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Quantiative analysis of silicate minerals by secondary ion mass spectrometry and laser microprobe mass analysis—A comparative study
Sprcfrc&imico Acco, Vol. 388. No. 5/6. pp. 843-851. 1983. Pnntcdin GreatBrilnin.
0584-Ss47/8313.00+ .oo Pergamon Press Ltd.
Quantitative analysis of silicate minerals by secondary ion mass spectrometry and laser microprobe mass analysis-A comparative study J. M.
BEUSEN, P. SURKYN,
R. GIJBELSand F.
ADAMS
Department of Chemistry, University of Antwerp (UIA). Universiteitsplein 1, B-2610 Wilrijk, Belgium (Received
17 December 1982)
Abstract-The local thermal equilibrium model applied to homogeneous silicate minerals was tested for secondary ion mass spectrometry in different experimental conditions and for laser microprobe mass analysis using the LAMMA-SOO transmission type instrument and the LAMMAreflection instrument. Semi-quantitatively the relative intensities of positive ions were found to follow a similar dependence on ionization potential.
INTRODUCTION IT HAS
long been known [1] that the degree of ionization aM. of sputtered particles decreases exponentially for increasing ionization potential (IP) of the corresponding element M, thus aM + cc exp[ - /Y(I P)]. ANDERSEN and H INTHORNE [2] have interpreted p as 1/kTi where 7; is the “ionization temperature”[3], i.e. a local thermal equilibrium is assumed (LTE model). Although the existence of such an equilibrium is very doubtful, e.g. in view of the very short time scale of the individual cascades (- lo- I3 s) [4,5], it cannot be denied that the concept has proven useful for quantitative interpretation of sputtered ion mass spectra. The LTE-model assumes that the dissociation reaction M” *M + +e- is in thermodynamic equilibrium and that its dissociation constant can be calculated using the Saha-Eggert ionization equation. This equation can be written as: log%
0
WO(IP - AE)
= 15.38+log
Ti
- log N,
(1)
where n,,and nM+ are the number densities of atoms of element M in two adjacent ionization states, 2 is the partition function of the free electron, B,. and B,, are the internal partition functions of species M o and M + , IP is the ionization potential (eV) of the lower ionization stage, AE the ionization potential depression (eV) due to Coulomb interactions of the charged particles, N, the number of electrons per cm3 (electron density) and T is the absolute “ionization” temperature (K). The LTE-model has been applied to SIMS (secondary ion mass spectrometry) measurements on several types of samples such as metals, metal-alloys, silicate glasses and fused rock powders[2]. MORGAN and WERNER modified the LTE-model using only one fitting parameter and demonstrated its applicability in SIMS measurements of low alloy steels [3], halide matrices [6], minerals [7] and glass silicate standards [8]. Recently the applicability of this procedure for quantification of Laser Microprobe Mass (LAMMA) spectra has been investigated for silicate glasses [9, lo]. Comparison with SIMS LTE analysis shows that similar T values were obtained although the scatter of the data points was considerably higher in the case of LAMMA. [ 1] G. BLAISEin Marerid
characrerization using ion beanu, Ed. P. THoMAsand A. CACHARD,Plenum, New York (1978). [2] C. A. ANDERSENand J. R. HINTHORNE,Anal. Chem. 45, 1421 (1973).
[3] A. E. MORGAN and H. W. WERNER, Anal. Chem. 48,699 P. SIGMUND, Appl. Phys. Lert. 25, 169 (1974). [5] P. SIGMUND, Appl. Phys. Let?. 27, 62 (1975).
(1976).
[4]
[6] A. E. MORGAN and H. W. WERNER, J. Chem. Whys. 68, 3900 (1978). [7] A. E. MORGAN and H. W. WERNER, Mikrochim. Acta II, 31 (1978). [8] A. E. MORGAN and H. W. WERNER, Anal. Chem. 49, 927 (1977). 843
J. M. BEUSEN et al.
844
Because of the very localized interaction of the laser microprobe with a sample (( 1 pm), this technique can be expected to be quite subject to sample heterogeneity effects, e.g. inclusions. On the other hand, it is true that the reproducibility is unfavourably influenced by the poor performance of the fast transient recorder, which is used to collect the single shot
mass spectra [9,11] and by other effects (differences in laser energy and its absorption in the sample, particle size effects, etc.). In order to arrive at a better understanding of the reason for the considerable scatter of the LAMMA data [9] and in order to test again the applicability of the LTE model for laserinduced positive ion mass spectra, it was decided to select from a number of minerals of wellknown chemical composition only those that had been proved to be homogeneous within the precision of the measurements [ 121. In addition, the results of the LTE approach in LAMMA will be compared with those of positive SIMS at different working conditions. EXPERIMENTAL Instrumentation
The SIMS measurements were performed with a CAMECA IMS-300 instrument described in detail by MORRIKIN~~~SLODZIAN[ 131. Installation of an electrostatic sector after the magnetic prism allows it to increase the mass resolution to about 3000. The CAMECA IMS-300 secondary ion microscope is connected with a PDP 1l/O3 computer using CAMAC as an instrument interface [14]. A detailed description of the LAMMAtransmission instrument (Leybold Weraeus) is given elsewhere [15, 163. It is composed of three major components: an optical microscope, a Q-switched frequency quadrupled Nd-YAG laser (A = 265 nm, pulse duration 15 ns) and a mass spectrometer of the time of flight (TOF) type. The mass spectra are stored in a very fast transient recorder. There the spectrum is available for visualization and computer interpretation with a MING1 1 microcomputer system. Recently, a reflection geometry instrument (LAMMALeybold Heraeus) became available [ 171. Selected measurements were also performed with this apparatus. Samples
The minerals used in this study are augite, hornblende and lepidolite. The augite is a pyroxene from Store Arti (Langessundfjord, Norway) while the hornblende is an amphibole from the ultramafic complex of Finer0 (Italy). The lepidolite is a mica from VarutrGk (Sweden). Bulk chemical analyses of these minerals were performed by different methods such as atomic absorption spectrometry, energydispersive X-ray fluorescence spectrometry and instrumental neutron activation analysis. Fluorine was determined in lepidolite by means of wet chemical analysis using an ion-sensitive electrode whereas gravimetry was applied for silicon. Water in hornblende was determined by an automated Karl-Fischer titration method. The results are summarized in Table 1. The spatial distribution of the elements of interest was examined by electron microprobe X-ray analysis and by ion microscopy (spatial resolution z 1 pm). During depth profiling in secondary ion mass spectrometry (SIMS), stable secondary ion signals were obtained (depth resolution _ 20 nm). The above minerals were found to exhibit homogeneous distributions for the elements Li, F, Na, Mg, Al, Si, K, Ca, Fe, Rb, Zr and Cs[l2]. Sample preparation
The sample preparation procedure for SIMS depends on the nature of the mineral studied. Selected fragments of augite and hornblende, approximately 5 x 5 x 3 mm, were mounted with Sn-Bi eutectic in a metal ring and polished with diamond paste. Lepidolite was mounted in a specially designed sample holder with a cleavage surface facing outwards. After ultrasonic cleaning with CCI, an electrically [9] P. SURKYNand F. ADAMS,J. Trace Microprobe Techniq. I, 79 (1982). U. HAAS,P. WIESER and R. WURSTER, Fresenius Z. And. Chem. 308,
[lo]
270 (1981).
