Ablative and transport fractionation of trace elements during laser sampling of glass and copper

SPECTROCHIMICA ACTA PART B

ELSEVIER

Spectrochimica Acta Part B 52 (1997) 2093-2102

Ablative and transport fractionation of trace elements during laser sampling of glass and copper P.M. Outridge*, W. Doherty, D.C. Gregoire Analytical Chemistry Laboratories, Geological Survey of Canada, 601 Booth St., Ottawa, K1A OE8, Canada

Received 24 February 1997; accepted 23 September 1997

Abstract

The fractionation of trace elements due to ablation and transport processes was quantified during Q-switched infrared laser .sampling of glass and copper reference materials. Filter-trapping of the ablated product at different points in the sample introduction system showed ablation and transport sometimes caused opposing fractionation effects, leading to a confounded measure of overall (ablative + transport) fractionation. An unexpected result was the greater ablative fractionation of some elements (Au, Ag, Bi, Te in glass and Au, Be, Bi, Ni, Te in copper) at a higher laser fluence of 1.35 x 104 W cm -2 than at 0.62 x 104W cm -2, which contradicted predictions from modelling studies of ablation processes. With glass, there was an inverse logarithmic relationship between the extent of ablative and overall fractionation and element oxide melting point (OMPs), with elements with OMPs < 1000°C exhibiting overall concentration increases of 20-1340%. Fractionation during transport was quantitatively important for most certified elements in copper, and for the most volatile elements (Au, Ag, Bi, Te) in glass. Elements common to both matrices showed 50-100% higher ablative fractionation in copper, possibly because of greater heat conductance away from the ablation site causing increased element volatilisation or zone refinement. These differences between matrices indicate that non-matrix-matched standardisation is likely to provide inaccurate calibration of laser ablation inductively coupled plasma-mass spectrometry analyses of at least some elements. © 1997 Elsevier Science B.V. Keywords: Laser ablation; Mass transport; Fractionation; Glass; Cu

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

There is increasing recognition that the process of laser ablation is associated with significant alteration (fractionation) o f at least some elements relative to their concentration in the original target. Chan et al. [1] reported that ablation with KrF excimer and N d : Y A G lasers operating at ultraviolet wavelengths produced enriched Bi and Cu in the vapour phase, and high residues of Ca and Sr in the ablation craters of a B i - S r - C a - C u target. Fryer et al. [2] documented the * Author to whom correspondenceshould be addressed. Fax: (613) 943-1286; E-mail: [email protected]

relative fractionation of 60 elements in the NIST 610 Glass SRM, finding that low-to-intermediate melting point elements, such as Ag, Au, Bi, Cd, Pb, Sb, Te, T1 and Zn, generally showed the greatest fractionation. Scanning electron microscopy and electron dispersive X-ray ( S E M - E D X ) analysis of the ablation product from two refractory materials (NIST610 Glass and mammalian tooth sections) disclosed the presence of particulates highly enriched in various low to intermediate melting point elements (Au, Ag, Bi, Cu, Fe, Ni, Pb and Zn), which were homogeneously distributed at trace concentrations in the original samples [3]. This finding was strongly suggestive of zone refinement during ablation (i.e. the migration and zonation

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of elements in the molten material around each ablation spot), and corroborated the earlier work on this subject by Cromwell and Arrowsmith [4]. Fractionation of elements during mass transport has also been postulated [ 1], but not demonstrated. While the existence of ablative fractionation has thus been established, there is little quantitative data on the extent of fractionation among different types of matrices, the relative importance of ablative and transport fractionation, and variations between elements [2]. It has been established that careful modulation of laser energy, either by stage translation during ablation [4] or selection of Q-switched rather than free-running laser [5], may reduce fractionation. However, one methodological problem is that most data on this phenomenon have been gathered by concurrent inductively coupled plasma-mass spectrometry (ICP-MS) or ICP-OES measurement of the sample carrier gas during ablation. Matrix-normalised element intensities which increase with decreasing laser energies have often been cited as evidence of fractionation (for example, Refs. [4,6]). But this evidence is not unequivocal nor quantitative. By producing variable amounts of matrix ablation, varying laser energies may also be associated with differing degrees of element-dependent signal suppression, or mass transport efficiency [7], which are potential confounding factors in the interpretation of fractionation data. The concurrent ablation-measurement approach also does not allow for the separate quantification of ablative and transport fractionation effects. Physical separation of the ablation and measurement steps, by trapping the ablation products for subsequent measurement in solution form, is a useful approach to this problem. Possible non-spectroscopic interferences during measurement can be more easily controlled than during simultaneous ablation measurement. Chan et al. [1] utilised this approach by trapping ablation products in a dilute nitric acid solution prior to analysis by solution nebulisation ICP-MS. The present study used chemically inert inline filters to trap the product from multiple ablations of glass and copper standard reference materials, before it reached the ICP torch. Subsequent acid digestion and solution nebulisation ICP-MS analysis of unablated fragments of the original sample, and of filters which had been positioned at the ablation cell outlet and at the torch (2.6 m distant), enabled the

degree of fractionation during ablation and mass transport to be separately quantified.

