Anal. Chem. 1988. 60. 1637-1639
1637
amplified signal is about the same as the MCPD signal, and of course'che large reflected signals shown in Figure 4 are greatly reduced.
w
Ions Figure 5. Oscilloscoplc display of the signal pulse whh a 50-0 impedance cable between the MCP and the vacuum flange. Scope sensitivity was 50 mVlcm vertical and 10 nslcm horizontal.
CONCLUSION The performance of a movable, homemade MCPD assembly is comparahle to that of a commercial design mounted onto a special flange (12). A 50-R cable with good vacuum characteristics coupled to a 5 0 4 vacuum feedthrough allows flexibility in research studies while preserving signal pulse integrity in the special ion optical configurations where the MCPD cannot he positioned close to a vacuum port. This flexibility becomes important as more studies in TOF-MS are performed. For example, placing the detector at various locations along the ion path is especially useful when an ion reflector (13)is being installed and tested. Some experimental apparatus may require impedances other than 50 R, hut similar techniques can be applied. ACKNOWLEDGMENT We gratefully acknowledge the assistance of J. F. Homer in the electronic measurements. LITERATURE CITED
I
Ions Flgure 6. Oscilloscopic display of the amplified pulse shown in Figure 5. Scope sensitivity was 500 mV/cm vertical and 10 nslcm horizontal.
subsequent reflected pulses. This effect can cause the artifact peaks seen in other experiments with TOF-MS (11). When the MCPD anode is connected with 5 0 4 cabling, the reflections are m y elimiited, as shown in Figure 5 With the same voltage on the MCPD, the amplitude of the signal pulse decreases from -170 (Figure 3) to -140 mV (Figure 5). This decrease in signal amplitude is expected because of the voltage divider effect of the source impedance of the detector and the impedance of the transmission line. The rise time of approximately 1 ns in Figure 5 indicates that the acceptable pulse characteristics are reproducible with a definite improvement over those in Figure 3. The fast rise time and narrow pulse width shown here yield good results for TOF-MS, i.e., no artifact peaks or peak tailing induced by the detector (9). Figure 6 shows the signal pulse shape after amplification of the MCPD signal from Figure 5. The rise time of the
(1) '"Applicationsof Microchannel Plates.'' Varian. Application Note. 1982. (2) Wira, J. L. N w l . Inshum. M e w s 1979, 162, 587. (3) Girard, J.: Bolore. M. Nuci. Instrum. Methods 1977, 140, 279. (4) Gabor. G.: Schimmerling, W.: Greiner. D.: Bieser. F.; Lindshom. P. Nucl. Inshum. Methods 1975. 130, 65. (5) Borman. S. Anal. Chem. 1987, 5 9 , 701A. (6) Sundqvisl. 0.: Macfariane. R. D. Mass Specworn. Rev. 1985, 4 . 421. (7) Beck. 0. Rev. Sci. Inshum. 1976, 47, 849. (8) White, M. G.; Rosenberg, R. A,; Gabor, G.: Poliakofl, E. D.: Thonon, G.: Southworth, S. H.; Shirley, D. A. Rev. Sci. Insmnn. 1979. 50,
,?e* ._"I. (9) Huang, L. Q.: Conremlus, R. J.; Junk, G. A,; Muk. R. S. A m i . Chem.
1988, 60, 1490.
(10) Ryder. John D. Network Lines and FieMs. 2nd 4.:Prenlice-Hall: Enalewood Cliffs.NJ. 1958: ChaOler 6.
RECEIVED for review November 17, 1987. Accepted March 11,1988. Ames Laboratory is operated for the US.Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. This research was supported by the Director for Energy Research, Office of Basic Energy Sciences, the Advanced Research and Technology Development (ARTD) Program of the Office of Fossil Energy Research, Morgantow Energy Technology Center, R. Letcher, Project Leader, and the Materials Preparation Center of the Ames Laboratory.
