Analytica Chimica Acta, 188 (1986) 111-118 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

DETERMINATION OF TOTAL TIN IN GEOLOGICAL MATERIALS BY ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY

ERIK LUNDBERG* and BJdRN BERGMARK

Department of Analytical Chemistry, University of Ume&, S-901 87 UmeB (Sweden)

(Received 26th March 1986)

SUMMARY

A graphite-furnace atomic-absorption spectrometric method is described for the deter- mination of total tin in geological materials. Samples are decomposed by fusion with lithium metaborate and the melt is dissolved in diluted (1 + 9) nitric acid. Spectral and non-spectral interferences are minimized by a combination of platform volatilization, “normal” heating rate, addition of ammonia as chemical modifier, use of integrated absorbance values and Zeeman background correction. Results are reported for six reference materials showing good accuracy and a precision of 12% at the 3 fig g-r level. The detection limit for tin in the original materials is 0.7 rg g-r.

Traces of tin are commonly found in many silicate rocks, soils and sedi- ments, partly as a constituent of the silicate lattice [l-3], and partly in the form of cassiterite, tin(IV) oxide [ 1, 21. Knowledge of the relative distribu- tion of tin in these two forms is important in studies related to the geo- chemistry of tin, and in geochemical prospecting [ 1,3,4] .

Whereas cassiterite in geological samples can be converted into tin(IV) iodide and separated by volatilization from the bulk of interfering ele- ments by heating with ammonium iodide [2, 5-71, total tin can only be released by fusion with an alkaline flux such as lithium metaborate [8-lo]. Methods that have been used for determining tin in geological materials in- clude spectrophotometry [ 21, flame atomic absorption spectrometry (a.a.s.) [4] , polarography [ 111, X-ray fluorescence spectrometry [ 121 and neutron activation analysis [13]. Considering tin concentrations of l-10 pg g-l, all of these methods, with the possible exception of neutron activation, are sub- ject to considerable error because they are insufficiently sensitive or selec- tive. However, low levels of tin can be determined by a.a.s., as the hydride [7-lo], through aspiration of an organic tin extract into a flame [5] or by dispensing the extract into a graphite furnace [ 141, or by inductively-coupled plasma atomic emission spectrometry following hydride generation [6]. Most of these methods, including hydride-generation a.a.s. [8, lo], require a multistage process to complete the analysis with minimal interference effects. Hence, there is a need for a reasonably simple and rapid method for determining low levels of tin in geological samples. The difference between

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the total tin determined by fusion with lithium metaborate and the cassiterite tin would give an estimate of the tin held in the lattice of silicates.

This paper describes a method involving fusion of the sample with lithium metaborate to release the total tin, dissolution of the melt in nitric acid and subsequent, direct determination of tin by graphite-furnace a.a.s. Optimiza- tion of the analytical parameters to minimize interference effects are dis- cussed, and six geological reference materials are used to demonstrate the accuracy and precision attainable with the method.

EXPERIMENTAL

Instrumentation and reagents All experiments were done with a Perkin-Elmer Zeeman-3030 atomic ab-

sorption spectrometer, equipped with an HGA-600 graphite furnace, an AS-60 autosampler and a PR-100 printer. The instrumental parameters are summarized in Table 1.

A 1000 pg ml-’ stock solution of tin was prepared by dissolving 0.1000 g of tin (99.9%; AnalaR, BDH) in 10 ml of concentrated hydrochloric acid and 1 ml of 90% formic acid (gentle heating), followed by dilution to 100 ml with distilled water. Standard solutions were prepared by dilution with (1 + 9) nitric acid, and were stored in acid-washed polyethylene bottles. Lithium metaborate (purum; Merck) was purified before use as described by Suhr and Ingamells [ 151.

