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Characterization of a d.c. arc plasma jet in argon as an atomization and excitation source for atomic spectroscopy
AwI~ticcc CllirtIicci Acrcr, 70 (1974) 17-24 ‘C;Elscvier Scientific Publishing Company.
CHARACTERIZATION ATOMIZATION AND SCOPY
PHILlP
MERCHANT,
Deporrrmwl
(Rcccivcd
II_/’C’iwrhw);
5th October
17 Amsterdam
OF A D.C. EXCITATION
JR* and CLAUDE Utliwrsiry
- Printed
in The Netherlands
ARC PLASMA JET IN ARGON AS ANSOURCE FOR ATOMIC SPECTRO-
VEILLON
of Houstotl.
Hoctstm.
Texas
77004 ( U.S.A.)
1973)
and excitation sources Renewed interest in d.c. arc plasmas as atomization has occurred in recent years. The high plasma temperatures and inert atmosphere offer important advantages for emission spectrometry, such as efftcient atomization, a high degree of excitation. reduced compound formation, and decreased quenching of excited species by molecules. In addition, aqueous solutions can be handled directly, although it is frequently desirable to desolvate the sample aerosol before introduction into the plasma device in order to improve the stability of the ‘discharge. Most plasma devices suffer from two limitations: the high background emission from the plasma necessitates the use of a high-dispersion monochromator, and the problem of getting most of the sample into the high-temperature plasma ,region is difficult. The first, of course, applies to any emission technique where high background emission is present, while the second appears to be a problem with most plasmas having a we!! defined high-temperature boundary which the sample aerosol particles must penetrate. If an arc plasma is forced to pass through an orilice, the outer layers of the plasma near the orifice wall are cooled. This cooling decreases ionization and causes the cross-sectional area of the plasma to decrease, increasing the current density (and effective temperature) within the arc. With the d-c. arc plasma jet, this thermal pinch effect can increase the plasma temperature at relatively low overall arc current, which simplifies the power supply required. Practical applications of these devices for spectrochemical analysis have been described by Margoshes and Scribneria2 and Korolev and Vainshetin3. Owen* modified the Margoshes and Scribner design for improved stability. More recently, Valente and Schrenk’ described a novel design for a plasma jet and gave relative emission intensity and detection limit data for several elements. Marinkovic and Vickers’ described a long-path stabilized d.c. arc device and reported radial atomic distributions and detection limits for several refractory elements. Elliott’ described the design and performance of a commercially available low-current plasma jet, and reported temperatures in the plasma core and the flame-like region above the horizontal cathode. The purpose of this investigation was to characterize the d.c. arc plasma jet, or rather one version of it, as completely as possible. The high apparent * Present
address:
Exxon
Chemical
Co., Houston,
Texas
77029.
P. MERCHANT,
18
C. VEILLON
temperatures reported in the literature for these devices and the inert atmosphere used suggest that the atomization and excitation efficiency would make this an ideal emission source for spectrochemical analysis, especially for refractory elements which are not easily determined in chemical flames. Many of the parameters investigated in this study. ‘such as the effect of gas flow rate, ionization. the effect of arc current. electron density, etc., gave results that were predictable or expected. Two studies, namely of apparent temperatures and relative atomic concentrations, gave results that were somewhat surprising. A sharp drop in apparent temperature in the plasma tail-flame region above the cathode was observed, and it. appears that only a relatively small fraction of the sample aerosol particles penetrates the plasma boundary and undergo atomization. EXPERIMENTAL
The main components of the plasma jet are shown schematically in Fig. 1. The cross-hatched components are the graphite anode, cathode and orifice ring. The dark portion is a phenolic plastic (Bakelite) insulator, and the remainder is constructed of brass. The water-cooled anode and cathode are made from 6.35 and 3.18-mm o.d. graphite rod, respectively, while the orifice ring is made from 12.7-mm o.d. graphite rod drilled to form a 6.35-mm i.d. orifice. Argon and deso1vate.d sample aerosol enter the electrically isolated chamber tangentially and exit through the orifice. The plasma was initiated and maintained by a O-15 A, d.c. arc power supply (Model 3020. Tomorrow Enterprises, Portsmouth, Ohio). The cathode center was 10 mm above the top of the orifice ring and the tip of the cathode was about 3 mm from the anode ‘axis. A vertical plasma column of about 2 mm the cathode, o.d. extended above the orifice ring, then sharply bent toward forming a very low intensity “flame” extending vertically above the bend. ’
CATHOOE
ARGON
SAh.E Hz0
IN
-
Fig. 1. Schematic
diagram
of the d.c. arc plasma jet.
