Characterization of an Inductively Coupled Plasma with Xylene

Oct 1, 1997 - Direct injection high efficiency nebulizer-inductively coupled plasma mass spectrometry for analysis of petroleum samples. Kaveh Kahen ,...
6 downloads 8 Views 263KB Size
Download PDF
Anal. Chem. 1997, 69, 3921-3929

Characterization of an Inductively Coupled Plasma with Xylene Solutions Introduced as Monodisperse Aerosols Alexandru C. Lazar and Paul B. Farnsworth*

Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602

The optimum operating parameters of the inductively coupled plasma (ICP) with organic solvents are different from those with aqueous solutions. With organic solvents, the ICP is operated at higher power to overcome plasma cooling due to the organic solvent, and aerosol desolvation is necessary in order to reduce solvent load into the plasma. The monodisperse dried microparticulate injector (MDMI) offers the possibility of controlling solvent load by controlling the frequency with which droplets are introduced into the plasma. A test solution that contained 0.5 mg/L Ba in xylene was used to study the influence of MDMI operating parameters on the behavior of the ICP with an organic solvent. The spatial and temporal distribution of the solvent in the ICP was determined for different droplet production frequencies and MDMI oven temperatures, and the optimum operating conditions were established. The best detection limit for Ba in xylene was 1.5 ng/mL, or 0.16 pg (in 200 droplets). The inductively coupled plasma (ICP) is a well-established atomization, ionization, and excitation source that is used in combination with either atomic emission spectrometric (AES) or mass spectrometric (MS) detection for trace elemental analysis. The determination of trace elements in organic solutions is important in the analysis of petroleum products, in the monitoring of wear metals in used aircraft lubricating oils, and in solvent extraction. The behavior of the ICP with organic solvents is different from that with aqueous solutions.1-3 The decomposition of organic compounds in the plasma requires more energy than the decomposition of water, and, consequently, the plasma is usually operated at higher power with organic solutions than with aqueous solutions.4 When used with conventional nebulizerspray chamber combinations, volatile organic solvents create high solvent loads in the plasma, which can cause soot formation on the wall of the torch or even extinguish the plasma. The vaporization characteristics and the effect of the physical properties of the solvents on the nebulization and transport efficiency of the organic aerosols have been described by Boorn and coworkers.5 The importance of controlling the amount of solvent introduced into the plasma has also been emphasized.6,7 The (1) Boumans, P. W. M.; Lux-Steiner, M. C. Spectrochim. Acta 1982, 37B, 97126. (2) Wier, D. G.; Blades, M. W. J. Anal. At. Spectrom. 1994, 9, 1311-1322. (3) Wier, D. G.; Blades, M. W. J. Anal. At. Spectrom. 1994, 9, 1323-1334. (4) Boorn, A. W.; Browner, R. F. Anal. Chem. 1982, 54, 1402-1410. (5) Boorn, A. W.; Cresser, M. S.; Browner, R. F. Spectrochim. Acta 1980, 35B, 823-832. S0003-2700(97)00269-2 CCC: $14.00

