(25) (26) (27) (28)
K. Kitagawa and T. Takeuchi, Anal. Chim. Acta, 60, 309 (1972). S.Murayama, Spectrochim. Acta., Part 8,25, 191 (1970).
H. C. Hoare and R. A. Mostyn, Anal. Chem., 39, 1153 (1967). G. F. Kirkbright, A. F. Ward, and T. S.West, Anal. Chim. Acta, 62, 241 (1972). (29) G. F. Kirkbright, A. F. Ward, and T. S.West, Anal. Chim. Acta, 84, 353 (1973). (30) S.Fukushima, Mikrochim. Acta, 1959, 596. (31) R. Herrmann, C. Th. J. Alkemade, and P. T. Gilbert, "Chemical Analysis by Flame Photometry", Interscience, New York, N.Y. 1963. (32) C. Th. J. Alkemade. Anal. Chem., 38, 1252 (1966). (33) V. A. Fassel and D. A. Becker, Anal. Chem., 41, 1522 (1969). (34) C. D. West and D. N. Hume, Anal. Chem., 36, 412 (1964). (35) A. C. West, V. A. Fassel, and R. N. Kniseley, Anal. Chem., 45, 2420 (1973). (36) M. Servigne and M. Guerin de Montgareuii, Chim. Anal., 36, 115 (1954). (37) R. E. Popham and W. G. Schrenk, "Developments in Applied Spectroscopy", E. L. Grove and A. J. Perkins, Ed., Plenum Press, New York, Vol. 7A, 1969, p 189. (38) C. Th. J. Alkemade and M. H. Voorhuis, 2.Anal. Chem., 163, 91 (1968).
(39) (40) (41) (42) (43) (44) (45) (46) (47) (48) (49)
J. D. Cobine and D. A. Wilbur, J. Appl. fhys., 22, 835 (1951). S.Lanz, W. Lochte-Holtgreven, and G. Traving, Z.fhysik, 176, 1 (1963). J. Janza, Czech. J. fhys., 817, 761 (1967). A. I. Leont'ev and E. A. Tsalko, Teplofiz. Vys. Temp., 7, 715 (1969); High Temp., 7, 654 (1970). J. D. Chase, J. Appl. fhys., 42, 4670 (1971). G. Pforr, froc. 14th Coll. Spect. lnt., Debrechen 1967, Adam Hilger, London, 1968. II 667. D. J. &lnicky, R. N. Kniseley, and V. A. Fassel, Spectrochim. Acta, fad 8,30, 511 (1975). R. M. Barnes and R. G. Schleicher, Spectrochim. Acta, Part 6, 30, 109 (1975). W. B. Barnett, V. A. Fassel, and R. N. Kniseley, Spectrochim. Acta, Part 8,23, 643 (1966). D. J. Kainicky, Ames Laboratory, unpublished data, 1975. J. M. Mermet and J. Robin, Anal. Chlm. Acta, 70, 271 (1975).
RECEIVEDfor review January 30, 1975. Accepted April 2, 1976.
Continuum Source Atomic Absorption Spectrometry with High Resolution and Wavelength Modulation Andrew T. Zander and Thomas C. O'Haver Department of Chemistry, University of Maryland, College Park, Md. 20742
Peter N . Keliher* Department of Chemistry, Villanova University, Villanova, Pa. 19085
A continuum source atomic absorption spectrometer has been assembled using a commercially available echelle monochromator which was modified for wavelength modulation with a quartz refractor plate. A 200-W Hg-Xe arc and a 150-W VIX-UV Eimac lamp were used as the continuum sources. Nine elements were studied with the continuum source high resolution, wavelength modulated AA (CEWM-AA) system. The standard curves are linear over sufficiently wide ranges to be analytically useful. They compare well with standard curves for background corrected (H2 lamp) AAL (AALBC(H2)) and with previous high resolution AAC. Sensitivities (characterlstic concentrations) were poorer only by a factor of 2-5 even though the system was not optimized speciflcally for sensitivity. For the nine elements studied, CEWM-AA detection limits were better in seven cases compared to AALBC(H2). Compared to uncorrected line source AA, the CEWM-AA detection limits were less than a factor of 10 poorer.
A recent review of analytical flame spectrometry (1)concluded the section on continuum source atomic absorption (AAC) by stating that it was surprising that no commercial AA instrumentation utilizes a continuum source as the primary source. The use of continuum sources for routine AA analysis offers a number of attractive advantages, which have been enumerated frequently (2-11). However, the disadvantages of AAC relative to atomic absorption using line sources (AAL) have kept AAC from being developed into the powerful technique which it can be. The development of AAC has centered mostly on improving sensitivities ("characteristic concentrations") and detection limits by using long path atomizer systems, high resolution detection systems, and wavelength modulation. Table I lists the pertinent characteristics of AAC systems reported, including this work. Detection limits for the ethanolic solutions used on the long path method of Fassel, Mossotti, Grossman, 1166
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
and Kniseley (7) were between one and two orders of magnitude poorer than for AAL. McGee and Winefordner (8) reported 21 detection limits by long path AAC; most of these detection limits were less than an order of magnitude poorer than for AAL and in a number of cases were better. The major limitations of the long path AAC experiments are that they do not reduce the problem of non-analyte absorption and, furthermore, may be more susceptible to chemical interference than a conventional burner system. High intensity sources (those with power outputs greater than that of the commonly available 150-W Xe and 200-W Hg-Xe sources) have not been extensively applied to AAC. They would produce increased absolute signals and better detection limits (to the extent that the detection limits are shot noise limited), but again would not improve the nonanalyte absorption problem. Wavelength modulation techniques have been used to increase the AAC signal-to-noise ratio (9-11). By modulating the wavelength of the monochromator, Le., rapidly scanning back and forth over a small spectral interval AA, a signal proportional to the change in spectral intensity over the interval is generated. The ac signal amplitude is proportional to the wavelength derivative within AA (12). When using a lock-in amplifier t o detect the ac component of the photosignal, the second harmonic ( 2 F , twice the frequency of modulation) is proportional to the wavelength second derivative. Snelleman (9) has shown that low frequency additive noises in flame spectrometric systems are discriminated against by a wavelength modulated detection system. Typical noises of this sort are flame background absorption flicker, flame continuum emission flicker, and continuum source flicker noise. Good source stability is a particularly critical requirement in AAC. The signals obtained in AAC are inherently low; noise which would be considered typical for AAL would be considered very large in AAC. This requires that the source stability must be higher in AAC to obtain good signal-to-noise ratios.