[I l] S. S. SIMONin Microbeam Analysis, p. 390. (1982). [12] J. M. BEUSENand R. GIJBELS, in Leaching-Di&siwl, Ed. S. S. AUGUSTITHIS, Theophrastus, Athens (1983). [13] G. H. MORRISONand G. SLODZIAN,Anal. Chem. 47, 932A (1975). [I43 M. VANCRAEN, P. VANESPEN and F. ADAMS,Rev. Scient. Instrum. 53, 1007 (1982). [15] [16]
R. WECHSUNG,F. HILLENKAMP,R. KAUFMAN,R. NITSCHE~~~ H. VOGT,Scanning Heelron Microsc. I,61 1 (1978). H. J. HEINEN, R. WECHSUNG,H. VOGT, F. HlLLENKAMPand R. KAUFMANN,Biorechnische Umschau 2. 346 (1978).
[17]
P. FEIGL. B. SCHUELERand F. HILLENKAMP,fnr. J. Muss Specfrom.
fan Phgs. 47, 15 (1983).
845
Quantitative analysis of silicate minerals Table I. Chemical composition of the minerals (weight %)
te
Augi
Si02 A'203
Hornblende
Lepidolite
52.7
44.1
50.27
1.1
14.7
24.56
30.0
7.7
0.34
NgO
0.6
16.8
0.03
cao
5.6
11.3
0.14
9.8
2.6
0.34
0.2
0.2
10.00
Fe203
5.80
Lip0 Na20 !?O Rb20
2.00
cs20
0.45
Ti02
0.35
0.2
0.05
MnO
0.75
0.1
0.48
Cr203 h-02
0.13 0.6 2.2
H20+ F
Total
1.12 6.25
101.7
100.1
101.8
conducting layer of gold (30-40 nm) was vapor-deposited on the surface. For augite no gold layer was needed since negligible electrical charging was observed. For LAMMAanalysis the samples were crushed in an agate mortar to micrometer-size particles. By bringing the powder into contact with a FORMVAR coated electron microscope grid particles are attached to the plastic surface. This grid can directly be mounted in the sample holder of the instrument. LAMMAmeasurements were performed with the samples as prepared for SIMS. Procedure SIMS. The primary ions used were either Ar+, 0;
or O-, with a nominal energy at impact of6.0,5.5 and 14.5 keV respectively. The primary ion current density ranged between 0.5 and 0.8 mAcm_‘. Augite was bombarded with both positive (Ar’, 0;) and negative ions, hornblende with Ar+ and O-. Lepidolite could only be analysed with O- ions due to its extremely high specific electrical resistivity [8]. The majority of the measurements was made in a residual vacuum of 51 lo-’ Torr. For augite and hornblende, measurements during Ar+ bombardment were also carried out under oxygen flooding. Positive secondary ions emitted from the central 55 pm of the eroded area, were mass separated and collected by an electron multiplier. The energy bandwidth of the secondary ions through the spectrometer was limited to 20 eV. The average relative standard deviations of the elemental ion intensities ratioed to the “AI+ signal was about 2.9 y0 for augite, 2.5 y0 for hornblende and 3.5 u/nfor lepidolite. Normally, only M + secondary ion currents were taken into account, except for these elements which emit appreciable amounts of monoxide ions e.g. Si, Ca and Zr. Due to the complexity of the silicate matrices and the limited mass resolution (- 400), mass spectral coincidences had to be taken into account using natural isotopic abundances e.g. 48Ti+ was corrected for 4”Ca+ contribution, *“SiO+ for 44Ca+, etc. Following a suggestion by MORGAN and WERNER[~] the measured secondary ion intensities were multiplied by the square root of the mass number of the isotope considered to correct for instrumental mass discrimination. This always led to an improvement of the closeness of the LTE ht. LAMMA. The LAMMA-5OOinstrument was used in the standard operation mode: energy 5 10 pJ, extraction potential 3000 V, lens potential 1150 V, reflector potential 125 V. To obtain an average composition of a sample, 30 microparticles were analysed. In every spectrum the peak area of an isotope X was related to that of the reference isotope “AI. The average relative standard deviation of the intensity ratios ranged from 7 ‘;;,for 39K in lepidolite and 40Ca in hornblende
