Spatial profiling of analyte signal intensities in inductively coupled plasma mass spectrometry

Spectrochimica Acta Part B 59 (2004) 291–311

Review

Spatial profiling of analyte signal intensities in inductively coupled plasma mass spectrometry Alison E. Holliday, Diane Beauchemin* Department of Chemistry, Queen’s University, Kingston, ON, Canada K7L 3N6 Received 15 September 2003; accepted 19 December 2003

Abstract While much of the spatial profiling research in ICP-MS has been devoted to optimization of sampling position as a function of power and aerosol carrier gas flow rate, the evolution of knowledge of ICP-MS can be traced through the publications of these fundamental studies. Axial profiling, which provides information on the energy and time needed to form a given ion, is also useful when establishing the optimal operating conditions, and can be used to assess if a matrix induces earlier desolvation. However, radial profiling can provide valuable insight into the predominant ionization mechanism(s) in the ICP, which can, in turn, facilitate the selection of an efficient internal standard. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Spatial profiling; Operating conditions optimization; Inductively coupled plasma mass spectrometry (ICP-MS)

1. Introduction Although instrumentation only became commercially available in the mid-1980s, inductively coupled plasma mass spectrometry (ICP-MS) is now frequently portrayed as a mature technique w1x. Indeed, due to its high sensitivity and multielemental capability, it has been accepted into routine use in government and industrial labs, despite several problems with spectroscopic and non-spectroscopic interferences (also called matrix effects) w2x. Furthermore, recent advances have been made in the reduction of spectroscopic interferences using, for example, high resolution ICP-MS, collision cells or dynamic reaction cells; this has increased the number of isotopes available for routine analysis, and further expanded the number of applications for the technique (w2x and references therein). Still, fundamental studies can help uncover the sources of the remaining problems (such as matrix effects); this should allow the identification of suitable remedies, and further expand the usefulness of ICP-MS. For instance, a review of the technical aspects for optimized performance of ICP-MS *Corresponding author. E-mail address: [email protected] (D. Beauchemin).

covered different sources of matrix effects: analyte transport into the ICP, ion production in the ICP, ion transport from the ICP through the sampling orifice and ion transport from the interface to the MS detector w3x. Fundamental studies in ICP-MS can also help identify possible energetic species, transport processes and energy transfer mechanisms, which affect ion production in the ICP w4x. The importance of these studies is highlighted by the fact that local thermodynamic equilibrium (LTE) does not strictly exist in the ICP since, using plasma profiling in ICP-OES, the measured temperatures corresponding to different processes (such as gas kinetic temperature and ionization temperature) differ considerably from one another w4x. Furthermore, non-LTE conditions have been observed in the ICP using Thomson scattering w5x, which makes the determinations of electron number density (ne) and electron temperature (Te) from absolute emission line intensities using ICP-OES questionable. In fact, one difficulty with ICP-OES spatial profiling is that a single area cannot be monitored in the plasma; rather, the emission across a full line through the plasma, either radial or axial, is collected at the same time. Estimations, based on the assumption that the plasma is perfectly cylindrical, must then be used to give a better

0584-8547/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2003.12.018

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picture of smaller areas of the plasma. This leads to errors, particularly in the central, most analytically useful, region of the plasma w6x. Computer tomography, which involves the collection and compilation of emission data along many different lines w7x, can also be used. However, even if adequate spatial resolution can be achieved using ICP-OES, the technique still cannot give the information needed to evaluate conditions for ICP-MS. This is because the measured emitted light can only originate from excited ions and atoms returning to their ground state. In ICPMS, however, ions in both ground and excited states are monitored. Therefore any plasma processes used to excite ions (which allow their detection with ICP-OES) are not as influential in ICP-MS. For example, the peak widths of the ion cloud resulting from single drops of solution were reported to be wider when measured by OES than by MS, despite the fact that the MS signals were acquired downstream of the location of the OES measurements w8x. This is because ionic emission occurs over a wider width of the plasma than the ICP-MS signal, which originates from a narrow region in the center of the plasma w9x. Furthermore, ion emission usually begins 2–3 mm upstream of where a MS signal can be measured w8x. Nonetheless, simultaneously acquired OES and MS signals allowed the determination of the gas velocity, i.e. the velocity at which ions travel through the plasma, which was shown to be independent of analyte mass, as expected w8x. However, Thomson scattering allows the unambiguous determination of both ne and Te without any assumptions of LTE and without perturbing the plasma, as long as the laser power (used to excite electrons in the plasma) is limited w6x. However, it provides no information on ion densities. Furthermore, a successful Thomson scattering experiment requires complicated and expensive instrumentation, which has limited its application to analytical plasmas w6x. The same is true of laser-induced fluorescence (LIF) measurements of ion spatial profiles in the ICP, which may give similar results to ICP-MS measurements but require a complicated instrumental set-up. In LIF, only the laser beam causes excitation of the ground state ions to the excited state, i.e. while the plasma conditions are responsible for initial ionization, they are not a factor in the fluorescence-producing ion excitation. In contrast to ICP-OES, where only excited state ions are monitored, only ions that were in the ground state prior to exposure to the laser can be monitored in LIF. If the fluorescence intensity is directly proportional to the number density of ground state ions, and if the fraction of ions in the ground state is relatively unchanged by experimental parameters w10,11x, then the results will be applicable to ICP-MS. Indeed, ionic LIF measurements have shown that significant matrix effects originate in the ICP w9,10x and

that they are worst near desolvating droplets or vaporizing particles w11x. Comparative LIF measurements made to the ion beam upstream from the sampler, andy or downstream from the skimmer also provided information on the radial spread of the ion beam and how it was affected by matrix w12,13x. Such experiments have shown that, although the addition of matrix reduced the throughput of analyte, either a focusing or widening of the ion beam could concurrently result, depending on the mass of the matrix w12x. Furthermore, significantly less suppression was concurrently observed at the tip of the sampler (outside the interface) compared to that downstream from the skimmer; this pointed to the first vacuum stage or the skimming process as contributors of non-spectroscopic interference w13,14x. The axial ion density distribution behind the skimmer also appeared to be dependent on mass, as evidenced by a more rapid drop of the signal from a light analyte than from a heavy one w13x. Time-resolved measurements using tandem detection by LIF and MS, in combination with sample introduction using a monodisperse, dried microparticulate injector (MDMI) for the reproducible introduction of individual droplets into the ICP, provide a powerful tool for investigating fundamental processes in the ICP. The single droplet size provided by the MDMI eliminates the complicating effects from introducing an aerosol with a range of droplet sizes, allowing a sharper definition of the ionization region. Such measurements have clearly demonstrated an increase in diffusion of the ion cloud in the ICP with sampling depth, as well as a inverse square root dependence of its width with analyte mass w8x. They have also shown that space-charge effects seem to be localized (temporally and spatially) in the region where the analyte ion cloud overlaps with the matrix ion cloud w15x. Although time-resolved measurements do not involve a physical spatial profiling of the ICP, they provide another way of obtaining spatial information. The ratio of the MS signal to the LIF intensity also allows an estimation of ion transmission efficiency from the ICP to the MS detector w9x. Such studies have shown that ion transport efficiency was lower near either incompletely desolvated droplets or vaporizing particles than away from them w9x. Unfortunately, not only is the MDMI not available commercially, but, as mentioned earlier, the LIF set-up is not trivial. Yet, relatively little use has been made of ICP-MS alone for diagnostic studies of the ICP, despite the fact that, as pointed out by Douglas and Houk (two pioneers of ICP-MS) in the early days of the technique, it can provide information corroborating or complementing that obtained by optical detection, or even totally inaccessible by the latter w4x. For example, 36Arq cannot be measured by ICP-OES w16x. The measurement of this second-most abundant isotope of Ar can prove useful when there is difficulty in monitoring the highest abun-

