Table II. Ammonium Ion Interference with Chelate Analysis (no chelate present) Copper in filtrate, mQ/l. ["4CI]
Unheated sample
Blank 1x 10-4~
1.04 1.10
x 10-3~ 1 x 10-2M
44.80
1
2.08
Heated sample
0.00 0.00 0.00 0.00
detected in the filtrate and the solubilization of copper by EDTA was stoichiometric. In the unheated samples, measurable amounts of iron were found in the filtrate and the concentration of copper in the filtrate was less than that which would result from quantitative solubilization by EDTA. For each point, however, the sum of the molar concentrations of iron and copper solubilized was equivalent to the amount of EDTA in the sample. In the unlikely case of a chelating agent which did solubilize iron in preference to copper, both iron and copper could be analyzed in the filtrate, with the sum of their molar concentrations equalling the total chelating capacity. Although calcium has not been shown to interfere with the analysis of any chelating agents, this potential interference is eliminated by using sodium carbonate for pH adjustment. Calcium chelates are generally much weaker
than copper chelates. However, because of the relatively high solubility of calcium hydroxide, some calcium chelates might predominate over the corresponding copper chelates a t pH 10. The presence of carbonate ion lowers the concentration of soluble calcium below any possible interfering level. Theoretically the ammonia complexes of copper(I1) are not strong enough to cause interferences except a t very high concentrations of ammonium ion. In practice, however, low levels of ammonium ion do interfere if the solutions containing the precipitates are not heated. This step eliminates ammonium-ion interference as shown in Table 11. Even in the absence of any interferences, high blanks are obtained if the heating step in the procedure is omitted. Incomplete removal of colloidal copper hydroxide by filtration probably is responsible. Heating not only eliminates the high blanks, it also facilitates filtration and enables use of the same filter for a number of different samples. Received for review October 24, 1972. Accepted January 22, 1973. This research was supported by the United States Department of the Interior Office of Water Resources Research Allotment Grant A-049-MO and the University of Missouri Environmental Trace Substances Center.
Comparison of Lock-In Amplification and Photon Counting with Low Background Flames and Graphite Atomizers in Atomic Fluorescence Spectrometry M . K. Murphy,' S. A. Clyburn, and Claude Veillon2 Department of Chemistry, University of Houston, Houston, Texas 77004
A photoelectron pulse counting system is compared with a conventional phase-sensitive ("lock-in") amplifier system for use in atomic fluorescence spectrometry. Minimum detectable concentrations and relative sensitivities for Zn, Cu, and Bi are determined, using conventional, low-intensity hollow cathode lamps, a 150-W Xe continuum lamp, and high-intensity electrodeless discharge lamps as primary excitation sources. Four atomization systems having little or no background emission are employed. A sheathed Ar/02/H2 flame, a slotted graphite rod, and a graphite tube system are employed with continuous sample introduction, and a graphite rod system is employed with small individual samples. The photon counting system is superior to the lock-in amplifier system when low-intensity sources are employed, giving detection limits comparable to those reported for atomic absorption flame spectrometry. With high-intensity sources, results with the two measurement systems are about equal.
In instruments used for atomic emission, absorption, and fluorescence spectrometry, signals are derived from photomultipliers and the anode current is measured, by 1468
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dc, ac (tuned or wide-band), or phase-sensitive ("lock-in") amplifiers. The last has become the most popular for improved signal-to-noise ( S / N ) ratios and is used in almost every commercially available atomic absorption instrument ( I ) , as well as by most researchers in analytical atomic spectrometry. In applications where small signals must be recovered from large noise signals, lock-in amplifiers allow a trade-off of time of measurement for improved S/N. In atomic emission spectrometry, fluctuations in the background emission of the atomization source ( e . g . , flame) constitute the major noise components, while in atomic absorption spectrometry, fluctuations in the primary light source, shot noise, and atomization system fluctuations are frequently the major noise components. In atomic fluorescence spectrometry, one is frequently trying to measure very small signal levels, but not necessarily in the presence of other large noise signals, so noise in the detector-amplifier-readout system often predominates. When used for atomic fluorescence spectrometry, atomization systems having very low back1 D e p a r t m e n t of Chemistry, California I n s t i t u t e o f Technology, Pasadena, Calif. 91109. 2 A u t h o r to w h o m a l l correspondence should be directed.
