Selective-ion monitoring

In a process similar to that described in the previous item, the stored data can be used to identify not just a series of compounds but specific ones. For example, any compound containing a chlorine atom is obvious from its mass spectrum, since natural chlorine occurs as two isotopes, Cl and Cl, in a ratio of. 3 1. Thus its mass spectrum will have two molecular ions separated by two mass units (35 -i- 2 = 37) in an abundance ratio of 3 1. It becomes a trivial exercise for the computer to print out only those scans in which two ions are found separated by two mass units in the abundance ratio of 3 1 (Figure 36.10). This selection of only certain ion masses is called selected ion recording (SIR) or, sometimes, selected ion monitoring (SIM, an unfortunate... 

Selected-ion monitoring (SIM). Describes the operation of a mass spectrometer in which the ion currents at one (or several) selected m/z values are recorded, rather than the entire mass spectrum. The use of the terms multiple-ion detection (MID), multiple-ion (peak) monitoring (MPM), and mass fragmentography are not recommended. 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

Solid-phase microextraction (SPME) was used for headspace sampling. The FFA were extracted from the headspace with PA, Car/PDMS, and CW/DVB fibers. It was examined whether addition of salt (NaCl) and decreasing the pH by addition of sulphuric acid (H SO ) increased the sensitivity. FFA were analyzed using gas chromatography coupled to mass spectrometry in selected ion monitoring. 

This set-up, or a very similar one, has been used to determine different group of pollutants in environmental waters (45, 83, 93). For example, with 10 ml of sample the limits of detection of a group of pesticides were between 2 and 20 ng 1 (92) in tap and river water, with this system being fully automated. Figure 13.19 shows the chromatograms obtained by on-line SPE-GC-MS under selected ion-monitoring conditions of 10 ml of tap water spiked with pesticides at levels of 0.1 pig 1 (92). 

Figure 15.8 Multidimensional GC-MS separation of urinary acids after derivatization with methyl chloroformate (a) pre-column cliromatogram after splitless injection (h) Main-column selected ion monitoring cliromatogram (mass 84) of pyroglutamic acid methyl ester. Adapted from Journal of Chromatography, B 714, M. Heil et ai, Enantioselective multidimensional gas chromatography-mass spectrometry in the analysis of urinary organic acids , pp. 119-126, copyright 1998, with permission from Elsevier Science.

Selected ion monitoring of mass 127 is used to determine the concentration of formaldehyde. 

Approximate retention time (min) Amino acid For selected ion monitoring... 

Watson, J. T., Hubbard, W. C., Sweetman, B. J., and Pelster, D. R. Quantitative analysis of prostaglandins by selected ion monitoring GC/MS. Advan. in Mass Spectr. in Biochern. Med II. New York Spectrum Publications, 1976. 

Retention times and suggested ions for selected ion monitoring (SIM) of TBDMS derivatives of AAs are given in Table 9.1 and the GC separation is shown in Figure 9.1. Amino acids should be derivatized prior to separation. The TBDMS derivatives are preferred and stable for at least 1 week. 

Table 9.3. Selected ion monitoring of some PTH-AAs as the TBDMS derivatives...

Selected ion monitoring (SIM) Describes the operation of a MS in which the ion currents of one or several selected m/z values are recorded, rather than the entire mass spectrum. 

The Use of Selected-Ion Monitoring to Examine the Number of Terminal Galactose Moieties on a Glycoprotein 143... 

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. 

In conventional mass spectromefiy, quantitative determinations are often carried out by using selected-ion monitoring (SIM), i.e. by monitoring the intensities of a small number of ions characteristic of the analyte of interest (see Section 3.5.2.1 below). 

What are the potential advantages of selected-decomposition monitoring (SDM) over selected-ion monitoring (SIM) ... 

The great advantage of SDM over SIM is that it usually confers even greater selectivity onto the analysis and reduces the amount of chemical noise observed. Selected-ion monitoring is concerned with particular... 

In this chapter, the main aspects of mass spectrometry that are necessary for the application of LC-MS have been described. In particular, the use of selected-ion monitoring (SIM) for the development of sensitive and specific assays, and the use of MS-MS for generating structural information from species generated by soft ionization techniques, have been highlighted. Some important aspects of both qualitative and quantitative data analysis have been described and the power of using mass profiles to enhance selectivity and sensitivity has been demonstrated. 

To understand the circumstances in which particular features of mass spectrometry, such as high-resolution measurements, MS-MS and cone-voltage fragmentation, selected-ion monitoring and selected-decomposition monitoring, may be nsed to address particular analytical problems. 

What is selected-ion monitoring and what are the advantages of using this techniqne ... 

Selected ion monitoring is a technique in which only the ions at a limited number ofmjz ratios chosen to be characteristic of the analyte(s)... 

Figure 5.8 Electrospray and transformed electrospray spectra of the light- and heavy-chain antibody fragments of recombinant ritnximab obtained by LC-MS analysis. Reprinted from 7. Chromatogr., A, 913, Wan, H. Z., Kaneshiro, S., Frenz, J. and Cacia, J., Rapid method for monitoring galactosylation levels dnring recombinant antibody production by electrospray mass spectrometry with selective-ion monitoring , 437-446, Copyright (2001), with permission from Elsevier Science.

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