JOURNAL OF ANALYTICAL CHEMISTRY

Vol. 58

No. 7

2003

THE INTERNATIONAL SYMPOSIUM 637

New Hyphenated Method for the Preconcentrationand Extraction of Arsenic

for Its Flow-Injection Determination

L. N. Moskvin*, A. V. Bulatov*, G. L. Grigor’ev*, and G. I. Koldobskii**

* St. Petersburg State University, Universitetskii pr. 2, Petrodvorets, St. Petersburg, 198904 Russia** St. Petersburg State Technological Institute (Technological University), St. Petersburg, Russia

Arsenic is among the chemical compounds belong-ing to the hazard class 1 from the environmental stand-point. The maximum permissible concentration (MPC)of arsenic in the water of reservoirs for utility anddrinking purposes is 50

µ

g/L, which imposes corre-sponding requirements on procedures for the determi-nation of arsenic in water.

The most popular photometric methods for thedetermination of arsenic based on measuring the absor-bance of the reduced form of molybdoarsenic acid inaqueous and organic solutions is unselective, and theattained detection limits do not meet the requirementsof environmental analytical control and monitoring.Recently proposed procedures for the determination ofarsenic with the use of tetrazolium salts as photometricreagents [1] provide the determination of down to 1 mg/Larsenic; however, in this case the analytical proceduresdo not meet the requirements of environmental moni-toring at the MPC level and are unsuitable for the deter-mination of background concentrations of arsenic innatural water. Therefore, along with the search for newphotometric reagents for the determination of arsenic,the development of methods for its preconcentrationremains an urgent problem.

The aim of this work was to develop a method forthe preconcentration and extraction of arsenic for itsphotometric determination with tetrazolium salts.

The compound of arsenic with tetrazolium salts forphotometric determination is formed by the interactionof the reagent with arsenic hydride in aqueous–organicsolutions. Therefore, for the preconcentration andextraction of arsenic, we developed a combined two-stage method including its reaction chromatomem-brane gas extraction in the form of hydride and the sub-sequent liquid-absorption extraction from the gas phase

into a water–dimethyl sulfoxide solution of the reagentretained as the stationary phase in a chromatographiccolumn with silica gel. The colored reaction productthat formed in the stationary phase is eluted with iso-propanol for subsequent photometric determination.The determination of arsenic can be implemented inboth the batch and the flow-injection modes of photo-metric analysis.

Among the compounds that were previously exam-ined as reagents for arsenic, 2,3,5-triphenyltetrazoliumchloride was found to be the most preferable [1].

We compared the molar absorptivities of arseniccompounds with different tetrazolium salts and the sta-bilities of the absorbances of water–dimethyl sulfox-ide–ethanol solutions of compounds for photometryand found that the replacement of 2,3,5-triphenyltetra-zolium chloride with 5-(1,3-benzdioxol-5-yl)-2-(4-iodophenyl)-3-phenyl-2H-tetrazolium chloride leads toan increase in the sensitivity of the analytical reactionby a factor of 3.5.

For the procedure for the flow-injection photometricdetermination of arsenic under the selected conditions,we attained a determination limit in water of 10

µ

g/L atthe volume of the sample of 20 mL and a time of a sin-gle analysis in the flow-injection mode of 10 min.

ACKNOWLEDGMENTS

The work was supported by the Russian Foundationfor Basic Research, project no. 99-03-32654.

REFERENCES

1. Lazarev, A.I., Lazarev, V.I., and Kharlamov, I.P.,

Zavod.Lab.

, 1980, vol. 46, no. 4, p. 291.

Use of Preconcentration for Sample Preparation in X-ray Fluorescence Analysis

E. N. Maiorova

NPO Spektron, St. Petersburg, Russia

Preconcentration in sample preparation for X-rayfluorescence analysis on Spektroskan spectrometers isused in procedures for the determination of metals innatural, waste, and potable water (group determination)and the determination of gold in ores and rocks afterdecomposition.