An unmagnified image of the plasma was focused onto the entrance slit of the monochromator by a 50-mm diameter, 200-mm focal length fused silica lens. Entrance and exit slit width was 0.020 mm, and to localize the region viewed, the entrance slit height was stepped down to 1 mm. The monochromator
A D.C. ARC PLASMA
JET
19
used was a Czerney-Turner type. with 250-mm focal length mirrors and an J;/4 grating blazed for 300.0 aperture. A 1200 grooves mm- ‘. 52 x 52 mm ruled-area nm was employed, resulting in a reciprocal linear dispersion of 3.2 nm mm-‘. An EMI 9783B photomultiplier was employed, and the signal was amplified by a phase-sensitive amplifier tuned to 330 Hz (Mode1 120, Princeton Applied Research Cbrporation, Princeton. New Jersey). A mechanical chopper modulated the radiation from the plasma jet (emission measurements) or the external source (absorption measurements) at 330 Hz. and also generated the reference signal for the amplifier with the circuit described previously M.Output from the synchronous amplifier was displayed on a 25-cm strip-chart recorder. Plasmas generally tend to become unstable when appreciable quantities of water are introduced’ into them. such as in the form of aqueous sample aerosol particles. To minimize this effect, a desolvation system was used to introduce the sample as dry aerosol particles. This system consisted of a pneumatic nebulizer spraying into a heated chamber and followed by a condenser. and has been described previously 9. The nebulizer was operated at an argon pressure of 2.8 kg and a sample solution cm- 2, which resulted in a gas flow rate of 3 I min-’ The -measured overall efficiency of this sample uptake rate of 1.9 ml mill-‘. introduction system was 33%. so that the net rate of sample introduction into the plasma jet device was 0.63 ml min - ‘. No OH band emission was observed from the plasma. In the characterization of this d.c. arc plasma jet, various parameters were investigated as a function of the region of the plasma viewed. Unless otherwise specified, an arc current of 9A was used. The optical arrangement described earlier allowed measurements from small volumes of the plasma which corresponded to areas of about 0.02 x I mm and horizontally as thick as the plasma. In order to observe these various regions, the entire jet assembly was displaced horizontally and vertically by an adjustable mount. In addition to apparent temperature measurements as a function of plasma region and argon flow rate, similar measurements were made for the degree of ionization, electron density and relative line intensities. Measurements of atomic emission and absorption were also made. RESULTS
AND DISCUSSION
Reif et al.’ ’ have critically reviewed spectroscopic temperature measurements and have shown that the values obtained depend on the method employed, the energy states used, temperature gradients and the concentration distribution correspond of the thermometric species. They found that the values observed to an apparent temperature and not to an average temperature within the measured volume. Significant errors and uncertainties are possible, depending on the method used and the source of data for the various parameters needed in the temperature calculation. As discussed by Reif et al. lo , different apparent temperatures would be measured when lines arising from different electronic levels are used.
P. MERCHANT.
20
C. VEILLON
Therefore, for present purposes, the temperatures shown are apparent temperatures measured by the methods described below and they may not be directly comparable to temperatures measured by others in plasmas. In order to determine the apparent excitation temperature in various plasma regions, the line-pair, intensity-ratio method’ 1 was employed. The argon 415.86 and 425.94 nm atomic lines, and the 434.8 1, 437.97 and 457.94 nm argon ion lines were used. Statistical weights and transition probabilities for these lines were obtained from Olson12. When the effective temperature along the plasma axis was measured vertically from the orifice, the results shown in Fig. 2 were obtained, where the data points represent the average values of the apparent temperatures calculated from the relative intensity measurements of the argon atomic and ion lines. The 415.86 and 425.94 nm atomic line pair gave one set of values for apparent temperature. and the ion line combinations 434.8 l/437.97, 434.8 l/457.94 and 437.97/457.94 nm,’ gave three sets of apparent temperatures. The relative intensities at each wavelength were corrected for differences in monochromator transmission and detector response. The four apparent temperatures thus obtained agreed within about 300 K and the averages were used for the data represented in Fig. 2.
IUOUO
HEIGHT
Fig. 2. Excitation
temperature
distribution
IN
PLASM.
in the plasma
rim
at 9 A and 3 I min-’
argon
flow.
