Determination of phosphorus at the parts per trillion level by laser-induced thermal lensing colorimetry

Abstract

The laser-induced thermal lensing effect was applied to molybdenum blue colorimetry of phosphorus measurement. An Ar+ laser pumped rhodarnlne 101 laser was used as the heat and probe source. The signal from a silicon photocell with a 1 mm2 photosensitive surface, which was used as a laser radiation detector, was processed with an Inexpensive personal computer. The detection limit is 5 pg of P/mL which is determined mainly due to the deviation In the color development with the molybdenum blue method. The present method was applied to determine phosphorus in seawater and lake water.Methods for detecting trace phosphorus (nanogram level) in environmental samples Eire very limited. For exEimple, the spectroscopic technique did not reach the detection limit of 1 ng of phosphorus/mL in spite of many investigations using chemiluminescence, ICP, DCP, or AAS. Even nowadays, the most sensitive method is the classic molybdenum blue colorimetry based on the reduction of molybdenum in phosphor-molybdate. In combination with solvent extraction, determination of phosphorus at sub-part-per-billion levels is possible. However, even with this method, the concentration of phosphorus in some natural waters cannot be measured. In the environmental water system, the concentration of phosphoric acid necessary to induce eutrophication is sometimes less than a detectable amount. Among the various attempts to improve molybdenum blue colorimetry, the present study applied thermal lensing colorimetry using Ar+ laser pumped dye laser.LEiser-induced thermal lensing effect was first investigated by Gordon et al. (1), and it is pointed out that thermal lensing colorimetry is an important way to monitor small absorbance. Recently, Dovichi and Harris studied the theoretical aspects of various applications of thermal lensing colorimetry to analytical chemistry (2-4). Imasaka and Ishibashi reported the detection of iron by the thermal lensing effect (5, 6).Haushalter and Morris (7) applied thermal lensing colorimetry to monitor enzymatic reaction involving dopamine which could not be sensitively detected by other spectrometric methods, including fluorometry. Because of its high sensitivity, the feasibility for laser-induced thermal lensing colorimetry in environmetal analysis cannot be ignored. In a previous paper (8), we reported on thermEd lensing colorimetry of nitrite ions wherein color development was based on the diazo-coupling reaction. The detection limit of nitrite ions was thus improved about 500 times over ordinary colorimetry. In the present paper, determination of parts per trillion levels of phosphorus by the thermal lensing effect is discussed, along with signal processing by a personal computer. This method was applied to the analysis of phosphorus in seawater and lake water.

Comparison of single- and dual-beam configurations for thermal lens spectrometry

“…The earliest studies of the thermal lens effect (I, [17][18][19] utilized a single laser beam which served both functions of forming the lens in the sample and probing its presence. The single beam configuration has persisted in the development of analytical instrumentation and applications of the thermal lens effect (2,3,6-9, [11][12][13].…”

MAGIC method for subnanomolar orthophosphate determination in freshwater

“…Precision in low nanomolar to subnanomolar natural samples was not reported, however, and its reagent blank is high (equivalent to ~144 nM, giving 0.2 absorbance units) and arsenate interference on SRP colorimetry was not addressed. Other methods, including laser-induced thermal lensing colorimetry (Fujiwara et al 1982) and gas chromatography (Hashimoto et al 1987), are very laborious and involve complex instrumentation, whereas the preconcentrating method of liquidliquid extraction using organic solvents (Koroleff 1983) is subject to high blanks.…”

Underway monitoring of nanomolar nitrate plus nitrite and phosphate in oligotrophic seawater

“…All these preconcentration procedures are time consuming, easily contaminated, and unsuitable for automated analysis. Another technique relies on expensive instruments such as laser-induced thermal lensing (Fujiwara et al 1982) and HPLC (Haberer and Brandes 2003), which again are not feasible for direct shipboard measurements. Using chemiluminescence detection (Yaqoob et al 2004) and microsequential injection (Wu and Ruzicka 2001), highly sensitive determination of phosphate in fresh water samples have been achieved, but to the best of our knowledge, these methods have not been successfully applied to oceanographic studies.…”