LASER THERMAL LENS EFFECT AND ITS APPLICATION TO TRACE SOLUTES DETERMINATION

(1)

HAL Id: jpa-00223252

https://hal.archives-ouvertes.fr/jpa-00223252

Submitted on 1 Jan 1983

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

LASER THERMAL LENS EFFECT AND ITS APPLICATION TO TRACE SOLUTES

DETERMINATION

Y. Deng, R. Sheng, M. Wang

To cite this version:

Y. Deng, R. Sheng, M. Wang. LASER THERMAL LENS EFFECT AND ITS APPLICATION TO

TRACE SOLUTES DETERMINATION. Journal de Physique Colloques, 1983, 44 (C6), pp.C6-565-

C6-568. �10.1051/jphyscol:1983693�. �jpa-00223252�

(2)

J O U R N A L DE PHYSIQUE

Colloque C6, suppl6rnent au nO1O, Tome 44, octobre 1983 page C6-565

LASER THERMAL LENS EFFECT AND I T S APPLICATION TO TRACE SOLUTES DETERMINATION

Y. Deng, R. Sheng and M. Wang

Department of Chemistry, University of Wuhan, Wuhan, China

Resume

-

L'effet de lentille thermique pour du CU(AA)~ dans plusieurs sol- --

vants organiques est observ6. La concentration minimale detectable est 12 ppb de Co, ce qui correspond 2 0,06 ng de Co dans le volume du faisceau, en utilisant un laser He-Ne de 1,2 mW et une cellule de 5 cm.

Abstract - The laser thermal lens effect of C U ( A A ) ~ in several organic solvents is observed. A minimal detectable concentration of 12 ppb Co corresponding to 0.06 ng Co in in-beam volume, is achieved using a 1.2 mW He-Ne laser and a 5 cm cell.

In 1965, GORDON et al.(l) first reported the laser thermal lens effect-Sin- ce then, a number of workers has studied and reviewed the physical properties of this effect and its application (2-10). More recently, thermal lens spectrophotometry has been used for cqlorimetric determination of inorganic ions.DOVICH1 and HARRIS(^^) have first used a 4 mW He-Ne laser for the determinat. ion of trace levels of Cu(I1) ion with EDTA. The determination of Fe(I1) ion has been reported with bathophenan- throlinic disulfonate using an ~ r + laser(12). The signal in the conventional spec- trophotometer is measured indirectly as the difference between the incident and transmitted radiation. It is therefore difficult to improve the detection sensitivity of apparatus. Howere, thermal lens spectrophotometry could measure directly the absorbed radiation energy and thus achieve very high sensitivity. This is demonstra- ted in our study.

The laser thermal lens effect of CU(AA)~ in five organic solvents is observed using a He-Ne laser (X=632.8 nm). The time required by reaching steady state, the dependence of detection sensitivity on the position of sample in the light path and some factors influencing the effect are studied.

The experimental apparatus is shown in Fig.1. When the laser beam pass through the sample solution, a defocussing negative lens is formed and adispersed

T : 1.5 mW He-Ne laser with TEMm mode.

L : focussing double convex lens.

S : light barrier S1,S2 : pinhole C : light splitter.

SC : sample cell..

Dl, D2 : photosensitive triode detector.

SC-16 : UV record oscilloscope.

Fig.1

-

Schematic diagram of experimental apparatus.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983693

(3)

C6-566 JOURNAL DE PHYSIQUE

l i g h t s p o t can be observed a t t h e o p t i c a l d i s t a n c e of s e v e r a l meter from t h e sample c e l l . The l a s e r t h e r m a l ( 1 e n s ) e f f e c t can be d e t e c t e d by measuring t h e l i g h t

i n t e n s i t y ( i n t h e c e n t r e of t h e beam) a t Z > > z . I t can be expressed a s ( I -I,)/I, o r A I / I , , where 10 i s t h e p h o t o e l e c t r i c c u r r e n t a t t = O and I m 1s t h e p h s o - e l e c t r i c c u r r e n t a t t h e s t e a d y s t a t e of thermal l e n s .

Cu(AA)2 i s p r e p a r a t e d and p u r i e d by t h e method of r e f e r e n c e (13).PAN-Co(ll1) -CHC13,solut~on i s p r e p a r a t e d by t h e method of r e f e r e n c e ( 1 4 ) . A l l o t h e r s o l v e n t s a r e a n a l y t r c a l l y pure.

The f o l l o w i n g r e s u l t s a r e o b t a i n e d .

1 . The time r e q u i r e d by reaching s t e a d y s t a t e i s determinated by t h e photo- thermal p r o p e r i t y of s o l v e n t and d i s t a n c e of sample from beam w a i s t . The c a l c u l a t e d and observed thermal l e n s formation c u r v e s a r e shown i n Fig.2. Steady s t a t e i s main- t a i n e d a t t > 5 tc.

Fig.2 - Thermal l e n s formation curve.

a ) c a l c u l a t e d , tc = 100 m s

,

O = -1.00 b) observed, Cu(AA) -CHC1 s o l u t i o n O=-1.06

2 3

tc : c h a r a c t e r i s t i c time of s o l v e n t f o r h e a t d i f f u s i o n

O : d e f l e c t i o n a n g l e of thermal l e n s .

