Free gases exhibit sharp and highly structured absorption spectra. Since spectra of solid materials are typically very dull, high-resolution spectroscopy allows selective detection of gases even when these are dispersed within porous and highly scattering solids such as wood, polystyrene foam, biological tissue and pharmaceutical tablets. This concept was developed at our division, and introduced to the scientific community via Optics Letters in 2001. We base our gas sensing on tunable diode laser absorption spectroscopy (TDLAS or TDLS). While conventional absorption spectroscopy is carried out with well-defined beamlines and known interaction pathlengths, this task involves dealing with diffuse light, unknown and uncontrollable pathlengths, severe backscattering, and devastating optical interference. These characteristics are further discussed below and motivate that the technique, since its introduction, is designated as gas in scattering media absorption spectroscopy (GASMAS).

An introduction to GASMAS, as well as a detailed description of various important aspects, is given in the PhD thesis of Tomas Svensson (see publications below).

One of our prioritized research areas is the application of GASMAS to pharmaceutical materials. We have published results showing that we can detect and quantify gas content inside highly scattering pharmaceutical solids. Since the optical pathlength is unknown and not well-defined due to multiple scattering in these samples, we employ photon time-of-flight spectroscopy (TOFS) to determine the optical pathlength. By doing so, we offer a new way of assessing porosity.

There are several applications of this approach.The pharmaceutical industry and pharmaceutical science in general, employ numerous techniques to characterize physical properties of solid materials. When it comes to porosity, the standard technique is mercury intrustion porosimetry. This technique is bulky, expensive, time-consuming, hazardous, and destructive to the sample. In contrast, GASMAS relies on compact instrumentation, and is suitable for fast and non-destructive testing.

Note also that GASMAS is sensitive not only to variations in porosity, but also variations in scattering (and thus structure in the widest sense). This means that GASMAS has the potential of being a sensor for several important properties of pharmaceutical materials, such as hardness and dissolution. Due to its speed and non-destructive character, it is particularly suited process analysis (process analytical technology, PAT).

Optical porosimetry and investigations of the porosity experienced by light interacting with porous media
T. Svensson, E. Alerstam, J. Johansson and S. Andersson-Engels
Optics Letters35, 1740-1742 (2010)

Laser spectroscopy of gas confined in nanoporous materials 
T. Svensson and Z. Shen
Applied Physics Letters 96, 021107 (2010)

Clinical system for non-invasive in situ monitoring of gases in the human paranasal sinuses
M. Lewander, Z. Guan, K. Svanberg, S. Svanberg, and T. Svensson
Optics Express 17, 10849-10863 (2009)

Pharmaceutical and biomedical applications of spectroscopy
in the photon migration regime
T. Svensson, PhD Thesis (2008)

VCSEL-based oxygen spectroscopy for structural characterisation of pharmaceutical solids
T. Svensson, M. Andersson, L. Rippe, S. Svanberg, S. Andersson-Engels, J. Johansson, and S. Folestad
Applied Physics B90, 345-354 (2008)

High sensitivity gas spectroscopy of porous, highly scattering solids
T. Svensson, M. Andersson, L. Rippe, J. Johansson, S. Folestad, and S. Andersson-Engels
Optics Letters33, 80-82 (2008)

Non-intrusive optical study of gas and its exchange in human maxillary sinuses
L. Persson, M. Andersson, T. Svensson, M. Cassel-Engquist, K. Svanberg, and S. Svanberg
Proc. SPIE: Diagnostic Optical Spectroscopy in Biomedicine IV6628, 662804 (2007)

Noninvasive characterisation of pharmaceutical solids by diode laser oxygen spectroscopy
T. Svensson, L. Persson, M. Andersson, S. Svanberg, S. Andersson-Engels, J. Johansson, and S. Folestad
Applied Spectroscopy 61, 784-786 (2007)

Analysis of gas dispersed in scattering media
M. Sjöholm, G. Somesfalean, J. Alnis, S. Andersson-Engels, and S. Svanberg
Optics Letters 26, 16-18 (2001)