Anal. Chem. 1980, 52, 642-646
642
it is possible to view only one side of the flame, or the center may be blocked to prevent the interfering light from reaching the photometer. There are also several disadvantages in using the primary flame in the inverted configuration. The H P O formation region is large. Since it is experimentally difficult to measure the emission from the entire zone, optimum sensitivity cannot be easily achieved. Also, since any aspirated organic materials fuel the flame, both the shape of the primary reaction zone and the position of the brightest HPO emission (when viewed side-on) change as a function of the solvent composition. Concentrations of acetone and acetonitrile of 30% and 5070 by volume, respectively, when aspirated a t 6 mL/min, caused flame instability. Aspirating solutions containing higher than 50% of these solvents at this rate would extinguish the primary flame. However, at aspiration rates of 2 mL/min and less, there was no stability problem. These disadvantages do not exist when viewing the secondary flame. As long as the primary flame stays lit, the secondary flame geometry remains virtually unaffected by the solvent composition. All of the advantages of the inverted primary flame still apply to the upper flame except for spectral interferences: as with any normal flame, a photometer must “look” through a reaction zone to see H P O within. The use of the inverted ai-hydrogen flame or its secondary flame has several potential hazards. If the secondary flame should be accidentally extinguished, hydrogen will be released. Also, phosphorus, when present, will be released in the form of phosphine. Carbon will be released in the form of CO. Organic nitrogen may be released as hydrogen cyanide (especially if nitriles are aspirated in the sample). No special precautions other than routine venting are requiren33as long as the secondary flame is lit.
components of aqueous/organic liquid samples is greatly reduced by burning the ai-hydrogen flame “inside-out”. Either the primary or secondary flame may be used for the photometric measurement. Some quenching still occurs, especially when organic nitrogen-containing compounds are aspirated. Acetonitrile was the worst solvent encountered in this respect. However, if this flame configuration were used as the basis for an HPLC detector, methanol could be substituted for acetonitrile in most applications in order to allow phosphorus-selective detection. Ion pair chromatography of phosphonic acids may also be possible with phosphorus-selective detection using tetraalkylammonium pairing ions.
CONCLUSIONS
RECEIVED for review August 20,1979. Accepted January 11,
T h e quenching of H P O formation or emission by matrix
ACKNOWLEDGMENT The contributions of N. J. Holzschuh in gathering experimental data, of H. Ankenbauer and the MVL Machine Shop, and of R. J. Lloyd in assisting with the gas chromatography measurements are gratefully acknowledged.
LITERATURE CITED Salet, G. Ann. Phys. 1869. 737, 171. Syty, A.; Dean, J. A. Appl. Opt. 1968, 7 , 1331. Veiilon, C.; Park, J. Y. Anal. Chim. Acta, 1972, 60, 293-301. Brody, S. S.; Chaney, J. E. J . Gas Chromatogr. 1966, 4 , 42-46. Julin, B. G.; Vandenborn, H. W.; Kirkhnd, J. J J. Chromatogr. 1975, 172, 443-453. (6) Chester, T. L.; Lewis, E. C.; Benedict, J. J. Unpublished work, The Procter & Gamble Company, Miami Valley Laboratories, 1978. (7)Dagnall, R . M.; Thompson, K . C.: West, T. S. Analyst(London) 1968. 93, 72-78, (8) Draeger, 0. West German Patent 1 133918 (1962). (9) Van der Smissen, C. E. U S . Patent 3213747 (1965). (10) Patterson. P. L.: Howe. R. L.; Abu-Shumavs, A. Anal. Chem. 1978, 50. 339-344. (11) Haraguchi, H.; Winefordner, J. D. Appl. Spectrosc. 1977, 37, 195-199. (1) (2) (3) (4) (5)
1980.
Non-Dispersive Atomic Fluorescence Spectrometer for the Direct Determination of Metals D. S. Gough” and J. R. Meldrum CSIRO Division of Chemical Physics, P.O. Box 160, Clayton, Victoria 3 168, Australia
An atomic fluorescence spectrometer for the determination of elemental concentrations in metal samples is described. The atomic vapor produced from the sample by cathodic sputtering is irradiated by intense lamps and atomic fluorescence is detected at right angles to the incident light. The amplifier of the spectrometer incorporates a feedback loop so that compensation is made for fluctuations in the lamp intensity during a series of measurements. Detection limits and reproducibility of measurement have been determined for various elements in alloys of iron, aluminum, and copper. The precision of the measurements is typically f2% for minor and trace constituents in the samples. Detection limits are in the range 1-100 ppm.
