Suppression of in-cell generated interferences in a reaction cell ICP-MS by bandpass tuning and kinetic energy discrimination

Transcription

1 Suppression of in-cell generated interferences in a reaction cell ICP-MS by bandpass tuning and kinetic energy discrimination Bodo Hattendorf* and Detlef Günther ETH Zürich, ETH Hönggerberg, CH-8093 Zürich, Switzerland. bodo@inorg.chem.ethz.ch; Fax: 141 (0) ; Tel: 141 (0) Received 30th September 2003, Accepted 6th February 2004 First published as an Advance Article on the web 5th March 2004 The efficiency of kinetic energy discrimination and bandpass tuning for the suppression of potentially interfering product ions, formed by ion molecule reactions in a dynamic reaction cell of an inductively coupled plasma mass spectrometer is compared. Suppression of the oxide ions from Sc 1,Y 1,La 1 and Th 1, formed in reactions with oxygen impurities in non-reactive gases, have been used as test system to determine the suppression efficiency of the in-cell generated ions in dependence of the operating parameters of a dynamic reaction cell. Kinetic energy discrimination was investigated by successively lowering the pole bias voltage of the reaction cell quadrupole below the pole bias of the analyser quadrupole to create a potential barrier of increasing height. For bandpass tuning, the transmission window of the reaction cell quadrupole was successively narrowed to determine the cut-off m/z, where precursors of the product ions are rejected. The efficiency of interference suppression and the elemental sensitivity are in all cases decreasing with m/z of the precursor ions. Both approaches allow the suppression of the in-cell generated ScO 1 and YO 1, while LaO 1 and to a greater extent ThO 1 cannot be fully eliminated without compromising elemental sensitivity significantly. LaO 1 and ThO 1 are observed at levels, which are by a factor of two and six higher than in standard operation of the ICP-MS. Elemental sensitivity is higher by approximately a factor of five and the abundance of the cell produced ions is reduced more effectively with the bandpass tuning approach when compared to kinetic energy discrimination. DOI: /b312155k Introduction Reaction and collisions cells are used in various applications and in different designs in ICP-MS (for a comprehensive review see Tanner et al. 1 ). Depending on the chemical properties of the gas, ion molecule reactions occur in the collision or reaction cell, which alter the composition of the ion beam. Ionic reaction products may also be transmitted to the mass analyser and interfere with elemental ions of interest. Whether a specific interference is formed depends mostly on the gas used. There is a wealth of information available, which can be used to evaluate if a given ion molecule pair may react and which products occur (e.g. refs. 2 and 3). Available kinetic data (e.g. refs. 4 and 5) can be further used to estimate relative efficiencies of individual reactions. Nonetheless, the complex composition of the ion beam from the ICP source and the wide distribution of ion energies, make an unambiguous evaluation difficult. Since the pioneering work of Rowan and Houk 6 it is known that reaction products remain the major concerns before any application of ion molecule reaction in elemental analysis by ICP-MS. Difficulties arise from the presence of impurities in the reaction gas (often not exactly specified with respect to specimen and concentration), which can lead to a variety of cell generated species even in the case of high purity gases. 7 Inside the reaction volume, interferences can be formed via a variety of pathways. Charge transfer reactions (with the dominating Ar 1 ions) that produce molecular ions of the reaction gas (or fragments of it) can be considered the major source of new species in the ion beam. However, the molecular weights of typical reactive components in the gas occur in a mass region were elemental ions are not greatly affected. More problematic therefore are condensation and association reactions, which lead to stable polyatomic ions at the m/z of analyte ions Oxide ions may be considered the most critical group in this respect because oxygen (elemental or in a compound) can be considered the most abundant reactive component in the commonly used gases and many elemental ions are forming stable oxide ions in reactions with water vapour or molecular oxygen. 