Low Pressure Inductively Coupled Plasma Ion Source for Molecular and Atomic Mass Spectrometry: The Effect of Reagent Gases GAVIN O’CONNOR, LES EBDON AND E. HYWEL EVANS* Department of Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, UK PL 4 8AA A low pressure inductively coupled plasma (LP-ICP) source, covering the range in-between, has been equated to that for sustained at only 6 W and utilising 6 ml min-1 helium, has the holy grail. However, a more realistic approach of using been investigated as an ionisation source for molecular and one ionisation source to provide alternately molecular and atomic mass spectrometry.Iodobenzene and dibromobenzene atomic information, but not covering the whole range, has were introduced to the LP-ICP via gas chromatography and been achieved by a number of research groups.1 yielded purely atomic ion signals for the iodine and bromine Plasma sources for MS have generally been used for elemenpresent, with detection limits of 4 and 76 pg for iodobenzene tal analysis.This association can be mainly attributed to the and dibromobenzene, respectively. The addition of nitrogen to success of the atmospheric argon ICP, which combines a high a LP helium ICP increased the molecular ion signal for thermal temperature source with almost complete atomisation chlorobenzene, with a detection limit of 2 pg. However, the and ionisation. One disadvantage of the atmospheric ICP is addition of nitrogen did not aid the production of molecular that it is diYcult to sustain plasmas using gases other than ions of iodobenzene and dibromobenzene.A study of the eVect argon. An MIP can be formed using a variety of gases, of skimmer spacing and forward power revealed considerable including helium,1,2 which has a higher ionisation potential spatial separation of ionisation processes within the expansion than that of argon and so leads to a more ionising plasma. chamber of the molecular beam interface.On the addition of However, plasma sources have also been used to provide both 0.07 ml min-1 isobutane the LP-ICP yielded mass spectra molecular and atomic mass spectra. Shen and Satzger3 have similar to those obtained by an electron impact source. shown that it is possible to form molecular ions, indicative of However on the addition of more isobutane only the molecular the analyte compound, using an atmospheric pressure helium ions (M+) for chlorobenzene, iodobenzene and dibromobenzene MIP.In this work the analyte was introduced into the after were observed. The detection limits for the instrument plume of the plasma so did not experience the full force of the operating in the molecular mode were 100, 140 and 229 pg for source. Reduced pressure MIPs have also been investigated chlorobenzene, iodobenzene and dibromobenzene, respectively. for providing both atomic and molecular mass spectra.4–6 However, in these studies a pure compound or vapour was Keywords: L ow pressure inductively coupled plasma; helium generally introduced into the source, which provides little plasma; mass spectrometry; reagent gas; element selective information on how such a source would behave if used for detection; molecular ion trace level determinations.ICPs, operated at reduced pressure and sustained with Mass spectrometry (MS) is a continuously growing area of argon, have been used for the production of atomic mass analytical chemistry.Proof of this is the ever increasing number spectra, using gaseous and vapour sample introduction.7–9 of analytes being qualitatively and quantitatively determined, Evans et al.10 have investigated the use of a low pressure (LP) in a wide selection of matrices. However, the increased use of helium ICP, at powers between 4 and 40W and 1 mbar MS can be partly attributed to the proliferation of sources pressure, for the production of mass spectra similar to those now available.The use of soft ionisation techniques, such as obtained with an EI source, for a series of organometallic and chemical ionisation (CI), fast atom bombardment (FAB), halogenated species introduced by gas chromatography (GC). matrix-assisted laser desorption/ionisation (MALDI) and elec- On increasing the power and pressure of this source it was trospray ionisation (ESI), have allowed ionisation of fragile, possible to increase the degree of fragmentation until at 150 W long chain, high molecular weight hydrocarbons