Anal. Chem. 1999, 71, 427-432
Microliter Sample Introduction Technique for Microwave-Induced Plasma Mass Spectrometry Min Huang,*,† Toshihiro Shirasaki,‡ Atsumu Hirabayashi,† and Hideaki Koizumi†
Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185-8601, Japan, and Hitachi Science Systems, Ltd., 882, Ichige, Hitachinaka, Ibaraki 312, Japan
Nitrogen-MIP-MS has an advantage over conventional inductively coupled plasma mass spectrometry (ICPMS) in that it is free from Ar-related spectral interference when measuring the concentration of elements, for example As and Se. Therefore, MIP-MS is a powerful technique for simultaneous determination of multiple elements. However, the ionization efficiencies in the MIP are lower for elements with high values of ionization potential (IP). Our goal was to improve the sensitivity of MIP-MS for As and Se by optimizing the sample introduction technique. A new nebulizer, called the sonic spray nebulizer (SSN), was developed to introduce a liquid sample into the MIP-MS with a flow rate at the microliter per minute level. Our results showed that plasma characteristics could be significantly affected by the sample introduction method used. Compared with a conventional concentric nebulizer (CCN), the detection limits were improved. By using a sample uptake rate that was only one-seventh of that for the CCN, i.e., a SSN rate of 50 µL/min, detection limits were improved by a factor of about 7 for As, Se, and Au, which have very high IP values. Detection limits were also improved for Te, Be, Zn, Sb, and Cd but remained almost the same for elements with low ionization potentials. A mechanism that accounts for this sensitivity enhancement was discussed. Typically, relative standard deviation was found to be less than 2%. In addition, the formation of oxide ions of rare earth elements was reduced by a factor of more than 100 when the SSN was used. The SSN system is capable of introducing samples at a rate as low as 1 µL/min with good precision, which is highly desirable in many cases. Analysis to ensure the quality of drinking water is currently a major priority. Arsenic and selenium in drinking water are two elements that cause great concern, and arsenic poisoning is becoming a major health risk around the world. These two elements must be strictly controlled to provide safe drinking water, and in 1993 the World Health Organization lowered its recommended acceptable concentration in drinking water to 10 ng/mL. To meet that challenge, a sensitive technique for simultaneously determining ultratrace As and Se, as well as other elements, is needed. † ‡
Hitachi, Ltd. Hitachi Science Systems, Ltd.
10.1021/ac980725+ CCC: $18.00 Published on Web 12/05/1998
© 1999 American Chemical Society
ICPMS is a highly sensitive technique that has many applications in a variety of fields.1-3 However, cluster ions formed in an argon plasma may severely interfere with the determination of some elements including As and Se. For instance, 38ArH+, 40Ar+, 41Ar+, 36Ar16O+, 40Ar15N+, 40Ar16O+, 40Ar18O+, 40Ar35Cl+, and 40Ar + 2 have been found to interfere with the determination of 39K+, 40Ca+, 41K+, 52Cr+, 55Mn+, 56Fe+, 58Fe+, 75As+, and 80Se+, respectively. With the improvements made in the sampling interface and ion lenses in recent years, the effects of background spectra and some sources of interference have been reduced.4 In addition, under optimized operating conditions enabling use of a “cool plasma”, the Ar+ concentration in the central channel of the plasma drops, causing a decrease in the number of unwanted argon polyatomic ions.5 Nevertheless, the failure to take spectroscopic overlaps into account is still considered to be the largest source of inaccuracies in ICPMS analysis. A separation process is often required, making simultaneous determination impossible. High-resolution magnetic-sector ICPMS provides a resolving power of >5000 and is capable of separating most element ion peaks from interfering polyatomic ions, which means it can also provide exceptionally low detection limits.6 However, magneticsector mass spectrometers are expensive and are applied mainly in the semiconductor industry, geology, the nuclear industry, and reference material analysis laboratories. Argon-associated interference can be avoided by using a helium7-9 or nitrogen10-12 microwave-induced plasma (MIP) ion source. The nitrogen plasma is much more economical and therefore more practical for routine analysis. Typical interferences (14N2+, 14N16OH+, and 14N216OH+ on 28Si+, 31P+, and 45Sc+, respectively) with the nitrogen plasma mainly occur in the lowmass range, which allows a determination of many important elements free from the spectral interference. It is extremely useful (1) Houk, R. S. Anal. Chem. 1986, 58, 97A-105A. (2) Evans, E. H.; Giglio, J. J.; Castillano, T. M.; Caruso, J. A. Inductively Coupled and Microwave Induced Plasma Sources for Mass Spectrometry; Royal Society of Chemistry: Herts, U.K., 1995. (3) Montaser, A., Ed. Inductively Coupled Plasma Mass Spectrometry; WileyVCH: New York, 1998. (4) Hieftje, G. M. J. At. Anal. Spectrom. 