Crossed-beam Thermal Lens Research Articles

Integrated Micro-Optics for Microfluidic Detection.

A method of embedding micro-optics into a microfluidic device was proposed and demonstrated. First, the usefulness of embedded right-angle prisms was demonstrated in microscope observation. Lateral-view microscopic observation of an aqueous dye flow in a 100-μm-sized microchannel was demonstrated. Then, the embedded right-angle prisms were utilized for multi-beam laser spectroscopy. Here, crossed-beam thermal lens detection of a liquid sample was applied to glucose detection.

Determination of light-absorbing layers at inner capillary surface by cw excitation crossed-beam thermal-lens spectrometry

A thermal-lens spectrometric unit suitable for selective quantitative measurements of light-absorbing layers adsorbed onto the inner surface of a quartz glass capillary is described. The quantitative description of the thermal-lens signal generated in a quartz glass capillary with a light-absorbing layer at the inner surface of capillary is developed, which is based on the description for the thermal-lens experiment in the layered solids presented elsewhere. The accuracy of calculations is demonstrated by the comparison of predicted results with the experimental data and those predicted by the conventional theory. The data achieved prove the accuracy of calculations both for the time dependent thermal-lens signal and for the lock-in amplifier signal under variation of the spectrometer configuration for capillaries having an adsorbed layer. The proposed technique is used for the investigation of chromate/2,10-ionene and 4-aminoazobenzene adsorption at capillary walls. The estimates of the minimum light absorption detectable at capillary walls are at a level of 1 × 10 −5 abs. units; the linear range of the thermal-lens signal from the inner surface layer no less than three orders of magnitude is predicted.

Photostability of Uranine via Crossed-Beam Thermal Lens Technique

Uranine is a diagnostic aid in ophthalmology and is used as an immunohistological stain. A photostability study on this important compound using a crossed-beam thermal lens (TL) technique was carried out. The study is based on the photodegradation (PD) behavior and rate regarding some experimental parameters such as the incident laser power, wavelength, modulation frequency, and sample concentration. The effects of such parameters on the TL signal and PD rate are discussed in details. It has been found that the PD rate is proportional to the power of the pumping laser and the concentration of the sample within the investigated range. However, the modulation frequency is found not to influence the PD rate. The photochemical quantum yield has been measured using potassium ferrioxalate actinometry and it was found to be very low.

Continuous-Wave-Laser versus Pulsed-Laser Excitation for Crossed-Beam Photothermal Detection in Small Volume Applications: Comparative Features

Crossed-beam thermal lens spectrometry can be implemented using continuous-wave- (cw) laser or pulsed-laser excitation. In both cases, the signal depends on the position of the sample with respect to the probe beam waist, the size of the excitation beam, the beamsize ratio into the sample, and the power or energy of the excitation beam. However, due to differences in the rate of formation and relaxation of the thermal lens, both methods exhibit distinct key features. Optimization of the experimental setup and understanding the thermal lens signal are more complicated under cw-laser excitation than with pulsed-laser excitation. Unlike that observed under pulsed excitation, the effect of the excitation beam waist, of the sample size, and of the flow rate are closely related to the effective size of the thermal element and depend on the chopping frequency. Although the intrinsic sensitivities are almost the same, the performance can significantly differ depending on the chopping frequency or pulse repetition rate, which should be high enough to allow fast data collection and efficient signal averaging.

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.

Investigation of the optimum optical design for pulsed-laser crossed-beam thermal lens spectrometry in infinite and finite samples

Optimization of the optical design for pulsed-laser crossed-beam thermal lens (PLCBTL) spectrometry has been investigated. Experiments have been carried out with large samples as well as for very small samples in a microchannel and using different lens combinations to focus the probe and excitation beams. The results have been interpreted in terms of the influence of the excitation beam size as well as the degree of mode-mismatching of the excitation and probe beams on the optimum sample position and on the amplitude and decay of the photothermal signal. A semi-empirical formula that describes the influence of the sample position with respect to the probe beam waist has been established. We have shown that the optimum signal is inversely proportional to the waist of the excitation beam and is independent of the sample size as long as the size of the excitation beam is smaller than the microchannel. Time-resolved experiments have also shown that when the excitation beam is smaller than the sample, the signal decay depends not only on the size of the excitation beam but also on the mode-mismatching factor. Otherwise, the temporal characteristics are closely related to the size of the microchannel.

