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AUTOMATED QUANTITATIVE ELECTRON MICROPROBE ANALYSIS OF PARTICULATE
MATERIAL
P. van Dyck, H. Storms, R. van Grieken
To cite this version:
P. van Dyck, H. Storms, R. van Grieken. AUTOMATED QUANTITATIVE ELECTRON MICRO-
PROBE ANALYSIS OF PARTICULATE MATERIAL. Journal de Physique Colloques, 1984, 45 (C2),
pp.C2-781-C2-784. �10.1051/jphyscol:19842179�. �jpa-00223853�
AUTOMATED QUANTITATIVE ELECTRON MICROPROBE ANALYSIS OF PARTICULATE MATERIAL
P. Van Dyck, H. Storms and R. Van Grieken U n i v e r s i t y of Antwerp, Belgium
Resume
-
Une sonde e l e c t r o n i q u e , equipee d'un spectrometre non-dispersif de rayons-X e t d ' u n systeme de d i g i t a l i s a t i o n du s i g n a l des e l e c t r o n s retro- d i f f u s e s , peut permettre de mesurer e t d ' a n a l y s e r de nombreuses p a r t i c u l e s d ' une f a ~ o n automatique. Un programme d ' o r d i n a t e u r ae t e
developpe a f i n d ' u t i l i s e r l e s donnees sur l a forme e t l e s mesures des p a r t i c u l e s pour des analyses q u a n t i t a t i v e s automatiques e t pour comparer l e s c a r a c t e r i s t i q u e s des d i f f e r e n t e s methodes de c o r r e c t i o n pour l e s e f f e t s de m a t r i c e . A b s t r a c t-
An automated e l e c t r o n microprobe, equipped w i t h an energy-dis- p e r s i v e X-ray spectrometer and an a d d i t i o n a l backscattered e l e c t r o n s i g n a l d i g i t a l i z a t i o n system, can a l l o w r a p i d s i z i n g and major element a n a l y s i s on numerous p a r t i c l e s . A software package has been developed t o e x p l o i t t h e p a r t i c l e s i z e and shape i n f o r m a t i o n t o achieve q u a n t i t a t i v e a n a l y s i s o f s i n g l e p a r t i c l e s , and t o compare t h e performance o f t h e d i f f e r e n t m a t r i x c o r r e c t i o n procedures.I n the i n c r e a s i n g l y important f i e l d o f p a r t i c u l a t e m a t e r i a l a n a l y s i s e l e c t r o n micro- probe X-ray microanalysis (EPXMA) can o f f e r a good p r e c i s i o n , h i g h speed and wide elemental range. New commercially a v a i l a b l e systems i n combination w i t h EPXMA can now a u t o m a t i c a l l y l o c a t e and s i z e up i n d i v i d u a l p a r t i c l e s by making use o f t h e back- s c a t t e r e l e c t r o n s i g n a l , and subsequently analyse them v i a energy-dispersive X-ray d e t e c t i o n . However, t h e a n a l y s i s r e s u l t s were h i t h e r t o q u a l i t a t i v e only. I n t h e present study, a computer a l g o r i t h m has been developed t h a t makes use o f t h e s i z i n g i n f o r m a t i o n t o c o r r e c t the measured X-ray i n t e n s i t i e s a u t o m a t i c a l l y f o r t h e p a r t i c l e m a t r i x e f f e c t s . This new software package was s u c c e s s f u l l y t e s t e d . I t s implementa- t i o n leads t o f u l l y computer c o n t r o l l e d s i z i n g and q u a n t i t a t i v e elemental a n a l y s i s o f i n d i v i d u a l p a r t i c l e s .
EQUIPMENT
Partricneswere sized and analysed w i t h a JEOL JXA-733 e l e c t r o n probe X-ray microana- l y z e r , equipped w i t h t h e TN-2000 energy-dispersive X-ray d e t e c t i o n system o f Tracor Northern and two M T l l tape u n i t s . The r e s u l t i n g spectra, w i t h t h e corresponding p a r t i c l e s i z i n g data, were t r a n s f e r r e d v i a magnetic tapes t o a VAX 11/780 mainframe computer, w i t h software developed a t t h i s l a b o r a t o r y f o r processing.
AUTOMATED SIZING AND QUANTITATIVE ANALYSIS
-
I n t h e p a r t i c l e d e t e c t i o n and s i z i n g r o u t i n e t h e e l e c t r o n beam scans t h e t a r g e t . surface. A p a r t i c l e ' i s "detected" when the d i g i t a l i z e d backscattered e l e c t r o n s i g n a l exceeds a chosen threshold. The path l e n g t h d u r i n g which t h e s i g n a l exceeds t h e t h r e s h o l d i s i n t e r p r e t e d as one diameter o f t h e p a r t i c l e . The mean diameter o f the p a r t i c l e i s the average r e s u l t o f 16 d i f f e r e n t l y o r i e n t e d cross-section measurements.
