Time-resolved measurements with single droplet introduction to investigate space-charge effects in plasma mass spectrometry

Time-Resolved Measurements with Single Droplet Introduction to Investigate SpaceCharge Effects in Plasma Mass Spectrometry Ian I. Stewart and John W. Olesik Laboratory for Plasma Spectrochemistry, Laser Spectroscopy and Mass Spectrometry, Department of Geological Sciences, The Ohio State University, Columbus, Ohio, USA

An investigation of the space-charge induced effects of high concentrations of Pb1 matrix ions on Li1 analyte ions in inductively coupled plasma mass spectrometry (ICP-MS) is presented using a vertically oriented mass spectrometer with single droplet introduction. Greater reproducibility and stability in droplet-to-droplet sample introduction using the monodisperse microparticulate injector (MDMI) was achieved with the vertical orientation. Typical variation (%RSD) in the droplet-to-droplet arrival times, and mass spectrometry peak analytical areas are better than 5%. With this precision, a more quantitative description of the space-charge effect on a single cloud of ions is obtained. Both radial and axial space-charge effects were found to occur in the ion beam. Radial effects result in a loss in intensity because of poor transmission or collisions at surfaces within the mass spectrometer. Axial effects modify the kinetic energy distribution of background ion beam components (e.g., 16O1 and 40Ar1) and sampled ion cloud constituents (e.g., 7Li1). However, axial effects do not appear to generate significant broadening of sampled ion clouds within the mass spectrometer. At the point of charge separation and high ion-beam charge density, the ion cloud maxima for Li and Pb are not coincident. This is because of mass dependent diffusion in the ICP as the ion clouds approach the sampling orifice. Space-charge induced ion loss occurs predominantly at a localized region after the Li1 sampled cloud peak maximum. When the Pb concentration in the sample is sufficiently high the 7Li1 sampled signal has a bimodal peak shape. The existence of the dip and its relative location in the bimodal 7Li1 sampled signal suggests that space-charge effects are localized to the region of high charge density occurring just after charge separation. (J Am Soc Mass Spectrom 1999, 10, 159 –174) © 1999 American Society for Mass Spectrometry

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he transport of ions from the plasma, into the mass spectrometer, and to the detector is a critical process in inductively coupled plasma mass spectrometry (ICP-MS) [1, 2]. However, the diverse sample matrices encountered in inorganic analysis often generate chemical matrix effects that change transport processes and degrade analysis accuracy. In particular, analyte signal suppression within the mass spectrometer has been linked to the presence of high concentrations of matrix elements that are efficiently ionized in the plasma and transported with the analyte into the mass spectrometer [3– 6]. The magnitude of the suppression is more severe as the mass of the matrix ion increases relative to the analyte ion. Mass dependent decreases in the transport efficiency from the plasma to the mass spectrometry detector are consistent with space-charge effects. The concept of space charge in ICP-MS can be understood by considering the various processes that

Address reprint requests to John W. Olesik, Department of Geological Sciences, Laboratory for Plasma Spectrochemistry, Laser Spectroscopy and Mass Spectrometry, Ohio State University, 275 Mendenhall Laboratories, 125 South Oval Mall, Columbus, OH, 43210. E-mail: [email protected]

occur when ions are sampled from the plasma and transported to the mass spectrometry detector. First, a “quasineutral” plasma consisting of neutral atoms, ions, and electrons, flows from atmospheric pressure through a typical sampler–skimmer interface and two stages of pumping into the mass spectrometry vacuum chamber. As the plasma flows through the interface, highly mobile electrons will separate from the bulk flow, resulting in a charge separation and the formation of a positive ion beam consisting of neutral atoms and positive ions. The exact location of the charge separation remains unknown, however it is thought to occur within the skimmer cone [7]. The consequence of forming a positive ion beam of high charge density results in a region of high coulombic charge repulsion or space charge, which deflects ions off the beam axis (radially), reducing their transmission efficiency. The mass dependence of the space-charge effect is related to ion kinetic energy where light ions with low kinetic energy are more readily deflected off axis than heavy ions with greater kinetic energy. Being dynamic in nature, space charge is highly dependent on time and both absolute and relative ion number densities.

© 1999 American Society for Mass Spectrometry. Published by Elsevier Science Inc. 1044-0305/99/$20.00 PII S1044-0305(98)00136-6

Received July 7, 1998 Revised September 24, 1998 Accepted September 24, 1998

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Numerous researchers have employed various strategies to gain insight into reducing the severity of space-charge effects. Tanner et al. [8, 9] utilized a three aperture interface to reduce the overall ion current, and hence space-charge effect. The use of ion optics to moderate the space-charge effect has resulted in the development of several different and perhaps inconsistent strategies. In particular, Turner has suggested the use of high extraction potentials to minimize the chemical matrix effect [10], whereas others have suggested lower extraction potentials [11, 12]. Work by Wang et al. [13], and Sheppard et al. [14], indicate that optimization of the ion-optic lens voltages for the analyte of interest (in the presence of matrix elements) is necessary to minimize the severity of the chemical matrix effect. Ross and Hieftje suggest removing the lens system altogether [15]. There have been several attempts to gain direct experimental evidence on sampling dynamics and space-charge effects occurring downstream of the skimmer orifice. Using fluorescence measurements, the work by Farnsworth et al. [16] has shown great potential. Other methods have employed targets which are designed to collect beam constituents and measure element dependent broadening by examining the deposition profile. These methods, however, are time consuming and require cautious interpretation [17–19]. Theoretical approaches presented by Tanner using computer based mathematical modeling to iteratively calculate space-charge effects have shown great promise [20, 21]. The model has been used to simulate ion beams of currents from 1 to 1500 mA for a number ion-optic configurations and applied voltages [20, 21]. Although the model predicts space-charge effects that are similar to those observed experimentally, it requires an arbitrarily assigned charge separation function and does not account for ion scattering or collisional damping. Other procedures presented recently continue to develop a more accurate picture of the location and origins of space-charge effects within the mass spectrometer [22, 23]. Recently, French et al. [24] designed a laminar flow furnace with an on-demand, piezoelectric based drop generator (micropump) to introduce individual droplets into the plasma. This is often referred to as a monodisperse dried microparticulate injector (MDMI). There are two very important considerations in their design: (1) the piezoelectric element allows droplets to be generated in a reproducible manner at a specified frequency, producing a train of monodisperse droplets, each 60 mm in diameter, and (2) the size of the droplets exiting the furnace can be controlled by the carrier gas flow rate through the laminar flow furnace, and the furnace temperature. By controlling the size of the droplet exiting the furnace, the MDMI allows the user to select the size of the droplets entering the plasma. Therefore, this allows reproducible control of the location in the plasma where droplet desolvation, vaporization, atomization, excitation, and ionization occur [25–

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28]. Consequently, the MDMI has been used to gain a greater understanding of these fundamental processes and probe the origins of matrix effects based upon high concentrations of concomitant elements [26, 27, 29], acids [29], and organic solvents [30] at the single droplet level through optical detection. The consistent generation of ion clouds within the plasma provides a basis for reproducible transmission of ion clouds from the plasma into the mass spectrometer. By using optical emission signals generated from the droplets in the plasma to trigger the mass spectrometry signals, time resolved measurements can be recorded. Previously, Dziewatkoski et al. [31] have used the MDMI to study the effects of diffusion broadening of ion clouds within the plasma and used the corresponding mass spectrometry peak widths to calculate diffusion coefficients. They also identified mass-dependent flight times which lead to ion cloud separation within the mass spectrometer. By controlling the location of the vaporization point of the droplet within the plasma, the effect of solution matrix on the analyte ion cloud shape and its transport through the mass spectrometer can be studied and compared to droplets containing no matrix. Using this approach, Olesik et al. [32] have gained direct experimental evidence on space-charge effects in ICP-MS. In particular, they observed broadening in analyte ion clouds in the presence of large concentrations of lead, which they attributed to space charge within the mass spectrometer. They also reported distortion of the Li1 peak shapes in the presence of high concentrations of lead which they attributed to space charge. However, no definite explanation for the location of the dip which creates the bimodal appearance could be given. Although the authors reported strong evidence for the existence of space charge, they also indicated the need for greater stability and reproducibility in their system before a more quantitative assessment could be made. Allen et al. [33] also used a horizontally mounted MDMI to investigate space-charge effects in ICP-MS. Their system however, did not use time resolved measurements based upon an emission trigger within the plasma. Instead they used a twin quadrupole, which enabled them to look at signals from both the analyte and matrix element simultaneously on individual ion clouds produced by an MDMI. With this unique capability, they report peak broadening and distortions of Li1 peak shapes in the presence of lead, although different than those reported by Olesik et al. [32]. In addition, they were also able to provide evidence indicating that the space-charge effect can occur in regions between the skimmer and extraction lens and between the extraction lens and second lens of their system. Poor reproducibility in peak shape and arrival time, on a drop-to-drop basis has been linked to the horizontal orientation of the MDMI-ICP-MS system by several researchers [31–33]. With the horizontal orientation it is necessary to operate the MDMI at high nebulizer gas flow rates and furnace temperatures. This is necessary

