Anal. Chem. 1994,66, 3400-3407
Low-Pressure Inductively Coupled Plasma Ion Source for Molecular and Atomic Mass Spectrometry E. Hywel Evans,’ Warren Pretorlus,t Les Ebdon, and Steve Rowland Department of Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, U.K.
A low-pressureinductively coupled plasma system is described
which can be used for both molecular and atomic mass spectrometry. Element-selective detection of organotin, organolead, organoiron, and organohalide compounds was possible using this source coupled with gas chromatography. Detection limits were as follows: tetraethyllead, 13 pg; tetrabutyltin, 35 pg; ferrocene, 33 pg; iodobenzene, 25 pg; bromobenzene, 50 pg; and chlorobenzene, 500 pg. By decreasing the power and plasma gas flow it was possible to sustain a helium plasma using only the carrier gas from the gas chromatograph, and mass spectra were obtained for chlorobenzene,bromobenzene, and iodobenzene that were similar to those obtainable using an electron impact ionization source. The molecular ion for chlorobenzenewas observed to 10 ng injected on-column. The effect of power on the mass spectrum was investigated. By adjusting the plasma gas flow and forward power, it was possible to influence the degree of fragmentation of the organic species. The potential exists for the development of a variable ion source with the ability to adjust the degree of fragmentation simply and rapidly. Inductively coupled plasmas (ICPs) have been used extensively as ion sources for elemental mass spectrometry (MS). Other plasmas such as microwave-induced plasmas (MIPS) and glow discharges (GDs) have been used less commonly. Conventionally, procedures such as electron impact (EI), chemical ionization (CI) fast atom bombardment (FAB), thermospray, and electrospray have been utilized as ion sources for molecular MS. However, the appropriate ion source will depend on the analyte to be studied and the degree of fragmentation which is desired. For instance, for molecular weight determinations of proteins, FAB would be the method of choice; and for total elemental determinations, the ICP would be the best source. The conclusion that can be drawn from this is that, to gain the appropriate degree of fragmentation, it is necessary to use the correct ion source. Evidently, it would be advantageous if a single ion source could fulfill all of the roles cited above. Microwave-induced plasmas have been investigated as a possible “soft” ionization source for molecular MS14 for the production of molecular fragment ions, although the most common use of a MIP is as a source of monoatomic ions. Heppner’ investigated a MIP formed with helium or hydrogen t Present addresss: Port Elizabeth Technikon, Private BagX6011,Port Elizabeth 6000, South Africa. (1) Heppner, R. A., Anal. Chem. 1983.55, 2170. (2) Poussel, E.; Mermet, J. M.;, Deruaz, D.; Beaugrand, C. Anal. Chem. 1988, 60, 923. (3) Olson, L. K.; Story, C. W.; Creed, J. T.; Shen, W.; Caruso, J. A. J. Anal. At. Specrrom. 1990, 5, 471. (4) Shen, W.; Satzger, R. D. Anal. Chem. 1991, 63, 1960.
