Determination of minor and trace elements in silicate rocks by inductively-coupled plasma emission spectrometry

Short Comnnmication

DETERhlINATION OF MINOR AND ROCKS BY INDUCTIVELY-COUPLED

TRACE ELEXIEXTS PLASMX EXliSSION

IX SILIC’_A’I’E

SPECTROMETRY

Surtznrary. Samples (0.5 g) are decomposed with miscd acids in a scnlctl tcf’ton vessci. r2t’tw suit;lble treatment. txirium. cohit. chromium, copper. lithium. nickel. axwdium. vanadium and zirconium are determined sequentially. The rnethocf is satisstrontium. factory for a variety of stundnrcl silicate materials.

The ind~tctivel?l-coLlt,led plasma (i.c_p.) seems to be the best escitation source in emission stwctrochemical analysis for major, minor and tract elements in silicates heca~w2 of its high sensitivity, w%k dJwamic range and comparative freedom from themicai and ionization interferences f 1. 2f _ Solution viscosity tends to affect results El, 31 but it has been shorvn [-I] that decomposition with a mixture of hycfrochtoric and hyctrofhroric acids in a sealed tcffon vessel is satisfactory for the intcrfw-encc-free determinatiotl of major and minor clcments in silicates. The present colnm~lnit:ation deals with the determination of minor and trace elements by i.c.p. emission sprtctronittry after decomposition of the silicate in a sealed teflon vessel. Almost all minor and trace elements are completely dissolved by the txoposcct method. Barium, scandium, strontium, vanadium and zirconium, whir:h arc not easily determined by atomic absorption spectrometry. NH be dctermincd accurately and reproducibly. Irzstrtrmentation and operating pwa~netcls. The itistrutncntatioti was the same as that employed t)reviously [4 J_ 0t)erating conditions \vere also identical, except for the ca_rrier gas flow rate. The rate (O.S5 1 min-‘) had to be increased. for stable nebulization, because the salt concentrat.ion of the sample solution was higher than that. obtained in analysis for major and minor elements.

Reagents ancl star&a& soktions_ Stock solutions of ahm~inttm, iron, magnesium, sodium and potassium were prepared f4] _Commerciallyavailable standard stock solr~tions for atomic absorption spectrometry (1000 ppm) were used for Ba, Co. Cr, Cu, Li, Ni, Sr and V. Senndium stock soiution was prepared by dissolving scandium oxide (99%) in nitric acid. Zirconium met.ai powder (99%) was dissolved in nitric and hydroffttoric acids, and the acids were espeiled by heating the solution with sulfuric acid. Hydrochloric, nitric, sulfuric and perchioric acids were snpcr-special grade, and hydrofhtoric acid was reagent grade. All chemicals were obtained from \Yako Pure Chemical Co_ \\‘orking standards were prepared by suitable dilution and appropriate misirqg of stock solutions. The concentrations of minor and trace elements in these standards, which contained as t-m&is efements ahuninum (600 ppm). iron (600 ppm), calcium (500 13pn1ft magnesium (400 pI3rn). sociiurn (200 ppmj nncf. potnssitani (200 ppm), were in the range 5.0-0.05 ppm, with corresponding Mnnk solutions. Each of these mised stontlard solutions contained 4 ml of hydrochloric rtcict and f?. ml of perchlorie neid per 100 g of solution. X11 dilutions were tionc ~ilvinwtrieally on 3 top-pm bala~~ce. Decomposition. To the teflon vessel [ 4. 51 were added 0.5 g of powciered sample (accurately weighed), 4 ml of hydrofluoric acid and 2 ml of aqua rcgin. After the vessel had been sealed and left to stand for 16 h at room temperature. the eotitcnls were transferred to a tefion beaker and evaporated nearly to dryness at 95°C on a hot-plate. The solution was fumed with 2 ml of per&loric acid at 190°C to remove tetraffuorosiiicate and escess of hydrofi,uoric aeici, and then cooled. After addition of 2 mI of concentrated hydro&ioric acid and 30 ml of water. the resicttw was clissotvcd by !wating at S?‘C. Finally the solution was transferred to a polypt-ol>yiene bottle, calcium,

diiuted The

with xater to 50 g. and ~v-ri$ecl. choice of spectral lines is discussed

belols.

