Ion microprobe trace-element analysis of silicates ... · Si also has a broader energy distribution than would be suggested by the Ge-Sn-Pb tie line. The Zr ÷ distribution is substantially

Chemical Geology, 83 (1990) 11-25 11 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Ion microprobe trace-element analysis of silicates: Measurement of multi-element glasses

R.W. Hinton Department of Geology and Geophysics, Grant Institute, University of Edinburgh,

Edinburgh, Scotland, EH9 3JW (Great Britain)

(Accepted for publication January 30, 1990)

ABSTRACT

Hinton, R.W., 1990. Ion microprobe trace-element analysis of silicates: Measurement of multi-element glasses. In: P.J. Ports, C. Dupuy and J.F.W. Bowles (Guest-Editors), Microanalytical Methods in Mineralogy and Geochemistry. Chem. Geol., 83:11-25.

At present, ion microprobe determination of trace elements in silicates is dependent upon the availability of homoge- neous well-characterised standards of similar major-element composition to the unknown. Our ability to predict yields where no standard is available and finally move towards standardless quantitative analysis requires modelling of ion yield behaviour. A table of ion yield data for low- and high-energy ions of > 60 elements measured in the NBS 610 standard glass is presented, and includes ion yields for doubly charged and oxide ions. The variations in ion yield with atomic number give relatively smooth patterns especially for high-energy ion yields. Notably, a periodic relationship exists in ion yield behaviour. Variations which occur under different analytical (beam density?) conditions are related to the mass of the ion. Corrections can be applied to remove these effects. Unfortunately Si +, while being the most obvious element to use for normalisation in silicates to remove operator/laboratory artifacts, is one of the most sensitive elements to changes in analytical conditions.

1. Introduction

It has been demonstrated that the ion microprobe can give quantitative analyses of silicates if an unknown sample is analysed with reference to a standard of similar major-element chemistry (Reed, 1989). However, no physical model has yet been proposed which adequately describes secondary ion generation nor are there any empirical laws to permit standardless trace-element analyses of un- knowns. Further, no consistent method has been used (or is always practical) for trace- element analysis in different laboratories. In silicates, ion yields are usually normalised to that of Si ÷ to remove some of the effects of

ondary ion transmission. Analyses are also variations in primary beam current and sec- often made using energy filtering both to sup- press molecular ion species and to reduce secondary ion tuning and matrix artifacts. Few papers report ion yield information for large numbers of elements; therefore, no intercom- parison of matrix or laboratory artifacts can readily be made. Work is under way (R.W.H., in prep. ) in this laboratory to measure a vari- ety of mineral and glass standards to give a data base of ion yields (principally at high ion energy) such that methods of quantitative analysis can be explored. As part of this project measurements have been made on NBS 610 glass (which is doped with ~500 ppm of 61 elements) using a Camrca ® ims-4f instrument.

0009-2541/90/$03.50 © 1990 Elsevier Science Publishers B.V.

12 R.W. HINTON

2. Secondary ion energy distributions

The absolute count rate and ratios of one element to another are in part dependent on the energy of the ions recorded. A slit located after the electrostatic sector determines the range of energies accepted by the mass spectrometer. The positioning of the energy "window" in relation to the secondary ion energy distribution can be varied by changing the ac- celerating voltage applied to the sample (the absolute energy of the accepted ion being constant ). Most ions leave the sample surface with an energy of 0-20 eV; however, some have energies of > 100 eV. The decrease in count rate with increasing energy is rapid; however, since molecular ions decrease much more rapidly than elemental ions, energy filtering is often used to discriminate against these species. Large variations exist between the energy distributions of different elements (Rudat and Morrison, 1979 ); therefore, if the energy window does not permit passage of ions of all energies, some fractionation of one element relative to another must occur. To illustrate these effects, analyses were made on borosilicate glasses in which the high level (0.7 wt.%) of the doped element made measurement with

a very narrow energy window feasible. While not giving identical absolute ion yields to the NBS glass the ion yield patterns are very similar.

The effect of ion energy distribution is best illustrated by comparison of alkali ion yields with those of Si, since the energy distribution o fK ÷ is narrow compared to Si ÷ (Fig. 1 ). The measured K+/Si ÷ ratio is dependent on both the width and the position of the energy window. The K ÷/Si + ratio decreases rapidly as the energy increases from 0 to 50 eV (Fig. 1). Above this value the ratio becomes less sensitive to small changes in energy. If the energy window is set at 1 eV the maximum K+/Si + ratio is 65. If the window is large (200 eV), and essentially all ions are recorded, this ratio falls to 25 (Fig. 2 ). Thus, depending on the energy window used any ratio between these two values could be measured. If a very narrow window is selected ion yields increase from K ÷ to Rb ÷ to Cs ÷ as might be expected from the decrease in ionisation potential from K to Cs (Storms et al., 1977 ). However, since Cs has a narrower energy distribution than K and Rb if ions of all energies are considered, the apparent ion yield for Cs ÷ becomes similar to that of Rb ÷ and K ÷. Similarly, the low-energy ion

10 5

10.4

t ,3 I'-" z 10 3 0

10 2

10 1 -20

A K ÷

0 40 80

/ a

1.0

I

120 -20 0 120 1 I I I I

40 80

ENERGY (eV)

100

x,"

10 r ~ ....1 i i i

) -

z o

Fig. 1. Energy dis t r ibut ions for Si + and K + (A) and change in K ÷ /S i + ratio with energy (B) for borosil icate glass. Energy window ~ 1 eV.