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Analytical Chemlstty, Vol. 66, No. 20, October 15, 1994
at between 30 and 150 W of forward power and at a pressure of between 10 and 200 Torr, in conjunction with GC sample introduction and mass spectrometry. Compounds such as hexane and toluene were injected into a MIP using GC, though no actual chromatography was performed. The compounds were almost totally destroyed in the MIP and broken down to constituent elements which recombined to form simple polyatomic forms such as CH, CH4, and CO in the MIP tail flame. These compounds were then extracted into a mass spectrometer where they were ionized by using E1 and mass spectrometrically analyzed. Poussel et ala2utilized a MIP formed with argon, xenon or krypton and maintained at a power of between 25 and 50 W, at pressures of between 2 X to 6 X mbar. Compounds such as cyclohexane and dodecane were introduced into the tail flame of the MIP discharge, where they were fragmented and ionised. These species were extracted into a mass spectrometer through a skimmer cone of 1.2-mm diameter and yielded spectra similar to those obtained using E1 ionization. Similar work was performed by Olson et al.,3 who obtained spectra for chlorobenzene and toluene similar to the E1 spectra. They also introduced compounds through the whole length of a MIP discharge and found that the molecular ions disappeared, with a consequent increase in the intensity of the monoatomic ions. These experiments were performed by introducing the pure compound itself, or headspacevapor, so were not representative of analysis at trace levels. Atmospheric pressure ionization (API) sources have also been developed utilizing a MIP4 or glow d i s ~ h a r g e .The ~ latter workers noted that the predominant ions produced using the API source were the M+ and (M + 1)+ ions, while the same glow discharge source used under reduced pressure produced molecular ions accompanied by fragmentation. However, these workers used continuous sample introduction, and no work was performed with transient signals. The need for element-selective techniques for the detection and speciation of metals in the environment has lead to the development of numerous chromatographic techniques being coupled to element-selective detector^."^ The most promising element-selective detectors for coupling to chromatography are ICPMS, MIPMS, and MIP or ICP atomic emission detection (AED). Most of the chromatographic work with coupled plasma MS systems has focused on the use of high-performance liquid chromatography (HPLC) coupled to plasma mass ( 5 ) Chien, B. M.; Michael, S.M.; Lubman, D. M. Anal. Chem. 1993.65, 1916. (6) Ebdon, L. C.; Hill, S.J.; Ward, R. W. Anolysf 1987, 112, I . (7) Ebdon,L. C.; Hill, S. J.; Ward, R. W. Analysf 1986, 111, 1113. (8) Vela, N. P.; Olson, L. K.; Caruso, J. A. Anal. Chem. 1993.65, 585A. ( 9 ) Hill, S.J.; Bloxham, M. J.; Worsfold, P. J. J. Anal. A f .Spectrom. 1993, 8, 499.
0003-2700/94/0366-3400$04.50/0
0 1994 American Chemical Society
spectrometry sources, but relatively little work has been done on gas chromatography (GC) systems.1° Gas chromatography coupled to a plasma detector is generally limited to volatile organometallic compounds (e.g., alkylleads, alkyltins, etc.) and halogenated or other heteroatomic volatile organic compounds. The work carried out with GC/plasma-MS to date can be divided into studies using helium MIPMS and those using argon ICPMS. The use of atmospheric capillary GC/ICPMS has been applied to organotin and organolead compounds;' 1,12 more recently a high temperature GC/ICPMS system has been developed for the analysis of metalloporphyrins in geological ~amp1es.l~ These systems require only slight modifications to the ICPMS instrument and do not require any form of solvent venting. Low-pressure MIPShave also been used for element-specific analysis of organic compounds.1417 In that work, GC was coupled to low-pressure MIPMS for the element-selective determination of P, S , and C1 in a variety of organic compounds including pesticides; however, no molecular fragmentation studies were performed. Other workers have used a furnace to introduce a dry aerosol.18 Initial investigations of low-pressure (LP) ICPMS systems, using argon1g.20and heliumz1 as the plasma gases, have demonstrated the applicability of the technique for elementselective detection of organic compounds using GC sample introduction. In the work described in the present study a helium/argon LP-ICP has been used to obtain both atomic and molecular ions by introducing subnanogram amounts of compounds, via GC, directly into the plasma discharge. By adjusting the plasma gas flow and forward power, it was possible to influence the degree of fragmentation of the organic species. The preliminary results presented here suggest that LP-ICPMS is capable of being tuned for either elemental or molecular MS.