T h e technique of cathodic sputtering is a convenient method for the production of atomic vapors from metal samples. T h e vapor produced is contained in an inert atmosphere, usually argon, which minimizes the possibility of chemical 0003-2700/80/0352-0642$01 . O O / O
reaction such as the formation of compounds. Vapors sputtered from solids have been analysed by emission (1-4), absorption (5-8) and fluorescence spectrometry (9). In an earlier paper (7),instrumentation was described for analysis of metals by atomic absorption in which a sputtering cell replaced a flame in a commercial atomic absorption spectrometer. The apparatus was convenient to use and the precision of the measurements was high. This paper describes a spectrometer in which the sample is atomized by cathodic sputtering but analysed by atomic fluorescence, The advantages of this spectrometer are that the detection limits achieved are approximately an order of magnitude lower than for absorption measurements, and the light path is easily purged with dry argon for the detection of elements whose resonance lines lie in the vacuum ultraviolet. The spectrometer is of the nondispersive type described by Larkins ( I O ) with approximately 1:l imaging of the fluoresced radiation on the detector, so that fluorescence is detected over a large solid angle. T h e light source intensity is measured continuously and a feedback loop within the amplifier provides compensation for variations in @ 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980
I
I
1
I
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I
643
I
nIc
ARGON
t OUT
I I
I
I I
T I
c
% M
I
I
1
D
-
I I
1
DIGITAL GEN E RATOR
-I
AMPLIFIER
Figure 1. Schematic diagram of spectrometer
intensity during a series of measurements.
EXPERIMENTAL Apparatus. A schematic diagram of the spectrometer is shown in Figure 1. Light emitted by the lamp A is split into two paths
by the beam-splitter B1. The light source intensity is monitored using light reflected from the mirror M (radius of curvature 17 cm) and the beam recombiner B2 to the photomultiplier E. Fluorescence excited in the sputtering cell C is detected by the same photomultiplier. The chopper D is designed so that the two signals are 180° out of phase and these signals are separated by the amplifier which is described below. The beam-splitter B1 and recombiner B2 are made from high quality quartz (Suprasil) so that only a small fraction of the light is reflected. Lenses L1 and L2 are made from calcium fluoride to permit the transmission of spectral lines in the vacuum ultraviolet. The entire light path is enclosed in an aluminum box which can be purged with dry argon. The lamp A is a metal version of the boosted glow-discharge lamp with interchangeable cathode described by Sullivan (11). This lamp is typically 30-100 times as intense as a commercial hollow-cathode lamp. For nondispersive fluorescence measurements, it is essential that the cathode of the lamp does not contain impurities which are present in the sample. For this reason Marz grade pure metals (Materials Research Corporation, Orangeburg, N.Y.) were used in the cathode of the lamp. The lamp requires three separate power supplies, one operated at approximately 30 mA (peak) and 700 V to sputter atoms from the cathode and provide some excitation, a filament heater run a t 2 V and 5 A to provide a source of electrons, and a supply run at 600 mA and 40 V connected between the anode of the lamp and the filament to provide further excitation of the sputtered atoms. The sputtering cell is similar to one described previously (7) but is in cruciform shape so that the fluorescence signal can be measured at right angles to the incident light beam. The lamp and the sputtering cell require a constant flow of pure, dry argon at predetermined flow rates and pressures. Each requires a vacuum pump (Edwards ED1001 and a gas-control unit (12). A Hamamatsu Rl66 photomultiplier is used for elements whose resonance lines lie below 300 nm, and for the elements Ag, Cu, Cr, V, the detector is an EM9783B photomultiplier. Because
the detector is enclosed in a light-tight box, and measurements are made after the glow-discharge emission has decayed substantially, it is not necessary to use any light filter. Electronics. In order to eliminate most of the noise induced in the detector by emission from the sputtering cell, the glowdischarge is modulated and fluorescence measured a few milliseconds after the glow-discharge is switched off. The atomic vapor decays much more slowly than the emission so that the measurement is made when the emission intensity has decreased by three or four orders of magnitude but. the atomic vapor concentration has decayed by only 20% of its maximum value. Because emission from the glow-discharge can be high enough to cause fatigue in the photomultiplier, the mechanical chopper is used to prevent this emission reaching the detector. The signals reaching the photomultiplier during a measurement cycle are composed of two separate but phase-related components, the signal components and the lamp intensity components. The timing of the sampling pulses used to measure both the fluorescence signal and the lamp intensity is shown in Figure 2, which also indicates key waveforms generated in the amplifier and digital circuitry. The master oscillator operates a t approximately 330 Hz and is used to trigger the lamp power supply. All other waveforms and trigger pulses generated are sub-multiples of this frequency and are shown in Figure 2. From Figure 2b it can be seen that the sputtering discharge is modulated at approximately 5 Hz. The emission from the glow-discharge 2c is blocked by the chopper 2d so that the photomultiplier is not overloaded. Figure 2e shows the output from the photomultiplier during that part of the cycle when the signal is measured. During time t l , the light source is switched off, so that the residual emission from the glow-discharge pulse can be measured (using sample pulses h and i). The atomic fluorescence signal is measured within time t 2 (using sample pulse 9). This measurement contains a component of signal due to scattering of lamp radiation from reflecting surfaces (measured by sample pulse 20, and residual emission from the glow-discharge. The sample pulses f-i are fed into circuits incorporating differential sample-and-hold amplifiers which isolate the atomic fluorescence signal. The lamp intensity is measured during the time when the sample is sputtered. This occurs twice during a measurement cycle as shown in Figure 2j.
