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Anal. Chem. 1986, 58,2558-2561
Thermospray Interfacing for Flow Injection Analysis with Inductively Coupled Plasma Atomic Emission Spectrometry Sir: Sample presentation to inductively coupled plasma atomic emission spectrometry (ICP-AES) via flow injection analysis (FIA) has been of great recent interest (1-3),resulting in part from the capabilities of this approach for rapid, precise introduction of small-volume samples into a powerful element-selective detector. Other advantages of FIA-ICP-AES include the potential for simple, automated dilution, matrix modification, and standards addition, as well as minimization of problems for ICP-AES with samples having high dissolved solids content or variable viscosity (I). Despite the potential for these attractive features, FIA-ICP-AES has found little practical application, owing in part at least to the limitations of conventional sample introduction systems ( 2 , 4 , 5 ) . With FIA, steady-state (continuous) signals are generally not observed and analyte dispersion occurs, resulting in reduced detection. Discrete sample presentation to ICP-AES from high-pressure liquid chromatography (HPLC) is similarly hindered, preventing the routine application of this powerful approach to metal speciation studies (4-6), a problem of significant environmental importance (7). Conventional ICP-AES systems for continuous sample introduction typically employ pneumatic nebulizers, coupled with aerosol conditioning chambers to reduce plasma solvent loading and fiiter out large aerosol droplets. Anal@ transport to the plasma source is characteristically only 1-270 for 1 mL/min or more of liquid sample consumption. Therefore, most of the analyte is not available for signal production, drastically limiting detection capabilities. Improvements in analyte transport should be of particular benefit to discrete sample presentation methods (FIA and HPLC). As a means of alleviating this limitation, Lafreniere, Rice, and Fassel (2,B) have designed a direct injection nebulizer (DIN) which is ideally 100% efficient at transporting introduced analyte into the ICP. This DIN is essentially a microconcentric pneumatic nebulizer placed a t the base of the plasma. Unusually high gas injection velocities are required, along with relatively low totalsample flow rates (120 ML/min), reducing analyte residence time within the plasma and the total analyte mass available. In addition, since no aerosol conditioning is possible with this approach, large droplets, which might not produce useful signal, could constitute a significant portion of the analyte mass transported, although aerosol measurements have not been reported to date for the DIN, Consequently, no major improvements in detection limits were reported for the DIN compared to the conventional nebulizers. Glass frit nebulizers also reportedly possess high analyte transport efficiencies ( 4 , 9), however once again only a t low sample flow rates (50 ML/min). The total analyte mass presented to the ICP therefore is low and detection limits are not much improved. Memory effects, which would broaden the discrete sample plugs of FIA or HPLC, are also reported (9).
Thermospray has been a highly successful approach for interfacing liquid chromatography to mass spectrometry (IO, 11). With thermospray, the liquid HPLC effluent is forced through an electrically heated stainless steel capillary at aqueous flow rates up to or above 2 mL/min. This process provides precisely controlled vaporization of the solution, producing a superheated aerosol carried in a supersonic jet of vapor. Nonvolatile species, such as analyte salts, are preferentially retained in the droplets of the aerosol; thus thermospray acts as a nebulizer and preliminary desolvation
apparatus. The aerosols produced by thermospray appear to have desirable properties for sample introduction into an ICP. In a recent preliminary communication, Meyer, Roeck, and Vestal (12) replaced a concentric pneumatic nebulizer from a conventional nebulizer/spray chamber combination with a thermospray vaporizer for the continuous introduction of liquid samples a t 0.5 mL/min. Detection enhancements typically around a factor of 5 were observed for a wide variety of elements with thermospray, compared to the concentric nebulizer. Flow rates were apparently limited to 0.5 mL/min as the lower level for operation of the present thermospray design, and as an upper limit to solvent loading to the ICP. The enhancements observed were presumed to result from a higher population of aerosol droplets below the cutoff diameter for the aerosol conditioning chamber employed in that study. In this communication, we report preliminary data for the application of thermospray vaporization to sample nebulization for FIA-ICP-AES. Included in this discussion will be the effects of higher liquid sample flow rate, as well as further desolvation and solvent condensation. Results are compared to a conventional cross-flow nebulizer/spray chamber combination. EXPERIMENTAL SECTION