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
1167
RECORDER
ECHELLE
SOURCE
Figure 1. CEWM-AA instrument schematic diagram
Snelleman (9) first used wavelength modulation (WM) for AAC. The detection limit obtained for Cu (324.7 nm) was 15 times better by wavelength modulated AAC (WMAAC) over dc detected AAC. The Cr (357.9 nm) detection limit obtained by WMAAC was 3 times better than dc detected AAL. The WMAAC system gave detection limits comparable to other AAC systems (7,8).Snelleman pointed out an added important advantage of WM: it effectively discriminates against broadband background interferences. By detecting a t twice the frequency of modulation, only those signals generated from spectral curvature within AA will be seen. Elser and Winefordner (10) extensively studied medium resolution AAC using wavelength modulation and optical chopping (double modulation AAC-DMAAC). The mechanical optical chopper was added to compensate for analyte thermal emission. With this system, it was shown that the signal for DMAAC is 5 times lower than for normal AAC, but the SIN ratio for DMAAC is 5 times larger. This indicates that the WM system effectively lowers the noise observed. Measurements also verified that the system was photon noise limited, which would be expected if lower frequency noise is discriminated against. Sensitivities were not reported, but analytical curves showed the predicted shape and dynamic range. Detection limits were typical for an AAC system. No attempts were made to test the background correction capability of the system. Instrumentation capable of very high resolution has been used to increase the sensitivity and improve the linearity of analytical curves for AAC (11,13-15). Winefordner (16) has shown that as the spectral bandwidth of the monochromator approaches the absorption profile width in the flame, the atomic absorption sensitivity will increase almost linearly with decrease in the spectral bandwidth. Nitis, Svoboda, and Winefordner (11) and Veillon and Merchant (15) used Fabry-Perot interferometers to attain very narrow spectral bandwidths for AAC. While reported sensitivities and analytical curves were similar to AAL results, detection limits were still poorer by more than an order of magnitude. Keliher and Wohlers (13,14,17)used an echelle monochromator to obtain AAC sensitivities and analytical curves. The sensitivities were only two to five times poorer than published AAL values. Cochran and Hieftje (18)have attained spectral bandwidths equal to the analyte absorption profile widths by modulating the continuum source with a flame system into which reproducible droplets of the element of interest are pulsed. The effect of this is to vary the intensity of the analytical wavelength at the frequency of droplet formation. The Xe source/modulated flame combination becomes a source with a line width effectively equal to the absorption profile width. A medium resolution monochromator was used. Working curves showed good linearity and dynamic range with sensitivities comparable to AAL results. The chief limitation of the system for adaptation to routine analysis is the complexity of the droplet generator. Nitis, Svoboda, and Winefordner ( 1 1 ) used an oscillating Fabry-Perot interferometer to gain the advantages of wave1168
*
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
length modulation and high resolution for AAC. T h e analytical curves obtained were significantly different from conventional analytical curves. Detection limits were comparable to all other AAC detection limits. The background correction advantage of WMAAC was stressed but no experiments verifying it were reported. The Fabry-Perot system is more complicated to adjust, more likely to need adjustment, and is much more thermally unstable than conventional wavelength resolving instruments. The very promising results of echelle AAC(AACE) combined with its ease of operation makes an echelle monochromator the instrument of choice for further development of AAC. Its compact, two-dimensional spectrum makes its adaptation to multielement detection systems simple (19). Wavelength modulation techniques are shown here to improve AAC results significantly. T h e modification of most monochromators for wavelength modulation is a simple matter which does not measureably affect the performance of the monochromator. The combination of a high resolution echelle monochromator modified for wavelength modulated continuum source AA (CEWM-AA) offers a number of impressive advantages. Sensitivities and linearities of working curves are shown comparable to AAL results. Signal-to-noise ratios and detection limits are improved markedly. The system will effectively always give continuous, automatic background corrected results since it is inherent in the modulation technique. The simplicity with which the single source CEWM-AA system is operated is in marked contrast to the longer, more detailed, and more critical adjustments necessary to operated conventional two-source background correctors. It should be possible to do reliable trace element analyses on real samples with such a system. It may be possible to correct for direct spectral overlap interferences by using the line-nulling technique (20).This capability is not realizable on any other form of background correction system. The potential for developing the CEWM-AA system into a fully background corrected, multielement AA instrument appeared to be incentive enough to begin evaluating the characteristics of the system, despite the conclusions of a recent review of multielement atomic spectroscopic methods (21) that AAC is not likely to be important in multielement work.
EXPERIMENTAL The echelle monochromator used (Model 102, Spectrametrics, Inc., Andover, Mass.) has been described previously ( I 7). Wavelength modulation was performed by a refractor plate modulator (20). The quartz refractor plate (Spectrosil, 1 X 1X 1hinch) was mounted vertically about 2 cm in front of the exit slit block. In this position, the entire beam of radiation passed through the plate. The beam does not pass through the exact center of rotation of the plate. This has the effect of slightly altering the calibration of the monochromator wavelength dial, but it has no effect on the range of movement of the entrance slit image during modulation. The flat quartz plate caused some defocusing of the slit image due to the change in optical path length (22).The amount of increase in the spectral bandpass caused by this defocusing was not considered a critical limitation, and so no effort was made to use a ground plate. No further modifications to the monochromator were necessary. The slit widths available were: 10, 25,50,100, and 200 pm. The photodetector was a Hamamatsu R446 photomultiplier tube. An 80Hz sinusoidal signal from the function generator (Beckman Model 910, Beckman Corp., Palo Alto, Calif.) was amplified by a conventional audio amplifier and used to drive the torque motor. The modulation of the quartz plate in this manner moves the image of the entrance slit back and forth across the exit slit. The ac signal from the photomultiplier was detected by a lock-in amplifier (Ithaco 353, Ithaca, N.Y.)which takes its reference signal from the function generator. The lock-in was set to detect the 2F signal of 80 Hz. The output time constant used was 300 ms, unless otherwise specified. All other variables of operation were optimized for maximum signal. The out-
Table 11. Instrument Components Component
Monochromator Photomultiplier tube PMT power supply Function generator Torque m o t o r amplifier Torque m o t o r Refractor plate Modulation frequency Lock-in amplifier Photometric amplifier Chopper Burner Source
Recorder
Type
Spectrametrics, Andover, Mass. Hamamatsu R446 Spectrogram LPA-1, Spectrogram Corp., North Haven, Conn. Beckman 9010, Beckman Corp., Van Nuys, Calif. Model 1448, VM Corp., Benton Harbor, Mich. MFE R4-077, MFE Corp., Salem, N.H. inch Spectrosil, 1 X 1 X 80 Hz Ithaco 353, Ithaco Co., Ithaca, N.Y. Spectrogram LPA-1 Spectrogram Varian Techtron AB-41 (10c m slot) Hanovia 200W Hg-Xe Hanovia 27799 Power Supply, Hanovia Lamp Div., Newark, N.J. Eimac 150W VIX-UV Xe Eimac P2505-2 Power Supply, Varian Eimac Div., San Carlos, Calif. Heath E U 2 0 8 Servo-Recorder, Heath Corp., Benton Harbor, Mich.
