Use of a Mono-Fiber Optrode in Remote and in Situ Measurements by the Raman/Laser/Fiber Optics (RLFO) Method

In recent years, the coupling of optical fibers to an optical spectrometer has been increasingly investigated due to its possibilities for remote and in situ multi-mea- surements? -s In terms of Raman spectroscopy, some Ra- man/laser/fiber optics (RLFO) systems are now proposed by manufacturers, 7,s marking a new step in development. Because of its selectivity and nondestructivity, this method offers tremendous potential for applications in different domains of human life including industry, med- icine, and the environment. Among its industrial appli- cations, process on-line control, the monitoring of radio- active or hostile sites, etc., can be found. In medicine, the potential applications of this method seem to be even more spectacular. By inserting flexible optical fibers through natural openings or small incisions in the human body, one can perform immediate in vivo chemical anal- yses. With the delivery of laser energy inside the body for surgery and therapy, this method may soon combine diagnosis and treatment. Many studies have been made in these directions. 9-17 The simplest and most successful configuration to be found for the RLFO method is a mono-fiber optrode. This geometry has the advantage of providing perfect overlap between the excitation and collection fields at the end of fiber, and thus neither adjustments nor ad- ditional components are necessary. Unfortunately, the exploitation of this optrode is not straightforward. The principal drawback is the presence of the very intense Raman spectrum of the fiber itself. This problem makes detection with mono-fiber optrodes possible only for very short fiber lengths. In approaches to overcome this dif- ficulty, multiple-fiber optrodes have been proposed. In these optrode configurations, the excitation and collec- tion beams are transported by different fibers. This ge- ometry allows the participation of the fiber spectrum in the observed RLFO spectra to be significantly reduced.

Use of a Mono-Fiber Optrode in Remote and in Situ

Measurements by the Raman/Laser/Fiber Optics

(RLFO) Method

N G U Y E N Q U A N G H U Y , M I C H E L J O U A N , and N G U Y E N Q U Y D A O *

Laboratoire de Chimie et Physico-Chimie Molgculaires, URA D0441 CNRS, Ecole Centrale Paris, 92295 Chdtenay-Malabry Cedex, France

A new versatile mono-fiber optrode based on the combined use of a dual-

fiber optrode and a short mono-fiber is proposed The quantitative an-

alytical possibility of this optrode configuration is tested on benzene/

heptane mixtures with the use of a P L S procedure, and good results are

obtained The new all-purpose mono-fiber optrode is extremely simple

Index Headings: Mono-fiber optrode; Raman/laser/fiber optics; Quan-

titative chemical analysis; PLS procedure

I N T R O D U C T I O N

In recent years, the coupling of optical fibers to an

optical spectrometer has been increasingly investigated

due to its possibilities for remote and in situ multi-mea-

surements? -s In terms of Raman spectroscopy, some Ra-

man/laser/fiber optics (RLFO) systems are now proposed

by manufacturers, 7,s marking a new step in development

Because of its selectivity and nondestructivity, this

method offers tremendous potential for applications in

different domains of human life including industry, med-

icine, and the environment Among its industrial appli-

cations, process on-line control, the monitoring of radio-

active or hostile sites, etc., can be found In medicine,

the potential applications of this method seem to be even

more spectacular By inserting flexible optical fibers

through natural openings or small incisions in the human

body, one can perform immediate in vivo chemical anal-

yses With the delivery of laser energy inside the body

for surgery and therapy, this method may soon combine

diagnosis and treatment Many studies have been made

in these directions 9-17

The simplest and most successful configuration to be

found for the RLFO method is a mono-fiber optrode

This geometry has the advantage of providing perfect

overlap between the excitation and collection fields at

the end of fiber, and thus neither adjustments nor ad-

ditional components are necessary Unfortunately, the

exploitation of this optrode is not straightforward The

principal drawback is the presence of the very intense

Raman spectrum of the fiber itself This problem makes

detection with mono-fiber optrodes possible only for very

short fiber lengths In approaches to overcome this dif-

ficulty, multiple-fiber optrodes have been proposed In

these optrode configurations, the excitation and collec-

tion beams are transported by different fibers This ge-

ometry allows the participation of the fiber spectrum in

the observed RLFO spectra to be significantly reduced

Received 5 April 1993; revision received 13 July 1993

* A u t h o r to whom correspondence should be sent

Consequently, the length of the fiber used in remote analyses can be increased However, this benefit does not mean t h a t the multiple-fiber optrodes have no obstacles The optical collection efficiency is markedly lower than

