Photothermal Spectrometry Research Articles

Thermo-Optical Characterization of Cu- and Zr-Modified TiO2 Photocatalysts by Beam Deflection Spectrometry

Cu/Zr-modified TiO2 photocatalysts were prepared in the form of nanopowders and characterized by photothermal spectrometry, UV–Vis spectrophotometry and X-ray diffraction (XRD) to investigate the effect of Cu/Zr content on their thermo-optical and transport properties. Adding Cu (0.05%) caused a change in the light absorption range limit, which reduced from 3.25 eV for pure TiO2 to 2.85 eV for Cu-modified TiO2. The decrease in energy band gap was accompanied by a 19.5% decrease in the charge carrier lifetime, which is not favorable for photocatalysis. The decrease in charge carrier lifetime can be minimized by additional modification of TiO2 with Zr (1%), which showed insignificant effects on the energy band gap of the investigated materials. Furthermore, modification of TiO2 with Zr affected the material’s structure and increased its specific surface area, which improved the adsorption of degraded compounds as well as the absorption of light. Altogether, these effects resulted in higher photocatalytic degradation rate constants of the investigated TiO2-based photocatalyst. It was also found that modification of TiO2 with Cu and/or Zr increases both the material’s thermal diffusivity and conductivity due to changes in the band gap and structure of material. Beam deflection spectrometry (BDS) has demonstrated high potential in materials’ characterization which stems from its high sensitivity and precision.

Consideration of Temperature and Slip Correction for Photothermal Spectrometry

Temperature was considered to estimate the minimum detectable absorption coefficient of aerosol particles from photothermal spectroscopy. Light energy absorbed by subsequent emission from the aerosol results in the heating of the aerosol sample and consequently causes a temperature change as well as changes in thermodynamic parameters of the sample. This thermal effect is the basis of photothermal spectroscopy. Photothermal spectroscopy has several types of techniques depending on how the photothermal effects are detected. Photothermal interferometry traces the photothermal effect, refractive index, using an interferometer. Photoacoustic spectroscopy detects the photothermal effect, sound wave, using a microphone. In this study, it is suggested that the detection limit for photothermal spectroscopy can be influenced by the introduction of a slip correction factor when the light absorption is determined in a high temperature environment. The minimum detectable absorption coefficient depends on the density, the specific heat and the temperature, which are thermodynamic properties. Without considering the slip correction, when the temperature of the environment is 400 K, the minimum detectable absorption coefficient for photothermal interferometry increases approximately 0.3% compared to the case of 300 K. The minimum detectable absorption coefficient for photoacoustic spectroscopy decreases only 0.2% compared to the case of 300 K. Photothermal interferometry differs only 0.5% point from photoacoustic spectroscopy. Thus, it is believed that photothermal interferometry is reliably comparable to photoacoustic spectroscopy under 400 K.

