Anulytica Cllinticu Actct. 68 ( 1974) 297-304 #‘J Elsevicr Scientific Publishing Company, Amsterdam - Printed in The Netherlands

297

MOLECULAR EMISSION CAVITY ANALYSIS-A NEW FLAME ANALYTICAL TECHNIQUE*

PART II. THE DETERMINATION OF SELENIUM AND TELLURIUM

R. BELCHER, T. KOUlMTZIS** and A. TOWNSHEND

Departnmrt of Chemistry, The Utricersity of Birnliughnnt, PO Box 363. Birtnir~glrcwn BIS 2TT (E~~yimd)

(Reccivcd 24th July. 1973)

In Part I’ of this series, a new flame device that enabled small samples to be analysed in cool flames was described. In particular, the determination of sulphur by measurement of S, emission was discussed. It was also indicated that it is possible to determine various other elements, many of which can be determined only with diffkulty, by means of conventional nebulization into a cool flame. Typical examples of such elements are selenium and tellurium: aspiration of an aqueous solution of a selenium compound into a hydrogen-nitrogen diffusion flame or a hydrogen-

I I I ’

300 40.3 500 600. 400 500 600

wavelength Mm) Wavelength (nm)

Fig. 1. (A), Spectrum obtained from SeO, by MECA with a hydrogen-nitrogen flame; (B) Flame background.

Fig. 2. Tellurium spectra obtuined from TcOl: (A) blue emission above cavity. (B) green emission inside cavity, for a tlamc composed of S I N2 rnin-‘. 5 I air min-* nir and 4 I H, min-‘.

* This paper is dcdicatcd to Professor D. Monnier on the occasion of his 70th birthday. l * Prcscnt address: Department of Chemistry. University or Thcssaloniki, Greece.

298 R. BELCHER, -I-. KOUIMTZIS. A. TOWNSHEND

nitrogen-air flame gives no emission attributable to a selenium species, and aspira- tion of an aqueous tellurium solution into a hydrogen-nitrogen flame gives only a faint blue emission.

When selenium powder was placed in the specially designed cavity of the MECA instrument’ and a hydrogen-nitrogen flame was used, a weak blue emission was observed. The intensity of the emission was greatly enhanced when air was also introduced into the flame. Selenium compounds such as selenium dioxide and sodium selenate gave the same emission spectrum (Fig. l), very similar to that obtained by previous workers - . 2 4 Telluric acid gave rise to a green emission from the cavity, with a faint blue emission in the, hydrogen-nitrogen-air flame above the cavity. The spectra of both emissions are shown in Fig. 2. The spectrum of the green emission resembled that obtained previously’, which was described as possibly due to the Te, and/or TeO species.

DETERMINATION OF SELENIUM

Optimizutiort oj’ Flame Conditions The addition of air to the hydrogen-nitrogen flame had a similar effect on

selenium emission (Fig. 3) as on sulphur ‘. The enhanced emission at higher air

Air flow rote (I min-‘1

Fig. 3. Elkct of air added to a llamc of hydrogen (4 1 min -I) diluted with nitrogen (5 I min-‘) on emission from sclcnous acid iit 41 1 nm.

content is not due solely to increased temperature, because maximal emission intensity occurred6 at a cavity temperature of about 3 15”, which was readily achieved without air in the flame gases. It should be noted that selenium dioxide sublimes at 315” whereas selenium volatilizes at 688”. Thus it would appear that the oxygen promotes the formation of (or at least resists the reduction of) readily volatile selenium dioxide, which is subsequently converted to the emitting species within the cavity.

The position of the cavity in the flame was fairly critical. Under the flame conditions used, the most intense emission was obtained with the orifice of the cavity 23 mm above the centre of the burner.

SELENIUM AND TELLURIUM BY MECA 299

2. 4

20

I

Y ‘2

1 I 0 10 20 30 40 50 0 20 20

6

Time after introduction of cavity to the flume (9 ’ Time after introduction (9

Fig. 4. Effect of time of exposure to optimul hydrogen-nitrogen-uir llamc on emission of: (A) I pg of sclcnium (as sclenous acid) at 411 nm: (B) 10 /cy of tellurium (as tclluric ncid) at 500 nm: (C) incandcsccnt background from cavity at 500 nm.

Fig. 5. Emission rcsponsc from various amounts of sclcnium (us sclcnous acid) in the optimal hydrogcn- nitrogen-air flame. Numbers indicate /cg of SC.

The change in emission intensity, measured at 411 nm, with time for a sample of selenium dioxide in the cavity is shown in Fig. 4. The response for various amounts of selenium dioxide is shown in Fig. 5. A plot of peak height uerslls amount of selenium added as selenium dioxide was linear for 0.4-3.0 clg of selenium. For larger amount’s of selenium, the calibration graph flattened off, possibly because of self- absorption. Less than 0.4 pg of selenium gave no response. The standard deviation for the determination of 2.0 ,ug of selenium (as SeO,) was 0.1 ,~g (7 results).