I O N M I C R O P R O B E T R A C E - E L E M E N T A N A L Y S I S O F S I L I C A T E S 13

100

0 I--

LIJ > l--

.J W

0 _I W

Z

10

1.0

0-I

, I01~ , ~ I O N S

K Co Ti V CrMnFeCoNi Cu Fig. 2. M+/Si + ratios for period-4 elements in borosili- care glasses measured for low-energy (0 + 1 eV), high-energy (77 + 19 eV) and for all ions ( - 8 0 to + 120 eV).

yields for the alkalis measured on NBS 610 show that Rb > K = Cs; this order is in part due to differences in the energy distributions of these elements. Ion yields for period-4 elements in borosilicate glasses (for low-, total- and high-energy ions) also demonstrate the effects of changing the energy of the ions filtered into the mass spectrometer. The patterns of low-energy and all ions relative to Si ÷ (Fig. 2 ) are similar. However, since Si ÷ has a very broad energy distribution, all yields fall, relative to this element, as the energy window is increased. As the ion energy is increased, K + falls much faster than the other elements leav- ing Ca ÷ with the highest apparent ion yield. Ti ÷ with the broadest energy distribution only falls by a factor of 2.5 relative to Si ÷ compared to a factor of 100 for K +. In general, the pat-

tern of ion yields becomes smoother as energy is increased. The smooth curve through period-4 elements is common to all glasses and minerals measured in this laboratory. It fol- lows from the previous discussion that any change in an element's ion energy distribution between matrices can lead to a change in relative ion yields. Measurements at high mass resolution are often made with a narrow energy window to reduce ion optical aberrations caused by variations in the energy spread of ions entering the mass spectrometer. If a very narrow energy window setting is used, the range in energies, and changes in the position of the maximum of the energy distribution must also be considered. However, the total range of ion energy maxima measured in the borosilicate glasses using a 1-2 eV window (including B ÷, Mg ÷ , A1 +, Si + and elements in Groups Ia-IVa, periods 4 -6 ) is < 4 eV and this effect is therefore small.

100

÷ bl

o 10

a

g

No

Cs

i nQ"

Sb~ ~r.Ti ,Mo Lo

[ x \

Si Zr

,z £,t ~-£'~-.

.,sin

x

x

"K

1000

loo 9

z O

u

Th

0 . 1 , i , , , , , i . . . . . . . . . . 0 20 4.0 60 80

ATOMIC NUMBER

Fig. 3. Ratio of ion yields at low (0__ 19 eV) relative to high ( 77 _+ 19 eV) energy against atomic number for NBS 610 glass. The absolute change in intensity is given on the right-hand axis.

14 R.W. HINTON

Despite the above-mentioned difficulties in determining the effect of energy distributions the changes in intensity between low and high energy (Fig. 3 ) are reasonably systematic. The changes between low (0_+ 19 eV) and high (77-+ 19 eV) energy are shown normalised to Si + (left-hand axis) and as absolute ratios (right-hand axis ). The Si + count rate drops by a factor of 30 between low and high energy. Nearly all other metallic elements have narrower energy distributions and fall by greater amounts. The ions of the alkalis, the alkaline earths, Group-IIIb elements A1, Ga and In, and Group-IVb elements Ge, Sn and Pb all give linear arrays on this logarithmic plot, high- lighting the fact that regularities do occur in ion yields of elements with similar chemistries. Within each period, the general trend is for the elements with the greatest oxygen bond strength to have broadest energy distributions. Thus, in the period-4 elements, K with the weakest oxide bond strength has the narrowest energy distribution and Sc and Ti have the strongest oxide bond strengths and broadest distributions. The distributions also narrow as bond strengths decrease from Ti to Zn. The formation of ions occurs during the breaking of bonds especially oxygen bonds (Yu, 1987), and the energy distribution profiles may be a strong indication of the actual bond strengths in a complex matrix. Fluorine has an unusually narrow energy distribution due to formation of a significant number of F ÷ ions by electron desorption (Williams and Gillen, 1987 ). The lightest elements Li, Be and B have relatively broad energy distributions compared to higher-mass members of their groups. Si also has a broader energy distribution than would be suggested by the Ge-Sn-Pb tie line.

The Zr ÷ distribution is substantially broader than that of Ti +, Hf ÷ would therefore be expected to have a very broad energy distribution (Fig. 3 ). In the borosilicate glasses, Hfhad the broadest energy distribution of any element measured.

3. Analytical conditions

Measurements were made with a 5-30 nA 160- beam of 14.5-keV energy focussed to a ~ 20-35-/zm spot. Two sets of conditions were used: (1) high-energy secondary ions ( ~ 77 + 19 eV ); and (2) low-energy secondary ions (0_ 19 eV). Ion count rates were recorded on a 17-stage electron multiplier and were dead time corrected ( < 2% effect). Ho- mogeneity was tested by making step scans across the glass fragments. A scan for 7Li, 9Be, liB, 20Ca2+, 47Ti ' 885r ' 89y, 9OZr ' 93Nb ' 133Cs '

138Ba, 139La and 14°Ce gave variations only

slightly greater ( + 0.5%) than that expected from counting statistics for all elements with the exception of Li and B. 11B and 7Li had variations of ___ 3.2% and +_ 3.3% respectively where _+0.3 and + 1.0 would have been expected from counting statistics (all errors quoted are 1 tr). This is possibly due to inhomogeneity of these elements within the glass. A scan of the rare-earth elements (REE) (as REE 2+ ) also showed no variations greatly in excess of counting statistics.