5,11,12 It has been observed that Sc 1,Y 1,La 1 and Th 1 react with unknown impurities in neon and form the corresponding MO 1 and other molecular ions. However, the elements were not chosen because they lead to severe interferences in elemental analysis. In fact only YO 1 and LaO 1 may be considered problematic interferences on 105 Pd 1 and 155 Gd 1, while ScO 1 interferes only with the minor isotope 61 Ni (and to much lesser extend with 62 Ni and 63 Cu) and ThO 1 does not interfere with any stable isotope. They were rather chosen because they are mostly mono-isotopic ( 138 La has only 0.09% relative abundance), which simplifies the interpretation of spectra, and provide a reasonable coverage of the mass range and ion energies respectively. An elegant way to eliminate interferences of this type has recently been demonstrated by Jackson et al., 13 who initiated collision induced dissociation of a stable oxide ion (TaO 1 ) after excitation in an ion trap. The linear multipole ion guides, most commonly used in ICP-MS, however, usually lack the possibility to store and excite ions to the kinetic energies required 14 and other means of interference control are necessary. This work has investigated the two most commonly used approaches for the control of cell generated interferences in ICP-MS, kinetic energy discrimination (KED) and bandpass tuning (BT), with respect to their efficiency in interference suppression and elemental sensitivity. KED is widely used in linear multipole ion guides to manipulate the energy distribution of the ions that leave the reaction volume. It is based on a differentiation between cell and ICP generated ions due to their difference in kinetic energy. Ions that were produced inside the reaction volume carry only a fraction of the kinetic energy of the precursor ions (i.e. ICP generated ions that enter the reaction cell). 6,10,15,16 Polyatomic 600 J. Anal. At. Spectrom., 2004, 19, This journal is ß The Royal Society of Chemistry 2004

2 species furthermore have larger collision cross sections (higher polarizability compared to elemental ions and eventually a dipole moment) which lead to more collisions inside the reaction volume and greater loss of kinetic energy. A potential barrier after the reaction cell can therefore be used to define the minimum kinetic energy an ion must carry to leave the reaction cell and to be detected by the MS. This mode can be characterized as a post-reactive suppression of cell generated ions (the suppression of the reaction product occurs after its formation) and its efficiency is thus depending on the overlap of the kinetic energy distributions of the ICP and cell generated ions at the exit of the reaction cell and the height of the potential barrier. The approach can also be used to discriminate against ICP generated polyatomic ions with large collision cross sections, which experience more collisions and loose a greater fraction of their kinetic energy inside the cell. 17 The term bandpass tuning originates in the mass selective properties of the quadrupole ion guide. In the dynamic reaction cell (DRC), the quadrupole ion guide is operated at low mass resolution, which results in a wide range ( bandpass ) of m/z that have stable trajectories. 18 Thus, rejecting the precursors of cell generated ions by selecting a bandpass width that excludes their m/z from the DRC will intercept the reaction before the formation of the product ions can occur (pre-reactive suppression). The bandpass boundaries are defined by the Mathieu parameters a and q (RPq and RPa), which can be adjusted individually for each m/z during the mass scan. 19 When RPa is set to zero (rf only operation), the quadrupole acts as a high-pass filter and RPq defines the low m/z boundary of ions with stable trajectories. Non-zero RPa introduces a stability boundary at high m/z and increases and sharpens the low m/z boundary. 1 Increasing RPq and RPa, however, not only affects the transmission of elements at the m/z boundaries but may also reduce the sensitivity for elements inside the stability region. The highest transmission is usually found for intermediate RPq and a substantial suppression occurs at the boundaries of the first stability region, which limits the usable range. Suppression of cell-generated MO 1 can be achieved by selecting a value for RPq, which corresponds to a bandpass boundary that is less than 16 m/z below the currently analysed m/z. Under ideal conditions, the precursor ions of the cellgenerated ScO 1, YO 1, LaO 1 and ThO 1 should thus be rejected from the reaction cell at values for RPq of 0.67, (m/z ~ 61, ScO 1 ), 0.77 (m/z ~ 105, YO 1 ), 0.81 (m/z ~ 155, LaO 1 ) and 0.85 (m/z ~ 248, ThO 1 ) when RPa is set to zero. The