© 1997 American Chemical Society

organic solvent load has a strong influence on the analytical performance of the system. Compared to aqueous solutions, organic solvents produce higher background fluctuations and cooler plasmas, with a consequent reduction in analyte excitation efficiency. Ng and Caruso8 used an electrothermal carbon cup for sample introduction into the ICP. First, the analyte was separated from the solvent during the drying step, after which the analyte was vaporized and introduced into the plasma. The sensitivity of the system with organic solvents was lower than that with aqueous solutions, which was explained by the higher tendency of organic solvents to soak into the wall of the carbon cup. In order to reduce the radio frequency (rf) power required for organic solvents, which has an impact on the cost of the rf power supply, Ng et al.9 evaluated the performance of a MAK torch with organic solvents. The rf power level for stable plasma conditions was dependent on the nature of the solvent and the outer argon flow. The use of a direct injection nebulizer (DIN) for organic sample introduction was investigated by Avery and co-workers.10 Sample consumption was substantially reduced, and no waste was generated, but the detection limits were about 1 order of magnitude higher than those obtained with aqueous solutions. The increase in detection limits was attributed by the authors to the cooling effect of the solvent on the plasma. Due to the high solvent vapor load, organic solvents used with ICP-MS can form carbon deposits on the cooled sampling cone.11 In order to eliminate soot formation, oxygen is sometimes added into the carrier argon stream. The addition of oxygen has to be carefully controlled because too much oxygen accelerates the degradation of the sampling cone. The addition of oxygen can also increase interferences in the mass spectra. In recent years, a novel sample introduction system has been developed. The monodisperse dried microparticulate injector (MDMI)12 is capable of introducing single droplets into the inductively coupled plasma. The analytical performance of an MDMI-ICP-AES with aqueous solutions has been evaluated by using Ba as a test analyte.13 The limit of detection was comparable (6) Maessen, F. J. M. J.; Kreuning, G.; Blake, J. Spectrochim. Acta 1986, 41B, 3-25. (7) Pan, C.; Zhu, G.; Browner, R. F. J. Anal. At. Spectrom. 1990, 5, 537-542. (8) Ng, K. C.; Caruso, J. A. Anal. Chem. 1983, 55, 2032-2036. (9) Ng, R. C.; Kaiser, H.; Meddings, B. Spectrochim. Acta 1985, 40B, 63-72. (10) Avery, T. W.; Chakrabarty, C.; Thompson, J. J. Appl. Spectrosc. 1990, 10, 1690-1698. (11) van Heuzen, A. A. ICP-MS in the Petroleum Industry. In Applications of Inductively Coupled Plasma Mass Spectrometry; Date, A. R., Gray, A. L., Eds.; Blackie and Son Ltd.: Glasgow, 1989; Chapter 7, pp 169-188. (12) French, J. B.; Etkin, B.; Jong, R. Anal. Chem. 1994, 66, 685-691. (13) Lazar, A. C.; Farnsworth, P. B. Appl. Spectrosc. 1997, 51, 617-624.

Analytical Chemistry, Vol. 69, No. 19, October 1, 1997 3921

to that obtained with ultrasonic nebulizers, but sample consumption was about 3 orders of magnitude lower, and no liquid waste was created. Some of the MDMI characteristics observed in the study of aqueous samples were also appealing for work with organic solutions. By controlling the droplet production frequency, one can easily control the solvent load into the plasma, and the solvent load is not dependent on the volatility of the solvent. Reduced organic solvent load eliminates soot formation and plasma stability problems. The monodisperse droplets can be dried to a desired degree, which suggests that the partially dried particle might be spatially separated from the solvent vapor cloud, allowing for the use of lower power in the operation of the ICP. Taking into account the potential advantages of the MDMI, we decided to investigate the ICP with organic solutions introduced into the plasma as monodisperse droplets. This experiment was designed to reveal the influences of specific operating parameters of the MDMI on the behavior of an MDMI-ICP-AES system with organic solvents with low or no oxygen content. For this group of solvents, xylene is a good representative compound, and it is one of the most widely used diluents in oil analysis. Our test solution contained 0.5 mg/L Ba in xylene. Even though we evaluated the system by monitoring the emission from the atomic, ionic, and molecular species in the plasma, some of the information obtained about the MDMI-ICP system might be useful in the operation of an MDMI-ICP-MS system with organic solvents. EXPERIMENTAL SECTION Instrumentation. The experiments were performed with a Plasma Therm 40.68 MHz rf generator (Model HFL 2000G, Plasma Therm, Kresson, NJ). The MDMI was operated in the vertical position. The MDMI used in these experiments was a protoype device developed at the Univerity of Toronto Institute for Aerospace Studies with funding from Sciex Corp. The singlepiece quartz injector tube, with 5-mm i.d. and 1.8-mm tip i.d., was mounted in a one-piece torch body (Model 310-02, Precision Glassblowing, Englewood, CO). The flow rate of the carrier argon gas through the oven of the MDMI was regulated by a mass flow controller (Model FMA-117, Omega Engineering, Stanford, CT). The plasma was imaged with unit magnification on the entrance slit of the 1-m scanning monochromator (Model 2061, McPherson, Acton, MA) with an adjustable two-lens assembly.14 Radiation at the selected wavelength was detected by a photomultiplier tube (PMT, Model R456, Hamamatsu, Japan), with an applied voltage of 750 V. The signal from the PMT was amplified by a current amplifier (Model 427, Keithly Instruments Inc., Cleveland, OH) and monitored with a digital oscilloscope (Model 4094, Nicolet Inc., Madison, WI). The experimental setup is presented in Figure 1. Sample Preparation. A 1000 mg/L Ba stock solution was prepared from anhydrous barium chloride (Spectrum Chemical Mfg. Corp., Gardena, CA) dissolved in spectrograde methanol (Fisher Scientific, Fair Lawn, NJ). The 0.5 mg/L Ba test solution was obtained from a 10 mg/L methanolic solution by dilution of the stock solution with reagent grade xylene. Procedures. Time- and space-resolved emission intensities of the C I line at 247.8 nm, the C2 band at 516.5 nm, and the Ba II line at 455.4 nm were recorded in order to determine the spatial (14) Padgett, L. R.; Farnsworth, P. B. Appl. Spectrosc. 1988, 42, 608-614.

