simultaneous determinations of several elements will be useful for unique solid samples as well as for fundamental studies of atomization processes. For general purposes, this technique has certain limitations and, therefore, the spectrometer will mainly be used for special applications. All absorbance measurements have a rather narrow operating range with adequate accuracy. When several elements are to be determined simultaneously, the variations in concentration between the elements are limited and appropriate dilution of a sample can be a problem. For a special application, this can be overcome by selecting resonance lines with suitable sensitivities. Besides, the computer can provide different scale expansions for the elements. The combinations of elements are limited by the availability of multielement hollow cathode lamps and by the portion of the spectrum available. With the grating used, this portion amounts to approximately 100 nm. It is not possible to select wavelengths closer than 2 nm because of the design of the slits, As the number of elements increase, the mentioned limitations become more pronounced and, therefore, a practical upper limit of five elements is expected.
ACKNOWLEDGMENT The authors thank Svante Jonsson and Lars Lundmark for
help with the construction of the equipment used and Michael Sharp for linguistic revision of the manuscript.
LITERATURE CITED (1) B. V. L'Vov, Talanta, 23, 109 (1976). (2) E. F. Palermo, A. Montaser, and S. R. Crouch, Anal. Chem., 46, 2154 (1974). (3) F. L. Fricke, 0. Rose, Jr., and J. A. Caruso, Anal. Chem., 47, 2018 (1975). (4) K. W. Busch and G. H. Morrison, Anal. Chem., 45, 712A (1973). (5) R. E. Santini, M. J. Milano and H. L. Pardue, Anal. Chem., 45, 915A (1973). (6) Y. Talmi, Anal. Chem., 47, 658A (1975). (7) Y. Talmi, Anal. Chem., 47, 699A (1975). (8) J. D. Winefordner, J. J. Fitzgeraid, and N. Omenetto, Appl. pectrosc., 29, 369 (1975). (9) R. Mavrodineanu and R. C. Hughes, Appl. Opt., 7, 1281 (1968). (10) A. Danielsson and P. Lindblom, fhys. Scr., 5, 227 (1972). (11) A. Danlelsson, P. Lindbiom, and E. Soderman, Chem. Scr., 6, 5 (1974). (12) W. G. Elliott, Am. Lab., 2, 67 (1970). (13) G. Lundgren, L. Lundmark, and G. Johansson, Anal. Chem., 46, 1028 (1974). (14) G. Lundgren and L. Lundmark, Anal. Chem., in press. (15) F. D. Posma, H. C. Smit. and A. F. Rooze, Anal. Chem.. 47, 2087 (1975).
RECEIVEDfor review March 17,1976. Accepted July 13,1976. This work was supported by grants from the Swedish Work Environment Fund.
Direct Analysis of Metals and Alloys by Atomic Absorption Spectrometry D. S. Gough Division of Chemical Physics, CSIRO, P.O. Box 160, Clayton, Victoria, Australia 3 1 6 8
Metal samples can be analyzed by maklng atomic absorption measurements on atomlc vapors produced from the solld by cathodic sputtering in an argon glow dlscharge. The Pyrex sputtering chamber can be easily interchanged with the flame atomlzer of a conventional atomic absorptlon spectrophotometer. A dual-modulatlon ampllfler provldes automatlc compensationfor backgroundabsorptlon and for any variatlon in the Intensity of the atomic spectral lamp. Alloys of iron, aluminum, copper, and zlnc have been analyzed, and the followlng elements were determined: antimony, berylllum, cadmlum, chromlum, copper, Iron, magnesium, manganese, molybdenum, nickel, lead, sllver, slllcon, titanium, vanadium, and zlnc. The time per analysis varles from about 2-3 min for brasses and Iron alloys to 5 min for zlnc and 10 for alumlnum alloys. The reproduciblllty depends on the matrlx sputtered and is typlcally f l % for Iron- and copper-base alloys, f2% for alumlnum-base, and f 3 % for rlnc-base alloys. Detection limits are in the range 0.0003 to 0.04%.