The emission-time response from various organic and inorganic selenium compounds varied with the constitution of the compound, in a similar way to the response from sulphur compounds I. Thus, separate calibration graphs are necessary for each selenium compound introduced into the cavity.

After four or five selenium determinations in the same cavity, the inner surface of the cavity becomes black and shiny. If the cavity is pretreated by running several samples of selenium so that this type of surface is achieved, the determination of selenium becomes reproducible. Acidic test solutions attack this pretreated surface and make the determinations less reproducible. The effect of acids may be alleviated by adjusting the pH of the test solution to above 8 with ammonia. An excess of ammonia does not affect the emission intensity of aqueous selenium dioxide solu- tions. Similarly ammonium fluoride, nitrate and chloride do not interfere in amounts lo-fold by weight compared to the weight of selenium. Tellurium, sulphur,

300 R. BELCHER, T. KOUIMTZIS. A. TOWNSHEND

arsenic and phosphorus can be tolerated when present in amounts up to 40 times the weight of selenium. Metal ions delay the appearance of the emission from selenite or selenate ions, in a similar way to their effect on sulphate ions’.

Eliminariott oJ’ irzterJererzces When this investigation was carried out, some of the devices reported

previously’ for removing interference effects, such-as selective volatilization, had not been investigated fully. Thus, in order to eliminate the interference effects arising from the varying volatilities of different selenium species, and from other elements that interfere spectrally, methods of separating and isolating selenium before the application of MECA were studied.

Where the effect is a general effect of an organic matrix, and is not due to specific interfering elements, the samples may be burned in an oxygen flask, and the resulting solution measured directly by MECA, aqueous selenium dioxide solutions being used as standards. This relatively rapid technique was used success- fully to determine very small quantities (0.02 and 0.04%) of selenium in shampoo formulations and also to determine percentage amounts of selenium in organo- selenium compounds (Table I); the carbon and hydrogen analyses for these com- pounds are also given together with selenium determinations carried out by atomic absorption spectrophotometry. Table I also shows that organo-selenium compounds containing arsenic or bismuth can be analysed by the MECA technique without separating selenium from those elements. When other metal ions are present in commensurate amounts, this technique may not be applicable.

If such interfering species are present, selenium must be selectively removed from the sample solution. The most effective way of separating selenium from inorganic interferences was found to be by reduction to elemental selenium and flltration through a very fine filter paper. By use of suitably small filtration equipment,

TABLE I

DETERMINATION OF SELENIUM IN ORGANOSELENIUM COMPOUNDS - Col,lpolold

L,, &I

Se (‘X,)”

A 3 --,

(W-U,SCC~, cnlc. 47.2 3.3 26 found 47.1 3.5 26 25

AstScC(=SefN(C,H,)& CalC. 22.5 3.8 59 found 21.9 3.9 57 57

As(ScC(=Sc)N(CH,C,H,),), talc. 46.1 3.6 40 found 46.4 3.9 39 38

As(ScCt-Se)NfCHIC(CHB)J)l)a talc. 33.5 5.6 49 found 33.2 5.8 48 48

Bi(ScC(=Se)N(C,HS),)a talc. 19.3 3.2 51 found 19.s 3.5 48 49

O,N-C,H,-ScCN GllC. 37.0 1.8 35 found 36.7 1.7 34 3s

c

27.5

57

41

49

50

33

e A: Or flask, direct injection. B: 0, flask, precipitation. C: atomic absorption.

SELENIUM AND TELLURIUM BY MECA 301

as little as 0.3 c(g of selenium can be quantitatively collected. If a cellulose-based filter (e.g. Millipore VMWP, 0.2~pm pore size) is used for filtration, it may be burned in an oxygen flask and a few 111 of the resulting solution injected directly into the cavity.

Filtration through a fine glass-Iibre filter, which quantitatively retains particles greater than 1 jtrn in diameter, gives an equally efficient collection of selenium, with the advantage that the non-combustible filter can be inserted into the cavity and the selenium emission measured directly. The use of normal paper in the cavity results in a green emission when the cavity is placed in the flame, whereas the glass libre gives no emission. Asbestos paper was also investigated, but in the cavity it gave appreciable emission from elements such as sodium.

The efficiency of the selenium precipitation-collection method was checked by analysing the organo-selenium compounds by this method after oxygen flask combustion. Table I shows that there is no significant difference between these results’and those obtained by direct injection of the solution from the oxygen flask combustion, which indicates that recovery is quantitative. Filtration takes about 30 min. The precipitation technique was also applied to the determination of selenium in the mixture of selenium and sulphur sometimes known as ‘selenium sulphide’. A sample reported as containing 41.0-42.5°/0 Se was found to contain 44.6’;d Se, Similarly, synthetic solutions of trace selenium in concentrated sulphuric acid (5 ml) were analysed; the results are given in Table II.