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operate in a vertical manner. A description of the new design and a discussion of improvements to signal stability and reproducibility including figures of merit, will be given. The goal of the paper is to provide a quantitative picture of space-charge phenomenon in terms of axial and radial distortions in ion clouds as they are transported from the plasma to the detector. In addition, insight into the origins of the changes in Li1 peak shapes because of the presence of high concentrations of Pb1 will be provided. This paper is the first of three; a second paper will concentrate on ion transport and mass dependent flight times within the mass spectrometer [34], and a third on sampling dynamics in ICP-MS [35]. Figure 1. Schematic diagram of the instrument used to make time resolved ICP measurements. Highlighting the plasma source, mass spectrometer, optical emission detection system, and the simultaneous detection interface.

to generate stable droplet trajectories from the micropump through the furnace and into the plasma. Unstable trajectories can result in collision with the wall of the center tube or off-axis introduction into the plasma. More importantly, a stable trajectory is only maintained within a narrow range of nebulizer gas flow rates and furnace temperatures. Because variability in these parameters is limited, it adversely limits the range of drop sizes entering the plasma and the operators control over the location where droplet desolvation and vaporization occur relative to the mass spectrometers orifice. Experience has shown that when the MDMI is operated in a vertical manner (as with optical measurements), the reproducibility in measurements and range of desolvated droplet sizes that can be used, is greatly improved [25, 28]. In this article, we report MDMI-ICP-MS results obtained on our mass spectrometer that has been rebuilt to

Experimental A schematic of the instrumental system is given in Figure 1 with typical operating conditions listed in Table 1.

ICP-MS The mass spectrometer employed for all measurements was a laboratory modified three stage Extranuclear (now ABB Extrel, Pittsburgh, PA) EMBA II quadrupole mass spectrometer (011-1 power supply with a model 012-15 High-Q head). Significant modifications have been made since previous reports [31, 32]: (1) the support for the mass spectrometer and vacuum system has been rebuilt so that it is operated in a vertical manner, (2) the original interface has been replaced with an ELAN 5000 interface with a pneumatically actuated gate valve, and (3) a new cylindrical three lens ion-optic system was developed (in-house) and is depicted in Figure 1. There is no photon stop in the new design. The new system, although not fully optimized,

Table 1. Instrumental and experimental parameters ICP-MS Plasma applied power Plasma reflected power Plasma gas Outer gas flow rate Intermediate gas flow rate Mass spectrometry vacuum (operating) Ion-optic Lens 1 Ion-optic Lens 2 Ion-optic Lens 3 Quadrupole rod offset ICP-OES PMT voltage Entrance slit width Exit slit width Trigger MDMI Carrier gas flow rate Furnace temperature Droplet production frequency

1 kW (unless otherwise specified) ,10 W Argon (99.996%) 15 L/min 1.4 L/min Stage 2: 1 3 1024 torr, stage 3: 1 3 1025 torr 260 V 2130 V 25 V 0 V (unless otherwise specified) 21000 V 105 mm 100 mm Specified Varied: 0.65–0.80 L/min 500°C 800 Hz

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gives roughly an order of magnitude improvement in sensitivity for most elements. A modified 40 MHz Plasma-Therm HFL-1500G ICP generator incorporating a center tapped load coil arrangement and manually tuned impedance matching system (Plasma-Therm AMNPS-2) was used for all measurements. The ICP incorporated an ELAN 5000 torch with a custom center tube adapter. The ICP source was mounted on an x, y, z translation stage, giving movement in three dimensions with a precision of 0.01 mm. Maximum motion is 100 mm along the vertical (z) mass spectrometry sampling axis, and 50 mm in the horizontal (x, y) directions.

MDMI The MDMI consists of a droplet generator (Microdrop, GmbH, Germany) and laminar flow furnace similar to those described previously [24, 25, 31, 32]. It is oriented vertically and mounted on the translation stage below the ICP torch providing it with the same translational movement. The micropump produces droplets ;60 mm in diameter. A two piece center tube design was employed, consisting of a short (12 cm, 2.0 mm i.d.) custom made center tube (Precision Glassblowing, Englewood, CO) mounted in the torch and inserted into a socket at the end of a fixed center tube (30 cm) that is part of the laminar flow furnace. Care must be taken to ensure that the two pieces are aligned properly so that the droplets are efficiently transported from the droplet generator into the plasma. Argon carrier gas flowing through the furnace was controlled by a mass flow controller (Brooks Instrument Division, Model 5850E).

Optical Emission Detection A 1:1 image of the plasma was focused onto the collection end of a horizontally mounted fiber-optic bundle (General Fiber Optics, part no. 24-0.1/6.0-0.25/ 3.0-1.5) using a 15 cm focal length, plano-convex lens (Oriel, Model 41775). The image and object distances were adjusted to compensate for chromatic aberration. The collection end of the fiber-optic bundle (1 3 24 horizontal array, with 105 mm diameter fibers) was mounted in a translation stage for vertical travel, allowing emission signals to be collected from the top of the load coil to the tip of the mass spectrometry sampling orifice (;12 mm) in horizontal slices perpindicular to the plasma axis. The other end of the fiber-optic bundle was mounted at the entrance of a 0.2 m monochromator (PTI, model 01-001, Princeton, NJ).

Detection Electronics Emission signals from the photomultiplier tube of the monochromator were sent to a current-to-voltage converter (Keithley, Model 427) having a gain of 104 V/A and a rise time of 30 ms. Similarly signals from the mass spectrometry detector (ETP, model AF553 or AF512H,

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Ermington, AUS) were sent to a second current-tovoltage converter (Keithley, model 428), having a gain of 105 V/A and a rise time of 30 ms. Voltages from the current-to-voltage converters were sent to separate channels of a digital oscilloscope (Nicolet, Model 3091). Normally, the signals were sampled at a rate of 2 ms/point with the emission signal triggering acquisition and storage of the optical emission and mass spectrometry signals. Data for both channels were captured and transferred to a Gateway 486/33 PC via an RS232 interface at 19200 baud using software written in ASYST.

Data Processing Signal information such as amplitude, area, full width at half maximum (FWHM), and full width at the base, were determined using Peak Fit (SPSS, Chicago, IL) software. In some cases, the mass spectrometry waveforms presented are the average of six individual peak waveforms that are first matched in time based upon their emission triggers [32] and then shifted so that the centers for all peaks are aligned.

Sample Solutions All solutions were prepared by diluting concentrated (10,000 mg/mL) commercial standard solutions (Radian, Austin, TX) to 100 mg/mL in 2% HNO3. Before use sample solutions were normally degassed for 10 min by sonication and then passed through a 10 mm filter (Millipore, Number LCWP-013, Bedford, MA) to remove any fine particulates. Degassing and filtering are necessary to prevent gas build up and blockage within the micropump, both of which will stop the pumping action.