Differential Laser-Enhanced Ionization Spectrometry in Flames Nikita B. Zorov,* Oleg I. Matveev, a n d Alexandr A. Gorbatenko
Department of Chemistry, Moscow State University, 119899 Moscow, USSR Laser-enhanced ionization (LEI) spectrometry has proven to be a highly sensitive method for trace elemental analysis in flames (1-10). The detection limits are comparahle to, and sometimes exceed those of, atomic ahsorption (AA) spectrometry in a graphite furnace. In the LEI experiment, the sample is aspirated into the flame and a resonance transition of the atomic species of interest is excited by a dye laser. Optical pumping selectively increases the probability of ionization of aualyte atoms. An electric field is applied across the interaction region within the flame to accelerate the charges to a detection electrode. The resultant current is
found to he proportional to the concentration of and* a t o m in the sample. The conventional LEI experiment uses a two-electrode configuration that suffers different disadvantages. Careful shielding of the flame and several regions is often required to eliminate electromagneticnoise from the laser (11). If other species in the sample possess transitions that are nearly degenerate with that of the analyte atom, then the enhanced ionization process becomes nonselective. In such cases, the dye laser must he scanned over transitions of both the analyte atom and the interfering species (12-14); this correction
0003-2700/8810360-1637$01.50/00 1968 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 15, AUGUST 1, 1988
1
C
r
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T
O
AMPUFIER
7
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Figure 1. Schematic diagram of the differential LEI signal detection system: 1, burner; 2, laser beams; 3, flame; 4, high-voltage electrode; 5, detection electrodes (cathodes); 6, high-voltage supply; 7, highfrequency pulsed transformer.
Figure 2. Amplitude-frequency characteristics of the high-frequency pulsed transformer: 1, signal is fed to L, and measured on L,; 2, differential scheme.
procedure is time-consuming and tends to introduce errors. Here, we report development of a differential LEI technique that minimizes these effects. The new approach utilizes a three-electrode system in conjunction with a high-frequency transformer and a two-color optical excitation scheme.
EXPERIMENTAL SECTION Apparatus. The laser system and the signal amplification and processing electronics have been described elsewhere (15). An acetylene-air flame from a 15 mm diameter Mekker burner head was used. Figure 1 illustrates the three-electrode, differential detection apparatus. All electrodes were of iridium wire (35 mm x 0.5 mm diameter). The detection cathodes (5) were symmetrically displaced 10 mm to the side and 5 mm below the high-voltage anode ( 4 ) . The two cathodes were connected to opposite ends of the primary windings of the pulse transformer (7) which was constructed on a torroidal ferrite core (40 mm 0.d. and permeability p = 2000). The primary consisted of a total of 68 turns with a grounded center tap; note that L1 and L, have opposite helicities. The secondary consisted of 27 turns. The anode was biased to +1300 V dc, and the LEI signal was capacitively (C3 = 220 pF) coupled to the preamplifier. The resistors R1 and Rzwere equal to 20 i 0.1 kQ. The inductance L1 = L2= 2 mH. The ballast resistor R3 = 1 MQ. Amplitude-frequency characteristics of the high-frequency pulsed transformer were investigated using SK4-59 spectrum analyzer with build-in constant amplitude, high-frequency signal generator, which allows a change in frequency from 10 kHz to 100 MHz. The input resistance of SK4-59 was 50 s2. Reagents. Stock solutions of calcium and rubidium (1 and 10 mg/mL) were prepared from chlorides of "high purity" grade. Water was purified by being twice distilled in a fused-silicastill and then demineralized with a Milli-Q system (water resistivity = 20 MQ-cm). Calcium and rubidium solutions were prepared from stock solutions and diluted with high-purity water.
