Detection methods mass spectrometry

Either gas chromatography (GC) or liquid chromatography (LC) can be used as a separation technique coupled with a variety of detection methods. Mass spectrometry (MS) is one of the most popular means of detection. When using GC-MS, a capillary column should be used, while any suitable LC column can be used for LC-MS. It is advisable to obtain a print-out of the chromatogram so that the shapes of individual peaks can be assessed. Electronically produced data using integrators should be treated with some suspicion and always examined visually to check the selected baseline, start- and end-points of peak integration, etc. 

There are several methods in use for producing these clusters. Particle bombardment or laser vaporization of a graphite surface leads to direct formation of ions that can be detected by mass spectrometry. These are normally of relatively small size (n<30). By laser vaporization of graphite into a molecular beam neutral... 

Several methods are available for the analysis of trichloroethylene in biological media. The method of choice depends on the nature of the sample matrix cost of analysis required precision, accuracy, and detection limit and turnaround time of the method. The main analytical method used to analyze for the presence of trichloroethylene and its metabolites, trichloroethanol and TCA, in biological samples is separation by gas chromatography (GC) combined with detection by mass spectrometry (MS) or electron capture detection (ECD). Trichloroethylene and/or its metabolites have been detected in exhaled air, blood, urine, breast milk, and tissues. Details on sample preparation, analytical method, and sensitivity and accuracy of selected methods are provided in Table 6-1. 

Another consideration when choosing a detector is whether it is important to preserve the separated analytes, either for use or for further analysis. Some methods, such as evaporative laser scattering detection and mass spectrometry, destroy the sample during the measurement. Other methods, such as fluorescence or radiochemical detection, may require chemical labeling of the analytes ... 

Great care has to be taken in the analytical characterization of synthetic cyclic peptides.[73] The major side reactions during cyclization are epimerization of the C-terminal amino acid residue and cyclodimerization. Cyclodimers can be detected by mass spectrometry, although the analysis is not trivial, because artifacts do occur in some ionization techniques such as ES-MS as a result of aggregation.1 1 Ll 121 Real dimers can be detected as double-charged particles with mlz values identical to the cyclic monomers, but with a mass difference of 0.5 amu in the resolved isotope signals. The mass difference of the corresponding monomer is 1 amu. The cyclodimerization has received some attention as a direct method for the synthesis of C2-symmetrical cyclic peptides.[62 67 94113 115]... 

For the measurement of a-iduronidase activity, a novel substrate was developed that was detected by mass spectrometry. It was possible to combine this method with similar assays for Niemann-Pick type A/ , Krabbe, Gaucher, Pompe, and Fabry disease. However, separate incubation is necessary, and some additional work-up procedures are usually required to purify the sample before mass spectrophotomet-ric analysis [68]. 

Although a great variety of analytical techniques have been applied to the simultaneous determination of methylxanthines in various matrices, HPLC is the one most frequently used nowadays. Most of the methods are based on reversed-phase HPLC, using ACN, MeOH, or THF in acetate or phosphate buffer as mobile phase and UV spectrophotometric detection (256 -270). Some RP-HPLC methods were proposed in combination with solid-surface room-temperature phosphori-metric detection (271), mass spectrometry (272), or amperometric (273) detection. The separation can also be achieved by RP ion-pair or ion-interaction HPLC (274-277) or micellar HPLC (278). In contrast, in recent years few normal-phase HPLC methods (279) were reported (see Table 5). 

Stenhoff et al. [117] determined enantiomers of omeprazole in blood plasma by normal-phase liquid chromatography and detection by atmospheric-pressure ionization tandem mass spectrometry. The enantioselec-tive assay of omeprazole is using normal-phase liquid chromatography on a Chiralpak AD column and detection by mass spectrometry. Omeprazole is extracted by a mixture of dichloromethane and hexane and, after evaporation, redissolution and injection, separated into its enantiomers on the chiral stationary phase. Detection is made by a triple quadrupole mass spectrometer, using deuterated analogs and internal standards. The method enables determination in plasma down to 10 nmol/1 and shows excellent consistency suited for pharmacokinetic studies in man. 

The HPLC/MS technique used in EPA Method 8321 is best suited for analysis of thermally unstable compounds that are hard to analyze with conventional GC methods, such as organophosphorus pesticides, chlorinated herbicides, and carbamates. In this technique, the detection with mass spectrometry provides the ultimate selectivity. The sensitivity for each individual compound depends on the interferences in a given environmental matrix and on the chemical nature of the analyte. 

The book includes several chapters on vapor and trace detection chemiluminescence, mass spectrometry, ion mobility spectrometry, electrochemical methods, and micro mechanical sensors, such as microcantilevers. Other chapters deal with bulk detection techniques neutron techniques, nuclear quadrupole resonance, X-ray diffraction imaging, millimeter-wave imaging, terahertz imaging, and laser techniques. Special chapters are devoted to personnel portals and to biological detection. 

The pyridone surface species has a C=0 stretching band at 1634 cm-1,3 Hydrogen gas has been detected by mass spectrometry (210), and the formation of this surface compound has been established by chemical methods by Boehm (215). This surface reaction points to the existence of strongly basic OH" ions held to certain sites on alumina surfaces, their number being of the order of magnitude of 1013/cm2 (121). Additional evidence for the existence of these reactive and strongly basic OH" ions on aluminas comes from surface reactions observed on adsorption of nitriles and ketones (see Section IV.F) and of carbon dioxide (see Section IV.G). These reactions may, thus, be valuable for the detection of the corresponding sites that most probably have to be considered as acid-base pair sites. 

Several examples are known in which the separation itself was done by using a planar technique, and the detection by using direct visualisation. X-ray detection or mass spectrometry after transfer of the spots to another plane. In one example the separation was done by two-dimensional (2D) slab electrophoresis and then applied to 2D-TLC directly combined with mass spectrometry 111. -115]. The methods cannot be directly applied for 2D polyacrylamide gel electrophoresis because an essential portion (S-Ll f) of the polyacrylamide gel is water, and therefore a 9 x 1.1 cm plate of polyacrylamide contains 15 ml water 1116. This is the reason that the gel cannot be directly inserted into the high-vacuum source of the mass spectrometer. To avoid the complications of freeze-drying and other methods of removing the water and to keep the substances in the gel as well as the form of the gel, the separated substances should be transferred to a plate. Nitrocellulose was found to have the properties required and this method was used to detect bradykinin and dynorphine using SIMS (Secondary /on Mass Spectrometry). A detailed description is given for TLC-SIMS (115]. 

K. B. Tomer, HPLC detection by mass spectrometry, in HPLC Detection, Newer Methods (G. Patonay, ed.), VCH, New York, 1992. 

Keywords High performance liquid chromatography (HPLC), Analytical biotechnology, Sample preparation, Diode-array-detection (DAD), Mass spectrometry (MS), Validation, Preparative chromatography, Competitive isotherm parameters, Perturbation peak (PP) method... 

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