One observes in Fig. 2 that the apparent temperature is quite high near the oritice ring, perhaps owing to a higher current density, then becomes relatively constant up to the cathode position at 10 mm, then rapidly decreases above this point in the “flame” above the cathode. A schematic representation of the actual appearance of the plasma, its bending toward the cathode and the “flame” above this region is illustrated in Fig. 3. This very rapid drop in apparent temperature of about 5000 K immediately above the cathode region is in sharp contrast
A D.C.
ARC
PLASMA
-
Fig. 3. Illustration the cathodc.
21
JET
of the
2100 2150 2200
OK OK OK
8300
OK
8700
OK
8870
OK
plastm;~ cow
bcridiny
toward
the c;~thodo.
and
tho Il~~nuAikc
rcginn
Ltbovc
with the temperatures reported by Elliott’ for ;I similar plasma device. Our apparent temperatures in the plasma agree quite well with those of Elliott. yet in the flame-like region above the cathode Elliott reports a much slower temperature drop with vertical distance. decreasing to a temperature of about 4000 K well above the cathode region. Because the conditions and dimensions used in both devices are similar. these vastly different apparent temperatures in the region above the cathode must be due to the different electrode sizes. materials and gas sheathing of the electrodes. and to differences in the apparent temperature measurement procedures in these two cases. The work of Reif er LIP.‘” would indicate that the latter reason is probably the most significant in the differences observed. Elliott’s data were based on measurements of Ti. V. Cd and Fe lines, and it was indicated that the plasma was not in local thermal equilibrium near well with those the cathode. While the data presented in Fig. 2 agree reasonably of de Galan * 3 for plasmas in air. an increase in temperature near the cathode was not observed, presumably because of increased radial diffusion in the bent arc 6 . In the cathode region (10 mm), the apparent excitation temperature was also measured by using the relative intensities (corrected for wavelength difference) of the 610.4 and 670.7-nm lithium atomic lines. transition probabilities from Corlissand Bozman16 and statistical weights obtained from data given by Moore”. The resulting apparent temperature was almost 1000 K higher than the 7400 K measured when the argon lines were used, the discrepancy probably being due in part to the different sources of data and different atmospheres used. Reif et al . lo have shown that differences of this magnitude ( 13’?$ in this case) can easily occur and in many cases are very much larger. Above the cathode region, the apparent temperatures measured with the lithium atomic lines were within 200 K of those obtained with argon. while in the plasma region below the cathode reliable relative intensity measurements of the lithium lines could not be made owing to the intense plasma background emission in this wavelength region. At a height above the orifice of 8 mm. the apparent plasma temperature as a function of argon flow rate is shown in Fig. 4. Up to a flow rate of the apparent temperature is constant, then decreases at higher about 3 1 min-I, flows. Presumably this is due to a simple cooling effect at the high flow rates, as
I’. MERCHANT.
22 10.000
.?
C. VEILLON
c
l3aoo
ki z s E r E: s t ii
6000
4000
2000
I
I
I
1
1
2
3 APMN
FLOW
Fig. 4. El’kct or gas flow rate on excitation
RATE,
I
I
4
5
1 /mln
tcmpcruture
in the plasma
(8mm).
is also evidenced by the very similar curve obtained for the degree of ionization as a function of flow rate shown in Fig. 5 for the 10 mm region near the cathode. The degree of ionization was calculated from the Saha equation” and the relative intensities of the 425.94-nm argon atomic line. the 434.8 I-nm argon ion line, statistical weights and transition probabilities from Olson”, and by assuming the ion and atomic partition functions to be approximately equal. The relative intensity of the 425.94-nm argon atomic line is directly proportional to the arc curr’ent for currents above 7 A. The same effect was observed for temperature. electron density and degree of ionization as a function of arc current.
ARGON
FLOU
Fig. 5. Effect of gas flow rute on ionization
RATE.
l/min
in the plasma.
A DC.
ARC PLASMA
JET
23
One of the limitations cited earlier for the plasma jet is the high background emission from the high temperature plasma region, which makes a high-dispersion monochromator necessary to achieve good line-to-background ratios in emission measurements. For example, Valente and Scbre-nk5 used a 3.4-m Ebert monochromator having a reciprocal linear dispersion of 0.25 nm mm- *, Marinkovic and Vickers’ used a l-m Czerney-Turner monochromator in the second order (0.4 nm mm-‘), and Elliott’ used an extremely high-dispersion echelle spectrometer’ (CU. 0.05 nm mm - ‘). The low-dispersion monochromator employed in this investigation resulted in the best line-to-background ratios being obtained in the flame-like region above the cathode. but the relatively ,low apparent temperatures observed in this region resulted in relatively low sensitivity (and high detection limits in this case) by atomic emission. The minimum detectable concentrations (based on S/N= 2) obtained for several elements by atomic emission are shown in Table I. TABLE
I
DETECTION (Ellkctivc
LIMITS
tcmpcraturc
Etuissiori
---.
