Thermooptical detection in microchips: From macro‐ to micro‐scale with enhanced analytical parameters

Abstract

In this paper, we compared the methods of photothermal spectroscopy used in different spatial scales, namely thermal-lens spectrometry (TLS) and thermal-lens microscopy (TLM) to enhance the performance parameters in analytical procedures. All of the experimental results were confirmed by theoretical calculation. It was proven that the design for both TLM and TLS, despite a different scale for the effect, is governed by the same signal-generating and probing conditions (probe beam diameter at the sample should be equal to the diameter of the blooming thermal lens), and almost does not depend on the nature of the solvent. Theoretical and experimental instrumental error curves for thermal lensing were coincident. TLM obeys the same law of instrumental error as TLS and shows better repeatability for the same levels of thermal-lens signals or absorbances. TLS is more advantageous for studying low concentrations in bulk, while TLM shows much lower absolute LODs due to better repeatability for low amounts. The behavior of the thermal-lens signal with different flow rates was studied and optimum conditions, with the minimum contribution to total error, were found. These conditions are reproducible, are in agreement with the existing theory of the thermal response in thermal lensing, and do not significantly affect the design of the optimum scheme for setups. TLM showed low LODs in solvent extraction (down to 10(-8) M) and electrokinetic separation (10(-7) M), which were shown to be governed by discussed instrumental regularities, instead of by microchemistry.

Determination of Trace Cr(VI) with Diphenylcarbazide by μFIA–Thermal Lens Microscopy

“…39 In microfluidic chips and capillaries, the plots of the signal amplitude on the flow rate show a shape with a rather distinct maximum. [40][41][42] This is accounted for by the fact that heating and cooling at high modulation frequencies do not result in complete dissipation of the heat during the cooling half-period of measurements, and the incremental heating during a measurement cycle is lower than in the case of complete dissipation of heat. At low flow rates, the flow removes this residual heating at the end of the measurement cycle, the incremental heating governing the signal increases, and signal grows up.…”

“…43 In this study, we obtained the results expected from the viewpoint of this heating model and based on our previous findings. 34,42 For the modulation frequency of 1 kHz, the dissipation time (half-period of the cycle when the excitation beam is closed by the chopper) is 500 μs. The thermal behaviour of a thermal-lens experiment is governed by the characteristic time constant of the thermal lens, t c = ω where ω is the excitation beam radius and D T is the thermal diffusivity.…”

“…The heating period is long enough for attaining the thermal equilibrium (usually 50t c ), however, the cooling half-cycle is governed by thermal effusivity and requires a longer time for the total dissipation of the thermal profile (ca 100t c ). 39,42 These conditions can be attained by selecting lower modulation frequencies (which decrease the S/N ratio) 39 or switching to the asynchronous mode (heating and cooling half-cycles are independent), which is not always advantageous for fast in-flow measurements and cannot be based on lockin schemes. 44 Thus, for a 1 kHz modulation frequency and a lock-in detection, a maximal S/N is obtained for given optical scheme.…”

Comparison of Performance Parameters of Photothermal Procedures in Homogeneous and Heterogeneous Systems

“…Finally, because the sensitivity coefficient (calibration slope) and LOD are governed by different factors, mainly the sensitivity and reproducibility, 14,15,55,56 we compared the photothermal procedures by the minimum value of the reproducibility RSD and by the SE ratio, Eq. (5), which highlights changes in the reproducibility.…”

“…14 Moreover, simultaneous consideration of the accuracy of TLS and reaction conditions at the nanoscale level also enhances the selectivity. 15 However, the dependence of the TLS sensitivity on the thermophysical properties of the solution poses a problem: What is more important, an increase in the strength of the photothermal effect (photothermal sensitivity) or an increase in the analytical sensitivity due solvent selection as a chemical component?…”