Anal. Chem. 1907, 5 9 , 2203-2206
Table V. Results Obtained by Using the PAIRSPLUS Subroutine of PAWMI and intIRpreta PAIRSPLUS results PAWMIb intIRpret positives: true
200
216
false
77
46 40 %
improvement negatives: true false improvement
3809 68
3840 52
total decisions
4154
4154
24 %
a The training set consisted of 62 compounds frequently found at hazardous waste sites (3). The test mixtures consisted of 67 fourcomponent mixtures of chlorobenzene, l,l,l-trichloroethane (TCE),toluene, and benzene. “These data do not match those previously reported (8) because the data set has been altered.
spectrum of the pure compound to the spectrum of the mixture. In intIRpret, rule peaks are chosen by STO, and weighted for frequency of occurrence ( k l ) ,intensity (k2),and for the cross-term (It3). Rules are entered automatically by AUTOGEN and compiled into PAIRS. The software system is several orders of magnitude faster than when peaks were entered manually, immune from mistakes made when complex data is entered manually, and is based on results that are consistently applied regardless of the operator or data set. These data show a 40% decrease in false positive results and a 24% decrease in false negative results when intIRpret is compared to PAWMI. Some additional improvements in results can be expected after completion of a study of the optimal values of window widths, window weighting factors, and the relative weights of k l , k,, and k3. However, a certain degree of uncertainty will remain in the direct interpretation of the infrared spectra of mixtures due to peak shifts in solution, the similarity of the spectra of structurally similar compounds, and the inability of the peak picking routines to recognize the presence of peaks that appear as unresolved shoulders or in poorly resolved envelopes.
ACKNOWLEDGMENT The authors thank Greg Kinnes for his help in preparing
2203
the mixtures and acquiring the IR spectra and Mary Weed for preparation of figures.
LITERATURE CITED Puskar, M. A.; Levine, S. P.; Turpin, R. I n Protecting Personnel at Hazardous Waste Sites, Levine. S. P., Martin, W. F., Eds.; Butterworths/Ann Arbor: Woburn, MA, 1985: Chapter 6. Gurka, D. F. “Project Summary: Interlaboratory Comparison Study: Methods for Volatile and Semivolatile Compounds”, Environmental Monitoring Systems Laboratory, Las Vegas, NV, June 1984; EPA600/54-84-027. Hallstedt, P. A.: Puskar, M. A.; Levine, S. P. J. Hazard. Waste Hazard. Mater. 1988, 3(2), 221-232. Eckel, W. P.; Trees, D. P.; Kovell, S. P. “Distribution and Concentration of Chemicals and Toxic Materials Found at Hazardous Waste Dump Sites”, Proceedings of the National Conference on Hazardous Waste and Environmental Emergencies, May, 1985. Mayhew, J. D.; Sodaro, G. M.; Carroll, D. W. A Hazardous Waste Site Management Plan ; Chemical Manufacturers Association: Washington, D.C., 1982. “The Hazardous and Solid Waste Amendments of 1984”; Congr. Rec. 1984, (Oct 3), H11103. Puskar, M. A.; Levine, S. P.; Lowry, S. R. Anal. Chem. 1986, 58, 1156-1 162. Puskar, M. A.; Levine, S. P.; Lowry, S. R. Anal. Chem. 1988, 58, 1981-1 989. Puskar, M. A.; Levine, S. P.; Lowry, S. R. Environ. Sci. Techno/. 1987, 27, 90-96. Woodruff, H. B.; Munk, M. E. J. Org. Chem. 1977, 42, 1761-1767. Woodruff, H. 8.; Munk, M. E. Anal. Chim. Acta 1977, 95, 13-23. Woodruff, H. B.; Smith, G. M. Anal. Chem. 1980, 52, 2321-2327. Woodruff, H. B.; Smith, G. M. Anal. Chim. Acta 1981, 133, 545-553. Tornellini, S. A.; Saperstein, D. D.; Stevenson, J. M.; Smith, G. M.; Woodruff, H. B. Anal. Chem. 1981, 53, 2367-2369. Tomellini, S. A.; Stevenson, J. M.; Woodruff, H. B. Anal. Chem. 1984, 56, 67-70. Tomellini, S. A.; Hartwick, R . A.; Stevenson, J. M.; Woodruff, H. B. Anal. Chim. Acta 1984, 162, 227-240. Biaffert, T. Anal. Chim. Acta. 1984, 167, 135-148. Zupan, J.; Munk, M. E. Anal. Chem. 1985, 5 7 , 1609-1616. Trulson, M. 0.; Munk, M. E. Anal. Chem. 1983, 55, 2137-2142. Frankel, D. S.Anal. Chem. 1984, 56, 1011-1014. Lowry, S. R.; Huppler, D. A. Anal. Chem. 1983, 55, 1288-1291. Jurs, P. C.; Isenhour, T. L. Applications of Pattern Recognition; Wiley: New York, 1975. Rasmussen, G. T.; Isenhour, T. L.; Lowry, S. R.; Ritter, G. L. Anal. Chim. Acta. 1978, 103, 213-221. de Haseth, J. A.; Woodruff, H. B.;Lowry, S.R.; Isenhour, T. L. Anal. Chim. Acta 1978, 703, 109-120. Saperstein, D. D. Appl. Spectrosc. 1988, 40(3), 344-348. Computer Supported Data Bases ; Zupin, J., Ed.; Howard Ltd.-Wiley Co.: New York 1986. Jurs, P. C. I n computer Software Applications in Chemistry; Jurs, P. C., Ed.; Wiley: New York, 1986; Chapter 16. Kwok, K-S; Venkataragahaven, R.; McLafferty, F. W. J. Am. Chem. SOC. 1973, 95, 4185-4194. Atwater, B. L.; Stauffer, D. B.; McLafferty, F. W.; Peterson, D. W. Anal. Chem. 1985, 5 7 , 899-903.
