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The analysis of some diving gas mixtures by microwave-induced plasma optical emission spectroscopy
OSW-8547186 83.OO+O.C@ Persamon Journals Ltd.
The analysis of some diving gas mixtures by microwave-induced plasma optical emission spectroscopy* MORAG MCKENNA, IA~NL. MARR,MALCOLMS. CRESSER and EVALAM Departments of Chemistry and Soil Science, University of Aberdeen, Old Aberdeen, AB9 2UE, U.K. (Received 26 August 1985; in revised
firm 1 October
1985)
Abstract-A helium MIP has been investigated as a possible source for the emission spectrosoopic analysis of helium-based gas mixtures containing 02, Nt and Ar, such as are used in deep-sea diving. The reduced-pressure
plasma (ca. 15 torr) in a &wave cavity is shown to be much superior to an a~osphe~pr~s~e plasma with a Beenakker cavity for this application, and justifies the use of pumping equipment. Emission lines in the near ir. are used, giving adequate sensitivity and linearity up to l-2 % of minor components.
1. INTRODUCXION DEEP-WATERdivers
are sustained on mixtures of oxygen in helium, the so-called Heliox mixtures, because of the problems associated with the solubility of nitrogen in blood under high pressures. The volume percentage of oxygen is reduced as the total working pressure increases at greater depths, to keep the oxygen partial pressure near 0.5 bar. Very often nitrogen is also present, in hyperbaric chamber systems on land, for example, simply because the chambers are initially full of air, and because for very deep-water operations, the so-called Trimix of he~~/oxygen/ni~ogen is sometimes used. Further, since underwater work may involve welding pipelines and other structures, often using argon-shield welding techniques, this gas may also be present. At high pressure it is a narcotic and therefore undesirable. The authors’ aim was to develop a simple procedure for the determination of these three gases in helium, preferably one giving a continuous and simultaneous readout. The standard method for gas analysis is gas chromatography (GC) but while it can give excellent precision and, if good standards are available, accuracy too, it does not give a continuous readout. Further, the separation of argon and oxygen is difficult, generally requiring sub-ambient temperatures. Infrared spectrometry is suitable for small polyatomic molecules (and for CO) but not for the permanent gases. The microwave induced plasma (MIP) is particularly suitable as an element-sync detector for GC with helium or argon serving as both carrier for the chro~t~phy and support gas for the plasma. These two gases have simple spectra, providing a clean background for measurement of the emission from the sample elements. Helium plasmas at atmospheric or reduced pressure have been utilized for many years for the analysis of nonmetallic, metallic and organometallic compounds after GC separation [l-8]. The performance of MIP’s as GC detectors has been compared with that of ICPs by CARNAHANet al. [9]: the MIP scores well in terms of low power and helium consumption as well as in its good sensitivity. The use of MIP-optical emission spectroscopy (OES) has not been restricted to detection systems for GC, however. KOLLOTZEKet al. [lo], for example, analysed natural *This paper is dedicated to the memory of Gordon F. Kirkb~gh~ a first-rateresearcher,teacher [I] C. A. BACHEand D. J. LISK, Anal. C/zem. 39, 786 (1967). R. M. DAGNALL,T. S. WEST and P. WHITEHEAD,Ad. Chem. 44,2074
[2] [3] [4] [S] [6] [7] [S] [9] [lo]
good friend.
(1972). C. I. M. BEENAKKER,Spectrocitim. Actu 32B, 173 (1977). J. P. J. VANDALEN, P. A. DE LEZENNECOULANDERand L. DE GALAN, Anal. Chim. Acta 94, 1 (1977). B. D. QUIMBY, P. C. UDEN and R. M. BARNES,Anal. Chem. 50, 2112 (1978). J. L. GENNA, W. D. MCANINCHand R. A. REICH, J. Chromatog. 238, 103 (1982). T. H. RISBY and Y. TALMI, Crit. Rev. Anal. Gem. 14, 231 (1983). K. B. OLSEN, D. S. SKLAREWand J. C. EVANS,Spectrochim. Acta 4OB, 357 (1985). J. W. CARNAHAN,K. J. MULLIGANand J. A. CARUSO,Anal. Chim. Acta 130,227 (1981). D. KOLLOTZEK,P. TSCHOPELand G. TOLG, Spectrochim. Acta 39B, 625 (1984). 669
MORAGMCKENNA et al.
