Support Gases

Nitrous oxide is purchased in cyhnders containing 15,000 htres which provides some 12 hours of operation of an N2O/C2H2 flame. When removed from the tank at 15-20 L/min expansion of the gas in the pressure regulator can freeze the diaphragm rendering it inoperative. Heated regulators for N2O are recommended for AAS applications. As with C2H2, a portion of the N2O in the cylinder is liquefied so that the drop in head pressure with consumption will not be hnear. The areas where this gas is stored or used shoiild be well ventilated as this gas is an asphyxiant and a potential hazard in a confined space. 

Many laboratories prefer to obtain their air from a compressor rather than purchasing cylinders. A compressor capable of dehvering at least 30 L/min at a pressure of 40 psi is recommended. A water and oil trap between the compressor and the instrument is a necessity. A cylinder containing 6,200 L will last some 5 hours and the O2 content is somewhat variable from cylinder to cylinder. 

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Background emission by the flame (Figure 8.23) includes contributions from molecular species and continuum radiation from incandescent particles and depends upon the combination of fuel and support gases used. The sample solvent and matrix will further augment background radiation. 

Gas iromatography (GC) Gas-Liquid (GLC) Gas, e.g. N2, H2, He Liquid-film coated on solid support Gases and adequately volatile liquids (mol. wt. <300) of adequate thermal stability, e.g. hydrocarbons and their derivatives, solvents, inorganic gases, essential oils, some steroids and vitamins. 

Type of detector Category Support gases Selectivity Detectability Dynamic range... 

Combustion supporting gases include air, oxygen, ozone and carbon monoxide. General evaluation methods for the ability of a gas to support combustion have not been established. When a gas is compressed, it becomes likely to burst its container, but general evaluation methods for bursting have not been established. The bursting depends on characteristics of the vessel in which the gas is stored rather than on the characteristics of the gas itself. 

Support gases, e.g. carrier gas (helium), reference gas for the TCD (helium), hydrogen and air for the HD, as well as calibrated standard gases (specially prepared calibration samples) are supplied at the required pressure by separate lines. They are placed and stored in a purpose built divided into two separated cubicles (flatnmable and non flammable gases). The gas chromatograph is commanded by HP Chemstation A.6.03 software. 

Sensor array detection also is needed because these devices can be micro-fabricated with very low dead volumes they require no support gases for their operation, and they can be fabricated with a variety of selectivities, which can be used for vapor recognition and for the deconvolution of overlapping peaks. This can reduce the resolution requirements for the colunm. Sensor detectors usually have lower sensitivity than do detectors incorporated in laboratory gas chromatographic instruments. Low detector sensitivity, coupled with the very low concentrations often associated with air monitoring, requires the use of a sorption preconcentrator for sample enrichment prior to separation and detection. 

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