Application of Interferometry to Simultaneous Multielement Atomic Emission Spectrometry Richard Pruiksma, James Ziemer, and Edward S. Yeung* Ames Laboratory-ERDA and Department of Chemistry, lowa State University, Ames, lowa 500 10
THEORY
free spectral range (FSR) is the separation between wavelengths transmitted simultaneously by the Fabry-Perot, and is simply given by FSR = X2/2L. The resolution of the interferometer, is determined by the finesse, F , such that A/AA = 2LF/X. The finesse is a function of surface reflectivity, parallelism, flatness, diffraction, and collimation of the beam, and is particular to the optical arrangement used. In our analytical scheme, we make use of the fact that most common elements have a limited number of emission lines within a certain wavelength region, say the visible spectral range of 400-700 nm. If one now uses a FabryPerot interferometer to look a t this entire wavelength region, then only emission lines nlhl = 2L, n 2 X 2 = 2L, . . . , etc. will be transmitted simultaneously, where A1 and A2 can be from the same or different elements. I t is in the latter case that spectral interference prohibits simultaneous determination. However, it can be seen that since A 1 and X2 are usually not whole numbers, changes of any two lines simultaneously transmitted for a given L are very small indeed. Thus, for multielement analysis, as long as the total number of emission lines from all elements in that spectral region is not too large, spectral interference will not be a problem. The exact number of allowable lines is ultimately determined by the finesse of the interferometer. Obviously, it is virtually impossible to set a Fabry-Perot interferometer to a particular L to transmit light of wavelength X with a particular n. However, this is not necessary in practice. By using a scanning interferometer such that the separation can be varied from L to L AL, where AL is equal to or larger than the longest wavelength of interest, every emission line can be covered during the scan. The shorter wavelengths may be repeated during the scan, and must be counted more than once towards the total number of lines. In such a scan, unless for some L nlX1/2 = n*X2/2, the various emission lines will appear a t a different point. For incident light not normal to the parallel optical plates, the transmission properties will change such that maxima will occur a t L = nX cos 0/2, where 0 is the angle from normal incidence. This implies that the incoming light must be well collimated to obtain the best resolution and thus the best selectivity. Collimation in general decreases the optical throughput of the system, in turn lowering the sensitivity. Fortunately, the optical throughput (6) of the Fabry-Perot interferometer is typically better than ordinary spectrometers, so that this is not a serious problem. In any case, this means that the optical system will have different configurations depending on whether maximum selectivity or maximum sensitivity is desired. For operation in various different regions of the spectrum, it may be necessary to change the Fabry-Perot plates.
The theory of Fabry-Perot interferometry has been reviewed recently (6),and will not be discussed in detail here. Briefly, when two flat and parallel optical plates separated by a distance L are illuminated by monochromatic light of wavelength X a t normal incidence, then transmission is a maximum whenever L = nX/2, where n is an integer. The
The heart of the optical system is a commercial scanning FabryPerot interferometer. A Trope1 CL-100 system is used throughout. The cavity spacing of the etalon is adjustable from 0.005 to 1.0 cm, and can be aligned mechanically via micrometers and electronically via piezoelectric crystals. Scanning is performed with a piezo-
A technique for simultaneous multielement analysis based on a scanning Fabry-Perot interferometer and atomic emission is described. Detection limits are determined for various common elements using the oxygen-hydrogen flame as a test case. The results show that semitrace to trace levels in solution can be analyzed. This technique is limited to groups of elements which contain relatively few total emission lines in a certain wavelength region. Spectral interference can, in general, be eliminated by the proper choice of the interferometer spacing. Analysis of tap water and parameters essential to this optical scheme are discussed. The main advantages of this technique are the high luminosity and the ability to perform analysis in the millisecond time scale.
