JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, DECEMBER 1986, VOL. 1 429

Determination of Trace Amounts of Nickel and Cobalt in Silicate Rocks by Graphite Furnace Atomic Absorption Spectrometry: Elimination of Matrix Effects with an Ammonium Fluoride Modifier

Rokuro Kuroda, Toshihiko Nakano, Yasuharu Miura and Koichi Oguma Laboratory for Analytical Chemistry, Faculty of Engineering, University of Chiba, Chiba, Japan

Trace amounts of nickel and cobalt have been determined in a variety of standard rocks by graphite furnace atomic absorption spectrometry after fusion with a mixture of lithium carbonate and boric acid. The presence of ammonium fluoride in the reaction medium served to remove severe matrix effects, allowing aqueous hydrochloric acid solutions to be used as calibration standards. Results are given for a variety of standard rock samples. The sensitivities are 14 and 25 pg for nickel and cobalt, respectively, with respect to 1% atomic absorption. Keywords: Atomic absorption spectrometry; cobalt and nickel determination; rock analysis; graphite furnace; ammonium fluoride modifier

The decomposition of silicates by fusion with lithium fluxes has found increasing use during the past decade, particularly in atomic absorption and X-ray fluorescence spectroscopy.192 Lithium metaborate is generally used when dissolution of the sample is necessary after decomposition. This solution tech- nique has been widely advocated by Ingamells,3 who used it for the rapid photometric analysis of silicate rocks for major and minor elements, and has been adapted to flame atomic absorption ~pectrometry.~7 Omangs proposed a method for the determination of Si, Al, Ti, Fe, Ca, Mg, Na and K in various silicates by atomic absorption spectrometry, in which a mixture of lithium carbonate and boric acid was used for decomposition, allowing rapid dissolution of the fusion cake with hydrochloric acid to be achieved. Barredo and Diez9 used the same flux and added EDTA to improve the stability of the rock solution for measurements by atomic absorption spec- trometry. The same decomposition method has been applied to silicate rock analysis for major and minor elements by spectrophotometric and atomic absorption spectrometric flow injection analysis. 1 ~ 3

With the advent of modern electrothermal atomic absorp- tion spectrometry, the use of the lithium fluxes has been extended to the trace analysis of rocks and related materials

In this work we attempted to determine trace amounts of nickel and cobalt in silicate rocks by graphite furnace atomic absorption spectrometry coupled with the fusion technique. Ammonium fluoride was used as a matrix modifier to remove the matrix effects that seriously impaired this determination. Results are given for the determination of both metals in a variety of standard rocks and these results are compared with the recommended values.

Experimental Reagents

A standard solution of nickel (1.002 mg ml-1 of Ni in 1 M hydrochloric acid) for atomic absorption spectrometry was obtained from Kanto Chemical Co. (Tokyo, Japan). The standard solution of cobalt (1.004 mg ml-1 of Co in 1 M hydrochloric acid) was prepared by dissolving 0.404 g of cobalt chloride hexahydrate in 1 M hydrochloric acid and diluting to 100 ml with the same acid. This solution was standardised by titration with EDTA disodium salt using xylenol orange as the metal indicator. Ammonium fluoride solution was prepared by dissolving 3.6 g of ammonium fluoride in 1 M hydrochloric acid and diluting to 100 ml with the same acid. This was prepared just before use.

Apparatus

A Shimadzu Model AA-646 atomic absorption spectrometer was fitted with a deuterium background corrector, a GFA-4 furnace atomiser and a Model U-135 chart recorder. Back- ground correction was carried out for all measurements. The radiation sources used were a Hitachi HLA-4S 2083004 multi-element hollow-cathode lamp (Cr, Cu, Fe, Mn, Ni) for nickel and a Hamamatsu TV L-233-27NU single-element hollow-cathode lamp for cobalt,

The settings for the spectrometer were as follows: lamp current, 9 mA for cobalt and nickel; wavelength, 240.7 nm for cobalt and 232.0 nm for nickel; band width 0.19 nm; and readout as peak height. The atomisation programmes are summarised in Table 1. The protection gas used was argon. A regular high-density graphite tube (200-54520) and a pyrolytic

