Tunable Diode Laser Spectroscopy Theory - Ametek 5100 HD User Manual

Tunable Diode Laser Spectroscopy Theory

1-2 | Model 5100 HD Analyzer
Tunable Diode Laser Absorption Spectroscopy (TDLAS) is a spectroscopic
technique that uses a diode laser as the light source. The technology is extremely
selective. The laser line is very narrow in frequency so it can probe the analyte
gas without interference from other gases. It works according to the fundamental
principles of Beer's Law. The Law indicates that for a constant path length, the
intensity of the incident light energy traversing an absorbing medium diminishes
exponentially with concentration.
Ln [I°/I] = S*L*N
Where:
I is the measurement of beam intensity when
tuned to the absorbing wavelength the analyte
I° is the reference measurement or beam inten-
sity when tuned away from the analyte absorb-
ing wavelength
S is the fundamental absorption line strength
and is a fixed constant
L is the path length of the beam through the
sample and is a fixed constant
N is the number of analyte molecules contained
in the beam path passing through the sample
So, by measuring the intensity of the laser both be-
fore and after absorption by the analyte, the number
of analyte molecules in the sample can be deter-
mined since all of the other terms in the equation are
constant.