Atomic emission spectrometry has long been established as a versatile multielement analytical tool. There are two main approaches to simhltaneous analyses, namely photographic and direct-reading. In the first approach, calibration for quantitative work is generally difficult and the useful working range is limited. In the second approach, the complexity, and thus cost, of the detection system can increase rapidly as more channels are monitored. In both cases, a high resolution spectrometer is needed to avoid spectral interference. The high resolution available in Fabry-Perot interferometers makes them very suitable for eliminating spectral interferences. Analytical applications of this technique have been in the measurement of atomic line profiles (1-41, and in combination with a continuum source for atomic absorption spectrometry ( 5 ) . However, the free spectral range of interferometers is usually small to achieve high resolution, so that overlapping orders cannot be avoided. This somewhat defeats the usefulness in eliminating spectral interference, unless a medium resolution spectrometer is used in conjunction with the interferometer. Such an optical arrangement provides little improvement over systems based on a single high resolution spectrometer, except in rare cases where ultimate resolution is necessary. In what follows, we shall present a novel scheme for simultaneous multielement atomic .emission analysis based on a scanning Fabry-Perot interferometer. I t is unique in that no spectrometers are used in conjunction with the interferometer. The concept is tested using an oxygen-hydrogen flame for a number of elements. The results show that detection in the semitrace to trace levels are feasible. Factors essential to the proper design of the system are also discussed.
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Table I.
FT
I
F7k -;t
A’
Ftj
Ll
1
,
AMP
Detection Limits”
Element
Wavelength, A
Relative detection limit, ppm solution
Na Ca Ba Mn Sr Cr Rb Li
5890 4.227 5536 4031 4607 4254 4202 6708
0.02c 2 8000 4000 20 120 200 400
Absolute detection limitb, ng
0.004 0.4 1400 700 4 20 34 70
a Defined for SIN = 2. This results from imaging about lho of the flame. If the light collection is more efficient, the detection limits will improve. Lower limit determined by residual concentration of sodium in deionized water. Actual detection limit is better than indicated.
SCOPE H.V.
Flgure 1. Interferometer for multielement analysis M. parabolic mirror; F, flame; T, telescope; A l , A2. apertures, 1- and 0.5mm, respectively: L1, L2, L3, collimating lenses; FP, Fabry-Perot interferometer; P, photomultiplier: H.V., power supply: AMP, sawtooth amplifier: C,
electronic alignment control electric drive, with a movement of 0.75 pm per 300 V. The cavity mirrors are coated for operation in the visible, with a reflectivity finesse of 50 or better throughout the range. A schematic of the experimental arrangement is shown in Figure 1. A telescope (20X) and a parabolic mirror collect light from a small portion of the flame and image it onto a diaphragm. A set of condensing lenses further images the light onto a second diaphragm. A third lens then collimates the light to pass into the interferometer. Since the flame is not a point source, it is necessary to use such an imaging system to produce reasonably parallel light Adjustment of the lenses and apertures is quite critical to achieve good resolution but, once adjusted, the system can be used repeatedly without change. Alignment of the Fabry-Perot interferometer is done with a high pressure sodium lamp in place of the flame. Detection is accomplished with an Amperex 56AVP photomultiplier and traced on a Tektronix 7904 oscilloscope. The sawtooth output from the oscilloscope is amplified to provide a 0-150 V ramp and is connected to the scanning crystal of the interferometer. To smooth out the output signal, an integrating capacitor is used, but the time constant is chosen so that the spectral scan is not affected. All measurements were made with a Beckman diffusion burner and a 02-Hz flame. Although it is not expected that this atom source can fully benefit from the Fabry-Perot optical system (vide infra), it is chosen to evaluate the concept because of its well-established behavior, and because of the relative ease in estimating atomic concentrations in the emission zone. Typically, ?ha of the entire flame is imaged on the first diaphragm. Solutions were made with deionized water and reagent grade salts. Blanks are always run as checks. Lines belonging to individual elements are identified by using a tunable narrow-band interference filter (lo-A pass, Oriel No. 7155) in the optical path.