Atomic absorption methods of chemical analysis are largely confined to the analysis of solutions. This paper reports progress in the development of methods for the direct analyses of metals and alloys in which some of the solid sample is converted into an atomic vapor by cathodic sputtering in an argon glGw discharge. The technique of cathodic sputtering has previously been used in the analyses of samples by absorption (1-7), emission (8-12) and fluorescence spectroscopy (13).This paper is a sequel to an earlier paper in which the sputtering technique was used in conjunction with the fluorescence technique (13). In that paper it was reported that 1826
some difficulties were encountered when sputtering aluminum alloys due to a tenacious oxide layer which forms on the surface of the metal. With the sputtering chamber then employed, the atomic vapor concentration was too low to permit accurate absorption measurements. It was therefore necessary to use a high-intensity hollow-cathode lamp (14) in conjunction with the fluorescence technique. This paper reports modifications to the design of the sputtering cell which enable aluminum alloys to be sputtered immediately, and which enable higher densities of atomic vapor to be produced so that a simple sputtering attachment can be added to a conventional atomic absorption spectrophotometer in the place of a flame atomizer. Under certain conditions, low levels of background absorption are observed. This background is attributed to agglomerates of atoms which absorb radiation in a broad band through the visible and ultraviolet spectrum. These agglomerates are formed only when there is a high density of metal atoms in the presence of an inert gas, and are present in most of the work reported here. All results in this paper were obtained using an amplifier which provides automatic correction for both background absorption and light-source fluctuations. The operation of the amplifier, and the method whereby background correction is achieved are discussed later in this paper,
EXPERIMENTAL Apparatus. A photograph of the apparatus is shown in Figure 1. Sputtering Cell. The sputtering chamber (Figure 2) consists of a Pyrex tube approximately 15 cm long and 3.5 cm in diameter with quartz windows at either end. It has a 5-cm diameter flat in the middle of the top side on which an O-ring seals the specimen to the cell, thus enabling specimens to be rapidly interchanged. An anode and a pumping port are provided, and also a gas inlet tube fitting into a
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
Recess O4mm r O 5mm
0.1m m
view
Figure 1. Photograph of the apparatus (a)Sputtering chamber, (b)dual-modulation amplifier, (c)sputtering power supply, (d) gas control box
AA
Figure 3. Schematic diagram of the hollow silica annulus admitting gas
into the sputtering chamber. The diagram is not to scale. The gap and recess dimensions have been enlarged for clarity
RopIaceobIe Stl,ca Annulus
'
Anode
1
Pump
Figure 2. Schematic diagram of the sputtering chamber hollow silica annulus on which the specimen sits. A schematic diagram of the replaceable silica annulus is shown in Figure 3. It has three main features. (i) The small recess in the top surface of the annulus is less than the width of the cathode dark space in the glow discharge for the operating conditions used in these experiments. The discharge is thereby confined to the hole through the center of the annulus (15).The recess is also too small to allow sputtered vapor to diffuse into the region between the top surface of the annulus and the cathode, and so the cathode remains electrically isolated from the sputtering cell, even when the cell becomes coated with sputtered metal, or when a metal cell is used. (ii) The flow of pure argon gas a t the surface of the specimen efficiently sweeps away any gaseous impurities, reducing the reaction with readily oxidized elements such as aluminum. In previous work (13),alloys of aluminum and zinc could not be analyzed because the reaction of impurities with the surface of the specimen was too great to allow efficient or reproducible sputtering. However, with the gas flowing a t the surface of the specimen, these elements are sputtered immediately and reproducibly. (iii) The gap in the hollow annulus near the surface of the specimen admits gas into the chamber a t a very high speed. For example, a t a flow rate of 0.3 l./min and a pressure of 5 Torr, the gas speed through this slit is approximately 3 X lo4cm/s, which is the same order as the average thermal speed of metal atoms in the cell. Therefore, few atoms diffuse into this gap. In a static system, a large percentage of the atoms produced in a glow discharge diffuse back to the cathode or condense on nearby sinks. In the apparatus described here, the gas flow past the specimen prevents some diffusion to the cathode or nearby sinks, resulting in an increase in the atom concentration in the sputtering cell (see Figure 4). The sputtered atoms are swept into the body of the chamber a t a speed of about 900 cm/s-a speed much greater than the normal diffusion speed of the metal vapor in argon gas a t the number densities encountered in this work. The atoms are concentrated in the center of the sputtering chamber where the gas speed is high since the effect of lateral diffusion is relatively small. Figure 4 shows a comparison of the spatial distribution of ground-state atoms
Static System
0.01 1.0 Distance
0.5
0
0.5
1.0
From Centre of Sputtering Chamber
(cm.)