put from the lock-in was recorded on a chart recorder (Heath EU208 Servo-Recorder, Heath Co., Benton Harbor, Mich.). Two sources were used: a 200-W Hg-Xe continuum lamp, which has been described previously (211, run a t 7 A (Hanovia 27799 Power Supply, Hanovia Corp.) and an Eimac 150-WXe VIX-UV continuum lamp run a t 12 A (P25OS-2 Eimac Power Supply, Varian Eimac Div., San Carlos, Calif.) which has been recently described (23). The source radiation was focused at the center of the air/acetylene flame, which was then focused through a second lens onto the entrance slit. The burner system used was taken from a Varian Techtron AA-4, with a Techtron AB-41 (10-cm slot) burner head. Gas flow was controlled by a Techtron GCU-2 gas control unit. For normal AAC experiments, the torque motor was not energized. A mechanical chopper (Spectrogram Corporation, North Haven, Conn.) was inserted between the source and the flame and run a t 400 Hz. The photomultiplier signal was sent to a photometric amplifier (Spectrogram LPA-1) which also contained the PMT power supply. The amplifier output was sent to the chart recorder. Absorbance was calculated from the recorded %T values. The instrument used for line source AA and background corrected line source AA was an IL Model 353 AA/AE spectrophotometer. A triple slot burner for air-acetylene flames was used. All instrumental parameters were set according to manufacturer’s specifications for optimum response. A Westinghouse H2 continuum HCL (WL-23490) was used for background correction. Relative Source Intensity. The Eimac 150W VIX-UV Xe lamp proved to be a far better source for the CEWM-AA system than the 200-W Hg-Xe lamp. The VIX-UV lamp has been shown to have a significantly higher usable photon flux than standard high pressure Xe arc lamps (23).This is due primarily to the more efficient light gathering design of the Eimac lamp. This was also found to be the case in a comparison of the 150-W VIX-UV Eimac and a standard type 200-W Hg-Xe arc lamp. The Hg-Xe arc used for echelle AAC was mounted inside a copper water-cooling jacket allowing the radiation to pass through a 1-cm diameter hole in the jacket (13).The light gathering efficiencyof this arrangement was low; no specific condensingsystem was designed into the cooling jacket. The Eimac VIX-UV lamp used has been described elsewhere (23). It was operated a t a fixed current of 1 2 A. Spectra of the two lamps obtained on a Jarrell-Ash 0.5-m Ebert monochromator are shown in Figures 2 and 3. The relative intensity scales on these figures have the same units. Insert B to Figure 2 is a
w .5
>
F .4
5 .3
500
400 WAVELENGTH (nm)
300
200
Figure 2. Spectrum of 200-W Hg-Xe arc lamp
8o01.
I
I
200
300
400
500
600
WAVELENGTH (nm)
Figure 3. Spectrum of 150-W VIX-UV Eimac Xe arc lamp: (A) 200-600 nm, (B) 200-250 nm scale expansion of approximately ten times of the 200- to 250-nm region to show that there is usable intensity in this region. The echelle monochromator does not have scanning capability. The photomultiplier tube had an S-19 response. The Hg-Xe arc lamp has no usable intensity below about 225.0 nm. This excludes its use for analysis of certain elements at their most sensitive analytical lines, notably As, Se, Zn, and Pb. The Hg-Xe spectrum is highly structured. In the WM mode, the presence of line structure of width similar to the size of the modulation interval would result in 2F signals; these source originated signals act as base-line offset. The high resolving power of the echelle effectively isolated source structure in the Hg-Xe so that no source originated signals were observed for any element determined. The Eimac VIX-UV Xe lamp has usable intensity down to 200.0 nm, and possibly even lower although experiments below 200.0 nm have not yet been conducted to verify this. The Eimac spectrum is noticeably free of structure in the ultraviolet region. Relative intensities across the spectra were measured on the echelle with the lamps positioned as they were used for AAC experiments. Conditions of photomultiplier voltage, amplifier gain and time constant, slit width and slit height, and warm-up time were the same for intensity measurements of both sources. The 200-W Hg-Xe lamp was a t least an order of magnitude less intense relative to the 150-W VIX-UV Eimac Xe lamp. The low efficiency light gathering array used for the Hg-Xe lamp can account for much of this low relative intensity. It is expected that the Hg-Xe lamp is actually only 2-4 times less intense than the Eimac, as found for a similar type Xe arc lamp (23). For both sources, the analytical curves exhibited the same shape, linearity, and range. The sensitivity for the elements determined were the same for both sources. The detection limits improved by approximately the square root of the relative source intensity. This is the increase expected for a system which is predominantly shot noise limited. The Eimac lamp was easier to mount, align, and operate than the Hg-Xe lamp. But the burner had to be partially shielded from the Eimac because the flame was distorted by the cooling air from the lamp housing which blows out along the optical axis. Wavelength Modulation Interval Calibration. It is desirable, but not critical, to know the magnitude of AA, the interval of modulation, in wavelength units (nm). Knowing the image displacement allows direct comparison of AA t o the spectral bandpass. To a first approximation, the optimum 2F signal will be obtained when Ah is ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
1169
Table IV. Modulation Interval Calibration
Table 111. Line Pairs Used for Calibration Line pair
336.9908 336.9809 309.2842 309.2713 302.0640 302.0489
Ne1 Ne1 A1 AI1 FeI FeI
Separation, nm 0.0099 0.0129 0.0151
237.3362 Al 237.3132 A1
0.0230
336.9809 Ne1 336.9544 Fe
0.0260
equal to twice the spectral bandpass (9).This approximation is based on the assumption that the radiant intensity distribution across the spectral bandpass will be basically the instrumental function of the monochromator. The values of a few experimentally determined absorption profiles in a flame (13)indicate that the spectral bandpass of the 50-pm slit used will be about 2-3 times wider than any particular elemental flame absorption profile. The spectral distribution then should be the instrumental function; and the optimum AA should be close to twice the spectral bandpass. The system is optimized by adjusting the torque motor amplifier until the optimum 2F signal is obtained. It was observed that this adjustment was relatively broader than observed on other systems ( I O , 20). For our purposes, obtaining the maximum signal was of primary concern. Conducting further CEWM-AA experiments, for example line-nulling to reduce spectral overlaps (201, would require a more exact calibration of AA than that performed here. Five pairs of closely spaced atomic lines were used to calibrate the torque motor amplifier input helipot, in nm. This precision helipot is used to control the magnitude of the signal from the function generator. The lines and their separations are given in Table 111. Standard Pe and A1 hollow cathode lamps were used as sources. The calibration was carried out following the procedure of Epstein and O'Haver (20). Calibration of the CEWM-AA system is not as direct a procedure as on a conventional medium resolution monochromator because the reciprocal linear dispersion (dl/dX) of the echelle varies by a factor of 4 over the range of the monochromator. Determining the AA for a particular optimized helipot setting requires incorporation of a factor accounting for the change in dl/dA with wavelength. The AA values calculated for the elements determined are given in Table 111. These values are for optimized 2F signals. Ah is approximately 19%larger than the value of twice the spectral bandpass calculated for each wavelength used. In the WM mode, the image of the incident beam is displaced (for non-axial beams passing through the refractor plate) longitudinally a small distance. This causes the image at the exit slit to be slightly defocused ( 2 2 )because of a change in optical path length. The effect of the defocusing will be to increase the effective spectral bandpass. The difference between the value of twice the spectral bandpass and AA can be accounted for in the inexactness of the method of calibration and by the defocusing effect of the refractor plate. The differenceobserved is consistent with previous results (20). The effect of the presence of the refractor plate on the monochromator performance was checked. The AAC sensitivity for any element should not change with the addition of the refractor plate if the spectral bandpass is not significantlyaffected. A series of Ca standards run by AAC with and without the plate in position showed no change in sensitivity. It can be assumed that any effect of the presence of the plate is negligible, as expected ( I O ) . 