044
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980
d
L-ONE LIGHT SOURCE CYCLE (a)
SPUTTERING CYCLE
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SIGNAL DEMODULATOR :TYPICAL SIGNAL LEVELS)
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SAMRE
Figure 2. Waveforms generated and sequence of operations
-,
E REF
PHOTOMULTRIER SIGNAL
TO POWER SUPPLIES GENERATOR CHOPPER
Figure 3.
Schematic diagram of the amplifier
Amplifier. Figure 3 shows the layout of the amplifier. The multiplexed current signal from the photomultiplier is converted t o a voltage and fed into a common amplifier whose output is accepted into either the fluorescence signal channel, or the lamp intensity channel. The gain of this amplifier is a function of the lamp intensity and is automatically adjusted to compensate for lamp intensity fluctuations in the following manner. The dc output voltage from the lamp demodulator is proportional to the lamp intensity. This voltage is compared with a reference voltage in the error amplifier. Any difference resulting from this comparison is reflected in the output resistance of the lamp/photocell module (MLT726A) which modifies the gain of the common amplifier. The gain of the signal channel can be adjusted to any suitable level and can be set using a standard sample so that the read-out indicates concentration directly. The digital circuit generates all the trigger pulses necessary to synchronize the operation of the amplifier, power supplies, and chopper drive. Measurements were made on the amplifier in order to determine its ability to compensate for light-source drift. The sputtering cell (C in Figure 1)was replaced by a mirror to reflect light into the photomultiplier. Light from the lamp was attenuated by 50% (using a wire mesh
filter) and the signal reflected from the mirror was measured. This signal was found to be within 0.5% of its original reading, indicating that adequate compensation is provided in the amplifier for lamp intensity fluctuations. Operating Conditions. The argon gas in the lamp is maintained a t a constant pressure of about 0.27 kPa (2 Torr) and a flow of 0.1 L/min. In the sputtering cell, the argon gas is maintained a t 0.66 kPa ( 5 Torr) pressure and a flow of approximately 0.3 L/min. The sputtering current is kept to a level such that no marked curvature is found in the calibration graph; in the analysis of low-alloy steels currents of 20 mA (peak) for the determination of Mn and Ni and 40 mA (peak) for Cr and Mo were found suitable. If more than one standard sample is available, some curvature will be tolerable and higher currents can be used, leading to a larger dynamic range. Preparation of Samples. A flat, scratch-free surface of 30-mm diameter is required so that the specimen forms a vacuum seal with an O-ring on top of the sputtering cell (7). A satisfactory surface was obtained by rubbing the specimen on a piece of wet-and-dry paper (320 grit) using alcohol as a lubricant. The steel standards used were British Chemical Standards low-alloy steels issued by the Bureau of Analysed Standards (Middles-
ANALYTICAL CHEMISTRY, VOL. 52. NO. 4, APRIL 1980
BRASS
Table I. Detection Limits for Elements in Various Matrices matrix material low-alloy steel low-alloy steel low-alloy steel low-alloy steel low-alloy steel A1
analyte element
645
detection limit,' ppm
Ni Si cu co
10 40
V
100
2 10
20 cu 10 Ni Al 4 Cr A1 Fe Al 100 10 brass Bi Ni 4 brass brass 7 Fe brass Pb 20 brass Sn 60 1 brass Ag Fe 20 cu Ni 1 cu 1 cu Ag ' Detection limit is defined ( 1 3 )as the concentration of analyte element for which the fluorescence intensity has a value equal to three times that of the standara deviation of a series of 1 0 readings measured on a blank sample. Figure 4. Profiles of surfaces sputtered for 3 min under typical operating conditions, viz., 40 mA (peak), 600 V, 5 Torr. (a) Cartridge brass. (b) Low-alloy steel
Table 11. Reproducibility of Fluorescence Signal' % analyte
matrix
element
AI A1 Fe Fe brass brass
0.03% N i 0.10% c u
reproducibility as % RSD
0.30% Cu 0.12% Ni 0.05% Ni 0.11% Pb
2.6
2.3 1.6 2.0 6.0 6.6
