Electroinduced thermal lens spectrometry

A new technique for the generation of a thermal lens effect is considered. In this technique, the action of focused radiation from an inducing laser is replaced by an analogous action of electric current, which forms a thermal lens at a certain site of a liquid by the generation of a high local current density in a small liquid volume. The main regularities of this technique for thermal lens signal generation are considered, and an expression is derived to relate the detected signal (a relative change in probing radiation intensity at the detector) and the electrolyte concentration. A cell is proposed for electroinduced thermal lens detection. The dependence of the thermal lens signal on the applied voltage and on the concentration of the test electrolyte is determined for a model electrolyte (NaCl), and sensitivity characteristics are found to be consistent with theoretical estimations made in this work.     

Studies of surfactant/water systems near the critical micellar concentration using thermal lens spectroscopy

A novel application of photothermal spectroscopy to the study of surfactant-water systems near the critical micellar concentration is reported. The thermal lens signal was induced by a slightly soluble dye and was measured with a dual-beam thermal lens spectrometer.For the two surfactants considered: nonyl phenol and Triton X-100, sharp variations of the thermal lens signal were observed at the critical micellar concentration (CMC), namely an increase for nonyl phenol and a decrease for triton X-100. These effects are arguably related to micelle formation.Our work serves as an initial assessment of the potential of the technique for the study of disperse systems of a higher complexity or dark systems where conventional techniques are impossible to apply.     

Thermal lens spectroscopy in liquid argon solutions: (Deltav = 6) C-H vibrational overtone absorption of methane

A dual-beam thermal lens technique has been used to obtain the absorption spectrum of the (Deltav = 6) C-H stretch of liquid methane and methane in liquid argon solutions. The lowest concentration detected was 1 x 10(-3) (mole fraction) of CH(4) in liquid Ar with a continuous wave laser power of 20 mW. The thermal lens signal is linear with the mole fraction of methane up to 1 x 10(-2) but not for higher concentrations. Considering the system CH(4)-Ar as an ideal solution, the factors that contribute to the thermal lens signal were calculated as a function of the concentration of methane. A mechanism of energy transfer based on the gas-phase results could explain qualitatively the dependence of the magnitude of the signal on the mole fraction of methane.     

A single and simple mathematical expression of the signal for cw-laser thermal lens spectrometry

Since the discovery of the thermal lens effect, several theoretical models have been put forward for cw-laser thermal lens spectrometry and different expressions of the thermal lens signal have been derived for various optical configurations including single-beam or dual-beam situations. This review focuses primarily on the successive mathematical expressions reported so far for both steady-state and time-resolved measurements, with the aim of comparing them in order to propose a single and convenient relation that accurately accounts for the sensitivity and the temporal behaviour of the thermal lens efrect.     

Near infrared thermal lens spectrometry for the real-time monitoring of supercritical fluid extraction

A commercially available supercritical fluid extractor provided with carbon dioxide was coupled to a dual-beam thermal lens spectrometer with a pumpprobe coaxial configuration, pumped by a pulsed Nd-YAG laser operating at the fundamental wavelength of 1064 nm. As a preliminary step, several compounds were studied in batch regime using carbon tetrachloride as solvent, in order to observe the influence of overtones and combinations involving distinct chemical bonds on thermal lens spectrometry (TLS). Several factors related with supercritical fluid extraction (SFE) under hydrodynamic conditions were studied in order to establish their influence both in the extraction yield and thermal lens signal magnitude obtained. The advantages and limitations of the hyphenated SFE-TLS technique proposed are discussed, and the possibility of on-line detection in SFE with a pulse thermal lens spectrometer was demonstrated.     

Cw-laser thermal lens spectrometry in binary mixtures of water and organic solvents: composition dependence of the steady-state and time-resolved signals

The thermal lens effect obtained in binary liquid systems composed of water and ethanol, propanol and acetonitrile has been investigated. The dependence of dn/dT upon the solvent volume fraction follows polynomials up to sixth order and cannot be precisely predicted using the additive rule. The sensitivity of the thermal lens method upon the addition of organic solvent in water varies as the temperature-dependent refractive index gradient to thermal conductivity ratio of the mixture provided that the signal is sampled correctly. Otherwise, especially when steady-state experiments are carried out, the thermally induced concentration gradient, known as the Soret effect, can change the thermo-optical properties of the solution locally in the irradiated area and produce an additional signal. This effect depends on the solvent and is maximum at low solvent composition. At the critical solvent volume fraction of 0.1-0.15, the Soret component may represent up to 25% of the pure thermal lens signal and has a time constant which is 200-400 times greater than the characteristic time constant of the thermal lens.     

