Evaluation of inductively coupled plasma emission spectrometry as an

Kimberley A. Forbes,1 Jodi F. Vecchiarelli,2 Peter C. Uden,* and Ramon M. ... GRC Tower A, University of Massachusetts, Amherst, Massachusetts 01003-0...
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Anal. Chem. 1990, 62,2033-2037

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Evaluation of Inductively Coupled Plasma Emission Spectrometry as an Element-Specific Detector for Supercritical Fluid Chromatography Kimberley A. Forbes,l Jodi F. Vecchiarelli? Peter C. Uden,* and Ramon M. Barnes Department of Chemistry, Lederle GRC Tower A, University of Massachusetts, Amherst, Massachusetts 01003-0035

A Capillary supercritical fluid chromatograph (SFC) was Interfaced to an inductively coupled plasma (ICP) atomic emisdon spectrometer (AES). The chromatographic system, interface, and plasma torch deslgn are described. The argon plasma was optimized for Si I 251.6 nm emlssion to detect separated organosillcon compounds selectively. The linear dynamk range for octamethylcydotetradloxanewas between 7.24 and 145.0 ng of Si injected, with a detection limit of 5.8 ng of Si injected (57.9 ppm SI). The precision of the measured peak heights of three replicate Injections of 75 ng of Si was less than 5 % . The effects of the moMie phase (CO,) pressure on the plasma stablitty and sensltMty are presented.

INTRODUCTION Chromatography with a supercritical fluid mobile phase was suggested more than 25 years ago ( I , 2 ) . The difficulties of handling the required pressures and temperatures of these fluids with the instrumentation existing at that time limited development, but as technology advanced in both capillary gas chromatography (GC) and high-performance liquid chromatography (HPLC), useful techniques that could be utilized in supercritical fluid chromatography (SFC) were provided (3). The first practical capillary SFC instrument was described in 1981 (41, and since then SFC has been considered as a complementary technique to GC and HPLC, and has been applied in a variety of areas including the analysis of polymers (5), glycerides (6, 7), pesticides ( 8 ) , cholesterol (91, and complex hydrocarbon mixtures (IO). SFC detection may be either on-column ( 4 ) or a t atmospheric pressure after a decompression stage. LC-like spectroscopic detectors, including variable-wavelength UV absorbance (IO),fluorescence (II), and infrared absorbance (12, 13), have been applied as on-column detectors for SFC. GO-like detectors have been applied after decompression of the supercritical mobile phase to a gas. The flame ionization detector (FID) was used for packed (14,15)and capillary SFC columns (7,9),yielding relatively low background signals (3). Other detection methods used in GC have been investigated for SFC, including supercritical fluid chromatography/Fourier transform infrared (SFC/FTIR) spectroscopy (16, 17), supercritical fluid chromatography/mass spectrometry (SFC/ MS) (18, 191,and SFC followed by ion mobility (20),thermionic (21),and dual flame photometric detection (22). Plasma atomic emission sources offer many advantages as chromatographic detectors, including multielement detection capability, high sensitivity, and inherent selectivity. These features enable nonideal chromatography to be tolerated and metal and nonmetal speciation to be performed. Additionally,

* To whom correspondence should be addressed.

Present address: Rorer Central Research, Fort Washington, PA

19034.

Present address: Union Carbide Corp., Tarrytown, NY 10591.

