Rotating Arc Plasma Jet for Emission Spectrometry W.
E. Rippetoe’
and T. J. Vickers2
Department of Chemistry, Florida State University, Tallahassee, Fla. 32306
A novel plasma jet device is described in which the arc column burns between a pointed thorlated tungsten cathode (upper electrode) and a circular graphite disc anode. Both electrodes are protected by an argon atmosphere. The arc column is caused to rotate rapidly on the disc electrode by a tangential gas stream, thus greatly extending the life of the electrode and improving interaction of the arc column with the sample aerosol introduced axially through the disc electrode. Spectrometric measurements are made in the plume above the cathode. Measurements reported include: a qualitative measure of atomization efficiency based on the effect of phosphate on the emission intensity of calcium; the excitation temperature in the plume as a function of experimental variables; the effect of variables on arc rotation rate; background emission from the plume; and detection limits for AI, Cd, Cr, and V.
The revival of interest in the field of atomic emission spectrometry during the past decade has been prompted by the development of numerous electrical discharge plasma sources. The majority of work has been devoted to two sources: the induction coupled radiofrequency plasma and the dc arc plasma. This work deals with an investigation of a low power dc arc plasma jet for spectrochemical analysis. A plasma jet is characterized by a flow of partially ionized gas that is forced through a relatively small orifice a t high velocity. The extruded plasma has a flame-like appearance, and spectroscopic observations are made in the plume. Many dc arc plasma sources which are referred to as plasma jets might more accurately be termed gas stabilized dc arcs. Examples of these are the various transfer electrode devices (1-4) in which spectroscopic observations are made off axis of the arc discharge. A more useful, and more fundamental, distinction between plasma sources can be based on whether the region of observation is a current carrying plasma or noncurrent carrying plasma (This distinction was suggested to us by M. Margoshes, who attributed it to Kaiser.). The distinction is crucial to understanding the properties of the source. The source described in the present report falls in the category of a noncurrent carrying plasma. Many recent reports of dc arc plasmas indicate difficulties that have plagued such devices since their inception ( 5 - 7 ) . Despite the high temperature available, interference effects occur, such as the depressing effect of phosphate on the intensity of calcium emission. This may be due to several factors. Among these are an insufficient sample residence time and the inability of the aerosol to penetrate the plasma because of the large thermal gradient. Although many recent reports emphasize the utility of the dc plasmas as a direct replacement for chemical combustion flames, the detection limits are often considerably poorer than with flames. The poor detection limits may be due to a number of factors: poor discharge stability, insufficient sample residence time, a high level of background emission, Present address, Deaconess Hospital, 311 Straight Street and Clifton Avenue, Cincinnati, Ohio 45219. Author to whom correspondence should be directed. 2082
and the inability of the aerosol to penetrate the plasma. A recent report characterizes a device utilizing potassium chloride to alter the arc discharge and consequently overcome several of these limitations (8). Although potassium chloride reduces the plume background intensity and facilitates entry of the aerosol into the discharge, constant spraying of KCl complicates the experimental arrangement. Consequently, a low power plasma jet device was designed with particular regard to overcoming previous limitations. Rotation of the arc discharge and a durable electrode system enhance short and long term stability. In addition, the interaction of the aerosol with the plasma is improved without the introduction of potassium chloride. 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 plume on numerous experimental variables; the effect of several parameters on the arc rotation rate; a background spectral scan of the plume; and detection limits for several elements.
