The Total Ion Current TIC

From a mass spectrometry perspective, the pump must be pulse free, i.e. it must deliver the mobile phase at a constant flow rate. Pulsing of the flow causes the total ion current (TIC) trace (see Chapter 3) - the primary piece of information used for spectral analysis - to show increases in signal intensity when analytes are not being eluted and this makes interpretation more difficult. 

The fundamental piece of information on which the subsequent spectral analysis is based is the total-ion-current (TIC) trace. Such a trace, obtained from the LC-MS analysis of a pesticide mixture, is shown in Figure 3.13, together with the UV trace recorded simultaneously. For the purposes of this discussion, the HPLC and MS conditions used to generate the data, other than the fact that electrospray ionization was used, are irrelevant. 

Assuming the sequence of the parent protein is known, it is not necessary to redetermine the whole sequence merely to locate, and sequence, that/those polypeptide(s) that have undergone modification. This can be done by examination of the total-ion-current (TIC) trace before and after protein hydrolysis for the appearance of new polypeptides or to use mass spectrometry methodology to locate those polypeptides that contain certain structural features. Examples are provided here of both methodologies. 

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.

A stream-splitter may be used at the end of the column to allow the simultaneous detection of eluted components by destructive GC detectors such as an FID. An alternative approach is to monitor the total ion current (TIC) in the mass spectrometer which will vary in the same manner as the response of an FID. The total ion current is the sum of the currents generated by all the fragment ions of a particular compound and is proportional to the instantaneous concentration of that compound in the ionizing chamber of the mass spectrometer. By monitoring the ion current for a selected mass fragment (m/z) value characteristic of a particular compound or group of compounds, detection can be made very selective and often specific. Selected ion monitoring (SIM) is more sensitive than TIC and is therefore particularly useful in trace analysis. 

Surface water samples often contain surfactants and their metabolites. After Cis-SPE combined with selective elution [7,9,10] the metabolites, PEG and PPG, were observed in the ether fraction (PPG) or in the combined methanol-water and methanol (PEG) fractions, respectively. They could be ionised in the form of their [M + NH4]+ ions applying ESI-FIA-MS(-I-) in combination with ammonium acetate for ionisation support. ESI-LC-MS(-I-) resulted in an excellent separation of both metabolites, as presented in the total ion current (TIC) trace in Fig. 2.9.6(7) together with selected mass traces of PEG (m/z 300, 344 and 388) and PPG (m/z 442, 500, 558) (Am/z 44 and 58) in Fig. 2.9.6(l)-(6) [36],... 

What is a reconstructed ion chromatogram (RIC) and how does it allow a specific compound to be located in the total-ion current (TIC) trace from an LC-MS analysis ... 

This information is plotted by the computer in alternative ways and it is important for the analyst to be aware of the ways in which the data system is operating. In the normalized or percentage relative abundance (%RA) method, which is commonly used, the height of each peak is shown as a percentage of the biggest peak in the spectrum. The total ion current (TIC) is the sum of all the detector responses for each scan plotted against time and this is equivalent to a GLC trace. This information is particularly useful in quantitative analysis. 

Fig. 2.83. Mass chromatogram extracted from the total ion current (TIC) of a wine. Reprinted with...

The total ion current (TIC) can either be measured by a hardware TIC monitor before mass analysis, or it can be reconstructed by the data system from the spectra after mass analysis. [27] Thus, the TIC represents a measure of the overall intensity of ion production or of mass spectral output as a function of time, respectively. The TIC obtained by means of data reduction, [28] i.e., by mathematical construction from the mass spectra as successively acquired while the sample evaporates, is also termed total ion chromatogram (TIC). For this purpose, the sum of all ion intensities belonging to each of the spectra is plotted as a function of time or scan number, respectively. 

This type of ionisation has been discussed earlier in this chapter. The mass spectrum of psoralen under El conditions is shown in Figure 9.21 along with the corresponding GC trace, which is produced by the total ion current (TIC) across the scan range of the mass spectrum. The molecular ion M - is in this case in good abundance at m/z 186 and other ions arise as follows m/z 158 (M+—CO), m/z 130 (M+-2CO), m/z 102 (M+-3CO). 

The MS tests of the first nib prototypes were conclusive a spray was observed after application of an HV on the liquid and the signal was stable for the various tested conditions. Figure 5.5 shows the total ion current (TIC) spectrum recorded over a period of 2 min using a 5 pM Gramicidin S sample... 

The mass spectrometer may be operated in various modes. In the total ion current (TIC) monitoring mode, it sums the currents from all fragment ions as... 

The total ion current (TIC) is the current due to the total number of ions passing through the analyzer (Fig. 4-4). It is necessary to measure this because part of... 

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