1092
Anal. Chem. 1985, 57, 1092-1095
Determination of Aqueous Chloride by Direct Nebulization into a Helium Microwave Induced Plasma Kevin G. Michlewicz and Jon W. Camahan* Department of Chemistry, Northern Illinois University, DeKalb, Illinois 601 15
The determlnatlon of chloride in aqueous solutlon by direct solution nebulization Into a robust atmospheric pressure hellum mlcrowave induced plasma Is described. The system utlllzes a commerclal500-W microwave generator, a modlled TMoloresonator cavity, and a demountable, slotted plasma torch to malntaln an ellipsoidal shaped helium plasma with a length of approxlmately 20 mm and a diameter of 7 mm. By use of thls plasma, several nebuilzatlon systems are investlgated to determlne the best system for chloride analysts. Wlth a MAK C nebulizer and MAK spray chamber (wlthout baffle), a detectlon llmlt of 7 ppm and a llnear range of 70-21000 ppm are obtalned. Also, matrix effects of certaln easily Ionizable elements are Investlgated.
Direct trace elemental analysis of solutions by atomic spectroscopic techniques has been limited predominantly to the determination of metals and metalloids. This becomes apparent observing detection limits for various spectroscopic techniques. These techniques have involved producing a solution mist which is then introduced into the spectroscopic source. Monitoring the concentration of various elements is typically done by elemental emission (1-4), absorption ( 5 , 6 ) , or fluorescence (7,8)techniques. Elemental determinations of nonmetals by atomic spectrometric techniques has been somewhat less successful. Resonance emission lines of most nonmetals reside in the vacuum ultraviolet (vacuum UV) regions of the spectrum (9), a region not ideal for routine spectrochemical analysis. Direct analysis of nonmetals in solution has been performed by observing spectral lines in the vacuum ultraviolet region of the spectrum with an inductively coupled plasma atomic emission spectroscopic (ICP-AES) source ( 1 0 , l l ) . While detectabilities appear promising with detection limits of 1, 3, and 10 ppm for I, Br, and C1, operation in the vacuum UV spectral region imposes constraints which may not be easily overcome. Vacuum monochromators and nonabsorbing optics must be used. Certainly, the ideal observation region for atomic emission spectrometry for any element is from 200 to 1000 nm, where vacuum constraints are absent and usual atomic emission spectroscopic systems are operational. For nonmetals introduced in the gas phase (gas chromatography, electrothermal vaporization, or volitile analyte species generation) elemental emission from nonresonant transitions has been observed in UV-visible-near-IR spectral region with both ICP-AES (12-18) and the helium microwave-induced plasma (He-MIP) (19-25). However, intense emission at these lines has not been observed during direct solution nebulization. Presumably, these atom reservoirs have not been capable of desolvating, atomizing, and exciting the nonmetals to the energy levels required for the observation of transitions in this spectral region. However, in recent years, efficient methods have been developed to couple microwave power to the plasma source (26, 27). Several modifications have since followed (28-30). The result is that atmospheric helium plasmas of up to 500 W may easily be maintained. 0003-2700/85/0357-1092$01.50/0
We recently described a He-MIP system, with a demountable, slotted torch and a cavity modified from that previously reported, capable of sustaining stable, robust helium microwave induced plasma during aqueous solution nebulization (30). The report herein details our analytical investigations of the direct determination of chloride in aqueous solution by He-MIP-AES. Pneumatic and fritted nebulizers were examined to determine the better sample introduction system. The optimum experimental conditions, linear range, and detection limit are presented for chloride. Analytical potentialities are also discussed.