EXPERIMENTAL SECTION Low-Pressure Plasma. All experiments were performed with a modified inductively coupled plasma mass spectrometer (VG PlasmaQuad 2, Fisons Instruments Elemental, Winsford, UK). The instrument was modified in several respects. First, the standard sampling cone was replaced with a low-pressure sampler machined from aluminium (Machine shop, University of Plymouth), with a 2-mm orifice and an ultratorr fitting for a 1/2-in. pipe. Using this design, it was possible to form a (10) Pretorius, W.; Foulkes,M.; Ebdon, L.; Rowland,S.HRC & C, J. High Resolut. Chromatogr. Chromarogr. Commun. 1993, 16, 157-160. (11) Kim, A.; Hill, S.;Ebdon, L.; Rowland, S. HRC & C, J. High Resolut. Chromatogr. Chromatogr. Commun. 1992, 15, 665-668. (12) Kim, A.; Foulkes M. E.; Ebdon, L.; Hill, S.;Patience, R. L.; Barwise, A. G.; Rowland, S.J. J . Anal. At. Spectrom. 1992, 7, 1147. (13) Pretorius, W. G.; Ebdon, L.; Rowland, S.J. J. Chromatogr., in the press. (14) Creed, J. T.; Davidson, T. M.; Shen, W.; Caruso, J. A. J. Anal. AI. Spectrom. 1990, 5, 109. ( 1 5 ) Creed, J. T.; Mohamad, A. H.; Davidson, T. M.; Ataman, G.; Caruso, J. A. J . Anal. At. Spectrom. 1988,3, 923. (16) Story, W. C.; Olson,L. K.; Shen, W.; Creed, J. T.;Caruso, J. A. J. Anal. At. Spectrom. 1990, 5, 467. (17) Story, W. C.; Caruso, J. A. J . Anal. At. Spectrom. 1993, 8, 571. (18) Eberhardt, K.;Buchert, G.; Herrmann, G.;Trautmann, N. Spectrochim. Acta 1992, 478, 89.
(19) Evans, E. H.; Caruso, J. A. J. Anal. At. Spectrom. 1993, 8, 427. (20) Evans, E. H.; Pretorius, W.; Ebdon L.; Worsfold, P. J.; Rowland, S.Presented at the XXVIII Colloquium Spectroscopicum Internationale, York,UK, June 29-July 4, 1993; paper FL6.1. (21) Castillano, T. M.; Giglio, J. J.; Evans, E. H.; Caruso, J. A. J . Chromatogr.. submitted for publication.
Table 1. Typlcai Operating Condnlons Used for LP-ICPMS element-selective molecular detection fragmentation
forward power (W) reflected power (W) argon plasma gas flow (L m i d ) helium GC gas flow (mL min-l) skimmer/sampler spacing (mm) pressure in interface (mbar) pressure in torch (mbar)
200
15-50
35 1.o -3.5 4.5 2.1
7-30
13
0 -3.5 4.5
not measd not measd
vacuum seal between the low-pressure torch and sampler. The torch was constructed from quartz and was simply a 140mm-long quartz tube of l/2-in. o.d., with a l/4-inU-o.d.side arm through which the plasma gas could be introduced. Second, the pumping rate at the interface was increased from 44 to -75 L min-l by the addition of a second pumping port at 135O to theoriginal port, linked to thesamerotaryvacuum pump (Edwards E1M-18, Edwards High Vacuum, Crawley, Sussex, UK). Typical operating conditions are shown in Table 1. Gas Chromatography. A gas chromatograph (HRGC 5300, Carlo Erba) was interfaced to the rear of the low-pressure torch by means of a heated transfer line maintained at a temperature of 200 OC. The GC column passed through the transfer line and into the torch so that it was positioned 10 mm behind the rearmost turn of the ICP load coil. The vacuum seal between the column and the torch was made using Swagelock and ultratorr fittings. Two types of capillary column were used: a DB-1 HT, 0.32 mm X 15 m with a 0.1-pm film thickness (J&W, Fisons, Loughborough, UK); and a DB-5, 0.32 mm X 20 m with a 0.1-pm film thickness (J&W). The GC program was typically 50-200 OC at 20 "C/min with a column head pressure of 54 kPa using helium as the carrier gas, which resulted in a gas flow of 3.5 mL min-l. The instrumental setup is shown in Figure 1. Data AcquisitionParameters. Data were acquired by single ion monitoring and multiisotopic peak jumping. For elementselective detection it was only necessary to acquire data