Techniques That Enhance Sample Volatilization

Electron impact mass spectrometry (El) and chemical ionization mass spectrometry (Cl) remain the most commonly employed mass spectrometric techniques due to their relatively low cost and simplicity. Negative chemical ionization mass spectrometry (NCI) is used less often, but remains useful. 

Many natural products, however, such as peptides - especially those containing arginine - oligosaccharides, nucleotides, and others with high molecular weights, cannot be successfully analyzed by these more common methods. More sophisticated techniques of ionization have therefore been developed, capable of analyzing compounds with molecular weights of 1000 to 5000. These techniques have made the question of whether ionization occurs prior to or subsequent to desorption somewhat moot. 

FD-MS has been shown to be an excellent method for recording the mass spectra of compounds showing both molecular and fragment-ion formation (1, 76, 88, 95, 108, 161, 372). Compounds that do not produce good mass spectra due to instability even under field desorption conditions - such as zwitterions - can be 

Several recently developed techniques to enhance sample desorption capitalize on the transfer of kinetic energy to the sample through molecular collisions to produce desorbed positive and negative ions as well as desorbed neutral molecules. The more useful of these bombardment techniques include secondary ion mass spectrometry (SIMS), fast atom bombardment mass spectrometry (FAB-MS), and californium-252 plasma desorption mass spectrometry (252 Cf-MS). The sample can be deposited on an inert surface and directly bombarded in the solid state, or more commonly it is dissolved (or suspended) in an inert and nonvolatile medium such as glycerol. 

The utility of Cf-MS (242, 243) is based on the fact that 3% of the radioactive decay of Cf is spontaneous fission, the remaining 97% being a-decay. More than 40 ion pairs have been reported as the products of the spontaneous fission, such as and with kinetic energies of 79 and 104 MeV, 

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Gas chromatography with open-tubular capillary columns (GC) is a powerful separation technique that is particularly suitable for determining (semi-)volatile compounds. Application to less volatile analytes is also possible provided that these analytes are first transformed into volatile derivatives. Since its introduction by Golay [1], many examples have illustrated the potential of GC for accurate identification and quantification of individual analytes in many types of difficult real-life mixtures. The excellent resolution provided by present-day onedimensional (ID) GC, combined with its accuracy and robustness, makes this technique the preferred separation approach in a variety of application areas. However, the improved detection capabilities provided by state-of-the-art detectors have also shown that, in many cases, aroma, food, petrochemical, and environmental samples are much more complex than was assumed a decade ago. Improved knowledge about the composition of such samples demands enhanced resolution — that is, adding an extra separation/identification capability over that achieved with ID GC. For example, considering that ID GC separation often... 

Soxhlet extraction (EPA SW-846 3540) is a very efficient extraction process that is commonly used for semivolatile petroleum constituents. In the method, the solvent is heated and refluxed (recirculated) through the soil sample continuously for 16 hours, or overnight. This method generates a relatively large volume of extract that needs to be concentrated. Thus, it is more appropriate for semivolatile constituents than for volatile constituents. Sonication extraction (EPA SW-846 3550) can also be used for semivolatile compounds, and as the name suggests, involves the use of sound waves to enhance analyte transfer from sample to solvent. Sonication is a faster technique than Soxhlet extraction and can require less solvent. 

The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

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