EXPERIMENTAL SECTION Major components of the helium plasma maintenance system consisted of a 500-W microwave generator (2.45 GHz), a modified TMolo cavity, and a demountable, slotted plasma torch. This system has been reported recently (30). This system was operated without modification except that the Pyrex coolant jacket of the slotted torch was replaced with one of quartz. During experimenta involving calibration curves and detection limits, the 0.35-m scanning spectrometer was replaced with a 0.75-m Echelle Spectrospan I11 (Spectrametrics, Andover, MA) spectrometer. With the Echelle spectrometer, slit dimensions were optimized at the 479.5-nm line and found to be 300 pm X 200 pm for both the entrance and exit slits. The photomultiplier tube was a Hammamatau R758 (Middlesex, NJ) operated at 900 V. Because of the extremely low solution uptake rates when pneumatically nebulizing with helium (30),the solution was delivered to the nebulizers with a Gilson Minipuls 2 (Middleton, WI) peristaltic pump. Several nebulizer/spray chamber configurations are investigated to determine optimum signal to background ratios at the 479.5 nm C1 I1 emission line. Each configuration was optimized in terms of nebulizer pressure, solution introduction rate, and plasma gas flow rate. Axial as well as radial viewing of the plasma was examined. Detection limits and linear ranges are presented with both spectroscopicsystems (0.35-mholographic and 0.75-m Echelle spectrometers). RESULTS AND DISCUSSION Plasma Viewing Configuration. The diameter of the elipsoidal-shaped He-MIP was 7 mm. The visible portion of the plasma extended from the cavity base to 4 mm beyond the faceplate. (The cavity was 11mm deep and the faceplate was 4 mm thick.) Preliminary radial and axial spectroscopic viewing was performed during the nebulization of a 700 ppm chloride (as MgC12) solution scanning the 480-nm spectral region. For emission profiles discussed in these studies, the system was operated a t a power of 480 W, a plasma gas flow of 17.5 L/min, a coolant air flow of 25 L/min, and a pumped solution introduction rate of 0.93 mL/min with the MAK C nebulizer and MAK spray chamber. Radial observations of the plasma showed no C1 I1 emission. However, a plasma emission profile was made a t the 486.1-nm H I line. This is shown in Figure 1. As expected, the signal intensity decreases progressing from the cavity faceplate to the tip of the plasma. The inability to view the most intense portion of the plasma (within the cavity) precludes analytical chlorine observations in this configuration. 0 I985 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
SJ SIB
-
14 -
-- 0.30
12 -
. 80 -
10 -
8 - - 0.20
60 -
6-
40 -
20
~
16 -
g100 -
3
Center of Plasma Axial Observation
18 -- 0.40
\
I4O[ 120
1083
4-
-
- - 0.1
I " " " " '
0.0
2.0
3.0
4.0
Position of Plasma
(mm)
1.0
Flgure 1. Radlal profile of relative hydrogen emlsslon at the 486.13-nm line from the cavity faceplate to 4 mm beyond the faceplate. 486.133H(I)
1 , 2.0
0.5
0.00
0.5 -
Figure 3. Axlal profile of signal (open dlamonds) and signal to back-
ground ratio (solld diamonds). Table I. Nebulizer/Spray Chamber Characteristics nebulizer
manufacturer
comments
MAK 415
Sherritt Gordon Mines, Ltd., Fort Saskatchewan, Alberta Sherritt Gordon Mines
manufacturer lists 1.5 mL/min 200 psi helium (1.52 L of He/min) (crossflow design) 1.0 mL/min a t 200 psi helium (0.76 L of He/min) (crossflow) 0.6 mL/min at 200 psi I helium (0.38 L of HeJMin) (crossflow) concentric tube
MAK C
Sherritt Gordon Mines
J. Meinhard and Associates, Santa Ana, CA concentric tube TR 50C2 J. E. Meinhard and Associates fritted disk after a design by 1.0 in. frit performed Layman and superiorly Lichte (31), 2.0, 1.0, and 0.5 in. diameter coarse disks TR 30C1
With the same plasma conditions, axial observation did allow elemental emission of chloride to be observed. A spectral scan in the wavelength region of interest is shown in Figure 2. Adjacent to the 486.1-nm hydrogen line the 479.5-, 481.0-, and 481.9-nm C1 I1 lines are seen with relative intensities of 100, 62, and 32. These ratios are similar to those reported by Tanabe et al. (32). As shown in Figure 3, the axial emission profile of the 479.5-nm C1 I1 line is not symmetric about the plasma center as might be expected. Although the entire plasma is 7 mm in diameter, chlorine emission is seen only across a 4 mm region in the center of the plasma. Visually, this region appears white while the plasma is pink outside this region. The most intense emission is seen 0.25 mm from the center of the plasma. That the most intense region of chlorine emission is not seen at the exact center of the plasma may be due to imperfections in the flow characteristics of the torch and/or sample not being routed through the exact center of the plasma. Therefore, the viewing position was optimized near the center of the plasma for all of the following experiments. Sample Introduction. The analytical characteristics of several nebulizer/spray chamber combinations were investigated. These items are listed in Table I. Several combinations of these sample introduction systems were optimized in terms of signal to background ratio at the 479.5-nm C1 I1
1.0
Position of Plasma (mm)
MAK B
Flgure 2. Spectrum obtained nebulizing a 700 ppm chloride solutlon (as MgCi,) into the HeMIP. Plasma viewed axially.
