Anal. Chem. 1988. 60. 2542-2544
2542
Table 11. Ion Activities and Their Ratios at Equivalent Potentials
interferent iodide nitrate bromide chloride acetate
potential change, mV
interferent activity, M
-50.0 -30.0 -14.0
4.67 x 10-3 1.14 X 4.08 X
-14.0
1.34 X
-7.0
4.57 X
dibasic orthophosphate activity, M 1.19 x 10-3 2.59 X 5.22 X 5.22 X 1.51 X
activity ratioo 3.92 44.0 78.1
257 302
Ratio = (interferent activity)/(dibasic orthophosphate activi-
Our bis(p-chlorobenzy1)tin dichloride based membrane electrode possesses selectivity for dibasic orthophosphate that is clearly superior to previous anion-selective polymer membrane electrodes. Based on the excellent selectivity, low detection limits, and favorable lifetimes of this membrane electrode, development of practical continuous monitor systems for orthophosphate may soon be possible.
ACKNOWLEDGMENT We wish to thank Professor Louis Messerle of the Department of Chemistry at the University of Iowa for his assistance in the synthesis and utilization of organotin compounds.
ty).
LITERATURE CITED
to the potential in Tris buffer. Ratios that are greater than one clearly indicate that the electrode is responding to a larger extent to dibasic orthophosphate under the specified conditions than to the interferent. Additional ratios of this type can easily be derived for other activities by inspection of the responses in Figure 1. Although the electrode's selectivity for orthophosphate over sulfate was not measured directly, it can be reasoned that this selectivity is considerable given the low limit of detection for dibasic orthophosphate found by calibration in the sulfate-containing Tris buffer. Usable calibration curves for dibasic orthophosphate are obtained over a 28-day period when electrodes are stored in the working buffer a t room temperature between measurements. After approximately 2 weeks of use, however, the detection limit begins to gradually deteriorate and slightly shorter linear ranges are observed. Detection limits below the millimolar activity level are observed even after 28 days.
Shkinev, V. M.; Shpigun, L. K.; Spivakov, B. Y.; Trepalina, V. M.; Zarinskii, V. A.; Zolotov, Y. A. 2%. Anal. Khlm. 1980, 35, 2137. Shkinev, V. M.; Shplgun, L. K.; Spivakov, V. A.; Zolotov, Y. A. Zh. Anal. Khlm. 1980. 35, 2143. Midgley, D. Ion-Sel. Nectrw'e Rev. 1988, 8, 3. Kinugawa, Z.; Sisido, K.; Takeda, Y. J. Am. Chem. Soc. 1961, 83,
538. Arnold, M. A.; Glarier, S. A. Talsnta 1988, 35, 215. Butler, J. N. Ionic Equilibrium A Mathematical Approach ; Wesley: Reading, MA, 1964 Chapter 12. Pure Appl. Chem. 1978, 48, 127.
Scott A. Glazier Mark A. Arnold* Department of Chemistry The University of Iowa Iowa City, Iowa 52242 RECEIVED for review March 29, 1988. Accepted August 23, 1988. Support for this work from the National Institute of Dental Research (DE07996) is greatly appreciated.
TECHNICAL NOTES Laminar-Flow Torch for Helium Inductively Coupled Plasma Spectrometry Hsiaoming Tan, Shi-Kit Chan, and Akbar Montaser*
Department of Chemistry, T h e George Washington University, Washington, D.C. 20052 Helium inductively coupled plasmas (He ICPs), operated
at atmospheric pressure ( 1 4 , possess two advantages compared to Ar ICPs for atomic emission spectrometry (AES) and mass spectrometry (MS). First, for the elements tested so far, the detection powers for the He ICPs are superior to those for an Ar discharge. Second, the emission background spectra of the He ICPs are quite simple in the red and the near-infrared regions, thus reducing the spectral interference problems encountered with the determination of halogens and other nonmetals. Relatedly, certain mass spectral interferences noted in the detection of monoisotopic elements are eliminated when helium is used as the plasma gas instead of argon. For the most recent studies of He ICPs (2-6), we used a tangential-flow torch to form an annular plasma at forward power of 1500 W with a total helium gas flow of 8 L/min. The present study is concerned with the formation and preliminary characterization of a He ICP using a laminar-flow torch. The total helium gas flow for this torch is less than 2 L/min. Studies of plasmas formed in laminar-flow torches are important because of the possibility to reduce one major source of noise resulting from the rotation of the plasma gas in
tangential-flowtorches. Previous studies on Ar ICP discharges have documented the advantages of laminar- versus tangential-flow torches for AES (7-11).