2. Methods 2.1. Instrumentation

The experimental work was carried out with a Perkin-Elmer Sciex Model 320 laser sampler and Elan 5000A inductively coupled plasma- mass spectrometer. Samples for ablation were enclosed in an air-tight glass sampling cell, with an Ar throughflow to carry ablation products from the cell to the ICP torch via 2.6 m of 0.25 inch diameter Tygon tubing. The Model 320 sampler employs a Nd:YAG laser operating at the fundamental wavelength of 1064 nm, and with a constant pulse frequency of 10 Hz and pulse width of 8-9 ns. The only laser variable in this study was the flashlamp input energy, which was controlled via the commercial system's software. The laser was operated in Q-switched mode, at a Qswitch delay of 280/zs (as per software settings). Ablation spot size, which was annular, was determined to be 160#m diameter. Laser power was measured during continuous ablation using a Mentor MA10 Indicator and MC 2501 Calorimeter (Scientech, Boulder, CO). 2.2. Experimental design

In separate experiments, NIST 610 Glass and NIST C1253 Phosphorised Copper were ablated at two different flashlamp input energy levels: 70J (the maximum power level) and 40 J (just above the point at which bulk ablation in glass commences). These input energies correspond to fluence levels of 1.35 and 0.62 x 104W cm -2, respectively, assuming the energy is evenly distributed across the ablation spot. The annular shape of the spot in this instance means that the effective fluence is higher than these values. However, since it was not possible to measure the exact width of the annulus, a closer approximation of the fluence was not obtainable. The ablation product was swept out of the sampling cell by Ar gas flowing at 0.9 1 min -~, and trapped on in-line 0.2/zm pore filters (Isopore Track-etched Membrane, 47 mm diameter, Millipore) positioned at the cell outlet and,

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2.3. Filter trapping efficiency and gas flow

during separate ablations, at the ICP torch inlet 2.6 m distant from the cell. Each filter trapped the products of multiple numbers of 1 min ablations (30-70 depending on experimental treatment), each ablation situated at different randomly selected sites on the sample. The stage was not translated during each ablation.There were three to five replicate filters for each of the four experimental treatments: 70 J flashlamp energy, filter at cell outlet; 70 J, filter at torch; 40 J, at cell outlet; 40 J, at torch. After ablation, filters were stored in new polycarbonate petri dishes until analysis. The following elements were analysed as their isotopes in the glass samples: 59Co, 71Ga, 85Rb, 93Nb ' l°7Ag ' 128Te' 139La' 140Ce' ]46Nd' 149Sm' 153Eu' 157Gd' J63Dy' 172yb ' 181Ta' 197Au' 205T1' 209Bi' 232Th' 238U; and in the copper samples: 9Be, 59Co, 6°Ni, 65Cu, 75As' 78Se' 1°TAg' llaCd ' 118Sn' 121Sb' t28Te ' 197Au' 208pb' 209Bi"

Using a ball gauge placed downstream of the filter holder, it was determined that the filters had no discernible effect on Ar gas through-flow, even at flow rates 30% above the usual rate of 0.9 1 min -1. Subsequently, laser ablation ICP-MS analyses of NIST 610 Glass and of a copper coin with and without the filters in-line showed that filter trapping efficiency in terms of signal intensity was > 99% for virtually all elements monitored (Fig. l(a) and l(b)). Slight increases in the intensity of the Ar dimer ('Ar2') ion showed that the filters had only a minimal effect on plasma configuration.

2.4. Analytical procedures To determine the composition of the original SRM materials, fragments of the NIST glass and copper standards (N = 5 each) were chipped or drilled out, washed in 10% HNO3 and D.D. water, dried and weighed. Sample weights ranged from 0.58-0.74 mg for NIST glass, 0.73-1.53 mg for NIST copper. By comparison, back calculations based on the final concentrations (see below) of Hf and Cu in the digests of glass and copper ablation products, respectively, suggest that approximately 10-20 ng of glass and copper ablation products were trapped by the filters.