1, Measurement: Calculation of Absorbance. The signals obtained a t the output of the lock-in amplifier for a wavelength modulated absorption experiment are proportional to ( I , - I ) . It is necessary, however, to have a measure of absorbance, A , which is related to ( I , - I) by: A = -log(l - ( I , - I)/I,). To calculate values of absorbance, then, a value for I , must be known independently. I , is obtained by disabling the modulation (turning off the torque motor) and inserting a mechanical chopper between the source and the flame. When the chopper is turned on, the signal from the photomultiplier tube will be an ac signal which corresponds to alternating periods of 100% T and 0% T, at the chopper frequency. With the lock-in amplifier referenced to the chopper, the output will correspond to a value of I,. It was observed that the frequency of chopping did not have to be the same as 2F, the frequency a t which wavelength modulated signals are detected. The I , value is independent of frequency. 1170
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
Element, A nm
Ah, nma
2SBP, nmb
Ah/2SBP
Zn 213.8 Pb 216.9 Cd 228.8 Ni 232.0 Fe 248.3 Mn 279.5 Pb 283.3 Mg 285.2 Cu 324.7 Ca 422.7
0.0086 0.0087 0.0091 0.0092 0.0106
0.0072 0.0062 0.0078 0.0078 0.0084 0.0096 0.0098 0.0098 0.0114 0.0152
1.19 1.21 1.17 1.18 1.26 1.23 1.20 1.19 1.17 1.11
0.0118 0.0118 0.0119 0.0133 0.0168
a Ah = (dl/dh) T @-(n- l ) / l ,where d l / d h = reciprocal linear dispersion, T = thickness of quartz $late, @ = angular displacement of plate, n = index of refraction of quartz. b Twice the spectral bandpass, a t slit width of 5 0 p m ; SPB = (hW/2f tan p ) + h z / ( 2 w tan p cos p), where W = geometric slit width, f = focal length, p = grating blaze angle, and w = grating width.
However, the waueform of modulation is of impcrtance. The chopper produces a nearly square-wave modulation of the photosignal, as viewed on an oscilloscope.Wavelength modulation, on the other hand, produces a very different distorted-sine waveform. These two different waveforms will give different lock-in amplifier output voltages. As a result, the ratio of I, - I to I, must be corrected for this waveform effect. Calculations show that symmetrical square-wave modulation of a voltage signal E , measured by a wide-band front-end lock-in amplifier with a multiplier (switching type) synchronous detector, results in an output dc voltage equal to 0.50E. Sinusoidal wavelength modulation of either a Gaussian or a triangular distribution of height E produces a lock-in output signal equal to 0.274 E . Thus our measuremeqts of ( I , - I)/Z,, are low by a factor of 0.274/0.50 N 0.55. For this reason, all measured values of ( I , - Z ) / Z , are multiplied by (0.55)-' = 1.8 before being converted to absorbance. Stray Light Checks. Stray light effects generally lower the observed absorbance because the stray light is not absorbed by the analyte. Errors due to stray light are particularly serious near the wavelength limits of the optical components. Because the Hg-Xe arc and Eimac Xe lamps used have very high ultraviolet and visible intensities, the echelle monochromator was checked for stray light. Using the ASTM method for estimating stray radiant energy ( 2 4 ) , approximately 5% of the radiation reaching the detector a t 230.0 nm was found to be stray radiation. At 195.0 nm, stray radiation amounted to 30% of the signal. Stray light is a somewhat troublesome factor in wavelength modulated AA. The stray light observed does not have narrow structural character to it so that in the WM mode, it is effectivelydiscriminated against at all wavelengths used. But because of the manner in which I , is measured, I, is affected by stray light. This effect can be seen in Figure 4. Curve B is a plot of ABS vs. concentration for which the measured stray light a t 213.8 nm was subtracted from the measured I , value before the absorbance was calculated. The two curves have basically the same shape, although B tends to bend less a t high concentrations. The calculated sensitivity is improved by approximately 8%because the absorbances increase when I , is decreased; but the slopes of the two curves are essentially the same. Use of a solar blind phototube and added baffling within the echelle may be required to successfully minimize the stray light problem at low wavelengths. A disadvantage of the CEWM-AA system is apparent here: the calculation of absorbance is not a direct step. In previous wavelength modulated AAC studies, either the absorbance was not sought (9), or the logarithm of the signal was used as an approximation of the absorbance (IO, II). This is a valid procedure when the percent absorption is small; then log ( % A ) = -log T . But in the CEWM-AA system, the absorption signals are markedly higher than with previous systems. The small signal approximation does not hold, and so actual absorbances must be calculated. They are influenced by whatever affects the I, measurement. A double modulated system ( I O ) (wavelength modulation and optical chopping) would allow a more direct approach to obtaining an absorbance output. This approach was not followed principally due to the lack of the added electronic components required. Another way to obtain an output directly proportional to concentration is t o divide the ac signal electronically by the total output of the photomultiplier ( 2 5 ) .These methods do not eliminate the effect of stray radiation; they merely simplify the conversion of the photosignal to an absorbance value.
,I
1.0
PPM
10.0
Figure 4. Zn standard curves: (A) Absorbance calculated without compensating for stray light; (B) Absorbance calculated after stray light compensation
I
PPM Mn
Figure 6. Mn (279.5 nm) analytical curves: (a) AAL, (b) AALBC, (c) CEWM-AA, (d) AACE
10.
ABS
/ 01
01
10
10 0
PPM Cd
Figure 5. Cd (228.8 nm) analytical curves:
(a) AAL. (b) AALBC, (c)
CEWM-AA, (d) AACE Noise Character of the System. The predominant character of the noise observed with the modified echelle-wavelengthmodulated (CEWM-AA) system with the two continuum sources used was checked. After the lamps were allowed to stabilize, the peak-to-peak noise on the recorded lock-in amplifier output was measured at 422.7 nm and a t 260.5 nm. A neutral density filter of known nominal cbsorbance was then inserted between the source and monochromator, and the peak-to-peak noise was measured again. For systems which are shot noise limited, the decrease in noise observed will be approximately equal to one half the nominal absorption of the filter used to reduce the intensity of radiation. The amount of decrease of the noise with the filter in place was close to the statistical limits of the expected decrease. The predominant noise in the CEWM-AA system appears to be shot noise. With the Hg-Xe lamp, the noise is observed t o be as large as it is due to the relatively lower output of this continuum source, and the high photomultiplier voltage used. Noise measurements were taken on the CEWM-AA system and on the IL353 AA spectrophotometer used as a comparison instrument. Time constants (bandwidths) of 1and 10 s were used (1.2 and 10 s for the IL353). The amount that the noise decreased as the bandwidth was decreased for the CEWM-AAsystem was indicative of white noise character (26);the noise dropped approximately the square root of the bandwidth change. As the bandwidth of the IL353 changed, the noise decreased an amount significantlydifferent from the square root of the change of bandwidth to indicate that the noise character was not predominantly white. This is reasonable when considering the numerous noise sources in a typical AA system (27). Contributions to the observed noise from source instability, flame flicker, analyte emission flicker, and shot noise, for example, will result in a noise character not necessarily dominated by one source. The wavelength modulated system specifically discriminates against all low frequency additive noise sources (e.g., source instability, flame flicker) allowing the few high frequency sources of noise to predominate. This makes the CEWM-AA system more amenable to simple electronic filtering than would a normal AA system.