FIA and ICP-AES Apparatus. Liquid carrier streams were generated with an Autochrom (Milford,MA) OPG/S low-pressure solvent mixing system having a Model M500 dual piston pump with a microbore pump head. These liquid streams were subsequently pulse dampened and pumped through a Rheodyne (Catati, CA) Model 7125 syringe-loading sample injector fitted with a 200-pL injection loop. It should be noted that pulsedampening was essential to the prevention of unstable (pulsed) vaporizer performance. Sample and carrier liquid were then transported to either the cross-flow or thermospray nebulizers using the same length of 0.25 mm i.d. X 1.6 mm stainless steel tubing. The emission from a Leeman Labs (Lowell,MA) Model 2.5 ICP was focused (1:0.7) onto the 1-mm entrance aperature of a McPherson (Acton, MA) Model 270 monochromator using a 7.6 em focal length quartz spherical lens (Esco, Oak Ridge, NJ). Wavelength modulation using a post-entrance-slit quartz refractor plate was used for dynamic background correction. All gas flows were regulated with standard rotameters except the nebulizer flow, which was controlled by a Tylan (Carson, CA) Model FC260 mass flow controller. Further operational details are listed in Table I. Nebulizers. The conventional sample introduction system supplied by Leeman Labs with the ICP was used as a reference and consisted of a fixed cross flow nebulizer with a mixer-paddle spray chamber. The basic thermospray vaporizer probe and temperature monitor/controller were obtained from Vestec (Houston,TX). The controller and probe employed the improved design features for temperature regulation which are described elsewhere (11). The original vaporizer probe from Vestec used a brazed joint between the capillary tube and an outer 6.4 mm 0.d. stainless steel tube for electrical contact and as a flow barrier. In order to make our vaporizer more robust in corrosive atmospheres (vaporized dilute acids),the braze contact for our vaporizer was replaced with a 1.6 mm X 6.4 mm Swagelok (Crawford Fitting, Solon, OH) stainless steel reducing union, drilled through to a 1.6 mm i.d. to allow the capillary tube to slide through. The capillary stainless steel tube was insulated from the outer tube, beyond the contact within the fitting using an appropriate length of Pyrex tubing. The capillaries used for the vaporizer in these studies were 0.127
0003-2700/86/0356-2558$01.50/0 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986
Table I. Plasma ODerating Conditions and Experimental Facilities
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I
551 0
Leeman Labs, Inc., Model 2.5 0.8 kW for aluminum 1.0 kW for remainder of elements 18 L/min Ar 0 L/min Ar
plasma power coolant flow rate auxillary flow rate nebulizer flow rate cross flow
0.4 L/min Ar for aluminum 0.5 L/min Ar for remainder 0.4 L/min Ar for aluminum thermospray carrier 0.5 L/min for remainder Tylan Model FC260 with R020A mass flow controller read out controller McPherson Model 270 monochromator 0.35 m focal length 2400 grooves/mm, holographic grating slit widths 20 pm wavelength modulation 185 Hz 14-15 mm above the load coil viewing height photomultiplier tube R928 at 750 V Keithley Model 485 picoammeter preamplifier Stanford Research Systems Model lock-in-amplifier SR510 Nicolet Model 3091 digital storage digital storage device oscilloscope Apple IIe with Nicolet Apple/31 data storage device software HPLC pump and controller Auto Chrom OPG/S system with M500 dual-piston pump, 2% nitric acid carrier stream Rheodyne Model 7125 with 200-pL injector injection loop 1 2 . 8 mm t o 6 . 4 m m R e d u c i n g U n i o n
4 6 mm i.d.
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Flgure 1. Thermospray probe interface. All components were fashioned from standard wall Pyrex tubing except the vaporizer probe and reducing unions.
