1554
Anal. Chem. 1907, 59, 1554-1557
(4) Sepanlak, M. J.; Vargo, J. D.; Kettler, C. N.; Maskarinec. M. P. Anal. Chem. 1984, 56, 1252. (5) Yang, Y. Anal. Chem. 1984, 5 6 , 2336. (6) Pang, T. J.; Morris, M. D. Anal. Chem. 1985, 5 7 , 2153. (7) Blalkowski. S. E. Anal. Chem. 1986, 58, 1706. (8) Allison, S. W.; Gillles, G. T.; Magnuson, D. w.; Pagano, T. s. Opt. 1985, 24, 3140.
(9) Light Transmission in Optlmi Flber; Noda. K. Korona: Tokyo, 1981. (10) Matsumura, H. Nlkkel.€bchonics 1981, (July 6). 154. (1 1) Sheldon, S. J.; Knight, L. V.; Thorne, J. M. Appl. Opt. 1982, 21, 1663.
RECEIVED for review August
26, 1986. Resubmitted February 18, 1987. Accepted March 4, 1987.
A Couple of Optical Fibers for Thermal Lens Spectrophotometry Totaro Imasaka, Kazuhiko Nakanishi, and Nobuhiko Ishibashi* Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan
The constructed thermal lens spectrophotometer conslots of only a couple of optkal flbers. The enhancement factor, tfie relatlve sensltlvity In comparison wlth conventional spectrophotometry, Is 15-42 wlth the 200-mW argon Ion laser, the mlnlmum detectable absorbance belng 7.6 X IO-'. The observed enhancement factor Is only several percent of the theoretlcal value. I t is ascrlbed to a small core dlameter of the angle-mode fiber, causing a large beam divergence and a small effective path length. The present thermal lens system may be useful when the sample path length is llmlted as a Uetector for hlgh-performance llquld chromatography or when the s a m e to be detected is In a hazardous envlronment, for example, In a hlgh-temperature and hlgh-pressure reaction vessel.
Thermal lens spectrophotometry has been used for detection of very small amounts of absorption (1,2). In the conventional system the laser beam is focused by a lens into the sample cell placed a t 3l/, times of the confocal distance (3). The dual beam system requires a probe beam, and the optical system is more complicated. In this system the laser beams are coaxially aligned by a beam splitter, and the probe beam is isolated by a filter after passing through the sample cell. However, it allows the use of a simple lock-in amplifier for signal detection, while a transient digitizer or a microcomputer is usually necessary for the single-beam system. It is noted that a sample cell is essential for these systems, since the thermal lens effects induced before and after the focal point are completely canceled (4). The direct incidence method using an unfocused probe beam is a podified dual beam system (5). It requires no sample cell; therefore the sample species in liquid or in atmosphere can directly be detected. The system alignment is not critical because of the unfocused probe beam. Furthermore, it gives a 1.9 times larger enhancement factor than that of the conventional thermal lens system and gives a longer sample path length (6). However, it is still more complicated than conventional absorption spectrometry. In this study we use a couple of optical fibers for transmission of the laser beam and for detection of the thermal lens effect. The laser beam from the optical fiber diverges as if it is focused at the end surface of the optical fiber. No focusing lens, no sample cell, and no other optical components such as a beam splitter or pinhole are necessary. It is noted that the background signal from the adsorbed species on the end surface of the fiber is essentially negligible since the 0003-2700/87/0359-1554$01.50/0
thermal lens effect a t the focal point should give no appreciable signal. This system is probably the most simple thermal lens spectrophotometer ever constructed. Therefore, the present approach may be promising for practical applications. In this paper we also propose the characteristics of the optical fiber required for more sensitive detection of absorption.
