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Trace-level Analysis by Differential Spectroscopy using Spectral Subtraction

Tunable Diode Laser Absorption Spectroscopy (TDLAS) instruments measure the concentration of a target analyte molecule in gas samples based on the Lambert-Beer law, whereby analyte molecules present in the gas sample absorb laser light energy in direct proportion to their concentration.    

Some chemical constituents present in a process gas stream will absorb light energy at or near the wavelength used to measure the target analyte molecule. In such cases the light energy measured by the TDL analyzer detector is attenuated by this background absorption effect.     

SpectraSensors developed and patented* a spectral subtraction technique that enables trace level (sub-ppm) measurements of H2O, H2S, or NH3 to be made when a process gas sample contains very low levels of an analyte and/or background analytical interferences.   

In operation the TDL analyzer performs a sequence of steps to obtain a “zero” spectrum and “process” spectrum that are used to calculate analyte concentration by spectral subtraction. The zero spectrum is obtained by passing the process gas sample through a high-efficiency dryer or scrubber which selectively removes the trace analyte without altering the process gas composition and background absorbance. The analyzer records the resulting dry spectrum of process gas without the H2O, H2S, or NH3 analyte.

The analyzer then automatically switches the sample gas flow path to bypass the dryer or scrubber and collect the process spectrum. Subtraction of the recorded zero spectrum from the process spectrum generates a differential spectrum of the trace analyte which is free of background interferences. The concentration of H2O, H2S, or NH3 can be calculated from the differential spectrum.

Differential Spectroscopy based on spectral subtraction extends the performance range of TDLAS analyzers in applications involving process gas streams with complex background interferences.   


Differential Spectroscopy

Differential Spectroscopy using Spectral Subtraction

A particular advantage of the spectral subtraction technique is that it can be used to dynamically correct for changes in process gas stream temperature, pressure and composition when spectral distortion from such changes exceed a user defined preset value.  This is accomplished by initiating a zero spectrum update to collect a new zero spectrum for spectral subtraction from the process spectrum that exhibits different background absorption as a result of a change in the process gas stream.         

*U.S. Patents; 7,704,301 B2 / 7,819,946 B2 / 7,679,059,059 B2 / 8,152,900 B2 / 8,500,849 B2