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A microwave induced plasma system for the maintenance of moderate power plasmas of helium, argon, nitrogen and air
Specrrochimicn Acra,Vol.4OB,No.
Printed in GreatBritain.
3,pp.493499,1985.
0584-8547/85 s3.00+.00 Pergamon PressLtd.
A microwave induced plasma system for the maintenance of moderate power plasmas of helium, argon, nitrogen and air KEVING. MICHLEWICZ, JOHN J. URH and JON W. CARNAHAN Department
of Chemistry, (Received
Northern
Illinois University,
DeKalb,
IL 60115, U.S.A.
10 April 1984; in revised form 28 June 1984)
Abstract-A microwave induced plasma system capable of maintaining stable plasmas of each of the gases helium, argon, nitrogen and air is presented. The system is capable of operation at powers of up to 500 W. The TM,,, cavity design is similar to that previously described in the literature with some modifications. A demountable torch facilitates centering of diXuse plasmas of helium, nitrogen and air by providing 6 flows directed tangentially within the quartz tube. This torch was not useful for argon plasmas. Toroidal argon plasmas were maintained with a threaded quartz tube arrangement. The heat generated by these plasmas was dissipated by an outer sheath ofcoolant air. Details of the design and preliminary characterization of each plasma system is presented.
1. INTRODUCTION MICROWAVE induced plasmas (MIP) have been investigated for analytical use since the 1965 application of the low pressure helium and atmospheric pressure argon plasmas [l, 21. However, literature in this area did not begin to proliferate until after 1976 when BEENAKKER introduced a TM,,, microwave resonator design capable of efficient power transfer to the plasma [3]. This design allowed the operation of an atmospheric pressure helium plasma. Subsequently, investigations have proceeded in the areas of analysis of gas chromatographic eluates, nebulized solutions and chemically and thermally vaporized analytes. Several excellent reviews are currently available in the literature [4-91. Recent reports describe a modification of BEENAKKER’S apparatus to maintain argon [lo] and helium [ 1l] plasmas with applied powers of up to 500 W. The Ar and He plasmas were used for the analysis of nebulized solutions and chromatogiaphic elutes respectively. However, with these systems it was not possible to maintain plasmas of alternate gases such as nitrogen or air. A series of talks by DEUTSCH~~~HIEFTJE~~recent F.A.C.S.S. meetings have described the operation and analytical characteristics of a nitrogen MIP [ 121. Power applied by the generator was limited to less than i‘OOW. This limitation was predominately due to thermal degradation of the torch at the irigher temperatures produced at higher powers. This report details the design and operation of an MIP system capable of maintaining stable plasmas of air, nitrogen, helium and argon at powers up to 500 W. This is facilitated by alteration of the internal diameter of the resonator previously described [lo] and the use of an air cooled, tangential flow, demountable torch. Spectral characterizations are presented and analytical implications are discussed.
[1] [2] [3] [4]
[S] [6] [7] [8] [9] ilO] :l 1] .12]
J. MCCORMICK, S. C. TONG and W. D. COOK, And. Chem. 37, 1470 (1965). A. BACHE and D. J. LISK, Anal. Chem. 37, 1477 (1965). I. M. BEENAKKER, Spectrochim. Acta. 31B, 485 (1976). T. ZANDER and G. M. HIEFTJE, Appl. Specrrosc. 35, 357 (1981). R. K. SKOGERBOE and G. N. COLEMAN, Anal. Chem. 48, 611A (1976). P. C. UDEN, Anal. Proc. 18, 189 (1981). T. H. RISBY and Y. TALMI, CRC Crit. Rev. Anal. Chem. 14, 231 (1984). J. W. CARNAHAN, K. J. MULLIGAN and J. A. CARUSO, Anal. Chim. Acta 130, 227 (1981). J. W. CARNAHAN, Am. Lab. 15(8), 31 (1983). D. L. HAAS, J. W. CARNAHAN and J. A. CARUSO, Appl. Spectrosc. 37, 82 (1983). J. W. CARNAHAN, Ph.D. thesis, University of Cincinnati, (1983). R. D. DEUTSCH and G. M. HIEFTJE, Papers presented at Annual Meeting of the Federation of Analytical Chemists and Spectroscopy Societies, Philadelphia, 1981; Papers Nos 222 and 223; 1982, Paper No. 50; 1983, Paper No. 420. A. C. C. A.