The chemical modifier, ammonia solution, was prepared by (1 + 4) dilu-

TABLE 1

Instrumental parameters

Stage Temp. Ramp (“G) (s)

Hold (s)

Gas flow (ml min“)

1 160 2 200 3 1000 4 300 5 2500 6 2650

Wavelength (nm) Spectral bandwidth (nm) Lamp source (power) Sample volume (~1) Modifier volume (~1) Integration time (s)

20 20 300 10 10 300 30 20 300

1 15 300 la 5 0 1 3 300

266.3b 0.7

EDL (12.5 W)c 20 20

4.5-5.0

aRamp = 0 for experiments with maximum heating rate; absorbance readings are taken at this stage bThis line is about 20% more sensitive than the 224.6~nm line in Zeeman a.a.s. CA Varian power supply was used for the Perkin-Elmer electrodeless discharge lamp (EDL).

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tion of ammonia liquor (p.a., Merck). The diammonium hydrogenphosphate magnesium nitrate modifier (referred to as the (NH4)2HP04 modifier) was prepared by dissolving 1.0 g of (NH,), HP04 (p.a. Baker) and 0.10 g magne- sium nitrate (p.a., Merck) in 100 ml of 0.05 M nitric acid. The ascorbic acid/ iron modifier (referred to as the ascorbic acid modifier) was prepared by dis- solving 5 g of ascorbic acid (p.a., Merck) and 0.02 g of iron(II1) nitrate non- ahydrate (p.a., Merck) in 100 ml of 0.05 M nitric acid. All other chemicals used were of analytical-reagent grade, and the argon was SR-grade.

Materials Reference materials MRG-1 (gabbro), SY-2 and SY-3 (syenite), and SO-4

(chemozemic A horizon soil) were obtained from CANMET (Canada Center for Mineral and Energy Technology) and materials AN-G (anorthosite) and BE-N (basalt) from CRPG (Centre de Recherches Petrographiques et Geo- chimiques, France).

Pyrolytically-coated graphite tubes with pyrolytic graphite platforms were used in the Perkin-Elmer HGA-600. Preliminary experiments were done with home-made, tungsten-impregnated [ 161, standard graphite platforms, which had performance characteristics similar to those of pyrolytic graphite plat- forms. Before use, pyrolytically-coated tubes were conditioned by heating four times to 2650°C with a ramp time of 60 s and a hold time of 0 s, fol- lowed by twice heating to 2650°C with a ramp time of 1 s and a hold time of 5 s.

Procedure The sample (0.250 g) was carefully mixed with 0.250 g of lithium meta-

borate in an agate mortar and transferred to a 15-ml platinum crucible. The mixture was fused in a muffle furnace (type 1300, Thermolyne Corp.) at 950°C for 15 min. After cooling, the bead formed was transferred to a lOO-ml teflon vessel equipped with a magnetic stirring bar, and 30.0 g of (1 + 9) nitric acid was added. The vessel was heated to 50°C on a hot-plate (fitted with a magnetic stirrer motor) for 20-30 min to dissolve the bead completely. The clear solution thus obtained was further diluted three times with diluted (1 + 19) nitric acid directly in an autosampler vial.

The tin content of the dissolved lithium metaborate blank as well as that in each of the dissolved reference materials was determined by means of the standard additions technique. The blank value was subtracted prior to calcu- lation of the tin concentrations in the solid reference materials. All results given in Table 2 and in the figures are based on peak-area values and repre- sent the means of at least three determinations.

RESULTS AND DISCUSSION

Sample decomposition Fusion with lithium metaborate as a flux has been shown to decompose

geological materials, bringing both lattice-bound and cassiterite tin into solu-

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TABLE 2

Determination of tin in six geological reference materials

Reference material

Tin concentration (pg g-‘)