EXPERIMENTAL SECTION Apparatus. Figure 1shows the block diagram of the thermal lens system using a couple of optical fibers. The laser beam is modulated by a beam chopper and introduced into the singlemode optical fiber (Fujikura, SM 6/125 or SM 10/135) by an objective lens (Nikon, X10) for a microscope. The sample cell used is a 1-cm square quartz cell with a drain (Fujiwara, T-15-W-10). The optical fiber for light introduction is inserted into the sample cell from the drain port. The sample path length, the distance between the end surface of the fiber and the inner wall of the cell, is adjusted by changing the position of the fiber. The signal intensity at the beam center is measured by using a multimode optical fiber (Fujikura,G 50/125) placed outside the sample cell. The distance between the fiben is adjusted by changing the position of the fiber for light detection. The light intensity is measured by a photodiode (Hamamatau Photonics, S780-8BQ). The decay signal is recorded by a transient digitizer (Autnics, S2lO) equipped with a signal averager (Autnics, F601). The decay curve was displayed by a plotter, which was controlled by a microcomputer (Sord, M223 Mark 111). Reagents. The iodine was dissolved in carbon tetrachloride and used as a sample solution. RESULTS AND DISCUSSION Analytical Curve. The thermal lens signal was measured by changing the concentration of iodine in carbon tetrachloride. The experiment was carried out by using the 488-nm line of the argon ion laser (200 mW) and the 6-pm fiber (2 m). The laser power that reached the sample cell was 130 mW. The coupling efficiency from the laser to the optical fiber, which was determined by measuring the light intensity before and after the optical fiber, was 80% for the present system. Residual loss may be due to reflection and absorption at the surfaces of the optical components. The constructed analytical M, the detection limit being curve was straight to 3 X 1.2 x IO4 M for I,. The minimum absorbance detected was 7.6 x 10-5. Laser Power. Figure 2 shows dependence of the signal intensity on the laser power. The observed curve is slightly sigmoidal. We ascertained that a straight line could be obtained for the conventional thermal lens system. This phenomenon could not be understood in our preliminary study, but we later found that this unexpected result came from a change in the beam diameter of the argon ion laser. Above 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
4 0) '7 optF;3 ; JJJ Objecti;
1555
Optical Fiber
Lens
Mirror
I
4
O'
6
8
10
Distance (mm)
Flgure 3. Dependence of thermal lens system on the distance between
two optical fibers.
Signal Averager ~GP-IB
I
1
Computer
Flgure 1. A couple of fibers for thermal lens spectrophotometry.
- 0.10.
20
0
b
.u
0.20.
P
15 ,f I
E"
P
0.15. > c .v) 0
P P
62
E 8 0.10- I
gs
w
0'05'
IO $
5 r 5
0
2
4
6
8
1
0
w
0
Length (mm) Flgure 4. Dependence of thermal lens signal on sample path length.
o/o
Laser Power (mw)
Flgure 2. Dependence of laser power on signal Intensity: core diameter, 6 pm; exciting source, argon Ion laser (488 nm); sample path length, 2 mm.
200 mW only the beam diameter increased, while the intensity at the beam center remained unchanged. Thus the beam diameter at 500 mW (1.3 mm) was 1.5 times larger than the diameter a t 200 mW (0.85 mm). It allows a tighter focus of the laser beam at high laser power and improves the coupling efficiency. However, further increase in the beam diameter, that is decrease in the beam spot size a t the waist, does not improve the coupling efficiency, since beam divergence exceeds the critical angle of the fiber (see ref 7 ) . It is probably the reason why the slope decreases above 500 mW. Distance between Fibers. The thermal lens signal was measured by changing the distance between the optical fibers. The sample path length was adjusted to 2 mm in this study. The result is shown in Figure 3. The intensity increases with increasing the distance, but it is saturated above 7 mm, which corresponds to a / 2 w = 0.054, where w is a beam radius of the laser at the end surface of the detection fiber with a core diameter of a. This distance is considerably shorter than that for the conventional thermal lens system. It may be ascribed to a very small core diameter and resulting large beam divergence of the laser. If the 50-pm multimode fiber for light detection was replaced by the 10-pm fiber, the distance between the fibers could be reduced to 2 mm. Such an optical system is quite useful for miniaturization of the detector. Effective Path Length. Dependence of the signal intensity on the path length of the sample is shown in Figure 4. The distance between the fibers for light introduction and detection was adjusted to 10 mm in this study. The signal intensity increases with increasing the sample path length, but it is completely saturated above 6 mm. This result is due to the short confocal distance (2, = 31 pm), resulting in a short effective path length. In the conventional theoretical models (3, 8),the sample path length is assumed to be sufficiently short in comparison with the confocal distance. In such
