life applications to complicated extracts. 

LC/MS is used as a multi-residue analytical method. The recovery of imidacloprid from tomato was 90-105% for 0.05 and 0.5mgkg . The LOD for imidacloprid was < 10 pg kg in the full-scan mode and 1 pg kg in the selected-ion monitoring (SIM) mode. ... 

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