without the and 10 mbar pressure total fragmentation occurred.Kohler total destruction of the analyte molecular ion, hence allowing and Schlunegger11 used a Penning ionisation source to provide molecular weight determination. At the other end of the a tuneable degree of fragmentation for a series of gaseous ionisation source spectrum are the harsh ionisation sources. organic compounds. This source was investigated for both These include inductively coupled plasmas (ICP), microwave positive and negative ion formation and was said to give induced plasmas (MIP) and glow discharge (GD) sources.spectra similar to those obtained with EI, GD and ICP sources. These sources are generally used to totally atomise analyte Olson et al.12 have used an rf GD source, with GC sample compounds, in the case of ICP, MIP and GD sources, allowing introduction, for the speciation of a series of organotin and ultratrace elemental analysis.Electron impact (EI) sources are organolead compounds, and observed molecular fragment intermediate sources, used to fragment organic compounds, peaks from the analyte compounds. From these studies it has providing structural information on the analyte. This depenbecome obvious that an LP plasma source is capable of being dence on ion source has led to many laboratories purchasing operated in a tuneable mode. Recently a specially designed a selection of sources and employing the relevant experts to instrument has been assembled to further investigate the use operate them.These extra capital and employment costs on of an LP-ICP as a tuneable source.13 top of instrumental running costs have made MS a costly field To date, the analytical figures of merit for the LP plasma and hence unattractive to many potential users. sources have been determined using the source in its atomic The search for a universal ionisation source, capable of operating as both a harsh and soft ionisation source and mode.In order to obtain molecular spectra, large quantities Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1263–1269) 1263of the analyte have been introduced into the source. Thus, the 20 °C min-1 with a helium carrier gas flow rate of 3 ml min-1. A diagram of the instrumental set-up is shown in Fig. 1. source has been used for trace elemental analysis but has not been used for providing molecular fragment ion information on trace level analytes.It has been suggested that the partial Data Acquisition Parameters pressure of the analyte in the LP-ICP is a contributing factor in molecular fragment ion formation.13 This would explain the Data were acquired on a Hewlett-Packard MS workstation, non-linear relationship between concentration and molecular with HP59970A (Version 3.1) software, which was interfaced ion signals in the LP-ICP. to the MSD.The ions were detected using two diVerent MS In the present study some of the problems associated with operating modes. For structural information on the analytes the non-linear nature of calibration, for molecular fragment the instrument was operated in scanning mode, where the ions, in an LP-ICP have been addressed. Initial studies on the mass range 60–800 m/z was monitored. For quantitative deteruse of reagent gases in the LP-ICP suggest that by altering mination of the analytes the instrument was operated in the composition of the plasma gas alone, it is possible to utilise selective ion monitoring (SIM) mode.In this mode of operation the LP-ICP as a soft ionisation source, yielding spectra similar the molecular ions, halogen ions and phenyl ions of the to that of a CI source, or as a harsh ionisation source which analytes were monitored. provides only elemental information, such as an atmospheric ICP. Furthermore the source can be operated in a tuneable Reagents and Standards mode between hard and soft ionisation regimes. Standards were diluted in pentane (HPLC grade, Rathburn Chemicals, Walkerburn, UK) to the required concentration.EXPERIMENTAL Chlorobenzene, iodobenzene and dibromobenzene were Low Pressure Plasma Mass Spectrometer obtained from Aldrich (Gillingham, UK). Nitrogen (99.9%) and isobutane (99%) were obtained from Air Products (Crewe, A detailed description of the design and optimisation of the Cheshire, UK).GC–LP-ICP-MS system used in this study has been given previously.13 In brief, a Hewlett-Packard (Stockport, Cheshire, UK) mass selective