RESULTS AND DISCUSSION Detection Limits. Before multielement data Can be properly interpreted, results on individual elements must be available. In Table I, we have listed the observed detection limits (as determined by the point where SIN = 2) for 8 common elements, together with the emission line used. of that in Table I still proIn general, a concentration duces a distinguishable signal if the signal is averaged by using the multiple sweep capability of the oscilloscope. This confirms that detection in the semitrace-to-trace levels is possible. Detection in most cases is limited by flame noise. The turbulent nature of the Beckman burner causes temporal variations in the volume consumption rate; hence, the signal shows fluctuations over the scan. Elements known to exhibit molecular bands (e.g., BaO in the case of barium) have poor detection limits. The multiline molecu668
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lar emission raises the background “noise” level and limits the usefulness of the atomic line(s). In such cases, signal averaging and/or the addition of suitable cut-off filters may be necessary. It is not possible to use molecular bands for analysis in this scheme. The tabulated detection limits must be viewed as specific to the optical arrangement in Figure 1, which was designed to test the concept of multielement analysis, rather than optimized for maximum sensitivity. Because of the large spatial spread of the flame, only a small portion of it can be collected and imaged onto the aperture A l . Furthermore, the continuous nature of atomic flames cannot be advantageously coupled to the relatively short sampling times of the Fabry-Perot. The conventional Beckman burner is thus a poor atomization source for this technique in actual analysis. However, it is perfectly adequate for the evaluation of the experimental scheme in this study. In any case, one can estimate the absolute sensitivity of the technique. The solution flow rate into the flame was found to be about 1.0 ml/min. Typical scan rates allow recording of a peak in 10 ms. The last column in Table I shows such estimated absolute detection limits. Such detection limits can most likely be achieved in pulsed atom sources. We have also obtained calibration curves for sodium using the 5890-A line. Rb was used as an ionization buffer, and an interference filter was used to eliminate Rb emission. Plots of the signal vs. concentration were linear for the range 0.1 to 10 ppm, within experimental error. Further, either the peak height or the peak area can be used without significant difference. In general, the use of peak height is preferred because of the lower probability of spectral interference. Multielement Analysis. Once the system is aligned for operation, it is ready to accept multicomponent mixtures. Initial alignment is best done with sodium emission, since it is most intense and since the doublet structure can easily be recognized and used to measure the degree of optimization. Figure 2 shows a signal trace of a solution mixture of sodium and strontium. The oscilloscope photograph is a combination of 3 traces, showing excellent trace-to-trace repeatability of the scanning technique. The small notch on the rightmost sodium peak, as well as fluctuations along the scan, are all due to unsteady solution flow into the flame. In principle, these can be eliminated with a better atomization source. Obviously, the initial alignment never guarantees suitability for multielement analysis because there is still a finite chance for spectral overlap at the initial setting of the separation L . The chances for spectral interference increase as more total lines are involved in the spectral range. When molecular lines are significant for a certain element, it will mask out signals due to other elements. Molecular
Figure 2. Superposition of 3 signal traces with 8 ppm Na and 4 ppt
Figure 3. Single signal trace with 8 ppm Na and 4 ppt Sr.