In the cIeterrnitla~iot1 of trace cfemcnts by i.c.lr. emission s~~e~tr#rnet~‘. the analytical lines must be chosen carefuli? to avoid spectral interferences. The Iines generafiy recommended for the determination of the ten trace elements are shown in Tabie 1 together with their detection limits. The detection limits are the concentrations corresponding to an intensity twice the standard deviation of the background obtained from the blank solution containing only matrix elements. In the absence of spectral interferences. concentrations five times the detection Iimit could be ctetcrmilled with a relative standard deviation of about 10%. The powdered silicate samples (0.5 g) could be decomposed with small amounts of hycirofluoric acid and aqua regia in the teflon vessel at room temperature after 16 h [ 51. In the present work. fluorosiiicate was removed, to decrease the salt. concentration of the sampie solution and so provide more reproducible nebuiization. The effects of temperature on the decompo-

TABLE

1

Anal>-ticnl Species

lines and detection

limits

_L\nnlytical line (nm)

Detection in silicate

Ba

455.4

CO Cr CU Li

345.3 367 .i 327 .-I GY0.P

0.17 8.6 0.59 0.91 0.19

sition

limit (ppm)

Species

Ni

.4nalytical line (nm)

Detection

limit

in silicate

(ppm)

Sr v

34 1 ..5 361.4 -lOi .i 310.2

1 .s 0.06s 0.013

Zr

349.6

0.31

SC

0.6i

\vere examined for all the elements listed at room temperature and at 50. 90 and 13O’C. For the decomposition at 90 and 13O’C, a novel design of teflon-lined stainless-steel bomb 161 was used. The effect of heating \vas not significant. except for zirconium in JG-1 and chromium in AIRG-I. fot which the contents in leached fractions increased with an increase in temperature. Xeverthelcss, even at 130%, the recoveries did not reach 100%; they xverc only 53% for zirconium and S3R for chromium_ Complete decomposition of various rock samples at room tetr.peraturc for rleterminations of copper. nickel, zinc and cadmium by atomic absorption slwctrometry \vas confirmed earlier [ 51. Thus. decomposition at room temperature is rccomt~nendecl for convenience. The calibration qaphs obtained were linear up to 5 ppm for synthetic solutions. The results for a wide variety of rocks are summarized in Table 2 \\-ith accuracies and re~~roducibilities obtained from 6 analyses. The results arc in reasonable agreement Lvith reported values. Features of individual clcments ‘are as follo\vs. Bariu,n. Barium is a minor el~~mcnt in most silicates. hut is not easily bccausc of chemical and determined by atomic absorption spectrometry ionization interferences. \\‘ith the I)lasnia. sensitivity is good and there is no interference. CoDalt. Spectral interfcrcwx5 tn3kc it difficult to dc~termitit trxe amounts of cobalt by the plasma method, as is clear from significant. differences between found and rq)ortcd values in JB-1 and hlRG-1 (Table 2). The atomic lines ahove 3:tO nm \vere less sensitive, but suffered less spectral interference than the ionic lines hclow 260 nrn. Chrorniccm. Chromium ionic lines nre sensitive , and sipnificant slwctr:tl intcrft~rences \vere not found at the 26S.‘i-nm line. I~o~~ever. chromium in MRG-1 and =\cI-V-l could not he brought into solution cotnpletcly by the method proposrcl, and the rq)roducibilitits \vercb poor for the other i\~o samples: the causes of these problems are not clear. Copper_ Though the 324.7-nm line is more sensitive than thcl 34’i.-I-ntn line, the latter was preft~rrecl hcaitsc of less intct-fcreticc from other clctnt~nts present, cscept in the case of JG-1 _

436 TABLE

B

Analytical

results

with

accuracies

and precisions

Reported

value

for minor

and trace elements

in standard

rocli samples Rock

Metal ---.-_-.--_

JC-1

Ba co Cr CU Li Xi SC Sr V Zr

JB-I

Ba co Cr Cu Li Xi SC Sr 1’ Zr

>\IRG-I

Ba co Clcu Li Xi SC Sr \’ Zr

AGV-1

Ba CO Cr CU Li Xi SC Sr v Zr

(ppm)

Value

166” 6.4 53-i 3.9 9-! P.” 6.5 1.91.1 3 -1 111

463 -

490” 39.1 405 56.7 11 .-I 133 “6 4352 211 1.53 .5.5b Si -120 135 1 3-00 -1s t?GO 5*20 100 1 “Us= l-! .1 12.2

59.i 12 18.5 13.4 65-i 135 I).,--.,

&Reported by -Ando et al. [i 1. ‘Reported [9 I. d351 .5 nm used as analytical line.

found

(ppm)

-

Recovery (5) 100 -

Rs.d. (S) 39 -._ -

51 .l -

9-i -

16.6 -

S6.1 i.9 5.4 lS5 41.9 29.0

92 96 s3 101 lo-! 26

3s 10.0 3-S 0.6

-!S6 -15.6

99 lli

4OG 56.1 1O.i l-10 26.3 43-1 21% 114

100 101 91 10-I 101 100 1 0.2 93

1 .l 3.3 6.6 1.9 2.6 *> 3 -.-

s5

-16.; 1‘25 30s 110 3.9 196 50.3

141

i.3

51

533 110 1200 13.s 7.4 G1.2 12.1

l.i.-ld 13.0 6% 124 23s by Abbey

_. Qi 11.6

1.” 1 .I 2.G ‘) A._3 -1.1 8. I 1.1

10-I 98 9s 105 1O-1 103 110

1 .-I 6.2 0.S 2.1 0.G ‘> _.-?