3.1. High-energy secondary ions

Most ions have an energy of between 10 and 20 eV. While very few occur with energies of

10 6 AI ÷

105 100"/,_ - :

104.

~ 10 3

10 2

101

100' V ~,

2' ' ' 4.'0 'o ' ' - 0 0 20 6 0 100 120

ENERGY (eV)

Fig. 4. 27A1+ signal vs. the measured ion energy, with the energy window at 38 eV (A) and 1 eV (B). The selected energy window under energy filtering conditions is indi- cated in curve B.

ION MICROPROBE TRACE-ELEMENT ANALYSIS OF SILICATES 15

TABLEI

Ion yields for NBS 610 silicate glass

Element High-energy ion yields .( 77 + 19 eV)

(M+/Si+)H M2+/M+ M O + / M + % isotope in ( )< 10 -3) ( X 10 -2 ) peak measured

Low-energy ion yields (0 + 19 eV)

(M+/Si+)L M2+/M + (X10 -4)

Abundance (ppm)

Li 1.32 ~<0.008 Be 1.27 ~<0.15 B 0.576 ~<0.16 O 0.003 0.96 F 0.004 - Na 0.88 0.25 Mg 1.88 < 5.0 AI 2.18 1.1 Si 1.00 < 0.91 P 0.093 - S < 0.022* - CI 0.009 - K 1.14 ~<0.32 Ca 3.26 16 Sc 3.02 6.4 Ti 2.56 2.0 V 1.70 0.44 Cr 1.22 ~<0.50 Mn 1.05 ~< 0.20 Fe 0.83 ~< 1.6 Co 0.489 ~< 0.05 Ni 0.284 - Cu 0.146 < 0.30 Zn 0.043 - Ga 0.418 0.41 Ge 0.224 < 0.90 As 0.016 < 1.0 Se ~<0.011" < 50 Rb 0.98 0.78 Sr 2.90 24 Y 3.47 8.2 Zr 2.23 2.2 Nb 1.39 0.34 Mo 0.91 < 0.20 Ag 0.039 < 4.0 Cd ~<0.012" - In 0.216 1.2 Sn 0.236 ~< 3.0 Sb 0.026 < 3.0 Te 0.004* < 90 Cs 0.66 2.6 Ba 2.04 32 La 2.27 17 Ce 2.21 - Pr 2.47 15 Nd 2.59 13 Sm 2.78 13 Eu 2.92 14 Gd 2.75 9.3 Tb 2.50 8.1

1.3

0.028 0.27 - 88

0.35 0.64 -

1 . 1

- 20

- 76

- 87 - 61

< 0.024 1.4 3.8 5.3 4.8 1.8

~< 5.4 39 < 18 22

~< 0.054 -

- 32

4.9 19 20 16 13

12

6.74 <0.001 0.946 0.17 0.446 < 0.060 0.001 3.7 0.041

19.2 0.072 4.99 3.6 4.65 2.5 1 . 0 0 5.9 0.043 ~< 2.1

0.010" ~< 1.7 29.7 0.061

9.96 150 5.44 45 4.81 9.9 3.59 1.3 3.64 < 2.6 3.23 2.63 1.66 0.24 0.86 0.60 0.058

< 0.22* 2.35 4.0 1.32 ~< 13

~<0.02"

33.0 0.22 1 1 . 2 3 1 0

4.49 93 1.74 21 1.12 1.9 1.55 <4.2

2.72 0.35 1 . 5 0 2.4 0.060* < 10

28.8 0.48 10.7 630 4.01 270 3.96 4.38 290 4.69 260 5.87 180

500 500 351

45.2% 500

10.1% 500

1 . 0 3 %

32.7% 500 500 500 461

8.36% 500 437 500 500 485 458 390 459 444 433 500 500 500 500 426 512 411 500 500 500 254 500 500 500 500 500 500 500 514 524 500 505 500 500 500 500

16

T A B L E I (continued)

R.W. HINTON

E l e m e n t H i g h - e n e r g y i o n y i e l d s ( 7 7 + 19 e V ) L o w - e n e r g y i o n y i e l d s ( 0 + 19 e V )

( M + / S i+ ) n M 2 + / M + M O + / M + % i s o t o p e in ( M + / S i + )L M 2 + / M + ( X 1 0 - 3 ) ( X 1 0 - 2 ) p e a k m e a s u r e d ( X 1 0 - 4 )

A b u n d a n c e

( p p m )

D y 2 . 5 0 7 .4 9 .2 - - - 5 0 0

H o 2 . 5 3 5 .6 . . . . 5 0 0

E r 2 .45 4 .4 7.8 - - - 5 0 0

T m 2 . 4 2 3 .9 5.5 - - - 5 0 0

Y b 2 . 4 7 3.5 4 .2 - - - 5 0 0

L u 2 .05 2 .6 6 .8 - - - 5 0 0

H f 1.29 5.6 16 86 - - 5 0 0

T a 0 .71 0 . 8 9 - 80 - - 5 0 0

W 0 . 3 8 0 . 5 8 - 71 - - 5 0 0

R e ~< 0 . 0 3 0 - - 4 4 - - 5 0 0

TI 0 . 0 5 5 * . . . . . 5 0 0

P b 0 .061 < 5 .0 - - 0 . 6 2 2 5 0 4 2 6

Bi ~<0.02" < 3 .0 . . . . 5 0 0

T h 1.36 16 - - 0 .91 - 4 5 7

U 1.34 8.1 - - 0 . 9 7 68 461

- = n o t d e t e r m i n e d o r < 10% in " % i s o t o p e in p e a k m e a s u r e d " c o l u m n ( a s n o t e d i n t e x t ) .