Pulsed-laser mode-mismatched crossed-beam thermal lens spectrometry within a small capillary tube: effect of flow rate and beam offset on the photothermal signal

Pulsed-laser crossed-beam thermal lens spectrometry within a small capillary tube has been investigated using static as well as flowing samples. It is shown that when both the excitation and probe beams are perfectly aligned, time-resolved and peak signals are adequately described by the theoretical relations previously reported for infinite samples. Depending on flow velocity and/or the lateral offset between excitation and probe beam axis, the optical element formed by the excitation beam may have a diverging or converging effect on the probe beam. However, optimization of the photothermal signal with pulsed excitation and flowing samples does not require offsetting the pump and probe beams and the peak signal is almost independent on the flow rate. The performance of the method has been checked with samples injected in a flowing stream. The response is linear over three orders of magnitude and the limit of detection obtained for cobalt nitrate in ethanol corresponds to an average absorbance of 5×10 −7 inside the capillary.

Enzyme immunoassay using a crossed-beam thermal lens technique

An enzyme immunoassay based on the use of crossed-beam thermal lens detection is described. In this assay, poly- N-isopropylacrylamide, a water-soluble, thermally precipitating synthetic polymer, was used as a carrier to minimize non-specific binding. The enzyme substrate of the horseradish peroxidase that was employed was 3,3′,5,5′-tetramethylbenzidine. The color development of the enzyme–substrate reaction was stopped by SDS and Na 2SO 3 to achieve a stable blue solution. The background reduction and stabilization made it possible to use a crossed-beam thermal lens technique as the measurement method. This method was demonstrated to be applicable by determination of hepatitis B surface antigen in human serum. A detection limit of 0.15 ng/ml was obtained. This was more sensitive than that of the commercially available ELISA method.

Pulsed laser induced mode-mismatched crossed-beam thermal lens measurements

In this article, an accurate Fresnel diffraction model for a pulsed laser induced thermal lens effect in mode-mismatched crossed-beam configuration is presented. The effect of a lateral offset between the two beams is included in the model. This model is used to calculate the diffraction field of the probe beam at the detection plane from the very near- to the far-field, therefore the evolution of the intensity profile of the probe beam from the near- to the far-field is clearly illustrated. Numerical calculations predict and experimental results demonstrate that the thermal lens (TL) signal maximum is obtained when the probe beam propagates about a characteristic distance away from the sample position, while the characteristic distance is determined by the radius of the excitation beam in the sample and the wavelength of the probe beam. At large size mismatch of two beams and near-field detection cases, the TL signal is approximately independent on the sample location relative to the waist of the probe beam. The theoretical and experimental results are of great importance to configuration optimization of an extremely sensitive and compact TL instrument, which is the trend in the development of analytical instruments.

Dual-laser crossed-beam thermal lens spectrophotometer pumped with a semiconductor diode-array laser

A simple and compact dual-laser crossed-beam thermal lens spectrophotometer pumped with a semiconductor diode-array laser, emitting up to 100 mW of power at 818 nm, is described. The pump laser aperture was located at a very small distance from the cell wall and was not focused. The He-Ne probe beam was made to pass near the cell wall, almost grazing the wall inner surface, crossing the pump beam at right-angles with respect to the beam propagation direction, but parallel to the major axes of the narrow elliptical lobes emitted by the array. Owing the particular spatial characteristics of the pump beam, a thermal lens signal with a complex spatial distribution was obtained. Univariant and simplex optimization of the set-up were performed using CuSO 4 and phosphomolybdenum blue solutions. Limits of detection of absorbance were 3.9×10 −4 in CCl 4, 3.4× −4 in 2-butanol, 1.1×10 −3 in ethyl acetate and 3.0×10 −3 in water.

