The analysis of geological samples by slurry nebulisation inductively coupled plasma—mass spectrometry (ICP-MS)

Chemical Geology, 77 (1989) 53-63 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

53

The analysis of geological samples by slurry nebulisation inductively coupled plasma-mass spectrometry (ICP-MS) KYM E. JARVIS and J.G. WILLIAMS ICP-MS Unit, Department of Chemistry, University of Surrey, Guildford, Surrey GU2 5XH (Great Britain) (Received November 23, 1988; revised and accepted May 1, 1989 )

Abstract Jarvis, K.E. and Williams, J.G., 1989. The analysis of geological samples by slurry nebulisation inductively coupled plasma-mass spectrometry (ICP-MS). Chem. Geol., 77: 53-63. New techniques for the analysis of geological samples by slurry nebulisation inductively coupled plasma-mass spectrometry (ICP-MS) are described. Although samples in the form of solutions are commonly used for ICP-MS analysis, many rocks and minerals are resistant to acid attack. Slurry nebulisation avoids some of the problems associated with sample digestion by introducing the sample into the ICP as a suspension of finely ground particles. Within the error of the measured data, no significant matrix effects were observed. No sample matrix matching is required and calibration can be carried out via standards prepared in the suspending agent used for the slurry preparation. Thirteen widely available reference materials were analysed for eighteen elements. These elements include those which may be lost during an open acid digestion (Se, Cr and As) and those present in insoluble mineral phases (Nb and Ta). Uranium and thorium were also included as they are difficult to analyse by other techniques. Some modifications to the standard ICP-MS operating conditions were found to be necessary. Both inter- and intra-sample precision was investigated and is considered by the authors to be adequate for routine analysis. Measured values for most elements analysed compare favourably with recommended values.

1. I n t r o d u c t i o n

For many years, a number of powerful analytical techniques have existed for the elemental analysis of rocks and minerals. Those which analyse the sample in the solid form, for example X-ray fluorescence (XRF), typically have inadequate detection limits for many important elements. Other techniques commonly rely on the introduction of samples in the form of solution. This may preclude the analysis of volatile elements lost during the dissolution stage (either by open acid digestion or fusion) or elements contained in mineral phases which are only partially soluble in even the most ag0009-2541/89/$03.50

gressive acids. Closed digestion procedures using high-pressure vessels may be limited by the acids used, e.g. hydrofluoric acid residue in final solution. Thus a great deal of important data would be made available if samples could be introduced undigested into an ultra-sensitive analytical instrument. This work describes the first of a two-part study using slurry nebulisation inductively coupled plasma-mass spectrometry (ICP-MS) for the analysis of a number of elements which provide important information for petrogenetic studies. I C P - M S can potentially be used for the analysis of most elements in the periodic table, and has the additional capability of isotope ra-

© 1989 Elsevier Science Publishers B.V.

54

tio measurement. [see Date and Gray (1989) for a number of recent reviews]. In general, sensitivity is greater for the heavier elements, although detection limits are low throughout the mass range (e.g., Date and Jarvis, 1989). Work by Williams et al. (1987) demonstrated the feasibility of slurry nebulisation I C P - M S for three standard reference soils for a number of major and minor elements. The agreement between measured I C P - M S values with ICP-AES (inductively coupled plasmaatomic emission spectrometry) and reference data was, in general, good. For this study elements were selected using three criteria: (1) We have examined some of those elements which would be either totally or partially volatalised during an open acid digestion (e.g., Se, Cr, As). (2) We consider those elements which are generally present in insoluble mineral phases (e.g. Nb, Ta). (3) U and Th have been included because they are difficult to analyse by many other techniques (e.g., insensitivity of X R F and complex spectra of ICP-AES), and they are important as incompatible elements in many igneous and metamorphic systems, and may show marked redox chemistries in some sedimentary and hydrothermal environments. Slurry atomisation has been used recently as a mode of sample introduction in electrothermal atomisation atomic absorption spectrometry for the analysis of soils (Hinds et al., 1988) and dried foods (e.g., Olayinka et al., 1986). Comparable techniques have also been applied to the analysis of coals and clays by ICP-AES (e.g., Ebdon and Parry, 1987; Ebdon and Wilkinson, 1987; Ebdon and Collier, 1988a, b; Sparks and Ebdon, 1988).