Al All B Bi CU In Li MO
AT 12-mm HEIGHT
396.2 267.6 249.7 306.8 324.7 303.9 670.8 390.3
IN PLASMA
2200 K)
Ahwpriori ---_----_-
3.8 2.2 4.1 1.5
1.o 1.2 0.5 3.3
-.
2. I 1.0 2. I 1.5 0.3 0.5 0.6 0.9
Even with the relatively low apparent temperatures in the region above the cathode, one would at first expect that the sample atomic concentration might be appreciable, owing to the atomization capabilities of the plasma region, the efficiency of the sample introduction system used and the inert atmosphere, which would result in good analytical sensitivity by atomic absorption spectrometry. Determinations of these same eight elements by atomic absorption (Table I) r&ulted in detection limits comparable or slightly better than those in emission, indicating that perhaps very little of the sample is present in this region. Similar absorption measurements within the plasma core (at 2 and S mm heights) supported this observation. Apparently, only a small fraction of the dry sample aerosol particles penetrates the plasma boundary and experiences the high temperatures therein. While these results are not conclusive, especially since absolute atomic concentrations are very difficult to measure, they support similar observations and conclusions by Marinkovic and Vicker@ and Dickinson and Fassel16. It would
P. MERCHANT.
24
C. VEILLON
then appear that the very good detection limits reported by others5-’ are largely and not necessarily to the high due to the use of high-dispersion spectrometers. overall atomization efficiency of these devices. An intense background emission does not IX’,’ SCJdegrade the S/N ratio and thus the detection limit. However. when ;I weak line emission is superimposed on an intense continuum. the analytical . . * sensltlvlty is a function of the dispersion of the rnonochromator. In this case. the lower sensitivity will frequently result in poorer detection limits as well. Atomic absorption measurements outside. but adjacent to. the plasma core did not give measurable ubsorpGon signals, indicating that any dry sample aerosol particles not penetrating the plasma boundary are not atomized to any appreciable extent. Clearly. further study is needed on these devices to determine the fraction of sample actually entering the plasma and undergoing atomization. and to establish means of inwcasing the atomic conccntrution within the plasma and region above the cathode.
in part
The work was by the Robert
supported A. Welch
in part by the Foundation.
National
Science
Foundation
and
SUMMARY
A low-current d.c. arc plasma jet in argon is churacterizcd. Apparent temperatures observed in the plasma flame region are considerably lower than those reported previously by other workers with similar devices. It appears that much of the sample aerosol does not enter the high-temperature plasma region.
REFERENCES I M. Margoshcs tend B. I-. Scribncr. Sp~~~.~rcd~iru. clc-/cc. I5 ( 1959) 138. 2 M. Margoshcs and B. F. Scribncr. .I. Rrs. Nctr. hr. Srtrml.. 67A ( 1063) Ml. 3 V. V. Korolcv and E. E. Vninshtcin. J . rlrrctl. Chcr,,. USSR. I5 ( 1959) 73 I. 4 L. E. Own. ..I/‘/I/. Spvrtv1s.. I5 ( I96 I ) 150. 5 6 7 8 9 IO II
S. E. Valcnw mcf W. G. Schrcnk. .dpp/_ Spvr,os.. 7-I (1970) 197. M. Marinkovic and T. J. Vickcrs. .-lpp/. Sp~~cvws.. 2.5 ( 1971 ) 3 19. W. G. Elliott. Armvkwrr Ldwrctror~~. August. I97 I. p. 45. C. Vcillon and M. Margoshcs. Spc*r,ochiru. Acvcr. 2.7B ( 1068) 50.3. C. Vcillon and M. Maryoshcs. Spwrrodliul. Auu. 2.3B ( 196X) 553. I. Rcif. V. A. Fasscl and R. N. Knisclcy. Sp~wrwdrir~~. Ar’ft/. 288 ( 1973) 105. P. W. J. M. l3oumans. Tlwwy f!/’ Sprc,troclrcrIri~.tr/ Esc.ifcctio/l. PICIIUIII Press. p. 106.
W~ishinglon. DC.. 1962. p. ISO. I5 C. E. Moore. N. 13. S. Ciwdtw No. 467. Vol. I. 1949, p. 20. I6 G. W. Dickinson mid V. A. Fasscl. Amrl. Clrcw.. 41 (1969) 1021.
