Analytico Chimica Acta, 111 (1979) 169-176 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

SEMI-AUTOMATED METHOD FOR THE DETERMINATION OF BISMUTH IN ROCKS

C_ Y. CHAN*, M. W. A. BAIG and A. E. PITTS

Geoscience Laboratories, Ontario Geological Suroey. Ministry of !\‘atural Resources, 77 Grenuilk Street. Toro,lto, Ontario (Canada)

(Received 20th June 1979)

A rapid and accurate method for the determination of bismuth in rock samples is described. Automated equipment is used to generate bismuth hydride from solutions of rock samples prepared by digestion with a mixture of hydroffuoric and perchloric acids. The evolved hydride is carried to a heated quartz tube by a stream of argon, and the atomic absorption of bismuth recorded. Thiosemicarbazide and l,lO-phenanthroline are used as masking agents to minimize interferences from copper and nickel. As little ZLS 20 ng Bi g-’ can be determined; the average r_s.d. is 5.4,0. Q Results obtained for sis USGS standard rocks are in close agreement with the recommended values obtained by an isotope dilution technique.

There has been growing demand in recent years for quantitative trace measurements of bismuth, a sulfide-associated element, in geological materials. The bismuth data are important in support of geochemical surveys, where bismuth plays a role as an indicator of various mineral deposits and in mineral deposit studies- In high-temperature hydrothermal deposits, bismuth is very often associated with gold and silver f l] _ The occurrence of bismuth in lead sulfide ores is unique. The amount of bismuth in galena is usually about lo-- 20 ppm, in exceptional cases even 1 OO- -1000 ppm ; it increases with increasing temperature of formation of the mineral deposits [l] _ In the upper crust of the earth, bismuth concentrations are estimated to lie in the range O.Ol- 0.10 ppm [ 11. In order to distinguish bismuth of high content from its background level in rocks, a sensitive analytical method is desirable_

Bismuth can be measured calorimetrically (21 based on its reaction with sodium diethyldithiocarbamate to form a yellow complex followed by solvent extraction; it can also be determined by conventional flame atomic absorption spectrometry. Neither procedure has enough sensitivity to measure the small enrichments of bismuth at the level near and slightly greater than the crustal abundance. The electrothermal atomization technique [3] is prone to matrix interferences although it provides better sensitivity. Ficklin and Ward [ 41 developed a flameless atomic absorption method which can determine as little as 50 ppb in 0.2g samples of rock or soil sample. In their procedure, the sample is fused with sodium hydrogensulfate and the

fusion product is leached with (1 + 4) HCl and treated with ammonium l- pyrrolidinedithiocarbamate to form a complex which is extracted into MIBK. Aliquots of the solution are analyzed for bismuth by the graphite-furnace atomization technique. A sin-&r approach was applied by Kane [5] who used aqueous rather than organic sample solutions for analysis with a graphite- furnace atomizer after the sample had been decomposed and bismuth extracted_ The method is tedious, time-consuming and unsuitable for routine analyses of large numbers of samples. Thermal neutron activation analysis and the substoichiometric isotope dilution technique have been adopted successfully for accurate measurements of trace bismuth in rock samples; the results on USGS standard rocks have been reported on several occasions

[61- The hydride generation approach for determining bismuth in aqueous

samples has been studied by several workers [ 7-9]_ Fernandez [ 71 collected the generated bismuth hydride in a balloon reservoir and subsequently determined bismuth by atomic absorption spectrometry (a.a.s.). Thompson [8] introduced the hydride directly into a silica tube mounted in an air- acetylene flame for a.a_s_ Both methods were sensitive, but few interferences were studied. Smith [9] investigated the chemical interferences inherent in the hydride generation method systematically; some elements, particularly copper and nickel, interfered strongly, but methods of avoidance were not elaborated. The masking effects of thiosemicarbazide on copper and of l,lO-phenanthroline on nickel are well known [lo] _ Kirkbright and Taddia [ 11 ] successfully used these compounds to minimize interferences from Cu, Ni, Pt, and Pd in the a-as. determination of arsenic after hydride generation_ The semi-automated hydride generation- atomic absorption method described in this paper for the determination of bismuth in rocks employs these masking agents and is based on previous esperience with a semi-automated method for antimony determinations [ 12] _

ESPERIhlENTAL

Reagents All reagents used were of analytical grade; water was glass-distilled_ The

acids used were hydrofluoric acid (Baker 49%), hydrochloric acid (l3aker 38W), perchloric acid (Baker 60%) and nitric acid (Baker 70%).

