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PRESENT STATUS OF X-RAY FLUORESCENCE ANALYSIS OF TRACE ELEMENTS USING
SYNCHROTRON RADIATION
P. Chevallier
To cite this version:
P. Chevallier. PRESENT STATUS OF X-RAY FLUORESCENCE ANALYSIS OF TRACE EL-
EMENTS USING SYNCHROTRON RADIATION. Journal de Physique Colloques, 1987, 48 (C9),
pp.C9-39-C9-47. �10.1051/jphyscol:1987905�. �jpa-00227200�
PRESENT STATUS OF X-RAY FLUORESCENCE ANALYSIS OF TRACE ELEMENTS USING SYNCHROTRON RADIATION
P. CHEVALLIER
Universite Paris VI and LURE, F-91405 Orsay Cedex, France
INTRODUCTION
Whenever physicists develop a new machine it seems t h a t chemists t r y and make an analytical tool out of it, and sometimes with great success. This has a l s o happened with the application of Synchrotron Radiation (S.R.) t o X-ray fluorescence analysis.
Since the pioneer work of C. Sparks a t Stanford (1) a l i t t l e more than ten years ago t h i s technique was tested i n most Synchrotron Radiations centers and we wish t o give a general view of the present s t a t u s i n t h i s f i e l d .
By now there i s a growing need, i n every human f i e l d of a c t i v i t y , f o r the deter- mination and characterization of elements a t t r a c e concentration, t h a t i s below one
p a r t per million by wieght. For q u a l i t a t i v e a s well a s quantitative analysis X-ray fluorescence i s often used a s a signature of elemental composition. Coupled with other techniques it w i l l soon be able t o give i n t e r e s t i n g chemical information, even a t low concentration, thanks t o the intense and perfectly well adapted Synchrotron Radiation sources.
PRINCIPLE OF X-RAY FLUORESCENCE ANALYSIS
I n principle X-ray flourescence analysis i s a very simple technique. A given sample is placed i n a chamber. A beam s t r i k e s t h i s sample i n order t o create inner- s h e l l vacancies and a detector records the X-ray fluorescence spectrum.
This is a q u a l i t a t i v e method of analysis a s each X-ray l i n e i s c h a r a c t e r i s t i c of a given element (or should be). It i s a l s o quantitative since the number of recorded X ray i s proportional t o the concentration of t h e emitting element. Moreover it i s q u i t e universal except f o r very l i g h t elements (2<12) because of t h e l r too low fluo- rescence yield and uneasy t o detect c h a r a c t e r i s t i c X-ray l i n e s . Another general and q u i t e interesting feature i s t h a t it is often a non destructive analytfbal method.
The v a r i e t y of beams (photons, electrons, heavy charged p a r t i c l e s ranging from proton t o uranium) and of detectors now available gave r i s e t o many d i f f e r e n t com- binations, each owner claiming h i s t o be the best. In f a c t , a s long a s we a r e only concerned with the i d e n t i f i c a t i o n of a p a r t i c u l a r element a t the lowest concentration possible, physical reasons should help and decide which r e a l l y i s the best arrangement
SEARCH FOR SENSITIVITY USING SYNCHROTRON-RADIATION Gen&
c o n a i d W n s
Whatever the experimental s e t up w i l l be we s h a l l end with an X-ray spectrum. For high s e n s i t i v i t y we want the peak area of a given X-ray l i n e t o be the l a r g e s t a s possible. Unfortunately many other r a d i a t i v e processes of i n t e r a c t i o n occur with the whole sample, i n competition with X-ray fluorescence of the searched t r a c e elements.
Consequently the fluorescence peak always sits on a continuous background t h a t should be minimum.
One can then calculate minimum detectable l i m i t s (M.D.L.) by comparing the fluo- rescence i n t e n s i t y (S) with the square r o o t of t h e background (B). S t a t i s t i c a l consi-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987905
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PHYSIQUEd e r a t i o n s t h e n t e l l s u s t h a t i n o r d e r t o i d e n t i f y a g i v e n element t h e r a t i o of t h e s e q u a n t i t i e s should exceed a g i v e n number, t h e v a l u e of which s t r o n g l y depends on t h e u s e r s optimism.