0

-

^COO ⁴⁰⁰ t ( n 5 ~

2 . The dependence of thermal l e n s e f f e c t of C U ( A A ) ~ - C H C ~ ~ on t h e p o s i t i o n of sample c e l l i n t h e l i g h t p a t h i s shown i n Fig.3. It can be seen t h a t AI/I, i s maximum a t Z=8 cm. T h i s r e s u l t i s c o n s i s t e n t w i t h t h e c a l c u l a t e d v a l u e 8.2 cm.

F i g . 3

-

A 1 / I m % z r e l a t i o n s h i p .

3. The a b s o r p t i o n enhancement (E) i s s t r o n g l y af-fectea by che s o l v e n t used, l a s e r power and wave l e n g t h . T h e i r e f f e c t s a r e shown i n Table I , Fig. 4 and 5 , r e s - p e c t i v e l y .

I t can be seen from Table I and F i g . 4 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 AI/I,(PA)-I and dn/dT The E i s g r e a t e r t h e more l a r g e p r o d u c t of d n / d t . k-I. The ^{E / P} of water i s o n l y 0.21 and i t s use i s l i m i t e d .

F i g . 5 shows t h a t f o r a g i v e n s o l v e n t , The E depends l i n e a r l y on l a s e r power i n t h e range of lower power.

(4)

The relationship between AI/I, and sample absorbance (A) is approximately linear in certain ranges of AI/I,

.

A approximately linear relationship is found for Cu(AA) -CHC13 when AI/I, is 0.4 - 2.0

2

Table I

-

Effect of some solvents on absorption enhacement.

Fig. 6 - A I / I , % A relationship Solvent

CC14 C H C ~ ~ Benzene Methylbenzene Methanol

IS

10 5 .

0

Cu(AA)

-

CHCl solution 3

(1) Data taken from reference (5) ; (2) calculated by dn/dT

.

^(hk)-l^;

(3) measured exprerimentally, p= 1.25 mW, [ Cu(AA) 2] = 1.48 x 10-3~.

dn/d~. k-1 ( X 10-~crn/rn~)

5.66 5.06 4.44 4.21 1.95

.

^I-^{t p - '}

.

^(rnw4!

. // ²

^0-4

dn/dT (1) (X I O - ~ / O C )

5.8 5.8 6.4 5.6 3.9

z 4 6 a8 I. 0 1.2

P

tmw)

-$ C.'

⁽

^x

¹⁰- 4 ~ / ~ w l )

Fig. 4 - Effect of solvent on E Fig. 5

-

^AI/Ia%P relationship.

k (1) (m~/cm O C )

1.025 1.146 1.440 1.330 2.010

E/P (2) I(AI/Iw) .I/PA(~) (rnw-l)

8.94 8.00 7.02 6.65 3.08

(mw-l) 15.82

13.09 11.49 10.25 3.71

(5)

JOURNAL DE PHYSIQUE

Fig. 6 indicates that laser thermal effect can be used to detect trace solutes. The experimental determination of trace PAN-Co(II1) is made using a 1.2 mW He- Ne laser and a 5 cm cell. The working curve is shown in Fia.7. It shows that in the range of 12-50 ppb,the curve has a good 1inearity.A minimum detectable concentration of 12 ppb Co is achieved, corresponding to 0.06 ng in in-beam volume. Later, the above apparatus is improved by using a lock-in amplifier and a 4 mW He-Ne laser. In this case a minimum detectable concentration of 1.8 ppb Co is achieved. The sensitivity is about 6 times higher than the earlier..Compared with the conventional spectro- photometric analysis of Co(II1) with PAN, the sensitivity is considerably more high.

-

^{Fig. 7}-

Co (111) -PAN working curve P = 1.2 mW

,

¹= 5 cm

Our calculations have indicate that if the detection sensitivity of AI/I, is 0.005, it is possible to measure the absorbance as low as 3 x cm-l, using a 20 mW He-Ne laser and a 10 cm cell. If the absorbance (A) of substance to be determinated in CHC13 is 2 - 5 x I O ' ~ the minimum detectable concentration would be 10-I0M.

The study shows that laser thermal lens spectrophotometry is a new useful technique for ultratrace analysis.

REFERENCE

1. J.P. GORDON et al.,J. Appl. Phys.

36,

(1965)' 3.

2. S.A. AKHMANOV et al. , IEEE, J. Quantum ~lectronics

9e-4

(19681, 568.

3. C. HU et al.

,

Appl. Opt.

12,

(19731, 72.

4. J.R. WHINNERY, Acc. Chem. Res.

1,

(1973), 225.

5. D. SOLIMINI, J. Appl. PhyS. 37, (1966), 3314.

6. M.E. LONG et al.

,

Science

191,

(1976), 183.

7. J.H. BRANNON et al. , J. Phys. Chem.

82,

(1978), 705.

8. A.J. TWA OWSKI et al., Chem. Phys.

3,

(1977), 259.

9. K. DATEE, Opt. Commun

4,

(1971), 238.

10. K.T. BAILY et al.

,

"Laser in Chemistry" Ed. by I.A. WEST, pp.257-264.

11. N.J. DOVICHI et al.

,

Anal. Chem. 51, (1979), 728. - 12. IMASAKA et al.

,

Anal. chim. Acta, 115, (19801, 407. - 13. K.G. CHARLES et al.

,

J. Phys. Chem.

62,

(1958), 440.

14. G. GOLDSTEIN et al. , Anal. Chem. 31, (1959), - 192.