WB) 40:3-p
493
494
KEVING. MICHLEWICZ et al. 2. EXPERIMENTAL
2.1. Resonator cavity The cavity design is illustrated in Fig. 1. The design was similar to the cavity described by HAAS et al. [lo] with the following modifications. (1) The cavity diameter was increased to 88.8 mm from the original 86.5 mm value to facilitate the effect of the dielectric constant of nitrogen. Nitrogen plasma ignition and maintenance was impossible without this modification. (2) The torch was inserted into a 13.2-mm hole in the center of the cavity. Initially the hole diameter was 19.2 mm to accommodate a larger torch, however, plasmas could not be ignited with this design. This effect is presumably due to alteration of the microwave field. Placement of a 13.2-mm id. copper insert in the floor and faceplate allowed plasma ignition and maintenance. (3) The coupling loop was constructed of a 3.2-mm dia. copper rod soldered at a right angle to a 6.4mm dia. rod and grounded to the cavity floor. A 12-mm long lo/24 screw passing through the cavity bulk and attached to the 6.4mm rod produced solid contact. This modification allowed ease of cavity disassembly for polishing and repair. Typically, oxidized copper on the surface is removed by polishing the internal dimensions of the cavity with steel wool. This is done only as a preventative measure to ensure good surface conductivity. Fortunately, cavity repair has not been required. 2.2. Torch Two plasma torch designs were utilized. In the plasma region the material of choice was quartz. This material was the most suitable due to the properties of good thermal stability and resistance to cracking. The torches were similar in design and function to those previously used for the inductively coupled plasma (ICP) and the MIP [ 131. 20cm --
Fig. 1. Modified TMolo cavity. [13]
A. BOLLO-KAMARAand E. G. CODDING,Spectrochim. Acta. 36B, 973 (1981).
MIP system for the ~nt~~~~
of stable plasmas
495
The plasma gas was directed tangentially in the plasma region by means of 6 equally spaced (1 mm deep-45”) slots (slotted torch)or a doubly threaded design (threaded torch). The latter torch is similar to that of BOLLO-KAMARA and COWING[13]. The slotted torch is shown in Fig. 2. With the threaded torch the slotted central tube was replaced by a snugly fitting tube which was doubly threaded from the plasma gas inlet to the plasma region. With both designs, the inner tubes were bonded to the plasma cont~nment tube by heating. A second significant change in these torch configurations from previous MIP torches is the provision for a coolant flow to dissipate the heat generated by the plasma. Air could be used for the coolant flow with all plasma gases since it did not come into contact with the plasma. However, this yields significant gas mixing in the afterglow region. Analytical effects of this mixing are currently under investigation. A coolant flow of 12 1min” ’ was found to be more than sufficient in all cases. The helium, nitrogen and air plasmascould be centered with either torch. However, the slotted torch provided better plasma stability under the conditions utilized in these experiments. Apparently a more uniform gas flow was produced with this arrangement. The argon toroidal shaped plasma was considerably more stable with the threaded torch. Details concerning the torch selection for the argon plasma are discussed in Section 3.5. 2.3. Plasma ignition The plasmas were initiated by placing a copper wire (attached to a rub& or quartz handle) in the plasma region and applying a power of 300-400 W. Upon plasma ignition slight adjustment of the tuning stubs to tune the system to 0 W reflected (as read at the generator) was generally required. Retuning was minimal after initial adjustments. Tuning stub settings were found to be similar for the ignition of helium, argon and nitrogen plasmas. All of these plasmas were operated with the tuning stubs set at approximately the same position. Ignition of the air plasma required some adjustment from the optimum settings with the other plasmas.
Teflon
1
’
cm
Fig. 2. Slotted plasma torch.
496
KEVING. MICHLEWICZ ef al. Table 1. Specifications of equipment
Microwave generator Microwave power transmission TMolo cavity Plasma torch Nebulizer/spray chamber Monochromator
Optics Photomultiplier i/V converter-amplifier Chart recorder
Micro-Now (Chicago) Model 420 B. Capable of producing 500 W (operated at l&500 W forward with less than 5 W reflected). 3 ft-FT4-50T coaxial cable with 44 SW end connectors (Andrews Corp., Orland Park, IL). Similar to that described by HAASet al. [lo] with cavity depth of 11.2mm and diameter of 88.8 mm. Detailed in Fig. 1. Similar to ICP torch with provision for separate flows of coolant air, plasma gas and nebulizer gas. See text and Fig. 2 for further details. Meinhard (Santa Ana, CA) C-30/Apphed Research Laboratories (Sunland, CA) upward conical design. 0.35 m EU-700 GCA/McPherson (Acton, MA) scanning monochromator (f/6.4) operated with 50 ym entrance and exit slits, and a 12OO_groovemm-’ grating blazed at 250nm. A Model 700-51 scanner driver was used to scan from 20&6OOnm at 0.5 rims-‘. 1 in., lO-cm focal length lense; with object distance of 25.5 cm; image distance 16.0 cm. RCA lP28 (Lancaster, PA). Variable gain amplifier similar to that described by MCCARTHYef al. [14]. Omniscribe (Austin, TX) Model B-51 17-5 strip chart recorder.