detector (MSD) was modified to enable it RESULTS AND DISCUSSION to analyse and detect ions from the LP-ICP. This was achieved To date LP-ICPs have been sustained with mainly argon or by using a custom made ion sampling interface. The LP plasma helium gas. The 1 1 min-1 argon LP-plasma has been utilised was sustained using a modified rf generator and matching for the production of atomic mass spectra, totally atomising network, in a 140 mm long quartz tube of 1/2 od, with a 1/4 analytes introduced to the source via a GC instrument.The od side arm to which a calibration vial containing perfluorohelium LP-ICP has been used as a dual mode ionisation tributylamine (PFTBA) was attached. The quartz plasma torch source producing both atomic and molecular ion mass spectra, was connected to the ion sampling interface via an LP sampling depending on the gas flow, plasma power and torch pressure cone (Machine shop, University of Plymouth), which was used.However, in the fragmentation studies relatively large machined from aluminium, had a 2 mm orifice and an Ultraamounts of analyte (>50 ng on-column) have been required torr fitting for a 1/2 pipe. This enabled a vacuum seal to be to facilitate the fragment ion formation. Also, the response formed between the LP torch and the ion sampling interface.obtained from the fragments produced by the LP-ICP was not The reagent gases were added to the plasma gas via the side linearly related to the analyte concentration. These two phenarm tube of the quartz torch. The amount of gas added was omena together are suggestive of the analyte playing a major controlled using a scaled needle valve (Edwards High Vacuum, role in the fragmentation and ionisation process, i.e., the Crawley, West Sussex, UK). Typical operating conditions are analytes were self ionising above a certain concentration.In shown in Table 1. order to suppress this phenomenon in LP-ICP-MS it was decided to investigate the eVect of reagent gases on analyte Gas Chromatography signal and molecular fragment formation. A gas chromatograph (PU 4550, Pye Unicam, Cambridge, UK) fitted with an on-column injector, was interfaced to the Nitrogen Addition LP-ICP-MS instrument by way of a heated transfer line Nitrogen was added to a 3 ml min-1 helium plasma via the maintained at a constant temperature of 250 °C. The GC side arm tube of the quartz torch.The amount of nitrogen capillary column used was a DB5 0.32 mm×30 m with a added was controlled using a scaled metering valve. The flow 0.1 mm film thickness (J & W, Fisons, Loughborough, UK). rate of the nitrogen gas through the valve was measured at The capillary column was passed through the heated transfer atmospheric pressure, for a series of needle valve settings, and line and into the LP torch, the vacuum seal being made using a combination of Ultra-torr and Swagelock fittings.This configuration has previously been described in more detail.10 One microlitre of a mixed standard was injected on-column and the GC programme was typically 40–110 °C at Table 1 LP-ICP-MS operating conditions Mass spectrometer Modified Hewlett-Packard MSD L ow pressure plasma— Forward power/W 6 Reflected power/W 0 Pressure/torr— Torch 0.2 Interface 0.03 Analyser <10-6 Fig. 1 Schematic diagram of the GC–LP-ICP-MS system. 1264 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12the flow of gas through the orifice of the needle valve, while diVered greatly depending on the skimming distance and fragment ion studied. Fig. 3 shows the eVect of power on the operating under LP conditions, was then calculated using Poiseuille’s relationship.14 signal intensity for fragment ions for PFTBA with a sampler– skimmer distance of 7 mm.This shows that at this skimming It would be expected that the helium–nitrogen plasma would diVer in temperature from a helium only LP-ICP, due to the distance the optimum power for the 69 and 219 m/z ions was between 6 and 8 W. The 69 m/z fragment ion optimises at the diVering thermal conductivity of nitrogen compared with helium, but also because of the diVering ionisation potentials highest power, between 7 and 8 W, whilst the 502 m/z ion showed a rapid decrease in signal as the power was increased.of the gases. If a change in the plasma gas kinetic temperature occurred then the ion flux through the sampler and skimmer As the power was increased the 502 m/z fragment of