Photothermal Spectrometry in Small Liquid Channels

A novel apparatus for performing photothermal lens spectroscopy is described. The apparatus uses a low-volume cylindrical sample cell, a chopped or pulsed excitation laser, and a continuous probe laser. The full volume of the sample is irradiated with constant, e.g., non-Gaussian, irradiance beam produced by the excitation laser. Constant irradiance excitation source does not directly produce the photothermal lens element in the sample. The lens element is formed by thermal diffusion from the irradiated sample volume, through the sample cell walls. Under continuous irradiation, thermal diffusion results in a parabolic temperature change profile. The apparatus has been found to work with cells designed to contain sample volumes from 6 μL down to 24 nL. Larger and smaller volume cells are practical but signals produced in the smallest sample cells exhibit deviation from that expected based on the theory. This is attributed to probe laser diffraction by the circular aperture of the sample cell. (Received on August 9, 2001, Accepted on......) Introduction Photothermal lens spectrometry is a novel means to measure optical absorption. It is one of several ultra-sensitive photothermal spectroscopy methods applicable to trace analysis. The applicability to trace analysis is due to the high sensitivity and small sample volumes needed for an analysis. There are several physical effects that limit sensitivity and influence the accuracy of measurements obtained using photothermal lens spectrometry. First, the photothermal lens signal is related to the lens formed the sample as a consequence of light absorption and subsequent energy transfer to the matrix. The optical element produced when laser beams are focussed into a homogeneous sample is not a simple lens. The aberrant nature of the photothermal lens results in signal magnitudes that are somewhat smaller than expected and also more difficult relate to sample absorbance. Second, sample irradiation can produce large temperature changes. In theory, the on-axis temperature change approaches infinity at long times for any excitation power when using continuous laser excitation sources. In practice, large temperature changes distort the thermal lens perturbation due to density differences and convection heat transfer or even boiling may occur. The later distorts the signal and ruins the analytical utility. Other factors resulting in inaccurate absorbance determination are related to the photophysics of sample excitation and solvent (matrix) relaxation. Nonlinear optical absorption and bleaching, transient volume changes, and excited state analyte absorbance can occur when using high irradiance laser excitation sources. These effects may generally be avoided only by using low irradiance. This is often accomplished by choosing continuous over pulsed lasers for sample excitation. An apparatus utilizing a two-laser photothermal lens apparatus that is immune to many problems associated with photothermal lens spectrometry and capable measuring optical absorbance in very small samples is described in this work. The apparatus uses a nanoliter-volume sample cell with a visible laser used as the excitation source. The sample cell is constructed from high thermal conductivity metal and has a large surface area and relatively large heat capacity. A simplified version of the theory for describing the photothermal lens developed in a cylindrical sample cell is given. A model for relating signals to sample absorbance is given and experimental evidence is presented. Timedependent photothermal signals are detected The technique exhibits high sensitivity and can be used for ultra-sensitive detection. Analyte excitation is often performed using laser light sources. The resulting photothermal lens is measured using a probe laser. Excitation and probe lasers may be the same. Laser light sources produce a small spot size at a focus. The small spot size allows one to probe extremely small sample volumes. Theory Theoretical time-dependent temperature changes and inverse photothermal lens focal lengths for both standard and cylindrical sample cells are described in the literature. The cylindrical sample cell used here has a large thermal conductivity compared to the solvent. Little temperature change occurs at the sample cell wall because of the high thermal conductivity ratio. The solution to the thermal diffusion equation for a zero temperature change at the cell wall temperature change is

Signal optimisation in cw-laser crossed-beam photothermal spectrometry: influence of the chopping frequency, sample size and flow rate

Optimisation of the optical design for cw-laser crossed-beam thermal lens spectrometry in infinite and finite samples has been investigated using different excitation beam waists and various lens combinations. The characteristics of the photothermal signal depending on the position of the sample with respect to the probe beam waist, the chopping frequency, the sample size and the flow rate have been considered. Depending on the irradiation duration, the size of the thermal element at the measurement time can be much greater than the waist of the excitation beam. As a result, the optimum sample position is closely related to the probe beam to thermal element size ratio and therefore depends on the chopping frequency and of the sample size. At low frequencies, the size of the thermal element is almost independent of the degree of focusing of the excitation beam because a smaller beam waist induces a faster thermal expansion. As a result, the amplitude of the optimum signal does not depend on the waist of the excitation beam. In contrast, at high frequency, the size of the thermal element remains closer to the size of the excitation beam and the signal is inversely proportional to the waist of the excitation beam as previously demonstrated under pulsed-laser excitation. Moreover, at moderate flow velocities, the signal is significantly enhanced because the negative effect produced by the displacement of the thermal element across the probe beam axis is more than compensated by a decrease of the effective thermal time constant due to radial mixing.

Photothermal spectrometry for detection in miniaturized systems: relevant features, strategies and recent applications

There is a growing interest in using miniaturized analytical devices because they allow to execute the different steps of an analytical process within very short times and with drastic reduction in the amounts of solvents, reagents and samples. As for capillary electrophoresis, these systems require detectors which are sensitive, versatile and adaptable to very small detection volumes. In this respect, photothermal spectrometry which is complementary to fluorescence seems to be a promising detection method. This review describes the basic principle of photothermal spectrometry along with the related methods based on colinear-beam or crossed-beam configuration of the pump and probe lasers. Two experimental set ups especially designed for microfluidic systems as well as for capillary electrophoresis are described. Their characteristics and key features are discussed and the main applications are outlined.

