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(23) k y a l , s. S.;Rains, D. W.; Huffaker, R. C. Anal. Chem. 1988, 6 0 , 175- 179. (24) Genfa. 2 . ; Dasgupta, P. Anal. Chem. 1989,67,408-412. (25) Willason, S. W.; Johnson, K. S. Mar. Siol. 1986,97,285-290. (26) Gardner, W. S.; Miller, W. H., 111. Can. J . Fish. Aquat. Sci. 1981, 38, 157-162. (27) Gardner, W. S.;Vanderploeg, H. A. Anal. Chem. 1982, 5 4 , 2129-2130. (28) Imai, K.; Toyo'oka, T.; Miyano, H. Analyst 1984, 709, 1365-1373. (29) Moore, S.:Stein, W. J. J . Bid. Chem. 1951, 192, 663-681. (30) Gupta, A. R.; Sarpal, S . K. J . fhys. Chem. 1967, 71, 500-506. (31) Gardner, W. S.; Chandler, J. F.; Laird, G. A.: Scavia, D. J . GreatLakes Res.1986,12,161-174.
Eliason, A. H.; Walden, 6.; Rowe. G. T.; Teal, J. M. Limnol. Oceanogr. 1976,27, 164-170. Gardner, W. S.; Fitzgerald, S. Unpublished results, Great Lakes Environmental Research Laboratory, 1990.
RECEIVED for review September 25,1990. Accepted December 18, 1990. This research was partially supported by the Nutrient Enhanced Coastal Ocean Productivity Component of the NOAA Coastal Ocean Program. This paper is GLERL Contribution No. 131.
A Completely Modulated Sample Introduction System for Atomic Emission Spectrometry Ruth E. Wolf,*?'Rodney K. Skogerboe, and Jeffrey J. RosentreterZ
Chemistry Department, Colorado State University, Fort Collins, Colorado 80523 INTRODUCTION In conventional atomic emission spectrometry (AES), the optical emission from the flame is converted to an ac signal by a chopper placed between the flame and the detector. While this arrangement reduces noise originating from the detection and amplification electronics, noise components originating in the flame are also chopped and detected along with the analytical information of interest. In addition, AES also suffers from spectral interferences caused by the overlap of molecular bands and atomic lines of species present in the sample matrix or in the flame itself with the analytical line of interest. These types of problems have limited the use of flame AES, particularly in cases where the analytical line of interest lies in a region of high source background. One approach that has been used to reduce these types of interferences is to convert only the actual analytical information of interest to an ac signal, while keeping the flame background emission at a dc level. In AES this may be done by periodically introducing the anal* into the flame by using sample modulation. This results in an ac analytical signal, which can then be distinguished from the dc-background signal by using synchronous demodulation. The use of sample modulation to reduce flame emission background and improve detection limits has been investigated for nearly 30 years (1-9). The previous sample modulation systems have had limited success. Systems which used a single nebulizer and a total consumption burner invariably resulted in modulating the flame background, as well as the analyte emission (2,5). This modulation was caused by the alternate introduction of air and aqueous aerosol pulses to the flame, which caused changes in the flame temperature. The use of a premixed flame and spray chamber to minimize flame temperature fluctuations was examined by Steele and Hieftje (8). Unfortunately, the use of a single nebulizer resulted in modulation of the flame background. As a result, interfering OH band emission could be reduced, but not entirely eliminated. In addition, detection limits were improved only by a factor of 2, which was attributed to a reduction in the detected flame flicker noise. Other systems used dual nebulizers in conjunction with total consumption burners (3, 6). Dual nebulizers were used to *Author to whom correspondence should be addressed. Present address: Environmental Chemistry Unit, EG&G Idaho, Inc., P. 0. Box 1625, Idaho Falls, ID 83415-4123. Present address: Chemistry Department, Idaho State University, Pocatello, ID 83209.
maintain a constant supply of aerosol to the flame; thus, avoiding modulation of the flame background emission. Unfortunately, these systems were undesirably complicated and difficult to use. In addition, these systems required the use of total consumption burners, which exhibit performance poorer than that of premixed flames. The sample modulation system designed for this work uses a conventional premixed air-acetylene burner and spray chamber. A dual Babington nebulizer assembly is easily fitted onto the spray chamber and requires only minimal adjustments. This system is capable of improving detection limits by factors as high as 15 and virtually eliminating flame background emissions.