2. Experimental 2.1. Sample preparation Samples were dried at 105 °C for 24 hr. Sample preparation was similar to that described by

K.E. JARVIS AND J.G. WILLIAMS

Williams et al. (1987). A 0.20-g sample was weighed into 30-ml plastic bottles; 10 g of zirconia beads {2.5 m m Glen Creston ® ) and 2 ml of a dispersing agent [0.05% (w/v) tetra sodium pyrophosphate, Na4P2Ov] were added. The thirteen standard reference materials were shaken on a laboratory flask shaker for 15 hr. (see below). The slurries were transfered to 100ml volumetric flasks and made to volume using 0.05% Na4P2Ov, thus giving a total solids concentration with respect to the sample, of 0.2% (500 × dilution factor). Previous studies (e.g., Ebdon and Collier, 1988a, b) suggest that, at a particle size of < 8 ~m, a solid particle will behave like a liquid droplet during pneumatic nebulisation and thus undergo the same vapourisation, dissociation and ionisation processes in the plasma. Therefore, prior to preparation of the standard reference materials, four test samples (granite, pyroxene diorite, oolitic limestone, sandstone ) were processed using the sample preparation scheme outlined above, and their grain-size distributions were measured using a Micromeritics ® Sedigraph. The four test samples were shaken for 2, 6 and 15 hr. each, to establish the time required to obtain a suitable grain-size distribution in a range of rock types with varying mineralogy and induration. The grain-size distribution varied greatly with rock type, as shown in Fig. 1 (e.g., diorite and limestone). It is notable that after only 2 hr. grinding, the diorite sample still contains ~ 15% of particles of > 60 #m diameter. In addition, > 10% of the sample settled before measurements were started, indicating that this proportion was > 70 ]~m in diameter. This may reflect the high modal abundance of a relatively dense and resistant mineral phase such as ilmenite or magnetite, known to be present in this sample. It was concluded that since modal mineralogy of samples is not always known prior to analysis, and the particle-size distribution after short grinding times may be highly variable, a minimum of 15 hr. (or overnight) grinding was

SLURRY NEBULISATION ICP-MS

55

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Fig. 1. Grain-size distribution graphs for four test samples: sandstone, oolitic limestone, pyroxene diorite and granite.

required. All samples used for this paper were therefore shaken for 15 hr. 2.2. Instrumentation

The samples were analysed by ICP-MS, a relatively new technique developed by A.L. Gray and A.R. Date at the University of Surrey (Gray, 1985). Most of the published work on ICP-MS uses pneumatic nebulisation of solutions to introduce samples into the ICP. However, the instrumental operating conditions required for slurry nebulisation are somewhat

different to those typically used for solution analysis. The instrument used was a VG ® Elemental PlasmaQuad ICP-MS. Details of the operating parameters are shown in Table I. A De Galan ® high-dissolved-solids nebuliser (Babington ® Vgroove type) was used with a pumped sample uptake rate of 2 ml min.- 1. Following the work of Williams et al. (1987) and Ebdon and Collier (1988b) a 3-mm-diameter torch injector was used in place of the standard 1.5-mm-diameter injector. The increased cross-sectional area results in a decrease in the linear velocity, and a

K.E. JARVISAND J.G. WILLIAMS

56 TABLE I I C P - M S operating conditions used for slurry nebulisation Forward power Reflected power Plasma Coolant gas flow rate Nebuliser gas flow rate Auxiliary gas flow rate Sample uptake rate Torch injector diameter Nebuliser Spray chamber Ion lens settings

1.5 k W <5W Ar 13 1 m i n . - ' 1.1 1 m i n . - ' 0.5 1 m i n . - 1 2.0 ml m i n . 3 mm De Galan ®, V-groove, high dissolved solids water-cooled single-pass spray c h a m b e r m a i n t a i n e d at 13 ° C optimised on ~gCo a n d 23sU

corresponding increase in residence time of the sample in the plasma. The nebuliser gas flow rate was adjusted to 1.1 1 min. -1, a flow rate which gives maximum sensitivity (whilst minimising polyatomic, oxide and doubly charged ion levels) for the 3-ram-diameter injector. Although this results in a slight increase in velocity, compared with the lower flow rates used for the standard system, the residence time in the plasma is still substantially greater than that achieved using the standard set-up. In addition to changes in the nebuliser gas pressure, the forward RF (radio frequency) power was increased to 1.5 kW. 2.3. Calibration