I
1.5
spray chamber
manufacturer
comments
MAK Standard
Sherritt Gordon Mines operated with and without baffle insert (for MAK Nebulizers) upward Conical manufactured at N.I.U. after the design by Applied Research Laboratories (Sunland, CA) (for Meinhard downward Conical manufactured at N.I.U. Design by ARL (for Meinhard nebulizers) Scott manufactured at N.I.U. after a design by Plasma Therm (Kresson, NJ) (for Meinhard nebulizers)
emission line. Parameters varied in these optimizations include power applied by the microwave generator, plasma gas flow rate, nebulizer pressure, and sample introduction rate
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
Table 11. Optimized Nebulizer/Spray Chamber Conditions nebulizer
nebulizer MAK 415 MAK B MAK C MAK C glass frit Meinhard C-30
Meinhard C-50
spray chamber standard standard standard w/o insert “Scott” “down conical” “up conical” “Scott” “down conical” “up conical”
pressure, psi
analyte uptake, mL/min
plasma flow, L/min
power,
W
signal
background
signal/ background
200 200 200 200 20 30 30 30 50 50 50
2.80 2.80 2.80 2.80 0.02 2.90 2.90 2.90 2.80 2.80 2.80
21.0 21.0 21.0 21.0 17.5 17.0 17.0 17.0 17.0 17.0 17.0
480 480 480 480 500 460 460 460 460 460 460
15.1 37.1 45.1 180.0 58.0 21.4 8.9 8.5 17.9 10.3 10.3
44.3 56.6 59.5 129.5 50.0 51.3 42.1 40.5 49.1 38.3 33.9
0.341 0.656 0.758 1.39 1.16 0.417 0.21 0.21 0.364 0.269 0.304
0.2
c
Sample Introduction Rate (mL/min)
Flgure 4. Peristaltic pump rate vs. signal to background ratios for
representatlve nebulization systems.
as varied with the peristaltic pump. Results for each sample introduction system are listed in Table 11. As is seen from the listed data, the MAK C nebulizer without the baffle insert offered the best signal to background ratio. While the 1.0-in. glass frit nebulizer was comparable, long-term drift in frit nebulizer performance caused abandonment of its use. The influence of sample uptake rate upon signal to background was observed by varying the peristaltic pump rate and monitoring signal/ background ratios. Increased signal/ background ratios were seen from 0.6 mL/min to a maximum a t about 2 mL/min. The signal decreases from 0.4 to 0.6 mL/min. These trends are shown in Figure 4. Reasons for this minimum are not clear at present. Plasma Gas Flow and Applied Power. With the optimum sample introduction system (MAK C nebulizer and MAK spray chamber without the baffle insert), power and plasma gas flow rate were optimized. Signal to background ratios are graphically illustrated in Figure 5. The general trend is that signal increases with greater applied powers. Increasing plasma gas flow rates increases SIB to a maximum at 17.5 L/min. Thereafter, the S I B decreases. This behavior may be explained if increased applied power produces an increase in the plasma species causing chlorine excitation and reduced flow rate yields increased analyte residence time. Linearity and Detection Limits. Detection limits obtained with the 0.35-m holographic and 0.75-meter Echelle monochromators were 32 ppm and 7 ppm, respectively. The detection limit is defined as the signal corresponding to 2 times the standard deviation of the background noise (10 points at 12-s intervals). These limits were obtained with 210 ppm and 70 ppm chloride solutions. The calibration plot for chloride with the Echelle spectrometer is shown in Figure 6. This plot shows linear response from 21 OOO to 70 ppm with a correlation coefficient of 0.9997. While 35 ppm was observable, the signal was relatively small compared to the noise. Therefore, some loss of linearity is exhibited a t this concentration. To the
400
460
480
Power (W) Flgure 5. Effect of power on slgnal to background ratios at various
flow rates.
k
Calibration Curve for CI (Echelle)
-
2.00 ‘2
2
.-0
8
, I 1.00
;
: 1.00
2.00
3.00 log (ppm)
4.00
5.00
Flgure 6. Calibration curve for chlorine (as MgCI,). The demonstrated linear range is from 70 to 21 000 ppm. Detection llmits are 7 ppm.
knowledge of the authors, this is the best analytical data which has been obtained in the visible region of the spectrum for the direct nebulization of chloride in terms of both detection limits and linear ranges. Matrix Effects. The effect of added metals on analytical signal was determined. As is shown in Figure 7, potassium, sodium, and calcium all exhibit analytical signal depression at the M level. This trend increases with increasing metal concentration. This effect is more apparent with alkali metals of low ionization potential. This indicates that electrons play
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, M A Y 1985
1095
LITERATURE CITED
0
0 005 0010 0 015 Added Metal Concentration (M)
Figure 7. Effect of various metals on the 479.5-nm C i signal. The concentration o f chloride is 4 ppt.
I1
emission
a role in C1+ emission quenching. However, because nonmetal ion excitation mechanisms are unclear at present, it is impossible to speculate on the exact role electrons play in this scheme. Current studies are examining these processes.