To reduce possible contamination, prior to ablation the surfaces of both SRMs were ground with new 600 grit paper, and immersed in 10% HNO3 and distilled deionised (D.D.) water. The ablation cell, filter holder and transfer tubing were washed out in distilled water, and N = 4 blank filters placed in-line with the Ar flowing for 30 min each to sample any intra-system contamination. All reagents used were ultra-high purity grade (Seastar Acids and Bases, Vancouver, British Columbia).

le+7 -

A

F - - - q Without filter

FlU

le+7 -

B

F i l t e r ~ l le+6 -

le+6 Ct. O

O. o 1e+5 -

le+5-

O}

o'~ 1e+4 -

.~_ le+4

.E ._~ le+3 -

le+3 le+2 le+2

le+1

|

1 e+0

|

, i

Si

Mn

Zn

Sr

i

Ag

Te

La

Ta

Pb

U

At2

to+,

ii I i

I|l

i

Mn Co

I

Ni

Cu Zn

T ~.7

T 99.3

m |

,

,

,

Sr Ag Sn Pb Ar2

Fig. 1. Trapping efficiency of filters during ablation of (A) NIST 610 Glass and (B) copper coin. (Bars represent mean -+ SD (N = 3) integrated intensities of elements during ablation of the samples, with and without filters in-line. Mean percentage retention indicated on the top of each bar.)

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Filters containing the ablation products, and blank filters, were dissolved with 10 ml a m m o n i a (20%) and heated to dryness in c o v e r e d Teflon beakers. Thereafter, the glass and copper samples were brought into solution with different procedures for each material. For the glass fragments and the samples containing glass ablation products, 10 ml of a 10:1:1 mixture of H F : H C I : H C I O 3 was added and heated to dryness, followed by 1 0 m l of H 2 0 : H C I : H C 1 0 3 (6:2:1), also taken to dryness. The sample was brought back into solution with 10 drops HC1, 1 ml H N O 3 and 1 drop HF, and finally m a d e up to 10 ml v o l u m e with D.D. water. The N I S T c o p p e r fragments and filters containing copper ablation products were treated with 2 ml HCI and gently heated to dryness. 5 ml of aqua regia was added and reduced o v e r heat to a p p r o x i m a t e l y 1 ml, f o l l o w e d by 1 drop of HF, and the solution heated for a further 10 min. E a c h sample was brought up to 10 ml final v o l u m e with D.D. water. All solutions were spiked with Ru (10/xg 1-~ final concentration) as an internal standard.

For digest solution analyses, Elan 5 0 0 0 A instrument settings were o p t i m i s e d against a 50/~g 1-~ solution of Mg, Ru and Pb. Analyses o f filter and fragment digests were calibrated with a series of m u l t i - e l e m e n t standard solutions, the accuracy o f which were c h e c k e d regularly against N I S T 1643d Trace Elements in Water. C o p p e r concentrations in the N I S T Cu f r a g m e n t and filter digests were determined separately by I C P - O E S ( P e r k i n - E l m e r ' O p t i m a ' 3000) on a 1:5 dilution o f the original digests. Non-spectroscopic interferences were likely to be m i n i m a l because o f the low sample concentrations in the digests, while spectroscopic interferences w e r e minimised by selection of appropriate isotopes for the elements of interest (see above). R e c o v e r i e s of certified e l e m e n t s in the N I S T glass fragments were generally g o o d (Table 1). R e c o v e r y o f certified e l e m e n t s f r o m the copper fragments were reasonable, with the e x c e p tion o f Au (Table 2). H o w e v e r , since the unablated fragments and ablation products on filters were subj e c t e d to the same dissolution procedures, Au data

Table 1 Measured concentrations, certified and information values, and procedural recovery of certified elements in NIST 610 Glass fragments Element

Measured

Certified (Information)

Recovery (%)

Mn Co Ni Ga Rb Sr Nb Ag Te La Ce Nd Sm Eu Gd Dy Yb Hf Ta Au TI Pb Bi Th U

525 _+ 29 390 ± 23 464 ± 37 486 ± 25 452 ± 23 524 +_ 29 484 ± 27 257 ± 11 203 ± 12 402 _+ 12 424 _+ 21 412 ± 23 424 ± 21 407 + 23 416 ± 24 405 ± 25 422 +_ 20 434 ± 21 470 _+_26 31 _+ 3 56 ± 6 439 ± 6 372 ± 18 406 ± 19 438 _+ 25

485 ÷ 10 (390) 459 ± 4 (500) 426 _+ 1 516 + 1 (500) (254 ± 10) (500) (500) (500) (500) (500) (500) (500) (500) (500) (500) (500) (25) (62 ± 3) 426 ± 1 (500) (500) 462 _+ 1

108 101 106 102 103 95

Units in #g g-t DW; measured data given as mean ± SD of N = 5 replicates; certified values as mean ± 95% confidence limits.

