Mass spectrometric detection MS

Mass spectrometry is becoming the most common detector for the analysis of VOCs in soil samples, particularly as bench top instruments have increased in performance and decreased in price. Very often, the nature of the contaminants present is unknown and MS is used to identify individual compounds in the sample. Mass spectrometry may be used either in full scan mode or selected ion monitoring, depending on the nature of the compounds to be analysed and whether or not identification and quantitation are required. The use of selected 

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G. S. Frysinger and R. B. Gaines, Comprehensive two-dimensional gas chromatogr aphy with mass spectrometric detection, (GC X GC/MS) applied to the analysis of petr oleum , /. High Resolut. Chromatogr. 22 251 -255 (1999). 

The ion spray liquid chromatography/mass spectrometry (LC-MS) interface coupled via a postsuppressor split with an ion chromatography (IC) has been used in the analysis of alcohol sulfates. The IC-MS readily produces the molecular weight while the tandem mass spectrometric detection IC-MS-MS provides structural information [305]. 

Confirmation of suspected residue findings relies on the various chromatographic principles of cleanup and determination (GPC, NP-LC, GC), and is further supported by re-analysis of the final extract(s) on a GC stationary phase of different polarity, providing modified selectivity, or by the use of GC with specific mass spectrometric detection [GC/MS or gas chromatography/tandem mass spectrometry (GC/MS/MS)]. 

Alternative approaches consist in heat extraction by means of thermal analysis, thermal volatilisation and (laser) desorption techniques, or pyrolysis. In most cases mass spectrometric detection modes are used. Early MS work has focused on thermal desorption of the additives from the bulk polymer, followed by electron impact ionisation (El) [98,100], Cl [100,107] and field ionisation (FI) [100]. These methods are limited in that the polymer additives must be both stable and volatile at the higher temperatures, which is not always the case since many additives are thermally labile. More recently, soft ionisation methods have been applied to the analysis of additives from bulk polymeric material. These ionisation methods include FAB [100] and LD [97,108], which may provide qualitative information with minimal sample pretreatment. A comparison with FAB [97] has shown that LD Fourier transform ion cyclotron resonance (LD-FTTCR) is superior for polymer additive identification by giving less molecular ion fragmentation. While PyGC-MS is a much-used tool for the analysis of rubber compounds (both for the characterisation of the polymer and additives), as shown in Section 2.2, its usefulness for the in situ in-polymer additive analysis is equally acknowledged. 

By coupling GC with MS as the detection system (either ion trap or quadrupole mass spectrometer), structural information can be obtained regarding unknown or unexpected components. Another advantage of using mass spectrometric detection is that it can distinguish analytes tagged with stable isotopes, offering the possibility to use them as... 

For the quick characterisation of polydisperse surfactants with relative high molecular weight distributions matrix-assisted laser desorption/ionisation (MALDI)-time of flight (TOF)-MS represented an interesting alternative since low mass compounds did not interfere with the mass spectrometric detection of the compounds of interest. For example, the mass spectrum of C12-APG (Fig. 2.7.8) exhibited equally spaced signals with Am/z 162 corresponding to sodiated adduct ions of the mono- (m/z 371) to heptaglucosides (m/z 1343) [7]. 

Whilst these methods are informative for the characterisation of synthetic mixtures, the information gained and the nature of these techniques precludes their use in routine quantitative analysis of environmental samples, which requires methods amenable to the direct introduction of aqueous samples and in particular selective and sensitive detection. Conventionally, online separation techniques coupled to mass spectrometric detection are used for this, namely gas (GC) and liquid chromatography (LC). As a technique for agrochemical and environmental analyses, high performance liquid chromatography (HPLC) coupled to atmospheric pressure ionisation-mass spectrometry (API-MS) is extremely attractive, with the ability to analyse relatively polar compounds and provide detection to very low levels. 

Owing to the anionic character of LAS, an electrospray ionisation (ESI) interface operated in negative ion mode is particularly attractive for the mass spectrometric detection of this surfactant type. Consequently, a great part of the atmospheric pressure ionisation-mass spectrometry (API-MS) work on LAS is devoted to the application of (— )-ESI-MS. 