Figure 4. Spatial distribution of atoms in a sputtering cell 0.08% Cr in Fa
*
e
- f
-
+
(*-
I
-SCALE EXPAND 15 rc1
t
I-
1
tI -
I
7 Figure 5. 0.08% chromium in iron with scale expansions of 1, 20, and 50 using the dual-modulation amplifier with a time constant of 2 s
using a fast flow of gas past the cathode with that obtained in a static system. Dual-Modulation Amplifier. In a sputtering chamber, trace elements produce very small absorbance readings. However, the background noise level is very low, being due almost entirely to shot-noise induced by the light source (emission from the sputtering cell will contribute substantially to the total noise if a strong line from one of the matrix elements falls within the spectral bandpass of the
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
1927
*
--A-
0
monochromator
atomic spectral lamp modulated at -320Hz
sputtered sample vapour modulated at IOHZ
photomultiplier
amplifier and demodulator
sampling period for absorption
Io
sampling period
Flgure 6. Schematic diagram showing the operation of the dual-modulation amplifier . ~
....
. .
Table I. Reproducibility of Results as Relative Standard Deviation of Absorbance on 10 Runs with the Specimen Removed, Linished, and Replaced between Each Run
I
I
Analyte element
.
s:
n
3.1% Ni 1.2% Cr 1.0%Si 1.13%Fe 2.74% Mg
a
5.13% Si 0.46%Be 2.8% A1 0.1% P b 0.61% Cu 3.9% A1
0
Figure 7. Absorbance measurements for 0.01 % cadmium in brass (a) with compensation for hollow-cathode lamp drifts and background absorption, (b) with no compensation for background absorption and light-source fluctuations.
monochromator). It is possible therefore to amplify the output signal to make accurate measurements (see Figure 5). This amplification can result in large drifts in the light-source intensity and in high background absorption signals. An amplifier has therefore been developed which automatically compensates for light-source drifts and background absorption. A schematic diagram of the operation of the amplifier is shown in Figure 6. The hollow-cathode lamp is modulated at some predetermined frequency (say 320 Hz) while the sputtering discharge is modulated at a sub-multiple of this frequency (10 Hz). The resultant multiplexed signal is amplified and then separated into two components. The hollow-cathode lamp signal measured during the “sputtering-off” cycle is used to adjust the gain of the amplifier to correct for lamp drifts. The lamp signal measured in the “sputtering-on” cycle is then proportional to percent transmission. Approximately 20 ms are required for an atom cloud to build up to 90% of its final concentration when a discharge is initiated. The same time is required for the atom concentration to decay to 10%on switching off the discharge, and so the first half of each “on” and “off” cycle is gated out to obtain a high amplitude of modulation. Although sputtering is free from some of the background effects which can occur in a flame, such as scatter from refractory particles and molecular absorption, some background nonatomic absorption can occur. The degree of attenuation of the incident beam depends on the matrix sputtered and is typically 0.25-1% for iron, copper, and aluminum, and can be 2% or even higher for silver. It has been shown (16, 17) that, where there is a high concentration of metal atoms in the presence of an inert gas, some aggregation of the metal vapor can occur. The metal particles so formed absorb radiation in a broad band with some wavelength dependence, through the ultraviolet and visible region of the spectrum (18-20). Absorption curves have been obtained with sputtered silver (21) which are similar to the optical absorption curves of silver particles with diameters around 5 nm obtained by Berry and Skillman (20). It is concluded, therefore, that at high atom concentrations in a sputtering chamber (typically 1012-1013atoms/ 1928
Matrix
Re1 std dev, %
