The determination of selenium by atomic-absorption spectrometry: A review

Tulunra, Vol. 28..pp. 637 to 646. 1981 Printed m Great Britain. All rights reserved

Copyright

0039-9140/81/090637-lOM2.00/0 0 1981 Pergamon Press Ltd

THE DETERMINATION OF SELENIUM BY ATOMIC-ABSORPTION SPECTROMETRY: A REVIEW M. VERLINDEN*, H. DEELSTRA and E. ADRIAENSSENS~ University of Antwerpcn (U.I.A.), Department of Pharmaceutical

Sciences, Universiteitsplein,

1, B-2610 Wilrijk, Belgium (Receioed 22

December

1980. Accepted 11 March 1981)

Summary-A comprehensive review is given of the determination of selenium by the various atomicabsorption spectrometry methods that have been developed, covering the use of various flame and electrothermal ionization methods, hydride techniques, preconcentration and separation, and giving an appraisal of the results.

The development of atomic-absorption spectrometry (AAS) since 1960 has brought about a real revolution in the field of trace analysis.’ This relatively simple, fast, sensitive and reasonably precise technique has to a large extent replaced analytical methods such as calorimetry and fluorimetry which could sometimes be rather time-consuming. AAS still remains attractive to analysts in spite of the availability of potentially more sensitive techniques such as neutron-activation analysis or spark-source mass spectrometry. Moreover, these techniques require expensive equipment and are not readily available to most analytical laboratories. Traces of selenium can be defermined by fluorimetry2.3 but the method is cumbersome, slow and subject to many interferences, which led to attempts to determine selenium by flame AAS, but the first attempts were not very successful, and offered no improvement over other techniques. As a result of persistent, research however, determination of selenium by AAS can now easily sustain comparison with other methods. This paper outlines the evolution of determination of selenium by AAS to give a better understanding of its capabilities and to illustrate the work of investigators from all parts of the world. DETERMINATION BY CONVENTIONAL FLAME AAS Allan

was the first to describe the determination

of

selenium

by AAS. He used a hollow-cathode lamp (HCL) and the less sensitive 204-nm line. For aqueous standards and an air-acetylene flame a 5-ppm selenium solution gave 1% absorption. Sprague et al.5 used the more sensitive selenium resonance line at * Research Assistant of the National Fund for Scientific Research (Belgium). t Metallurgic Hoboken-Overpelt NV.

196.1 nm, and obtained 1% absorption for a 1-ppm standard. They were also the first to apply flame AAS ‘, for the determination of selenium in a real sample (a copper alloy). Rann and Hambly6 and Allan’ described two major difficulties limiting the successful use of flame AAS for determination of selenium. First the stability and intensity of the hollow-cathode lamp are poor and the life span is short. Secondly the main selenium resonance line is in the far ultraviolet region of the spectrum, where a very high proportion of the radiation is absorbed by air and flame gases. Rann and Hambly6 and Dagnall et a1.8*9described the construction and use of microwave-excited electrodeless discharge lamps (EDLs) for selenium which give higher intensity, better stability and longer life than the hollow-cathode lamps. However, the construction of these EDLs remained problematic”.’ ’ and it was not until 1973 that they became commercially available.‘2-14 Allan’ had found that a hydrogen-air flame absorbed 50% of the incident radiation at 196.1 nm and an air-acetylene flame absorbed 70%. Dagnall’ used a nitrogen-hydrogendentrained air) flame with a threeslot burner and was able to reduce absorption by the flame to 20%. However, because of the low temperature of this flame (300-5OOO)thermal breakdown of selemum cor$ounds tii&t be incomplete and interferences from ions and molecules would be likely to occur. Kahn and Schallis” used an argon-hydrogen4entrained air) flame with a premix burner and a three-slot burner head. The background absorption from this flame at 196.1 nm was only 13%, but the argon-hydrogen flame appeared to be much more subject to matrix effects than the air-acetylene flame. They aiso tried to improve their results by adding oxygen to the flame but this caused a radical decrease in the absorption by selenium standards. Chakrabarti16 compared the sensitivities obtained at different wavelengths with air-acetylene and air-hydrogen 631

M. VERLINDEN,H.

638

DEELSTRAand

flames and determined optimum experimental conditions. He studied the effect of interfering cations and anions at lOOO-foldratio to selenium in an air-acetylene flame. Kirkbright et ai.” used nitrogen sheathing of an air-acetylene flame to reduce the background radiation. The background absorption (at 196.L nm) was also reduced, from 70 to 28”/, and the detection limit lowered by a factor of three from 3.6 to 1.2 pg/ml. Interferences were the same as with a conventionai air-acetylene flame, however. K~rkbright and Ranson’* introduced a nitrogen-separated fuel-rich nitrous oxide-acetylene flame which still further lowered the background absorption. The increased temperature of this flame and its reducing properties permitted atomization of refractory oxides and gave considerable freedom from chemical interferences, although a loss of sensitivity was observed. Several workers investigated interferences that are encountered in the cooler flames such as nitrogenhydrogen or argon-hydrogen flames. Nakahara er a1.19studied the effects of 28 cations and 4 commonly used acids on the determination of selenium by AA’S with an air-hydrogen flame, and later” claimed that interferences in the argon-hydrogen flame could be minimized by adding excess of tin (as stannous chloride). Others, however, found this method not entirely successfu1.*’ Severne and Brooks22~23dealt with interferences by separating selenium from the matrix by co-precipitation with arsenic and dissolving the precipitate in nitric acid before atomizing it in the nitrogen-hydrogen flame. Reichel and 131eakleyz4separated traces of selenium from a copper matrix by co-precipitation with lanthanum hydroxide. MulfordZ5 used ammonium pyrrolidinedithiocarbamate (APDC) and solvent extraction into a solvent such as methyl isbutyl ketone (MIBK) to separate and concentrate selenium and aspirated the extract directly into the flame. Chakrabarti16 used sodium diethyldithiocarbamate (DDTC) to form Se(DDTC)& and extracted it into MIBK. This extraction improved the sensitivity by a factor of 1.5 for an air-hydrogen flame and 2.4 for an air-acetylene flame. Chambers and McClellanz6 evaiuated several complexing agents and organic soivents; the organic phase was stripped with aqueous cyanide solution and the aqueous solution was atomized in an air-acetylene or nitrogen-(entrained air)hydrogen flame.