Digestion mistrtre_ Mis hydrofluoric acid, perchloric acid and water in the ratio of 2:2:1_ Store the mixture in a polyethylene bottle.

Tetraizydroborate solution_ Dissolve 3-O g of sodium tetrahydroborate in 300 ml of water, adding one pellet of sodium hydroxide for each 100 ml of the solution_ The solution is stable for at least a week in a refrigerator_

Masking reagent. Dissolve O-5 g each of thiosemicarbazide and l,lO- phenanthroline in 100 ml of 0.1 M HCl solution_

Standard solutions_ For the stock solution (1000 ppm Bi), dissolve 1.1148 g of Bi203 in (1 + 4) HCl, and dilute to 1 1 with the same acid. Prepare working

lil

;c.., r3:e

:rr; CYlrl -11

3.90

3.00

0.32

!.20

1.2C

2.5’)

Fig. l_ Flow diagram of Autoanalyzer--a.a.s. system for determination of bismuth.

standards containing 1.0, 2.0, 3.0, 5.0 and 10.0 ng Bi ml-’ by serial dilutions of the stock standard with a solution containing (1 + 4) HCl and (1-C 9) HClO,.

Apparatus The flow diagram of manifold used is shown in Fig. 1. The Varian Model AA6 atomic absorption spectrometer was equipped

with a bismuth hollow-cathode lamp and a Model A-25, l-10 mV variable- range, strip-chart recorder_ A Technicon Sampler II and a Proportioning Pump I were used for sampling and mixing of the reagents_ A gas--liquid separator separated the hydride and the waste solution_ An impinger half- filled with concentrated sulfuric acid served to remove moisture and to homogenize the hydride-argon mixture_ The quartz tube (10 cm long, 0.6 cm id. with an inlet tube fused into the centre), wound with a 22-gauge chrome1 A heating wire and insulated with a layer of wrapped asbestos string, was mounted on the burner of the spectrometer_ The temperature of the quartz tube atomizer was controlled at 850 + 20°C by a variable trans- former_

The optimum operating parameters for a-as. are as follows: wavelength, 223.2 nm; lamp current, 8 mA; slit width, 50 I_tm; instrument damping, C (maximum); expansion, 6.0; recorder span, 1 mV full-scale; chart speed, 50 cm h-' ; atomizer temperature, 850 +_ 2O”C, argon flow rate, 300 ml min-’ ; sampling time, 45 s; wash time, 45 s.

Procedures Decomposition of samples. Weigh out 0.200 g of rock sample and transfer

to a 30-ml Teflon beaker. Digest the sample with 5 ml of the acid digestion mixture on a hot plate at low heat for ca. 1 h until white fumes of perchloric

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acid appear and the volume is reduced to 1-Z ml. Avoid heating the contents to dryness. Coo! and dilute the contents with a little water; add 6 ml of (If 1) HCI. Transfer the contents with water, rinsing into a plastic test tube cali- brated at 15 ml. Make up to volume with water. Mix the solution thoroughly and allow the residue to settle. Prepare a reagent blank simultaneously. Transfer the solution to a sample cup for subsequent a_a.s. determination as described below.

Determination of bismztth. Select the a-as. instrumental parameters indicated above and set up the hydride generation equipment as in Fig. 1 using the appropriate tube manifold_ Mount the quartz tube on the ‘burner, with its side arm connected to a tygon tubing leading to the hydride generator_ After the bismuth lamp has warmed up, align the quartz tube with the light beam to allow masimum radiation to reach the detector. Obtain the required temperature of the quartz tube atomizer by switching on the pre-set variable transformer_ Turn on the proportioning pump with all the reagent tubes dipped in the water. Introduce the argon gas immediately with its flow rate regulated by a flowmeter. As soon as the system has stabilized, insert the reagent tubes into their corresponding solutions, and establish a base-line signal. The standards, samples and blank solutions which have been loaded and held in the sampler can then be run continuously. Record the absorption signals. Measure the peak heights of the standards, and draw a calibration graph. The calibration curve is linear at least up to 10 ng Bi ml-‘_ Typical recorder tracings on a series of standards are depicted in Fig. 2. Read the concentrations of bismuth in the samples from the graph after subtracting the peak height of the blank_ For a sample weight of 0.200 g and a volume of 15 ml, ng Bi g -’ in rock corresponds to ng Bi ml-’ X 75 ml g-l_

Fig. 2. Typical recorder tracings of bismuth. The numbers on the peak correspond to ng Bi ml-‘.