EXPERIMENTAL PROCEDURES A N D FACILITIES DC Arc Plasma Jet Device. A cross-section schematic diagram of the dc arc plasma jet is given in Figure 1.T h e arc burns between a pointed 3.18-mm diameter thoriated tungsten cathode, 1 (Airco, 2?6 thorium). and 2, a 12-mm diameter circular disc graphite anode (Ultracarbon No. 861, U-2-11), The cathode is pointed using a machinist’s grinder to an angle of approximately 20’. The anode configuration is modified as follows: the 3-mm diameter center hole is enlarged to 5 mm, and a 7.5-mm hole is recessed half the thickness of the electrode. The recessed outer area enhances the discharge stability by providing a slightly larger circumference of rotation than the entry orifice. Thus, the sprayed aerosol and discharge do not interact with the same portion of the anode. Both electrodes are electrically isolated from the brass case, 3 and 6, by removable ceramic inserts, 4 and 5 (Lavite). By providing for removable ceramic inserts, the exit orifice a t the cathode can readily be investigated for optimum diameter. In addition, electrical isolation provides for much safer operation of the device. A permanently inserted ceramic sleeve isolates the remainder of the cathode from the brass case. Electrical contact is established to the circular disc anode by a 3.18-mm diameter stainless steel screw which is threaded through a Nylon insulation sleeve. Contact to the cathode is established through an alligator clip. Brass segments 3 and 6 are water cooled. T h e interelectrode spacing is determined by a PlexigLas spacer, 9. A tangential stream of inert gas is also introduced through segment 9 as a coolant and to induce rotation of the arc discharge. A very low flow of inert gas is introduced around the cathode to reduce electrode consumption. Sample aerosol is introduced via a quartz glass tube “injector” (5-mm 0.d. and 3-mm i.d.), 8, which is housed in a circular glass filled Teflon connector, 7. Injection of the sample aerosol improves the performance considerably by elimination of the degradation of the bottom surface of the anode that occurs when the aerosol is allowed to drift through the entrance orifice. The plasma jet device is mounted against a pressed phenolic resin base. T h e base is secured to a vertically and horizontally adjustable stand. A dc power supply is used to supply the plasma jet (Zeebac 2001-B01).The open circuit voltage is 300 volts with a 0-15 ampere current range. Aerosol Introduction. Aerosol generation is provided by a direct impingement ultrasonic nebulizer (Tomorrow Enterprises). Solution samples are introduced across the face of the piezoelectric
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
CERAMIC
BRASS GLASS FILLED TEFLON GLASS GRAPHITE (ANODE) TUNGSTEN [CATHODE) DISCHARGE A
t
500
6
8 IO 12 14 16 ELECTRODE SEPARATIONhm)-+
Figure 2. Power dissipated in the discharge as a function of eiectrode separation for a tangential flow of ( A ) helium, (B)argon Figure 1. Cross section of dc arc plasma jet (1) cathode, (2) anode, (3, 6) case, (4, 5)insulation, (7) aerosol delivery connector, (8)aerosol injector, (9) Plexiglas spacer
crystal by a peristaltic pump (Cole-Parmer) a t a rate of 1.3 ml/ min. The crystal is operated a t a frequency of 1.7 MHz by a custom designed power supply (Tomorrow Enterprises). An argon flow of 0.54 l./min is used to carry the sample aerosol to the plasma jet device. A small amount of a surfactant (Triton X-100) is added to aqueous samples to increase nebulization efficiency and provide for more constant spraying. Desolvation of the sample aerosol is accomplished using a heated chamber (15 cm long, 3.2-cm 0.d.) with Vigreux-type projections and a small bubble-type condenser. T h e sample aerosol is heated using nichrome wire wound around the heating chamber. Approximately 405 watts of power are dissipated by dropping 78 volts across the measured 15-ohm resistance wire. Without correcting for the temperature increase in the source when desolvation is used, the -OH band emission is reduced approximately 85%. Consequently, less than 15% of the water in the sample is introduced into the arc discharge when using the desolvation apparatus. The nebulizer efficiency was estimated using the desolvation apparatus hy measuring the condensate from the condenser for 10-ml quantities of solution uptake. The system was thoroughly wetted prior to any determinations. The average nebulizer efficiency is 17.5% for a constant argon flow of 0.54 l./min. Rotation of the Arc Discharge. After the discharge is ignited and the tangential flow