E. ADRIAENSSENS

Watling 27 described the use of a slotted quartz tube in conjunction with different flames for the determination of selenium, antimony, arsenic and mercury. The use of this tube resulted in much enhanced absorbances for selenium in an air-acetylene flame. No interferences were observed and the precision was considerably better than for the ordinary flame. Khalighie et al.*’ used a water-cooled silica tube placed at the tip of the primary reaction zone of an air-acetylene flame to trap the analyte compounds formed in the lower part of the flame. These compounds were released and atomized by increasing the temperature of the tube. The atomic absorption in a zone near the surface was measured. Several atomization mechanisms were postulated?’ Another interesting atomic-absorption method was developed by Lau and Lott.30 As in the fluorimetric methods, selenium was reacted with 2,3-diaminonaphthalene (DAN) to give the piazselenol, which was then reacted with an excess of aqueous palladium(I1) chloride after extraction into chloroform. The resulting Pd(DANSe)$& complex was then nebulized in an air-acetylene flame and the palladium absorption signal measured. Table 1 gives a survey of the sensitivities and detection limits obtained for selenium with several flame AAS systems. Appli~tions of flame AAS worth mentioning are the determination of traces of selenium in stainless steel,j’ organometaltic compounds in rubber-manufacturing processes,32 and in animal feed premix.” BOAT OR WIRE IN FLAMESYSTEMS AAS detection limits can be improved by methods which eliminate the nebulizer (which has c 10% et& ciency as a means of transporting sample into the flame), such as the tantaIum-boat sampling system,33’34 or by use of electrothermal atomization. Only volatile elements such as As, Pb, Cd, Te, Se can be determined with the boat system, because of the relatively low temperature attainable. At waveIengths shorter than 200 nm, introduction of the boat into the flame causes pronounced effects and the use of deuterium-lamp background-compensation is mandatory in this region. For selenium, absolute detection limits of 10 ng (in 1 ml of sample) have been obtained, but the precision is poor. 34 GolembeskiJS has used the tech-

Table 1. Flame Air-acetylene Air-hydrogen Argon-hydrogen/ nitrogen-hydrogen Nitrous oxideacetylene

Sensitivity, ~~rn~~~ ~sar~t~~n

Detection limit,

OS-3 0.4-l 0.15-1.1

0.2-3 I 0.05-1.15

1.5-6.5

wm

1.8-2.5

References 5,6,8,12,13,t5,18,21 16 &l&21-23 12,18

The determination of selenium by atomic-absorption spectrometry

639

organic form or a metabolized form in rat whole blood. These authors found copper and iron to be incapable of stabilizing either form of selenium whereas silver and nickel were satisfactory. Most of the applications of graphite-furnace AAS dealt with below are concerned with water anajysis,46,47.50-55 samples,s6-61 metallurgical foods48,62-6S or body fluids.4g*66*67 Baird er &50*51 used carbon-rod atomization to determine traces of selenium in waste waters. They used a nitric acid-perchloric acid digestion to oxidize organic material. The presence of concentrated nitric acid was necessary to suppress the intensity of the background perchloric acid peak, and prevent depression of the selenium peak. At l-g/l. concentrations, alkali-metal halides caused significant background absorption. Henn46*47‘noted severe enhancement or depressant, effects on the signal from 50-pg/ml selenium samples, by 26 out of 31 cations studied at the lo-gg:/ml level. Errors ranged from 610,/,depre~ion by tin to 278% enhancement by vanadium. Nickel and molybdenum enhanced the signal by 85 and 243% respectively. Further increase of the interferent metal concentration to 100 &ml or more caused very severe background absorption, exceeding the compensating capabilities of a deuterium lamp background-corrector. Cationic interferences were eliminated by removal of the ions by ion-exchange. Remaining interferences from anions were dealt with by adding molybdenum to the solution. The moly~enum not only reduced ELE~ROT~ERMAL ATOMIZATION matrix interferences but also extended the charring L’vov described the graphite-furnace technique as temperature range to 1400’. Martin er a1.54devised a destruction procedure for early as 1959.40.4* It was modified later by Massuse in the dete~ination of selenium in water, waste mann.42*43 Even before instruments for graphite-furwater, sediment and sludge by electrothermal AAS, nace AAS became commercially available, around 1970, several workers had been applying the new using a nitric acid-hydrogen peroxide digestion and technique for the determination of selenium. adding nickel nitrate to both samples and standards. MatouSek44 found a detection limit of 72 pg in 2 ~1. They found that many suppressive effects of metals and anions could be partly compensated for by the As Ediger4’ pointed out, some matrix-analyte combipresence of nickel at a concentration of 1%. Neverthenations present problems owing to the extreme volatiless the standard-additions method often remained lity of the analyte, as is the case of selenium. The mandatory, even for environmental water samples.54 analyte may then be partly or completely lost during the ashing of the matrix. He therefore introduce the Weicher et al.s6 tried to optimize e~ectrot~erm~ principle of matrix modification, by adding nickel to a AAS for the rapid determination of selenium and other volatile metals in high-temperature alloys. They sample in which volatile elements were to be deterfound, like other workers, that the total acid concenmined. The nickel-selenium compound, probably tration should be limited1 to 10% v/v to minimize nickel selenide, was thermally stable up to 1200”. This background effects. For several alloys the matrix effect was not unique to nickel; addition of copper interferen~s were severe and led the authors to recwas also reported to allow the use of higher charring ommend matrix-matching of standards and samples. temperatures. Henn used molybdenum.46*47 He noted that much higher charrmg temperatures (up to 1400”) Marks et al.‘* introduced solid alloy samples into the furnace and induced atomization without a precould be used without loss of selenium and explained this behaviour by postulating the formation of a atomi~tion heating cycle. Selenium was determined in chromium metal by Hamner et ais and in copper heteropolymolybdate anion containing a selenium atom in its centre. Szydlowski, however, found the use by Mullen, ” who separated selenium from the copper matrix by co-precipitation with ferric hydroxide, of molybdenum to yield poorer sensitivity than the collected the precipitate on a filter paper disc, and use of either nickel or copper.48 Saeed et a1.49 studied analyzed portions of the disc by furnace AAS. Ohta the effect of copper, iron and silver on the thermal and Suzukis7 constructed a molybdenum micro-tube stability of radioactive selenium present either in in-

