Anal. Chem. 1983, 55, 146-148
146
R : Cj4HZg INTACT CATION I
h H 2 5 II
II
I
HPLC
BENZALKONIUM CHLORIDES
Figure 1. HPLC separatlon of benzalkonium chloride mixture (I, 11).
, ~ o
BENZALKONIUM CHLORIDES
POSITIVE ION LDMS
spectrometry can be applied to analysis of mixtures of organic compounds using quasi-molecular ions. It is interesting to note that if one attempts mixture analysis from the fragment ions a t mlz 240 and 212 a ratio of 1I:I = 4.0 results. This clearly differs from the HPLC ratio, indicating that one may have to exercise caution when using peaks other than those for quasi-molecular ions for quantitation. One might argue that the intensity of the mlz 304 intact cation may have a contribution from the other (mlz = 332), as a result of a neutral loss of 28 mass units. Such a loss can be excluded, because fragmentation through a four-membered transition state, which is usually expected for quaternary ammonium salts, would not lead to loss of neutral fragment of 28 mass units. Even if it did occur, one would expect the same type of fragmentation from I1 which is chemically similar and of higher intensity than compound I. Such is not observed. The analysis of the benzalkonium chloride mixture by LMS is in excellent agreement with the HPLC result, within the limit of experimental error (&lo%). This first attempt of quantitative analysis of organic compounds shows that LMS using the LAMMA 500 has potential for quantitative analysis of organic mixtures. LMS has several advantages over other techniques for quantitation. First, it requires only a small amount of sample (micrograms or less). Second, no special sample preparations are required. Third, analyses are fast. Fourth, the microprobe capabilites of the LAMMA-500 have the potential for quantitative analysis of organic inclusions in an organic matrix.
ACKNOWLEDGMENT C14H29
INTACT CATION
51
I
C12H25 II
We thank N. Brake and K. Cornelius for their help with the HPLC work. Registry No. I, 139-08-2;11, 139-07-1.
'I
LITERATURE CITED
Figure 2. Laser desorptlon mass spectrum of benzalkonium chloride mixture (I, 11).
The base peak at mlz = 91 is the familiar benzyl (tropylium) ion, as would be expected. We have used the intensities of the intact cation peaks for quantitative calculation. The ratio measured by LMS for the benzalkonium chlorides is 111 = 2.9 & 0.3. This is within experimental error of the value of 3.0 derived from HPLC data. This is a clear demonstration that laser desorption mass
(1) Hercules, D. M.; Day, R. J.; Balasanmugarn. K.; Dang, T. A.; Li, C. P. Anal. Chem. 1982, 5 4 , 280A. (2) Talrni, Y.; Dutta, K. P. Anal. Chlm. Acta 1981, 132, 111-118. (3) Kaufmann, R.; Hillenkamp, F.; Wechsung, R. Med. Prog. Techno/. 1979, 6, 109-121. (4) Schroder, W. H. Z . Anal. Chem. 1981, 308, 212-217. (5) Seydel, U.; Lindner, B. Z . Anal. Chem. 1981, 308, 253-257.
Kesagapillai Balasanmugam David M. Hercules* Department of Chemistry University of Pittsburgh Pittsburgh, Pennsylvania 15260 RECEIVED for review August 2, 1982. Accepted October 14, 1982. This work was supported by the National Science Foundation under Grant CHE-8108495.
Fiber Optic Probe for Remote Raman Spectrometry Sir: When combined with UV-Vis spectrophotometry, optical fibers are useful waveguides for directing radiation of the sample and returning the partially absorbed light for detection. A recent report discussed the application of fibers for remote fluorescence, where the sample may be located a great distance from the spectrometer ( I ) . However, the poor transmission characteristics of silica fibers in the infrared region prevent their use in IR absorption spectrometry. We report here a probe for Raman spectrometry, where the advantages of fiber optics are combined with the structural
information inherent in Raman spectra. Fiber optics have been used previously for collection of Raman scattering ( 2 ) and for holding samples for Raman spectrometry ( 3 , 4 ) . In the present work, both the excitation beam and scattered light are carried by fibers, so the sample may be located far away from the spectrometer, in a hostile environment if necessary. In addition the probe itself is very simple and rugged and may be used for routine analysis. The apparatus is shown in Figure 1 and is based on 200 pm diameter multimode fibers of common use in communications.
0003-2700/83/0355-0146$01.50/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983 Collection
147
Fiber
Input Fiber
Sample
Spectrometer Call e c t i o n Lens
c
F = 45mm
1
Flgure 1. Schematic of fiber probe. Laser output is focused on input fiber and exits into solution as a cone, shown in the insert (a= 27').