EXPERIMENTAL SECTION 1. Instrumentation and Operating Conditions. Except for
the laminar-flow torch, the ICP-AES system and the operating conditions for the spectrometer are described elsewhere (12,13). Briefly, most experimental data were acquired with an intensified photodiode array spectrometer using a slit width of 50 pm. However, measurements of rotational temperature (Trot)and electron number density (ne)required the use of a photomultiplier tube to utilize the maximum resolution of our spectrometer. The slit widths were 5 and 10 pm for measurements of Trotand ne, respectively. The aperture of the imaging optics was set at 25 mm diameter, and a red filter (catalog no. CS2-63,2424, Corning Class Works, Corning, NY) was placed in front of the entrance slit of the spectrometer to eliminate possible spectral interference from the higher order spectra (3). Pure helium (99.997%,MG Industries, Valley Forge, PA) was used to form and sustain the He ICP. To introduce sample into the plasma, the injector gas was replaced with a gas mixture containing 105 pL/L of SFB,99 &/L of CC12F2,and 96 pL/L of CRrF3in helium (Certified gas mixture, Matheson Gas Products,
0003-2700/S8/0360-2542$01.50/0 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
PLASMA QUARTZ TUBE
Table I. Generating and Optimized Operating Conditions for He ICPs Formed in Laminar- and Tangential-Flow Torches
~
INSERT WITH GAS SHEATH
parameter INJECTOR TUBE TORCH BASE
2543
plasma gas flow, L/min injector gas flow, L/min forward power, W reflected power, W obsvn height above load coil, mm load coil (top turn grounded)
-
SWAGELOK FITTING: NY-400-3.4TMT PLASMA GAS INLET TYGON TUBING INJECTOR GAS INLET
......: Flgure 1. Schematic diagram of (A) the laminar-flow torch, (B) its four major components shown separated in space, and (C) the top view of the gas sheath and the injector tube.
East Rutherford, NJ). The presence of nitrogen and hydrogen impurities in helium allowed determination of Trotand ne, respectively. For excitation temperature (T,,,) measurement, the thermometric species was the chlorine atoms produced from the atomization of the gaseous mixture. 2. Laminar-Flow Torch. Figure 1 shows the schematic diagram of the laminar-flowtorch developed in this study. The major components were (a) a torch base made from Plexiglas, (b) an insert with a gas sheath, (c) a sample injector tube, and (d) a high precision quartz tube (13 i 0.02 mm i.d., 15 i 0.02 mm o.d., 70 mm long, Wilmad, Buena, NJ) and its associated holder. To sustain a stable plasma over an extended period, items 2 and 3 were made from MACOR machinable glass ceramic (Corning Glass Works, Corning, NY). To obtain a laminar flow, the plasma gas was directed through the torch base into the gas sheath having 24 holes (0.4 mm diameter, 4 mm depth) arranged circumferentidy around the highly smooth insert, Figure 1B,C. The sample injector tube (4 mm 0.d.) resided tightly within the insert (12.4 mm o.d.), and possessed a 0.5 mm i.d. at the injection port. Six small screws were used to attach the gas sheath to the torch base. The holder for the quartz tube was in turn connected to the gas sheath by means of six screws which also served as alignment devices for centering the quartz tube around the insert. Nitrile O-rings (Parker, Lexington, KY) were used to closely fit together the four major components. In operation, helium plasma gas emerged from the gas sheath into the annulus between the quartz tube and the insert, and flowed into the quartz tube in a laminar fashion. Sample was transported through the injector tube into the He ICP. The length of this plasma extended approximately 1 cm outside the quartz tube, as compared to 6-7 cm for the plasma formed in the tangential-flow torch (2).