Brass
0.7 0.9 2.2 2.5 0.9 2.6 2.1 1.0 1.6
Zn
3.3
Zn
2.8
Fe
Fe Fe AI A1 A1
cu
Brass
em3),some aggregation of the metal atoms occurs and results in the formation of small particles. The number of particles can be reduced to an insignificant number by decreasing the sputtering current and gas flow rate to reduce the number of atoms in the sputtering chamber. However, where a high sensitivity is required, the number of atoms must be maximized and, under these conditions, some form of background correction is necessary. Background correction is achieved using the dual-modulation amplifier. The particles formed in the sputtering cell are large, typically 5 nm in diameter, and diffuse very slowly through the argon gas. Therefore, if the sputtering discharge is modulated a t a frequency of say 10 Hz, then the atomic vapor will be modulated, but not the slowly diffusing particles. Thus the atomic absorption can be separated from the nonatomic absorption by adjusting the amplifier gain in the “off’ cycle to give 100%transmission. The absorption measured in the “on” cycle will then be due only to atomic absorption (see Figure 6 for a schematic diagram of the amplifier). Figure 7 shows absorption measurements of 0.01% cadmium in brass with and without compensation for background absorption and light-intensity drifts. The effectiveness of the background correction was clearly demonstrated in determinations of vanadium in iron. The concentrations of vanadium for several samples were close to the detection limit of the apparatus, and the absorbances measured were in the range 0.0005-0.002. In such a case, the calibration curve of concentration vs. absorbance was linear and passed through the origin, providing confirmation that correction for the background is complete. Other Apparatus. The monochromator used was a Varian Techtron model AA3 (focal length 50 cm, grating 5 X 5 cm, 638 lines/mm). A 2-stage rotary backing pump, Edwards ED 100,was used to pump the sputtering chamber to a low pressure and to maintain a flow of gas at the required pressure. The gas used was a special dry argon containing maximum impurity levels of 6 ppm oxygen, 1 ppm hydrogen, 5 ppm carbon dioxide, 30 ppm nitrogen, and 20 ppm water. The gas purity was tested by installing purification columns of titanium sponge to remove nitrogen and oxygen, copper oxide to take out hydrogen, and molecular sieves (Linde 5A) to remove moisture and carbon dioxide. However, the effect of these impurities on the sput-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
T a b l e 11. Detection Limits Obtained for Elements in Various Matrices Analyte element Cr Cr
cu cu
V Mn Mn
Ni 0
2
4
Ni Mo
6
Si
TIME IN MINUTES
Figure 8. Simultaneous warm-up traces for magnesium and iron in
aluminum tering efficiency is insignificant with the exception of water vapor, and the only gas-purification column used in this work was a molecular-sieve trap. The sputtering power supply is current-regulated and delivers up to 100 mA at 800 V. The gas control box is described elsewhere (22), and is designed to give a constant flow of gas at a constant pressure. Procedure. The specimen must have a flat, smooth surface of not less than 30-mm diameter so that it will seal against the O-ring shown in Figure 2. The back surface of the specimen must also be flat to ensure effective water cooling, which is necessary to prevent thermal vaporization, outgassing, oxidation, and self-diffusion within the specimen. A sufficiently flat surface can be produced by holding the specimen against a belt sander for a short time. The gas flow rate is normally set at about 0.3 l./min and the gas pressure is typically 5 Torr. The total time required for an analysis is several min. About 20 s are required for the gas in the sputtering chamber to be adjusted to the required pressure, and several minutes for the sputtered sample to give a steady signal. The