Results and Discussion Stability and Reproducibility The main goal of mounting the MDMI-ICP-MS instrument vertically is to improve signal stability and reproducibility. To appreciate the improvements, the steps required for generation of an mass spectrometry signal will be described below, with reference to Figure 1. Figure 2 contains a typical time resolved signal trace, obtained when the MDMI-ICP-MS is operated in the new vertical orientation. The micropump located at the base of the laminar flow furnace generates droplets 60 mm in diameter, containing 100 mg/mL La at 800 Hz. The droplets enter the laminar flow furnace held at 500°C and are carried to the plasma by the hot Ar carrier gas (0.70 L/min). As the droplets pass through the furnace, they desolvate, shrinking to diameters of ;25 mm at the center gas tube exit [36]. As the droplets travel from the center gas tube to the base of the plasma (;3 mm), they will continue to desolvate. The final droplet desolvation occurs within the plasma. The emis-

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Figure 2. A typical time resolved ICP-MS peak trace of a 100 mg/mL La and 100 mg/mL Na sample solution introduced at 800 Hz. For clarity and reference only the first Na(I) 590 nm emission trigger peak is shown and corresponds to the maximum emission intensity which was located at 3.65 mm below the sampling orifice. The 139La1 mass spectrometry six peak trace is a typical representation of the reproducibility and stability obtainable with the current instrumental system.

sion trigger corresponding to the point of maximum atom line intensity [Na(I) 590 nm] for the La1 mass spectrometry signal in Figure 2, is located at 6 mm below the mass spectrometry sampling orifice and 18 mm above the exit for the center gas tube. Under these conditions, a droplet with a 25 mm diameter requires ;15 mm of travel within the plasma to fully desolvate, vaporize, and atomize. Peak atom emission intensity has been shown to occur only a short distance from particle vaporization in the plasma [25] and so provides an indication of the particle vaporization point. Typically, this distance is approximately 1–2 mm [25], but may vary depending on particle size, composition, plasma gas temperature, and plasma gas velocity. For example, Dziewatkoski et al. [31] report distances of 2.5–3.5 mm for solutions containing 150 mg/mL Li, Mg, Mn, and 200 mg/mL Sr, and a plasma gas velocity of 24 m/s. Peak atom emission intensity is used as the emission trigger for all measurements discussed below, unless otherwise noted. The time between the maximum peak intensity for the emission trigger and the maximum peak intensity for the La1 mass spectrometry peak shown in Figure 2 is called the delay time. It refers to the time required for the La1 ions to travel 6 mm through the plasma, into the mass spectrometry sampling interface, through the ion optics and quadrupole mass filter, and strike the detector. Within the plasma, the La1 ion cloud will diffuse resulting in broader less concentrated ion clouds entering the mass spectrometer. The extent of diffusional broadening is based on the ion diffusion coefficient and time spent in the plasma, which then depends upon the sampling depth (distance from vaporization point to

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mass spectrometry sampling orifice) and plasma gas velocity. For matrices where the total dissolved solids is approximately 10 –50 mg/mL, the mass spectrometry peak shapes are thought to be similar to the on-axis plasma ion emission peak shape at, or just prior to, the mass spectrometry sampling orifice. At first approximation, this has been confirmed by comparison with the ion emission peak profile recorded in the plasma just prior to the mass spectrometry sampling orifice [31, 32]. Within the mass spectrometer, normal off-axis broadening in the expansion, space-charge induced broadening, and poor transmission efficiency through the ion optics and mass filter, can alter the peak shapes observed at the detector. As discussed by Olesik et al. [25], the major source of signal fluctuation is because of variations in the dropletto-droplet vaporization point within the plasma. The fluctuations arise from jitter that is induced by turbulence in the center tube gas delivering the droplets to the plasma, and turbulence within the plasma itself. As a result, vaporization, atomization, excitation, and ionization times will vary, and contribute to peak to peak variation recorded at the mass spectrometry detector. Turbulence in the plasma gas after the vaporization point can also contribute to additional variation in the mass spectrometry signal, although probably to a lesser extent. The reproducible train of La1 mass spectrometry signals (Figure 2) suggests minimal variation in the drop-to-drop vaporization point within the plasma for this time scale. Table 2 lists short term (1 trace, corresponding to Figure 2) and long term (12 traces) averages and standard deviations for La1 peak amplitude, FWHM, FW at the base, and peak area. Both the short term and long term precision of these measurements are quite good and are generally better than 5% for all measurements and normally on the order of 2%–3%. The results are comparable to Olesik et al. [25], who reported short term precision of 1%– 6% for emission peak heights (or areas). It is superior to the 10%–20% precision reported for similar measurements using horizontal orientation MDMI-ICP-MS [31, 32]. It should be noted that the data reported in Table 2 represent typical values, and that normal day-to-day operation in this mode generally gives a precision of 5% for most elements. Values better than 1% and greater than 5% are possible. Large variation in precision (greater than 10%) is readily identified on the oscilloscope and generally indicates a problem with the micropump, poor gas regulation, or gross pump–furnace–torch misalignment. All of these may be corrected. The average delay time between the emission trigger and La1 mass spectrometry signal was 217 6 5 ms (2.5% RSD) when the emission was viewed 4 mm below the sampling orifice. As expected, the precision in timing is consistent with that reported for the peak parameters described above (Table 2). The average peak-to-peak arrival time for one trace is calculated using two methods; the first averages the time between

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Table 2. Typical MDMI ICP-MS signal reproducibility and stability for 100 mg/mL Laa Amplitude (V)

FWHM (ms)

FW base (ms)

Analytical area (31026)

1 Trace (6 peaks) Average S.D. % RSD

0.260 0.005 2.0

153 3 1.9

344 7 2.0

44.7 0.8 1.8

12 Traces (72 peaks)b Average S.D. % RSD

0.281 0.013 4.6

149 3 1.7

334 6 1.8

46.7 1.7 3.6

a

Vaporization point is 6 mm below the sampling orifice. Acquired over 30 min.

b

each successive peak and the second averages the time between each peak and the first peak divided by the number of peaks the peak of interest is from the first peak (i.e., peak 2 5 1, peak 3 5 2, etc.). This results in values of 1.26 6 0.03 ms (2.3% RSD) and 1.27 6 0.02 ms (1.8% RSD), corresponding to frequencies of 795 and 788 Hz, respectively. The signal stability and reproducibility obtainable in the vertical orientation has shown great improvement over the original horizontal orientation. At this point, any further improvements to signal stability and reproducibility would come from minimizing gas turbulence originating in the MDMI and ICP torch, as well minimizing noise pick-up from the rf generator.

Ion Sampling and Transport in MDMI-ICP-MS Time-resolved ICP-MS signals from sampled analyte ion clouds are affected by diffusion in the plasma and mass dependent acceleration through the ion optics. There is a constant flow of Ar1 plasma ions and other background plasma species (O1, for example) into the mass spectrometry creating a continuous beam. Superimposed on this beam are sampled clouds of analyte ions. Each cloud of analyte ions is generated from a single droplet of sample. This creates, a “quasicontinuous” ion beam, different in properties than that of “more” continuous ion beams encountered in the normal operation of ICP-MS [1, 2, 7], and of truly modulated beams encountered in time of flight (TOF) experiments [36]. The acceleration of ions through the ion optics and quadrupole mass spectrometer lead to ion flight times from the sampling orifice to the mass spectrometry detector that increase with mass. The transport of individual ion clouds from the plasma to the mass spectrometry detector therefore represents a unique case of sampling dynamics and ion transport. Sampled Ion Cloud Signals: The Influence of Diffusion in the ICP. Although diffusion of ions in the ICP results in a symmetric ion cloud in space, the signal acquired from a fixed location in space as a function of time, leads to an asymmetric sampled ion cloud signal (Figure 3b, c). The asymmetry results from significant diffusion of an

Figure 3. (a) A cartoon depicting diffusional ion clouds as they travel from the vaporization point to the mass spectrometry sampling orifice. (b) Theoretical shape of the ion clouds as a function of time as observed from the mass spectrometry sampling orifice 6 mm from the vaporization point along the plasmamass spectrometry axis, eq 1 was used to generate the data. (c) Individual overlaid time-resolved ICP-MS signals for 7Li1 and 208 Pb1 from individual solutions containing 100 mg/mL Li in 2% HNO3 and 100 mg/mL Pb in 2% HNO3. The two waveforms are synchronized and overlaid based upon their individual emission triggers, being Li(I) 670 nm and Pb(I) 261 nm located at 3 mm below the sampling orifice.

