Characterization of the Plume of a Direct Current Plasma Arc for Emission Spectrometric Analysis W. E. Rippetoe, E. R. Johnson, and T. J. Vickers’ Department of Chemistry, Florida State University, Tallahassee, Fla. 32306
A low power dc plasma arc device for emission spectrometry with solution samples is described, and the plume region of the discharge is characterized. The introduction of potassium chloride into the discharge enhances the analytical utility of the device by reducing background emission in the plume and improving aerosol introduction into the plasma. The dependence of excitation temperature in the plume on the concentration of potassium chloride sprayed, on the arc current, and on vertical position is reported. Detection limits are :eported for seven elements (AI, Cd, Cr, Fe, Pb, V, W). For a single set of operating conditions, the detection limits are as good as or better than the best reported flame emission values.
relatively large electrode spacing. This results in a relatively long arc discharge and, consequently, a long sample residence time. In addition, potassium chloride is introduced with the sample aerosol to facilitate entry of‘ the aerosol into the discharge. This report includes: a qualitative measure of atomization efficiency based on the effect of phosphate on the emission intensity of calcium; the temperature dependence of the extruded plasma or “plume” on numerous experimental variables; the background emission of the plume; and the detection limits, analytical curves, and precision data for a number of elements.
EXPERIMENTAL FACILITIES AND PROCEDURES Low power plasma arc devices were introduced as excitation sources for spectrochemical analysis by Margoshes and Scribner ( I ) and independently by Korolev and Vainshtein (2) in 1959. Modifications, improvements, and further characterization of the plasma arc have been described by various authors in subsequent reports. The most recent reports ( 3 - 5 ) emphasize the utility of the plasma arc as a direct replacement for chemical combustion flames in spectrometric measurements. Despite the considerable promise of the plasma arc for this purpose, there are limitations to the devices so far described which are manifested by the rather disappointing (compared to flames) detection limits reported and the observation of atomization interferences, such as the depressing effect of phosphate on calcium emission. The plasma arc devices so far described exhibit a relatively (again, compared to flames) high level of background emission which probably, in part, accounts for the disappointing detection limits. The incomplete sample atomization associated with the occurrence of interference effects appears anomalous when the high temperature of the arc discharge is considered. At least two factors are likely to contribute to this phenomenon: insufficient residence time of the sample in the discharge and failure of the aerosol to enter the discharge due to the large thermal gradient. Addition of elements of low ionization potential to an arc discharge has been shown to alter many of the operating characteristics (6, 7 ) . An increase in the electron density of the plasma, a decrease in the plasma temperature, a decrease in the radial temperature gradient, and an increase of the residence time of the sample in the plasma are among the changes noted. Marinkovic and Vickers demonstrated that adding potassium chloride to the sample aerosol results in a much higher atom concentration of the analyte in the central portion of the arc discharge (8). This report describes results obtained with a plasma arc device constructed with particular regard to overcoming the limitations described above. This device is a modification of the design described by Marinkovic and Dimitrijevic (9).The device used in the current report incorporates a I
Author to whom correspondence should be directed.
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DC Plasma Arc Device. A cross section diagram of the dc plasma arc device is given in Figure 1. Figure 2 shows photographs of the device in operation. The arc burns between two 6.35-mm diameter graphite electrodes, 1 (the cathode), and 2 (the anode) (UltraCarbon V50-7). The discharge is formed in the channel created in the openings in the brass segments 3 and 4 and the Pyrex glass cylinder, 7. Segments 3,4, 5, and 6 are water cooled. Segments 3 and 4 are electrically isolated from the electrodes. Each of the electrodes is secured by a 2.38-mm stainless steel bolt through which electrical contact is also established. An argon stream of 2.5 l./min is introduced into segments 3 and 4 around the electrodes to reduce consumption of the