transmission boundaries, however, get blurred when of the quadrupole is operated at the relatively high pressures used for ion molecule reactions, 1 which will limit the efficiency of suppression of the cell-generated ions. Experimental Instrumental An ELAN 6100 DRC ICP-MS (PE/SCIEX, Ontario, Canada) was used in this work. The instrument is equipped with a dynamic reaction cell (DRC) in front of the analyzer quadrupole. The DRC consists of a bandpass quadrupole, located in a cylindrical reaction volume (Fig. 1). When operated in standard mode, no gas is added and the reaction volume is evacuated through additional apertures (Cell Vent) into the high vacuum region of the analyser quadrupole. The quadrupole inside the dynamic reaction cell then acts as an rf only ion guide with a wide transmission bandpass. In the pressurized (DRC) mode, the cell vent is closed and the reaction gas is added via two independently controlled transfer lines, which are joined before the reaction cell. The transmission of the cell quadrupole can be set to specific m/z ranges by adjusting the bandpass width in concert with the analyser quadrupole. A potential bias is applied to the entrance and exit Fig. 1 Schematic view of the dynamic reaction cell and reaction gas supply (not to scale). aperture (Cell Path Voltage, CPV), the DRC quadrupole rods (Cell Rod Offset, CRO) and the analyser quadrupole rods (Quadrupole Rod Offset, QRO). The bias is adjusted individually for vented and pressurized conditions to compensate for changes in focus position at the entrance aperture and the change of ion kinetic energy, during collisions in the gasfilled reaction volume. An additional axial field (AFT) can be applied to the reaction cell, which permanently accelerates ions towards the exit aperture of the cell. 20 This field was off in these experiments order to avoid perturbation of the ion kinetic energies. One gas supply line (A) is equipped with an additional Getter before the mass flow controller to further reduce the level of oxygen and water vapour in the gas. Solenoid valves at the entrance to the instrument act as additional seal between atmosphere and the high vacuum system. The sample introduction systems consisted of a quartz cyclonic spray chamber and a glass concentric nebuliser, operated in pumped mode with a sample uptake rate of 0.8 ml min 21. The instrumental settings used in the experiments are listed in Table 1. Reagents and standards All sample solutions were prepared from 1000 mg ml 21 single element stock solutions (CPI International, USA) and diluted using ultrapure water (Millipore, USA) and 1% (v/v) HNO 3, purified by subboiling distillation. Table 1 ICP parameters Operating conditions of the ICP-MS Low oxide setting High oxide setting Rf power 1350 W 1250 W Nebulizer gas 0.94 L min L min 21 Auxiliary gas 0.9 L min L min 21 Plasma gas 16 L min 21 Sampler cone 1.1 mm Pt Skimmer cone 0.9 mm Pt Autolens On DRC parameters Standard mode DRC mode Cell gas flow a Off H 2 : 2 ml min 21 He: 0.5, 3 ml min 21 Ne: 0.5, ml min 21 Ar: 0.5 ml min 21 Cell rod offset 213 V 21 V b Rejection 0 0 parameter a: Rejection b parameter q: Cell path voltage 224 V 233 V Quadrupole 21 V 25 V rod offset AFT Off a Software settings. b Starting values, optimised settings are given in Table 3. J. Anal. At. Spectrom., 2004, 19,

3 Table 2 Ion signals for La 1 at different operating conditions and intensity ratios for detected molecular species Standard mode ICP oxide level DRC mode a ICP oxide level m/z Low High Low High Fig. 2 Formation of additional polyatomic ions in the pressurized dynamic reaction cell. Insert shows the spectrum recorded in standard mode. 200 mg L 21 La, H 2 : %, low oxide settings (Table 1). The formation of polyatomic and cluster ions of La 1 was studied using hydrogen of a purity of a % and % (Linde, D). The comparison between bandpass tuning and kinetic energy discrimination efficiency was performed using impurities in helium ( %, PanGas, CH) neon (99.996% Linde, D) and argon (99.996%, PanGas, CH). Results and discussion Formation of polyatomic ions inside the reaction volume The formation of LaO 1 and higher polyatomic and cluster ions was investigated in a series of experiments under different operating conditions (see Table 1). Two sets of ICP operating conditions were used. One (low oxide setting, Table 1) is characterized by a LaO 1 /La 1 ratio, which is typical for optimised operating conditions (#1.3%). The other setting causes an unfavourably high formation of LaO 1 (9.9%, high oxide setting). In both modes of operation