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been assigned to spatial positions I, II, and III to illustrate this (Figure 4a). As the ion cloud’s spatial center approaches the mass spectrometry orifice, diffusion occurs continuously, resulting in an expansion (broadening) of the ion cloud and a decrease in the concentration in the spatial center of the ion cloud (point II in Figure 4b). As spatial point III approaches the sampling orifice (Figure 4c) diffusion will result in an increase in ion concentration (compared to the concentration at spatial point I when it reached the sampling orifice) because of diffusion of ions outward from the ion cloud center. The sampled ion cloud signal observed at the sampling orifice as a function of time is then a tailing, asymmetric peak (Figure 3b). The sampled ion signal, measured as a function of time at the sampling orifice, can be described by eq 1 [31]: C~ y, t! 5

Figure 4. A cartoon depicting the dynamic effect of diffusion on the ion cloud as it flows through the mass spectrometry sampling orifice at three different times.

ion cloud in the ICP as it is sampled by the mass spectrometer. From the time ions on the front side of the ion cloud in the ICP (closest to the mass spectrometry sampling orifice) are sampled by the mass spectrometer to the time ions on the trailing side of the ion cloud reach the sampling orifice, significant diffusion can occur. Consider the cartoon shown in Figure 4 depicting a Li1 ion cloud entering the mass spectrometry sampling orifice (or a fixed point in space where an emission intensity is measured). Being symmetric in space, the Li1 ion cloud will have equivalent ion concentrations on the side closest to the mass spectrometry orifice (point I in Figure 4a) and farthest from the mass spectrometry orifice (point III in Figure 4a) with the highest concentration located in the center of the ion cloud (point II in Figure 4a). Values have arbitrarily

S

2~ y9 2 vt! 2 C0 3/2 exp 8~Dt! 4Dt

D

(1)

where C 0 refers to the initial concentration, D refers to the diffusion coefficient, t refers to time, y9 refers to the distance between the sampling orifice and the vaporization point in the plasma, and v refers to the plasma gas velocity. The equation assumes that instantaneous vaporization occurs from a point source and that the ICP-MS signal is proportional to the concentration of ions at the sampling orifice. The experimental parameters for eq 1 are defined in Figure 3a (and below), and the results are plotted in Figure 3b. The width, asymmetry, and time of maximum signal for the sampled ion cloud signal depends on the ion diffusion coefficient in the ICP and the time between vaporization and sampling of the signal (i.e., the distance from the vaporization point to the sampling orifice times the gas velocity). The distance from the vaporization point to the sampling orifice (y9), is 6 mm. The emission trigger was 4 mm below the sampling orifice when the data in Figure 3c were acquired. Based on the approximations of Olesik et al. [25] and Dziewatkoski et al. [31] the vaporization point is estimated to fall 2 mm below the maximum atom emission intensity (trigger). For a plasma gas velocity of 24 m/s [31], the total diffusion time for the spatial center of both ion clouds would be ;250 ms. The diffusion coefficient for Li1 in the plasma is reported to be 84 cm2/s [31], the value for Pb1 (14 cm2/s) however, is obtained by extrapolation (diffusion coefficient vs mass1/2) of the data presented by Dziewatkoski et al. [31]. If the rate of diffusion is sufficiently high relative to the velocity of the ion cloud traveling to the mass spectrometry sampling orifice, then the maximum sampled ion signal will appear before the spatial center of the ion cloud passes through the sampling orifice (as is the case for Li1 in Figure 3b). Note that the peak in the sampled Li1 signal occurs before the peak in the

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sampled Pb1 signal, and the Li1 sampled ion cloud will be broader and more diffuse than the Pb1 sampled ion cloud at the mass spectrometry sampling orifice because Li1 ions diffuse more rapidly in the ICP than Pb1 ions. Therefore, Li1 ions should arrive at the sampling orifice and generate a detectable signal before Pb1 ions. At some time later, Pb1 ions will begin arriving at the mass spectrometry sampling orifice. The results obtained from modeling eq 1 (Figure 3b) clearly show (1) that at the sampling orifice the maximum intensity for the Li ion cloud appears before the maximum intensity for the Pb ion cloud and (2) that both peaks have asymmetric profiles with Li being more pronounced than Pb. Once the spatial center of the ion cloud has passed the mass spectrometry sampling orifice (250 ms after vaporization) the remaining portion of the ion cloud will pass through, having an elongated or tailed appearance because of diffusion away from the mass spectrometry sampling orifice. The maximum sampled 7 1 Li and 208Pb1 signals occur 210 and 244 ms respectively, after vaporization. Experimental 7Li1 and 208Pb1 sampled ion cloud signals acquired from two separate, single element (cation) 100 mg/mL solutions are shown in Figure 3c. The emission triggers [Li(I) 670 nm and Pb(I) 261 nm] were located 4 mm below the sampling orifice. The signals were aligned in time based on the peak atom emission signals so that the mass spectrometry signals could be compared directly. The offsets in the peaks in Figure 3b compared to Figure 3c will be discussed below. Both the 7Li1 and 208Pb1 sampled ion clouds are asymmetric. However, the 7Li1 sampled ion cloud has more pronounced tailing. In addition, the 7Li1 sampled ion cloud appears broader than the 208Pb1 sampled ion cloud. Comparison of the experimental ion-cloud peak shapes (Figure 3c) with the simulated ion cloud peaks shapes (Figure 3b) shows a fairly good agreement in terms of asymmetry. However, the Pb1 sampled ion cloud (Figure 3c), does not appear to be much narrower than the Li1 sampled ion cloud as predicted by the model (Figure 3b). Because the emission signal is observed from a fixed point in the plasma (;100 mm in height) analogous emission cloud peak shapes should also be observed, this is corroborated by the asymmetric Li(I) emission trigger shown in Figure 3c. The peak shapes observed by Allen et al. [33] were much broader than reported here. In addition they also show sampled ion cloud peak shapes that are similar for both Li1 and Pb1 (Figure 2 of [33]). The later point is unexpected in light of the effect of diffusion in the plasma described above. Furthermore, their data indicates that ion loss from the Li1 cloud occurs primarily on the rising portion or “front end” of the cloud in the presence of Pb1. Allen et al. [33] suggest that this may be attributable to the fact that different instruments will have different ion transmission characteristics. In addition, they also present data illustrating the significant effect of ion-optic lens voltage on the peak shape observed.

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Figure 5. Cartoon illustration of (a) ion cloud separation during transmission from the interface to the detector of the mass spectrometer and (b) potential sources of space charge in an argon ion beam with two separating ion clouds.

Mass Dependent Ion Transport from the Sampling Orifice to the Mass Spectrometry Detector. Ion cloud separation within the mass spectrometer is illustrated in Figure 5a and has been described previously [31–33]. Consider two ion clouds that overlap in the plasma and enter the mass spectrometer together. Within the sampling interface region (skimmer) ions from both clouds overlap in time as depicted. However, once a positive ion beam is formed, the ions in each sampled cloud will travel through the ion-optics and mass filter at different rates. Ions of different mass but similar initial velocities (that of the Ar expansion gas) will have different initial kinetic energies and therefore be accelerated differently by the voltages applied to the ion-optic system [31]. As a result, the sampled ion clouds will be partially or entirely separated when they reach the mass spectrometry detector, as illustrated in Figure 5a. It should be noted that the cartoon depicted in Figure 5a is used only to promote the concept that ion clouds (as detected) appear to separate in time based on different flight times within the mass spectrometer. In reality, individual ion clouds can have regions that exist in the plasma, in the sampling interface, in the ion optics, in the mass filter, and at the detector simultaneously at a given instant in time. This is illustrated by the fact that emission is still detected in the plasma at the same time that ions from the same ion cloud are recorded at the mass spectrometry detector (see Figure 6 for example).

