The
Malaysian Journal of Analytical Sciences Vol 15 No 2 (2011): 265 – 272
Determination of platinum
group elements in
terrestrial rocks using
nickel sulfide fire assay
isotope dilution ICP-MS
(Penentuan Unsur
Kumpulan Platinum Dalam Batuan Menggunakan
Pencairan Isotop ICP-MS Assay Api Nikel Sulfida)
B.S. Wee1,2, M. Ebihara2, A.K.H. Wood1
1Malaysian Nuclear Agency, Bangi,
43000 Kajang,
Selangor, Malaysia.
2Tokyo Metropolitan University,
Minami-ohsawa
1-1, Hachioji-shi, 192-0397 Tokyo, Japan.
Abstract
Platinum
group elements (PGE: Ru, Rh, Pd,
Os, Ir, Pt) are highly siderophile elements (HSE)
and their abundances in terrestrial rocks are low (~ ppb). This group of
elements is important for understanding of geochemical processes, essential in
mineral exploration, environmental studies, and identification of impact
craters. Determination of PGE is
very challenging and a reliable analytical technique is required to obtain high
quality data. This study aims to utilize the fire assay isotope dilution
inductively coupled plasma mass spectrometry (ICP-MS) for the determination of
platinum group elements in terrestrial rock samples (WPR-1, WMG-1, and UMT-1).
The nickel sulfide fire assay is applied to preconcentrate PGE from terrestrial rocks. All samples (about 0.2
– 0.5 g) were spiked with enriched isotopes (except Rh) prior to fire assay
procedure in order to achieve isotope equilibrium. The NiS beads obtained were
dissolved in HCl and the residue containing PGE
was then dissolved prior to ICP-MS measurement. Isotopic ratios of PGE were measured and concentrations were
calculated using the isotope dilution equation. The recoveries are in the range
of 60 – 85% for all PGE except Os,
which is prone not to be quantitatively collected due to the formation of OsO4.
However, the lost of analytes does not affect the accuracy of the data obtained
once the equilibrium between elements in samples and spikes is established.
Actually, an excellent agreement between our data and literature values was
confirmed for Os data. The measurement of PGE
using ICP-MS requires correction from monoatomic and polyatomic ions having the
same mass-to-charge ratio as those of analytes of interest. Instrument memory
effect is evident in Os measurements and flushing with HCl and TMSC were
adequate to reduce the memory effect. Low sample blanks (< 12 pg/g) and
detection limits (< 15 pg/g) were achieved. In conclusion, the present
method is able to determine PGE
with reasonable accuracy despite of small sample size and is very favorable for
application to terrestrial samples.
Keywords: platinum
group elements, terrestrial rocks, fire assay, isotope dilution, ICP-MS
References
1.
K.H.
Wedepohl (1995). The composition of the continental crust. Geochim. Cosmochim. Acta 59,
1217 – 1232.
2.
R.L.
Watters Jr., K.R. Eherhardt, E.S. Beary, and J.D. Fassett (1997). Protocol for
isotope dilution using inductively coupled plasma-mass spectrometry (ICP-MS)
for the determination of inorganic elements. Metrologia 34, 87 – 96.
3.
A.A.
van Heuzen, T. Hoekstra, and B. van Wingerden (1989). Precision and accuracy
attainable with isotope dilution analysis applied to inductively coupled plasma
mass spectrometry: theory and experiments. J.
Anal. Atomic Spec. 4, 483 – 489.
4.
J.R.
De Laeter, J.K. Böhlke, P. De Bièvre, H. Hidaka, H.S. Peiser, K.J.R. Rosman,
and P.D.P. Taylor (2003). Atomic weights of the elements: Review 2000. Pure Appl. Chem. 75, 683 – 800.
5. G. Ravizza, and
D. Pyle (1997). PGE and Os
isotopic analyses of single sample aliquots with NiS fire assay
preconcentration. Chem. Geol. 141, 251 –
268.
6.
T.
Meisel, and J. Moser (2004). Reference materials for geochemical PGE analysis: new analytical data for Ru, Rh, Pd,
Os, Ir, Pt and Re by isotope dilution ICP-MS in 11 geological reference
materials. Chem. Geol. 208, 319 – 338.