2.3 Data acquisition and QCL system
Before measuring the QCL spectra, a background spectrum of a roughened
gold substrate was obtained. This background spectrum provided a good
and smooth reference trace, due to the lack of MIR signals from the
employed solid matrix. The samples were placed in the wells of metal
holders (2.54 cm in diam., 10 mm deep). The sample surface was flattened
to create a smooth surface for accurate measurements. Duplicate spectra
were collected at ten (10) different locations on the surface, resulting
in a total of twenty (20) spectra per sample. These were used for the
calibration and test analyses. This process was used for the neat
samples, and each concentration in the test mixes samples.
The spectra were obtained in reflectance (R) mode at a distance of
approximately 15 cm, using a LaserScan™ MIR pre-dispersive spectrometer
(Block Engineering, Marlborough, MA, USA) equipped with three tunable
MIR lasers with a tuning range from 990 to 1111, 1111 to 1178 and 1178
to 1600 cm-1, the scan time was 0.5 s approximately on
each laser for a total time scan of 1.5 s. The average power typically
varied between 0.5 to 10 mW across the 600 cm-1 total
tuning range with 100:1 transverse electromagnetic polarization
(TEMoo) and beam divergence of < 2.5 mrad in
the x-axis and < 5 mrad in the y-axis. The spectrometer had a
ZnSe lens of 3 in. in diameter, which was used to focus the MIR beam, to
collect the reflected light, and to focus the light onto a
thermoelectrically cooled mercury-cadmium-telluride (MCT) detector. The
wavelength accuracy and precision were 0.5 cm-1 and
0.2 cm-1, respectively. The spectroscopic system
worked best at a laser head to target distance of 15 ± 3 cm. Each laser
diode formed an elliptical spot of 4 x 2 mm2 at the
focal plane of the ZnSe lens due to the difference of beam divergence in
the axes.