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Anal. Chem. 1988. 60.2537-2540
Table I. Solubility of Hydrocarbons in Water
hydrocarbon
water solubilitv mn/L lit. value measd valuea
o-xylene
168
ethylbenzene toluene benzene n-hexane n-octane n-heptane
130
558 1780 10 0.66 2.9
176 f 8 172 f 6 538 f 15 1650 f 20 14 k 1
1.25 f 0.03 2.9 f 1.9*
a Mean and standard deviation. *Interfering contaminant in water.
variation involving direct purging to a capillary column. Their method avoids potential problems associated with the use of a sorbent trap and reduces the overall time required for analysis, but retains the conventional purge cycle. The purpose of this paper is to suggest an alternative approach that eliminates the purge cycle and permits direct injection of water samples to a capillary column. Tubing made of Nafion (Du Pont) and in particular Perma-Pure dryers are widely used in cryotrapping methods for capillary column analysis of gaseous samples (3)and Cochran ( 4 ) has suggested using a Perma-Pure dryer in place of the trap in the conventional purge and trap method. In using these dryers, we have noted that the bulk of the water vapor is removed within the first few centimeters of the Nafion tubing, suggesting that the normal dryer containing 30 cm or more of tubing should have a much greater capacity for water removal than is utilized in drying gaseous samples. We believed that this capacity might be great enough to permit direct injection of water samples of reasonable volume. To test this hypothesis, we modified an existing combination Nafion tubing and cryotrap capillary GC inlet by installing a heated injection port upstream of the tubing. The dryer assembly [Perma-Pure MD-125-48 (F)]was 122 cm long and was operated with dry nitrogen purge gas a t ambient temperature. The inlet was attached to a Varian Model 3700 GC equipped with a Hewlett-Packard 3388 integrator, a fusedsilica capillary column (50 m X 0.32 mm, OV-1, HewlettPackard), and a flame-ionization detector. Aqueous samples were vaporized in a stream of helium (60 cm/min) in the heated injection port (85 "C),after which they passed through the Nafion tubing (ambient temperature). The hydrocarbon components of the aqueous samples were then collected on the cryogenic trap (-180 "C). A flush period of 5 min was allowed to ensure complete transfer of the vapors to the cryogenic trap. This is comparable with the purge period used by Pankow and Rosen. A series of measurements of the solubilities of hydrocarbons was performed to test this procedure. Samples of water and
various pure hydrocarbons were sealed in vials having septum caps and were allowed to equilibrate for several days with occasional gentle agitation. Samples of the water phase were withdrawn through the septum caps and were analyzed with results as shown in Table I. The probable error associated with the literature values cited in Table I is unknown, but based on previous experience (5)is likely to be of the order of 3-5%. The solubilities determined by the current method are therefore in excellent agreement with the literature values. For the purposes of these measurements, adequate sensitivity was obtained by using 50-pL water samples. The detection limit with this sample size was approximately 2 kg/L. Multiple injections or slow injection of larger samples could be used for sample concentration where needed. The limiting factor in determining sample size was not the capacity of the dryer but rather the dimensions of tubing fittings a t the inlet end of the dryer. With samples larger than 50 KL, swelling of the Ndion at the inlet end was sufficient to cause temporary blockage of the sweep gas passage. In no case was there any evidence for penetration of water vapor to the cryogenic trap. The method has not been tested extensively but appears to be limited only by compatibility of the analytes with Ndion. Very polar compounds such as alcohols and amines are known to be absorbed by the Nafion, but chlorinated hydrocarbons are passed quantitatively ( 3 ) . Although our work involved the use of a cryotrap, out of convenience, there is no reason why the direct on-column approach of Pankow and Rosen could not be used in conjunction with the method suggested here. Such an approach would eliminate both the purge and the trap cycles of water analysis, resulting in improved convenience and speed of analysis. Registry No. o-Xylene, 95-47-6; ethylbenzene, 100-41-4; toluene, 108-88-3; benzene, 71-43-2; hexane, 110-54-3; octane, 111-65-9;heptane, 142-82-5;water, 7732-18-5.