actual sputtering time required depends on the matrix being sputtered, and varies from about 1min for copper-base alloys to about 8 min for aluminum-base alloys. The sputtering current also depends on the alloy being examined, mainly because it is necessary to keep the voltage below 800 V to avoid breakdown of the arrester recess (see Figure 3) or excessive heating of the specimen. The iron specimens used were British Chemical Standards low-alloy steels, issued by the Bureau of Analysed Standards (Middlesbrough, Teeside, England). The aluminum alloys were spectrochemical standards from Alcoa (New Kensington, Pa.), and all others were from the U.S. National Bureau of Standards. They were all in the form of disks of diameters large enough to span the O-ring shown in Figure 2. The operating conditions varied with the alloy being sputtered but, in each case, the sputtering chamber was operated in the abnormal (voltage depending upon current) mode of the glow discharge which ensures a constant sampling area of the specimen. (In the normal mode, the discharge does not cover the entire cathode and any variations in the surface conditions of specimens may result in a variation in the area sampled.) The typical operating pressure was between 4 and 6 Torr at a flow rate of 0.2-0.3 l./min. The operating currents were typically 50 mA for iron-, zinc- and copper-base alloys, and 80 mA for aluminum-base alloys. These conditions result in a maximum removal rate of sample of 0.0004 g/min for iron-base alloys and 0.0003 g/min for aluminum, but these conditions can be varied to suit the measurements t o be made. A typical warm-up trace for magnesium and iron in aluminum is shown in Figure 8. Traces obtained for iron-base alloys have a similar shape to magnesium in aluminum but reach equilibrium more quickly.
RESULTS T h e reproducibility of t h e results for elements in several matrices a r e presented in T a b l e I. A series of 10 readings was taken for each element a n d t h e specimen was removed, linished, and replaced after each reading t o present a fresh surface t o be sampled. T h e relative s t a n d a r d deviation for each series varies from 1%for iron-base alloys t o 3% for zinc-base alloys. Calibration curves passing through t h e origin were
Mg Zn Cr Cr cu CU
Mn Mn Ni Ni Ti Fe Fe Si Zn
Ni Fe
Si Be Cd Ag Mn Mn
Ni Ni Fe Fe
Sb Pb Sb Mg
Cr
Cr Cd cu Cu Mn Mn
Ni Ni Pb
Si Fe Fe
Matrix Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe A1 A1 A1 A1 A1 A1 A1 A1 A1
A1 A1 A1 A1
A1 cu cu cu cu Brass Brass Brass Brass Brass Brass Brass Brass Brass Brass Brass Brass Zn Zn Zn Zn Zn
2n Zn Zn Zn Zn 2n Zn Zn Zn
Wavelength, nm 357.87 357.87 324.75 324.75 318.54 279.48 279.48 232.00 232.00 313.26 251.61 285.21 213.86 357.87 357.87 324.75 324.75 279.48 279.48 232.00 232.00 364.27 248.33 248.33 251.61 213.86 232.00 248.33 251.61 234.86 228.80 328.07 279.48 279.48 232.00 232.00 248.33 248.33 217.58 217.00 217.58 285.21 357.87 357.87 228.80 324.75 324.75 279.48 279.48 232.00 232.00 217.00 251.61 248.33 248.33
Lamp current,a mA
Detection limit, 6
7 131400 3 4.51’500 20 7 251600 7 121600 7 10 3 7 7 131400 3 4.51500 7 251600 10 121600 20 7 161600 15 6 10 7 15 7 3
0.003 0.001 0.005 0.001 0.009 0.012 0.006 0.012 0.003 0.016 0.04 0.001 0.008 0.005 0.001 0.008 0.002 0.012 .0.007 0.019 0.007 0.03 0.04 0.01 0.08 0.001 0.005 0.011 0.013 0.0003 0.0005 0.0006 0.002 0.0009 0.004 0.0007 0.008 0.002 0.018 0.02 0.006 0.0002 0.0003 0.0001 0.0007 0.0009 0.0003 0.002 0.0005 0.003 0.001 0.006 0.006 0.007 0.001
5 7 251600 10 121600 7 161600 10 6 121650 3 7 131400 3 3 4.5/500 7 251600 7 1216 ’ 00 6 15 5 16/600
%
a Where one number is given, it refers to a hollow-cathode lamp. Where two numbers are given, the first refers to the cathode current and the second to the booster current of the high-intensity lamp described in reference (24). The detection limit is defined as the concentration required t o give signal equal to the peakto-peak noise with a time constant of 2 s. ~