P.M. Outridge et aL/Spectrochimica Acta Part B 52 (1997) 2093-2102

2097

Table 2 Measured concentrations, certified and information (in parentheses) values, and recovery of certified elements in NIST C1253 Phosphorized Copper fragments Element

Measured

Certified (Information)

Be

7.7 _+ 0.4 367 _+ 21 431 _+ 25 96.6 _+ 6.5 411 + 21 211 + 41 509 _+ 36 73.6 -+ 5.1 454 _+ 32 122 _+ 8 185 _+ 10 10.8 _+ 6.1 256 _+ 25 58.5 _+ 4.0

(12)

-

495 _+ 20 (500) 99.4 432 + 116 164 _+ 6 495 _+ 24 74 _+ 4 (470) 140 _+ 10 199 _+ 5 74 _+ 0.5 244 _+ 2 70 _+ 5

74 97 95 129 103 99 87 93 15 105 84

Co

Ni Cu (%wt) As Se Ag Cd Sn Sb Te Au Pb Bi

Recovery (%)

Units in/zg g-~ DW; measured data given as mean -+ SD of N = 5 replicates; certified values as mean -+ computed error limits.

from the fragments and filter products may be compared. 2.5. Data handling and statistical treatment Initially, it was hoped to ablate sufficient material onto the filters to determine sample weights reliably with a microbalance, and thus express element concentrations on a unit mass basis. However, the electrostatic charge on the post-ablation filters rendered this approach unsuccessful. Consequently, element concentrations in the original fragments and in the ablation products were normalised against a common element in order to quantify any fractionation effects. For copper samples, trace elements were ratioed against the matrix element (Cu), while data for glass samples were normalised against Hf, because the matrix element (Si) was eliminated during the HF digestion step. Hf was chosen because it is one of the most refractory elements present in the glass SRM, and likely to be less affected by volatilisation or zone refinement than other elements. The glass and copper data were also normalised against a common element, Co, in order to compare between matrix types. The absolute fractionation values calculated from normalised concentrations will change with the use of different normalising elements. However, so long as common elements are used for normalisation, comparisons of fractionation effects between laser energy levels, matrix types and elements are valid.

Normalised element concentrations were arc-sine transformed [8], and the concentrations in unablated SRM fragments statistically compared against those in the ablation products by means of a one-way analysis of variance. Significant differences between mean concentrations were determined with the Student-Newman-Kreuls test, and are stated in the text at P < 0.05. For each element, ablative fractionation was calculated as the percent difference in Hf-, Cu-, or Co-normalised concentrations between the original SRM fragments and ablation products at the sample cell outlet. Transport fractionation was calculated as the percentage difference in concentration between ablation products at the cell outlet and torch inlet. Overall fractionation was the difference between the original SRM and ablation products at the torch inlet. 3. Results 3.1. NIST 610 GLASS Table 3 presents Hf concentrations and Hf-normalised concentrations of 20 elements in the unablated fragments and ablation products of NIST 610 Glass. In Fig. 2, the Hf-normalised concentrations are presented as calculated fractionation data, with fractionation values representing the percent change in concentration due to ablation, transport, or overall

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Table 3 Concentrations of Hf, and Hf-normalised concentrations of 20 other elements, in digest solutions of the ablation products and unablated fragments of NIST 610 Glass SRM Elements

Glass SRM fragments (N= 5)

Hf (/xgl -t) 30 + 3.8 Hf-normalised element concentrations Co 0.90 ± 0.04 Ga 1.12 ± 0.05 Rb 1.04 ± 0.04 Nb 1.12 _+ 0.04 Ag 0.59 ± 0.02 Te 0.47 ± 0.02 La 0.93 ± 0.03 Ce 0.98 ± 0.03 Nd 0.95 ± 0.02 Sm 0.98 ± 0.01 Eu 0.94 ± 0.02 Gd 0.96 ± 0.02 Dy 0.93 _+ 0.03 Yb 0.97 ± 0.02 Ta 1.08 ± 0.02 Au 0.07 ± 0.01 T1 0.13 ± 0.01 Bi 0.86 ± 0.02 Th 0.94 _+ 0.00 U 1.01 ± 0.01

70 J, cell outlet (N= 4)

40 J, cell outlet (N = 5)

70 J, torch inlet (N= 5)

40 J, torch inlet (N= 4)

1.67 ± 0.33

0.61 ± 0.06

0.84 ± 0.34

0.59 ± 0.20

0.86 ± 0.09 1.08 _+ 0.11 0.96 ± 0.03 1.06 ± 0.06 0.64 _+ 0.11 0.90 ± 0.32 0.82 ± 0.0.7 0.98 ± 0.10 0.86 ± 0.05 0.92 + 0.06 0.88 ± 0.07 0.90 ± 0.08 0.90 _+ 0.06 0.92 ± 0.07 1.06 ± 0.07 0.66 ± 0.18 0.13 _+ 0.01 1.19 _+ 0.35 0.95 ± 0.11 0.98 -+ 0.02