RECEIVED for review March 6, 1987. Accepted May 26, 1987. This work was supported by Grant 1-R01-OH02066-01 from the National Institute for Occupational Safety and Health of Centers for Disease Control.
Low-Pressure Laser Spectroscopy with Flame Atomization W. B. Whitten,* L. B. Koutny, T. G. Nolan, and J. M. Ramsey Oak Ridge National Laboratory, Analytical Chemistry Division, P.O. Box X , Oak Ridge, Tennessee 37831 -6142 We have developed a low-pressure interface that permits high-resolution laser spectroscopy to be performed with an air-acetylene analytlcai burner. The reduced pressure in the measuring cell effectively eliminates collision broadening so that laser techniques for Doppler-free spectroscopy can be fully exploited. The interface has been tested with a form of saturation spectroscopy. Spectral resolution of 50 MHz has been demonstrated for sodium.
Flame atomization with an analytical burner is commonly
used in conjunction with various spectroscopic techniques, ranging from flame emission ( 1 ) and atomic absorption spectroscopy (2) to the more complicated laser-based methods. The latter include laser-induced fluorescence (3), laser-enhanced ionization (4),polarization saturation spectroscopy (5), and degenerate four-wave mixing (6). All of these techniques exhibit excellent sensitivity for trace elements in aqueous solution, typically in the part-per-billion range or less. The spectral resolution, however, is limited by the homogeneous broadening of the optical transitions due to collisions in the atmospheric pressure flame. Line widths of about 5 GHz are
0003-2700/87/0359-2203$01.50/00 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 17,
SEPTEMBER 1, 1987
commonly observed (7). Such resolution is inadequate for some purposes, such as where isotopic selectivity is required. The isotopic spectral shift for most elements is less than 1 GHz. The purpose of this investigation is to explore the use of a low-pressure interface to an air-acetylene burner so that the high spectral resolution of Doppler-free laser spectroscopy could be achieved while retaining the simplicity and efficiency of flame atomization. The apparatus to be described has permitted the resolution of hyperfine features in the sodium D lines with a spectral resolution of better than 50 MHz. Recognizable spectra can be obtained with a concentration of 100 ppb sodium in water. Previous spectroscopic measurements at low pressure with flame atomization used a flame within the vacuum chamber (8). Such operation, however, would preclude the use of commercially available burners without substantial modification or would generate an impractical load on the vacuum system. We have chosen instead to sample a portion of the flame gases and expand them into an evacuated chamber for the measurements. Such sampling has been shown to be effective for combustion diagnostics, both with laser spectroscopy (9, 10) and with mass spectrometry (11). Mass spectrometer interfaces to plasma torches for elemental analysis have been demonstrated by Gray (12) and by Houk and co-workers (13), who interfaced a quadrupole mass spectrometer to an inductively coupled plasma torch. A similar interface has also been used for atomic emission spectroscopy (14). We use a commercial air-acetylene burner operating in air at atmospheric pressure. A portion of the flame passes through a small orifice into an evacuated spectroscopy cell for study. Pressures of less than 1 Torr can be attained with a large but conventional mechanical roughing pump. Optical acc:ess to the cell is through two quartz windows and a light pipe leading to a photomultiplier. A form of saturation spectroscopy using beam overlap modulation and fluorescence detection was chosen for this study. The