S. N. Bendrysheva

Thermooptical detection in microchips: From macro‐ to micro‐scale with enhanced analytical parameters

In this paper, we compared the methods of photothermal spectroscopy used in different spatial scales, namely thermal‐lens spectrometry (TLS) and thermal‐lens microscopy (TLM) to enhance the performance parameters in analytical procedures. All of the experimental results were confirmed by theoretical calculation. It was proven that the design for both TLM and TLS, despite a different scale for the effect, is governed by the same signal‐generating and probing conditions (probe beam diameter at the sample should be equal to the diameter of the blooming thermal lens), and almost does not depend on the nature of the solvent. Theoretical and experimental instrumental error curves for thermal lensing were coincident. TLM obeys the same law of instrumental error as TLS and shows better repeatability for the same levels of thermal‐lens signals or absorbances. TLS is more advantageous for studying low concentrations in bulk, while TLM shows much lower absolute LODs due to better repeatability for low amounts. The behavior of the thermal‐lens signal with different flow rates was studied and optimum conditions, with the minimum contribution to total error, were found. These conditions are reproducible, are in agreement with the existing theory of the thermal response in thermal lensing, and do not significantly affect the design of the optimum scheme for setups. TLM showed low LODs in solvent extraction (down to 10–8 M) and electrokinetic separation (10–7 M), which were shown to be governed by discussed instrumental regularities, instead of by microchemistry.

Optimization of Instrumental Parameters of a Near-Field Thermal-Lens Detector for Capillary Electrophoresis

The optical scheme of a near-field dual-beam mode-mismatched thermal-lens detector for capillary electrophoresis with a crossed-beam configuration employing a multimode HeCd laser (325 nm) as an excitation source was optimized. It is shown that a multimode laser can be successfully used as an excitation source in thermal lensing with minimal deviations in thermal responses from Gaussian excitation sources. An equation for diffraction thermal-lens theory for near-field measurements is deduced, and the experimental results agree with the deduced equation. The temperature rise in the capillary was estimated, and the exponential decrease of the signal with time for static conditions and low flow velocities was explained. The optimum configuration of the detector from the viewpoint of the maximum sensitivity and beam sizes was found. The detector provides a significant improvement in the detection limits for model compounds absorbing at 325 nm (nitrophenols) compared to the results obtained with a commercial absorbance detector operating at the same wavelength.

Advances in thermal lens spectrometry

The main examples of the application of thermal lens spectrometry and thermal lens microscopy as highly sensitive power-based thermooptical (photothermal) methods of molecular absorption spectroscopy to photometric, chromatographic, and electromigration methods of analysis and to solving problems related to the use of microfluid chips are considered. The fundamentals and characteristics of the thermooptical methods and instruments and the problems and prospects of their development are discussed. Examples are provided for illustrating the sensitivity and selectivity of combined methods and multidisciplinary studies performed with the aid of thermal lens spectrometry.

Indirect thermal lens detection for capillary electrophoresis

Thermal lens detection with a 325.0nm He-Cd excitation laser is used for thermooptical indirect detection in combination with the capillary electrophoretic separation of organic anions. The optimization of indirect thermooptical detection is discussed. With Mordant Yellow 7 (an azo dye) chosen as a probe ion limits of detection for 1-heptane-, 1-pentane-, 1-butane-, 1-propanesulfonic, and acetic acid at a level of nx10(-7)M were achieved with a separation electrolyte containing 50muM of the probe ion and 5mM Tris pH 9.90. A further increase in the detection sensitivity (twofold decrease in the limit of detection ) was obtained with a separation electrolyte containing a volume fraction of 20% acetonitrile.

Compact photothermal-refractometric detector for high performance liquid chromatography based on a polarization interferometer

A combined photothermal-refractometric detector for high-performance liquid chromatography is proposed. Five metal complexes as 4-pyridylazo-2-naphthol chelates and three sugars were investigated as model substances. For these compounds, detection limits, repeatability, and reproducibility of determination were calculated. Obtained results were compared with results for traditional detectors, which show that photothermal detection has higher sensitivity than photometric and other absorption detectors.