t h a t of mono-fiber optrodes, and the adjustment is more difficult The Raman spectrum of optical fibers in these multiple-fiber optrodes now reaches the spectrometer because of reflections at the measurement sites As a result of this factor, the contribution of the fiber Raman signal in each RLFO spectrum varies enormously and uncontrollably from one spectrum to another This lim- itation may cause problems for the quantitative appli- cations of this method Different optrode configurations using two or more fibers have been developed or sug- gested Various studies, such as the optimization of ad- jacent-fiber optrode efficiency, is the utilization of a dou- ble-core fiber 9,19 or of a bundle of fibers where one excitation fiber is surrounded by six or more collection fibers, 17,2°,21 the introduction of GRIN lenses 22,23 and op- tical filters 24 to increase the optical collection efficiency and to remove the fiber spectrum, etc., have been carried out Thanks to these developments, several important improvements have been obtained, and analyses can now

be performed with long fibers Nevertheless, the major disadvantage of these optrodes is t h a t their size is sig- nificantly increased

It is clear that multiple-fiber optrodes are useful for remote analysis while mono-fiber optrodes are more suit-

able for in situ and in vivo applications

In a preceding paper, 25 we announced the possibility

of using mono-fiber optrodes for Raman measurements This paper presents quantitative analytical results of such a system

M E T H O D The quantitative analysis was carried out with the use

of the partial least-squares (PLS) regression This meth-

od was chosen for its performances and rapidityY G-33 Moreover, it has been shown that this method is able to determine both analyte concentrations and sample tem- perature at the same time, 34 which is useful for various applications In addition, the capability of this method

to determine the chemical composition in mixtures of very similar chemical formula has also been demonstrat-

ed 35,36 Our version of PLS was developed in order to take into account the contribution of the fiber Raman signal

in each RLFO spectrum 34 In this procedure, the fiber signal participation in each RLFO spectrum of the cal- ibration set is estimated and is entered into the model

Spectroscopy

Intensity

121 ~ Mean intensity for a

~o ~ - - ~ ~ 7 ~ "

30 ~ - - ~ x ~ / 7 ~ _ ~ -

Wavenamber (cm -1 )

Before mean intensity shifting

Intensity

Wavenumber (cm -1 )

After mean intensity shifting

FIG 1 Mean intensity shifting

calculations as another component The so-called "fiber

explicit modeling" makes it possible to use the "raw"

RLFO spectra directly without subtraction of the fiber

spectrum This capability is very important in those cases

where multiple-fiber optrodes are used and optical filters

are not applicable because of optrode dimension require-

ments Since the PLS method is described in detail else-

where, 26-2s it is not presented here

Spectrum Treatments Before any calculations are ini-

tiated, it is usually necessary to pretreat the spectral

information to remove the irrelevant or useless data

Two pretreatment techniques used in this work are pre-

sented below

mean calibration spectrum should be subtracted from

all spectra before their delivery for calibration and anal-

ysis Using this technique, one can make the spectrum-

Laser ~ Excitation fiber

Raman spectrometer

Collection fiber

Holographic beam splitter

Mono-fiber

MiNor DILOR Super Head

Sample

FIa 2 Instrumental setup for mono-fiber o p t r o d e R L F O s p e c t r o m -

etry

1.600 10 4 1.400 104 1.200 104 1.000 104

8000

6000

4000

.~

2000

O5od

FIG 3

' ' 1 ' ' 1 I ' ' ' ' l ' ' ' ' l ' ' ' ' l ' ' ' '

region used for analysis

1000 1500 2000 2500 3000 3500 4000

wavenumber (era" ~)

