680
Anal. Chem. 1990, 62,680-683
To determine the sensitivity of SERS at a Ag electrode for selected DNA bases, standard solutions of adenine were injected under stopped flow conditions and the intensity of the 732-cm-' band was integrated for 5 s. Stopped-flow conditions were utilized because of the difficulty in coordinating manual adjustment of the potentiostat and the applied potential as the analyte entered the cell. The intensity showed a monotonic relationship with concentration and a detection limit of 510 pmol. In addition, under stopped-flow conditions, adenine, thymine, and cytosine limits of detection were evaluated with a photomultiplier tube (RCA C31034-RF) and yielded 175, 233, and 211 pmol, respectively. Even though the cell volume reported here is at least a factor of 5-10 smaller than any other design reported in the literature, it is not optimum, and only a very small fraction of the total cell volume is being interrogated by the laser beam. Also, since SERS is a surface phenomenon, a very small fraction of the total analyte in the cell is being probed. With further improvements in cell volume and the ratio of cell volume to electrode area, it should be possible to considerably reduce the limits of detection below those reported here. This research has clearly proven the rapid response to adsorption and desorption controlled by potential modulation at the silver electrode. The technique yields good reproducibility. The concentration and volume ranges are rapidly approaching those necessary for modern liquid chromatographic systems. The need for a solute property detector yielding qualitative and quantitative information is of extreme importance in complex biochemical separations, and we feel SERS at a Ag electrode may prove beneficial as a detector for HPLC and FIA. Continuing research in this laboratory is directed toward the utilization of gold electrodes, which have proven a very stable SERS active substrate (20-22). In addition further
miniaturization of the cell and computer control of the potentiostat are presently under development.
ACKNOWLEDGMENT We thank Charles Nittrouer and Princeton Instruments for the use of the optical multichannel analyzer and associated data acquisition hardware and software. LITERATURE CITED Fleishmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. Surface Enhanced Raman Spectroscopy; Chang, R. K., Furak, T. E.; Eds.; Plenum Press, New York, 1982. Fleishman, M.; Hill, 1. R. Compr. Treatise Electrochem. 1984. 8 , 373. Jeanmire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1. Suh, J. S.; Moskovits, M. J. Am. Chem. SOC. 1988, 108, 4711. Moskovits, M.; Suh, J. S. J. Fhys. Chem. 1984, 88, 1293. Joo, T. H.; Kim, M. S. Chem. Fhys. Lett. 1984, 112, 65. Chou, Y. C.; Liang, N. T.; Tse, W. S. J. Raman Spectrosc. 1986, 17, 481. Gao. P.;Gosztola, D.; Leung. L.; Weaver, M. J. Elechoanal. Chem. Interfacial Electrochem. 1987, 233, 21 1. Weaver, M. J.; Hupp, J. T.; Barz, F.; Gordon, J. G., 11; Philpolt, M. R. J . Electroanal. Chem. Interfacial Electrochem. 1984, 160, 321. Vo-Dinh, T.; Uzeil, M.; Morrison, A. L. Appl. Spectrosc. 1987, 41. 4. Lasema, J. J.; Campiglia, A. D.; Winefordner, J. D.Anal. Chem . Acta 1988, 208, 21. Garrell, R. L. Anal. Chem. 1989, 61, 401A. Birke, R. L.; Lombardi, J. R. Spectrmlectrochemisby, Theory and Practice; Gale, R. J., Ed.; Plenum hess: New York, 1988. Ni, F.; Thomas, L.; Conon, T. M. Anal. Chem. 1989, 6 1 , 888. Berthod, A.; Laserna, J. J.; Winefordner, J. D. Appl. Spectrosc. 1987, 4 1 , 1137. ForcB, R. K. Anal. Chem. 1989, 60, 1987. Freeman, R. D.; Hammaker, R. M.; Meloan. C. E.; Fateley, W. G. Appl. Spectrosc. 1988, 42, 456. Leung, L. W. H.; Weaver, M. J. J. Am. Chem. Soc.1987, 109, 5113. Ganell, R. L.; Beer, K. D. Spectroch/m. Acta 1988, 43, 617. Patterson, M. L.; Weaver, M. J. J. Fhp. Chem. 1985, 89, 1331.
RECEIVED for review August 11, 1989. Revised manuscript received December 26, 1989. Accepted January 2, 1990.