Table 1. Analytical conditions

Drying Ashing Atomisation Cleaning

Ni c o Ni c o Ni c o Ni c o Temperature/"C . . . . . . 150 500 700 2400 2600 2800 2900 Tim& 30 20 4 3 Mode . . . . . . . . Ramp Step Step Step gas flow-rate/lmin-1 . . . . 1.5 1.5 0 1.5

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430 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, DECEMBER 1986, VOL. 1

graphite tube (200-54525) (Shimadzu) were used for cobalt and nickel , respectively.

Procedure Place a 200-mg sample in a 30-ml capacity platinum crucible. Add 500 mg of 1 + 1 lithium carbonate - boric acid and mix. Dry the contents for 5 min with a gentle flame and then fuse the mixture for 15 min at 1000 "C. Dissolve the cooled melt in 1 M hydrochloric acid (with magnetic stirring) and dilute to exactly 50 ml with the same acid. Store in an air-tight polyethylene bottle.

Take a 5-ml aliquot of the rock solution and transfer it into a polystyrene vial of 13 ml capacity with a screw-cap. Add exactly 5 ml of the ammonium fluoride solution to the vial with an Eppendorf type of pipette and mix well. Inject 10 p1 of the mixture into the furnace and proceed according to the programme given in Table 1.

Results and Discussion It can be expected that the fusion method will not suffer any matrix effects, because a uniform content of lithium borate should act as a spectrochemical buffer to minimise interfer- ences due to differences in the matrices. With respect to nickel and cobalt, however, the calibration graphs obtained with the blank flux solution and 1 M hydrochloric acid solution differed considerably and the values obtained for several standard rocks with both calibration standards were biased significantly from the recommended values. Therefore, we decided to modify the matrices with ammonium fluoride, which reacts with both silicic acid and the borate, leading to their

-Ashing - -Atomisation -+ 0.2

B I

I I I I

1000 1500 2000 2500 Temperature/"C

Fig. 1. Effect of ashing temperature (2400 "C atomisation) and atomisation temperature (500 "C ashin ) on the signal from nickel. A( A ) In 1 M HC1- 0.5 M NH4F solution $0.30 ng Ni); B( A) in 1 M HC1 - 0.5 M NH4F solution containing lithium borate flux (0.30 ng Ni); C ( 0 ) in 1 M HCl - 0.5 M NH4F solution containing a silicate rock (AGV-1; andesite) and lithium borate flux. Volume of solution injected, 10 p1. Solutions prepared in accordance with the Procedure regarding the concentration of each constituent

0.2

0) u C rn + g 0.1 n a

0

- Ashing - -Atomisation -

I I I

500 1000 1500 2000 2500 Tem peratu re/"C

Fig. 2. Effect of ashing temperature (2600 "C atomisation) and atomisation temperature (700 "C ashin ) on the signal from cobalt. A( A) in 1 M HCI - 0.5 M NH4F solution 6.80 ng Co); B( A) in 1 M HCl - 0.5 M NH4F solution containing lithium borate flux (0.80 ng Co); C(0) in 1 M HCI - 0.5 M NH4F solution containing a silicate rock (JB-1; basalt) and lithium borate flux. Volume of solution injected, 10 PI. Solutions prepared in accordance with the Procedure regarding the concentration of each constituent

volatilisation during the ashing process. Fig. 1 shows the effect of ashing and atomisation temperatures on the response from nickel in (A) 1 M hydrochloric acid, (B) 1 M hydrochloric acid + flux and (C) a rock sample solution that is 1 M in hydrochloric acid and contains flux. In all instances ammo- nium fluoride was added prior to the atomic absorption measurements. From the ashing and atomisation curves it appears that the ashing and atomisation of nickel proceed similarly in the presence of fluoride for (A) nickel alone, (B) nickel and flux and (C) a rock sample solution (standard rock AGV-1; andesite). The peak height for the rock solution at 2400 "C (ashing at 500 "C) as calibrated with the standard solutions (A and B) reasonably accounts for its nickel content calculated from the recommended composition. Nickel can be determined accurately on the basis of a calibration graph constructed using a series of aqueous nickel standard solu- tions.