Especially helpful in igniting the air plasma was to supply a spark to the flow of air with a copper wire attached to a tesla coil. Once proper tuning was determined, ignition of the air plasma was routine. 2.4. Instrumentation Specifications of the equipment
utilized are listed in Table 1
3. RESULTS AND DISCUSSION 3.1. General
behavior of plasmas
Although specifics vary with plasmas of particular gases, many aspects of the plasma behavior were similar. With the described system, all plasmas were stable over the periods used to gather the reported data. This time period varied from 2 to 6 h of plasma operation. Although some minor tuning adjustments (moving the attached stubs) were necessary, power fluctuations were generally less than 10 W and reflected powers did not exceed 10 W. In the studies which required variation of the applied power, minor retuning was required. A procedure which was found to yield consistent ease of alteration of this operational parameter was to vary the forward power 2C30 W, retune to 0 W reflected power, and continue this procedure until the desired forward power was obtained. As flow rate did not seem to significantly effect tuning, only minimal retuning was required when altering the plasma gas flow rates by as much as a factor of 2. Within the flow ranges indicated as stable in these studies, the maximum reflected power seen with plasma gas flow adjustment rarely exceeded 20 W. Significant differences were noted in the minimum plasma gas flow rates needed to maintain a stable plasma with and without solution nebulization. Operation of the air and nitrogen plasmas with plasma gas and coolant flows only (no nebulized solution) allowed support gas flows of less than 2 1min- ’ to be utilized while maintaining stable, suspended plasmas at 250 W. This value was 6.6 1min- 1 with a 270-W helium plasma. At flows of less than 9.9 ml min - i, the argon plasma caused the torch to glow red. At lower flow rates, nitrogen and air plasmas became elongated significantly and the plumes extended up to 7 cm beyond the torch. At higher flows, the visible portion of the plasma tended to become much shorter and was restricted to within the cavity to 1 cm beyond the faceplate. Flow rate variations did not significantly affect the size of the helium plasma. The argon plasma retained a constant size at high flows but began to propagate back up the plasma gas introduction tube with lower flows. With all plasmas, a maximum flow rate was seen for which the plasma could be maintained. Above this flow, the plasmas went out. This [14] J. P. MCCARTHY, M. E. JACKSON,T. H. RIDGWAYand J. A. CARUSO53, 1512 (1981).
MIP system
491
for the maintenanceof stableplasmas
behavior appeared to be simply a matter of residence time of the plasma gas species within the microwave field. It appeared that some minimum time was required for support gas excitation. During solution nebulization the minimum necessary support gas flow was found to be significantly greater. Gas flow rates and water uptake rates are tabulated in Table 2. When the air, nitrogen or helium plasma support gas flow was adjusted to below a certain value, the plasma tended to deviate from its solid elipsoidal shape to one in which the axial appearance was no longer circular, but distorted and unstable. At these flows flicker noise inceased significantly and the analytical potential was degraded. This is the flow rate designated as unstable in Fig. 3. For the argon plasmas, the designated flow rate is that at which the torch began to glow due to excess heating. 3.2. Characteristics of the nitrogen plasma The minimum flow rate required for the stable maintenance of the pure nitrogen plasma was approximately 1.5 1min - 1 at 500 W. During solution nebulization the minimum plasma gas flow required to maintain a stable plasma increased to 6.5 1min- 1 at 250 Wand increased slightly with increasing power. This trend is depicted on Fig. 3a. The spectrum of the nitrogen MIP during solution nebulization revealed species of OH, NO, CN, NH, N, and N& Band assignments were obtained from Refs [15-181. At present it Table 2. Nebulizer Nebulizer
gas
conditions*
Gas flow rate (I min - ‘)
Water uptake rate (ml min _ ‘)
1.15 1.00 2.53 1.34
0.34 0.38 0.031 0.45
Air Argon Helium Nitrogen *All nebulizers
operating
operated
at 30 psig.
2 t 01 150
I
I
I
250
350
450
Applied
Fig. 3. Minimum nitrogen plasma, A. Cl51
Foward
Power
500
(watts)
flows for stable plasma maintenance during solution nebulization. (b) suspended air plasma, (c) suspended helium plasma, (d) toroidal
(a) Suspended argon plasma.
