185
Anal. Chem. 1982, 5 4 , 165-168
column to plasma distances small. In considering the choice between tiansporting analyte predominantly in liquid or predominantly in aerosol form, the peak broadening which occurs in liquid transport must be balanced against the signal loss which may occur in aerosol transport. If mobile phase flow rates are to be altered, the insensitivity of peak height measurements to such changes with the external placement of the spray chamber can be advantageous. The transport efficiency with a conventional ICP pneumatic nebulizer is extremely low (typically 1.5%), making the ICP very wasteful of anal*. Improvement of transport efficiency, without the usual negative aspects of such processes (8) is clearly desirable if sensitivity is to be bettered. Extrapolation of data obtained in this study to organic matrices, where solvent evaporation plays a much greater role in aerosol transport processes ( I I ) , cannot be made at this time. It is worth noting that the data in this study, although obtained for LC!/ICP interfacing, are directly transferable to
flow injection analysis with ICP detection.
LITERATURE CITED (1) Gast, C. H.; Kraak, J. C.; Poppe, H.; Maessen, F. J. M. J . J . Chroma-
togr. 1979, I85,549. (2) Fraley, D. M.; Yates, D.; Manahan, S. E. Anal. Chem. 1979, 51, 2225. (3) Morita, M.; Ilehira, F.; Fuwa, K. Anal. Chem. 1980, 52,351. A.; Barnes, R. M. Anal. Chem. 1981, 53, 364. Miyanzakl, (4) (5) Hausier, D. W.; Taylor, T. T. Anal. Chem. 1981, 5 3 , 1223. (6) Hausler, D. W.; Taylor, T. T. Anal. Chem. 1981, 5 3 , 1227. (7) Novak, J. W.; Lillie, D. E.; Boorn, A. W.; Browner, R. F. Anal. Chem. 1980, 52,579. (8) Smith, D. D.; Browner, R. F. Anal. Chem., in press. 8.; Smith, P. B. Anal. Chim. Acta 1972, 59, 341. Greenfield. (9) (10) RuiiEka. J.; Hansen, E. H. Anal. Chim. Acta 1978, 99, 37. (1 1) Boorn, A. W.; Crasser, M. S.; Browner, R. F. Spectrochlm. Acta, fat? B 1980, 35,823.
RECEIVED for review August 24, 1981. Accepted November 4,1981. This work was supported by the National Science Foundation under Grant No. CHE-80-19947.
Sequential Slew Scanning Monochromator as a Plasma Emission Chromatographic Detector for Determination of Volatile Hydrides Mlchael A. Eckhoff, James P. McCarthy, and Joseph A. Caruso' Department of Chemistry, Universiv of Clnclnnati, Cincinnati, Ohio 4522 1
A sequentlal determlnation of chrornatographlcallyseparated hydrides has been developed to demonstrate the use of a sequentlal slew scanning monochromator as a plasma emlssion Chromatographic detector. Hydrides of Ge, As, and Sb are generated, !separated on a column of Chromosorb 102, and Introduced Unto an Inductively coupled plasma. The sequential slew scanning monochromator monitors each resolved chromatographic peak at a different atomic emission wavelength. The detection limlts are 0.004 pg (0.20 ppb) for Ge and 0.05 pg (2.5 ppb) for As and Sb. Relative standard devlations at the 1 pg (50 ppb) level range from 2 to 5%. Accuracy of the method Is demonstrated by analysis of EPA water quality catntrol reference standards.