(1) "Handbook of Commercial Scientific Instruments. ' C.Veillon and W. W. Wendlandt, Ed., Vol. 1, Marcel Dekker, New York. N.Y.. 1972.
ground emission, such as chemical flames a t wavelengths below 2800 A ( 2 ) ,separated flames (3-7), and graphite atomizers (8-12), place the major emphasis in detecting low elemental concentrations on the ability to measure very low light levels. In these situations, photoelectron pulse counting (frequently referred to as “photon counting”) offers significant advantages over analog voltage amplification (13).The principal advantages of photon counting are as follows: ( a ) extremely high sensitivity and ability to measure very low-light levels, (b) relative insensitivity to changes in detector gain, (c) ability to discriminate against dark currents originating from leakage and/or the dynode chain, (d) high long term stability, and (e) lower susceptibility to l / f noise (14, 15). In this study, photon counting and lock-in amplification are compared for use with four low-background atomization systems. These systems consist of an Ar/OZ/Hz flame, three graphite atomizers, a heated graphite rod for small sample volumes, and two heated graphite systems for use with continuous sample introduction. Virtually all of the development with graphite atomizers has concentrated on the use of single-shot samples of very small volume ( 1 1 ) . While these systems are excellent in situations where sample is limited, they must be used with small sample volumes, leading to decreased precision and matrix effects in some cases.
EXPERIMENTAL Apparatus. Lock-In Amplifier. A model HR-8 phase sensitive amplifier (Princeton Applied Research, Princeton, K.J.) with a Type D preamplifier and a 6.5 X lo5 ohm load resistor was used. Source radiation was modulated a t 330 Hz by a mechanical chopper driven by a synchronous motor. An overall detector voltage of 750 V resulted in optimum S / N ratios in most cases and was used throughout. All measurements are reported using a 10-sec time constant, unless otherwise indicated. Photon Counter. The photoelectron pulse counting system consisted of a Model 1105 data converter console (nonsynchronous ratemeter) coupled to the output of a Model 1120 amplifier discriminator (SSR Instruments, Santa Monica, Calif.). A selected RCA type 1P28 photomultiplier detector having a dark count rate of about 20 sec-1 at 18 “C and 600 V overall was used with both systems. Log plots of dark count rate cs. overall voltage and light count rate (rate -100 tiines dark count by exposure to constant intensity radiation) cs. voltage resulted in an optimum S/N ratio at 600 V overall, which was used throughout with the photon counting system. At this point, the slope of the dark count curve is twice the slope of the liight count curve. The discriminator level was adjusted to the manufacturer’s specifications. An integration time constant of 10 sec was used, unless otherwise indicated. Prescaling (count suppression) was used for scale expansion where appropriate. Sources. Excitation sources consisted of conventional hollow cathode lamps of Zn, Cu, and Bi operated at I/z their rated maximum current (Jarrell-Ash, Waltham, Mass.), a 150-U’ high pressure Xe continuum source (Hanovia, Newark, N.J.) powered by a Model 422-848 supply (American Instrument Co., Silver Spring, R . C. Elser and J . D Winefordner, Appi. Spectrosc.. 25, 345 (1971). G . F. Kirkbright and T. S West. Appi. Opt.. 7 , 1305 (1968) G. F. K i r k b r i g h t , M . Sargent. and T. S. West, Taianta. 16, 1467 ( 1 969) R . M . Dagnailetai.. Anel. Chem.. 42, 1029 (1970) T. L. Martin, F. M . Harnm, and P. 6 .Zeeman. Ana/. Chim. Acta. 53, 437 (1971). R . F. Browner and D. C Manning, Anal. Chem.. 44, 843 (1972). H . Massrnann, Spectrochim. Acta. Part E , 23, 215 (1968) T. S. West and X . K . Williams, Anal. Chim. Acta, 45, 27 (1969). B. V. L v o v , Pure Appl. Chem.. 23, 211 (1971). M . D Amos et a / , Ana/. Chem., 43, 21 1 (1971). K . 1 Aspila. C. L. Chakrabarti, and M. P. Bratzel, J r . , Anal. Chem.. 44, 1718 (1972). J . D . Ingle. J r . , and S. F,. Crouch, Anal. Chem.. 44, 785 (1972). H . V . Mairnstadt. M . L. Franklin, and G . Horlick, Anal. Chem.. 4 4 1 8 ) . 63A (1972). J . D . Ingle. J r . , and S . F:. Crouch, A n a / . Chem.. 44, 777 (1972).