conditions the signal intensity must be proportional to the path length. The theoretical model assuming a long path length sample is also proposed elsewhere (9, IO). In this model the signal intensity tends to saturate above b / Z , = 10, where b is the sample path length. Our result is qualitatively understandable from this theoretical prediction, but the completely saturated point ( b / Z , = 190) seems to be much larger than the expected value. Discrepancies might occur for several reasons: (1)the thermal lens effect induced by light passing through the cladding has a longer effective path length so that this contribution is not negligible when the sample path length is sufficiently long, (2) in the present configuration the ratio of the beam diameter at the focal point (wo and 2, becomes small since the laser beam is tightly focused. In this case longitudinal thermal diffusion is appreciable, and the present model assuming only radial thermal diffusion may probably be inadequate for complete discussion of the present result. The calculated enhancement factor is also shown in Figure 4. The enhancement factor decreases with increasing the sample path length. Thus the present system has a limited performance when a long sample cell can be used to improve the sensitivity. However, it is quite useful when the sample path length is limited as a detector for high-performance liquid chromatography (HPLC). Enhancement Factor. The enhancement factor for the thermal lens system using various single mode optical fibers is listed in Table I. The coupling efficiency for the 2 m long and 6-pm core fiber is 80%. But, it may include the light transmitted through the cladding, since the efficiency decreases with increasing the length of the fiber. However, the achieved enhancement factor for the 2-m fiber is larger than that for the 30-m fiber. It implies that the laser beam passing through the cladding is also effectively used to form a thermal lens effect. This result is contrary to the previous result in ref 7 , in which the laser beam from the cladding is more poorly focused by a lens and induces only a small thermal lens effect. It is noted that the effective path length is so short in the present thermal lens system that the laser beam from the
1556
ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
Table I. Enhancement Factor of Thermal Lens Spectrophotometrya output
ratio of
observed/theo-
core diameter, pm
fiber length, m
coupling efficiency, 70
power,* mW
6
2 30 2 30
80 45 85 80
130 70 140 130
10
enhancement factor
retical
29 (780)' 15 (420)
3.7 3.6 4.9
42 (850) 33 (780)
4.2
The 488-nm line of the argon ion laser is used as an exciting source. *The output power was measured at the cell position. 'Theoretical value, sample path length, 2 mm.
.-cn
-= v)
/L I
O8 0
50 100 Time (ms)
Figure 5. Transient thermal lens signal: core diameter, 6 pm; sample, 2X M I, in CCI,; laser, argon ion laser (488 nm, 200 mW); path length, 2 m. The theoretical curve is drawn by assuming T = 5.3 ms.
cladding with a large beam diameter may give an appreciable thermal lens signal because of its long effective path length. The coupling efficiency is improved for the 10-pm fiber, since the laser beam is properly focused into the core of the single mode fiber. Furthermore, the enhancement factor for the 10-pm fiber is slightly better than those for the 6-pm fiber. It may be ascribed to the longer effective path length. Since beam divergence for the 10-pm fiber is much smaller, the confocal distance is 2.7 times longer than that for the 6-pm fiber. We notice that the observed enhancement factors are 20-30 times smaller than the theoretical values. This is probably due to the short effective path length. It is apparent that the theoretical model assuming the short sample path length is not valid in the present thermal lens system. Nevertheless, it should be emphasized that this approach using a couple of optical fibers for thermal lens spectrophotometry still has a great sensitivity in comparison with conventional absorption spectrometry. A couple of optical fibers are sometimes used as a waveguide for measurement of light absorption in aqueous solution or biological fluid. No researchers have pointed out that the signal is power dependent due to the thermal lens effect. It might probably be due to the use of a conventional incoherent light source or the use of a laser with a small output power. We point out that the absorption measurement is not accurate even when a laser with moderate output power (several milliwatts) is introduced into the organic solution through a single-mode fiber. The effect is smaller for the aqueous solution due to the small enhancement factor. However, the 100-mW laser induces serious error even for the aqueous sample and is inadequate to use for conventional absorption spectrometry. Transient Signal. The transient thermal lens signal is shown in Figure 5. The observed time constant, calculated by assuming the parabolic lens model (8),was 5.3 ms, corresponding to a beam radius of 40 pm at the sample position. In the present configuration the beam radius at 3lI2times the confocal distance is 4.4 pm. It is known that the signal intensity rapidly decreases with decreasing the distance between the sample cell and the focal point, while the intensity only gradually decreases with increasing the distance. Therefore, the effective beam radius might be substantially larger than that expected from the conventional optically thin lens model.