method was developed by Duffey et al. (15). A laser beam with an intensity close to the saturation intensity is retroreflected by a vibrating mirror. The ac portion of the laser-induced fluorescence of the sample vapor is monitored as the frequency of the laser beam is scanned (the detector alternately sees the fluorescence excited by the overlapping beams and by the two separated beams). Because of the Doppler effect, groups of atoms with different velocity components parallel to the beams will be resonant at different laser frequencies. The fluorescence will be modulated when atoms in a particular velocity group are resonant with both beams. (The frequencies of the two beams will in general be different in the rest frame of the atom since they are Doppler-shifted in opposite directions.) The resonance may occur in two ways. A principal or Lamb-dip resonance takes place when the laser frequency matches the energy of a transition of the atoms with zero velocity along the beams. Crossover resonances, where some velocity group is resonant with one beam for a particular transition and with the other beam for a different transition, can occur when the photon energy is the mean of the two transition energies. The crossover resonances are possible, of course, only if the Doppler broadening is comparable to the splitting of the energy levels. The sign of the modulation of the fluorescence will depend on whether or not states excited by one beam relax into lower levels that can be excited by the other beam. There will be a negative contribution when a common lower level is shared by both laser beams, positive otherwise. A comprehensive theory for these saturation phenomena in the alkali metals has been presented in a number of papers by Nakayama (16-18). His theory covers a number of different polarization configurations and includes the effect of optical pumping. A summary of the results
SODIUM Di LINE
SODIUM
D2
LINE
7 189
MHz
1
F-2
-
7-
-
F=2
I I
1772 3s2s1/,
3 2 s 1/2
MHz
I772 VIHZ
I
,
I
1-
Figure 1.
1
1
Energy levels for the sodium D, and D* lines.
pertinent to our experiment follows. The energy levels for the sodium D lines are shown in Figure 1. In addition to the fine structure splitting of the and P J j alevels, the interaction of the valence electron with the nuclear spin produces the hyperfine splitting that is shown, with levels corresponding to the total angular momentum, F. Transitions between levels are subject to the selection rules, 1F = 0, fl, and AmF = fl for circularly polarized light, AmF = 0 ( F # 0) for linear polarization. The calculated response (16)is the sum of many terms, each involving three transitions-a transition from a ground-state Zeeman level to an excited state by one laser beam, relaxation from that excited level to the previous or different ground state, and finally a transition from that level to an excited state by the other laser beam. The processes can involve two, three, or four different levels. The ac fluorescence intensity observed under our conditions due to a group of atoms with a particular velocity component is described by the relationship (16)
We have modified the equation slightly and changed the notation because the beams are of equal intensity in our experiment. In the above expression, mL and mR index the Zeeman sublevels of a particular transition with a prime indicating an upper level. The pLVare dipole matrix elements between the i and j levels. We use AWI,R
=
IWL
- wRI
(2)
where wL and wR are the frequencies of the transitions excited by the beams from left and right and WLR
=
(WI.
+ WR)/2
(3)
f is the total transition probability for relaxation from the excited state; Le., the sum of the p 2 from a given Zeeman level to all allowed lower levels. D represents the Doppler broadening, given by
D
= exp[-(AqR/2hU)2]
(4)
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1. 1987
n
TO VACUUM PUMP
2205
n
I Flgun 9. Schematk &gam ofme opUd arrangement. The sample Rwrescence 15 monitored h tw a plpa met Ls not shown. Beam deflection is exaggerated.
WINDOW
WINDOW O4NG
- -.-
NOZZLE PLATE L
8
-
88 d
I
I 2
1
BURNER INSERT
I 3
4
FREQUENCY (GHz)
npun4. satuatiar specbvn01s o d l ~ m0 , nne: sodm c ~ ~ c ~ n l r ~ l l o n . 400
-re
ppm; pressure, 0.12 Ton.