J. M. BEUSENet al.
846
up to 205 y0 for ‘sSi in augite. The large scatter on most of the intensity ratios made the application of corrections for spectral interferences irrelevant. Contrary to the SIMS analyses no correction for instrumental mass discrimination was taken into account. The LAMMAinstrument was used in the standard operating conditions specified by the manufacturer. The EiVZmodel. If in Eqn (1) the partition functions and ionization potential are known, the ratio of the number of singly charged ions to neutral atoms of an isotope is determined by the ionization temperature and the electron density. This ratio can be set equal to the ratio of ion intensity measured vs concentration, both relative to an internal reference matrix isotope, in this study 27A1 (relative sensitivity coefficient RSC = [I~./C,]/[I,,*/C,,]). This isotope was chosen above YSi because of its higher sensitivity and reproducibility in LAMMA analysis. Whereas the ionization potentials of the atoms can be directly taken from the literature, the partition functionsare more problematic to obtain. For the common elements the data calculated by DRAwrNand FELENB~K [ 181 were used. A number of them were not listed for the high “temperatures” encountered in this study, so a linear extrapolation was carried out. In contrast to ANoERsENand HINTHORNE[~]we used in this study only one fitting parameter to define the relationship between the RSC and I P - A E. The optimum T-value was calculated by iteration until the T-value used for the partition functions coincided to within 1% with that derived from the slope. The N,-value was then simply calculated from the intercept of the straight line with the ordinate, in other words, N, was not treated as a “temperature” dependent parameter during the iteration procedure [19]. RESULTS AND DISCUSSION
Augite
In Table 2 the results for the parameters 7;-and N, (ionization “temperature” and electron density) are summarized as calculated for a number of experimental conditions from SIMS Table 2. Results for au&e
&WA-5OC I
Secondary
Isjo+
+ I
Si OH+
Ions:O-20
eV
0.17
0.22
0.11
0.14
1.5
2.7
1.3
1.6
Ti(K)
8770
9120
NJc~-~)
4.1
ISi+
Izro+
IZr+
Meanerror
‘SiO+
factor
101’
5.1
1.27
Y810 101
9.7
8520
10’7
1.AJw-loal
1.9
1017
1.39
1.23
1.56
Secondary
Ions:40-60
eV
20.15
8706 7. ,2 1016. 1.58’
li !800 5. 8 1018 2.35
+ ‘SiOH+ 0.08 ISi+ 0.Y
10130 2.2
1018
1.46
-
-
i
-
=Exccpt Si Datas Cl81 H. W. DRAWINand P. FELENBOK,
for Plasmas in Local ThermodynamicEquilibrium, Paris (1965). 1191 D. S. SIMONS,J. E. BAKERand C. A. EVANS,Jr., Anal. Chem. 48, 1341 (1976).
Gauthier-Villars,
Quantitative analysis of silicate minerals
841
and LAMMA measurements. An error factor is defined as the experimental value of the relative sensitivity coefficient, divided by that required for an exact fit to the straight line in a plot of log (RSC) vs IP - AE. The mean error factor for all elements is indicated in Table 2. The individual data for each element are also displayed as an RSC corrected for partition functions vs ZP - AE in Figs. 1 and 2 for SIMS and LAMMA. AE was kept constant at 0.1 eV. For LAMMA, the low dynamic range prevented Na from being measured simultaneously with the other elements, as its intensity was too large. 3gK+ is interfered by NaO+ in the SIMS spectra. The maximum error is a factor of 2 according to high resolution measurements of sodium salts, but probably considerably less in the actual measurements owing to the larger energy bandpass of the collected secondary ions. “NaO+ does not interfere with the 39K+ signal in the LAMMA spectra according to measurements on pure sodium salts. Normally, only M + signals were used but for Si and Zr appreciable contributions of oxide, and hydroxide species had to be taken into account. The extent of these molecular ion contributions appears from Table 2. Even for the oxygen rich augite (ca. 50 y0 0), oxygen flooding appears to have a non-negligible effect on the relative abundance of the monoxide (cl)
I
I
I
I
I
I
I
I
45
5
55
6
65
7
75
6
7
75
6
I 7
I 75
I 0
IP-AE
(eV)
(b)
+r m .S
08 04
;
0
CT -0.4 3 -0
6
45
5
55
6
IP-AE
65
(eV)
Cc)
cn -043 -06-I
2 I 45
I 5
I 55
I 6
I 65
_
IP-AE(eV)
Fig. 1. Plot of log (RX (B,,&~BM+) vs If -AE of augite. (a) Ar+ bombardment, 0-20eV secondary ions; (b) Ar+ bombardment, O2 bleed-in, O-20 eV secondary ions; (c) 0; bombardment, O-20 eV secondary ions. SA(B)%/S-6-M
J. M.
BEUSEN et al. (a)
I 45
I 5
I 55
I 6
I 65
I 7
I 75
I 65
I 7
I 75
I 6
IP-AE(eV) (b)
-2-
-3
I 45
I 5
I 55
I 6
*eS, . 8
I P - AE (eV)
Fig. 2. Plot of log (RX (B~a/28~ 4) vs IP - AE of augite. (a) O- bombardment, &20 eV secondary ions; (b) LAMMA-500.
ions. The relative abundance of ZrO+/Zr’ in the LAMMA spectra is rather inaccurate but consistent with the ones obtained in SIMS for secondary ion energies between 0 and 20 eV. Secondary ion energies between 2040 eV decrease the relative intensity of the oxide ions, as expected [3,6-81. In Figs 1 and 2, the intensities of Zr+ and ZrO+ were added to yield a data point for this element which corresponds more closely with Eqn 1. For Si and Ca the same procedure was followed although the fit did not improve significantly. Figures 1 and 2 indicate a low RSC for Ti, possibly because the TiO+ intensity is non-negligible compared to that of Ti+. A correction was impossible due to spectral interferences in the mass range of the TiO+ species. For LAMMAthe average relative deviation from Eqn 1 is considerably larger than for the SIMS measurements in any of the experimental conditions selected in spite of the fact that Si was omitted from the average deviation and for the calculation of N, and T. The abnormally low Si+ intensity in the LAMMA-spectra not only occurred for augite but also for the other minerals (see below) and for materials such as silicate glasses and fly-ash [93. The intensities of the atomic ions as observed in SIMS may vary in different experimental conditions, e.g. as the energy bandpass of secondary ions is increased from @-20eV to 60-80 eV, the M+ intensities are considerably reduced (Si, Ca and Na by a factor 7, 15 and 43 resp.). Oxygen flooding in the sample chamber has practically no effect on the intensities of %+ and 56Fe+, when the sample is bombarded with Ar+ primary ions, whereas the 23Na+, 27Alt and 40Ca+ signals slightly decrease to 90%, 75 ‘;; and 67 ‘i;; respectively, of their original value. This indicates that for SIMS the LTE parameters depend on the experimental conditions used. For LAMMA, instrumental conditions also influence the LTE-parameters. In the first place there is the power dissipation into the sample. On the same augite samples considerably higher 7; and N, values were obtained with the LAMMAinstrument. In the second place variation of instrumental parameters such as extraction lens or reflector potential give rise to significantly different LAMMA_spectra[20]. A direct comparison of the data [2OJ E. MICHIELS and R. GIJLIELS, submitted
to Spectrochim. Acta. B.