* T o t a l c o u n t s r e c o r d e d fo r i s o t o p e m e a s u r e d < 500 .

< 10 e V a significant proportion of ions have energies of > 20 eV (Fig. 4). The positioning of the energy window is essentially the same as that of Zinner and Crozaz ( 1986 ). The energy window position was initially set by closing the window to ~ 1 eV and physically moving it to obtain the max imum intensity for A1 + secondary ions (Fig. 4). The energy window was then opened to 38 eV. The sample voltage was raised until the A1 ÷ count rate fell to 10% of that observed at 0 eV. This accurately measures the position of the low-energy cut-off and estab- lishes the position of the energy window to within + 1 eV. The sample voltage is then set at 100 V below this value; the typical energy of ions recorded was + 77 eV with a window of + 19 eV. Measurement of ion yields of high- energy ions were made at low mass resolution by computer-controlled peak stepping through all masses where stable isotopes occur (211 masses). Peaks were measured for 10 s for each mass. Corrections for drift in intensity with time were made by fast switching through 14 selected masses from B to U. Corrections were

made for the presence of metal oxide (MO ÷ ) ions using simultaneous equations where ap- propriate. Doubly charged ions (M 2 ÷ ) were corrected using abundances determined at ½ unity mass. Masses where simple complexes of the major elements in the glass occur, e.g. CaNa, CaA1, were avoided. The corrections applied to the elements given in Table I were usually much less than 10%. Where corrections exceed 10%, the proportion of the element in the recorded peak(s) is given. Preci- sions are probably better than _+ 5%. The values in Table I are marked with an asterisk where counts recorded for the isotope(s) used for abundance measurements were < 500 counts. Since molecular ions form only very low levels of doubly charged species, ions at ½ mass units are almost exclusively doubly charged odd mass isotopes of elements. Intensities of the doubly charged positive ions (with the exception of Ca, Ce, Th and U) were measured for the most abundant odd mass for at least 30 s per mass. The 2 + ion intensities for Ca, Ce, Th and U were measured from the mass spec-


trum. Where 2 + peaks were not detected, the upper limit for Mg+/M ÷ given in Table I is based on an assumed 2 + count rate of 0.1 cps. Background is <0.005 cps.

3.2. Low-energy secondary ions

Ion yields were measured at low ion energy (0+ 19 eV) using high mass resolution (M/ AM~ 5000 ) to reduce molecular interferences. Count rates were recorded by manually maximising the intensity and counting for 3 s. In cases where the elemental peak was on the tail of a molecular peak, or where identification was not clear, the values are given as upper limits only. Elements which have significant unresolved molecular peak overlaps (e.g., heavy REE, HREE) were not measured. Where total counts were < 500, the analysis is marked with an asterisk. The difficulty in maximising sharp high mass resolution peaks produces larger errors than low mass resolution measurement; the precision of the low-energy measurements is probably _+ 10%. Since most ions occur at low ion energy, > 80% occurring within the energy window setting used (except Hf ), the tables of low-energy ion yields are very similar to total yields for ions of all energies. The 2 + ions were recorded manually at low mass resolution using the most abundant odd mass. Fe e÷ and Mn 2+ could not be recorded due to the presence of Si~ ÷ and SiAl 2÷, respectively.

4. Secondary ion yields

Ion yields relative to Si ÷, for low- and high- energy ions, together with 2 + / 1 + and M O + / M ÷ ratios are given in Table I. Also included are the abundances used for the ion yield cal- culations [NBS plus REE from Michael (1988 )], thus any new determinations of the NBS standard can be used to correct the ion yield data given here. The count rates, per nA, for Si ÷ at low energy under the above-men- t ioned conditions were 2.0- 10 4 cps %- 1 n A - l

and at high ion energy 7.0.104 cps %- z nA- z Plots of ion yields, relative to Si +, against atomic numbers for low- and high-energy ions are given in Figs. 5 and 6, respectively.

It is immediately apparent that secondary ion yields are periodic in nature. At low energies (essentially equivalent to the total ion yield over all energies) the alkali metals are the most intense species; ionisation for these elements may exceed 25% (Williams, 1985). Perhaps surprisingly, the relationship between low-energy ion yields is similar to yields that were ob- tained for pure metals (Storms et al., 1977). However, the ion yields for metals do have a much greater range than that of silicates.

Ionisation potentials have a similar periodic nature; however, the correlation between yield and ionisation potential is poor. It has been shown that analyt ical artifacts can lead to changes in relative ion yield (as for the alkali metals); however, these effects are too small to explain the lack of correlation. For instance, elements in the same period with similar ionisation potentials, such as Ca + and Ga +, have very different yields. Similarly, elements from the same group with similar ionisation potential such as Ti, Zr and Hf can also have very different yields. The correlation observed for ion yields of weakly bonded elements against ionisation potential in a silica matrix (Wilson and Novak, 1988) does not appear to apply to silicate glasses.