The f i n g e r p r i n t o r energy-dispersive X-ray spectrum o f the p a r t i c l e i s accumulated w i t h the e l e c t r o n beam p o s i t i o n e d i n t h e c e n t r e o f t h e p a r t i c l e .
To d e r i v e q u a n t i t a t i v e data the spectrum should f i r s t o f a l l be deconvoluted. The importance o f t h i s step i s very o f t e n overlooked i n automated f e a t u r e a n a l y s i s a l - g o r i thms. For example, d i f f e r e n c e s between X-ray i n t e n s i t i e s o u t p u t t e d from the powerful AXIL-program ( A n a l y s i s o f X-ray spectra by I t e r a t i v e Least-squares f i t t i n g ) developed i n t h i s l a b o r a t o r y / l / , and the i n t e n s i t i e s c a l c u l a t e d by the peak deconvolu-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842179
C2-782 JOURNAL DE PHYSIQUE
t i o n r o u t i n e s a l r e a d y present on t h e system (which are based on t h e f i l t e r i n g p r i n c i - p l e ) amounted t o 21 % f o r Na and 16 % f o r C1 f o r a simple NaCl spectrum, where peak i n t e r f e r e n c e o v e r l a p i s absent. I t i s obvious t h a t those e r r o r s are r e f l e c t e d i n t h e f i n a l r e s u l t s . Since, however t h e AXIL peak deconvolution r o u t i n e i m p l i e s a s i g n i f i c a n t c a l c u l a t i o n time, i t was decided n o t t o use t h e LSI 11/23 computer a v a i - l a b l e w i t h t h e EPXMA instrument and t h e o p e r a t i n g system. Instead, t h e X-ray spec- trum o f each l o c a l i z e d and s i z e d p a r t i c l e i s t r a n s f e r r e d t o a tape medium t o g e t h e r w i t h p a r t i c l e s i z i n g i n f o r m a t i o n . A f t e r completion of the t o t a l sample f i e l d search w i t h t h e microprobe, a l l t h e raw spectra a r e deconvoluted o f f - l i n e on a VAX 11/780 computer, u s i n g t h e AXIL-program.
I n the n e x t step one has t o convert peak i n t e n s i t i e s t o weight f r a c t i o n s , w h i l e t a - k i n g i n t o account t h e well-known p a r t i c l e m a t r i x e f f e c t s : v a r y i n g a n a l y s i s volumes, p a r t i c l e dependent X-ray path l e n g t h s and X-ray a b s o r p t i o n e f f e c t s , e l e c t r o n side- s c a t t e r e f f e c t s and secondary fluorescence by c h a r a c t e r i s t i c and continuum r a d i a t i o n . To c o r r e c t f o r t h i s m a t r i x e f f e c t s t h e m o d i f i e d ZAF-version o f Armstrong and Buseck /2/ was p r e f e r r e d over t h e peak-to-background procedure/3,4/ because i t i s more amply developed and wide spread/5/ and can be combined b e t t e r w i t h t h e a v a i l a b l e EPXMA setup. I n t h e p a r t i c l e ZAF-correction scheme o f Armstrong and Buseck /2/ o n l y t h e X-ray a t t e n u a t i o n c o r r e c t i o n i s considered i n d e t a i l , w h i l e the o t h e r p a r t i c l e e f f e c t s a r e supposed t o be n e g l i g i b l e o r t o be s i m i l a r f o r d i f f e r e n t elements, hence cancelled o u t i n t h e n o r m a l i z a t i o n o f t h e r e s u l t s . The general form o f the absorp- t i o n c o r r e c t i o n f a c t o r f (p,D) i s
,
Y ~ ( P Z ) ,X~(PY,PZ)1
@(Pz) e x @ - ~ g ( ~ x , PY,PZP)I d ( p x ) d ( ~ y ) d ( p z ) pz=o PY = yl(pz) PX = Xl ( P Y ~ P Z )
f ( L O ) = (1)
@(Pz) d(pz)
where A. = area o f t h e p o r t i o n o f t h e e l e c t r o n be,am passing through the t o p surface o f the p a r t i c l e
a = v e r t i c a l thickness o f t h e p a r t i c l e
@(pz) = X-ray production f u n c t i o n , depending on t h e d e n s i t y pand l i n e a r depth z ( I n our procedure t h e @(pz) equations o f B a s t i n e t a l . / 6 / a r e used) p = 1 i n e a r a b s o r p t i o n c o e f f i c i e n t o f t h e considered r a d i a t i o n
D = h o r i z o n t a l diameter o f the p a r t i c l e
g (px, py, pz, D) = e f f e c t i v e path l e n g t h f u n c t i o n
For t h e two i n n e r i n t e g r a t i o n s (over px and p y ) Armstrong and Buseck/2/ d e r i v e d general expressions f o r d i f f e r e n t p a r t i c l e models, i . e . geometrical p a r t i c l e types.