LITERATURE CITED Methods for the Determination of Organic Compounds in Finished Drinking Water and Raw Source Water; Physical and Chemical Methods Branch, EMSL: Cincinnati, OH, 1986; Method 524.2. Pankow, J. F.; Rosen, M. E. Environ. Sci. Technoi. 1988p 22, 396-405. McClenney, W. A,; Pleil, J. D.; Holdren, M. W.; Smith, R. N. Anal. Chem. 1984, 56, 2947. Cochran, J. W. HRC C C , J. High Resolut. Chromatogr. Common. 1~137. . .. , i. o .., 573-575. .. - .. ..
Coutant, R. W.; Lyle, L.; Callahan, P. J. "Validation of the Water Solubility Tests"; final report from Battelle Columbus Division to the Office of Pesticides and Toxic Substances, U.S. Environmental Protection Agency, Contract No. 68-02-5043, Feb. 1981.
Robert W. Coutant* G . William Keigley Battelle Columbus Division 505 King Avenue Columbus, Ohio 43201
R ~ c for m review May 16,1988. Accepted August 23,1988.
Experimental Comparison of Single- and Double-Fiber Configurations for Remote Fiber-optic Fluorescence Sensing Sir: Interest in the application of fiber optics to chemical remote sensing is growing rapidly (1-7). Generally, fiber optics are utilized to guide radiation to and from a remote sample containing an analyte of interest. This species can then be quantitated by using traditional spectroscopic methodologies
such as absorption or fluorescence spectrometry. Many unique methodologies for detecting nonabsorbing and nonfluorescing species have been reported (1-7). Most fiber-optic chemical sensors (FOCS), or optrodes, for the detection of fluorescence employ either the single- or double-fiber probe configuration.
0003-2700/88/0360-2537$01.50/00 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Excitation Mount /
f.0. #
I
\ /
f.0.
L1
#2
F/0.75 Concave Mirror
Fiber Holder
I I
.c
Figure 1. Schematic of the laser/fiber-optic instrumentation. Fiber optics 1 and 2 are both 1 m
in length and are stripped of all cladding at the
probe tip.
To date, no quantitative comparison of these two basic configurations has appeared. We are interested in remote fiber fluorometry (RFF) and in this work describe a simple experimental scheme for measuring the relative fluorescence collection efficiencies of single- and double-fiber probe systems. This ratio is important as it ultimately limits the relative signal levels obtainable from these systems. A single-fiber optrode for fluorescence measurements uses the same fiber to transmit the excitation radiation to the sample and to guide the collected signal radiation (the wavelength-shifted luminescence) to the detection system. The detection system consists of an emission monochromator or filter (with appropriate entrance optics), photomultiplier tube (PMT), a signal processor, and a readout device. An optical interface or coupler capable of spatially separating these two radiation components a t the proximal end of the fiber optic is required. Double-fiber optrodes for fluorescence measurements utilize one fiber to guide the excitation radiation to the sample and a second fiber to collect and guide the emission radiation back to the detection system. It is also possible to use one excitation fiber and two or more emission fibers (8, 9). Such systems are conceptually similar to the double-fiber configuration. Several researchers have also used bifurcated fiber-optic bundles (10, 11),in which half the fibers carry the excitation radiation while the other half return the emission radiation to the detection system. This approach may be prohibitively expensive for remote sensing over long distances. Also, it is difficult to optimize the angle between excitation and emission fibers. A number of different optical interfaces for use with single-fiber systems have been reported (12, 13). In addition, modifications to enhance the signal obtained with the single-fiber configuration (12,13) and the double-fiber configuration (8, 14) have been presented. The optical efficiencies of the single-fiber configuration (13) and double-fiber configurations (8, 14) have been theoretically treated. Still lacking, however, is any quantitative comparison between the two configurations. Factors that must be weighed when choosing the appropriate configuration include cost, complexity, ruggedness, size (of the FOCS), and detectability. For fixed excitation conditions and a given type and size of fiber optics, the detect-
ability for a specific analyte varies between optrode configurations due to differences in the fluorescence emission collection efficiency, the magnitude of the background signal (and hence blank noise), and the efficiency of the single-fiber configuration optical coupler. Overall, the net signal obtained will be a function of both probe collection efficiency and the efficiency of this necessary optical coupler, which can be easily measured. By quantitation of the relative collection efficiencies of the two basic probe configurations (single and double fiber), it is then possible to calculate the overall relative signal levels obtainable when using different couplers and sources.