SOURCE
LENS ATMlIUTION CELL,
LOCK-IN U P L I F I E R OR PHOTON COUNTER
(T;)
REAMUT
p \
LENS A
Schematic of apparatus showing optical system details (top view)
Figure 1.
Md.), and temperature-controlled electrodeless discharge lamps (16) containing the appropriate element. Optical System. A schematic of the instrumental system employed is shown in Fiaure 1. The extreme sensitivitv of the photon counting system made light leaks and stray light in the monochromator quite evident. Light leaks were eliminated by sealing all openings and flanged joints with opaque tape. Those joints sealed with O-rings, such as the detector housing, were completely light tight. Room light reaching the entrance slit also appeared as undispersed radiation reaching the detector. This problem was eliminated by the shield around lens A and the light trap (Figure 1). The interiors of the shield and trap were coated with flat black paint having extremely low reflectivity (101-C10 Velvet Coating, 3M, S t . Paul, Minn.), resulting in complete insensitivity to room light. The diameter and focal length of the fused silica lens A was chosen to match the f/8 aperture of the 0.5 m. 16 A/mm Ebert monochromator (Model 82-000, Jarrell-Ash, Waltham, Mass.). A monochromator spectral bandwidth of 6.4 A was employed for all measurements, using curved, 20 mm high slits. Lens B (5-cm diameter, 10-cm focal length, fused silica) was used to focus an unmagnified image of the source a t the focus of lens A on the optical axis. Atomization Systems. A sheathed Ar/OZ/Hz burner similar to the one described previously (17) was employed as an atomization cell for an initial comparison of the photon counter and lock-in systems with various sources. This flame exhibits low background emission, its temperature can be varied by adjusting the Oz/Ar flow ratio, and it minimizes quenching by molecular species like XZ. A heated chamber-condenser sample introduction system (18) was employed with this atomization system, as well as with the heated graphite systems employing continuous sample introduction. Spike-Rod Atomizer. A graphite (Ultra Carbon, grade UFIS, Bay City, Mich.) rod atomizer utilizing samples in the 1-10 p1 range was constructed as shown in Figure 2. Samples were measured into the bottom of the sample cavity with a 10-pl syringe, evaporated to dryness by passage of a low current through the rod, then atomized by passage of high current. Peak fluorescence was recorded at time constants of 300 msec and 1 sec for the lock-in and photon counting systems, respectively. The region observed was immediately above the sample cavity, the height and rod power being adjusted to give the maximum S/h’ for each element and conditions. A gradual deterioration of the rod occurred as samples were run, decreasing the cross-sectional area of the rod, resulting in a gradual increase of the rod temperature at a given power setting. This did not affect the sensitivity of the measurements but caused a gradual change in the background signal. The rod could be restored to its original characteristics by introducing methane into the sheath gas and operating the rod at high power levels. A deposit of extremely hard material (which had the same appearance as pyrolytic graphite) built up on the rod, increasing its cross-sectional area and greatly decreasing the rate of deterioration of the rod. Slot-Rod Atomizer. With the spike-rod atomizer system shown in Figure 2, small sample volumes must be used. In cases where one is not limited in available sample. or where equivalent or greater sensitivity can be obtained by continuous sample introduction, allowing one to dilute small samples, one can again trade (16) R. F. Browner, M . E. Rietta, and J . D . Winefordner, abstracts,
Pittsburgh Conference on Anaiytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 7, 1972, paper no. 136. (17) C. Veillon and J. Y . Park, Anal. Chim. Acta. 60, 293 (1972). (18) C. Veillon and M . Margoshes, Spectrochim. Acta. Part 6. 23, 553 (1968). A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973
