Total ion trace

Examination of polyol migration from test samples Samples 5, 6, 8 and 9 in the form of pouches were subjected to ten-day testing at 40 °C with the food simulants distilled water, 3% acetic acid, 10% ethanol and olive oil. The three aqueous simulant extracts (1 ml of each) were diluted 1 1 with acetonitrile prior to analysis. Olive oil extracts (5 g) were shaken and extracted with 3 ml of acetonitrile prior to examination, using the same conditions. No evidence was found from the total ion traces to suggest migration of any unreacted polyols under the ten days at 40 °C test condition. 

Figure 3. Stepped collision energy scanning LC-ESMS of a 150 pmol osteopontin tryptic digest comparing the total-ion trace with that of the sum of the two phosphate-selective markers (m/z 63 and 79).

The example presented in Fig. 8.20 shows the principle of operation. The UV and Total ion trace (Mass spec trace) show the presence of six components. 

By scanning the temperature in a filament pyrolyser, the technique allows the separation of non-polymeric impurities from a polymer or composite material. Time-resolved filament pyrolysis has a series of useful applications as an analytical tool or even in some structure elucidations. As an example, it can be used [48] to differentiate the existence of more labile groups in a polymer structure. A typical variation of the total ion trace in a time-resolved pyrolysis MS for a composite material is shown in Figure 5.4.3. 

Figure 5.4.3. Total ion trace (TIT) of a time-resolved filament pyrolysis MS for a composite material.

The volatile components or the labile groups from a polymer will be released at lower temperatures (or at the beginning of the heating time), while the polymer backbone will decompose at higher temperatures. Each point in the total ion trace (TIT) has an associated mass spectrum that allows the characterization of this process. Thermal analysis (TA) associated with the MS analysis of the evolving pyrolysates is a useful tool for polymer analysis, but the main focus in this process is not relat to an actual pyrolysis. 

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. 

If the belt moves too quickly, in relation to the rate of deposition, sample will not be deposited on all parts of the belt. This results in the production of an uneven total-ion-current (TIC) trace and a distortion of the mass spectra obtained, with consequent problems in interpretation, particularly if library searching is employed. 

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 5.27 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 5.49 (a) Total-ion-current trace, and (b) the reconstructed ion chromatogram of mjz 510.2 0.5 (monooxygenated metabolites) from LC-MS analysis of human microsomal incubation of Glyburide. Reprinted with permission from Zhang, H., Henion, J., Yang, Y. and Spooner, N., Anal. Chem., 72, 3342-3348 (2000). Copyright (2000) American Chemical Society. 

A reconstructed ion chromatogram is a plot showing the variation in intensity of an ion of a particular m/z ratio as a function of analysis time, while the total-ion-current trace shows the variation in the intensity of all ions being produced as a function of analysis time. Simplistically, the TIC will show an increase as a compound elutes from an HPLC column and is ionized. If an ion with a particular m/z value is found to be diagnostic of a compound or series of compounds of interest, then an RIC of this m/z will show where its intensity increases and, therefore, where a compound of interest may have eluted. The mass spectrum at this point can then be examined for further confirmation that it is of significance. 

Total-ion-current trace A plot of the total number of ions reaching the mass spectrometry detector as a function of analysis time. 

FIGURE 5.4 Chromatograms of 2DLC (affinity/SEC/MS). Bottom trace is affinity separation with UV detection and 2 min fraction specified. Middle trace is MS total ion chromatogram showing protein elution and salts diverted to waste. Top trace in MS extracted ion chromatogram of protein of interest. 

As an example of the form of the information that may be derived from a pyrolysis-MS, Figure 26 [69] shows the structure of the polycarbonate (PC) and the EI-MS spectra of pyrolysis compounds obtained by DPMS of poly(bisphenol-A-carbonate) at three different probe temperatures corresponding to the three TIC (total ion current) maxima shown in Figure 27(b) Figure 27 compares the MS-TIC curve with those obtained from thermogravimetry. (The TIC trace is the sum of the relative abundances of all the ions in each mass spectrum plotted against the time (or number of scans) in a data collection sequence [70].)... 

Figure 13 Detection of 6 ppm (w/v) of a mixed ethoxylated amine by ESI (electrospray) LC-MS. Total ion current (TIC) trace.

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. 

Fig. 11.17. Simulated mass chromatograms resulting from precursor ion and constant neutral loss tandem mass spectra (middle and bottom traces), illustrating the selectivity that those MS/MS scan modes can bring to chromatographic analyses. The top trace in the figure represents a total ion chromatogram obtained using a conventional single stage of mass analysis.

Fig. 21.14. Temperature-programmed capillary GC-MS total ion chromatograms for kerosene (upper trace) and Moth-Knox pesticide (lower trace). Note the similarity in the pattern of peaks, with the exception of the large peak in the pesticide sample (at a retention time of about 12.5 min). The mass spectrum and the retention time of this peak both corresponded to a standard of naphthalene.

Figure 2.2 shows the total ion current trace and a number of appropriate mass chromatograms obtained from the pyrolysis gas chromatography-mass spectrometry analysis of the polluted soil sample. The upper trace represents a part of the total ion current magnified eight times. The peak numbers correspond with the numbers mentioned in Table 2.1 and refer to the identified compounds. The identification was based on manual comparison of mass spectra and relative gas chromatographic retention times with literature data [34, 35] and with data of standards available. In some cases unknown compounds were tentatively identified on the basis of a priori interpretation of their mass spectra (labelled tentative in Table 2.1). 

Because no pretreatment of the samples was carried out, the peaks present in the total ion current trace reflect components generated by pyrolysis of primary compounds ( real pyrolysis products ) and components that are present as such in the sample and simply evaporate ( free products ). If desired these two types of products may be differentiated using wires with a Curie temperature of 358°C [36], It was demonstrated in separate analyses (not shown here) that most compounds were not generated by pyrolysis but were present as such in the sample and thermally extracted . Compounds 1-8 and 10-17, 27, 37, 38, 54 and 65 were only present in pyrolysis gas... 

Fig. 11.4 shows the total ion current trace and some mass chromatograms obtained by flash evaporation pyrolysis gas chromatography-mass spectrometric analysis of the polluted sediment sample. All compounds present in this complex mixture were not listed. A selection was made to exemplify several aspects of the screening approach. The peak number correspond with the numbers in Table 11.1. Identifications were based on the same criteria as mentioned above. Although several components were shown to be real pyrolysis products, all the compounds are present as such in the sample and resulted from simple thermal extraction from the wire. This was shown in separate analyses using ferromagnetic wires with a Curie temperature of 358°C. 

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