nique for the determination of selenium in rocks, and extended detection down to 0.06 ppm with good precision. Delves has modified the sampling-boat system36 but the absolute detection limits for selenium were poorer: 100 ng (in IO0 ~1).37 Using the selenium line at 206.3 nm, Clark et ~1.~’ obtained a detection limit of 22 ng (5 pl), the detection limit being taken as a signal equal to the noise. A technique somewhat similar to the Delves-cup technique was described by Lund et a1.39They preconcentrated the analyte of interest by electrolytic dep&ition on a platinum wire, which they then placed in an air-acetylene flame for atomization. To improve the sensitivity a quartz tube was placed above the filament, with a central hole facing it. Alkali and alkaline-earth metal salts, which usually cause severe background absorption in flame AAS, were thus not only removed before atomization but also acted as supporting electrolytes in the preconcentration step. The stand~d-additions method proved mandatory for calibration. The detection limit for selenium was superior to that obtained with the Delves-cup technique: 5 ng/ml with a 2-min electrolysis. The main disadvantages of these non-nebul~er techniques are the very small number of elements for which they can successfully be used and the poor precision for elements with resonance lines at wavelengths shorter than 200 nm.

640

M. VERLINDEN, H.

DEELSTRA and E. ADRIAENSSENS

for the determination of selenium in metallurgical samples. Extraction of Se from the sample with 3,3’-diaminobenzidine proved to yield a more accurate determination. Meyer et al.61 determined traces of selenium in copper, copper alloys, silver, gold, lead and bismuth after heating at approximately 1100” in a stream of oxygen, the SeOz formed being trapped in a cooled receiver, dissolved in nitric acid and determined by furnace AAS. In the field of food analysis, and analysis of biological samples in general, Ihnat has performed extensive and critical studies on the use of carbon-furnace atomization62-64 for the determination of selenium. Samples were digested with a nitric acid-perchloric acid-sulphuric acid mixture. The influence of several acids was studied and it was concluded that the presence of traces of perchloric acid in sulphuric acid resulted in poor absorbance signals. Nitric and perchloric acids were superior matrices and their optimum concentrations were established to be 0.5 and 1M respectively (in the presence of nickel). Recovery experiments indicated that sample matrices could seriously interfere with the selenium signal. These interferences were obviated by isolating selenium from the matrix by reduction and precipitation with ascorbic acid. The filter and precipitate were digested and nickel was added before by graphitefurnace AAS for selenium. The addition of nickel greatly improved the recovery and precision. Shum et aL6’ used graphite-furnace AAS for the determination of selenium in fish and food products. AAS Szydlowski4’ optimized a graphite-furnace method for the analysis of complex carbohydrate matrices. He used a nitric acid-perchloric acid digestion and relied on solvent extraction to minimize interference. The selenium was reacted with 2,3-DAN under standardized conditions, and the piazselenol was extracted into decahydronaphthalene. Copper was added to ensure the complete breakdown of the chelating agent during atomization and the prevention of volatilization of the analyte. A similar extraction technique has been applied by Ntve and of selenium in Hanocq 67 for the determination human plasma and erythrocytes. After digestion, the Se(IV) is reacted with 4-chloro-l,Zdiaminobenzene, which is soluble in toluene. Ishizaki66 used dithizone to chelate selenium and extracted the complex into carbon tetrachloride. Blood and other biological samples were first decomposed by oxygen-flask combustion. Interfering cations were removed by passing the digest through a column of cation-exchange resin. Kamada et al.” studied the extraction behaviour of Se(IV) and Se(V1) with DDTC, APDC and dithizone. They concluded that DDTC extraction at pH 3.545 into chloroform is best for the selective separation of Se(IV) and Se(V1). Saeed et aL4’ determined selenium in serum without destruction of the organic matrix. They transferred small aliquots of serum directly into the atomizer

graphite-furnace electrothermally. suitable for the blood, because that arose from

tube, added nickel and atomized They noted that the method was undetermination of selenium in whole of the serious spectral interferences the large amounts of iron present.

THE VOLATILE

HYDRIDE

TECHNIQUE

Hatch and Ott” described a “vapour generation” technique for the determination of mercury in solution by AAS. It employed a reduction procedure with stannous sulphate. The mercury vapour was removed from the solution with a stream of air inttoduced into a quartz-windowed atomization cell. Holak used a similar technique for the determination of arsenic,6g based on the generation of arsine, which had been established for years as an official method for separating arsenic from a solution.” Holak trapped the volatile arsine in a tube cooled with liquid nitrogen, desorbed it and swept it into the airacetylene flame of an AA spectrometer. This technique was then used to determine selenium,‘l the hydride being atomized in an argon-hydrogen-(entrained air) flame. The technique offered several potential advantages. All the selenium present was converted into the hydride and introduced at once into the atomization device. Moreover it was hoped that chemical interferences could be minimized by this separation of the analyte from the sample matrix. Numerous papers have since dealt with the hydrideevolution technique, with applications to the determination of selenium in natural waters.s3.72-80 environmental samples,81*82 blood,s3 urine,*’ foods,62.*‘s* animal feed and forage samples,*g-gl metallurgical samples, g2-g4 glassg6 and petroleum.” Methodology

Arsenic(V) and selenium(V1) are first reduced to arsenic(II1) and selenium(IV) by letting potassium iodide and stannous chloride react with the acidified sample, then zinc metal is added and hydrogen, arsine and hydrogen selenide are evolved. The hydrides are swept from the solution by an inert gas stream. The potassium iodide and stannous chloride solutions have been used with different concentrayamamoto99 and Church tions. 62.71.73-75.80.97-103 and Robison”’ omitted the potassium iodide and were able to generate hydrogen selenide equally well. The stannous chloride appreciably accelerates the reduction process, however.” Fiorino et aLs4 stated that the addition of sodium iodide might even prevent the generation of H2Se and this was confirmed by observations by Yamamoto et aLlo and by Clinton.83 Walker et al.” demonstrated that the catalytic effect of tin(I1) on the Zn/HCI reduction was only required for weakly acidic media. They observed loss of selenium if acidic solutions with stannous chloride present were left standing for a certain time before the Zn/HCI reduction. They suspected that elemental selenium was formed and agglomerated, thus deplet-

