Mass spectrometry detection systems

Mass Spectrometer. The mass spectrometer is the principal analytical tool of direct process control for the estimation of tritium. Gas samples are taken from several process points and analy2ed rapidly and continually to ensure proper operation of the system. Mass spectrometry is particularly useful in the detection of diatomic hydrogen species such as HD, HT, and DT. Mass spectrometric detection of helium-3 formed by radioactive decay of tritium is still another way to detect low levels of tritium (65). Accelerator mass spectroscopy (ams) has also been used for the detection of tritium and carbon-14 at extremely low levels. The principal appHcation of ams as of this writing has been in archeology and the geosciences, but this technique is expected to faciUtate the use of tritium in biomedical research, various clinical appHcations, and in environmental investigations (66). 

Figure 2.13—Detection by mass spectrometry. TIC chromatogram obtained with a mass spectrometer as a detection system. The instrument is capable of obtaining hundreds of spectra per minute. The above chromatogram corresponds to the total ion current at each instant of the elution profile. It is possible to identify each of the components using its mass spectrum. In many instances, compounds can be identified with the use of a library of mass spectra. (Chromatogram of a mixture of 71 volatile organic compounds (VOCs), reproduced by permission ofTekmarand Restek, USA.)...

Flavor chemists have traditionally relied on mass spectrometry in conjunction with gas chromatography (GC/MS) to identify the structures of volatile flavor components in heated food systems. Mass spectrometry provides the molecular weights of fragment ions, which are useful for deducing-molecular structure. The MS detection limit is on the order of 1CT g, however detection limits for target compound analysis or chemical class detection via selected ion monitoring can be much lower. Extensive libraries of mass spectra are available even so, many new flavor compounds can often not be identified from MS data alone. 

The main advantage of OT CEC is that separation efficiency can be doubled using this type of column. The trade-off is that the OT columns can easily be overloaded and therefore require a sensitive detection system. The small diameter of these columns precludes the use of UV detection, and fluorometric detection or mass spectrometry (MS) needs to be used. The use of fused silica capillaries with a bubble cell at the detection window has been reported as an alternative to employ UV detection. This features limit, to a certain extent, the range of practical applications of OT CEC. 

The common procedure to generate silylated pyrolysates is to perform pyrolysis in a filament system followed by off-line derivatization with BSTFA. The chromatographic separation was done on a DB-5 column (60 m long, 0.32 mm i.d., 0.25 pm film thickness) using a temperature gradient between 50° C and 300° C with detection by mass spectrometry. The compounds identified by mass spectral library search in the pyrograms from Figures 12.3.3 and 12.3.4 are listed in Table 12.3.2. 

In the next steps which could also be thermally induced, the aromatic systems (3080, 1600, and 1515 cm-1) decompose, while acetylenes (3320 and 2255 cm-1) are formed. At the same time isocyanate species are detected (2270 cm-1), which decompose upon further irradiation, or by reaction with other species (e.g., water to form amines). This decomposition is at least partially thermal, because at low repetition rates (0.086 Hz as compared to 10 Hz) the decrease of the isocyanate band is less pronounced. In the following steps nitrile (2230 cm-1) and aliphatic hydrocarbons (CH) are formed (2950 cm-1), as shown in Fig. 65. The increase of the peak area of the aliphatic CH compounds is slower and nearly linear with pulse numbers, suggesting that these species are formed continuously, probably through combination reactions. The volatile products detected by mass spectrometry and... 

Mass spectrometiy (MS) is a powerful tool for identifying compounds that are related. Coupled to an LC system, mass spectrometry can often detect the similarities and relatedness of materials corresponding to chromatographic peaks by their characteristic fragmentation pattern and/or by the fact that they contain particular atoms, e.g.. Cl and Br, that have characteristic isotope ratios. More simply, it may just be that two compounds have a similar molecular weight or molecular weights separated by expected or explicable differences ... 