the mass spectrum shows only LaO 1, LaOH 1 and LaOH 1 2 at m/z 154 through 157 (see insert in Fig. 2). When the cell is pressurized with nonpurified hydrogen via line B (no Getter), an increasing number of polyatomic and cluster ions are observed and even in the low-oxide setting the LaO 1 /La 1 ratio approaches 100% (Fig. 2). New signals occur with dominating peaks at m/z 173, 191 and 209, which indicate the addition of at least 3 water molecules to LaO 1 inside the reaction volume. Other apparent product ions occur at m/z 171, attributed to LaO 1 2, 172, 190 and 208 (LaO(OH)(H 2 O) 1 n, n ~ 1 3), and at m/z 175, 177, 193, 195, 211 and 213, which may originate from LaO- (H 2 O) n (H 2 ) 1 m (n ~ 1 3; m ~ 1, 2). For both modes of operation an approximately two-fold increase in transmission for the La species (sum of the intensities, Table 2, not corrected for mass bias) is observed when the cell is pressurized. This increase is attributed to the loss of kinetic energy during collisions, which allow a better confinement of the ions at the axis of the multipole ( collisional focussing 21 ). A great fraction of elemental La 1, however, has undergone a reaction and cannot be detected as elemental ion. This fraction is much greater for the high-oxide mode (only 30% of La 1 is detected as elemental ion) than in the low oxide mode (55% La 1 ). This is likely caused by a smaller average kinetic energy of the ICP generated La 1 in the high oxide mode (lower plasma temperature), which leads to a higher collision frequency and correspondingly higher reaction rates. Another possibility could be a higher concentration of reactants in the cell due to increased entrainment from the plasma in the high oxide mode. The latter, however, would require an at least threefold increase in the concentration of oxygen (estimated from the average abundance of the water clusters w m/z 170 for low and high oxide settings), which is not considered very likely under these conditions cps cps cps cps sum b cps cps cps cps Intensity ratios relative to m/z 139 (%) n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d n.d. n.d a 2mLmin 21 H 2, not-purified, RPQ: 0.45, RPa: 0. b Sum of all ion signals, which have been assigned to La 1 species. c n.d.: not detectable. Gas purification Fig. 3 shows the signal for La 1 and the LaO 1 /La 1 -ratio when the gas supply is changed from line B (no Getter) to line A (with Getter). A significant increase of LaO 1 is observed immediately after changing the supply line. This indicates that reactants (impurities) have been accumulated in line A after the mass flow controller either from the reaction gas or from diffusion of ambient air through the PFA tubing. This is not reflected in the La 1 signal, because only a small fraction of La 1 is consumed in these reactions, even though the increase in the LaO 1 /La 1 ratio is significant. The impurities are constantly swept out with the flow of the purer gas. Consequently less La 1 is consumed in reactions and the ion signal for La 1 increases. However, an almost steady state is reached only after more than 10 min. The residual reactants may be introduced into the cell via the gas supply (diffusion through the tubing or equilibrium concentration in the getter) or from entrainment of oxygencontaining species through the ion optics. The dominating memory effects for impurities are expected to occur in the gas supply lines (large surface/volume ratio) and not in the reaction Fig. 3 Reduction of cell generated LaO 1 after a change of the reaction gas supply from line B to line A (at t ~ 150 s). The arrow indicates the LaO 1 /La 1 ratio observed in non-pressurized mode. 20 mg L 21 La, 2 ml min 21 H %, low-oxide settings (Table 1). 602 J. Anal. At. Spectrom., 2004, 19,

4 volume itself. Nonetheless, the formation of cell generated LaO 1 cannot be suppressed completely just by purification of the reaction gas. Even though the LaO 1 /La 1 ratio continuously approaches the level observed in standard mode, we always observed at least a three times higher formation rate of LaO 1. Suppression of cell generated ions Gas purification can only reduce the formation of polyatomic ions, when they are caused by reactive impurities which can be removed by an appropriate filter and not when the reaction gas itself forms stable molecular ions. 1,8 In order to investigate how efficient these cell generated ions can be removed by kinetic energy discrimination or bandpass tuning, the reaction cell was pressurized, mostly using neon at a flow of 0.5 ml min 21, through the non-gettered