Adjacent to the input fiber is the collection fiber which accepts light from a similar cone. The other end of the collection fiber is positioned at the slit image of the spectrometer such that its output is focused on the input slit. Both fibers were 2 m long and the cladding was Intact except for 2 cm removed at the sample end. Silica core diameter was 200 p m . 3000
The 200-pm silica core is coated with a 100 pm thick sheath of silicone rubber and then a 100-ym protective coating of nylon. The laser beam (5145 A) was focused into the input fiber with a 45-mm lens, with about one-third of the input light being transmitted by the fiber. Two centimeters of cladding was removed from the sample end of both input and collection fibers, and the exposed ends were placed adjacent to each other with the axes of the fibers approximately parallel. The other end of the cnllection fiber was positioned at the slit image of the spectrometer (5). With the monochromator tuned to a Raman line of the sample, the laser end of the input fiber and output end of the collection fiber were adjusted to maximize the signal. During the adjustments and during use of the probe, the position of the fibers in the sample was unimportant. The laser light exits the input fiber into the sample as a cone whose size is determined by the numerical aperture of the fiber. In this case the cone had an angle a t its apex of 54' (a = 27'), so that its base was about 1 cm wide after traveling 1cm in the sample. The collection fiber gathered scattered light from a similar cone, and the scattered light was analyzed as usual by the spectrometer. A Raman spectrum for acetonitrile using the fiber optic probe is shown in Figure 2A and is compared with a conventional spectrum obtained with the usual 90" sampling geometry. All parameters were identical for the two spectra except for laser power, which was 0.8 W for Figure 2B and 1.8 W (incident on the input fiber) for Figure 2A. After an adjustment was made for this power difference, the spectrum obtained with fibers is 13% as intense at that obtained conventionally. The small feature at about 1100 cm-l in Figure 2A was caused by a mercury line in the room lights which was collected by the fiber. Spectra were also obtained with a single fiber acting both as input and collection fiber, but a large silica background was observed in the region 200-700 ern‘.' and peaks in the C-H stretch region were observed which originated from the silicone cladding. Several improvements in this initial design are apparent, which will augment the value of the probe. An array of six collection fibers oriented1around the input fiber will increase the sensitivity over that of a conventional sampling arrangement. The six colllection fibers could be oriented as a row at the spectrometer, to match the slit image. This improvement in collection optics, combined with more efficient coupling of the laser to the input fiber should allow the sensitivity to exceed that of conventional collection optics. E n
2000
AV
,
IO00
3
cm-1
Flgure 2. (A) Raman spectrum of neat acetonitrile using the fiber optic probe described in Figure 1. Laser power incident on the input fiber was 1.8 W (5145 A) of which about 600 mW entered the sample. Spectral slit width was 5 cm-'. (B) Spectrum of neat acetonitrile obtained with conventlonal 90' collection geometry. Sample was contained in a 20-mL vial, slit image was oriented along focused beam. All other conditions were as given in A, except input laser power was
0.8 W.
capsulation of the sampling end of the fibers will permit a very rugged and corrosion resistant probe to be constructed, provided the encapsulation material has a refractive index less than that of the fiber (n < 1.49). Several advantages of the fiber optic probe exist for many areas of application of Raman spectrometry. First, the sample may be distant from the spectrometer, since losses in the fibers are very small (ca. 1% /m). Second, no special sample positioning is necessary once the fibers have been coupled to the laser and spectrometer. The probe is simply inserted in the sample, by an unskilled operator if necessary, with no alignment required. Third, the probe can be very small, consisting of encapsulated fibers with a total diameter of less than 1mm. Applications to samples with limited accessibility are possible, in areas such as medicine, chemical processing, and the petroleum industry. Fourth, the probe may be positioned in hostile environments not amenable to conventional Raman, such as polymer melts, high-temperature reactors, etc. Only silica and an encapsulation material (e.g., Teflon) need be in contact with the sample and the probe could be mounted in the wall of a reactor or pipe. Fifth, the sensitivity may exceed that of conventional sampling techniques, resulting in broader applicability. Finally, it is possible to have several probes multiplexed to the same spectrometer, so several locations in a plant or research facility could make use of a single spectrometer. While UV-Vis absorption and fluorescence spectrometry permit sensitive monitoring of known components, Raman spectrometry allows the fingerprinting of species present and is therefore structurally specific. In addition, the inherently high resolution of Raman spectra often permits the analysis of several components in a mixture simultaneously. The simplicity and versatility of the fiber optic probe described here should broaden the applicability of Raman spectrometry to a variety of analytical problems, especially as there is no need for sampling or sample preparation.
Anal. Chem. 1983, 5 5 , 148-150
148
Note: Just at the time of submission of this manuscript, a commercial device was announced (“Optrode”,Oriel, Stamford, CT) which is similar to that described here, but intended for fluorescence spectrometry. The advantage of using the device for Raman is that the fibers conduct well in the wavelength region used for Raman, whereas they conduct the UV light usually necessary for fluorescence much less efficiently. In addition, a review of optical waveguides applied to spectroscopy has appeared (6).
LITERATURE CITED Borman, S. A. Anal. Chem. 1981, 53, 616A. Eysel, A. Spectrochim. Acta, PartA 1971, 27A, 173. Walrafen, G. E.; Stone, J. Appl. Spectrosc. 1972, 26, 585. Stolen, R. H. I n Proceedings of the Internatlonal Conference on Light Scatterlng in Solids, 3rd; Balkanskl, A,, Ed.; 1975; p 656. Chem. Abstr. 1978, 85, 133491. (5) Reld E. S.; Cooney, R. P.; Hendra, P. J.; Flelschmann, M. J . Nectroanal. Chem. 1977, 80, 405.