849
Quantitative analysis of silicate minerals
obtained with LAMMA and SIMS is therefore difficult. Nevertheless, the results of Table 2 agree reasonably well between both methods. Hornblende
LTE parameters obtained for different SIMS working conditions and for LAMMAand LAMMAstandard measurement conditions are summarized in Table 3. For SIMS, 54Fe+ was used to monitor iron as s6Fe+ was strongly interfered by *‘CaO+ and 3gKOH+. Monoxide corrections were not carried out for Ti and Cr owing to mass spectral interferences in the *‘TiO* and 52Cr0+ regions, but were taken into account for Si and Ca. Under Ar+ bombardment the ratios (I,o++I,o,+)/I,, were 0.15 and 0.11 respectively for these elements. The largest deviation from Eqn 1 occurred for Fe, probably through an unresolvable spectral interference of the All cluster ion. The other elements give a mean error factor of 1.41. This deviation is somewhat reduced when the energy bandpass is set at 50-70 eV, probably as a result of suppression of molecular interferences. & and N, are increased when oxygen is admitted in the sample chamber or when negative atomic oxygen is used as a primary ion beam. The precision of the mode1 increases considerably. The LTE-parameters increase drastically in the LAMMAinstrument because of a more efficient laser energy dissipation in the sample. The applicability of the model improves dramatically, as appears from Fig. 3, where results for SIMS with O- bombardment and LAMMAand 1000 for standard measurement conditions are given. Lepidolite
On lepidolite only negative oxygen bombardment was possible in SIMS because of the very low electrical conductivity. Monoxide corrections were carried out for silicon and calcium. 3oSi was used, as 2eSi+ suffered from a large interference due to 27AlH+. For low energy secondary ions a K value of 10,590 & 530 K could be derived. The closeness of the fit was much worse than for the other materials with an average deviation of nearly 100%. ‘Li behaves erratically with a.negative deviation from Eqn (1) by a factor of 3.6. The result for fluorine with an ionization potential of 17.42 eV i.e. a factor of 2 above the range for the other elements examined, is 280 times too large. LAMMA with the transmission instrument gives a high q value of 10,500 K but with an average deviation from the LTE plot of a factor of 2.7. The largest deviation again occurs for Si with an RSC that is a factor of 8 too low. The “temperature” does not increase significantly with the reflection type geometry and neither does the precision but the deviation for silicon is considerably reduced. Fluorine could not be measured with the LAMMAinstrument whereas with the LAMMAit was almost a factor of 2 x lo* too high, compared to the intensity expected from the model. CONCLUSIONS
The physical significance of the LTE-approach in SIMS, and also in LAMMA is limited, as was already stressed in the literature [l, 193. Nevertheless, the comparison of the data
Table 3. Results for hornblende LAEFIA
SIMS
Ar+ (O-20 eV)
Tj
(K)
Ne (~rn-~)
9340 4.0
1017
Ar++O2 (O-20
ev)
10540 1.1 1018
A?+
0-
(50-70
eV)
98 10 6.3
1017
(O-20
“5004’ ev)
10270 8.4
80 1000”
1017
9770 3.3
12800 1017
3.4
1018
Mean error factor
1.41
1.28
1.35
1.20
4.60
1.11
J. M. BEUSEN et al.
850
(cl)
I P - AE (eV)
(b)
0804-
04-
.
-084
I
I
I
I
I
I
I
45
5
55
6
65
7
75
2% . I
t3
I P - LIE (eV) ICI
-04-084
1
I
I
I
I
I
I
I
45
5
55
6
65
7
75
8
I P-AEleV)
Fig. 3. Plot of log (RSC (B,&ZBy+) vs IP-AE for hornblende. (a) O- bombardment O-20 eV secondary ions; (b) LAMMA-500; (c) LAMMA-1000.
obtained on the samples discussed in this paper, indicates that, at least semi-quantitatively, the relative intensities of positive ions obtained in both methods follow a similar dependence with the ionization potential. The dependence breaks down for some elements in extreme conditions, e.g. for Si in LAMMA measurements of the augite or for F in the SIMS and LAMMAmeasurements of lepidolite (high ionization potential). The T,-values obtained with both methods are comparable semi-quantitatively. They increase in the ranking order augite, hornblende, lepidolite for both methods. For LAMMA, the increase in “temperature” could possibly be related to the efficiency of energy coupling of the laser pulse with the sample, a factor dependent on sample composition and structure. The considerably larger deviations from Eqn (1) obtained with LAMMA do not depend on the poorer applicability of the LTE-approach with this method as is illustrated by the excellent results obtained with the reflection type LAMMAinstrument on hornblende (Fig. 3). Also sample heterogeneity can be precluded for the minerals selected in this study[12]. Rather it must be associated with other factors: the inherent lack of repro-
Quantitative analysis of silicate minerals
851
ducibility of the method [21], and spectral interferences since this instrument provides less resolution than SIMS apparatus, which can provide for high mass resolution or molecular ion discrimination. From these and other measurements [22,23] it appears that the relative molecular ion intensities are comparable for SIMS and LAMMA spectra. When the “apparent temperature” increases LAMMA results tend to agree better with the LTE-model while molecular interferences drop in importance. Acknowleclyemenrs-This research was carried out within Research Project 80/85-10 of the Interministrial Commission for Science Policy Belgium. The LAMMA-IOOO measurements were performed through the courtesy of Leybold-Heraeus, Cologne, F.R.G.
[21] E. DENOYER, R. VAN GRIEKEN, F. ADAMS and D. F. S. NATUSCH, Anal. Chem. 54, 26A (1982). [22] E. MICHIELS, A. CELIS and R. GIJBELS, Inr. J. Mass Spectrom. lon Phys. 47, 23 (1983). [23] C. PLOG. L. WIEDMANN and A. BENNINGHOVEN, Surj. Sci. 67, 565 (1977).