' Expressed as the relative standard deviation on 1 0 runs with the sample removed and rubbed on wet-and-dry paper between runs. brough, Teeside, England), the aluminum alloys were spectrochemical standards from Alcoa (New Kensington, Pa.), and the other standards were from the U.S. National Bureau of Standards. They were all in the form of disks. Some low-alloy steel samples were prepared for flame atomic absorption measurement by dissolving 1 g of the sample in a 4:l v/v mixture of hydrochloric and nitric acids and diluting to a 1% w / v solution. Manganese and nickel were determined using an &-acetylene flame, and chromium using a nitrous oxideacetylene flame.
RESULTS AND DISCUSSION T h e samples used for the measurement of detection limits and precision were t h e spectrochemical standards described previously. Detection limits for elements in several matrices are shown in Table I. T h e limit of detection is defined (13) as the concentration of analyte giving a net fluorescence signal above the blank signal of 3 times the standard deviation of the blank signal. Unfortunately, for most of the samples measured here, Table 111. Analysis of Low-Alloy Steels' Cr, % sample this work flame AAS 1
2 3
4 a
0.58 0.46
0.59 0.47
0.03 0.69
0.03 0.69
no true blank specimen was available; in these cases the standard deviation was derived from the signal given by the sample with the lowest analyte concentration. For each determination of detection limit, measurements were made for different analyte concentrations and a curve of fluorescence signal vs. concentration was plotted; extrapolation of this curve to zero concentration gave the blank signal. A check of this blank signal was made by sputtering a pure sample of the element corresponding to the major constituent of the alloy. In all these replicate measurements, the discharge was switched off for 30 s between readings and then switched on for 20 s before beginning a 10-s integration of the signal. T h e reproducibility of the fluorescence signal for several elements in various matrices was measured t o demonstrate the precision available from the spectrometer (Table 11). A number of low-alloy steels were analyzed using the fluorescence spectrometer, with a single standard (low-alloy steel BCS/SS 404). T h e fluorescence signal for each sample was taken as directly proportional to the analyte concentration in that sample. In each determination the sample was sputtered for 2 min before beginning a 10-s integration. The samples were also analyzed by flame absorption for Cr, Mn, and Ni and by X-ray fluorescence for Mo. The data are shown in Table 111. Some background signal is usually present, i t s contribution to the total signal being dependent on the system under observation. This background is thought to be due to scattered radiation from aggregates of metal vapor (7, and is very small. The maximum background signal observed was equivalent to
Mn, %
Ni, %
this work
flame AAS
this work
flame AAS
0.64 0.91 0.86 1.55
0.66 0.90 0.86 1.61
2.81 0.52 0.01
2.87
0.24
0.51 0.02 0.23
Mo, 7c this work XRF 0.47 0.18
The standard used was BCSISS404 (certificate values: Cr 0.68%, Mn 0 . 5 2 % , Ni 0.46%, M o 0.33'%).
0.46
0.17
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646
Anal. Chem. 1980, 52, 646-650
the signal produced by a concentration equal to ten times the detection limit. The technique of sputtering appears to be useful for the study of surface layers since the layer sampled is only a few micrometers in depth. Figure 4 shows the profiles of the surfaces of brass and iron after sputtering for 3 min under typical operating conditions. These profiles were obtained using a surface texture measuring instrument (Talysurf 10, Rank Taylor Hobson, Leicester, England). It can be seen that the bottom of the sputtered crater is essentially flat, indicating that the sputtering technique may be useful for the measurement of concentration gradients through a specimen. The present instrument gives about a 10-fold improvement in limits of detection over those obtained in earlier work in which nondispersive florescence from sputtered vapors was measured (9). This can be attributed to a better design of the sputtering cell and to more sophisticated electronics which allows the measurement of fluorescence signal without the intense emission from the glow-discharge. Automatic compensation for lamp intensity drift in the present instrument enables precise measurements to be made within a few minutes of switching on the lamp. Circuit diagrams are available on request.