PFTBA was quickly broken down. The increase in the 219 and 69 m/z would also change. The physical processes that describe this phenomenon have been described in detail elsewhere.1,10,15,16 signals for the PFTBA suggests that the 502 m/z ion was further fragmented, hence increasing the signals of the lighter It has been our experience that for an LP-helium ICP the experimental optimum pressure and flow conditions were fragments.However, above 8 W forward power even the smaller molecular fragments began to disintegrate, which diVerent to those calculated using theory,13 hence it was decided to experimentally optimise the ion sampling con- should add to the atomic ion signals, though these could not be monitored because of the high background signals between ditions.For the optimisation study PFTBA was introduced into a helium–nitrogen LP-ICP. Three fragment ions of 12 and 32 m/z. Optimisation of the helium carrier gas flow and the nitrogen PFTBA, at 69, 219 and 502 m/z, respectively, were continuously monitored while the plasma forward power and the sampler– reagent gas flow for the production of stable molecular ions was then performed by introducing 10 ng of chlorobenzene skimmer spacing were optimised.Fig. 2 (a)–(c) shows the resulting plots of the signal intensity versus skimmer–sampler into the plasma via the GC instrument. The molecular ion for chlorobenzene (112 m/z) and the phenyl ion (77 m/z) were spacing and forward power for these fragment ions. The points labelled ‘A’ correspond to maxima on each plot. The plots are continuously monitored for three repeat injections of the 10 ng ml-1 standard. The helium carrier gas flow had little eVect shown in two dimensions only to help reveal the pertinent features. If the plots were to be shown in three dimensions they would reveal three-peak plots with the central peaks being the most intense for the 69 and 219 m/z fragment ions.This phenomenon has been described previously for helium only LP-ICP-MS13 and suggests the formation of several ‘shock’ regions behind the sampler. For the helium–nitrogen plasma the fragment ions at 69 and 219 m/z [Fig. 2 (a) and (b)] gave rise to maximum signal intensity at a skimming distance of 7 mm, with less intense peaks at 4 and 10 mm.However, the fragment ion at 502 m/z [Fig. 2 (c)] yielded no maxima between 5–9 mm and instead yielded maxima at 3 and 10 mm. This may be because the higher mass fragment at 502 m/z underwent a diVerent ionisation process compared to the lower mass fragments or was ionised in a diVerent part of the plasma or interface. Alternatively, the helium–nitrogen plasma may simply cause the molecular ion at 502 m/z to Fig. 3 Plot of normalised signal intensity versus plasma power for further fragment into smaller molecular species. the fragment ions of PFTBA at 69, 219 and 502 m/z, at 7mm skimming distance. It is also evident that the optimum plasma operating power Fig. 2 Surface contour plots showing the eVect of plasma power and skimming distance on the signal intensity of PFTBA at: (a) 69 m/z; (b) 219 m/z; (c) 502 m/z. The points labelled ‘A’ indicate intensity maxima.Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1265on the chlorobenzene signal (Fig. 4), with both the phenyl and eVects, or that the decreased thermal conductivity of the nitrogen may have stabilised the LP-ICP. molecular ion remaining fairly constant in intensity between 2 and 5 ml min-1. However, as the carrier gas flow was increased Once the optimisations were completed, an investigation of the analytical figures of merit (given in Table 2) was performed. the signals became increasingly unstable, indicated by the increased standard deviations of the chlorobenzene signals, The figures of merit were obtained by SIM for the molecular ion of chlorobenzene (112 m/z).The detection limit of 2 pg shown in Fig. 4. The optimum carrier flow rate was found to be 3 ml min-1. The signal for the molecular ion decreased suggests that the LP helium–nitrogen plasma is capable of providing structural information on the analyte even at ultra above 6 ml min-1 helium.This could be due to the extra gas increasing the electron and ion number density and thermalis- trace levels. Also the nitrogen addition improved the linear range of calibration, with calibration over three orders of ing the plasma. This would lead to a greater amount of collisions