Selective Spectral-Line Modulation Technique for High Sensitivity Continuum-Source Atomic Absorption Spectrometry Ronald L. Cochran and Gary M. Hieftje" Department of Chemistry, Indiana University, Bloomington, Ind. 4 740 1
A selective spectral-line modulation technique for perlormlng continuum-source atomic absorptlon is described and Its operation discussed. In the new technlque, dlscrete droplets of a metal salt standard solution are perlodlcally introduced Into a "source modulator flame". The atom clouds produced wlthln this flame move through the radiation path of a continuum source, thereby selectively modulatingonly those atomic llnes characterlstlc of the atoms within the cloud. This combination of continuum source and "source modulator flame" replaces a modulated line source within an otherwise typlcal atomic absorption spectrometer. The system Is shown to enable multielement analysis and exhlbits relative freedom from narrow-line spectral interferences. Broad-band spectral interferences can also be corrected wlth little dlfflculty. Analytical curves and sensitivities obtained wlth the system for several elements are presented. Extensions of the system and its applications are briefly dlscussed.
The use of a spectral continuum as a primary source for analytical atomic absorption spectrometry (AA) was largely ignored until 1962 when Gibson et ai. ( I ) first explored the possibility. Since that time, the inherent advantages offered by continuum-source atomic absorption spectrometry (AAC) have stimulated considerable interest in the technique (1-9). The most important of these advantages include single-source qualitative and multielement analysis capabilities, simplified background correction, and increased selection of usable lines for each element. The use of a continuum source in AA has been evaluated theoretically in several publications (2-5). From these studies it can be concluded that only when a spectrometric system of extremely narrow spectral band-pass is used (on the order of 0.02 A) will the sensitivity and analytical curve linearity be comparable to those provided by line-source atomic absorption spectrometry (AAL). Consequently, when a common, medium resolution monochromator is employed in AAC, an adherence to the Beer-Lambert law can be expected only for very small absorbance values, where errors resulting from the broad band-pass become insignificant. In addition, substantial decreases in sensitivity relative to AAL are predicted for AAC. McGee and Winefordner (6) and Fassel et al. (7) have experimentally evaluated continuum-source atomic absorption in which a medium resolution monochromator was used. In both cases attempts were made to enhance the technique through the use of flame types uncommon to AA, through the use of methods to increase the absorbing path length, and by use of scale expansion. Regardless of these efforts, resultant detection limits were 10-100 times worse than accepted AAL detection limits (assuming use of optimum flame for each element). Analytical working curves showed linear ranges confined to low absorbances for all but a few elements. Keliher and Wohlers (8,9) have shown that the high resolution attainable with an Echelle-grating monochromator can produce good sensitivity and linearity in AAC. However, the expense of Echelle spectrometers or of more conventional 98
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
types of high resolution monochromators has stimulated the search for other means of producing high resolution AAC systems. One method of achieving the required spectral resolution involves selectively modulating the amplitude of a desired narrow spectral region of the output of the continuum source. By use of photoelectric detection followed by selective signal processing at the modulation frequency, the modulated spectral region can be exclusively detected, thereby defining the effective spectral band-pass of the system. Alkemade and Milatz (IO) first applied this technique to atomic spectrometry, employing a double-beam method of selective spectral modulation to enhance the spectral resolution of a nondispersive, flame photometric system. Bowman et al. (11)have utilized selective spectral modulation to isolate resonance from nonresonance lines emitted by a dc-operated, high-intensity, hollow cathode lamp. The lamp radiation was passed through a pulsating atomic vapor cloud formed within a modulated cathodic sputtering cell, where selective absorption of the resonance lines by the vapor caused the exclusive modulation of those lines. A similar kind of selective modulation has been applied to AAC by Marinkovic and Vickers (12) and by Mossotti et al. (13).In both studies the flow of sample solution into the analytical flame of an AAC system was modulated, and the signal was detected at the modulation frequency. In this way the absorption line width of the sample atoms defines the effective bandwidth of the spectrometric system. The results of this narrowing were shown to include substantial increases in signal-to-noise ratio as well as increased analytical curve linearity. Unfortunately, along with the reduction in bandwidth come some less desirable aspects of the sample modulation approach to selective modulation. Perhaps the most severe drawback is the restriction imposed on both flame type and burner design. In general, the popular nebulizer-spraychamber systems, using laminar, premix flames, are not suitable for sample modulation because of their relatively slow response times. Although