electrodes and to lower the voltage drop by excluding air. To ignite the dc plasma arc, auxiliary graphite rods are inserted through the 2.38-mm diameter hole of each cylindrical electrode and a third graphite rod is inserted through the vertical channel to establish electrical contact. When the arc has been struck, the graphite rods are removed. The interelectrode spacing can be varied by the size of the glass cylinder, 7, used in the device. The position of the glass cylinder is maintained by a circular brass ring, 11, attached to segment 4. The plasma arc device is mounted against a pressed phenolic resin base. The base is secured to a vertically and horizontally adjustable stand. Two 6.35-mm diameter Bakelite bolts are used to hold the plasma arc apparatus together. A relatively inexpensive dc power supply of the type normally used in spectrographic analysis is used to supply the dc plasma arc device, (Zeebac 2001-BO1). Open circuit voltage is 300 V with a 015 A current range. Aerosol Introduction. Two indirect pneumatic nebulizers are used to simultaneously spray the analyte and a potassium chloride solution. A glass Y junction is used to combine the two aerosols. The combined aerosol is directed into segment 8 which tangentially introduces it into the arc discharge. Segment 8 is constructed of Pyrex glass. A snug fit against the plasma jet arc device is assured by segment 9, a circular Teflon adaptor with a 1.59-cm centered hole. A flow of 4.9 l./min argon at 40 psig is used as a nebulizing gas for each sprayer. Each of the chamber type nebulizers is constructed by removing the jacket from a Beckman No. 4020 total consumption burner and attaching the remaining portion of the burner to a glass spray chamber of 50-ml volume. A 1-cm length of 2-mm i.d. plastic tubing is slipped over the tip of the nebulizer used for spraying the potassium chloride solution to eliminate clogging. Nebulization efficiency, defined as the volume of aerosol reaching the atom reservoir divided by the total volume aspirated, is approximately 5.0% for potassium chloride and 8.0% for water. The volume reaching the atom reservoir was estimated by subtracting the volume discarded from the volume aspirated. Possible inaccuracy due to adhesion of droplets to the spray chamber was
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
T a b l e I. Operating Conditions DC plasma a r c 11 amperes Operating current I 2 volts Operating voltage 0.8 kW Net power dissipated 2.5 cm Interelectrode spacing 1.2-cm diameter base with Extruded plasma 3.5-cm height in a conical dimensions configuration 4.9 1./min argon to spray Gas flow rates analyte 4.9 l./min argon t o spray potassium chloride 2.5 l./min argon circulation around electrodes Spectrometer 1.0-m focal length (JarrellCzerny-Turner Ash Division Model No. I8 - 4 62 ) Curved unilateral slits Slits 50-wm entrance slit 50-gm exit slit 15-um exit slit heiFht Grating 1180 rupngs/mm, blazed at 2000 A, first order 8 A/mm, first order Reciprocal linear dispersion Photomultiplier RCA 1P28, S-5 response Variable high voltage Supply Photomultiplier Power SUPPlY (John Fluke Mfg. Co., . Model 412B) 160-Hz modulation f r e Mechanical Chopper quency T.ock-in amplifier tuned to 160 Hz (Princeton Applied Research, Model HR-8 with Type A preamplifier) -mA galvanometric s t r i p chart recorder (Texas
reduced by spraying the appropriate ROhJtiOn for B short time prior to the measurements. The solution uptake rate is 3.1 mllmin. Operating Conditions, A number of dependent operating variables required optimization for a meaningful investigation of the dc plasma arc. Consequently, the Simplex technique ( l o ) ,a geometrical approach of minimal complexity, was employed for the optimization. The experimental variables investigated were arc current, slit width, slit height, and the arc optical path. The optimization criterion was the signal-to-noiseratio of the emission intensity of a 10 pglml calcium solution. A Nova 1200 digital computer was used to collect data and perform the mathematical manipulations directed by the Simplex method. Once these variables were optimized, other relevant variables could be investigated. Optimization for calcium resulted in B set of operating Conditions applicable t~ all elements examined. Table I describes the operating conditions used in this study. Reagents. Standard dilutions from 1000 pglml stock solutions were used for all studies. All solutions were prepared with deionized water. The 2.OM potassium chloride solution was prepared by dissolving 149.8 g of 99.8% pure reagent (J. T. Baker Chemical Co.) in deionized water and diluting to 1liter. All solutions were stored in polyethylene bottles.