EXPERIMENTAL SECTION Figure 1 is a block diagram of the sample modulation system developed for this study. A standard Varian-Techtron premixed air-acetylene burner (Model AB-51) and spray chamber assembly is used. A new spray chamber end cap was machined from PFTE (Teflon). Two holes 7.93 mm in diameter were drilled 10 mm from either side of the center point of the cap, angling inward by 5 O , to accommodate two 6.35-mm stainless steel Swagelok bulkhead union fittings. Two identical Babington nebulizers, similar to the one described by Suddendorf and Boyer (IO),were constructed as shown in Figure 2. The nebulizers were machined out of pieces of solid stainless steel rod 1.11cm in diameter and 1.588cm in length. One end of each piece was drilled and tapped with an 8 mm x 1.25 mm bottoming tap to a depth of 7.93 mm. The opposite end was milled to an angle of 30°, and a V-shaped groove 2 mm wide and 1 mm deep was machined down the length of the nebulizer face. A hole 0.04 cm in diameter was drilled through the center of the groove completely through to the threaded end. The nebulizers were screwed onto the bulkhead union fittings inserted through the spray chamber end cap. The other ends of the fittings were connected to the nebulizer air supply tubes coming from a three-way solenoid valve (Angar Scientific, No. AL-025312). The purpose of the solenoid valve is to switch the nebulizer air supply between the two nebulizers. The switching frequency of the solenoid valve is controlled by a function generator (Heath, Model EU-BlA),which also supplies a reference signal to the lock-in amplifier, and can be varied from 0.1 to 100 Hz. Sample introduction is achieved through 20-gauge stainless steel needle tubing fitted through a small hole drilled in the spray chamber end cap above each nebulizer. The tubing is bent so the tip fits in the top of the V-groove,and solution flows evenly down the length of the groove. Sample solutions are pumped at flow rates ranging from 3 to 6 mL/min by using a multichannel peristaltic pump (Buchler Multistatic,Model 2-6200). The peristaltic pump delivers solution to both nebulizers simultaneously, while the solenoid valve
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Figure 1. Block diagram of sample modulation system.
sample in
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Flgure 2. Design of dual Babington nebulizer assembly.
switches the air supply between the two nebulizers at the desired frequency, allowing only one nebulizer to produce aerosol at a time. Sample modulation is achieved by running the analyte solution through one nebulizer and a blank solution through the other, so that the flame alternately receives blank and sample aerosols at the switching or modulation frequency. To assure that each nebulizer delivers the same amount of aerosol to the flame, a standard solution of the desired element is pumped into each nebulizer separately and the flow rates of the corresponding peristaltic pump channels are adjusted to obtain the same size signal from each nebulizer, to within i l % . The typical time required for such an adjustment is approximately 5 min. The flow rates of the flame gases are maintained between 2.0 and 2.75 L/min of acetylene and 8.0 and 10.5 L/min of air, depending on the flame conditions required for the element analyzed. Nebulizer air is supplied by a separate air tank and is maintained at a flow rate of 12 L/min. A simple optical and detection system is used, consisting of a quartz lens (25-mm diameter, 110-mm focal length) to focus the flame onto the entrance slit (12 mm X 100 pm) of a 0.35-m monochromator (Heath, Model EU-700). The signals are detected by a photomultiplier tube (PMT, Model 1P28A) operated at 600 V by a high-voltage power supply (Heath, Model EU-701-30) and sent to a lock-in amplifier (Ithaco, Model 353). The output of the lock-in amplifier is sent to either a strip-chart recorder (Houston Instrument, Model B5117-5AE) or a storage oscilloscope (Tektronix,Model 5403). To obtain data for comparison purposes, an optical chopper rotating at 20 Hz is placed between the flame and the monochromator allowing the same detection system to be used. When the optical chopping mode of operation is employed, the solenoid valve switching circuit is turned off. This allows nebulizer air to only pass through the normally open channel of the valve, supplying air to only one nebulizer, which operates continuously. All sample solutions used are prepared by dilution of appropriate stock solutions prepared according to standard procedures using reagent grade chemicals (11).
RESULTS AND DISCUSSION System Performance. Preliminary studies indicated that a 100% depth of modulation is achieved by using modulation frequencies between 0.1 and 1.3 Hz. At frequencies above 1.3 Hz, the depth of modulation rapidly decreases, until only a 10% depth of modulation is observed at 4 Hz. Further ex-
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Figure 3. (A) Spectral scan obtained by using optical chopping of OH band emission interference on Mg (500 mg/L) and Cu (500 mg/L). (B) Spectral scan of same region as in A, using sample modulation.
