Use of Light Frequency Modulation of Continuum Source in Atomic Absorption Photometry SIR: The limit of detection in atomic absorption (fluorescence) spectrometry is a function of the ratio of the measured signal and the background noise (1-3). The main sources of noise in these methods are: a. contamination during analytical procedure by the element to be found and by irregularities in transport and atomization of the sample b. fluctuations of the primary light source intensity c. fluctuations due to the light scattering and nonspecific absorption in the absorbing medium and fluctuations of its own light emission d. the noise of the detector system. Only the last three noise sources may be diminished by appropriate choice of measuring apparatus. The fluctuations of the primary light source may be eliminated with a doublebeam instrument, as has been reported by Alkemade and Milatz (4). The influence of irregularities in the continuum absorption of the atomizing medium (flame, etc.) has been eliminated (5)by the combination of a continuum source and a line source. Whereas the light from the line source is absorbed mainly by the analyzed element, the light from the continuous source, which has the same optical path through the flame and monochromator, is primarily influenced only by continuum absorption and scattering. The compensation is achieved by comparison of both intensities. The disadvantage of this ingenious method is not only the need of fitting a continuum light source to every particular line source, but also the demand for a very high stability of both sources. These instabilities may be compensated either by use of two additional channels with detectors (four channels altogether) or, with the one detector scheme, by successive measurement of the beams coming from both sources directly and through the flame. Obviously, in this second case, the actual measuring time is diminished further. PROPOSED METHOD
The continuum or broad-line source is used as the primary light source. Its light is modulated by a chopper (see Figure 1) and then is passed through an oscillating interference filter. Transmission of this filter is a periodic function of wavelength (see Figure 2). At the same time, the wavelengths of the maxima of transmission are oscillating with an amplitude which equals their distance. The filtered beam is then directed into the absorbing medium, which reduces its overall intensity as a result of nonspecific absorption. The narrow line absorption, corresponding to the analyzed element, changes the beam intensity with a frequency corresponding to the oscillation frequency of the interference filter. The signal from the detector is attenuated by two narrow-band amplifiers, one adjusted to the chopper frequency, the other to the oscillating frequency of the optical filter. In the next step, the
(1) H. Kaiser, Z . Anal. Chem., 209, 1 (1965). (2) J. D. Winefordner and T. J. Vickers, ANAL.CHEM.,36, 1939 (1964). 13) . _J. M. Mansfield. J. D. Winefordner, and C. Veillon. ibid.,. 37,. 1049 (1965). (4) - SOC.Am.. . 45.. . , C. T. J. Alkemade and J. M. W . Milatz.. J . Oot. 583 (1955). ( 5 ) S. R. Koirtyohann and E. E. Picket, ANAL, CHEM., 37,601 (1965).
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ANALYTICAL CHEMISTRY
Absorbing Medium
Ow.
Light Source
Intaf. Filter
Narrow Band AmpliRen Monochromator Detector
OUtPUt Voltage Comparator
Chopper
Figure 1. General arrangement for detection with lightfrequency modulation ratio of these output voltages is measured. Finally, the output voltage may be log-transformed and integrated in the normal manner. THEORY OF THE OSCILLATING INTERFERENCE FILTER
The oscillating interference filter may be based on various principles (birefringence interference filter, spherical (6), or the classical flat Fabry-Perot interferometer, etc.). Although there are several types, only the simplest-i.e., an ideal flat Fabry-Perot interferometer-will be treated here. The transmittance of this interferometer, T, for the near axial beam is described by the Airy equation:
T
=
TR2/[(1-R)2
+ 4 R sinZ(27rdn/X)]
(1)
where T, is transmittance of the mirror-coating, R is the reflection coefficient of the mirror coating, d is the geometrical distance between mirrors, n is the refractive index of the medium between the plates, and X is the wavelength of the transmitted radiation. Thus, the transmittance is a periodic function of wavelength. The distance between two adjoining maxima (in wavelengths units] is about X2/2dn. For example, if d = 2 mm and A = 4000 A, the distance equals 0.8 A. If the geometrical plates distance is varying periodically from d to ( d X/2n), then the transmittance peaks are also shifting with an amplitude which equals their distance. Not only the distance between maxima, but also the shape of transmittance peaks may be chosen. The halfwidth of the transmittance peak is obtained from the above Equation 1 and is given by: X z ( l - R ) / 4 ~ n d R l / ~ . For example, if R = 0.7, d = 2 mm and X = 4000 A, the halfwidth equals 0.053 A. Probably the optimal modulation will be obtained with an oscillating filter having the halfwidth of the transmittance peak nearly equal to halfwidth of spectral line and with the distance of adjoining maxima identical with this value. The mathematical treatment of a similar case-the scanning of atomic transition profiles with an interferometer-has been published recently (7). Various techniques (8, 9) for vibrating one of the etalon plates have been described. Frequencies in the range of kHz are easily attainable.