Multichannel crossed-beam thermal lens measurements for absolute absorbance determination with pulsed laser excitation

Multichannel crossed-beam thermal lens measurements for absolute absorbance determination with pulsed laser excitation

Laser detectors for capillary liquid chromatography

The high spatial coherence of cw gas lasers is useful in the development of small volume detectors for capillary liquid chromatography. Three detectors are considered in this manuscript. The first is a very simple, subnanoliter refractive index detector based upon the diffraction of a laser beam by a fluid filled tube. Detection limits of a few nanograms of sugars separated upon a reversed phase Chromatographic column are obtained. The second detector utilizes the crossed-beam thermal lens to produce an absorbance detection limit of 6 × 10 −8, measured across the 80-μm square bore flow cuvette. This detector is combined with gradient elution, reversed phase capillary liquid chromatography for sub-femtomole amino acid determination. Last, a light scatter detector provides nearly seven orders of magnitude of dynamic range for capillary hydrodynamic chromatography of sub-micrometer diameter particles.

Subfemtomole detection limit for amino acid determination with laser-induced crossed-beam thermal lens detection.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSubfemtomole detection limit for amino acid determination with laser-induced crossed-beam thermal lens detectionThomas G. Nolan and Norman J. DovichiCite this: Anal. Chem. 1987, 59, 23, 2803–2805Publication Date (Print):December 1, 1987Publication History Published online1 May 2002Published inissue 1 December 1987https://doi.org/10.1021/ac00150a017RIGHTS & PERMISSIONSArticle Views84Altmetric-Citations36LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (380 KB) Get e-Alerts Get e-Alerts

Submicrometer resolution images of absorbance and thermal diffusivity with the photothermal microscope

The photothermal microscope is a laser-based technique which utilizes the crossed-beam thermal lens to measure simultaneously both absorbance and thermal diffusivity within small probe volumes. The microscope provides images with spatial resolution better than 1microm(3) in a simple optical design. High resolution images of histological and botanical samples are presented.

Crossed-beam thermal-lens detection for 0.25-mm diameter microbore liquid chromatography: Separation of 2,4-dinitrophenylhydrazones

The crossed-beam thermal-lens detector is applied to capillary liquid chromatography for the separatin of picomole amounts of 2,4-dinitrophenylhydrazones. A relatively inexpensive instrument is described, which utilizes a 3-mW helium—cadmium pump laser and a low-power helium—neon probe laser. Detection limits of 120 fmol of acetone (injected) are obtained.

Ultrasensitive analysis with the crossed-beam thermal lens

The crossed-beam thermal lens is a two-beam laser technique that produces a high-spatial resolution measurement of weak optical absorbance. The technique has been used for scanning microscope, nonintrusive flow measurements, and liquid chromatographic detection. The authors report the detection of 120 analyte molecules within a 2×10-10 cm3 volume of liquid. The possibility of detecting and counting single antibodies is considered.

Simple model for the time dependence of the periodically excited crossed-beam thermal lens

A time-dependent model is presented for the periodically excited crossed-beam thermal lens. In order to obtain good agreement between the theory and data for chopped excitation, it is necessary to consider the finite time required to block and unblock the pump beam. Unfortunately, a closed form expression has not been developed which considers the excitation waveform for a chopped Gaussian beam. On the other hand, a useful approximation has been developed based on a trapezoidal excitation function. This model is verified with respect to time, absorbance, alignment, and thermal diffusivity.

Time-resolved crossed-beam thermal lens measurement as a nonintrusive probe of flow velocity

A simple theory is generated for crossed-beam thermal lens and photothermal deflection in flowing sample streams. This theory is used to analyze time-resolved crossed-beam thermal lens data. The result of the regression analysis may be used as a good probe of flow velocity in the 0.2–30-mm/sec range in liquid streams. Faster flow velocity should be measurable in gas streams.