Mass selective detectors, liquid chromatography

Chemical Analysis. Gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC) techniques were used to analyze 4-chlorophenol and its oxidation intermediates. For GC-MS analysis, the samples were acetylated in pyridine. The samples were first evaporated to dryness. Then 200 xL of pyridine and 200 (xL of acetic anhydride were added to the dry residue. The samples were heated at 65 °C for 2-3 h to ensure the complete acetylation reaction, and then gently evaporated to dryness in a nitrogen stream. Finally, the residue was redissolved in 0.1 mL of hexane for GC analysis. A GC (HP model 5890) equipped with mass selective detector (HP model 5971) and SPB-5 capillary column (Supelco Co., PA., 25- X 0.2-mm i.d. X 0.33-p.m film thickness) was used. To separate different intermediate products, various oven-temperature programs were performed. The GC-MS interface line was maintained at 300 °C. The mass-... 

The capability of HPLC has been greatly extended by the commercialisation of affordable liquid chromatography-mass spectroscopy (LC-MS) instruments. The mass selective detector enables unknown samples to be analysed and the antioxidants present to be identified. The LC-MS technique therefore compliments GC-MS and, with it, enables the complete molecular weight/volatility/polarity range to be covered. 

The mass-selective instability mode of operation permits the selection and trapping of all ions created over a specified period with subsequent ejection to the detector.26 Ions with different m/z values can be confined within the ion trap and scanned singly by application of voltages that destabilize the orbits of the ions and eject them to the detector. Ion trap instruments interface readily with liquid chromatography, ESI,15 and MALDI.27 The motions of the ions and the dampening gas (e.g., helium) concentrate around the middle of the ion trap, thereby diminishing ion loss through collisions with electrodes. 

In the last twenty years, many of the developed and validated high performance liquid chromatography methods with conventional diode array or fluorescence detectors (DAD, FLD) were improved and substituted by new hyphenation with mass spectrometric instrumentation and/or NMR, especially for the analyses of raw materials derived from Natural sources. The main goal of this coupling is achieved by improvement of selectivity and sensitivity of new instrumental configurations [7], Furthermore, with these configurations it is possible to obtain, in only one analysis, the complete chemical structure elucidation, identification and quantification of targeted compounds. 

The mass spectrometer is a very sensitive and selective instrument. However, the introduction of the eluent into the vacuum chamber and the resulting significant pressure drop reduces the sensitivity. The gas exhaust power of a normal vacuum pump is some 10 ml min-1 so high capacity or turbo vacuum pumps are usually needed. The gas-phase volume corresponding to 1 ml of liquid is 176 ml for -hexane, 384 ml for ethanol, 429 ml for acetonitrile, 554 ml for methanol, and 1245 ml for water under standard conditions (0°C, 1 atmosphere). The elimination of the mobile phase solvent is therefore important, otherwise the expanding eluent will destroy the vacuum in the detector. Several methods to accomplish this have been developed. The commercialized interfaces are thermo-spray, moving-belt, electrospray ionization, ion-spray, and atmospheric pressure ionization. The influence of the eluent is very complex, and the modification of eluent components and the selection of an interface are therefore important. Micro-liquid chromatography is suitable for this detector, due to its very small flow rate (usually only 10 p min - ). 

The number of detectors that are sensitive and selective enough to be applied online with LC is limited because the solvents used are not compatible, as in the case of immunochemical detection after reversed- or normal-phase LC. The technology of coupling is still under development and not yet available in a large number of laboratories not specialized in techniques such as LC-MS. Therefore, LC separations are frequently followed by offline detection. Confirmatory analysis of suspected liquid chromatographic peaks can be made possible by coupling liquid chromatography with mass spectrometry. Atmospheric-pressure chemical ionization LC-MS has been employed for the identification of six steroid hormones in bovine tissues (448). 

The successful combination of mass spectrometry with gas chromatography (GC-MS) and, subsequently, with liquid chromatography (HPLC-MS) allowed not only the determination of urea pesticides in food but also the identification of their residues at trace level. Mass spectrometry is a technique that can be used as a general detector, with cyclic scanning. The selectivity and sensibility of analysis can be enhanced using characteristic ions of the molecule, with selected ion monitoring (SIM). Urea pesticides have been determined by HPLC-MS directly (175-180), without the thermal instability problems of GC analysis. 

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