Several detection systems are utilized in CE for the analysis of nucleoside and nucleotide mixtures. The performances of UV-visible absorption, conductance, electrochemical, a- P radiochemical and fluorescence detectors and mass spectral interfacing have been compared recently. Although UV-visible absorption is generally considered as not very sensitive, low limits of detection (LODs) of 8x10 mol 1 have been reported for purine metabolites using this method. The conductivity technique suffers from poor sensitivity. Electrochemical detection has a higher sensitivity, but its usefulness is limited by the fact that only electroactive species can be detected. Detection by mass spectrometry (MS) leads to poor sensitivity and implies expensive instrumentation. Radiochemical detection has been applied to a- P-labeled thymidine, cytidine, and adenosine... 

Amador TA, Elisabetsky E, Souza DO (1996) Effects of Psychotria colorata alkaloids in brain opioid system. Neurochem Res 21 97-102. doi 10.1007/BF02527677 Verotta L, Peterlongo F, Elisabetsky E, Amador TA, Nunes DS (1999) High-performance liquid chromatography - diode array detection - tandem mass spectrometry analyses of the alkaloid extracts of Amazon Psychotria species. J Chromatogr A 841 165-176. doi 10.1016/ 80021-9673(99)00298-8... 

No single analytical techniqne can provide detailed characterization of complex mix-tnres of closely related molecnles as is frequently encountered with polymeric systems. Mass spectrometry (MS) is one of the most promising becanse of its high sensitivity and mass resolution, and especially because of its ability to characterize individual oligomeric molecules of a polymer distribution. Many polymers and polymeric blends are too complex to be detected and analyzed directly by MS techniques, and condensed-separation methods are employed but suffer the drawbacks discussed earlier. 

The advantages of FIA-HPLC systems for the simultaneous determination of these preservatives have been clearly described above. However, the simultaneous separation of additives by HPLC is hindered by polarity differences. GC, with or without derivatization, is also employed for the selective determination of food preservatives [32]. The Association of Official Analytical Chemists (AOAC) official GC method [33] for preservatives in foods involves several extractions, evaporation, derivatization to a trimethylsilyl ester, and flame ionization detection (FID). Mass spectrometry (MS) is a sensitive and selective technique that is gradually gaining ground in the determination of these additives by GC, but involves sample pretreatments similar to that of the AOAC method. 

Unlike other members of this group, the DNA strand scission by C-1027 occurs without activation by a thiol or nucleophile [207, 208]. The formation of radical intermediates causing DNA cleavage by the antibiotic C-1027 was confirmed by spin trapping. The signals of the spin adducts with 2-methyl-2-nitrosopropane were recorded by ESR spectroscopy. The spin adducts were also detected by mass spectrometry (Scheme 3.1) [208]. It was shown that the antibiotic C-1027 is an equilibrium mixture of forms 3.407 and 3.410, the latter in very low concentrations. The process of hydrogen atom elimination from the DNA and the formation of aromatic structure 3. 411 were found to proceed in two steps first, slowly from atom C and then from atom C in a fast step [208, 209]. These data help us to understand the mechanisms of DNA strand scission by the antibiotic and thus contribute to the development of new anticancer drugs with specific delivery systems [208, 209]. Maduropeptin 3.408 is also able to form diradicals without activation [210]. 

The first mass spectrometric investigation of the thiazole ring was done by Clarke et al. (271). Shortly after, Cooks et al., in a study devoted to bicydic aromatic systems, demonstrated the influence of the benzo ring in benzothiazole (272). Since this time, many studies have been devoted to the influence of various types of substitution upon fragmentation schemes and rearrangements, in the case of alkylthiazoles by Buttery (273) arylthiazoles by Aune et al. (276), Rix et al. (277), Khnulnitskii et al. (278) functional derivatives by Salmona el al. (279) and Entenmann (280) and thiazoles isotopically labeled with deuterium and C by Bojesen et al. (113). More recently, Witzhum et al. have detected the presence of simple derivatives of thiazole in food aromas by mass spectrometry (281). 

The mass spectrometer (ms) is a common adjunct to a chromatographic system (see Mass spectrometry). The combination of a gas chromatograph for component separation and a mass spectrometer (gc/ms) for detection and identification of the separated components is a powerful tool, particularly when the data are collected usiag an on-line data-handling system. QuaUtative information inherent ia the separation can be coupled with the identification of stmcture and relatively straightforward quantification of a mixture s components. 

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