TABLE II

DETERMINATION OF SELENIUM IN CONCENTRATED SULPHURIC ACID

pg Sc ml-’ taken 0.40 0.60 0.80 found 0.36 0.6 1 0.74

0.42 0.66 0.8 I

The methods devised are readily applicable to other types of sample. It should be possible to determine selenium in sulphur after nitric acid dissolution, or in mineral samples after acid digestion and distillation from hydrobromic acid.

DETERMINATION OF TELLURIUM

Like selenium, tellurium emission is more intense when a reasonable amount of air is added to the flame. As the cavity heats up, two peaks occur when the emission from telluric acid is measured at 500 nm (Fig. 4)“. The first peak, which occurs at a cavity temperature of CCC. 500”, could arise from the volatilization of tellurium dioxide, which is reported to occur at 450”. This peak is much smaller than the second peak, and does not allow less than 10 ;cg of tellurium to be determined. The second peak occurs at a cavity temperature of CQ. 780*, and is superimposed on the incandescent emission of the cavity, which is increasing rapidly at this stage6. Measurement of this peak therefore requires that the background emission be taken into account. If this is done, as little as 1 pg of tellurium can be determined.

302 R. BELCHER. T. KOUIMTZIS. A. TOWNSHEND

DISCUSSION

The determination of sefenium by most flame techniques is relatively in- sensitive. Atomic absorption, with a conventional nebulization system, has a maximal sensitivity of only 0.5 pg ml -* for 1% absorption of the 196.0-nm selenium fine. with triple-pass optics *‘. Moreover, the determination is subject to numerous interferences from other elements 7*8 The use of a nitrogen-separated nitrous oxide- I acetylene ffame doubles the sensitivity, and might eliminate many of the inter- ferences”. The carboh rod atomizer provides a sensitivity for selenium of 32 pg for l”/J absorption and is free of interference from many metals”.

An indirect method has been published in which selenium is converted to naphtho-(2-3-cl)-2sefeno-f,3-diazole, and extracted as its paIfadium(I1) complex into chloroform’ ’ ; the palladium is determined by atomic absorption spectrometry. The method is more than an order of magnitude more sensitive than direct atomic absorption; interfering metals are removed by ion exchange.

The equipment used for MECA measurements in this investigation was not designed to achieve high sensitivity. It is believed that a more suitable optical detection system will give greater sensitivies than those reported here. Under the present conditions, however, more than 0.4 pg of selenium or 1 llg of can be readily determined.

EXPERIMENTAL

The spectroscopic equipment and experimental tecfmique used same as described previously I. A stainless steel cavity with an aperture of 5 mm and a volume of 45 /il was used throughout.

tellurium

were the diameter

Standard selenium solution ( 1000 p.p.m.): Dissolve exactly 1 g of elemental selenium powder in 5 ml concentrated nitric acid, and dilute to 1 1 with water. Add ammonia solution to give pH 8 before dilution is completed.

All tellurium experiments were carried out with an aqueous ZOOO-p.p.m. solution of telfuric acid.

Burn the accurately weighed sample (2-5 mg) in a 250”ml oxygen flask containing 7 ml of water. After dissolving the combustion gases, make the solution ammoniacal with a few drops of concentrated ammonia liquor. Make up the volume to exactly 10 ml (or 2 ml’ for fess than 0.2 mg of selenium), evaporating the solution if necessary.

If interfering elements are absent, inject exactly 5 /tl of the solution from a syringe onto the interior surface of a warm cavity. After exactly 1 min, during which all the water evaporates, place the cavity in the flame, and measure the maximal emission intensity, as described previously’. Determine the amount of selenium present by reference to a calibration graph prepared by measuring the emission from exactly 1, 2,3, and 4 111 of stock selenium solution under the same conditions (Fig. 5).

SELENIUM AND TELLURIUM BY MECA 303

If interfering elements are present, take an aliquot of the lo-ml solution, containing 0.4-4 ,ug of selenium, dilute if necessary, and mix with concentrated hydrochloric acid so that the acidity exceeds 6 M. Add 2-3 drops of 10% hydro- xylammonium chloride solution, or bubble sulphur dioxide for 15 min. Heat at 70” for a few min. Filter the hot suspension through a glass-fibre filter disc (24 mm diam.; Whatman GF/C) supported on an asbestos sheet disc in a Millipore filtration apparatus. Wash the precipitate with a few ml of 9 M hydrochloric acid followed by hot water. Dry the filter paper in a desiccator over silica gel and transfer to the cavity so that it fits the contour of the cavity, with the selenium deposit towards theaperture. Measure the selenium emission as above, and determine the amount of selenium present by reference to a calibration graph prepared by measuring the emission of 1, 2, 3 and 4 1~6 of selenium taken through the precipita- tion procedure, and filtered onto the glass libre paper. The calibration must be done with selenium on the paper because of the slightly different peak intensities obtained with and without the paper. This could arise from the changed thermal contact between sample and cavity surface and. the light reflection from the white paper.