99 9s 61 103

1.6 7.5 -1.5

103 9-I 9s 100 99 106

et al. [S] _ “Reported

32

1.6 1 .‘i s.l 3.7 2.1 1 .i

3.9 1,~ Flanagan

43-i

Lithium. As for other alkali metals, there is no suitable ionic line and atomic lines must be used. The emission intensity of lithium at 6702 nm is fairly sensitive and there is no significant spectral interference SO that agreement with reported values is good. ~Vickel. Ionic lines arc unsuitable for the determination of nickel in silicates as in the case of cobalt. For the determination of nickel in AGV-1, zirconium emission at 341.466 nm interferes \vith the nickel emission at 341.4ii nm. Thus, 351.5 nm was used in this case. Scandilcnz. Scandium can bc determined very sensitively by i.c.p. emission qwctromctry. Even at the trace level. the results are in fair agrwmclnt with reported values. Strorztircnl. ionization interference was not obscrwd; thcx results ag-cc ~~11 with reported values and rtproducibility is sood. were often ol)sc~nwl \vith vanadium Vat~adi~or~ _ Spectral intcrfc~rcnccs .2, 31 1.1 atId 311 .S nm lines \vere invcastiionic lines. The 292.4. 309.3. 310 gatcd. The 310.Znm line \vas found to bcb suitablt> bccausc of less s;l)cctral intcrferenw, esccpt from large amounts of nickel. %ircorriirm. Zirconium is difficult to determine by atomic abwrption spectromctr!: because of the formation of thr monoritlc in ch(w~ic*al flamc~: the ionic linc>s eivc good stlnsitivity. The* most s:cnsitivc iti i.c.I>. spectromctry. 3-13.S23-nm line cannot Iw itsccl Iwcuusc of spectral interfwww from tlw Fe .343.5X1-nni line.

The

inductivtlly-coul,lctl plasma is sttitablc for thrl dctcrmination of rninc~~ c~lc~nwnts in silicate rocks. For Ixirium. c:hromiuni. sc::intliirin. vanadium and zirconium. \vhich xc difficult to tfctcrniint~ fly fl:unca atonlic sensitivity is high and cliclmical and ionization absorption slwc*tronictry. iritc~rferrnccs ‘arc’ not obscwc~d. I lowever. a slwcrronictcr with grt~at rc.iolvin!g power is necessary in order to rcduw q)ectral intcrfcwnccs. The 1nx~l)owci nicthod for the dc~cotii~~osition of silicate> rocks gives c*otiii)lc~tt~ dissolution of almost ill1 minor and tract- clcwcnts. .\liswl standard solutions arc sirnl)l>. ~~rc’pu’cd in thcb same matrix solution. and

tract

The

authors

csl)wss

their

qatitucle

to Dr. 1. Kojima

for

iisc~1’iil

disviissio~ls.

REFERENCES 1 S. Grcenficltl, H. 31. ~lcCe:tcltin :tncl P. B. Smith, Anal. Chitn. Art;~. S.1 jl9TC;) 67. 2 L). J. Krtlnicky, V. A. Ftl~~l :~nd R. N. Knisclry. t\ppl. Speclrosc., 31 (1977) 137. 3 H. Ucttida and H. hhtsui, Bunko Kenkyu, 2’7 (19iS) 1 10. 1 H. Uchitla, T. Ucliida and C. Iida, Anal. Chim. AcLa. 10s (19i9) Si. 5 T. Uchich, RI. Naguse, I. Kojima and C. Iitla, Anal. Chim. .kt;t, 9-l ( 197i ) Zii.T,. (I C. Iida. T. tichicla and I. Kojima, Atxtl. Chini. Xch, 113 (19SO) 3ti5. 7 A. Antlo, H. Kurasawa, T. Ohmori ant! E. T:tkccl;t, Geochem. J., S ( 19i-1) 175. S S. Al>lwy. A. H. Gillicson and G. Perrault. A. Report on the Coll;tbor;ttivr! ..\n;tlysis of Thrcr C:tnntli;tn Rock Sampirs for USC as CcrliI’iccI Rrfcrmce Xlntrri:~ls, 197.5. 9 F. .J. Flntxtgnn, Geochim. Cosmochim. Acta. 3'7 (1973) 1 IS9.