gas supply line (Gas B in Fig. 1). Sc, Y, La and Th were introduced at a concentration of 20 mg L 21. The efficiency of their suppression is in this context defined as the MO 1 /M 1 ratio found in the pressurized cell relative to standard mode operation. Kinetic energy discrimiation The DRC allows adjusting the potential barrier height before the analyser quadrupole in two ways: It is either possible to vary the bias voltage at the MS quadrupole (quadrupole rod offset, QRO) or at the DRC ion guide (cell rod offset, CRO). In our experiments the analyser quadrupole bias was held constant at 25 V and CRO was changed. This approach was chosen because it provides a fixed potential difference between the analyser quadrupole and detector subsystem to the ICP source and should lead to comparable detection conditions. It must nonetheless be kept in mind that reduction of CRO will lead to an increase in ion kinetic energy within the reaction volume. The higher kinetic energy, however, will reduce the number of collisions and therefore the efficiency for exothermic reactions. Due to this, the approach chosen could overestimate the efficiency of KED for the suppression of cell generated ions in this case. In the case of endothermic reactions, on the other hand, this approach might lead to a higher production of cell generated ions. The corresponding reaction products, however, carry much smaller kinetic energy and should generally be suppressed more efficiently by KED. Fig. 4 shows the effect of the increasing potential barrier, as CRO is made more negative. While ion signals are not greatly affected before a CRO of 23 V, a significant, but in its magnitude strongly mass dependent, decay is observed for all ions at more negative settings. The transmission of the elemental ions is decreasing by one ( 232 Th 1 ) to two orders of magnitude ( 45 Sc 1 ) when CRO is reduced from 23 Vto25 V. At the same time, the abundance of the cell generated ions in the mass spectrum is decreased even further by up to two orders of magnitude. The suppression efficiency of the cell generated ions is depending on m/z (i.e. kinetic energy of the ions extracted from the ICP). Cell generated ScO 1,YO 1 and LaO 1 can be suppressed by one to two orders of magnitude when CRO is set below 28 V and the oxide ratios approach or reach the level observed in standard mode. Cell generated ThO 1,on the other hand cannot be suppressed efficiently (reduction of ThO 1 /Th 1 by only a factor of two). While cell generated ions from ScO 1 and YO 1 can be virtually eliminated, LaO 1 and especially ThO 1 are present at elevated levels for all values of CRO used. Compared to standard mode operation, LaO 1 and ThO 1 are enhanced by a factor of two and eight respectively. The kinetic energetic loss of cell and ICP generated ions is also influenced by the mass of the gas targets in the reaction cell. A higher collision frequency (i.e. gas density) or a heavier collision target will lead to a greater energy loss within the Fig. 4 Effect of kinetic energy discrimination with neon as collision gas on elemental sensitivity, and abundance of oxide ions. The analyser quadrupole was held fixed at 25 V in all experiments. Cell gas flow: 0.5 ml min 21 neon. reaction volume. This energy loss is more pronounced for the larger polyatomic ions due to their higher collision frequencies. This effect is demonstrated in Fig. 5 for the ThO 1 /Th 1 ratio and the ion signals of Th 1 in dependence on CRO. Since the concentration of the reactive species in the gases used is not exactly known, the absolute formation of ThO 1 in the cell is not necessarily reflecting the different reaction efficiencies. Of greater interest is therefore the efficiency at which the product ions can be suppressed by KED. In general the MO 1 formation rate increases with increasing gas flow and with the heavier gases, (potentially higher concentration of impurities and increased reaction rates due to more efficient thermalization 1 ). Almost no suppression is observed when He is used at 0.5 ml min 21 because the number of collisions and the fractional energy loss per collision is small. 19 Increasing the helium gas flow to 3 ml min 21 results in a similar dependence like using 0.5 ml min 21 neon, apart from the higher initial formation rate of ThO 1. Due to the greater energy loss of ThO 1 compared to Th 1, cell generated ThO 1 can be suppressed by a factor of approximately two (CRO: v24 V) with 3 ml min 21 helium. Argon as collision gas in comparison leads to an order of magnitude suppression of cell-generated ThO 1. At a flow rate of 0.5 ml min 21 argon, however, we also observe a suppression of elemental sensitivity for Th 1 by almost an order of magnitude (even more pronounced for the lighter isotopes), caused by scattering losses in the heavier collision gas. J. Anal. At. Spectrom., 2004, 19,