The determination

of selenium by atomic-absorption

spectrometry

641

are mainly determined by the design of the apparing the selenite in solution.97 Yamamoto and KumamaruLo4 noticed that the presence of more than 0.04% atus.85~86~110Most workers have simply optimized stannous chloride impoverished the sensitivity by 30:/, their own particular system, whether laboratory-made or commercially available. 85*91*117 Hydrochloric acid and also gave poorer reproducibility. They therefore decided to use the Zn/HCI system as sole reductant. (0.4-6M) has usually been the acid of choice, although Granular zinc,71.99,101 zinc tablets,lo4 and zinc dust sulphuric acid is equally suitable.62~85~116~‘17Many have been used. Several or slurry 62~73~80~97~100*103 workers have recognized that increasing the analyte workers have found the sensitivity to depend upon solution volume has an adverse effect on peak absorbthe reaction time between the addition of zinc to the ance measurements.92~93*100~110~117The selenium acidified sample and flushing of the hydride vapours must also be present in the quadrivalent form, and from the generator flask into the atomization cell. any selenate must be quantitatively converted into Cutter’8 discussed Goulden and Brooksbank tried aluminum powder selenite. ‘4~75~78~83~91~93~109.119 instead of zinc, and found reaction times of a few several ways of doing this and found heating the solminutes necessary for complete reduction. However, ution in a boiling water-bath, at a hydrochloric acid others have pointed out that waiting too long to concentration of 4M, to be the only suitable method. release the hydrides from the generator flask may This has also been stated by other authors.75,1 l8 cause a decrease in sensitivity, because of partial dissolution of the hydride in the liquid phase.lOO*‘O1 Instrumentation Besides the Zn/HCI reduction system other reducing Reaction chambers have included large test-tubes, agents such as TiCI,/Mg/HCI have been used.lo5 Erlenmeyer flasks, wash-bottles or pear-shaped flasks. Schmidt and Royer” were the first to report the The hydride evolved has been transferred in two pringeneration of hydrides by reduction with sodium cipal ways: either the hydride is conveyed directly borohydride. Sodium borohydride was found to be into the atomization cell as it is generated, or some superior to zinc with regard to reduction yields and form of storage is used before the transfer to the atocontamination of the blank. Ever since, it has been mizer. In the former case the system acts as a continuous-flow system; in the latter case all of the hydride the reductant of choice. It has often been used as pellets53~72~82~8*~*9.92~ formed is introduced in a burst into the atomizer, and 94.104-108 but McDaniel et a!.*’ found that pellets a “transient” sharp signal is observed. Goulden and were only 4&600/, as efficient as borohydride solBrooksbank called this a “batch operation”.74 utions; they attributed this to the very fast and localMost of the early procedures included some form ized generation of hydrogen when pellets are dropped of collection of the generated hydride. The hydride into an acidic solution. Attempting to automate the was collected in a rubber balloon,’ 1-73*93*98~102,106 determination of arsenic by arsine generation, Vijan plastic bag or a pressurized a collapsible and Wood” were the first to use solutions of sodium chamber g9~104~105or it was simply stored in the borohydride instead of pellets. Soon after, Thompson head-spice of the generator.’ OoqlO1 and Thomerson”’ used a 1% solution of sodium boroAfter the introduction of sodium borohydride as hydride for the generation of hydrogen selenide. The the reductant, continuous-flow systems became popuconcentrations used have varied widely, ranging from lar, because the reduction reactions proceeded much 0.5 to 80, 76-79.80-82.84.86.90,91,93.95.96,104,109-124 faster. In some cases the excess of hydrogen generated Sodium borchydride solutions need to be stabilized served as a propellant for the transfer of the hydride by alkalization, but the concentration of alkali should to the atomization cell, but in most cases inert gas not be too high lest the solution viscosity becomes so streams were used to carry the hydride to the high that homogeneous dispersion of the hydrogen ce11.~4~85~go~91*100~101 Moreover, flushing the system bubbles is no longer certain, which would be reflected with an inert gas in order to remove air had proved in poor precision. Concentrations of l-2’? sodium necessary. Yamamoto99 demonstrated that purging hydroxide stabilize the solutions satisf~ctorily.8s’1 ” the air from the generation/atomization system was Comparative studies of the different reduction sys- necessary in order for precise results to be obtained. tems have been made.81~‘04 Yamamoto and KumaFrom the literature it is apparent that gas-flow adjustmaru concluded that both the Zn/HCl and the ments during the sweeping phase are critical in most sodium borohydride systems yielded equally good open-ended atomization systems.76.1a9*125 In most sensitivity and precision.104 Using radiotracers, cases argon has been used as carrier gas although McDaniel et a1.*l investigated the reduction efficiency nitrogen is cheaper and gives comparable results; of several hydride-generation methods and found the helium has also been used.‘**“’ Many authors have Zn/HCI systems very inefficient, 88% of the selenium stressed the importance of keeping the dead volume present ‘remaining unreacted. Sodium borohydride between the generation solution and the atomization yielded higher recoveries, although conversion into cell as small as possible in order to obtain a highly the hydride and transfer to the atomizer were not concentrated atom cloud in the atomization ce.11.85J10 quantitative in most systems. The efficiency of hydride The design of most apparatus has undergone modigeneration is strongly dependent on the use of optifications to facilitate the addition of reagents, decrease mized chemical and physical parameters, but these the duration of an analysis and diminish errors due to

642

M. VERLINDEN.H.

DEELSTRAand E. ADRIAEN~SENS

numerous manipulations in, the course of the analysis. Automated designs have been described.74.76*gl.l l1 In recent years several authors have demonstrated the importance of taking into account the kinetics of the reduction of different elements to their covalent hydrides. They propose collection procedures for hydrides that are slowly evolved, while hydrides that are formed very rapidly or that are unstable should be transferred directly for atomization. Chapman and DalelIz introduced a versatile gas-handling system that is inserted between the generator flask and the atomization cell, and that leaves the operator with the choice of collecting the hydride (advised for selenium or arsenic) or using the direct transfer mode. Most investigators, however, have preferred cold-trap procedures in which the hydride is stripped from the solution. dried and collected in a liquid-nitrogen trap.78*81~1Q8*1 1o.126 After a minute the cold trap is removed from the liquid nitrogen and warmed, and the hydride is vaporized and transferred at once into the atomizer. Because the boiling point of hydrogen is lower than that of nitrogen, such methods remove the excess of hydrogen generated, so the hydride is much less diluted when introduced into the atomization cell and better sensitivities should be obtained.“’ As pointed out by Siemer ‘lo these procedures eliminate reduction and stripping kinetics as possible sources of variation in the overall performance of the method. Cold-trap methods always include a drying step because any moisture would condense and clog the trap tube, and impair the revolatilization of the hydride.s’ Atomizers The first applications of hydride generation usually involved atomization in an argon-hydrogen-(entrained air) flame. The excess of hydrogen generated along with the hydride often perturbed the flame, changing its composition and thus also causing a change in the absorption of the flame. Though use of the deuterium background-corrector was found to reduce the blank signals,98 in almost every other study with this flame background correction was judged unnecessary. ” The relatively cool and nonabsorbing argon-hydrogen-(entrained air) flame from a Boling triple-slot burner has been used by many 102, 103. investigators, ‘53.62. ‘l-73.77.82.86,92,94,97-100, 105-108~111*119-121 as have nitrogen-hydrogen--(enhelium_ trained air) 75~83~84~88.89~93.118 and even hydrogen-(entrained air) flames.78~101 In some cases the diffusion flame has been confined by a tube. ~4~78~80~90~91~110~127