periments showed that this decrease in the depth of modulation is due to the mixing of the blank and analyte aerosols in the spray chamber at higher modulation frequencies. Th6idata also indicate that the mean spray chamber residence time of analyte aerosol is very near 1 s. The residence time thus determines the degree of mixing at any one modulation frequency. Signal to noise (SIN)ratio studies also indicate that the optimum modulation frequency for this system, where the S I N ratio is at a maximum, is 1 Hz. As a result of these experiments, all subsequent investigations were carried out at a modulation frequency of 1 Hz. Furthermore, the temporal behavior of the flame background with changes in the fuel/ oxidant ratios, described by Steele and Hieftje (8), was avoided with the dual-nebulizer system. Elimination of Flame Background Emission. The ability of sample modulation to eliminate flame background emission from species such as OH, CH, and C2is demonstrated in Figures 3-6. Figure 3A shows a typical spectrum of the OH band and continuum emjssion in an air-acetylene flame from 280 to 340 nm, when optical chopping is used. The OH background emission in this region is quite intense and interferes with the analytical lines of several elements, including Cu and Mg, as is illustrated by Figue 3A. Even at high concentrations of Cu and Mg (500 mg/L), the analytical lines are barely distinguishable above the intense OH emission from 281 to 345 nm. Spectrum B in Figure 3 shows the same region as spectrum A when sample modulation is used instead of optical chopping. All other instrumental parameters, including amplifier gain, are the same as in Figure 3A. These spectra demonstrate the ability of sample modulation to virtually eliminate the OH background emission and enhance the
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 5, MARCH 1, 1991 ca CH
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signal-to-background ratios for Cu and Mg. The spectra in Figure 4 illustrate the ability of the sample modulation system to allow analytical lines to be identified with ease. The Cu line in spectrum A, from a 20 mg/L Cu solution, is nearly obscured by the OH bands from 320 to 330 nm, whereas, in spectrum B, the two Cu lines at 324.7 and 327.4 nm are the
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Figure 6. (A) Spectral scan from 410 to 435 nm of 20 mg/L Ca solution using optical chopping. (6)Spectral scan from 410 to 435 nm of 20 mg/L Ca solution using sample modulation.
most prominent features in the spectrum and are easily identified. Figure 5A shows the swan (Cz)bands observed in a fuel-rich flame between 540 and 570 nm when optical chopping is employed. The Cz bandhead at 554.0 nm nearly obscures the Ba line from a 100 mg/L Ba solution at 553.6 nm. Figure 5B shows that sample modulation can also be used to eliminate the flame background emission due to the presence of C2 in the flame. The Ba line in spectrum B is easily distinguished from the background, since the interfering swan band emissions have been removed. Figure 6A shows the CH emission commonly observed in flames between 420 and 435 nm when optical chopping is employed. The CH background can be especially intense in flames when organic solvents are used and may interfere with the determination of several elements, including Ca (422.7 nm), as is illustrated by spectrum A. The CH emission is completely eliminated (Figure 6B) by using the sample modulation technique. This may be especially useful in improving detection limits when organic solvents or matrices are employed. Improvements in Detection Limits. Figures 3-6 clearly illustrate that background emissions due to species such as OH, Cz, and CH present in the flame can be eliminated through the use of sample modulation. This reduction in the overall flame background signal leads to an increase in the signal-to-background ratio, which should result in improvements in detection limits. Figure 7 shows that the use of sample modulation results in an increase in the SIN ratios for Ca at 422.7 nm, as compared to conventional optical chopping. The results obtained for Ca are similar to those for the other five elements tested. The detection limits obtained by using both the optical chopping and sample modulation methods, for all six elements examined, are listed in Table I. Overall, the use of sample modulation results in improvements in detection limits by factors between 2 and 15, depending on the element. For elements with lines in regions of high source background, such as Ba, Ca, Cu, and Mg, the improved detection limits are mainly a result of the elimination of detected background emission due to molecular species formed in the flame, such as OH, Cz, and CH. The elements for which minimal improvements were observed, Fe and Cd, do not suffer from any appreciable interferences due to flame background emission at the wavelengths used. In these cases, the improvement is due to a reduction in the detected flame flicker noise, as noted in the results reported by Steele and Hieftje (8).
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Table I. Comparison of Detection Limits Using Sample Modulation and Optical Chopping for Ba, Ca, Cd, Cu, Fe,and Mg analytical line, nm
element
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Elimination of Interfering Concomitant Element Emission. Sample modulation can also be used to eliminate interfering background emission due to species introduced into the flame with the analyte. Figure 8A shows the spectrum obtained while a solution containing 100 mg/L of both Ba and Ca is modulated with distilled water. The resulting emission from the CaOH formed in the flame cannot be removed, because the Ca is introduced with the analyte as a concomitant. This CaOH emission completely obscures the Ba line at 553.6 nm. However, when the solution containing both Ba and Ca is modulated with a blank solution containiig the same amount of Ca, the CaOH emission is completely eliminated, as is illustrated in Figure 8B. By matching the Ca concentrations in the analyte and blank solutions, the concentration of Ca in the flame remains constant as the flame alternately receives blank and sample solutions. As a result, the CaOH emission is not modulated and is, therefore, not detected at the modulation frequency with the Ba signal. Although this method requires that the concentration of Ca in the sample solution be known or can be measured, this can easily be determined by atomic emission at 422.7 nm, and a suitable blank can be prepared. Comparison of the Ba peak height in Figure 8B with a spectrum obtained from a 100 mg/L Ba solution modulated with distilled water (Figure 5B) shows that the background correction for the CaOH emission using sample modulation is quantitative. CONCLUSIONS The use of sample modulation to correct for flame background in AES has several advantages over the alternate technique of wavelength modulation (12-15). The technique of wavelength modulation consists of scanning over a small spectral region encompassing the analytical line of interest
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Figure 8. (A) Spectrum of 100 mg/L Ba and Ca modulated with distilled water. (B) Spectrum of 100 mg/L Ba and Ca modulated with 100 mg/L Ca blank.