N. WRIGHT and C. A. WINKLER, Ache Nitrogen. Academic Press, New York (1968). R. STRIGANOV and N. S. SVENTILSKI, Tables ofSpectral Lines ofNeutrnl and Ionized Atoms. Plenum Press, New York (1968). [I71 K. P. HUBER and G. HERZBERG, Molecular Spectra and Molecular Structure. Van Nostrand Reinhold, New York (1979). Cl81 G. HERZBERG, Spectra ofDiatomicMolecules. Van Nostrand, New York (1950).
Cl61A.
498
KEVIN G. MICHLEWICZet al.
is not clear if the bands of NH and CN arise from the combination of afterglow entrained atmospheric or coolant hydrogen and carbon containing molecules with N2 or if a contribution to these bands arise from impurities in the support gas. Diatomic bands of nitrogen include transitions of the second positive system (second vibrational level of neutral N,) and the first negative system (first vibrational level of N:). The latter species is particularly interesting for its high energy (15.57eV in the ground N: state) and the implications for excitation of elements to these energy levels. N: was seen in both the pure nitrogen plasma and during solution nebulization. 3.3. Characteristics of the air plasma The minimum flow rates required for the stable maintenance of the air plasma was less than 2 1min- ’ at 250 W. During solution nebulization the minimum gas flow required to maintain a stable plasma increased to 10.4 lmin-r at 250 W and to 16.1 lmin-’ at 450 W. This is illustrated at Fig. 3b. As expected, the spectrum of the air plasma is similar to that of the nitrogen spectrum. The only notable difference is that the NO bands are significantly enhanced with the air plasma. 3.4. Characteristics
of the helium plasma
Helium exhibited somewhat different behaviour both physically and spectroscopically as might be expected from the density and the available energies of the gas. Minimum required flow rates for the pure helium system were 7.5 1min - ’ at 250 Wand increased to 13.0 1min - ’ at 440 W. The trend during solution nebulization is depicted in Fig. 3c for a stable helium plasma with a 30 psig drop across the Meinhard C-30 nebulizer. However, the solution uptake rate at this pressure was only 0.03 1 ml min - i. The low uptake rate was due to the low density of helium (ca 0.1 times that of argon). Examination of helium plasma spectra showed the presence of several species whose energy was greater than that of metastable helium (19.7 eV). Also present are intense lines due to hydrogen (486.1 and 433.7nm) from the nebulized water. The presence of the intense emission of the helium and hydrogen species indicated that a great amount of energy may be available for transfer to the analyte during the solution nebulization experiment. Possible excitable elements include metals, metaloids and nonmetals. 3.5. Characteristics of the argon plasma The argon plasma was particularly interesting in terms of plasma formation and characteristics. With the slotted torch the argon plasma extended back from the plasma tube to the area of argon introduction. Plasmas created under these conditions tended to be noisy and unstable. However, use of the threaded torch allowed argon plasma maintenance within the cavity region. With this torch, flow rates similar to those of the slotted torch caused linear flow rates to be greater. This flow rate exceeded the plasma propagation rate back into the argon gas supply. This avoided plasma formation in the area of argon introduction. However, this plasma exhibited a toroidal shape with the most intense portion of the plasma on the perimeter when viewed axially. Under no conditions could the argon plasma be centered as could the air, nitrogen and helium plasmas. However, 170 W argon plasmas (no nebulized solution) were stable with flows as low as 1.4 lmin-‘. Below this flow rate the plasma tended to propogate back up the threaded insert and became unstable. With solution nebulization, minimum support gas flow rates were 3.4 1min- ’ at 145 W and increased to 8.9 1min - ’ at 490 W. 4.
CONCLUSION
An MIP system capable of stable plasma maintenance with nitrogen, air, helium and argon has been presented. Plasmas of each gas were stable with both pure plasma gas and during solution nebulization. Intense elemental emission of metals has been observed during solution nebulization with each plasma. Possible gains which may be accrued from these plasmas include the direct spectroscopic determination of nonmetals with the helium and nitrogen plasmas. Secondly, a cost effective approach for the determination of metals in
MIP system for the maintenance of stable plasmas
499
solution may arise with the air and/or nitrogen plasmas. Although the argon plasma behaved somewhat differently, the unique toroidal shape may provide the basis for efficient plasma entrainment of the analyte. These pathways are presently being explored in our laboratories. Aclinowledgements-The technical assistance of M. CREWS,K. DUNN, L. GREGERSON, E. HYLANDand T. H. E. RIDGEWAY is greatly appreciated. The authors would also like to thank the Northern Illinois University Graduate School for a 1983 Summer Research Grant and Sigma Xi for a Grant-in-Aid of Research.
