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ogenated hydrocarbons, esters, aldehydes, ketones, and others) behave in a similar manner within the context of such relationships. Although compound classes such as alcohols and amines demonstrate different behaviors, within each class the behavior is nevertheless predictable. The availability of these relationships should be beneficial to researchers concerned with predicting retention volume values for compounds whose behavior on Tenax-GC has not as yet been studied.
Registry No. n-Pentane, 109-66-0; n-hexane, 110-54-3; nheptane, 142-82-5;n-octane, 111-65-9;n-decane, 124-18-5;n-dodecane, 112-40-3; benzene, 71-43-2; toluene, 108-88-3; xylene, 1330-20-7; p-xylene, 106-42-3; ethylbenzene, 100-41-4; nprnpylbenzene, 103-65-1; cumene, 98-82-8; trimethylbenzene, 25551-13-7;styrene, 100-42-5;a-methylstyrene, 98-83-9;butadiene, 106-99-0; chloromethane, 74-87-3; dichloromethane, 75-09-2; chloroform, 67-66-3; carbon tetrachloride, 56-23-5; chloroethane, 75-00-3;1,l-dichloroethane, 75-34-3; 1,2-dichloroethane,107-06-2; l,l,l-trichloroethane, 71-55-6; 1,1,2-trichloroethane, 79-00-5; 1,1,1,2-tetrachloroethane,630-20-6; 1,1,2,2-tetrachloroethane, 79-34-5; vinyl chloride, 75-01-4; 1,l-dichloroethene,75-35-4; 1,2dichloroethene, 540-59-0; trichloroethylene, 79-01-6; tetratrichloroethylene, 127-18-4; bromomethane, 74-83-9; trichlorofluoromethane, 75-69-4; dichlorodifluoromethane, 75-71-8; chlorotrifluoromethane, 75-72-9;aUyl chloride, 107-05-1;fluorobenzene, 462-06-6; chlorobenzene, 108-90-7;bromobenzene, 108-86-1;odichlorobenzene, 95-50-1; m-dichlorobenzene, 541-73-1; p-dichlorobenzene, 106-46-7;1,2,4-trichlorobenzene, 120-82-1;phenol, 108-95-2;o-cresol, 95-48-7; n-cresol, 108-39-4;p-cresol, 106-44-5; rn-chlorophenol, 108-43-0;2,4,64richlorophenol, 88-06-2;methyl acetate, 79-20-9;ethyl acetate, 141-78-6;propyl acetate, 109-60-4; butyl acetate, 123-86-4; isopropyl acetate, 108-21-4; methyl acrylate, 96-33-3; ethyl acrylate, 140-88-5; acrolein, 107-02-8; acetaldehyde, 75-07-0; acetone, 67-64-1; methyl ethyl ketone, 78-93-3;methyl isobutyl ketone, 108-10-1;3-methyl-2-butanone, 563-80-4; cyclohexanone, 108-94-1; 2-heptanone, 110-43-0; 4heptanone, 123-19-3;acetophenone, 98-86-2; methanol, 67-56-1; ethanol, 64-17-5; 1-propanol,71-23-8; 2-propanol, 67-63-0; l-bu2-methyl-1-propanol, tanol, 71-36-3;2-methyl-2-propanol,75-65-0; 78-83-1; 1-methyl-1-propanol,78-92-2; 1-octanol, 111-87-5;allyl alcohol, 107-18-6;acetic acid, 64-19-7; 1-propionic acid, 79-09-4; 1-butanoic acid, 107-92-6; 1-pentanoic acid, 109-52-4;acetic anhydride, 108-24-7; methylamine, 74-89-5; ethylamine, 75-04-7; n-propylamine, 107-10-8;n-butylamine, 109-73-9;n-pentylamine, 110-58-7;n-hexylamine, 111-26-2;benzylamine, 100-46-9;di-nbutylamine, 111-92-2;tri-n-butylamine, 102-82-9;pyridine, 11086-1;aniline, 62-53-3; acetonitrile, 75-05-8; acrylonitrile, 107-13-1; epichlorohydrin, 106-89-8; ethylene oxide, 75-21-8; ethyl mercaptan, 75-08-1; nitrobenzene, 98-95-3; Tenax-GC, 24938-68-9. LITERATURE CITED (1) Janak, J.; Ruzickova, J.; Novak, J. J. Chromatogr. 1074, 9 9 , 689.