1996, 11, 613-621. (5) Eiden, G. C.; Barinaga, C. J.; Koppenaal, D. W. J. At. Anal. Spectrom. 1996, 11, 317-322. (6) Vanhaecke, F.; Riondato, J.; Moens, L.; Dames, R. Fresenius’ J. Anal. Chem. 1996, 355, 397-400. (7) Creed, J. T.; Davidson, T. M.; Shen, W.-L.; Brown, P. G.; Caruso, J. A. Spectrochim. Acta 1989, 44B, 909-924. (8) Wu, M.; Carnahan, J. W. J. Anal. At. Spectrom. 1992, 7, 1252-1258. (9) Jin, Q.; Zhang, H.; Wang, Y.; Yuan, X.; Yang, W. J. Anal. At. Spectrom. 1994, 9, 851-856.
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for simultaneously determining the concentrations of As, Se, and other elements in water when using MIP-MS, because a separation process is not necessary. However, the excitation temperature of the N2-MIP is in the range of 5400-5500 K, about 1500 K lower than that of an Ar-ICP.12 Subsequently, the ionization efficiencies for As, Se, and other elements with high ionization energies are considerably lower than those in an Ar-ICP. As a result, the detection limits for As and Se in MIP-MS are not as good as they could be with a higher excitation temperature. To achieve a higher plasma temperature in the MIP and thus better sensitivities, we have reconsidered the sample introduction technique. The conventional concentric nebulizer (CCN), which has a typical sample uptake rate of 0.5-1.5 mL/min, is the most widely used nebulizer in plasma emission spectrometry or plasma mass spectrometry, because of its simplicity and low cost. However, this nebulizer has poor nebulization efficiency and it is difficult to introduce samples that are limited in quantity, expensive, and/or hazardous. Recently, the direct injection nebulizer (DIN),13,14 the microconcentric nebulizer (MCN),15 the high efficiency nebulizer (HEN),16-18 and the oscillating capillary nebulizer (OCN)19 were developed. More recently, the direct injection high-efficiency nebulizer (DIHEN), which combines the advantages of both the DIN and the HEN, was also reported.20 In this work, a microliter sample introduction technique is employed to improve sensitivity for elements with high ionization potentials based on our belief that the solvent is overloaded in the MIP when a CCN is used. Loading too much solvent may substantially alter some of the fundamental characteristics of MIP, such as the plasma temperature. A highly efficient nebulizer with a sample consumption rate at the microliter per minute level can overcome this problem. We have used a sonic spray technique,21 which was originally developed for the ionization of organic compounds in LC/MS, and have adapted the sonic spray nebulizer (SSN) to make it compatible with MIP-MS. We evaluated the analytical characteristics of the nebulizer and compared these characteristics with those of a CCN system. On the basis of their ionization potentials, Se, As, Au, Te, Zn, Be, Sb, Cd, Co, Cr, Fe, Ba, and Pb were selected for our evaluation of the SSN. Our ultimate purpose is to develop a highly sensitive and less subject to spectral interference methodology that is to allow simultaneous determination of As, Se, and other elements without a tedious and time-consuming separation. (10) Okamoto, Y. Anal. Sci. 1991, 7, 283-288. (11) Oishi, K.; Okumoto, T.; Iino, T.; Koga, M.; Shirasaki, T.; Furuta, N. Spectrochim. Acta, Part B 1994, 49, 901-914. (12) Ohata, M.; Furuta, N. J. At. Anal. Spectrom. 1997, 12, 341-347. (13) Lawrence, K. E.; Rice, G. W.; Fassel, V. A. Anal. Chem. 1984, 56, 289292. (14) Wiederin, D. R.; Smith, F. G.; Houk, R. S. Anal. Chem. 1991, 63, 219-225. (15) Vanhaecke, F.; Van Holderbeke, M.; Moens, L.; Dams, R. J. Anal. At. Spectrom. 1996, 11, 543-548. (16) Tan, H.; Meinhard, B. A.; Meinhard, J. E. Recent Investigations of Meinhard Concentric Nebulizers. Presented at the 19th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, PA, September, 1992. (17) Olesik, J. W.; Kinzer, J. A.; Harkleroad, B. Anal. Chem. 1994, 66, 20222030. (18) Liu, H.; Montaser, A. Anal. Chem. 1994, 66, 3233-3242. (19) Browner, R. F. Presented at the 21st Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, St. Louis, MO, October 1995; Paper 8. (20) McLean, J. A.; Zhang, H.; Montaser, A. Anal. Chem. 1998, 70, 1012-1020. (21) Hirabayashi, A.; Sakairi, M.; Koizumi, H. Anal. Chem. 1994, 66, 45574559.