Church

and

RobisonlOl

noted

that the use of helium instead of argon as carrier gas reduced flame noise and increased the sensitivity twofold. Ihnats5 pointed out that the flow-rates of the flame gases need adjustment according to the kind of reduction system used. Zn/HCI reduction requires high argon flow-rates to dilute the excess of hydrogen generated by the reduction. Flames used in conjunction with the sodium borohydride system were easier

to balance; the lower flow-rates resulted in a 30% enhancement of the sensitivity in comparison with a “stiff” flame. Fiorino,84 Corbin7’ and Ihnats5 used shielded flames in order to minimize drafts from the ambient air, thus improving flame stability and suppressing baseline noise. Background absorption in the flame often necessitated the use of a deuterium corrector and influenced the signal to noise ratio unfavourably. DBdina and RubeHka”’ have studied the atomization mechanism of hydrogen selenide in a tubeconfined fuel-rich hydrogen-oxygen diffusion flame. They found that atomization was not achieved by thermal decomposition, but was due to free radicals generated in the flame. They showed that selenium atoms were lost from the flame mainly by reactions on the quartz surface of the tube. On the basis of both theoretical and experimental data they also studied the relation between the amount of hydride present and the absorption signal. Chu et a1.12’ were the first to describe the use of an electrically heated silica tube for the thermal decomposition of arsine in an argon atmosphere. This method is twice as sensitive as decomposition in an argon-hydrogen-(entrained air) flame because of the longer residence time of the atom cloud in the optical path and the much reduced noise levels. Schmidt and Royer72 noted that use of a quartz furnace tube heated by an argon-hydrogen-(entrained air) flame from a single-slot burner significantly eliminated noise. Thompson and Thomerson”’ used a stoichiometric air-acetylene flame to heat a 17 cm long open-ended silica tube. The hydrides were introduced through a side-arm in the centre of the tube. The excess of hydrogen was prevented from igniting at the ends of the tube by a transverse stream of nitrogen. This type of cooling system has also been used by others. 12’ In other cases, as in some commercially available instruments, quartz windows have been fitted to the quartz tube, so that a closed circuit is obtained. Some authors have remarked, however, that as they are part of the optical system, these windows need to be extremely clean in order to allow maximal transmission of light in the far ultraviolet region of the spectrum.1099129 A slight loss in sensitivity when quartz windows are used has been reported.74*91*109Nakashima7’ used nitrogen-hydrogen-(entrained air) flame to heat his quartz tube. Vijan and Wood” tried to improve the sensitivity by increasing the length of an electrically heated silica tube in which an (entrained air)-hydrogen flame was burning. Contrary to expectation at lo-cm quartz tube generated signals that were 25% higher than those from a 16- or 2%cm tube. Goulden and Brooksbank74 also used an electrically heated quartz furnace, in which an argon-hydrogen-(entrained air) flame was burning, but Siemer et a1.90s110 did not use electrical heating. Electrically heated silica tubes without flames have been increasingly adopted. 76,95.112-114.119.122-124.129.130 Tempera-

