Study of Xelnon-Mercury Arc as a Continuous Source for Atomic Absorption Spectrometry C.W. Frank,' W. G.Schrenk, and C . E. Meloan Kansas Agricultural Experiment Station, Department of Chemistry, Kansas State University, Manhattan, Kan.
THEBASIC CONCEPT of atomic absorption has been theoretically defined for many years, but was not described as an analytical tool until 1955 by Walsh (1). The void may have been due to the lag in instrumentation in relation to the theory to the system. Because the first work involved the use of the hollow cathode source, and there are many good reasons for using this, the area of atomic absorption has been more or less confined to this type of source. Some attempts to deviate from this pattern have been published, notably, by Fassel and Mossotti (2), Ivanov and Kozyreva (3), and Ginsburg and Satarina ( 4 ) . This prior work on continuous sources has been involved only with narrow aspects and several of the conclusions drawn have been vague. Recently, a more comprehensive study has been published (5) which shows the utility of the continuous source. This study also concerns the feasibility of using a continuous source on a representative group of elements so as to properly evaluate and compare the hollow cathode with a continuous sourcea xenon-mercury arc. EXPERIMENTAL
Apparatus. Data were obtained using a Jarrell-Ash 0.5meter grating with Jarrell-Ash electronics, including chopper, a.c. amplifier, Sargent recorder (Model S-72150), and a 1P28 photomultiplier tube as detector. A Hanovia xenonmercury 200-watt, 9.5-amperes arc, No. 901B-1, was used as the source, coupled with a Hanovia power supply (No. 27800). The attenuator used was a Beckman Model 95490 reference beam attenuator. The position of each is given in Figure 1. All data reported were obtained by using one large-bore Beckman burner No. 4090 in a single pass system. The fuels Present address, Department of Chemistry, University of Iowa, Iowa City, Iowa 52240 1
(1) A. Walsh, Spectrochim. Acta, 7 , 108 (1955). (2) V. A. Fassel and V. G. Mossotti, ANAL.CHEM., 35, 252 (1963). (3) N. P. Ivanov and N. A. Kozyreva, Zh. Aizalit. Khim., 19 (lo), 1266 (1964). (4) V. L. Ginsburg and G. I. Satarina, Zauodsk. Lnb., 31 (2),249 (1964). (5) V. A. Fassel, V. G. Mossotti, W. E. L. Grossman, and R. N. Kniseley, Spectrochim. Acta, 22, 347 (1966).
n
b
Figure 1. Block diagram of apparatus A-xenon-mercury arc, L-lens, C-chopper, BA-beam attenuator, S-sample, M-monochromator, R-readout were oxygen and acetylene, controlled by Jarrell-Ash regulators and monitored by G.I. flow meters, No. F1300. Solutions. All solutions were prepared from reagent grade chloride salts for both the interfering elements and the elements being studied. Solutions were made from the reaction with HCl where it was practical. Standard solutions of 100, 1000, and 10,000 p.p.m. were prepared and all other solutions were prepared by dilution of these. Procedure. The basic procedure is the same as any other atomic absorption process with one exception. It is now necessary to determine the point of greatest gain on the amplifier using a combination of photomultiplier voltages, amplifier gain, and beam attenuation. This is done as follows: with the flame on, aspirating a sample, the beam attenuator is set so that the photomultiplier is not at its limit. Then the photomultiplier voltage is varied until the maximum absorption is indicated. Now the gain is adjusted to determine whether a further improvement can be made. This cycle is repeated until no further improvement in sensitivity can be obtained. Usually this requires only one adjustment, and any subsequent changes that may be needed as different elements are examined are made with the beam attenuator only. RESULTS AND DISCUSSION
Four elements-Ca, Na, Cu, and Zn-were chosen for the study at resonance lines given in Table I. These metals were selected first because they represent an alkali, and alkaline earth, and transition element and second because data already exist using the hollow cathode source for these elements, thus providing for a better komparison. Six parameters were thought to be significant enough to warrant an investigation of their magnitude. These were the electronics, the optimum fuel-to-oxygen ratio, the correct burner height, sensitivity, precision of measurement, and interfering ions.
Table I. Electronic Conditions for Measurement of Na, Ca, Cu, and Zn
Element Na Ca Zn cu ~~
~
534
Photomultiplier voltage 480 480 480 480
~
ANALYTICAL CHEMISTRY
Resonance lice
(4
5889.95 4226.73 2138.56 3247.54
Amplifier gain
Recorder scale (mV>
Source current (amps)
2.5,5 5 2.5,5 1.25,2.5
7.5 7.5 9.0 7.5
/
60
/
50;
100.
n
.
