Total ion count

Figure 8.4 Typical total ion count (TIC) of a bitumen extract from the exterior surface of an archaeological shard obtained by GC-MS. IS = internal standard (C34 w-alkane). This is identical to the output of a GC.

Typical total ion count (TIC) of a bitumen extract from... 

As an example, the novel setup was applied to the characterisation of coffee headspace as a complex food system Basically, an aliquot of the headspace is trapped in defined time periods on several Tenax adsorbents for characterisation by GC-MS. Figure 15.17 shows the simultaneously recorded total ion counts of the EI-MS (top frame) and PTR-MS (bottom frame) for VOCs trapped on the first Tenax cartridge. The GC-separated pure compounds are identified... 

It is always possible to reduce the order of an analytical technique. This is indicated in Fig. 10.1c by two primed, dashed lines. A good example is GC-MS, where the MS is operated in total ion count mode, only as a chromatographic detector. 

Propagation of errors using isotope dilution ICP-MS has been considered to determine how to optimize the measurements [201]. Comparison of analysis results from external calibration versus isotope dilution can be used to assess the quality of external calibration results and the effectiveness of internal standards with external calibration [202,203]. Because isotope ratio precision depends on the total ion count rate, the use of high-efficiency sample introduction to generate larger signals can improve isotope ratio precision and, therefore, analysis precision [204]. 

Constituents in each class are listed in order of increasing retention tiine on a J W DB 1701 fused silica capillary G.C. column. Quantities were estimated by a comparison of the total ion count for the mass spectrum of each peak with that of an n-octeinol internal standard +, 0.1-1 Pg/L ++, 1-10 pg/L +++, 10-100 pg L ++++, 100-500 pg/L. For compound structures see Figure 1. A, previously identified in this laboratory B, tentatively identified fron interpretation of the mass spectrum C, synmetrical peak enhancement and identical mass spectrum on ooinjection with c n authentic saiiple D, carparison of mass spectrum with published spectrum E, ccmparison of mass spectrum and retention time with that of material available cottiitercially or synthesized in the... 

Normalization is a very important step, as it aims to reduce experimental variance. Normalization is most often performed by dividing each spectrum by a normalization factor (Figure 2G). The most popular normalization factor is calculated as the total ion count (TIC), which is the sum of all ion intensities in a spectrum. Several studies discovered that in MSI the assumptions for TIC applicability hold true only for very homogeneous tissues. In heterogeneous samples, more robust normalization factors based on the median or the TIC with exclusion of very localized mass signals have been proposed (35-37). 

Another way to alleviate the problem of lack of molecular weight information resides in the use of so-called "soft ionisation techniques". These techniques have been developed to provide the mass spectrometrist with means to obtain mass spectra that almost invariably contained molecular weight information. In addition, in many instances, the limit of detection is enhanced as a result of the lesser number of ion species being produced for a same quantity of ionised material (less ion species for the same total ion count). However, these techniques suffer from reproducibility difficulties and, generally, offer less structural information. These factors might account for the fact that libraries for such mass spectra were never tabulated. 

Spectroscopy by UV-Vis and MS, when in total ion count mode, yields comparable detection limits for flavonoids, in the range of 10 ng. When single ion monitoring (SIM) mode is used, MS analysis provides better detection limits, usually below 1 ng. " SIM mode, however, causes loss of valuable information concerning fragmentation pattern, which is very important for the identification of many compounds. 

FIGURE 8 Single ion reconstruction traces (top trace = total ion count) of volatiles released from Camp Berteau montmorillonite at 39(fC. Note the different volatile compositions at different times, e.g., 3 and 10 minutes (dashed lines). (Reprinted from Ref. 65.)... 

A shell program, MASK has been added to the LLNL standard GC-MS data analysis code M CODE to protect the confidentiality of information not relevant to the CWC. This program has been demonstrated to work well where high total ion counts are available. A subroutine for blind background subtraction needs to be added for cases where the peak is only slightly above baseline. 

For data pre-processing, the data set is normalized to each pixel s total ion counts and then mean-centered. Image PCA calculation is then performed by the MIA toolbox and the resulting principal components (PC) are generated. Scores image and loading plots of selected PCs are then created by the software. 

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