3922 Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

Figure 1. Experimental setup: 1, sample reservoir; 2, MDMI; 3, torch; 4, collection optics; 5, monochromator; 6, PMT; 7, amplifier; 8, digital oscilloscope; 9, computer. Table 1. Experimental Conditions plasma power, kW argon flow rate, L/min plasma gas auxiliary gas carrier gas monochromator slit width, µm slit height, mm MDMI pulse amplitude, V pulse width, µs frequency, Hz oven temperature, °C no. of scans averaged HALC, mm

1.00 and 1.25 16 0.8 1.0 100 1 90 40 20 and 200 110 and 135 30 0-20

and temporal distribution of the solvent in the plasma and the influence of the solvent on the analyte emission intensity. We studied the effects of the operating conditions of the MDMI by collecting data at different oven temperatures, droplet production frequencies, and plasma powers in a viewing range of 0-20 mm above the load coil (HALC). Note that, for each of the three operating parameters, two values were studied, to which we will refer in subsequent discussion as the high and low values. The numerical values for the three parameters are listed in Table 1, along with other critical instrument settings. The passage of the ionic vapor cloud through the observation axis generated emission peaks that varied in intensity as the height of the observation axis was changed. From the recorded waveforms, the emission intensity was measured at the top of the peak and at the baseline, between peaks. The difference between these two values, which represents the net peak intensity, was also determined. The continuum emission intensity from the plasma was obtained for the different operating conditions of the system, without the introduction of droplets into the ICP, at the same wavelengths at which the C I, C2, and Ba II signals were acquired. RESULTS AND DISCUSSIONS Droplet Volume. All previous experiments with the MDMI reported in the literature have been performed with aqueous solutions. In order to evaluate the effects of the physical properties of the liquids on the droplet production mechanism, we measured the volumes of the droplets produced by the micropump with three different solvents. The times required to

Table 2. Droplet Diameters, Droplet Volume Ratios (Vorg sol/Vwater), Viscosities at 20 °C, and Viscosity Ratios (ηorg sol/ηwater) for Methanol, Xylene, and Water solvent

diameter (µm)

Vorg sol/ Vwater

viscosity (mPa s)

ηorg sol/ ηwater

water xylene methanol

76.7 67.1 59.8

1 0.89 0.63

0.890 0.760 0.544

1 0.854 0.611

Figure 3. C I emission waveform showing the presence of a small leading peak before the appearance of the peak corresponding to the droplet: plasma power, 1 kW; carrier gas flow rate, 1 L/min; MDMI frequency, 20 Hz; toven ) (a) 110 and (b) 135 °C.

Figure 2. C I emission at 1 Hz MDMI frequency. HALC, 7 mm; toven ) 110 °C; carrier gas flow rate, (a) 1 and (b) 0.65 L/min.

empty a fixed length of capillary tubing were used to determine the volumes of methanol, xylene, and water droplets. The estimated droplet diameters, along with the viscosity of the liquids, are presented in Table 2. Variations in droplet size can be explained by examination of the droplet production process. The micropump consists of a glass tube surrounded by a piezoceramic sleeve.12 A compressive pulse from the piezoceramic causes a small and sudden decrease in the internal volume of the glass tube, and a certain amount of liquid is expelled as a droplet. In order to reduce the amount of solution flowing backward and to dampen the oscillation induced in the liquid by the pulse, a restriction is placed at the inlet of the capillary. When the glass tube expands to its initial dimensions after the pulse, the liquid is aspirated from both ends, i.e., from the tip and from the aspiration tubing. Surface tension pulls the meniscus of the liquid back to its initial position at the tip of the pump. The amount of solution that flows backward through the restriction during the compressive pulse depends on the viscosity of the liquid. Low viscosity allows more solution to flow backward, and smaller droplets are expelled at the tip of the pump. Table 2 contains the organic