RLFO spectrum recorded for 12% benzene/heptane sample

to-spectrum variation predominant, as should be the case for the spectral information concerning the concentra- tion of the components under investigation In particu- lar, this technique was found to be useful for the RLFO applications in reducing the influence of the fiber signal? 4

procedure, the mean intensity of the useful part of the spectrum is subtracted from each intensity This treat- ment is intended to solve the problem of intensity shift- ing, as illustrated in Fig 1 It may also be useful in reducing the fiber spectrum effect in the RLFO quan- titative applications, as will be seen later

E X P E R I M E N T A L

Apparatus The experimental setting for mono-fiber optrode RLFO spectrometry is shown in Fig 2 It couples the new commercial DILOR RLFO 260V system with a short fiber used as a mono-fiber optrode The DILOR RLFO 260V system is an inexpensive instrument spe- cially constructed for remote and on-line applications It

is equipped with the DILOR Super Head, which is a retro-Raman dual-fiber optrode, s This head is linked to the laser and Raman spectrometer by two 10-m-long, 100-#m-core optical fibers Longer lengths up to 100 m are also available from the catalog or on customer de- mand The high performance of lenses, holographic beamsplitter, and optical filters ensures good collection efficiency and effective rejection of fiber Raman emission

At the end of the Super Head, the laser beam is focused into the other fiber The mono-fiber optrode used is a 50-cm-long PCS 600-#m-core fiber with one end fixed on the Super Head and the other end dipped directly into the liquid sample contained in a small flask The Raman spectrometer is a multi-channel type which detects the whole Raman spectrum (from 150 to 3500 cm -1) in one single scan by means of a 700-intensified photodiode array (the resolution is about 30 c m - 0 The 514.5-nm radiation excitation is delivered by an air-cooled argon- ion laser, with 25 mW on the sample

Quantitative Analytical Procedure Quantitative anal- ysis was performed on benzene/heptane solutions This mixture was chosen for illustration since quantitative analysis of hydrocarbon fuel mixtures is important for the petroleum industry The molar fractions of benzene ranged from 1 to 13 %, calculated from the mass ratios

1.300 104

1.o38 lO 4 i A ~ benzene

.~ 5125

(b)

(c)

800 850 900 950 1000 1050 1100 1150 1200

wavenumber (cm" x) Flc 4 RLFO spectra recorded for (a) 12% benzene, (b) 7.5% ben-

zene, and (c) 2% benzene in heptane Region used: 800-1200 cm-L

The pseudo-noise observed is the photodiode array response

600

4 0 0

200

0

.~ 0 -200

-400800' 850 900 950 1 0 0 0 1 0 5 0 1 1 0 0 1 1 5 0 1200

wavenumber (cm 1)

FIG 6 R L F O s p e c t r u m of 12 % b e n z e n e / h e p t a n e , obtained after m e a n intensity shifting a n d m e a n centering

Forty samples were prepared and recorded with an in-

tegration time of one second and an accumulation num-

ber of 240 These 40 samples were divided into two sets;

21 samples were gathered in the calibration set to build

the model, and the 19 remaining samples formed the

verification set to check the model validity As it is not

necessary to use the entire spectrum, but only the spec-

tral ranges where characteristic bands exist for both com-

pounds, a 400-cm-1 broad spectral region (800-1200 cm -1,

or 58 points for each spectrum) was used, to reduce the

calculation time cost The spectra were mean centered

and also mean intensity shifted before their introduction

into the calculations The precision of the results ob-

tained can be evaluated by the Standard Error of Pre-

diction:

S E P = ~ ~ (Cpred Ctrue) 2

An example of an RLFO spectrum recorded for a 12 %

benzene sample with the working window is shown in

Fig 3

RESULTS AND DISCUSSION

Figure 4 shows some of our RLFO spectra in the region

used for analysis In this figure, one can note a difference

7000 i i D I J i 4

4750

"~ 2500

.2

250

- 2 0 0 0 t L L ~ n i i

-800 850 900 950 1 0 0 0 1 0 5 0 1 1 0 0 1 1 5 0 1200

wavenumber (¢m 1)