plasma detectors suffer from fewer spectral interferences and have a wider dynamic range than flame-based detectors (23). The first use of a supercritical fluid sample introduction system for inductively coupled plasma (ICP) was reported in a correspondence by Olesik et al. (24). The limitation in the development of plasma chromatographic detectors is the interface which must minimize band broadening. In HPLC, the conventional nebulizer and spray chamber systems used for liquid sample introduction show typical transport efficiencies of less than 2%. The unique properties of supercritical fluids eliminate the need for the nebulizer/spray chamber interface, since as the fluid leaves the chromatographic column and restrictor, it becomes a gas a t atmospheric pressure and will transport essentially 100% of the sample in a readily atomized form. Fujimoto et al. reported the interfacing of ICP to a packed column SFC system (25). A laboratory-made nebulizer with a built-in restrictor was attached directly to the ICP torch, and the system was evaluated with ferrocene and its derivatives. No reports of the use of the ICP as a detector for capillary SFC have appeared, but the above studies suggest that capillary interfacing may show equal or better analytical performance to the packed-column case. Recently, Galante et al. (26) and Luffer et al. (27) coupled the surface-wavesustained-microwave-induced-plasma (surfatron) to a capillary SFC system and detected sulfur-containing polyaromatic compounds. Element-specific detection for sulfur was in the near-infrared region because the sulfur atom lines were more intense than those in the visible region, which also suffered from severe spectral interferences from intense Cz band emission resulting from the carbon dioxide mobile phase (26). In general the small size and low power, which make the microwave-induced helium plasma (MIP) detectors of the TMolotype most attractive for GC detection, but less so for HPLC detection hecause of mobile phase impact, reduce their applicability for SFC. The argon ICP however suffers from the same limitations for elemental speciation in SFC as in other chromatographies, namely poor or absent detectability for many nonmetallic elements. Development of different complementary plasma detection systems for SFC is thus important. This paper discusses the coupling of an argon ICP to a capillary SFC. Various restrictor designs were investigated, and overall system parameters are discussed. The effects of the mobile phase on plasma stability, sensitivity, selectivity, and spectral background are presented, and optimization for the detection of silicon-containing compounds is reported. Automobile and fuel sensors and emission control devices are susceptible to silica poisoning arising from contamination of unleaded fuels (28). Inductively coupled plasma atomic emission spectrometry (ICP-AES), alone, may be used for quantitative assay of the silicon content in a sample, and the chemical identity of the cyclic silicone contaminant, octamethylcyclotetrasiloxane, was obtained by high-resolution silicon-29 nuclear magnetic resonance (NMR) spectroscopy (28). The use of a capillary supercritical fluid chromatogra-

0003-2700/90/0362-2033$02.50/00 1990 American

Chemical Society

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ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990 ICP TORCH ASSEMBLY

CAPILLARY RESTRICTOR

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n n

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Figure 1. Schematic diagram of the supercritical fluid chromatographlinductively coupled plasma. phy/inductively coupled plasma atomic emission spectrometry (SFC/ICP-AES) system could provide a method to separate and detect on-line various organosilicon compounds present in the fuels.