Sr
From left to right, Na (5890 A), Sr (4607A), Na (5896 A)
From left to right, Sr (4607 A), Na (5890 A), Na (5896 A). Noise is primarily from molecular emission
lines can sometimes be eliminated by suitable choice of cut-off filters in the optical path to limit the spectral region of detection without much sacrifice in sensitivity or ease of operation. The same applies to mixtures involving too many elemental lines to be resolved. In Figure 2, one can also see that the Sr line and the Na (5890-h;) line are slightly overlapping. At the relative concentrations shown on the photograph, quantitative measurements using the peak heights are not significantly affected. However, if the relative concentrations of the two elements are very much different from this, the smaller signal peak will tend to be masked by the larger peak, thus affecting analysis. In applications where atomic emission from matrix elements are important, suitable optical filtering may be needed. Since the finesse is what determines the maximum number of resolvable lines in the scan (we are concerned with the level of resolution where atomic linewidths are negligible), the importance of such interference depends on the optical design. The width of the peaks in Figure 2 is determined not by reflectivity or flatness finesse, but by the lack of collimation of the light incident on the Fabry-Perot interferometer. Better collimation can be achieved by using smaller diaphragms but, in turn, sacrificing spectral throughput and thus sensitivity. In the case where the initial alignment gives overlapping lines, one can in general go to a different order of the interferometer to avoid spectral interference. This is illustrated in Figure 3, which shows exactly the same scan as Figure 2 a t a different order. In Figure 3, the Sr emission line is between the sodium doublet, such that interference is minimized. One can, in fact, calculate the relative line positions for succeeding orders given the observation a t a particular order. Starting from the separation of the Fabry-Perot plates in Figure 2 , if one now decreases the separation by 0.59 pm, the sodium lines will appear in roughly the same location on the scan. One is looking a t the (n - 2)th order rather than the n t h order. Since the Sr wavelength is 0.46 pm, for this second separation, it will require the piezoelectric scanning crystal to travel 0.13 pm more than before during the scan in order that the (n - 2)th order of the Sr line is transmitted. Since the complete scan (10 divisions on the picture) represents 0.4 pm total travel, one predicts that the Sr line will appear roughly 3 divisions later than before. The net result is shifting the relative positions of the Sr and the Na lines by 3 divisions to the right, which is exactly what is seen in Figure 3. When the overlapping lines are closer together in wavelength than the Sr-Na pair, more orders must be skipped to provide separation. When more lines are involved, this remedy for spectral interference may not be possible a t all.
Figure 4. Analysis of tap water The three rightmost peaks correspond to Na (5890 A), Ca (4227 A), and Na (5896 A)
Applications. Figure 4 shows a scan taken with tap water in the laboratory. The peaks on the extreme left result from lower orders of the interferometer. The three main peaks correspond to the sodium doublet a t 5890 A and 5896 h;, and the Ca 4227-h; line between the two. From our calibration plots, those correspond to 26 ppm sodium and 80 ppm calcium. Agreement with independent measurements (Ca by atomic absorption and Na by atomic emission, also Ca by EDTA titration) is good, within experimental errors. T a p water is only one of many potential applications of this technique. Others that look promising include the analysis of Na, K, and Ca in serum and in urine, and the determination of certain groups of elements in alloys. In general, if 3 or so elements are of interest, it is quite likely that this technique can be adapted for analysis. The actual feasibility studies must be made on the real sample, to isolate all sources of interference. From Figurs 2 , the trace-to-trace reproducibility is good enough for signal averaging to enhance the detection limits. Typical scan rates in the kHz range are possible in such cases. The background continuum emission and solution consumption fluctuations can be thus suppressed. Only in the case where discrete molecular emission dominates will signal averaging be of little use. In actual applications of this technique, it is inconvenient to use an oscilloscope for scanning and for recording. However, it should not be difficult to have ramp generators to provide the sweep. Synchronized detection of each peak can be achieved by coupling level-sensing logic gates to trigger sample-and-hold amplifiers for recording the signal. The main advantage of this over other simultaneous multielement schemes is that only one phototube is used instead of many. An immediate result of our study is the suggestion of the proper design parameters in using such a technique. The ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
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use of an electric spark source will certainly be a great improvement. The spark localizes the emission such that collection efficiency as well as collimation can be much improved. The spark also can take advantage of the short duration of the detection scan so that absolute sensitivity can be maximized. The trade-off between sensitivity and resolution must be optimized on a case-to-case basis for real samples. We do not expect that the Fabry-Perot interferometer can replace routine emission spectrometers for most applications. However, in specific situations, it offers advantages not present in other optical systems, such as the high luminosity and the short detection period, although more extensive studies must be performed to benefit fully from these advantages. In general, one can expect simultaneous analysis of three elements in the worst cases and of ten elements in the most favorable cases with a high-finesse system.
LITERATURE CITED I
H. C. Wagenaar and L. de Galan, Spectrochim. Acta, Part E, 28, 157
(1973). G. F. Kirkbright and M. Sargent, Spectrochim. Acta, Part E, 25, 577
(1970). G. F. Kirkbright, 0. E. Trocolli, and S.Vetter, Spectrochim. Acta, Part E,
28, l(1973). G. F. Kirkbright and 0. E. Troccoli, Spectrochim. Acta, Part E. 28, 33
(1973). G. J. Nitis, V. Svoboda, and J. D. Winefordner, Spectrochim. Acta, Part E, 27, 345 (1972). P. Jacquinot, Rep. Progr. Phys., 23, 268 (1960).