The determination

of selenium by atomic-absorption

spectrometry

643

between 850 and 950” have proved suitable . mination of the hydride with an argon-hydrogen-(enfor the thermal decomposition of hydrogen sele- trained air) flame is only a third as sensitive. 1’ 9 Owing nide 76.117,125,130 to the large sample volumes that can be used in the Several authors have observed deterioration of the hydride methods, compared to a graphite-furnace inner surface of the quartz tube,“2~117~‘23~‘25~131 solution technique, the relative detection limits for the which impoverishes the sensitivity and precision. former can be reduced to the pg/ml range. As the Some workers have overcome this by inserting small maximum sample volume is mainly determined by the silica tubes within the original tube so that a new design of the generator, absolute detection limits, may surface is exposed to the hydrides and exchange or reflect the analytical capabilities of a given design “renewal” of the surface is made feasible.’ 12,1z3 All better than do the relative detection limits and usually range from 0.5 to 60 ng. The detection limit for real these hydride-generation procedures used hydrochloric acid to provide the acidic medium for the evo- samples instead of standard Solutions is higher and generally ranges from 25 to 80 ng.62+*6 Siemer”’ lution of hydrogen. Verlinden13’ noticed that using showed that cold-traps do not necessarily improve hydrochloric acid could shorten the life of a quartz absolute detection limits, because poor efficiency of tube to as little as 24 hr of continuous use. She prothe hydride transfer to the atomizer is a factor often posed the use of sulphuric acid instead, thus prolongoverlooked. However, if efficient hydride transfer is ing the life of the tube by at least a factor of 20 and guaranteed, a twofold improvement in sensitivity is considerably decreasing the day-to-day variability. obtained, mainly owing to the separation of the huge Knudson and Christian”’ used a graphite (carbon) volume of hydrogen generated during the reduction furnace for combustion of the hydrides, and atomizreaction, thus minimizing dilution of the hydride.“’ ation of HzSe in a heated carbon furnace has been Preconcentration methods for water samples have used by others. 53**1Fricke et al.“6 have used hydride extended the method to the pg/ml range.79s80 generation in conjunction with detection by plasma The precision of the determination step depends on emission spectrometry. Thompson113 passed the hydride into an argon-hydrogen-(entrained air) flame the concentration level of the analyte and on the generation and atomization systems used. Reported and used atomic-fluorescence for the detection. values usually range between 1 and So/, relative stanSensirivity, detection limit, precision dard deviation. The overall relative standard deviation can be considerably greater however.86 Because of the wide range of hydride generators and atomizers and the resulting differences in optiInterjerences mized parameters, it is difficult to compare published data with regard to analytical performance of the Almost all papers on hydride-generation AAS deal hydride-generation technique. Moreover, there is to some extent with possible interferences. Smith”’ made the first systematic study on the influence of 1 much confusion about definition of the term detection mg of each of 48 cations on the determination of 2 pg limit, so users of the data should always check the definition applied. of selenium. Sodium borohydride was used as the Ihnat and Millers6 have reported an extensive colreductant, with an argon-hydrogendentrained air) laborative study undertaken to evaluate the overall flame. Yamamoto and Kumamaru”” compared the performance of the method, regardless of the type of effect of diverse ions with both the zinc and the sodium borohydride reduction systems and found the generator that was used. They investigated the precision of the various stages, the overall reproducibility degree of interference of given ions to depend on the and the interlaboratory precision of the method. They nature of the reductant used, some ions giving more interference in the borohydride system than in the concluded that the method showed little bias but was unacceptable with regard to precision. Their results Zn/HCl system and vice versa. Smith”’ found that Fe3+, Co’+, Cd2+, and Pb2+ suppress the selenium showed the existence of laboratory-dependent systematic errors. Raptis er al. 124 have also demonstrated signal moderately whereas Ag+, Cu2+, Ni’+, Hg’+, that the hydride-evolution method may be afflicted Sn2+, Cr20:-, I-, MnO; and NO; can interfere strongly. Pierce and Brown compared the interference with systematic errors, which do not depend on the method of sample preparation but are assumed to be of several anions and cations in the method using caused by interferences from concomitant ions in the borohydride reduction and combustion in an electridigest. They advise the use of the method of standard cally heated quartz furnace114 and in the argon-hydadditions and the development of separation techrogen flame. I’9 The quartz tube system was less subniques to further isolate selenium from the matrix. ject to interferences. The sensitivity and the detection limit seem to be In almost every paper that has included obserless dependent on the nature of the reduction system vations on the influence of acids the strongly supusedlo than the kind of atomization system. The senpressing effect of nitric acid and sulphuric acid has sitivity obtained by hydride evolution and combusbeen recognized. These effects are more pronounced tion of the hydride in an electrically heated quartz in closed-furnace systems than in systems that use an tube is similar to that obtained by the graphite-furopen-ended tube.“’ Meyer et a1.‘23 investigated a nace solution technique ( - 1 pg/l.) whereas the deternumber of cross-interferences, using a commercially tures

M.

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VERLINDEN, H. DEELSTRA and

available hydride-generatio,n system coupled with a closed electrically heated quartz cell. Suppression of the selenium signal generally seemed to depend on the absolute concentration of interferent rather than on the concentration ratio between selenium and the interfering ion. The influence of the acidity and the volume of the sample was fully investigated. The condition of the inner surface of the quartz tube proved to be important.lz3 Verlinden and L3eelstra’29 studied the effects of other hydride-forming elements on the determination of selenium. Tin and bismuth interfered severely, mercury, antimony, and germanium were considered to be moderately interfering, whereas arsenic and tellurium interfered only at high concentration levels. Most of the interferences in the hydride AAS methods occur during the reduction step and can be explained by the formation of very insoluble selenides or by preferential reduction of metals present in the matrix. If these metals are reduced to their elemental state they can cause depression of the selenium signal by co-precipitating selenite or decomits evoluposing the hydride or impairing tion 107.122.123~129 Several authors have tried to overcome interferences by applying masking agents or by separating selenium from the matrix. Meyer et ~1.‘~~noticed that increasing the acidity of the hydride-generation solution reduced interferences from copper and silver ions. Increasing the volume of the evolution medium led to reduction of the suppressive effect of nickel and cobalt but not that of silver or arsenic(II1). Kirkbright has described the use of thiosemicarbazide and l,lO-phenanthroline to minimize the suppressive effects of copper on arsine generation and to obviate those of nickel. palladium and platinum, but in the case of selenium these masking agents were found to be ineffective.‘20.121 However. he was able to restrict these interferences by forming compounds more stable than the selenides. Thus addition of controlled amounts of tellurium(IV) led to the formation of stable copper, palladium or platinum telluride rather than the corresponding selenide. The addition of eihylenediaminetetra-acetic acid counteracts the effects of nickel and cobalt, but is only partially effective for copper, cadmium or 1ead.‘22 Rob&t has demonstrated that precipitation of selenium with lanthanum is incomplete when metals sulphide concentrates are analysed. He therefore recommended fusing the sample with sodium peroxide, leaching the melt with water and filtering off copper and nickel.‘25 Use of the method of standard additions is essential in ca~s.123~124~130~132 many

CONCLUSION

Comparative studies53.62.1lo.‘19 as well as this survey of the literature have shown that in the past decade AAS has been established as an adequate method for the determination of selenium in the sub-ng/ml range. The best absolute sensitivity and de-

E.

ADRIAENSSENS

tection limit are obtained with the graphite-furnace technique, the median values being 50 and 100 pg respectively. The hydride-generation technique allotis the use of large sample volumes and therefore offers the most favourable relative sensitivity and detection limit: 0.8 and 0.25 ng/ml respectively.h3 Interferences during the atomization step are severe in the graphite-furnace technique and make the use of background-correction mandatory, and may even then still be severe. Interference during atomization rarely occurs in the hydride-evolution method, but interferences during the reduction step, especially by copper. nickel and the noble metals, are common. Interferences are the principal limitation of both methods and for some samples the sensitivity and accuracy can be considerably impaired. With both methods it is possible to determine nanogram quantities of selenium. The hydrogen selenide AAS methods offer the convenience and the rapidity required for routine analysis. Acknowledgement-Marleen Verlinden wishes to express her thanks to the National Fund for Scientific Research (Belgium), whose grant supported this work. REFERENCES

1. 2. 3. 4. 5.