A relatively smooth pattern is observed for ion yields of the period-4 transition elements (Figs. 4 and 6). It should be pointed out that if ion yields were plotted against mass the smooth pattern would be lost. In particular, the smooth curve Fe--,Co--,Ni-,Cu would be bro- ken. In all minerals and glasses analysed in this laboratory smooth trends in ion yields have been observed for ions of Group-IV elements. This is despite absolute changes relative to Si ÷ and changes in Ca ÷ relative to Fe +. The general pattern for high-energy ions is a decrease in ion yields through the transition elements, an increase for the Group-IIIb elements, and a

18 R.W. HINTON

10 2 . . . .

101 Ca Bo

"C_ Mr~Fe -Co In u~ Z r l M o n

o G e 10 o _s~ Th~

L

10-I ,,J

I S b P

I F A, 1{~ 2 eCl

O} i I , , I i , . i I

ATOMIC NUMBER Fig. 5. Ion yields, relat ive to Si +, for low-energy ions (0 + i 9 eV) against a tomic number for NBS 610 glass.

101

10 0 2

n -

S LU > -

z 10 -1 o

~o-2

L, ~ e No

B

Ca ~Sc

si K'L ~ c~"

Ct

Zr

Sr~(~ Zr

Rb ~

.Ge

As

in# 'sn

AgiD

Sb

Eu

l

I

I I I

l l

Tle, BPb

L Th~

i I i i i i i I J I

ATOMIC NUMBER

Fig. 6. Ion yields, relative to Si +, for high-energy ions (7? _+ 19 eV) against atomic number for NBS 610 glass.

ION MICROPROBE TRACE-ELEMENT ANALYSIS OF SILICATES | 9

rapid decrease from Group IVb towards the inert gases.

While the variations within periods can be seen to be smooth no obvious relationship exists between periods. If the ions of Group-IV- VI elements are considered, as the period increases the ion yield relative to Si decreases. This decrease cannot be defined by a smooth curve which is common to elements from all groups. However, an empirically derived mass- dependent correlation can be applied to the ion yields of period-4-6 elements such as that a relationship between yields and atomic number can be established. The correction is:

(M+/Si+)coR =

(M+/Si+)MEAs[K+{(1-K)28}/m] (1)

where m = mass; and K is a constant (0.3). No simple function which is related to either

atomic number or mass alone has been found to give consistent relationships between ion yields of groups of elements.

A plot of corrected ion yields against atomic number (Fig. 7 ) demonstrates that despite artifacts due to ion yield measurement, ionisation potentials, partitioning between elemental and oxide ions, etc., the ion yields of period- 4-6 elements are systematic. Thus the ion yield of one element can be determined if two others of the group are known. This also holds where the mass ranges are great, as for Ti +, Zr ÷ and Hf ÷. The correction is most significant below mass 60, thus the Group-IIIb elements, Ga +, In ÷ and T1 +, lie close to linear arrays both before (Fig. 6) and after (Fig. 7) correction. However, the correction ion yields do lie closer to a linear array than the uncorrected yields. Table II gives ion yields calculated with eq. 1 and assuming a logarithmic relationship between corrected ion yield and atomic number for period-6 Group-I-Via elements (based on yields for Group-IV and -V elements). With the exception of Group-Il ia and -IVb elements, the fit is remarkably good. If the yields for the latter groups are considered it would

101

L~ o

,7',

'R, LU >-

Z 0

a LU

b w

o L )

100

1(~ 1

L I~ Mgq CQ

c, i ~-~Z ~ ~ - -~T0 • ~ ~ ~ Vlo ~0 : /

Ct

, l I 1o-2 1o 20 ~o

~." -. IVb

\ m--b~ = ' : 2 - . . Ag~ Ti~'PPb

Sb Re~ I i I I |

~0 50 60 70 8 0 ATOMIC NUMBER

Fig. 7. Corrected ion yields, relat ive to Si + (see text) against a tomic number .

Th~

i

90

20

TABLE II

Predicted ion yields assuming log relations between corrected ion yields and atomic number for periods 4-6

Group Known Unknown Calculated Observed

Ia K-Rb Cs 0.66 0.66 Ila Ca-Sr Ba 2.05 2.04 Ilia Y-La Sc 4.39 3.02 IVa Ti-Zr Zr 1.24 1.29 Va V-Nb Ta 0.72 0.71 Via Cr-Mo W 0.40 0.38 IIIb Ga-In TI 0.055 0.055 IVb Sn-Pb Ge 0.46 0.22

appear that the period-4 elements Sc and Ge are anomalous when compared to patterns of yields through periods. Sc ÷ would be expected to be greater than Ca +, and Ge + only slightly lower than Ga ÷. The anomalies could be caused by incorrect concentrations given for the glass. The ion yields for the period- 1-3 elements are not simply related to other elements in their groups. While ionisation potentials play some part in this, the electronic structure as manifested in the element's chemistry, would also appear to be important. The ion yield of AI + would appear to be closer to that of the Group-IIIa elements; similarly B + behaves more like Si ÷ rather than other members of its group. Li + might therefore be expected to be- have more like Mg + and Be ÷ like A1 +.

5. Ion yield variations

The ability to make standardless analyses is in part dependent on our ability to either control, or compensate for, machine artifacts. While the ion yields in Table I are recorded using the highest beam density (best focus) and are reasonably reproducible, variations still occur. When the primary beam is focussed onto the sample a large amount of oxygen is introduced and sputtering occurs. Since the ion yields are enhanced by the presence of oxygen, both that already in the sample and that introduced in the primary beam, the beam must be

R.W. HINTON

left on the sample long enough to permit the oxygen concentration to stabilise. Further, since each element may initially be sputtered at a different rate, the concentration at the surface will change until the ions sputtered become representative of the sample composition. The time to reach equilibrium may be different for each of these effects.