Hemichrome Determination by Thermal Lensing with Polyethylene Glycols for Signal Enhancement in Aqueous Solutions

The thermal lens technique is proposed for the determination of total hemoglobin in the form of reversible hemichrome. The conditions were optimized (concentration of sodium dodecyl sulfate, 2?mM) to attain the maximum sensitivity with the use of polyethylene glycols as signal enhancers. For polyethylene glycols with molecular weights 1500–35000 Da in a concentration range of 5–15% w/w (5–25?mM), the influence on thermal lens signal enhancement was estimated. It is shown that the use of 5% w/w polyethylene glycol 2000 provides the maximum increase in the thermal lens enhancement factor (by 40%) in comparison with unmodified aqueous solutions. The detection limit of iron(II) tris(1,10-phenanthrolinate) as a model system is 60?nM. Under these conditions, the thermal lens detection limit of hemichrome is 10?nM, which shows a 15-fold enhancement compared to spectrophotometry. Modification of the medium with polyethylene glycols decreases the limit of detection of hemichrome determination by 15% in comparison with unmodified aqueous solutions due to better reproducibility for the range of concentrations from 0.02 to 0.9?µM.     

Thermal lens spectrum of organic dyes using optical parametric oscillator

The wavelength dependence of thermal lens signal from organic dyes are studied using dual beam thermal lens technique. It is found that the profile of thermal lens spectrum widely differ from the conventional absorption spectrum in the case of rhodamine B unlike in the case of crystal violet. This is explained on the basis of varying contribution of nonradiative relaxations from the excited vibronic levels.     

Sensitivity improvement in capillary electrophoresis using organo-aqueous separation buffers and thermal lens detection

It is shown that organo-aqueous separation buffers show much promise when used in capillary electrophoresis separations with photothermal (thermal lens) detection systems. Acetonitrile–water and methanol–water mixtures were selected, as conventionally used in capillary electrophoresis. It is shown that, despite more sophisticated experimental conditions (significant heat outflow from the capillary body) and peak detection, the theoretical ratio of the thermal lens signal for a binary mixture to the thermal lens signal for an aqueous solution (or the corresponding ratio obtained experimentally under bulk batch conditions) can be used to predict the sensitivity of thermal lens detection in capillary electrophoresis. The limits of detection for 2-, 3-, and 4-nitrophenols selected as model compounds in 70% v/v acetonitrile separation buffers are 1×10⁻⁶ M, 1×10⁻⁶ M and 3×10⁻⁷ M, respectively, and are therefore decreased by a factor of six compared to thermal lens detection in aqueous separation buffers. The overall increase in the thermal lens detection sensitivity in a 100% ACN buffer is a factor of 13.      

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.     

Integration of a microextraction system solvent extraction of a Co-2-nitroso-5-dimethylaminophenol complex on a microchip

A newly designed microchannel for solvent extraction was fabricated in a quartz glass chip and applied to solvent extraction of a Co-2-nitroso-5-dimethylaminophenol complex. The aqueous solution of Co complex and toluene were introduced into the microchannel, and the Co complex extracted in toluene was detected by thermal lens microscopy (TLM). The Co complex was quickly extracted into toluene when the flow was stopped. The observed extraction time, ca. 50 s, was almost equivalent to the value calculated using the diffusion distance and diffusion coefficient. The dependence of the TLM signal on the concentration of the Co complex showed good linearity in the range of 1 x 10(-7) - 1 x 10(-6) M.     

Thermal lens spectrometry in aqueous solutions of Brij 35: investigation of micelle effects on the time-resolved and steady-state signals

This work investigates the effect of micelles on the time-resolved and steady-state thermal lens signals in aqueous solutions. The temperature gradient produced subsequently to non-radiative relaxation of the sample induces migration of micelles towards the colder region of the irradiated area along with an opposite flow of solute molecules. This phenomenon, known as the Soret effect, produces an additional probe beam signal with a rise time that is much longer than the thermal time constant and depends on the surfactant and solute concentrations. Extrapolation of the mass-diffusion constant at zero solute concentration allowed the determination of diffusion coefficients that are close to those derived by other methods for Brij 35 micelles in water. It is also shown that the surfactant has only a small effect on the thermal lens signal and that the enhancement produced by micelles with respect to pure water originates mainly firom the Soret effect. It follows that interpretation of experimental data without discriminating both components of the probe beam signal can lead to erroneous values of dn/dT.     

Two-Laser Thermal Lens Spectrometry with Signal Back-Synchronization

A two-laser dual-beam thermal lens spectrometer based on continuous-wave lasers, which implements the mode with signal back-synchronization, is developed with the aim to develop instrumentation for thermal lens spectrometry. This mode offers a potentially higher sensitivity of measurements in comparison with the lock-in mode of thermal lens measurements and variation of conditions depending on the measurement medium (solvent, or dispersed system, or solids). The wider possibilities of the back-synchronization mode in thermal lens spectrometry both for solving problems of chemical analysis and in some related fields are shown.     

Trace level vanadium determination using thermal lens spectrophotometry

Trace level vanadium determination is reported using a dual beam thermal lens spectrometer. The thermal lens was generated using an argon ion beam laser (pump beam) which was focused into a sample cuvette. The thermal lens signal (TLs) was monitored with a He-Ne laser beam and a photodiode detector. Multichannel averager software was developed for processing the transient TLs. The optimal set up, ensuring maximum sensitivity and linear calibration graphs was obtained using experimental design techniques. Under optimized conditions, the detection limits for aqueous and ethanol-water (2+3 v/v) and (4+1 v/v) vanadium complex solutions were, respectively, 0.0071 mg/l, 0.0065 and 0.0039 mg/l.     