RESULTS AND DISCUSSION Effect of Potassium Chloride on the Arc Discharge. As noted in the introduction, species of low ionization po-
9
C
0 BRASS
0 GLASS
m
GRAPHITE
LFi
@@B TEFLON ARC DISCHARGE Figure 1. Cross section of the dc plasma arc device
Figure 2. Photograph of the dc plasma arc in operation
0DISTINCT
ZONE
DIFFUSE ZONE
Figure 3. Schematic diagrams of the arc discharge (excluding the plasma plume) for various concentrations of potassium chloride (a)absence of KCI, ( b ) 0.4M KCI, ( c )1.OM KCI. ( d ) 1.5M KCI. (e) ?.OM KCI
tential, such as potassium chloride, have a significant effect on a n arc discharge. Figure 3 presents in schematic fashion the effect of increasing t h e concentration on the appearance of the arc discharge (excluding the plasma plume) of the d c plasma arc. Increasing the potassium chloride concentration results in enlargement in the width of the discharge. As the volume of the discharge increases, it also hecomes more and more diffuse in appearance. A limiting concentration of 2.OM potassium chloride results in a stable, completely diffuse arc configuration. This is referred t o as the diffuse discharge mode. Operation with less than 2.OM KCI results in the distinct mode of discharge. Table I1 outlines the effect of increasing the potassium chloride concentration on the operating voltage and current of the d c arc plasma jet. Ohm’s law was used t o calcnANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
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Table 11. Effect of Concentration of Potassium Chloride on Operating Current and Voltage Concentration of IK1, moiariv
Operating cur'cnt, amperes
operating i o l t a q e , volts
10.2 10.8 10.8 10.8 10.8 11.0
82 75 75 75 75 72
0 0.25 0.4 0.7 1 .o
2 .o
' y /
1
91
>
c
ec
I
I
2
I 4
I 6
I
I
I
8
IO
12 14 16
I
I
VERTICAL POSITION (mm ABOVE DEVICE)
Figure 5. Log of relative background intensity as a function of vertical position in the plasma plume while spraying ( a )Deionized water ( b )2.OM potassium chloride
L LL
02
PO,/Ca 06
MOLAR RATIO-
IO 0
Figure 4. Effect of increasing phosphate concentration on the emission intensity of calcium at the 4226.7 A line while spraying ( a )2 OM potassium chloride and ( b ) 0.4M potassium chloride
late the resistance of the arc discharge. The device exhibits a resistance of 8.3 ohms in the absence of potassium chloride and 6.5 ohms when 2.OM potassium chloride is aspirated. Assuming a cylindrical shape and using the interelectrode distance of 2.5 cm, the volume of the arc discharge may be estimated. In the absence of potassium chloride, the discharge is approximately 2 mm in diameter resulting in a calculated volume of 79 mm3. The 6-mm width of the discharge in the diffuse mode indicates approximately a 650-mm" volume. The current density of the discharge decreases from 140 mA/mm3 in the absence of potassium chloride to only 17 mA/mm3 when spraying 2.OM potassium chloride. The depression of the emission intensity of calcium by phosphate was used to qualitatively estimate the atomization efficiency of the dc plasma arc device. Figure 4 shows the effect of increasing phosphate concentration on the intensity of the 4226.7-A emission line of calcium while aspirating 0.4M and 2.OM potassium chloride, respectively. When 2.OM potassium chloride is sprayed, producing the diffuse arc mode, no depression in the intensity of the 10 bg/ml calcium is noted. An interference effect is noted when the device is operated with a distinct mode discharge. The occurrence of an interference effect in such a high temperature environment suggests failure of the sample aerosol to enter the arc discharge. With the device operating in the diffuse mode, the sample aerosol apparently enters the arc discharge. Relative Background Emission Intensity. A relatively high intensity of background emission is characteristic of the majority of plasma arc devices. T o achieve acceptable signal-to-noise ratios, it is usually necessary to carefully select the region of the arc observed. For example, Murdick 438
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and Piepmeier ( 5 ) limited the region viewed in their plasma arc device to an area only 2 mm in diameter and varied the position of observation to optimize the signal-to-noise ratio for each element examined. The relative background intensity of the dc plasma arc device used in this study has been characterized. Figure 5 presents the relative background intensity of the device as a function of the observation path in the plasma plume. Measurements were made with the plasma arc operating in both the distinct and diffuse discharge modes-that is, while aspirating deionized water and 2.OM potassium chloride, respectively. Measurements were made with a slit height of 1 mm and a t a wavelength of 2483.3 A. As shown in Figure 5, the relative background intensity is over ten times greater with the device operating in the absence of potassium chloride than when operating in the diffuse mode. Relative atom distributions were measured in the plasma plume for operating in the diffuse and distinct discharge modes. Atomic absorption of a 