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Table 1. Instrumental Conditions and Measurement Parameters of MIP-MS microwave frequency microwave power plasma gas flow rate carrier gas flow rate sample uptake rate spray chamber spray chamber temperature mass analyzer sampler cone skimmer cone data acquisition mode mass species measured
a
2.45 GHz 1.3 kW 13 L/min 1.0 L/mina (1.3 L/min for the CCN) 50 µL/mina double-pass 2 °Ca quadrupole analyzer copper, 0.8 mm diameter, 0.5 mm long copper, 0.4 mm diameter, 0.3 mm long peak hopping 9Be+, 52Cr+, 56Fe+,59Co+, 64Zn+, 75As+, 80Se+ , 114Cd+, 121Sb+, 130Te+, 138Ba+, 197Au+, 208Pb+
Unless otherwise mentioned.
EXPERIMENTAL SECTION Instrumentation. The instrument used in this work was a high-power microwave-induced plasma mass spectrometer (P6000, Hitachi Ltd.). The nitrogen plasma (2.45 GHz, up to 1.5 kW) with a donut-shaped structure was produced by using an Okamoto cavity. The mass spectrometer was a quadrupole type, with a threestage vacuum system. A more detailed description is given elsewhere.11 Operating conditions listed in Table 1 were used in this work, unless otherwise specified. The position of the MIP torch could be finely adjusted in three dimensions to allow ions to be taken efficiently from the plasma into the mass analyzer through the sampler cone. The MIP torch consisted of two concentric tubes, rather than the three tubes in an ICP torch. The sampling depth was defined as the distance from the sampling cone to the end of the outer tube of the plasma torch. We measured the variation of the sampling depth as a function of the signal intensity for both the SSN and the CCN. The ion focus lenses and other parameters were optimized to get a maximum signal-to-background ratio before the measurement. Sonic Spray Nebulizer. In the original sonic spray ionization (SSI) device,21 a silica capillary was fixed in the middle of an orifice formed in a duralumin material. The sample solution was pumped through the silica capillary and dispersed into a fine aerosol by nitrogen gas passing through the orifice at high speed. A nitrogen gas flow rate as high as 3 L/min was employed to ionize organic analyte in the solution. We adapted the SSN as shown schematically in Figure 1. To make the nebulizer suitable for introducing a sample into the MIPMS, the flow rate of the nebulizer gas was reduced to about 1 L/min by reducing the orifice diameter from 400 to 250 µm. Also, the orifice was formed in polyamide material instead of duralumin to avoid contamination from the alloy. A silica capillary (50 µm i.d., 150 µm o.d.) supported by a stainless steel tube was put in the orifice. The stainless steel tube was fixed with a ferrule. The capillary was adjusted until its end was located in the middle of the orifice to guarantee good nebulization efficiency and stability. The sample solution could be precisely fed at an uptake rate as low as 1 µL/min by using a syringe pump (Micro-feeder, JP-S1, Furue Science, Inc.) and a 1 mL syringe (Hamilton gastight syringe-1000, Hamilton Ltd.). To avoid any contamination from the metal needle of the syringe, the needle was replaced with a
Figure 1. Cross-sectional view of the sonic spray nebulizer (SSN).