Laser-Induced-Fluorescence Detection of Sodium Atomized by a Microwave- Induced Plasma with Tungsten Filament Vaporization Yuji Oki,* Hisanori Uda, Chikahisa Honda, Mitsuo Maeda, Jun Izumi,' Takashi Morimoto,' and Masazumi Tanoura'
Department of Electrical Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan
The laser-lnduced-fluorescence(LIF) technique is applied to the detection of Na atoms in pure water for a concentration range down to pg/cms. Na compounds are dissociated by a mlcrowave-krduced plasma of He wlth a tungsten filament vaporization system. CaUbrating the absolute denslty at the observing region, the efficiency of thk atomizer Is estimated. I t Is shown that the atomizer can generate hlgh atomic denslles by fHament heating. For example, the number denslty reaches lo7 atoms/cms for a lO-wL sample of 1 pg/cm3.
The sensitivity of laser-induced-fluorescence (LIF) spectroscopy with a tunable dye laser is excellent especially for *Nagasaki R & D Center, Mitsubishi Heavy Industries, Ltd.,1-1 Akunoura-machi, Nagasaki 850-91, Japan
the detection of atomic species. In an extreme case, the detection of individual atoms is possible when using a vapor cell or an atomic beam (1-4). Applications of these techniques to the atomic analysis are expected to provide sensitivity much higher than traditional methods. The first application of LIF to the frame atomic fluorescence spectroscopy was reported Fraser and Winefordner in 1971 (5, 6). Detection limits of metals (Ca, Na, Sr, Mg, etc.) lower than 1 ng/cm3 were reported by Weeks et al. (7);furthermore, lower detection limits were obtained with a graphite furnace. For example, Hohimer and Hargis obtained 20 pg/cm3 in Cs (8) and 0.5 pg/cm3 in Ti (9). In LIF, it is not difficult to detect atoms with concentrations as low as 105-106 atoms/cm3 in an atomic vapor cell. The potentiality of the LIF method in the detection sensitivity is extremely large. However, the atomizer usually limits it.
0003-2700/90/0362-0680$02.50/00 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 7, APRIL 1. 1990
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,
~
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Figure 1. Instrument design.
Therefore, it is important to evaluate the performances of the atomizer, that is, the atomizing efficiency, the background radiation, the fluorescence quenching, and so on. In this paper, we report the LIF detection of Na atoms atomized by a microwave-induced plasma (MIP) of He with a tungsten filament vaporization system (10)with a detection sensitivity on the order of pg/cm3. We also have measured the absolute number of dissociated Na atoms by comparing the fluorescence intensity of three Na vapor cells buffered with He, have evaluated the atomizing efficiency, and have discussed the detection limit of this method.
EXPERIMENTAL SECTION Figure 1 shows the experimental setup. A continuous wave (CW) dye laser, pumped by an argon ion laser (Spectra Physics 164)and tuned hy a tuning wedge and two solid etalons, was used for excitation of the n2 line of Na atoms. The laser spectral bandwidth was -3.3 pm. The fluorescence was detected with a photomultiplier tube (PMT)through an interference fdter (IF) with a spectral bandwidth of 1nm and a maximum transmission of 70%. The LIF signal was recorded hy a pen recorder through a lock-in amplifier (NF Carp., LI-573) or directly recorded hy a storage oscilloscope. If n e c e s w , ND filters were used to prevent PMT saturation. The atomizer was constructed from a Beenakker-type microwave cavity (10,a quartz discharge tube, and a fdament chamber. The filament was a looped tungsten wire of 0.1 mm diameter mounted in an acrylic chamber. He carrier gas flowed through the discharge tube which was open to the atmosphere. The discharge tube (6.4 mm id., 10 mm 0.d.) was made of synthesized quartz having a Na concentration was less than 0.05 ppm in order to avoid fluorescence from atoms sputtered from the wall. The microwave plasma was generated by means of a cavity (83mm diameter, 10 mm thickness) powered by a magnetron through a coaxial cable. The analytical procedure is as follows. Water samples of 10 rL were placed on the filament, and slowly evaporated hy heating with a current of 1.1 A for 1 min while flowing He carrier gas. After the MIP was turned on with a magnetron current of 40 mA (SOW), the filament was flash heated by a 0.0235-F capacitor charged to 17 V. The discharge eurrent had an exponentialshape with a decay time of 100 ms. When the dye laser was tuned to the D~line of Na, fluorescence at the same wavelength was ohserved over a range of about 10 mm downstream from the microwave cavity for about 100 ms after filament flashing. Since there was no severe background light from the MIP in this wition, floorescencecould he o b ~ ~ with e d a good signal to noise (S/N) ratio. To calibrate the ahsolute Na number density in the atomizer, we prepared three Na vapor cells made of Pyrex glass, whose internal He buffer gas pressures were 160,350, and less than 1 Torr, respectively. The atomizer was replaced with these vapor cells set in an oven with a temperature controller, and the fluorescence intensity was measured with the same optical configuration of the detection system as in the previous experiment. RESULTS AND DISCUSSION Detection of Sodium Atoms. Figure 2A shows an example of the oscilloscope waveform of the LIF signal. After 300 ms
Flgure 2. An example of oscilloscope traces of LIF signal and MIP background: sample concentration, 10 nglcms.