ACKNOWLEDGMENT The authors thank P. Larkins for the flame AAS analyses and W. Mannens, of Broken Hill Proprietary Company, Melbourne Research Laboratories, for the X-ray fluorescence measurements.
LITERATURE CITED (1) W. Grimm, Spectrochim. Acta, Part B, 23, 443 (1968). (2) R. A. Kruger, L. R. P. Butler, C. J. Liebenberg, and R. G. Bohmer, Analyst (London), 102, 949 (1977). (3) K. Naganuma, M. Kubota, and J. Kashima, Anal. Chim. Acta, 9 8 , 77 (1978). (4) M. E. Waitlevertch and J. K. Hurwitz, Appl. Spectrosc., 30, 510 (1976). (5) B. M. Gatehouse and A. Walsh, Spectrochim. Acta, 16, 602 (1960). (6) A. J. Stirling and W. D. Westwood, J . Phys. D . , 4 , 246 (1971). (7) D. S. Gough, Anal. Chem., 4 8 , 1926 (1976). (8) D. C. McDonald, Anal. Chem.. 4 9 , 1336 (1977). (9) D. S. Gough, P. Hannaford, and A. Waish, Spectrochim. Acta, Part 6 , 28, 197 (1973). (10) P.. L. Larkins, Spectrochim Acta, Part 6 , 26, 477 (1971). (11) J. V. Sullivan, Anal. Chim. Acta, 105, 213 (1979). (12) P. L. Larkins, Anal. Chim. Acta, to be submitted. (13) IUPAC Commission on Spectrochemical and other Optical Procedures for Analysis, Spectrochim. Acta, Part 6 ,33, 242 (1978).
RECEIVED for review August 14, 1979. Accepted December 26, 1979.
Photoacoustic and Spectrophotometric Quantitation of Copper Phthalocyanine Films David Hursh' and Theodore Kuwana" Department of Chemistry, The Ohio State University, Columbus, Ohio 432 10
A single beam photoacoustic spectrometer is described which used a xenon arc lamp source providing between 8 X and 2.5 X W of radiant power in the visible region. The sample celVmicrophone detector sensitivity for a thermally thin V/W of absorbed sample on glass at 206 Hz was 8 X power and response was linear in the range of 2 X to 2 X lo-' W of absorbed power. The photoacoustic spectra of thin films of copper phthalocyanine on glass agreed serniquantitatively with their optical absorption spectra and monolayer coverages were detected. The photoacoustic detection W of absorbed power which corresponded limit was 1 X to an optical absorbance of 4 X
of a dye and to compare the results obtained by PAS with those obtained by conventional optical absorption spectrophotometry. Thin films of copper phthalocyanine on glass were examined and, owing to the high molar absorptivity of this dye, a wide range of optical densities were prepared in which the sample thickness was much less than the sample's thermal diffusion length (,us) for all samples examined. The thermal diffusion length of a material i is defined according to Rosencwaig (32) by the equation:
where cyi is the thermal diffusivity of material i in cm2/s and is the chopping frequency in radians/s. A previously reported calibration method (33)employing a platinum black thin film resistor was used to measure the PAS detector system's sensitivity, linearity, and frequency response as well as the amplitude of the incident light modulation. The platinum black thin film resistor used for this calibration was especially useful since it was thermally thin and its substrate thermally identical to the substrate of the copper phthalocyanine films. Rosencwaig (32)derived the following equation which describes the complex amplitude of the pressure fluctuation for a thermally thin optically absorbing sample on a transparent
w
Photoacoustic spectroscopy has been shown to be a sensitive method for obtaining absorption spectra of solids ( I - I O ) , liquids (1I-15), and gases (16-20). I t is particularly advantageous for samples which cause severe light scattering problems such as powders (21-25) and biological samples (26-31). The major disadvantage which has prevented PAS from becoming a popular analytical technique for the study of solids is the difficulty of quantitation as well as defining standards for measuring spectrometer performance. The objective of this paper is to quantitatively describe the performance of a noncommercial laboratory built photoacoustic spectrometer which was used to examine thermally thin films Present address:
Pa. 18848.
E. I. du P o n t de Nemours and Co., Tonawanda, 0003-2700/80/0352-0646$01 .OO/O
0 1980 American Chemical Society