EXPERIMENTAL SECTION Instrumentation. The instrumental configuration used is shown in Figure 1. The excitation source is a 2.5-mW heliumneon laser (Melles Griot, X of 632 nm). The optical coupler is designed around a f/0.75 convex mirror (5.2-cm diameter) with a 3-mm hole in its center. Maxlight, PCS fiber optics (plastic-clad silica fiber optic, core/cladding diameter of 860 pm with a 600-pm core, numerical aperture -0.33) were used throughout. The excitation mount for two fibers was constructed in-house utilizing two Newport FPH-DJ fiber-optic chucks and a Newport LP05-XY translator. This mount holds the two fibers at the same vertical height and allows for xpz translation as a pair. The distance between the center of two fibers was -5.7 mm. The excitation fiber (f.0. #2) was positioned two focal lengths (78 mm) from the mirror and the angle defined by f.0. #2, the mirror hole, and Eo. $3 was -4.2’. The distal (sensing)ends of the fiber optics (the optrode) were stripped of cladding and held at a 20’ angle by a Teflon holder constructed in-house. The detection system consists of a f/0.75 glass aspherical lens, a PTR Optics “Minichrome” monochromator (reciprocal linear dispersion of 20 nm/mm, set to 732 nm, effective aperture of f / 4 , 1-mm emission and excitation slits), a 660-nmcutoff filter (located between the exit slit of the monochromator and the PMT), and an RCA 4840 PMT powered by a Keithley high-voltage power supply (Model 244). The PMT signal was monitored with a DVM after current to voltage conversion and amplification. The sample cell was a 10-mL beaker over which the fiber-optic holder fit. This system provided a light-tight enclosure and reproducible optrode positioning in the sample solution at ca. 2.5 cm above the bottom of the beaker. At this depth, the sample volume was determined to be large enough so as to have no effect on signal levels. Procedures. The efficiency of the optical interface for the single fiber configuration was evaluated as follows. The sensing
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Table I. Experimentally Determined Values of E , R f ,and RC P run
E
Rr
Rd
A A'
0.68
0.60 0.86 0.83 0.34
0.89
B B'
0.40 0.49 0.25
2.14
RCf(paired) 1.52
1.69 1.36
1.53
OTest solution: 100 ng/mL HIDC dye. end of fiber optic 2 was illuminated by green light from a tungsten lamp and band-pass filter combination. The radiant power exiting at point 4 (a4)was then measured with a photodiode (PTR Minichrome Silicon Detector). The radiant power exiting fiber optic 3 at point 6 (@e) was then maximized by optimizing the off-axis angle of the mirror (relative to fiber 3) and the vertical position of both fiber terminations. The efficiency E is defined as For fluorescence measurements, the laser beam was first adjusted to pass through the hole in the mirror and directly impinge only on optical fiber 2 at point 4. Fluorescence measurements were then obtained from one dye solution (emission maximum at 732 nm) using both optrode configurations by simply switching fibers 1 and 3 at the input to the detection system. These measurements were then repeated for a blank solution. The fluorescence signal ratio (Rf) is calculated as where S6 is the blank corrected fluorescence signal from the single-fiber optrode and SI is the blank corrected signal from the double-fiber configuration. From Rf and E , the corrected fluorescence ratio Rcf is calculated as
Rcf = R f / E
(3)
The corrected fluorescence ratio is a direct measurement of the relative fluorescence collection efficiencies of the two configurations. Fibers 1 and 2 were then exchanged and another set of measurements taken (obtaining new values of E, Rf,and Rd). This pairing of measurements helps compensate for any variations in the transmission characteristics of the two fiber optics (e.g., surface inhomogeneities). The paired fluorescence measurements were repeated twice, between which the fibers were repolished and reorientated in the holder. Performing these replicate measurements helps compensate for any bias due to variability in the orientation of the fibers (at the probe tip) in the final ratio. Reagents. HIDC iodide laser dye (1,1,3,3,3',3'-hexamethylindodicarbocyanine iodide) was obtained from Sigma and 1.96 X lo-' M (100 ng/mL) and 9.80X M (500 pg/mL) solutions were prepared with reagent grade DMSO (Baker).