EXPERIMENTAL SECTION Instrumentation. A schematic diagram of the instrumentation is shown in Figure 1. The components of the system are described below. Chromatographic Equipment. A gas chromatograph (Perkin-Elmer 3920B, Norwalk, CT) was modified for use in SFC. A syringe pump (Varian 8500, Varian Aerograph, Walnut Creek, CA) was used to deliver carbon dioxide (supercritical fluid grade, Scott Specialty Gases, Plumsteadville, PA) mobile phase, which was filtered at the outlet of the pump through a 2-pm filter (Valco Instrument Co., Inc., Houston, TX). The pressure of the C02 delivered by the pulseless syringe pump could be controlled manually, or pressure programming could be performed with a computer (Apple IIe, Apple Computer, Inc., Cupertino, CA). The fused silica capillary column used was coated with poly(dimethylsiloxane)(20 m X 0.2 mm id., 0.05-mm film thickness, J+W Scientific, Inc., Folsom, CA). The column temperature was controlled by the GC oven. Samples were injected with a microvalve (Valco) with a 0.1-pL electronically actuated loop. Six inches of 1/16 in. 0.d. X 0.02 in. i.d. stainless steel tubing were connected between the injection valve, through the injection port opening into the oven, and into the side of the straight run on a 1/16 in. tee (Valco ZT1). The other two ports in the tee had reducers (Valco IZR1.5LFS.4) installed. A retention gap (0.375 mm 0.d. X 50 pm i.d. uncoated fused silica) was passed through the tee and the */16 in. tubing, then butted flush with the end of the tubing at the injector valve port. The other end of this retention gap was connected to the head of the capillary column with a 1/32 in. union (ValcoZU.5FS.4). In the side of the splitter tee, a 10-15 pm i.d., 140 pm 0.d. piece of fused silica (Polymicro Technologies, Phoenix, AZ) was used as the splitter vent restrictor. The length of this split vent capillary tubing and the length of the restrictor used to give the required decompression to atmospheric pressure for detection could be adjusted to obtain a usable split ratio. A 182.5-cm splitter vent restrictor with a 17.5 cm long straight capillary restrictor composed of the same fused silica tubing as the splitter vent restrictor was used. The splitter vent restrictor could be tailored to the flow characteristics of the restrictor, and various combinations of the two restrictors were investigated. The results of these studies, including the use of a 2 mL/min integral restrictor as described by Guthrie and Schwartz (29), are included. Organosilicon standards (Petrarch Systems Inc., Bristol, PA) were made in HPLC grade toluene (Fischer Scientific, Fair Lawn, NJ). Compounds used included hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4),decamethylcyclopentasiloxane (D5), 1,7-dichlorooctamethyltetrasiloxane, decamethyltetrasiloxane, octamethyltrisiloxane, 1,1,3,3,5,5,7,7-octamethyltetrasiloxane, cyclopentamethylenedimethylsilane, cyclotrimethylenedimethylsilane, and cyclotetramethylenedimethylsilane. Interface. The interface and plasma torch are shown in Figure 2. The plasma was sustained in the torch designed by LaFreniere et al. (30) for use as a DIN (direct injection nebulizer) (Cetac DIN-200, Questron Corp., Princeton, NJ). However, since the supercritical fluid leaving the capillary restrictor becomes a gas

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HEATED TRANSFER LINE TO SFC OVEN

Figure 2. Interface for supercritical fluid chromatographAnductively coupled plasma. in the atmospheric pressure plasma, the nebulizer is not required. Thus, the capillary restrictor was positioned concentrically inside an alumina tube in place of the capillary fused silica tubing used for nebulization. The alumina tube (0.8 mm i.d., 1.6 mm 0.d.) supported and positioned the restrictor in the torch and also provided insulation to help prevent charring of the polyimide coating on the restrictor. The DIN torch design allowed three gas flows to be used, even without the nebulizer functioning. The outer and intermediate gas flows served their usual function to sustain the plasma discharge. The central gas flow through the inner nozzle of the DIN torch was used to carry the column effluent into the plasma. A heated transfer line, similar in design to those used for GC/MIP (31),was constructed to transport the sample from the SFC oven to the ICP. Temperature control of the capillary column was maintained by passing it through 60 cm of resistively heated '/8-in. copper tubing, which was insulated and contained in heat tape. The capillary column was connected to the restrictor by a '/&n. butt connector. To provide support for the delicate restrictor, a 6-cm section of 1 cm i.d. aluminum tubing was used as a sleeve, connected by specially constructed unions from the end of the copper tubing to the base of the torch. This sleeve supported the end of the SFC column, butt connector, and the beginning of the restrictor. The sleeve and the unions used to provide connection to the torch were wrapped in heat ribbon to maintain constant temperature control. Thermocouples were used to monitor the temperature along the transfer line and a t the base of the torch. The transfer line was maintained at 70 "C, the base of the torch was approximately 80-90 "C, and the oven was 60 OC.

The alumina tube was carefully positioned in the center of the connecting unions with graphite tape, prior to connecting the restrictor to the column. The restrictor was fed through the alumina tube, unions, and into the sleeve, which was snot yet connected to the end of the copper tubing. The butt connection to the column was made, and then the sleeve was connected to the insulated copper tubing containing the column. The restrictor could then be pulled back so as to be contained totally inside the alumina tube, which was then carefully fed into the center of the ICP torch, protecting the delicate restrictor. The alumina tube was positioned in the center of the nozzle and helped to position the restrictor in the torch.

ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990

The proximity of the restrictor to the ICP was critical. The 6 cm long support sleeve permitted adjustment of the distance between the restrictor and the top of the nozzle. The distance between the end of the restrictor and the plasma must be small to minimize the dead volume, but it must not be too small because the restrictor can be fused shut. The tip of the restrictor was placed 10 mm below the discharge region of the ICP, to provide optimal transport from the restrictor into the central region of the plasma. The tip of the restrictor was positioned approximately 2 mm above the tip of the nozzle, centered symmetrically,so that the column effluent was swept efficiently into the ICP. Inductively Coupled Plasma Atomic Emission Spectrometer. The ICP power generator (Plasma Therm, Inc., 40.68-MHz rf Generator Type HFS 5000D,Kresson, NJ) was operated at 850 W, unless noted otherwise. Simplex optimization (32) was performed to determine the rf power level and gas flow rates to be used for detection of Si I 251.6 nm. The DIN torch was used with an outer gas flow rate of 24.1 L/min, intermediate gas flow rate of 1.2 L/min, and a central gas flow rate of 0.6 L/min. An image of the plasma was formed with quartz optics (Oriel 2 in. diameter, 200 mm focal length, 1:l image) onto the entrance slit of the monochromator (Heath Model EU-700-56,0.35-mfocal length. Czerny-Turner scanning monochromator,with programmable filter attachment, McPherson Instruments, Acton, MA). The entrance and exit slit widths were 50 pm. The grating had 1180 grooves/mm and was blazed at 250 nm. The voltage of the photomultipliertube (RCA 1P28A) was maintained by the Heath Photomultiplier Module Model EU 701-30 at 650 V. Output was recorded by the Heath Log/Linear Current Module EU-20-28 (Heath Co.). The plasma was observed 14 mm above the load coil, unless stated otherwise.

RESULTS AND DISCUSSION Optimal Interface Design for Capillary SFC/ICP. The most important component in the design of the capillary SFC/ICP system was the column restrictor. The flow of the column effluent to the detector is determined by the flow characteristics of the capillary restrictor used to achieve decompression and the splitter vent restrictor. Various restrictor designs were investigated to provide the decompression necessary to introduce the eluent from the SFC column into the atmospheric pressure ICP. A straight capillary restrictor of relatively large internal diameter (25-50 pm i.d.1 did not provide sufficient back pressure to maintain supercritical conditions on the column. Although signals were observed for Si I and P I, all samples eluted in the void volume and the separation of various silicon containing compounds could not be accomplished (33). Although a 2 mL/min integral restrictor provided significant back pressure to maintain supercritical conditions on the column, its 2-pm orifice resulted in a mobile phase flow rate that was too low to allow transport of significant amount of sample into the ICP. No analyte signals could be observed. Similar results were observed when the splitter vent restrictor length was much less than the length of the restrictor to the ICP. When a relatively long (86 cm) 5 pm i.d. straight capillary restrictor was used in combination with a 12 in., 14 Fm i.d. straight capillary as the splitter vent restrictor, too much of the sample exited the splitter vent, and no signal was observed. Details of the investigations of various restrictor combinations are reported elsewhere (34). To allow the shortest length capillary to be used as a restrictor to achieve decompression prior to introduction into the ICP, the SFC column was passed through the transfer line. Heating of the transfer line was controlled to maintain constant temperature throughout the column. A 15.6 cm long, 5 pm i.d. fused silica capillary was connected to the end of the SFC column by using a butt connector which was supported in the sleeve, as described. The short length of the 5 pm i.d. capillary restrictor resulting in less resistance to flow of the mobile phase to the plasma. However, carbon emission at 247.9 nm was not observed, and no visual color changes in