Selective chromatographic detectors provide information on the chemical nature of the eluting compounds and generally increase the sensitivity of the method. Element-specific detection using atoimic absorption and fluorescence spectrometry has greatly simplified chromatographic separations of metal-containing species in complex samples (1-3). The development of the inductively coupled plasma (ICP), direct current plasma (DCP),and microwave-induced plasma (MIP) as stable spectroscopic sources has renewed interest in atomic emission spectrometry (AES) for element-selective detection in chromatography (4,6).These plasmas produce intense atomic emission from most elements of the periodic table; thus, plasma eniission spectrometry is a particularly versatile method of chromatographic detection. The multielemlent capabilities of plasma emission detection in gas and liquid chromatography have previously been dem0003-2700/82/0354-0165$0 1.25/0
onstrated. By use of a monochromator, sequential multielement detection was achieved by making successive injections and monitoring each injection at a different emission wavelength (6-9). Simultaneousmultielement detection of a single chromatographic injection has been achieved by use of a polychromator (10-1.2). Although polychromators can simultaneously monitor the atomic emission at 20-30 different wavelengths, the high cost and limited flexibility of these systems would prohibit their use for routine multielement chromatographic detection. Another approach to multielement plasma emission chromatographic detection may be the use of sequential slew scanning (SSS)monochromators(13-16). These instruments use computer-controlled, stepper motor operated gratings to rapidly slew between analytical lines and acquire data only at preselected wavelengths, thus providing rapid sequential analysis at atomic emission wavelengths. The sequential determination of trace element constituents in continuously nebulized solutions has been demonstrated for ICP/SSS monochromator systems (17,18). Since chromatographically separated analytes are sequentially presented to a detector, SSS monochromatorsmay be well suited for plasma emission chromatographic detection. Given a sufficient amount of time between chromatographic peaks, the SSS monochromator could monitor one wavelength as an analyte elutes and then slew to another line to monitor the next peak at a different wavelength. In this manner, multielement monitoring of a single chromatographic run could be achieved with a singlechannel spectrometric detector. A computer-controlled SSS monochromator has been developed in this laboratory for the analysis of continuously nebulized solutions by inductively coupled plasma atomic 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
CaClz (Baker Reagent Grade). A U-tube for condensing the hydrides in liquid nitrogen was constructed from Teflon tubing (4.5 mm i.d.) and filled to the extent of 25 cm with Teflon shavings. The chromatographic columns consisted of lengths of Polypenco Nylaflow pressure tubing (3 mm i.d.) which were packed with Chromosorb 102 and operated at ambient temperature. The solenoid valve (General Valve Corp., E. Hanover, NJ, Model 9-51-901)was interfaced to the computer to provide automatic control of the chromatographic gas flow.
ZENERA-:?
Figure 1. System diagram: V i , V3, and V4 are three-way valves; V2 is an on/off valve; and F1 and F2 are flow meters.
emission spectrometry (18).To demonstrate the feasibility of using this instrument as a plasma emission chromatographic detector, a sequential determination of chromatographically separated hydrides using the ICP/SSS system has been developed. The analytical performance of the ICP/SSS as a chromatographic detector has been evaluated and is reported here.
EXPERIMENTAL SECTION Reagents. The hydride generating reagent consisted of a 4% (w/v) solution of sodium borohydride (Aldrich Chemical Co., 98%) in aqueous 5 % (w/v) sodium hydroxide (MC/B Reagent Grade). This solution was filtered through medium-porosity fritted glass to improve its stability (19). The hydrides were generated from 20-mL aliquots of 7% (v/v) HCl (Baker Analyzed Reagent). The acid aliquots were spiked with 500 pL of multielement standards prepared by appropriate dilution of 1000 ppm Ge, As, Sn, and Sb stock solutions (Spex Industries, Metuchen, NJ). The EPA Quality Control References for Water Quality Testing were dduted per instructions and acidified to 7% HC1, and 20-mL aliquots were analyzed directly. System Description. A system diagram is shown in Figure 1. VI, V3, and V4 are three-way valves; V2 is an on/off valve; and F1 and F2 are flow meters. The ICP/SSS system,consisting of a radio frequency generator, matching network and torch enclosure (Plasma-Therm Model HFP-2500D, Kresson, NJ), 0.85-m double monochromator (Spex Industries, Metuchen, NJ), and Intel 8080 based microcomputer, has been previously described in detail (18). BASIC computer programs used to operate this instrument when analyzing continuously nebulized solutions have been modified to take chromatographic data. The functions of the chromatographic programs are explained in the System Operation section. The ICP was operated at 1000 W forward and 0-5 W of reflected power, with 13 L/min Ar cooling gas. The plasma observation height was 15 mm above the load coil. The entrance, intermediate, and exit slits of the double monochromatorwere 25,250, and 75 pm, respectively. Solutionscontainingthe elements of interest must be nebulized into the plasma to calibrate the wavelength of the SSS monochromator. To facilitate the changeover from liquid nebulization to gas chromatographic introduction into the ICP, an interface was constructed from a three-wayglass valve with Teflon stopcock and a spray chamber. Thus, gas or liquid introduction could be chosen by means of valve GLV. Generationand separation of the hydrides were performed with a system similar to that of Hahn et al. (20). The hydride generator was a semiautomated type first described by Fiorino and coworkers (21). The drying tube was packed with &mesh anhydrous
