Photothermal experiments on condensed phase samples of agricultural interest : optical and thermal characterization

A rapidly increasing number of photothermal (PT) techniques has had a considerable impact on agriculture and environmental sciences in the last decade. It was the purpose of the work described here to develop and apply new PT techniques in this specific field of research.Chapter I is a general introduction with an overview of PT techniques used in this research. Two different photoacoustic (PA) techniques used for optical characterization of a variety of condensed phase samples are discussed in chapters 2 and 3. The possibilities for thermal characterization of samples are described in chapter 4.In chapter 2 classical PA spectroscopy with microphone detection was used to obtain spectra in the visible region (350-700 nm) of powdered (light scattering) food samples such as flours, coffee and spices (chapter 2.1). The outcome of these experiments suggest the feasibility of PA spectroscopy for quality control in the food-processing industry. Another PA cell was designed (chapter 2.2) and used in the IR region (10 μm). The final PA experiment was concerned with study of various carboxylic acids, alcohols and alkanes at 3.39 μm (chapter 2.3).The feasibility of optothermal window (OW) method, an elegant approach to determine the optical absorption coefficient of condensed phase samples is described in chapter 3. The method was not only extended to 9-11 μm but also proved capable of investigating opaque samples (liquids and gel) which are otherwise not amenable to conventional IR spectroscopies.The content of trans fatty acids in several margarine samples was measured with the OW technique and its performance compared to that of GLC, GLC + TLC and FTIR. The data obtained with the different methods were generally in a good agreement. The improved OW cell resulting in substantial reduction of the background signal, was then used to study extra virgin olive oil (chapter 3.2) adulterated by known adulterants (sunflower oil (4.5%) and safflower oil (6%)). The achieved limit of detection (LOD) was comparable to those reported in literature for other techniques (FTIR-ATR, GLC, HPLC and mass spectroscopy).Many biological samples contain water that itself exhibits a strong absorption in IR. Quantitative measurements on such specimens are all but trivial. The new OW sensor was shown capable of direct and quantitative measurements (chapter 3.3 and 3.4) of lactose, corn starch and sulfate in water. The performance of the OW method was slightly inferior to that of FTIR. On the other hand, unlike the OW method, FTIR-ATR could only provide quantitative results for corn starch samples. As to the study of sulfate in water, the limit of detection (I mmolL <sup>-1</SUP>) achieved with the OW method is one order of magnitude better than that of ATR.In the fourth and last chapter, two different PT techniques (photopyroelectric method and photothermal beam deflection) were used for thermal characterization of a candy (a model for a glassy sugar system) and different packaging materials. The thermal diffusivity of a candy at room temperature found by PPE and PTBD was 14.7 x10 <sup>-8</SUP>and 12.0 x10 <sup>-8</SUP>m <sup>2</SUP>S <sup>-1</SUP>respectively. In addition, the PPE technique in the standard configuration, was used to measure temperature dependence of the thermal diffusivity in the -30°C to 70°C temperature range. The glass-rubber transition, underwent by the sample in this temperature range was observed and compared to the results obtained by differential scanning calorimetry (DISC).The PPE technique was also used to obtain the temperature behaviour of thermal diffusivity of low density polyethylene and polyvinylchloride foils (used for packaging purposes). Such behavior was found dependent on the thermal history of the sample. The "untreated' foils exhibit values for thermal diffusivity that were consistently lower (7-20%) than those obtained for the same samples when heated to 70°C. The increase in thermal diffusivity is associated with structural changes (i.e. glass transitions) taking place in the material during thermal annealing. Additional measurements by differential scanning calorimetry (DSC) confirmed the validity of PPE results. It was shown that the PPE method is more sensitive than the standard DSC in detecting changes in thermal parameters.Finally, the PPE method allows one in principle to obtain the temperature behavior of all static and dynamic thermal parameters provided one of the remaining thermal parameters (thermal conductivety, thermal effusivity and volume specific heat) is available at a given temperature.<strong>Outlook</strong>Results of a research described in this thesis show the feasibility of PT techniques for applications to a wide range of condensed phase samples. The methods used here constitute only a part of techniques developed and used in our laboratory. The low-cost and compact OW device is easy to handle and moreover offers the possibility for on-line studies of optically opaque and thermally thick samples that are normally not accessible by other techniques. It is anticipated that development of infrared diode lasers will increase the potential of the OW method because desired wavelengths characterized by the highest spectral contrast will become available. The sensitivity of the OW method was shown comparable to that of FTIR and therefore additional developments might eventually make the OW approach a candidate technique for quantitative analysis throughout the entire infrared region. The analytical potential of PT schemes becomes more obvious when they are used as detectors in combination with separation techniques such as GLC, HPLC or capillary electrophoresis.The potential of photopyroelectric technique was demonstrated by obtaining thermal diffusivity values for different kind of samples. The PPE method is able to provide data on aging effect, structural stability and crystaline polymorphism. The technique is fast, sensitive and reproducible, requires small amounts of sample for analysis while providing more information than existing, classical methods used currently in thermal research. The only drawback of PPE is the necessity for good thermal contact between sample and sensor. When one is interested to obtain thermal diffusivity data in a non-contact manner the photothermal beam deflection (zero crossing method) is a valuable tool. Accurate (5% error determination of thermal diffusivity is achievable due to intrinsically low errors in measurement of frequency and of zero crossing position.In conclusion, based on the results of the work described here, one can anticipate that in the years to come the PT methods, alone or combined with existing techniques, will most likely play a more important role for variety of applications in agricultural and environmental sciences.