I n a s i m p l i f i e d manner we c a n w r i t e , f o r a t h i n t a r g e t , t h a t t h i s r a t i o depends on t h e f o l l o w i n g f a c t o r s :
where N z and NT = number of atoms of element Z and of t h e whole sample per cm 2 uFZ = f l u o r e s c e n c e c r o s s s e c t i o n f o r element Z
OB = background p r o d u c t i o n c r o s s s e c t i o n I = number of i n c i d e n t p a r t i c l e s on t h e sample
0
Z = o v e r a l l d e t e c t o r e f f i c i e n c y . Then M.D.L. a r e o b t a i n e d by r e s o l v i n g :
- '" - % fi . g i v e n number.
5
This simple e q u a t i o n w i l l h e l p u s t o d i s c u s s t h e i n t e r e s t of S.R. e x c i t e d X-ray f l u o r e s c e n c e over o t h e r p a r t i c l e s .
Tahget and d&ecton in6luence
The f i r s t and l a s t term of e q u a t i o n 2 a r e independent of t h e c h o i c e of t h e e x c i t i n g beam. We s h a l l o n l y mention t h a t t h e sample backing should be a s t h i n a s p o s s i b l e t o keep NT small. Care must a l s o be t a k e n t h a t t h e elemental c o n s t i t u t i o n o r t h e s t r u c t u r e of t h i s backing c a n be of s i g n i f i c a n t importance on t h e background i n t e n s i t y .
It appears a l s o t h a t t h e d e t e c t o r o v e r a l l e f f i c i e n c y i s of primary i n t e r e s t and should b e l a r g e . For t h i s r e a s o n most experimental s e t up now u s e S i ( L i ) d e t e c t o r s t o r e c o r d t h e f l u o r e s c e n c e spectrum.
High i n t e m L t y and dtcawbach
I n t e n s i t y (we mean t h e number of i n c i d e n t p r o j e c t i l e ) should b e v e r y h i g h a s M.D.L. o n l y improves w i t h i t s s q u a r e r o o t . The c h o i c e of t h e beam e n t e r s t h e n i n t o c o n s i d e r a t i o n and we must wonder wether o r n o t t h e sample w i l l support without any damage t h e d e s i r e d h i g h i n t e n s i t y .
Charged p a r t i c l e s would seem t o have t h e f a v o r because t h e i r beam a r e u s u a l l y w e l l c o l l i m a t e d and moreover they can b e r a t h e r e a s i l y focused on small samples (we a r e n o t speaking about microprobes). T e c h n i c a l l y i n t e n s i t y of t h e o r d e r of a p A c a n b e o b t a i n e d o n small a r e a s , t h a t means 10'2 t o 1013 p r o j e c t i l e s p e r second. Conventional X-ray s o u r c e s cannot compete b u t because of i t s i n t e n s e beam of v e r y low angular d i v e r g e n c e S.R. becomes a p e r f e c t t o o l .
When c o n s i d e r i n g t h e sample a b i l i t y t o s u p p o r t such h i g h f l u x S.R. becomes even more i n t e r e ~ t i n g . I n Table 1 we have r e p o r t e d t h e c a l c u l a t e d energy ( i n keV) l e f t i n v a r i o u s 1
urn
t h i c k t a r g e t s when one c h a r a c t e r i s t i c X-ray l i n e i s produced by d i f f e r e n t p r o j e c t i l e s of d i f f e r e n t e n e r g i e s . The i n t e r e s t of monochromatic photons i s obvious and can b e e a s i l y understood. The photon which has n o t i n t e r a c t e d p a s s e s through t h e sample w i t h o u t l e a v i n g any energy whereas a charged p a r t i c l e l o o s e s i t s energy d u r i n g i t s showing down i n a q u i t e s h o r t r a n g e wether o r n o t it h a s c r e a t e d a u s e f u l i n n e r - s h e l l vacancy.Energy l e f t ( i n keV) &n d i f f e r e n t lum t h i c k t a r g e t s by v a r i o u s p r o j e c t i l e s f o r t h e p r o d u c t i o n of one c h a r a c t e r i s t i c X-ray.