PROCEDURE Initialization and Wavelength Calibration. Upon initiating the computer program, the user enters the wavelength counter reading of the spectrometer and the number of analytical wavelengths to be monitored during the chromatographic run. The wavelengths and corresponding element symbols are then entered in order of chromatographic elution. The wavelengthsused in this work for Ge, As, Sn, and Sb were 3039.06, 2780.22, 3175.05, and 2877.92 A, respectively. Valve GLV is turned to liquid introduction and a solution containing all the elements of interest at a concentration of approximately 50 ppm is nebulized into the plasma. Once the wavelength calibration procedure is initiated, the SSS monochromator slews first to an argon reference line and then to each analytical wavelength in order of chromatographic elution. The computer acquires data over a 0.3-nm region about each analytical line. From the peak maximum in each region, the computer determines the number of motor steps to each analytical wavelength relative to the initial argon reference line. Upon completion of the wavelength calibration procedure, valve GLV is turned to gas introduction and the user may proceed with chromatographic analysis. Initialization of Chromatographic Data Acquisition Program. At this point, the user must select a data acquisition rate and acquisition time range for each wavelength to be monitored. A data acquisition rate is entered as the number (>0.25) of seconds per acquired point. An acquisition time range is established by entering starting and finishing times (in seconds). Careful selection of these two parameters can save computer memory, reduce computer time spent in data manipulation, and improve the appearance of the chromatogram. Slower data acquisition rates are chosen for wavelength regions where broad peaks are expected, thus reducing the number of points acquired and resulting in a peak which appears to be sharper. Acquisition time ranges are chosen so that the retention time of each chromatographic peak falls roughly halfway between the starting and finishing time in its wavelength region. If two peaks are sufficiently resolved, the end of the first acquisition time range and the beginning of the second may be chosen to leave a gap in the data acquisition. Thus, data of little analytical interest need not be acquired. When all the acquisition rates and time ranges have been entered, the SSS monochromator finds the reference argon line and slews to the first wavelength region. Once the monochromator is positioned at the first analytical wavelength, the acquisition of chromatographic data may be initiated by depressing the appropriate key on the keyboard. Hydride Generation. Initially, valve V1 is turned to helium flow because argon condenses in liquid nitrogen; valve V2 is on because the solenoid is normally off; valve V3 is turned to permit gas flow through the U-tube; and valve V4 is turned to vent. A 20-mL acid aliquot is introduced into a reaction tube, the aliquot is spiked with an appropriate amount of analyte, and the reaction tube is connected to the hydride generator. The U-tube is immersed in liquid nitrogen for 30 s, after which valve V3 is turned toward the hydride generator. Upon activation of the hydride generator, 15 mL of the sodium borohydride solution is delivered to the reaction tube over a 15-s interval. Following a 5-s delay, the delivery tube is cleared
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982 RET. T I I ~ E S
Ai
5,64?E+6
1,9JlE+5
5,943Et5
20731.7
167
AT. TIMES T(GE) = 119.5
I M3!1.06
a.
3175.E
27lbl.22 l!S
SN ET. TIES
M39,06 A-
GE
1,65@+8
2780,22 As 9,1UE+6
3175,05
2877,92 SB
SN
2.455E+7
1.443E+6
Figure 3. Chromatogram of hydrides generated from a solutlon con10 pg of Ge, As, Sn, and Sb.
taining
A 2,819Et6
3039,06
2780,22
GE
AS
3175.05 SN
Figure 2. Effect of data acqulsitlon rate and acqulsltlon tlme range
on chromatogram appearance. by the addition of 10 mL of water. The volatile hydrides are swept through the drying tube which removes any water vapor and are condensed in the U-tube. Upon completion of the generator reaction cycle, valve V3 is turned back to gas flow and the hydrogen reaction byproduct is vented to the atmosphere with a 20-s flow of helium at 500 mL/min. Then valve V1 is turned to argon flow and the helium is purged from the system for 10 s. Valve V4 is then turned tow,ardthe chromatographic column and valve V2 is turned off so that the chromatographicflow is controlled by the solenoid. Chromatographic Separation and Detection. The condensation tube is transferred to a hot water bath (75-80 OC) and the data acquisition program is initiated. The computer signals the opening of the solenoid valve to provide a chromatographic flow of 325 mL/min and the revolatilized hydrides are swept from the U-tube into the chromatographic column. The computer acquires data over the predetermined time range and tlhen the solenoid valve is closed and the SSS monochromator begins slewing to the next analytical wavelength. As the monochromator slews, the solenoid valve is opened for a time equal to the difference between the end of the previous acquisition time range and the beginning of the next range and in then closed. When the monochromator is correctly positioned at the next wavelength and the time interval between ranges is completed, the solenoid valve is opened and data are acquired over the next time range. Turning the chromatographicflow on and off ensures that no data are lost while the SSS monochromator slews to the next wavelength. This process is repeated until data have been acquired at all tlhe analytical wavelengths. The chromatographic data are then numerically smoothed and presented to the analyst via a video display. Peak areas are automatically integrated; however, the user may reposition the integration limits and have the areas recalculated. Hardcopy of the chromatogram may be obtained by means of a digital plotter. The computer program then gives the analyst a choice of performing a duplicate chromatographicanalysis, setting new data acquisition rates and time ranges, or returning to wavelength initialization and calibration.