homemade plastic connector that was attached with a Teflon tube. The flow rate of the nebulizer gas could be adjusted from 0.2 to 1.8 L/min by a thermal mass flow controller (model 3650, Kofloc Ltd.), with an error of 2.0% of the flow scale used. Reagents. Standard solutions of Se, As, Te, Be, Zn, Sb, Cd, Cr, Fe, Co, Ba, and Pb in a 0.1% HNO3 medium were prepared by diluting stock solutions (1000 µg/mL, SPEX Plasma Standard, SPEX industry, Edison, NJ) to the concentrations required for determination with deionized water (18 MΩ) and high purity nitric acid. A 1% HNO3 medium was necessary to keep the Au solution stable. Blank solutions of deionized water with 0.1% and 1% HNO3 were also prepared. RESULTS AND DISCUSSION Nebulizer Optimization. In the SSN, gas pressure from the gas inlet to the orifice is not greatly reduced, due to a spacious gas pathway. Therefore, it is not necessary to employ a very high gas pressure, as is used in the HEN nebulizer. The gas pressure used here was 5 atm. Furthermore, the SSN was directly inserted into a spray chamber end cap and sealed with two internal O-rings in our system, which makes the attachment or removal of the SSN very convenient. In the CCN, the sample uptake rate is controlled through the nebulizer gas flow. In the SSN, however, the nebulizer gas and the sample uptake rate can be adjusted independently without any mutual limitations. This flexibility of the SSN makes parameter optimization more convenient and efficient. It also makes the SSN capable of introducing a solution at a rate less than 0.1 mL/min, because the small diameter of the capillary greatly reduces selfaspiration. As mentioned, the flow rate of the nebulizer gas could be controlled from 0.2 to 1.8 L/min in our system, while the sample uptake rate can be varied from 1 to 150 µL/min. Stable nebulization was observed with a gas flow rate of 0.4 L/min, but a higher gas flow rate is preferred for better nebulization efficiency. Figure 2a shows the variation in the signal intensity as a function of the nebulizer gas flow rate. At about 1.0 L/min, the maximum intensity was obtained. Also, the background was dramatically reduced when the gas flow rate was increased to 1.0 L/min, as shown in Figure 2b. We attribute this coincidence to the formation of the donut structure in the plasma. The background was measured at a mass number of 150 because no spectral interference existed at that mass number. It was found that the background was not mass dependent. Therefore, it was
Figure 2. Effect of the nebulizer gas flow rate on the signal intensity and the background. The sample liquid flow rate was controlled at 50 µL/min.
most likely caused by photon emission from the plasma. When the flow rate is less than 1.0 L/min, the gas would not pass through the plasma by forming a central channel, causing a stronger photon emission in the sampling area. Therefore, the analyte would spread throughout the plasma, rather than be concentrically located along the channel as occurs under normal ICP or MIP operating conditions, and this dispersion will lead to a low signal intensity and a high background level. Thus, we used a gas flow rate of 1.0 L/min in this work unless otherwise specified. The relationship between the signal intensity and the sample uptake rate at the optimized gas flow rate is shown in Figure 3. The intensity increased linearly with the sample uptake rate when the rate was less than 40 µL/min. The signal was still intensified slightly from 40 to 90 µL/min and then saturated. When the sample uptake rate was higher than about 120 µL/min, we found the signal intensity would not increase further and began to decrease. Obviously, a higher gas flow rate is preferred for better nebulization. However, limitations on the gas flow rate due to the cooling effect on the plasma characteristics and the dilution effect on the analyte have to be taken into account and a compromise has to be made. As stated, the stable nebulization of a low-flow-rate sample solution is one of the main targets of this work. We evaluated the stability through 10 successive measurements of a 10 ng/mL standard solution. The relative standard deviations (RSDs, %) were Analytical Chemistry, Vol. 71, No. 2, January 15, 1999
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Table 2. Comparison of the Detection Limits (DL, ng/mL) Obtained with the Two Nebulizersa elements IP, eV DL(CCN) DL(SSN) relative factor absolute factor
Se 9.75 0.11 0.015 7.2 50.4
As 9.81 0.050 0.007 7.0 49
Au 9.23 0.14 0.022 6.4 45
Te 9.01 0.065 0.014 4.7 33
Zn 9.39 0.014 0.004 3.8 27
Be 9.32 0.003 0.001 3.6 25
Sb 8.46 0.017 0.005 3.4 24
Cd 8.99 0.030 0.009 3.5 25
Co 7.86 0.001 0.001 1.0 7
Fe 7.87 0.002 0.002 1.0 7
Pb 7.42 0.007 0.007 0.90 6.3
Cr 6.77 0.002 0.002 0.84 5.9
Ba 5.21 0.002 0.002 1.0 7
a The relative factor was defined as the ratio of the CCN system’s detection limit to that of the sonic spray system. The absolute factor was calculated by further considering the different sample uptake rates: 350 µL/min for the CCN system and 50 µL/min for the sonic spray system.