T i m e (s)
Flgure 3. LIF envelope pulses for varlous He flow rate: sample concentration, 1 nglcm3. of flash heating the filament, the LIF signal appeared. The plasma discharge was turned on and off with a frequency of 60 Hz, because the power supply of the magnetron had a half-wave rectifier at ac 60 Hz. Therefore, the LIF signal, as well as the backpound plasma radiation, showed a pulse shape with a repetition frequency of 60 Hz. In Figure 2B, the LIF and the MIP radiation pulses are shown. Na compounds are atomized only when the plasma is tumed on. However, the LIF signal decreased at the tail of the MIP pulse. This may mean that the dissociated atoms in the ground state decreased, because of the increase in ionized and excited atoms. With an increase in the plasma power, stationary fluorescence was observed without the sample. The fluorescence (called 'residual fluorescence") was probably due to the Na atoms sputtered from the wall of the discharge tube. It could not be removed by washing the tube. In the present experiment, the plasma power was decreased to the level at which this fluorescence could he ignored. Figure 3 shows the envelope of LIF signals at various He flow rates. With a decrease of the flow rate, the delay time between the heating pulse and the signal and the half-width of the signal pulse increased, which indicates that the sample is carried by He. The delay time agreed with the value calculated from the He flow rate. The product of the peak intensity and the half-width of the LIF signal was almost constant in the flow-rate range of 1500-2600 mL/min. The amplitude of the signal depended on the total length of the discharge tube L and the position of cavity. After the optimization process, we determined L = 300 mm. The cavity was placed a t the center of the tube. The rate equation for excitation of a two-level system by the monochromatic flux I is given by
dn, I - = -&)(BI2nl dt e
- Bzln2)- Az,nz
(1)
where n, and h a r e the populationa in the lower and the upper levels; Apl, BIZ,and Bp1are Einstein coefficients for sponta-
682
ANALYTICAL CHEMISTRY, VOL. 62, NO. 7, APRIL 1, 1990
-
106-
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0 He Zs). Thus we can expect a high-intensity LIF signal even for the very low atomic density. For example, eq 6 predicts the number of the signal photoelectrons at the P M T at 108/s for the Na density of lo5 atoms/cm3, V = 0.1 cm3, Azl-l = 16 ns, dQ = 0.1 sr, vl = 0.30, and qz = 0.10. It should be noted, therefore, that the S / N ratio is not determined by the shot noise of the PMT detector. The LIF intensity was strong enough to be seen by the naked eye even a t a concentration of 10 pg/cm3. The fluctuation of the background radiation directly depended on stability of the plasma and the laser, so that the S / N ratio can be improved by stabilization of the plasma and the laser. It is difficult to know the absolute density n from V , by using eq 6. We determined the n value in the atomizer by the aid of reference vapor cells. The absolute Na density in the vapor cell can be calculated from the cell temperature. The absolute density is calibrated by replacing the atomizer
Anal. Chem. 1990. 62,683-689
with a vapor cell in an oven and comparing the LIF intensity
IF in the same system. Figure 5 shows the PMT output voltage VF as a function of the Na density n for three vapor cells with different buffer gas pressures. The Fairbank formula was used for the conversion from temperature to Na number density (1). VF was proportional to n over a number density range from lo7to 10" ~m-~ The . detection limit lo7cm-3 is relatively high, because it is determined by stray light and no attempt to decrease it has been made. The VF value for the vapor cell with a He gas pressure of 1 atm is 50% smaller than that of the evacuated cell because of collisional quenching by He. From Figure 5, the output PMT voltage of 1mV corresponds to the atomic density n of 1.68 X lo7 atoms/cm3 at 1 atm. We calculated the total number of dissociated atoms N passed through the observing region in the atomizer by the following formula: N = nsvS (7) where s is the cross section of the discharge tube, u is the He velocity, and S is the integrated pulse form of the LIF signal in mV; consider the absolute density and the atomization efficiency for the LIF signal shown in Figure 2A obtained for a concentration of 10 ng/cm3. The maximum number density is 1.18 X 10" atoms/cm3, because the peak voltage in Figure 2A is 700 mV and a 10% ND filter is used. Integration of the envelope of the LIF signal in Figure 2A provides S = 280 mV s; s = 0.043 cm2,u = 108 cm/s and n = 1.68 X lo7 atoms/(cm3 mV). Therefore, N = 2.62 X 10" atoms. We define the atomizing efficiency (3, as number of dissociated atoms N = number of atoms (ions) in sample water (8)
883
Since the number of atoms in a 10 FL sample of water of 10 ng/cm3 is 2.7 X 10l2,(3, is calculated to be 9.7%. However, shown in Figure 2A, since the plasma was turned on and off, the integration should not be done along the envelope but the pulse shape. In that case, the efficiency is given by (3, = 1.6%. The feature of this type of atomizer is that the instantaneous number density obtained in the observation region is large, because rapid injection of the sample is attained by flash heating. According to the results mentioned above, a peak density of 1.18 X lo7 atoms/cm3 is obtained for the sample of 1 pg/cm3. Therefore, we can expect atomic detection of less than 1 pg/cm3 by decreasing the noise sources.
