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LASER PHOTOACOUSTIC DETERMINATION OF TRACE SUBSTANCES
Y. Deng, G. Chen, R. Sheng, M. Wang
To cite this version:
Y. Deng, G. Chen, R. Sheng, M. Wang. LASER PHOTOACOUSTIC DETERMINATION OF TRACE SUBSTANCES. Journal de Physique Colloques, 1983, 44 (C6), pp.C6-569-C6-572.
�10.1051/jphyscol:1983694�. �jpa-00223253�
JOURNAL DE PHYSIQUE
Colloque C6, suppl6ment au nO1O, T o m e
44,
octobre 1983 page C6- 569LASER PHOTOACOUSTIC DETERMINATION OF TRACE SUBSTANCES
Y. Deng, G. Chen, R. Sheng and M. Wang.
Department of Chemistry, University of Wuhan, Wuhan, China
Resum&
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Un appareil experimental pour la determination photoacoustique de tra- ces de Co sous forme de PAN-Co(II1) est present&. Quelques paramdtres experimentaux sont Btudies. Une bonne courbe de travail est obtenue pour la detection quantitative de traces de Co.Abstract
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An experimental apparatus for the photoacoustic determination oftra- ce Co in the form of PAN-Co(II1) is described. A good working curve isobtained for quantitative detecting for trace Co.
The photoacoustic spectroscopy using laser as light source has very high sensitivity. Its principle andtheory were reviewedby several authors ( 1 - 3 ) . The method is being applied increasingly widely to the analysis of trace substances (4- 20). In this paper, the quantitat~ve determination of trace substances by solid-sta- te photoacoustic spectroscopy is studied, using a He-Ne laser and a He-Cd laser as light sources and a solid complex PAN-Co(II1) as a reseach object.
The block diagram of the experimental apparatus used in this work is shown in Fig. 1.
1. ~ e - C d laser ; 2. ~ e - N e laser ;
3. pinhole ;
4. total reflective mirror for He-Ne laser ;
5. beam splitter ; 6. lens ;
7. chopper ;
8. power source for the chopper ;
9. photoacoustic cell and micro- phone ;
10. preamplifier for the electric siqnal from microwhone ;
11. lock-in amplifier ;
Fig. 1 - Block diagram of apparatus for laser 12'
'-'
with pen ;photoacoustic determination. 13. laser powermeter
14. power source for He-Ne laser 15. power source for He-Cd laser.
It is arranged in such a way that it is capable of making both the single- beam and double-beam determination.. The photoacoustic cell used are made of pyrex glass. The schematic diagram of single-microphone cell structure is shown in Fig.2.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983694
C6-570 JOURNAL DE PHYSIQUE
In t h e s i n g l e microphone p h o t o a c o u s t i c c e l l a s e n s i t i v e microphone t h a t i s used t o d e t e c t t h e p h o t o a c o u s t i c s i g n a l (PA) i s f i x e d on t h e sample s i d e of c e l l . Double microphones a r e a l s o used, b u t t h e y a r e f i x e d on t h e o p p o s i t e s i d e s ver- t i c a l t o t h e i n c i d e n t l a s e r beam. I n o r d e r t o reduce t h e i n t e r f e r e n c e of ambient n o i s e t h e p h o t o a c o u s t i c c e l l s i s b u r i e d i n sand. The background n o i s e of t h e c e l l s i s 15-60 nV, depending on t h e s t r e n g t h of ambient n o i s e .
1. i n c i d e n t window 2. microphone
3 . rod f o r sample e n t r y 4 . sample
5. s e a l i n g cover 6 . vacum s e a l i n g wax.
F i g . 2 - Schematic diagram of single-microphone c e l l s t r u c t u r e .
The pA s i g n a l produced by t h e e x c i t i n g l i g h t can be d e t e c t e d by t h e microphone and t r a n s l a t e d i n t o an e l e c t r i c s i g n a l which i s f e d i n t o t h e lock-in a m p l i f i e r through t h e p r e a m p l i f i e r . The pA s i g n a l i s picked up from n o i s e , and t h e n f e d i n t o t h e y-axis of t h e X-Y r e c o r d e r . The s t r e n g t h of pA s i g n a l (S ) s t a n d s f o r t h e amount o f t h e t r a c e substance d e t e c t e d . PA
The following a s p e c t s a r e s t u d i e d by t h e above a p p a r a t u s .