New
York.
1966.
CHARACTERIZATION ATOMIZATION AND SCOPY
PHILlP
MERCHANT,
Deporrrmwl
(Rcccivcd
II_/’C’iwrhw);
5th October
17 Amsterdam
OF A D.C. EXCITATION
JR* and CLAUDE Utliwrsiry
- Printed
in The Netherlands
ARC PLASMA JET IN ARGON AS ANSOURCE FOR ATOMIC SPECTRO-
VEILLON
of Houstotl.
Hoctstm.
Texas
77004 ( U.S.A.)
1973)
and excitation sources Renewed interest in d.c. arc plasmas as atomization has occurred in recent years. The high plasma temperatures and inert atmosphere offer important advantages for emission spectrometry, such as efftcient atomization, a high degree of excitation. reduced compound formation, and decreased quenching of excited species by molecules. In addition, aqueous solutions can be handled directly, although it is frequently desirable to desolvate the sample aerosol before introduction into the plasma device in order to improve the stability of the ‘discharge. Most plasma devices suffer from two limitations: the high background emission from the plasma necessitates the use of a high-dispersion monochromator, and the problem of getting most of the sample into the high-temperature plasma ,region is difficult. The first, of course, applies to any emission technique where high background emission is present, while the second appears to be a problem with most plasmas having a we!! defined high-temperature boundary which the sample aerosol particles must penetrate. If an arc plasma is forced to pass through an orilice, the outer layers of the plasma near the orifice wall are cooled. This cooling decreases ionization and causes the cross-sectional area of the plasma to decrease, increasing the current density (and effective temperature) within the arc. With the d-c. arc plasma jet, this thermal pinch effect can increase the plasma temperature at relatively low overall arc current, which simplifies the power supply required. Practical applications of these devices for spectrochemical analysis have been described by Margoshes and Scribneria2 and Korolev and Vainshetin3. Owen* modified the Margoshes and Scribner design for improved stability. More recently, Valente and Schrenk’ described a novel design for a plasma jet and gave relative emission intensity and detection limit data for several elements. Marinkovic and Vickers’ described a long-path stabilized d.c. arc device and reported radial atomic distributions and detection limits for several refractory elements. Elliott’ described the design and performance of a commercially available low-current plasma jet, and reported temperatures in the plasma core and the flame-like region above the horizontal cathode. The purpose of this investigation was to characterize the d.c. arc plasma jet, or rather one version of it, as completely as possible. The high apparent * Present
address:
Exxon
Chemical
Co., Houston,
Texas
77029.
P. MERCHANT,
18
C. VEILLON
temperatures reported in the literature for these devices and the inert atmosphere used suggest that the atomization and excitation efficiency would make this an ideal emission source for spectrochemical analysis, especially for refractory elements which are not easily determined in chemical flames. Many of the parameters investigated in this study. ‘such as the effect of gas flow rate, ionization. the effect of arc current. electron density, etc., gave results that were predictable or expected. Two studies, namely of apparent temperatures and relative atomic concentrations, gave results that were somewhat surprising. A sharp drop in apparent temperature in the plasma tail-flame region above the cathode was observed, and it. appears that only a relatively small fraction of the sample aerosol particles penetrates the plasma boundary and undergo atomization. EXPERIMENTAL
The main components of the plasma jet are shown schematically in Fig. 1. The cross-hatched components are the graphite anode, cathode and orifice ring. The dark portion is a phenolic plastic (Bakelite) insulator, and the remainder is constructed of brass. The water-cooled anode and cathode are made from 6.35 and 3.18-mm o.d. graphite rod, respectively, while the orifice ring is made from 12.7-mm o.d. graphite rod drilled to form a 6.35-mm i.d. orifice. Argon and deso1vate.d sample aerosol enter the electrically isolated chamber tangentially and exit through the orifice. The plasma was initiated and maintained by a O-15 A, d.c. arc power supply (Model 3020. Tomorrow Enterprises, Portsmouth, Ohio). The cathode center was 10 mm above the top of the orifice ring and the tip of the cathode was about 3 mm from the anode ‘axis. A vertical plasma column of about 2 mm the cathode, o.d. extended above the orifice ring, then sharply bent toward forming a very low intensity “flame” extending vertically above the bend. ’
CATHOOE
ARGON
SAh.E Hz0
IN
-
Fig. 1. Schematic
diagram
of the d.c. arc plasma jet.