670
for cadmium and copper using a toroidal argon MIP and obtained low detection limits for a range of elements. Trace levels of water present in solid samples have been determined recently using a combination of thermal gas evolution and helium MIP-OES by HANAMURA et al. [l 1J. In these cases, only trace amounts of determinant are present in the plasma so that the operating conditions and temperature are una&cted by the presence of the elements being determined, Some use has also been made of MIP-OES in direct gas analysis: TAYLORet aL f12] determined carbon-, oxygen-, nitrogen- and hydrogen~o~t~ning impurities in argon at fractional parts per million levels using an atmospheric-pressure MEP, taking advantage of the excellent sensitivity which it offers. SERRAVALL~ and Rrssv [13], on the other hand, preferred a low pressure MIP for the determination of traces of vinyl chloride in air, measuring the Cl(I1) emission, aad injecting discrete samples of air into the helium gas stream to the plasma. They reported severe quenching effects due ta the oxygen in the air-a problem which has been faced in this present investigation, since our interest lies in the analysis of gas mixtures with argon up to 5 %, and oxygen and nitrogen each up to 20% in helium. Diving gas mixtures of such composition can sustain a MIP at either atmospheric or reduced pressure provided that a suitable cavity is used. The possibility of using MIP-OES for their analysis has been investigated to see what problems, if any, would arise when high levels of dete~n~ts are present in binary and multi-component mixtures. Alternative excitation m~h~isms which might be invoked to explain the nature of interferences in such systems have been discussed in a review by ZANDER and HIEFTJE[ 145 and also by MATWSEK et al. [IS-J. Emission spectroscopy using a glow discharge was suggested more than 40yr ago for continuous quantitative analysis of breathing atmospheres for nitrogen in the presence of oxygen, water and carbon dioxide: [16]. An unfortunate aspect of the meter developed by LILLYand ANDERSON[16] was the curvilinear calibration. Further work was done in the 1950s using OES for the analysis of respiratory gases [17-191. Under appropriate conditions, satisfactory performance as a continuous carbon dioxide analyser was achieved. With oxygen in the presence ofcarbon dioxide or nitrogen dune, the calibrations were linear, but changing the carbon ~oxide-t~nitrogen ratio in a th~~om~nent mixture changed the shape of the oxygen calibration graph. The analysis of mixtures containing inert gases using such methods has not been reported. waters
2, EXPERIMENTAL 2.X. Apparatus
For the atmospheric-pressure plasma a TM 010 cavity (after Been&&), model 218 L from Electromedical Supplies was used. The plasma was viewed axially, either through the open end of the quartz discharge tube, or through a quartz window cemented to the end af the tube. For the reducedpressuse plasma a $-wave cavity, model 214 L was used. In either case the cavity was coupled through a reflected-power meter to a ticrotron 200Mk 2 microwave generator (both from ELectrcrmedical Supplies) supplying 20-200 W at 2450 MHz. An image of the plasma was focussed by a &sed silica lens onto the cr~trance slit of a Techtron AA4 monochromator. With entrance slit set at 25 pm the resolution was approximately al nm. At wavelengths from 200 to 5OOnm an IP28 photomultiplier was used as detector, but for longer wavelengths a silicon photodiode with integral amplifier (Radiospares 308-067) was preferred: its spectral sensitivity peaks near 900 nm and reaches beyond 1100 nm. Spectral scan speeds between 3 and
[Ii]
[la] [13] [14-j [Is] [l&]
KIRSCH and J. D. WINEFORDNER, Anai. Chem.57,9 (198s). H. E. TAYLOR,J. H. GIBSON and R. K. SKOOERBOE, Anal. Chem. 42, 876 (1970). F. A. SERRAVALLO and T. H. Rmeu, Ad C&m. 48,673 (1976). A. T. ZA~DERand G. M. HE~FUE,A&. Spectrmc.35, 357 (1981). J. P. Marousix, B. J. GRR and M. SELBY,Pmg. A&. At. Spectxa 7,275 (1984). T. C. LILLYaad T. F. ANDERSON, National ResearchCwxii, Divisionof Medical Scietxq ttepbrt 299,2%h
S. HANAMURA, B.
[17] C. S. WH~TFZ,L, C. Wmms [lg] C. S. WHITE,L, C. Wmms
and E. E. FLETCXER, J. At&t. Me& 27,232 (1956). and E. E. FLETCHER, J. Aviat. Med. 27,414 (1956). [19] C. S. WHITE, L. C. WATKINSand E. E. FLETCHER, J. Aviat. Med. 28,406 (1957).
MIP-OES analysis of gas mixtures
671
Table 1. Wavelengths of the emission lines 106.5nm
He(I)
771.4nm
OfI) MI)
811.5~ 868.1nm
N(I)
Snm min-’ were employed when scanning was required, and the output was presented on a Servoscribe chart recorder. Table 1 lists the atomic emission lines found to be most useful. Many of the gas-line components were common to both atmospheric and reduced-pressure plasma systems. Gas was fed via a flow meter through a length of 3 mm o.d. copper tubing flattened to restrict gas flow, which could then be varied by altering the applied gas pressure to the discharge tube, a piece of quartz tubing 6.5 mm o.d., 1.5 mm i.d. and 15 cm long. A Tesla coil was used to initiate the discharge. The vacuum system was constructed from 3 mm copper tubing and Swagelok compression fittings, and is shown diagramatically in Fig. 1. The pressure in the plasma tube was measured with a mercury manometer and a travelling microscope. Many problems were encountered initially with leaks in the system, which were evident as quite intense nitrogen and oxygen emission in samples of high-purity helium. The copper tubing had to be treated with care since repeated bending caused work-hardening and produced tiny hairline fractures. One attempt to overcome this problem involved replacing all the copper with nylon tubing: the emission from nitrogen and oxygen increased, since the diffusion of these gases through the plastic was even greater than through the microscopic fractures. The line was rebuilt again in copper, each piece being carefully bent to shape and then used without further adjustment. Copper was chosen in preference to stainless steel because it is softer and more easily bent-especially at the 3-mm o.d. size-and because it seals more easily in compression fittings to give a vacuum-tight joint. Connections to the quartz discharge tube were also made with compression fittings, but with silicon “o”-rings in place of brass ferrules. New components in the line were degassed by gentle heating with hot air from a hair drier while being continuously evacuated: in this way large blanks for oxygen, due to water, can be eliminated. 2.2. Preparation of gas mixtures Gas mixtures were prepared on a high-pressure mixing line which allowed the addition of individual pure gases to be monitored by pressure--on a 12 in. precision gauge reading to 300 bar, readable to 0.3 bar-and more accurately, by weight-using a Mettler model PE 16000 top-loading electronic balance reading up to 16 kg with a precision of 0.1 g. Aluminium sample cylinders were evacuated on the line using a rotary oil pump, then charged with each gas in turn from large stock cylinders, with the final pressure reaching 70 bar. Mixtures with l-10 % of Oz. Nz or Ar were prepared directly in this way, then diluted gravimetrically, after being left horizontal for 24 h to mix thoroughly, to produce the lower concentrations. 2.3. Analysis of the gas mixtures by GC The composition of the various mixtures, relative to the highest standard, was confumed by GC analysis. This was necessary because of the possibility of contamination by air at some stage. For the
flattened
copper
Photodiode thick walled -) slbca tube manometer m
gascylinder
*
vacuum pump
needle valve
Fig. 1. Schematic diagram of the MIP-OES apparatus.
recorder
MORAG MCKENNAet al.