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dance isotope, 40Ar, due to the large amount of argon present in the plasma (resulting in detector saturation, as well as presenting problems with the detection of small changes). In contrast to ICP-OES, ions do not have to be excited to an upper electronic state prior to detection. Furthermore, greater spatial resolution can be obtained, as ions are extracted from a localized region of the ICP that is immediately in front of the sampler. Although this region is about two sampler orifice diameters deep w4x and eight sampler orifice diameters wide w17x, calculations by Douglas and French w17x indicate that the ions detected downstream of the skimmer actually come from a much narrower region, ‘‘corresponding only to the center-line flow of the sampler’’ w17x. Moreover, the flow of the expanding plasma, which is assumed to be similar to that of a neutral gas expansion, is fast enough to prevent significant mixing or diffusion of the sampled gas in the interface region. Therefore, with a sampler of 1.14-mm orifice diameter, flow through the skimmer was calculated to come from a region of approximately 0.8-mm diameter in the plasma w17x. In addition, due to space-charge effects, only the ions in the center of this flow can be focused on the MS detector, and so radial spatial resolution should be better than 0.8 mm. It was in fact demonstrated to be on the order of 0.25 mm or less, since substantial changes in signal could be seen after moving the torch just 0.25 mm in a radial profile of Rhq signal w17x. Thus, even though gas flow entrainment into the sampling orifice has been shown to have a most pronounced effect within 3 mm of the sampling orifice w18x, because the center line is actually monitored, spatial profiles of ion abundances can be readily and directly obtained by translating the plasma across the sampler. The resulting high spatial resolution w19x rivals that achieved by LIF (w17x and reference therein). Furthermore, in contrast to optical techniques, no mathematical deconvolution is required to obtain radiallyresolved ion abundance data w19x. Although both OES and LIF give a more direct view of ion populations in the plasma, unencumbered by signal changes that may occur during ion sampling or focusing, ICP-MS can still provide valuable information on the densities of ions in the ICP, as long as sampling perturbations can be identified and corrected w4x. A sound knowledge of the mechanisms of analyte ionization and matrix effects would likely lead to greater analytical advances in ICP-MS, as there may be further potential for increasing sensitivity while decreasing interferences. It is, however, difficult to successfully manipulate conditions when the mechanism of action remains largely unknown. Spatial profiling of analyte and background species in the plasma can give important information about plasma processes, as the presence or absence of correlations between profiles of species gives

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a strong indication of possible interactions within the plasma. 2. Physical requirements for spatial profiling in ICPMS Since ion transmission can change with ion lens voltages, either in terms of myz dependence or in terms of changes as a function of sample introduction conditions (sampling depth, etc.), a single set of voltages should be used throughout the profiling. This also eliminates the problem of hysteresis in the ion optics. However, this is not possible on instruments suffering from a secondary discharge, as discussed in Section 2.3. Furthermore, since significant changes in ion transmission efficiency through the sampler cone can result from changes in RF forward power, aerosol carrier gas flow rate, torch shield configuration, and sample composition w14x, constant conditions should preferably be used throughout the profiling. 2.1. Translation stage Torch positioning is generally a component of the signal optimization process in ICP-MS. For the majority of users, this is the only contact with the positioning controls that are the basis for spatial profiling studies. Indeed, spatial profiling of a plasma is usually done by moving the torch relative to the sampler cone in either a radial (x or y) or axial (z) direction (Fig. 1). The z direction is often referred to as the sampling depth. Only ICP-MS instruments where the torch is installed on a three-dimensional translation stage, which is either manually- or computer-controlled, will allow spatial profiling. With only one dimension of motion, only partial spatial profiling can be performed (such as depth profiling).

Fig. 1. Schematic representation of the ICP torch and the MS interface, with the three directions for profiling.

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2.2. Size of increments in spatial profiling As discussed above, the ion beam that enters and is analyzed by the mass spectrometer is not from a single point in the plasma. Rather, it originates from a volume of plasma estimated as a cylinder with 0.25-mm diameter and 2-mm depth w17x, which gives a sample volume of 0.14 mm3. This volume is small enough to allow the acquisition of valuable information from the ICP, which is approximately 14-mm wide and a few centimeters long. Nonetheless, spatial profiling will be a convolution of the probed 0.14-mm3 volume and the experimental distribution. In fact, the high spatial resolution of ICPMS combined with the sharpness of the curves of signal vs. aerosol gas flow rate makes ICP-MS highly sensitive to changes in aerosol gas flow rate or in sampling position w19x. Small changes in the plasma, which effectively change the sampling position, such as those resulting from the passage of large intact wet droplets through the plasma or the periodic oscillation of the shape of the plasma, can result in significant signal fluctuations w19x. Collecting multiple points across a small portion of the profile (for example, one point every 0.1 mm in the radial directions or one point every 0.5 mm in the axial direction) is required to compensate for these fluctuations. In fact, even in the absence of fluctuations, these multiple sampling points are necessary to obtain a better view of the ion distributions in the plasma. This is because the sampled volume is not homogeneous, and the ion distribution may not be centered within the volume. For example, the radial distribution would only be centered within the volume at a position of radial maximum or minimum signal; thus, the ion distribution is normally expected to be skewed. Any heterogeneity within the sample volume can only be seen, however, by doing overlapping measurements. If the torch was moved 0.3 mm to sample a completely different volume of the plasma, and the researcher must assume that no variationyoscillation of signal occurs within that 0.3 mm. If there was a change of signal, then a skew would appear (Fig. 2), which would depend on the choice of starting point. The details of changes within the sampling volume are lost. If, however, the average signal is calculated using overlapping steps, the trace more closely reflects that of the true ion distribution. By moving 0.1 mm, a portion of the volume is already accounted for in the previous data measurement, and the change in signal reflects the difference between the new 0.1 mm being sampled and the 0.1 mm that is no longer being sampled in this new measurement. Thus, a finer spatial resolution of signal can be obtained. This gives an increased ability to detect trends in the data and variations within the signal profile (Fig. 2), especially those present on a smaller scale. This is a very similar argument to that used for the collection of transient

Fig. 2. Effect of sampling frequency on a synthetic radial profile. Full line: original synthetic profile; dashed line: three-point average every point; dotted line: three-point average every three points.

signals, wherein the collection of more points across a profile gives more credence to the shape (and thus height and area) of the peak. On some instruments, the torch movement is automated. However, in many cases, it is probably better to move the torch stage ‘manually’, i.e. point-by-point as opposed to a continuous scan. This is due to the fact that the automated movement may (1) be relative to some undefined point; (2) be irreproducible, even on the short term, particularly if the torch is frequently moved or replaced; (3) suffer from hysteresis if movement is over too great of a distance, as the instrument does not ‘know’ to move more slowly. Moving in tiny increments, with settling times, is a way to avoid these difficulties, although it can be more time-consuming and, of course, requires more user–instrument interaction. Signal drift may also cause a greater problem over the longer timescales required. Furthermore, it is more difficult to accomplish on some ICP-MS instruments where the torch stage, which must literally be moved manually, is entirely within a torch box with safety closures.

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2.3. Absence of a secondary discharge Before performing any spatial profile, care should be taken to minimize any secondary discharge between the plasma and sampling orifice to ensure that representative ion distributions are obtained. The secondary discharge, which can modify the density, identity and energy of ions w20x, is indeed dependent on sampling depth as well as on plasma operating conditions, especially the aerosol gas flow rate (w19x and references therein). For instance, in the presence of a residual secondary discharge, ion energies increased with an increase in aerosol gas flow rate but decreased with an increase in forward power, whereas the reverse effects would have been observed in its absence w21x. Therefore, instruments with a load coil geometry that minimizes the secondary discharge most readily allow spatial profiling of ion abundances in the ICP. On other instruments, some sort of shield, which is inserted between the induction coil and the torch, is required. Unfortunately, this shield may also affect the ICP. Therefore it may not be possible to obtain representative spatial profiles of ion distribution under typical operating conditions with these ICP-MS instruments. A shielded torch also cannot be used at very high RF powers. For instance, the shield was reported to melt at 2 kW with an air–Ar plasma, and thus could not be used; this resulted in an unstable plasma discharge when the sampling position was higher than 15 mm w22x. On some commercial instrumentation, the ion optics lens voltage must be re-optimized to get optimal signal when changing sampling depth w23x. That is because the plasma potential varies from one plasma location to another w16x. Hence, if the lenses were tuned while sampling the central channel, then radial profiles would only be representative of the actual ion distribution in the region of the central channel; artificial depression could be observed in the induction region because the plasma potential would then be significantly different w16x. Similarly, axial profiles obtained after tuning for sampling within the initial radiation zone (IRZ w24x) were artificially steep when the sampling depth was within the normal analytical zone (NAZ w24x) w16x. 2.4. Software requirements Instrumentation problems are one of the main obstacles to doing spatial profiling. Commercial instrumentation is not, in general, constructed for fundamental studies. Instead, it is increasingly made for robust and hands-off high sample throughput analyses, and so ‘unusual’ parameters (that is, parameters that would usually not be changed for use in an industrial setting) are difficult to alter. This can be hardware or software related. For example, a seemingly common software deficiency is the need for multiple methods if multiple

