Total ion monitoring

Using MS detection relaxes the constraints on LC resolution, because additional separation occurs in the mass domain. In principle, LC-MS may yield a complete 2D distribution of a polymer according to chemical composition and molar mass. If MS detection is employed, the efficient cleaning in the LC step makes it possible to use total ion monitoring and even to identify unknown compounds from the sample. As extracts often contain interfering compounds, mass spectrometry in selective ion mode is a practical detector. Fully automated multidimensional LC-MS-MS-MS systems are available. 

Figure 5. Presumed Structural and Optical Isomers Contained in DPG and Isolation of Isomers by Glass Capillary GC-MS (Total Ion Monitoring). (Reproduced with permission from Ref. 23. Copyright 1985 The Society of Fermentation Technology, Japan.)...

The use of the MS total ion monitor (TIM) output as a detector is recommended to assure the detection of all eluting substance in the sample. It is a "universal" detector, responding to all eluting species. 

A total ion monitor (TIM), though not part of the interface, is often useful for the GC MS operation. The TIM is an ion detector positioned in the analyzer tube between the ion source and the magnet and is adjustable to collect a certain percentage of the total ions formed. This detector response is registered on a dual pen strip chart recorder with that of the GG flame detector (Figure 6). There may be some difiFerences in peak ratios because of differences in detector responses to the various compounds the mass spectra are taken at the TIM peak maxima since the ions are then most concentrated. Pesticide residue analysis by GG MS... 

Basically the same Py-GC technique developed to study the tacticity of PS was applied to determine the tacticity of various stereoregular poly(methyl methacrylate) (PMMA) samples by separating the associated diastereomeric-tetramers of which identification was carried out by a directly coupled Py-GC /MS system. Figure 3.7 shows a typical pyrogram of PMMA observed in the total-ion monitor (TIM) by a Py-GC/MS system in Cl mode, where various minor peaks of MMA dimers, trimers, tetramers, and pentamers are recognized in addition to the main monomer one. 

Pyrogram of syndiotaclicity-rich poly(methyl methacrylate) observed in total-ion monitor by Py-GC/MS in Cl mode at 500°C, from Ref. 18. Column fused silica capillary (HP Ultra 1,0.2 mm I. D. X 25 m, 0.33 /an of immobilized PDMS) programmed from 50 to 280°C at 4°C/min. Sample... 

MS can work in total ion monitoring (TIM) mode when the full mass range from which specific ions can be extracted is observed. If sensitivity needs to be improved, MS can work in the selected ion monitoring (SIM) mode when only a few selected ions were observed. 

GC/MS separation of mixtures of the compounds are usually performed on capillary columns with low and mid polarity and a length in the range of 30 50 m, with a total separation time of20 40 min, and temperature ramping from 40 to 300 °C. Total ion current (TIC) profiles are often obtained using ion trap or quadrupole analysers. Quantification is performed by selected-ion monitoring (SIM) detection using calibration curves. 

Figure 5.19 Formation of amino acids on ice surfaces irradiated in the laboratory (Nature Nature 416, 403-406 (28 March 2002) doi 10.1038/416403a-permission granted). Data were obtained from analysis of the room temperature residue of photoprocessed interstellar medium ice analogue taken after 6 M HCl hydrolysis and derivatization (ECEE derivatives, Varian-Chrompack Chirasil-L-Val capillary column 12 m x 0.25 mm inner diameter, layer thickness 0.12 pirn splitless injection, 1.5 ml min-1 constant flow of He carrier gas oven temperature programmed for 3 min at 70°C, 5°C min-1, and 17.5 min at 180°C detection of total ion current with GC-MSD system Agilent 6890/5973). The inset shows the determination of alanine enantiomers in the above sample (Chirasil-L-Val 25 m, single ion monitoring for Ala-ECEE base peak at 116 a.m.u.). DAP, diaminopentanoic acid DAH, diaminohexanoic acid a.m.u., atomic mass units.

Isotope dilution gas chromatography-mass spectrometry has also been used for the determination of ppb of total chromium in seawater [181-183]. The samples were reduced to ensure Cr111 and then extracted and concentrated as tris (l,l,l-trifluoro-2,4-pentanediono) chromium (III) [(Cr(tfa)3>] into hexane. The Cr(tfa)2 mass fragments were monitored into a selected ion monitoring (SIM) mode. 

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. 

Total or partial ion suppression is a well-known LC-MS effect, which is induced by coeluting matrix components that can have a dramatic effect on the intensity of the analyte signal. As can be observed in Fig. 1, analyte suppression occurs as a consequence of the different matrix interferences present in waste-water samples, making the identification and/or quantification process difficult or unfeasible. Even when working under selection ion monitoring (SIM) conditions, these matrix effects can cause ion suppression in the detection of some analytes that are present at low levels of concentration, as seen in this figure. Several papers have reported this effect [30-32] and different alternatives to overcome these problems, such as the inclusion of a size-exclusion step [33] or sequential SPE [28], have been applied for the determination of pesticides in... 

Fig. 21.10. Total ion (TIC) and selected ion monitoring (at m/z = 104) chromatograms of the headspace of the acceptable and unacceptable flavor samples. (Redrawn/redrawn from J. Chromatogr., 351, R.A. Sanders, and T.R. Morsch, Ion profiling approach to detailed mixture comparison. Application to a polypropylene off-odor problem, 525-531, Copyright (1986) with permission from Elsevier.)...

Ser SIM SIR SRM TFA Thr TIC ToF Trp TSP Tyr UV V vol/vol Val serine selected-ion monitoring selected-ion recording selected-reaction monitoring trifluoroacetic acid threonine total-ion current time-of-flight tryptophan thermospray tyrosine ultraviolet volt volume by volume valine... 

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