Investigation of adsorption of nanogram quantities of iron(II) tris-(1,10-phenanthrolinate) on glasses and silica by thermal lens spectrometry

Thermal lens spectrometry is used for studying adsorption equilibria in aqueous solutions at the level of nanogram quantities of iron(II) tris-(1,10-phenanthrolinate) as a model system. The kinetics of the sorption of the chelate on silica is studied and adsorption isotherms are built. Thermal lensing is used as a method for direct determination of the chelate concentration adsorbed on a quartz surface. The detected amount is 4.1x10(-15) mol at the area irradiated by the excitation beam. The adsorption of iron(II) tris-(1,10-phenanthrolinate) on laboratory glassware at the nanogram level is characterised by measuring the residual concentration of the sorbate in solution. A procedure for handling and cleaning the laboratory glassware for determining nanogram amounts of iron in aqueous solutions is proposed. The sensitivity of thermal lensing both in measuring adsorption on silica and glass and quartz surfaces is 100-fold higher than diffuse-reflectance measurements under the same conditions.

Criteria for Assessing the Effect of the Composition of Mixed Media on Analytical Sensitivity in Thermal Lens Spectrometry

The effect of the main parameters of solvent mixtures (water-acetone, water-ethanol, water-acetonitrile, water-ethylene glycol, and chloroform-acetone) of various compositions on the detection limit and analytical range was studied, and the selection criteria for the composition of mixed media were discussed in the context of trace determination by thermal lens spectrometry. It is shown that the strength of the thermal lens effect in the medium cannot serve the measure of its effect on the sensitivity and reproducibility of the determination. The lowest detection limits were reached in the water-ethanol (8 times lower in comparison to water), water-acetone (3 times), and water-ethylene glycol (2.5 times) mixtures with a volume ratio of components of 1: 4, while in water-acetonitrile mixtures (22 times), with a component ratio of 3 : 1.

Effect of the Composition of Aqueous-Organic Mixtures on the Sensitivity of Thermal Lens Measurements

Thermooptical properties of aqueous solutions of methanol, acetonitrile, dimethyl sulfoxide, ethylene glycol, glycerol, 1,4-dioxane, and sucrose were studied, and it was found that the analytical thermal lens signal depends on the nature of the organic component, most of all, on the polarity and molecular size. The sensitivity coefficient of thermal lens measurements is increased to a maximum extent in methanol solutions (by 7.3 times at the concentration 50 vol %) and acetonitrile (by 8.8 times at 26 vol %). It was found that a small concentration of water slightly affects the thermooptical properties of polar organic solvents.

Differential kinetic thermal lens determination of aniline and 4-nitroaniline

Thermal lens spectrometry was used for the differential kinetic determination of aniline (over the concentration range of 8 × 10−4–3.2 × 10−3 M) and 4-nitroaniline (2 × 10−4–1.6 × 10−3 M) present in combination in a single sample based on the oxidation reaction with periodate ions in an acidic medium (this determination is not possible with the spectrophotometric monitoring of the rate of reaction). The thermal lens procedure (λe = 488.0 nm; 80 mW) was characterized by good performance characteristics in the determination of aniline (cmin = 3 × 10−4 M; cd = 8 × 10−4 M) and 4-nitroaniline (cmin = 7 × 10−5 M; cd = 2 × 10−4 M), simplicity, and rapidity.

Effect of a solvent on the parameters of the analytical signal, detection limit, and analytical range of the determination in analytical thermal lens spectrometry

The effect of the main parameters of the solvents (water, acetone, methanol, ethanol, acetonitrile, dimethyl sulfoxide, dichloromethane, toluene, and chloroform) that are most frequently used in the analytical practice on the characteristics of the thermal lens effect (increase in temperature because of optical heating and in the size of the region involved in heating) and on the detection limit and the lower determination limit of the thermal lens determination was considered, and the criteria of the selection of the medium for thermal lens experiments were discussed. It was demonstrated that the gain in these characteristics of the thermal lens determination in the given medium in comparison with the aqueous medium does not necessarily coincide with the strength of the thermooptical effect in this medium. It was demonstrated that the optimum conditions of measurements in the thermal lens detection are controlled not only by the absorption of the analyte and the reagents, but also by the intrinsic absorption of the solvent. Recommendations were given on the selection of the solvent for analytical thermal lens spectrometry.