RECEIVEDfor review October 1, 1975. Accepted January 15, 1976. Prepared for the U S . Energy Research and Development Administration under Contract No. W-7405eng-82. The Alfred P. Sloan Foundation is acknowledged for a Research Fellowship granted to E.S.Y.
Microgram Determination of Boron in Surface Waters by Atomic Emission Spectrometry F. D. Pierce*’ and H. R. Brown’ Uterco, 3059 Lola Circle, Salt Lake City, Utah 84 109
A semiautomated method for the analysis of boron in surface water specimens by atomic emission spectrometry is described. The technique employs a nonmechanical concentration step, an automated boron concentration by methanol distillation, and an automated aspiration-analysis procedure. Sixty specimens can be distllled and 180 dlstilled specimens can be analyzed per hour. The detection limit and the sensitivity provided by the described technique is 0.002 mg/l. and 0.004 mg/l. boron, respectively.
The demand for pollutant surveillance of surface waters continuously requires faster and more sensitive methods of chemical analysis. Boron, being one of many inorganic pollutants for which surveillance has been focused, has provided a challenge to analytical chemists in pollution monitoring laboratories. The use of an automated boron analytical technique could provide an answer to expedite chemical analysis. The Environmental Protection Agency has certified the Circumin method for the analysis of boron in surface waters and the potentiometric boron method has achieved wide acceptance in monitoring laboratories. Because of the chemical manipulations required or the large volume of specimen needed by these two methods for requested sensitivities, they have not yielded well to automation. Afghan, Goulden, and Ryan ( 1 ) described an automated fluorescent technique for boron analysis capable of analyzing 10-20 specimens per hour. This method was complicated and required the construction of special equipment. Being encouraged by the existence of an automated boron analytical procedure, we continued our experimentation. Schlesinger (2) Permanent address, Utah State Division of Health, Bureau of Laboratories, 44 Medical Drive, Salt Lake City, Utah 84113. 670
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
noted the formation of trimethyl borate from a reaction between methanol and boric oxide, borax, and boric acid. This reaction was confirmed by Steinberg and Hunter ( 3 ) . They added that trimethyl borate was stable in the absence of moisture but that it would hydrolyze in the presence of water to methanol and boric acid. Using this approach, Hayashi et al. ( 4 ) , described the removal of boron from steel solutions using a repetitive methanol distillation and distillate collection in a caustic solution. The semiautomated technique described in this paper uses a methanol distillation for the concentration of boron from surface water specimens. The use of two synchronized Technicon samplers was described by Pierce et al. ( 5 ) .A separate aspiration-analysis technique is employed in the described method for the analysis of boron. This method uses standard Technicon equipment and is capable of preparing 60 samples per hour and analyzing 180 samples per hour after specimen preparation. A nonmechanical 4X concentration step is employed prior to specimen introduction into the automated distillation sequence. The technique offers for the analysis of boron, a detection limit of 0.002 mg/l. boron and a sensitivity of 0.004 mg/l.
EXPERIMENTAL Apparatus. The specimens were concentrated in 250-ml Teflon
beakers (Bel-Art Products) and were reduced to a concentrated volume in 50-ml polypropylene volumetric flasks (Nalgene). The automation was accomplished using a Technicon I1 proportioning pump (Technicon Instrument Corporation) and two Technicon I1 samplers. A Technicon heating bath was used for solution heating. The manifold configuration used is shown in Figure 1. A concentrated specimen was placed in two sequentially located 8.5-ml polyethylene sample cups in sampler 1’s sample tray. The action of the first sampler was dictated by a modified 60/h 2:l cam which allowed a specimen to be withdrawn from two sequentially located sample cups. This modified cam was described by Pierce et al., in Reference 5 . The concentrated specimen was withdrawn using a