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sampling positions are desired. While this presents no difficulty for a typical analysis, wherein only the optimum sampling position is utilized, it can mean hundreds of methods (replicates except for spatial coordinates) are necessary to accomplish a spatial profile. On some instruments, one can autocollect spatial profiles as part of the optimization process. However, the data used to create these graphs can be inaccessible, and, as it is for optimization purposes only, the number of isotopes or ratios that can be monitored simultaneously is limited. Again, as these measurements are not intended for quantitative purposes, replicates are not collected, and the swift movement of the torch may lead to less reproducible profiles than those found using point-by-point data collection. However, instruments that are designed so that the torch does not move a consistent distance when the software input changes by one (unitless) position would require some means of continual external calibration to allow spatial profiling to take place. Only on some custom-made instruments can a computer-controlled x–y stage do rapid scans with 0.15 mm radial resolution in 10 min with apparently no problems with reproducibility w16x. 2.5. Other considerations In theory, one might have to take the local gas temperature into account when considering the volume of plasma sampled. Under very different conditions, the gas temperature might change significantly. Since the local speed of sound is dependent on the gas kinetic temperature Tgas as 6Tgas w17x, and the flow through the sampler is directly dependent upon the speed of sound w17x, the sampled ion number density might also change dramatically, which might skew the results somewhat. However, axial profiles obtained under fixed plasma operating conditions, but with sampler and skimmer orifices of different diameters, have shown that the profiles were very similar, albeit with an increase in space charge effects as a result of the increased flux of ions through the larger orifices w25x. This is likely because, although a larger plasma volume is sampled through the larger orifices, only the center-line still reaches the mass spectrometer. Nonetheless, the situation may be different when comparing cold plasmas to normal plasmas. 3. Spatial profiling of argon plasmas 3.1. Fundamental characteristics of an Ar ICP Table 1 summarizes spatial profiles of background ions from an Ar ICP. These studies confirmed what was intuitively assumed from the temperature distribution in the plasma, i.e. that the population of Arq (which cannot be measured by ICP-OES w16x) is lower in the

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Table 1 Summary of spatial profiling studies of background ions from an Ar ICP in ICP-MS Type of profiles

Background ions

Observations

Reference

Radial and axial profiles of ion kinetic energy (KE) and signal

40

Ion signals constantly decreased, while kinetic energy increased, with increasing sampling depth. Radially, constant KE of Arq within "4 mm from the center; highest near "7–8 mm (roughly corresponds to one skin depth from outer boundary of ICP).

w21x

Background spectra at different sampling depths for a torch with 23-mm i.d.

14

Nq, 16Oq, Ar14 Nq, 40 Ar16Oq

Air entrainment: more of a problem at higher depths for 23-mm torch than conventional one (increased 14Nq and ArNq but lower ArOq and similar 16Oq)

w26x

Maps of signals in a 6=6 mm area of the ICP

36

Radial profile of 36Arq showed two local maxima, i.e. one on each side of the central channel region of the plasma.

w16x

Radial profiles of ion signals

Arq

Two maxima on both sides of the central channel in a dry plasma, i.e. Arq signal lowest on the torch axis because of the cooler central gas stream; approximately 3-mm wide central channel.

w27x

Axial and radial profiles of ion signals to see the influence of water vapor on ICP-MS measurements without changes in ‘ion generating’ factors such as aerosol carrier gas flow rate (CGFR)

36

Arq minimal in central channel of dry plasma (diameter ;1.5 mm) while ArHq fairly uniform radially across the plasma, with peaks at outer edge. 36 Arqand 36ArHq increased with sampling depth. Dry plasma had larger diameter than wet plasma. Arq and ArHq both had local maxima in center of wet plasma, albeit very small for Arq. Higher ArHq peak attributed to additional source of hydrogen, i.e. water in the aerosol.

w28x

Axial profiles of ion signal

ArOq

With a shielded torch, ArOq increased with sampling depth. At all depths, adding Ar to the spray chamber decreased ArOq (and other Ar polyatomic ions) but increased NOq, Oq 2 , and metal monoxides.

w29x

Radial profiles of ion signals

myz 27, 54, 56, 80

q Arq have a fairly flat distribution across the plasma 2 and ArO except on its outer edge, where the signal falls steeply. CNq and ArNq have a bimodal distribution with a minimum in the central channel. These distributions did not significantly change with preevaporation (aerosol from spray chamber passed through a heated tube prior to entering plasma).

w30x

Radial profiles of ion signals in central channel

myz 54, 56

Both ArNq and ArOq displayed a broad distribution across the central channel with a maximum in the center at 16.6 mm above the load coil. The distribution flattened as depth increased. With Ar added through a sheathing device (between the spray chamber and torch), the ArNq profile changed to an upward facing parabola.

w31x

Radial profiles of ion signal

Arq 2 (myz 76)

The signal intensity of 76Arq 2 showed two local maxima, one on each side of the central channel region of the plasma.

w32x

Effect of non-spectroscopic interferences on the spatial distribution of background ions. Both matrix elements similar in mass and different in ionization potential (IP), and similar in IP but different in mass were considered.

Arq 2 (myz 78)

Arq 2 increased with depth, until 25 mm, then decreased rapidly EIE* matrix shifted Arq 2 profile like the analyte profiles (see Table 2), but with more severe suppression. Cl suppressed Arq 2 (without shift), perhaps due to competitive ArClq formation. Significant suppressions by I and Cs attributed to space-charge effects. Two local maxima, one on each side of central axis, in radial q profiles of Arq 2 , except at high depths where Ar2 is nearly constant across central 2 mm of plasma. Matrix element suppressed Arq 2 at all radial positions, especially in the center.

w33x

Effect of non-spectroscopic interferences on spatial distribution. Both matrix elements similar in mass and different in IP, and

40

Greater suppression of ArOq axial profiles than for analytes (see Table 2) in the presence of non-EIEs. Less suppression than for analytes in presence of EIEs. Suppression correlated with matrix mass, which supports presence of space-charge effects.

w33x

q

Ar , H2O

q

40

Arq

Arq, ArHq

36

Ar16Oq

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Table 1 (Continued) Type of profiles

Background ions

Observations

Reference

ArO , unlike metal oxides (see Table 3), went through a maximum before continuously decreasing with depth. q

similar in IP but different in mass were considered. Effect of non-spectroscopic interferences on spatial distribution of ion

12

Similar depth and radial profiles of COq 2 to those of analytes (Table 2) in presence of matrix, with the exception of the Na matrix, which induced a slight enhancement at low depths. Broadened radial profiles in presence of matrix, with greater suppression along the central axis.

w33x

Radial profile of ion signal

Arq

Arq in a wet plasma shows a bimodal distribution with a minimum centered on the torch axis.

w34x

Radial profiles of ion signals in a cold plasma (1.4 kW, 1.3 lymin high aerosol CGFR)

38

Arq, Ar14 Nq, 40 Ar16Oq, 40 Ar16OHq, 40 Arq 2 , 15 16 q N O , 13 16 q C O

38 Arq and Ar polyatomics all had distinct peaks in outer plasma regions, which steadily decreased towards the plasma center. Almost zero Arq in the center. Only NOq had local maximum in the center. From outer plasma to its center, background ions decreased in order of decreasing first IP, i.e. Ar polyatomics first and NO last. EIE matrix suppressed NOq similarly to analytes (see Table 2); matrix with lower IP, K, suppressed to greater extent.

w35x

Radial profiles of ion signals in presence of different matrices

38 Arq, myz 27, 52, 55, 56, 76

Bimodal distribution of Arq and all Ar polyatomics (with maxima on either side of central axis) at all depths except the highest, where there is a much less pronounced central axis minimum. Arq maxima are further out than those for Arq 2 . Global minimum of myz 27 (non-argide) along the central channel at lower depth; but similar profile to analytes (bell-shaped) at the highest depth.

w36x

*

C16Oq 2

40

Easily ionized element.