Mossotti et al. explored the use of several different flame types, only a short path length, total consumption burner could be easily used. The burner-modulator used by Marinkovic and Vickers provided a 4-cm path length, but dictated the use of an air-hydrogen diffusion flame. Because of the low temperature of most nonturbulent diffusion flames, the use of such a system for real samples is somewhat impractical. In the present paper a novel technique is described in which the desired selective modulation is produced not by the sample solution but by the periodic introduction of a controlled, secondary modulating solution. A discrete droplet method of introducing this modulating solution, combined with conventional pneumatic nebulization of the analytical sample, maintains the versatility of flame and burner type typical of conventional AAL. The analytical performance of the selective spectral-line modulation (SLM) system is evaluated for the elements, calcium, copper, iron, lithium, and sodium. Typical analytical curves extend linearly over at least an order of magnitude concentration range, when plotted on
I XENW
@-+ I :ek/
+
_.___
ZZF ANALVTICAL
WAVELENGTH
L2
1
FLAME A
''
a
Y 2
SOURCE MODULATOR FLAME
L MONOCWROUATOR
z c
s
P w
Figure 1. Schematic diagram of the SLM experimental system (- -) Electrical path; (-)
optical path
a
< WAVELENGTH
log-log coordinates. SLM sensitivities are up to 100 times better than those found with AAC, but are somewhat than those using sharp line sources. The relative freedom of the system from both line and broad-band spectral interferences and its multielement analysis capabilities are also demonstrated. Future work involving the system is briefly discussed.
CHARACTERISTICS OF SELECTIVE SPECTRALLINE MODULATION Because of the importance of selective spectral-line modulation to this work and because of its unusual characteristics, it is worthwhile to briefly examine this modulation process from the standpoint of the present application. Our approach to the use of selective modulation can be best understood with reference to the schematic diagram of the experimental system shown in Figure 1. T h e basic configuration of the SLM instrument shown in Figure 1 is similar to that found in conventional atomic absorption spectrometers, and is comprised of a primary source system, an analytical flame, a medium-resolution monochromator, a photodetector, and a readout array. However, the line source ordinarily employed in AAL has been replaced by the combination of a continuum lamp and a modulated atom reservoir (labeled "Source Modulator Flame"). This novel combination creates a versatile new kind of source for atomic absorption spectrometry. The modulated atom reservoir can in principle consist of any confined volume within which the atomic concentration can be periodically raised and lowered. Consequently, possible modulated atom reservoirs range from flames to various electrically powered atom cells, with the suitability of a given reservoir being based on such characteristics as the degree to which the atom concentration can be periodically altered, the rates a t which the concentration can be changed, and the constraints placed on usable elements and combinations of elements. T o understand the operation of this combination source, let us envision the spectrum of the continuum lamp as it appears to the analytical flame. This spectrum will alternate between a) the original spectrum emitted by the continuum lamp and b) a similar spectrum from which have been removed the atomic absorption lines characteristic of the atoms in the modulated atom reservoir. The spectral region (as defined by the monochromator's spectral band-pass) immediately surrounding one such isolated absorption line would appear to the analytical flame (or at the monochromator's exit
Figure 2. Simulation of selective modulation process (A) Spectral region with no analyte aspiration. (B) Spectral region with analyte
aspiration. (- -)Nomodulating vapor in light path; (-)with light path: (s) spectral band-pass of monochromator
modulating vapor in
slit when the analytical flame is transparent in that spectral region) much as in Figure 2A. In Figure 2A the dashed line represents the viewed spectrum with no modulating atomic vapor in the light path, while the solid line represents the spectrum with atomic vapor intercepting the light beam. Within the observed spectral region, only the absorption line will appear and disappear a t the modulation frequency; all other spectral components remain unchanged by the modulation process. Thus, by placing at the monochromator's exit slit a detection System that is synchronized to the appearance and disappearance of the modulating atomic vapor, a signal proportional to the shaded area in Figure 2A will result. Clearly, this detected signal approaches in spectral bandwidth the signal ordinarily obtained from a modulated (chopped) hollow cathode source. If atoms of the same kind as those in the modulated atom reservoir are introduced into the analytical flame, the spectral region containing the atomic line of interest would be changed in the way portrayed in Figure 2B. When atoms are absent from the modulated atom reservoir, the radiation transmitted by the analytical flame exhibits the absorption characteristics of the atoms in the analytical flame (dashed line). However, when modulating atomic vapor interrupts the light path, the depth of the absorption line is increased, as depicted by the solid line in Figure 2B. Obviously, the