gas set a t some appropriate rate to sustain rotation (around 3 l./min for Ar), rotation of the arc discharge is initiated by momentarily increasing the sample gas flow rate. This perturbs the discharge, and a stable rotation is attained by returning the sample gas flow rate to about 0.5 l./min. The rotation of the discharge is monitored using a light-sensitive transistor (General Electric X-19). T h e phototransistor was placed behind a 1-mm slit onto which a 1:l image of the nearest edge of the rotating arc discharge was focused using a 10-cm focal length lens. The signal from the phototransistor is monitored using a storage oscilloscope. A peak, indicating response of the phototransis-
Table I. Operating Conditions DC arc plasma jet Operating cur rent 11 amperes Operating voltage 30 volts Net power dissipated 0.33 KW Interelectrode spacing 1.3 cm Exit orifice diameter 5 mm G a s flow rates 0.54 l./min argon to spray analyte 6 . 4 1. /min argon tangential Amplifier Recorder
flow DC nanoamrneter (9) 10-V strip chart recorder
(Hewlett-Packard Model 6 80)
tor, occurs when the discharge is in line with the slit and after the discharge has traversed 180’. T h e second peak is smaller and thus may readily be distinguished from the “in focus” peak. Operating Conditions. Table I describes the operating conditions used in this study. The spectrometer and detector have been previously described (81, except that 30-wm slits were used in this work. Reagents. Standard dilutions from 1000 pg/ml stock solutions were used for all studies. All solutions were prepared with deionized water. One-tenth ml of Triton X-100 was added to each 250 ml to improve performance of the ultrasonic nebulizer. All solutions were stored in polyethylene bottles.
RESULTS AND DISCUSSION Electrical Properties. Voltage drops, current levels, and, consequently, the amount of power dissipated were measured as a function of the type of inert gas used and the electrode gap. The inert gases used were helium and argon. The electrode gap is adjusted by varying the size of the Plexiglas spacer through which the tangential gas flow is introduced. Figure 2 shows the effect of electrode separation on the power dissipation for helium and argon tangential flow gases. A constant current level setting was used. Argon was introduced a t a 0.54 l./min flow rate through the anode orifice for all measurements. A tangential flow of either helium or argon was used a t a rate sufficient to sustain arc rotation. Argon was introduced a t 6.3 l./min whereas a 20 l./min flow rate of helium was used. As shown by Figure 2, the power dissipated in the discharge is much higher for a tangential gas flow of helium. This is a consequence of the higher voltage drop for helium. The greater voltage drop for helium is primarily due to the much higher ionization potential (24.46 eV) for helium than for argon (15.68 eV) and the significantly higher flow rate for helium. The excitation temperature in the plume (noncurrent carrying) region of the plasma jet has been shown to depend on the current and, thus, the power dissipated in the arc (current carrying) discharge (8).Thus, it is generally desirable to maximize the power dissipated for a given system. However, helium was not used for a number of reasons. The discharge stability, as noted by voltage fluctuations, is much worse when using helium. Aside from being quite expensive, the high flow rate of helium required to sustain rotation contributes to a relatively low excitation temperature in the plume as is shown later. Rotation of t h e A r c Discharge a n d Atomization Efficiency. When rotating, the arc discharge exhibited by this device assumes a conical shape as depicted in Figure 1. The discharge burns between the upper inside edge of the 7.5mm diameter recessed portion of the graphite anode and the pointed thoriated tungsten cathode. Rotation of the discharge improves short and long term stability and facili-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2083