For the analysis of solutions by ICP-MS, at concentrations of < 2000/~g m1-1, calibration with simple aqueous standards is usually sufficient with no matrix matching required (e.g., Jarvis, 1988; Date and Gray, 1989). For the analysis of slurry samples, standards were simply prepared in 0.05% w/v Na4PzO7, no further matrix matching was performed. Calibration standards were prepared from commercially available 1000-/~g-ml-1 stock solutions. Since some elements are more stable in a chloride matrix (e.g., Cr) or cause precipita-

tion when added to a chloride matrix (e.g., Ag) two standards were used in addition to a reagent blank. The first contained 100 ng m l - ' of Be, B, Ge, As, Se, Nb, Mo, Ag, Cd, Ta, Bi, Th and U (from nitrate stock) and the second 100 ng m1-1 ofCr, Sn, Sb, Te and W (from chloride stock) both in 0.05% w/v Na4P2OT. In addition to initial calibration prior to sample analysis, both standards were run after every two samples to allow a correction for signal drift to be made. A slurry sample was aspirated continuously for 10 min. to stabilise the system prior to calibration and analysis (Jarvis et al., 1988). This is necessary because a large initial drop in signal occurs as sample condenses on the sampling cone. However, an equilibrium can be established between condensation and evaporation. If this "priming" process is adopted, signal drift due to further cone blockage is minimised and can easily be corrected for using a drift monitor (in this case the two calibration standards were used). In some studies using slurry nebulisation ICP-AES (e.g., Ebdon and Collier, 1988a), a magnetic stirrer has been used to agitate the slurry samples during analysis. This was found to be impracticable for the geological samples used in this study since it resulted in the removal of magnetic mineral phases, such as magnetite, during mixing. In preference, to insure that the slurry particles remained in suspension and well mixed, sample bottles were placed in a small ultrasonic tank during analysis. The choice of isotopes used for analysis (Table II) is partly dependent on the sample matrix, but more important considerations are the potential formation of refractory oxide species, doubly charged ions and polyatomic ion interferences (Gray and Williams, 1987). The choice of isotopes for many lighter elements is restricted by possible interferences from polyatomic ions, particularly those involving Ar (the plasma support gas) and oxygen (which is present in both the sample and dispersing agent). Se is perhaps the worst affected light

57

SLURRYNEBULISATIONICP-MS TABLE II Isotopes used for slurry nebulisation analysis by ICP-MS Element

Isotope

Abundance

Comments

(wt.%) Be B Cr Ge As Se

9 11 52,53 74 75 82

100 8O 83.8,9.5 36.4 100 9.0

Nb Mo Ag Cd

93 95,98 109 114

100 15.9, 24.4 48.2 28.8

Sn Sb Te Ta W Bi Th U

118,120 121,123 128 181 184 209 232 238

24.1, 32.8 57.3, 42.7 31.8 100 30.7 100 100 99.3

monoisotopic most abundant mean value used 72Ge interference from a6Ar2 monoisotopic S°Se interference from 4°Ar4°Ar;7SSe interference from 4°AraSAr;82Kr under 82Se thus high blank (see detection limit data in Table II) monoisotopic possibility of 94ZrlH on 95M0" '°7Ag interference from 91Zr1°0"; 1°gAginterference from 92Zr1~O'H* 1'2Cd interference from 96Zr160*; '*~Cd interference from 9'5Zr'60";possible overlap of ~4Sn mean value used mean value used 'a°Ba overlap with ~a°Te monoisotopic most abundant isotope monoisotopic monoisotopic most abundant isotope

*Zr contamination from grinding beads.