RESULTS AND DISCUSSION Chromatograma for the separation of GeH,, AsH3,and SnH, on a 3.5-ft column of Chromosorb 102 are shown in Figure 2. The hydrides were generated from a solution containing 10 hg of each analyte. The analytical wavelengths (in ang-
I
1,441EtS
26972 1 I
RET, TlhES T(GE) = 52.5 T(AS) = 83,6 T(SB) = 345
Figure 4. Chromatogram of hydrides generated from a solution containing 0.5 pg (25 ppb) of Ge, As, and Sb.
stroms) and element symbols are given below each region. The data in each region are scaled to the maximum screen height; thus, peak maxima (in ADC counts) are given above each region for intensity comparison. The beginning and finishing times (in seconds) for each acquisition range are given in the upper right and left corners of each region. Figure 2A is a chromatogram obtained with continuous data acquisition from 1to 500 s at an acquisition rate of 1s/point. In the germanium region (3039.06 A), the initial change in the base line is caused by movement of the plasma when the chromatographic flow is turned on. This background shift is observed in each region when the solenoid valve opens. The two small peaks obtained at this wavelength are not GeH,; rather, the first is probably residual He, which causes an impedance mismatch, and the second is most likely COz,another byproduct of the hydride reaction. Figure 2B is the chromatogram of an identical run, only the acquisition time ranges are not continuous, and the data acquisition rates for the Ge, As, and Sn regions are 0.5, 1, and 2 s/point, respectively. With these conditions, only the germanium peak is observed in the first region, and the width of the tin peak appears to be reduced. This demonstrates how data of little interest are eliminated and the appearance of chromatograms is improved by careful selection of data acquisition rates and acquisition time ranges. The separation of the hydrides of Ge, As, Sn, and Sb on the same 3.5-ft column of Chromosorb 102 is shown in Figure 3. It is worthy to note that had a splitter been used as in earlier work, Se could have been included within the same time frame (20). Again,the hydrides were generated from a solution containing 10 pg of each analyte. Since this separation required 20 min, a separation of GeH4,ASH, and SbH3was used to characterize the analytical performance of the ICP/SSS system as a chromatographic detector. These three hydrides can be resolved on a 2.5-ft column of Chromosorb 102 in less than 10 min. A chromatogram of GeH,, AsH3, and SbH3, generated from a solution containing 0.5 pg of each analyte is shown in Figure 4.
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
CALIBRATION CURVES (GE;AS;SB)
18
Table I. Analysis of Water Quality Reference Standards
sample element 1
2 a
-
4
-3
-2
-1 0 1 LOG (MICROW)
2
Figure 5. Calibration curves resulting from triplicate determinations. Calibration curves for Ge, As, and Sb are presented in Figure 5. Each point represents the average result of triplicate determination. The calibration curves are linear for 2 to 3 orders of magnitude with correlation coefficients of 0.9994, 0.9996, and 0.9993 for the Ge, As and Sb lines, respectively. The detection limits (defined as the average blank value +3a) are 0.004 pg (0.20 ppb in 20 mL) for Ge, and 0.05 g (2.5 ppb) for both As and Sb. These results are reasonable considering that band broadening within the chromatographic column is very pronounced at ambient temperature. Detection limits might also be improved by using more sensitive atomic emission wavelengths which are located lower in the UV region (i.e., As at 1936.96 A). Unfortunately, the response of the SSS monochromator system below 2200 A is degraded primarily because the gratings are blazed at 3000 A. However, SSS monochromators are flexible in terms of wavelength selection; thus, working at alternate wavelengths causes no problems for analyst. The precision of this technique, in part, depends upon the ability of the SSS monochromator to reproducibly slew to each analytical wavelength. As motor steps may be lost when the grating direction is changed, it proved necessary to use an exit slit width (75 pm) which was 3 times larger than the entrance slit width (25 pm). Although this procedure somewhat degrades the resolution and signal-to-noiseratio, these slit widths provided a spectral window which the SSS monochromator could reproducibly hit. Relative standard deviation for 10 determinations at the 1-pg (50 ppb) level were 2.3% for Ge, 5.0% for As, and 4.6% for Sb. As the condensationfrevolatilization procedure and the chromatography also contribute to the imprecision of the method, these relative standard deviation values indicate that the SSS monochromator slews to each wavelength with sufficient accuracy. The accuracy of the method was evaluated by analyzing EPA Quality Control References for Water Quality Testing. These samples had to be spiked with Ge in our laboratory because the levels were not monitored in the references. As seen from the results in Table I, agreement between the analytical results and the reference values is good.