PPM
Cu
Figure 7. Cu (324.7 nm) analytical curves:
(a) AAL, (b) AALBC, (c)
CEWM-AA, (d) AACE Sample Emission Correction. A limitation of the present instrumental system is that any signal arising from analyte emission is not rejected. This is because the source is neither pulsed nor mechanically chopped as is done generally in AAS. Addition af a mechanical chopper is possible (IO),but was not done here strictly as a matter of expediency. Analyte emission in the CEWM-AA system results in a 2F component in the photosignal which is 180' out of phase with the 2F component due to analyte absorption and thus produces a negative lock-in output voltage. To correct for these signals, the source was blocked while the sample was aspirating. Any emission signal obtained was added to the measured intensity transmitted. In most cases, the analyte emission signals were negligible.
RESULTS AND DISCUSSION Analytical curves for three representative elements studied are given i n Figures 5-7. T h e composite figures show t h e analytical curves for the four AAS methods: a) AAL, b) AALBC(H2), c) CEWM-AA, a n d d) AACE. T h e AAL and AAL using H2 continuum background correction analytical curves show the expected linearity, sensitivity, a n d dynamic range. T h e s e d a t a were obtained o n t h e Instrumentation Laboratory Model 353, double beam, d u a l channel AAIAE spectrophotometer. The curves represent typically obtained data using routine operating procedures a n d parameters. T a b l e V lists the sensitivities obtained; the sensitivity is calculated a s that concentration (pglml) of element in aqueous solution which produces a n absorbance of 0.0044 (1%a b sorption). T h e sensitivities for AAL a r e equal t o or slightly ANALYTICAL CHEMISTRY, VOL. 48, NO. 0, JULY 1976
1171
-~
Table VI. AACE and CEWM-AA Sensitivities
Table V. AAS Sensitivities @g/ml)a Element A nm
Zn 213.8 Pb 216.9 Cd 228.8 Ni 232.0 Fe 248.3 Mn 279.5 Pb 283.3 Mg 285.2 Cu 324.7 Ca 422.7
AALBCAAL~
AALC
0.01 0.06 0.005 0.05 0.05 0.01 0.23 0.004 0.03 0.05
0.014 0.15 0.005 0.08 0.07 0.04 0.49 0.004 0.04 0.09
( ~ , ) d
0.01 0.15 0.006 0.09 0.06 0.04 0.34 0.004 0.04
...
AACE
... ...
0.11 1.4 0.56 0.18 5.8 0.06 0.40 1
CEWMA A ~
AACE (13,21)
AACEa
Zn 213.8 Pb 216.9 Cd 228.8
...
...
0.17 0.7 0.3
0.11 1.4 0.56
0.14
0.18
1.5 0.02 0.20 0.20
5.8 0.06 0.40 1
0.05 0.73 0.11 0.49 0.40 0.15 1.6 0.03 0.34 0.59
Concentration which gives an absorbance equal to 0.0044; also IUPAC “characteristic concentration”. b Manufacturer’s listings. C Obtained during routine operation. d Slit width increased in accordance with manufacturer’s instructions. e Applies to both Hg-Xe and VIX-UV Eimac sources, except for Zn and Pb 216.9 nm. a
poorer than the manufacturer’s listing; this is not unexpected and is the result of following strictly routine operating procedures rather than carefully optimizing each element individually. Cases in which the sensitivity in the background correction mode AAL is different from normal AAL reflect the changed parameters (e.g., slit width) required for background correction mode optimization. The normal AAC analytical curves show the curvature, shifted dynamic range, and poorer sensitivity expected for AAC. These data were obtained using the high resolution echelle monochromator; it compares well with previous high resolution studies ( 1 3 , 1 7 )taking into account the difference in slit widths utilized. I n this study, the slit width chosen was not that which would optimize solely the sensitivity, as has been done previously (13, 17). The choice of slit width depended upon several factors. A primary consideration was that the data would be compared directly with those obtained in the wavelength modulation mode. T o ensure proper comparisons, then, the choice of slit had to be made with respect to optimum conditions for both AAC and CEWM-AA. In CEWM-AA, it is expected that the sensitivity will increase as the spectral slit width decreases, as with normal AAC. A determination of the sensitivity a t each of four slit widths for both AAC and CEWM-AA showed that the sensitivity for each method does increase in the same way. Optimization of the system for the best sensitivity favors the smallest slit available. But in the WM mode, the detection limit has been predicted to be independent of slit width (9);therefore, the detection limit will not be affected when optimizing the light throughput, to obtain the highest absolute signals, by using large slits. It was observed, though, that because of alignment difficulties and greatly reduced light levels when trying the smallest slit, the detection limit was adversely affected. Optimization of the system for the best detection limit favors a large slit. The optimum slit width in terms of absolute signal, noise level, ease of alignment, sensitivity, and detection limit was determined to be 50 pm. The slightly worse AAC sensitivities obtained, Table VI, and the vertically shifted AAC and CEWM-AA analytical curves, Figures 5-7, reflect the difference in slit widths used here (50 pm) and used previously for optimum sensitivity only (10 and 25 pm) (13, 17). The CEWM-AA analytical curves in Figures 5-7 show a high degree of linearity and dynamic ranges closely equivalent to AAL methods. Linearity and dynamic range of elements is poorest by CEWM-AA only for those elements with analytical wavelengths below 230 pm, in which region the continuum source intensity drops rapidly. The sensitivities for CEWM-AA, Table V, are the same as, or slightly better than, 1172
Element, h n m
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
...
Ni 232.0 Fe 248.3 Mn 279.5 Pb 283.3 Mg 285.2 Cu 324.7 Ca 422.7 b
...
a This work, same monochromator as in Slits 50 pm.
CEWM-AA~
0.05 0.73 0.11 0.49 0.40 0.15 1.6 0.03 0.56 0.59 (21 ), slits 5 0 pm.
Table VII. Comparison of Detection Limits (pglml) for Four AAS Methods Element , A nm
Zn 213.8 Pb 216.9 Cd 228.8 Ni 232.0 Fe 248.3 Mn 279.5 Pb 283.3 Mg 285.2 Cu 324.7 Ca 422.7
AALQ
0.001 0.01 0.001 0.005 0.01 0.002
...
0.0003 0.004 0.002
AALb
0.004 0.07 0.005 0.046 0.03 0.014 0.18 0.0018 0.018 0.036
AALBC(H,)
0.015 0.26 0.012 0.24 0.14 0.072 0.66
0.008 0.062
...