between the analyte and electrons, which in turn magnitude possible. This is a vast improvement compared to helium only LP-ICP-MS for which calibration was not poss- would lead to increased fragmentation of the analyte causing a reduction in the molecular signals.This theory would seem ible. Fig. 6 shows a SIM chromatogram at 112 m/z for a 100 pg on-column injection of chlorobenzene illustrating the excellent to be confirmed by the addition of more helium gas. On the addition of 7 ml min-1 of helium the molecular ion decreased signal to noise obtained. These results suggest that addition of nitrogen to the whilst the phenyl ion signal increased.This may be indicative of the molecular ion fragmenting and adding to the phenyl LP-ICP-MS instrument would cure the problems observed previously.10,13 However, on the addition of analytes with ion signal, however, the precision was poor so it was not possible to draw a firm conclusion. Above 7 ml min-1 helium retention times greater than 2 min, only the atomic signals were observed. This was thought to be due to the influence of the phenyl and molecular signals disappeared, which could be because the ions became totally atomised, yielding only atomic the tail of the solvent peak on chlorobenzene due to the short retention time of the latter.In order to minimise this eVect it information. Alternatively, these eVects could be due to changing the chromatographic conditions. was decided to investigate the use of isobutane as the reagent gas. The eVect of the nitrogen gas added to a 3 ml min-1 helium plasma on the chlorobenzene signal is shown in Fig. 5. The signal for the molecular ion peak at 112 m/z for chlorobenzene Isobutane Addition was relatively unaVected by the nitrogen as the signal remained fairly constant up to 2.1 ml min-1. With nitrogen flows above Isobutane was added to the LP helium plasma in a similar 2.1 ml min-1 the signals for both the molecular ion and the manner to nitrogen and its eVect on the molecular, phenyl and phenyl ion were reduced by over 50%. Again this may be due atomic ion signals for a series of halobenzenes was investigated.to the increased electron and ion density in the plasma further The analytes were injected, approximately 10 ng each fragmenting the analyte. A point of interest is the stability of on-column, as a mixed standard. Fig. 7 (a) shows the eVect of the analyte signals, even above a combined gas flow of the isobutane on the signals for 10 ng on-column injection of 7 ml min-1. This suggests that the unstable signals obtained chlorobenzene.The atomic signal for chlorine has not been on adding helium carrier gas were due to chromatographic shown because fragment ions from the reagent gas interfered with ion signals below 58 m/z. With a 3 ml min-1 helium only plasma, the molecular ion of chorobenzene was the parent ion. On the addition of the reagent gas both molecular ion and phenyl ion signals were greatly increased. However, as the reagent gas partial pressure was further increased the phenyl ion peak disappeared, leaving only the molecular ion.This is Table 2 Analytical figures of merit for chlorobenzene using a 0.43 ml min-1 nitrogen, 3 ml min-1 helium LP-ICP Single ion monitoring, mass monitored 112 m/z Linear range studied/decades 3 Slope/counts pg-1 99 r2 (regression coeYcient) 0.985 Slope of log–log plot 1.012 Detection limit*/pg 2 Fig. 4 EVect of helium carrier gas flow rate on the signal intensity of RSD† (%) 8.5 a 10 ng on-column injection of chlorobenzene, in a nitrogen–helium LP-ICP.* LOD=3s/slope. † RSD (%) for five replicate 10 pg injections. Fig. 5 EVect of nitrogen reagent gas flow rate on the signal intensity Fig. 6 Chromatogram of a 100 pg on-column injection of chlorobenzene for a helium–nitrogen (3.0 and 0.43 ml min-1) LP-ICP using of a 10 ng on-column injection of chlorobenzene, in a 3 ml min-1 helium LP-ICP. selected ion monitoring at 112 m/z. 1266 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Fig. 8 Total ion chromatogram for 10 ng on-column injection of chlorobenzene, iodobenzene and dibromobenzene for a helium–isobutane (3.0 and 0.07 ml min-1) LP-ICP.Fig. 7 EVect of isobutane reagent gas flow rate on the signal intensity of the molecular and fragment ions of (a) chlorobenzene, (b) iodobenzene and (c) dibromobenzene in a 3 ml min-1 helium LP-ICP. consistent with the isobutane–helium