MS as detector

The flame ionization detector (FID) is the detector most often used in steroid analyses. For very low concentrations of steroids, the application of ECD is needed. Thermal conductivity detectors (TCDs) cannot be used in the analysis of steroids because of their very low sensitivity. For steroidal alkaloids, a nitrogen-specific detector (NPD) has also been used. By the use of dual detector systems (e.g., FID and NPD), closely related nitrogen-containing and non-nitrogen-containing steroids can be easily differentiated. The application of MS as detector was already discussed in a previous entry in this encyclopedia." By using a GC/MS system, the identity of the peak(s) can be determined in an undisputed manner. ... 

Water is evaporated and trapped from aqueous or moist samples as well. Most of which is disposed of by the dry purge step, particularly when using Tenax adsorption traps with low water retention. Residual moisture can still be transferred to the GC column during the desorption step (Madden and Lehan, 1991). As the resolution of highly volatile substances on capillary columns would be impaired and the detection by the mass spectrometer would be affected, additional devices are used to remove water. In particular, where the P T technique is used with ECD or MS as detectors, reliable water removal is necessary. Different technical solutions working automated during the desorption phase are in use with the P8dT instruments of different manufacturers. 

Hieftje and co-workers have given an excellent overview of the applications of ICP-TOF-MS, as detector for chromatography and capillary electrophoresis (CE), for elemental speciation studies. For the interested reader, speciation using ICP-MS is covered in detail in Chapter 7 of this book. 

So far, only a handful of papers have been published using MC-ICP-MS as detector for coupling techniques in speciation analysis in order to benefit from the superior isotope ratio precision. Several factors have to be taken into account when using MC-ICP-MS instrumentation ... 

Confirmation of the identities of nitrosamines generally is accompHshed by gas chromatography—mass spectrometry (gc/ms) (46,87). High resolution gc/ms, as well as gc/ms in various single-ion modes, can be used as specific detectors, especially when screening for particular nitrosamines (87) (see Analytical LffiTHODS Trace and residue analysis). 

In addition, the appHcation of the mass spectrometer (ms) as a detector for gas—Hquid chromatography has made the positive identification of peaks possible. High performance Hquid chromatography (hplc), which involves various detectors, can be used to measure hydrophilic and hydrophobic organic compounds in water. 

Infrared (in) spectrometers are gaining popularity as detectors for gas chromatographic systems, particularly because the Fourier transform iafrared (ftir) spectrometer allows spectra of the eluting stream to be gathered quickly. Gc/k data are valuable alone and as an adjunct to gc/ms experiments. Gc/k is a definitive tool for identification of isomers (see Infrared and raman spectroscopy). 

The application areas for LC-MS, as will be illustrated later, are diverse, encompassing both qualitative and quantitative determinations of both high-and low-molecular-weight materials, including synthetic polymers, biopolymers, environmental pollutants, pharmaceutical compounds (drugs and their metabolites) and natural products. In essence, it is used for any compounds which are found in complex matrices for which HPLC is the separation method of choice and where the mass spectrometer provides the necessary selectivity and sensitivity to provide quantitative information and/or it provides structural information that cannot be obtained by using other detectors. 

Gas and liquid chromatography directly coupled with atomic spectrometry have been reviewed [178,179], as well as the determination of trace elements by chromatographic methods employing atomic plasma emission spectrometric detection [180]. Sutton et al. [181] have reviewed the use and applications of ICP-MS as a chromatographic and capillary electrophoretic detector, whereas Niessen [182] has briefly reviewed the applications of mass spectrometry to hyphenated techniques. 