Printed in GreatBritain.
3,pp.493499,1985.
0584-8547/85 s3.00+.00 Pergamon PressLtd.
A microwave induced plasma system for the maintenance of moderate power plasmas of helium, argon, nitrogen and air KEVING. MICHLEWICZ, JOHN J. URH and JON W. CARNAHAN Department
of Chemistry, (Received
Northern
Illinois University,
DeKalb,
IL 60115, U.S.A.
10 April 1984; in revised form 28 June 1984)
Abstract-A microwave induced plasma system capable of maintaining stable plasmas of each of the gases helium, argon, nitrogen and air is presented. The system is capable of operation at powers of up to 500 W. The TM,,, cavity design is similar to that previously described in the literature with some modifications. A demountable torch facilitates centering of diXuse plasmas of helium, nitrogen and air by providing 6 flows directed tangentially within the quartz tube. This torch was not useful for argon plasmas. Toroidal argon plasmas were maintained with a threaded quartz tube arrangement. The heat generated by these plasmas was dissipated by an outer sheath ofcoolant air. Details of the design and preliminary characterization of each plasma system is presented.
1. INTRODUCTION MICROWAVE induced plasmas (MIP) have been investigated for analytical use since the 1965 application of the low pressure helium and atmospheric pressure argon plasmas [l, 21. However, literature in this area did not begin to proliferate until after 1976 when BEENAKKER introduced a TM,,, microwave resonator design capable of efficient power transfer to the plasma [3]. This design allowed the operation of an atmospheric pressure helium plasma. Subsequently, investigations have proceeded in the areas of analysis of gas chromatographic eluates, nebulized solutions and chemically and thermally vaporized analytes. Several excellent reviews are currently available in the literature [4-91. Recent reports describe a modification of BEENAKKER’S apparatus to maintain argon [lo] and helium [ 1l] plasmas with applied powers of up to 500 W. The Ar and He plasmas were used for the analysis of nebulized solutions and chromatogiaphic elutes respectively. However, with these systems it was not possible to maintain plasmas of alternate gases such as nitrogen or air. A series of talks by DEUTSCH~~~HIEFTJE~~recent F.A.C.S.S. meetings have described the operation and analytical characteristics of a nitrogen MIP [ 121. Power applied by the generator was limited to less than i‘OOW. This limitation was predominately due to thermal degradation of the torch at the irigher temperatures produced at higher powers. This report details the design and operation of an MIP system capable of maintaining stable plasmas of air, nitrogen, helium and argon at powers up to 500 W. This is facilitated by alteration of the internal diameter of the resonator previously described [lo] and the use of an air cooled, tangential flow, demountable torch. Spectral characterizations are presented and analytical implications are discussed.
[1] [2] [3] [4]
[S] [6] [7] [8] [9] ilO] :l 1] .12]
J. MCCORMICK, S. C. TONG and W. D. COOK, And. Chem. 37, 1470 (1965). A. BACHE and D. J. LISK, Anal. Chem. 37, 1477 (1965). I. M. BEENAKKER, Spectrochim. Acta. 31B, 485 (1976). T. ZANDER and G. M. HIEFTJE, Appl. Specrrosc. 35, 357 (1981). R. K. SKOGERBOE and G. N. COLEMAN, Anal. Chem. 48, 611A (1976). P. C. UDEN, Anal. Proc. 18, 189 (1981). T. H. RISBY and Y. TALMI, CRC Crit. Rev. Anal. Chem. 14, 231 (1984). J. W. CARNAHAN, K. J. MULLIGAN and J. A. CARUSO, Anal. Chim. Acta 130, 227 (1981). J. W. CARNAHAN, Am. Lab. 15(8), 31 (1983). D. L. HAAS, J. W. CARNAHAN and J. A. CARUSO, Appl. Spectrosc. 37, 82 (1983). J. W. CARNAHAN, Ph.D. thesis, University of Cincinnati, (1983). R. D. DEUTSCH and G. M. HIEFTJE, Papers presented at Annual Meeting of the Federation of Analytical Chemists and Spectroscopy Societies, Philadelphia, 1981; Papers Nos 222 and 223; 1982, Paper No. 50; 1983, Paper No. 420. A. C. C. A.