RESULTS AND DISCUSSION Other experiments showed that an approximate 20' angle between fibers gave the optimum signal-to-background ratio (S/B) for the double-fiber configuration and this angle was used for all measurements. This is consistent with the model of Plaza et al. (14). Table I lists the experimental values obtained for E, Rf,and Re Runs A and A are paired as are runs B and B'. The mean values of Rcf for each paired run are 1.52 and 1.53 (A and B) and the mean of the two means is 1.52 with a relative standard deviation (RSD) of 0.7%. The RSD calculated from the four (unpaired) values is 36%. Before the paired measurement scheme was implemented, 11 unpaired experiments were conducted and yielded a mean value for Rcf of 1.40 (34% RSD), which is in good agreement with the value obtained from the paired measurement scheme. The significant scatter in the values of E determined (37% RSD) is attributed to imprecision in adjusting the positions of the laser and mirror between measurements. An optical
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table was not used for the temporary optical setup employed nor were the laser or mirror mounted on micrometer-controlled translation stages. The scatter in the individual values of Rf is due to scatter in E and to the differences in transmission characteristics of the two fiber optics employed. By the division of each value of R f by the corresponding value of E (eq 2), the error introduced by the variability of E is compensated. Thus, the scatter of Rd is due primarily to the differences (e.g., transmission characteristics) between the two fibers. With the two fibers switched, the measurements repeated, and the value of RCfreported as the mean of these paired measurements (A & A', B & B'), this source of error is also minimized, as evidenced by the marked decrease in uncertainty (paired vs unpaired). For example, if the fiber optic with better transmission characteristics is used as f.0. #1,the single value of Rd obtained will be less than the true value (i.e., the value that would be obtained by using equivalent fiber optics). Likewise, the experimental value of Rcfwill be greater than the true value if f.0. #1exhibits lower transmittance than f.0. #2 (the data for runs A & A' reflect this effect). For the fiber used, the fluorescence collection efficiency of the basic single-fiber configuration is only about 50% greater than that of the basic double-fiber configuration. Thus, the efficiency of an optical coupler used with the single-fiber c o n f i i a t i o n must be ca. 70% to provide the same signal level as the double-fiber configuration. Single-fiber couplers are generally based on either dichroic beam splitters (12,13,15) or geometric couplers utilizing prisms and/or simple mirrors (12, 13, 16, 17). The simplest geometric coupler consists of a planar mirror with a small hole in its center. Source radiation is focused through the hole onto the fiber optic, while most of the radiation exiting the fiber is reflected to the detection system. When used with laser sources, these devices can be designed such that their efficiency is limited primarily by the reflectivity of the mirror and thus are theoretically capable of collecting ca. 90% of the radiation exiting the fiber optic. This assumes that losses due to the hole (
