A. A. Zhirkov

Criteria for Assessing the Effect of the Composition of Mixed Media on Analytical Sensitivity in Thermal Lens Spectrometry

The effect of the main parameters of solvent mixtures (water-acetone, water-ethanol, water-acetonitrile, water-ethylene glycol, and chloroform-acetone) of various compositions on the detection limit and analytical range was studied, and the selection criteria for the composition of mixed media were discussed in the context of trace determination by thermal lens spectrometry. It is shown that the strength of the thermal lens effect in the medium cannot serve the measure of its effect on the sensitivity and reproducibility of the determination. The lowest detection limits were reached in the water-ethanol (8 times lower in comparison to water), water-acetone (3 times), and water-ethylene glycol (2.5 times) mixtures with a volume ratio of components of 1: 4, while in water-acetonitrile mixtures (22 times), with a component ratio of 3 : 1.

A microplasma analyzer for the determination of alkali and alkaline-earth metals in small volumes of samples of complex phase composition

A microplasma analyzer is developed for the determination of potassium, lithium, calcium, and magnesium in small volumes of samples of complex phase composition on an example of putrefactive lymphocytic wound exudates and articular and amniotic fluids. The operation of the device is based on a drop spark discharge between liquid electrodes, one of which is a drop of a studied solution applied onto the cathode. This scheme allows the reduction of sample volume for a single determination to 15 µL.

Spectrophotometric and thermal lens determination of aluminum with 3-sulfo-5-nitro-4′-diethylamino-2,2′-dihydroxyazobenzene

The conditions of the spectrophotometric and thermal lens determination of aluminum with sulfo-5-nitro-4′-diethylamino-2,2′-dihydroxyazobenzene have been compared. The limit of spectrophotometric detection of aluminum in aqueous solutions has been found to be 8 ng/mL. On the basis of the conditions of spectrophotometric determination, the conditions for thermal lens determination have been proposed (532.0 nm, exciting radiation power of 42 mW); they provide a decrease of the detection limit down to 0.6 ng/mL and an increase of the sensitivity coefficient by an order of magnitude. It has been shown that, in the case of the thermal lens determination of aluminum in water-organic mixtures (50 vol % of dimethyl sulfoxide or 30 vol % acetonitrile), the sensitivity coefficient is respectively 9.1 and 6.3-fold higher as compared with the thermal lens determination in water. As a result, the detection limits are reduced 2.5 and 10-fold, respectively. Aluminum has been determined by thermal lens spectrophotometry in Moscow’s tap water using the standard addition method, its concentration being 0.79 ± 0.07 mg/L, which is above the threshold limit value of the aluminum content of drinking water.

Effect of the Composition of Aqueous-Organic Mixtures on the Sensitivity of Thermal Lens Measurements

Thermooptical properties of aqueous solutions of methanol, acetonitrile, dimethyl sulfoxide, ethylene glycol, glycerol, 1,4-dioxane, and sucrose were studied, and it was found that the analytical thermal lens signal depends on the nature of the organic component, most of all, on the polarity and molecular size. The sensitivity coefficient of thermal lens measurements is increased to a maximum extent in methanol solutions (by 7.3 times at the concentration 50 vol %) and acetonitrile (by 8.8 times at 26 vol %). It was found that a small concentration of water slightly affects the thermooptical properties of polar organic solvents.

Effect of polyethylene glycols on the sensitivity of the thermal lens determination of cobalt with nitrosonaphthols of various structures (nitroso-R-salt and 2-nitroso-1-naphthol)

The effect of polyethylene glycols (PEGs) with molecular weights of 2000, 6000, 13000, and 20000 on the sensitivity of the thermal lens determination of cobalt(III) using nitroso-R-salt and 2-nitroso-1-naphthol is studied. At the polymer concentration as low as 10%, the sensitivity coefficient significantly increases and the detection limit decreases by a factor of 1.5–2 with respect to the determination in an aqueous medium without PEG. This effect is accompanied by an increase in sensitivity due to an increase in the absorbance of the cobalt complexes in PEG solutions at the operating wavelength. Under optimal conditions, the detection limits for cobalt(III) are 6 and 10 ng/mL with nitroso-R-salt and 2-nitroso-1-naphthol, respectively.