solvent-to-water viscosity ratios. The values agree well with the observed droplet volume ratios. The volume of the produced droplets can affect the analytical performance of the system. Large droplets contain more analyte species, which produce higher signal levels in the plasma. At the same time, large droplets contain more solvent, which can cause plasma cooling and background shifts. Droplet Desolvation. The transformation of the droplet into an analyte vapor cloud requires first the evaporation of the solvent. The solvent starts to evaporate in the oven and is distributed between the liquid and the gas phases. The relative amounts of solvent in the two phases at the moment the droplet enters the plasma has a pronounced influence on the spatial and temporal behavior of the analytical and background signals. The total amount of a given solvent injected into the ICP per unit time is constant for a constant MDMI frequency, but the amount of solvent in the microenvironment of the analyte particle in the plasma varies with the distribution of the solvent between the two phases. The droplet spends a relatively long time in the oven while it travels a length of ∼30 cm, with a velocity of approximately 1 m/s (at 1 L/min carrier gas flow rate and 135 °C oven temperature). Solvent vapors are carried away from the droplet by diffusion. The solvent that enters the discharge in the liquid phase evaporates rapidly and has a more pronounced effect on the plasma region near the analyte particle than does the vapor created in the oven. The waveforms in Figure 2, acquired at a droplet production frequency of 1 Hz, show the C I emission pattern originating from a single droplet at two different nebulizer Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

3923

Figure 4. Net C I emission peak heights (arbitrary units) vs HALC (mm): (a) 110 °C, 1.00 kW; (b) 110 °C, 1.25 kW; (c) 135 °C, 1.00 kW; (d) 135 °C, 1.25 kW.

gas flow rates. The emission corresponding to the solvent that enters the plasma as vapor appears as a very broad peak lasting over 500 ms. In contrast, the peak produced by the solvent that enters the plasma as a liquid has a width of less than 1 ms. This narrow peak is off-scale in Figure 2. Examination of the emission on a shorter time scale gives a clearer picture of the peak arising from the liquid droplet, which appears as the narrow peak at time zero in Figure 3. We attribute the small leading peaks in Figure 3 to solvent vapor that travels through the oven at a higher rate than the liquid droplet. Several factors can contribute to different rates of travel for the solvent vapor and liquid through the MDMI oven and the tubing leading to the ICP torch. The liquid droplet is slowed by gravity relative to the gaseous species in the laminar flow through the MDMI oven. The magnitude of the this effect depends on the droplet diameter, which is decreasing during the droplet’s passage through the oven. Radial diffusion of the solvent vapor places it in laminar flow streams with velocities that decrease with increasing distance from the tube center. Finally, some vapor undoubtedly interacts with the tube walls, particularly in the unheated section of tubing that connects the MDMI oven with the ICP torch. The relative magnitudes of these factors that affect the liquid and vapor rise velocities change as the oven temperature, initial droplet size, and nebulizer flow rate are changed. Such changes are evident in the differences between parts a and b of Figure 3. The interactions among the three variables are sufficiently complex that we did not attempt to model them quantitatively. 3924 Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

The qualitative picture, however, is quite clear. The narrow peak at time zero arises from the liquid droplet, a conclusion supported by the observation that all of the analyte emission coincides with this peak. The remainders of the profiles in Figure 3 are from solvent vapor produced before the droplet enters the plasma. The temporal profiles are in the form of a relatively fast rising edge, followed by a long tail. The tails are significant, because at droplet introduction frequencies above a few hertz, the solvent tail from one droplet overlaps the leading edge from the next. At high droplet introduction frequencies, a significant dc solvent concentration is created that can affect both background levels and analyte signal amplitudes. The continuous vapor concentration increases with increasing droplet frequency, while the amount of liquid solvent in each droplet remains unchanged. Vertical Emission Profiles. C I Emission at 247.8 nm. The information about the liquid contained in the droplets at the moment they enter the plasma can be obtained from the net height above the baseline of the C I emission peaks. The vertical profiles of the atomic carbon emission are presented in Figure 4. The net emission intensity is high at low oven temperature, when the size of the droplets is relatively large at the exit of the oven and more material contributes to the C I net emission intensity (compare Figure 4a to c and b to d). The high rf power increases the plasma temperature, which causes an increase of the net atomic carbon emission intensity (compare Figure 4a to b and c to d). The net emission intensity is affected by the MDMI frequency only when oven temperature and plasma power are low (Figure 4a). The low oven temperature and high droplet fre-