672
two-component mixtures a laboratory-built instrument was used, fitted with a thermistor bead katharometer detector and molecular sieve 5A columns. Chromatograms from 2-ml samples were recorded on a Pye Unicam PU 4810 integrator, and the amounts of each gas were calculated from the areas of the peaks by comparison with those for a gravimetric standard with 10% each of nitrogen, oxygen and argon in helium (on a mole, i.e. volume % basis). When both argon and oxygen were present the analyses could not be carried out at room temperature. For these samples, a Perkin-Elmer F-30 gas chromatograph was used, with the 5A columns immersed in a cold bath at 0°C: the retention times, especially for nitrogen, are long, but the argon and oxygen are adequately separated.
3. RESULTS 3.1. The atmospheric plasma By using a TMolo cavity after Beenakker’s design, a plasma could be sustained in either helium or argon at atmospheric pressure. Tuning the cavity was quite difficult and ignition was often troublesome, especially when oxygen was present; when the oxygen content exceeded 5 %, ignition became impossible at the input power of 100 W being used. 3.1.1. Efict of applied power on emission intensity. Figure 2 shows that as applied power is increased the emission intensity also increases, but not linearly. No measurements were made at applied powers below 75 W since at lower powers the plasma becomes tinstable. GOODE [20] has reported similar behaviour. Sensitivity was adequate with the atmospheric-pressure plasma and stability acceptable at 75 W, which applied power was used for most of the subsequent work. This also facilitated comparison with the measurements made on the lowpressure plasma. 3.1.2. E&ct of heliumjlow rate. Figure 3 shows that the emission intensity from both argon and oxygen increases as the flow rate is increased. At higher flow rates the increase in emission intensity with flow rate is less pronounced. GOODE [20] also considered the effect of flow rate, but at higher flow rates than those used here. 3.1.3. Linearity of calibration graphs. Emission calibration graphs were prepared for oxygen and argon in helium using binary gas mixtures for the atmospheric-pressure helium MIP. Typical curves are shown in Fig. 4: the responses are decidedly non-linear over most of the range, though for levels below 0.1% it would seem that linearity (i.e. a slope of unity on the log-log graph) is approached. It should be pointed out that these signals are corrected for
70
80
90
100 110 120 130 140 150 Incident power (WI
Fig. 2. Relative emission signal (linear scale) as a function of applied power for the atmosphericpressure plasma. (0) 777.4nm for O2 at 1% in helium. (0) 811.5 nm for Ar at 1% in helium. [20] S. R. GOODE,N. P. BUDDIN,B. CHAMBERS, K. W. BAUGHMANand J. P. DEAVOR,Spectrochim.Acta 4OB,317
(1985).
673
MIP-OES analysis of gas mixtures
0.1 0.2 03
0.4 05
0 6 0.7 0 8 0.9
Flow (Lmin-'I
Fig. 3. Relative emission signal (linear scale)as a function of gas flow rate for the atmospheric-pressure plasma. Details as in Fig. 2.
I 0.0
I
O-03
I
/
0.1
03
,
1.0
I
10
Mole % minor component
Fig. 4. Log-log calibrations for the a~osphe~c-prep plasma. (0) 777.4 nm for O2 in O1 in helium mixes, (e) 811.5 nm for Ar in Ar in helium mixes.
background, and at the lower levels the correction becomes large and its variability causes difficulty in measurement of the sample signal. The background signal is due to oxygen in the air diffusing back into the discharge tube. At higher levels the quenching of emission from either minor component, and also from the helium itself, became serious, resulting in the very pronounced curvature of these graphs to lower signals than would have been expected. Because of the difficulties in tuning and igniting the atmospheric-pressure helium MIP, and the undesirable curvature of the calibration graphs, it was decided to investigate the potential of a low-pressure MIP for this application. 3.2. The reduced-pressure pkls?na By using the 214L #-wave cavity a plasma could be sustained in either helium or argon over a wide range of applied powers, with low retlected powers. Ignition was easy-indeed very often the plasma ignited spontaneously when the microwave power was switched on. 3.2.1. E’ct ofincident power on emission intensity. Figure 5 shows the effect of varying the applied microwave power over the range 20-130 W on the intensity of the helium emission at 706.5 nm in a pure helium plasma. Similarly shaped curves were obtained for the emission of argon at 811.5 nm, oxygen at 777.4 nm and nitrogen at 868.6 nm when mixtures of these gases
Moru~ MCKENNAet al.