A. Walsh, Spectrochim. Acta, 1955, 7, 108. J. H. Watkinson. Anal. Chem.. 1966, 38. 92 C. A. Parker and L. G. Harvey, Analyst, 1962,87, 558. J. E. Allan, Spectrochim. Acta, 1962, 18, 259. S. Snranue. D. C. Manning and W. Slavin, At. Abs. New’s1.,~9&, 20, 27. 6. C. S. Rann and A. N. Hambly, Anal. Chim. Acta, 1965, 32, 346. 7. J. E. Allan, 4th Australian Spectroscopy Conference, August 1964. 8. R. M. Dagnall, K. C. Thompson and T. S. West, At. Abs. New& 1967, 6, 117. 9. ldem, Talanta, 1967, 14, 551. 10. R. M. Dagnall and M. D. Silvester, Anal. Chem.. 1971, 44.204. 11. D. 0. Cook, R. M. Dagnall and T. S. West, Anal. Chim. Acta, 1971, 56, 17. 12. W. B. Barnett, At. Abs. News/., 1973, 12, 142. 13. W. B. Barnett, J. W. Vollmer and S. M. Denuzzo. ibid., 1976, 15, 33. 14. G. E. Bentley and M. L. Parsons, Anal. Chem., 1977, 49, 551. 15. H. L. Kahn and J. E. Schallis, At. Abs. News/., 1968, 7, 5. 16. C. L. Chakrabarti, Anal. Chim. Acta, 1968, 42, 379. 17. G. F. Kirkbright, M. Sargent and T. S. West, At. Abs. Newsl., 1969, 8, 34. 18. G. F. Kirkbrieht and L. Ranson. Anal. Chem.. 1971. 43, 1238. 19. T. Nakahara, M. Munemori and S. Musha, Anal. Chim. Acta, 1970, SO, 51. 20. T. Nakahara, H. Nishino, M. Munemori and S. Musha. Bull. Chem. Sot. Jaoan. 1973. 46. 1706. 21. S. Ng, M. Munroe and W: M&harry, j. Assoc. 08 Anal. Chemists. 1974, 57, 1260. 22. B. C. Severne and R. R. Brooks, Anal. Chim. Acta, 1972, 58, 216. 23. Idem, Tajanta, 1972, 19, 1467. 24. W. Reichel and B. G. Bleakley, Anal. Chem., 1974,46, 59. 25. C. E. Mulford, At. Abs. News/., 1966, 5, 88.

The determination of selenium by atomic-absorption 26. J. C. Chambers and B. E. McClellan, Anal. Chem., 1976,48, 2061. 27. R. J. Watling, Anal. Chim. Acta, 1977, 94, 181. 28. J. Khalighie, A. M. Ure and T. S. West, ibid., 1979, 107, 191. 29. Idem, ibid., 1980, 117, 257. 30. H. K. Y. Lau and P. F. Lott, Talanta, 1971, 18, 303. 31. E. A. Peterson, At. Abs. Newsl., 1970, 9, 129. 32. G. R. Supp and I. Gibbs, ibid., 1974, 13, 71. 33. H. L. K&I, G. E. Peterson and J. E. Schallis, ibid., 1968, 7, 35. 34. H. L. Kahn, ibid., 1968, 7, 40. 35. T. Golembeski, Talanta, 1975, 22, 547. 36. H. T. Delves, Analyst, 1970, 95, 431. 37. J. D. Kerber and F. J. Fernandez, At. Abs. News/., 1971, 10, 78. 38. D. Clark, R. M. Dagnall and T. S. West, Anal. Chim. Acta, 1972, 60, 219. 39. W. Lund, Y. Thomassen and P. Dovle, ibid., 1977, 93, 53. 40. B. V. L’vov, Inzhener.-Fiz. Zh., Akad. Nauk Belorus. SSR, 1959, 2, 44. 41. Idem,xSpecrrochim. Acta, 1961, 17, 761. 42. H. Massmann, Z. Anal. Chem., 1967, 225, 203. 43. Idem, Spectrochim. Acta, 1968,23B, 215. 44. J. P. MatouSek, Am. Lab., 1971, 3, June, 45. 45. R. D. Ediger, At. Abs. News/., 1975, 14, 127. 46. E. L. Henn, Anal. Chem., 1975,47, 428. 47. Idem, A.S.T.M. STP 618, 1977, 54. 48. F. J. Szydlowski, Ar. Abs. News/., 1977, 16, 60. 49. K. Saeed, Y. Thomassen and F. J. Langmyhr, Anal. Chim. Acta, 1979, 110, 285. 50. R. B. Baird, S. Pourian and S. M. Gabrielian, Anal. Chem., 1972,44, 1887. 51. R. B. Baird and S. M. Gabrielian, Appl. Spectrosc., 1974, 28, 273. 52. J. C. Guillaumin, At. Abs. News/., 1974, 13, 135. 53. B. B. Mesman and T. C. Thomas, Anal. Lett., 1975, 8, 449. 54. T. D. Martin, J. F. Kopp and R. D. Ediger, At. Abs. News/., 1975, 14, 109. 55. T. Kamada, T. Shiraishi and Y. Yamamoto, Taianta, 1978, 25, 15. 56. G. G. Welcher, 0. H. Kriege and J. Y. Marks, Anal. Chem., 1974,#, 1227. 57. K. Ohta and M. Suzuki, Anal. Chim. Acta, 1975, 77, 288. 58. J. D. Mullen, Talanta, 1976, 23, 846. 59. R. M. Hamner, D. L. Lechak and P. Greenberg, At. Abs. New& 1976, 15, 122. 60. J. Y. Marks, G. G. Welcher and R. J. Spellman, Appl. . Spectrosc., i977, 31, 9. 61. A. Meyer, Ch. Hofer and G. T6lg, Z. Anal. Chem., 1978, 290, 292. 62. M. lhnat, J. Assoc. 08 Anal. Chemists, 1976, 59, 911. 63. M. lhnat and R. J. Westerby, Anal. Left., 1974,7,257. 64 M. Ihnat. Anal. Chim. Acta, 1976, 82, 293. 65. G. T. Shum, H. P. Freeman and J. F. Uthe, J. Assoc. Off: Anal. Chemists. 1977. 60. 1010. 66. G Ishizaki, Talan&, 19;s. 25, 167. 67. J. N&e and M. Hanocq, Anal. Chim. Acra, 1977, 93, 85. 68. W. R. Hatch and W. L. Ott, Anal. Chem., 1968, 40, 2085. 69. W. Holak, Anal. Chem., 1969, 41, 1712. 70. Oficial Methods of Analysis, 11th Ed., Association of Official Analytical Chemists, Washington D.C., 1970. 71. F. J. Fernandez and D. C. Manning, At. Abs. Newsl., 1971, 10, 86. 72. F. J. Schmidt and J. L. Royer, Anal. Lett., 1973,6, 17. 73. J. S. Caldwell, R. J. Lishka and E. F. McFarren, J. Am. Water Works Assoc., 1973, 5, 731.