1469
Top V i e w
Side V i e w
n SAMPLE CAVITY (3 x 4 mn) GRAPHITE
I .. ... ... ... .... yd .... . . . . ... ... ... ... ..t . . . . . .‘ 1 ;. . . . . . . . . . . . . . . . . . . . . . . . .I1
f--
1
-1
/-\
ROD ( 6 nm)
BAKELITE
ARGON
STOPPER
Figure 2. Spike-rod atomization system
Side View
Top V i e w
n I I
nn SLO1
11
i. .I . I! tARGON
QUARTZ
Iul lu1
Is1 ARGON + S M P L E AEROSOL
Figure 3. Slot-rod atomization system
time in improving the S / N ratio. To test this, the graphite rod atomizer shown in Figure 2 was modified, as shown in Figure 3. Continuous sample introduction was used, and the sample introduction system (18) removed a sufficient portion of t h e aqueous phase so that only slightly increased rod deterioration was noted. With this design, sample aerosol is forced to pass through the high temperature region in the slot. A pyrolytic graphite (Union Carbide Corp., Chicago, Ill.) slot rod was also used, with the slot cut through the conduction planes so that the inner surfaces of the slot achieved a much higher temperature than the outer rod surface, reducing scattered background radiation reaching the detector. Pyrolytic graphite, while considerably more expensive than spectrographic grade graphite, was extremely durable and resistant to deterioration by sublimation and oxidation. During the course of this investigation, about 10 slot-rods were used u p and had to be replaced. 1470
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973
With approximately the same usage on the pyrolytic material. almost no deterioration was observed. As with the spike-rod. the slot-rod system does not yield complete atomization in many cases, because of the short period of time t h a t t h e sample atoms spend in the high temperature region and the fact that many of them do not experience the rod temperature. Graphite Tube Furnace. T o reduce this problem, the system shown in Figure 4 was constructed. The upper and lower halves of the water-cooled chamber are electrically isolated, except for conduction through the graphite tube. Sample aerosol and argon enter the chamber tangentially and exit up through the heated tube. Constant compression of the rod for reliable electrical constant is maintained by two external spring clamps holding the chamber halves together. A thin layer of silicone rubber (RTV102, General Electric, Waterford, N.Y.) is used to seal the outer circumference of the chamber where the halves meet. Graphite tubes of uniform cross section, as shown in Figure 4, resulted in a short, nonuniform high-temperature zone due to cooling of the tube ends. Tapered rods, with a larger middle cross section and thinner ends produced a longer, more uniform hightemperature zone and increased tube life. The best results were obtained by heating a thin-walled graphite tube in the presence of methane gas. a s described above for the spike-rod atomizer. resulting in a tube having nonuniform cross section but a long. uniform heated zone. The coating built-up on the tube in this manner was very hard, had the appearance of pyrolytic graphite. and made the rod extremely durable. As its electrical properties slowly changed over a long period of time due to sublimation and/or oxidation, the methane treatment could be repeated and the tube restored. For all of the heated graphite atomization systems, power was derived from a 220-V primary. 8-V secondary, 3-kW transformer (Light Electric Corp., Newark, N . J . ) . The primary voltage was controlled by a 0-240-V, 10-A variable transformer (Superior Electric Co., Bristol, Conn.) and the secondary current monitored with a 0-5 A ac ammeter (Model 433, Daystrom, Inc., Weston Instruments Div., Newark. S . J . ) and a 1OO:l ratio current transformer (No. 1-111301, Simpson Electric Co., Chicago, Ill.). Depending on the dimensions of the graphite and the power required, currents in the 200-500-A range were used, a t secondary voltages between 2 and 4 V ( i . e . . power levels in the 500-2000-W range). Secondary voltage was monitored with an ac voltmeter.