AACE
... ... 0.10 1.4 0.54 0.14 3.0 0.03 0.67 2.0
CEWMAAc
0.033 0.44 0.074
0.10 0.077 0.031 0.15
0.002 0.018
0.018
Manufacturer’s listings. b Typically obtained detection limits during routine operation. C 1 5 0 - W VIX-UV Eimac Xe lamp. Q
those for normal AAC and within a factor of 10 of AAL methods. Detection Limits. AAC Methods. Detection limits are dependent upon several parameters. In AAC, the slit width is not one of these parameters; the detection limit is predicted to be independent of the slit width. As for any shot noise limited measurement, the more intense the source, the better will be the detection limit. The method in which the sample vapor is presented for interaction with the incident radiation will also affect the detection limit. The utilization of multipass optics or long path absorption tubes and nonflame atomization systems will allow a greater amount of sample-radiation interaction per unit time, and will lead to better detection limits. The use of high resolution techniques has been shown to increase sensitivity and thus might be effective in increasing the SIN ratio and give better detection limits. The fact that high resolution techniques alone do not give significantly different AAC detection limits compared to medium resolution AAC reflects the fact that this factor is not of major consideration in determining AAC detection limits. Table I lists the detection limits for the various methods and combinations of methods of doing AAC which have proved successful. McGee and Winefordner (8)utilized a long path absorption tube, and so the detection limits would be expected to be better than for normal AAC methods; it is seen that they are better. Snelleman (9) utilized a higher intensity 500-W Xe source and so would also be expected to show better detection limits; this is not seen. The CEWM-AA method gives detection limits better than all other methods reported, even the long path method. Clearly, the CEWM-AA detection limits could be improved even more by using the long path approach. These detection limits are less than an order of magnitude poorer than those for AAL which is not background corrected.
Comparison of AAS Methods. Table VI1 lists the detection limits obtained hy four AAS methods; normal AAL, H2 continuum background corrected AAL, normal echelle AAC, and CEWM-AA. The difference between the manufacturer’s listings and our experimental AAL detection limits is that our data were taken during routine operation in which the primary goal was not simply to find the best detection limit. No information was available on the time constant used by the manufacturer or the definition adhered to for detection limit. The difference between AAL and AALBC reflects the noise which is added to the system when a continuum HCL is operated to obtain background correction (28). The noisier signal from the continuum source channel is subtracted from the primary signal resulting in a quadratic addition of the noises from the two signals. A 30% increase in noise, with the consequent SNR decrease, leads to the poorer detection limits (28). The AALBC and CEWM-AA detection limits are most directly comparable because the total analytical result from each method is background corrected. The AALBC method was operated on a state-of-the-art AA spectrophotometer which was well serviced and calibrated. The sources were essentially brand new. Optimization of the background corrector was completed in accordance with the manufacturer’s instructions. The background correction mode has been used to compensate for severe backgrounds in a number of complex matrices, up to the point a t which chemical effects became dominant. T h e CEWM-AA system is equivalent in capability t o AALBC(H2). The detection limits for the nine elements studied in a background corrected mode are better by CEWM-AA in seven cases. Elements with wavelengths below 230.0 ym have detection limits only about a factor of three poorer. Background Correction. A serious disadvantage of AAC methods for real sample analysis is the relatively more severe nonatomic (background) absorption interference, caused by the fact that the decreased sensitivity for atomic absorption is not accompanied by a corresponding decrease in nonatomic absorption. Background absorption interference which would be small or negligibie in AAL could become quite severe in AAC. Thus, it is simply not enough to say that scale expansion and low-pass filtering or integration can make AAC fully competitive with AAL. A matrix solution of 1%Zr is known to cause severe light scatter when nebulized into an air/acetylene flame. The Zr is not appreciably atomized in this flame. AAL not corrected for background, was used to measure the scatter-caused absorbance signals a t 232.0 nm (Ni), 283.3 nm (Pb), and a t 422.7 nm (Ca). Absorbances between 0.005 and 0.010 were recorded. The nonanalyte absorbance was corrected by AALBC(H2) only a t 232.0 nm and 283.3 nm. The low output of the continuum does not allow background correction at 422.7 nm. The nonanalyte absorbance was corrected by CEWM-AA completely a t all three wavelengths. A somewhat more complex matrix solution of 1%Cr was also tested. This solution causes a measurable amount of scatter. A point-by-point background absorption check (29) shows that there is also some broad-band structure to the nonanalyte background; maxima can be seen around 265 and 290 nm. Measured absorbances for this solution, by AAL, a t 232.0 nm (Ni), 279.5 nm (Mn), 283.3 nm (Pb),and at 422.7 nm (Ca) were between 0.010 and 0.020. The nonanalyte absorbance was corrected by AALBC(H2) at all but 422.7 nm. The nonanalyte absorbance was corrected by CEWM-AA a t all wavelengths. Figure 8 shows absorption recordings of a series of P b solutions and solutions of l%Cr and 2% Cr. Both traces have the same scales; maximum absorbance shown is about 0.030. The upper trace was taken by AACE. There is definite baseline
AACE
2%Cr l%Cr
10
20
CEWMAA
Figure 8. AACE and CEWM-AA absorption signals for 10, 20,50p p m Pb and for 1 % Cr and 2 YO Cr solutions
drift, a significant amount of noise, and the nonanalyte signals are of the same magnitude as the standard signals. By AAL, the standard signals are more than a factor of ten larger than the nonanalyte signals. The lower trace was taken by CEWM-AA. The sensitivity has not been affected; but the SIN ratio is markedly improved, even using a time constant lower by a factor of 1.7. The nonanalyte absorption signals are completely eliminated, not simply lost in a relatively larger amount of noise. The CEWM-AA method was operated on an improved echelle monochromator. Although the characteristics of the monochromator have been optimized for FES, and not AAC as elucidated (14), it has been utilized satisfactorily for AAC. The addition of the wavelength modulator effectively eliminates low frequency additive noise from the system, the major contributor of which is the source fluctuation noise, especially if the 200-W Hg-Xe lamp is used, and consequently allows superior detection limits. Optimization of the CEWM-AA system for effective background correction is only a matter of turning on the modulator and setting AA to a predetermined value. The system is effectively always operating for background correction because it is inherent in the method. The system is effectively always operating a t optimum conditions; no special attention to balancing various parameters is required. CEWM-AA is a much easier method to use for background correction. It is able to give automatic, continuously operating background correction, and this advantage must be taken into account when comparing instrumental systems. The simplicity with which the single source CEWM-AA system is set into operation can be set against the generally longer, more detailed, and more critical adjustments which must be carried out to optimize a two-source corrector. When analyzing for a number of elements, CEWM-AA has an even larger edge in that tuning-in the next analytical line is the only readjustment necessary to continue background corrected analysis. AALBC requires rebalancing of the corrector, sometimes, as in our case, with a flame off/flame on optimization. Total analysis time is reduced using CEWM-AA. I t is possible that the CEWM-AA approach may be more accurate in correcting for background in some special cases. Most backgrounds encountered in AA are assumed to be independent of wavelength across the spectral bandpass used. The occurrence of any deviation from this constancy may lead to a miscorrection depending upon the nature of the background (29). Such a case can arise for a matrix with a high salt content and which has absorption lines close to the analyte line. Background correction is needed to eliminate signals from scatter and molecular absorption. But with the size of spectral bandpass normally used for background corrected AAL, interfering aasorption lines may fall within the spectral bandpass, while not directly overlapping the analyte line. The presence of these lines within the spectral bandpass will cause ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
1173
Table VIII. Cu Analyses, ppm SpectroSample
photometrya
Flame A A b
CEWM-AA
NBS SRM (proposed) Tomato Leaves 10.8 f 0.1 11.5 i 0.2 10.9 k 0.1 NBS SRM (proposed) Pine Needles 2.9 I0.1 3.2 * 0.4 3.0 k 0.3 a Chelate exchange reaction of Cu with Zn dibenzyldithiocarbamate in 1 N HCIO,; private communication, R. Burke and B. Diamondstone, NBS, Gaithersburg, Md. b Background corrected with the nonabsorbing line method; private communication, M. S. Epstein, NBS, Gaithersburg, Md.