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the argon plasma were apparent. In order to increase the flow of C 0 2 to the plasma further, the splitter vent was plugged with an end-capped fitting. Thus, all of the flow was directed out of the restrictor to the detector. Emission was observed for both the C 1247.9 nm and the C I 193.1 nm lines. The carbon emission increased as the COP pressure increased. The C2 and CO+ molecular bands were not observed upon scanning the background region. All of the carbon introduced into the ICP probably was decomposed into carbon a t these C 0 2 introduction rates, which, although not measured directly, were estimated to be ca. 50 pL/min at a pressure of 80 atm. Toluene was injected into the SFC and the C 1247.9 nm line was monitored. No signal resulting from the solvent was detected over the background at this wavelength; this was not unexpected since the contribution from COz is much larger than that of toluene. Strong CN bands were observed in the spectrum, since the ICP torch had no extension, and their presence served as a good indication of pressurization of the SFC system. A study was performed to determine where in the plasma the maximum carbon emission at 247.9 nm was observed. The forward power was 1 kW, reflected power was tuned to 0 W, the outer gas flow rate was 22 L/min, intermediate gas flow rate was 1.2 L/min, the central gas flow rate was 0.63 L/min, and the COz pressure was 200 atm. The maximum carbon emission was observed at observation heights of 11.0-13.5 mm above the load coil. The intensity of the carbon emission and the noise in the background of the plasma increased as the delivery pressure of the carbon dioxide was increased. The signal noise was largest while the syringe pump was adjusting to the desired pressure. Once the pump reached the desired pressure, the plasma became stable. With the splitter vent plugged and the 5 pm i.d. capillary restrictor in the torch, various organosilicon compounds were injected at several different power levels, observation heights, and gas flow rates. No response was detected by Si 1251.6 nm. Ferrocene was also injected, but no Fe I response at either 238.0 or 259.7 nm was detected. The supercritical fluid mobile phase was transported into the plasma, but insufficient eluent entered the plasma to detect analyte emission. The flow to the ICP was increased by installing a 17.5 cm long, 14 pm i.d. straight capillary tubing in the interface. The flow of COz out of this restrictor was verified prior to insertion of the restrictor in the torch. At 80 atm pressure, the flow rate to the ICP was estimated at ca. 50 pL/min from calculation based upon measurement of split vent dimensions and vented flow using a simplified form of Poiseuille’s law. When the delivery pressure of the COz was about 100 atm, an ice ball was observed a t the tip of the restrictor, indicating JouleThompson cooling of the supercritical fluid leaving the capillary. The tip of the capillary was positioned about 2 mm above the top of the inner quartz nozzle in the DIN, about 10 mm below the plasma, to ensure optimal sample transport. Emission of the C I a t 247.9 nm and a t 193.3 nm increased as the delivery pressure of the COPmobile phase was increased. The l-kW (approximately 50 W reflected power) argon ICP was stable for COz pressures of 80-250 atm. The central channel of the argon plasma was green owing to the C2 molecular band emission. The channel became wider and greener as the COz pressure increased. The plasma was extinguished when the COP pressure was increased to 300 atm. The argon plasma was easily ignited with the 14 pm i.d. capillary restrictor in the torch and the COZ pressure at e 1 0 0 atm. With the splitter vent plugged, 3.5-pg of hexamethylcyclotrisiloxane (D3)was injected. The Si 1251.6-nm line was monitored in a 1.1-kW plasma at an observation height of 13 mm (outer gas flow rate 24 L/min, intermediate gas flow rate

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1.2 L/min, central gas flow rate 0.60 L/min, and the preheated gas flow around the restrictor 0.6 L/min). The SFC oven and the transfer line temperatures were held a t about 70 "C, and the torch base was about 100 "C. The relatively high quantity injected overloaded the capillary column, resulting in poor peak shape; however, a Si I signal was detected in the effluent. Two other organosilicon compounds, D4 and D5, were injected under the same conditions. Under isobaric SFC operation, the retention time of D5was greater than D4 or D3.

