(US5412467-A)      The analysis comprises: (a) directing a sample of the gas through an electric discharge source; (b) applying an alternating source of power across the source and adjusting a variable frequency oscillator to provide a preselected excitation frequency with the power source having a peak voltage sufficient to sustain an electric discharge and to generate a wide radiation spectrum of emissive radiation from the gas stream; (c) filtering the radiation spectrum to form an optical signal with a narrower radiation emission bandwidth corresp. to a stronger emission wavelength(s) of a preselected impurity gas or vapour to be analysed; (d) converting the optical signal to an electrical signal; (e) selectively amplifying the electrical signal within a narrow frequency range centred at twice the excitation frequency; and (f) analysing the signal to determine the concn. level of the impurity gas or vapour under analysis.    USE/ADVANTAGE -   Used for analysing a continuously flowing gas stream using gas emission spectroscopy to detect a presence of gas of vapour impurities in a gas stream at low concn. levels. The sensitivity of detection of gas impurity is increased to a min. detection level (MDL) of below 20 ppb.    (US5831728-A)      Method for analysing a continuously flowing gas stream using gas emission spectroscopy to detect at least one gas or vapour impurity in the gas stream at low concn. levels, comprises (a) directing a sample of the gas stream through an electric discharge source, (b) applying an alternating source of power across the electric discharge source at a preselected excitation frequency with the alternating power source having a peak voltage sufficient to sustain an electric discharge and to generate a wide radiation spectrum of emissive radiation from the gas stream, (c) filtering the radiation spectrum to form an optical signal having a narrow radiation emission bandwidth corresponding to the stronger emission wavelength(s) of a preselected impurity gas or vapour to be analysed, (d) converting the optical signal into an electrical signal, (e) selectively amplifying the electrical signal within a narrow frequency range centered at twice the excitation frequency, and (f) analysing the se lectively amplified electrical signal to determine the concn. level of the impurity gas or vapour under analysis, in which (g) the electric discharge source is a silent electric discharge tube.    Also claimed is a gas emission spectrometer for such analysis.    USE -   Gas emission spectrometer for analysing low concns. with high accuracy.    ADVANTAGE -   The method of analysis uses emissive radiation filtered to form a narrow bandwidth optical signal converted and amplified within a narrow range at twice the excitation frequency, increases the sensitivity of detection of any gaseous impurity in the gas sample by an order of magnitude.