repetitively, at a desired frequency. When the wavelength of the monochromator is modulated over a spectral interval where the light intensity is increasing (or decreasing), an ac signal will be generated at the modulation frequency. If a lock-in amplifier is tuned to 2 times the modulation frequency, only changes which occur at this frequency will be detected. Thus, a constant or broadly sloping background will not be detected. However, any background emissions which result in both an increase and decrease in the light intensity over the modulated region will be detected. This limits the ability of wavelength modulation to correct for background emissions that appear as sharp l i e s in close proximity to the analytical line, especially if more than one interfering line is involved. A second disadvantage is that the band-pass of the instrument is effectively doubled during wavelength modulation, degrading the resolution of the monochromator (13). In addition, during wavelength modulation, the spectrometer wavelength setting is changing continuously and is, therefore, at the optimum position to measure the line or background intensity only a small fraction of the time. The net result is a loss in
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average signal input to the amplifier, which leads to a deterioration in the S I N ratio. This deterioration in the Sf N ratio explains the lack of improvements in the detection limits reported by Koirtyhann et al. (14). The sample modulation system described here is capable of eliminating all types of flame background emissions, including complex line and band structures caused by flame, solvent, and concomitant species. Interferences caused by direct spectral overlap can also be removed; this is not possible when wavelength modulation is used. In addition, when sample modulation is used, light losses, such as those seen in wavelength modulation, are avoided, since the analytical wavelength is interrogated at all times. Finally, the resolution of the monochromator in sample modulation is not affected by the modulation procedure, as it is in wavelength modulation. Future work with the sample modulation system will include further investigations into its usefulness in flame spectrometry, including using its aerosol mixing capabilities to study specific radiation interference effects. The present work has been restricted to flame AES. However, this apill be just as effective for background correction when proach w other techniques, such as inductively coupled plasma atomic emission spectrometry (ICP-AES), are used.
LITERATURE CITED (1) Herrmann, R.; Lang, W.; Riidiger, K. Z . Anal. Chem. 1984, 206, 24 1-246. (2) Riidiger, K.; Gutsche, B.; Kirchhof, H.; Herrmann, R. Ana/yst 1989, 9 4 , 204-208. (3) AntiEJovanoviE, A.: BojoviE, V.: MarinkoviE, M. Spectrochim. Acta 1970, 2 5 8 , 405-410. (4) MarinkoviE, Momir; Vickers, T. J. Anal. Chem. 1970, 4 2 , 1613-1618. (5) Mossotti, Victor G.; Abercrombie, Frank N. Appl. Spectrosc. 1971, 2 5 , 331-341. (6) BojoviE, V.; Antie-JovanoviE, A. Spectrocbim. Acta 1972, 2 7 8 , 385-390. (7) Herrmann, Roland; Alkemade, C. T. J. Chemical Analysis by Flame Photometry; Interscience: New York, 1963; pp 13-36. (6) Steeie, A. W.; Hieftje, G. M. Appl. Spectrosc. 1886, 4 0 , 357-363. (9) Steele, A. W.; Hieftje, G. M. Appl. Spectrosc. 1988, 4 0 , 1127-1131. (10) Suddendorf, Ronald F.; Boyer, Kenneth W. Anal. Chem. 1978, 5 0 , 1769-1 771, (11) Dean, J. A.; Rains, T. C. Flame Emission and Atomic Absorption Spectrometry; M. Decker: New York, 1969; Vol. 11, Chapter 13. (12) Snellman, W.; Rains, T. C.; Yee, K. W.; Cook, H. D.; Menis, 0. Anal. Chem. 1970, 42, 394-398. (13) Epstein, M. S.;O'Haver, T. C. Spectrochim. Acta 1975, 308, 135-1 46. (14) Koirtyhann, S. R.; Glass, E. D.; Yates, D. A.; Hinderberger, E. J.; Lichte, F. E. Anal. Chem. 1977, 49, 1121-1126. (15) Bezur, L.; Marshall, J.; O'Haway, J. M. Spectrochim. Acta 1984, 398, 787-605.
RECEIVED for review August 10, 1990. Revised manuscript received December 17,1990. Accepted December 20, 1990.