an overcorrection since there will be a significant absorption of continuum channel radiation by the interfering absorption lines. A sample of this nature is a Ni alloy in which trace Cd (228.8 nm) is to be determined. Background correction is required to alleviate scatter interference, but over-correction occurs because of the number of Ni lines which fall within the spectral bandpass (29, 30). Cd does not have a close nonresonance line to do a two-line background correction. A twoline correction will not be accurate because of the complexity of interfering lines present. A conventional approach is a long, tedious separation procedure with anion and cation exchange columns (31).The high resolution of the CEWM-AA system affords a spectral bandpass almost two orders of magnitude narrower than that required by conventional AA units used in the background corrected mode. This capability effectively eliminates all interfering absorption lines from the Ni matrix, allowing the wavelength modulated system to accurately correct for scatter and molecular absorption in the determination of Cd. Sample Analysis. Two NBS proposed standard reference materials, Tomato Leaves and Pine Needles, were analyzed for Cu (324.7 nm) on the CEWM-AA instrument. The results in Table VI11 show that CEWM-AA is equivalent in accuracy and precision compared to other recognized means of analysis.
CONCLUSIONS We have shown that the method of continuum source, high ' comresolution, wavelength modulated AA, CEWM-AA, is parable to fully background corrected line source AA. The frequently enumerated advantages of using a continuum source as primary source for AA have been realized. More importantly, the disadvantages of using a continuum source have been alleviated or greatly minimized. Detection limits for the elements studied are superior at seven of ten analytical lines. The analytical curves extend over sufficiently wide ranges to be experimentally useful. Futhermore, the data are fully background corrected for broadband interferences. The correction occurs in the normal course of analysis and does not require added instrumental adjustments to accomplish. Disadvantages of the present system include the added step of atomizing a standard solution to find the analytical line. However, correctly tuning-in the analytical line is not more time consuming than changing a hollow cathode lamp. The present system requires an added step to account for analyte emission. Modification of the system to do this automatically is straightforward ( 1 0 ) . Previous AAC studies were able to utilize a small signal approximation, log (%A) E -log T , to obtain outputs proportional to concentration. But with this system, the large signals obtained do not allow the small signal approximation; and so actual absorbances must be calculated. But this calculation is not a straightforward step. An independent value 1174
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
of I , must be obtained and the present method of making this measurement is affected by factors not influencing ( I , - I ) , t h e absorption signal. Consequently, it is unclear what the correct value of the sensitivity is. The practical effect of this is vertical shifting of the analytical curve, although linearity of the curve is only slightly altered in the high concentration region. The utility of sensitivity figures is an open question in light of conflicting statements in this regard (13,32).Proposed developments of this system will lead to some clarification of this question. CEWM-AA has substantial potential for further development. Substitution of the flame by some version of furnace atomizer will make a promising combination. Encouraging results for background correcting high salt content samples on a nonflame atomizer have already been obtained (33). Studies of absorption backgrounds, broadband and structured, absorption profile determinations, and minimization of line overlaps can be accomplished in a direct manner. All continuum methods also possess the inherent ability to be used for multielement analysis; but of all methods, the CEWM-AA method is more simply adaptable to multielement approach. This is because the echelle produces a compact, two-dimensional spectrum. Recent design changes to the commercially available echelle monochromator take advantage of this feature by equipping it with a 20-channel P M T direct reader cassette (34).Wavelength modulation of this new monochromator would require repositioning the refractor plate behind the entrance slit. T h e lock-in amplifier, which has more capability than that required for these purposes, would be replaced by an integrated-circuit synchronous detector (35) for each operational channel. Data reduction requirements would be of the same order as for any multielement instrument. Such equipment requirements are not more extensive than those already available on microprocessorequipped conventional AA units.
ACKNOWLEDGMENT We are grateful to Thomas H. Doyne, Villanova University, for his encouragement. One author (ATZ) gratefully acknowledges Mr. and Mrs. D. McElwee, Villanova, Pa., for their generous support during the experimental portion of this work.
LITERATURE CITED (1) J. D. Winefordner, V. Svoboda, and L. J. Cline, Crit. Rev. Anal. Chem., 1, 233 (1970). (2) A. Walsh, Spectrochim. Acta, 7, 108 (1955). (3) J. H. Gibson, W. E. Grossman and W. D.Cooke, Proc. Feigl. Anniv. Symp., p 296, Elsevier (1962). (4) N. P. lvanov and N. A. Kozyreva, Zh. Anal. Khim., 19, 1266 (1964). (5) V. L. Ginsburg and G. I. Satarina, Zavod. Lab., 31, 249 (1964). (6) C. W. Frank, W. G. Schrenk, and C. E. Meloan, Anal. Chem., 39,534(1967). (7) V. A. Fassel, V. G. Mossotti, W. E. Grossman, and R. N. Kniseley, Spectrochim. Acta, 22, 347 (1966). (8) W. W. McGee and J. D.Winefordner, Anal. Cbem., 37, 429 (1967). (9) W. Snelleman, Spectrochim. Acta, Part8, 23, 403 (1968). (IO) R. C. Elser and J. D. Winefordner, Anal. Chem., 44, 698 (1972). (11) G. J. Nitis, V. Svoboda, and J. D.Winefordner, Spectrochim. Acta, Part 5, 27, 345 (1972). (12) T. C. O'Haver and G. L. Green, Am. Lab., p 3 ) , 15 (1975). (13) P. N. Keliher and C. C. Wohlers, Anal. Chem., 46, 682 (1974). (14) C. C. Wohlers, Ph.D. Thesis, Villanova University. 1975. (15) C. Veillon and P. Merchant, Appl. Spectrosc., 27, 36 1 (1973). (16) J. D.Winefordner, Appl. Spectrosc., 17, 109 (1963). (17) P. N. Keliher and C. C. Wohlers. Anal. Chem.. 48. 140 (1976). (18) R. L. Cochran and G. M. Hieftje, FACSS 2ndNational Meeting, Indianapolis, Ind., 1975, Paper No. 163. (19) P. N. Keliher and C. C. Wohlers. Anal. Chem., 48, 333A (1976). (20) M. S. Epstein and T. C. O'Haver, Spectrochim. Acta, Part 8,30, 135 (1975). (21) J. D.Winefordner, J. J. Fitzgerald, and N. Omenetto, Appl. Spectrosc., 29, 369 (1975). (22) W. Snelleman. T. C. Rains, K. W. Yee, H. D.Cook, and 0. Menis, Anal. Chem., 42, 1993 (1975). (23) R. J. Perchalski, J. D.Winefordner, and B. J. Wilder, Anal. Chem., 47, 1993 (1975). (24) ASTM E387-69T, Manual on Recommended Practices on Spectropho-
tometry, 3rd ed., American Society for Testing and Materials, Philadelphia, Pa.. 1969. R. N. Hager, Jr., Anal. Chem., 45, 1131A(1973). C . P.Thomas, Ph.D. Thesis, University of Maryland, 1972. J. Ingle, Anal. Chem.,46, 2161 (1974). M. S.Epstein, T. C. Rains, and 0. Menis. 15th Eastern Analytical Symposium and 12th National SAS Meeting, New York, 1973, Paper I f . J. Y. Marks, R. J. Spellman. and E. Wysocki, FACSS 2nd National Meeting, Indianapolis, Ind., 1975, Paper No. 202. G. R. Harrison, "MIT Wavelength Tables," MIT Press, Cambridge, Mass., 1969. M. Kirk, E. G. Perry, and J. M. Arritt, Anal. Chim. Acta, 80, 163 (1975). M. L. Parsons, 6.W. Smith, and G. E. Bentley, "Handbook of Flame Spectroscopy", Plenum Press, New York, 1975. T. C. O'Haver, 28th Annual Summer Symposium on Analytical Chemistry, June 1975, Knoxville. Tenn. P. N. Keliher, Res. lDev., 27(6), 26 (1976).