Figure 5. Baseline C I emission intensity between droplet peaks (arbitrary units) vs HALC (mm): (a) 110 °C, 1.00 kW; (b) 110 °C, 1.25 kW; (c) 135 °C, 1.00 kW; (d) 135 °C, 1.25 kW.

quency produce high continuous solvent vapor concentration and large liquid droplets. At low plasma powers, the cooling effect of the solvent vapor is enough to reduce the efficiency with which the liquid droplets are vaporized and excited. High plasma power can compensate for plasma cooling, and the net peak heights are similar for different droplet production frequencies (Figure 4b). The curves in Figure 5, obtained by plotting the C I emission intensity between peaks as a function of HALC, give information about the solvent evaporated in the oven. The baseline emission intensity increases with increasing oven temperature (compare Figure 5a to c and b to d). This increase complements the decrease in net peak intensities shown in Figure 4, and reflects a shift of the solvent from the liquid to the vapor phase. The effect of plasma power on the C I emission intensity between peaks is similar to its effect on net peak emission intensities. The MDMI frequency has a dramatic influence on the baseline emission intensity at the C I emission wavelength. The emission increase at high frequency is a confirmation of our earlier discussion about the increase of the dc solvent vapor concentration in the carrier argon flow at high frequencies. C2 Emission at 516.5 nm. The C2 emission spans a large spectral range and can interfere with the analyte emission lines. The diatomic carbon molecules exist in significant concentrations only in the relatively cool regions that exist either low in the plasma or in the vicinity of a vaporizing droplet. From the vertical profiles of the C2 emission, one can determine the plasma location above which spectral interferences from C2 band emission disappear. The diatomic carbon emission can also give information

about the distribution of the solvent as a function of the operating parameters of the system. The vertical profiles of the net C2 emission peak heights are presented in Figure 6. The dependence of the net peak intensity on oven temperature is similar to the dependence observed for the C I emission (Figure 4). An increase in oven temperature causes a decrease in the volume of the liquid droplet, and the peak heights are smaller at high oven temperature (compare Figure 6a to c and b to d). The influence of rf power is different from that seen in the C I vertical emission profiles. As plasma power is changed from its low to its high setting, the net C2 emission peak intensity decreases because the diatomic carbon molecules are dissociated to a higher degree, and the “blue tongue” disappears at low HALC values (compare Figure 6a to b and c to d). The influence of the droplet production frequency on the emission intensity is complex. The local number density of the solvent molecules in the plasma is higher when more solvent is present in the droplet. Solvent evaporation and decomposition have a pronounced plasma cooling effect.13,15 The decomposition of the solvent and the decomposition and the excitation of the C2 molecules are temperaturedependent processes. The emission intensity is the result of the excitation of the locally present diatomic carbon molecules. The vertical profiles of the C2 emission intensity between peaks are presented in Figure 7. Intense baseline emission was observed close to the load coil at the high droplet frequency and was particularly strong at the high oven temperature. It is worth (15) Olesik, J. W. Appl. Spectrosc. 1997, 51, 158A-175A.

Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

3925

Figure 6. Net C2 emission peak heights (arbitrary units) vs HALC (mm): (a) 110 °C, 1.00 kW; (b) 110 °C, 1.25 kW; (c) 135 °C, 1.00 kW; (d) 135 °C, 1.25 kW.