I
0
100
200 Time (ms)
300
Flgwe 6. Transient decay curve for background signal: core diameter, 6 pm; laser, argon ion laser (488 nm, 200 mW). The focusing signal (a) appears when the distance between the focusing lens and the optical fiber is adjusted to be slightly longer than the optimum value and the defocusing signal (b) is observed at the shorter distance.
Furthermore, the theoretical curve is well fitted at a longer time scale after beam introduction. I t reflects the thermal lens effect at a large beam radius, which provides a longer time constant in analysis of signal decay. Background Signal. The decay curve shown in Figure 6 is frequently observed for the background signal. This signal has a relatively long decay time. It seems to originate from light absorption by the solid or from adsorbed species on the solid surface because of the large thermal diffucison constant. In this work the lens position was carefully adjusted to minimize these background signals. Of course, this background could be reduced by increasing the modulation frequency. The blank signal was observed even when the sample cell was removed. Thus it was ascertained to originate neither from the species adsorbed on the sample cell nor from impurities in the solvent. In the experiment the signal tended to increase with increasing the length of the fiber. This implies that it is caused by light absorption in the optical fiber. If so, the present method may provide a new analytical technique to measure very small light absorption by impurities in the optical fiber. It is noted that absorption loss is quite difficult to determine by measuring transmission loss of the fiber since main loss is not light absorption but Rayleigh scattering originating from fluctuation of refractive index in the fiber core. Suggested Parameter for the Optical Fiber. The thermal lens spectrophotometer using a couple of optical fibers has a limited performance in sensitivity because of the short effective path length. To improve the sensitivity, the core diameter of the optical fiber should be increased. This gives a large effective path length and is useful for sensitive detection of the sample. The suggested parameters for the optical fiber are listed in Table 11. If the optical fiber with a 13.6-pm core diameter was available, the effective path length could be increased to 10 mm. In this case the laser beam should be more loosely focused into the core of the optical fiber for mode matching. For the manufacturing of such an optical fiber, the difference in refractive indexes between the core and the cladding should be designed to be small, and the numerical aperture may decrease. The alignment between the laser and the optical fiber is at present critical due to slight mode mismatching. The fiber
Anal. Chem. 1987, 59, 1557-1563
Table 11. Optical Parameter for Fiber Required for More Sensitive Thermal Lens Spectrophotometry' effective p a t h length,
Z,, pm
wo, pm
a, pm
NA
31 160
2.2
10
6 13.6
65
1000
0.062 0.027 0.011
mm 2
4.9 13
34
A, % 0.087 0.017 0.0026
" E x c i t i n g source, Ar !aser (488 nm); 2w0 = 0 . 7 2 ~ . T h e refractive index is assumed t o be 1.5. T h e effective p a t h l e n g t h means t h e distance t h a t t h e singal i n t e n s i t y does n o t increase even when t h e p a t h l e n g t h increased t h i s value.
is manufactured for use at 850 nm (6 pm) or 1300 nm (10 pm); therefore transmission of the 488-nm line of the argon ion laser causes considerable mode mismatch as shown in Figure 4 of ref 7. It also increases the background signal originating from the chemical species adsorbed on the fiber surface. We expect that a single-mode optical fiber designed for the individual laser lines will be developed in the near future. Application. The semiconductor laser may have great advantage for the present application. It has already been used as a light source for thermal lens spectrophotometry of phosphorus (6). Overtone vibrational bands of hydrocarbons are located in the near-infrared region, and they can easily be detected by semiconductor laser thermal lens spectrophotometry ( 1 1 ) . We expect that such a detection system is quite useful for remote sensing of the chemical products in extremely hazardous surroundings, such as in a high-pressure vessel or in a high-temperature reactor.