central channel and maximal in the annulus (torroidal) region, especially low in the plasma, even in a dry plasma. This bimodal radial distribution was also observed for Ar-containing polyatomic ions w36x. One possible exception is ArHq, which was reported to be fairly uniform across a dry plasma, and which displayed maxima in the center and on the outer edges of the ICP when a wet aerosol was introduced w28x. However, the distribution of polyatomic ions is highly dependent on sampling depth. For instance, ArOq exhibited a bimodal profile at low sampling depth but a very broad peak centered on the central channel of the plasma at high depth w36x. Similarly, the background signal at myz 27 (likely from CNq) exhibited a minimum in the central channel at low depth but a bell-shaped distribution at high depth w36x. Nonetheless, the fact that Ar-containing polyatomic ions, in general, have a similar radial distribution to that of Arq suggests that they all have Arq as a precursor ion and are formed by its collision with neutral species (Ar, O, C, OH, etc.). Furthermore, the fact that the Arq maxima are further out than those for the Arq 2 indicates that Arq 2 may arise from the collision of neutral Ar with Arq originating from the toroidal zone. (It is also possible that Arq 2 is significantly formed by threebody association in the vacuum expansion, in which case its signal might show a dependence on the product wArqxØwArx; this, however, cannot be verified by ICPMS alone.) Similarly, the fact that the analyte ion radial

distributions (Table 2) are completely different (i.e. bell-shaped and centered on the central channel) strongly suggests that charge transfer from Arq is not their dominant ionization mechanism in the ICP under typical operating conditions w36x. Since no other profile matched those of analyte and analyte-containing ions, their predominant ionization mechanism is likely through electron impact ionization, which has been independently identified as the most likely analyte ionization mechanism w9,19x. Similarly, the fact that the axial profile of ArOq is different from that of analyte oxide ions (Table 3) suggests that the dominant process for their formation is different. Indeed, not only does ArOq peak at a significantly higher depth w33x, but an addition of Ar through the spray chamber, which effectively cools the plasma while decreasing the residence time in the plasma, decreased ArOq while increasing metal oxide ions w29x. Although the authors of this last paper suggested that ArOq arose from the collision of Ar with Oq, it is also possible, as discussed above, that it resulted from the collision of Arq with O. The situation is different in so-called cold plasmas, where the aerosol carrier gas flow rate is increased andy or RF power is decreased andyor the internal diameter of the injector is increased to substantially decrease the residence time of the aerosol in the plasma. Physically, such a plasma has a wider and more diffuse central channel than that found under normal operating condi-

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Table 2 Summary of spatial profiling studies on the effect of an Ar ICP operating parameters on analyte ion signals in ICP-MS Type of profiles

Analytes

Radial and axial profiles of ion kinetic energy (KE) and signal

59

Radial and axial profiles

115

Co

q

Inq Uq

238

Observations q

Reference

Co constantly decreased, while KE increased, with depth. Radially, constant KE within "4 mm from center, but bellshaped Coq profile in this region.

w20x

Bell-shaped radial profiles of 115Inq become lower and flatter at greater sampling depth. 115 q In and 238Uqsignals decreased smoothly along the z axis.

w37x

Plots of signal vs. aerosol CGFR at different sampling depths

Liq, Naq, Rbq, Csq, Vq, Mnq, Znq, Cdq

At a given forward power, highest overall signal at lower depth for alkali elements. Increasing CGFR increased optimal depth. Slightly different optimal CGFR for each element at each depth. Aerosol CGFR had much more effect on signal than axial position, but, because of interdependency, CGFR must be optimized for each depth and vice versa.

w38x

Plots of signal vs. auxiliary gas flow rate at different sampling depths

First row transition metals, Liq, Naq, Rbq

Changing the auxiliary gas flow rate had only a very small effect on signal intensity at analyte optimal sampling depth. The auxiliary gas flow rate could be used to fine tune the torch axial positioning.

w38x

Plots of signal as a function of power and aerosol CGFR at different sampling depths

Niq, Pdq, Ptq, Srq, Baq, Liq, Naq, Csq, Rbq

The higher the sampling depth, the greater the CGFR required to reach the optimal analyte signal intensity at any given power. In contrast to Ref. w38x, for most elements, the same approximate parameters (power, CGFR, and sampling depth) can be used for analysis. As in Ref. w38x, the best sampling position is at lower depth for alkalis.

w39x

Plots of signals as a function of aerosol CGFR at five sampling depths

Scq in presence of up to 1000 ppm Na

Aerosol CGFR had a larger impact on the degree of nonspectroscopic interference than sampling depth. Suppression at all sampling depths, but less severe at smaller depths; however, signal intensity was not optimal at this depth, and altering the CGFR could not increase signal to that which was seen at higher depths.

w40x

Plots of signal as a function of aerosol CGFR at different sampling depths

24

Mgq, Coq, 114 Cdq, 138 Baq, 208 Pbq

Clear maximum in analyte signal as a function of CGFR at each depth, but not much difference in maximum signal intensity over several millimeters at lower sampling depths. With increasing sampling depth came increasing signal loss, and the maximum signal shifted out to slightly higher CGFR. Very similar optimal sampling position for all elements studied.

w41x

Plots of signal vs. aerosol CGFR at three different sampling depths

133

The optimal CGFR increases with increasing depth. At a given forward power, increasing depth decreased the ion signal, regardless of the CGFR.

w42x

Influence of easily ionized matrix elements on radial profiles of analyte signal intensity

7 Liq in presence of up to a 570 molar ratio of Cs to Li

Matrix-induced suppression decreased with increasing distance from the center of the radial profile (where analyte sensitivity is optimum). A small enhancement was seen at the most extreme position (where analyte signal was very low), possibly due to matrix-induced analyte diffusion or altered matrix effect from increased Ar to matrix ratio. Increasing sampling depth decreased the matrix effect, possibly because of different rates of diffusion of matrix and analyte ions.

w43x

Plots of ion signals, oxide ratio and doubly-charged ratio vs. aerosol CGFR at different sampling depths in a 9-mm (micro) torch and a conventional 18-mm torch

140

Ceq, Cdq, 208 Pbq

For micro and conventional torch, optimal CGFR increased with sampling depth, except for Ce, which remained constant with the micro torch, presumably because of the dominance of oxide formation. Different optimal sampling depths for different elements with smaller torch. Elements that form oxides and doubly charged ions had higher optimal sampling depths. Same optimal radial position for all elements with conventional torch but different optimal positions with smaller torch.

w44x

Plots of ion signals, oxide ratio and doubly-charged

59

Increasing depth changed optimum power, which became more element-dependent (different CGFRs were required at different

w45x

59

Csq, Ceq

140

114

Coq, 88Srq, Baq,

138

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299

Table 2 (Continued) Type of profiles ratio vs. aerosol CGFR at different sampling depths in a 13-mm (mini) torch

Analytes

Observations

140

sampling depths). Only at elevated powers did elements follow the trend of increasing optimal CGFR with increasing depth: the plasma could then manage the solvent load. In small or conventional torches, optimal sampling depth increased with both atomic mass and forward power.

q

Ce , Uq, 208Pbq

238

Plots of ion signals, oxide ratio and doubly-charged ratio vs. aerosol CGFR at different sampling depths in a 23-mm torch

59

Maps of analyte ion signals in a 6-mm by 6-mm area of the plasma

Coq, Baq, 140 Ceq,

Reference

Optimal sampling depth was mass-dependent at 1.25 kW, with higher 59Coq signal at greater depth. At higher powers, same optimal depth for all elements. At 1.35 kW Uq was less dependent on depth than other analytes, a reversal of the mass dependency found at 1.25 kW. As with a conventional torch, the optimal CGFR increased with increasing sampling depth.

w26x

Baq

Bell-shaped radial distribution of Baq is narrowest and steepest at the top of the IRZ; it is wider and less intense within the IRZ, and it clearly broadens within the NAZ, suggesting that turbulent flow then begins to dominate.

w23x

Radial and axial profiles of ion signals

59

Coq, Ceq

Coq is concentrated within 1 mm of the central axis at low sampling depth; its profile broadens with sampling depth, demonstrating diffusion of the central channel into the annulus. Higher optimal axial position for Ceq than for Coq, as a longer residence time (or higher sampling depth) is required to break-up CeO.

w27x

Axial profile of ion signals at different aerosol CGFRs and powers

7

Liq, 63Cuq, Cdq, 208 Pbq, Yq

Maximum signals shift to higher depths with increased CGFR, but their intensities are fairly constant over a wide range of flow rates. Increasing power shifts the maximum signal to lower depth without changing its intensity. Lower optimum sampling depth for Liq than for other analytes.

w46x

Axial profile of ion signal on a 40 MHz custom-made instrument

Cuq

Desolvation shifted the optimal sampling depth to lower depths and led to higher signal intensity at the same CGFR.

w47x

Axial and radial profiles of ion signals

57

Humidifying a dry plasma shifts optimal sampling depth to higher depths. Adding small amount of water increased signals (vs. those in dry plasma). Adding more water decreased and broadened the signal. Dehumidifying a wet aerosol lowered optimal sampling depth.

w28x

Axial profiles of ion signals

Laq, Pbq

Optimal depths increased with CGFR. Laq was optimal at higher depth than Pbq. Increasing CGFR (and sampling depth) increased Pbq because of greater amount of sample being introduced. Increasing CGFR decreased Laq because of concurrent increase in LaOq (see Table 3); optimal Laq position (i.e. with smallest LaOq yLaq) is not optimal for Pbq. Adding Ar sheathing gas greatly decreased Laq, which shifted to slightly higher depth.

