Sample X-ray Si (Li) Detector Energy-dispersive TXRF Substrate Large solid angle (High detection efficiency) Collecting whole XRF spectra simultaneously Advantages Low energy-resolution Limitation of counting-rate Scattering background Disadvantages High energy-resolution Good signal to background ratio Advantages Low detection-efficiency Disadvantages Substrate Analyzing Crystal ( Johansson ) Scintillation Detector Wavelength-dispersive TXRF Sample X-ray F i g . 1 . C o m p a r i s o n b e t w e e n c o n v e n t i o n a l e n e r g y - d i s p e r s i v e a n d t h e p r e s e n t w a v e l e n g t h - d i s p e r s i v e T X R F . T h e m a i n i d e a o f t h e p r e s e n t r e s e a r c h i s t h e employment of the Johansson-type spectrometer instead of a Si(Li) detector in TXRF experiments. The expected problem is low efficiency for ultra trace element analysis, but the present downsized spectrometer can solve it. Trace metals sometimes play quite significant roles in spite of the extreme small amounts in which they exist, not only in industrial research but also in e n v i r o n m e n t a l a n d b i o m e d i c a l s c i e n c e s . T h e sy nc hr ot ro n X- ra y fl uo re sc en ce te ch ni qu e [1 ] is a po we rf ul pr ob e fo r tr ac e me ta ls , an d in pa rt ic ul ar , total-reflection X-ray fluorescence ( TXRF ) [2] using a m i r r o r - p o l i s h e d s u b s t r a t e a s a s a m p l e s u p p o r t , ca n de te ct tr ac e me ta ls wi th ve ry hi gh se ns it iv it y. S o f a r , a n e n e r g y - d i s p e r s i v e s p e c t r o m e t e r b a s e d on a Si(Li) detector has been employed, because of i t s h i g h d e t e c t i o n e f f i c i e n c y a n d t o l e r a b l e e n e r g y resolution (130 ~ 170 eV for 5 ~ 10 keV X-rays) for separating X-ray fluorescence from the neighboring e l e m e n t s . H o w e v e r , t h e r e e x i s t o b v i o u s l i m i t s i n Detection of Trace Metals by Means of an Efficient Wavelength-dispersive X-ray Fluorescence Spectrometer d e t e c t i o n p o w e r ; t h e b i g g e s t p r o b l e m i s t h e sc at te ri ng ba ck gr ou nd , th e lo w- en er gy -s id e ta il of which severely restricts the detection of weak X-ray flu ore sce nce sig nal s. Ther efo re, one sho uld not e t h a t u p g r a d i n g t h e d e t e c t i o n p o w e r i s n o t a l w a y s s t r a i g h t f o r w a r d , e v e n w h e n b r i l l i a n t s o u r c e s a r e available. A new idea comes with the use of wavelength- d i s p e r s i v e ( W D ) s p e c t r o m e t e r s t o i m p r o v e t h e signal to background ratio by eliminating scattering X - r a y s w i t h e n h a n c e d e n e r g y r e s o l u t i o n ( F i g . 