WB) 40:3-p
493
494
KEVING. MICHLEWICZ et al. 2. EXPERIMENTAL
2.1. Resonator cavity The cavity design is illustrated in Fig. 1. The design was similar to the cavity described by HAAS et al. [lo] with the following modifications. (1) The cavity diameter was increased to 88.8 mm from the original 86.5 mm value to facilitate the effect of the dielectric constant of nitrogen. Nitrogen plasma ignition and maintenance was impossible without this modification. (2) The torch was inserted into a 13.2-mm hole in the center of the cavity. Initially the hole diameter was 19.2 mm to accommodate a larger torch, however, plasmas could not be ignited with this design. This effect is presumably due to alteration of the microwave field. Placement of a 13.2-mm id. copper insert in the floor and faceplate allowed plasma ignition and maintenance. (3) The coupling loop was constructed of a 3.2-mm dia. copper rod soldered at a right angle to a 6.4mm dia. rod and grounded to the cavity floor. A 12-mm long lo/24 screw passing through the cavity bulk and attached to the 6.4mm rod produced solid contact. This modification allowed ease of cavity disassembly for polishing and repair. Typically, oxidized copper on the surface is removed by polishing the internal dimensions of the cavity with steel wool. This is done only as a preventative measure to ensure good surface conductivity. Fortunately, cavity repair has not been required. 2.2. Torch Two plasma torch designs were utilized. In the plasma region the material of choice was quartz. This material was the most suitable due to the properties of good thermal stability and resistance to cracking. The torches were similar in design and function to those previously used for the inductively coupled plasma (ICP) and the MIP [ 131. 20cm --
Fig. 1. Modified TMolo cavity. [13]
A. BOLLO-KAMARAand E. G. CODDING,Spectrochim. Acta. 36B, 973 (1981).
MIP system for the ~nt~~~~
of stable plasmas
495
The plasma gas was directed tangentially in the plasma region by means of 6 equally spaced (1 mm deep-45”) slots (slotted torch)or a doubly threaded design (threaded torch). The latter torch is similar to that of BOLLO-KAMARA and COWING[13]. The slotted torch is shown in Fig. 2. With the threaded torch the slotted central tube was replaced by a snugly fitting tube which was doubly threaded from the plasma gas inlet to the plasma region. With both designs, the inner tubes were bonded to the plasma cont~nment tube by heating. A second significant change in these torch configurations from previous MIP torches is the provision for a coolant flow to dissipate the heat generated by the plasma. Air could be used for the coolant flow with all plasma gases since it did not come into contact with the plasma. However, this yields significant gas mixing in the afterglow region. Analytical effects of this mixing are currently under investigation. A coolant flow of 12 1min” ’ was found to be more than sufficient in all cases. The helium, nitrogen and air plasmascould be centered with either torch. However, the slotted torch provided better plasma stability under the conditions utilized in these experiments. Apparently a more uniform gas flow was produced with this arrangement. The argon toroidal shaped plasma was considerably more stable with the threaded torch. Details concerning the torch selection for the argon plasma are discussed in Section 3.5. 2.3. Plasma ignition The plasmas were initiated by placing a copper wire (attached to a rub& or quartz handle) in the plasma region and applying a power of 300-400 W. Upon plasma ignition slight adjustment of the tuning stubs to tune the system to 0 W reflected (as read at the generator) was generally required. Retuning was minimal after initial adjustments. Tuning stub settings were found to be similar for the ignition of helium, argon and nitrogen plasmas. All of these plasmas were operated with the tuning stubs set at approximately the same position. Ignition of the air plasma required some adjustment from the optimum settings with the other plasmas.
Teflon
1
’
cm
Fig. 2. Slotted plasma torch.
496
KEVING. MICHLEWICZ ef al. Table 1. Specifications of equipment
Microwave generator Microwave power transmission TMolo cavity Plasma torch Nebulizer/spray chamber Monochromator
Optics Photomultiplier i/V converter-amplifier Chart recorder
Micro-Now (Chicago) Model 420 B. Capable of producing 500 W (operated at l&500 W forward with less than 5 W reflected). 3 ft-FT4-50T coaxial cable with 44 SW end connectors (Andrews Corp., Orland Park, IL). Similar to that described by HAASet al. [lo] with cavity depth of 11.2mm and diameter of 88.8 mm. Detailed in Fig. 1. Similar to ICP torch with provision for separate flows of coolant air, plasma gas and nebulizer gas. See text and Fig. 2 for further details. Meinhard (Santa Ana, CA) C-30/Apphed Research Laboratories (Sunland, CA) upward conical design. 0.35 m EU-700 GCA/McPherson (Acton, MA) scanning monochromator (f/6.4) operated with 50 ym entrance and exit slits, and a 12OO_groovemm-’ grating blazed at 250nm. A Model 700-51 scanner driver was used to scan from 20&6OOnm at 0.5 rims-‘. 1 in., lO-cm focal length lense; with object distance of 25.5 cm; image distance 16.0 cm. RCA lP28 (Lancaster, PA). Variable gain amplifier similar to that described by MCCARTHYef al. [14]. Omniscribe (Austin, TX) Model B-51 17-5 strip chart recorder.