An unmagnified image of the plasma was focused onto the entrance slit of the monochromator by a 50-mm diameter, 200-mm focal length fused silica lens. Entrance and exit slit width was 0.020 mm, and to localize the region viewed, the entrance slit height was stepped down to 1 mm. The monochromator
A D.C. ARC PLASMA
JET
19
used was a Czerney-Turner type. with 250-mm focal length mirrors and an J;/4 grating blazed for 300.0 aperture. A 1200 grooves mm- ‘. 52 x 52 mm ruled-area nm was employed, resulting in a reciprocal linear dispersion of 3.2 nm mm-‘. An EMI 9783B photomultiplier was employed, and the signal was amplified by a phase-sensitive amplifier tuned to 330 Hz (Mode1 120, Princeton Applied Research Cbrporation, Princeton. New Jersey). A mechanical chopper modulated the radiation from the plasma jet (emission measurements) or the external source (absorption measurements) at 330 Hz. and also generated the reference signal for the amplifier with the circuit described previously M.Output from the synchronous amplifier was displayed on a 25-cm strip-chart recorder. Plasmas generally tend to become unstable when appreciable quantities of water are introduced’ into them. such as in the form of aqueous sample aerosol particles. To minimize this effect, a desolvation system was used to introduce the sample as dry aerosol particles. This system consisted of a pneumatic nebulizer spraying into a heated chamber and followed by a condenser. and has been described previously 9. The nebulizer was operated at an argon pressure of 2.8 kg and a sample solution cm- 2, which resulted in a gas flow rate of 3 I min-’ The -measured overall efficiency of this sample uptake rate of 1.9 ml mill-‘. introduction system was 33%. so that the net rate of sample introduction into the plasma jet device was 0.63 ml min - ‘. No OH band emission was observed from the plasma. In the characterization of this d.c. arc plasma jet, various parameters were investigated as a function of the region of the plasma viewed. Unless otherwise specified, an arc current of 9A was used. The optical arrangement described earlier allowed measurements from small volumes of the plasma which corresponded to areas of about 0.02 x I mm and horizontally as thick as the plasma. In order to observe these various regions, the entire jet assembly was displaced horizontally and vertically by an adjustable mount. In addition to apparent temperature measurements as a function of plasma region and argon flow rate, similar measurements were made for the degree of ionization, electron density and relative line intensities. Measurements of atomic emission and absorption were also made. RESULTS
AND DISCUSSION
Reif et al.’ ’ have critically reviewed spectroscopic temperature measurements and have shown that the values obtained depend on the method employed, the energy states used, temperature gradients and the concentration distribution correspond of the thermometric species. They found that the values observed to an apparent temperature and not to an average temperature within the measured volume. Significant errors and uncertainties are possible, depending on the method used and the source of data for the various parameters needed in the temperature calculation. As discussed by Reif et al. lo , different apparent temperatures would be measured when lines arising from different electronic levels are used.
P. MERCHANT.
20
C. VEILLON
Therefore, for present purposes, the temperatures shown are apparent temperatures measured by the methods described below and they may not be directly comparable to temperatures measured by others in plasmas. In order to determine the apparent excitation temperature in various plasma regions, the line-pair, intensity-ratio method’ 1 was employed. The argon 415.86 and 425.94 nm atomic lines, and the 434.8 1, 437.97 and 457.94 nm argon ion lines were used. Statistical weights and transition probabilities for these lines were obtained from Olson12. When the effective temperature along the plasma axis was measured vertically from the orifice, the results shown in Fig. 2 were obtained, where the data points represent the average values of the apparent temperatures calculated from the relative intensity measurements of the argon atomic and ion lines. The 415.86 and 425.94 nm atomic line pair gave one set of values for apparent temperature. and the ion line combinations 434.8 l/437.97, 434.8 l/457.94 and 437.97/457.94 nm,’ gave three sets of apparent temperatures. The relative intensities at each wavelength were corrected for differences in monochromator transmission and detector response. The four apparent temperatures thus obtained agreed within about 300 K and the averages were used for the data represented in Fig. 2.
IUOUO
HEIGHT
Fig. 2. Excitation
temperature
distribution
IN
PLASM.
in the plasma
rim
at 9 A and 3 I min-’
argon
flow.
One observes in Fig. 2 that the apparent temperature is quite high near the oritice ring, perhaps owing to a higher current density, then becomes relatively constant up to the cathode position at 10 mm, then rapidly decreases above this point in the “flame” above the cathode. A schematic representation of the actual appearance of the plasma, its bending toward the cathode and the “flame” above this region is illustrated in Fig. 3. This very rapid drop in apparent temperature of about 5000 K immediately above the cathode region is in sharp contrast
A D.C.