RESULTS AND DISCUSSION 1. Plasma Formation and Operating Conditions. The conditions for generation and optimized operation of the laminar-flow He ICP are shown in Table I. For comparison, compromised operating conditions are also given for the tangential-flow He ICP used in our previous studies (2-6). In both cases, He plasmas are formed directly from pure helium gas in one step. In our experience, the use of a reversed load coil (top turn grounded) simplifies formation of the He ICPs, especially with this laminar-flow torch. Also, an insulated carbon rod may be held inside the laminar-flow torch for approximately 5 s to facilitate plasma formation. The He ICP formed in the laminar-flow torch was operated a t a total gas flow of only 1.6 L/min, about 5 times less than the helium gas flow required for the tangential-flow torch (2-6). To form an annular plasma, the injector gas had to be introduced gradually, and the final flow could not exceed 0.1 L/min, otherwise the plasma transformed into a filament-type
1.5 0.1
7 1
700
1500 5
2 2
25
41/2 turns, 26 mm i.d. in both
cases, coils made from in. 0.d. copper tubing insulated with heat-shrinking polyolefin tubing
24 HOLES, 0 4 m m , d
SMALL SCREWS
laminar-flow tangential-flow torch torch
impedance matching network series capacitance, pF shunt capacitance, pF
10 to 300 800 to 900
10 to 300 750 to 850
discharge that shifted or bent toward the quartz tube. Best plasma symmetry was achieved when (a) mass flow meters were used to control the gas flows and (b) the torch was positioned at the exact center of magnetic field inside the load coil. Because plasma symmetry in this laminar-flow He ICP was more sensitive to the uniformity of the radio frequency (rD field compared to those in tangential-flow torches, we used an X-Y translation stage to position the torch within the optimum location inside the load coil. Because of its low gas consumption, the He ICP formed in the laminar-flow torch could not be operated at forward power level greater than 700 W unless the quartz tube was cooled externally. The optimum observation height for this plasma was 2 mm above the load coil as compared to 25 mm for the He ICPs generated in tangential-flow torches (2-6). 2. Fundamental Plasma Characteristics. Three fundamental properties were measured for the He ICP: T,,, T,,, and ne. Because of the lack of local thermodynamic equilibrium for the current He ICPs, T,, and Trotcannot be viewed as unique measures of energy characteristics of the discharges. Excitation temperature reflects the ability of the plasma in populating particular excited levels, while rotational temperature is an indicator of gas temperature. Excitation temperature was measured by the atomic Boltzmann plot method (14) using C1 as the thermometric species. The parameters used for T,,,calculations are given elsewhere (5). To minimize array registry problems (12, 15, 16) associated with the detector, peak areas measurements, rather than peak intensities, of the C1 spectral lines were used in this work for temperature estimation. Thirteen pixels were used at each line to calculate the peak area. For the determination of Trot,the P and R branches of the (0,O) band of the first negative system of N2+were used (5,17). The electron number density was determined via least-squares fittings of the H-/3 line at 486.13 nm to the theoretical Stark-broadened profile using an improved algorithm described in a recent study (13). Table I1 shows the fundamental properties of the He ICPs generated in laminar- and tangential-flow torches under the optimized conditions listed in Table I. In general, comparable properties were measured, except for the rotational temperature which was about 400 K lower for the He ICP sustained in the laminar-flow torch due to the use of lower forward power. At power levels greater than 700 W, the quartz tube of the laminar-flow torch began to erode. Because of its low gas temperature, the analytical performance of the laminarflow He ICP is evaluated below for gaseous samples only. 3. Analytical Performance. Table I11 shows the signal-to-background (S/B) ratios, the detection limits, and the percent relative standard deviations (% RSD) of the back-
2544
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Table 11. Electron Number Density" and Excitation and Rotational Temperaturesb of He ICPs parameter electron no. density, cm-3 excitation temp, K rotational temp, K P branch R branch
laminarflow torch
tangentialflow torch
5.6 X 1013 6380
5.6 X 1013 6100
1525 1565
2100
1900
" For ne calculation ( 1 3 ) , electron temperature and Doppler temperature were assumed to be 10000 and 2500 K, respectively. The precision of ne value was *5%. bPrecision of measurements for T,,, and Trotobtained from the R branch, and T,,, estimated from the P branch were *3%, *7%, and *3%, respectively. Table 111. S/B Ratios, Detection Limits (ng/s), and Percent RSDs of Background Intensities for Gaseous Sample Injected into He ICPs laminar-flow torch element (nm) F 1685.6 C1 1837.5 Br 1827.2 S 1921.2 C 1833.5
SIB
DL"
13 23
3 0.5
21
1.6 2.6
70 RSD
tangential-flow torch 7 0
SIB
1.0
4
2
0.8 2.1
3
0.6
7 8 0.3
2
0.7
1
DL"
RSD
12 2 2 8 2
1.0 0.6 0.9 0.3
0.5
the same. These results indicate that for the gaseous sample injection, the He ICP sustained in the laminar-flow torch provides analytical performance similar to those of the plasma formed in tangential-flow torch, but a t significantly lower helium gas flow rate and rf power.