mm i.d. by 1.6 mm 0.d. stainless steel obtained from Alltech (Deerfield, IL). The thermospray vaporizer probe was interfaced to the plasma torch using an arrangement depicted by Figure 1. The expansion chamber, into which the thermospray aerosols were injected, was wrapped with heating tape. The inside diameter of the expansion chamber was chosen to be large in order to minimize aerosol collisional losses within the chamber. This arrangement allowed provision for an argon carrier flow for the thermospray aerosols, postprobe heating at moderate temperatures for additional desolvation, and a 50-cm West condenser for partial solvent removal. The effluent from the condenser was transferred into the torch box to the sample injection tube of the ICP using a minimum length of 6.4 mm 0.d. Teflon tubing. Solvent waste from the condenser was pumped away with a peristaltic pump. The delay time for signal response after injection was 7 s for 1mL/min liquid flow. Reagents. Standard solutions were prepared by dissolving base metals or Spex (Metuchen, NJ) sample compounds in Baker (Phillipsburg, NJ) Ultrex acids followed by appropriate dilution with deionized, distilled water or dilute nitric acid, as required. Procedures. The operating conditions in Table I were chosen to optimize signal-to-noiseratios for the continuous introduction of standards with the cross-flow nebulizer. Viewing heights were optimized for each set of experiments and varied within the listed range. The injection volume employed throughout these studies (200 pL) was chosen to provide signal intensities within 5% of the peak intensities observed for continuous introduction with the cross-flow nebulizer.
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Data points reported are the result of at least five separate injections, using peak intensity measurements. Detection limits were estimated by using the slope method described by Boumans (13) and a 3a value. RESULTS AND DISCUSSION Thermospray Temperature and Flow Rate. The gradual heating process that occurs within the vaporizer capillary results in significant variation in temperature along the probe. This process has been modeled (10);it has been shown that for LC/MS the most meaningful temperature measurement is that for the probe tip. The tip temperature determines the degree of vaporization of the solution, which is affected by the liquid flow rate and composition (11). For LC/MS, optimal performance occurs when the tip temperature results in partial but nearly complete vaporization of the solution (10). At this temperature, mean aerosol droplet diameters are presumably smallest, and deposition of nonvolatiles is not significant. For sample introduction to ICPAES, these factors would be similarly favored and therefore similar optimum temperatures would be anticipated. Figure 2 shows the effect of tip temperature on SNR for injected standards. The optimum temperature (145 "C) compares well with predicted results for this type of probe at this flow rate (1 mL/min) with LC/MS (11). Similar profiles were observed for higher flow rates with slightly higher optimum temperatures. Pump pressures increased as probe temperatures and liquid flow rates were increased. An intermediate value of -1.2 MPa was typically observed a t 1 mL/min and 145 "C for the probes tested; a pulse-dampened, high-pressure pump was therefore a requirement for accurate, pulse-free solvent metering. At temperatures significantly higher than the optimum for any liquid flow rate, signal peak shapes were poorly reproducible and pump pressures gradually increased with probe use. Operation at these higher temperatures was therefore discouraged. The jet exiting the vaporizer contains both aerosol droplets and supersaturated vapor. The free expansion of this jet would lead to rapid cooling and condensation of part of the vapor, potentially enlarging the aerosol droplets in the jet. Since transport of the aerosol would be degraded by increasing particle diameters, we chose to investigate the effects of postprobe heating to minimize this condensation effect. In the present arrangement, the surface temperature of the expansion chamber was limited to 110 OC. At higher temperatures thermal coupling with the vaporizer was significant enough to cause unstable vaporizer performance. However, this modest postheating was sufficient to yield significant improvements compared to the unheated case.
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986
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Table 11. Comparison of Detection Limit Estimates (ng mL-') for FIA
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Figure 3. Typical signal-to-noise Calibration data: thermospray with postprobe heating (m), thermospray with postprobe heater off (+), cross flow (X); 200-pL injections, 1 mL/min 2% HNO,, 327.4-nm
analytical wavelength, 145 OC probe temperature. 100
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Flgure 5. Raw signal data for repeated injections of 100, 50, and 10 wg/mL Ni solutions: 200-pL injections, 1 mL/min 2% HNO, flow rate, 145 O C probe tip temperature, 341.5-nm analytical wavelength. 10
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Flgure 4. Typical signal only calibration data, same conditions as in Figure 3.
