Sample handling gaseous samples

There are different modes of handling liquid, solid and gaseous samples in a GC which will be discussed briefly here ... 

Formation of gaseous chemical species during sample handling. 

Gaseous formaldehyde Indoor and ambient air UV-Vis 0.08 ppbv 300 Alternated gas collection (STOP)/sample handling (GO) involving two parallel diffusion scrubbers [144]... 

Radiolysis and photolysis products were analyzed by injecting samples into an F M gas chromatograph provided with a flame ionization detector. A 1/4-inch o.d. Teflon column 2 ft. long, packed with Porapak Q, was used to separate the components from the column in a reasonably short time. In some cases fractions were trapped out from the helium stream at the column outlet and analyzed on a C.E.C. 21-103C mass spectrometer. Conventional high vacuum techniques were used in handling all gaseous samples and products. 

Infrared spectrophotometry is a familiar established analytical technique which provides identification of compounds by fingerprint spectra, of which a vast library is available. Both liquid and gaseous samples may be easily analysed and therefore modifications of established sample handling techniques have enabled both GC and HPLC instruments to be readily interfaced. Ideally, scan times of less than 1 s are required to be able to record each peak and peak shoulders. Instrument sensitivity is sufficient so that on the fly recording of spectra can be obtained from GC and HPLC eluants which contain nanograms of sample per ml mobile phase, for example, 10 ng sample in 100 pi GC-IR sample cell. Fourier transform infrared (FTIR) instruments are able to meet these criteria but until recently the instrumentation and computer system have been too expensive for routine use. The new generation of... 

There are sample introduction systems that can handle slurries of particles suspended in hquids. Powders can be injected directly into the plasma for analysis. Lasers, sparks, and graphite furnaces (exactly the same as AAS graphite furnaces) are used to generate gaseous samples from sohds for introduction into the plasma. Hydride generation for As and Se and cold-vapor Hg introduction are used for ICP as for AAS these two techniques were discussed in Chapter 6. 

The virtues of ICP/MS include efficient production of positive atomic ions robustness of the plasma (tolerance of sample matrix material in the ion source) sample introduction at atmospheric pressure ability to handle solid, liquid or gaseous sample forms and speed of analysis. Sample preparation requires much less chemical purification than for TIMS, AMS or radiation detection by beta- or alpha-spectrometry. 

Several atomic absorption analytical problems are best handled by producing a gaseous sample external to the sampling cell. Elements frequently sampled in this manner include mercury, arsenic, selenium, and antimony. 

Currently, arc and spark source methods are mostly limited to the elemental analysis of solids because liquid and gaseous samples are handled so much more conveniently by the plasma emission methods that we have discussed in the previous sections. 

If the material to be studied is in the gaseous state, special difficulties arise because the sample is not visible. When handling gaseous specimens, the required material is drawn from a given volume and then tested, or a sample of the gas is caught and analyzed as a whole. 

The gaseous sample introduction line is connected directly to the central tube of the plasma torch, eliminating the need for the conventional nebulizer and spray chamber. Additional equipment to handle gas mixing and dilution at controlled flow rates may be required. 

For oxidation by atmospheric oxygen a sample in an EPR tube may simply be opened to air and stirred. However, reaction with any other gas than air requires special handling of the sample on the manifold of a Schlenk line. Examples are oxidation by pure oxygen, reduction by hydrogen, and also the reaction by any gaseous substrate or inhibitor such as CO, C02, NO, N20, etc. Basically, there are two different experimental approaches mixing with a solution in which the gas is dissolved or mixing with a pressurized atmosphere of the gas. 

Potassium bromide windows have been used by Boreskova et and by Liengme and Hall . The latter cell is a modification of the design by Little et Sample discs can be moved between the ir windows and a furnace, with the cell connected by a flexible metal tube to a vacuum and gas handling system. An identical ir cell is also placed in the reference beam. Because the gaseous atmosphere is present in both cells, it is automatically subtracted by the instrument and only the spectrum of adsorbed species is recorded. 

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