614
I
I
I
I
I
20
40
60
80
I
1
I
100 120 140
Input power Iw)
Fig. 5. Relative emission signal (linear scale) as a function of applied power for the reduced-pressure plasma. ( x ) 706.5 nm for He in pure helium. (0) 777.4 nm for 0s in 1% O2 in helium. ( q!)777.4 nm for0,in1~0,+1~Arinhelium.(A)811.5nmforArin1~Arinhelium.(o)811.5nmforArin1~ Ar + 1% 0, in helium.
in helium were excited. The reflected power was found to reach a minimum (always less than 5 W) for applied powers in the range 7&90 W for all the gas mixtures. A power setting of 75 W was chosen as a compromise between sensitivity and efficiency. 3.2.2. Effecctofpressure. The effect of pressure in the discharge tube on the helium emission intensity is illustrated in Fig. 6 for a pure helium plasma and is in agreement with published findings [21]. It is clear that while decreasing the pressure improves the sensitivity, the precise control of the lower pressures becomes both more difficult and more critical. The combination of a fine metering valve (Whitey B-2MG) down-line from the plasma tube and a length of severely constricted copper tubing up-line from it enabled a working pressure of 15 mm Hg to be maintained to +O.l mm. 3.2.3. Linearity of response. Mixtures covering a range between 0.01 and 10 % v/v of argon, nitrogen or oxygen in helium were prepared and the emission measured at various wavelengths. Figure 7 shows the calibration curves obtained for the preferred wavelengths. In each case the upper limit for linear response lies around 1%. Further, when both argon and oxygen are present the emission from each is decreased. The effect this has on the calibration graphs can be seen in Fig. 8. Figure 5 shows that this effect cannot be eliminated simply by increasing the applied microwave power 4. DISCUSSION Bearing in mind the ultimate aim of developing an analytical detection system for in situ analysis of breathing mixtures for diving, the need for a vacuum pump is restrictive. The atmospheric pressure plasma was thus initially more attractive for this application. With the atmospheric plasma the need for precise pressure control also disappears. These investigations have shown, however, that in addition to the difficulties of tuning and ignition there are severe problems due to the ingress of air back along the discharge tube, and into the plasma region, giving rise to very large “blank” signals and causing all response to be non-linear. Observation of this phenomenon with an ICP used as a GC detector had earlier persuaded BROWN and FRY [22] to use an extended plasma tube to minimise the ingress of air. We attempted to overcome the effect by cementing a window on the end of the discharge tube and [21] K. W. BUSZHand T. J. VICKERS, Spectrochim. Acta 28B, 85 (1973). [22] R. M. BROWNand R. C. FRY, Anal. Chem. 53, 532 (1981).
MIP-OES analysis of gas mixtures
I
0 Fig. 6.Relatiye emission
I
1
I
I
I
67.5
I
10 20 30 40 50 60 PRESSURE (TORR)
signal (linear scale) as a function of pressure in the reduced-pressureplasma. (0) 706.5 nm for He in pure helium,
0.0’
’ 0 03
I
0.1
I
I
8
/
I
0.3 1.0 3 IO 30 Mole% minor component
Fig. 7. Log-log calibrations for binary mixtures in the reduced-pressureplasma. (0) 777.4 nm for Oz in O2 in helium mixes. (0) 811.5 nm for Ar in Ar in helium mixes. (0) 868.1 nm for N1 in N1 in helium mixes.
leading the gas away through a side arm and a long piece of flexible tubing. There was little improvement and the atmospheric plasma was abandoned. For emission spectroscopy to be a useful tool a linear relationship between concentration and emission signal is clearly an advantage. At least at low ~n~ntrations the reduced pressure plasma gives linear responses, but only up to around 1% v/v. BRASSEMet ~2. {23j found that for argon, nitrogen or hydrogen in helium the response was linear up to around lx, but at much lower pressures, and getting worse as the pressure increased. The use of emission lines at longer wavelengths seems to be satisfactory and the spectral interferences should be minimised since this part of the spectrum is less crowded. The complexity of e.g. the Nz emission spectrum in the near U.V.can be seen from the work described by BROIDA and CHAPMAN [24]. In the case of argon, the large number of lines [23] P.
BRASSEM,F.
J. M. J. MAESSENand L. DE GALAN, Speccrochim.Acta 314 537 (1976).
MORAGMCKENNAet al.
676
001
0.1
1,o
Mole % Ar
Fig. 8. Log-Log calibrations for argon at 811.5 nm in the reduced-pressure pIasma, for threecomponent mixtures in helium. (0) with 0.01% Oz. (e) with 0.1% O2 (Cl) with 1.0% 0,.
appearing close together, e.g. around 4OOnm, makes measurement of individual line intensities, as would be needed for measurement of plasma temperatures, very difficult. The lines listed in Table 1, lying between 700 and 900 nm, can be detected with good sensitivity by the small silicon photodiode which, by comparison with red-sensitive photomultipliers has the advantages of being much cheaper, smaller, and requiring much simpler power supplies. Since we envisage building a multi~~nnel instrument with several detectors, each with an appropriate interference titer to isolate the respective emission lines, the choice of photodiode over photomultiplier was obvious. In earlier reported studies [20,21, 231 the lines used were He(I) 388.9 nm; Ar(1) 416 nm; and N at 337.1 nm. BRASSEM [23] explained the variation in emission intensity with concentration of minor component in terms of changes in excitation conditions, particularly in electron temperature. No work has been published on the emission from multi-component gas mixtures: the limited results presented here on three-component mixtures suggest that the interferences will be complex. More work on these systems must be carried out before the suitability of microwave emission spectroscopy for the analysis of diving gas mixtures can be finally established; this work is now underway. Preliminary results obtained in our laboratory employing gas dilution systems with pure helium to extend the working range and reduce the interferences have been ~~~~ng. Ackaowiedgements-The authors are indebted to the SERC Marine Technology Directorate for financial support for this study, to Gas & Equipment Ltd for supplies of helium and, along with Wharton Williams Ltd and Sub Sea Offshore Ltd, for financing a preliminary study carried out by Mrs. EVA LAM.