P r o j e c t i l e Energy (KeV)
T a r g e t A 1 (K) Cu (K) Au (L3)
This i s a n o t h e r i n t e r e s t i n g p o i n t f o r photon e x c i t a t i o n . P h o t o i o n i z a t i o n c r o s s s e c t i o n s a r e u s u a l l y l a r g e , much l a r g e r t h a n i n n e r - s h e l l i o n i z a t i o n c r o s s s e c t i o n s by charged p a r t i c l e s of u s u a l energy f o r f l u o r e s c e n c e a n a l y s i s . Moreover f o r a given energy t h e p h o t o i o n i z a t i o n c r o s s s e c t i o n s c l a e s l i k e
z4
whereas i n n e r - s h e l l i o n i z a t i o n p r o b a b i l i t y d r o p s v e r y r a p i d l y , n e a r l y a s 2-4. Remembering t h e u n i v e r s a l abundance of elements d r o p s by more t h a n 1 2 o r d e r s of magnitude between hydrogen and uranium t h i s behavior of t h e p h o t o i o n i z a t i o n c r o s s s e c t i o n i s most i n t e r e s t i n g i n t r a c e element a n a l y s i s .This a p p e a r s even more s t r i k i n g l y when c o n s i d e r i n g t h e t h i c k t a r g e t X-ray y i e l d . For a s u i t a b l e chosen energy no more t h a n 3 o r 4 i n c i d e n t photons a r e n e c e s s a r y t o g e t one Ka l i n e of n e a r l y any element whereas 100 t o 10 000 charged p a r t i c l e s a r e needed f o r t h e same r e s u l t .
Bachghound and %A o h i g h Photon
F i n a l y y t h e background under t h e considered f l u o r e s c e n c e peak should be a s low a s p o s s i b l e . I n o r d e r t o s e e whether t h i s background c a n be reduced o r n o t we must f i n d i t s o r i g i n . For t h i s d i s c u s s i o n we s h a l l r e f e r t o f i g u r e 1 r e p r e s e n t i n g t h e r e s u l t of c a l c u l a t i o n s from J a k l e v i c ( 2 ) . The X-ray f l u o r e s c e n c e of a same sample c o n s i s t i n g of 250 ng of elements A l , S, C a , Fe, Cu, Br d e p o s i t e d on a 25 mg cmV2 carbon backing
( t h a t i s a I0 ppm concentration) is recorded with a S i ( L i ) d e t e c t o r of 200 eV energy r e s o l u t i o n . E x c i t a t i o n i s done by e l e c t r o n protons and continuous X-ray spectrum
( l i k e t h a t of S.R. o r W anode X-ray t u b e ) . 5
5
For e l e c t r o n s ( F i g u r e l b ) one only s e e s a continuous background and i n t h i s c a s e M.D.L. should be s e t more l i k e l y around 100 ppm. T h i s background i s due t o t h e brans- s t r a h l u n g accompanying t h e slowing down of t h e i n c i d e n t e l e c t r o n s a s w e l l a s t h e e j e c t e d e l e c t r o n s d u r i n g c o l l i s i o n s . I t cannot be avoided b u t i n p r i n c i p l e minimized when u s i n g lower energy e l e c t r o n s ( b u t i o n i z a t i o n c r o s s s e c t i o n s w i l l a l s o d e c r e a s e ) and samples w i t h low Z matrix.