Observed variations in the NBS 610 ion yields with t ime probably reflect both sputtering rate and oxygen implantation artifacts. For example, when the primary beam starts sputtering a sample, the Si + ion signal stabilises very quickly and that of the alkaline earths much more slowly. Ion yield measurements made before the yields stabilise are low for all elements relative to Si + except for the alkali metals. When measurement is made after the beam has been on the sample for 3 min., compared with 30 min., the largest change is for the heaviest element U (U+/S i ÷ increases by 25%) and the lowest for AI (AI+/Si + increases by 4%). The ion yields for alkaline earths increase more than that for neighbouring elements (Sr+/Si + by 14% compared to Y+/Si + by 12% and Ba+/Si + by 21% compared to Ce +/Si + by 18%). Li +, despite being much lighter than Si+, also increases with t ime relative to this element. In general, once a stable signal is recorded the effects are largely related to mass and for neighbouring elements the difference is < 2%.

The alkali metals give higher ion yields at the surface which fall with time. This may in part be due to surface contaminat ion during polish- ing. However, images produced while the intensity is enhanced, reflect the underlying variations in these elements. The surface increase may be caused by a high sputter rate for these elements (their narrow energy distribution and low oxide bond strengths suggest that their binding energy is likely to be low) or the movement of alkalis due to charging. Pecul- iarly, the alkalies can be imaged during initial sputtering of the gold or carbon coat (prior to


its removal) perhaps due to charge driven movement from the sample below.

The concentration of oxygen in silicates is relatively constant; therefore, the amount of oxygen in the volume sputtered will be related to how much is introduced by the primary beam (beam density) and the sputter rate of the particular material. Deline and Evans ( 1978 ) and Deline et al. ( 1978 ) have demonstrated that as the sputter rate increases, the amount of oxygen which is present in a sample decreases. Therefore, the ion yield also decreases. Thus, even if two matrices have identical chemistries but sputter at different rates, the two will not give the same yields. Measure- ments made on pyroxenes and glasses of py- roxene bulk chemistry by Ray and Hart ( 1982 ) gave different relative ion yields for elements in the two matrices. It was suggested that this was due to differences in sputter rate between glasses and minerals. Analyses of a pure diop- side crystal and glass in this laboratory gave identical ion yields for Ca +, Mg ÷ and Fe ÷ relative to Si ÷ for both phases. It is questionable if the pyroxenes and glasses of Ray and Hart (1982) were sufficiently close in composition (especially in their Fe concentration) to permit the conclusion that anything, other than chemistry, caused the differences.

The sputter rate for a standard glass should be constant, therefore the day-to-day variations are probably due to the one parameter which is difficult to control precisely: the beam density. Individual operators focus the beam to different degrees of sharpness/density; fur-

TABLEIII

Variations in ion yield, relative to Si +, for Group-IIa elements under different analytical conditions

Mode Ca + Sr + Ba + + Ba(calculated )

Spot 3.26 2.90 2.04 2.05 Spot 3.14 2.78 1.93 1.94 Defocussed 3.06 2.67 1.77 1.83

spot 25-/tm raster 3.04 2.35 1.42 1.42

ther, as the source ages, the beam density usually decreases. Analyses of NBS 610 made on different days using spot, defocussed spot and raster modes for a limited number of elements gave large variations in ion yields relative to Si +. The largest variation was in the heaviest element analysed; a U+/Si ÷ ratio 54% lower than those given in Table I was measured. Variations in elements with low mass (between 20 and 50) did not exceed 15% in the same analysis. However, as with the variations observed during beam stabilisation, Li ÷ increased substantially ( ~ 25% ) relative to Si +. These mass-dependent artifacts related to beam density are best illustrated by the yields of Ca + , Sr ÷ and Ba ÷ relative to Si ÷. If the corrected ion yields (see p. 19 ) are plotted against atomic number the linear relationship between the alkaline earths is maintained; the slope defining this relationship simply increases as the oxygen content decreases. Therefore, the predic- tion of ion yields (Table III) is unaffected by these artifacts. Before comparison can be made between different matrices the variations given by a single matrix must be modelled, even if the cause of these variations is not fully understood. The best fit to observed variations in ion yields, relative to Si +, is given if the M+/Si + ratios change by a power law relative to the values given in Table I:

(M+/Si +)TABLE = [ (M+/Si +)MEAs] × 10 -mK (2)

where m is mass; and K is a constant. K can be determined using M+/Si + of any accurately measured heavy mass. If determined using U+/Si + then

K=238 -~ log[ 1.34-1 (U+/Si+)MEAS ] (3)

Similar expressions could be used for other elements. Eq. 3 is only valid for masses > 40. The increase in ion yields for elements below this mass (with the exception of Li) is a constant; it is essentially the same as that given for mass 40. The Li ion yield increases as if it were equivalent to mass 90. Thus changes in ion

22 R.W. HINTON

v)

LIJ >

n-

O U LIJ

>-

10 0

i0 "1

10-2

10--"

10-&

• B e

10-5

M2"/~"

• 0+_19eV • 77 t19eV

AI Mg ;"

t l i

No:

CG

I

Sc

Ti

V Ooe

•Co

S r

h I I I I FL

Rb

Zr

L Nb O, Sn

J i

eln

Cs

Bao , i r t

L t I

" Hf

W

ePb

U

lo 2'0 3o ~o s'o 60 7'0 8'0 90 ATOMIC NUMBER

Fig. 8. Ion yields of 2 + ions, relative to Si +, against atomic number for low- (0 + 19 eV) and high- (77 + 19 eV) energy ions for NBS 610 glass. The ion yield of the normalising elements (Si ÷ ) falls by a factor of 30 between low and high energy.

yields caused by different operating conditions can be quantified if a few elements are re- analysed.