The analysis of some diving gas mixtures by microwave-induced plasma optical emission spectroscopy* MORAG MCKENNA, IA~NL. MARR,MALCOLMS. CRESSER and EVALAM Departments of Chemistry and Soil Science, University of Aberdeen, Old Aberdeen, AB9 2UE, U.K. (Received 26 August 1985; in revised
firm 1 October
1985)
Abstract-A helium MIP has been investigated as a possible source for the emission spectrosoopic analysis of helium-based gas mixtures containing 02, Nt and Ar, such as are used in deep-sea diving. The reduced-pressure
plasma (ca. 15 torr) in a &wave cavity is shown to be much superior to an a~osphe~pr~s~e plasma with a Beenakker cavity for this application, and justifies the use of pumping equipment. Emission lines in the near ir. are used, giving adequate sensitivity and linearity up to l-2 % of minor components.
1. INTRODUCXION DEEP-WATERdivers
are sustained on mixtures of oxygen in helium, the so-called Heliox mixtures, because of the problems associated with the solubility of nitrogen in blood under high pressures. The volume percentage of oxygen is reduced as the total working pressure increases at greater depths, to keep the oxygen partial pressure near 0.5 bar. Very often nitrogen is also present, in hyperbaric chamber systems on land, for example, simply because the chambers are initially full of air, and because for very deep-water operations, the so-called Trimix of he~~/oxygen/ni~ogen is sometimes used. Further, since underwater work may involve welding pipelines and other structures, often using argon-shield welding techniques, this gas may also be present. At high pressure it is a narcotic and therefore undesirable. The authors’ aim was to develop a simple procedure for the determination of these three gases in helium, preferably one giving a continuous and simultaneous readout. The standard method for gas analysis is gas chromatography (GC) but while it can give excellent precision and, if good standards are available, accuracy too, it does not give a continuous readout. Further, the separation of argon and oxygen is difficult, generally requiring sub-ambient temperatures. Infrared spectrometry is suitable for small polyatomic molecules (and for CO) but not for the permanent gases. The microwave induced plasma (MIP) is particularly suitable as an element-sync detector for GC with helium or argon serving as both carrier for the chro~t~phy and support gas for the plasma. These two gases have simple spectra, providing a clean background for measurement of the emission from the sample elements. Helium plasmas at atmospheric or reduced pressure have been utilized for many years for the analysis of nonmetallic, metallic and organometallic compounds after GC separation [l-8]. The performance of MIP’s as GC detectors has been compared with that of ICPs by CARNAHANet al. [9]: the MIP scores well in terms of low power and helium consumption as well as in its good sensitivity. The use of MIP-optical emission spectroscopy (OES) has not been restricted to detection systems for GC, however. KOLLOTZEKet al. [lo], for example, analysed natural *This paper is dedicated to the memory of Gordon F. Kirkb~gh~ a first-rateresearcher,teacher [I] C. A. BACHEand D. J. LISK, Anal. C/zem. 39, 786 (1967). R. M. DAGNALL,T. S. WEST and P. WHITEHEAD,Ad. Chem. 44,2074
[2] [3] [4] [S] [6] [7] [S] [9] [lo]
good friend.
(1972). C. I. M. BEENAKKER,Spectrocitim. Actu 32B, 173 (1977). J. P. J. VANDALEN, P. A. DE LEZENNECOULANDERand L. DE GALAN, Anal. Chim. Acta 94, 1 (1977). B. D. QUIMBY, P. C. UDEN and R. M. BARNES,Anal. Chem. 50, 2112 (1978). J. L. GENNA, W. D. MCANINCHand R. A. REICH, J. Chromatog. 238, 103 (1982). T. H. RISBY and Y. TALMI, Crit. Rev. Anal. Gem. 14, 231 (1983). K. B. OLSEN, D. S. SKLAREWand J. C. EVANS,Spectrochim. Acta 4OB, 357 (1985). J. W. CARNAHAN,K. J. MULLIGANand J. A. CARUSO,Anal. Chim. Acta 130,227 (1981). D. KOLLOTZEK,P. TSCHOPELand G. TOLG, Spectrochim. Acta 39B, 625 (1984). 669
MORAGMCKENNA et al.