1 ) [ 3 ] . S i n c e t h e r e i s u s u a l l y a t r a d e - o f f b e t w e e n resolution and efficiency, one promising candidate is a spec trom eter with down size d Joha nsso n-ty pe f o c u s i n g o p t i c s w i t h m o d e r a t e e n e r g y r e s o l u t i o n ( ~ 1 0 e V f o r 5 ~ 1 0 k e V X - r a y s ) [ 4 - 7 ] . A l t h o u g h a n o t h e r w a y t o a c c o m p l i s h t h i s m i g h t b e t o u s e conventional optics with flat crystals [8,9], the loss of det ect ion eff ici enc y can be a pro ble m for tra ce a n a l y s i s . F i g u r e 2 s c h e m a t i c a l l y s h o w s a wa ve le ng th -d is pe rs iv e TX RF sp ec tr om et er , wh ic h i s e q u i p p e d w i t h a G e ( 2 2 0 ) a n a l y z i n g c r y s t a l 99 K-B mirrors Experimental hutch Clean booth Hall SR 55 m 45 m 40 m 35 m Ring 6 m 3.4 m Outside the experimental hutch Inside the experimental hutch Receiving slit YAP: Ce scintillation detector Sample V a c . c h a m b e r SR Curved crystal Johansson Ge (220) 4 axes for scanning X-ray energy Entrance slit Rowland circle R = 120 mm (flexible) Fig. 2. Schematic view of the downsized Johansson TXRF spectrometer developed at Nat’l Inst. for Materials Science ( NIMS ), Tsukuba. F i g . 3 . E x p e r i m e n t s s e t u p a t B L 4 0 X U . C o m b i n a t i o n o f h i g h - f l u x u n d u l a t o r b e a m a n d e f f i c i e n t X R F s p e c t r o m e t e r . (Rowland radius 120 mm ) and a YAP: Ce detector [ 1 0 ] w i t h a 0 . 0 7 m m r e c e i v i n g s l i t . T h e m o s t i m p o r t a n t feature is reasonably high detection efficiency with only a small loss of energy resolution [6]. Experiments have been performed at beamline BL 40 XU wi th qu as i- mo no ch ro ma ti c X- ra ys fr om a h e l i c a l u n d u l a t o r s o u r c e ( I D g a p 1 0 . 8 m m , f u n d a m e n t a l p e a k 1 0 k e V ) a n d f o c u s i n g o p t i c s based on K-B mirrors ( Fig. 3 ). The beam size used i s 3 0 μ m × 3 0 μ m . F i g u r e 4 s h o w s t y p i c a l T X R F s p e c t r a f o r a d r o p o f l i q u i d ( 0 . 1 μ l ) c o n t a i n i n g 2 0 ppb Fe, Co and Ni. The scan requires 5 sec/point and the tot al mea sur ing tim e is 10 - 15 min . The e n e r g y r e s o l u t i o n i s a r o u n d 6 ~ 7 e V , a r o u n d 2 0 times better than that obtained using a conventional e n e r g y - d i s p e r s i v e T X R F s p e c t r o m e t e r . T h i s 50 m 4 axes for alignment and positioning of the sample Detector Sample Crystal analyzer 100 5 sec/point X-ray Sample Fe, Ni, Co 20ppb 0.1 μ l 6350 6400 6450 6900 7000 7100 7400 7500 7600 7700 5000 10000 15000 20000 25000 30000 0 Fe K α 2 FeK α 1 FWHM 5.71 eV FWHM 6.62 eV Co K α 2 CoK α 1 NiK α 1 NiK α 2 FWHM 7.02 eV CoK β 1,3 FeK β 1,3 Energy ( eV ) Intensity (counts) c o n t r i b u t e d t o r e d u c i n g t h e i n f l u e n c e o f s c a t t e r i n g b a c k g r o u n d a n d X - r a y f l u o r e s c e n c e f r o m t h e n e i g h b o r i n g e l e m e n t s . S i n c e t h e e n e r g y r e s o l u t i o n i s e n h a n c e d o n e c a n s e e e v e n c h e m i c a l e f f e c t s b y l o o k i n g c a r e f u l l y i n t h e a r e a ar ou nd K β sp ec tr a [7 ], wh ic h ex hi bi t so me satellite lines. Figure 5 shows another result for Ni in a 0 . 1 μ l d r o p . T h e t e c h n i q u e s h o w s g o o d linear relation in a wide dynamic range. The ab so lu te de te ct io n li mi t ob ta in ed is 0. 