2. hawing of tm sample dramber and k~~wessum Int&cs.
with photon wavenumber k = w/c and most probable velocity u = (2ksT/M)1/zdetermined by the temperature, Boltzmann constant, and atomic mass. The transition line width for a given velocity group is determined by the Lorentzian, I
.P = o - oLR - iy
(5)
where y is the homogeneous broadening, assumed for convenience to be the same for all transitions. The sum in eq 1 is over all allowed transitions. Only linearly polarized transitions are included for excitation but both linear and circular are allowed for the relaxation transitions. The Kronecker delta, aij,. that is 1when i = j and 0 otherwise, gives a negative contnbution when the two beams interact with a common ground state. The sum should be over all of the Doppler subgroups as well. For low homogeneous broadening, however, only atoms in a narrow velocity range, if any at all, will be resonant a t a given laser frequency. The calculations need be made only for the resonant frequencies in this case. The summations for the n1lines of all of the alkali elements and for some of the 9lines have been tabulated by Nakayama (17,18).We have extended these calculations to the sodium nZ line in the zero line width approximation.
EXPERIMENTAL SECTION The airacetylene burner is a Varian Techtron slot burner that has been modified to produce a circular flame. The new burner head consists of a bundle of 40 stainless steel tubes each 0.13cm in diameter, cemented with Torr-Seal epoxy adhesive in an aluminum sleeve that mates with the O-ring seal in the burner. The total circumference and cross sectional area of the tubes are approximately those of the original burner slot to minimize the chance of flashback (19). The vacuum chamber is the principal component of the experimental apparatus. An exploded diagram is shown in Figure 2. The burner operates in air blow the chamber with the circular insert that is shown in the figure. The flame expands through a small bole in the nozzle plate into the measwing chamber. The nozzle plate was first made of nickel but aluminum was found
to be satisfactory for these experiments and much easier to machine. Apertures ranging from 0.15 to 0.94 m m were drilled in the nozzle plate itself. Smaller apertures, to 0.025 mm, were laser-drilled in stainless steel and then cemented over a larger opening in the plate. The measuring chamber is a water-mled copper block, 6.5 cm by 6.5 em by 5 em high with quartz windows for the two laser beams. Fluorescence is monitored with a 1P28 photomultiplier tuhe at the end of a light pipe. On the rear of the sample c h a m h , not shown in the figure, is a vacuum gauge. The chamber is pumped with a 500 L min-l mechanical pump. The chamber pressure ranged from 0.1 to 0.8 Torr, with the drilled nozzle apertures. The optical configuration is shown schematically in Figure 3. A stabilized single frequency ring dye laser pumped by an argon ion laser is the excitation source. The laser beam passes through the sample cell, then is retroreflected and spatially dithered in a vertical plane by a small mirror glued to the edge of a I-cm paper cone speaker. The return beam also passes through the cell, alternately overlapping with or slightly displaced from the first beam. The vertical excursions of the retroreflected beam must be small enough that the light pipe can collect the fluorescence from both beams when they are separated. The speaker voice coil is driven hy the square wave output of a signal generator at 250 Hz. The signal from the photomultiplier is synchronously detected with a lock-in amplifier and digitally recorded. The laser is typically scanned over a 6GHz range in a time of several minutes. For the present measurements,solutions of NaCl in triply distilled water were aspirated into the burner.
RESULTS AND DISCUSSION A saturation spectrum of the various hyperfine and cxossover resonances of the scdium n1line is shown in Figure 4. The concentration of sodium in the aspirated water wm about 400 ppm. The nozzle diameter was 0.15 mm and chamber pressure was 0.12 Torr. The resonances are grouped into three prominent features corresponding to the two SI/* ground state levels split by 1.78 GHz and the crosaover lines midway between them. Each group has three linestransitions involving the two PlIzlevels separately and the
2206
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
-
s -
--$
~ _ _
_._~
7I
1 C
7
2
4
FREQLENCY ( G l z ,
Figure 5. Saturation spectrum of sodium same as those given in Figure 4.
