Linda Persson

Spectroscopic studies of wood-drying processes.

By the use of wavelength-modulation diode laser spectroscopy, water vapor and oxygen are detected in scattering media nonintrusively, at 980 nm and 760 nm, respectively. The technique demonstrated is based on the fact that free gases have extremely sharp absorption structures in comparison with the broad features of bulk material. Water vapor and oxygen measurements have been performed during the drying process of wood. The results suggest that the demonstrated technique can give information about the drying process of wood to complement that of commercially available moisture meters. In particular, the time when all the free water has evaporated from the wood can be readily identified by a strong falloff in the water vapor signal accompanied by the reaching of a high-level plateau in the molecular oxygen signal. Furthermore, the same point is identified in the differential optical absorption signal for liquid water, with a sharp increase by an order of magnitude in the ratio of the signal intensities at 980 nm and 760 nm.

Gas monitoring in human sinuses using tunable diode laser spectroscopy

We demonstrate a novel nonintrusive technique based on tunable diode laser absorption spectroscopy to investigate human sinuses in vivo. The technique relies on the fact that free gases have spectral imprints that are about 10.000 times sharper than spectral structures of the surrounding tissue. Two gases are detected; molecular oxygen at 760 nm and water vapor at 935 nm. Light is launched fiber optically into the tissue in close proximity to the particular maxillary sinus under study. When investigating the frontal sinuses, the fiber is positioned onto the caudal part of the frontal bone. Multiply scattered light in both cases is detected externally by a handheld probe. Molecular oxygen is detected in the maxillary sinuses on 11 volunteers, of which one had constantly recurring sinus problems. Significant oxygen absorption imprint differences can be observed between different volunteers and also left-right asymmetries. Water vapor can also be detected, and by normalizing the oxygen signal on the water vapor signal, the sinus oxygen concentration can be assessed. Gas exchange between the sinuses and the nasal cavity is also successfully demonstrated by flushing nitrogen through the nostril. Advantages over current ventilation assessment methods using ionizing radiation are pointed out.

Temporal correlation scheme for spectroscopic gas analysis using multimode diode lasers

The reliability of diode lasers used in spectroscopic applications is limited by their intrinsic multimode and mode-jump behavior when wavelength-tuned by current or temperature. We report on a scheme for gas analysis based on temporal correlation between absorption signals from an unknown external and a known reference gas concentration, simultaneously recorded when the diode laser wavelength is temperature-tuned across absorption features of the gas of interest. This procedure, which does not require any knowledge of the exact spectrum, also eliminates light intensity fluctuations due to mode competition. The method is illustrated for atmospheric oxygen absorption applied to diffusion measurements.

Flexible lock-in detection system based on synchronized computer plug-in boards applied in sensitive gas spectroscopy

We present a flexible and compact, digital, lock-in detection system and its use in high-resolution tunable diode laser spectroscopy. The system involves coherent sampling, and is based on the synchronization of two data acquisition cards running on a single standard computer. A software-controlled arbitrary waveform generator is used for laser modulation, and a four-channel analog/digital board records detector signals. Gas spectroscopy is performed in the wavelength modulation regime. The coherently detected signal is averaged a selected number of times before it is stored or analyzed by software-based, lock-in techniques. Multiple harmonics of the modulation signal (1f, 2f, 3f, 4f, etc.) are available in each single data set. The sensitivity is of the order of 10(-5), being limited by interference fringes in the measurement setup. The capabilities of the system are demonstrated by measurements of molecular oxygen in ambient air, as well as dispersed gas in scattering materials, such as plants and human tissue.

Simultaneous detection of molecular oxygen and water vapor in the tissue optical window using tunable diode laser spectroscopy

We report on a dual-diode laser spectroscopic system for simultaneous detection of two gases. The technique is demonstrated by performing gas measurements on absorbing samples such as an air distance, and on absorbing and scattering porous samples such as human tissue. In the latter it is possible to derive the concentration of one gas by normalizing to a second gas of known concentration. This is possible if the scattering and absorption of the bulk material is equal or similar for the two wavelengths used, resulting in a common effective pathlength. Two pigtailed diode lasers are operated in a wavelength modulation scheme to detect molecular oxygen ~760 nm and water vapor ~935 nm within the tissue optical window (600 nm to 1.3 mum). Different modulation frequencies are used to distinguish between the two wavelengths. No crosstalk can be observed between the gas contents measured in the two gas channels. The system is made compact by using a computer board and performing software-based lock-in detection. The noise floor obtained corresponds to an absorption fraction of approximately 6x10(-5) for both oxygen and water vapor, yielding a minimum detection limit of ~2 mm for both gases in ambient air. The power of the technique is illustrated by the preliminary results of a clinical trial, nonintrusively investigating gas in human sinuses.