plasma acting as a conventional CI source where the partial pressure of the reagent gas often determines the analyte spectra obtained.Unlike the nitrogen reagent gas, the eVects of the isobutane were consistent throughout the chromatographic run. Fig. 7 (b) and (c) show the eVect of the isobutane on iodobenzene and dibromobenzene which had retention times of 1.1 and 2.0 min, respectively. For the 3 ml min-1 helium plasma, with a 10 ng on-column injection, the only ions that were observed were the atomic signals Fig. 9 Mass spectra scans obtained from an isobutane–helium (0.07 and 3.0 ml min-1) LP-ICP for 10 ng on-column injection of (a) chlorob- for the halogens.When 0.07 ml min-1 of isobutane reagent enzene, (b) iodobenzene and (c) dibromobenzene. gas was added the phenyl and molecular ions were observed. This yielded spectra very similar to those obtainable by EI source MS. On the addition of more isobutane the phenyl and and in the case of iodobenzene no MH+ peak was visible. This, along with the reduction in molecular and fragment ion atomic halogen signals were no longer observed and only the compound molecular ion remained, yielding spectra similar to signals on increasing the isobutane partial pressure, suggests that the isobutane was not behaving as a proton transfer those expected from CI source MS.However, on the addition of greater than 1 ml min-1 isobutane the molecular ion signals reagent gas. Also, no quasimolecular ions were observed. On the addition of 0.07 ml min-1 of isobutane the major reagent started to reduce in intensity.The halogen and phenyl ions for the analytes did not increase on the reduction of the molecular ion was 57 m/z. This is consistent with the loss of a proton from the isobutane, however, it has already been shown that ions. This suggests a reduction in the ionisation power of the source, because if the ionisation power increased one would protonation of the analytes was not the dominant ionisation process. As the reagent gas concentration was increased the expect to see a corresponding increase in the signal of the lower mass fragment ions as the molecular ion decomposed.most abundant reagent ion changed from 57 to 43 m/z, which is consistent with the loss of a methyl group from isobutane. Fig. 8 shows a total ion chromatogram for a 10 ng on-column injection of chlorobenzene, iodobenzene and This suggests that as more isobutane was added the plasma ionisation processes were getting harsher, because greater dibromobenzene, obtained using a 0.07 ml min-1 isobutane–3 ml min-1 helium LP-ICP.The resulting mass fragmentation was observed, however, analyte fragmentation exhibited the opposite trend. An alternative explanation may spectra for each compound are shown in Fig. 9 (a)–(c). The predominant ionisation mechanism for isobutane in CI is be that the ionisation process of the helium plasma was suppressed by the presence of the isobutane, and that as more proton transfer. However, the mass spectra of the halobenzenes studied [Fig. 9 (a)–(c)] show little sign of protonation with the isobutane was added a greater amount of energy was required to form the reagent ions, thereby leaving less energy to ionise MH+ peak being less than one third the intensity of the M+, Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1267Table 3 Analytical figures of merit for chlorobenzene, iodobenzene and dibromobenzene, using a 0.25 ml min-1 isobutane, 3 ml min-1 helium LP-ICP Analyte Chlorobenzene Iodobenzene Dibromobenzene Selected ion monitoring, mass monitored 112 m/z 204 m/z 236 m/z Linear range studied/decades 3 3 3 Slope/counts ng-1 84 645 28 405 4905 r2 (regression coeYcient) 0.9925 0.9848 0.9925 Log–log slope 0.85 0.74 0.70 Detection limit*/pg 100 140 229 RSD† (%) 12 6 5 * LOD=3s/slope.