670
for cadmium and copper using a toroidal argon MIP and obtained low detection limits for a range of elements. Trace levels of water present in solid samples have been determined recently using a combination of thermal gas evolution and helium MIP-OES by HANAMURA et al. [l 1J. In these cases, only trace amounts of determinant are present in the plasma so that the operating conditions and temperature are una&cted by the presence of the elements being determined, Some use has also been made of MIP-OES in direct gas analysis: TAYLORet aL f12] determined carbon-, oxygen-, nitrogen- and hydrogen~o~t~ning impurities in argon at fractional parts per million levels using an atmospheric-pressure MEP, taking advantage of the excellent sensitivity which it offers. SERRAVALL~ and Rrssv [13], on the other hand, preferred a low pressure MIP for the determination of traces of vinyl chloride in air, measuring the Cl(I1) emission, aad injecting discrete samples of air into the helium gas stream to the plasma. They reported severe quenching effects due ta the oxygen in the air-a problem which has been faced in this present investigation, since our interest lies in the analysis of gas mixtures with argon up to 5 %, and oxygen and nitrogen each up to 20% in helium. Diving gas mixtures of such composition can sustain a MIP at either atmospheric or reduced pressure provided that a suitable cavity is used. The possibility of using MIP-OES for their analysis has been investigated to see what problems, if any, would arise when high levels of dete~n~ts are present in binary and multi-component mixtures. Alternative excitation m~h~isms which might be invoked to explain the nature of interferences in such systems have been discussed in a review by ZANDER and HIEFTJE[ 145 and also by MATWSEK et al. [IS-J. Emission spectroscopy using a glow discharge was suggested more than 40yr ago for continuous quantitative analysis of breathing atmospheres for nitrogen in the presence of oxygen, water and carbon dioxide: [16]. An unfortunate aspect of the meter developed by LILLYand ANDERSON[16] was the curvilinear calibration. Further work was done in the 1950s using OES for the analysis of respiratory gases [17-191. Under appropriate conditions, satisfactory performance as a continuous carbon dioxide analyser was achieved. With oxygen in the presence ofcarbon dioxide or nitrogen dune, the calibrations were linear, but changing the carbon ~oxide-t~nitrogen ratio in a th~~om~nent mixture changed the shape of the oxygen calibration graph. The analysis of mixtures containing inert gases using such methods has not been reported. waters
2, EXPERIMENTAL 2.X. Apparatus
For the atmospheric-pressure plasma a TM 010 cavity (after Been&&), model 218 L from Electromedical Supplies was used. The plasma was viewed axially, either through the open end of the quartz discharge tube, or through a quartz window cemented to the end af the tube. For the reducedpressuse plasma a $-wave cavity, model 214 L was used. In either case the cavity was coupled through a reflected-power meter to a ticrotron 200Mk 2 microwave generator (both from ELectrcrmedical Supplies) supplying 20-200 W at 2450 MHz. An image of the plasma was focussed by a &sed silica lens onto the cr~trance slit of a Techtron AA4 monochromator. With entrance slit set at 25 pm the resolution was approximately al nm. At wavelengths from 200 to 5OOnm an IP28 photomultiplier was used as detector, but for longer wavelengths a silicon photodiode with integral amplifier (Radiospares 308-067) was preferred: its spectral sensitivity peaks near 900 nm and reaches beyond 1100 nm. Spectral scan speeds between 3 and
[Ii]
[la] [13] [14-j [Is] [l&]
KIRSCH and J. D. WINEFORDNER, Anai. Chem.57,9 (198s). H. E. TAYLOR,J. H. GIBSON and R. K. SKOOERBOE, Anal. Chem. 42, 876 (1970). F. A. SERRAVALLO and T. H. Rmeu, Ad C&m. 48,673 (1976). A. T. ZA~DERand G. M. HE~FUE,A&. Spectrmc.35, 357 (1981). J. P. Marousix, B. J. GRR and M. SELBY,Pmg. A&. At. Spectxa 7,275 (1984). T. C. LILLYaad T. F. ANDERSON, National ResearchCwxii, Divisionof Medical Scietxq ttepbrt 299,2%h
S. HANAMURA, B.
[17] C. S. WH~TFZ,L, C. Wmms [lg] C. S. WHITE,L, C. Wmms
and E. E. FLETCXER, J. At&t. Me& 27,232 (1956). and E. E. FLETCHER, J. Aviat. Med. 27,414 (1956). [19] C. S. WHITE, L. C. WATKINSand E. E. FLETCHER, J. Aviat. Med. 28,406 (1957).
MIP-OES analysis of gas mixtures
671
Table 1. Wavelengths of the emission lines 106.5nm
He(I)
771.4nm
OfI) MI)
811.5~ 868.1nm
N(I)
Snm min-’ were employed when scanning was required, and the output was presented on a Servoscribe chart recorder. Table 1 lists the atomic emission lines found to be most useful. Many of the gas-line components were common to both atmospheric and reduced-pressure plasma systems. Gas was fed via a flow meter through a length of 3 mm o.d. copper tubing flattened to restrict gas flow, which could then be varied by altering the applied gas pressure to the discharge tube, a piece of quartz tubing 6.5 mm o.d., 1.5 mm i.d. and 15 cm long. A Tesla coil was used to initiate the discharge. The vacuum system was constructed from 3 mm copper tubing and Swagelok compression fittings, and is shown diagramatically in Fig. 1. The pressure in the plasma tube was measured with a mercury manometer and a travelling microscope. Many problems were encountered initially with leaks in the system, which were evident as quite intense nitrogen and oxygen emission in samples of high-purity helium. The copper tubing had to be treated with care since repeated bending caused work-hardening and produced tiny hairline fractures. One attempt to overcome this problem involved replacing all the copper with nylon tubing: the emission from nitrogen and oxygen increased, since the diffusion of these gases through the plastic was even greater than through the microscopic fractures. The line was rebuilt again in copper, each piece being carefully bent to shape and then used without further adjustment. Copper was chosen in preference to stainless steel because it is softer and more easily bent-especially at the 3-mm o.d. size-and because it seals more easily in compression fittings to give a vacuum-tight joint. Connections to the quartz discharge tube were also made with compression fittings, but with silicon “o”-rings in place of brass ferrules. New components in the line were degassed by gentle heating with hot air from a hair drier while being continuously evacuated: in this way large blanks for oxygen, due to water, can be eliminated. 2.2. Preparation of gas mixtures Gas mixtures were prepared on a high-pressure mixing line which allowed the addition of individual pure gases to be monitored by pressure--on a 12 in. precision gauge reading to 300 bar, readable to 0.3 bar-and more accurately, by weight-using a Mettler model PE 16000 top-loading electronic balance reading up to 16 kg with a precision of 0.1 g. Aluminium sample cylinders were evacuated on the line using a rotary oil pump, then charged with each gas in turn from large stock cylinders, with the final pressure reaching 70 bar. Mixtures with l-10 % of Oz. Nz or Ar were prepared directly in this way, then diluted gravimetrically, after being left horizontal for 24 h to mix thoroughly, to produce the lower concentrations. 2.3. Analysis of the gas mixtures by GC The composition of the various mixtures, relative to the highest standard, was confumed by GC analysis. This was necessary because of the possibility of contamination by air at some stage. For the
flattened
copper
Photodiode thick walled -) slbca tube manometer m
gascylinder
*
vacuum pump
needle valve
Fig. 1. Schematic diagram of the MIP-OES apparatus.
recorder
MORAG MCKENNAet al.