~~
obtained for each of t h e elements examined, indicating an absence of background absorption. T h e width of a spectral line emitted by a hollow-cathode lamp is very much narrower t h a n t h e width of a n absorption line i n a flame (23). When t h e atomic vapor is produced by
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
1929
r
length. At present the path length is about 1-2 cm but it could be increased to 10 or 20 cm by measuring absorption along the /Of axis of the plume of atoms swept down the center of the sputtering cell by the fast gas flow (Figure 4); an increase in 0.5 b, sensitivity would therefore follow. Alternatively. a high-intensity source could be used and the atomic vapor analyzed by atomic fluorescence. This has the added advantage of al04lowing simultaneous multielement analysis (26,27). If an analysis of the bulk of the sample is required, then the sample must be homogeneous since only the first few micrometers of the surface are normally sputtered. The sampling area is 0.64 cm2so segregation on a micro scale should not lead to spurious results. Sputtering is an ideal technique for removing thin layers from a surface in a well-controlled manner (28).The composition of these layers can then be measured by atomic absorption spectroscopy (29). Matrix Effects. Few matrix effects were observed over a Concentration p q h l wide range of concentrations from trace amounts to alloying I 7.5 10 concentrations. There is, however, a matrix effect for some 1 2 3 4 5 elements in aluminum at the surface of the metal. Two pheConcenirotion X Ni nomena combine to produce this effect-the aluminum surface is readily oxidized on exposure to air, and a number of Figure 9. Calibration curves for nickel elements will not dissolve to any extent in aluminum. In the Spectral line 232.0nm, monochromator slit width 50 pm. a, = hollow-cathode case of iron, all the iron will be present as FeA13 crystals. These lamp current 5 mA, flame absorber: b, = hollow-cathode lamp current 10 mA, crystals are far more conducting than the oxide layer which flame absorber: as = hollow-cathode lamp current 5 mA. sputtering-chamber absorber; b, = hollow-cathode lamp current 10 mA, sputtering-chamber abcovers most of the alloy. Consequently, when a discharge is sorber initiated, the current density on these FeAls crystals is high. The iron is sputtered rapidly until the FeA13 crystals have cathodic sputtering, however, the absorption line width will been depleted. The slow-sputtering oxide layer is eventually be about the same as the width of the emission line. Under removed to reveal FeA13 crystals in a matrix of aluminum. normal operating conditions, curvature in the calibration This may take 4-6 min (see Figure 8 for simultaneous traces graph will be evident a t lower absorbances than with a flame of magnesium and iron in aluminum). At this equilibrium, the atomizer (see Figure 9). However, for trace elements, the absputtered vapor is expected to be representative of the bulk, sorbance is low and the calibration graph linear. For minor and is certainly reproducible. Similar behavior is observed for constituents, it is usually possible to reduce the sputtering silicon in aluminum where the silicon does not go into solid current and the gas flow rate so that the absorbing atom solution in the aluminum. concentration is not too high. For major constituents, it is necessary to use a less sensitive spectral line. Detection limits for several elements in various matrices are shown in Table 11. The specimen was chosen in each case ACKNOWLEDGMENT so that the concentration of the element to be analyzed was The author thanks A. Walsh for his encouragement and as near to the limit of detection as possible. The time constant many helpful suggestions throughout this project, and also for the