Butler, L. D.; Burke, M. F. J. Chromatogr. Sci. 1076, 14, 117. VMal-Madjar, C.; Gonnord, M.-F.; Benchah, F.; Guiochon, G. J. Chromatogr. Sci. 1078, 16, 190. Gallant, R. F.;King, J. W.; Levins, P. L.; Plecewicz, J. F. Characterization of Sorbent Reslns for Use in Envlronmentai Sampling; U.S. EPA60017-78-054; U.S.Environmental Protection Agency: Washington, DC, March 1978. Brown, R. H.; Purneii, C. J. J . Chromatogr. 1970, 178, 79. Eiceman, G. A.; Karasek, F. W. J. Chromatogr. 1080, 200, 115. Vejrosta, J.; Roth, M.; Novak, J. J. Chromatogr. 1081, 277, 167. Krost, K. J.; Peilizzari, E. D.; Walburn, S.G.; Hubbard, S . A. Anal. Chem. 1082, 5 4 , 810. van der Straeten, D.; van Langenhove, H.; Schamp, N. J. Chromatogr. 1085, 331, 207. Zaranski, M. T.; Bidleman, T. F. J. Chromatogr. 1087, 409, 235. Cropper, F. R.; Kaminsky, S. Anal. Chem. 1063, 35, 735. Raymond, A,; Guiochon, G. J. Chromatogr. Sci. 1075, 13, 173. Senum, G. I. Environ. Sci. Technol. 1081, 15, 1073. Littlewood, A. B. Gas Chromatography;Academic: New York, 1970. Kiselev, A. V.; Yashln, Y. I. Gas-Adsorption Chromatography; Plenum: New York, 1969. Adamson. A. W. Physical Chemistry of Surfaces: Wiiey: New York, 1982. Brunauer, S.:Copeland, L. E.; Kantro, D. L. I n The Solid-Gas Interface; Flood, E. A., Ed.; Marcel Dekker: New York, 1967; Vol. 1. Brunauer, S.;Emmett, P. H.; Teller, E. J . Am. Chem. SOC.1038, 6 0 , 309. Hoare, M. R.; Purnell, J. H. Trans. Faraday SOC. 1055, 52, 222. Rose, A.; Schrodt, V. N. J. Chem. Eng. Data 1083, 8 , 9. Westcott, J. W.; Bldleman, T. F. J. Chromatogr. 1081, 210, 331. Conder, J. R.; Young, C. L. Physicochemical Measurement by Gas Chromatography: Wiley-Interscience: New York, 1979. Sakcdynskii, K.; Panlna, L.; Klinskaya, N. Chromatographia 1074, 7 , 339. Handbook of Chemistry and Physics, 51st ed.; Weast, R. C., Ed.; Chemical Rubber Co.: Cleveland, OH, 1970; p D-146. Tenney. H. M. Anal. Chem. 1058, 30, 2. Desty, D. H.; Whyman, B. H. F. Anal. Chem. 1057, 2 9 , 320. Brazhnikov, V.; Sakodynski, K. J. Chromatogr. 1068, 3 8 , 244. Daemen, J. M. H.; Dankelman, W.; Hendrlks, M. E. J. Chromatogr. Sci. 1075, 13, 79. Karickhoff, S . W. J. Hydraul. Eng. 1084, 110, 707. Pellizzari, E. D., personal communication, 1988. Butler, L. D. Ph.D. Thesis, Unlverslty of Arizona, 1978. Sawyer, D. T.; Brookman, D. J. Anal. Chem. 1068, 4 0 , 1847.
James F. Pankow Department of Environmental Science and Engineering Oregon Graduate Center 19600 N.W. Von Neumann Drive Beaverton, Oregon 97006
RECEIVED for review August 7,1987. Accepted December 22, 1987. This work was funded in part with federal support from the United States Geological Survey (USGS) under Grant 14-08-0001-A0410 and with the support of the Northwest Environmental Research Center (NWERC). The contents do not necessarily reflect the views or policies of USGS, nor does the mention of trade names or commercial products constitute endorsement for use.