6. Doubly charged (2+) ions

The doubly charged ions have similar energy distributions to elemental ions. The ratio of doubly charged ions to elemental ions is therefore not greatly changed by increasing the secondary ion energy (Table I). For most practical purposes these species cannot be discriminated against using energy filtering. A plot of 2 + ion yields for both low- and high- energy ions relative to Si +, against atomic number, is given in Fig. 8. Note that the absolute change in the count rates between M 2+ measured at low ion energy relative to high ion energy is 30 times the (Si normalised) difference shown in Fig. 8. The periodic nature of

the 2 + yields is apparent. The alkaline earths, having the lowest second ionisation potential, have the highest 2 + yields. Some of the 2 + yields are very high and could cause significant overlap in the mass spectrum. Ba 2+, in particular, has yields approaching that of Si +. The correlation of the 2 + ion yields with second ionisation potential, as with the 1 + ions, is qualitative and can only be used as a general guide. The alkalies, for example, have a very high second ionisation potential and would not be expected to yield recordable 2 + ion yields. However, the increase from K 2+ to Cs 2+ would be expected from the decrease in the second ionisation potential with increasing period for the alkalies.

Potentially all the REE, except Ce, could be measured as REE 2 + at 1 unity mass using the odd mass isotopes. Measurements can be made at low mass resolution and low energy since no

ION MICROPROBE TRACE-ELEMENT ANALYSIS OF SILICATES

TABLE IV

Ion yields for REE2+/Si+ measured at low (0+ 19 eV) and high (77 + 19 eV) ion energy

Element (REE2+/S i+ )L ( REE2+/S i+ )H (0+ 19 eV) (77+ 19 eV)

La 0.11 0.039 Ce (0.051 ) Pr 0.13 0.037 Nd 0.12 0.034 Sm 0.11 0.036 Eu 0.11 0.041 Gd 0.046 0.026 Tb 0.037 0.020 Dy 0.036 0.019 Ho 0.028 0.014 Er 0.021 0.011 Tm 0.017 0.0094 Yb 0.016 0.0086 Lu 0.0021 0.0053

significant molecular interferences occur (Metson et al., 1984). The absolute yields for REE 2+ relative to Si + at low and high ion energy are given in Table IV. At low ion energy the 2 + ion yields drop approximately by a factor of 10 from La to Yb. Lu 2 + is anomalously low compared to the other REE (approximately a factor of 6 lower than Yb) . This is also true for the Lu + ion relative to Yb + at low energy (Reed, 1983). If the comparison is made between the absolute ion yield of REE 2 ÷ at low energy vs. REE ÷ measured at high energy (Table II ), with the exception o f L a ÷ and Pr +, the REE are more efficiently measured as REE + at high ion energy.

7. Oxide formation

Since the silicate matrix always contains ap- preciable oxygen and oxygen is implanted from the pr imary beam, the presence of MO + species is a major problem in trace-element analysis. At low masses the oxides can be separated by operating at high mass resolution; however, if large numbers of trace elements are re- quired, the ability to maintain the mass calibration on low-intensity peaks makes this type

23

of recording difficult. Oxides are considerably reduced by energy filtering; however, they are not completely el iminated and still can cause significant overlap. A thorough study has been made of the oxides of the REE in order to correct for the light REE (LREE) overlap onto the HREE (Reed, 1983; Zinner and Crozaz, 1986; Fahey et al., 1987 ). The formation of REE-oxides has been related to oxide bond strength, modif ied for the ionisation potentials of the element and oxides (Reed, 1983; Fahey et al., 1987):

Ekio = E M o -- IM - - I M o (4)

where E~to is the modified bond strength; EMO the bond strength; and IM and IMO the ionisation potentials of the element and oxide, respectively. A plot of M O + / M + against Egto (Morgan and Werner, 1978) for high-energy ions is given in Fig. 9. The correlation between MO + / M + and E ~ o for the REE is very good, however, it is apparent that the curves for M + / MO + are periodic in nature. Curves drawn

161

162

~0-3

Hfo

/ " " Yb.Eu

Moo

Ti

J eSc

Ce,

Sie

3-0 4'0 5'0 6"0 7"0 8"0 9'0

E~O Fig. 9. MO+/M ratios against modified oxide dissocia- tion energy, Egto = (EMo q- IM -- IMO).

24 R.W. HINTON

parallel to the period-6 ratios fit the period-4 and -5 data reasonably well. A mass term cannot be introduced to explain the observed changes with period without destroying the good correlation observed for the REE-O+/ REE ÷ ratios.

In mineral analyses CeO+/Ce ÷ is always significantly higher than LaO+/La ÷ and the ion yield o fLa ÷ higher than Ce ÷ (Fahey et al., 1987 ). Unusually, the CeO ÷/Ce ÷ ratio of NBS 610 glass is similar to that of the LaO ÷/La ratio. However, similar M O + / M ÷ ratios would be expected from the EMo-Values. The ion yield of Ce ÷ is also unusually high, when compared to La ÷ , suggesting that in this glass matrix the CeO+/Ce + partitioning is different from that observed in other silicates and glasses (Fahey et al., 1987).