However, the t o t a l i n t e g r a l cannot be converted t o a closed form s o l u t i o n . Hence a numerical i n t e g r a t i o n has t o be performed over pz. This necessitates again the use o f a f a s t computation system l i k e the VAX 11/780. I n s p e c t i o n o f e q . ( l ) r e v e a l s t h a t t h i s m a t r i x c o r r e c t i o n depends on t h e p a r t i c l e diameter, d e n s i t y and thickness,and t h e model used. Therefore automatic procedures have t o be developed and t e s t e d t o accound f o r these unknown f a c t o r s . Since t h e average p a r t i c l e diameter i s known from the s i z i n g step, o n l y t h r e e parameters remain t o be t r e a t e d .
F i r s t l y , t h e d e n s i t y of t h e p a r t i c l e can simply be obtained by w e i g h t i n g t h e densi- t i e s of t h e pure elements ( o r corresponding oxides) according t o t h e i r concentrations.
A simple t e s t f o r t e n common randomly selected g e o l o g i c a l m a t e r i a l s , such as a l b i t e , diopside, s p i n e l , e t c . , revealed t h a t t h e r a t i o between c a l c u l a t e d and r e a l d e n s i t y amounts t o 0.998 _+ 0.084. This procedure i s q u i t e s a t i s f a c t o r y ; indeed t h e densi- t y i n f l u e n c e s the l i m i t o f t h e o u t e r i n t e g r a t i o n and t h e u n c e r t a i n t y on t h i s l i m i t , induced by t h e e s t i m a t i o n of t h e l i n e a r p a r t i c l e thickness a, w i l l be much more s i g n i f i c a n t .
production y i e l d a s a function of p a r t i c l e t h i c k n e s s ) d i d not give any successful r e s u l t s a s y e t , i t was decided t o adopt t h e a b i t r a r y approach of Armstrong and Buseck/2/. They assumed t h a t p a r t i c l e s w i l l e v e n t u a l l y f i n d a most s t a b l e p o s i t i o n on t h e i r l a r g e s t surface. Hence t h e v e r t i c a l t h i c k n e s s w i l l be smaller than t h e automatically measured diameter. Armstrong and Buseck supposed t h a t a p a r t i c l e thickness-to-diameter r a t i o of 0.5 would be most useful i n t h e p a r t i c l e a n a l y s i s procedure/2/.
The t h i r d unknown f a c t o r i s t h e p a r t i c l e model, which i n f l u e n c e s t h e e f f e c t i v e path length function g i n e q . ( l ) . Armstrong and Buseck/2/calculated g-functions f o r t h e following models: f l a t - t o p group ( r i g h t r e c t a n g u l a r prism, tetragonal prism, c y l i n - d e r ) , peak-top group ( t r i a n g u l a r prism, square pyramid), rounded ton group (hemis- phere, s p h e r e ) . Up t o now t h e only information t h a t t h e automatic s i z i n g s t e p y i e l d s with r e s p e c t t o t h e p a r t i c l e model i s defined a s t h e projected p a r t i c l e a r e a divided by n/4-times t h e square of t h e average horizontal diameter. Since t h i s f a c t o r only r e f l e c t s t h e two dimensional projected shape of t h e p a r t i c l e , i t i s not possible t o automatically d i s c r i m i n a t e between t h e d i f f e r e n t p a r t i c l e models. The c r i t e r i u m t o observe t h e c h a r a c t e r i s t i c X-ray v a r i a t i o n , o r t h e d i g i t a l i z e d topo- graphic b a c k s c a t t e r s i g n a l , across a p a r t i c l e and s e l e c t automatically a s p e c i f i c p a r t i c l e model based on t h i s v a r i a t i o n , i s s t i l l under i n v e s t i g a t i o n .
RESULTS AND DISCUSSION
Table 1 shows t h e average r e s u l t s obtained from t h e automated EPXMA-analysis of 70 spherical g l a s s p a r t i c l e s (NBS standard # 610), with diameters ranging from 2.5 t o 14 pm, when various conventional ZAF-correction algorithms a r e applied and f o r the modified p a r t i c l e ZAF c o r r e c t i o n with d i f f e r e n t assumed p a r t i c l e models.