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tion rate. mum tangential flow rate of 20 l./min is necessary to sustain arc rotation when using helium. The high flow rate is apparently necessary due to the relatively low density of helium. Consequently, helium is not a good tangential flow gas for this plasma jet configuration. In order to maximize the arc rotation rate, the experimental results indicate the tangential gas flow rate should be increased and the sample gas flow rate should be decreased. The latter criterion is met by desolvating the aerosol and using a sprayer that operates on a low gas flow. An ultrasonic nebulizer meets this requirement particularly well. The depressant effect of phosphate on the intensity of calcium emission at the 4226.7-A line is used as a qualitative measure of atomization efficiency. Figure 5 shows the effect of increasing phosphate concentration on the intensity of calcium emission for a nondesolvated aerosol and an estimated arc rotation rate of 19300 rpm. There is a 32% reduction in the calcium emission intensity for a P/Ca molar ratio of 1.0. This signal depression at such a high temperature apparently indicates inability of the analyte to penetrate the plasma discharge. This may seem anomalous considering the relatively fast rotation rate of the arc discharge. However, if the analyte residence time in the discharge is estimated, the plasma may not appear a cone to the injected analyte. Expressions by Kirkbright and Ward (10) were used to estimate the analyte residence time in the arc discharge. When a total sample gas flow rate of 1.36 l./min, a column temperature of 10000 K, and a 1.3-cm interelectrode spacing are used, the residence time in the discharge region is 0.34 msec. Thus, for a rotation rate of 19300 rpm, the analyte would experience only 0.1 rotation as it passes through the 1.3-cm arc discharge. However, when the sample aerosol is desolvated, and a higher tangential flow rate is used, there is no depression in the intensity of calcium emission. For these conditions, the arc rotation rate was 37000 rpm. The absence of an interference effect is apparently due to improved interaction of the aerosol with the rotating arc column. A residence time of 0.92 msec may be calculated for a sample gas flow rate of 0.54 l./min, temperature of the entering aerosol of 302 K, and an interelectrode separation of 1.3 cm. Thus an analyte particle experiences approximately 0.8 arc rotation while passing through the arc discharge. These calculations are perhaps a “worst case” estimate since the temperature experienced by the analyte is considerably lower than 10000 K at the central base of the cone and approaches this value only as the analyte moves toward the cathode. As the tangential flow rate is increased, resulting in an increase in the arc rotation rate, the atomization efficiency is improved. However, the increase in argon flow rate decreases the concentration of analyte in the plume and may
ANALYTICAL CHEMISTRY, VOL.47, NO. 13, NOVEMBER 1975
\I 2
0
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Figure 6. Excitation temperature in the plume for a wet aerosol
as a function of
height
result in a decrease in plume temperature due to the larger volume of gas that is heated in a given time. Both of these factors would reduce analyte emission intensity. Thus, the signal intensity depends on several related variables. Results indicate the desirability of increasing the arc rotation rate by decreasing the sample gas flow rate rather than increasing the tangential gas flow rate. Excitation Temperature in the Plume. Since the atom reservoir is used to thermally excite the analyte in atomic emission spectrometry, the excitation temperature is of considerable importance. Signal intensity, freedom from interferences, and, in general, the analytical utility are related to the excitation temperature. Consequently, the excitation temperature has been measured in the plume as a function of optical position, arc current, the type of inert gas used, diameter of the exit orifice a t the cathode, and desolvation of the sample aerosol. Most measurements of excitation temperature were made using the relative intensity of 10 Fe(1) lines in the spectral region of 3719.5 to 3763.8 8, ( 1 1 ) . Excitation temperature measurements were also performed for comparison using six Ar(1) lines (12, 13). Fe excitation temperature measurements were made while aspirating 500 pg/ml Fe solution and scanning the spectral region at 12.5 A/min. Ar excitation temperature measurements were made while aspiratin deionized water and scanning the six emission lines a t 5 /min. The peak height of each emission line, Fe(1) or Ar(I), was used to measure the relative intensity. A linear least squares program was employed for both methods of temperature measurements. The excitation temperature was then calculated from the slope of the “best fit” line. A calibrated tungsten filament lamp (Eppley Laboratory Model No. EPT-1109) operated a t 35 A was used to correct for variation in monochromator efficiency and photomultiplier response for the six argon emission