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RESULTS AND DISCUSSION

Interferences The influence of various ions was studied at concentrations equal to or

greater than those found in the samples to be analyzed. The conclusions were as follows: the method is free of interference from large concentrations of the major constituents of rocks, namely Al, Fe, Ca, Mg, Na, K and Ti; anions such as chloride, fluoride, nitrate, sulfate and phosphate do not interfere; some trace elements exhibit severe interferences if the analysis is done without additional reagents. Although the tolerance limits and the corresponding concentrations of Te, Pt, Au and Ag are relatively low, the occurrence of these elements at interfering levels in natural rocks is very uncommon_ Copper and nickel are potential interferences because they occasionally occur at high levels in natural rocks and minerals. Attempts were made to overcome these interferences by introducing chelating and complexing reagents_ Reagents such as EDTA, thiocyanate, citrate and oxalate were found to be ineffective. Potassium iodide was effective for copper, but not for nickel. The application of thiosemicarbazide and l,lO- phenanthroline for controlling the interferences proved successful as can be observed from Table 1. The tolerance limit was increased tenfold for nickel, and the interferences of copper and platinum were eliminated completely.

Precision, accuracy, sensitivity and detection limit Several geochemical standard reference samples were analyzed. The

results are shown in Table 2, together with the recommended values reported in the literature [ 61. The precision is satisfactory and the present results agree

TABLE 1

Elemental concentrations having no interference effect on bismuth (5 ng ml-‘)

Element concentrationa Equivalgnt content Element Concentrationa 042 ml-‘) in rock 04-z ml-‘)

Equivagnt content in rock

CA 8000 Fe 8000 MtZ 4000 K 4000 Na 4000 Al 4000 Li 1000 Ba 1000 Zn 400 Pb 400 Ti 400 Cd 400 Cr 400

60% 60% 3tx-o 30% 309-o 30% 7.5% 7 -5%

3% 3% 3% 3% 3%

Mn

cu Pt Ni CO As Se Sb SII Te AU

AF.

400 400 (0.2) 400 (0.2)

6OC(6) 100 (20)

40 40 40 40 0.4C(O.l) 0.03=(0.01) lC(O.1)

35% 3% 3% 4500 PPrn

7500 ppm 3000 PPm 3000 ppm 3000 ppm 3000 ppm

30 PPm 2 rwm

75 ppm

a Values in parentheses are the tolerance limits in the absence of masking reagents.

bBased on 0.2 g of sample in 15 ml of solution CTolerance limit.

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TABLE 2

Results for bismuth (ppb) in standard reference rock samples

Sample This work -.

ppbb Rs.d. (%)

Recommended value”

(ppb)

USGS QLO-1 63 3.6 66.3 RGM-1 273 a.9 283 MAG-1 381 3.2 384 SIX-1 2'72 2.6 276 BHVO-1 22 12.1 18.S STM -1 172 13.1 250= sco-1 384 3.4 - SGR-1 s50 3.3 -

GSC SY-2 154 2. 5 -

SY-3 293 1.9 -

MRG-1 73 10.4 -

aValues quoted from ref. [6], which were obtained by isotope dilution. bMean of 5 determinations. CThe value is not recommended because of poor precision in results as described by Greenland et al. [ 61.

closely with the values recommended. The average relative standard deviation over the bismuth concentration range 20--900 ng g-’ is 5.4%_ Samples SY-2, SY-3 and MRG-1 are reference materials supplied by the Geological Survey of Canada (GSC) and no recommended values are available.

Two reference mineral samples, supplied by GSC, with high bismuth content were analyzed by the present method and by a flame method. The results obtained (Table 3) show good agreement. Despite the high dilution involved, the deviation of the results from possible mechanical errors is minimized in the proposed method. Sample PR-1 was also analyzed by a calorimetric method with diethyldithiocarbamate with similar results.