0584-Ss47/8313.00+ .oo Pergamon Press Ltd.
Quantitative analysis of silicate minerals by secondary ion mass spectrometry and laser microprobe mass analysis-A comparative study J. M.
BEUSEN, P. SURKYN,
R. GIJBELSand F.
ADAMS
Department of Chemistry, University of Antwerp (UIA). Universiteitsplein 1, B-2610 Wilrijk, Belgium (Received
17 December 1982)
Abstract-The local thermal equilibrium model applied to homogeneous silicate minerals was tested for secondary ion mass spectrometry in different experimental conditions and for laser microprobe mass analysis using the LAMMA-SOO transmission type instrument and the LAMMAreflection instrument. Semi-quantitatively the relative intensities of positive ions were found to follow a similar dependence on ionization potential.
INTRODUCTION IT HAS
long been known [1] that the degree of ionization aM. of sputtered particles decreases exponentially for increasing ionization potential (IP) of the corresponding element M, thus aM + cc exp[ - /Y(I P)]. ANDERSEN and H INTHORNE [2] have interpreted p as 1/kTi where 7; is the “ionization temperature”[3], i.e. a local thermal equilibrium is assumed (LTE model). Although the existence of such an equilibrium is very doubtful, e.g. in view of the very short time scale of the individual cascades (- lo- I3 s) [4,5], it cannot be denied that the concept has proven useful for quantitative interpretation of sputtered ion mass spectra. The LTE-model assumes that the dissociation reaction M” *M + +e- is in thermodynamic equilibrium and that its dissociation constant can be calculated using the Saha-Eggert ionization equation. This equation can be written as: log%
0
WO(IP - AE)
= 15.38+log
Ti
- log N,
(1)
where n,,and nM+ are the number densities of atoms of element M in two adjacent ionization states, 2 is the partition function of the free electron, B,. and B,, are the internal partition functions of species M o and M + , IP is the ionization potential (eV) of the lower ionization stage, AE the ionization potential depression (eV) due to Coulomb interactions of the charged particles, N, the number of electrons per cm3 (electron density) and T is the absolute “ionization” temperature (K). The LTE-model has been applied to SIMS (secondary ion mass spectrometry) measurements on several types of samples such as metals, metal-alloys, silicate glasses and fused rock powders[2]. MORGAN and WERNER modified the LTE-model using only one fitting parameter and demonstrated its applicability in SIMS measurements of low alloy steels [3], halide matrices [6], minerals [7] and glass silicate standards [8]. Recently the applicability of this procedure for quantification of Laser Microprobe Mass (LAMMA) spectra has been investigated for silicate glasses [9, lo]. Comparison with SIMS LTE analysis shows that similar T values were obtained although the scatter of the data points was considerably higher in the case of LAMMA. [ 1] G. BLAISEin Marerid
characrerization using ion beanu, Ed. P. THoMAsand A. CACHARD,Plenum, New York (1978). [2] C. A. ANDERSENand J. R. HINTHORNE,Anal. Chem. 45, 1421 (1973).
[3] A. E. MORGAN and H. W. WERNER, Anal. Chem. 48,699 P. SIGMUND, Appl. Phys. Lert. 25, 169 (1974). [5] P. SIGMUND, Appl. Phys. Let?. 27, 62 (1975).
(1976).
[4]
[6] A. E. MORGAN and H. W. WERNER, J. Chem. Whys. 68, 3900 (1978). [7] A. E. MORGAN and H. W. WERNER, Mikrochim. Acta II, 31 (1978). [8] A. E. MORGAN and H. W. WERNER, Anal. Chem. 49, 927 (1977). 843
J. M. BEUSEN et al.
844
Because of the very localized interaction of the laser microprobe with a sample (( 1 pm), this technique can be expected to be quite subject to sample heterogeneity effects, e.g. inclusions. On the other hand, it is true that the reproducibility is unfavourably influenced by the poor performance of the fast transient recorder, which is used to collect the single shot
mass spectra [9,11] and by other effects (differences in laser energy and its absorption in the sample, particle size effects, etc.). In order to arrive at a better understanding of the reason for the considerable scatter of the LAMMA data [9] and in order to test again the applicability of the LTE model for laserinduced positive ion mass spectra, it was decided to select from a number of minerals of wellknown chemical composition only those that had been proved to be homogeneous within the precision of the measurements [ 121. In addition, the results of the LTE approach in LAMMA will be compared with those of positive SIMS at different working conditions. EXPERIMENTAL Instrumentation
The SIMS measurements were performed with a CAMECA IMS-300 instrument described in detail by MORRIKIN~~~SLODZIAN[ 131. Installation of an electrostatic sector after the magnetic prism allows it to increase the mass resolution to about 3000. The CAMECA IMS-300 secondary ion microscope is connected with a PDP 1l/O3 computer using CAMAC as an instrument interface [14]. A detailed description of the LAMMAtransmission instrument (Leybold Weraeus) is given elsewhere [15, 163. It is composed of three major components: an optical microscope, a Q-switched frequency quadrupled Nd-YAG laser (A = 265 nm, pulse duration 15 ns) and a mass spectrometer of the time of flight (TOF) type. The mass spectra are stored in a very fast transient recorder. There the spectrum is available for visualization and computer interpretation with a MING1 1 microcomputer system. Recently, a reflection geometry instrument (LAMMALeybold Heraeus) became available [ 171. Selected measurements were also performed with this apparatus. Samples
The minerals used in this study are augite, hornblende and lepidolite. The augite is a pyroxene from Store Arti (Langessundfjord, Norway) while the hornblende is an amphibole from the ultramafic complex of Finer0 (Italy). The lepidolite is a mica from VarutrGk (Sweden). Bulk chemical analyses of these minerals were performed by different methods such as atomic absorption spectrometry, energydispersive X-ray fluorescence spectrometry and instrumental neutron activation analysis. Fluorine was determined in lepidolite by means of wet chemical analysis using an ion-sensitive electrode whereas gravimetry was applied for silicon. Water in hornblende was determined by an automated Karl-Fischer titration method. The results are summarized in Table 1. The spatial distribution of the elements of interest was examined by electron microprobe X-ray analysis and by ion microscopy (spatial resolution z 1 pm). During depth profiling in secondary ion mass spectrometry (SIMS), stable secondary ion signals were obtained (depth resolution _ 20 nm). The above minerals were found to exhibit homogeneous distributions for the elements Li, F, Na, Mg, Al, Si, K, Ca, Fe, Rb, Zr and Cs[l2]. Sample preparation
The sample preparation procedure for SIMS depends on the nature of the mineral studied. Selected fragments of augite and hornblende, approximately 5 x 5 x 3 mm, were mounted with Sn-Bi eutectic in a metal ring and polished with diamond paste. Lepidolite was mounted in a specially designed sample holder with a cleavage surface facing outwards. After ultrasonic cleaning with CCI, an electrically [9] P. SURKYNand F. ADAMS,J. Trace Microprobe Techniq. I, 79 (1982). U. HAAS,P. WIESER and R. WURSTER, Fresenius Z. And. Chem. 308,
[lo]
270 (1981).