lines used in excitation temperature determinations. Figure 6 shows a plot of excitation temperature in the plume as a function of distance above the exit orifice for a nondesolvated aerosol. Figure 7 represents the same measurement using a desolvated aerosol. Both measurements are made for 1-mm plume segments. Argon is used for both aerosol introduction and the tangential gas flow. A 5-mm diameter exit orifice was used in all experiments unless specified otherwise. The maximum excitation temperature is significantly higher for the desolvated sample aerosol with an available temperature of slightly greater than 5000 K. Excitation temperature was also measured as a function of optical path using helium as a tangential flow gas. The 20 l./min flow rate necessary to induce rotation resulted in
1
Figure 7. Excitation temperature in the plume as a function of height for a desolvated aerosol
w 3 4 6
7
8 9 IO 11 12 ARC CURRENT & M E R E S )
Flgure 8. Excitation temperature in the plume as current
13
a function of arc
an excitation temperature of 3840 K at a 2-mm distance above the exit orifice. The power dissipated using helium is almost 700 watts. For an argon flow rate of 6.4 l./min, only 370 watts are dissipated with a resultant excitation temperature of 3800 K a t the same optical position. A number of factors contribute to these results. Among these is the necessity of heating a much larger volume of gas when using helium. This would reduce excitation temperature in the plume. In addition many of the physical properties of the two gases are quite different. The thermal conductivity of helium is significantly higher than that of argon. Radiation losses are apparently greater for argon than helium as qualitatively noted by the brighter plasma when using argon. Moreover, the signal intensity decreases when helium is used as a tangential flow gas. The plume or viewing region is apparently affected by a change in the total gas flow rate. As the gas flow rate is increased, the amount of time spent in the plume for a particular volume of gas, and hence the analyte, correspondingly decreases. Thus, when helium is used as the tangential flow gas, the concentration of analyte in the viewing region is reduced because of the significantly higher flow rate. Figure 8 shows a plot of excitation temperature as a function of arc current for a desolvated sample aerosol. The observation path is 4 mm above the exit orifice viewing a 1-mm segment of the plume. There is an increase in excitation temperature with arc current as previously observed (8). This increase in excitation temperature in the plasma plume with current occurs because of the increase in power dissipation for a constant flow rate of inert gas. Excitation temperature in the plume was also measured as a function of the diameter of the exit orifice a t the cathode. When a 4-mm diameter exit orifice is used, the maximum excitation temperature and slope of the temperature
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
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Table 11. Detection Limits Spectral ltne m A
Element
Al Cd
3961.5 2288.0" 3578.7 4379.2
Cr V a Second order.
Excitation
Detection
potential, eV l i m i t . f i q l r n l
3.1 5.4 3.5 3.1
0.02 2 .o 0.07 0.10
IKC~/IH 0
2
10 2.6 1.9 1.5
11
vs. distance plot is virtually identical to the result obtained using a 5-mm diameter exit orifice. The same tangential and sample gas flow rates were used for both measurements. However, when a 6.5-mm diameter exit orifice is used, a higher tangential flow rate is necessary to sustain rotation. The maximum plume temperature available was 3100 K. A 3-mm diameter exit orifice was also investigated, but the close proximity of the discharge resulted in melting of the ceramic insert. These results indicate that the excitation temperature in the plume depends chiefly on the volume of gas that is heated in a given time. A thermal pinch effect is apparently not a significant factor in determining excitation temperature in the plume of this device. The excitation temperature was determined at a 3-mm height above the exit orifice using six Ar(1) emission lines. Deionized water was aspirated into the desolvation apparatus, and a 1-mm segment of the plume was observed. The excitation temperature is 4477 K, a value in good agreement with the 4520 K temperature obtained using Fe emission lines at the same viewing region. The similarity of the excitation temperatures for both methods of measurement indicates a similar distribution of argon and iron atoms in the plume. In