5
*-*---
0
LO IO 20 30
20
IO
30
40
50
60
---_---70
80
90
Concentration (ppm)
40
50
6 3
Figure 3. Calibration curves for copper and zinc at optimum conditions (absorbance measured in mm of recorder chart)
(Concentration(pprn)
Figure 2. Calibration curves for sodium and calcium at optimum conditions (absorbance measured in mm of recorder chart)
cu
- - -Zn
Ca
_ _ _ _ - - Na
Electronics. This parameter has already been discussed under the experimental section. This, along with optical alignment, was the most critical adjustment to make. When these above parameters were obtained, they were suitable for all of elements tested. {Conditionsfor the elements are shown in Table I. Fuel-02 Ratio. Table I1 is a comparison of the optimum acetylene-oxygen ratios. The flow rates are somewhat the same for all the elements; however, the importance of this adjustment can be ascertained by the fact that a 100% absorbance change is found when the flow rate of oxygen in the calcium system is changed by 400 mliminute. Because the pressures can be regulated within fine limits, the fuel ratio ceases to be a problem once the optimum values are established. Burner Heights. Burner heights are reported as the distance from the top of the burner to the middle of the source beam. Flame profiles were recorded but are not reported because of the inhomogeneity of intensity throughout the source. The cross section of the source at the point of going through the flame was 2 rectangle 12 mm. long and 2 mm. wide, the greatest interisity being at each end. The beam attenuator should cause negligible distortion as it was placed directly after the first condensing lens (see Figure 1). Relative absorbances were determined between 3 and 7 mm above the burner. In general, the apparent concentration of absorbing atoms reached a wide maximum, then decreased relatively slowly as the burner was raised or lowered from this position. Burner heights for the four elements were: Na, 4.3 mm; Ca, 4.8 mm; Zn, 6.8 mm; and Cu, 6.8 mm. There exists a great difference in burner heights of the transition elements as compared with the 01 hers. Sensitivity and Precision. Calibration curves for each of the elements are shown in Figures 2 and 3. The linearity seems to be similar to that obtained with a hollow cathode. Sensitivities are calculated according to the definition of Slavin et al. and are given in Table 111. Precision results on Cu and Na are included. The sensitivity of the source, using the conditions described, is somewhat less than that reported for the hollow cathode, but is still practical for most
Table 11. Optimum c ~ H 2 - 0Ratios ~ for Na, Ca, Cu, and Zn Flow (ml/min) X 10-8 Element GH2 0 2 Na 7.8 3.3 Ca 7.7 2.9 Zn 7.8 3.3
cu
Ca 280
e
3.1
Table 111. Sensitivities (ppb) Na cu 100
Precision on No. of runs Concn. level Av." Dev."
Zn 800
200
cu
Na 5 0 . 8 ppm 9.6 =t .72
6 1 . 6 ppm 11.0 f 1.3
Measured in chart divisions.
Table IV. Interferences in Per Cent Decrease or Increase on 50 ppm of Element by 500 ppm of Cation or 5000 ppm of Anion Interfering agents Ca Na cu Zn ca ... +I4 ... ... L5 0 ... ... 0 Na 0 ... ... 0 Alb -13 - 10 0 ... co +9 ... 0 ... cu t-13 ... ... ... Cr -24 -2 ... - 13 Cd 0 ... -8 0 cs -8 - 10 ... I
(a,
( 6 ) W. Slavin, S . Sprague, and D. S . Manning, Atomic Absorption Newsletter, 18, February 1964, p. 2.
7.1
SO4-2
NOa-
-48 -24
Pod-a
-11
c1Mg Sr
Li
...
...
... .
a
1 gram of KI added.
b
400 ppm.
.
I
... ... - 10 -6 +8 - 10 +8
+4
.
.
+I7
0
... - 17 ... ...
...
0
... 0
...
...
...
~~
VOL. 39, NO. 4, APRIL 1967
535
analysis systems. The authors believe that the sensitivity could be increased by the use of a multiburner system, and in the above cases a multipass arrangement along with a more sophisticated slit and lens system. Interferences. The concentrations of the interfering agents in Table IV are 500 ppm of cation and 5000 ppm of anion. The amount of interference was as expected but is difficult to compare with that of the hollow cathode. At first analysis,
interferences are expected to be somewhat higher than those of a sharp line source because a broader band of energies is available for absorption.