† RSD (%) for five replicate 380 pg injections. the analyte and resulting in molecular ion production of the analyte. This suggests that the source was not acting as a conventional CI source and that a number of ionisation mechanisms may be taking place.This is consistent with other plasma sources where a number of non-equilibrium properties are used to describe the plasma ionisation characteristics. This is a well known phenomenon because most plasmas do not exhibit thermal equilibrium even at atmospheric pressure, let alone at reduced pressure. Charge transfer is a well known ionisation mechanism in helium plasmas, and if ionisation was occurring via charge transfer in a helium–isobutane plasma one would expect a small degree of fragmentation and ionisation due to the low ionisation potential of isobutane (10.57 eV).17 The figures of merit for the helium–isobutane plasma operating in the molecular mode are shown in Table 3.Helium Addition In the initial studies performed using an LP helium ICP the dependence of molecular ion formation on the analyte concen- Fig. 10 Mass spectra scans obtained from a 6 ml min-1 helium tration was suggestive of chemical ionisation processes pre- LP-ICP for a 50 ng on-column injection of (a) iodobenzene and dominating in the plasma.If the helium was acting as a reagent (b) dibromobenzene. gas for conventional CI the expected predominant ionisation process would be charge transfer. The rate of charge transfer Dublin, Ireland) in place of the needle valve. A 6 ml min-1 is dependent on the partial pressure of reagent gas and analyte helium LP-ICP was then studied for the production of elemen- in the source.The survival of molecular ions in a charge tal mass spectra for iodobenzene and dibromobenzene. Fig. 10 transfer source is also dependent on the internal energy of the (a) and (b) show the resulting mass spectra scans (60–240 m/z) ion. If the internal energy is large (>5 eV) a great deal of of a 50 ng on-column injection of the standards and shows the fragmentation would be expected. The internal energy of a existence of only the atomic signals for the iodine (127 m/z) molecular ion can be calculated using eqn.(1):18 and bromine (79 and 81 m/z) even at this relatively high Eint=RE(X+)-IP(M) (1) concentration. This shows that the 6 ml min-1 helium plasma where RE(X+) is the recombination energy of the reagent ion (24.6 eV for helium) and IP(M) is the ionisation potential of the analyte molecule. This would lead to an internal energy of over 13 eV for the molecular ions of the halobenzene series studied, with ionisation potentials between 9–11 eV.Hence, extensive fragmentation of the analyte molecules would be predicted using a dense helium plasma. However, in conventional CI MS it is not unusual to observe molecular ions for organic molecules, with ionisation potentials less than 10 eV, when using helium as the reagent gas. Therefore, the presence of the rf magnetic field may induce collisional energy exchange between excited electrons and the analyte, increasing the ionisation power of the plasma.Hence, by increasing the helium partial pressure in the LP-ICP, the rate of charge exchange would increase. This should lead to greater fragmentation, and eventually atomisation, of the analyte molecules, leaving only the atomic ions to be detected. This suggests the possibility of utilising a low flow helium LP-ICP-MS for atomic MS. To test this hypothesis a helium make up gas was added to Fig. 11 Extracted ion chromatograms for a 50 ng on-column injection the plasma gas, via the side arm of the LP torch.The helium of (a) iodobenzene at 127 m/z and (b) dibromobenzene at 81 m/z, using a 6 ml min-1 6W helium LP-ICP-MS instrument. was introduced using a mass flow controller (Unit Instruments, 1268 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 4 Analytical figures of merit for iodobenzene and dibromobenzene, using a 6 ml min-1 helium LP-ICP Analyte Iodobenzene Dibromobenzene Selected ion monitoring, mass monitored 127 m/z 81 m/z Linear range studied/decades 3 3 Slope/counts pg-1 235 129 r2 (regression coeYcient) 0.999 0.999 Log–log slope 0.890 0.881 Detection limit*/pg 4 76 RSD† (%) 8 12 * LOD=3s/slope.† RSD (%) for five replicate 100 pg injections. operating at only 6 W forward power can atomise and ionise only molecular ions of the analytes, as it would in CI source MS, at low level concentrations. the halobenzenes.Fig. 11 (a) and (b) show extracted ion chromatograms for 50 ng on-column of iodobenzene and dibromoben- An LP plasma sustained at 6 W and utilising only 6 ml min-1 of helium has been used to totally atomise both zene at 127 and 81 m/z, respectively. The chromatograms show extensive peak tailing which is thought to be due to the analyte iodobenzene and dibromobenzene, producing atomic mass spectra. This proves that a GC–LP-ICP-MS system is capable atomic ion interacting with the wall of the plasma torch, whereas this does not occur for the