Because of the higher temperature operation of the thermospray nebulizer, solvent vapor concentrations would certainly be higher than those for conventional nebulizers. Excessive vapor loading is known to degrade signal intensities for ICP-AES, presumably by quenching plasma energy ( 4 , 5 ) . In addition, condensation eventually would build up on cooler transfer lines, resulting in perodic injection of large droplets that would extinguish the plasma. Consequently, we employed a condenser for removal of excess solvent. For this arrangement, maximum response was observed a t 1.0 mL/min. At lower flow rates, decreased input of analyte mass per unit time resulted in reduced response. At higher flow rates, the condenser employed was not efficient in removing solvent resulting in rapid formation of condensate in the transfer line to the ICP, eventually extinguishing the plasma. Consequently, it is felt that the optimum flow rate observed here results from limitations of our present apparatus and does not necessarily describe a fundamental property of thermospray sample introduction. Operation a t higher flow rates with adequate solvent removal should increase total mass transport and provide further signal improvements. Comparison of Nebulizers, Signal measurements were made for the cross-flow nebulizer, the thermospray sample introduction system with postprobe heating at 110 "C (particle desolvation), and the thermospray system with the heater turned off. Figure 3 compares typical SNR results for these three systems, using copper as the analyte. Slopes of calibration curves increased from the cross-flow system, to thermospray without additional heating, to thermospray with postprobe heating. Interestingly, these enhancements in SNR resulted from both signal increases and noise reductions, as shown by a decreased difference in slopes for signal mea-
surements alone (Figure 4), compared to the SNR data. At this point the reason for these noise reductions is not certain. However, intuitively one might anticipate such reductions resulting from a narrowing of the particle size distribution of the input aerosols; efforts to characterize thermospray aerosols are in progress. In accordance with these SNR improvements, corresponding detection limit improvements were estimated for each of the elements studied and listed in Table 11. The improvements in limits of detection for thermospray were close to an order of magnitude or better, for each of the elements tested. Precision and Memory Effects. Figure 5 depicts a typical series of injections of repeated and differing concentrations for the thermospray nebulizer. As can be seen, the reproducibility of results for each concentration is good with an average % RSD of 3%. Typical RSD's for the cross-flow case were 6%. Once maxima were achieved, the signal levels fell rapidly back to the base line (within 30 s); no significant memory effects were observed when the probe tip temperature was maintained at 145 "C. Tests done at higher operating temperatures with 1 mL/min exhibited significantly different results. Bimodal signal intensities often were observed suggesting that some portion of the analyte was flash vaporized and deposited on the inner walls of the capillary. In addition, peak intensities and widths were not reproducible. Operation at these temperatures often resulted in rapid increases in pressure drop along the capillary and, ultimately, complete plugging of the capillary. Once this occurred, no chemical and/or thermal treatment was found useful to reverse the process. CONCLUSION Thermospray offers a very controlled means of continuously generating aerosols from liquid samples at relatively high flow rates. With the addition of postprobe heating and solvent removal, thermospray becomes a very stable nebulizer for ICP-AES, which is suitable for reproducible analysis of microliter volumes via flow injection techniques. Since the thermospray nebulizer requires a high-pressure pump (large
Anal. Chem. 1986, 58,2561-2563
dead volume) for solution introduction, this system is ideally suited for discrete sampling techniques such as FIA and HPLC. The thermospray system provides significant detection enhancement compared to conventional sample introduction systems. This enhancement can allow lower concentrations to be detected or provide adequate detection for low concentrationswith smaller sample volumes. These results are particularly encouraging, considering the observed limitations of our present postprobe system, and demonstrate the great potential of thermospray for making ICP-AES a practical detector for FIA and HPLC. Future studies will include the characterization of aerosol and transport properties, matrix/solvent effects, and HPLC applications. We are also modifying our present system to allow higher liquid flow operation and investigating improved probe designs. ACKNOWLEDGMENT The authors thank Marvin Vestal and Vestec Corp. for loan of the thermospray unit. LITERATURE CITED (1) Greenfield, S. Spectrochlm. Acta, Pari 8 1983, 386,93. (2) Lafrenlere. K. E.; Rice, G. W.; Fassel, V. A. Specfrochim. Acta, Pari 6 1985, 408, 1495.