F m 5 T h e same s p e c t r a as in Fig 4, after m e a n intensity shifting

in the contribution of the fiber Raman signal in each of our RLFO spectra However, it should be noted that, with the use of this mono-fiber optrode, the variations

in the contribution of the fiber Raman signal in RLFO spectra should not be very great, compared with those

of a multiple-fiber optrode In this optrode configuration, the Raman signal of the fiber reaches the spectrometer mainly by retro-Raman diffusion, and the reflection in the measurement site has only a slight effect This slight effect of the fiber spectrum may be seen as an intensity shift from one to another RLFO spectrum, and the mean intensity shifting should be exploited here The RLFO spectra obtained after such a procedure are shown in the Fig 5 In these spectra, no significant difference in con- tribution of the fiber Raman signal could be observed Consequently, the mean-centering treatment enables the fiber spectrum effect to be reduced (Fig 6) The explicit fiber modeling procedure is not necessary here

Quantitative analysis has been carried out in two cases The first one uses only mean centering; the other also includes the mean intensity shifting The results ob- tained are compared in Table I A small difference in these results shows t h a t the fiber spectrum has only a slight effect on the quantitative analysis Furthermore, this slight effect may be effectively eliminated by using the mean intensity shifting The predicted values ob- tained for this case are plotted against the real values in Fig 7 The regression equation calculated from these values is y = 0.05 + 0.998 x, which is very close to the ideal line y = x

The good results obtained prove the reliability of this mono-fiber optrode configuration They also show that the use of longer fibers is possible Moreover, the focal point diameter of the Super Head is about 20 ~m and allows the injection of the laser beam into a 100-#m-core fiber As the fiber spectrum depends only on the fiber

T A B L E I Results obtained for benzene/heptane mixtures, with and

without mean intensity shifting

Mean Correla- intensity tion coef- Regression Case shifting S E P ficient equation

1 no 0.19 0.997 y = - 0 0 5 _+ 1.O07x

2 yes 0.13 0.999 y = 0.05 + 0.998x

14

12

O

10

8

6

"~ 4

2

Ideal line

O

R e a l c o n c e n t r a t i o n s

FIG 7 Prediction results obtained for verification set Pretreatments

used: mean intensity shifting, mean centering

length, but not on the fiber diameter, the use of a 100-

~m-core fiber instead of the present 600-ttm one should

enhance the signal-to-noise ratio by an order of magni-

tude The applications of this mono-fiber optrode to bio-

medical analysis using 1-m-long, 100-ttm-core fiber are

at present being investigated, this time with a near-in-

frared excitation

C O N C L U S I O N

The possibilities of the new mono-fiber optrode for

remote and in situ quantitative analysis have been dem-

onstrated in the present study for a case of chemical

analysis With its simplicity and small dimensions, the

mono-fiber optrode can easily be introduced into the

human body as well as other sites of difficult access

Therefore, it is very useful not only for in situ but also

for in vivo detection In such applications, the Super

Head can be kept away from the spectrometer, and thus

remote analyses can be obtained The same fiber can also

be used for both the detection and the therapeutic laser

beams The coupling of this optrode with an inexpensive

industrial and biomedical Raman spectrometer makes it

possible to look forward to successful applications of the

RLFO method in industry and medicine

ACKNOWLEDGMENTS

The authors wish to thank Dr E Da Silva, Director of DILOR S.A.,

for helpful discussions and for the loan of an INDURAM RLFO Raman

spectrometer

1 R L McCreery, M Fleischmann, and P Hendra, Anal Chem 55,

146 (1983)

2 Nguyen Quy Dao, M Jouan, P Plaza, and J Robieux, Brevet Fran~ais 84.15047 (1984)

3 P J Hendra and G Ellis, J Raman Spectrosc 19, 413 (1988)

4 D D Archibald, L T Lin, and D E Honigs, Appl Spectrosc 42,

1558 (1988)

5 J P Alarie, D L Stokes, W S Sutherland, A C Edwards, and

T Vo-Dinh, Appl Spectrosc 46, 1608 (1992)