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For simplicity, the above discussion has assumed that all the ions reach the terminal velocity of the expansion gas (Ar), and therefore mass dependent flight times are primarily because of differences in the ion velocity induced by different rates of acceleration in the electrostatic fields. It is possible, that the terminal velocity is not truly achieved for all ions because of velocity slip. This would also contribute to mass dependent flight times. Although it is unclear exactly how much of the ion cloud is entrained at the mass spectrometry orifice, only ions from a small radial zone on the plasma-mass spectrometry axis are thought to be transported efficiently to the mass spectrometry detector [7]. In this manner it is assumed that the MDMI-ICP-MS sampled signal is obtained from the center of the ion cloud along the plasma-mass spectrometry axis. The dynamic nature of the “quasicontinuous” beam raises some interesting questions regarding the nature of space-charge and ion transport within the mass spectrometer (Figure 5b). For example, to what extent does ion cloud separation mitigate space-charge effects? Does space-charge broadening and loss occur equally in axial (along the ion beam) and radial directions (perpendicular to the ion beam), or do major constituents of the background ion beam (e.g., Ar1 and O1) act to constrain axial ion-cloud broadening? Similarly, does ion cloud separation and coulombic repulsion alter the kinetic energy distribution of ions within the beam? To answer these questions it is necessary to consider and examine the peak shapes for all of the ion beam components.

Lithium Ion Transport With And Without Lead Ion Matrix The research presented below focuses on the effect of lead on sampled Li1 ion signals. The lithium and lead system was chosen for three reasons. (1) Both are efficiently ionized in the plasma. (2) There is a large mass difference between lithium and lead. This should produce a severe space-charge effect. (3) The effect of Pb on sampled Li1 ICP-MS signals has been studied previously using a horizontal MDMI-ICP-MS [32, 33], providing a useful frame of reference for our results. A high concentration of a concomitant element(s), such as Pb, results in an earlier appearance of atoms and ions in the ICP produced from a single drop of sample [27, 31]. A change in the vaporization height is of great concern because it affects the total diffusion time and therefore the shape of the sampled ion cloud entering the mass spectrometer. For example, when the vaporization height decreases in the plasma, there will be more time for diffusion before the ion cloud reaches the mass spectrometry sampling orifice. To keep a constant time for diffusion, the nebulizer gas flow rate was adjusted slightly when necessary to maintain a fixed vaporization height. The emission trigger [Li(I)

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Figure 6. Time-resolved ICP-MS signals for (a) 7Li1, 16O1, and 40 Ar1 from a solution of 100 mg/mL Li in 2% HNO3 and (b) 7Li1, 16 1 40 O , Ar1, and 208Pb1 from a solution of 100 mg/mL Li and 1000 mg/mL Pb in 2% HNO3. The ICP-MS signals were triggered using the Li(I) 670 nm emission signal corresponding to the maximum intensity located at 6 mm below the sampling orifice.

670 nm, maximum intensity] was then located at the same spatial location within the plasma for all measurements. Normally only slight changes in the nebulizer gas flow rate were required (e.g., 0.75 to 0.76 L/min). Figure 6a illustrates the 7Li1, 16O1, and 40Ar1 sampled ion current signals as a function of time when the ions produced from one droplet of a 100 mg/mL Li, 2% HNO3 solution are sampled by the mass spectrometer. Each trace is acquired separately (but each from a simultaneously acquired emission trigger) from different droplets of the same sample. The trigger signal is used to indicate time 0, and all sampled ion signals are shown as a function of time relative to the trigger. As shown in Figure 6a, 40Ar1 provides a strong background current. However, at a time (20.20 ms) well in advance of the emission trigger located 6 mm below the sampling orifice, the 40Ar1 background decreases until it reaches a minimum (0.11 ms) whereupon it begins increasing to the original steady state background level (0.24 – 0.80 ms). The steady state portion of the 16O1 trace is most likely the result of the water vapor loading that originates from droplet desolvation within the laminar flow

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furnace and to a lesser extent oxygen entrainment from the air. For example, the total water loading (liquid and vapor) from droplets with a 60 mm initial diameter produced at 800 Hz is 5.43 mg/min. The measured droplet diameters exiting the furnace held at 500°C are reported to be ;25 mm [37], therefore 5.04 mg of water enters the plasma per minute as vapor with only ;0.39 mg/min entering as liquid. At STP this would correspond to 0.006 L/min water vapor, a modest contribution to the total gas flow. The low, broad peak in the 16 1 O signal from 20.20 to 0.30 ms corresponds to the oxygen liberated during desolvation of the remaining water on the droplet. It is unclear why the 16O1 signal peaks before the 7Li1 sampled mass spectrometry signal in this example. Because desolvation usually precedes vaporization, atomization, excitation, and ionization in time, it is possible that a velocity difference exists between the desolvating water vapor and the droplet/ particle. If the particle was slower, the 16O1 signal would peak before the 7Li1 analyte signal. The 7Li1 sampled mass spectrometry signal reaches a maximum ;100 ms after the Li(I) emission trigger located 6 mm below the sampling orifice. The peak is noticeably asymmetric and is fairly broad at the base because of the high diffusion coefficient for Li1 in the plasma (84 cm2/s) [31], as discussed previously. Examining all three traces (Figure 6a) an interesting correlation is drawn. The beginning of the dip in the 40 Ar1 mass spectrometry signal corresponds to the beginning of the increase in the 16O1 signal and suggests that as the droplet is desolvating, it is simultaneously cooling the plasma locally and reducing the ionization efficiency of Ar. The first ionization potentials of Ar, O, Li, and Pb are 15.76, 13.62, 5.39, and 7.42 eV, respectively [38]. Because of the high ionization potential for Ar, it tends to be a sensitive indicator of plasma temperature. Lazar et al. [30] have also presented evidence supporting the effects of droplet cooling in the plasma using the MDMI with emission detection. Other possibilities for the decease in Ar1 ion intensity include displacement of argon by the increased partial pressure of vaporized water and/or atomized oxygen. Charge exchange reactions or Penning ionization reactions could also be occuring which simultaneously decrease the argon ion concentration while increasing the oxygen ion concentration. At this point it is unclear whether or not the background 40Ar1 and 16O1 signals affect the 7Li1 sampled mass spectrometry peak shape or vice versa. Olesik has suggested that droplet cooling can also effect the transmission efficiency of ions from the plasma to the detector [27]. A significantly different result is observed for the individual ion traces when a solution of 100 mg/mL Li with 1000 mg/mL Pb is used (Figure 6b). Here, the 208 Pb1 signal dominates the trace, with the maximum peak intensity striking the detector ;150 ms after the 7 1 Li maximum intensity. It is interesting to note that the 7 1 Li sampled ion cloud has changed significantly, being less intense and adopting a bimodal distribution. Sim-

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ilar shapes have been observed previously by Olesik et al. [32], however, because of poor reproducibility they had to compare normalized intensities. Therefore, space-charge induced axial broadening of the ion cloud could not be discriminated from broadening because of a loss in ion transmission that becomes more severe as the sampled matrix ion current increases. In this study, it was possible to distinguish the effects of these two processes, as discussed below. The 40Ar1 and 16O1 signal traces are also significantly affected by the presence of high Pb1 ion currents (Figure 6b). In particular, the steady state 40Ar1 signal is more depressed, the dip is more pronounced and occurs later in time, and there seems to be an Ar “pinch” that arrives at the detector between the 7Li1 and 208Pb1 sampled signal. It is difficult to explain the depression in the steady state 40Ar1 signal in the presence of 1000 mg/mL Pb matrix, and as yet this is still a matter of investigation. The larger, broader, more rounded dip in the 40Ar1 signal is attributable to Pb1 matrix induced space charge. Of note is the fact that the dip occurs much later in time (0.20 ms) than that of the 40 Ar1 signal in Figure 6a (0.11 ms). The broadness of the dip reflects either a spatial (broadness of Pb ion cloud) or temporal dependence (contact time), of the 208Pb1 ion cloud on the space charge, or both. The observed 40 Ar1 pinch suggests that the 208Pb1 ion cloud is exerting an axial repulsive force on nearby, lighter 40 Ar1 ions. This pinch may be further influenced through its location between the 7Li1 sampled ion cloud maximum and the 208Pb1 sampled ion cloud maximum. The 16O1 trace in Figure 6b also exhibits marked differences from that of Fig. 6a. In the presence of Pb1 there is a significant dip in the 16O1 signal (1.50 ms). Also, the maximum signal is more sharply peaked (0.80 ms) and seems to correspond in time with the 7Li1 signal maximum. Both of these features are similar to the axial and radial space-charge effects described for Pb1 on the 40Ar1 signal.