element since its two principal isotopes 8°Se and 78Se are coincident with the two Ar dimers, 4°Ar4°Ar and 4°Ar38Ar, respectively. In addition, S2Se (used for this study) is coincident with 82Kr. After an investigation of blank solutions and both bottled gaseous and liquid Ar, it was concluded that the Kr was derived from the liquid Ar supplies. As a result, the blank values for 82Se are relatively high, and detection limits are consequently poor (see Table IV). The midmass elements (mass > 8 0 ) a r e generally free from polyatomic ion interferences. However, since zirconia beads were used as a convenient grinding medium, significant Zr contamination of the samples occurred. Elements with high oxide bond strengths, such as Zr ( 759 kJ tool- ' ) do not undergo complete dissociation in the plasma and may be subject to recombination in the plasma. This can lead to an interference at M+O ~6 (i.e. masses 106, 107, 108, 110 and 112). This is not a serious problem except for the analysis of Ag, which has only two stable iso-

topes 1°rAg and l ° 9 t g . The former has an oxide interference from 91Zr160 and the latter a small interference from 93Nb'60 but more importantly from 92Zr1601H, which using this type of sample preparation is very large. Oxide interferences can usually be corrected for since oxide formation is a relatively constant factor (typically 1.5% of the parent ion signal for Zr). However, a correction was not successfully made in this case (see p. 60) and the reason for this is unclear. The heavier elements are generally free from interferences but can form doubly charged ions.

2.4. Samples A number of relatively well-characterised standard reference materials were prepared in order to assess precision and analytical accuracy. Although the technique has obvious attractions particularly for the analysis of "difficult" samples (e.g., refractory minerals), such

58

K.E.JARVISANDJ.G.WILLIAMS

TABLE III

3.1. Detection limits

Description of standard reference materials with their distributors analysed by ICP-MS

C.C.R.M.P. (Canadian Certified Reference Materials Project), Canada: MRG-1 SY-2

gabbro syenite

N.B.S. (National Bureau o/Standards), U.S.A.: NBS-688

basalt

N.R.C.C. (National Research Council o/Canada), Canada: MESS-/

marine sediment

G.S.J. (Geological Survey o/Japan), Japan: JB-1 JG-I

basalt granite

N.I.M. (National Institute o/ Metallurgy), South Africa: NIM-G NIM-L NIM-N NIM-S

granite lujavrite (syenite) norite syenite

U.S.G.S. (United States Geological Survey), U.S.A.: AGV-1 G -2 W-1

andesite granite dolerite

well-characterised sample types are not readily available as standard reference materials and therefore cannot be used in such an assessment. The reference materials used here therefore cover a range of igneous and sedimentary rock types (Table III), which, while data for all elements are not available for all samples, allows a reasonable assessment of analytical accuracy to be made. 3. Results A number of important analytical parameters were assessed, including elemental detection limit, precision and accuracy.

I C P - M S is becoming widely accepted for geochemical analysis, particularly because of the very good detection limits, which are principally a result of a very low and stable background signal. Due to the changes in instrumental operating conditions necessary for the analysis of slurry samples, the detection limits were redetermined using the conditions under which these analyses were carried out (Table IV). For comparison, detection limits measured in a 1% HNO3 matrix are given. It was anticipated that the detection limits would be poorer under the modified operating conditions because of: (a) the Na4P207 matrix; and (b) the use of a 3-ram-diameter torch injector which punches a significant hole through the centre of the plasma. However for most elements, the detection limits compare favourably between the two matrices while a few are poorer (Table IV). TABLE IV A comparison of ICP-MS 3a detection limits {ng ml- 1) Element

Simple aqueous (A) (1.5-mm injector)

Na4P~O7matrix (B) (3-mm injector)

9Be liB ~2Cr 74Ge 7~As S~Se 93Nb 9SMo 1°gAg 114Cd ~°Sn 121Sb 12STe ~SlTa ls4W 2°9Bi 2~2Th 238U

1.16 3.78 0.11 0.96 2.84 0.28 0.11 0.70 0.32 0.57 0.43 0.28 3.06 0.09 0.11 0.11 0.04 0.02

0.72 11.1 1.52 0.32 0.20 10.6" 0.92 0.15 0.09 0.14 0.12 0.08 0.24 0.03 0.07 0.96 0.02 0.04

*The variation in detection limit between the two matrices is a result of variable Kr levels in the liquid Ar supply.