A recovery study was done by adding known amounts of bismuth (as Bi203) to the rock samples and applying the entire procedure. The added

TABLE 3

Comparison of results for bismuth in two GSC reference samples ~- Sample

Zn-Sn-Cu-Pb ore. NP-1

Molybdenum ore, PR-1

Bi (%) -- ___-

Air-acetylene Hydride Calorimetric Recommended flame generation value

0.024 0.024 - 0.02-I

0.113 0.108 0.106 0.111

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TABLE 4

Recovery of bismuth added as Bi20, to typical rock samples

Rock sample

(O-2 g) Bi (w)

Added Founda Recovery

Recovery (%)

Amphibolite 0 25 -

75 102 77 102.6

Syenite, SY-3 0 65 -

75 138 73 97.3

=Average of 3 results.

bismuth was recovered quantitatively as shown in Table 4, and the matrix effect was negligible.

The sensitivity, defined as the concentration producing 1% absorption, of this procedure is O-90 ng Bi ml-‘_ The practical detection limit, defined as the concentration giving a signal three times the standard deviation of the overall blank, is 0.3 ng Bi ml-’ equivalent to 20 ng g-’ of rock.

General remarks The absorbance does not vary appreciably with the following factors: (I)

concentration and type of the acid in the sample solution; (2) concentration and type of the acid in the background solution fed in line 5 in Fig. 1; (3) concentration of sodium tetrahydroborate; (4) temperature and length of the quartz tube. Control of these variables need not be critical. in contrast, the absorbance does change significantly with change of flow rate and type of the carrier gas. Strict regulation of the gas flow is thus required_ Argon as a carrier gas provided better sensitivity than nitrogen with a gain of about 2GcTo. The impinger filled with concentrated sulfuric acid acts as a gas mixer and moisture absorber_ It has the effect of homogenizing the gas misture and hence reducing the noise of the signal. Perchloric acid in the digestion mixture can be replaced by sulfuric acid which is just as effective_ To digest samples containing organic materials, a small amount of nitric acid should be added to the digestion misture. -4 batch of 40 samples can be digested simultaneously on a hot plate with ease; complete digestion is normally attainable within an hour. It was observed that bismuth contamination occurred frequently if the sample solutions were kept in glass test tubes. Bismuth is present in most types of glassware as an additive in glass manu- facturing, and if the digested samples are kept in contact with the glass tubes, the residual hydrofluoric acid that may occasionally exist in minute quanti- ties will etch the glass and leach out bismuth, thus causing serious contamin- ation. To prevent this from happening, plastic test tubes were used. When the system is ready to run, it is good practice to condition it first by repeat- edly analyzing a sample solution or a bismuth standard solution containing

176

3000 pg Fe ml-’ until a constant signal is attained prior to the actual analyses_ The analysis rate is 40 digested samples per hour.

This paper is published by permission of the Director, Ontario Geological

Survey.

REFERENCES

1 V. M. Goldschmidt, Geochemistry, Oxford University Press, London, 1958, p_ 480. 2 E. B. Sandell, Calorimetric Determination of Traces of Metals, Vol. 3, 3rd edn., Inter-

science, New York, 1959, p_ 338. 3 B. R. Culver, Analytical Methods for Carbon Rod Atomizers, Varian Techtron Pty.

Ltd., Springvale, Australia, 1975. 4 W. H. Ficklin and F. N. Ward, J. Res. U.S. Geol. Surv., 4 (1976) 21’7. 5 J. S. Kane, Anal. Chim_ Acta, 106 (1979) 325. 6 L. P. Greenland, E. Y. Campbell and F_ J. Flanagan, Descriptions and Analyses of

Eight New USGS Rock Standards, Geological Survey Professional paper 640, 1976. pp_ 45,59.

7 F. J. Fernandez, At. Absorpt. Newsl., 12 (1973) 93. 8 K. C. Thompson, Analyst, 99 (1974) 595. 9 A_ E. Smith, Analyst, 100 (1975) 300.

10 D. D. Perrin, Masking and Demasking of Chemical Reactions, Wiley-Interscience, New York, 1970, pp_ 35,37.

11 G. F. Kirkbright and M. Taddia, Anal. Chim. Acta, 100 (1978) 145. 12 C. Y. Chan and P. N. Vijan, Anal. Chim. Acta, 101 (1978) 33.