1557
Electronic engineers are now developing a local area network (LAN) using an optical fiber and a semiconductor laser for data communication in a factory and a social community. We would like to propose a new analytical technique which can be directly coupled with a data communication system; the present thermal lens method is performed by simply separating the optical fiber for data communication and by placing a couple of fibers in the sample monitoring space. This simple approach may provide a very sensitive analytical tool without using any special analytical equipment. The optical-fiber sensor coupled with a semiconductor laser may be also involved in this category.
ACKNOWLEDGMENT The authors wish to thank Yuji Kawabata for his helpful discussion about the optical fiber. LITERATURE CITED Harris, J. M.; Dovichi, N. J. Anal. Chem. 1980, 52, 695A. Imasaka, T.; Ishibashi, N. TrAC. Trends Anal. Chem. (Pers. Ed.) 1982, 1, 273. Sheldon, S. J.; Knight, L. V.; Thorne, J. M. Appl. Opt. 1982, 21, 1663. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1980, 52, 2338. Higashi, T.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1983, 55, 1907. Nakanishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1985. 57,
1219. Nakanishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. previous paper in this issue. Hu, C.; Whinnery, J. R . Appl. Opt. 1973, 12, 72. Carter, C. A.; Harris, J. M. Appl. Spectrosc. 1983. 3 7 , 166. Bialkowski, S. E. Anal. Chem. 1986, 58, 1706. Imasaka, T.; Sakaki, K.; Ishibashi, N., Hakozaki, Japan, unpublished work, 1986.
RECE~VED for review August 26,1986. Resubmitted February 18, 1987. Accepted March 4, 1987.
Iron(1) Chemical Ionization for Analysis of Alkene and Alkyne Mixtures by Tandem Sector Mass Spectrometry or Gas Chromatography/Fourier Transform Mass Spectrometry David A. Peake, S.-K. Huang, and Michael L. Gross*
Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588
Slmple mixtures of alkene or alkyne Isomers were analyzed by using Fe' chemical lonlzatlon and tandem mass spectrometry (MS/MS). The approach Is successful over a ca. 1:lO dynamic range. However, mixtures of Isomers over greater concentratlon ranges and the presence of congeners require chromatographic separatlon prior to MWMS analysis. One means of overcoming the requlrements of fast scannlng and Mgh resolution In conventlonal sector mass spectrometry is Fourier transform mass spectrometry (FTMS) In a pulsed valve mode. Ionlzatlon was by multlphoton dissociation/ multiphoton lonlzatlon of Fe(CO), to produce nearly exclusively gas-phase Fe'. Detectlon llmlts are In the nanogram range. A problem Is encountered for alkenes havlng 12 or more carbons because the hlgh reactlvlty of Fe' leads to consecuthre reactlons with the hydrocarbon, particularly In the low-pressure regime of an FT mass spectrometer cell.
The use of metal ions as reagents for chemical ionization (CI) mass spectrometry is a new and developing area of current research (1-8). Cationization of organic molecules with alkali 0003-2700/87/0359-1557$0 1.50/0
ions to provide molecular weight information was first demonstrated with the reaction of Li+, produced by field ionization, and 1-hexanol(1). Subsequently, the molecular weights of olefins (2),peptides (3),and saccharides (4)were determined by cationizing the sample molecules with alkali ions in the gas phase. The reactivity of transition-metal ions with organic molecules (5) has stimulated further interest in metal ion CI. Unlike alkali ions, transition-metal ions provide both structural and molecular weight information (6-8). Recently, several transition-metal ions were found to be useful CI reagents for differentiating ketones (6), locating double bonds in olefins (7), and recognizing classes of compounds (8). In our first paper on this subject (3, a method of locating double bonds in olefinic compounds by collisionally activating Fe(olefin)+ adducts was introduced. In this paper, Fe+ CI is evaluated for the analysis of principally isomeric alkene and alkyne mixtures. Two approaches are compared: (i) multicomponent analysis of collisionally activated decomposition (CAD) spectra, whereby mixtures of isomeric octynes are analyzed, and (ii) separation of olefins by capillary gas chromatography prior to reaction with Fe+ in the cell of a 0 1987 American Chemical Society