SLURRY NEBULISATION ICP MS

59 T A B L E VI

3.2. Precision Both inter- and intra-sample precision were assessed. This was achieved by analysing reference material NIM-L three times (Table V), and three separate preparations of SY-2 once each (Table VI). "Within sample" precision is better than 5% RSD for most elements. Precision is particularly good for the volatile elements As and Sn. "Between sample" precision is a little poorer, although typically better than 10% RSD. Those elements which display poorer precision are present at the lowest concentration, e.g. Sb and W. 3.3. Accuracy Analytical accuracy is best assessed by comparison of measured values with recommended or published data. Although relatively wellcharacterised materials were chosen for analysis, data for some elements are not" available. Where values are available, some are based on TABLE V Intrasample precision for reference lujavrite N I M - L Element

Mean (ttgg 1)

SD (~tg g - l )

RSD (%)

Be B Cr Ge As Nb Mo Cd Sn Sb Ta W Bi Th U

24.2 8.37 12.0 0.76 1.88 932 0.75 0.93 6.90 0.36 18.8 7.72 1.76 58.3 18.1

0.82 0.62 0.42 0.02 0.03 22 0.08 0.01 0.16 0.06 0.62 0.19 0.22 2.4 0.80

3 7 4 3 2 2 10 1 2 17 3 2 12 4 4

Reference (/.tg g - ' ) 20 10

960 4 7 22 66 14

Reference values from Govindaraju ( 1984 ). SD --- s t a n d a r d deviation; RSD = relative s t a n d a r d deviation; - -- no reference value available. Mean value calculated for n -- 3 (triplicate analysis of single sample ).

Intersample precision for reference syenite SY-2 Element

Mean (pgg-1)

SD (~g g - , )

Be B Cr Ge As Nb Mo Cd Sn Sb Ta W Bi Th U

21.1 92.8 9.19 1.56 15.2 26.7 0.40 0.67 4.92 0.32 1.78 1.95 3.38 326 255

0.49 7.10 0.19 0.05 0.08 0.57 0.08 0.21 0.09 0.07 0.09 0.51 0.72 16 14

RSD (%) 2 8 2 3 < 1 2 21 32 2 21 5 26 21 5 6

Reference (#g g - l ) 23 85 12 18 23 3 4 0.2

380 290

Reference values from Govindaraju ( 1984 ). SD = s t a n d a r d deviation; RSD = relative standard deviation; - = no reference value available. M e a n value calculated for n = 3 (analysis of three sample preparations).

determinations made by a single laboratory using only one technique and cannot be regarded as true "recommended" values and comparison with these data cannot be used as a true measure of accuracy. The values obtained by slurry nebulisation ICP-MS are shown in Table VII. In general, measured values compare favourably with reported values, particularly for the heavier elements. In addition As, Be, B, Nb, Sn and Sb show reasonably good agreement at levels of > ~0.5/zg g-1. Of the 17 elements measured, three (Ag, Cd and Bi) display a systematic error. The determination of Ag is hampered by the large amount of Zr contamination derived from the grinding medium (see p. 57). Refractory Zroxide or -hydroxide interferences occur on both of the Ag isotopes and it must be concluded that the accurate determination of Ag in the presence of high Zr levels is not possible. A similar problem occurs with l'lCd which has an interference from 94Zr16OIH. The reason for a sys-

60

K.E. JARVIS AND J.G. WILLIAMS

TABLE VII (continued)

T A B L E VII Trace-element concentrations in ttg g - 1in 13 international standard reference rock samples by slurry nebulisation I C P MS This work