for one selected mass at a time; however, for the molecular fragmentation study the mass range between m/z 60 and 209 was monitored. Data acquisition parameters for both elementselective detection and molecular fragmentation are given in Table 2. It should be noted that the software supplied with the instrument was designed for multielement analysis of monoatomic ions, so was not ideally suited for a full mass range, data-capture experiment. Reagentsand Standards. Standards were diluted in hexane (HPLC grade, Rathburn Chemicals, Scotland, UK) to the required concentration. Tetrabutyltin, tetraethyllead, iodobenzene, chlorobenzene, bromobenzene, and ferrocene were obtained from Aldrich Chemicals (Gillingham, UK). N
RESULTS AND DISCUSSION Element-SelectiveDetection. Optimization of Plasma Gas. The plasma was sustained with argon gas introduced through the side arm of the low-pressure torch. The plasma gas flow was a critical parameter with respect to the analytical utility of the low-pressure ICP. The effect of plasma gas flow on the signals for bromobenzene and tetraethyllead, when determined Analyticalchemistry, Vol. 66, No. 20, October 15, 1994
3401
I Figure 1. GC/LP-ICPMS instrumentation used In thls work: A, skimmer; 6,low-pressure sampler; C, vacuum fitting: D, ICP load coli; E, tip of QC column; F, plasma gas; G, quartz torch; H, heated transfer line; I, gas chromatograph.
Table 2. Maw Spectrometer Data Acqukltion Parameters element-selective molecular detection fragmentation
mode no. of masses points per mass mass range (mlz) dwell time (ms) no. of sweeps per time slice no. of channels a Not
single ion 1 n/a@ n/a 164 nla 4096 max
peak jumping 150 1
60-209 1.28 2 nla
applicable.
0
02
0.4
0.6
0.8
1
1.2
Argon gas flow (Vmin)
in element-selective mode at m/z 79 and 208, respectively, are shown in Figure 2. Evidently the effect on signal was dependent on the type of compound studied. The signal for tetraethyllead was not detectable below a flow rate of 0.6 L min-l, whereas the bromobenzene signal was unaffected and was considerably enhanced when the flow was reduced to 0.2 L min-l. Several explanations may be considered to explain the contradictory behavior of the two compounds. First, the increase in signal at m/z 79 when the gas flow was reduced to 0.2 L min-1 could have been caused by the formation of polyatomic ions. However, a similar effect was observed for the iodobenzene signal at m/z 127 so this seems unlikely to be the explanation. Evidently, it would be useful to ascertain the effect of the gas flow on ionization, ion sampling, and throughput in order to try and explain the contradictory behavior of bromobenzene and tetraethyllead. At a gas flow of 0.2 L m i d the plasma was quite diffuse and extended from the tip of the GC column. As the gas flow rate was increased, the base of the plasma was pushed further forward and the plasma itself became progressively denser and brighter, until it was confined completely within the load coil at a gas flow greater than 0.8 L min-l. Hence, the effect of the gas flow was to condense the plasma at higher flows. Unfortunately, another consequence of reducing the gas flow was an apparent reduction in the gas temperature (Le., the quartz torch no longer glowed red and did not soften below a gas flow of 0.8 L min-l), with a probable decrease in the ionization temperature. A lower ionization temperature would explain the apparent decrease in the number of Pb+ ions formed, though not the increase in Br+. 3402
Analytlcel Chemlstty, Vol. 66,No. 20, October 15, 1994
Figure 2. Effect of plasma gas flow on peak area signal: (A) Br+ (m/z 79) from bromobenzene; (6)Pb+ (m/z 208) from tetraethyllead.