H20 OUT
COOLING COILS f-- ARGON + SAMPLE
Figure 4.
AEROSOL
Graphite tube furnace (cross section)
Secondary power leads consisted of 2 AWG neoprene welding cable, which permitted continuous operation for several minutes, followed by a similar cool-down period. For longer periods of continuous operation, heavier leads are suggested. Means of measuring the graphite temperatures were not available and power settings for successive runs were reproduced by adjusting to the same wattage.
RESULTS AND DISCUSSION Some preliminary measurements with the Ar/02/H2 flame using low intensity line and continuum sources were made with the photon counting system and the lock-in amplifier. Some of these data are illustrated in Table I. The blank count rate consists of the flame background emission plus the detector dark count (15-20 sec-1). For all of the Zn and Cu imeasurements, the 2139 and 3247 b, lines were used, respectively. While only one set of data is shown for the lock-in system, these results are typical; the detection limits and relative sensitivities were one to several orders of magnitude better in each case when the photon counting system was used. Of greater significance are the sensitivities and detection limits obtained with the flame atomization system and the low intensity excitation sources when using the photon counter. For Cu and Zn, the detection limits are comparable to the best values obtained with conventional atomic absorption instrumentation, even though the fluorescence excitation source used was a conventional hollow cathode lamp operated a t half the manufacturers’ rated current. Somewhat higher detection limits were obtained for Zn when using the Xe continuum source, presumably due to its low intensity in this wavelength region. Detection limits obtained for Bi, while higher than those observed in conventional atomic absorption spectrometry, are quite low considering the source intensity and relatively low atomization efficiency of the flame system used. The intensity of the Bi hollow cathode source was very low, as evidenced by the superior sensitivities obtained with the Xe continuum source at 2231 A. These preliminary results indicated that the combination of the photon counting system with a more efficient atomization system arid little or no background radiation should yield good sensitivity, even with low-intensity sources, and excellent sensitivity with high-intensity excitation sources. With the spike-rod atomizer, the signal for Cu and Zn samples of a given size increased as the rod current was increased until a plateau was reached-ie., further current increases did not yield greater signals, only a more rapid rise and fall of the signal. Being more volatile than Cu, Zn could be atomized a t lower power settings. In the case of Bi, the signal increased gradually as the rod current was increased, and a true plateau was never reached
Table I. Detection Limits and Relative Sensitivities with Flame Atomization MeasureBlank ment count rate, Sensitivity,c Detection Element systema Sourceb sec-’ Fg/mi limit,d p g / m i Zn
Zn Zn
cu cu
Bie Bif Bie Bif
LI PC PC PC PC PC PC PC PC
HC HC
Xe HC
Xe HC HC
Xe Xe
... 37 30 335 230 51 85 38 110
0.71 0.00027 0.013 0.022 0.029 19 19
4.4 3.5
0.035 0.001 0 0.01 1 0.0036 0.01 1 7.5 6.2 0.88 1.8
a LI = lock-in amplifier: PC =.photon counting system. HC = conventional hollow cathode lamp: Xe = 150-W Xe continuum. Concentration yielding a signal of 10% of full scale under conditions used to measure the detection limit. Concentration where S / N = 2. e At 2231 A. f At