Reference value

Standard AGV - 1: Be B Cr Ge As Nb Mo Ag Cd Sn Sb Ta W Bi Th U

2.35 12.8 11.4 1.30 0.20 14.4 1.60 0.75 0.86 4.06 3.88 0.90 0.64 0.54 6.56 1.98

2 7 12 1.25 O.84 15 3 0.104 0.061 4.2 4.4 0.92 0.53 0.054 6.5 1.89

Standard G-2: Be B Cr Ge As Nb Mo Ag Cd Sn Sb Ta W Bi Th U

2.34 6.18 8.24 0.93 < DL 9.90 0.46 1.38 0.50 1.66 < DL 0.85

2.4 2.2 9 1.14 0.27 13 1 O.O45 0.025 1.6 0.087 0.88 0.15 0.041 24.6 2.04

Standard J B - I : Be B Cr Ge As Se Nb Mo Ag Cd

1.66 10.9 384 1.89 1.96

1.6 12 405 1.3 2.3 26 31 34.4 0.05 0.103

This work

Reference value

Standard JB-1 (cont.): Sn Sb Ta W Bi Th U

1.76 0.35 1.61 19.9 2.10 10.6 2.21

1.8 0.23 4.4 19.4 0.031 9 1.8

Standard JG-I: Be B Cr Ge As Se Nb Mo Ag Cd Sn Sb Ta W Bi Th U

1.36

3 6 53 1.3 0.3 15 1.5 0.04 0.023 4.1 2.9 1.3 0.52 13.5 3.3

Standard M E S S - 1: Be B Cr Ge As Nb Mo Ag Cd Sn Sb Ta W Bi Th U

1.93 44.2 61.3 1.59 11.2 14.7 2.20 2.34 0.89 3.20 0.72 1.32 2.20 0.93 19.4 5.69

1.9 71 1.7? 11 (20) 2.2? 0.59 3.98 0.73 2 3 20 (5)

Standard MRG- 1: Be B Cr

1.18 6.10 440

0.60 13 450

61

SLURRY NEBULISATION ICP-MS

T A B L E VII (continued) This work

T A B L E V I I (continued) Reference value

Standard M R G - I ( c o n t . ) : Ge As Nb Mo Ag Cd Sn Sb Ta W Bi Th U

1.30 1.00 26.5 1.31 1.83 0.45 4.44 0.73 0.83 0.38

Be

Cr

270 1.64 2.75 5.40

Ge As Nb Mo Ag

Cd Sn Sb Ta W Bi Th U

0.7 20 3 [1.01.1, 1 . 0 3 " , 0.84 *2 ] 0.14 3.2 0.4 - [0.27 .1, 0.24 .1, 0.54 *2 ] 1 0.3

-

330 [260"~], 2 3 0 - 3 7 7 *4 2.33, 2.68 *4 5 *4

< DL

-

0.85 0.20 1.00 < DL 0.25 0.28

0 . 0 8 7 - 0 . 4 7 *4 0 . 2 4 6 - 0 . 3 8 *4 0.33, 0 . 3 1 - 0 . 4 6 *4 0.37, 0 . 2 8 - 0 . 3 4 *4

4.81 10.2 6.89 1.77 5.70 37.0 1.95 1.13 0.56 5.70 0.66 4.19 1.()6 0.85 44.2 16.0

7 7 12 15 53 3 4 0.6 4.5 52 15

Standard N I M - L : Be B

24.2
Cr Ge As Nb Mo Ag Cd Sn Sb Ta W Bi Th U

12.0 0.76 1.88 932 0.75 9.45 0.93 6.90

10 960 4 7 22 66 14

0.2 .3, 1.2 *4

Standard N I M - G : Be B Cr Ge As Nb Mo Ag Cd Sn Sb Ta W Bi Th U

R e f e r e n c e value

Standard N I M - L ( c o n t . ) :

Standard NBS-688: B

This work

20 -

Standard N I M - N : Be B Cr Ge As Nb Mo Ag Cd Sn Sb Ta W Bi Th U

1.04 4.76 28.0 1,67 < DL 0.68 < DL 0.76 < DL 0.28 < DL 0.09

1 30

-

0.6

Standard N I M - S : Be B Cr Ge As Nb Mo Ag Cd Sn Sb Ta W Bi Th U

1.66 15.4 5.52 < DL < DL 0.46

1.4?

13 12

(4)

0.6

0.9?

0.6?

62

K.E.JARVISANDJ.G.WILLIAMS

TABLE VII (continued) This w o r k

4. Conclusions

Referencevalue

Standard SY-2: Be B Cr Ge As Nb Mo Ag Cd Sn Sb Ta W Bi Th U

19.6 92.9 7.95 1.49 13.0 20.6 < DL 28.4 0.56 4.25 < DL 1.52 0.48 0.50 232 174

23 85 12 18 23 3

4 0.2

380 290

Standard W-l: Be B Cr Ge As Nb Mo Ag Cd Sn Sb Ta W Bi Th U

1.52 12.2 85.5 1.34 1.23 6.21 0.47 1.11 1.57 2.69 0.78 1.27 0.47 < DL 4.87 1.34

0.7 13 114-120 1.75 1.9-2.2 8-9.5 0.57-0.75 0.069-0.081 0.15-0.17 2.6-3.2 1.0-1.05 0.5 0.48 0.046-0.048 2.4 0.57-0.58

Reference values from Govindaraju (1984), Gladney et al. ( 1983) and National Research Council of Canada (MESS/ ). ICP-MS data are for single determinations only except NIM-L (three determinations, see Table V) and SY-2 (three sample preparations, see Table VI). *'Hall et al. (1987; ICP-MS); *2Terashima (1980); *3Crock et al. (1983); *4Gladney et al. (1987); ?--uncertain; ( )=additionaluncertainty;
tematic error for Bi is at present unclear. All known interferences have been checked for and none were found to produce the error recorded. Se was not detected in any sample.