RESULTS AND DISCUSSION Curve 1 of Figure 2 shows the response observed at L3 when the constant-amplitude high-frequency signal from generator SK4-59 is fed only to L1.Curve 2 illustrates the differential response obtained when this same signal is fed to both L1and L2. Up to 3-4 MHz, the differential scheme suppresses the noise by more than 2 orders of magnitude. The last scheme provides great benefit because most of the electromagnetic noise from laser is produced in this frequency range. The extent of cancellation depends on the signals in L1 and L2 having equal amplitudes and being n radians out of phase: symmetrical positioning of the cathodes and high precision and symmetrical construction of the L1and L2 relative to the L3 windings are essential. The usefulness of the differntial LEI technique is demonstrated by the determination of a low concentration of Ca atoms in the presence of a large excess of Rb atoms. The
0.02b
t
-(Io2
3 TJME~S)
2
1
F l g m 3. S@af waveforms of lo3pg/mL Rb: 1, Conventional scheme of detection; 2, the same signal is fed to primary winding L, and measured on secondary winding L;, 3, differential signal. The measurements are made at calcium's 422.67-nm line.
conventional LEI determination of trace Ca in excess Rb is plagued by loss of selectivity because of the proximity of the 4p lPoand Rb 5s ?S1/2 6p 2Pol/2transitions a t Ca 4s2 'S 422.67 nm and 421.6 nm, respectively. For example, with the laser tuned to the Ca 4s2 'S 4p lPotransition, a lo3 pg/mL Rb solution yields a signal equivalent to that obtained from kg/mL Ca. Figure 3 shows the LEI temporal signal waveform when lo3 pg/mL Rb solution is aspirated into a flame which is irradiated 4p by a dye laser tuned to a resonance with the Ca 4s2 lPotransition at X = 422.67 nm. Curve 1 illustrates the case when this irradiated volume is in the vicinity of only one of the detection electrodes (see ref 5 in Figure 1). The waveform was measured by conventional methods (5-7) using only a 1-kQ ballast resistor R3 and dc blocking capacitor C3 without use of a high-frequency pulse transformer. Curve 2 depicts the LEI waveform in differential mode with the use of a highfrequency transformer, when the flame region was irradiated near only one of the detection electrodes. If the flame volumes near both detection electrodes were irradiated with beams generated from the same dye laser with equal power density and the high-frequency pulse transformer was used, a differential LEI waveform results (curve 3). The positions of these beam regions were chosen so that the distance between
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Anal. Chem. 1888, 60, 1639-1642
experiment by a factor of 200 such that it should now be possible to determine 5 X low5pg/mL Ca in the presence of lo3 pg/mL Rb. Figure 4 illustrates LEI calibration curves for aqueous solutions of Ca ranging in concentration from 3 x IO4 pg/mL to 10 pg/mL. Curve 1 depicts the experimental observations in the case of samples containing only Ca. Curves 2 and 3 compare the results of conventional (2) and differential (3) two-color LEI measurements for the same concentrations of Ca in the presence of lo3 pg/mL Rb. Inspection of these calibration curves amply demonstrates the superiority of the differential LEI technique.
5t
1-X 2- e 3-A
-3
-4
-2
-1
1639
ACKNOWLEDGMENT We are extremely grateful to Robert J. Miller for valuable help during the preparation of this manuscript. Registry No. Ca, 7440-70-2; Rb, 744-17-2. 1
0
LITERATURE CITED Figure 4. Calibration curves for the determination of Ca in aqueous sokrtion: 1, conventional LEI signal detection (R3 = 20 kQ); 2, differntial scheme for the determination of Ca in the presence of io3 pglmL Rb. The flame volume near one of the detection electrodes was irradiated by the lasers (A, k2);3, differential scheme for the determination of Ca in the presence of lo3 pg/mL Rb, as shown In Figure 1 (the laser beam at A, was directed to the first detection electrode and the laser beams at A, and A, were directed to the second one).
