796
Anal. Chem. 1987, 5 9 , 796-799
CORRESPONDENCE Supercritical Fluid-Based Sample Introduction for Inductively Coupled Plasma Atomic Spectrometry Sir: Sample introduction often limits the performance of inductively coupled plasma (ICP) atomic spectrometry (1,2). Conventional nebulizer/spray chamber systems are noisy and inefficient. Typically, less than 2% of the sample entering the nebulizer reaches the plasma because the majority of the sample consists of droplets too large to be easily atomized. The large droplets are removed by a spray chamber. The inefficiency and noise characteristic of conventional sample introduction systems are particularly detrimental when analyzing small sample volumes. The combination of two powerful analytical techniques, chromatography and ICP atomic spectrometry, is often limited by the interface coupling them. The sample introduction process may not only be inefficient but aIso result in broadening of the chromatographic peaks. We report here a sample introduction system utilizing the unique properties of supercritical fluids with the potential to overcome the most severe limitations of the nebulizer/spray chamber system. By use of the supercritical fluid-based system, the ICP can be used for elemental analysis of (1) small sample volumes via direct injection into a supercritical fluid, (2) supercritical fluid chromatographic effluents, and (3) species extracted from solids. Supercritical fluids, such as COz, are gases a t room temperature and atmospheric pressure but effective solvents at temperatures and pressures above their critical point. The densities and solvating properties of supercritical fluids are similar to liquids. However, the viscosities of supercritical fluids are more similar to gases than liquids (3). As a result, shorter analysis times are required for supercritical fluid chromatography (SFC) than for HPLC to obtain similar resolution. Thermally labile compounds are amenable to separation by SFC because low temperatures can be used, similar to high-performance liquid chromatography (HPLC). In our system the sample is injected or extracted into the supercritical fluid. The sample-containing fluid is then carried in a narrow capillary or chromatographic column to a point just below the plasma. When the fluid exits the capillary it becomes a gas in the atmospheric pressure plasma. As a result, the sample is introduced into the plasma in an easily atomized form fiiely dispersed in the gas with virtually 100% efficiency. We report the first use of a supercritical fluid sample introduction system for ICP analysis of metal-containing organic species. Also described are investigations of the effect of the addition of C02on atomic and ionic emimion in the ICP, using conventional aqueous aerosol sample introduction.
with a 2.0-pm filter (Valco, Model ZUFR1). The filtered C02 flowed into a four-port injection valve (Valco, Model ACI4WHC.2) with a sample injection volume of 0.2 pL. An injection is triggered by a switch in the Valco digital valve interface which controls solenoids for the gas actuator of the injection valve. A 1 m long, 250 pm i.d. capillary tube was used to carry the supercritical fluid from the injector to a point less than 10 mm below the discharge region of the ICP. The capillary was surrounded by a 0.25 in. diameter copper tube heated by resistive heater wire (Ace Glass, Model 12065-25). The temperature of the argon inside the copper tube was measured with an ironconstantan thermocouple. A temperature controller (Omega, Model 6102) regulated a variable transformer which supplied power to the heater wire to maintain the desired temperature. The incoming argon gas supply was preheated by passing it through ten 1 f t diameter coils of copper tubing warmed by a heating tape controlled by a variable transformer. The fluid was released into the plasma through a 1to 15 pm restriction. The restriction was made in the end of the capillary by partially melting the tip in a propane/oxygen flame. The inner diameter of the quartz capillary was reduced from 250 pm over a distance less than 5 mm. Supercritical fluid flow rates of 20-300 pL/min were used,depending on the pressure and restrictor size. Flow rates were determined by measuring the time from injection of the sample to the peak in emission intensity detected in the plasma. The ICP power supply (Plasma Therm Model 2500) was operated at a power of 1.25 kW, unless noted otherwise. The argon plasma, supported in a low-flow torch (Precision Glassblowing of Colorado, Plasma Therm low flow type), was used with an outer flow rate of 8.0 L m i d , intermediate flow rate of 1.0 L min-', and a central flow rate of 0.6 L min-'. An image of the plasma was formed at the entrance slit of a 1-m spectrometer (McPherson Model 2061 with 1200 g/mm holographic grating) with a magnification of 0.33 using a 1-m focal length spherical mirror (Opco Laboratory). The reflective optical system has been described previously (4). The entrance and exit slit widths were 50 pm. Current signals from the photomultiplier tube (RCA 1P28A)were converted to a voltage using a current amplifier (KeithleyModel 427). Intensity-time data were digitized and collected by a Tecmar Lab Master analog-to-digitalconverter board in an IBM PC microcomputer. Spectra and vertical emission profiles were obtained with an optical multichannel analyzer (Princeton Applied Research Corporation OMA I11 system: Model 1460 console, Model 1254 SIT vidicon detector with scintillator, and Model 1216 detector controller). Cyclohexane (EM Science) solutions of ferrocene (Eastman Kodak) were used as test samples. Solutions of CaC12.2H20in deionized, distilled water were used to study the effect of C 0 2 on plasma excitation conditions.