Effect of Space Charge on the Sampled Li1 Ion Signal Figure 7 shows the effect of increasing amounts of Pb on the mass spectrometry-sampled 7Li1 ion cloud peak shape generated from a 100 mg/mL Li solution in 2% v/v HNO3. In all cases the Li(I) emission trigger was at 4 mm below the sampling plate. The raw 7Li1 mass spectrometry-sampled signals are shown in Figure 7a. The ratio of the 7Li1 mass spectrometry-sampled signals in the presence of Pb1 to the 7Li1 signal in the absence of Pb1 is shown in Figure 7b. Three important observations can be made from Figure 7. (1) There is a significant loss of 7Li1 sampled signal as the Pb concentration is increased. For a sample containing 1000 mg/mL Pb, the 7Li1 mass spectrometry-sampled signal amplitude at the point of maxiumum loss (Figure 7b) decreases by as much as 90%. It

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sampled ion cloud is not as large as previously suggested [32] from a comparison of normalized signals. (3) There appears to be a slight enhancement in the 7Li1 mass spectrometry-sampled signal on the rising edge of the peak when Pb is present in the sample. The 7Li1 mass spectrometry-sampled signal is the product of the Li1 concentration at the sampling orifice and the transmission of Li1 ions through the sampling orifice and skimmer as a function of time and the ion transmission efficiency from the skimmer to the mass spectrometry detector. The number of Li1 ions sampled depends on the production of Li1 ions in the plasma and diffusion of Li1 ions in the ICP as the ion cloud is sampled by the mass spectrometry (as ions reach the sampling orifice). The transmission efficiency of ions from the skimmer to the mass spectrometry detector depends on the space-charge induced losses that in turn should become more severe as the Pb1 sampled ion current increases. As discussed above, the 7Li1 mass spectrometry-sampled ion cloud is asymmetric because of diffusion in the ICP and as a result of significant diffusion during the time that the ion cloud is being sampled, the maximum 7Li1 mass spectrometry-sampled signal occurs before the physical center of the Li1 ion cloud reaches the sampling orifice. The asymmetric 7Li1 mass spectrometry-sampled signal behavior with 1000 mg/mL shown in Figure 7a is a natural consequence of the Pb1 sampled current dependent space-charge induced loss in ion transmission efficiency. In the absence of Pb1 the 7Li1 signal is more symmetric. Furthermore, the time when the minima in the ratios (indicating most severe space-charge induced loss of Li1 ions) occur is relatively independent of Pb concentration. The apparent movement of the peak 7Li1 mass spectrometry-sampled signal to earlier times as seen in Figure 7a, is a consequence of timedependent decreases in the 7Li1 ion transmission efficiency, as shown in Figure 7b. As discussed previously (Figure 3 and associated discussion), the sampled Li1 ion cloud flight time from the sampling orifice to the mass spectrometry detector is shorter than that for the Pb1 ion cloud. Therefore, the time when the maximum space-charge induced ion transmission loss is observed relative to the flight time of the Pb1 mass spectrometry-sampled ion cloud can

Figure 7. (a) Time-resolved 7Li1 ICP-MS signals in the presence of 0, 100, 500, and 1000 mg/mL Pb for solutions of 100 mg/mL Li in 2% HNO3. The time scales of the four individual traces are synchronized based upon correlation with the maximum emission intensities for the Li(I) 670 nm trigger located at 3 mm below the sampling orifice. (b) Ratios of the signals with Pb matrix (100, 500, 1000 m/mL) presented in (a) to the signal with 100 mg/mL Li in 2% HNO3.

is interesting to note however that because of the separation of the maximum intensities of Li1 and Pb1 sampled ion clouds during (diffusion), the maximum amplitude of the 7Li1 mass spectrometry-sampled signal decreases by only 32% (Table 3). Similarily, the total 7 1 Li mass spectrometry-sampled signal peak area decreased by more than a factor of 2 (Table 3). In previous research [32, 33] the precision was insufficient to make these comparisons. (2) The time-resolved 7Li1 mass spectrometry-sampled signal is not dramatically broadened at high Pb concentrations. Therefore, axial (along the ion beam direction) space-charge broadening of the

Table 3. The effect of Pb matrix concentration on the MDMI Li1 mass spectrometry signal characteristicsa,b Pb matrix concentration (mg/mL) 0 100 500 1000 a

Amplitudec (max) (V)

Amplitudec (dip) (% of max)

FWHM (ms)

Analytical area (31026)

Delay timed (ms)

Li1–Pb1 peak separatione (ms)

0.100 0.091 0.086 0.068

100.0 57.5 22.5 9.8

72 60 42 36

8.55 7.11 5.40 3.93

99 84 71 60

123 128 155

Average of six peaks, all % RSDs are better than 5%. Based on the data presented in Figure 7. max refers to maximum peak amplitude, dip refers to amplitude at the position of maximum signal loss (Figure 7b). d From maximum emission intensity of the Li(I) 670 nm trigger. e Based on the time difference between the maximum intensity of the Li1 mass spectrometry signal and the Pb1 mass spectrometry signal. b c

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provide some insight into when and where within the instrument, the space-charge effects occur. The dip in Figure 7b occurs near the time when the maximum Pb1 concentration is sampled but well before the sampled Pb1 ion cloud reaches the mass spectrometry detector. Consistent with the modeling by Tanner [21], this implies that the space-charge effect is most pronounced shortly after charge separation (i.e., in the early stages of ion transport) and is corroborated by direct and indirect evidence presented below. The experimentally measured time from the peak Li1 signal to the peak Pb1 signal because of both diffusion in the sampling process and mass dependent flight times for Li1 and Pb1 is 106 ms (Figure 3c). This is consistent with the data presented in Figure 8 (discussed below), where for Li1 in excess of Pb1 the separation is 108 ms and space-charge loss of the sampled Li1 ion cloud is not thought to significantly influence the apparent separation of the respective peak maximum intensities. Where the solution concentrations are similar (or the Pb concentration is in excess of Li) and space-charge loss of the sampled Li1 ion cloud starts becoming significant, the difference in the peak maxima is larger (e.g., 123 ms for 100 mg/mL Li and Pb see Table 3). Diffusion during the sampling process results in the appearance of Li1 and Pb1 peak maximum separated in time by 32 ms (Figure 3b). Based on eq 1, the relative difference does not change significantly (i.e., 2–5 ms) between for vaporization points between 4 and 10 mm below the sampling plate. Therefore, although slight differences in the actual vaporization height between samples of various matrices may effect the absolute flight times, the relative flight times are much less sensitive. The actual difference in Li1 and Pb1 peak separation time because of mass dependent flight times is therefore 74 ms. Assuming no velocity slip or ion scattering, the total flight times through the ion-optics and mass filter can be modeled with Simion using a scaled representation of our mass spectrometer, and the appropriate voltages [34]. The initial velocity of the Li1 and Pb1 ions (and KE) are determined by the argon plasma gas temperature [7]. At 4200 K the total flight time for Li1 from a region just inside the skimmer to the mass spectrometry detector is calculated to be 44 ms, whereas the total flight time for Pb1 is 116 ms yielding a difference in flight time of 74 ms. For comparison at T 5 3000, 5000, and 7000 K the differences in flight times are 87, 67, and 55 ms respectively. Although the agreement between the theoretically determined flight time separation and experimental data seems reasonable, caution should be employed concerning absolute interpretation. Consider now, the data presented in Figure 7a, b. The time difference between the point of maximum sample loss for Li1 in the presence of 100, 500, and 1000 mg/mL Pb and the peak maximum intensity for Li1 without Pb is on average 33 ms. This number is very

