Process and apparatus for determining the quality of impurities in a gas by chromatography in gas phase and utilization for measuring the quantity of doping impurities in silane

A method of determining trace levels of at least one gaseous impurity loaded in a pure gas having a first liquefaction temperature, comprising:

• cooling with a cryogen a first column filled with a first adsorbent capable of adsorbing said at least one impurity at a first low temperature greater than said first liquefaction temperature;
• passing said gas containing said at least one impurity through said first column at said first low temperature to adsorb said at least one impurity;
• raising the temperature of the first column while passing through said first column a carrier gas having a second liquefaction temperature lower than said first liquefaction temperature to desorb said at least one impurity;
• passing said carrier gas carrying said desorbed impurity to a first chromatographic adsorber;
• conducting a first gas phase chromatography analysis to analyze said at least one impurity with a first level of accuracy;
• passing the carrier gas through the first chromatographic adsorber to desorb said at least one impurity;
• cooling with a cryogen a second column filled with a second adsorbent capable of adsorbing said at least one impurity at a second low temperature lower than said first liquefaction temperature and greater than said second liquefaction temperature;
• passing said carrier gas carrying said desorbed impurity through the second column to adsorb said impurity;
• raising the temperature of the second column while passing therethrough the carrier gas to desorb said impurity;
• passing said carrier gas carrying said desorbed impurity to a second chromatographic adsorber having a detection device;
• and
• conducting a second gas phase chromatography analysis to analyze said impurity with a second level of accuracy higher than said first level of accuracy of about 10.sup.-5 ppmv.