Determination of selenium in shampoo formulations or irl ‘selenium sulpltitle Carry out an oxygen flask combustion on an amount of sample containing

0.6-S mg of selenium. Dissolve the evolved gases in 5-7 ml of water, and make up to 10 ml with water. Inject exactly 5 ~41 of this solution into the cavity, and proceed as described above. for interfering elements absent.

Determination of seleru3m in sulphwic acid Dilute a volume of concentrated sulphuric acid containing less than 4 116

of selenium with twice the volume of water. Add to this solution one third of the volume of concentrated hydrochloric acid, and 0.5 ml of 10% tartaric acid solution to prevent precipitation of antimony. Pass sulphur dioxide for 15 min and continue as described above with the precipitation, filtration and determination of selenium.

The authors thank Mr. S. Bogdanski for recording the tellurium spectra. They also thank Fisons Pharmaceuticals Ltd. for the provision of synthetic shampoo samples, and the Inorganic Chemistry Department, University of Thessaloniki and Dr. E. R. Clark, University of Aston in Birmingham, for provision of the organo- selenium compounds. Th. Kouimtzis thanks the Greek Ministry of National Eco- nomy for the award of a research scholarship.

SUMMARY

The determination of 0.4-4 pg of selenium by molecular emission cavity analysis is described. Selenium in organic compounds is determined after oxygen flask combustion. Metal ion interferences are eliminated by reduction of selenium to the element, filtration onto a glass-libre paper, and direct incorporation of the filter into the cavity. Applications to the determination of selenium in inorganic and organic compounds are described. The determination of ,ug-amounts of tellurium is also outlined.

304 R. BELCHER. T. KOUIMTZIS. A. TOWNSHEND

RkSUMk

Une mCthode est d&rite pour le dosage du sClCnium (0.44 i(g) par analyse d’bmission molCculaire. Le s2lCnium dans des composCs organiques est do& apr6s combustion dans l’oxygene. Lcs interferences m&lliques sont CliminCes par rCduction du sClCnium h 1’Ctat &lCmentaire, tiltration sur libre de verre et incorporation directe du liltre dans la cavitC pour 1’Cmission molCculaire. On dCcrit des applica- tions de ce dosage de sClCnium dans des compos&s inorganiques et organiques. Le dosage de microquantit6s de tellurc est Cgalement mention&.

ZUSAMMENFASSUNG

Die Bestimmung von 0.44 jig Selen durch Molekiilemissionsanalyse unter Verwendung eines Hohlraums wird beschrieben. Selen in organ&hen Verbindungen wird nach Verbrennung in einem Sauerstoffkolben bestimmt. Sttirungen durch Metallionen werden vermieden, indem das Selen zum Element reduziert, auf’Glas- faserpapier abfiltriert und das Filter direkt in den Hohlraum gegeben wird. An- wendungen auf die Bestimmung von Selen in anorganischen und organischen Ver- bindungen werden beschrieben. Die Bestimmung von jcg-Mengen Tellur wird eben- falls dargelegt.

REFERENCES

1 R. Belchcr. S. Bogdanski and A. Townshcnd. Awl. Chirrr. Acta. 67 ( 1973) I. 2 Mitika Miyanisi. Sci. Pctp. Itzsr. P/IJX C/I~~~I. Res.. Tokyo. 37 ( 1940) 955. 3 G. Salct, nw. Clrirtl. Phys.. 28 ( 1873) 5. 4 I-I. J. Emclcus and H. L. Riley. Pwc’. Ro_w/. Sot.. Loruh. 140A (1933) 37X. 5 R. M. Dagnall. B. Fleet and T. H. Risby, Taltrr~rtr, IX (197 I) 155. 6 S. Bogdnnski. Ph.D. thesis. Birmingham University. 1973. 7 C. S. Rann and A. N. Hambly. nmr/. Chiw. Acftr, 32 ( 1965) 346. 8 C. L. Chakrabarti. .dwl. Chiru. Acra. 42 (1968) 379. 9 G. F. Kirkbright and L. Runson, ,4utr/. Chw.. 43 (1971) 1238.

IO R. B. Baird, S. Pourian and S. M. Gabriel. ,htr/. Chrw., 44 (1972) 1X87. I I l-1. K. Y. Lau and P. F. Lott. Tdar~rcc. 1X (1971) 303.