The lower gas temperature may have resulted in incomplete dissociation of the tetraethyllead molecules, though a similar effect on the bromobenzene molecules would also be expected, and in any case, this seems unlikely due to the extremely unstable nature of tetraethyllead. Another important consideration is the effect of gas flow on the sampling and skimming processes of ions from the plasma into the mass spectrometer. Sampling/skimming interfaces for plasma-MS have been using criteria derived for molecular beam experiment^.^^ The plasma gas is sampled from a relatively high pressure environment through an orifice into a low-pressure expansion chamber. In the expansion chamber the gases undergo adiabatic expansion to form a barrel shock region. Within the barrel shock is the so-called “zone of silence”,which is thought to be representative of the sampled plasma gases. The barrel shock takes the form of a cone with its apex at the orifice. The base of the cone is a diffuse collisional region where the expanding gas meets the background gas known as the “Mach disk”. In order to obtain the greatest ion throughput into the mass spectrometer, the gases must be sampled within the zone of silence and upstream of the Mach disk. The position of the Mach disk is dependent on the pressure differential between the sampled gas and the gas in the expansion chamber; and the orifice (22) Douglas, D. J.; French, J. B. J. Anal. At. Specrrom. 1988, 3, 743. (23) Olivares, I. A,; Houk, R. S. Anal. Chem. 1985, 57, 2674. (24) Campargue, R. J . Phys. Chem. 1984,88, 4466.
5 4.8
1 Position of skimmmer ..........................................................................
4.4
32 3
min-l, the gas temperature was estimated to be -1900 K, because of the tendency for the quartz torch to glow red and soften. Douglas and FrenchZ2have calculated that the local plasma temperature does not depart from the source temperature until very close to the sampling orifice. Assuming that the source conditions prevail at the sampling orifice, then for the low pressure torch, T = 1900 K; PO= 13 mbar (1300 m. Using these Pa), NO= 4.96 X loz2m-3, and DO= 2 X values, then X = 8 X m and Kno = 0.04 (the Knudsen number for the sampling orifice). This indicates that the flow through the sampling orifice is transitional between viscous and molecular flow (1 < Kn < 0.01)for the low pressure plasma. C a m p a r g ~ has e ~ ~shown that the skimming distance which gives maximum beam intensity is given by
i
005
0 15
0.25 04 0.6 Argon gas flow (Ilmin)
08
1
Figure 3. Effect of plasma gas flow on the distance of the Mach disk downstream of the sampling orifice ( X , ) .
diameter,24 as shown in eq 1, where X , is the distance of the X , = 0.67Do(Po/P,)’/2
(1)
Mach disk downstream of the sampling orifice, DO is the diameter of the sampling orifice, Le., 2.0 mm, POis the pressure in the torch, and P1 is the pressure in the expansion stage of the interface. In this work, the pressure was measured in the absence of the GC column at the rear of the torch, where the GC transfer line would normally be interfaced, and at the side arm to the expansion chamber. The influence of helium from the GC column (3.5 mL m i d ) was ignored because it was considered to be negligible in comparison with the argon gas flow (between 0.05 and 1 L min-l). The position of the Mach disk was calculated by use of eq 1, and the effect of argon gas flow on its position is shown in Figure 3. It is evident that the gas flow had very little effect on the position of the Mach disk (i.e., the Po/P1 ratio remained relatively constant). The actual skimming distance used was 4.5 mm, so the expanding gases were skimmed downstream of the Mach disk, though the extent of the Mach disk itself is ill defined. This means that the skimmed ions were not necessarily representative of those sampled from the plasma source because of the liklihood of collisional processes occurring outside the zone of silence, and the onset of background gas penetration through the skimmer. The nature of the gas flow through an orifice can be charaterized by the Knudsen number
Kn = AID
M(x/Do) = 3 . 2 6 ( ~ / D ~-) ~0.61(~/Do)-~/~ ’~
(5)
where Mis the local Mach number, xis the skimming distance downstream of the sampler, and DOis the sampling orifice diameter). The local temperature is given byZZ
(2)
where h is the mean free path and D is the orifice diameter. The mean free path can be calculated by use of simple kinetic theory:26 h = 3q/mN(8kT/~m)’~’
where X , is the optimum skimmer distance, Kna is the Knudsen number at the sampling orifice, POis the pressure in the source, and PI is the pressure in the expansion stage. For the lowpressure plasma at a gas flow of 1 L min-’, X , is calculated to be 1.4 mm, which is much less than the distance of 4.5 mm used in this work. To describe the flow through the skimmer it is necessary to calculate the mean free path at the skimming distance used (4.5 mm in this case). The theory that describes flow through the skimmer has been derived for skimming processes within the zone of silence. As already stated, in this work skimming was performed outside the zone of silence, but it is neverthless instructive to calculate the skimming Knudsen number to gain some idea of the skimming processes. The local Mach number is given by22*27
(3)
where q is the viscosity of argon at the specified temperature, m is the mass of argon (6.642 X kg), and Nis the number density. For the low-pressure plasma at a gas flow of 1 L (25) Dushman,S. InScientifc Foundationsof Vacuum Technique,2nd ed.;Lafferty, J.M., Ed.; Wiley: New York, 1962; p 80. ( 2 6 ) Atkins, P. W. Physical Chemistry, 4th ed; Oxford University Press: Oxford, U.K., 1992; pp 722-745.