magnitude of this increased absorption is due to the combined effects of the analyte atoms and those in the modulated atom reservoir. Because the detected spectrum will alternate between the two limits shown in Figure 2B with the periodic appearance of the modulating atoms, the output of the synchronous detection system will be proportional to the shaded area in Figure 2B, which is reduced from that in Figure 2A. The combination source could make feasible single-source atomic absorption, thus greatly simplifying both qualitative and multielement quantitative analysis by AA. In cases where a broad range of elements must be determined, a substantial savings could be realized by such a source. And perhaps most important is that the combination source could be readily employed in existing AA spectrometers as a hollow cathode replacement. Quantitative Characteristics of SLM. It might not be intuitively obvious that selective spectral-line modulation AA ANALYTICAL CHEMISTRY, VOL. 49,
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would follow many of the same relationships that govern line-source AA. In order to further explain why the BeerLambert law is expected to be obeyed here, a more quantitative treatment will be given. To simplify this treatment, it will be assumed that the absorption line width of the modulating atoms is extremely narrow, and that the peak absorption value of that line is proportional to the total integrated absorption of the line. Of course, a completely rigorous treatment of the absorption process would require consideration of the total integrated area under the spectral absorption curves of the modulating and sample atoms; however, approximate treatment will suffice to reveal the basic quantitative features of SLM. Narrow-Band (Atomic) Absorption. Let us consider the radiant power arriving at the photodetector in a situation similar to that portrayed in Figure 2A, but with the simplifying assumption of an extremely narrow modulating absorption line. Here the observed spectral region is defined by the monochromator's spectral band-pass, s, and the absorption line of the modulating atoms is characterized by a transmittance, T,, at the peak wavelength, A. Let i t be assumed that the radiant power of the continuum source is effectively constant over the wavelength region, s, and let the radiant power have a value P Aa t X and an integrated value, P,, over all other wavelengths included within s. Under these conditions, the total radiant power received by the photodetector when no modulating atomic vapor is in the light path (dashed line in Figure 2A) is given by
Ptotal = PA+ Ps
(1)
However, when the modulating atomic vapor interrupts the light beam (solid line in Figure 2A), the power is reduced to
P,
=
+ P,
T,Px
As the modulating atomic vapor periodically intercepts the light beam, the amplitude of the resulting ac signal, I,, will be equal to the difference between Ptotaland P , or
I , = ( P A+ P,) - (T,P^
+ P,) = P A- T,Px
(3)
The quantity I , in Equation 3 corresponds to the blank (or 100%transmittance) signal in conventional AAL systems; I , might also be considered to be the effective spectral radiant power incident upon the analytical flame from the combination source. When analyte atoms of the same type as the modulating atoms and a t a given concentration are produced within the analytical flame (e.g., by aspiration of sample solution), they will display a transmittance, T,, at the wavelength A. This situation is similar to that shown in Figure 2B, where the detected radiant power in the absence of the modulating atoms (dashed line in Figure 2B) is reduced by the analyte absorption to
(4) The detected radiant power in the presence of the modulating atoms (solid line in Figure 2B) is attenuated through absorption by both the modulating and sample atoms, and is given by
Pa.,
=
TmTJ'A
+ Ps
(5)
As the modulating atoms pass in and out of the light beam, the amplitude of the ac signal, I , is just the difference between Pa and Pa?,, or
I = (T,Px
+ P,) - (T,T,Px + P,) = T,(Ph - TmPJ ( 6 )
Substitution from Equation 3 then simplifies Equation 6 to
I = T,I, 100
(7)
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
or
T , = I/I, From Equation 8, the transmittance (and thereby the absorbance) of the analyte atoms can be calculated from two measured signal amplitudes in a fashion similar to that employed in AAL. Because the absorbance values calculated in this way involve only the radiant power at the atomic line, and not the entire integrated radiant power over s, linear calibration curves analogous to those found in AAL are also to be expected within the narrow modulating line assumption. Effect of Slit W i d t h on SignallNoise Ratio i n SLM. Because the spectral half-width of the modulated atomic line is independent of monochromator slit width, the radiant power passed by the monochromator at an atomic line, PA, is given by
P A =KW where K is a constant (including monochromator acceptance angle and slit height), and W is the monochromator slit width. Equation 9, in general, expresses the throughput of a monochromator to radiation from any sharp line source. Substitution of Equation 9 into Equation 3 yields
I , = KW - T,KW = K W ( 1 - T,)
(10)
which indicates that I,, the 100% transmittance signal from the combination source, should increase in direct proportion to monochromator slit width. However, with this increase in ac signal amplitude comes a concomitant increase in shot noise. If it is assumed in Equation 1that P A