Especially helpful in igniting the air plasma was to supply a spark to the flow of air with a copper wire attached to a tesla coil. Once proper tuning was determined, ignition of the air plasma was routine. 2.4. Instrumentation Specifications of the equipment
utilized are listed in Table 1
3. RESULTS AND DISCUSSION 3.1. General
behavior of plasmas
Although specifics vary with plasmas of particular gases, many aspects of the plasma behavior were similar. With the described system, all plasmas were stable over the periods used to gather the reported data. This time period varied from 2 to 6 h of plasma operation. Although some minor tuning adjustments (moving the attached stubs) were necessary, power fluctuations were generally less than 10 W and reflected powers did not exceed 10 W. In the studies which required variation of the applied power, minor retuning was required. A procedure which was found to yield consistent ease of alteration of this operational parameter was to vary the forward power 2C30 W, retune to 0 W reflected power, and continue this procedure until the desired forward power was obtained. As flow rate did not seem to significantly effect tuning, only minimal retuning was required when altering the plasma gas flow rates by as much as a factor of 2. Within the flow ranges indicated as stable in these studies, the maximum reflected power seen with plasma gas flow adjustment rarely exceeded 20 W. Significant differences were noted in the minimum plasma gas flow rates needed to maintain a stable plasma with and without solution nebulization. Operation of the air and nitrogen plasmas with plasma gas and coolant flows only (no nebulized solution) allowed support gas flows of less than 2 1min- ’ to be utilized while maintaining stable, suspended plasmas at 250 W. This value was 6.6 1min- 1 with a 270-W helium plasma. At flows of less than 9.9 ml min - i, the argon plasma caused the torch to glow red. At lower flow rates, nitrogen and air plasmas became elongated significantly and the plumes extended up to 7 cm beyond the torch. At higher flows, the visible portion of the plasma tended to become much shorter and was restricted to within the cavity to 1 cm beyond the faceplate. Flow rate variations did not significantly affect the size of the helium plasma. The argon plasma retained a constant size at high flows but began to propagate back up the plasma gas introduction tube with lower flows. With all plasmas, a maximum flow rate was seen for which the plasma could be maintained. Above this flow, the plasmas went out. This [14] J. P. MCCARTHY, M. E. JACKSON,T. H. RIDGWAYand J. A. CARUSO53, 1512 (1981).
MIP system
491
for the maintenanceof stableplasmas
behavior appeared to be simply a matter of residence time of the plasma gas species within the microwave field. It appeared that some minimum time was required for support gas excitation. During solution nebulization the minimum necessary support gas flow was found to be significantly greater. Gas flow rates and water uptake rates are tabulated in Table 2. When the air, nitrogen or helium plasma support gas flow was adjusted to below a certain value, the plasma tended to deviate from its solid elipsoidal shape to one in which the axial appearance was no longer circular, but distorted and unstable. At these flows flicker noise inceased significantly and the analytical potential was degraded. This is the flow rate designated as unstable in Fig. 3. For the argon plasmas, the designated flow rate is that at which the torch began to glow due to excess heating. 3.2. Characteristics of the nitrogen plasma The minimum flow rate required for the stable maintenance of the pure nitrogen plasma was approximately 1.5 1min - 1 at 500 W. During solution nebulization the minimum plasma gas flow required to maintain a stable plasma increased to 6.5 1min- 1 at 250 Wand increased slightly with increasing power. This trend is depicted on Fig. 3a. The spectrum of the nitrogen MIP during solution nebulization revealed species of OH, NO, CN, NH, N, and N& Band assignments were obtained from Refs [15-181. At present it Table 2. Nebulizer Nebulizer
gas
conditions*
Gas flow rate (I min - ‘)
Water uptake rate (ml min _ ‘)
1.15 1.00 2.53 1.34
0.34 0.38 0.031 0.45
Air Argon Helium Nitrogen *All nebulizers
operating
operated
at 30 psig.
2 t 01 150
I
I
I
250
350
450
Applied
Fig. 3. Minimum nitrogen plasma, A. Cl51
Foward
Power
500
(watts)
flows for stable plasma maintenance during solution nebulization. (b) suspended air plasma, (c) suspended helium plasma, (d) toroidal
(a) Suspended argon plasma.