100 bg/ml Fe solution using the 2483.3-A absorption line was used to estimate the relative atom distribution. Results were essentially the same for both modes of operations. Thus, the reduction of background intensity observed in the diffuse mode of operation is accomplished without change in the concentration or distribution of atoms in the plasma plume. For several elements, relative emission intensities were compared for the distinct and diffuse modes of operation. The signals were of equal magnitude but the noise was much lower for the diffuse mode. I t seems likely that this signal-to-noise ratio improvement is primarily a consequence of the large reduction in background intensity obtained when spraying 2.OM KC1. Figure 6 compares the relative intensity of background emission for the dc plasma arc device to that of a nitrous oxide-acetylene flame. The plasma arc was operated in the diffuse discharge mode. The nitrous oxide-acetylene flame was operated in a slightly fuel rich configuration on a 5-cm slot burner (Techtron high temperature burner) with deionized water aspirated. Prolonged aspiration of KC1 resulted in flame instability due to clogging of the slot. However, when briefly aspirating KCl the intensity of background emission for the flame was the same as for deionized water. Optical and signal handling conditions were identical for both excitation sources. As shown by the spec-
a
b
5k25 5500 5375
5i50
I
5125 5000 4875 4730 4625
4500 4375 4250 4125 4000 3875 3750 3625 3500 f-- W A V E L E N G T H ( A N G S T R O M S )
3375
3250 3125
3000 2875
2750
Figure 6. Spectral scan of the background emission for (a)a slightly fuel rich nitrous oxide-acetylene flame aspirating water and (b)the dc plasma arc while aspirating 2.OM potassium chloride tral scans, the nitrous oxide-acetylene flame has several rather broad spectral regions with intense continuum or band emission. Aside from the OH band emission and several potassium emission lines, the arc plume has no unusable spectral regions. Excitation Temperature in the Plume. The temperature of the excitation source is of considerable significance in the atomic emission experiment. Signal intensity, as well as freedom from interferences, is related to the excitation temperature. Consequently, the excitation temperature has been measured for the extruded plasma as a function of optical position, arc current, and the concentration of potassium chloride sprayed. Measurements of the excitation temperature were made using the relative intensity of emission of 10 Fe lines in the spectral regions 3719.9 t o 3763.8 8, ( 1 2 ). Measurements were made while aspirating a 100 wg/ ml Fe solution and scanning the spectral region a t 12.5 A/ min. The peak height of each emission line was used as the measure of relative intensity. A linear least squares program was employed to determine the best line. The excitation temperature was then calculated from the slope of the line. Figure 7 shows a plot of the excitation temperature as a function of vertical position in the extruded plasma. Measurements were performed using a l - m m monochromator slit height with a 1:l image of the plasma formed on the entrance slit. The excitation temperature was also determined for the plasma arc while spraying various concentrations of potassium chloride in the range 0.4 to 2.OM. Despite the significant effect on the configuration of the arc discharge, as shown in Figure 3, the excitation temperature of the plume is unaffected by changing the KCl concentration. This apparent anomaly can be understood by realizing that the net power dissipated in the arc is nearly constant, regardless of tbe concentration of the sprayed potassium chloride. Conse tuently, even though the temperature in the arc column del reases when potassium chloride is introduced (91, the temperature of the plasma plume is unaffected. Figure 8 presents the dependence of the excitation temperature on the arc current. These measurements were con-
I
0 1 234567891Oll12131415
(mmABOVE ARC)
OBSERVATION PATH
Figure 7. Excitation temperature of the plasma plume as a function of vertical position (a 1-mm slit height was employed) 3800
c /Y
3200
1
1
6
I
I
I
1
I
7 8 9 IO II ARC C U R R E N T ( A M P S ) J
Figure 8. Excitation temperature of the plasma of arc current (a 20-mm slit height was used)
I
12
plume as a function
ducted using a full slit height of 20 mm and consequently represent an average temperature. Previous reports of arc discharges indicate little dependence of the excitation temperature on the arc current (12). The rationale for this is that the diameter of the discharge increases a t higher curANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
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439
Table 111. Detection Limits (pg/ml) Flame Elewent
Spectral l i n e ,nA"
A1 Cd
3961.5 3261.1 4254.4 3719.9 4057.8 43 79.2 4008.8
Cr Fe Pb V W
Exc ltation potential, eV
3.1 3.8 2.9
3.3 4.4 3.1 3.5
DC arc plasma i e t d
4Eb
0.009 2 .o 0.001 0.02 0.02
0.005 2 .o 0.005 0.05 0.2 0.01 0.5
0.01 0.3
AAc
AF C
0.1 0.02 0.005 0.005
5 .O
0.000001
0.01 0.02 3 .O
0.05 0.008 0.01
... ...
