Abstract
Broadband cavity enhanced absorption spectroscopy (BBCEAS) has been used
to make measurements on condensed phase species using a white LED and an
optical cavity formed by R ≥ 0.99 and R ≥ 0.999 mirror sets.
Measurements were made on a number of dyes dissolved in water and placed in
a 1 cm quartz cuvette at zero degrees angle of incidence between the cavity
mirrors. A comparison was made between a low cost uncooled Ocean optics
spectrometer as the detector and a cooled Andor spectrometer. The Andor
spectrometer was found to be between ~3.5 times and ~5 times more sensitive
than the Ocean optics spectrometer for similar levels of light intensity reaching
the detector with the R ≥ 0.99 and R ≥ 0.999 mirror sets respectively. The most
sensitive measurements were made with the R ≥ 0.99 mirror set on rhodamine B
at 555 nm which produced an αmin(t) value of 2.8 × 10-6 cm-1 Hz-1/2 and a LOD of
34 pM. When compared with previous liquid phase cavity measurements these
results were amongst the most sensitive whilst using a simple and low cost
experimental setup. Tests were also made on the robustness and accuracy of
the BBCEAS measurements and revealed an accuracy of ~5% limited by the
calibration procedure used whilst the cavity alignment was found to be
reproducible even if both mirrors were removed and replaced.
To increase the sensitivity of measurement, experiments were also performed in
a 10 cm quartz cuvette on the dyes rhodamine B and chocolate brown dissolved
in water. The longer base pathlength led to an increase in cavity losses from high
overtone absorption from water especially in the region 500 - 600 nm.
Consequently the most sensitive measurements were obtained with the R ≥ 0.99
mirrors at 455 nm with chocolate brown. An αmin(t) value of 4.2 × 10-7 cm-1 Hz-1/2
was obtained, which when compared with previous similar cavity studies
appeared to be the most sensitive liquid phase measurement reported. The
absorption coefficient of water was also calculated in the region 415 nm - 600 nm
and was found to be in reasonably good agreement with values determined
previously by other methods.
The first reported application of BBCEAS to the measurement of thin films
is also made. Rhodamine B thin films were made by drop coating onto glass
microscope slides and microscope coverslips. A sol-gel thin film was also
fabricated on a glass microscope slide. The most sensitive measurements as
determined by the minimum detectable loss per pass (∆Fmin(t)) was
obtained for the drop coated rhodamine B thin films deposited on
microscope coverslips and the R ≥ 0.99 mirror set. A value of ∆Fmin(t) =
3.3×10-6 was obtained. The results with the sol-gel thin films were worse
than expected due to the poor optical quality of the thin films. When
compared with previous cavity studies on thin films and solid/liquid
interfaces these results were amongst the most sensitive.
In summary this study has led to an improvement in the sensitivity of liquid
phase LED based BBCEAS and also an extension of the technique to the
measurement of thin films. When compared with other cavity based
condensed phase studies these results appear to be amongst the most
sensitive whilst using a simple and low cost experimental setup.
In summary this study has led to an improvement in the sensitivity of liquid
phase LED based BBCEAS and also an extension of the technique to the
measurement of thin films. When compared with other cavity based
condensed phase studies these results appear to be amongst the most
sensitive whilst using a simple and low cost experimental setup.