w22x

Radial profiles of ion signals

14 analytes over mass range 27–208

All have bell-shaped distribution within "2 mm from plasma center. The width of this peak did not significantly change with preevaporation (by passing the aerosol exiting the spray chamber through a heated tube prior to its entry into the plasma).

w30x

Radial profiles of ion signals in central channel

14 analytes over mass range 27–208

All have bell-shaped distribution within "1.5 mm from the plasma center. Increasing sampling depth broadened the distribution and decreased its maximum intensity. Adding Ar through a sheathing device did not change the distribution but decreased the maximum intensity.

w31x

Radial and axial profiles

Laq, Alq, Asq

Distribution within 3 mm of the central axis, with the maximum signal intensity on axis.

w32x

138

238

Uq

114

Feq

300

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Table 2 (Continued) Type of profiles

Analytes

Observations

Reference

Similar optimal depth for most singly-charged analyte ions. Exception: those forming refractory oxides, which require a longer residence time in the plasma, i.e. greater sampling depth. Time-resolved signal from single droplet vs. sampling depth

88

The peak height rises steeply at low depth, goes through a maximum, and decreases more slowly at higher depth as ions diffuse in the ICP. This diffusion had a inverse square root dependence with analyte mass.

w8x

Spatial distribution of analyte ions

14 analytes over mass range 27–208

In axial profiles, all elements behaved similarly except for oxide-forming elements, As, and Sb. Arsenic (with highest IP) had higher optimal depth. Antimony behaved similarly to oxide-forming elements, with maximum at higher depth. The greater the stability of the oxide, the higher the depth of its signal maximum.

w25x

Effect of non-spectroscopic interferences on the spatial distribution of analyte ions. Both matrix elements similar in mass and different in IP (K and Cl; Cs and I) and those similar in IP but different in mass (Na, K and Cs; Cl and I) were considered

27

Alq, 51Vq, Crq, 55 Mnq, 58 Niq, 59Coq, 63 Cuq, 64 Znq, 75Asq, 98 Moq, 121 Sbq, 139 Laq, 140 Ceq, 208 Pbq

Matrix-induced suppression depended on both analyte mass and matrix element mass in the axial profiles, and was more important at low depths as a result of a shift of the entire profile to lower depth. K and Cs suppressed more than Na, likely because of spacecharge effects. Suppression decreased with distance from the central axis and with increasing sampling depth, and increased with matrix element mass and decreasing matrix first IP. Cl (highest IP) induced some enhancement. Na enhanced signals at high depths with older sampling cones.

w25x

Radial profiles of matrix effects when using a direct injection high efficiency nebulizer (DIHEN). The radial movement of the torch was limited to "2 mm, an area that should encompass the central channel region.

7

On-axis enhancement by 0.9 M HNO3 and suppression by 5% MeOH for Li; suppression of Be by both matrices. Matrix effect decreased with increasing radial distance, except for increased Be suppression by HNO3 at higher CGFR. With 0.9 M H2SO4, decreasing CGFR increased suppression offaxis for Li, but had little effect on Be. Effects attributed to matrix-induced spatial aerosol redistributions (i.e. matrix changed density and size of droplets) and ionization interference (concurrent suppression of Arq 2 ).

w48x

Radial profiles of ion signal

Mnq

The analyte shows a bell-shaped distribution within the central channel, which is centered on the torch axis.

w34x

Radial profiles of ion signals in a cold plasma (1.4 kW power, 1.3 lymin aerosol CGFR)

16 isotopes over mass range 27–208, 51 16 V O, 98 Mo16O, 139 La16O, 140 Ce16O

Analytes with low IPs and low enthalpies of vaporization had profiles similar to NOq(see Table 1), with distinct central peak. Analytes with higher IPs (As, Sb, Zn) or higher enthalpies of vaporization (V, Mo) did not have a pronounced central peak. Ce, La, and V were almost all in the form of oxides in the central region. NOq was suppressed similarly to analytes by Na and K. Some analyte signal enhancement by up to 0.01 M Na or K in outer plasma regions may be attributed to enhanced electron-impact ionization or, at least partially, to ambipolar diffusion.

w35x

16 isotopes over mass range 27–208

Usual bell-shaped radial profile without matrix. With 0.01 M Na or 0.01 M K, enhancement (except for Pb) was greatest along central axis and at lower depths, and almost completely removed with increasing depth. This was attributed to increased electron impact ionization of analytes from increased electron concentration generated by EIE ionization. In most cases, K enhanced more than Na.

w36x

Radial profiles of ion signals in presence of different matrices

Srq

52

Liq (low IP), Beq (high IP)

9

tions w35x. Although a bimodal radial distribution is still observed for Arq and Ar-containing polyatomic ions, the distance between the two peaks is about twice that observed in an ICP under typical operating conditions, thereby confirming the broadening of the central chan-

nel. Bimodal radial distributions are also observed for COq and NOq, with NOq exhibiting a peak in the center of the plasma as well w35x. Analyte ions showed a similar profile to NOq. Furthermore, the NOq signal was suppressed by the matrix to the same extent as the

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301

Table 3 Summary of spatial profiling studies on the effect of an Ar ICP operating parameters on analyte oxides and hydroxides ion signals in ICP-MS Type of profiles

Polyatomic ions

Axial profiles

238

U ratio is fairly constant at lower depths.

w21x

Plots of signals as a function of aerosol CGFR at three sampling depths

BaOq, BaOHq

Shift of BaOq and BaOHq maxima to higher CGFR with increase in depth. Maxima for BaOq and BaOHq at higher CGFR than Baq.

w49x

Plots of signal vs. aerosol CGFR at three different sampling depths

140

Optimal CGFR increased with increasing depth. 140 CeOq decreased to a much greater extent than 140Ceq with an increase of depth. As a result, increasing sampling depth decreased oxide ratio if CGFR was kept constant.

w42x

Plots of signal vs. aerosol CGFR at different sampling depths in a 9-mm (micro) torch

140

For both of a 9-mm (micro) torch and a conventional 18-mm torch, CeOq yCeq was lower at higher sampling depth. Greater CeOq yCeq at lower sampling distances for micro torch. Trends seemed independent of the torch configuration; only the magnitude of the oxide ratios depended on the plasma.

w44x

Plots of signal vs. aerosol CGFR at different sampling depths in a 13-mm (mini) torch

140

Sampling depth did not alter the overall shape of plots of CeOq yCeq vs. CGFR at various powers, but the magnitude of the ratio decreased with increasing depth. Oxides were reduced with increasing sampling depth at all CGFRs investigated, as with a conventional torch.

w45x

Plots of signal vs. aerosol CGFR at different sampling depths in a 23-mm torch

140

CeOq yCeq was lowest at higher powers and lower CGFRs Similarities between signals at two depths suggested that Ceq is in equilibrium with CeOq at the lower depth. Oxide-ion ratios in a 1.25-kW plasma were slightly higher at greater depths presumably because of oxide reformation.

w26x

Maps of signals in a 6=6 mm area of the plasma. Axial profiles were also collected

BaOq, BaOHq

x–y profiles of BaOHq and BaOq were very similar to those of Baq. Hence, plots of BaOHq yBaq and BaOq yBaq were very flat in the middle of the profile, which suggest that each ion either arises from or dominates in the formation of the others. Maximum BaOq was at lower depth than Ba q in axial profiles.

w23x

Axial profiles of ion signals

CeOq

CeOq continuously decreases with sampling distance in contrast to Ceq, which goes through a maximum before decreasing.

w27x

Axial profile of ion signals

YOq

Maximum YOq is at lower depth than Yq in axial profiles.

w46x

16

U O

q

Ce16Oq

Ce16Oq

Ce16Oq

Ce16Oq

Axial profiles of ion signals

LaO

Effect of non-spectroscopic interferences on the spatial distribution of ions. Both matrix elements similar in mass and different in IP, or similar in IP (Na, K and Cs, Cl and I) but different in mass were considered Different sampling depths

q

Observations The

238

16

U O y

q

Reference

q 238

q

q

As for La , optimal depth for LaO increased with increasing CGFRs, although Laq optimal depth still greater than that of LaOq. Increasing CGFR increased LaOq. Adding Ar sheathing gas did not change the sample load but enhanced LaOq and shifted its profile to slightly higher depth.

w22x

LaOq, CeOq

Analyte oxides at lower depths than analyte, and their signal continuously decreased with depth. Profile of the sum of signal from analyte and analyte oxide was very similar to that of a non-oxide forming analyte, regardless of whether a matrix element was present or not. LaOq y(LaqqLaOq ) profile shifted to lower depth by EIE but not by matrix element with high IP. The shift is only at lower depths in Na, whereas K or Cs shifts the whole profile.

w33x

CeOq yCeq

The oxide ratio decreased as sampling depth increased.

w36x

analyte signal (see Table 2), and greater suppression was observed from the matrix with lower IP (i.e. K). This suggests preferential charge transfer with NOq as a contributing (maybe even dominant) ionization process. This might have important implications for the degree of ionization of analytes in a cold plasma, i.e. only analytes with an IP lower than that of NO (9.26

eV) might then be significantly ionized w50x. For instance, it would imply that using cold plasma conditions to knock down the interference from ArClq might still not enable the easy determination of As, which has an IP of 9.8 eV, and might undergo little ionization. A relationship between analyte signal and enthalpy of vaporization was also seen for non-oxide forming low

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Table 4 Summary of spatial profiling studies on the effect of an Ar ICP operating parameters on analyte doubly-charged ion signals in ICP-MS Type of profiles

Analytes 2q

Observations q

Reference q

q

As for Ba , BaO and BaOH , increasing depth shifts the Ba maximum to higher CGFR but to a smaller extent. At each depth, highest Ba2q at lower CGFR than Baq.