N. WRIGHT and C. A. WINKLER, Ache Nitrogen. Academic Press, New York (1968). R. STRIGANOV and N. S. SVENTILSKI, Tables ofSpectral Lines ofNeutrnl and Ionized Atoms. Plenum Press, New York (1968). [I71 K. P. HUBER and G. HERZBERG, Molecular Spectra and Molecular Structure. Van Nostrand Reinhold, New York (1979). Cl81 G. HERZBERG, Spectra ofDiatomicMolecules. Van Nostrand, New York (1950).
Cl61A.
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KEVIN G. MICHLEWICZet al.
is not clear if the bands of NH and CN arise from the combination of afterglow entrained atmospheric or coolant hydrogen and carbon containing molecules with N2 or if a contribution to these bands arise from impurities in the support gas. Diatomic bands of nitrogen include transitions of the second positive system (second vibrational level of neutral N,) and the first negative system (first vibrational level of N:). The latter species is particularly interesting for its high energy (15.57eV in the ground N: state) and the implications for excitation of elements to these energy levels. N: was seen in both the pure nitrogen plasma and during solution nebulization. 3.3. Characteristics of the air plasma The minimum flow rates required for the stable maintenance of the air plasma was less than 2 1min- ’ at 250 W. During solution nebulization the minimum gas flow required to maintain a stable plasma increased to 10.4 lmin-r at 250 W and to 16.1 lmin-’ at 450 W. This is illustrated at Fig. 3b. As expected, the spectrum of the air plasma is similar to that of the nitrogen spectrum. The only notable difference is that the NO bands are significantly enhanced with the air plasma. 3.4. Characteristics
of the helium plasma
Helium exhibited somewhat different behaviour both physically and spectroscopically as might be expected from the density and the available energies of the gas. Minimum required flow rates for the pure helium system were 7.5 1min - ’ at 250 Wand increased to 13.0 1min - ’ at 440 W. The trend during solution nebulization is depicted in Fig. 3c for a stable helium plasma with a 30 psig drop across the Meinhard C-30 nebulizer. However, the solution uptake rate at this pressure was only 0.03 1 ml min - i. The low uptake rate was due to the low density of helium (ca 0.1 times that of argon). Examination of helium plasma spectra showed the presence of several species whose energy was greater than that of metastable helium (19.7 eV). Also present are intense lines due to hydrogen (486.1 and 433.7nm) from the nebulized water. The presence of the intense emission of the helium and hydrogen species indicated that a great amount of energy may be available for transfer to the analyte during the solution nebulization experiment. Possible excitable elements include metals, metaloids and nonmetals. 3.5. Characteristics of the argon plasma The argon plasma was particularly interesting in terms of plasma formation and characteristics. With the slotted torch the argon plasma extended back from the plasma tube to the area of argon introduction. Plasmas created under these conditions tended to be noisy and unstable. However, use of the threaded torch allowed argon plasma maintenance within the cavity region. With this torch, flow rates similar to those of the slotted torch caused linear flow rates to be greater. This flow rate exceeded the plasma propagation rate back into the argon gas supply. This avoided plasma formation in the area of argon introduction. However, this plasma exhibited a toroidal shape with the most intense portion of the plasma on the perimeter when viewed axially. Under no conditions could the argon plasma be centered as could the air, nitrogen and helium plasmas. However, 170 W argon plasmas (no nebulized solution) were stable with flows as low as 1.4 lmin-‘. Below this flow rate the plasma tended to propogate back up the threaded insert and became unstable. With solution nebulization, minimum support gas flow rates were 3.4 1min- ’ at 145 W and increased to 8.9 1min - ’ at 490 W. 4.
CONCLUSION
An MIP system capable of stable plasma maintenance with nitrogen, air, helium and argon has been presented. Plasmas of each gas were stable with both pure plasma gas and during solution nebulization. Intense elemental emission of metals has been observed during solution nebulization with each plasma. Possible gains which may be accrued from these plasmas include the direct spectroscopic determination of nonmetals with the helium and nitrogen plasmas. Secondly, a cost effective approach for the determination of metals in
MIP system for the maintenance of stable plasmas
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solution may arise with the air and/or nitrogen plasmas. Although the argon plasma behaved somewhat differently, the unique toroidal shape may provide the basis for efficient plasma entrainment of the analyte. These pathways are presently being explored in our laboratories. Aclinowledgements-The technical assistance of M. CREWS,K. DUNN, L. GREGERSON, E. HYLANDand T. H. E. RIDGEWAY is greatly appreciated. The authors would also like to thank the Northern Illinois University Graduate School for a 1983 Summer Research Grant and Sigma Xi for a Grant-in-Aid of Research.