Optimal Conditions for Determination of SiliconContaining Compounds. Split Ratio. T o improve peak shape, the plug in the split vent was removed, and a splitter vent restrictor (14-pm i.d. x 182.5-cm length, fused silica straight capillary) was installed. The combination of this split vent restrictor with the 17.5 cm long restrictor (14 pm i.d. straight capillary) in the ICP torch resulted in venting approximately one-tenth of the sample, estimated as noted above using restrictor and vent dimensions and application of Poiseuill's relationship. The peak shape observed for the organosilicones was improved under the previous chromatographic conditions. Effect of Preheated Gas Flow. A fourth gas flow, which was preheated by passing it through -70-ft of copper tubing coiled in an oven which could be heated to 300 "C, was introduced at the beginning of the transfer line to provide better sample transport. This heated argon passed through the transfer line, flowed around the restrictor inside the alumina insulating tube, and into the ICP torch. Olesik reported that the velocity of the central gas should be high enough to ensure that the sample travels through the plasma boundary and into the center of the discharge (24). Additionally, the flow of this preheated gas around the restrictor should provide uniform heating and help minimize condensation. The Si I signals measured without the preheated gas flow around the restrictor were almost twice those measured with a gas flow rate of 0.6 L/min around the restrictor. In fact, any added gas flow around the restrictor resulted in a decreased response, even when the observation height and other gas flow rates were adjusted. This is in contrast to the observation of Olesik (24). In the SFC/ICP system described here, the extra gas flow around the capillary restrictor decreased sample residence time and, thus, decreased the emission signal. The DIN torch used had the alumina tube surrounding the capillary restrictor set concentrically inside the inner quartz nozzle, which reduced the effective size of the central nozzle (Figure 2). The velocity of the central flow which resulted was sufficient to ensure the sample traveled into the center of the plasma. A channel was visible in the plasma with the central gas flow rate as low as 0.55 L/min. Detector Response. The "spiking" problem that has been reportedly observed in FID signals when used with SFC (35) was not observed in this work. Spiking occurs frequently for polar solvents and high molecular weight solutes, which are likely to condense and result in noisy peaks as discrete solute particles enter the detector and produce a sudden response. Perhaps the lack of spiking in this work was due to the samples studied. Condensation also was not observed by Olesik and Olesik ( 2 4 ) . Simplex Optimization. A simplex optimization was performed with a 905 pg/mL solution of octamethylcyclotetrasiloxane (D,) (90.5 ng of Si injected). The delivery pressure of the C 0 2 mobile phase was 80 atm. The chromatographic oven was maintained a t 60 "C, the transfer line at 70 "C, and the torch base a t 100 "C. The intermediate flow rate was maintained constant at 1.2 L/min to ensure the plasma was an appropriate distance from the tip of the restrictor in the torch. The tip of the restrictor was withdrawn slightly until it was even with the top of the central nozzle