EXPERIMENTAL SECTION The instrumental system for generation and introduction of supercritical fluids into the ICP is shown in Figure 1. An airdriven piston pump (Haskell Model DSF-60-C) was used to provide pressures of 1100 to 3000 psi. A reservoir (Whitey, Model 304L-HDF2-40)was added to damp out pressure variations caused by the piston pump. The pressure was monitored by a pressure transducer (Schaevitz Engineering, Model P2503-009) and controlled by a manual adjustment on the pump. The critical temperature and pressure for C 0 2 are 31.3 O C and 1068 psi, respectively. Liquid C02 (Air Products, Coleman grade) was filtered
RESULTS AND DISCUSSION Interfacing of the supercritical fluid transport system to the ICP was relatively straightforward. The capillary was inserted in the central tube of a conventional, low-flow ICP torch. The central argon gas which normally carries the sample aerosol from a nebulizer/spray chamber into the plasma served two functions in the supercritical fluid sample introduction system. The velocity of the central gas must be high enough to ensure that the sample travels through the plasma skin and into the center of the plasma. If the velocity
0003-2700/87/0359-0796$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
600 ng Fe
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/NJECT/ON VAL VE
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Figure 1. Supercritical fluid sample introduction system.
is too low the sample skirts around the outside of the plasma and is not effectively atomized. A central argon gas flow rate somewhat less (0.6 L m i d ) than that normally used (0.8-1.0 L min-') was selected because the effective size of the central tube of the torch was reduced by the presence of the capillary. Secondly, the central argon gas is heated to the temperature desired for formation of the supercritical fluid. The flowing argon provides even heating of the capillary from the injector to its end just below the plasma. The supercritical fluid sample introduction system with ICP atomic emission detection was used without optimization of experimental parameters such as supercritical fluid or argon flow rates, incident power, observation height, or signal integration. Plasma conditions typically used with nebulizer/spray chamber sample introduction systems were employed. A detection limit of 60 pg of Fe was obtained from initial data. The detection limit was determined for a signal-to-noise ratio of 3. The standard deviation of the noise on the background emission was calculated from 300 data points acquired in the 6 s following injection (and before the sample reaches the end of the capillary). Intensity vs. time data are shown in Figure 2. Injection-to-injection reproducibility of the area under the intensity vs. time peak was found to be 1.6% for six successive injections of sample at a concentration 25 times the detection limit. Substantial improvements in the detection limit should be possible. The 60-pg detection limit for Fe corresponds to a concentration of 0.6 pg mL-' in the original sample. The detection limit for Fe in large volumes of aqueous solution introduced through a nebulizer is approximately 0.004 pg mL-' (5). Emission from the ICP was collected from a large portion of the plasma (5-23 mm above the load coil). The detection limit should be improved by viewing a smaller region of the plasma where the signal-tebackground ratio is optimum. Also, detection limits were calculated on the basis of noise on the observed signal collected at a rate of 50 Hz. Peak area measurements should be less effected by the integrated background noise. Unlike previous reporh noting observation of large clusters of sample molecules resulting in spikes in flame ionization detector signals when used with supercritical fluid chromatography (6),we have not seen evidence of such clusters.
8
Flgwe 2. Intensity vs. time data for injection of varied amounts of Fe.
A
n
I
n
Flgure 3. Ca I1 spectra (A) 1.25 kW without carbon dioxide, (6)1.25 kW with carbon dioxide, and (C) 0.70 kW without cerbon dioxide. The emission lines are as follows: left, Ca I1 396.845 nm; right, Ca I1 393.37 nm. All three spectra are plotted on the same intensity scale.