1. The e f f e c t of modulated frequency ( f ) and l a s e r power on SPA. The r e s u l t s o b t a i n e d show t h a t t h e r e i s a l i n e a r r e l a t i o n s h i p between SPA and f - I when f > 6 0 H z . I f f i s l e s s t h a n 60 H z , t h e r e l a t i o n s h i p i s n ' t l i n e a r . T h l s i s due t o t h e range of frequency response of t h e microphones used (60 H z '1. 10 K H z ) . The r e s u l t s a r e shown i n Fig.3.
The SPA measured i s d i r e c t l y p r o p o r t i o n a l t o t h e power of i n c i d e n t l a s e r , a s shown i n F i g . 4 .
F i g . 3
-
S p ~ 5 f-I r e l a t i o n s h i p .-
The increase of laser power can therefore raised the sensitivity of photoacoustic detection.
2. The effect of sample backing and carrier on S p ~ . In our work the two kinds of substrate are used : one is a glass rod with a polished end covered by a layer of carrier, and another is a piece of plain glass adhered to a glass tube, which is also covered by a layer of carrier. In each case, the thikness of carrier is 0.1-0.2 mm. The SPA produced by a same substrate are 150-240 !JV (for 8 mW He-Ne laser) and 200-300 p v (for He-Cd laser), corresponding to that produced by 2-3 ng Co in the form of PAN-Co(II1). In the determination of trace Co, the background si- gnal is removed by zero adjustement of the lock-in mplifier. The carrier used are silica gel (SiO2), BaS04 and Al2O3, which are carefully treated before use and are of homogenized grain size of about 5 Vm. In the cases of PAN-Co(II1)-Si02 and PAN- Co(II1)-BaS04 systems larger S p ~ can be produced (about 100 pV/ng Co). The SPA pro- duced by PAN-Co(II1)-A1203 systems is smaller, and the layer made of A1203 is also rougher-Therefore, it isn't used in further work.
3. The quantitative determination of trace Co. In this case the requirements of heat-thinness and good light transparency for the sample to be determinated can be met, and SPA can be expressed as a approximate formula :
SPA
'
K.
B1 = K'Cwhere B1 is the light absorption of sample, and C is the content of substance to be determinated. A good linear relationship between the SPA and contents of CO is found, as shown in Fig.5.
Fig. 5 - working curve for Co.8 m W He-Ne laser, f=32 Hz, silica gel carrier, single microphone cell.
4. The comparison of the detection sensitivity of double-microphone cell with that of single-microphone cell. The results show that the double-microphone cell has a additive effect, that is, the detected S p ~ is the sum of that from the two micro- phones, and it raises notonly the detect& sensitivity, but also S/N.
5. The comparison of the S p ~ produced by double-beam light with that by sin- gle-beam light. The absorption curve of PAN-CO(III)-CHC~~ solution is shownin Fig.6.
It has maximal absorbance at 452 nm and 585 nm. In our work, a He-Ne laser (633 nm, near 585 nm) and a He-Cd laser (442 nm, near 452 nm) are used to excite coaxaly the sample to be detected. The results measured are shown in Fig.7..
It fs seen from Fig.7 that the S p ~ produced by double-beam light is equal to the s w of that produced by each light beam, corresponding to the effect of increa- sing laser power.
In our experimental condition, it is possible to detect Sub-ng Co in the form of PAN-Co(II1). This shows that the laser photoacodstic spectroscopy is a very ef- fective tool for the determination of trace substances.
JOURNAL DE PHYSIQUE
Fig. 6
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Absorptfon curve of Fig. 7-
Spn of double-beam light.PAN CO(III)-CHC~~ solution. Curve 1 : H e - ~ e laser ('~5.3 mW) Curve 2 : He-Cd laser ( ~ 5 mW) Curve 3 : He-Ne laser
+
He-Cd laser f = 40 HZ , silica gel carrier, single microphonecell.Reference
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