molecular ion signals using of providing diVerent degrees of fragmentation for a series of halobenzenes. the same chromatographic conditions.The figures of merit for the 6 ml min-1 helium only The authors would like to thank: BP International (Sunbury LP-ICP-MS are shown in Table 4. The detection limits for the Group) for their kind donation of the HP 5970 MSD; the instrument operating in the atomic mode, reported in this NuYeld Foundation for the provision of an instrument devel- study, are comparable to those obtained by GC–LP-MIP-MS, opment grant; and the University of Plymouth for continuing namely 22, 0.1 and 3.5 pg for chlorotoluene, iodobenzene and financial support of G.O’C.bromononane, respectively.1 Studies of a GC–LP-ICP-MS system sustained with 0.5 l min-1 of helium have yielded element selective detection limits of 2.9 and 3.8 pg for chlorob- REFERENCES enzene and bromobenzene.1 In comparison, detection limits 1 Evans, E.H., Giglio, J. J., Castillano, T. M., and Caruso, J. A., given for the Hewlett-Packard MS instrument operating with Inductively Coupled and Microwave Induced Plasma Sources for an EI source are typically 10 pg, for SIM of the molecular ion Mass Spectrometry, ed. Barnett, N. W., Royal Society of of methyl stearate at 298 m/z. It is interesting to note that in Chemistry, Cambridge, 1995. 2 Chambers, D. M., Carnahan, J. W., Jin, Q., and Hieftje, G., the previous study10 no peak tailing for the atomic species was Spectrochim.Acta, Part B, 1991, 46, 1745. observed. With a 1 l min-1 argon LP-ICP there is a distinct 3 Shen, W., and Satzger, R. D., Anal. Chem., 1991, 63, 1960. central channel evident, much like a conventional atmospheric 4 Heppner, R. A., Anal. Chem., 1983, 55, 2170. pressure ICP. However, unlike an atmospheric pressure ICP 5 Poussel, E., Mermet, J. M., Deruaz, D., and Beaugrand, C., Anal.this is thought to be formed by the pressure drop at the 2 mm Chem., 1988, 60, 923. diameter sampler orifice pulling the central portion out of the 6 Olson, L. K., Story, W. C., Creed, J. T., Shen, W., and Caruso, J. A., J. Anal. At. Spectrom., 1990, 5, 471. plasma. This eVectively pulls the analyte ions into the centre 7 Evans, E. H., and Caruso, J. A., J. Anal. At. Spectrom., 1993, 8, 427. of the plasma and away from the torch walls. At very low gas 8 Castillano, T. M., Giglio, J. J., Evans, E. H., and Caruso, J. A., flows and pressures this eVect is not observed so it is likely J. Anal. At. Spectrom., 1994, 9, 1335. that the analyte interacts more with the walls of the torch. 9 Yan, X., Tanaka, T., and Kawaguchi, H., Appl. Spectrosc., 1996, 50, 2, 182. 10 Evans, E. H., Pretotius, W., Ebdon, L., and Rowland, S., Anal. Chem., 1994, 66, 3400. CONCLUSIONS 11 Kohler, M., and Schlunegger, U. P., J. Mass Spectrom., 1995, 30, 134. The GC–LP-ICP-MS system has been shown to be capable 12 Olson, L. K., Belkin, M., and Caruso, J. A., J. Anal. At. Spectrom., of providing a tuneable degree of fragmentation for a series of 1996, 11, 491. halobenzene compounds. The problems associated with poor 13 O’Connor, G., Ebdon, L., Evans, E. H., Ding, H., Olson, L. K., linear calibration range and high detection limits for the and Caruso, J. A., J. Anal. At. Spectrom., 1996, 11, 1151. molecular ions have been addressed and alleviated by the use 14 Physical Chemistry, Oxford University Press, Oxford, 4th edn., of reagent gases. 1990. 15 Niu, H., and Houk, R. S., Spectrochim. Acta, Part B, 1996, 51, 779. The addition of small amounts of nitrogen to the LP-ICP 16 Douglas, D. J., and French, J. B., J. Anal. At. Spectrom., 1988, increased the stability of the plasma and the detection limits 3, 743. for the molecular fragments of chlorobenzene were greatly 17 Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, improved. However, this eVect did not extend to the other 65th edn., 1984–1985. halobenzenes studied. 18 Chapman, J. R., Practical Organic Mass Spectrometry, Wiley, The addition of isobutane enhanced all the analyte molecular Chichester, 2nd edn., 1993. and fragment ion signals. The isobutane did not seem to be acting as it would in a conventional CI source as proton Paper 7/03733C transfer reactions were minimal. However, the isobutane ReceivedMay 29, 1997 Accepted August 20, 1997 seemed to reduce the ionisation energy of the plasma, yielding Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1269