Analytical Chemistry for Advanced Technologies. Water-content analysis by photothermal spectrometry with a near-infrared diode laser.

Photothermal spectrometry for water is described in which high-power diode laser with a wavelength coincident with strong vibrational overtone absorption in a near-infrared region was used. In the thermal lens technique, an apparatus consisting of only solid-state devices, such as optical fibers, a fiber-coupler and photodiode, was used. The optical fibers acted as not only optical paths, but also as collimation devices. A water content of 0.15% in acetonitrile could be detected, and a linear analytical curve was obtained in the range of 0%∼7% with a 1478 nm, 100 mW diode laser at an intensity modulation frequency of 20 Hz. A photoacoustic technique could be also applied to water in acetonitrile and water adsorbed on molecular sieves by using a cell equipped with a cylindrical piezoelectric ceramic. These techniques can be a versatile water-analysis tool for the process monitoring and control.

Molecular spectrometry—milestones for the millennium

This paper presents an overview of selected aspects of molecular spectrometry in terms of milestone developments from which conclusions about the future development of the subject can be drawn. The paper discusses these in terms of technological, phenomenal and applications milestones. Subjects such as photothermal spectrometry, bioluminescence, and clinical applications of near-infrared spectrometry are seen as exciting developments together with a trend towards miniaturised analytical instrumentation. Longer term predictions about single molecule/particle detection, FT-NIR and terahertz spectrometry are also made.

Métodos analíticos ultrasensíveis: lente térmica e técnicas correlatas

Among the emergent laser based spectrometric methods, thermal lensing and other photothermal techniques present a great potential for solving a variety of problems in the fields of chemistry, physics and biology. Their main advantages are high concentration sensitivity, sensibility to physical-chemical properties of the medium, excellent spatial resolution and noninvasive characteristics. In this article, theoretical principles, main applications and practical hints as well as fundamental limitations of these techniques will be carefully described. It is hoped that this will give the reader a clear picture of this field of investigation as well as provide to the ones who are not specialists in the area, the necessary background to understand, implement and use photothermal techniques. In the final sections the development frontiers of photothermal spectrometry will be discussed.

Application of photothermal spectrometry to planar chromatograpy (Review).

Application of photothermal spectrometry to planar chromatograpy (Review).

Progress toward a better understanding of signal generation in laser-excited photothermal spectrometry of homogeneous samples

This article examines some recent experiments and theories that have advanced our understanding of the physical effects following thermal relaxation of the excited state. Experiments which show that the temperature-dependent refractive index is composed of density and pure-thermal terms are discussed. The importance of the purely thermal term is that it allows ultra-fast pulsed excitation sources to be used in photothermal refraction spectroscopy. This opens up new areas of applications in ultra-fast measurement. The effect of and progress towards the elucidation of specific volume changes are reviewed. Finally, recent progress in understanding nonlinear optical absorption and its effect on the photothermal signal are summarized.

Ultrasensitive on-column detection of capillary electrophoresis using laser-induced capillary vibration method