The observed correlations (Fig. 9) can be used as a rough guide to M O + / M + ratios; however, data for AIO+/A1 ÷ and some transition elements in the borosilicate Coming ® glass standards (unpublished data of this laboratory, 1989) fall substantially below the observed curves.

8. Normalisation to Si ÷

The normalisation of ion yields to Si + gives ratios which are very sensitive to both matrix and machine artifacts. Its broad energy distribution leads to a progressive decrease in nearly all elemental ions relative to Si ÷ as energy is increased. It also appears to be the element that is most sensitive to changes which occur with time and with differing ion bombarding conditions. If two laboratories at tempt to analyse in a similar way, differences may still occur due to variations in beam density between different operators and /or instruments. Indeed slight changes in focussing across a sample could potentially lead to abundance changes. Unfortu- nately, since Si is the only element common to all silicates the use of this element cannot always be avoided. Certainly, normalisation to Mg +, A1 + or Ca ÷ would be expected to im-

prove comparison between operators' or laboratories' standards.

9. Conclusions

Tables of ion yields for low- and high-energy ion yields generated from a silicate matrix under oxygen bombardment are presented. Ion yields are a complex function of ionisation potential, bond strength, e lement/oxide partitioning and mass. This work highlights the regularities that exist in the behaviour of secondary ion yields. While the physical laws relating the secondary ion formation from different elements are not understood, empirical laws can be developed to predict ion yields for elements where no standard is available. The patterns of ion yields measured for the NBS 610 glass are also observed in other glasses and natural minerals (unpublished data of this laboratory, 1989). Unfortunately, the yields determined for natural mineral standards are not as comprehensive as those for glasses because of the variability of both the number of elements analysed and the accuracy of the bulk analysis. Day-to-day variations in ion yields measured for a single phase can largely be re- moved by re-analysis of a few elements with as large a mass range as possible. Standard tables for a given matrix can then be applied without the need to redetermine all elements in the standard (s).

An important next step in the understanding of processes of secondary ion formation is the analysis of multi-element standards in different laboratories under closely defined analytical conditions. As the individual artifacts which lead to differences in measured ratios for a single matrix become known, our ability to understand differences between matrices will improve.

Acknowledgement

The author acknowledges funding for the ion microprobe by NERC.


References

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Deline, V.R., Katz, W. and Evans, C.A., 1978. Mecha- nism of the SIMS matrix effect. Appl. Phys. Lett., 33: 832-835.

Fahey, A.J., Goswami, J.N., McKeegan, K.D. and Zin- ner, E., 1987.26A1, 244pu, 5°Ti, REE and trace element abundances in hibonite grains from CM and CV me- teorites. Geochim. Cosmochim. Acta, 51: 329-350.

Metson, J.B., Bancroft, G.M., Nesbitt, H.W. and Jonas- son, R.G., 1984. Analysis of rare earth elements in accessory minerals by specimen isolated secondary ion mass spectrometry. Nature (London), 307: 347-349.

Michael, P.J., 1988. Partition coefficients for rare earth elements in mafic minerals of high silica rhyolites: The importance of accessory mineral inclusions. Geochim. Cosmochim. Acta, 52: 275-282.

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Ray, G. and Hart, S.R., 1982. Quantitative analysis of silicates by ion microprobe. Int. J. Mass. Spectrom. Ion Phys., 44: 231-255.

Reed, S.J.B., 1983. Secondary-ion yields of rare earths. Int. J. Mass. Spectrom. Ion Phys., 54:31-40.

Reed, S.J.B., 1989. Ion microprobe analysis - - a review of geological applications. Mineral. Mag., 53: 3-24.

Rudat, M.A. and Morrison, G.H., 1979. Energy spectra of ions sputtered from elements by O + : A comprehensive study. Surf. Sci., 82: 549-576.

Storms, H.A., Brown, K.F. and Stein, J.D., 1977. Evalu- ation of a cesium positive ion source for secondary ion mass spectrometry. Anal. Chem., 49: 2023-2030.

Williams, P., 1985. Limits of quantitative microanalysis using secondary ion mass spectrometry. In: A.M.F. O'Hare (Editor), Scanning Electron Microscopy, II. SEM Inc., Chicago, Ill., pp. 553-561.

Williams, P. and Gillen, G., 1987. Direct evidence for coulombic ejection ofelectron-desorbed ions. Surf. Sci., 180:L109-L112.

Wilson, R.G. and Novak, S.W., 1988. Systematics of SIMS relative sensitivity factors versus electron affinity and ionisation potential for Si, Ge, GeAs, GaP, InP and HgCdTe determined from implant calibration standards for about 50 elements. In: A. Benninghoven, A. Huber and H.W. Werner (Editors), Secondary Ion Mass Spectrometry - - SIMS VI. Springer, Berlin, pp. 57-61.

Yu, M.L., 1987. A bond breaking model for secondary ion emission. Nucl. Instrum. Methods Phys. Res., B18: 542-548.

Zinner, E. and Crozaz, G., 1986. A method for quantitative measurement of rare earth elements in the ion microprobe. Int. J. Mass. Spectrom. Ion, Phys., 69: 17- 38.

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Ion microprobe trace-element analysis of silicates ... · Si also has a broader energy distribution than would be suggested by the Ge-Sn-Pb tie line. The Zr ÷ distribution is substantially