Figure 3. Relationship between the signal intensity and sample uptake rate.
found to be 1.50, 1.46, 1.50, 1.50, 1.42, 1.26, 0.93, and 1.34 for sample uptake rates of 1, 5, 10, 20, 30, 40, 50, and 60 µL/min, respectively. These results demonstrate that the nebulizer is capable of introducing a sample solution at a rate as low as 1 µL/ min. This is a highly desirable advantage for detecting species separated using capillary electrophoresis, as well as for analyzing samples whose quantity is very limited. Detection Limits. The detection limit, here, is defined as the concentration that produces a net intensity equivalent to 3 times the standard deviation of the background. The detection limits listed in Table 2 were calculated from the results of 10 successive measurements of the 0.1% HNO3 blank solution. Results obtained with a CCN are also given in the table. The sample uptake rate in our system was 50 µL/min, only one-seventh of that for the CCN. However, the detection limits for many elements were improved considerably by using the SSN, especially for the elements with high ionization potentials. Considering the different sample uptake rates of the two systems, we calculated that the absolute sensitivities for the elements in the table were increased by 6-50 times. Furthermore, Cr, Fe, Co, Ba, and Pb had similar improvement factors, averaging about 6.6, while the others had much higher factors. These results suggest that there is an inherent relationship between the improvement factor and the ionization potential value. It is worth noting that the significant improvement for As and Se should be especially useful for determining the concentration of toxic elements at the ultratrace level in surface water, groundwater, and drinking water. Currently, they are determined by combining a hydride-generation technique with inductively coupled plasma atomic emission spectrometry (ICP-AES) or atomic absorption spectrometry (AAS). However, the concentration of As and Se, as well as that of other elements for which the hydride 430
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generation technique is not valid, could be determined simultaneously by MIP-MS. Effect of Sample Introduction on the Plasma Characteristics. We next considered the mechanism that improved the detection limit. Improved nebulization efficiency could be one reason for the sensitivity increase. The unique configuration of the nozzle and the adjustment of the capillary end position, as described in the Experimental Section, enable a good nebulization performance. Along with the difference in nebulization behavior, the energy of the high-speed gas is more efficiently used in the SSN. The energy is partially consumed in the uptake of the liquid sample in the CCN system, while the full energy of the gas is used to collide with the liquid sample in the SSN because the sample is fed by a syringe pump. Therefore, the sonic spray technique provided much finer aerosols.22 The nebulization efficiency was improved by an estimated factor of 7, according to the results for Cr, Fe, Co, Ba, and Pb. However, the improvement in nebulization efficiency should affect all elements to the same extent. Therefore, there must have been some other reason responsible for the different improvement factors of different elements. The elements with high IP values are those that had high improvement factors, which strongly suggests the plasma temperature plays an important role in improving the ionization efficiency in our system. The size of droplets produced by a nebulizer affects not only the transport efficiency of the analyte but also the temperature and its distribution in various parts of the plasma. This is because the solvent evaporation and decomposition processes that occur in the plasma consume energy, consequently cooling the plasma. This effect could be severe when the sample uptake rate is high and relatively big drops are introduced into the plasma. More than 1 million drops of a sample with a wide range of drop sizes are introduced into the plasma every second by a CCN.23 Subsequently, the solvent consumes considerable energy, resulting in poor ionization efficiency for elements with high IP values. Generally, elements with an ionization potential of less than 7 eV should be completely ionized in the plasma, while others with a higher potential are only partially ionized. With a CCN, the ionization efficiencies of As, Se, and Au, whose respective ionization potentials are 9.81, 9.75, and 9.23 eV, were reported to be 49%, 31%, and 49%, respectively, in an ICP ion source,1 and lower values have been reported for an MIP source.12 Obviously, a higher plasma temperature is needed to ionize these elements efficiently. Therefore, a suitable sample introduction technique (22) Hirabayashi, A.; Fernandez de la Mora, J. Int. J. Mass Spectrom. Ion Processes 1998, 175, 277-282. (23) Olesik, J. W. Appl. Spectrosc. 1997, 51, 158A-175A.
