The Thermal Lens Detector

The cell can be made a few microlitres in volume and thus would be suitable for use with small bore columns. A sensitivity of 10 AU was claimed for the detector and a linear dynamic range of about three orders of magnitude (although by now this may have been increased). As with other bulk property detectors, the thermal lens detector would not be suitable for use with gradient elution. The use of lasers make the detector extremely expensive, however, the availability of a UV laser, should it be developed, might make the device more useful. 

Basically, the thermal lens detector is a special form of refractive index detector and, as such, has potential as a type of universal detector. There is also the possibility that it could be developed into a more sensitive detecting system than the conventional refractive index detector. 

Solvent flows continuously through a bridge network which consists of four matched stainless-steel capillaries. The differential pressure across the bridge is zero until the sample contained in the column eluent passes into capillary R3. 

As a consequence, the differential pressure begins to rise until it reaches a steady state value of A/ proportional to the specific viscosity of the solution. The differential pressure is monitored continuously on a strip chart recorder. The equation relating A/ to specific viscosity is  

A form of the viscosity detector has been used for exclusion chromatography, and prior experiments employing a single capillary have demonstrated the feasibility and usefulness of a continuous on-line viscosity detector in conjunction with a concentration detector (RI or UV). The sensitivity of the device, however, is rather poor, the minimum detectable concentration being about lO g/ml. To date, the instrument has only been used in a recent application in polymer chromatograjdiy. 

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Another detector based on refractive index change is the thermal lens detector. When a laser is focused on an absorbing substance, the refractive index may be affected in such a way that the medium behaves as a lens. This effect was first reported by Gorden et al. [4]. Ther-... 

In this arrangement the pulse and probe beams are colinear the probe beam is generated by a CW laser, and it falls on the photomultiplier tube D through a pinhole PH. When the thermal lens is formed in C, the divergence of the probe beam increases and the intensity seen by the detector D decreases. 

FIGURE 7.20 Schematic illustration of the thermal lens measurement. The excitation beam was focused by the objective lens. After excitation of some analytes, their radiationless relaxation caused the thermal effect and the formation of a concave thermal lens. The probe beam, which was also coaxially focused by the same objective lens, was collected by the photodiode detector defined by the pinhole. Any change in the amount of heat produced by the radiationless relaxation is manifested as the change in the photodiode output as a TLM signal [733]. Reprinted with permission from Elsevier Science. 

In contrast to the setup shown in Figure 5, our experiment uses two coaxial laser beams focused onto the sample C by a lens L (Figure 7). The dye laser beam (power 1 to 10 mW) creates the thermal lens in the sample, whereas the helium-neon laser is used only for monitoring development of the thermal lens. To avoid a thermal lens being induced by the helium-neon laser, a neutral density filter F2 reduces the power of its beam to 6 to 7 /xW in the sample. In front of the detector D, an interference filter F, blocks the beam of the dye laser, and a pinhole P is placed such that only light near the optical axis reaches the detector. To monitor the wavelength of excitation Ao, part of the dye laser beam is deflected by a glass plate Gj onto a... 

The excitation volume generated by a Gaussian profile laser beam constitutes a thermal lens, and hence a probe beam focuses or defocuses on passage through the thermal lens volume. Therefore, a different spot size of the probe laser beam is observed at the location of a photodetector the diameter change can be measured directly with a position-sensitive detector. 

Both molecular and atomic detectors have been used in combination with SCF extractors for monitoring purposes. Thus, the techniques used in combination with SFE are infrared spectroscopy, spectrophotometry, fluorescence spectrometry, thermal lens spectrometry, atomic absorption and atomic emission spectroscopies, mass spectrometry, nuclear magnetic resonance spectroscopy, voltammetry, and piezoelectric measurements. 

In case of small detector areas fluctuations of the local position of the laser spot on the detector will cause fluctuations of the measured output signal. These beam instabilities can be caused by thermal effects of the laser resonator which might result in a small tilt of the resonator mirrors, or by inhomogeneous heating of the active laser medium, which leads to thermal lens effects and affects the stability of the laser beam. 

This article provides some general remarks on detection requirements for FIA and related techniques and outlines the basic features of the most commonly used detection principles, including optical methods (namely, ultraviolet (UV)-visible spectrophotometry, spectrofluorimetry, chemiluminescence (CL), infrared (IR) spectroscopy, and atomic absorption/emission spectrometry) and electrochemical techniques such as potentiometry, amperometry, voltammetry, and stripping analysis methods. Very few flowing stream applications involve other detection techniques. In this respect, measurement of physical properties such as the refractive index, surface tension, and optical rotation, as well as the a-, //-, or y-emission of radionuclides, should be underlined. Piezoelectric quartz crystal detectors, thermal lens spectroscopy, photoacoustic spectroscopy, surface-enhanced Raman spectroscopy, and conductometric detection have also been coupled to flow systems, with notable advantages in terms of automation, precision, and sampling rate in comparison with the manual counterparts. 

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