3068 A.
up to power settings of 2 kW. At the higher power levels, rod incandescence increased the background signal (photon counter) and noise (lock-in amplifier), and the power level used corresponded to that where the best S / N ratio was observed. With Cu and Zn, power levels used were those just on the plateau. With the slot-rod atomizer, similar effects were observed, although considerably higher power settings were necessary to reach the plateau regions. Except in the case of Zn, the optimum S / N was found a t a power setting below that required to reach the plateau. With the graphite tube furnace, no background emission or scatter was observed. Power settings could be increased to the point where the plateau was observed with no increase in the observed noise or background, and the apparent tube temperature was considerably below that required with the other two systems. The results obtained with the spike-rod atomization system are shown in Table 11. Using hollow cathode sources, 5 pg of the respective elements could not be detected. With the Xe continuum source, Cu and Zn fluorescence could be observed with the photon counting system, but the sensitivity was quite low in comparison with the high-intensity electrodeless discharge lamps used for the remaining combinations. Sensitivities and detection limits are shown in concentration as well as absolute units to facilitate comparisons with the continuous sample introduction atomization systems. Except for Cu, the photon counting system results in a greater sensitivity than the lock-in amplifier system, and comparable detection limits for the 3 elements with the spike-rod atomization system and high intensity sources. Comparing the data in A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1 9 7 3
1471
Table II. Detection Limits and Relative Sensitivities with the Spike-Rod Atomizer Detection limit
Sensitivity Element
System
Source”
Zn
LI PC PC LI PC PC
EDL
Zn Zn cu cu cu
Bi Bi a
LI PC
Xe EDL EDL
Xe EDL EDL EDL
EDL = electrodeless discharge lamp.
Blank count rate, sec-’
... 24 93
... 2040 750 ... 250
Ccglmlb
9
8.0 x 7.5 x 1.0 x 5.6 x 5.0 X 5.5 x 3.6 x 3.9 x
lo-” 10-8 10-1’ 10-9
10-9 10-9 10-10
4 x 10-5 4 x 10-2 5 x 10-6 3 x 10-3 2.5 3 x 10-3 2 x 10-3 2 x 10-4
Ccglmlb
9
1.5 X 1.5 x 1.6 X 1.7 X
IO-’’
10-7 lo-” lo-@
2.5 X
9.5 x 10-9 2.1 x 10-9 6.6 X
8 8 8 9
x 10-6
x 10-2 x 10-6
x 10-3
1.3 5 x 10-3 I x 10-3 3 x 10-3
Based on sample dissolved in 2 ml for continuous sample introduction.
Table I l l . Detection Limits and Relative Sensitivities with the Slot-Rod and Graphite Tube Atomizers Sensitivity. pg/ml Element
System
Zn Zn
LI
Xe
...
LI
HC EDL
...
Zn
LI
Zn Zn
PC PC PC LI LI LI PC PC PC LI PC
Zn cu cu cu cu cu cu
Bi Bi a
Source
Blank count rate,a sec-’
Xe HC EDL
Xe HC EDL
Xe HC EDL EDL EDL
... 88 (25) 60(15) 220(30) ... ... ... - (24) -(15) 430(24)
... 120(20)
Slot
1.7 X 2.5 X l o + ’ 5.6 X 4.7 x 10-1 1.4 X 4.5 x 10-3 b b
1.3 X lo-’ b b 8.4 x 10-2 3.3 x l o - ’ 6.6 X
Detection limits. Fg/ml Tube
2.0 x 3.2 X 8.7 x 3.8 x 2.7 x 1.1 x 8.3 X 2.7 X 1.6 x 5.0 x 2.0 x 1.3x 1.5 X 1.1 x
lo-’ 10-1 10-7 10-3
10-3 10-7 lo-’ lo+’ 10-4 10-3 100 10-4 lo-’ 10-3
Slot
6.3 X 7.0 X 5.4 x 4.4 x 6.3 X 1.1 x
IO+’ loo 10-3
10-2 10-3
b b
7.9 x lo-’ b b 5.1 X lo-’ 4.2 X lo-’ 1 . 3 X lo-’
Tu be
1.0 x 1.4 X 2.7 x 6.1 x 3.8 x 2.9 x 9.0 x 1.0 x 8.0 x 7.5 x 4.0 X 3.2 x 2.9 x 1.2 x
10-1 10-1 10-7 10-3 10-3 10-7 10-1
IO+’ 10-5
10-3 loo 10-5 10-3 10-3
Values in parentheses are for the tube furnace and are essentially the dark count rate. These combinations were not investigated
Tables I and 11for the Xe continuum source, the two atomization systems give essentially equivalent results for Zn, while the spike-rod system is about twofold better than the flame system for Cu. However, when one compares the date in Table I for the low-intensity sources with the fact that most of the equivalent measurements with the spike-rod system were not even observed, it becomes apparent that the two atomization systems cannot be compared directly. Reproducibility of the spike method was equal to that of the syringe used, about &6%. Some of the results obtained with the slot-rod and graphite tube atomizers are summarized in Table 111. Detection limits and sensitivities for Cu and Bi with the Xe and hollow cathode sources were all above 0.1 Fg/ml with both measurement systems, except for Cu with the Xe source, photon counting