Fig. 2 shows similarly the ashing and atomisation curves for cobalt. In this instance the behaviour of cobalt is also the same for cobalt alone, cobalt with flux and a rock solution (JB-1; basalt) in the presence of ammonium fluoride. Cobalt can be determined accurately again on the basis of a calibration graph constructed from a series of aqueous cobalt standard solutions at an ashing temperature of 700 "C and an atomisation temperature of 2600 "C.

The addition of ammonium fluoride makes the excess of borate and silicic acid in the sample volatilise as boron trifluoride and silicon tetrafluoride , respectively. The matrices become simplified, being converted into a lithium chloride or fluoride. Similar decomposition and furnace atomic absorp- tion measurements without fluoride were conducted by Salles and Curtius.15 They found that the atomic absorption signal for lead and barium was higher in the blank than in aqueous media, and the opposite was found for cobalt, chromium and copper. The effect of silica was found to be significant particularly for barium, making it necessary to prepare barium calibration solutions containing silicic acid, and to match the concentrations of the samples. This is tedious and almost impracticable for the analysis of a variety of silicate rock samples.

The results obtained for many different types of standard silicate rocks are given in Tables 2 and 3 for triplicate independent determinations. The fused masses of these rocks are always dissolved completely in 1 M hydrochloric acid to give clear solutions and remain unchanged for a long period of time . lo This implies satisfactory destruction of carbonaceous

Table 2. Determination of Ni in standard rocks

Recommended$ or proposed value,

Rock* Average found, p.p.m.t p.p.m.17

JG-1 . . AGV-1 BCR-1 G-2 . . GSP-1 MAG-1 QLO-1 RGM-1 sco-1 SDC-1 SGR-1

. . 8.6, 7.1, 7.8( 7.8)

. . 15.9,18.1,13.2 (15.7)

. . 11.1,10.0,10.4(10.5)

. . 3.5, 4.1, 4.0( 3.9)

. . 9.4, 9.5, 8.6( 9.2)

. . 52.1,56.0,64.8(57.6)

. . 3.3, 3.3, 2.9( 3.2)

. . 3.6, 2.5, 3.2( 3.1)

. . 25.4,33.6,28.3(29.1)

. . 44.0,47.9,48.5 (46.8)

. . 24.6,23.2,27.7 (25.2)

6 15,17,18.5 10,13, 15.8 3.5,5, 5.1 9,10,12.5 54 5.5 6 30 36 34

* JG-1 = granodiorite; AGV-1 = andesite; BCR-1 = basalt; G-2 = granite; GSP-1 = granodiorite; MAG-1 = marine mud; QLO-1 = quartz latite; RGM-1 = rhyolite; SCo-1 = shale; SDC-1 = mica schist; SGR-1 = shale.

t Values of three independent determinations. $ Recommended values are given in italics.

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, DECEMBER 1986, VOL. 1 431

Table 3. Determination of Co in standard rocks

Rock* JB-1 . . JG-1 . . AGV-1 BCR-1 G-2 . . GSP-1 MAG-1 QLO-1 RGM-1 sco-1 SDC-1 SGR-1

Average found, p.p.m.t . . 37.3,38.0,41.0(38.8) . . 3.7, 4.2, 4.1 ( 4.0) . . 14.0,16.6,17.2(15.9) . . 29.4,33.8,38.2(33.8) . . 4.0, 5.1, 4.5( 4.5) . . 6.8, 8.1, 6.7( 7.2) . . 19.1,24.1,20.3(21.2) . . 7.3, 9.5, 6.7( 7.8) . . 2.7, 2.1, 2.3( 2.4) . . 10.4,11.3, 9.7(10.5) . . 16.8,20.0,20.6(19.1) . . 10.7,11.0, 9.1(10.3)

Recommended$ or proposed value,

p. p.m. 17

38.4 4 14.1, I S . 7 , I 6 36,36.3 4.6,5, 5.5 6.4,7, 7.8 20 7.4 2.3 I1 17 12.5

* JB-1 = basalt; JG-1 = granodiorite; AGV-1 = andesite; BCR-1 = basalt; G-2 = granite; GSP-1 = granodiorite; MAG-1 = marine mud; QLO-1 = quartz latite; RGM-1 = rhyolite; SCo-1 = shale; SDC-1 = mica schist; SGR-1 = shale.