+
(6) P. Conne, J. Phys. Radium, 19,262 (1958). ( 7 ) E. A. Ballik, Appl. Optics, 5,170 (1966). ( 8 ) D. J. Bradley, Proc. Roy. SOC.(London), A 262, 529 (1961). (9) J. Cooper and J. R. Grieb, J . Sci. Inst., 40, 433 (1963).
DISCUSSION
Using the proposed experimental system, a continuum source or a very broad line source may be used. A comparison of their light outputs with hollow cathode discharge tubes can be only approximated because absolute data about narrow line sources is lacking. The characteristics of a typical 900 W Xe arc lamps are: 11 mW cm-2A-1ster-1 at 2800 A, and 24 mW cm-2A-1ster-1 as measured in (IO); and the characteristics of a typical high pressure mercury arc lamp 200 mW cmA2A-l, have been reported in (11) from 4000 to 4100 A. The typical hollow cathode lamp emits (12) about 1 mW cm-2ster-1 in one line; it corresponds to about 1020 mW cm-2A-1sterd1. So, using the high pressure mercury arc lamp, a substantial increase of light intensity may be attained and the influence on the detector system noise diminished. In comparison with systems using a continuum source for atomic absorption in combination with high-resolving power monochromators (13), the effective light power increases further, because wide-band pass monochromators or first-order interference filters can be used. The possibility of matching exactly the wavelength width of the measuring beam to the width of the measured line also increases the detection sensitivity of the system, as the calibration curve becomes steeper (14). In comparison with nonoscillating narrow band pass monochromator and continuous source, the proposed system will be probably less sensitive to exact wavelength setting and to geometrical drift of systems using narrow slits. The influence of damping of spurious signals from source and flame background fluctuations will be verified experimentally. The proposed method may find applications
Wave1mgthr Broad-Line Source
Trmimitbnce of Interf. Filter
7
OK.
Spectrum o f Light Beam
t
Transmittme* of Abiorbina Medium
t
Light Beam Alter Akorbing Medium
Figure 2. Optical spectrum of various components in such procedures where these fluctuations are predominant, as in turbulent flames, graphite furnace, etc., and also where the universality of a high sensitivity is required.
VRATISLAV SVOBODA (10) N.Z.Searle, P. Giesecke, R. Kinmont, and C. J. Hirt, Appl. Optics, 3,923 (1964). (11) W.Going, 2.Phys., 131,603 (1952). (12) H. M. Crosswhite, “The Spectrum of Iron I,” John Hopkins Spectrosc. Report No.13,August, 1958. (13) V. A. Fassel, V. G. Mossotti, W. E. L. Grossman, and R. N. Kniseley, Spectrochim. Acta, 22, 347 (1966). (14) I. RubeSka and V. Svoboda, Anal. Chim. Acta, 32,253 (1965).
Institute for Research, Production, and Use of Radioisotopes Piistavni 24 Prague 7, Czechoslovakia
RECEIVED for review November 13, 1967. Accepted March 19, 1968.
VOL 40, NO. 0, JULY 1960
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