672
two-component mixtures a laboratory-built instrument was used, fitted with a thermistor bead katharometer detector and molecular sieve 5A columns. Chromatograms from 2-ml samples were recorded on a Pye Unicam PU 4810 integrator, and the amounts of each gas were calculated from the areas of the peaks by comparison with those for a gravimetric standard with 10% each of nitrogen, oxygen and argon in helium (on a mole, i.e. volume % basis). When both argon and oxygen were present the analyses could not be carried out at room temperature. For these samples, a Perkin-Elmer F-30 gas chromatograph was used, with the 5A columns immersed in a cold bath at 0°C: the retention times, especially for nitrogen, are long, but the argon and oxygen are adequately separated.
3. RESULTS 3.1. The atmospheric plasma By using a TMolo cavity after Beenakker’s design, a plasma could be sustained in either helium or argon at atmospheric pressure. Tuning the cavity was quite difficult and ignition was often troublesome, especially when oxygen was present; when the oxygen content exceeded 5 %, ignition became impossible at the input power of 100 W being used. 3.1.1. Efict of applied power on emission intensity. Figure 2 shows that as applied power is increased the emission intensity also increases, but not linearly. No measurements were made at applied powers below 75 W since at lower powers the plasma becomes tinstable. GOODE [20] has reported similar behaviour. Sensitivity was adequate with the atmospheric-pressure plasma and stability acceptable at 75 W, which applied power was used for most of the subsequent work. This also facilitated comparison with the measurements made on the lowpressure plasma. 3.1.2. E&ct of heliumjlow rate. Figure 3 shows that the emission intensity from both argon and oxygen increases as the flow rate is increased. At higher flow rates the increase in emission intensity with flow rate is less pronounced. GOODE [20] also considered the effect of flow rate, but at higher flow rates than those used here. 3.1.3. Linearity of calibration graphs. Emission calibration graphs were prepared for oxygen and argon in helium using binary gas mixtures for the atmospheric-pressure helium MIP. Typical curves are shown in Fig. 4: the responses are decidedly non-linear over most of the range, though for levels below 0.1% it would seem that linearity (i.e. a slope of unity on the log-log graph) is approached. It should be pointed out that these signals are corrected for
70
80
90
100 110 120 130 140 150 Incident power (WI
Fig. 2. Relative emission signal (linear scale) as a function of applied power for the atmosphericpressure plasma. (0) 777.4nm for O2 at 1% in helium. (0) 811.5 nm for Ar at 1% in helium. [20] S. R. GOODE,N. P. BUDDIN,B. CHAMBERS, K. W. BAUGHMANand J. P. DEAVOR,Spectrochim.Acta 4OB,317
(1985).
673
MIP-OES analysis of gas mixtures
0.1 0.2 03
0.4 05
0 6 0.7 0 8 0.9
Flow (Lmin-'I
Fig. 3. Relative emission signal (linear scale)as a function of gas flow rate for the atmospheric-pressure plasma. Details as in Fig. 2.
I 0.0
I
O-03
I
/
0.1
03
,
1.0
I
10
Mole % minor component
Fig. 4. Log-log calibrations for the a~osphe~c-prep plasma. (0) 777.4 nm for O2 in O1 in helium mixes, (e) 811.5 nm for Ar in Ar in helium mixes.
background, and at the lower levels the correction becomes large and its variability causes difficulty in measurement of the sample signal. The background signal is due to oxygen in the air diffusing back into the discharge tube. At higher levels the quenching of emission from either minor component, and also from the helium itself, became serious, resulting in the very pronounced curvature of these graphs to lower signals than would have been expected. Because of the difficulties in tuning and igniting the atmospheric-pressure helium MIP, and the undesirable curvature of the calibration graphs, it was decided to investigate the potential of a low-pressure MIP for this application. 3.2. The reduced-pressure pkls?na By using the 214L #-wave cavity a plasma could be sustained in either helium or argon over a wide range of applied powers, with low retlected powers. Ignition was easy-indeed very often the plasma ignited spontaneously when the microwave power was switched on. 3.2.1. E’ct ofincident power on emission intensity. Figure 5 shows the effect of varying the applied microwave power over the range 20-130 W on the intensity of the helium emission at 706.5 nm in a pure helium plasma. Similarly shaped curves were obtained for the emission of argon at 811.5 nm, oxygen at 777.4 nm and nitrogen at 868.6 nm when mixtures of these gases
Moru~ MCKENNAet al.