Capillary electrophoresis (CE), with its advantage of high resolution power, has attracted much attention. But CE has another advantage of needing only a small sample volume, which is expected to make it a useful tool for ultramicroscale analysis as well. However, conventional absorbance detectors are not sensitive enough because the optical light-path is that of the diameter of a capillary, which is several tens of microns. A highly sensitive detector is indispensable to application of CE to ultramicroscale analysis. Several sensitive detection methods have been developed, such as laser-induced fluorometry (LIF), electrochemical detection, mass spectrometry, etc. To make the best use of the high separation efficiency of CE, an on-column detection scheme, which can easily reduce the detection volume needed by using a focused laser beam as a probing beam, seems to be more preferable than other detection schemes including off-column or end-column detection schemes, and LIF seems to be more suitable than electrochemical detection and mass spectrometric detection. However, LIF cannot be applied to nonfluorescent analytes, and fluorescent derivatization is usually needed, which is not suitable for ultramicroscale analysis. Laser-induced photothermal spectrometry is another highly sensitive on-column detection approach, and differing from LIF, nonfluorescent species are also detectable. As a kind of photothermal effect, we found the capillary vibration induced by laser (CVL) effect [1], and proposed it as a highly sensitive on-column detection method of CE [2]. Like other photothermal detection methods, the CVL method is based on light absorption by a sample and successive heat generation due to nonradiative relaxation. This heat changes the capillary tension. Because the excitation laser beam is periodically irradiated onto the capillary, the tension fluctuation occurs periodically, resulting in vibration of the capillary. The amplitude of the capillary vibration has been found to be proportional to sample concentration. Therefore, quantitative analysis can be performed by detecting the amplitude of the capillary vibration.

Laser techniques for chemical analysis

In this review a wide range of laser-based techniques for chemical analysis is discussed with reference to the principles and practical utilities of each technique. Thus techniques for atomic spectrometry, elemental analysis, molecular spectrometry, LIDAR, photothermal spectrometry and mass spectrometry are considered. Where possible qualitative comparisons are made with alternatives for particular analyses, e.g. atomic absorption and emission spectrometry. Where alternatives do not exist e.g. LIDAR, the practical utility and applications of such techniques are discussed.

Laser spectroscopy on organic molecules.

Various laser spectrometric methods have been developed until now. Especially, laser fluorometry is most sensitive and is frequently combined with a separation technique such as capillary electrophoresis. For non-fluorescent compounds, photothermal spectrometry may be used instead. A diode laser is potentially useful for practical trace analysis, because of its low cost and long-term trouble-free operation. On the other hand, monochromaticity of the laser is essential in high-resolution spectrometry, e.g. in low temperature spectrometry providing a very sharp spectral feature. Closely-related compounds such as isomers can easily be differentiated, and information for assignment is obtained from the spectrum. Multiphoton ionization mass spectrometry is useful for soft ionization, providing additional information concerned with molecular weight and chemical structure. A short laser pulse with a sufficient energy is suitable for rapid heating of the solid surface. A matrix-assisted laser desorption/ion-ization technique is recently employed for introduction of a large biological molecule into a vacuum for mass analysis. In the future, laser spectrometry will be developed by a combination with state-of-the-art laser technology. In the 21st century, new laser spectrometry will be developed, which may be based on revolutionary ideas or unexpected discoveries. Such studies will open new frontiers in analytical laser spectroscopy.

Fuzzy Linear Regression Method in Ultratrace Analysis and Analytical Interpretation

Fuzzy sets theory was introduced for quantitative treatment of measured values in ultratrace analysis. The data obtained in extremely low concentration have uncertainties such as errors of measurement or errors arising from chemical handling of the samples. In place of the least squares method, a fuzzy linear regression method was applied to measured values expressed by the fuzzy numbers, and instead of a calibration curve, a calibration area was obtained. The validity and propriety of the calibration area were shown from simulation and experimental data of ultratrace analyses done using laser induced photothermal spectrometry. Analytical meanings of the shape of the calibration area were discussed.

Analytical characterization of laser photothermal transient deformation spectrometry

The radiationless relaxation process of excited molecules has developed as the basis for many laser photothermal spectroscopies. For example, laser photoacoustic spectrometry, laser thermal lens spectrometry, laser photothermal deflection spectrometry, laser photothermal refraction spectrometry and laser photothermal interference spectrometry have been applied to analytical chemistry and evolved into a kind of high sensitivity and high space resolution absorption spectrometries.

