VCSEL-based oxygen spectroscopy for structural analysis of pharmaceutical solids
Author
Summary, in English
We present a minimalistic and °exible single-beam instrumen-
tation based for sensitive tunable diode laser absorption spectroscopy (TD-
LAS) and its use in structural analysis of highly scattering pharmaceu-
tical solids. By utilising a vertical-cavity surface emitting laser (VCSEL)
for sensing of molecular oxygen dispersed in tablets, we address structural
properties such as porosity. Experiments involve working with unknown
pathlengths, severe backscattering and di®use light. These unusual experi-
mental conditions has lead to the use of the term gas in scattering media
absorption spectroscopy (GASMAS). By employing fully digital wavelength
modulation spectroscopy and coherent sampling, system sensitivity in am-
bient air experiments reaches the 10¡7 range. Oxygen absorption exhibited
by our tablets, being in°uenced by both sample porosity and scattering,
were in the range 8 £ 10¡5 to 2 £ 10¡3, and corresponds to 2-50 mm of
pathlength through ambient air (Leq). The day-to-day reproducibility was
on average 1.8% (0.3 mm Leq), being limited by mechanical positioning.
This is the ¯rst time sub-millimetre sensitivity is reached in GASMAS. We
also demonstrate measurements on gas transport on a 1 s timescale. By
employing pulsed illumination and time-correlated single photon counting,
we reveal that GASMAS exhibits excellent correlation with time-domain
photon migration. In addition, we introduce an optical measure of porosity
by relating oxygen absorption to average photon time-of-°ight. Finally, the
simplicity, robustness and low cost of this novel TDLAS instrumentation
provides industrial potential.
tation based for sensitive tunable diode laser absorption spectroscopy (TD-
LAS) and its use in structural analysis of highly scattering pharmaceu-
tical solids. By utilising a vertical-cavity surface emitting laser (VCSEL)
for sensing of molecular oxygen dispersed in tablets, we address structural
properties such as porosity. Experiments involve working with unknown
pathlengths, severe backscattering and di®use light. These unusual experi-
mental conditions has lead to the use of the term gas in scattering media
absorption spectroscopy (GASMAS). By employing fully digital wavelength
modulation spectroscopy and coherent sampling, system sensitivity in am-
bient air experiments reaches the 10¡7 range. Oxygen absorption exhibited
by our tablets, being in°uenced by both sample porosity and scattering,
were in the range 8 £ 10¡5 to 2 £ 10¡3, and corresponds to 2-50 mm of
pathlength through ambient air (Leq). The day-to-day reproducibility was
on average 1.8% (0.3 mm Leq), being limited by mechanical positioning.
This is the ¯rst time sub-millimetre sensitivity is reached in GASMAS. We
also demonstrate measurements on gas transport on a 1 s timescale. By
employing pulsed illumination and time-correlated single photon counting,
we reveal that GASMAS exhibits excellent correlation with time-domain
photon migration. In addition, we introduce an optical measure of porosity
by relating oxygen absorption to average photon time-of-°ight. Finally, the
simplicity, robustness and low cost of this novel TDLAS instrumentation
provides industrial potential.
Department/s
Publishing year
2008
Language
English
Pages
345-354
Publication/Series
Applied Physics B
Volume
90
Issue
2
Full text
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Document type
Journal article
Publisher
Springer
Topic
- Atom and Molecular Physics and Optics
Status
Published
Research group
- Biophotonics
ISBN/ISSN/Other
- ISSN: 0946-2171