Ge As Gc Sb
amt of element, ppb ref value this work 50.0
48.7
22.0
28.83 f 0.01 93 j: 2 92.3 * 0.9
100.0
97.5
i:
0.6a
Average of duplicate determinates.
CONCLUSION The SSS monochromator has been shown to be a viable multielement emission detector for chromatographic monitoring of hydrides. The detection limits and analysis time reported here could be improved with better chromatographic conditions. As a method for determining hydride-forming elements, this approach is limited by the fact that hydrides of Se, Te, Bi, and Pb are not quantitatively transferred through columns of Chromosorb 102 (although splitting techniques could be used at least for Se). However, the flexibility of the SSS monochromator in terms of wavelength selection provides the analyst with any number of different element-specificmodes of detection during a single chromatographic run. Thus, the SSS monochromator should find use in a variety of chromatographic applications which require multielement detection. ACKNOWLEDGMENT The authors thank the Environmental Protection Agency for providing the water quality control samples. LITERATURE CITED Fernandez, F. J. At. Absorpt. Newsl. 1977, 16, 33-36. van Loon, J. C. Anal. Chem. 1979, 51, 1139 A-1150 A. van Loon, J. C.; Radzlnk, 8.; Kahn, N.; Lichwa, L.; Fernandez, F. J.; Kerber, J. D. At. Absorpt. Newsl. 1977, 16, 79-83. Krull, I . S.;Jordan, S. J. Am. Lab. (Fairfield, Conn.) 1980, 21-33. Carnahan, J. W.; Mulllgan, K. J.; Caruso, J. A. Anal. Chlm. Acta 1981, 130,227-241. Beenakker, C. I . M. Spectrochim. Acta, Part B 1977, 32, 173-187. Uden, P. C.; Barnes, R. M.; DiSanzo, F. Anal. Chem. 1978, 50, 852-855. Lloyd, R. J.; Barnes, R. M.; Uden, P. C.; Elliot, W. G. Anal. Chem. 1978, 50, 2025-2029. Quimby, B. D.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1978, 50, 21 12-21 18. Windsor, D. L.; Denton, M. B. Appl. Spectrosc. 1978, 32, 366-371. Morita, M.; Uehlro, T.; Fuwa, K. Anal. Chem. 1980, 52, 349-351. Hausler, D. W.; Taylor, L. T. Anal. Chem. 1981, 53, 1223-1227. Spillman, R. W.; Malmstadt, H. V. Anal. Chem. 1976, 48, 303-311. Johnson, D. J.; Planky, F. W.; Wlndfordner, J. D. Anal. Chem. 1975, 4 7 , 1739-1743. Boumans, P. W. J. M.; Van 0001,G. H.; Jansen, J. A. J. Analyst (London) 1976, 101, 585-507. Edlger, R. D.; Wllson, D. L. At. Absorpt. Newsl. 1979, 18, 41-45. Floyd, M. A.; Fassel, V. A,; Wlnge, R. K.; Katzenberger, J. M.; D'Sllva, A. P. Anal. Chem. 1980, 52,431-438. McCarthy, J. P.; Jackson, M. E.; Rldgway, T. H.; Caruso, J. A. Anal. Chem. 1981, 53, 1512-1516. Fraser, J. L.; Knechtel, J. R. Analyst (London) 1978, 103, 104-106. Hahn, M. H.; Mulllgan, K. J.; Jackson, M. E.; Caruso, J. A. Anal. Chim. Acta 1980, 118, 115-122. Fiorino, J. A,; Jones, J. W.; Capar, S. 0. Anal. Chem. 1978, 5 8 , 120-123.
RECEIVED for review June 22, 1981. Accepted October 22, 1981. This research was partially supported by the National Institute of Occupational Safety and Health through Grant OH-00739.