CONCLUSION Determination of chloride in aqueous solution by direct nebulization atomic emission spectrometry in the visible region of the spectrum has been described. This technique shows much promise for the direct spectroscopic determination of other nonmetals in aqueous solution. We are currently investigating the analytical characteristics of other nonmetals as well as improvements in the modes of sample introduction into the helium plasma. ACKNOWLEDGMENT The authors thank Sherritt Gordon Mines for the gift of the MAK nebulization systems. Registry No. C1-, 16887-00-6; He, 7440-59-7.
Fassei, V. A.; Kniseiey, R. N. Anal. Chem. 1974, 4 6 , 111OA-1111A, lll6A-1118A, 1120A. Fassei, V. A,; Kniseley, R. N. Anal. Chem. 1974, 46, 1155A, 1158A, 1162A, 1164A. Bournans, P. W. J. M. Spectrochim. Acta, Part B 1983, 368, 747-776. Greenfield, S.;McGeachin, H. MqD.; Smith, P. B. Talanta 1978, 23, 1-14. Siaven, W. Anal. Chem. 1982, 5 4 , 685A-686A, 689A-690A, 693A694A. Koirtyohann S. R.; Kaiser, M. L. Anal. Chem. 1982, 5 4 , 1515A1516A, 1518A. 1520A. 1522A, 1524A. Winefordner, J. D.; Vickers, T. J. Anal. Chem. 1984, 36, 161-165. VanLoon, J. C. Anal. Chem. 1981, 5 3 , 332A-334A, 337A-338A, 340A, 344A. 346A, 350A, 352A, 354A, 358A, 361A. Mavrodineanu, R.; Boiteux H. “Flame Spectroscopy”; Wiiey: New York, 1980; pp 251-252. Nygaard, D.; Schleicher, R.; Chase, D.; Leighty, D. The Pittsburgh Conference and Exposition, Atlantic City, Paper No. 900, March 5-9, 1984. Nygaard, D. D.; Leighty, D. A. 1l t h Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, Paper No. 198, Sept 16-21, 1984. Windsor, D. L.; Denton, M. B. Anal. Chem. 1979, 54, 1116-1119. Windsor, D. L.; Denton, M. B. J. Chromatogr. Sci. 1979, 77, 492-496. Brown, R. M.; Fry, R. C. Anal. Chem. 1981, 53, 532-538. Northway, S. J.; Fry, R. C. Appl. Spectrosc. 1980, 34, 332-338. Hughes, S . K.; Fry, R. C.Anal. Chem. 1981, 5 3 , 111-1117. Fry, R. C.; Northway, S.J.; Brown, R. M.; Hughes, S. K. Anal. Chem. 1980, 5 2 , 1716-1722. Northway, S. J.; Brown, R. M.; Fry, R. C. Appi. Spectrosc. 1980, 34, 338-348. Bache, C. A.; Lisk, D. J. Anal. Chem. 1987, 39, 786-789. Carnahan, J. W.; Muiiigan, K. J.; Caruso, J. A. Anal. Chim. Acta 1981, 130, 227-24 I . Zander, A. T.; Hieftje, G. M. Appl. Spectrosc. 1981, 35, 357-371. Carnahan, J. W.; Caruso, J. A. Anal. Chim. Acta 1982, 136, 261-267 . - .. Aider, J. F.; Jin, Q.; Snook, R. D. Anal. Chim. Acta 1981, 120, 147-154. . . . . - .. Alder, J. F.; Jin, Q.; Snook, R. D. Anal. Chim. Acta 1981, 123, 329-333. ._. ... Estes, S. A.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1981, 5 3 , 1829-1837. Beenakker, C. 1. M., Spectrochim. Acta, Part B 1978, 318, 483-486. Beenakker, C. I. M.; Boumans, P. W. J. M.; Rommers, P. J. Phil@ Tech. Rev. 1980, 39, 65-77. Van Daien, J. P. J.; deLezene Coulander, P. A,; de Gaien, L. Spectrochlm. Acta, Part B 1978, 338, 545-549. Haas, D. L.; Carnahan, J. W.; Caruso, J. A. Appl. Spectrosc. 1983, 37,82-85. Michiewicz, K. 0.; Urh, J. J.; Carnahan, J. W. Spectrochim.Acta, Part S, in press. Layman, L. R.; Lichte, F. E. Anal. Chem. 1982, 5 4 , 638-642. Tanabe, K.; Haraguchi, H.; Fuwa Spectrochim. Acta, Part B 1981, 368, 119-127.
RECEIVED for review October 17,1984. Accepted January 28, 1985.