ACKNOWLEDGMENT The authors thank Shin'ichi Ichitsubo and Tatsuya Izuha for their assistance. LITERATURE CITED Fairbank, W. M., Jr.; Hansch, T. W.; Schawlow, A. L. J . Opt. Soc. Am. 1975, 65, 199-204. Gelbwachs, J. A.; Klein, C. F.; Wessel, J. E. ZEEEJ. Quantum. €/eo tron. 1978, OE-14, 121-125. Balykin, V. I.; Letokhov, V. S.; Mishin, V. 1.; Semchishen, V. A. JETP Lett. 1977, 26, 357-360. She, S. Y.; Fairbank, W. M., Jr. Opt. Lett. 1978, 2 , 30-32. Fraser, L. M.; Winefordner, J. D. Anal. Chem. 1971, 43, 1693-1897. Fraser, L. M.; Winefordner, J. D. Anal. Chem. 1972. 4 4 , 1444-1451. Weeks, S. J.; Haraguchi, H.; Winefordner, J. D. Anal. Chem. 1978, 50,360-368. Hohimer, J. P.; Hargis, P. J., Jr. Appl. W y s . Lett 1977. 30, 344-346. Hohimer, J. P.; Hargis, P. J., Jr. Anal. Cbim. Acta 1978. 97, 43-49, Kawaguchi, H.; Vallee, 8 . L. Anal. Chem. 1975, 47, 1029-1034. Beenakker, C. I. M. Spectrocbim. Acta 1978, 318, 483-486.
RECEIVED for review September 22,1989. Accepted December 21. 1989.
Prediction of Gas Chromatography Flame Ionization Detector Response Factors from Molecular Structures Andrew D. Jorgensen* Department of Chemistry, University of Toledo, Toledo, Ohio 43606 Kurt C. Picel' and Vassilis C. Stamoudis2 Argonne National Laboratory, Argonne, Illinois 60439 The prediction of flame Ionization detector response factors as a function of molecular structure components is evaluated with modem capillary column gas chromatography equipment that included an on-column injector. The effect on the standard carbon content based response by electronegative atoms is analyzed for various functional groups. This study updates much earlier work that characterized the decrease in signal response by using average correction factors for each functional group. The effective carbon number concept based on naphthalene as the internal standard was used. For 56 compounds containing a single functional group, predlctlons based on these average responses reproduced the actual response to within 1.7% on average. This model was then extended to bifunctional groups with similar success for several molecules. The effects of changes in temperature programmlng and concentration were found to be minimal within the range studied. Environmental Research Division. 'Environment, Safety and Health, Support Services Division.
INTRODUCTION High sensitivity, uniform response to hydrocarbons, and a broad linear range have made the flame ionization detector (FID) perhaps the most widely used detector in gas chromatography. The FID response of hydrocarbons is generally proportional to the mass of carbon present in the sample, but the degree of signal reduction due to partially oxidized carbon atoms in heteroatomic compounds varies markedly with heteroatom and bond types. Compounds that contain, for example, oxygen, nitrogen, or halogens give varied responses but can be quantified by the FID if corrections are made for the usual loss of sensitivity caused by the presence of these atoms in various functional groups. Correction factors can be determined either through the analysis of authentic standards or from accurate predictions of loss of response based on measurements made for compounds of related molecular structure. Predictive methods would be quite useful if they could accurately indicate the change in response caused by the presence of one or more heteroatomic functional groups. Correction factors have long been used for various function-
0003-2700/90/0362-0683$02.50/00 1990 American Chemical Society