2q

Plots of signals as a function of aerosol CGFR at three sampling depths

Ba

Plots of ion signal ratio vs. aerosol CGFR at different sampling depths

Ce2qy 140Ceq

Ce2qy 140Ceq increased and shifted to higher CGFR as sampling depth increased in a 9-mm torch, as in a conventional 18-mm torch.

w44x

Plots of ion signal ratios vs. aerosol CGFR at different sampling depths

Ba2qy 138Baq

With 13-mm i.d. torch, Ba2q y 138Baq did not depend on sampling depth, power, or CGFR, in contrast to with a conventional torch.

w45x

Plots of ion signal ratios vs. aerosol CGFR at different sampling depths

Ba2qy 138Baq

With 23-mm i.d. torch, at lower depths, Ba2q y 138Baq was approximately independent of forward power, whereas it dropped at higher depths with decreasing forward power. Ba2qy 138Baq was lower at lower sampling depths.

w26x

Maps of ion signals in a 6=6 m area of the plasma. Axial profiles were also collected

Ba2q

Ba2q x–y profiles very similar to but slightly narrower than those of Baq, which resulted in a large peak in the center of the Ba2qyBaq radial plot, and implied that Ba2q was concentrated in the central channel (presumably because of more likely ionelectron recombination where electron number density is higher, i.e. off-axis at low sampling depths). Similar radial characteristics suggest that Ba2q arose from Baq. Slightly sharper maximum of Ba2q was at similar depth to Baq, possibly from spurious effect of a secondary discharge.

w23x

Axial profile of ion signals

Y2q

Maximum Y2q at higher depth than Yq in axial profiles.

w46x

Effect of non-spectroscopic interferences on the spatial distribution of ions.

Ce2q

EIE matrix broadened the characteristic bell-shaped radial profile, which appeared flattened along the central axis. This broadening was not observed for singly-charged ions of similar myz, and was more extensive than for Ceq.

w25x

Different sampling depths

Ce2qyCeq

Ce2qyCeq increased slightly with sampling depth.

w36x

IP elements, indicating that the plasma is not hot enough to fully vaporize, never mind ionize, all analyte species. Tanner demonstrated that, in general, the axial profile of the oxide fraction MOq y(MqqMOq ) provides a good reference point within the plasma, as it is not shifted by changes in lens potentials, plasma operating conditions, and even by a secondary discharge w51x. Only processes that change the distribution of ions within the plasma, such as a shift in ion–atom equilibrium or earlier desolvation of the aerosol, will induce a shift in the oxide fraction. For example, the entire LaOq y(LaqqLaOq) profile was shifted to lower sampling depth by either 0.02 M K or Cs, but did not move in the presence of a matrix element with a high ionization potential w33x. This indicates that 0.02 M K or Cs induced earlier desolvation. However, only the lower depth portion of the same profile was shifted to lower sampling position by 0.02 M Na, which suggests that a shift in atom–ion equilibrium, rather than earlier desolvation as was previously suggested w25x, was responsible for suppression by Na w33x. The concentration of electrons is indeed expected to only increase in the plasma region where Na is ionized, i.e. low in the plasma.

w49x

While axial profiles provide information about the energy and time needed to form the species (for example, those found at lower depth require less energy and less time in the plasma) w16x, radial profiles also provide additional information that is not known intuitively. Indeed, although the central channel is twice as wide in a cold plasma compared to in a conventional plasma w35x, the analyte ion distribution was much narrower: within "0.6 mm from the center w35x as opposed to within "2 mm of the center of a normal plasma w36x. The distribution in the central channel of the cold plasma matched that of NOq, further suggesting that charge transfer from NOq may be a dominant ionization process of the analytes. One might also expect a narrowing of the central channel if the solvent from the aerosol is pre-evaporated prior to its entry in the plasma, since the plasma temperature should not decrease as much (less energy from the plasma would be required for the evaporation process). However, no significant difference was observed in the radial distribution of either background ions or analyte ions upon the insertion of a heated tube between the spray chamber and the torch w30x.

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303

w38,39x can likely be attributed to the translation device underneath the torch box. On the ELAN 250 and 500, the torch box was installed on a single rod, which was not centered under the torch box, and was pivoted from one end. In other words, the axis of the plasma was often not aligned with the axis of the mass spectrometer. In the experience of one of the authors (D.B.), this had a significant effect on the optimal sampling position as well as sensitivity, especially for high-mass elements such as Pb. Indeed, the installation of a three-dimensional translation stage under the torch box resulted in a significant increase in sensitivity for high-mass elements, which was observed twice (while D.B. was at the National Research Council of Canada and then again after she moved to Queen’s University). Similarly, w43x, who was the first to examine the influ´ Gregoire ence of easily ionized matrix elements on radial profiles of analyte signal intensity, had to estimate the distances for the radial profile curves that he obtained with an ELAN-250 instrument, assuming a triangular radial ion signal profile (whereas it is bell-shaped, see Table 2). Other discrepancies can be attributed to the fact that, in some of these studies, the axial signal maximum was at a lower depth than was accessible during the experi-

Fig. 3. Axial profiles of normalized signals of Ceq (triangles), Ce2q (squares), and CeOq (lozenges) under typical operating conditions.

Finally, the studies in Table 1 have implications for the selection of an internal standard. Indeed, several people w52,53x have proposed the use of background ions as internal standards since there is then nothing to add to the sample, which eliminates a potential source of contamination. However, as was seen above, the distribution of several background ions is significantly different than that of analyte ions, especially radially. Although these background ions can still be affected similarly to the analytes in the central channel, those with a similar radial distribution in the central channel, q such as COq in a cold 2 in a normal plasma or NO plasma, are likely to be most efficient if they can be used. In the case of COq 2 as an example, however, the carbon content of all samples and standards must either be constant or known exactly. 3.2. Adequacy of the translation stage The discrepancies between the results that were obtained by the same group on the same instrument

Fig. 4. Radial profiles of CeOqy(CeqqCeOq qCe2q ) ratios at different sampling depths under typical operating conditions: optimaly 2.5 mm (squares); optimal (=); optimalq1.5 mm (lozenges).

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Fig. 5. Axial profiles of oxide ratios under typical operating conditions: CeOqy(CeqqCeOq qCe2q ) (squares); LaOqy(Laqq LaOqqLa2q) (lozenges).

ments, since a constantly decreasing signal was observed with increasing sampling depth w21,37x. The conclusions reached may, therefore, be different from those achieved with a clear maximum in signal intensity. For instance, neither the maximum in the Uq signal, nor that of the UOq signal, were seen under the conditions used in the first ICP-MS spatial study w37x. 3.3. Critical plasma parameters The studies in Tables 2–4 demonstrate the interdependence of different plasma-related parameters, especially the aerosol carrier gas flow rate, the forward power, and the sampling depth. Although most of these studies were performed on first generation instruments, the very significant interaction of these three parameters is still true on newer instruments w54x. Increasing power will shift the IRZ away from the sampler. Since, in general, the optimal analyte ion signal is observed when the sampling orifice is 1–2 mm downstream of the tip