where TOis the source temperature, Txis the local temperature, M is the Mach number, and y is the specific heat ratio of argon (513). For the low-pressure plasma at a gas flow of 1 L min-1 and a skimming distance of 4.5 mm, then M = 2.33, Tx = 680 K, the number density at the skimmer N , = 1.6 X 1021m-3, and X=2X m. Hence, the skimmer Knudsen number K b = 2, assuming a skimmer orifice diameter of 1.0 mm. This indicates that molecular flow is occurring through the skimmer, though the assumption has been made that the theory is applicable when skimming downstream of the Mach disk. (27) Ashkenas, H.; Sherman, F.S. Rarified Gas Dynamics. In Proceedings, 4th International Symposium of Rarified Gas Dynamics: de Lceuw, J. H., Ed.; Academic Press: New York, 1966; Vol. 11, p 84.
Analfljcal Ch”.tty, Vol, 66,No. 20, October 15, 1994
3403
300 3
Table 3. Summary of Performance Parameters for the Low-Pressure Plasma at Different Gas Flows and Source Temperatures.
in source
at skimming position
gas source A N flow temp X, (X10-4 (~1022 (L min-l) (K) (mm) m) m-3) Kn 1.0 0.2 0.2 0.2
1900 1900 1500 1100
1.4 1.0 1.1 1.3
0.8 2.2 1.7 1.2
5.0 1.7 2.2 3.0
0.04 0.1 0.08 0.06
A N ( ~ 1 ~(XI020 3
m)
m-3)
Kn
2.0 5.9 4.4 2.7
16 5.5 6.9 9.4
2 6 4 3
0
2
4
6
8
Time (minutes)
0 X,, optimum skimming distance; A, mean free path; N , number density; Kn. Knudsen number.
_ _ _ _ _ _ _ _ _ ~
Table 4. Detectlon Llmlts Obtalned for a Selection of Standards, Using Element-SpecHlc Detection.
compound
m / z monitored
chlorobenzene ferrocene bromobenzene tetrabutyltin iodobenzene tetraethyllead
35 56 79 120 127 208
detection limit (pg) 5OOb 3 3c 50b 35 c 25b 13c
Argon plasma gas 1.0 L m i d ; Helium carrier gas -3.5 mL min-l. ratio. c Three times thestandard deviation of blank.
b Three times the signal-to-noise
0
Table 3 contains a summary of the critical parameters for ion sampling and skimming, calculated for different gas flows and source temperatures. As can be seen, there is not a great deal of difference in the mean free path, the number density, or the Knudsen number for the different gas flows and source temperatures, so it is difficult to explain the contradictory behavior of the two analytes in terms of ion sampling and skimming. However, since skimming was performed downstream of the Mach disk, the calculated values may not be representative of the actual conditions at the skimmer, and it is quite possible that further recombination and ionization reactions could be occurring. Another possibility is that, because of the relatively large sampling orifice diameter used and hence the small pressure differential between the source and the expansion chamber (