31 fg for Ni, and the concentration in a 0.1 μ l drop is 3.1 ppt, or further lower for a usual 1 ~ 50 μ l solution sample. The results are almost 1.5 ~ 2 decades better than the current best record performed with a Si(Li) detector [11]. T h e p r e s e n t t e c h n i q u e i s c o m p e t i t i v e w i t h t r a c e a n a l y s i s , s u c h a s A A S a n d I C P - M S Fig. 5. Performance of the present wavelength-dispersive TXRF spectrometer, ppt level detection limit with less than ∆ E = 10 eV. Intensity ( cps ) Ni 1ppb - 0.1 μ μ l liquid drop 800 600 400 200 0 FWHM 7eV 7420 7440 7460 7480 7500 7520 Energy ( eV ) X-ray Intensity ( cps ) 10 3 10 4 10 5 10 6 1 1000 100 10 (r = 0.99908) Concentration ( ppb ) ( Ni in 0.1 μ μ l solution) Fig. 4. Typical TXRF spectra for trace elements ( Ni, Co and Fe, 20 ppb each) in a micro drop (0.1 micro litter). Details of the experiment are covered in the main text. a d v a n t a g e i s t h e n o n - d e s t r u c t i v e n a t u r e f o r t h e m e a s u r e m e n t . N e w o p p o r t u n i t i e s f o r a d v a n c e d analytical applications could be opened up. ( F i g . 6 ) . B e s i d e s e x t r e m e l y h i g h s e n s i t i v i t y , capability of an al yz in g ve ry s ma ll a mo un t of s am pl es i s s i g n i f i c a n t f o r p r a c t i c a l a n a l y s i s . A n o t h e r Substrate 101 References [ 1 ] A . I i d a a n d Y . G o h s h i , H a n d b o o k o n S y n c h r o t r o n R a d i a t i o n , e d s . S . E b a s h i , M . K o c h and E. Rubenstein, Chap. 9, 4 (1991) 307. [ 2 ] R . K l o c k e n k a m p e r , “ T o t a l - R e f l e c t i o n X - r a y Fl uo re sc en ce An al ys is ”, Jo hn Wi le y & So ns , Ne w York (1997). [3] K. Sakurai, H. Eba, K. Inoue and N. Yagi, Anal. Chem. (2002) - in press. [ 4 ] K . S a k u r a i a n d H . E b a , J p n . J . A p p l . P h y s . Suppl. 38-1 (1999) 650. [5] H. Eba et al. , Anal. Chem. 72 (2000) 2613. Kenji Sakurai National Institute for Materials Science ( NIMS ) E-mail: sakurai @ yuhgiri.nims.go.jp Fig. 6. Comparison of relative and absolute detection limits. Combination of the undulator source and the present TXRF spectrometer can provide the most powerful tool for trace element analysis. AAS : atomic absorption spectrometry; ICP-MS : inductively coupled plasma-mass spectrometry. Concentration (g/g) AAS I C P - M S C o n v e n t i o n a l X R F 1 0 – 1 5 1 0 – 1 2 1 0 – 9 1 0 – 6 ( fg ) ( pg ) ( ng ) ( μ g) Absolute amount (g) Trace chemical characterization using K β spectra C o n v e n t i o n a l T X R F ( p p m ) ( p p b ) ( p p t ) 10 -3 10 -6 10 -9 10 -12 P r e s e n t d e t e c t i o n l i m i t ( S R - W D T X R F ) [6 ] K. Sa ku ra i et al . , Nu cl . In st ru m. Me th . A 46 7- 468 (2001) 1549. [7] K. Sakurai and H. Eba, Nucl. Instrum. Meth. B (2002) - in press. [8] N. Awa ji et al. , Jpn . J. App l. Phy s. 39 (20 00) L1252. [ 9 ] P . P i a n e t t a e t a l . , T h i n S o l i d F i l m s 3 7 3 ( 2 0 0 0 ) 2 2 2 . [10] M. Harada et al. , Rev. Sci. Instrum. 72 (2001) 4308. [11] P. Wobra usche k et al. , Spect rochi m. Acta B 52 (1997) 901. 102
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