noting from these data that all C2 emission from material that enters the plasma as vapor has disappeared at a height above the load coil of 4 mm. Ba II Emission at 455.4 nm. In contrast to the C I and C2 measurements, the measurements of the Ba II intensity reflect only the changes in the point at which the Ba is vaporized and the temperature that it experiences, and not the changes in the amount of material. At the low oven temperature, the initial Ba II emission occurs at a height above the load coil at which the C2 emission has not yet decayed completely. Figure 8 shows the waveform acquired at the Ba emission line (455.4 nm) and that obtained at 456.4 nm. The Ba II and C2 emissions from the same droplet are partially separated in time, but the peaks overlap to some extent. In Figure 9a,b, the first data points (at the lowest HALC value) were collected at the height above the load coil where C2 emission was not observable at 456.4 nm (1 nm from the Ba II line), avoiding the interference from the Swan band. For analytical purposes, this HALC is probably the best observation zone, because analyte emission has the highest intensity. The maximum emission intensity depends on the combination of the local excitation conditions and the local number density of analyte species. The analyte local number density decreases higher in the plasma due to diffusion. At high oven temperatures, the droplet is vaporized low in the discharge, and by the time the Ba vapor has reached the position in the plasma best suited for excitation, the vapor has diffused to such a large extent that the peak intensity is low (Figure 9c,d). At the low MDMI frequency, the net peak height is greater than that at the high frequency. The nature of the effect is not 3926 Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

clear. A possible explanation could be the plasma cooling produced by the high solvent load at high frequency. A drop in the continuum emission intensity between peaks was also observed at high droplet frequency, which is another evidence of plasma cooling. On the other hand, a comparison of Figure 9b with Figure 4b suggests that other factors also contribute to the drop in Ba II emission intensity. If the solvent were simply cooling the plasma, one would expect comparable drops in the C I emission intensity. The oven temperature has a more pronounced influence on the behavior of the system with xylene than with aqueous solutions, despite the small differences between the physical properties and the evaporation factors of the two solvents.4,5 Xylene has a higher boiling point than water (140 vs 100 °C) and a similar heat of vaporization (36 vs 41 kJ/mol). With aqueous solutions, oven temperatures in the range of 300-900 °C have been reported.12,13,16,17 With xylene solutions, for a carrier gas flow rate of 1 L/min, in order to obtain the maximum emission in an observation range of 5-20 mm HALC, the oven temperature has to be maintained around 100 °C. Oven temperature fluctuations with organic solvents have a severe effect on the axial position in the plasma where atomization and excitation occur. We tried to maintain the temperature constant by using a PID temperature controller, but due to the interference caused by the rf generator, relatively large temperature fluctuations were observed. In order to reduce the noise picked up by the thermocouple and the wire, (16) Olesik, J. W.; Hobbs, S. E. Anal. Chem. 1994, 66, 3371-3378. (17) Dziewatkoski, M. P.; Daniels, L. B.; Olesik, J. W. Anal. Chem. 1996, 68, 1101-1109.

Figure 7. Baseline C2 emission intensity between droplet peaks (arbitrary units) vs HALC (mm): (a) 110 °C, 1.00 kW; (b) 110 °C, 1.25 kW; (c) 135 °C, 1.00 kW; (d) 135 °C, 1.25 kW.

we inserted a filtering circuit18 between the thermocouple and the controller. Even with this configuration, the system was sensitive to the position of the controller relative to the rf power supply. The temperature fluctuations could not be completely eliminated, but they were substantially reduced. It should be noted in the discussion of these results that the MDMI system used in the experiments is an experimental design. Its performance is extremely sensitive to the placement of the micropump at the base of the oven. Simple removal and replacement of the pump can cause the vertical profiles reported in Figures 4-7 and 9 to shift by 2-3 mm. It was necessary to remove the pump between the recording of the solvent emission profiles and the analyte profiles, and the Ba II profiles are shifted downward with respect to the solvent profiles. The shapes of the curves, however, were unaffected. Detection Limit. The limit of detection for Ba in xylene with MDMI-ICP-AES was determined as the concentration that produces a signal equal to 3 times the standard deviation of the background. The Ba II emission intensity at 455.4 nm was as an ensemble average of 200 droplet waveforms. The signal was obtained from the height of the peak above the baseline, while the standard deviation of the background was determined from the baseline between peaks. The best detection limit for Ba, determined by averaging the signal from 200 droplets, which had a total volume of approximately 32 nL containing 0.16 pg of Ba, was 1.5 ng/mL. The optimum operating conditions are listed in (18) Qi, L. P.; Zhong, G. P.; Zheng, L. T.; Houk, R. S. Spectrochim. Acta 1988, 43B, 273-285.