in the torch. The observation height was 13.5 mm throughout the Si I studies. Power levels from 800 W to 1.15 kW were investigated. Below 800 W the ICP was not stable upon introduction of the supercritical fluid COz at 80-100 atm. Central gas flow rates from approximately 0.5 to 1.5 L/min were examined. The third parameter involved in the simplex optimization was the outer gas flow rate. Values from 23.5 to 24.3 L/min were investigated and were observed to effect the peak shape. Three injections were made for each set of conditions tested in the simplex program. The optimum conditions obtained for the determination of Si I at 251.6 nm by SFC/ICP with a C 0 2 delivery pressure of 80 atm were forward power 850 W, central gas flow rate 0.6 L/min, and outer gas flow rate 24.1 L/min. Detection Limits. The detection limit ( S I N = 3) for D, under optimum conditions was found to be approximately 5 ng of Si injected with the COz pressure at 80 atm. The possibility of monitoring the Si I emission at 288.1 nm was explored but the background and the noise were much higher in that region of the spectrum, due in part to molecular bands, resulting in poor response. In an attempt to improve the detection limit of D4,oxygen was added to the argon outer gas flow. The addition of 3-17% oxygen to the outer gas flow did not increase the response at Si 1251.6 nm. The response also was not enhanced by the addition of oxygen to the intermediate or central gas flows. Linearity and Precision. The linearity of the SFC/ICP system was measured for the detection of D4using the conditions determined by the simplex optimization with the C02 pressure maintained at 80 atm. Linearity was demonstrated over a concentration range of 7.24-145.0 ng of Si injected. Concentrations above this range resulted in column overload. The slope of the calibration curve for seven concentrations in the specified range is 6.9 f 0.1 mV/ng, with an intercept of -1.2 X and a correlation coefficient of 0.999. All statistics were calculated by using RS/1 (Version 12.1, BBN Software Products Corp., Cambridge, MA). The relative standard deviation of the peak heights for D4 was based on three replicate injections of each of the concentrations. The values ranged from 1 to 5% for concentrations greater than 14 ng of Si injected. A slightly higher value of 8% was determined for the injection of 7.24 ng of Si, which is closer to the detection limit. When the COz pressure was increased to 100-120 atm and maintained constant, linearity of Si I 251.6-nm emission was demonstrated for D4 between 7.24 and 145.0 ng of Si injected. The slope of the calibration curve for four concentrations in the specified range is 6.0 f 0.3 mV/ng, with an intercept of 0.12 and a correlation coefficient of 0.998. The decrease in slope of this calibration curve compared to that at 80 atm indicates a slight decrease in sensitivity occurs at the higher pressure. Similar results were reported in the determination of polyaromatic sulfur-containing compounds by SFC/MIP (27). The sharpest and most reproducible peaks were recorded for D, using a C02delivery pressure of 80 atm. For the elution of the higher molecular weight siloxane standard, 1,7-dichlorooctamethyltetrasiloxane, 100 atm COz resulted in the sharpest, most reproducible peaks. A mixture of these two tetrasiloxane standards was prepared and injected. The two compounds were not completely separated under isobaric conditions a t either 80 or 100 atm. To achieve better separation and good peak shape,the pressure was slowly adjusted manually to provide a simple pressure program from about 80 to 100 atm, and the resolution of the two siloxane standards was improved (Figure 3). No continuous controlled pressure programming facility was available, due to rf electronic interference with the SFC control system, but the manual ex-

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Gamble Corp., Cincinnati, OH, for their collaboration in assembling the SFC system used in this work and their helpful discussions about the interface.

LITERATURE CITED

u I

I

I

0

2

4

Time, minutes Flgure 3. Separation of two tetrasiioxanes: (1) octamethylcyclotetraskxane (89.8 ng of SI);(2) dichlorooctamethylslbxane (160.3 ng of Si).

periment indicated that background noise would generally increase during programming. Although the compounds studied were smaller than typical condidates for SFC, these siloxanes demonstrate the feasibility and selectivity of the ICP as a detection method for capillary SFC. These studies have shown that the argon ICP may be used to detect the emission of silicon-containing compounds in the UV-vis region of the spectrum as they elute in the supercritical mobile phase. While the carbon dioxide mobile phase does diminish sensitivity of the ICP somewhat, the molecular background emission is not increased significantly, as observed upon introduction of the supercritical carbon dioxide into the microwave induced plasma (26, 27). The response of the ICP to Si I 251.6-nm emission is linear, and the sensitivity provided, coupled with the chromatographic separation capabilities, should be suitable for many applications. Plasma conditions would require optimization with respect to flow rates, viewing height, etc. to determine detection limits and linear ranges for other elements. However, it may be noted that there is much less interference due to introduction of COPinto the ICP than in introduction to the MIP: thus potentially useful analytical ranges may be larger for some elements. The selectivity of the ICP demonstrates the capability to detect early eluting compounds which are normally not separated from the solvent peak. This work also demonstrates the use of supercritical fluids as a means of efficient sample introduction into the ICP.

ACKNOWLEDGMENT We gratefully acknowledge Thomas Chester and David Innis a t The Miami Valley Laboratory of the Procter and

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RECEIVED for review January 25, 1990. Revised manuscript received June 5,1990. Accepted June 19,1990. Support for this work was provided in part by Baxter Healthcare Corporation, 3M, and also by Merck, Sharp and Dohma Research Laboratories through a Merck Predoctoral Fellowship Award to K.F. Research was supported in part by the ICP Information Newsletter.