When one compares t h e r e s u l t s obtdined v i a t h e d i f f e r e n t ZAF-corrections t o t h e real value, i t appears hard t o recommend one p a r t i c u l a r procedure, because t h e r e i s nearly no d i f f e r e n c e between t h e r e s u l t s . All t h e procedures t e s t e d indeed represent a dramatic improvement compared t o t h e use of uncorrected normalized X-ray i n t e n s i t i e s . Of t h e p a r t i c l e models, t h e rounded top model d o e s n o t y i e l d t h e b e s t r e s u l t s . The reason f o r t h i s i s probably i n t h e f a c t t h a t a spot mode was used during t h e p a r t i - c l e a n a l y s i s . The e l e c t r o n beam h i t s a small p a r t of t h e s u r f a c e and t h e assump- t i o n s of Armstrong and Buseck i n t h e algorithm a r e not t r u l y f u l f i l l e d .
Since t h e computer software f o r a l l t h e models has duly been implemented now, i t w i l l s t r a i g h t f o r w a r d l y be p o s s i b l e i n t h e near f u t u r e t o e v a l u a t e t h e various proce- dures f o r p a r t i c l e a n a l y s i s by studying p a r t i c l e s of much smaller s i z e . A r a s t e r - scan o r s t a r - s c a n w i l l be used during a n a l y s i s t o give more r e l i a b l e information about t h e X-ray i n t e n s i t y f o r a c e r t a i n p a r t i c l e shape.
Table 1: Average r e s u l t s ( i n %) f o r 70 NBS 610 g l a s s standard spheres, obtained by using d i f f e r e n t conventional ZAF-correction models f o r bulk samples and considerina t h e article aeometrv*
A . conventional ~ ~ ~ l c o r r e c t i o n procedures- Element
Na A 1 S i Ca
C e r t i f i e d concentrations 10.2
1.70 32.9
7.6
Concentrations found via t h e algorithm of
Bas-kin e t a1 ./6/
11.3 f 0.01 - 1.67+ 0.01 32.3
+
0.16.84
+
0.01Duncumb-Reed/7/
11.1
+
0.11.68+ 0.01 32.3 k 0 . 1
6.87_+ 0.01
Love e t a1/8/
11.3
+
0.1 1.67+ 0.01 32.3+
0.16.84+ 0.01
Brown /9/
11.3
+
0 . 1 1.67+ 0.01 32.3+
0 . 16.85+ 0.01
C2-784 JOURNAL DE PHYSIQUE
h he
u n c o r r e c t e d n o r m a l i z e d X-ray i n t e n s i t i e s ( t a k i n g i n t o account oxygen by stoecheometric c o n s i d e r a t i o n s ) l e a d t o t h e f o l l o w i n g r e s u l t s :1.29 % Na, 1.03 % A l , 39.7 % S i and 8.21 % Ca.
B. P a r t i c l e ZAF-correction procedures/2/:
REFERENCES
1- A x i l X-ray A n a l y s i s Package, Users' Guide, Canberra I n d u s t r i e s I n c . , 45 Cracey Avenue, Meriden, CT 06450 (1981)
Element Na A1 S i C a
2- J. ARMSTRONG and P. BUSECK; ~ n a i . Chem., 47 (1975) 2178
3- J. SMALL. K. HEINRICH. D. NEWBURY and R. WKLEBUST. Scannino E l e c t r o n Microscoov.
. - .
SEM I n c .
;
AMF o ' H ~ ~ ~ , ~ I L 60666, I (1979) 801.4- P. STATHAM, Microbeam A n a l y s i s , Td. D. Newbury, San F r a n c i s c o Press. Inc., 547 Howard St., San Francisco, CA 94105 (1979) 247.
5- J. WERNISCH and E. KETTNER, Microchim. Acta, I (1982) 73.
6- G. BASTIN, F. VAN LOO and H. HEYLIGERS, submiTted t o X-ray Spectrometry 7- P. DUNCUMB and S. REED, Q u a n t i t a t i v e E l e c t r o n Probe M i c r o a n a l y s i s , Ed. K.
H e i n r i c h , NBS Spec. Publ. 298 (1968) 133.
8- G. LOVE, M. COX and V. SCOTT, J. Physics D: Appl. Phys., 11 (1978) 7 9- J. BROWN, Microbeam A n a l y s i s , Ed. D. Newbury, San ~ r a n c i s z Press. Inc.,
CA-94101-6800(1982) 151.
C o n c e n t r a t i o n s found assuming as a model R i g h t Rectangular p r i s m
10.0
+
0.11 . 5 4 j 0.01 32.9 2 0.1
7.372 0.01
Square pyramid 9.02
+
0.011.64 2 0.01 32.2 2 0.1
7.67 2 0.01 T r i a n g u l a r p r i s m
9.29 2 0.01 1.64
+
0.0133.2 0.1
7.43
2
0.01Hemisphere 9.76+ 0.03 1.462 0.01 33.1 2 0 . 1
7.312 0.01
Sphere 8.58k0.15 1.3820.04 34.8 2 0 . 1
6.012 0.05