5 Fig. 5 Effect of kinetic energy discrimination for different collision gases and pressures. Analyser quadrupole bias: 25 V. Bandpass tuning Fig. 6 shows the dependence of the cell generated oxide ions on RPq, when RPa is set to zero. Cell generated ScO 1 and YO 1 can be eliminated from m/z 61 and 105 at RPq of 0.7 and 0.8 respectively, which is close to but slightly higher than the predicted cut-off values for the precursor ions. The deviation from the predicted values is most likely due to the blurred m/z boundaries. 1 LaO 1 too is reduced significantly at RPq of 0.85 but the complete suppression of LaO 1 formation in the cell requires a higher effective low mass cut-off. ThO 1 cannot be attenuated at any value of RPq below The transmission of the elemental ions shows a similar trend upon variation of RPq. The decay at increasing RPq, however, is more pronounced for low m/z. Further suppression of the cell generated LaO 1 and ThO 1 can be achieved using non-zero RPa. Fig. 7 shows the dependence of Th 1 and ThO 1 on RPq for increasing values of RPa. While RPa has no significant influence on elemental sensitivity, the abundance of ThO 1 is significantly reduced with RPq set to 0.85 and RPa to Due to the narrow transmission window required to achieve rejection of the precursor ions, elemental sensitivity is also suppressed by a factor of 5 compared to RPq of 0.4, where the highest transmission is observed. The influence of collision gases of different mass and density on the suppression of cell generated ions in the bandpass tuning approach is shown in Fig. 8 for Th 1 and ThO 1. Changing from helium to neon at a flow of 0.5 ml min 21 leads to higher elemental sensitivity, caused by a better confinement of the ions through thermalising collisions. Scattering and/or reactive losses on the other hand become more important in argon and for the higher helium gas flows. Higher gas densities or a heavier gas also leads to more efficient suppression of the cell generated ions. Especially when using argon as collision gas or a higher gas density of helium, the decrease of the ThO 1 /Th 1 ratio begins at smaller values for RPq (0.75 and 0.85 respectively). This shift towards lower RPq for the cut-off value for the MO 1 /M 1 ratio is observed for all elemental ions studied here, when a heavier collision gases or higher gas density is used. The cause for the stronger suppression of MO 1 in a heavier gas or at higher gas density, however, is not so obvious. It is assumed that the cut-off value is lowered when the precursor ions experience more rf cycles in the quadrupole field. This may cause more precursor ions to be rejected from Fig. 6 Effect of bandpass tuning on elemental sensitivity, and abundance of oxide ions. RPa was zero in all experiments. Cell gas flow: 0.5 ml min 21 neon. the quadrupole before reaction can occur and the product ions are formed. Thus the blurring of the stability boundaries of the pressurized quadrupole appears to be reduced as the ions become more efficiently thermalized. However, a higher RPa value can achieve a more efficient suppression of the cell produced ions and retains a significantly higher elemental sensitivity. Fig. 7 Improved suppression of ThO 1 using non-zero RPa. 604 J. Anal. At. Spectrom., 2004, 19,

6 Fig. 8 Effect of bandpass tuning for different collision gases and pressures. RPa: 0. Table 3 Reaction cell parameters, elemental sensitivity (M 1 ) and oxide ratios for optimized conditions of kinetic energy discrimination and bandpass tuning Sc 1 Y 1 La 1 Th 1 Mass Diff. MO 1 (%) Kinetic energy discrimination CRO/V M 1 /cps mg 21 L MO 1 /M 1 (%) Bandpass tuning Rpa RPq M 1 /cps mg 21 L MO 1 /M 1 (%) Summary Table 3 summarizes the conditions, which provide the lowest abundance of the cell generated MO 1 but retain the highest elemental sensitivity, when using kinetic energy discrimination or bandpass tuning at a gas flow of 0.5 ml min 21 neon. Both approaches can essentially eliminate the cell generated ions from Sc 1 and Y 1. At higher m/z of the precursor ions the suppression is less efficient and the product ions are still detected at significant levels. Kinetic energy discrimination cannot considerably suppress the cell generated ThO 1 and the ThO 1 /Th 1 ratio is higher by