E l e c t r o n 20
I4 13
The spectrum o b t a i n e d under 3 MeV p r o t o n s e x c i t a t i o n (FIgure l a ) i s much more i n t e r e s t i n g a s X-ray peaks a l s o appear and one c a n s a y t h a t f o r elements Fe t o Br ( i n t h i s example) M.D.L. could be a s low a s I ppm. This i n c r e a s e i n s e n s i t i v i t y i s mainly due t o a d r a s t i c r e d u c t i o n i n background shape and o v e r a l l i n t e n s i t y . Bremsd
s t r a h l u n g due t o t h e slowing down of t h e p r o t o n becomes n e g l i g i b l e because of its rn-' dependence and t h e c o n t r i b u t i o n of t h e e j e c t e d e l e c t r o n s i s q u i t e low s i n c e t h e y u s u a l l y a r e e j e c t e d w i t h v e r y small k i n e t i c energy.
20
50 1900 1700
Proton
The l a s t spectrum ( F i g u r e l c ) f o r p h o t o e x c i t a t i o n , do show c h a r a c t e r i s t i c peaks b u t 10 ppm seems a l i m i t . I n t h i s c a s e t h e continuous background is due t o t h e
50
630 910
3000
170 31000 71000
5000
10000 21000
C9-42 JOURNAL DE PHYSIQUE
scattering (Rayleigh and Compton) of the incident beams on the sample. Of course the scattering cross sections are usually much smaller than photoionization cross sections but there are so many more atoms in the sample than the ones we are interested in that this background contributes to the great major part of the count rate when looking at trace elements.
Figure 1. X-ray fluorescence spectrum of a 10 ppm sample under various excitations (from Ref.2).
The situation changes drastically when using .monochromatic photons- as it appears from figure Id. In this case the scattered events are reduced to a sharp peak which energy is of course higher than that of the fluorescent lines- to be ohserved. Then the background is only due to the low energy tail of the total absorption peak observed with every Si(Li) "detector. Of course the signal is reduced but not as much as one could fear. Because of the E~3 dependence of the photoionization cross section it appears that most fluorescence events arise from those photons whose energy is just above the considered absorption edge while the others mostly contribute to scattering events and thus to the background. Then monochromatization ends up with much lower M.D.L.
Such monochromatic beam (or near monochromatic) can be obtained with properly filtered X-ray tubes. But S.R.has many unique characteristics that allow to take full advantage of this very intense beam :
(i) the low divergence of the beam, which is of the same order as the diffraction pattern of perfect crystals,allows for a very efficient monochromatization;
(ii) the high degree of horizontal polarization leads to a further reduction in scattering if the detector is placed in the plane of polarization and at 90* of the incident beam;
EXAMPLES
A more complete relation than equation 1 allows the calculation of M.D.L. This is a useless calculation as the results strongly depend on the machine used because the energy spectrum of S.R. spreads more or less in the high energy X-ray range and that the real beam divergence, its polarization and the electron source size changes dras- tically from a machine to another and even sometimes from a beam line to another. All these factors greatly affect the number of photons that can strike a given sample as well as the intensity of the unwanted scattered radiation.
In the following we shall present some typical results as a guide to get an idea of the power and promises of this technique.
Experimental set-ups
Different experimental set-ups have already been used in the various S.R. centers.
The simplest one consists of a set of slits that can define a very small aperture in the direct beam. Such a device is currently in use in Brookhaven (3) and because of the beam quality M.D.L. at the ppm level on very small samples are routingly reached. S.Sutton (4) has pointed out that under white beam excitation Bragg peaks are most likely to appear, mixed with the fluorescence spectrum, if microcrystals are present in the sample (as in geological samples). Such a phenomenon becomes quite improbable under monochromatic excitation and the observed spectrum will usually be free of such dangerous interferences. Nevertheless these Bragg peaks could happen to be of a certain aid to get some interesting structural informatibn of the sample.
Most of the other facilities use monochromatic excitation. The monochromator can consist of two parallel perfect crystals, often of the "channel cut'"
type. This is well adapted for the study of large samples, up to a few cm2, and Tery useful to get representative average quantitative results.
Curved crystals are also widely used. They allow an interesting focusing of the beam in the horizontal plane with a few tenfold increase on the i'ncident flux on the sample. This geometry is very interesting for the study of small samples, up to a few m2). Quite often mosaic crystals are then used to improve further the number of in- cident photons but with the drawback of a larger band pass.