[I l] S. S. SIMONin Microbeam Analysis, p. 390. (1982). [12] J. M. BEUSENand R. GIJBELS, in Leaching-Di&siwl, Ed. S. S. AUGUSTITHIS, Theophrastus, Athens (1983). [13] G. H. MORRISONand G. SLODZIAN,Anal. Chem. 47, 932A (1975). [I43 M. VANCRAEN, P. VANESPEN and F. ADAMS,Rev. Scient. Instrum. 53, 1007 (1982). [15] [16]
R. WECHSUNG,F. HILLENKAMP,R. KAUFMAN,R. NITSCHE~~~ H. VOGT,Scanning Heelron Microsc. I,61 1 (1978). H. J. HEINEN, R. WECHSUNG,H. VOGT, F. HlLLENKAMPand R. KAUFMANN,Biorechnische Umschau 2. 346 (1978).
[17]
P. FEIGL. B. SCHUELERand F. HILLENKAMP,fnr. J. Muss Specfrom.
fan Phgs. 47, 15 (1983).
845
Quantitative analysis of silicate minerals Table I. Chemical composition of the minerals (weight %)
te
Augi
Si02 A'203
Hornblende
Lepidolite
52.7
44.1
50.27
1.1
14.7
24.56
30.0
7.7
0.34
NgO
0.6
16.8
0.03
cao
5.6
11.3
0.14
9.8
2.6
0.34
0.2
0.2
10.00
Fe203
5.80
Lip0 Na20 !?O Rb20
2.00
cs20
0.45
Ti02
0.35
0.2
0.05
MnO
0.75
0.1
0.48
Cr203 h-02
0.13 0.6 2.2
H20+ F
Total
1.12 6.25
101.7
100.1
101.8
conducting layer of gold (30-40 nm) was vapor-deposited on the surface. For augite no gold layer was needed since negligible electrical charging was observed. For LAMMAanalysis the samples were crushed in an agate mortar to micrometer-size particles. By bringing the powder into contact with a FORMVAR coated electron microscope grid particles are attached to the plastic surface. This grid can directly be mounted in the sample holder of the instrument. LAMMAmeasurements were performed with the samples as prepared for SIMS. Procedure SIMS. The primary ions used were either Ar+, 0;
or O-, with a nominal energy at impact of6.0,5.5 and 14.5 keV respectively. The primary ion current density ranged between 0.5 and 0.8 mAcm_‘. Augite was bombarded with both positive (Ar’, 0;) and negative ions, hornblende with Ar+ and O-. Lepidolite could only be analysed with O- ions due to its extremely high specific electrical resistivity [8]. The majority of the measurements was made in a residual vacuum of 51 lo-’ Torr. For augite and hornblende, measurements during Ar+ bombardment were also carried out under oxygen flooding. Positive secondary ions emitted from the central 55 pm of the eroded area, were mass separated and collected by an electron multiplier. The energy bandwidth of the secondary ions through the spectrometer was limited to 20 eV. The average relative standard deviations of the elemental ion intensities ratioed to the “AI+ signal was about 2.9 y0 for augite, 2.5 y0 for hornblende and 3.5 u/nfor lepidolite. Normally, only M + secondary ion currents were taken into account, except for these elements which emit appreciable amounts of monoxide ions e.g. Si, Ca and Zr. Due to the complexity of the silicate matrices and the limited mass resolution (- 400), mass spectral coincidences had to be taken into account using natural isotopic abundances e.g. 48Ti+ was corrected for 4”Ca+ contribution, *“SiO+ for 44Ca+, etc. Following a suggestion by MORGAN and WERNER[~] the measured secondary ion intensities were multiplied by the square root of the mass number of the isotope considered to correct for instrumental mass discrimination. This always led to an improvement of the closeness of the LTE ht. LAMMA. The LAMMA-5OOinstrument was used in the standard operation mode: energy 5 10 pJ, extraction potential 3000 V, lens potential 1150 V, reflector potential 125 V. To obtain an average composition of a sample, 30 microparticles were analysed. In every spectrum the peak area of an isotope X was related to that of the reference isotope “AI. The average relative standard deviation of the intensity ratios ranged from 7 ‘;;,for 39K in lepidolite and 40Ca in hornblende