Line employed in this study. Ref. (14). Ref. ( 1 5 ) ,line sources used to excite transitions. d All results for one set of operating conditions.
Table IV. Precision of Results for Chromium Emission Concn of &le u g m l
0.03 0.05 0.3
1.o 10 .o 20 .o
100 .o
R t l \ t a d diL
6.69 6.12 3.42 3.53 3.10 2.22 0.60
rent, so that the energy available per unit volume of arc column remains nearly constant (1 ). However, the measurements reported here are for the plume, not the arc column, and it is to be expected that the plume temperature will rise in direct proportion to the power dissipated in the arc. Detection Limits, Analytical Curves, and Precision Data. Table I11 presents the detection limits obtained with the plasma arc for seven elements. The lines used are the most sensitive for each element studied. Corresponding detection limits for flame atomic absorption, flame atomic fluorescence, and flame atomic emission are given for comparison. The comparison results represent the best literature values for each technique. The detection limits reported for all analytical methods represent the concentration in pg/ml necessary to give a signal-to-noise ratio of 2. For the results obtained with the plasma arc, the noise is defined as the standard deviation of five consecutive emission measurements ( 1 3 ) . Detection limit values were obtained by determining the signal-to-noise ratio for approximately five concentrations from 10 to 20 times the detection limit. A plot of the signal-to-noise ratio us. concentration was then used to extrapolate to a concentration giving a signal-to-noise ratio of 2. A single set of operating conditions, described in Table I, was used for all elements examined. The comparison data are for individually optimized elements. Analytical curves were constructed for the elements examined for the concentration range 1 to 100 pg/ml (10 to 100 pg/ml for Cd). All but P b exhibited a linear plot. For Pb, slight curvature toward the concentration axis occurred above 20 pg/ml. All analytical curves cross the origin.
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
Table IV gives typical precision data for several concentrations of chromium as determined with the plasma arc. Values are the result of five consecutive determinations. CONCLUSIONS The introduction of 2.OM potassium chloride has been shown to sgnificantly alter the characteristics of a low power dc arc plasma device and to improve its analytical utility. Among the beneficial effects of the diffuse mode of operation are improved aerosol introduction into the plasma, as demonstrated by elimination of the depressing effect of phosphate on calcium emission; and reduced background intensity, with a consequent signal-to-noise ratio improvement. The excitation temperature of the plasma plume has been shown to be largely independent of potassium chloride concentration with a maximum temperature of about 3850 O K available in the plume. Detection limits obtained with the plasma arc have been shown to be superior to the best reported flame atomic emission values for a number of elements. Significantly, for possible multielement analysis applications, the detection limits reported for the arc were obtained with a single set of operating conditions, whereas the comparison values were obtained with individual optimizations. LITERATURE CITED M. Margoshes and B. F. Scribner, Spectrochim. Acta, 15, 138 (1959). V. V. Korolev and E. E. Vainshtein, 2. Anal. Khim., 14, 658 (1959). J. F. Chapman, L. S. Dale, and R. N. Whittem. Analyst (London), 98, 529 (1973). P.Merchant, Jr., and C. Veillon. Anal. Chim. Acta, 70, 17 (1974). D. A. Murdick, Jr., and E. H. Piepmeier, Anal. Chem., 46, 678 (1974). V. Vukanovic, "Emission spectroscopie," p 9, Berlin, Akad., 1964. P. W. J. M. Boumans, "Theory of Spectrochemical Excitation," Adam Hilger. London, 1966. M. Marinkovic and T. J. Vickers, Appl. Spectrosc., 25, 319 (1971). M. Marinkovic and B. Dimitrijevic. Spectrochim. Acta, Part E, 23, 257 (1968). S. N. Deming and S. L. Morgan, Anal. Chem., 45, 278A (1973). L. DeGalan and J. D. Winefordner, d. Quant. Spectrosc. Radiat. Transfer, 7, 703 (1967). H. Brinkman, Dissertation, Utrecht, 1937. J. D. Winefordner, M. L. Parsons, J. M. Mansfield, and W. J. McCarthy. Anal. Chem.. 39. 436 (1967). E. E. pickett and's. R . Koirtyohann, Anal. Chem., 41, (14), 28A (1969). J. D. Winefordner, V. Svoboda, and L. J. Cline, Crit. Rev. Anal. Chem., August 1970, pp 233-274.
RECEIVEDfor review September 3,1974. Accepted November 8, 1974. Work supported in part by funds from P H S Grant GM15996.