TECHNICAL NOTES Supersonic Jet Atomlc Spectroscopy with Flame Atomization Lance B. Koutny, William B. Whitten,* Thomas G. Nolan, and J. Michael Ramsey Oak Ridge National Laboratory, Analytical Chemistry Division, Oak Ridge, Tennessee 37831 -6142 We have recently shown that high-resolution laser spectra of flame-atomized samples can be obtained with a low-pressure sampling technique (1). We used a form of saturation spectroscopy to obtain Doppler-free spectra. Collision broadening was reduced to a negligible value by reducing the sample pressure to as low as 0.1 Torr. There were, however, sufficient velocity-changing collisions within the residual gas to produce
strong background pedestals. Also, additional spectral lines due to crossover resonances were present, with background pedestals as well. Since these resonances occur midway between primary resonances ( I ) , they produce no additional spectral information but instead are an unnecessary hindrance to quantitative high-resolution spectroscopy. The purpose of this note is to report an improved apparatus in which the
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sampled flame gases are measured in a supersonic expansion. Because of the low rate of velocity-changing collisions in the expansion, we can obtain spectra relatively free of the background pedestals previously observed. Furthermore, we can selectively detect the primary resonances and exclude some of the crossover resonances by spatially filtering the sample fluorescence. Thus,two major sources of potential interference in high-resolution spectral determinations, collision pedestals and crossover resonances, are removed. Hyperfine spectra of the sodium D~ and D2 lines measured in a supersonic expansion are presented and compared to measurements made under conditions similar to those described in ref 1. Spectroscopic applications of supersonic jets have been mostly confined to studies of molecules because of the high degree of rotational and vibrational cooling that can be obtained in the expansion (2,3). The high throughput and low collision rate in the expanded beam, however, are also desirable for high-resolution atomic spectroscopy. As the gas flows through an orifice into a low pressure sample chamber, random thermal motion of the atoms and molecules becomes converted to directed motion of the center of mass of the particles. The velocities of the particles are radially directed from the orifice with essentially equal magnitudes. Collisions are therefore infrequent once the cooling has occurred. In our previous measurements (I),the gas particles leaving the nozzle had a mean free path of less than 0.1 cm. Their motion was then largely randomized by the time they arrived in the laser beam 1-cm downstream from the nozzle. The chamber pressure in the present apparatus has been reduced by the addition of a 20-cm diffusion pump and the geometry of the nozzle plate has been modified so that the laser beam can pass less than 0.1 cm from the aperture if desired. An indication that the velocities of the sample atoms are indeed correlated comes from visual observation of the sample fluorescence as the laser is scanned in frequency. With a single laser beam crossing the sample cell, spatially resolved fluorescence zones corresponding to different hyperfine resonances can be observed, each occurring when atoms with a given velocity direction are Doppler-shifted into resonance (4). These observations show that the atomic velocities are directed radially from the orifice and also that the magnitudes of the velocities are within a narrow distribution.
EXPERIMENTAL SECTION Doppler-free saturation spectra of sodium atomized in an air-acetylene burner have been measured by a fluorescence intermoddation technique (5). The sample fluorescence is excited by two counterpropagating laser beams, one chopped at a frequency f l , the other at fi. The component of the fluoresencewith frequency f l + f z is monitored with a lock-in amplifier. Fluorescence at the sum frequency is observed only when atoms with the proper velocity are resonant with both beams. This velocity selection process produces a spectrum relatively free of the inhomogeneous broadening due to the Doppler effect. The chamber for the supersonic jet sampling is a slight modification of that reported in ref 1. The main difference is a modified aperture plate that lets us measure the fluorescence close to the expansion orifice. The orifice is drilled in a hollow pedestal that protrudes into the chamber rather than in a flat plate. This geometry was chosen for expedience, to demonstrate the benefits of the supersonic expansion with minimum modification of the existing sample chamber. The offset nozzle does require the capillary burner to be positioned closer to the chamber than with a flat aperture plate, approximately 0.2 cm, which degrades the burner performance to some extent. The nozzle plate is machined from aluminum with a mechanically drilled orifice 0.013 cm in diameter. The chamber is pumped by a 20-cm diffusion pump and 500 L m i d roughing pump. The air-acetylene burner with capillary insert is the same as discussed previously ( I ) . The sample solution for the present measurements is a solution of reagent grade sodium chloride in triply distilled water, with a sodium concentration of 1 g L-l.
959
k M T
TWO FREQUENCY CW DYE LASER
/
BS
-
I
.
Figure 1. Schematic diagram of the optical arrangement.
I
I
I
1
2
3
FREQUENCY (GHz.) Flgure 2. Saturation spectrum of expansion-cooled sodium, D, transitions.
A block diagram of the experiment is shown in Figure 1. The beam from a stabilized continuous wave (CW) ring dye laser (Rhodamine 6G dye) is split into two components of about equal intensity. The two beams are modulated by a mechanical chopper with two sets of holes to produce the two frequencies, f l and fi. An electrical signal at the frequency f i + f zis generated by the chopper driver for the lock-in reference signal. The region within the sample cell in which the laser beams cross the supersonic expansion is imaged by a 2.0-cm focal length lens onto a 0.08-cm by 1.0-cm slit in front of a 1P28 photomultiplier tube. The slit restricts the field of view of the photomultiplier to those atoms moving along the axis of the expansion, approximately perpendicular to the laser beams, thus discriminating against atoms that would undergo crossover resonances. The photomultiplier output is synchronously detected at frequency f l + f i t digitized, and stored by a laboratory computer as the laser is slowly scanned in frequency.
