TDLAS
TDLAS VS CRDS
The classic spectroscopy technique involves light being directed both through and around the sample at an interference frequency of the analyte . The difference in intensity is used to determine the concentration of analyte in the sample .
Cavity Ring-Down Spectroscopy ( CRDS ) is based on extending the principle of classic spectroscopy to an “ infinite ” path length . With ultra-high reflectivity mirrors , a pulse of light in the sample will decay to undetectable levels more quickly with a contaminant present than it would in the absence of a contaminant . The time taken for light to decay in the sample at an interference frequency is compared to the same frequency in a “ pure ” sample .
Instead of physically routing light around the sample , Tunable Diode Laser Absorption Spectroscopy ( TDLAS ) scans across a small range of wavelengths , such that some of its light passes through without being affected by the analyte .
This allows comparison between an affected and unaffected signal traveling the exact same path . Using a Herriott cell allows for a long laser path length with a relatively small footprint . The curved mirrors also refocus the laser on every bounce , keeping the signal intact , and provide resistance to misalignment .
TDLAS
CRDS Measurement type Instantaneous Instantaneous
Background gases DF-750 : N 2 , H 2 , He , Ar , O 2 , other inerts
DF-745 SGMax : CO , CO 2 , SF 6 , Ne , Kr , Xe , N 2 O , Fluorocarbons , NF 3
N 2 , He , Ar , H 2 , O 2 , CDA
DF-740 : NH 3 Sample flow 1 lpm 1 lpm Measurement detection range
Regular maintenance requirements
13 gas components , from 10-100 ppm
No regular maintenance required
6 gas components , from 1-5 ppm
May need to be zeroed over time due to loss of reflectivity of mirrors
Lower detection limit 55 ppt 100 ppt
Accuracy DF-750 gases : 200 ppt or 3 % of reading
DF-745 SGMax gases : 5 ppb or 5 % of reading
NH 3 : 5 ppb or 5 % of reading HCl : 500 ppt or 5 % of reading
100 ppt or 4 % of reading
Response time DF-750 : 3 Minutes to 90 % 2 Minutes to 95 %
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