spectrometry

645

74. P. D. Goulden and P. Brooksbank, Anal. Chem., 1974, 46, 1431. 75. M. Lansford, E. M. McPherson and M. J. Fishman, At Abs. Newsl., 1974, 13, 103. 76. F. D. Pierce, T. C. Lamoreaux, H. R. Brown and R. S. Fraser, Appl. Spectrosc., 1976, 30, 38. 77. D. R. Co&in and W. M. Barnard, At. Abs. Newsl., 1976, 15, 116. 78. G. A. Cutter, Anal. Chim. Acta, 1978, 98, 59. 79. S. Nakashika, Anal. Chem., 1979, 51, 654. 80. R. J. Watling, Spectrochim. Acta, 1980, 35B, 451. 81. M. McDaniel, A. D. Shendrikar, K. D. Reiszner and P. W. West, Anal. Chem., 1976, 48, 2240. 82. T. J. Knein. Health Lab. Sci.. 1977. 14. 53. 83. 0. E. Cl&&, Anal] st, 1977,102, 187. 84. J. A. Fiorino, J. W. Jones and S. G. Capar, Anal. Chem., 1976,48, 120. 85. M. Ihnat and H. J. Miller, J. Assoc. Ofl Anal. Chemists, 1977, 60, 813. 86. Idem, ibid., 1977, 60, 1414. 87. K. Kolar and A. Widell, Mitt. Gebiete Lebensm. Hyg., 1977,68, 259. 88. J. Flanjak, J. Assoc. Ofl Anal. Chemists, 1978, 61, 1299. 89. S. Ng and W. McSharry, ibid., 1975, 58, 987. 90. D. D. Siemer and L. Hagemann, Anal. Lett., 1975, 8, 323. 91. P. N. Vijan and G. R. Wood, Talanta, 1976, 23, 89. 92. H. D. Fleming and R. G. Ide, Anal. Chim. Acta, 1976, 83, 67. 93. R..V. D. Roh&t, Natl. Inst. Metall. Republ. S. A@., Rept. 1838, 1976. 94. M. Bedard and J. D. Kerbyson, Can. J. Spectrosc., 1976, 21, 64. 95. L. P. Greenland and E. Y. Campbell, J. Res. U.S. Geol. Survey, 1977, 5, 403. 96. R. Hermann, At. Abs. News/., 1977, 16, 44. 97. H. H. Walker, J. H. Runnels and R. Merryfield, Anal. Chem., 1976,48, 2056. 98. D. C. Manning, At. Abs. Newsl., 1971, 10, 123. 99. Y. Yamamoto, T. Kumamaru, Y. Hayashi and M. Kanke, Anal. Lett., 1972, 5, 717. 100. J. Y. Hwang, P. A. Ullucci, C. J. Mokeler and S. B. Smith, Am. Lab., 1973, 5, March, 43. 101. M. R. Church and W. H. Robison, Intern. J. Environ. Anal. Chem., 1974,3, 323. 102. H. C. Freeman and J. F. Uthe, At. Abs. Newsl., 1974, 13, 75. 103. A. M. de Kersabiec, B. Giraud and J. Nicolas, Compt. Rend., l976,282D, 2025. 104. Y. Yamamoto and T. Kumamaru, Z. Anal. Chem., 1976, 281, 353. 105. E. N. Pollock and S. J. West, At. Abs. Newsl., 1973, 12, 6. 106. F. J. Fernandez, ibid., 1973, 12, 93. 107. A. E. Smith, Analyst, 1975, 100, 300. 108. E. J. Knudson and G. D. Christian, Anal. Left., 1973, 6, 1039. 109. K. C. Thompson and D. R. Thomerson, Analyst, 1974,99, 595. 110. D. D. Siemer and P. Koteel, Anal. Chem., 1977,49,1096. 111. F. J. Schmidt, J. L. Royer and S. M. Muir, Anal. Lett., 1975, 8, 123. 112. J. F. Chapman and L. S. Dale, Anal. Chim. Acta, 1979, 111, 137. 113. K. C. Thompson, Analyst, 1975, 100, 307. 114. F. D. Pierce and H. R. Brown, Anal. Chem., 1976, 48, 693. 115. R. D. Wauchope, At. Abs. Newsl., 1976, 15,64. 116. W. Lautenschlager, J. M+assen and W. Oelschlager, Landwirtsch. Forsch., 1976, 29, 59. 117. M. Verlinden, J. Baart and H. Deelstra, Talanta, 1980, 27, 633.

646

M. VERLINDEN. H. DEELS~RAand E. ADRIAENSSENS

118. K. G. Brodie, Intern. Lab., 1977, March, 65. 119. F. D. Pierce and H. R. Brown, Anal. Chem., 1977, 49, 1417. 120. G. F. Kirkbright and M. Taddia, Anal. Chim. Acta, 1978, loo, 145. 121. Idem, At. Abs. Newsl., 1979, 18, 68. 122. G. Mausbach, GIT Fachz. Lab., 1979,23, 898. 123. A. Mever. Ch. Hofer. G. Ttila. S. Rantis and G. Knapp: 2: Anal. Chem:, 1979, 2&j, 337. L 124. S. Raptis, G. Knapp, A. Meyer and G. T61g, ibid., 1980,300, 18. 125. R. RobCrt and G. Balaes, Nat/. Insf. Metall. Republ. S. Afr., Rept. 2023, 1979.

126. F. N. Fricke, W. D. Robbins and J. A. Caruso, J. Assoc. Oft: Anal. Chemists, 1978, 61, 1118. 127. J. Dtdina and I. RubeSka, Spectrochim. Acta, 1980, 35B, 119. 128. R. C. Chu, G. P. Barron and P. Baumgarner, Anal. Chem., 1978, 44, 1476. 129. M. Verlinden and H. Deelstra, 2. Anal. Chem., 1979, 2%, 253. 130. J. R. S. Broughton and C. W. Fuller, Proc. Anal. Div. Chem. Sot., 1977, 14, 112. 131. M. Verlinden, to be published. 132. M. Verlinden, Ph.D. Dissertation, University of Antwerpen, 1981, in preparation.