ARC
PLASMA
-
Fig. 3. Illustration the cathodc.
21
JET
of the
2100 2150 2200
OK OK OK
8300
OK
8700
OK
8870
OK
plastm;~ cow
bcridiny
toward
the c;~thodo.
and
tho Il~~nuAikc
rcginn
Ltbovc
with the temperatures reported by Elliott’ for ;I similar plasma device. Our apparent temperatures in the plasma agree quite well with those of Elliott. yet in the flame-like region above the cathode Elliott reports a much slower temperature drop with vertical distance. decreasing to a temperature of about 4000 K well above the cathode region. Because the conditions and dimensions used in both devices are similar. these vastly different apparent temperatures in the region above the cathode must be due to the different electrode sizes. materials and gas sheathing of the electrodes. and to differences in the apparent temperature measurement procedures in these two cases. The work of Reif er LIP.‘” would indicate that the latter reason is probably the most significant in the differences observed. Elliott’s data were based on measurements of Ti. V. Cd and Fe lines, and it was indicated that the plasma was not in local thermal equilibrium near well with those the cathode. While the data presented in Fig. 2 agree reasonably of de Galan * 3 for plasmas in air. an increase in temperature near the cathode was not observed, presumably because of increased radial diffusion in the bent arc 6 . In the cathode region (10 mm), the apparent excitation temperature was also measured by using the relative intensities (corrected for wavelength difference) of the 610.4 and 670.7-nm lithium atomic lines. transition probabilities from Corlissand Bozman16 and statistical weights obtained from data given by Moore”. The resulting apparent temperature was almost 1000 K higher than the 7400 K measured when the argon lines were used, the discrepancy probably being due in part to the different sources of data and different atmospheres used. Reif et al . lo have shown that differences of this magnitude ( 13’?$ in this case) can easily occur and in many cases are very much larger. Above the cathode region, the apparent temperatures measured with the lithium atomic lines were within 200 K of those obtained with argon. while in the plasma region below the cathode reliable relative intensity measurements of the lithium lines could not be made owing to the intense plasma background emission in this wavelength region. At a height above the orifice of 8 mm. the apparent plasma temperature as a function of argon flow rate is shown in Fig. 4. Up to a flow rate of the apparent temperature is constant, then decreases at higher about 3 1 min-I, flows. Presumably this is due to a simple cooling effect at the high flow rates, as
I’. MERCHANT.
22 10.000
.?
C. VEILLON
c
l3aoo
ki z s E r E: s t ii
6000
4000
2000
I
I
I
1
1
2
3 APMN
FLOW
Fig. 4. El’kct or gas flow rate on excitation
RATE,
I
I
4
5
1 /mln
tcmpcruture
in the plasma
(8mm).
is also evidenced by the very similar curve obtained for the degree of ionization as a function of flow rate shown in Fig. 5 for the 10 mm region near the cathode. The degree of ionization was calculated from the Saha equation” and the relative intensities of the 425.94-nm argon atomic line. the 434.8 I-nm argon ion line, statistical weights and transition probabilities from Olson”, and by assuming the ion and atomic partition functions to be approximately equal. The relative intensity of the 425.94-nm argon atomic line is directly proportional to the arc curr’ent for currents above 7 A. The same effect was observed for temperature. electron density and degree of ionization as a function of arc current.
ARGON
FLOU
Fig. 5. Effect of gas flow rute on ionization
RATE.
l/min
in the plasma.
A DC.
ARC PLASMA
JET
23
One of the limitations cited earlier for the plasma jet is the high background emission from the high temperature plasma region, which makes a high-dispersion monochromator necessary to achieve good line-to-background ratios in emission measurements. For example, Valente and Scbre-nk5 used a 3.4-m Ebert monochromator having a reciprocal linear dispersion of 0.25 nm mm- *, Marinkovic and Vickers’ used a l-m Czerney-Turner monochromator in the second order (0.4 nm mm-‘), and Elliott’ used an extremely high-dispersion echelle spectrometer’ (CU. 0.05 nm mm - ‘). The low-dispersion monochromator employed in this investigation resulted in the best line-to-background ratios being obtained in the flame-like region above the cathode. but the relatively ,low apparent temperatures observed in this region resulted in relatively low sensitivity (and high detection limits in this case) by atomic emission. The minimum detectable concentrations (based on S/N= 2) obtained for several elements by atomic emission are shown in Table I. TABLE
I
DETECTION (Ellkctivc
LIMITS
tcmpcraturc
Etuissiori
---.