Noninvasive Characterization of Pharmaceutical Solids by Diode Laser Oxygen Spectroscopy

Characterization of solid pharmaceuticals, ranging from monitoring of solid-state reactions to understanding tablet dissolution, is of great interest for pharmaceutical science. The quality of the finished product highly depends on knowledge about the pharmaceutical materials used and the different unit operations involved in manufacturing of pharmaceuticals. Thus, availability of appropriate and reliable tools for measurements of physical and chemical properties of drug materials in situ during chemical and physical processing is a key for optimized processing. In early stages of pharmaceutical development a whole range of techniques for characterization of the solid state is available, addressing, for example, particle size and shape, density, porosity, calorimetry, thermo-mechanical properties, specific area, and crystallinity. However, most of these techniques are slow and not well suited for fast laboratory-based or process applications. Process analytical technology (PAT) is a term describing a holistic approach to pharmaceutical manufacturing based on in-depth understanding through advanced process sensors and modeling tools. In order to succeed with this, new tools are needed, e.g., with capability to directly measure physico-chemical attributes in situ of the mechanical process. In this context, tools based on spectroscopic techniques offer obvious advantages owing to their speed, compactness, versatility, and ability to perform noninvasive analysis. By employing the spectroscopic technique referred to as gas in scattering media absorption spectroscopy (GASMAS), it is possible to extract information related to gas dispersed within highly scattering (turbid) materials. In our case, the key is contrast between the sharp (GHz) absorption features of molecular oxygen located around 760 nm and the broad absorption features related to tablet bulk material. The technique is based on high-resolution diode laser absorption spectroscopy, and its main principle is illustrated in Fig. 1. Light is injected into a highly scattering sample, often utilizing optical fibers. The actual path length distribution of transmitted photons will depend on the scattering properties of the sample. Due to significant multiple scattering, the average photon path length greatly exceeds sample dimensions. For example, the average path length of photons that have traveled through a pharmaceutical tablet typically exceeds 10 cm. During passages through air-filled pores, photons in resonance with an absorption line in the A-band of molecular oxygen can be absorbed. Oxygen absorption can be distinguished from bulk absorption due to the extremely narrow absorption features (GHz) exhibited by free gases. To resolve such narrow features, high-resolution spectroscopy must be employed. The resulting absorption signal depends on both the oxygen content and the scattering properties (photons path lengths) of the sample. Indirectly, it is thus related to mechanical properties such as porosity and particle size. The GASMAS technique has previously been used to study the gas content in, for example, polystyrene foam, wheat flour, granulated salt, wood materials, fruit, and human sinuses. Gas exchange dynamics has been studied by placing samples of wood and fruit in nitrogen atmospheres and monitoring the re-invasion of oxygen. In this paper we show the potential of using GASMAS for determination of physical and structural parameters of pharmaceutical solids. We present results from a study of pharmaceutical tablets made from two different sieve fractions (particle size distributions) and with different compression forces. In addition, the prospects of a broader use of this technique for pharmaceutical analysis are discussed.

Laser spectroscopy of gas in scattering media at scales ranging from kilometers to millimeters

Free gases are characterized by their narrow line width, and they can conveniently be studied by laser spectroscopy. The present paper discusses the monitoring of such ambient pressure gases, which are dispersed in scattering media such as aerosol-laden atmospheres, solids, or liquids. Atmospheric work basically constitutes the well-known field of differential absorption lidar (DIAL), while the study of free gas in solids and liquids was initiated more recently under the name of GASMAS (GAs in Scattering Media Absorption Spectroscopy). We discuss the connections between the two techniques, which are extensively used in our labortory. Thus, we span the field from trace-gas mapping of gases in the lower atmosphere to gas studies in construction materials, food products, and the human body. We show that the basic ideas are very similar, while the spatial and temporal scales vary greatly.

Non-intrusive optical study of gas and its exchange in human maxillary sinuses

We demonstrate a novel non-intrusive technique based on tunable diode laser absorption spectroscopy to investigate human maxillary sinuses in vivo. The technique relies on the fact that free gases have much sharper absorption features (typical a few GHz) than the surrounding tissue. Molecular oxygen was detected at 760 nm. Volunteers have been investigated by injecting near-infrared light fibre-optically in contact with the palate inside the mouth. The multiply scattered light was detected externally by a handheld probe on and around the cheek bone. A significant signal difference in oxygen imprint was observed when comparing volunteers with widely different anamnesis regarding maxillary sinus status. Control measurements through the hand and through the cheek below the cheekbone were also performed to investigate any possible oxygen offset in the setup. These provided a consistently non-detectable signal level. The passages between the nasal cavity and the maxillary sinuses were also non-intrusively optically studied, to the best of our knowledge for the first time. These measurements provide information on the channel conductivity which may prove useful in facial sinus diagnostics. The results suggest that a clinical trial together with an ear-nose-throat (ENT) clinic should be carried out to investigate the clinical use of the new technique.

Monte Carlo simulations of optical human sinusitis diagnostics

We investigate the feasibility of using diode laser gas spectroscopy for sinusitis diagnostics. We simulate light propagation using the Monte Carlo concept, as implemented by the Advanced Systems Analysis Program (ASAPTM) software. Simulations and experimental data are compared for a model based on two scattering bodies representing human tissue, with an air gap in-between representing the sinus cavity. Simulations are also performed to investigate the detection geometries used in the experiments, as well as the influence of the optical properties of the scattering bodies. Finally, we explore the possibility of performing imaging measurements of the sinuses. Results suggest that a diagnostic technique complementary to already existing ones could be developed.

Monte Carlo simulations related to gas-based optical diagnosis of human sinusitis

We investigate the feasibility of using diode laser gas spectroscopy for sinusitis diagnostics. We simulate light propagation using the Monte Carlo concept, as implemented by the Advanced Systems Analysis Program (ASAP) software. Simulations and experimental data are compared for a model based on two scattering bodies representing human tissue, with an air gap in-between representing the sinus cavity. Simulations are also performed to investigate the detection geometries used in the experiments, as well as the influence of the optical properties of the scattering bodies. Finally, we explore the possibility of performing imaging measurements of the sinuses. Results suggest that a diagnostic technique complementary to already existing ones could be developed.