Quantification of boron in water by of spectrophotometry and thermal lens spectrometry using reaction with beryllon III

The conditions for the thermal lens quantification of boron in aqueous solutions with a detection limit of 0.3 ng/mL are found (λ = 532 nm, laser power 40 mW); this value of the detection limit is an order of magnitude lower than that attainable in conventional spectrophotometry. A 1: 1 composition of an aqueous ethylene glycol mixture is proposed, using which as a medium the detection limit for boron was reduced to 0.1 ng/mL. Using spectrophotometry and thermal lens spectrometry, boron was quantified in mineral water; the results agree with the data acquired by the reference method of inductively coupled plasma atomic emission spectrometry.

Electroinduced thermal lens spectrometry

A new technique for the generation of a thermal lens effect is considered. In this technique, the action of focused radiation from an inducing laser is replaced by an analogous action of electric current, which forms a thermal lens at a certain site of a liquid by the generation of a high local current density in a small liquid volume. The main regularities of this technique for thermal lens signal generation are considered, and an expression is derived to relate the detected signal (a relative change in probing radiation intensity at the detector) and the electrolyte concentration. A cell is proposed for electroinduced thermal lens detection. The dependence of the thermal lens signal on the applied voltage and on the concentration of the test electrolyte is determined for a model electrolyte (NaCl), and sensitivity characteristics are found to be consistent with theoretical estimations made in this work.

Effect of a solvent on the parameters of the analytical signal, detection limit, and analytical range of the determination in analytical thermal lens spectrometry

The effect of the main parameters of the solvents (water, acetone, methanol, ethanol, acetonitrile, dimethyl sulfoxide, dichloromethane, toluene, and chloroform) that are most frequently used in the analytical practice on the characteristics of the thermal lens effect (increase in temperature because of optical heating and in the size of the region involved in heating) and on the detection limit and the lower determination limit of the thermal lens determination was considered, and the criteria of the selection of the medium for thermal lens experiments were discussed. It was demonstrated that the gain in these characteristics of the thermal lens determination in the given medium in comparison with the aqueous medium does not necessarily coincide with the strength of the thermooptical effect in this medium. It was demonstrated that the optimum conditions of measurements in the thermal lens detection are controlled not only by the absorption of the analyte and the reagents, but also by the intrinsic absorption of the solvent. Recommendations were given on the selection of the solvent for analytical thermal lens spectrometry.

Effect of electrolytes on the sensitivity of the thermal lens determination

The effect of some electrolytes on the detection limits and sensitivity coefficients of the thermal lens determination of model compounds of different types in aqueous solutions is studied. A significant increase in the sensitivity of thermal lens determination in electrolyte solutions is shown. For example, the sensitivity coefficient is doubled in 0.6 M H2SO4 and 1.2 M HNO3, regardless of the analyte nature. It is shown that the effect of electrolyte on the sensitivity of a thermal lens determination cannot be assessed a priori.

Effect of electrolytes on the sensitivity of thermal-lens measurements in aqueous media

The thermooptical properties of aqueous solutions of strong electrolytes (H2SO4, HCl, NaCl, Na2SO4, and NaOH) are considered. The thermal lens signal depends on the nature of the electrolyte. The effect of an electrolyte is not the sum of the effects of the constituent ions. The largest gain in the sensitivity of thermal-lens measurements is achieved with sulfuric acid (sensitivity enhancement factor of 2 for 0.6 M H2SO4 versus water) and sodium chloride (sensitivity enhancement factor of 1.5 for 3 M NaCl), while the effect of hydrochloric acid is negligible.