J. M. BEUSENet al.
846
up to 205 y0 for ‘sSi in augite. The large scatter on most of the intensity ratios made the application of corrections for spectral interferences irrelevant. Contrary to the SIMS analyses no correction for instrumental mass discrimination was taken into account. The LAMMAinstrument was used in the standard operating conditions specified by the manufacturer. The EiVZmodel. If in Eqn (1) the partition functions and ionization potential are known, the ratio of the number of singly charged ions to neutral atoms of an isotope is determined by the ionization temperature and the electron density. This ratio can be set equal to the ratio of ion intensity measured vs concentration, both relative to an internal reference matrix isotope, in this study 27A1 (relative sensitivity coefficient RSC = [I~./C,]/[I,,*/C,,]). This isotope was chosen above YSi because of its higher sensitivity and reproducibility in LAMMA analysis. Whereas the ionization potentials of the atoms can be directly taken from the literature, the partition functionsare more problematic to obtain. For the common elements the data calculated by DRAwrNand FELENB~K [ 181 were used. A number of them were not listed for the high “temperatures” encountered in this study, so a linear extrapolation was carried out. In contrast to ANoERsENand HINTHORNE[~]we used in this study only one fitting parameter to define the relationship between the RSC and I P - A E. The optimum T-value was calculated by iteration until the T-value used for the partition functions coincided to within 1% with that derived from the slope. The N,-value was then simply calculated from the intercept of the straight line with the ordinate, in other words, N, was not treated as a “temperature” dependent parameter during the iteration procedure [19]. RESULTS AND DISCUSSION
Augite
In Table 2 the results for the parameters 7;-and N, (ionization “temperature” and electron density) are summarized as calculated for a number of experimental conditions from SIMS Table 2. Results for au&e
&WA-5OC I
Secondary
Isjo+
+ I
Si OH+
Ions:O-20
eV
0.17
0.22
0.11
0.14
1.5
2.7
1.3
1.6
Ti(K)
8770
9120
NJc~-~)
4.1
ISi+
Izro+
IZr+
Meanerror
‘SiO+
factor
101’
5.1
1.27
Y810 101
9.7
8520
10’7
1.AJw-loal
1.9
1017
1.39
1.23
1.56
Secondary
Ions:40-60
eV
20.15
8706 7. ,2 1016. 1.58’
li !800 5. 8 1018 2.35
+ ‘SiOH+ 0.08 ISi+ 0.Y
10130 2.2
1018
1.46
-
-
i
-
=Exccpt Si Datas Cl81 H. W. DRAWINand P. FELENBOK,
for Plasmas in Local ThermodynamicEquilibrium, Paris (1965). 1191 D. S. SIMONS,J. E. BAKERand C. A. EVANS,Jr., Anal. Chem. 48, 1341 (1976).
Gauthier-Villars,
Quantitative analysis of silicate minerals
841
and LAMMA measurements. An error factor is defined as the experimental value of the relative sensitivity coefficient, divided by that required for an exact fit to the straight line in a plot of log (RSC) vs IP - AE. The mean error factor for all elements is indicated in Table 2. The individual data for each element are also displayed as an RSC corrected for partition functions vs ZP - AE in Figs. 1 and 2 for SIMS and LAMMA. AE was kept constant at 0.1 eV. For LAMMA, the low dynamic range prevented Na from being measured simultaneously with the other elements, as its intensity was too large. 3gK+ is interfered by NaO+ in the SIMS spectra. The maximum error is a factor of 2 according to high resolution measurements of sodium salts, but probably considerably less in the actual measurements owing to the larger energy bandpass of the collected secondary ions. “NaO+ does not interfere with the 39K+ signal in the LAMMA spectra according to measurements on pure sodium salts. Normally, only M + signals were used but for Si and Zr appreciable contributions of oxide, and hydroxide species had to be taken into account. The extent of these molecular ion contributions appears from Table 2. Even for the oxygen rich augite (ca. 50 y0 0), oxygen flooding appears to have a non-negligible effect on the relative abundance of the monoxide (cl)
I
I
I
I
I
I
I
I
45
5
55
6
65
7
75
6
7
75
6
I 7
I 75
I 0
IP-AE
(eV)
(b)
+r m .S
08 04
;
0
CT -0.4 3 -0
6
45
5
55
6
IP-AE
65
(eV)
Cc)
cn -043 -06-I
2 I 45
I 5
I 55
I 6
I 65
_
IP-AE(eV)
Fig. 1. Plot of log (RX (B,,&~BM+) vs If -AE of augite. (a) Ar+ bombardment, 0-20eV secondary ions; (b) Ar+ bombardment, O2 bleed-in, O-20 eV secondary ions; (c) 0; bombardment, O-20 eV secondary ions. SA(B)%/S-6-M
J. M.
BEUSEN et al. (a)
I 45
I 5
I 55
I 6
I 65
I 7
I 75
I 65
I 7
I 75
I 6
IP-AE(eV) (b)
-2-
-3
I 45
I 5
I 55
I 6
*eS, . 8
I P - AE (eV)
Fig. 2. Plot of log (RX (B~a/28~ 4) vs IP - AE of augite. (a) O- bombardment, &20 eV secondary ions; (b) LAMMA-500.
ions. The relative abundance of ZrO+/Zr’ in the LAMMA spectra is rather inaccurate but consistent with the ones obtained in SIMS for secondary ion energies between 0 and 20 eV. Secondary ion energies between 2040 eV decrease the relative intensity of the oxide ions, as expected [3,6-81. In Figs 1 and 2, the intensities of Zr+ and ZrO+ were added to yield a data point for this element which corresponds more closely with Eqn 1. For Si and Ca the same procedure was followed although the fit did not improve significantly. Figures 1 and 2 indicate a low RSC for Ti, possibly because the TiO+ intensity is non-negligible compared to that of Ti+. A correction was impossible due to spectral interferences in the mass range of the TiO+ species. For LAMMAthe average relative deviation from Eqn 1 is considerably larger than for the SIMS measurements in any of the experimental conditions selected in spite of the fact that Si was omitted from the average deviation and for the calculation of N, and T. The abnormally low Si+ intensity in the LAMMA-spectra not only occurred for augite but also for the other minerals (see below) and for materials such as silicate glasses and fly-ash [93. The intensities of the atomic ions as observed in SIMS may vary in different experimental conditions, e.g. as the energy bandpass of secondary ions is increased from @-20eV to 60-80 eV, the M+ intensities are considerably reduced (Si, Ca and Na by a factor 7, 15 and 43 resp.). Oxygen flooding in the sample chamber has practically no effect on the intensities of %+ and 56Fe+, when the sample is bombarded with Ar+ primary ions, whereas the 23Na+, 27Alt and 40Ca+ signals slightly decrease to 90%, 75 ‘;; and 67 ‘i;; respectively, of their original value. This indicates that for SIMS the LTE parameters depend on the experimental conditions used. For LAMMA, instrumental conditions also influence the LTE-parameters. In the first place there is the power dissipation into the sample. On the same augite samples considerably higher 7; and N, values were obtained with the LAMMAinstrument. In the second place variation of instrumental parameters such as extraction lens or reflector potential give rise to significantly different LAMMA_spectra[20]. A direct comparison of the data [2OJ E. MICHIELS and R. GIJLIELS, submitted
to Spectrochim. Acta. B.
