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Widely used commodity gases such as nitrogen, oxygen and argon are obtained by cryogenically liquefying ambient air, then distilling it into separate fractions of various purity levels. Pressure swing absorption (PSA) methods are also used to produce air separation gases however the gas purity achieved is significantly lower.
Gas Purity is usually expressed in terms of “ number of nines”. For example, a gas described as ”five nines” is 99.999% pure or contains no more than 0.001% = 10 ppm v/v of defined impurities. A gas purity expression of 99.999% is actually obtained by an “impurity subtraction” process. This involves defining the most likely contaminants, measuring each targeted impurity, then subtracting this total from 100%. The reason for this indirect process is that direct purity measurements are not accurate enough to determine gas purity beyond about 99.99%. Therefore, in practice, several types of specialized instruments and sampling techniques are required to determine gas purity. In some cases routine analysis programs for total purity can miss impurities that are not normally expected and so not detected or measured.
Impurities routinely monitored in air separation & specialty gases include, for example: nitrogen, oxygen, argon, krypton, hydrogen, helium, carbon monoxide, carbon dioxide, nitrous oxide, water, methane, acetylene, non-methane hydrocarbons, sulfur oxides, nitrogen oxides, halogenated organics, particulates, etc.
ALI has the experience and resources needed to accurately determine the purity of air separation and other commodity gases. In addition to using standard GC’s and dedicated analyzers we can employ our sophisticated GC/MS & FTIR resources to check for any “unexpected” impurities.
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