Single-laser/crossed-beam thermal lens detection for short path length samples and flow injection analysis

Abstract

Thermal lens spectrometry (1-23) continues to receive widespread attention because of the simplicity, the sensitivity, and the applicability afforded by this technique. Recently, extensive efforts have been directed toward developing the thermal lens method into a practical detector for liquid chromatography (24)(25)(26)(27)(28)(29). These approaches appear to be feasible and promising for several reasons. First, under most chromatographic conditions, the disturbances to thermal lens caused by the flow are usually tolerable (24). Secondly, the coherent laser source, when focused, provides a diffractionlimited beam size that is compatible with the small deadvolume flow cells of the high-performance liquid chromatographs (13). And thirdly, fiied-wavelength UV laser radiation replacing the conventional mercury light source is readily obtainable, for example, from a frequency-doubled Ar+ laser.This source is capable of exciting a good many molecules eluting from the column. A recent study also reveals that a laser source of relatively low power (<lo mW) suffices for the development of thermal lens signals with good sensitivity (28). The implication of this result is that the thermal lens detector may be accessible to a wider range of chemists because of the reduced laser cost.Earlier, we reported a new dual-beam thermal lens photometer which is based on the utility of a single laser (30). We have also elaborated on the advantages associated with such a single-laser/dual-beam system. One of the inherent sources of noise, however, is the optical interference exhibited by the collinear pump and probe beams. The extent to which the interference can be alleviated depends upon the extinction ratios of the polarizer pair (30). Conceivably, this optical interference phenomenon would not occur in a crossed-beam configuration. Although crossing the pump and probe beams reduces the interaction length, such a configuration should be suitable for short path length samples or flow injection/ chromatographic systems in which the optical path length of the flow cell is typically short (13, 31, 32). For many years, the two-laser/crossed-beam configuration has been utilized in coherent anti-Stoke Raman spectrometry to achieve the proper phase-matching condition (33-35). It has also been ultilized in inverse Raman/Raman gain spectrometry (36,37), Fabry-Perot interferometry (38), thermal diffraction calorimetry (39), and photothermal deflection spectrometry (40-42). Nevertheless, the analytical potential of the crossed-beam arrangement, to our knowledge, has not been exploited for thermal lens spectrometry. In this paper, we report a crossed-beam thermal lens photometer which is also based on the utilization of a single laser. The pump beam is rejected from the probe beam spatially, in contrast to our earlier design in which separation of the two beams is based on their differences in polarization. The feasibility of this approach for 0.1-cm path length samples, as well as its application to the flow injection system, is demonstrated by...

Versatile instrument for pulse width measurement

“…the detection limit is area normalized to approximately 45 pg/mm2. This detection limit is similar to the silver-staining limit, reported as 20 pg/mm2 (24) to 42 pg/mm2 (15) in recent studies.…”

Rotoreflected laser beam thermal lens spectrometry

“…A major attribute arises from the inherent difficulties associated with measuring a small change in a large background signal, as in conventional spectrophotometry. Nevertheless, due to the many variables and degrees of freedom available in thermal lens arrangements, refinement in instrumental design and optimization of experimental conditions become possible, which have led to continuing improvements in its detection capabilities (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16).Of all the studies reported thus far, the majority of efforts appears to have concentrated on the reduction of thermal lens system noise, rather than on the increase of system sensitivity (4, 7,15). In this work, a simple thermal lens photometer is developed for the purpose of sensitivity enhancement.…”

“…A major attribute arises from the inherent difficulties associated with measuring a small change in a large background signal, as in conventional spectrophotometry. Nevertheless, due to the many variables and degrees of freedom available in thermal lens arrangements, refinement in instrumental design and optimization of experimental conditions become possible, which have led to continuing improvements in its detection capabilities (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16).…”

Matrix effects in thermal lensing spectrometry: determination of phosphate in saline solutions

“…The first analytical study involving thermal lensing was performed by Dovichi and Harris in 1979 (2). The technique has rapidly become a widely used analytical method with applications in trace analysis (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14), chromatographic detection (15)(16)(17)(18)(19)(20)(21)(22)(23), and flow injection analysis (24). Inherent in the analytical power of thermal lensing is the ability to detect ultratrace quantities of analyte.…”