+
the two laser beams and their respective detection electrodes was the same. Also the distance between each laser beam and the high-voltage electrode was the same. In the differential LEI experiment, one of the two interaction regions (see Figure 1) can be irradiated by a second laser beam tuned to resonance with the Ca 4p lPo 6d lDz transition at 468.5 nm. Rb has no transition in this wavelength region. The nonselective ionization signals resulting from 422.67-nm excitation at both interaction regions cancel in the transformer leaving only a Ca signal resulting from both 422.67- and 468.5-nm excitation. The extent of cancellation depends on symmetrical positioning of the interaction regions relative to their respective detection cathodes and on their being equal 422.67-nm power densities a t both interaction regions. The scheme improves the selectivity of the LEI
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(1) Green, R. B.; Keller, R. A.; Schenck, P. K.; Travis, J. C.; Luther, G. C. J . Am. Chem. SOC. 1978, 98, 8517-8516. (2) Turk, G. C.; Travis, J. C.; De Voe, J. R.; O'Haver, T. C. Anal. Chem. 1978, 50, 817-820. (3) Turk, G. C.; Travis, J. C.; De Voe, J. R.; O'Haver, T. C. Anal. Chem. 1979, 5 1 , 1890-1896. (4) Gonchakov, A. S.; Zorov, N. B.; Kuzyakov, Yu. Ya.; Matveev, 0.1. Anal. Lett. 1979, 12, 1037-1048. (5) Travls, J. C.; Turk, G. C.; Green, R. B. Anal. Chem. 1982, 5 4 , 1008A10.11A. ... (6) Zorov, N. 8.; Kuzyakov, Yu. Ya.; Matveev, 0.I. Z h . Anal. Khim. 1982. 37. 520-523. (7) Travis, J.C.; Turk, G. C.;De Voe, J. R.; Schenck, P. K.; Van Dijk, C. A. Prog. Anal. A t . Spectrosc. 1984, 7 , 199-241. (8) Axner, 0.; Magnusson, I. Phys. Scr. 1985, 3 1 , 587-591. (9) Chaplygin, V. I.; Kuzyakov, Yu. Ya.; Novodvorsky, 0.A,; Zorov, N. B. Talanta 1987, 3 4 , 191-196. (10) Axner, 0.; Magnusson, I.; Petersson, J.; Sjostrom, S. Appl. Spectrosc. 1987, 41, 19-26. (1 1) Axner, 0.; Berglind, T.; Heully, J. L.; Lindgren, I.; Rubinsztein-Dunlop, H. J . Appl. PhyS. 1984, 55, 3215-3225. (12) Turk, G. C.; De Voe, J. R.; Travis, J. C. Anal. Chern. 1982, 5 4 , 643-648. (13) Magnusson, T.; Axner, 0.; Rubinsztein-Dunlop, H. Phys. Scr. 1988, 33, 429-433. (14) Turk, G. C.; Ruegg, F. C.; Travis, J. C.; De Voe, J. R. Appl. Spectrosc. 1988, 40, 1146-1152. (15) Chaplygin, V. I.; Zorov, N. B.; Kuzyakov, Yu. Ya. Talanta 1983, 30,
505-508.
RECEIVED for review November 30, 1987. Accepted March 3, 1988.
Simple Technique for Constructing Thin-Layer Electrochemical Cells Chaojiong Zhang and Su-Moon Park* Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131 Thin-layer electrochemical cells (TLECs) have been used for rapid bulk electrolysis as well as for other purposes (1,2). The cell thickness typically ranges between 2 and 300 pm, smaller than the semiinfiiitive electrochemical diffusion layer thickness (2Dt)lI2,for a given experimental time. The theory for the electrochemical behavior in thin-layer cells is well established (1, 2). An optically transparent thin-layer electrode (OTI'LE) has an added feature in its ability to take spectra of electrogenerated species (3). OTl'LEs are constructed by using the gold minigrid, which is sandwiched between two microscope slides with counter and reference electrodes usually located outside 0003-2700/88/0360-1639$01.50/0
the sandwich ( 4 ) . These gold minigrids are partially transparent ( 6 0 4 0 % ) depending on the number of wires woven per inch. Other transparent electrodes used for assembling thin-layer cells include sputtered or evaporated platinum thin film (5, 6) and tin oxide doped with antimony oxide (7), all coated glass plates. Thin-layer electrochemical cells using these transparent electrodes are not always straightforwardly assembled, and they need to be reassembled after each experiment. In our current communication, we wish to report a very simple method of constructing TLECs employing a disk electrode and a piece of a glass pate. This cell can be used for elec0 1988 American Chemical Society