GC-AAS has found late acceptance because of the relatively low sensitivity of the flame graphite furnaces have also been proposed as detectors. The quartz tube atomiser (QTA) [186], in particular the version heated with a hydrogen-oxygen flame (QF), is particularly effective [187] and is used nowadays almost exclusively for GC-AAS. The major problem associated with coupling of GC with AAS is the limited volume of measurement solution that can be injected on to the column (about 100 xL). Virtually no GC-AAS applications have been reported. As for GC-plasma source techniques for element-selective detection, GC-ICP-MS and GC-MIP-AES dominate for organometallic analysis and are complementary to PDA, FTIR and MS analysis for structural elucidation of unknowns. Only a few industrial laboratories are active in this field for the purpose of polymer/additive analysis. GC-AES is generally the most helpful for the identification of additives on the basis of elemental detection, but applications are limited mainly to tin compounds as PVC stabilisers. 

Several studies have focused on the use of low-pressure and atmospheric-pressure MIP-MS as a detector for GC [334]. Atmospheric-pressure GC-MIP-MS systems have some limitations with regard to analysing many low-mass elements (P, S, Cl, etc.). Low-pressure MIP-MS is better equipped for this purpose. Both non-metals and metals have been analysed by GC-MIP-MS. 

A limitation in the use of API sources results from the frequent application of mobile-phase composition programming in pSFC. Pinkston el al. [411] have compared electrospray and electron impact for open-tubular and packed-column SFC-MS. Direct on-line coupling of SFC to FAB/MS (as well as SFC-ELSD) is also very promising to detect components which give no response in a UV detector [412]. 

The two remaining shortfalls with MALDI-MS analysis of whole bacterial cells are sensitivity and mixture analysis. The sensitivity for MALDI-MS analysis of whole-cell bacteria from our experiments and those reported by other laboratories is about 107 cells/ml. To realistically utilize MALDI MS as a tool that meets DoD detector sensitivity goals, this should be 103 cells/ml or lower. 

There are two different oligomer series present in all spectra. The oligomer series can be identified by calculating the masses of the end groups and assigning them to specific chemical structures (Pasch and Schrepp, 2003 Weidner et al., 2004). In the present example, the two species are the propionic amide-acid (R-am-ac) and the propionic amide-propionic amide polyamides (R-am-am-R). The use of MALDI-TOF MS as a structure-sensitive detector allows the resolution to be indirectly enhanced since several species coelute, as shown in Fig. 17.21. The polarity of the... 

Another approach in GC is that of using more power in the separation by doing GCxGC. In this approach, a second column is used with a different type of stationary phase than the primary stationary phase, and fast chromatography using TOF-MS as the detector is carried out [39]. This technique uses only TOF-MS as the detector since it has the most sensitivity for fast-eluting peaks. The method has been applied to complicated matrix analysis. 

Figure 13.5. Diagram of quadrupole MS used as detector for gas chromatography.

MS as a gas chromatographic detector has become much simpler to operate in the past decade. Nearly all control of the detector is performed through a data system with similar look-and-feel to other chromatographic data systems. Components such as ion sources now have less than 10 parts, making periodic cleaning and maintenance much simpler than in the past. Finally, the pricing of mass selective detectors is now similar to other selective detectors. It is likely that MS will eventually supplant most of the other selective detectors. 

We discussed the fundamentals of mass spectrometry in Chapter 10 and infrared spectrometry in Chapter 8. The quadrupole mass spectrometer and the Fourier transform infrared spectrometer have been adapted to and used with GC equipment as detectors with great success. Gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectrometry (GC-IR) are very powerful tools for qualitative analysis in GC because not only do they give retention time information, but, due to their inherent speed, they are also able to measure and record the mass spectrum or infrared (IR) spectrum of the individual sample components as they elute from the GC column. It is like taking a photograph of each component as it elutes. See Figure 12.14. Coupled with the computer banks of mass and IR spectra, a component s identity is an easy chore for such a detector. It seems the only real... 

Isotope dilution mass spectrometry (IDMS) is another method to overcome the problem of sample recovery [370-372]. The 13C-labeled isotope of the analyte is added to the sample at the commencement of the analysis and the ratio of the labeled and unlabeled compound is measured by MS. This technique eliminates the need for recovery measurements and automatically accounts for any losses in the determination [373]. The two major limitations of this method are the cost and availability of the labeled compounds and the need to use the MS as a detector. 

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