This work Recommended or reported values

MRG-1 3.2 i 0.42s 3.2Bb, 3.2P SY-2 4.6 * 0.87 4Bb, 4.80c SY-3 5.5 f 0.99 6?b, 5.50c so-4 1.1 f 0.28 l.l3C, 3c BE-N 3.0 * 0.34 l.ge, 2.4e AN-G <0.7 0.26e, 1.5e, 8e

aFive independent dissolutions. The results of the other materials are based on 3-4 dis- solutions. The precision is expressed as *l s.d. bSee Abbey [22 ] ; values are classified as “A”, “B”, or “?W, where “A” means the greatest reliability. CSee [ 231. dSee [ 241. %ee [251.

tion [8-10, 141. Exceptions were found to be materials with a high sulphur content (>lO%), which did not form a bead but rather a nonhomogeneous “gel” when fused with this flux. The flux/sample ratio used in the previous work referred to was 3:l. Because it was found here that lithium metaborate contributed to the non-specific absorbance signal during atomization, tests were made in which the fraction of lithium metaborate was decreased; it was found that ratios of 3:l and 1:l gave the same tin signal for the reference material MRG-1, whilst the non-specific signal was decreased by 40%. A ratio of 1:l is thus sufficient for the graphite-furnace a.a.s. determinations.

Spectral interferences Even though the geological samples were diluted about 360 times, many

sample constituents were still present in sufficient concentrations to cause high background signals. The nature of the background seems to be com- plex; even the Zeeman background corrector could not compensate accu- rately for it. This can be seen in Fig. 1, where “normal” and maximum heating rates during atomization are compared with respect to spectral inter- ferences. When the maximum heating rate was utilized for tin in SY-2 (Fig. lb), the resulting background absorbance was as high as 0.84 and partly overlapped the tin signal, causing the latter to cross the baseline and con- tinue below it. The resulting tin peak-height or peak-area signal will produce erroneous results, even if an “integration window” around the peak is set by means of the Zeeman 3030 software. However, by applying a lower heating rate (RAMP = l), the tin and background peaks could be separated in time to a greater extent (Fig. la). Because the tin signal then returns properly to the baseline, correct peak-area values can be obtained by selecting an integra- tion time of 5 s. It should be observed that the lower heating rate did not result in decreased sensitivity.

01 AA 0.4 BG

1

Fig. 1. Specific (AA; -) and non-specific (BG, . . .) absorbance traces obtained for refer- ence material SY-2 under different conditions: (a) normal heating rate (RAMP = 1) and gas stop; (b) maximum heating rate and gas stop; (c) normal heating rate and a 10 ml min-’ mini-flow during atomization. Scale expansion was done by means of the 23030 software (right-hand traces).

The influence of a mini-flow of inert gas (10 ml min-‘) during atomization on the specific and non-specific signals is shown in Fig. l(c). The tin signal de- creases slightly whereas, unexpectedly, the non-specific signal increases. Fewer spectral interferences were thus obtained when gas stop was used during atomization.

Non-spectral interferences It is well known that the determination of tin in many materials is difficult

because of matrix interferences, especially by chloride and sulphate [ 17,181. In order to decrease these interferences, platform vaporization/atomization in combination with a chemical modifier was applied to the geological samples. Vaporization from a platform not only decreased spectral and non- spectral interferences but also increased the peak-area sensitivity for tin by 40%, which is in line with findings reported elsewhere [ 18, 191.

Several modifiers have been proposed for tin [ 18, 20, 211. Figure 2 shows the specific and non-specific signals obtained for 1 ng of tin by using the modifiers ammonia, (NH4)*HP04 and ascorbic acid at optimum ashing tem- peratures. Ammonia has, by means of radioactive measurements, been shown

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0

. . . . . . . . . . . . . . . .

0 1 2 3

Tome (s)

0.2

Fig. 2. Specific (AA; -) and non-specific (BG; **e) absorbance traces obtained for 1 ng of tin with different modifiers: (a) ammonia; (b) (NH,)J-IPO,; (c) ascorbic acid. Ashing temperatures were 1000, 1200 and 13OO”C, respectively.