Measurement of thermal energy recovery using thermal lens spectrometry in fluorescence quenching experiments

This paper describes some limitations of the pulsed-laser thermal lens method in measuring the heat produced by non-radiative relaxation of excited fluorescent molecules. Simultaneous measurements of the fluorescence intensity and thermal lens signal have been carried out for fluorescein dianion in water and perylene in ethanol. It is shown that complete fluorescence quenching of fluorescein and perylene by potassium iodide and nitromethane, respectively, does not result in full recovery of the thermal energy. The results depend on either the operating mode of the excitation laser or the solvent composition.     

Matrix effects in thermal lens spectrometry: influence of salts, surfactants, polymers and solvent mixtures

In this paper, we present an overall view of the matrix effects that can change or alter the signal in thermal lens spectrometry and we report the main works published in this field. The addition of salts, surfactants and polymers in aqueous solutions or the use of solvent mixtures is often needed in a variety of applications either to enhance the sensitivity of the thermal lens method or more generally because such media are required in the separation process prior to thermal lens detection. In most cases, matrix effects result in small changes in the thermo-optical properties of the solution and small signal variations. However, most important signal alterations can arise from the Soret effect. In binary mixtures as well as in solutions with macromolecular species which are initially homogeneous, the temperature gradient will induce the migration of molecules and the formation of a concentration gradient. This results in the formation of a concentration-dependent refractive index gradient which adds to the temperature-dependent refractive index gradient and contributes to the formation of a new signal. This effect can seriously alter the analytical signal and lead to erroneous interpretation of the experimental data. In contrast, time-resolved measurements can help in separating both signal components and have allowed to derive mass-diffusion times and mass-diffusion coefficients for a variety of micelles and polymers.     

Monitoring supercritical fluid extraction by thermal lens spectrometry with pulsed laser excitation

A dual-beam thermal lens spectrometer (TLS) with a pulse pump–probe coaxial configuration was coupled to commercially available supercritical fluid extraction equipment using a high-pressure flow cell interface. Because of its feasible critical parameters, moderate cost and good thermooptical properties, carbon dioxide was used as supercritical solvent. Using trans-β-carotene as analyte, several factors related with the extraction process under continuos flow conditions were studied in order to establish their influence in the thermal lens signal magnitude. Under the optimum experimental conditions, the relative TLS signal area showed a linear relationship with the concentration of trans-β-carotene from 1.5×10⁻⁶ to 8×10⁻⁸ M (n=5, r=0.998) in the supercritical phase. Thus, the viability of on-line detection for supercritical fluid extraction with a pulse thermal lens spectrometer was demonstrated.     

pH and excitation wavelength dependence of the thermal lens signal of fluorescein in aqueous solutions

It is shown that the thermal lens signal of fluorescein in aqueous solutions depends on the molecular form of the solute even after correction for fluorescence. Possible factors that may affect the thermal lens response are photochemical reactions and vibration-to-translation energy transfer from the solute to the solvent. These are considered in relation to the existence of excited-state proton transfer reactions.     

Evaluation of lateral resolution of scanning surface microscopy by total internal reflection with thermal lens effect

We have developed a novel method for in situ and non-destructive surface analyses, or a total internal reflection with thermal lens spectroscopy (TIR-TLS), which has sufficient sensitivity to monitor phenomena in thin films, such as lipid bilayers. In this study, we applied TIR-TLS to microscopy for surface analyses, and we experimentally obtained its lateral resolution using the edge of a chromium film made by a photolithography technique. The obtained resolution was 20 microm, which was 60% of the diameter of an excitation beam at the interface. The estimated resolution with a simple model agreed with the experimental one, and from this model, TIR-TLS microscopy has the same resolution as that of ordinary optical microscopy. The microscopy by TIR-TLS was applied to a sample whose contrast was too weak to be visually seen, and an image of the sample was obtained without any loss of resolution.     

Effect of silver nano particles on the fluorescence quantum yield of Rhodamine 6G determined using dual beam thermal lens method

Nano structured noble metals have very important applications in diverse fields as photovoltaics, catalysis, electronic and magnetic devices, etc. Here, we report the application of dual beam thermal lens technique for the determination of the effect of silver sol on the absolute fluorescence quantum yield (FQY) of the laser dye rhodamine 6G. A 532 nm radiation from a diode pumped solid state laser was used as the excitation source. It has been observed that the presence of silver sol decreases the fluorescence quantum efficiency. This is expected to have a very important consequence in enhancing Raman scattering which is an important spectrochemical tool that provides information on molecular structures. We have also observed that the presence of silver sol can enhance the thermal lens signal which makes the detection of the signal easier at any concentration.