of the IRZ w19x, a smaller sampling depth or an increased aerosol carrier gas flow rate will be required to sample the same position. However, a change in aerosol carrier gas flow rate also changes the amount of aerosol being introduced into the ICP and, therefore, both the analyte and solvent loads. For example, with a Meinhard C-3 nebulizer and a Scott double-pass spray chamber, an increase in the aerosol carrier gas flow rate from 0.83 to 1.02 lymin nearly doubled the aerosol transport efficiency (i.e. the amount of aerosol exiting the spray chamber) w55x. The fact that at a given sampling depth the analyte signal increases as both the aerosol carrier flow rate and the power are increased w38,39,49x is, therefore not surprising: more analyte is being introduced and more power is required to cope with the increased solvent load. Similarly, increasing the aerosol carrier gas flow rate at a given forward plasma power will induce an increase in the optimal sampling depth w28,38,39,41,42,47,49x. This is because a longer residence time will then be needed in the ICP to compensate for the increase in flow rate (which effectively shifts the IRZ) as well as for the desolvation, vaporization, atomization and ionization of any additional amount of aerosol being introduced. As a result, plots of signal as a function of aerosol carrier gas flow rate are unlikely to correspond to ion distributions in the plasma under a fixed set of operating conditions. For example, plots of various Ba ion species as a function of aerosol carrier gas flow rate w49x suggest that BaOq is present lower in the plasma than Baq, which itself is lower in the plasma than Ba2q. Intuitively, this order seems reasonable: oxides should be broken before atoms can be singly and then doubly ionized, and the latter processes should be sequential since significantly more energy is required for the second ionization. However, axial profiles of the ICP under typical operating conditions, which were fixed throughout the profiling, show a different picture (Fig. 3). Although CeOq is indeed located lower in the plasma than Ceq, Ce2q is located in the same region as Ceq. Identical profiles to those of Fig. 3 were obtained for La. Furthermore, although these results were recently obtained using a Varian UltraMass 700, very similar results were reproducibly obtained, several years earlier, on a Perkin-ElmerySCIEX ELAN-500. This would suggest that the plasma temperature andyor electron density is maximum in that region, i.e. a further increase in residence time cannot be beneficial. As a finite plasma volume is sampled, it is also possible that the doubly charged ion is slightly higher in the plasma than the singly charged one, but still within the sampled plasma volume. Spatial profiles of fluorescence would be useful to clarify this. Nonetheless, the sampling depth will determine the proportion of oxide, as shown in Fig. 4, which is worst at low depth.

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305

Fig. 6. Radial profiles of La2qy(LaqqLaOq qLa2q ) ratios at different sampling depths under typical operating conditions: optimaly2.5 mm (squares); optimal (=); optimalq1.5 mm (lozenges).

Also, because of the plasma volume being sampled, analytes with a range of IP will undergo their maximum ionization within the same sampled cylindrical region of the plasma, i.e. they could not be axially resolved by the standard MS detection system. Only the easily ionized elements, which would ionize more rapidly in the plasma, have been observed at a lower depth than other analytes, such as transition metals w39x. Arsenic, with a high IP, is the only element for which the maximum signal was observed at a slightly higher depth than that of other elements w25x. In any case, because of the strong interdependence of the plasma operating parameters, optimizing by altering one of them at a time w38x is unlikely to give the best overall signal intensities, although it could give a local maximum for that particular fixed parameter. Furthermore, differences in findings in the literature are likely due primarily to differences in plasma operating parameters.

3.4. Effect of a secondary discharge Incorrect conclusions can be reached in the presence of a secondary discharge. For instance, in the early days of ICP-MS, the 238U16Oq y238Uq ratio was observed to be constant at lower depth w37x on some instruments that are now known to suffer from secondary discharge. Since, if the oxides were from the original solution, sampling at lower depths would have allowed less time for their dissociation and so should have given rise to higher ratios, the authors deduced that the source of the majority of oxides in ICP-MS was not undissociated refractory oxides from the original solution. Rather, they concluded that these oxides were formed in the boundary layer along the sides of the sampling cone and were entrained into the extracted gas flow. In fact, as shown in Fig. 5, higher oxide ratios are observed at lower depths on instruments free of secondary discharge. Again, this observation was true for both Ce and La,

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fairly flat except at low depth where it suddenly drops. Although these results were obtained on an UltraMass 700, similar profiles were observed with an ELAN-500. 4. Spatial profiling of mixed-gas plasmas

Fig. 7. Axial profiles of doubly charged ion ratios under typical operating conditions: Ce2qy(CeqqCeOqqCe2q ) (squares); 2q q q 2q La y(La qLaO qLa ) (lozenges).

and was reproduced on both an old ELAN 500 and a new UltraMass 700. This, combined with the fact that the oxide profile has a maximum at a lower depth than the profile of the corresponding analyte, strongly suggests that refractory analyte oxides survive the plasma, in agreement with the kinetic energy measurements of Tanner w56x and the calculations of Niu and Houk w19x. Even when adjustments (through lens tuning for example) are made to minimize the secondary discharge, the residual discharge may still significantly affect observations. This is particularly important for doublycharged ions since the discharge has been reported to exacerbate them. For instance, the radial profile of Ba2q yBaq was reported to have a peak in the center of the plasma at the optimal sampling depth w16x. Yet, as shown in Fig. 6, such a peak could only be observed at low depth on instruments where no adjustment of bias potential is required. These results (also seen for Ce) were reproducibly obtained on an UltraMass 700 and an old ELAN-500. Similarly, the axial profile of Ba2q y Baq was fairly flat except at a large sampling depth where it steeply increased w16x. Although this increase is likely a spurious effect of the secondary discharge not being minimized at that sampling depth, a different profile is observed on instruments with a load coil configuration that minimizes the discharge. As shown in Fig. 7, the profile for La2q y(LaqqLaOqqLa2q) is

Spatial profiling can similarly be very useful to characterize mixed-gas plasmas. These plasmas, which result from the addition of a foreign gas to one or more of the Ar gas flows, have different properties depending on the properties of the foreign gas and where it is introduced (w32x and references therein). For example, spectroscopic interferences from ArOq and ArClq can be drastically reduced with an addition of a small amount of nitrogen to the central channel (w32x and references therein). Nonetheless, a better fundamental characterization of these plasmas would help establish the most beneficial operating conditions. Table 5 summarizes spatial profiles of different ions in mixed-gas plasmas. As can be seen in this table, the addition of a small amount of hydrogen to the central channel of either a dry or a wet plasma presented distinct advantages. Analyte sensitivity improved in the dry plasma, and the rate of oxide dissociation increased in the wet plasma; this is evident from the shift of the LaOq and Laq maxima to lower sampling depths. This is likely a result of improved energy transfer from the annulus to the central channel as a result of the high thermal conductivity of H2 w23x. This is further supported by radial profiles (obtained with hydrogen added through a sheathing device), which are not only narrower (perhaps as a result of shrinking of the plasma) but also flatter over a good portion of the central channel in comparison to those recorded for an Ar plasma w31x. Furthermore, the upward-facing parabolic profile of ArNq, which is observed upon an addition of nitrogen to the central channel using the same sheathing device, suggests that the added gas remained largely as a sheath around the central channel w31x. However, radial profiles of ArHq should be acquired while H2 is introduced in the sheathing device to verify this observation. In any case, the configuration of the sheathing device (dimensions of inner and outer diameter, etc.) is likely an important factor that warrants further studies. The sheathing device, which does not change the sample load, can be used to change the efficiency of energy transfer between the annulus and the central channel because it allows the formation of a channel between the central channel and the plasma w23x. An addition of nitrogen to the aerosol carrier gas flow resulted in a plasma resembling a cold Ar plasma (see Table 1), with a wider and more diffuse central channel than that found under normal operating conditions of an Ar ICP w35x. However, the signals from background ions were more intense and the NOq peak was more

A.E. Holliday, D. Beauchemin / Spectrochimica Acta Part B 59 (2004) 291–311

307

Table 5 Summary of spatial profiling studies performed with a mixed-gas ICP in ICP-MS Type of profiles

Type of plasma

Analytes

Observations

Reference

Axial profiles of analyte ion signals

Up to 40 mlymin H2 added to aerosol gas flow of ETV coupled to 1-kW ICP

57

Plasma shrank and optimal depth shifted to higher values (especially at higher CGFR) upon addition of H2 to electrothermal vaporization-generated aerosol of a dry plasma. These are similar effects to plasma humidification (see Table 2). A small amount of H2 increased sensitivity, but more than 5 mlymin of H2 decreased it.

w28x

Axial profiles of analyte ion signals

20 mlymin H2 added to aerosol gas flow of a wet 1.2-kW plasma

Laq

Adding H2 to the carrier gas shifted LaOq and Laq to lower depths; the shift in LaOq was large enough to move its maximum to below that which can be sampled. Concurrent increase in Laq signal.

w29x

Plots of analyte signal vs. aerosol CGFR at different sampling depths

5% N2 added to outer gas of a wet 1.5-kW plasma

Asq

Intensity increased with 5% N2 but higher CGFR was required because of plasma size reduction. Extent of enhancement by N2 depended on sampling depth; greatest at lower depth. Greater dependence of signal on depth than in Ar plasma, but same trend of higher depth requiring higher CGFR.