Determination of beryllium(II) on solid substrates by photothermal spectrometry after selective complexation with Chrome Azurol S

Chrome Azurol S (CAS) has been used as a chromogenic reagent in the design of specific reagent strips for the determination by photothermal spectrometry (PTS) of beryllium(II) in aqueous samples collected on a membrane filter. This reagent must be carefully formulated for compatibility, stability, reaction speed, sensitivity and freedom from interferences. A linear relationship existed between PTS signal amplitude and Be(II) concentration for the concentration range of 0.1–15 μg g −1. The potential of this method for long-term sample storage is important in practical applications where the samples cannot be analyzed for Be(II) immediately after their collection on membrane filters in the field.

Rapid Recovery of Wide-Bandwidth Photothermal Signals via Homodyne Photothermal Spectrometry: Theory and Methodology

Homodyne photothermal spectrometry (HPS) is a very wide bandwidth signal recovery technique which uses many of the elements of lock-in detection at very low cost. The method uses a frequency sweep, with a high-frequency bandwidth of up to 10 MHz, to excite a linear photothermal system. The response sweep of the photothermal system is downshifted into a bandwidth of a few kilohertz by means of in-phase mixing with the excitation sweep with the use of a four-quadrant double-balanced mixer and a low-pass filter. Under conditions derived from theory, the filter output gives a good approximation to the real part of the photothermal system's frequency response, dispersed as a function of time. From a recording of this signal, the frequency and impulse response of the photothermal system are rapidly recovered at very high resolution. The method has been tested with the use of laser photopyroelectric effect spectrometry and provides an inexpensive, convenient method for the recovery of high-frequency photothermal signals.

Thermal-diffusivity measurements of ultrahigh thermal conductors with use of scanning photothermal rate-window spectrometry: Chemical-vapor-deposition diamonds.

A comprehensive study of a photothermal rate-window spectrometry for thermal-diffusivity measurements of ultrahigh thermal conductors, using either a dual-channel boxcar averager or a lock-in amplifier, is presented. Theoretical analysis of infrared radiometric transients and techniques to extract the diffusivities of materials from the transient are discussed. By exploiting the derivative signal nature of the rate-window methodology, one can measure the thermal diffusivity of the sample with superior signal-to-noise (S/N) ratio from the maximum position of the radiometric rate-window signal. Our measurements of thermal diffusivities of chemical-vapor-deposition diamonds made by the hot-filament process as a function of temperature between 60 and 300 K are the first such photothermal data obtained completely nonintrusively and they illustrate the unique potential of this measurement methodology for the nondestructive, noncontact photothermal investigation of ultrahigh thermal-conductor thermophysics, largely inaccessible by other diagnostic probes.

Amplitude and phase-modulation (AM-PM) wide-band photothermal spectrometry. II. Experiment

Amplitude and phase-modulation (AM-PM) wide-band photothermal spectrometry is experimentally demonstrated, using apparatus constructed from readily available components. Theoretical AM-PM wide-band waveforms were digitally synthesized, filtered, and used as excitation waveforms in a wide-band spectrometer using photopyroelectric effect detection. The frequency domain dynamic range of the AM-PM wide-band technique was clearly superior to linear FM photothermal spectrometry. Multiband spectrometry was also demonstrated, in which several arbitrary narrow-band regions or ‘‘slices’’ of the frequency response were recovered simultaneously. The procedure shows great promise for image enhancement in thermal-wave applications, where selective target features of interest may be studied in the absence of neighboring interference.

Amplitude and phase modulation (AM-PM) wide-band photothermal spectrometry. I. Theory

Amplitude and phase modulation (AM-PM) wide-band photothermal spectrometry is a new technique of excitation and signal recovery with extensive applications in thermal-wave imaging using linear photoacoustic and photothermal techniques. The excitation waveform consists of the superposition of two orthogonal amplitude-modulated frequency sweeps, and exhibits an optimally flat input autospectrum over an arbitrary bandwidth. Signal quality is greatly superior to earlier wide-band photothermal methods based on linear frequency sweep excitation. The new methodology also permits ‘‘multiband acquisition’’ in which several arbitrary narrow-band segments or ‘‘slices’’ of the photothermal frequency response are acquired, a capability that is missing in conventional linear FM measurements. This multiband acquisition capability has many potential applications for information recovery in thermal-wave imaging.