Perhaps this is due to the high atomization efficiency of the plasma or our capillary restriction design. We are now studying the transition from supercritical fluid to gas in more detail. The introduction of organic solvents often requires use of higher powers to maintain the plasma excitation conditions comparable to those when an aqueous aerosol is introduced (7,8).Similarly, the introduction of C 0 2 was found to affect the excitation conditions. We found that supercritical fluid flow rates of up to approximately 300 pL min-l could be introduced into ICP without extinguishing the plasma operated at 1 kW. However, the background noise level did increase when higher C 0 2 flow rates were used. In order to initially characterize the effect of COPon excitation conditions in the plasma, a conventional nebulizer/spray chamber was used. Aqueous solutions of calcium were introduced. Carbon dioxide was added to the aerosolcarrying argon between the spray chamber and the ICP at a supercritical fluid flow rate of appoximately 200 p L min-l. However, the carbon dioxide was not heated at the end of the
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ANALYTICAL CHEMISTRY, VOL. 59,NO. 5, MARCH 1, 1987
0
14.7
29.3
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Figure 4. Vertical emission profiles for Ca 11, 393.37 nm (A) 1.25kW without carbon dioxide, (B) 1.25kW with carbon dioxide, and (C) 0.70 kW with carbon dioxide. Each profile is plotted on its own normalized intensity scale.
capillary. Figure 3 parts A and B, shows the spectra obtained at a power of 1.25 kW with and without the addition of COz. The intensity of the Ca I1 emission is much lower when COz is added. The intensity of the Ca I1 emission when COz is added to a 1.25-kW plasma is similar to that when no COz is added to a 0.70-kW plasma, shown in Figure 3C. The shape of the Ca I1 emission vs. height profiles is also affected by the addition of COz, as shown in Figure 4. The addition of COz results in a substantial decrease in the emission low in the plasma, similar to the 0.70-kW plasma without COz. However, at regions high in the plasma the relative emission intensity is higher in the 1.25-kW plasma with COzthan in the 0.70-kW plasma without COz. (Note that the valley in the intensity vs. height profile at approximately 4 mm above the load coil is due to the top of the torch. The detected profiles were not corrected for detector element to element response variations.) Figure 5 shows the Ca I spectra with and without the addition of COP Again, the addition of COz results in a decrease in the intensity of the Ca I emission. Excitation conditions appear to be "cooler", as indicated by the great reduction in the Ar line intensities. However, the Ca I emission is lower in a 1.25-kW plasma with C02added than in a 0.70-kW plasma without COz. While the above data were collected under somewhat different conditions than those for the supercritical fluid sample introduction system, it does show that the effect of the addition of supercritical COz to the ICP is not simple. The atomization and/or excitation efficiency of both atoms and ions is affected. The formation of molecular species in the plasma when COz is added may be a t least partially responsible for the observed behavior. Emission bands observed at 415.8, 418.1, 419.7, and 421.6 nm are consistent with the presence of CN.
CONCLUSIONS Supercritical fluids were used to introduce sample into an inductively coupled plasma using conditions similar to those employed for conventional nebulized aqueous sample introduction. The method is a promising means of efficiently transporting small volumes of sample into the ICP. This work
Figure 5. Ca I spectra (A) 1.25kW without carbon dioxlde, (8) 1.25 kW with carbon dioxide, and (C) 0.70kW without carbon dioxide. The emission lines in (A) from left of spectrum are Ca I 422 nm, Ar I 420.07 nm, Ar 1419.83nm, Ar 1419.07 nm, and Ar 1418.19 nm. The bands in (B)are 421.6 nm, 419.7nm, and 418.1 nm, probably due to CN. Ail three spectra are plotted on the same intensity scale.
B
c 0
14.7
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HEIGHT ABOVE LOAD COIL
Flgure 6. Vertical emission profiles for Ca I, 422 nm (a) 1.25 kW without carbon dioxide, (b) 1.25kW with carbon dioxide, and (c) 0.70 kW with carbon dioxide. Each profile is plotted on its own normalized intensity scale.
indicates relative ease of interfacing supercritical fluid chromatography with the ICP. The interface between the chromatography and ICP will not degrade the chromatographic separation because there is virtually no dead volume. The interface is the end of the column itself. The largest problems with supercritical fluid sample introduction are the effect of the fluid on excitation conditions in the plasma and background emission. However, the effects appear to be no more severe than those observed when organic solvents are used with conventional nebulizer sample intro-
Anal. Chem. 1907, 59, 799-800
duction systems. Changes in both background and excitation conditions should be smaller when the ICP is used to detect eluents from supercritical fluid chromatography. The fluid flow rates will normally be lower for SFC and the solvent will be well separated from the analyte species. A number of improvements are now being made. These include automated peak area detection, simultaneous background subtraction using photodiode array and direct reading spectrometers, and automation of the sample injection. We are in the process of characterizing the spatial profiles (both lateral and vertical) of the emission signals from the injected samples. We are also determining optimum experimental conditions including flow rates and ICP power. Other supercritical fluids are also being tested including NzO, propane, and NH,, which are commonly used for supercritical fluid chromatography. Because of the low flow rates required, xenon is also a practical supercritical fluid. The effect of Xe on the plasma excitation conditions and supercritical fluid induced background should be much smaller than those resulting from COz. Xe has been shown to be a useful mobile phase for supercritical fluid chromatography (9).