0.82 ± 0.08 1.11 +_ 0.16 0.95 _+ 0.20 1.05 ± 0.07 0.49 ± 0.16 0.60 + 0.28 0.76 ÷ 0.13 0.88 _+ 0.08 0.84 ± 0.16 0.90 ± 0.10 0.90 ± 0.12 0.87 ± 0.15 0.89 ± 0.07 0.94 ± 0.11 1.01 ± 0.13 0.43 ± 0.10 0.12 _+ 0.03 1.03 ÷ 0.57 0.92 _+ 0.17 1.01 ÷ 0.06

1.02 ± 0.10 1.33 ± 0.10 1.26 _+ 0.09 1.19 ± 0.06 0.89 + 0.20 1.22 ± 0.55 0.94 ± 0.01 1.03 ± 0.04 0.96 ± 0.04 1.02 ± 0.02 0.99 ± 0.04 1.06 ± 0.12 0.94 ± 0.01 1.00 ± 0.05 1.19 ± 0.08 1.01 ± 0.61 0.15 ± 0.03 1.02 ± 0.28 0.95 ± 0.10 1.08 ± 0.03

1.10 ± 0.19 1.16 ± 0.32 1.17 _ 0.29 1.19 ± 0.23 1.08 ± 0.30 0.59 ± 0.21 0.89 ± 0.10 0.96 ± 0.06 0.91 +_ 0.14 0.93 ± 0.17 0.90 ± 0.15 0.91 ± 0.07 0.86 ± 0.13 0.88 ± 0.12 1.10 ± 0.16 0.50 ± 0.21 0.13 +_ 0.01 0.72 ± 0.17 0.96 ± 0.05 1.05 ± 0.05

Data shown as mean ± SD of five replicate fragments of the original SRM, and 4-5 replicate filters at each laser energy/filter position. f r a c t i o n a t i o n . M o s t e l e m e n t s , a n d p a r t i c u l a r l y the l a n t h a n i d e s , s h o w e d n o e v i d e n c e o f significant fract i o n a t i o n , since t h e i r c o n c e n t r a t i o n s in the a b l a t i o n

70 J. T h e m o s t v o l a t i l e e l e m e n t s e x h i b i t e d the r e v e r s e pattern, i.e. for A u a n d Te, a b l a t i o n w a s u s u a l l y a g r e a t e r s o u r c e o f f r a c t i o n a t i o n t h a n transport. A sec-

p r o d u c t s w e r e w i t h i n 10% ( n o s i g n i f i c a n t d i f f e r e n c e s ) o f t h o s e in the o r i g i n a l S R M . H o w e v e r , a n u m b e r o f low-to-intermediate melting point/boiling point (MP/ B P ) e l e m e n t s (Au, Ag, Bi, Co, Rb, Te) w e r e s u b j e c t to r e l a t i v e l y large o v e r a l l f r a c t i o n a t i o n , w i t h c o m p o s i tional c h a n g e s o f 3 0 % or m o r e d u r i n g a b l a t i o n a n d / or transport. Au a n d T e were the m o s t affected, w i t h o v e r a l l f r a c t i o n a t i o n v a l u e s o f 1 3 4 0 % a n d 160%, respectively, at 7 0 J laser i n p u t e n e r g y (a fluence o f 1.35 × 104 W c m 2). O t h e r e l e m e n t s ( L a a n d U ) disp l a y e d statistically s i g n i f i c a n t c o m p o s i t i o n a l c h a n g e s , h o w e v e r , the m a g n i t u d e o f f r a c t i o n a t i o n was relatively s m a l l ( < 13%). O n e n o t e w o r t h y feature o f the data w a s the g e n e r ally larger m a g n i t u d e o f t r a n s p o r t f r a c t i o n a t i o n c o m p a r e d to a b l a t i v e f r a c t i o n a t i o n for the m a j o r i t y o f e l e m e n t s , a l t h o u g h the c o n c e n t r a t i o n c h a n g e d u e to t r a n s p o r t f r a c t i o n a t i o n w a s o n l y statistically signific a n t for C o a n d A g at 4 0 J a n d Ag, L a a n d U at

o n d feature w a s the h i g h level o f v a r i a n c e in the data for e l e m e n t s e x h i b i t i n g the g r e a t e s t o v e r a l l f r a c t i o n a tion. T h e r e l a t i v e s t a n d a r d d e v i a t i o n s ( R S D = S.D./ m e a n x 1 0 0 % ) o f c o n c e n t r a t i o n s in a b l a t i o n p r o d u c t s r a n g e d f r o m 1 7 - 6 0 % ( m e d i a n o f 3 2 % ) for Au, Ag, T e a n d Bi, c o m p a r e d to 1 - 2 7 % ( m e d i a n o f 9 % ) a m o n g all o t h e r e l e m e n t s (see T a b l e 3). T h i r d l y , t h e r e w a s a m a r k e d l y g r e a t e r level o f a b l a t i v e a n d o v e r a l l fraction a t i o n o f the v o l a t i l e e l e m e n t s , e x c e p t Ag, at h i g h e r laser p o w e r .