RESULTS AND DISCUSSION The spectra of the two sodium D lines measured under supersonic conditions are shown in Figures 2 and 3. The spectrum in Figure 2 is of the D1line at 589.6 nm. Both the ground state and excited state are split by the hyperfine interaction into two levels. Thus there are four possible primary resonance transitions and all are allowed. There are in addition five crossover resonances where the Doppler effect causes certain atoms to be resonant with the two counterpropagating laser beams on two different transitions. These resonances occur when the laser is tuned midway between the frequencies of the two transitions. All of these resonances were observed in the D~ spectrum in ref 1. There was also considerable spectral distortion in the form of pedestals attributed to atoms that undergo velocity-changing collisions during their
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spatially confined to two regions f19.5 f 1.2O from the nozzle axis, where the atoms with the appropriate velocity cross the resonant laser beam. From this angle and the expression for the Doppler shift
FREQUENC" [GHz
1
Flgurb 3. Saturation spectrum of expansion-cooled sodium, D~ tran-
sitions. interaction with the two laser beams (6). The three central crossover resonances are not observed in the present experiment because of the spatial filtering of the fluorescence. The resolution of the slit is not sufficient to suppress the excited state crossover resonances that appear between each pair of primary resonances. The collision pedestals are also less evident in the present results due to the low collision rate in the supersonic expansion. The spectrum of the D2line, at 589.0 nm, is shown in Figure 3. It is in principle more complicated because three excited states can be accessed by allowed transitions from each of the lower levels. However, the spacing between the upper levels is too small to be resolved, so the observed lines are a combihation of all of the transitions from the two ground-state levels. Again, the six unresolved central crossover resonances involving the two ground-state levels have been removed by the spatial filtering. Visual observation of the fluorescence during the central c r m v e r reaonance of either D line lets us estimate the velocity of the sodium atoms in the expansion. At this resonance, the transitions are Doppler-shifted by half the ground-state splitting, or 0.886 GHz. We observe the luminescence to be
AU = -k*u (1) where w is the angular frequency, k the wave vector, and u the velocity, we calculate the speed of the sodium atoms to be (1.5 f 0.1) X lo5 cm s-l. We can also estimate the effective reduction in Doppler width produced by spatially filtering the fluorescence. For the present configuration with an image magnification of 4x and an excitation region 0.2 cm from the orifice, the Doppler width of a transition with no other velocity selection would be 250 MHz. This value could be reduced by narrowing the slit, increasing the magnification, or moving the excitation region farther downstream from the nozzle. The present investigation has shown that we can substantially improve the performance of low-pressure sampling of an analytical burner by making the measurements under supersonic expansion conditions. These conditions have been obtained in the present work by reducing the pressure in the sample chamber and moving the interaction region closer to the nozzle. The radial trajectories with small velocity spread in the supersonic expansion provide an ideal environment for high-resolution laser spectroscopy with optical velocity selection techniques. The present measurements were concerned solely with improving the spectral resolution of the measurements-no attempt was made to determine the limit of detection. A recognizable hyperfine spectrum was obtained with 100 ng mL-l sodium concentration in the earlier experiments (I). There should be no loss in sensitivity on going to supersonic conditions for a given-sized orifice since the pressure drop across the nozzle is the same for either the supersonic or thermalized regime. Registry No. Na, 7440-23-5. LITERATURE CITED (1) Whitten, W. B.; Koutny, L. B.; Nolan, T. G.; Rarnsey, J. M. Anal. Chem. 1987. 59. 2203-2206. (2) Smalley, R. E.; Wharton, L.; Levy, D. H. J . Chem. fhys. 1975, 63,
4977-4989.
(3) Levy, D. H. Aflflu. Rev. fhys. Chem. 1980, 31, 197-225. (4) Marlella, R., Jr. Appl. Phys. Lett. 1979, 35, 580-582. (5) Sorem, M. S.; Schawlow, A. L. Opt. Commufl. 1972. 5 , 148-151. (6) Dabklewicz, Ph.; Hansch, T. W. Opt. Commufl. 1981, 38, 351-356.
RECEIVED for review October 26,1987. Accepted January 12, 1988. This work was sponsored by the U.S.Department of Energy, Office of Energy Research, under Contract DEAC05-840R21400 with Martin Marietta Energy Systems, Inc.