Figure 8. Oscilloscope images acquired at (a) 456.4 and (b) 455.4 nm. At HALC ) 7 mm, the C2 emission overlaps the Ba II emission line.

Table 3. Our detection limit is close to the detection limits obtained by several groups with organic solutions.19 We did not determine the detection limit of our ICP-AES system with organic solvents and pneumatic nebulizer. With aqueous solutions and pneumatic nebulizer, the detection limit for Ba in our system was (19) Boorn, A. W.; Browner, R. F. Applications: Organics. In Inductively Coupled Plasma Emission Spectroscopy. Part II: Applications and Fundamentals; Boumans, P. W. J. M., Ed.; John Wiley & Sons, Inc.: New York, 1987; Chapter 6, pp 151-216.

Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

3927

Figure 9. Net Ba II emission peak heights (arbitrary units) vs HALC (mm): (a) 110 °C, 1.00 kW; (b) 110 °C, 1.25 kW; (c) 135 °C, 1.00 kW; (d) 135 °C, 1.25 kW.

Table 3. Optimum Conditions for the Detection Limit Obtained for Ba in Xylene plasma power, kW argon flow rate, L/min plasma auxiliary carrier MDMI oven temperature, °C droplet frequency, Hz monochromator slit height, mm slit width, µm HALC, mm

1.0 16 0.8 ∼1 95 20 2 50 15

considerably higher than literature values, which suggests that the MDMI coupled to a better ICP-AES system can provide lower detection limits than that reported in this paper. The MDMI-ICP-AES detection limit for Ba in aqueous solution was 30 times lower than that in xylene. We attribute most of this decrease in analytical performance to plasma cooling associated with liquid organic solvent present in the droplet at the moment it enters the plasma. The cooling of the region near the droplet appears to be more pronounced with organic solvents than with aqueous solutions. However, this picture of excitation is undoubtedly oversimplified, as suggested by the different responses of Ba II emission and C I emission to changes in the droplet introduction frequency. CONCLUSIONS The operating parameters of the MDMI have a stronger influence on the analytical performance of the system with organic 3928

Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

solutions than with aqueous samples. In order to obtain a high analyte signal, the MDMI parameters (oven temperature and carrier gas flow rate) have to be set in such a way that the droplets enter the plasma incompletely dried, and analyte emission appears first in a useful observation zone, in a range from 5 to 25 mm above the load coil. The maximum analyte emission intensity is close to the particle vaporization point because the analyte vapor cloud is at its maximum density. The optimum observation height is immediately above the point where the C2 emission disappears and does not produce any interference on the analyte emission. The solvent load into the plasma is proportional to the MDMI frequency. Because the solvent clouds from adjacent droplets overlap at high frequencies, the ratio of analyte to solvent decreases as the droplet frequency increases. The solvent cools the plasma, and analyte emission intensities have lower values at higher solvent load. For this reason, low MDMI frequencies give the best detection limits. When choosing the frequency, one has to take into account the averaging speed of the signal processing system and the required speed for analyses. There is a trade-off between the limit of detection and the speed of analysis. Initially, we had hoped to obtain a spatial separation of the solvent and the analyte, which would result in a higher excitation efficiency locally in the plasma. Although complete separation of the solvent from the analyte proved to be impossible, it was possible to clearly distingiush between the contributions of the liquid and vapor solvent to the background emission. The MDMI-ICP-AES system, operated at 1 kW forward power, offers detection limits similar to those of ICP with organic solvents and conventional nebulizers operated at higher rf power. With

the MDMI, the same equipment can be used with organic and aqueous samples. The low sample size required and the 100% sample introduction efficiency contribute substantially to the reduction of the organic waste generated during the analysis of organic solutions. The sample flow rate of the MDMI can be set similar to that of LC or CE, by varying the droplet production frequency. Unfortunately, the relatively large internal volume of the micropump (∼10 µL) prevents the successful coupling of the separation techniques with ICP. Currently, we are investigating the possibility of reducing the washout time of the system by making modifications on the design of the micropump.

ACKNOWLEDGMENT The authors thank the University of Toronto Institute for Aerospace Studies and Sciex Corp. for the donation of the MDMI, and Ray Jong for his assistance in delivering and setting up the instrument.

Received for review March 11, 1997. Accepted July 16, 1997.X AC970269+ X

Abstract published in Advance ACS Abstracts, September 1, 1997.

Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

3929