Because of the high beam quality reached with new machines, especially the incre- dible gain in source brilliance (photons mrad-I mm-2 s-I) more sophisticated arran- gement are on the verge to be used. These new generation facilities will often include multilayer devices for pre-monochromatization and/or focusing reflecting mirors assicated with sophisticated monochromators to get rid of harmonics in the monochro- matic beam, etc... The final goal in these investigations is to build a scanning microprobe which could compete very well with what is currently obtained with electrons and protons.
To our knowledge every facility uses Si(Li) detectors to record the fluorescence spectrum.
Typical spectrum
Figure 2 shows a typical spectrum obtained at LURE-DCI (in Orsay). The sample is a Ig cellulose matrix with 1 ppm elements from V to Zn. The detector (Si(Li) of 26 mm2 area) is located 20 cm apart from the sample, the incident beam intensity has been attenuated by a 0,5 m thick aluminium filter and nevertheles-s the exposure time is only of 100 s. The running conditions for the beam was 1.72 GeV and 200mA. The characteristic peaks corresponding to the Ka lines of these elements show up nicely indicating a well under the ppm level M.D.L. But the most apparent feature is the huge peak due to scattering processes. Had it not existed the detector could have been set much closer from the sample with a resulting important increase in fluorescence
C9-44 JOURNAL
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PHYSIQUEi n t e n s i t y and a c o n s e c u t i v e r e d u c t i o n i n M.D.L.
2 .lo3
F i g u r e 2. T y p i c a l spectrum of 1 ppm V
...
Zn i n 1 g c e l l u l o s e m a t r i x ( s e e t e x t f o r d e t a i l s ) .This spectrum shows c l e a r l y t h a t M.D.L. a r e i n f a c t imposed by t h e i n a b i l i t y of S i ( L i ) d e t e c t o r s and of t h e whole a s s o c i a t e d e l e c t r o n i c t o keep an a c c e p t a b l e energy r e s o l u t i o n under h i g h count r a t e . Although s l i g h t improvements can be expected w i t h new machines because of a r e d u c t i o n i n Rayleigh s c a t t e r i n g , new d e t e c t o r d e v i c e s must be thought o f f .
Improvements c a n be a l s o o b t a i n e d when u s i n g u l t r a t h i n samples d e p o s i t e d on t o t a l l y r e f l e c t i n g backing such a s q u a r t z o r u l t r a p u r e s i l i c o n , a s p o i n t e d o u t by W.L. Dorn (5). This l e a d s t o d i f f i c u l t sample p r e p a r a t i o n which cannot apply t o any kind of specimen. There seems t o he a l s o some problems i n g e t t i n g r e l i a b l e quan- t i t a t i v e r e s u l t s w i t h t h i s geometry.
Anyhow i t i s h o p e l e s s t o observe t r a c e elements a t a sub ppm l e v e l i f f l u o r e s c e n t major elements a r e p r e s e n t a t t h e same time i n t h e sample a s long a s S i ( L i ) d e t e c t o r s a r e t o be used. A new d e v i c e , t r y i n g t o c o n c i l i a t e t h e advantages of S i ( L i ) d e t e c t o r s (namely a multielement a n a l y s i s w i t h h i g h e f f i c i e n c y ) and t h o s e of wavelength d i s p e r - s i v e systems (high r e s o l u t i o n t o avoid most of t h e energy i n t e r f e r e n c e s between many c h a r a c t e r i s t i c X-ray l i n e s ) , h a s been t e s t e d a t LURE w i t h some success. It a s s o c i a t e s a f l a t mosaic c r y s t a l w i t h a p o s i t i o n s e n s i t i v e d e t e c t o r . The p r i n c i p l e i s shown i n f i g u r e 3. We s e e how such a c r y s t a l c a n r e f o c u s t h e f l o u r e s c e n c e spectrum emitted from a p o i n t s o u r c e a t c h a r a c t e r i s t i c p o i n t s according t o t h e wavelength. Simple geometrical c o n s i d e r a t i o n s show t h a t a photon of wavelength h l e a v i n g a p o i n t source w i t h a n a n g l e a i n r e g a r d t o t h e c e n t r a l r a y (we mean t h e one r e a c h i n g t h e c r y s t a l s u r f a c e w i t h t h e proper Bragg a n g l e ) , may f i n d i n s i d e t h e mosaic c r y s t a l a m i c r o c r y s t a l j u s t t i l t e d enough s o t h a t Bragg c o n d i t i o n s a r e met. This photon w i l l be refocused on a p o i n t , symmetric of t h e p o i n t source. For a small mosaic spread %(, i s t y p i c a l l y of t h e o r d e r of 1') and f u r t h e r small i n regard of t h e Bragg a n g l e t h e image s i z e can be c a l c u l a t e d t o b e
transmission competes quite well with the usual solid angle that can be found when using Si(Li) detectors.