D~
line. Conditions are the
crossover involving them both. The pedestals below the sharp lines are attributed to atoms that have undergone velocitychanging collisions during the interaction with the two beams. The stick spectrum below the experimental curve shows the intensities calculated by Nakayama (18) from eq 1. The agreement is very good. A spectrum of the sodium D~ line under similar conditions is shown in Figure 5. Because of the higher multiplicity of the P3,2level and the close spacing of the levels, we are unable to resolve the individual transitions corresponding to the upper levels and crossover resonances. However, the large features due to the ground state are evident. The stick spectrum from eq 1 is presented below the experimental curve for reference. The preceding results were obtained with samples of relatively high sodium concentration to explore the resolution capabilities of the apparatus. We have also looked a t samples of lower concentration to test the sensitivity of the technique. A D~ line spectrum of a sample with 100 ppb sodium in water is shown in Figure 6. The principal features are clearly visible a t this concentration. The sensitivity is sacrificed to some extent because of the velocity selection process that restricts the participating atoms to a small fraction of the sodium atoms within the cell. The apparatus in its present configuration has achieved a hundred-fold improvement in spectral resolution over the atmospheric-pressure flame while retaining respectable sensitivity. Although the sample density is reduced by the expansion, many contributions to the background, such as Rayleigh scattering and flame luminescence, are also reduced. Furthermore, because of the reduced saturation intensity at low pressures, particularly where optical pumping is taking place, saturation spectroscopy can be performed with laser powers in the milliwatt range, a level easily attained with single-frequency continuous-wave (CW) dye lasers. We anticipate some additional improvement in both resolution and sensitivity if the cell pressure can be further reduced so that the expansion is supersonic (20). The atoms would have a narrower velocity range so that the population in a given subgroup would be higher. The velocity-changing collisions
-!
I
I
I
I
1
2
3
4
I
0
5
FREQUENCY (GHz)
Figure 6. Saturation spectrum of sodium D:, line: sodium concentration, 100 ppb; pressure, 0.3 Torr.
that produce the pedestals should also be reduced under such conditions. The low-pressure interface we have described is by no means confined to the beam overlap modulation technique used in this investigation. The apparatus should be useful in reducing the homogeneous pressure broadening for almost any type of Doppler-free spectroscopy. The resulting spectral resolution should be adequate for performing isotopic analysis with flame atomization.
LITERATURE CITED Rarnirez-Munoz, J. I n Advances in Analytical Chemistry and Instrumentation; Winefordner, J. D., Ed.; Wiley: New York, 1971; Vol. 9. Fuwa, K.; I n Advances In Analytical Chemistry and Instrumentation; Wlnefordner, J. D., Ed.; Wlley: New York, 1971;Vol. 9. Weeks, S.J.; Haraguchi, H.; Winefordner, J. D. Anal. Chem. 1978, 5 0 , 360-368. Travis, J. C.; DeVoe, J. R. I n Lasers in Chemical Analysis; Hleftje, G. M., Travis, J. C., Lytle, F. E.. Eds.; Humana: Clifton, NJ, 1981. Tong, W. G.; Yeung, E. S. Anal. Chem. 1985, 5 7 , 70-73. Ramsey, J. M.; Whitten, W. 6. Anal. Chem. 1987, 5 9 , 167-171. Parsons, M. L.; McCarthy, W. J.; Wlnefordner, W. D. Appl. Spectrosc. 1966, 20, 223-230. Hollander, T. J.; Broida, H. P. Combust. Flame 1969, 73,63-70. Whitten, W. 6. Appl. Spectrosc. 1986, 4 0 , 104-106. D'Silva, A. P.; Iles, M.; Rice, G.; Fassel, V. A,, Ames Laboratory Report IS4856 UC-SOE, 1984. Young, W. S.;Rodgers, W. E.; Cullian, C. A,; Knuth, E. L. in Rarified Gas Dynamics; Seventh Symposium; Phi, D., Cercignanl, C., Nocilla, S.,Eds., Editrice Technic0 Scientifica: Pisa, 1971;Vol. I. Gray, A. L. I n Dynamic Mass Spectrometry; Price, D., Todd, J. F. J., Eds.; Heyden: Philadelphia, PA, 1975;Vol. 4. Ollvares, J. A.; Houk, R. S.Anal. Chem. 1985, 5 7 , 2674-2679. Houk, R. S.;Lim, H. B. Anal. Chem. 1986, 58, 3244,3246. Duffy, T. P.; Kammen, D.; Schawlow, A. L.; Svanberg, S.;Xia, H. R.; Xiao, G.-G.; Yan, G.-Y. Opt. Lett. 1985, IO, 597-599. Nakayama, S.J . Fhys. SOC.Jpn. 1981, 50, 609-616. Nakayama, S.Jpn. J . Appl. Phys. 1984, 23, 879-883. Nakayama, S.J. Opt. SOC.A m . B 1985, 2 , 1431-1437. Saturday, K. A.; Hleftje, G. M. Anal. Chem. 1977, 4 9 , 2013-2018. Levy, D. H. Annu. Rev. Phys. Chem. 1980, 197-225.
RECEIVED for review March 9,1987. Accepted May 21, 1987. This research was sponsored jointly by the Great Lakes Colleges Association/Associated Colleges of the Midwest and the U.S. Department of Energy, Office of Energy Research, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