almost an order of magnitude when compared to standard mode operation. KED is further accompanied by a substantial loss in elemental sensitivity under these conditions. Even though bandpass tuning can also not eliminate all cell generated MO 1, the suppression is much more efficient and LaO 1 and ThO 1 can be reduced to almost the plasma background level. It further provides an elemental sensitivity, which is higher by a factor of at least 5 compared to kinetic energy discrimination. Conclusion Cell generated polyatomic ions may have a significant impact on elemental analysis with ICP-MS instruments that are equipped with collision or reaction cells. The origin and identity of these reaction products are often not of great interest but need to be suppressed in order to eliminate analytical artefacts caused by spectral interferences. It could be shown that impurities in the reaction or collision gas have a significant impact on the formation of cell generated interferences and elemental sensitivity. Especially the presence of oxygen-containing impurities like water vapour and other highly reactive compounds like NH 3 can lead to a broad range of interferences, 8 which complicate multi-element applications using collision or reaction cell ICP-MS instruments. The efficiency for suppressing such cell generated polyatomic ions by kinetic energy discrimination and bandpass tuning is mostly depending on the relative mass difference between the precursor and product ions. For the bandpass tuning approach this is a direct result of the design of the quadrupole used as ion guide and its limited mass resolving properties in a pressurized cell. The mass dependence for kinetic energy discrimination on the other hand results from the mass related initial kinetic energy of the precursor ions that enter the cell. Provided that m/z(precursor) v m/z(product), it is only possible to suppress the cell generated ions efficiently when the mass (or energy) differences are sufficiently large. A significant overlap of the kinetic energy distributions exists for the precursor and product ions, which limits the effectiveness of kinetic energy discrimination for suppression of the cellgenerated ions. A better separation of the energy distributions can be achieved using heavier collision gases and/or higher gas densities. This approach, however also leads to a strong reduction of the elemental sensitivity due to scattering losses in the reaction cell. Bandpass tuning can eliminate the cell generated ions when the relative mass difference between product and precursor ion is greater than 18%. Due to this, it is not possible to fully eliminate oxide ions, which have a precursor of an m/z of above roughly 100. Higher RPq values or the use of heavier gases and/or a higher gas density can improve rejection of the precursor ions and thus reduces the abundance of cell-generated product ions. Both approaches, however, also compromise elemental sensitivity, which can to some extent be avoided by increasing RPa to Under optimized conditions the bandpass tuning approach provides superior analytical performance because it retains a significantly higher elemental sensitivity and provides more efficient suppression of cell generated oxide ions even at high m/z, when compared to kinetic energy discrimination. It shall be further stressed here that cell generated interferences will occur regardless of the type of collision or reaction cell, when even minute amounts of impurities are present in an otherwise non-reactive gas. Taking advantage of the increased elemental sensitivity of collisional focussing without applying kinetic energy discrimination or bandpass tuning can therefore lead to a drastically increased abundance of unpredicted and sample dependent cell-generated spectral interferences. Acknowledgements This work has been supported by ETH research proposal TH-12/99-2. References 1 S. D. Tanner, V. I. Baranov and D. R. Bandura, Spectrochim. Acta, B, 2002, 57, S. G. Lias, J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D. Levin and W. G. Mallard, J. Phys. Chem. Ref. Data, 1988, 17, 1. 3 NIST, webbook, 4 V. G. Anicich, J. Phys. Chem. Ref. Data, 1993, 22, D. K. Bohme, Ion molecule kinetics database, yorku.ca/profs/bohme/research/research.html. 6 R. T. Rowan and R. S. Houk, Appl. Spectrosc., 1989, 43, N. Yamada, J. Takahashi and K. Sakata, J. Anal. At. Spectrom., 2002, 17, B. Hattendorf and D. Günther, J. Anal. At. Spectrom., 2000, 15, B. Hattendorf and D. Günther, Fresenius J. Anal. 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