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Faske, A. J.; Snable, K. R.; Boorn, A. W.; Browner, R. F. Appl. Spectrosc. 1985, 39, 543. Browner, R. L.; Boorn, A. W. Anal. Chem. 1984, 56, 786A. Browner, R. F.; Boorn, A. W. Anal. Chem. 1984, 5 6 , 875A. Bushee, D.; Krull, I. S.; Savage, R. N.; Smith, S. B. J . Llq. Chromafogr. 1982, 5 , 693. Florence, T. M.; Batley, G. E. CRC Crlf. Rev. Anal. Chem. 1980, 7 7 , 219. Lawrence, K. E.; Rice, G. W.; Fassel, V. A. Anal. Chem. 1984, 56, 292. Layman, L. R.; Lichte, F. E. Anal. Chem. 1982, 5 4 , 638. Blakely, C. R.; Vestal, M. L. Anal. Chem. 1983, 55, 750. Vestal, M. L.: Fergusson, G. J. Anal. Chem. 1985, 57, 2373. Meyer. G. A.; Roeck, J. S.; Vestal, M. L. ICP I n f . Newsl. 1985, 70, 955. Boumans, P. W. J. M. Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectrometry; 2nd ed.; Pergamon Press: New York, 1984.
J. A. Koropchak* D. H. Winn Department of Chemistry and Biochemistry Southern Illinois University Carbondale, Illinois 62901 RECEIVED for review April 7, 1986. Accepted June 2, 1986. This work was supported in part by a grant from the Office of Research Development and Administration, SIU-C. In addition, financial support for D.H.W. was provided by the Coal Research Center, SIU-C.
Species Discrimination and Quantitative Estimation Using Incoherent Linear Optical Signal Processing of Emission Signals Sir: What follows is a description of a simple apparatus for optical spectroscopic signal processing. This apparatus may be best suited for application in background interference limited emission spectroscopy for selective species detection. Operation is based on the established techniques of linear incoherent optical signal processing (OSP) (1) and optimal weight function estimation techniques (2, 3). The spectroscopic processor is made up of four components: an element to disperse the wavelength-dependent information into space-dependent information, a spatially variant optical transmission mask to filter the spectroscopic information, optical and/or electronic elements to spatially integrate the transmitted intensity and convert it to an electronic signal proportional to integrated intensity, and an electronic postprocessor to perform simple algebraic computations. The transmission characteristics of the optical mask are central to the operation of the processor. This mask takes the place of the slit in the image plane of a conventional monochromator. The optical transmission of the mask represents a mathematical function of space and, therefore, of wavelength beyond a dispersion element. The operation of this mask on the spatially dispersed information is to produce a mathematical product between the spectroscopic information and an orthogonal weight function recorded in the mask. This product is represented by the intensity beyond the mask. The orthogonal weight function is formulated such as to maximize the signal-to-noise ratio of the signal estimate while being orthogonal to, and therefore independent of, interferences (2, 3). Linear OSP is basically a means to obtain a mathematical product of two values by passing light of an intensity representing one value through a transmission filter with a transmittance representing a second value. The intensity past the filter is the product. Extension of this procedure to obtain vector dot products is straightforward. One of the vectors is
defined as a spatially dependent intensity pattern. For example, this vector can be the output of a spectroscopic dispersion device. The intensity of each spatially dispersed wavelength or band of wavelengths represents an element of the vector. This dispersed intensity is imaged onto a spatially variant, achromatic transmission filter. The intensity at each wavelength band is imaged onto a different location on the transmission filter. The optical transmission of the filter defines the second vector. The transmitted intensity is a spatially ordered series of products of the intensities of the first vector elements with transmissions of the second. The vector dot product is the sum of these individual element product intensities. This summation or integration is performed by focusing the transmitted image to a spot that is smaller than the active area of a photodiode detector, or by using a large-area photodiode placed just after the mask. The current from this photodiode is directly proportional to the desired vector dot product. In an emission source, a species will emit over a characteristic spectrum, with an integrated emission intensity that is proportional to the quantity of this species in the source. Mathematically, the function that describes this emission is a wavelength-dependent function multiplied by a constant proportional to the amount of this species in the source. In many real situations, the total emission is made up of several overlapping spectra due to the many species in the source. An important factor in the formulation of the optical spectroscopic signal processor mask element is that it be such that the processed signal estimate is independent of spectral interferences. To optically process an emission signal made up of several component spectra, a vector function may be formulated such that it is orthogonal to the characteristic emission spectra due to interfering species ( 2 , 3 ) .The integrated product of this vector with the emission will result in a value that is independent of the amount of light emitted
0003-2700/86/0358-256 1$01.50/0 0 1986 American Chemical Society