3.2. N I S T C1253 Copper T a b l e 4 c o n t a i n s the C u c o n c e n t r a t i o n s a n d C u - n o r m a l i s e d c o n c e n t r a t i o n s o f 13 trace e l e m e n t s in C o p p e r S R M f r a g m e n t s a n d a b l a t i o n p r o d u c t s , w h i l e Fig. 3 p r e s e n t s the c o r r e s p o n d i n g f r a c t i o n a t i o n values. Significant a b l a t i v e f r a c t i o n a t i o n o c c u r r e d for a m a j o r i t y o f the certified e l e m e n t s in this S R M , w i t h Au, Be, Se,

P.M. Outridge et al./Spectrochimica Acta Part B 52 (1997) 2093-2102

2100 Table 4

Concentrations of Cu, and Cu-normalised concentrations of 13 other elements, in digest solutions of the ablation products and of unablated fragments of NIST C1253 Copper SRM Elements

Copper SRM fragments (N = 5)

70 J, cell outlet (N = 4)

40 J, cell outlet (N = 4)

70 J, torch inlet (N = 4)

40 J, torch inlet (N = 3)

Cu (/~g ml ')

31 _+ 5.3

2.40 _+ 0.66

1.84 +_ 0.41

3.63 ± 0.89

1.41 ± 0.24

0.94 5.50 17.0 8.51 13.1 9.47 1.20 8.98 1.16 9.76 2.66 2.50 1.63

0.37 5.52 14.5 7.49 12.0 7.14 1.10 9.29 0.98 6.67 1.94 2.26 1.14

1.06 ± 0.14 5.43 + 0.62 9.91 _+ 0.66 10.81 + 0.8 17.4 ± 6.1 10.3 ± 2.9 1.16 ± 0.22 7.63 ± 0.58 1.16 ± 0.18 12.72 ± 4.5 2.87 _+ 1.34 1.81 _+ 0.17 1.40 ± 0.21

0.87 7.04 16.6 8.77 14.5 10.1 1.64 9.58 1.06 10.8 1.94 4.07 1.64

Cu-normalised element concentrations ( x 10 -4) Be Co Ni As Se Ag Cd Sn Sb Te Au Pb Bi

0.08 3.84 4.50 4.31 2.23 5.34 0.77 4.75 1.28 1.93 0.13 2.67 0.61

± 0.01 ± 0.38 _+ 0.46 _+ 0.44 _+ 0.57 _+ 0.65 +0.89 _+ 0.46 _+ 0.14 _+ 0.18 ± 0.02 ± 0.20 + 0.04

± 0.22 + 0.47 ± 0.8 ± 1.21 _+ 1.6 _+ 0.99 _+ 0.27 _+ 1.62 _+ 0.17 _+ 1.11 ± 0.48 ± 0.61 + 0.32

- 0.08 ± 1.75 _+ 2.6 _+ 1.51 _+ 2.9 _+ 0.38 ± 0.04 _+ 1.18 ± 0.18 ± 0.30 ÷ 0.40 _+ 0.63 ± 0.07

+_ 0.34 _+ 1.64 _+ 0.4 ± 0.89 ± 4.0 ± 2.1 _+ 0.70 ± 1.36 _+ 0.06 +_ 2.7 ± 0.54 ± 1.85 _+ 0.20

Data shown as mean _+ SD of five replicate fragments of the original SRM, and 3 - 4 replicate filters at each laser energy/filter position.

2000 ~ A) 70J

1200-~ ~1600 ~ 40o

Ablation~ Transport ~ Overall **1I

•

t

-200

,

2ooo : B) iOJ

,

~

,

,

,

,

,

,

,

. . . . . . .

1600 /

600-

4OO

0

u -

Be Co Ni As Se Ag Cd Sn Sb "re Au Pb BI Fig. 3. Element fractionation during the ablation of NIST C1253 Phosphorized Copper, at (A) 70 J flashlamp input energy (1.35 × 104W cm-2), and (b) 40 J energy (0.62 x 104W cm-2). (Element concentrations normalised against Cu. See Fig. 2 for details.)

of the zone of bulk ablation (for example, Ref. [9]), and zone refinement, in which elements within the molten zone surrounding the ablating crater undergo selective migration into or away from the ablation spot, resulting in altered concentrations in the sample stream [4,10]. For NIST Glass, it is likely that oxides or silicates are the dominant form of most elements, because of the percentage level concentrations of Si in the SRM, and the absence of high concentrations of other possible reactive ligands. Ablative and overall fractionation data from the 70 J ablation series were plotted against a number of possible predictors of fractionation, including element BP, element oxide MP and BP, and element silicate MP. Data from the 70 J series was chosen because the ablative fractionation effect is more pronounced at this energy level. The most coherent pattern was found with element oxide MP (OMP), with an inverse logarithmic relationship between ablative and overall fractionation and oxide MP (Fig. 4). The interval of OMPs between 1000 and 1500°C appear to mark an inflection point, with elements possessing OMPs > 1500°C generally agreeing to within 10% of the concentration in the original SRM, while most elements with OMPs < 1000°C were fractionated by > 20%. Chan et al. [1] also reported an inverse relationship between fractionation and OMP for four elements. These associations with