system, and the tube furnace. From the data in Table 111, it is obvious that the graphite tube atomizer yields significantly better detection limits and sensitivities than the slot-rod atomizer. For Cu and Zn determinations with the graphite tube atomizer, sensitivities and detection limits are about 2 orders of magnitude better with the photon counting system than with the lock-in amplifier when using low intensity continuum or hollow cathode sources. When high-intensity electrodeless discharge lamp sources are employed, the two measurement systems give comparable results. Comparing the data in Table I11 with those for the flame atomizer (Table I), one notes that in some cases the observed sensitivity is higher with the flame system, while in other cases the graphite tube atomizer gives higher sensitivity. These differences are due primarily to the fact that the sensitivities reported are not necessarily the highest obtained, but the sensitivity observed under condi1472
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973
tions where the S/N ratio was a maximum. Comparing the detection limits obtainable with the flame and graphite tube atomization systems, one observes that the two atomization systems give comparable results with the lowintensity sources, except in the case of Cu with the photon counter and hollow cathode source, where the flame proved superior. However, with the limited flame data in Table I, comparisons between the two atomization systems cannot be accurately made a t this time. Comparing the graphite tube atomizer (Table 111) with the spike-rod atomizer (Table 11), one observes that the sensitivities and detection limits obtained with the tube atomizer are considerably better for Cu and Zn. This comparison assumes that the minimum detectable amount of each element was dissolved in 2 ml of solution for use with the continuous sample introduction system employed with the tube atomizer. For Bi, the detection limits are comparable when EDL sources are used.
CONCLUSIONS The atomization systems, elements, sources, and measurement systems employed in this study require 7 2 individual determinations of the optimum operating conditions and S / N ratio and subsequent determination of minimum detectable concentrations and relative sensitivities. Not all possible combinations are reported because of obvious superiority of other combinations. It would be of little value to predict an ultimately superior system on the basis of the data presented here, because other elements, interferences, matrix effects, etc. were not investigated. However, all of the data were obtained with the same experimental set-up under controlled conditions, and several general conclusions can be made.
In all of the cases investigated, the graphite tube atomizer is superior in sensitivities and detection limits to the slot-rod atomizer. The sensitivities and detection limits for Zn and Cu with the tube atomizer are superior to those obtainable with the spike rod-atomizer. For Bi, the sensitivity obtained with the spike-rod atomizer is greater than that obtained with the tube atomizer, while the minimum detectable concentration is about the same for both atomization and measurement systems. No clear superiority or trend could be found between the flame and graphite systems when low-intensity sources were used. The photon counting system resulted in detection limits superior to those obtained with the lock-in amplifier system when low intensity sources were employed. With high intensity line sources, comparable sensitivities and detection limits were obtained with both measurement systems, indicating that source flicker is the major noise component. With the photon counting system, detection
limits comparable to those obtained by conventional atomic absorption flame spectrometry can be obtained by atomic fluorescence spectrometry using conventional hollow cathode lamps and low-power Xe continuum sources of excitation.
ACKNOWLEDGMENT We wish to thank Betty R. Bartschmid for assistance in the preparation and operation of the electrodeless discharge lamp sources. Received for review September 12, 1972. Accepted January 29, 1973. This work was supported in part by the Robert A. Welch Foundation and in part by the National Science Foundation. Presented at the 11th National Meeting, Society for Applied Spectroscopy, Dallas, Texas, September 1972.