LC separation applying ion chromatography in combination with ion spray mass spectrometric detection was applied for the examination of a synthetic mixture of alkyl sulfonates (CnH2n+i-SO3 re = 8) and AS with different alkyl chain lengths in the selected ion monitoring (SIM) ESI-MS(—) mode [53], Selected ion current profiles provided the separation of the compounds. The ionic matrix constituents of the eluent were removed by a suppressor module prior to MS detection to improve the signal to noise (S/N) ratio. 

Bayliss and co-workers [10] combined ultra-high flow rates, parallel LC columns, a multiplex electrospray source, and mass spectrometric detection for the rapid determination of pharmaceuticals in plasma using four narrow bore (50 mm x 1 mm, 30 pm Oasis HLB) or capillary (50 mm x 0.18 mm, 25 pm Oasis HLB) HPLC columns with large particle sizes (to avoid high system back-pressure) in parallel with a multiple probe injector and a MUX MS interface. Small sample aliquots were injected directly into the system without sample pre-treatment procedure, obtaining very low limits of quantification (from 1 to 5 ng/mL). 

Bedard and May [452] used congener-specific GC with electron capture detection and mass spectrometric detection to determine the PCBs in sediments of Woods Pond (Lennox,MA). The congener distributions of all samples showed the hexa-, hepta-, and octachlorobiphenyls characteristic of Aroclor 1260, but key hexa- and hepta-CBs had decreased by as much as 45% relative to Aroclor 1260, and the tri-, tetra-, and penta-CBs had increased. GC-MS analysis revealed unusual tetra-, penta-, and hexa-CBs, many containing 2,4- and 2,4,6-chloro-phenyl rings, which are uncommon in higher Aroclors, and provided strong... 

Solubility screens using LC/MS detection do not require an ultra-pure sample of the test compound due to the selective detection of the mass spectrometer. Mass spectrometric detection offers high selectivity and low detection limits, which eliminates the need to develop complex chromatographic methods. The LC/MS-based solubility screen surpasses the traditional HPLC/UV-based equilibrium solubility assay with increased throughput, minimal manual intervention, and high sensitivity and selectivity. 

LC-MS/MS has dramatically changed the way bionalysis is conducted. Accurate and precise quantitation in the pg ml scale is nowadays possible however one has to be aware of certain issues which are specific to mass spectrometric detection such as matrix effects and metabolite crosstalk. With the current growing interest in the analysis of endogenous biomarkers in biological matrices, quantitative bioanalysis with MS has certainly the potential to contribute further in this field with the development of multicomponent assays. Modern triple quadrupole instruments have the feature to use very short dwell times (5-10 ms), allowing the simultaneous determination of more than 100 analytes within the timescale of an HPLC peak. Due to the selectivity of the MS detection the various analytes... 

The assay principle for MS-based enzyme inhibition assay is shown in Fig. 5.1. The assay is based on the mass spectrometric detection of reaction products of... 

Samples are analysed using gas chromatography coupled with mass spectrometry (GC-MS). This instmment separates the organic hydrocarbons in the extract with a very high resolution fused silica capillary column prior to mass spectrometric detection. Acquisition parameters have been customized to detect the target list of 162 hydrocarbons with the sensitivity required to obtain 1 part-per-trillion reporting limits which represents an innovation in these analytics. 

Undoubtedly, mass spectrometric detection has a substantial role to play in condensed-phase chromatographic analyses of toxic impurities. As in GC/MS, it can be highly sensitive, although this is probably more analyte-specific than in GC/MS. Selectivity can be gained by SIM on single quadrupoles or, if necessary, SRM on MS/MS instruments. What must be considered is the appropriate ionisation mode to be used in LC/MS. Most modern instruments use atmospheric pressure ionisation sources, including electrospray ionisation (ESI), atmospheric pressure chemical ionisation (APCI) and more recently atmospheric pressure photoionisation (APPI). 

The gas-liquid chromatography with mass spectrometric detection (GLC-MS) analysis of the electrolyzed solution has shown that thiophenol is the only reduction product and the S—S bond cleavage is quantitative. Such a mechanism of bond breaking was confirmed by electrochemical studies. In cyclic voltammograms, anodic and cathodic peak potentials were the same for thiophenol and diphenyl disulfides thus the same species were participating in these processes. Electrode reactions of diphenyl disulfide are given by the following equations [166] ... 

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