In the first case, the preconcentration of a sample isperformed to decrease the determination limit of ele-ments, because after preconcentration the amount ofthe element in the volume from which fluorescenceradiation is taken increases by several times. For exam-ple, in the determination of vanadium from an aqueous

638

JOURNAL OF ANALYTICAL CHEMISTRY

Vol. 58

No. 7

2003

THE INTERNATIONAL SYMPOSIUM

solution, the volume from which radiation is taken isabout 0.01 cm

3

for a Spektroskan spectrometer. In pre-concentration on a DETATA filter (the area at whichsorption occurs is about 3 cm

2

; the volume of the solu-tion from which preconcentration is performed is 100mL), this volume increases to 8 mL. Thus, on changingfrom an aqueous solution to a DETATA filter, the signalincreases by a factor of 800 (for vanadium) to 200 (forlead). The determination limits of the elementsdecrease by the corresponding factor. In an aqueoussolution, the determination limits of metals is 10 mg/L.The relative error in the determination is 40% or more.After preconcentration, the determination limit is0.7

µ

g/cm

3

. This means that after sorption on aDETATA filter from 100 mL of a solution with a con-centration of, e.g., bismuth of 0.05 mg/L, it can be

determined on a Spektroskan spectrometer with anerror below 30%.

In the case of the second problem, the aim of pre-concentration is not only to decrease the determinationlimit of gold, but also to improve the selectivity of theextraction of gold from the sample. After the decompo-sition of ore (rock), the solution commonly containslarge amounts of zinc, copper, and some other elementswith concentrations significantly higher than the con-centration of gold. After preconcentration, only goldremains on the sorbent, which makes it possible toimprove the accuracy of its determination.

The methodological support provided with Spektros-kan instruments is certified in the State Standards Com-mittee, and the instruments are supplied with the neces-sary additional equipment and expendable materials.

Separation and Preconcentration in Neutron Activation Analysis

G. G. Glukhov

Research Institute of Nuclear Physics, Tomsk State University, pr. Lenina 2a, Tomsk, 634050 Russia

Twenty-five years have passed since I first consid-ered the problem of preconcentration in activation anal-ysis [1]. In the last few years, the range of tools andmethods used in analytical chemistry in general andneutron activation analysis in particular was signifi-cantly extended. Nowadays, because of the use of moresophisticated detectors of radionuclides, computers forprocessing the data of neutron activation analysis in itsinstrumental version, and more powerful neutronsources, neutron activation analysis provides the deter-mination of from 30 to 45 radionuclides in differentmatrices in the concentration range from 10

–1

to 10

–8

g/gand lower.

However, neutron activation analysis in some ana-lytical tasks should be used after chemical (radiochem-ical) preconcentration, isolation, and separation.Among these tasks are primarily the determination oftrace elements in high-purity metals with strongly acti-vated matrices; the determination of noble metals inrocks, ores, and minerals in the concentration rangefrom 10

–8

to 10

–1

g/g; and the determination of impuri-ties in rare-earth elements and their concentrates.

Examples are provided by the determination ofrhodium in pure and alloying silver, metals in matriceswith high neutron-absorption cross sections, and traceimpurities in gold. Unfortunately, conventional chemi-cal preconcentration and separation methods cannot beused in these cases. This is explained primarily by thenuclear-physical properties of the analyte and the majorcomponents of the analyzed matrix.

We have developed several approaches and methodsfor solving this problem in the analysis of matrices ofnatural and artificial origin.

The general conclusion is that neutron activationanalysis in its instrumental or radiochemical versionstill remains a leading analytical method.

REFERENCES

1. Glukhov, G.G. and Smakhtin, L.A., Abstracts of Papers,

Vtoraya Vsesoyuznaya konferentsiya po

metodam kont-sentrirovaniya v analiticheskoi khimii

(2nd All-UnionConf. on Methods of Preconcentration in AnalyticalChemistry), Moscow, 1977, p. 13.

Fire Assay as a Method for the Preconcentration of Noble Metals

G. M. Kolesov and D. Yu. Sapozhnikov

Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia

Fire assay belongs to the earliest methods used toextract pure gold, silver, and other metals. Fire assay inmicrocruicibles is actively currently used for the pre-concentration (extraction) of silver, gold, and platinum-group metals for their subsequent determination.

The primary attention of the analysts working in fireassay was always focused on the sophistication of themain step, crucible melting, which in this case is mostoften preconcentration into a lead collector. However,this classical method for the extraction of noble metals