P.M. Outridge et al./Spectrochimica Acta Part B 52 (1997) 2093-2102

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Co Ga Rb Nb Ag To La Co NdSm Eu Gd Dy Yb Ta Au TI Bi Th U Fig. 2. Element fractionation during the ablation of NIST 610 Glass, at (a) 70 J flashlamp input energy ( 1.35 × 10 4 W cm 2), and (b) 40 J energy (0.62 × 104W cm-2). (Significant differences refer to differences in Hf-normalised concentrations; *, significant at P < 0.05. Ablative fractionation, percentage difference between original SRM composition and ablation products at sampling cell outlet. Transport fractionation, percentage difference between ablation products at cell outlet and torch inlet, 2.6 m distant. Overall fractionation, percentage difference between original SRM and ablation product at torch inlet).

Te and Ni displaying > 200% increases in concentration as a result of 70 J ablation, compared to the original SRM fragments. These elements also displayed significant or marked fractionation during transport ( > 100% concentration changes). As with NIST Glass, the magnitude of fractionation was generally greater at the higher laser power, with the exception of Be during transport. A comparison of the fractionation values across matrices, using Co-normalised data on four common elements (Table 5), showed roughly similar elementspecific fractionation patterns in each. However, the degree of ablative and overall fractionation was approximately 50-100% greater in NIST Copper than NIST Glass. Transport fractionation was similar in both. Co-normalisation of the data also confirmed the earlier finding that reduced fractionation was a feature of ablation at 40 J compared to 70 J in both matrices.

4. Discussion

These results provide unequivocal evidence regarding the magnitude of elemental fractionation during ablation and transport, and of quantitative differences in fractionation related to laser fluence levels, elements and matrix types. In some cases, the effects of ablative and transport fractionation were confounded, cancelling each other out and producing lower or negligible overall fractionation. This finding illustrates the value of separately quantifying ablative and transport fractionation, which was achieved by trapping the ablation product at different points in the sample introduction line. These data may be used to test hypotheses about the mechanisms underlying both ablative and transport fractionation. Two different mechanisms are currently proposed to account for ablative fractionation: volatilisation of non-refractory elements from sample areas outside

P.M. Outridge et al./Spectrochimica Acta Part B 52 (1997) 2093-2102

2101

Table 5 Fractionation values from Co-normalised data for elements common to NIST 610 Glass and NIST C1253 Copper Element

NIST 610 Glass

NIST C1253 Copper

Ablation

Au Ag Bi Te

Transport

Overall

Ablation

Overall

70 J

40 J

70 J

40 J

70 J

40 J

70 J

40 J

70 J

40 J

70 J

40 J

883 14 47 108

570 -13 58 75

47 47 23 32

0 8 -29 - 12

1341 68 80 174

570 -6 12 54

1264 26 81 267

918 0 30 166

34 9 -12 27

-21 10 11 27

1726 37 59 366

700 10 43 238

melting point are more consistent with zone refinement rather than volatilisation as the main ablative fractionation mechanism, because refinement putatively involves elements that are miscible in the molten portion of the ablating sample [4]. A previous study under nearly identical laser fluence conditions to those used here found high-concentration particulates of Au, Ag, Bi, Cu, Fe, Ni, Pb and/or Zn, either singly or in combination, in the ablation product from NIST 610 Glass and mammal tooth sections [3]. Several lines of evidence, especially the size and spatially-heterogeneous chemistry of the particles, suggested, as in the present study, that zone refinement, rather than volatilisation, was the most likely cause of this phenomenon. For NIST Phosphorised Copper, there was no clear relationship between the amount of fractionation and any single property of an element 1

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Element oxide M.P. (oc) Fig. 4. Ablative and overall fractionation as a function of element oxide MP in NIST 610 Glass. (Using data for 70 J ablation series. To facilitate plotting on a log scale, negative fractionation values have been set to a constant value of 0.1 I.)