conclusion, the excitation temperature in the plume can readily be varied by changing the viewing region or the arc current. Aerosol desolvation permits more efficient utilization of power for analyte excitation with a resultant increase in excitation temperature. Analytical Utility. Although the present study is primarily directed to the measurement of fundamental properties of a plasma jet, the long range goal is the development of an analytically useful device. Thus, the detection limits for several elements were measured as shown in Table 11. All solutions were made in 0.1M potassium chloride to suppress ionization. The enhancement in signal intensity for each element in 0.1M potassium chloride rather than deionized water is given as the enhancement factor I K C ~ / I HThe ~ ~detection . limit is defined as the concentration giving a signal-to-noise ratio of 2 where the noise is the standard deviation of five consecutive signals ( 1 4 ) . Signals were integrated for 30 seconds. Little individual optimization was performed, and all elements were investigated under identical conditions. A 2:l magnification of the source was used such that a 10-mm slit height selected a 5-mm plume segment. The background spectrum of the device was obtained while aspirating deionized water into the desolvation apparatus and scanning from 2500 to 6000 A as shown in Figure 9. There are numerous argon, tungsten, and thorium emission lines due to the relatively high excitation temperature of the plume. CONCLUSIONS In this work, a plasma jet device was examined which overcomes many limitations of previous designs. Rotation of the arc discharge and use of a durable cathode enhance short and long term stability. Rotation of the arc discharge is used to minimize a previously noted interference effect, 2086
I
I
Figure 9. Background spectral scan of the plume (2500-6000
A)
the depressant effect of phosphate on calcium emission. A maximum excitation temperature in the plume of over 5000 K is available. The discharge rotation rate depends on the tangential and sample gas flow rates. The plasma-aerosol interaction, and, consequently, the minimization of interference effects, is enhanced by increasing the discharge rotation rate. Although increasing the tangential flow rate increases arc rotation rate, a compromise of conditions is necessary to optimize signal intensity. Excitation temperature in the plume increases with current and is maximized by desolvating the aerosol. The diameter of the exit orifice affects excitation temperature, primarily as a consequence of the tangential argon flow needed to sustain rotation. Detection limits, although not particularly impressive, were obtained with one set of operating conditions and little optimization. The ability to operate for several hours without attention enables this device to be used with the same ease as a chemical combustion flame. Stability, the ability to minimize interference effects, and the versatile excitation conditions indicate this is an analytically useful configuration. ACKNOWLEDGMENT The authors acknowledge the assistance of Lou Owen provided through stimulating conversations and the loan of an ultrasonic nebulizer at a critical juncture in this study. LITERATURE CITED (1) M. Maraoshes and B. F. Scribner. J. Res. Nat. Bur. Std.. Sect. A. 67A. 561 ( 1 6 3 ) . L. E. Owen, Appl. Spectrosc., 15, 150 (1961). L. E. Owen, "Developments in Applied Spectroscopy," Vol. 1, Plenum, New York, 1962, p 143. E. F. Scribner and M. Margoshes, Proc. 9th Colloq. Spectrosc. Intern., Lyons, 7967, Vol. 2, Groupement pour I'Avancement des Methodes Spectrographiques, Paris, 1962. p 309. G. F. Chapman, L. S. Dale, and R. N. Whittem, Analyst (London), 98 529 (1973). D. A. Murdick and E. H. Piepmeier, Anal. Cbem., 46, 678 (1974). P. Merchant, Jr.. and C. Veillon, Anal. Chlm. Acta, 70, 17 (1974). W. E. Rippetoe. E. R. Johnson, and T. J. Vickers, Anal. Chem., 47, 436 (1975). T. C. O'Haver and J. D. Winefordner. J. Chem. Educ., 46, 241 (1969). A. F. Kirkbright and A. F. Ward, Talanta, 21, 1145 (1974). L. De Galan and J. D. Winefordner, J. Quant. Spectrosc. Radiat. Transfer, 7, 703 (1967). K. W. Busch, Ph.D. Dissertation,Florida State University, 1971. 6. D. Adcock and W. E. G. Plumtree, J. Quant. Spectrosc. Radiat. Transfer. 4. 29 (1964). J. D. Winefbrdner, M.'L. Parsons, J. M. Mansfield, and W. J. McCarthy, Anal. Cbem.. 39,436 (1967).
RECEIVEDfor review June 13, 1975. Accepted August 12, 1975. Work supported in part by funds from PHS Grant GM 15996.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