Slurry nebulisation I C P - M S is a useful multielement analytical technique for the quantitative analysis of a wide range of rock types without the necessity for a sample digestion. The technique provides an alternative means of analysis for volatile elements such as As, Sb and Sn which might otherwise be lost during a digestion procedure. The sensitivity of the technique allows for the determination of a number of heavy elements (U, Th, Nb and Ta) at relatively low levels in a range of rock types. The accuracy and precision of the technique have been examined in a range of standard reference materials including gabbro, basalt, andesite, granite and a marine sediment. M a n y of the elements studied here are difficult to analyse by other techniques, and the reliability of the "reference" values for elements such as Ge, Sn, Se and Cd, for example, is uncertain. Of the 170 reference materials documented by Govindaraju (1984), only 16 have recommended values for Cd and only 4 for Se. The precision for most elements is between + 5 - 1 0 % depending on the concentration. The accuracy of the technique is not easy to assess, but within the limits of the published data slurry nebulisation has been shown to be a viable and useful method of analysis. The use of an alternative grinding medium such as agate should eliminate interference problems due to the formation of Zr-oxides and -hydroxides (providing that the sample is not itself very high in Zr), and the accurate and precise analysis of Ag and Cd should then be possible in most silicate matrices. Acknowledgements

Technical assistance was provided by Ed McCurdy. Grain-size measurements were carried out at the Institute of Oceanographic Sciences-Deacon Laboratory, Wormley, Surrey. A critical review of this manuscript was made by Ian Jarvis. Support of the I C P - M S facility in the Department of Chemistry, University of

SLURRY NEBULISATION ICP-MS

Surrey is provided by the Natural Environment Research Council (NERC). This work was performed during the tenure of a NERC research fellowship by K.E.J., and J.G.W. acknowledges support from the Ministry of Defence. References Crock, J.G., Lichte, F.E. and Briggs, P.H., 1983. Determination of elements in National Bureau of Standards geological reference materials SRM278 obsidian and SRM688 basalt by inductively coupled argon plasmaatomic emission spectrometry. Geostand, Newsl., 7: 335340. Date, A.R. and Jarvis, K.E., 1989. The applications of ICPMS in the earth sciences. In: A.R. Date and A.L. Gray (Editors), The Applications of Inductively Coupled Plasma Mass Spectrometry, Ch. 2. Blackie, Glasgow, pp. 43-70. Ebdon, L. and Collier, A.R., 1988a. Direct atomic spectrometric analysis by slurry atomisation, Part 5. Analysis of kaolin using inductively coupled plasma atomic emission spectrometry. J. Anal. At. Spectrom., 3: 557-561. Ebdon, L. and Collier, A.R., 1988b. Particle size effects on kaolin slurry analysis by inductively coupled plasmaatomic emission spectrometry. Spectrochim. Acta, 43B: 355-369. Ebdon, L. and Parry, H.G.M., 1987. Direct atomic spectrometric analysis by slurry atomisation, Part 2. Elimination of interferences in the determination of arsenic in whole coal by electrothermal atomisation atomic absorption spectrometry. J. Anal. At. Spectrom., 2: 131134. Ebdon, L. and Wilkinson, J.R., 1987. Direct atomic spectrometric analysis by slurry atomisation, Part 1. Optimisation of whole coal analysis by inductively coupled plasma atomic emission spectrometry. J. Anal. At. Spectrom., 2: 39-44. Gladney, E.S., Burns, C.E. and Roelandts, I., 1983. 1982 compilation of elemental concentrations in eleven United States Geological Survey rock standards. Geostand. Newsl., 7: 3-226. Gladney, E.S., O'Malley, B.T., Roelandts, I. and Gills, T.E., 1987. Compilation of elemental concentration data for NBS clinical, biological, geological, and environmental

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