w57x

Axial profiles of analyte ion signals and ion signal ratios in presence of a secondary discharge

2-kW air–Ar ICP with 100% air in outer and aerosol carrier gas flows, and 100% Ar in auxiliary flow

Coq, Cdq, Yq, Pbq, Baq, Ceq

Each element had a different optimal depth. M2qyMq increased with increasing sampling depth in Ar plasma but was more stable over a range of depths in air-Ar ICP. Pb and Y had similar M2qyMq despite having quite different second IPs. Compared to Ar ICP, M2qyMqof elements with low 2nd IP was lower, while that of elements with high 2nd IP was higher. MOqyMq decreased with increasing depth in Ar plasma but was fairly constant in air-Ar ICP. Compared to Ar ICP, MOq yMq of elements with high oxide dissociation energy was similar, while that of elements with low dissociation energy was much higher.

w16x

Radial profiles of ion signals in central channel

Addition of 83 mlymin N2 to a 1.4-kW Ar plasma through a sheathing device inserted between the spray chamber and the plasma torch

13 analytes over mass range 27–208, myz 54, myz 56

Adding N2 decreased the maximum analyte signal (without flattening it), and a shoulder of smaller intensity appeared on each side of the central central peak. The ArNq profile had the shape of an upward facing parabola whose inflection point decreased with a decrease in sampling depth. ArOq had a bimodal distribution with a minimum on axis and local maxima "2 mm off axis; this might be attributed to entrainment of condensed aerosol from the sheathing device.

w31x

Radial profiles of ion signals in central channel

96 mlymin H2 added to 1.4kW Ar ICP through a sheathing device

13 analytes over mass range 27–208

Adding H2 decreased and flattened the maximum while narrowing the bell-shaped distribution of analyte ions compared to regular Ar plasma. The flat peak top was approximately 1-mm wide.

w31x

Radial profiles of background ion signals

1.4-kW ICP with 110 mlymin N2 added to 0.9 lymin aerosol gas flow (i.e. 12.5% N2)

38 Arq, 40Ar14 Nq, 40 Ar16Oq, 40 Ar16OHq, 40 Arq 2 , myz 31 (15N16Oq most likely), myz 29 (15N14 Nq, 14 N2Hq,

Same distinct spatial zones as in cold plasma, i.e. distinct peaks of Arq and Ar polyatomics in outer regions; signal steadily decreased towards center. Very small Arq in the center (slightly larger than in cold plasma). In comparison to cold plasma, much higher myz 29 signal with local minimum in plasma center of much higher absolute intensity. From outer plasma to its center, Ar polyatomics decreased first, then myz 29.

w35x

Fe

q

308

A.E. Holliday, D. Beauchemin / Spectrochimica Acta Part B 59 (2004) 291–311

Table 5 (Continued) Type of profiles

Type of plasma

Analytes

Observations

13

C O , H12C16Oq)

Only myz 31 showed local maximum in the center; this was attributed to NOq and was more pronounced than in cold plasma. NOq was suppressed by Na or K to the same extent as analyte (but less than in cold plasma), with matrix of lower IP, K, suppressing to greater extent. Distribution of ions characteristic of a ‘lukewarm’ plasma.

16

q

Reference

Radial profiles of analyte ion signals

1.4-kW ICP with 110 mlymin N2 added to 0.9 lymin aerosol gas flow (i.e. 12.5% N2)

16 isotopes in myz range 27-208, 51 16 V O, 98 Mo16O, 139 La16O, 140 Ce16O

Analytes with low IPs had similar radial profiles to NOq, with distinct central peak. Analytes with higher IPs did not have a pronounced central peak, although they were more ionized than in cold plasma. Compared to cold plasma, smaller proportion of analyte in form of oxides, and some doublycharged ions seen in central region. Suppression by Na or K less severe than in cold plasma. Matrix with lower IP, K, suppressed to a greater extent.

w35x

Radial profiles of background ion signals in presence of different matrices

5.9% N2 in the outer gas flow of a 1.4 kW ICP

38 Arq, myz 27, 52, 55, 56, 76,

Arq 2 and Ar polyatomics profiles were flattened and broadened with less bimodal appearance compared to Ar ICP. Their profiles more closely approximated the analyte profiles. Profile of myz 27 was still dissimilar to other background ions and analytes. Evident constriction effect upon N2 addition: Arq dropped off substantially, instead of being very close to a maximum at the edges, as in Ar plasma (see Table 1).

w36x

Radial profiles of analyte ion signals in presence of different matrices

5.9% N2 in the outer gas flow of a 1.4 kW ICP

16 isotopes in myz range 27-208, 51 16 V O, 98 Mo16O, 139 La16O, 140 Ce16O

Much lower analyte intensities in absence of matrix than in 1.2-kW Ar plasma under otherwise identical operating conditions. Bimodal analyte ion profiles seen, with slight maxima on either side of central axis. Slight matrix-suppression was seen in these maxima in the presence of matrix, but to a much reduced extent compared to Ar ICP. Oxide ratio decreased at all depths by over an order of magnitude compared to Ar ICP, with greatest oxide ratio still observed at low depth. Doubly charged ion ratio was 2–3 times that observed in Ar ICP.

w36x

pronounced in the center of the mixed-gas plasma w35x. The fact that analytes with low IPs had similar radial profiles to that of NOq, while only a small central peak could be observed for analytes with higher IPs than NO, suggests a charge-transfer ionization mechanism between NOq and analytes w35x. This is further supported by the fact that NOq was suppressed to the same extent as the analyte signal in the presence of Na or K. However, compared to a cold plasma, the suppression was reduced. In addition, there was a stronger relationship between analyte signal and IP than with enthalpy of vaporization, indicating that the mixed-gas plasma was able to vaporize elements to a greater degree than the cold plasma. This leads to the conclusion that this mixed-gas plasma was ‘lukewarm’ w35x.

However, the addition of nitrogen to the outer plasma gas of an increased power plasma, which drastically shrank the plasma, improved sensitivity if the sampling depth or aerosol carrier flow rate was re-optimized w57x. The plasma shrinkage effectively moved the IRZ away from the sampling cone, thereby requiring an increase in aerosol carrier gas flow rate or a lower sampling depth. Without such a re-optimization, much lower analyte signal intensities were observed than in a 1.2kW Ar plasma under otherwise identical operating conditions w36x. The addition of N2 changed the bell-shaped analyte radial profiles into bimodal ones, which have a similar shape to the flattened and broadened profiles of Arq 2 and argon-containing polyatomic ions. This observation, combined to the absence of correlation between

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analyte signal and IP (as was observed in the lukewarm mixed-gas plasma discussed in the previous paragraph), suggests that a predominant ionization mechanism in such a mixed-gas plasma may be charge transfer from Arq w36x. The location of the addition of the foreign gas may therefore also affect processes in the ICP. In any case, whenever mixed-gas or molecular gas plasmas (where Ar is completely replaced in one or more Ar flows) are used, care should be taken to watch for a secondary discharge, as it can be exacerbated by some gases, such as nitrogen w58x, which may completely invalidate the results. For example, the more stable doubly-charged ion ratio that was observed over a range of depths in the air–Ar ICP may well be a spurious result of the large secondary discharge that was present w22x. 5. Conclusions Spatial profiling has had an important role to play in the development of ICP-MS. From the determination of optimum sampling position as a function of plasma parameters, to the investigation of the effects of hardware configuration and sample composition, to the theoretical framework for proposed ionization mechanisms, spatial profiling has provided key information. The use of ionic fluorescence detection in tandem with MS detection would further enhance the fundamental information obtained. In combination with the knowledge gained from research into such areas as electron densities, electron temperatures, and ion kinetic energies within the plasma, spatial profiling may yield more advances, particularly in the field of mixed-gas plasmas. For example, full spatial profiling has yet to be done of a helium plasma or of a mixed-gas Ar–O2 plasma; the latter is of increasing importance in the analysis of biological materials and a requirement for the analysis of organic samples, such as petroleum naphthas and tars w59x. Furthermore, spatial profiling should become an essential part of every day ICP-MS operation. In particular, monitoring of the axial distribution of the oxide fraction MOq y(MqqMOq ) should be included in the quality control protocol, i.e. be done routinely, to decipher non-spectroscopic interferences occurring within the ICP from those originating within the sampling interface and mass spectrometer w51x. Furthermore, the sampling position of this profile should be used as a reference point through which different ICP-MS instruments could be better compared. For example, if different instruments were operated such as to sample the same portion of the, say, Ce oxide fraction profile, then differences in their figures of merit could be attributed to the design of the mass spectrometer (including the sampling interface). Stating the absolute sampling depth (typically the distance between the sampler and the load

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