Figure 3. New detector device for X-ray fluorescence analysis.
1- graphite mosaic crystal. 2- Position sensitive detector. 3- Slits (see text for details).
If a position sensitive detector is placed at the focus and perpendicular to the central ray we see that a large range of Bragg angle can be covered at the same time.
Then a multielement analysis (for neighbour element) is still possible. Eiorewer slits in front of the detector can prevent unwanted wavelengths to reach the detector. This is interesting to get rid of the scattered radiation but also of the fluorescence of major elements and avoid unnecessary saturation of the detector.
We present the spectrum of a spherule of extraterrestrial matter about 300 gun
diameter observed with a Si(Li) detector (figure 4b) under the same conditions.
Saturation from the scattered radiation and from the fluorescence of Pe and Ni was avoided and we can observe with high resolution characteristic lines due to Ge and Ga which are only at the 2 ppm level.
M.D.L. of a few ppb have been measured with this device as is attested from Fig.5 which shows a calibration curve obtained with a series of Co standards ranging from 20 ppb to I ppm in a 1 g cellulose matrix.
CONCLUSIONS
X-ray fluorescence analysis induced by S.R. offers already unbeatable M.D.L., even on bulk samples, compared with other fluorescence techniques using X-ray tubes, pro- tons or electrons. Due to the penetrating power of the photon this technique is per- fectly adapted to the study of medium or high Z elements in low Z matrix. In conjunct- ion with the low energy left in the sample it becomes ar ideal mean of investigation for biological samples.
This technique is still under improvements. Just like PIXE and its association with
R . B . S . and N.R.A. which did not become operational until the disponibility of solid
state detectors, synchrotron X-ray fluorescence analysis needs new detectors and real
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PHYSIQUElog N N i b
Fe A r
Figure
Concentration ( pp b )
Figure 5 . Calibration cruve for cobalt i n the 10-1000 ppb range.
Finally, X-ray fluorescence should be more widely associated with other fields of research. For example, in conjunction with XANES and EXAFS very interesting charac-
terizations of trace elements should be possible with the intense flux of new S.R.
machines.
REFERENCES
1 C.J.SPARKS Jr. X-rav fluorescence micromobe for chemical analysis:(
-
- PP 459-512 in Synchrotron ~adiation-~esearch ~.~inick,~:~oniach, Plenum Press, N.Y. 1980.2 J.M.jaklevic. Proceedings of the Energy REsearch and Developmept Administration:
X and Ray symposium ( Conf 760539 ) Ann Arbor, Michigan
4
May 1976 ) pp 1-63 B.M.gordon, K.W.Jones: Desing Criteria and Sensitivity Calculations for Multielemental Trace Analysis at the NSLS X-Ray Microprobe. N.I.M. B 10/11 p293 (1985)
4 S.B.Sutton, M.L.Rivers, J.V.Smith: Anal Chem. 58 p2167 (1986)
5 W.L.Dorn, A.Knoche1 : ESRF Sup 1 and 2 European Science Fondation. Stasbourg (May 1979)