Table 3. Comparison of the Intensity Enhancement Factor (the Ratio of Intensity at - 1 °C to That at 2 °C) between the SSN and CCN System element SSN CCN
Figure 4. Signal intensity variation with sampling depth.
that prevents excessive energy dissipation due to solvent vaporization and decomposition is needed. With a sample uptake rate on the microliter per minute scale and a very fine aerosol in the SSN, less energy is consumed than with the CCN. This enables a higher plasma temperature and subsequently higher ionization efficiency. The higher the ionization potential of an element is, the bigger the improvement factor that can be achieved with the SSN (Table 2). The different droplet sizes and different amounts of solvent loaded into the plasma by the two nebulizers affect the plasma temperature, as well as the temperature distribution. Considering the fate of a single droplet in the plasma is a helpful way to understand the dynamic factors responsible for the improvement with the SSN. It takes longer for a bigger particle to be vaporized, causing the analyte to be ionized gradually. Therefore, the analyte ions produced inside the plasma are distributed in a wider area along the central channel. In other words, the ions are “diluted”. On the contrary, a smaller particle is quickly vaporized within the plasma and the maximum ion concentration is in the region near the torch. With a CCN, fine particles as well as considerable numbers of big particles are introduced into the plasma. The poor ionization efficiency for the analytes in the bigger particles and the different ionization regions for the bigger and the smaller particles result in significant dilution of the analyte ion. However, with the SSN, ions are expected to be concentrated in a narrower region due to the narrower range of particle sizes, which will lead to similar particle behavior and a more homogeneous ionization region. To learn more about the ionization behavior of the analyte in the MIP, we also studied the relationship between the signal intensity and the sampling depth. Figure 4 shows the variation of arsenic. A similar result was observed for other elements. As mentioned, sampling depth was defined as the distance from the choke (the end of the outer torch tube) to the sampler cone. The data obtained for the CCN was scaled by a factor of 4 in the figure. The highest intensity was obtained at a sampling depth of 2.7 mm for the SSN and 3.3 mm for the CCN. The difference in the sampling depth reveals that the analyte could have been ionized within a shorter distance and a shorter time after being introduced into the plasma, due to the higher plasma temperature when the SSN is used. In other words, the two different nebulization techniques clearly led to differences in the temperature and the temperature distribution in the plasma.