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Figure 8. Li1 and Pb1 ion cloud separation during transport from the sampling interface to the mass spectrometry detector. A solution of 500 mg/mL Li and 100 mg/mL Pb in 2% HNO3 was used to generate the traces. The upper trace is from the current collected at the first ion-optic lens (L1) as a function of time. The lower trace is the sum of the 7Li1, 16O1, 40Ar1, and 208Pb1 ion traces collected at the mass spectrometry detector. The Li (I) was located 3 mm below the mass spectrometry sampling plate.

similar to the expected peak separation based on diffusion during the sampling process (32 ms) and indicates quite strongly that the space-charge induced losses observed (Figure 7) occur at or near the point of charge separation (i.e., near the skimmer). Although the results presented seem both reasonable and consistent with experimental data, caution is urged with regards to absolute interpretation. A more thorough discussion on the mass dependent flight times will be presented in a subsequent publication [34]. The results of an experiment to define the extent of ion cloud separation within the mass spectrometer are shown in Figure 8. A solution containing 500 mg/mL Li and 100 mg/mL Pb in 2% HNO3 was used to generate a reference TIC trace by recording the 7Li1, 16O1, 40Ar1, and 208Pb1 mass spectrometry signals (all the major ions). The various signals were synchronized in time relative to their emission triggers (located 3 mm below the sampling orifice), and then manually added. The result is plotted as the lower trace in Figure 8. The first ion-optic lens (Lens 1) located 25 mm from the base of the skimmer cone was held at ground and the current was recorded at this point. The result is plotted as the upper trace in Figure 8. It should be noted that by changing the first ion-optic lens (L1) from 260 to 0 V, the ion cloud flight times are altered. Previous investigations exploring the effect of ion-optic lens voltages on flight time suggested that over this range the differences would be quite small (10 ms) and therefore the comparison is still valid. Qualitatively the traces look similar, however, the peaks in the upper trace (recorded at Lens 1) are less well resolved. The time between the Li1 and Pb1 peak maxima for the ion current at Lens 1 is 45 ms, whereas

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the time between the two peaks at the mass spectrometry detector is 108 ms. A good portion of this separation (32 ms) is probably because of the fact that diffusion in the ICP, while the Li1 ion cloud is being sampled, causes the peak Li1 mass spectrometry-sampled signal to occur earlier than the arrival of the spatial center of the Li1 cloud at the sampling orifice. The remainder of the difference in the time between the observed peaks is because of the ion cloud separation which occurs in the mass spectrometry between the sampling orifice and the first lens. The data suggest that there is a partial separation of the Li1 and Pb1 ion clouds at the first lens of the ion-optic system. The separation observed (45 ms) occurs at a time greater than where the maximum space-charge effects are seen to occur in Figure 7b (33 ms) and suggests that the point where maximum spacecharge loss occurs is before this point (Lens 1). The rising edge of the 7Li1 mass spectrometrysampled signal appeared to increase as the Pb concentration was increased (Figure 7a, b). One explanation that could account for this, is that the portion of the 7Li1 ion cloud traveling in front of (and separate from), the Pb1 ion cloud, is accelerated forward along the axis because of a kinetic energy “kick” exerted by repulsion from the Pb1 ion cloud. This would also imply that the region of the 7Li1 ion cloud directly in front of the Pb1 ion cloud would have a different (greater) kinetic energy than the portion of the 7Li1 cloud directly overlapping the center of the Pb1 cloud. It would also imply that ion clouds can exert axial space-charge effects on other ion clouds. Direct evidence for axial space-charge effects will be described in more detail below.

Evidence For Axial Space-Charge Effects The 40Ar1 signal (Figure 6b) had a peak, described as an 40 Ar1 pinch above, only when the sample contained a high concentration of Pb. The existence of the pinch was attributed to a kinetic energy kick that the 40Ar1 received traveling immediately in front of an ion cloud of a heavier element (Pb1). In Figure 6a where the solution contained only Li1, no heavier ion than Ar1 had a significant ion current in the beam and no pinch was observed. A simple experiment was performed to identify the presence of kinetic energy variation within the Ar ion beam using solutions containing elements of lower mass than Ar1 and solutions containing elements of higher mass than Ar1. The quadrupole rod offset voltage was varied from negative to positive voltages, increasing the electrostatic stopping potential and reducing the transmission efficiency of positive ions through the quadrupole to the detector. Figure 9a shows the 40Ar1 signals for a solution containing 100 mg/mL Li in 2% HNO3 as a function of quadrupole rod offset voltage (RO). As the RO increases from 25.0 to 14.0 V the entire signal seems to decrease uniformly to 0% transmission, i.e., noticeable pockets of kinetic energy variation along the beam for this matrix are not apparent.

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Figure 9. Time-resolved ICP-MS signals for (a) 40Ar1 from a solution of 100 mg/mL Li in 2% HNO3 and (b) 40Ar1 from a solution of 100 mg/mL Na and 100 mg/mL La in 2% HNO3, as a function of quadrupole RO voltage. The ICP-MS signals were triggered using (a) Li(I) 670 nm and (b) Na(I) 590 nm emission signals, both corresponding to the maximum intensity located at 3 mm below the sampling orifice.

Figure 9b shows the 40Ar1 signals for a solution containing 100 mg/mL Na and 100 mg/mL La in 2% HNO3 as a function of RO. In this experiment there are three noticeable differences. (1) The dip in the 40Ar1 signal is more pronounced, indicative of space charge induced by the La1. (2) As the RO is increases a peak or pinch of 40Ar1 with higher kinetic energy emerges from the background of the leading edge of the 40Ar1 dip. (3) The 40Ar1 signal on the trailing edge of the dip seems to go from a slight enhancement to a more pronounced “rounded” appearance with increasing RO. Point (3) is important as it complements the kinetic energy pinch observed on the leading edge of the 40Ar1 dip. At 25.0 V RO, the 40Ar1 traveling behind the heavier 139La1 ion cloud “bunches up” and accumulates there. The kinetic energy of the 40Ar1, however, is less than the bulk 40 Ar1 in the ion beam because it is based upon a positive force natural to the forward motion of the 40 Ar1 beam and a negative force because of repulsion by the 139La1 ion cloud. Therefore as the RO increases the lower energy 40Ar1 on the trailing edge of the dip is lost faster and so the dip assumes a rounded rather than bunched appearance. The shift of the dip to longer transmission times in the 40Ar1 traces occurs because the higher RO slows the ions velocity through the quadrupole.

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sampled ion cloud (peak 2). The shift in kinetic energy distribution or kicks observed in both sets of data provide strong evidence for ion cloud–ion beam and ion cloud–ion cloud axial space-charge effects. Axial space charge however, seems to represent a unique case being constrained or directly influenced by the presence of other beam constituents.

Conclusions

Figure 10. (a) Normalized time-resolved ICP-MS signals for 7Li1 from a solution of 100 mg/mL Li in 2% HNO3 as a function of quadrupole RO voltage. The data has been shifted in time for easy comparison. (b) Graphical representation of the ratio of peak 2 amplitude over peak 1 amplitude (as defined by the inset), to illustrate the difference in kinetic energy between the two peaks.