Fig. 3. Influence of different concentrations of elements on the integrated signal for 1 ng of tin: (A) aluminium; (o) silicon; (0) iron. Appropriate amounts of the interfering ele- ments (e.g., 0, 25, 50, 75 and 100 mg of iron powder) were mixed with lithium meta- borate and fused as described in the Experimental section. To the dissolved bead, tin was then added to give a final concentration of 50 ng g-‘.

to stabilize tin up to 800°C [US]. From ashing curves examined for all the modifiers, it was concluded that effective stabilization was also achieved with the (NH4)2HP04 modifier, whereas the tin peak-area signal was about 10% lower when the ascorbic acid modifier was used. The large non-specific signal introduced by the (NH4)2HP04 modifier (background absorbance = 0.74; area = 0.68) excluded its application as a modifier for tin. The performance of the modifiers ammonia and ascorbic acid, when the reference material MRG-1 was vaporized at optimized conditions, was investigated. The tin peak-area values obtained were very similar for the two modifiers, but the ascorbic acid modifier suffered from the serious disadvantage that a large carbon residue accumulated on the platform causing irreproducible results. Hence, ammonia was found to be the most suitable modifier for dissolved geological samples; it not only stabilizes tin, but also removes chloride as ammonium chloride during ashing, and leaves no residue on the platform.

Figure 3 shows interference effects on tin from three main constituents of geological materials, silicon, iron and aluminium. These interferences are probably the reason why the slopes of the standard addition curves, based on peak areas, differ for each of the six reference materials. As a consequence of the different slopes, quantitation against aqueous tin standards was impos-

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2 Time (5) Time(s)

Fig. 4. Specific (AA; -) and non-specific (BG; * . . ) absorbance traces obtained for refer- ence material SY-2 with 0.2 ng of tin added. Comparison with Fig. l(a) shows that the addition contributes mainly to the second peak.

Fig. 5. Specific (AA; -) and non-specific (BG; *u.) absorbance traces obtained for a lithium metaborate blank (B; area = 0.005 A s) and the reference materral SO-4 (S; area = 0.020 As).

sible. When the standard addition technique was used, it was observed that the signal traces obtained for the additions often consisted of two peaks (Fig. 4). Evaluation by means of peak-height values would thus give erroneous tin concentrations, and therefore all analytical results reported are based on peak-area values.

Application to geological samples The values obtained for tin in some reference materials (five rocks and one

soil) are compared with recommended or reported values in Table 2. Although the testing is by no means exhaustive, the data show that the method is cap- able of yielding accurate results. That the determination of tin is troublesome is reflected in the individual results obtained by different laboratories and techniques for the reference material SY-2, on which the derivation of the recommended value was based [22] ; the data reported were 2.5, <lo, 2, 5.5, 4.1, <4, <7, 3, 10, 5.7, 3.1 4 and 4.4 pg g-l tin [22]. For the proposed method, the relative standard deviation at the 3 pg g-’ level was about 12% (Table 2), which can be regarded as acceptable, considering the low concen- tration and the interferences discussed above.

The detection limit, defined as the concentration in the diluted sample that corresponds to an integrated absorbance value twice that obtained for the lithium metaborate blank, is estimated to be 0.7 pg g-’ tin in a geological material. To demonstrate the overall performance of the method close to the detection limit, Fig. 5 shows signal traces for a blank and the reference ma- terial SO-4, which has been found to have a tin concentration close to 0.7 pg

-1 g *

The fairly rapid method described here for the determination of tin in geological materials should also be applicable to materials of diverse chemical composition such as sediments.

The authors are indebted to Dr. Bernhard Welz, Perkin-Elmer, FRG, for providing the Zeeman 3030 spectrometer, to Mr. Hans bstling, Perkin Elmer,

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Sweden, for the loan of a tin electrodeless discharge lamp, and to Dr. Wolfgang Frech for fruitful discussions. They also thank Mrs. Karin Olsson for skillful technical assistance and Mr. Douglas Baxter for revising the manuscript. This work was supported by the Swedish Natural Science Research Council.

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