849
Quantitative analysis of silicate minerals
obtained with LAMMA and SIMS is therefore difficult. Nevertheless, the results of Table 2 agree reasonably well between both methods. Hornblende
LTE parameters obtained for different SIMS working conditions and for LAMMAand LAMMAstandard measurement conditions are summarized in Table 3. For SIMS, 54Fe+ was used to monitor iron as s6Fe+ was strongly interfered by *‘CaO+ and 3gKOH+. Monoxide corrections were not carried out for Ti and Cr owing to mass spectral interferences in the *‘TiO* and 52Cr0+ regions, but were taken into account for Si and Ca. Under Ar+ bombardment the ratios (I,o++I,o,+)/I,, were 0.15 and 0.11 respectively for these elements. The largest deviation from Eqn 1 occurred for Fe, probably through an unresolvable spectral interference of the All cluster ion. The other elements give a mean error factor of 1.41. This deviation is somewhat reduced when the energy bandpass is set at 50-70 eV, probably as a result of suppression of molecular interferences. & and N, are increased when oxygen is admitted in the sample chamber or when negative atomic oxygen is used as a primary ion beam. The precision of the mode1 increases considerably. The LTE-parameters increase drastically in the LAMMAinstrument because of a more efficient laser energy dissipation in the sample. The applicability of the model improves dramatically, as appears from Fig. 3, where results for SIMS with O- bombardment and LAMMAand 1000 for standard measurement conditions are given. Lepidolite
On lepidolite only negative oxygen bombardment was possible in SIMS because of the very low electrical conductivity. Monoxide corrections were carried out for silicon and calcium. 3oSi was used, as 2eSi+ suffered from a large interference due to 27AlH+. For low energy secondary ions a K value of 10,590 & 530 K could be derived. The closeness of the fit was much worse than for the other materials with an average deviation of nearly 100%. ‘Li behaves erratically with a.negative deviation from Eqn (1) by a factor of 3.6. The result for fluorine with an ionization potential of 17.42 eV i.e. a factor of 2 above the range for the other elements examined, is 280 times too large. LAMMA with the transmission instrument gives a high q value of 10,500 K but with an average deviation from the LTE plot of a factor of 2.7. The largest deviation again occurs for Si with an RSC that is a factor of 8 too low. The “temperature” does not increase significantly with the reflection type geometry and neither does the precision but the deviation for silicon is considerably reduced. Fluorine could not be measured with the LAMMAinstrument whereas with the LAMMAit was almost a factor of 2 x lo* too high, compared to the intensity expected from the model. CONCLUSIONS
The physical significance of the LTE-approach in SIMS, and also in LAMMA is limited, as was already stressed in the literature [l, 193. Nevertheless, the comparison of the data
Table 3. Results for hornblende LAEFIA
SIMS
Ar+ (O-20 eV)
Tj
(K)
Ne (~rn-~)
9340 4.0
1017
Ar++O2 (O-20
ev)
10540 1.1 1018
A?+
0-
(50-70
eV)
98 10 6.3
1017
(O-20
“5004’ ev)
10270 8.4
80 1000”
1017
9770 3.3
12800 1017
3.4
1018
Mean error factor
1.41
1.28
1.35
1.20
4.60
1.11
J. M. BEUSEN et al.
850
(cl)
I P - AE (eV)
(b)
0804-
04-
.
-084
I
I
I
I
I
I
I
45
5
55
6
65
7
75
2% . I
t3
I P - LIE (eV) ICI
-04-084
1
I
I
I
I
I
I
I
45
5
55
6
65
7
75
8
I P-AEleV)
Fig. 3. Plot of log (RSC (B,&ZBy+) vs IP-AE for hornblende. (a) O- bombardment O-20 eV secondary ions; (b) LAMMA-500; (c) LAMMA-1000.
obtained on the samples discussed in this paper, indicates that, at least semi-quantitatively, the relative intensities of positive ions obtained in both methods follow a similar dependence with the ionization potential. The dependence breaks down for some elements in extreme conditions, e.g. for Si in LAMMA measurements of the augite or for F in the SIMS and LAMMAmeasurements of lepidolite (high ionization potential). The T,-values obtained with both methods are comparable semi-quantitatively. They increase in the ranking order augite, hornblende, lepidolite for both methods. For LAMMA, the increase in “temperature” could possibly be related to the efficiency of energy coupling of the laser pulse with the sample, a factor dependent on sample composition and structure. The considerably larger deviations from Eqn (1) obtained with LAMMA do not depend on the poorer applicability of the LTE-approach with this method as is illustrated by the excellent results obtained with the reflection type LAMMAinstrument on hornblende (Fig. 3). Also sample heterogeneity can be precluded for the minerals selected in this study[12]. Rather it must be associated with other factors: the inherent lack of repro-
Quantitative analysis of silicate minerals
851
ducibility of the method [21], and spectral interferences since this instrument provides less resolution than SIMS apparatus, which can provide for high mass resolution or molecular ion discrimination. From these and other measurements [22,23] it appears that the relative molecular ion intensities are comparable for SIMS and LAMMA spectra. When the “apparent temperature” increases LAMMA results tend to agree better with the LTE-model while molecular interferences drop in importance. Acknowleclyemenrs-This research was carried out within Research Project 80/85-10 of the Interministrial Commission for Science Policy Belgium. The LAMMA-IOOO measurements were performed through the courtesy of Leybold-Heraeus, Cologne, F.R.G.
[21] E. DENOYER, R. VAN GRIEKEN, F. ADAMS and D. F. S. NATUSCH, Anal. Chem. 54, 26A (1982). [22] E. MICHIELS, A. CELIS and R. GIJBELS, Inr. J. Mass Spectrom. lon Phys. 47, 23 (1983). [23] C. PLOG. L. WIEDMANN and A. BENNINGHOVEN, Surj. Sci. 67, 565 (1977).