In a similar manner, the kinetic energy distribution across the 7Li1 ion cloud in the presence of high concentrations of Pb1 was investigated. This experiment was performed to determine if the ion cloud from a heavier Pb1 matrix would accelerate the portion of the 7 1 Li ion cloud immediately in front of it and decelerate the portion of the 7Li1 ion cloud behind it, changing its kinetic energy. Figure 10a shows the normalized plot of 7 1 Li ion cloud traces from a solution of 100 mg/mL Li with 1000 mg/mL Pb in 2% HNO3. The plot has been shifted in time so that the maximum intensities of the first peak (peak 1) in the bimodal 7Li1 distribution overlap in time for easy comparison. The normalized data clearly shows that peak 1 decreases at a much slower rate than peak 2 in the distribution. The ratios of the peak 2 amplitude to peak 1 amplitude is plotted in Figure 10b to demonstrate this point graphically. The data in Figure 9b indicate a varied kinetic energy distribution along the 40Ar1 ion beam, and are consistent with axial space-charge effects induced by high mass ion clouds on the lower mass 40Ar1. Similarly, the data in Figure 10 indicates a bimodal kinetic energy distribution within the 7Li1 ion cloud. Ions on the front side of the sampled ion cloud (peak 1) appear to have a greater kinetic energy than ions on the back side of the

By orienting the MDMI-ICP-MS vertically, greater stability and reproducibility was achieved on a drop-todrop basis. Variation in short and long term reproducibility was typically better than 5% RSD for delay times, analytical areas, amplitude, FWHM, and FW base measurements. This provided the basis for a more quantitative assessment of space-charge effects of Pb1 on Li1 during transport from the sampling interface region of the mass spectrometer to the detector. Evidence for both axial and radial space-charge effects were observed directly in the “quasicontinuous” MDMI-ICP-MS ion beam. Radial effects account for loss in ion density in the 7Li1 ion cloud as a function of increasing Pb matrix concentration within the mass spectrometer. This is attributed to preferential off-axis broadening of the 7Li1 ion cloud that results in inefficient transfer of ions through the ion optics or collisions of the ions with surfaces (i.e., skimmer or ion-optic lens system) inside of the mass spectrometer. Loss in the 7 1 Li peak maximum intensity is also attributed primarily to radial effects rather than axial broadening of the ion cloud within the mass spectrometry. Axial space-charge effects appear to occur simultaneously with radial space-charge effects. However, the results of coulombic repulsion along the beam are complicated by its quasicontinuous nature. For example, instead of being able to broaden freely, a heavy mass analyte ion cloud must exert force against 40Ar1 species and other ions which comprise the ion beam. This imparts additional kinetic energy to 40Ar1 directly in front to the heavy mass ion cloud and reduces the kinetic energy of 40Ar1 directly behind the heavy mass ion cloud. Evidence for this was presented in Figure 9. As a result, the beam itself may act to retard axial broadening. Similarly, axial space-charge effects were found to occur between ion clouds traveling along the beam axis. Evidence for this was presented in Figure 10, where the 7 1 Li ion cloud was found to have a bimodal kinetic energy distribution. The leading peak in the 7Li1 distribution (peak 1 Figure 10b) was found to have a higher kinetic energy consistent with a kinetic energy kick from the 208Pb1 ion cloud traveling just after it in time. The conclusions reported here are consistent with the previous work of Allen et al. [33], who illustrate the significant effect of ion-optic lens voltage on the sampled ion cloud peak shape and therefore and implied a spatially dependent kinetic energy distribution within a sampled ion cloud in a mass spectrometer. In addition,

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the consistency of the observations from two completely different instrument configurations supports the interpretation of space charge presented here and by Allen et al. [33]. A second conclusion from this work is that the bimodal peak shapes observed with our MDMI-ICP-MS experiments are significantly influenced by dynamic diffusion processes occurring in the plasma during sampling. The Li1 mass spectrometry-sampled signal is an asymmetric, tailing peak due to diffusion in the ICP that occurs between sampling of the front side of the ion cloud and the back side of the ion cloud. At the point of charge separation and high ion-beam charge density the sampled ion cloud maxima for Li1 and Pb1 are not aligned (Figure 7b) and ion loss occurs predominantly at a localized region after the 7Li1 sampled peak maximum where the Pb1 ion current is maximum. At a Pb concentration of 1000 ppm in the original sample, Pb1 induced space-charge loss of Li1 ion transmission efficiency is high enough to generate a bimodal distribution (e.g., Figure 10). This behavior, because of the pulsed nature of analyte ion signals produced from individual drops of solution, has important ramifications, especially for correctly understanding the relative space-charge induced losses of ions whose sampled peak maximum are less separated from the Pb1 ion peak maximum during the sampling process (e.g., Sr1). In particular, does a greater extent of sampled analyte and matrix ion cloud overlap at the time of sampling result in; less, similar, or more severe space-charge induced ion loss? Preliminary investigations have indicated that the total 88Sr1 signal decreases more severely because of Pb1 induced space-charge losses than 7Li1 under similar conditions. As the analyte and matrix ion masses are more similar, the sampled ion peak widths and shapes will be more alike. This will be discussed in more detail in a second publication [34]. An indirect conclusion on the location and dynamics of space-charge induced loss of ion transmission efficiency can be made. It has been established that the Li1 sampled ion cloud is broader than the Pb1 sampled ion cloud, as expected based on their diffusion coefficients in the plasma. Furthermore, the peak in the Li1 sampled signal occurs before the peak in the Pb1 sampled signal, again because of diffusion in the ICP as the ion clouds move to the sampling orifice. Evidence from Figures 3, 7, and 10, suggests that this determines the location of the space-charge effect (i.e., off the 7Li1 peak maximum). Furthermore, the space-charge effect is localized (likely to times when the Pb1 sampled ion current is high) appearing as a dip and generating a bimodal 7Li1 peak shape appearance. If space charge occurs predominantly at one fixed location (temporally and spatially), then only ions experiencing significant overlap (in numbers) with the Pb1 ion cloud would be deflected strongly or lost. After this point, little significant loss should occur and the ion cloud peak shape should reflect this. If space-charge loss was not local-

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ized (largely), then as the Pb1 ion cloud separated from the Li1 ion cloud because of differences in the mass dependent flight times (and axial space charge) the Pb1 peak maximum would dynamically overlap the tailing portion of the 7Li1 ion cloud. This would then have the effect of a time dependent space-charge stripping of the tailing portion of the 7Li1 ion cloud which should result in a sharper loss in the tailing portion of the 7Li1 ion cloud or a much more pronounced and broader dip than is seen in Figure 7b. Although the above discussion suggests that the space-charge effect is localized to a defined area in time and space coinciding with the point of charge separation in the beam, absolute interpretation is limited by the accuracy in defining the actual spatial location of charge separation. Previous studies by Allen et al. [33] using a significantly different instrumental arrangement, however, have also provided strong evidence for the existence of space charge in the region between the skimmer and extraction lens, or between extraction lens and second lens in their system. Further experiments are now underway in this laboratory to test the effect of varying the instrumental arrangement (ion optics) and operating conditions. If space charge was localized within the skimmer cone and resulted in significant loss of the ions to collisions with the skimmer walls then placement and design of ion optics may be academic. If the effect of space charge was not as severe and resulted in “manageable” off axis broadening of the beam then the placement of ion-optic lens elements and their design could play a critical role in mitigating the severity of the space charge as suggested by Allen et al. [33]. A second, perhaps more important point concerns why the space charge occurs in a localized region. Most likely this because of a coincidence with the point of highest charge density in the beam, that rapidly attenuates itself through space charge and the natural consequence of beam expansion in the mass spectrometry vacuum, which would be consistent with the modeling experiments of Tanner [21]. Reports on experiments examining the time dependent current signals generated from an MDMI-ICP-MS system and collected immediately after the skimmer will be discussed in a subsequent paper.

Acknowledgments Some of the equipment used for these studies was purchased with funds from the National Science Foundation (CHE-9217170), the Biomedical Research Support Grant Program, Division of Research Resources, National Institute of Health (BRSG 2 S07 RR07072), and The Ohio State University. Additional support for the research was provided by the Perkin-Elmer Corporation and PE-Sciex. Special thanks to Barry French, Ben Etkin, and Ray Jong (University of Toronto Institute for Aerospace Science and Sciex, Division of MDS Health) for developing the MDMI and many subsequent, stimulating discussions.

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