6 Nguyen Quy Dao, M Jouan, and P Plaza, Les Cahiers de Spectra

2000 168, 8 (1992)

7 Ph Roussel and Ph Cleon, Analusis 20, 58 (1992)

8 DILOR RLFO 260V spectrometer technical data sheet (DILOR, Lille, France, 1993)

9 H Fevrier, H Saisse, P Plaza, and Nguyen Quy Dao, Brevet Fran-

~ais 85.09725 (1985)

10 Nguyen Quy Dao, M Prod'homme, P Plaza, and M Joyeux, C R Acad Sci Paris 302 II, 313 (1986)

11 Nguyen Quy Dao, P Plaza, and M Joyeux, Analusis 14, 334 (1986)

12 S Hameau, M Jouan, and Nguyen Quy Dao, Analusis 16, 173 (1988)

13 C Gomy, M Jouan, and Nguyen Quy Dao, Anal Chim Acta 215,

211 (1988)

14 D Phat, P Plaza, Nguyen Quy Dao, P N Vuong, and P G Steg,

in Proceedings of the 12th International Conference on Raman

Spectroscopy (John Wiley & Sons, Chichester, 1990), p 704

15 D Phat, P Plaza, P N Vuong, F Cheilan, and Nguyen Quy Dao,

in Proceedings of the 64th Scientific Sessions of the American

Heart Association, supplement to Circulation 84(4), II-725 (1991)

16 D Phat, P N Vuong, P Plaza, F Cheilan, and Nguyen Quy Dao,

in Proceedings of the Future Trends in Biochemical Applications

of Lasers, SPIE Vol 1525 (SPIE, Bellingham, Washington, 1991),

p 196

17 T D Nguyen Hong, D Phat, P Plaza, M Daudon, and Nguyen Quy Dao, Clin Chem 38, 292 (1992)

18 P Plaza, Nguyen Quy Dao, M Jouan, H Fevrier, and H Saisse, Appl Opt 25, 3448 (1986)

19 P Plaza, Nguyen Quy Dao, M Jouan, H Fevrier, and H Saisse, Analusis 15, 504 (1987)

20 K P J Williams, J Raman Spectrosc 21, 147 (1990)

21 C G Zimba and J F Rabolt, Appl Spectrosc 45, 162 (1991)

22 M Jiaying and L Zhong, Appl Spectrosc 45, 1302 (1991)

23 N V Tien, H Nguyen Quang, M Jouan, Nguyen Quy Dao, Opto

92, 554 (1992)

24 M L Myrick and S M Angel, Appl Spectrosc 44, 565 (1990)

25 Nguyen Quy Dao, M Jouan, H Nguyen Quang, and E Da Silva, Analusis 21,219 (1993)

26 P Geladi, B R Kowalski, Anal Chim Acta 185, 1 (1986)

27 A Lorber, L E Wangen, and B R Kowalski, J Chemometrics 1,

19 (1987)

28 K R Beebe and B R Kowalski, Anal Chem 59, 1007 {1987)

29 D M Haaland and E V Thomas, Anal Chem 60, 1193 (1988)

30 W Lindberg, J Persson, and S Wold, Anal Chem 55, 643 (1983)

31 D M Haaland, R G Easterling, and D A Vopicka, Appl Spec- trosc 39, 73 (1985)

32 M P Fuller, G L Ritter, and C S Draper, Appl Spectrosc 42,

228 (1988)

33 D M Haaland and E V Thomas, Anal Chem 60, 1202 (1988)

34 H Nguyen Quang, B Vu Thi, M Jouan, and Nguyen Quy Dao, Analusis 20, 141 (1992)

35 M B Seasholtz, D D Archibald, A Lorber, and B R Kowalski, Appl Spectrosc 43, 1067 (1989)

36 H Nguyen Quang, N V Tien, M Jouan, Nguyen Quy Dao, N

Zanier, V Ruffier-Meray, and E Behar, in Proceedings of the 13th

International Conference on Raman Spectroscopy (John Wiley &

Sons, Chichester, 1992), p 1030

2016 Volume 47, Number 12, 1993