Optimization of Instrumental Parameters in Flameless Atomic Absorption Spectrometry F. D. Posma, H. C. Smit, and A. F. Rooze Laboratory for Analytical Chemistry, University of Amsterdam, Nieuwe Achtergracht 166, Amsterdam, The Netherlands
Fundamental aspects of various noise sources in flameless atomic absorption have been considered and the measurement system appeared to be shot noise limited. The influence of time constants of the measurement system, modulation frequency of the primary source, and heating rate of the sample cell on both noise level and signal-to-noise ratio have been experimentally investigated. Furthermore, the effect of the bandwidth of the amplifier-recorder system on analytical parameters, such as detection limit, sensitivity, and linear working range of the calibration curves, has been studied. Detection limits for undistorted signals, optimum signal-to-noise ratio, and highly distorted signals are reported.
A large number of instrumental parameters affect the results of flameless AAS. Some of these parameters are similar to those in flame atomic absorption because the same equipment is used and consequently an adequate description can be found in literature (1-3). However, owing to th.e transient nature of signals occurring in flameless AAS, a number of parameters such as dominant time consi;ani;s of the amplifier-recorder system, modulation frequency, and heating rate affect the analytical performance characteristics, viz., S/N ratio, sensitivity, and linearity of the calibration curves. The effect of the response of the amplifier-recorder system on the resolution of peaks was demonstrated previously ( 4 ) ,while the influence of this factor on the calibration curve for lead was shown by Matouselc ( 5 ) . Theoretical considerations concerning the optimum signal-to-noise ratio with respect to the time constant cf the measurement system have been made by L’vov (6) for his furnace. Still, there is lack of experimental data regarding the optimum conditions of the instrumental parameters. T o enable a rational choice of the equipment required for flameless AAS, the analyst needs information concerning the time constants of the measurement system. Depending on the purpose in view, this will be either the time constant corresponding to optimum S/N ratio, or the response time required for undistorted signal reproduction. The main objective (of this study is to evaluate the instrumental parameters in flameless atomic absoprtion on the basis of theoretical and experimental data, viz., noise level, S/N ratio, arid calibration curves for various elements, including bcth volatile and refractory metals, and measured under var:.ousjexperimental conditions.
EXPERIMENTAL Apparatus. The cairbori rod atomizer Model 63 (Varian Techtron: was used in conjunction with a Varian Techtron AA5 spectrophotometer. The carbon-tube, coated with pyrolytic graphite was used as sample cell; the dimensions of this cell are as described by Matousek and Brodie ( 7 ) .Nitrogen was used as sheathing gas a t a flow rate of 4.5 l./min. As detector, a Hamamatsu R 213 photomultiplier was applied (load resistor 0.5 MR). Peak signals were registered with a response system consisting of a PAR Model 126 lock-in amplifier, a transient recorder (Biomation Model 802), and a Varian strip chart recorder Model .4-25. Noise sources and mag-
nitude of the noise levels were examined with a Hewlett-Packard correlator Model 3721 A. Sampling was accomplished with a 5-kl “Excalibur Autopette” fitted with disposable tips. Conventional hollow cathode lamps (Varian Techtron) were used as light sources operated a t the recommended current. Standards. Analytical grade inorganic reagents were dissolved in deionized and doubly distilled water and freshly diluted to the appropriate concentrations. Operation. The measurement system consisting of successively a lock-in amplifier, transient recorder, and a strip chart recorder has been applied in two different ways, Le., with and without the use of the transient recorder. The signal from the amplifier may either be stored in the memory of the transient recorder and, after completion, displayed on the strip chart recorder or, without the transient recorder, be recorded directly on the strip chart recorder. All measurements of noise sources and magnitude of noise levels pertain to the output of the amplifier. The modulation frequency of the primary source utilized during the investigation is 800 Hz unless otherwise reported. In all determinations a sample volume of 5 pl has been used.
RESULTS AND DISCUSSION Noise. T o the final measured noise level in flameless AAS, contributions come from various sources, viz., shot noise, thermal agitation noise, flicker noise, fluctuations of the primary source, mains hum, and internally generated noise in the amplifier. Experimental investigation of the noise spectrum by means of the correlator demonstrated, after careful installation and connection of the various parts of the measurement equipment, that mains hum (50 or 60 Hz) was of no account. According to the specification of the amplifier, the noise figure, using a load resistor of 0.5 MR, was better than 0.05 dB. This means that less than 1%of the noise present a t the input terminals of the amplifier is added as internally generated noise and, consequently, this noise source was negligible. However, for a time constant of 10 msec, the ac component a t the modulation frequency appearing in the output signal may contribute to the noise, because the rms ripple voltage is about 1%of the signal voltage for a firstorder low-pass filter. For a second-order low-pass filter, it is only and, consequently, it could be neglected. According to L’vov (3) ( p 5 3 ) measurement of intensity fluctuations of the resonance line of the hollow cathode lamp with various luminous fluxes reaching the photomultiplier proved that the noise in the photomultiplier tube did not depend on fluctuations in the primary source. l?erformance of this experiment for all hollow cathode lamps used in our study demonstrated an equal result. In all cases, a thousandfold increase of the luminous flux and, consequently, of the photo current by opening the monochromator slits, increased the rms noise voltage only about thirty times, indicating that the noise would probably be restricted t o shot noise. Flicker noise is important in the range 70 Hz and below as appears from (8). However, modulation of the primary source a t 800 Hz together with the use of a lock-in amplifier ensures that this noise source, insofar as it occurs after the modulation step, can also be omitted.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
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Table I. R M S Noise Levels for Various Hollow Cathode Lamps Using the Model 63 Carbon Rod Atomizer Workhead with a Carbon Tube in the Optical P a t h
Element
Lamp
Spectral
\I avelength,
current,
bandwidth,
nm
mx
nm
yoise level: '6 absorption
328.1 4 0.33 0.12 0.15 309.3" 10 0.17 Au 242.8 5 0.33 0.23 0.40 Be 234.8 5 0.33 0.19 Cd 228.8 5 0.66 0.78 co 240.7 10 0.08 5 0.33 0 -27 Cr 257.9 5 0.33 0.09 cu 324.8 8 0.17 0 :40 Fe 248.3 0.25 In 303.9 5 0.33 0.09 Li 670.7 8 0 -66 8 0.17 0 -47 Ni 232.0 0.37 Pb 217.0 7 0.99 8 0.33 0.50 Zn 213.9 a The line is actually a doublet which is not resolved by the used monochromator. The noise level was measured under the following conditions: modulation frequency 800 Hz. second-order lowpass filter with a time constant of 10 msec having an equivalent noise bandwidth of 12.5 Hz, and at a signal level of 100% transmission.
Thus, for practical purposes (i, is about A), the measurement system is shot noise limited and the S/N ratio depends mainly on the square root of the photo cathode current and the bandwidth and less on the factor a which is defined by the current gain. With typical values of F, M and Af being 0.3-0.4, loo%, and 10 Hz, respectively, Equation 3 can be simplified to
S/N = 1.7 X
Ag
Al
Two sources, shot noise and thermal or Johnson noise, remain to be investigated in detail. The current output of the photomultiplier is passed through a 0.5-MR load resistor and the resulting voltage is measured. The relative importance of both noise sources can be better understood from a study of the signal-to-noise ratio formula (9, IO) which has the form
GR&M