Al All B Bi CU In Li MO
AT 12-mm HEIGHT
396.2 267.6 249.7 306.8 324.7 303.9 670.8 390.3
IN PLASMA
2200 K)
Ahwpriori ---_----_-
3.8 2.2 4.1 1.5
1.o 1.2 0.5 3.3
-.
2. I 1.0 2. I 1.5 0.3 0.5 0.6 0.9
Even with the relatively low apparent temperatures in the region above the cathode, one would at first expect that the sample atomic concentration might be appreciable, owing to the atomization capabilities of the plasma region, the efficiency of the sample introduction system used and the inert atmosphere, which would result in good analytical sensitivity by atomic absorption spectrometry. Determinations of these same eight elements by atomic absorption (Table I) r&ulted in detection limits comparable or slightly better than those in emission, indicating that perhaps very little of the sample is present in this region. Similar absorption measurements within the plasma core (at 2 and S mm heights) supported this observation. Apparently, only a small fraction of the dry sample aerosol particles penetrates the plasma boundary and experiences the high temperatures therein. While these results are not conclusive, especially since absolute atomic concentrations are very difficult to measure, they support similar observations and conclusions by Marinkovic and Vicker@ and Dickinson and Fassel16. It would
P. MERCHANT.
24
C. VEILLON
then appear that the very good detection limits reported by others5-’ are largely and not necessarily to the high due to the use of high-dispersion spectrometers. overall atomization efficiency of these devices. An intense background emission does not IX’,’ SCJdegrade the S/N ratio and thus the detection limit. However. when ;I weak line emission is superimposed on an intense continuum. the analytical . . * sensltlvlty is a function of the dispersion of the rnonochromator. In this case. the lower sensitivity will frequently result in poorer detection limits as well. Atomic absorption measurements outside. but adjacent to. the plasma core did not give measurable ubsorpGon signals, indicating that any dry sample aerosol particles not penetrating the plasma boundary are not atomized to any appreciable extent. Clearly. further study is needed on these devices to determine the fraction of sample actually entering the plasma and undergoing atomization. and to establish means of inwcasing the atomic conccntrution within the plasma and region above the cathode.
in part
The work was by the Robert
supported A. Welch
in part by the Foundation.
National
Science
Foundation
and
SUMMARY
A low-current d.c. arc plasma jet in argon is churacterizcd. Apparent temperatures observed in the plasma flame region are considerably lower than those reported previously by other workers with similar devices. It appears that much of the sample aerosol does not enter the high-temperature plasma region.
REFERENCES I M. Margoshcs tend B. I-. Scribncr. Sp~~~.~rcd~iru. clc-/cc. I5 ( 1959) 138. 2 M. Margoshcs and B. F. Scribncr. .I. Rrs. Nctr. hr. Srtrml.. 67A ( 1063) Ml. 3 V. V. Korolcv and E. E. Vninshtcin. J . rlrrctl. Chcr,,. USSR. I5 ( 1959) 73 I. 4 L. E. Own. ..I/‘/I/. Spvrtv1s.. I5 ( I96 I ) 150. 5 6 7 8 9 IO II
S. E. Valcnw mcf W. G. Schrcnk. .dpp/_ Spvr,os.. 7-I (1970) 197. M. Marinkovic and T. J. Vickcrs. .-lpp/. Sp~~cvws.. 2.5 ( 1971 ) 3 19. W. G. Elliott. Armvkwrr Ldwrctror~~. August. I97 I. p. 45. C. Vcillon and M. Margoshcs. Spc*r,ochiru. Acvcr. 2.7B ( 1068) 50.3. C. Vcillon and M. Maryoshcs. Spwrrodliul. Auu. 2.3B ( 196X) 553. I. Rcif. V. A. Fasscl and R. N. Knisclcy. Sp~wrwdrir~~. Ar’ft/. 288 ( 1973) 105. P. W. J. M. l3oumans. Tlwwy f!/’ Sprc,troclrcrIri~.tr/ Esc.ifcctio/l. PICIIUIII Press. p. 106.
W~ishinglon. DC.. 1962. p. ISO. I5 C. E. Moore. N. 13. S. Ciwdtw No. 467. Vol. I. 1949, p. 20. I6 G. W. Dickinson mid V. A. Fasscl. Amrl. Clrcw.. 41 (1969) 1021.
New
York.
1966.