614
I
I
I
I
I
20
40
60
80
I
1
I
100 120 140
Input power Iw)
Fig. 5. Relative emission signal (linear scale) as a function of applied power for the reduced-pressure plasma. ( x ) 706.5 nm for He in pure helium. (0) 777.4 nm for 0s in 1% O2 in helium. ( q!)777.4 nm for0,in1~0,+1~Arinhelium.(A)811.5nmforArin1~Arinhelium.(o)811.5nmforArin1~ Ar + 1% 0, in helium.
in helium were excited. The reflected power was found to reach a minimum (always less than 5 W) for applied powers in the range 7&90 W for all the gas mixtures. A power setting of 75 W was chosen as a compromise between sensitivity and efficiency. 3.2.2. Effecctofpressure. The effect of pressure in the discharge tube on the helium emission intensity is illustrated in Fig. 6 for a pure helium plasma and is in agreement with published findings [21]. It is clear that while decreasing the pressure improves the sensitivity, the precise control of the lower pressures becomes both more difficult and more critical. The combination of a fine metering valve (Whitey B-2MG) down-line from the plasma tube and a length of severely constricted copper tubing up-line from it enabled a working pressure of 15 mm Hg to be maintained to +O.l mm. 3.2.3. Linearity of response. Mixtures covering a range between 0.01 and 10 % v/v of argon, nitrogen or oxygen in helium were prepared and the emission measured at various wavelengths. Figure 7 shows the calibration curves obtained for the preferred wavelengths. In each case the upper limit for linear response lies around 1%. Further, when both argon and oxygen are present the emission from each is decreased. The effect this has on the calibration graphs can be seen in Fig. 8. Figure 5 shows that this effect cannot be eliminated simply by increasing the applied microwave power 4. DISCUSSION Bearing in mind the ultimate aim of developing an analytical detection system for in situ analysis of breathing mixtures for diving, the need for a vacuum pump is restrictive. The atmospheric pressure plasma was thus initially more attractive for this application. With the atmospheric plasma the need for precise pressure control also disappears. These investigations have shown, however, that in addition to the difficulties of tuning and ignition there are severe problems due to the ingress of air back along the discharge tube, and into the plasma region, giving rise to very large “blank” signals and causing all response to be non-linear. Observation of this phenomenon with an ICP used as a GC detector had earlier persuaded BROWN and FRY [22] to use an extended plasma tube to minimise the ingress of air. We attempted to overcome the effect by cementing a window on the end of the discharge tube and [21] K. W. BUSZHand T. J. VICKERS, Spectrochim. Acta 28B, 85 (1973). [22] R. M. BROWNand R. C. FRY, Anal. Chem. 53, 532 (1981).
MIP-OES analysis of gas mixtures
I
0 Fig. 6.Relatiye emission
I
1
I
I
I
67.5
I
10 20 30 40 50 60 PRESSURE (TORR)
signal (linear scale) as a function of pressure in the reduced-pressureplasma. (0) 706.5 nm for He in pure helium,
0.0’
’ 0 03
I
0.1
I
I
8
/
I
0.3 1.0 3 IO 30 Mole% minor component
Fig. 7. Log-log calibrations for binary mixtures in the reduced-pressureplasma. (0) 777.4 nm for Oz in O2 in helium mixes. (0) 811.5 nm for Ar in Ar in helium mixes. (0) 868.1 nm for N1 in N1 in helium mixes.
leading the gas away through a side arm and a long piece of flexible tubing. There was little improvement and the atmospheric plasma was abandoned. For emission spectroscopy to be a useful tool a linear relationship between concentration and emission signal is clearly an advantage. At least at low ~n~ntrations the reduced pressure plasma gives linear responses, but only up to around 1% v/v. BRASSEMet ~2. {23j found that for argon, nitrogen or hydrogen in helium the response was linear up to around lx, but at much lower pressures, and getting worse as the pressure increased. The use of emission lines at longer wavelengths seems to be satisfactory and the spectral interferences should be minimised since this part of the spectrum is less crowded. The complexity of e.g. the Nz emission spectrum in the near U.V.can be seen from the work described by BROIDA and CHAPMAN [24]. In the case of argon, the large number of lines [23] P.
BRASSEM,F.
J. M. J. MAESSENand L. DE GALAN, Speccrochim.Acta 314 537 (1976).
MORAGMCKENNAet al.
676
001
0.1
1,o
Mole % Ar
Fig. 8. Log-Log calibrations for argon at 811.5 nm in the reduced-pressure pIasma, for threecomponent mixtures in helium. (0) with 0.01% Oz. (e) with 0.1% O2 (Cl) with 1.0% 0,.
appearing close together, e.g. around 4OOnm, makes measurement of individual line intensities, as would be needed for measurement of plasma temperatures, very difficult. The lines listed in Table 1, lying between 700 and 900 nm, can be detected with good sensitivity by the small silicon photodiode which, by comparison with red-sensitive photomultipliers has the advantages of being much cheaper, smaller, and requiring much simpler power supplies. Since we envisage building a multi~~nnel instrument with several detectors, each with an appropriate interference titer to isolate the respective emission lines, the choice of photodiode over photomultiplier was obvious. In earlier reported studies [20,21, 231 the lines used were He(I) 388.9 nm; Ar(1) 416 nm; and N at 337.1 nm. BRASSEM [23] explained the variation in emission intensity with concentration of minor component in terms of changes in excitation conditions, particularly in electron temperature. No work has been published on the emission from multi-component gas mixtures: the limited results presented here on three-component mixtures suggest that the interferences will be complex. More work on these systems must be carried out before the suitability of microwave emission spectroscopy for the analysis of diving gas mixtures can be finally established; this work is now underway. Preliminary results obtained in our laboratory employing gas dilution systems with pure helium to extend the working range and reduce the interferences have been ~~~~ng. Ackaowiedgements-The authors are indebted to the SERC Marine Technology Directorate for financial support for this study, to Gas & Equipment Ltd for supplies of helium and, along with Wharton Williams Ltd and Sub Sea Offshore Ltd, for financing a preliminary study carried out by Mrs. EVA LAM.