Application of Thin Metal and Carbon Films in Infrared Internal Reflectance Spectrometry James S. Mattson Division of Chemical Oceanography, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Fla. 33 149
Thin films of carbon or metal are vacuum-deposited on KRS-5 (TIBr-TII) internal reflection elements. Infrared internal reflection spectrometry can then be employed to observe chemical reactions taking place at the resulting metal or carbon surface. This method is applicable to redox reactions of and at the surface, adsorption of organic molecules from solution, etc. Examples of the applications of this technique are presented, including the the O~, solution oxidation of carbon films by K ~ K ~ O B / H ~ S adsorption of bovine albumin on a carbon surface, and the air oxidation of a copper film.
absorbing species in solution (1-6). Several groups have measured infrared spectra of adsorbed species on infrared transparent internal reflection elements (7-9), and there has been a considerable amount of work done in transmission infrared spectrometry of adsorbed species (10-13) contained as finely divided particles in or on infrared transparent matrices. Efforts have been made to measure surface functionalities on a layer of carbon by placing finely divided carbon in optical contact with a germanium internal reflection element (14). Poling (15-1 7) has conducted studies of iron and copper oxidation by polarized specular reflection infrared spectrometry, using thin films of iron and copper
Reactions at surfaces comprise the most important part of many fields of chemistry, including electrochemistry, catalysis, corrosion, etc. Indirect methods of following such reactions, such as voltammetry and methods involving the analysis of reactants and reaction products in the gaseous or solution phase, rarely yield positive information regarding the nature of the reaction complex a t the surface, where distinct complexes are thought to exist. Direct methods of analysis, such as ultraviolet, visible, and infrared spectrometry, tend to be less than ideally suited to the measurement of surface phenomena on metals, metal blacks, carbon, and carbon blacks, as Rayleigh scattering at shorter wavelengths adversely affects both transmission and specular reflection measurements, while the extinction coefficients of metals and carbon in the infrared are generally too high for any measurements of surface-bound species. Successful efforts at observing the metal/solution interface have been made in the past by employing thin metal film electrodes on optically transparent internal reflection elements for the measurement of ultraviolet and visible-
(1) H. B. Mark, Jr., and B. S. Pons, Anal. Chem., 38, 119 (1966). (2) B. S. Pons, J. S. Mattson, L. 0. Winstrom, and H. B. Mark, Jr., Anal. Chem., 39, 685 (1967), (3) W. N. Hansen, R . A. Osterydung, and T. Kuwana, J. Amer. Chem. SOC.,88, 1062 (1966). ( 4 ) W. N. Hansen, T. Kuwana, and R . A. Osteryoung, Anal. Chem., 38, 1810 (1966). (5) W. N. Hansen and A. Prostak, Phys. Rev., 174,500 (1968). (6) N. Winograd and T. Kuwana, J. Electroanal. Chem., 23, 333 (1969). (7) N. J. Harrick, J. Phys. Chem., 64,11 10 (1960). (8) G. E. Becker and G. W. Gobeli, J. Chem. Phys., 38,2942 (1963). (9) R . E. Baier, G. I . Loeb, and G. T. Wallace, Fed. Proc., Fed. Amer. SOC.Exp. fliol., 30, 1523 (1971). (10) L. H. Little, "Infrared Spectra of Adsorbed Species," Academic Press, New York, N.Y., 1966. (11) J. A. Cusumano and M. J. D. Low, J. Colloid lnterface Scl., 38, 245 (1972), (12) M. J. D. Low, A. J. Goodsei, and N. Takezawa, Environ. Sci. Techno/., 5, 1191 (1971). (13) A. J. Goodsel, M. J. D. Low, and N. Takezawa, Environ. Sci. Techno/., 6, 268 (1972). (14) J. S. Mattson and H. B. Mark, Jr., "Activated Carbon: Surface Chemistry and Adsorption from Solution," Marcel Dekker, New York, N.Y., 1971. (15) G. W. Poling, J. Electrochem. SOC.,116,958 (1969). (16) G. W. Poling, Corros. Scl., IO, 359 (1970). (17) G. W . Poiing, J . Colloid lnterface S o . . 34, 365 (1970). A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 8 , J U L Y 1973
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