7 Values of three independent determinations. $ Recommended values are given in italics.

materials found in mud and shales and of the chromite, ilmenite, tourmaline, chlorite minerals, etc., which are particularly resistant to acid dissolution. For comparison purposes, the working values17 for these standard rocks are also given in Tables 2 and 3. As several compilations have been published for presenting working values, two or three values are listed as recommended or proposed values. Because of difficulties in performing trace analyses, even the recommended or proposed values do not coincide with each other, although they are close, and it is not known which value is the most reliable. However, the present results are within the range of recommended and proposed values or are in reasonable agreement with the recommended or proposed values. The nickel content of QLO-1, RGM-1, SDC-1 and SGR-1 appears to differ from the proposed or recommended values. For the former two rocks, which are low in nickel, more data are required in order to establish recommended values. For SDC-1 and SGR-1 Abbey18 has given 47 p.p.m. for the former and 29 p.p.m. for the latter as usable values for

nickel; these values are close to those reported here. The relative standard deviation (n = 10) obtained by replicate atomic absorption measurements of a standard rock solution (SCo-1, Ni 30 p.p.m.) was 8.9%, and a value of 7.2% was obtained for cobalt (SDC-1, Co 17 p.p,m.).

The proposed method may have wide applicability to the determination of trace amounts of metals in silicate rocks, as can be seen from the variety of rocks listed in Tables 2 and 3. The sensitivities of the method are 14 pg of Ni and 25 pg of Co for 1% absorption. The method is rapid, taking only 20-30 min to complete the fusion, so that 1 h is sufficient for sample preparation prior to the atomic absorption measurements.

References 1. 2.

3. 4. 5 .

6. 7.

8. 9.

10. 11.

12. 13.

14. 15.

16.

17. 18.

Bennett, H., Analyst, 1977, 102, 153. Bock, R., “A Handbook of Decomposition Methods in Analytical Chemistry,” International Textbook Co., Glasgow , 1979. Ingamells, C. O., Anal. Chem., 1966, 38, 1228. Boar, P. L., and Ingram, L. K., Analyst, 1970,95, 124. Jeffery, P. G., and Hutchison, D., “Chemical Methods of Rock Analysis,” Third Edition, Pergamon Press, Oxford, New York, Toronto, Sydney, Paris and Frankfurt, 1981. Van Loon, J. C., and Parissis, C. M., Analyst, 1969, 94, 1057. Verbeek, A. A., Mitchell, M. C., and Ure, A. M.,Anal. Chim. Acta, 1982, 135, 215. Ornang, S. H . , Anal. Chim. Acta, 1969, 46, 225. Barredo, F. B., and Diez, L. P., Talanta, 1976, 23, 859. Kuroda, R., Ida, I . , and Kimura, H., Talanta, 1985, 32, 353. Kuroda, R., Ida, I . , and Ogurna, K. , Mikrochim. Acta, 1984, I , 377. Mochizuki, T., and Kuroda, R. , Analyst, 1982, 107, 1255. Mochizuki, T., Toda, Y., and Kuroda, R., Talanta, 1982, 29, 659. Bettinelli, M., Anal. Chim. Acta, 1983, 148, 193. Salles, L. C., and Curtius, A. J. , Mikrochim. Acta, 1983, 11, 125. Zhou, L., Chao, T. T., and Meier, A. L., Anal. Chim. Acta, 1984, 161, 369. Govindaraju, K., Geostand. Newsf., 1984, 8 , Special Issue. Abbey, S. , Geostand. Newsl., 1980, 4, 163.

Paper J6f40 Received May 27th, 1986 Accepted June 30th, 1986

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