ACKNOWLEDGMENT We thank D. W. Golightly and A. Dorrzapf, Jr., and R. W. Werre of U S . Geological Survey for their helpful comments and assistance during the course of this study. Registry No. He, 7440-59-7;F2, 7782-41-4;C12,7782-50-5;Br2, 7726-95-6; S, 7704-34-9; C, 7440-44-0. LITERATURE CITED S.; Montaser, A. Spectrochim. Acta, Part 6 1985, 408, 1467-1472. (2) Chan, S.;Van Hoven, R. L.; Montaser, A. Anal. Chern. 1986, 58, 2342-2343. (3) Chan, S.;Montaser, A. Appl. Spectrosc. 1987, 4 7 , 545-552. (4) Montaser, A.: Chan, S.:Koppenaal, D. W. Anal. Chem. 1987, 59, 1240-1243. (5) Chan, S.; Montaser, A. Spectrochlm. Acta, Part 6 1987, 426. 591-597. (6) Montaser, A.; Van Hoven, R. L. CRC Crit. Rev. Anal. Chem. 1987, 78, 45-103. (7) Wendt, R . H.; Fassel, V. A. Anal. Chem. 1985, 3 7 , 920-922. (8) Davies, J.; Snook, R. D. Ana/yst (London) 1985, 770, 887-888. (9) Davies, J.; Snook, R. D. J . Anal. At. Spectrom. 1988, 7 , 195-201. (10) Davies, J.; Snook, R. D. J . Anal. At. Specfrom. 1987, 2 . 27-31. (11) Davies, J.; Du, C. M. J . Anal. At. Spectrom. 1988, 3 , 433-439. (12) Ishii, I.; Golightly. D. W.; Montaser, A. J . Anal. At. Specfrosc., in (1)
(13) (14)
" Detection limit is defined as the concentration giving a signal equivalent to 3 times the noise, calculated from the standard deviation of 11 repetitive measurements of the background intensities. ground intensities measured for the determination of F, C1, Br, S, and C in a gaseous mixture when the sample mixture in helium was injected at the rate of 0.05 and 0.1 L/min into the plasmas sustained in the laminar- and tangential-flow torches, respectively. Except for the torches that were operated under the conditions shown in Table I, all experimental facilities and operating conditions for the spectrometers were
(15)
Chan,
press. Chan, S.; Montaser, A. Spectrochim. Acta, Part 6 , in press. Hasegawa, T.: Haraguchi. H. In Inductive& Coupled PIesmas in Anavial Atomic Specfromfry; Montaser, A., GoUghtly,D. W., Eds.; VCH Publishers: New York, 1987, and references cited therein. Winge, R. K.; Fassel, V. A.; Eckels, D. E. Appl. Specfrosc. 1988, 4 0 , 461-464.
(16)
Burton, L. L.; Blades, M. W.
(17)
Abdallah, M. H.; Mermet, J. M. Spectrochlm. Acta, Part6
5 13-5 19.
Spectrochim. Acta, Part 6 1987. 426, 1982, 376,
391-397.
RECEIVEDfor review April 25,1988. Accepted July 25,1988. A portion of this paper was presented a t the 1987 FACSS Meeting, Detroit, MI, Oct 1987. This work was sponsored by the U S . Department of Energy under Grant No. DE-FGO587-13659.