ACKNOWLEDGMENT Donation of the ICP power supply by Perkin-Elmer is appreciated. Loan of the SIT vidicon detector and controller by Princeton Applied Research Corp. is gratefully recognized. Registry No. COz,124-38-9;Xe, 7440-63-3; NzO, 10024-97-2; NH3, 7664-41-7;propane, 74-98-6. LITERATURE CITED (1) Browner, R. F.; Boorn, A. W. Anal. Chem. 1984, 56, 786A. (2) Browner, R. F.; Boorn, A. W. Anal. Chem. 1984, 56, 875A.
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(3) White, C. M.; Houck, R. K. HRC CC, J . High Resolut. Chromatogr.
Chromatogr. Commun. 1986, 9 4. (4) Olesik, J. W.; Bradley, K. R. Spectrochim. Acta, Part 8 , in press. (5) Boumans, P. W. J. M. Line Coincidence Tables for Inductively Coupled Plasma Emission Spectrometry. 2nd ed.; Pergamon: Oxford, 1980. (6) Richter, 8. E. HRC CC, J . High Resoiut. Chromatogr. Chromatogr. Commun. 1985, 8 , 297. (7) Blades, M. W.; Caughlin, B. L. Spectrochlm. Acta, Part8 1985, 406, 579. (8) Boorn, A. W.;%rowner, R. F. Anal. Chem. 1982, 5 4 , 1402. (9) French, S. B.; Novotny, M. Anal. Chem. 1986, 58, 164. ~
John W. Olesik* Department of Chemistry Venable and Kenan Laboratories 045A University of North Carolina Chapel Hill, North Carolina 27514
Susan V. Olesik Department of Chemistry The Ohio State University 140 West 18th Street Columbus, Ohio 43210 RECEIVED for review July 21, 1986. Accepted November 4, 1986. Funding was provided in part by BRSGS07 RR07072 awarded by the Biomedical Research Support Grant Program of the Division of Research Resources from the National Institutes of Health, a Du Pont Young Faculty Grant, the University of North Carolina Department of Chemistry, and the UNC University Research Council. Portions of this work were presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1986, and the Federation of Analytical Chemistry and Spectroscopy Societies Meeting, St. Louis, MO, Sept 1986.
A I D S FOR ANALYTICAL CHEMISTS Rotary-Type Injector for Capillary Zone Electrophoresis Takao Tsuda* Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan
Toshihide Mizuno and Junichi Akiyama Shimadzu, Nishinokyo-kuwabara-cho,Nakagyo-ku, Kyoto 604, Japan In capillary zone electrophoresis, deficiencies are present in injection methods that use either electroosmotic flow (1, 2 ) or suction (1-4). In an electroosmotic method, introduction is carried out by applying an electrical field between a column inlet and a sample reservoir. Thus, sample components are introduced by both electroosmotic flow, u(osm), and mobility, u(mob). The latter is generally dependent on the number of charges on a solute and its molecular size. Thus, the total amount of each solute injected depends on the nature of the solute. When (u(mob)Iof a solute is larger than lu(osm)l and each sign is different, the solute cannot be introduced into the column. sampleintroduction with suction, the end of the column dipped in the sample solution should be lifted up for every injection (1-3). Injecting an accurate amount, repeatedly, is difficult due to constant Positioning ofthe level ofthe cohmn. Therefore, it is better to use an internal standard for cakulating the total amount injected (3, 4 ) . If we use a rotary type injector, which is commercially
available for liquid chromatography, bubbles are generated inside the injector, causing the electrical current to stop. This may be due to the electrochemical reaction at the metal surface inside the injector. A rotary type injector is more favorable for the injection of an absolute amount of solute than injection methods that use electroosmotic flow and suction. We have devised a new rotary injector which can be used under high electrical field conditions.
EXPERIMENTAL SECTION A schematic diagram of the injector is shown in Figure 1. A (load position) and B (injection position) in Figure 1 show cross-sectional views of the rotary injector. C shows a rotary injector viewed from the left side of A. The injector consists of one robr, two stators, a d one central pin made from fine ceramic (Kyocera Co., Kyoto, Japan). The plates and Screws were made by using a tetrafluoroethylene resin. The rotor and stators have two flow passages. After one passage is filled with a sample solution (A in Figure l),the rotor is rotated 90' by hand (B in
0003-2700/87/0359-0799$01.50/0 0 1987 American Chemical Society