Se 1.60 1.20
As 1.78 1.14
Au 1.30 1.11
Te 1.31 1.16
Zn 1.63 1.08
Sb 1.36 1.13
Cd 1.31 1.10
Pb 1.03 1.04
Ba 1.13 1.03
Sometimes, solvent loading can greatly change the electron number density, ionization temperature, and other characteristics of the plasma. For example, the high solvent loading when an ultrasonic nebulizer is used can even make the plasma unstable. With a desolvation technique though, such as membrane separation or cryogenic cooling, the detection limits were improved.24 The size of droplets produced by an SSN is expected to be between the droplet size of ultrasonic nebulization and that of a CCN. Therefore, we also considered desolvation in this work. To enable desolvation, the spray chamber was cooled. Solvent removal efficiency depends on the aerosol size. It is easier to remove the water solvent from smaller droplets due to their relatively larger surface area, which enhances water volatilization and leads to a faster response to a temperature change in the ambient environment, compared with that of bigger droplets. As a result, the solvent can be removed more efficiently. In other words, we could obtain information on the aerosol droplet size of the CCN and the SSN by comparing their solvent removal efficiencies, i.e., the intensity responses to changes in the chamber temperature. The intensity responded more sharply to a falling temperature with the SSN than with the CCN (Table 3), although an enhanced signal was observed from both nebulizers when the chamber was cooled from 2 to -1 °C. In addition, there were sharper responses for As and Se than for Ba and Pb. This can also be explained in terms of the effect of the sample introduction technique on plasma temperature; that is, the solvent removal leads to a higher plasma temperature. Thus, we have demonstrated that the technique used for sample introduction significantly affects the temperature and temperature distribution in an MIP ion source. Our results agree well with those previously reported,25,26 where ionization behavior inside a plasma was studied by introducing a single, isolated droplet into an Ar-ICP. Oxide Ion Formation. The solvent is one contributor to the formation of oxide ions in the plasma, and the oxide ions may interfere with the determination of other analytes in ICPMS or MIP-MS. The oxide ions are more likely to form in a lowtemperature region, where the oxide ions could not decompose. A larger droplet cools the local region, creating cooler surroundings where the oxide ions form more easily. We measured BaO+/ Ba+, a ratio often used to evaluate formation of oxide ions. The ratio was 0.10% with the CCN when the chamber temperature was 2 °C, but no oxide ions were observed when the SSN was used. When the solution with the 1% HNO3 medium was introduced, the BaO+/Ba+ ratio at 2 °C was less than 0.10% for the SSN and was 2.4% for the CCN. The higher sample consumption and bigger (24) Walters, P. E.; Barnardt, C. A. Spectrochim. Acta 1988, 43B, 325-337. (25) Olesik, J. W.; Hobbs, S. E. Anal. Chem. 1994, 66, 3371-3378. (26) Dziewatkoski, M. P.; Daniels, L. B.; Olesik, J. W. Anal. Chem. 1996, 68, 1101-109.
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droplets with the CCN are responsible for the greater oxide ion formation when the CCN is used. When the 1% HNO3 solution was introduced, the main source of oxygen for the oxide ion formation came from highly volatile nitrogen acid. Therefore, as long as the acid medium is kept below 0.1% HNO3, the oxide ion can be ignored in the SSN. The oxide ion interference is more severe for rare earth elements that have many stable isotopes and successive mass numbers, leading to a very complicated correction being necessary or even making an accurate determination impossible. Also, rare earth elements are more likely to form oxide ions. Our preliminary results show that the ratios of CeO+/Ce+ and LaO+/La+ found by the SSN are 100 times lower than those obtained by the CCN. This characteristic may enable use of the SSN in the measurement of stable isotope ratios of rare earth elements in geochemistry.
microliter scale sample introduction technique, which we could use with the SSN, for MIP-MS allowed much more efficient ionization of As, Se, Au, and other elements with a high ionization potential than was possible with a CCN. Therefore, SSN/MIPMS is a powerful technique that enables the simultaneous determination of As, Se, and other elements at the ultratrace level. The improved sensitivity is due to the use of a very fine aerosol at a very low sample uptake rate and avoidance of a significant decrease in the plasma temperature due to extensive cooling caused by excessive solvent loading. The sonic spray nebulizer provides high sensitivity and allows the analysis of liquid samples at a microliter per minute scale consumption rate. This makes it very useful for the analysis of a sample with a limited volume available, as well as for elemental speciation combined with capillary electrophoresis or other separation techniques.
CONCLUSION Nitrogen-MIP-MS is a powerful trace analysis technique due to its high sensitivity and low susceptibility to interference. The sensitivity was enhanced by an optimized sample introduction technique. To evaluate a nebulizer though, not only its nebulization efficiency but also its effect on the plasma characteristics should be taken into account. A CCN has a high sample consumption rate, poor efficiency, and relatively large droplets, which together cause a substantial change in the plasma characteristics. A
ACKNOWLEDGMENT This work has been partly supported by CREST (Core Research for Evolutional Science and Technology) of the Japan Science and Technology Corp. (JST).
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Received for review July 7, 1998. Accepted October 27, 1998. AC980725+