measurement was 2 s. Since most of the noise is shot J. Meldrum for the design and construction of the dualnoise induced in the detector by the light source, a high-inmodulation amplifier. tensity source emitting intense narrow lines (24)was used to increase the signal-to-noise ratio for some elements. The improvement obtained in the achievable detection limit is shown in Table 11. LITERATURE CITED It will be noticed that the sensitivity of one element relative (1)6.J. Russell and A. Walsh, Spectrochim. Acta, 15, 883 (1959). to another in a sputtering cell is not the same as in a flame (2) 6.M. Gatehouse and A. Walsh, Spectrochim. Acta, 16, 602 (1960). atomizer. For example, chromium has a higher sensitivity than (3)A. Walsh, Proc. Xth Colloquium Spectroscopium Internationale, Washington, p 127 (1962). copper in a sputtering chamber. Generally, where there are J. A. Goleb and J. K. Brody, Anal. Chim. Acta, 28, 457 (1963). refractory oxides or compounds formed in the flame, the relA. J. Stirling and W. D. Westwood, J. Appl. Phys., 41,742 (1970). A. J. Stirling and W. D. Westwood, J. Phys. D, 4, 246 (1971). ative sensitivity in a sputtering cell will be greater. There is B. W. Gandrud and R. K. Skogerboe, Appl. Spectrosc., 25, 243 (1971). some population of the lower energy levels below about 1000 W. Grimm, Naturwissenschaffen, 54,586 (1967). cm-' (25),but most of the atoms in the region of viewing are W. Grimm, Spectrochim. Acta, Part B, 23,443 (1968). P. W. J. M. Boumans, Anal. Chem., 44, 1219 (1972). single and are in the ground state. The populations of the M. Dogan, K. Laqua, and H. Massmann, Spectrochim. Acta, Part B, 27,65 low-lying energy levels for the neutral iron atom and singly (1972). H. W. Radmacher and M. C. de Swardt, Spectrochim. Acta, Parts, 30,353 ionized iron atom were measured. It was found that under (1975). typical operating conditions 73% of the iron atoms were in the D. S.Gough, P. Hannaford, and A. Walsh. Spectrochim. Acta, Part B. 28, ground state and 1%ionized. The number of atoms in the 197 (1973). R. M. Lowe, Spectrochim. Acta, Parts, 26, 201 (1971). ground state varies from element to element depending on the A von Engel, "Ionized Gases", Oxford University Press, Oxford, 1955,p operating conditions and on the energy levels of the lower206. K. Kimoto and i. Nishida, Jpn. J. Appl. Phys., 6, 1047 (1967). lying states. o.6
I
DISCUSSION As shown in Table 11, the detection limits are at present around 0.01 to 0.001% but it is possible to increase the sensitivity of the measurements by increasing the absorption path 1930
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
Yu. I. Petrov, Opt. Spectrosc. (USSR), 27,359 (1969). U. Kreibig and C. v. Fragstein, 2. Phys., 224,307 (1969). U. Kreiblg, 2.Phys., 234,307 (1970). C. R. BerryandD. C. Skillman, J. Appl. Phys., 42,2818 (1971). R. M. Lowe, private communication. P. L. Larkins, to be published. A. Walsh, Spectrochim. Acta, 7, 106 (1955).
(24) R. M. Lowe, Spectrochim. Acta, in press. (25) P. Hannaford, unpublished work. (26)D. G. Mitchell and A. Johansson, Spectrochim. Acta, Part 6,25, 175 (1970). (27) R. M. Dagnall, G. F. Kirkbright,T. S. West, and R. Wood, Ana/yst(London), 97, 245 (1972).
(28) T. F. Fisher and C. E. Weber, J. Appl. Phys., 23, 181 (1952). (29) J. E. Greene and F. Segueda-Osorio, J. Vac. Sci. Techno/., 10, 1144 (1973).
for review
19, 1976* Accepted
19,
1976.
Noise at Detection Limit Levels in Atomic Absorption Flame Spectrometry Peter
I?.
Liddell
Varian Techtron my. Ltd., P.O. Box 222, Springvale 3 171, Victoria, Australia
The standard deviation in the absorbance at zero concentration is experimentally determined for ten elements in an airacetylene flame. The contribution of various noise sources to the total standard deviation is determined and the domlnant source at the detection limit is found to be photon noise, lamp flicker noise, or flame transmission noise.