(35) G.Horlick and K. R. Betty, Anal. Chem., 47, 363 (1975).
RECEIVEDfor review December 11, 1975. Accepted March 19,1976. From a dissertation to be submitted to the Graduate School, University of Maryland, by A. T. Zander, in partial fulfillment of the requirements for the Ph.D. degree in Chemistry. The financial support provided by the National Science Foundation, Grant No. ESR-75-02667 (NSF-RANN), and partial support for one author (PNK) by the Villanova Faculty Research Program is gratefully acknowledged. This paper was presented at the 27th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1, 1976. Paper Number 31.
Controlled Atmosphere Discontinuous Semiautomatic Dispenser for Carbon Rod Atomizer Vito Sacchetti, Gin0 Tessari,* and Giancarlo Torsi lstituto di Chimica Analitica, Universita di Bari, Via Amendola, 173-70 126 Bari, Italy
A semiautomatic dispenser for liquid solutions is described which makes possible a strict control of the atmosphere during the sampling operations. The dispenser was optimized with regard to the sampling mass reproducibility, by checking different capillary treatments. A typical relative standard deviation at l pl level of aqueous samples was 0.6 %.
When automatic procedures of sample introduction are sought for the carbon rod atomizer in atomic spectrometry three requirements should be met: i) small sample volumes (1-5 ~ 1 ) ii) ; critical sample positioning (narrow strip of graphite); iii) absence of containing walls. The concomitance of these three requirements rules out solutions such as those proposed by Pickford and Rossi ( I ) (too large a volume injected: 100 11.1) or by Molnar and Winefordner (2) (graphite tube acting as container for the nebulized sample). Maessen et al. ( 3 )used a commercial syringe (S.G.E.) with a device for accurste sample positioning in the case of an Argon-sheathed Mini-Massmann device open to the atmosphere. In the course of some investigations in this laboratory on the kinetics of atomic release from different substrates (4-7), it was found that in some cases the kinetic parameters of the metal release were critically dependent on the "inert" atmosphere surrounding the rod (8). The present investigation was carried out in order to work out a sampling device which, besides the above mentioned requirements: i) should permit a complete control of the atmosphere surrounding the rod, avoiding the opening of the atomization chamber after each measurement; ii) could be easily interfaced with a microcomputer in the fully automatized apparatus envisioned in this laboratory. The important features embodied in the device are: i) the use of a capillary of fixed volume to be filled by the sample solution; ii) the possibility of sampling different volumes of analyte solution by changing the piston tip, fitted with different capillaries; iii) the use of the same mechanical device for positioning the capillary and for generating the vacuum and the pressure needed to fill and to drain the capillary; and iiii) a variable buffer volume acting as a pressure regulator according to the volume and the viscosity of the solution to be sampled.
EXPERIMENTAL Sample Dispenser. A scale drawing of the device is presented in Figure 1. In Figure 2, a sketch is shown in which the actual dimensions have been altered to emphasize the parts which are critical to understanding how the device works. The hollow piston A, by its reciprocating motion, produces alternatively pressure or depression in chamber B. In Figure 2, the piston is shown in its extreme positions corresponding to: a) maximum pressure in chamber B and the tip of the capillary over the carbon rod; b) maximum depression in chamber B and the tip of the capillary dipping in the sample reservoir. The conditions prevailing in chamber B are transmitted to chamber A through the electrovalve C. The pressure (Figure 2a) is used for ejecting the sample on the carbon rod. Likewise, the depression is used for sucking the sample solution inside the capillary (MicrocapsDrummond Corp.), as usually ( 6 ) curved a t one end a t 90° by gentle heating. The solution volume inside the capillary is the dispensed quantity. The solution sucked in excess is collected a t the bottom of the piston (chamber A), and occasionally spilled through the stopcock F. The suction is interrupted by equilibrating, through electrovalve C, chambers A and B with the pressure inside the atomization chamber (dome). During the piston displacement, obviously chamber B is isolated, whereas the connection between chamber A and the dome is maintained. A buffer volume in parallel to chamber B has been added, with the aim of obtaining a pressure or depression difference between chamber A and the dome allowing: i) the suction of the minimum quantity necessary; ii) a gentle ejection of the sample drop avoiding any splashing. All the chambers and tubings were thoroughly washed with purified gas before a run of measurements, in order to have the same atmosphere inside the atomizer and the dispenser. In order to allow visual inspection in all the steps of the sampling process, the device was obtained from a block of transparent methacrylic material. The dispenser was screwed to a side window of the dome of a standard atomizer already described ( 3 , 6 ) .This coupling, as can be seen from Figure 1,forms a lever with a fairly long arm and, in order to make the applied force as small as possible, the device was driven by a light synchronous motor at 400 Hz.An endless screw was keyed to the shaft of the motor and coupled, through a helicoidal gear, to the circular rack moving the piston. T o suck the solution inside the capillary, the sample container D must be raised to such a height that the capillary tip dips in the liquid. As can be seen in Figure 1,the circular rack driving the piston, at the other extremity, acts on lever E raising the sample container D. This tight coupling between the motion of the capillary and the sample container secures the synchronism necessary to avoid the breaking of the fragile capillary. The force is applied to lever E through ball bearing L, to minimize the friction. The horizontal displacement of the rack is converted to a vertical displacement of the container by ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
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