or element compound, in part because of uncertainty about which element compounds are present, since the SRM composition and manner of manufacture suggested that elements may possibly be present as alloys with Cu, oxides, phosphates or phosphides, or in elemental forms. A key result of this study was quantification of transport fractionation. As a source of fractionation, mass transport was smaller in magnitude than ablation for most elements in NIST Copper, but greater than ablation for all except the most volatile elements (Au and Te) in NIST Glass. Because the latter data are based on Hf-normalised element concentrations, the existence of positive transport fractionation (concentration increases) indicates a decrease in Hf relative to other elements during transport. This result, together with consideration of the likely relative volatilisation efficiencies of Hf and other elements, suggests a possible transport fractionation mechanism, namely differential partitioning between vapour and particulate phases as a function of element volatility. Hf, being one of the most refractory elements present in NIST Glass, is not likely to occur in vapour form in the ablation stream to the same degree as other less refractory elements; rather, it should occur mainly in particulates. Other elements may occur in varying proportions of vapour and particulates, with particulates predominant in mass for all except the most volatile elements. It is proposed that refractory and less refractory elements may change proportionately to each other during transport as a result of particle deposition, with relatively greater loses of the more refractory (particulate-bound) elements. Adsorption or condensation of atomic vapours in the transfer tubing occurs during transport. However, mass transport calculations demonstrate that gravitational deposition, especially of particles > 5/,m, is the dominant cause

2102

P.M. Outridge et al./Spectrochimica Acta Part B 52 (1997) 2093-2102

of loss of material [ 11 ]. Several predictions regarding transport fractionation during ablation of glass may be made, based on this hypothesis. *

•

•

Refractory elements similar in MP or BP to Hf should show virtually no evidence of transport fractionation, because they are similarly found predominantly as particulates. As Fig. 2 shows, Th and Dy (the next most refractory elements after Hf) display the smallest transport fractionation of 20 elements. Ablation with higher laser energies should cause greater transport fractionation because of the potential for greater amounts of vapour to be generated. This statement is true for the majority of elements (see Fig. 2). The volatile elements most susceptible to ablative fractionation should display the greatest rates of transport fractionation. This result was demonstrated at both energy levels.

The data also allow testing of a prediction that greater laser power would reduce ablative fractionation, because it reduces the molten volume of sample relative to the bulk ablation volume [4]. Comparison of data from the 40 J (0.62 x 10 4 W s cm -2) and 70 J (1,35 × 10 4 W cm -2) ablation series showed for some elements that the reverse pattern occurs: higher fluence is more prone to fractionation effects during ablation and/or transport for Au, Ag, Bi and Te in glass, and for Au, Be, Bi, Ni and Te in copper. Nickel, Sn and Pb in copper showed less overall fractionation at higher power, in agreement with Ref. [4]. Chan et al. [ 1] also reported data contradicting the prediction. Excimer and Nd:YAG lasers operating at similar ultraviolet wavelengths were used to ablate a common superconducting target, with the Nd:YAG producing a 100-fold higher fluence on the sample surface. No difference in the extent of overall Bi and Cu fractionation was observed. One possible explanation is that, contrary to prediction, the volume of molten sample around an ablation crater is smaller at lower energy levels, with consequently less zone refinement

and volatilisation. The explanation is corroborated by the finding that a heat-conductive target (copper) gave rise to greater ablative and overall fractionation than a non-conductive matrix (glass). Ablation of conductive matrices like copper and other metals may lead to higher temperatures in areas surrounding the ablation site, thereby producing proportionately more refinement or volatilisation than in non-conductive targets. The difference between matrices also indicates that non-matrix-matched standardisation, as proposed by Feng [12], is unlikely to yield accurate calibration of LA-ICP spectrometric analyses of at least some elements in some sample/standard matrix combinations.

Acknowledgements Financial support was provided through the Industrial Partners Program between Perkin-Elmer (Canada) Ltd. and the Geological Survey of Canada. This paper is GSC Contribution No. 1996427.

References [1] W.-T, Chan, X.L. Man, R.E. Russo, Appl. Spectrosc. 46 (1992) 1025. [2] B.J. Fryer, S.E. Jackson, H.P. Longerich, Can. Mineral. 33 (1995) 303. [3] P.M. Outridge, W. Doherty,D.C. Gregoire, Spectrochim. Acta Part B 51 (1996) 1451. [4] E.F. Cromwell, P. Arrowsmith, Anal. Chem. 67 (1995) 131. [5] J.W. Hagar, Anal. Chem. 61 (1989) 1243. 16] P.M. Outridge, R.D. Evans, J. Anal. At. Spectrom. 10 (1995) 595. [7] Y. Huang, Y. Shibata, M. Morita, Anal. Chem. 65 (1993) 2999. [8] R.R. Sokal, F.J. Rohlf, Biometry,2nd edn., W.H. Freeman and Co., 1981, pp. 417-428. [9] S. Chenery, A. Hunt, M. Thompson, J. Anal. At. Spectrom. 7 (1992) 64% [10] E.F. Cromwell, P. Arrowsmith, Appl. Spectrosc. 49 (1995) 1652. [ 11] P. Arrowsmith,S.K. Hughes,Appl. Spectrosc.42 (1988) 1231. [12] R. Feng, Geochim. Cosmochim.Aeta 58 (1994) 1615.