Amplifier-Readout Noise. UAR. This is the standard deviation in the absorbance due to the electronic processing of the signal and any errors in the readout device. If these noise sources are assumed to be independent, their standard deviations add quadratically and the standard deviation in the measured absorbance of the blank solution is given by UA
There are a number of published works giving detailed theoretical analyses of noise in atomic absorption measurements, both over a broad absorbance range (1-6) and over the narrow absorbance range near the detection limit (7-11). Some experimental data have been published on the measurement of noise in the optimum working range (1-3, 6, 12-15). It is generally agreed (1,3,6,14)that most of the noise in this range is due to fluctuations in the absorption properties of the analyte, although analyte emission noise can also be important for some elements. Both of these noise sources depend on the concentration of the analyte and can be assumed to be negligible at the low concentrations corresponding to the detection limits. Factors which may affect the standard deviation in the absorbance, U A , at the detection limit are (5, 6 ) the following. Photon Noise. This consists of statistical fluctuations in the photocathode current, i, generated by light from the lamp and is sometimes called signal shot noise. The standard deviation in the absorbance due to photon noise, up, depends on i according to the relationship (5),
Lamp Flicker Noise, UL. This is the standard deviation in the absorbance due to fluctuations in the output intensity of the light source. Flame Transmission Noise, UF. This is the standard deviation in the absorbance due to fluctuations in the transmission of light through the flame. It includes the effect of aspirating a blank solution. Background Emission Noise, UBE. This is the standard deviation in the absorbance due to emission from the flame and the blank solution. It includes flicker noise caused by fluctuations in the emission intensity and photon noise caused by the increase in the intensity of light at the photomultiplier. However, if a modulated light source is used, any fluctuations which occur at a frequency less than the modulation frequency should be corrected for. Dark Current Noise, UD. This is the standard deviation in the absorbance caused by emission of electrons from the photocathode without the arrival of a photon.
= (Up2
+
UL2
+
UF2
+ UBE2 + UD2 + .AR2)1’2
(2)
In this paper, the magnitude of these noise sources is experimentally determined for a number of elements under standard operating conditions.
EXPERIMENTAL Apparatus. A Varian Techtron atomic absorption spectrophotometer, Model AA-GD, was used for all measurements. This instrument employs single beam optics with the light source modulated a t 285 Hz with a 50%duty cycle. In every case, the light source used was a Varian Techtron hollow cathode lamp. The instrumental conditions are listed in Table I. For the absorbance measurements, the digital readout on the instrument was used with fifty times scale expansion. For the %T measurements, a National chart recorder, Model VP653A, with a resolution of 1 WVor 0.001%Twas used. Procedure. For each noise measurement, a series of 101 consecutive 10-s integrations was obtained. This series was handled in the same way as an actual analysis consisting of alternating blank and sample measurements. Every second reading was treated as a sample measurement and the average of the preceding and following readings (treated as blanks) subtracted from it. This yielded 50 readings corrected for any long-term drift in exactly the same way as in an actual analysis. The standard deviation was then calculated from these readings. The following measurements were made for each element using this procedure. 1)The standard deviation in the absorbance was determined with the lamp on and the flame off, giving (up2 u~~ The photon noise here will be less than that in Equation 2 for those elements where the flame or the blank produce significant absorbance. 2) The standard deviation in the absorbance was determined with the lamp on, the flame on, and distilled water aspirating, giving UA. Prior to this step, the burner position and flame stoichiometry were adjusted for maximum absorbance using a standard aqueous solution of the element under investigation. 3) The standard deviation in the %T mode was determined with the lamp off, the flame on, and distilled water aspirating. The monochromator settings and photomultiplier supply voltage were the same as in step 2. The effect of this O%Tnoise on the noise in the absorbance mode at zero absorbance (100%T)was determined from the relationship ( 5 ) , u(%T) u (abs) = 0.4343 (3) 100 where dabs) and u(%T) are the standard deviations in absorbance
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
1931