RECEIVED for review November 4, 1965. Accepted January 27,1966. Work performed in partial fulfillment of the requirements for the Ph.D. Degree (C.W.F.) in Chemistry,
Simultaneous Determination of Tin and Lead Using Cyclic Stationary Electrode Polarography Sidney L. Phillips IBM Corp., Systems Development Division, Poughkeepsie, N . Y TIN AND LEAD may be determined simultaneously at a dropping mercury electrode in supporting electrolytes containing a high concentration of chloride ( 1 ) or bromide (2) ions. In the former method, the tin concentration is determined from the first wave, while the sum of tin and lead is estimated from the second wave. This sum is obtained from the second wave because the second tin reduction step takes place at the same potential as lead, and is superposed on the lead wave. In the latter method, only one tin wave is observed, and this is shifted to potentials more cathodic than that of lead. Under these conditions, it is possible to determine the tin and lead contents from the separated waves. In the present work, the simultaneous determination of tin and lead from a solution which does not require a large excess of halide ions is reported. When the supporting electrolyte is a formate buffer solution containing added pyrogallol, the lead wave is interposed between the two tin waves so that simultaneous determination is feasible. In addition, the analytical procedure utilizes an amalgam stripping technique so that the oxidation state of tin is unimportant and better resolution is obtained between the tin and lead peaks. EXPERIMENTAL Apparatus. All data were obtained using a Wenking Model 61R potentiostat (Brinkmann Instruments). The initial potential was obtained from the mercury battery low voltage power supply built into the instrument, while the triangular wave was obtained from an Erwin Holstrup Motor Potentiometer (Brinkmann Instruments). Experimental data were recorded as a voltage drop across a decade box load resistor by means of a Leeds and Northrup Type G Speedomax recorder, This instrument had a full scale pen response time of 1 sec and a chart speed of 20.2 inches per min. The hanging mercury drop electrode, cell, and associated equipment were similar to those shown by Alberts and Shain (3), except that the counterelectrode was a platinum sheet of about 2 cm2 area, and the electrode shield was not used. The cell was immersed in a water bath maintained at 25" C. Chemicals. All chemicals were of reagent grade and were used as received. Stock 0.0100M tin(1V) solutions were (1) D. E. Sellers and D. J. Roth, ANAL.CHEM., 38, 516 (1966). (2) T. Kitagawa and K. Nakano, Rev. of Polurog. (Jupun) 9 (3) (1961). (3) G. S. Alberts and I. Shain, ANAL.CHEM.,35, 1859 (1963).
536
ANALYTICAL CHEMISTRY
12602
prepared by dissolving the required amount of sodium stannate dihydrate in 0.1Msodium hydroxide, or by dissolving metallic tin in a heated mixture of 20 ml concentrated hydrochloric acid and 5 ml concentrated nitric acid. This latter solution was then cooled, transferred to a 200-ml volumetric flask and diluted to mark. Standard 0.010M Iead(I1) solutions were prepared by dissolution of lead acetate in 0.1M perchloric acid. A stock solution of 0.5M pyrogallol was prepared by dissolving a weighed amount of the solid in 0.1M perchloric. This last solution remained colorless for several weeks under ambient conditions, indicating negligible oxidation. The supporting electrolyte was a buffer solution composed of 2 M formic acid and 2M ammonium formate (4M in total formate), which gave a final pH of 3.7. All solutions were prepared from twice-distilled water, with the second distillation being made from an alkaline permanganate solution. Procedure. A 50-ml aliquot of the supporting electrolyte was deaerated by nitrogen bubbling for 8 to 10 min. Following this step, 2 ml of the pyrogallol solution and an appropriate aliquot of the tin and lead solutions were added to the electrolysis cell. The solution was mixed by bubbling with nitrogen for 5 min, after which the fritted glass inlet was raised above the liquid level so that nitrogen passed constantly through the upper portion of the cell. A mercury drop (0.067-cm radius) was then attached to the working electrode while applying the initial potential (0.0 volt us. SCE). The solution was allowed to remain quiet for 30 to 60 sec after attaching the mercury drop to minimize stirring effects caused by movement of the solution during this step. Cyclic stationary electrode polarograms were then recorded by application of the signal voltage and replicate results were obtained readily by attaching a fresh mercury drop and repeating the procedure. Data obtained in this manner for the variation in anodic peak currents as a function of bulk concentration are recorded in Table I. As shown in Figure 1, the stripping peak potentials for the first tin wave and the lead wave are about 100 mV apart. RESULTS AND DISCUSSION The electrochemical behavior of lead appears to be relatively uncomplicated in both the absence and presence of pyrogallol. Addition of pyrogallol to a concentration as high as 0.1M did not cause a shift in the lead peak, indicating negligible complexation. On the other hand, the behavior of tin(1V) is not as straightforward and appears to be complicated by a