(1) (2) (3) (4)
(6) Chabay, I . Anal. Chem. 1982, 5 4 , 1071A.
Richard L. McCreery* Department of Chemistry The Ohio State University Columbus, Ohio 43210
Martin Fleischmann Patrick Hendra Department of Chemistry University of Southampton Southampton SO9 5NH, England RECEIVED for review July 19,1982. Accepted October 20,1982. This work was performed while R.L.M. was on sabbatical at the University of Southampton. Partial support by the Alfred P. Sloan Foundation and the hospitality of the Chemistry Department at Southampton are acknowledged.
AIDS FOR ANALYTICAL CHEMISTS Cadmium Telluride y-Ray Liquid Chromatography Detector for Radiopharmaceuticals Richard E. Needham”’ and Michael F. Deianey Winchester Engineering and Analytical Center, U S . Food and Drug Administration, Winchester, Massachusetts 0 1890, and Department of Chemistry, Boston Universiw, Boston, Massachusetts 022 15
The separation of y-emitting radiochemical species has been an important aspect of radiopharmaceutical research and quality control. Recently, high-performance liquid chromatography (HPLC) has found promising application in this area (I, 2). Typical radiopharmaceuticals contain a low-energy y-emitting radioactive label (100-200 keV) which is chelated or covalently bonded to a carrier molecule of interest. Frequently encountered isotopes include technetium-99m (140 keV) and iodine-123 (159 keV). Detection of radiochemical species separated by HPLC has been accomplished by placement of a flow cell or coil in the central depression of a sodium iodide [NaI(Tl)] well-type scintillation detector, so that y-rays emitted from species in the flow cell are detected by the surrounding detector. A NaI(T1) well-type scintillation detector is not ideal for this application since (1)it is relatively large, typically a 3 in. (7.5 cm) diameter by 3 in. (7.5 cm) thick crystal with a 1in. (2.5 cm) diameter well, (2) it requires a large and inconvenient photomultiplier tube (PMT), which restricts its placement relative to a flow cell, and (3) it is generally necessary to surround the crystal with extensive shielding to reduce background count rates. A more satisfactory approach to the detection of y-rays after HPLC separation would be to have a small volume, compact y detector which could view an HPLC flow cell externally and with high detection efficiency. Cadmium telluride (CdTe) detectors are a relatively recent development which have found application as nuclear radiation dosimeters and probes ( 3 , 4 ) . CdTe detectors are suitable for the present application for a number of reasons: (1) the CdTe detector is a semiconductor detector, in which an electrical charge is produced directly from a y-ray interaction. The need for a P M T is therefore eliminated; (2) the active volume of the detector itself is quite small (5-10 mm3) with correspondingly compact external Author to whom correspondence should be addressed at Winchester Engineering and Analytical Center. This artlcle not subject to
packaging, permitting convenient placement of the detector in confined areas; (3) the small active volume results in relatively low background count rates and maximizes detection sensitivity for small source volumes, such as HPLC flow cells; (4)the y peak resolution is comparable to that of a NaI(T1) detector; this gives energy resolution sufficient to discriminate against background and scattered radiation; (5) the detection efficiency of CdTe, because of its higher effective atomic number (Zaff= 50), is higher than that of NaI(T1) (Zeff= 38) on an equal volume basis. This report presents our experience with a CdTe detector which we have mounted in a commercially available refractive index (RI) HPLC detector, allowing simultaneous monitoring of the bulk properties and y radiation of eluting radiochemical species.
EXPERIMENTAL SECTION A 2.6 X 2.6 X 2.0 mm “cubic”CdTe detector housed in a 5 mm diameter X 12 mm cylindrical aluminum housing with a 20 cm long miniature coaxial cable lead and BNC connector (Radiation Monitoring Devices, Inc., Watertown, MA) was used. An aluminum detector holder was constructed in-house to mount the detector to the heat exchanger coil of a Micromeritics Model 771 refractive index detector (Micromeritics, Norcross, GA). The detector lead was brought outside the Dewar flask housing of the RI detector to a Radiation Monitoring Devices Model PSP-1 preamplifier/bias supply. The preamplifier signal was routed sequentially by coaxial cable to an Ortec Model 572 spectroscopy amplifier (EG+G Ortec, Oak Ridge, TN), an Ortec Model 551 single channel analyzer, and an Ortec Model 441 rate meter. The recorder output from the ratemeter was used to drive one side of a Varian Model A-25 dual channel recorder (Varian Aerograph, Walnut Creek, CA), while the second side was driven by the RI detector. The CdTe detector was collimated by wrapping a 1.5 mm thick layer of lead foil around the cylindrical housing, in order to attenuate any extraneous radiation up to 150 keV by greater than 95%.
U S . Copyrlght. Published 1982 by the American Chemical Soclety
