NaC104 (pH 9 to 10.3, depending on sulfide concentration), 0.5 M NaOH, and 1M NaOH. In 1M NaOH, a maximum was observed on rapid dc polarograms for sulfide concentrations in the narrow range of 2 X 10-4 to 6 X lO-4M. Since this maximum prevented accurate evaluation of the limiting current, a supporting electrolyte of lower p H ( e . g . , 0.5M NaOH) is recommended if sulfide concentrations are expected to fall in this range. Supporting electrolytes of pH less than about 8 or 9 are not recommended because of the possibility of loss of sulfide as hydrogen sulfide.
CONCLUSIONS As discussed by Bark and Rixon ( I I ) , most methods available for the determination of sulfide have one or more disadvantages. These include practical disadvantages, such as strict time control, as well as interference from other anions. The silver sulfide ion-selective electrode represents a potentially simple means of sulfide ion determination. However, this electrode also responds to
cyanide (13, 14). Furthermore, the potential of the electrode depends on the ionic strength and pH of the sulfide solution, so that if it is to be used for direct potentiometry these parameters must remain constant. Although the direct method proposed in this paper is not as sensitive as some of the previously established techniques, it appears superior to many of these methods because of its simplicity, freedom from interference, and the independence of the limiting current on pH. For the determination of sulfide ion in natural samples, it is recommended that the sulfide be fixed by reacting it with zinc acetate and subsequently recovered by distillation, unless the sample is analyzed immediately after collection (16). The rapid polarographic method should be suitable for analysis of samples treated in this way if the evolved hydrogen sulfide is collected in a sodium hydroxide solution. Received for review May 1,1973. Accepted July 9.1973. (16) "Standard Methods for the Examination of Water and Wastewater," APHA. AWWA, WPCF, New York, N.Y., 13th ed.. 1971, Part 157, p 336.
Pulsed-Mode Atomic Fluorescence Utilizing a Demountable Hollow Cathode Excitation Source J. 0 . Weide and M. L. Parsons1 Department of Chemistry, Arizona State University, Tempe, Ariz. 85281
There have been several reports concerning the use of pulsed hollow cathode systems in flame spectrometry. Dawson and Ellis ( I ) originally applied this technique to an atomic absorption system, finding an increase in resonance line intensity. The radiation output over the pulse width in their system was observed to be several hundred times the original dc level with little increase in line width or self-reversal. Mitchell and Johansson (2, 3 ) originally applied this technique to atomic fluorescence measurements while developing a simultaneous multielement analysis technique. Their system incorporated a rotating interference filter wheel for element selectivity and a phase sensitive detector for signal detection. Barnett and Kahn ( 4 ) made a comparison of atomic absorption and atomic fluorescence utilizing a pulsed excitation system with synchronous demodulation (lock-in amplifier). Cordos and Malmstadt (5-8) showed that the relative standard deviation for their dual channel synchronous integrator measurements was less than 0.1%. Their atomic fluorescence system was To w h o m a l l correspondence should b e addressed. (1) J. B. Dawson and D. J. Ellis, Spectrochim. Acta, Part A, 23, 565 (1972). (2) D. G. Mitchell and A. Johansson, Spectrochim. Acta, Part B, 25, 175 (1970). (3) D. G. Mitchell and A. Johansson, Spectrochim. Acta., Part 6, 26, 677 (1971). (4) W. B. Barnett and H. L. Kahn, Ana/. Chem., 44, 935 (1972). (5) E. Cordos and H. V. Malmstadt, Anal. Chem., 44, 2277 (1972). (6) E. Cordos and H. V. Malmstadt, Anal. Chem., 44, 2407 (1972). (7) E. Cordos and H. V. Malmstadt, Anal. Chem., 45, 27 (1973). (8) H. V. Malmstadt and E. Cordos, Amer. Lab., Aug., 35 (1972).
composed of several hollow cathode excitation sources operating in sequential on-off modes. In a preliminary investigation, the authors (9) showed approximately three orders of magnitude improvement in the limit of detection for zinc when using the pulsed system as opposed to a conventional dc system. In this system, a pulse controlled dual-channel boxcar integrator was used to detect the pulsed atomic fluorescence signal. High-voltage pulses produce high intensity emissions from hollow cathode lamps. The resultant pulsed atomic fluorescence signals are converted by the photomultiplier detector and operational amplifier into voltage pulses. The signal and the background are integrated separately in the dual-channel boxcar integrator. The sampling gate of the boxcar integrator can be adjusted to receive maximum signal, whereas the reference gate is adjusted to monitor background between signal pulses. In both cases the noise level is decreased considerably when the signal is integrated by an RC-circuit, but is also dependent upon the time required to charge the capacitor. Thus, the integration time is directly related to analysis time. The final dc output of the boxcar is the difference between the integrated sample and background signals.
EXPERIMENTAL Apparatus. F i g u r e 1 shows a b l o c k d i a g r a m for t h e pulsed a t o m i c fluorescence system. A pulse generator (Tektronix-114) was used t o synchronize t h e excitation source w i t h t h e detection system. A continuous-flow l o w pressure h e l i u m atmosphere was used t o sustain t h e discharge within t h e demountable hollow (9) J, 0. Weideand M. L. Parsons, Anal. Left., 5 , 363 (1972)
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 14, DECEMBER 1973
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Table I. System Parameters Used for Measurement of Limits of Detectiona Elements
Ni
cu
Oxidant, I./min Hydrogen, I./min Burner height, c m Photomultiplier tube, V Recorder, mV-full scale Boxcar integrator Attenuation Integration time, msec Pulse delay, psec Pulse generator Width, psec Period, msec Amplitude, V Hollow cathode lamp Gas flow rate, cm3/min Gas pressure, Torr Background current, mAd
9.0b 9.6 7.0 1000 10
10.0 0.5 900 2
Zn 10.0 8.8 5.5 900 5
0.95 30 400
0.5 100 60
20 10 13 N AC NA 35
900 5
Ag 10.0 8.6 2.0 1000 20
0.95 30 840
0.95 30 700
16 10 7
20 10 15
14.1 4.0 38
5.2 7.5 90
8.0
Cd
Au
10.0b 8.6
10.0
1000 10
0.5 1000 2
Hg 10.5 9.6 5.0 1000 10
0.95 3 350
0.95 30 10
0.95 30 60
0.95 30 400
20 9 13
17 10 9
20
a 12
18 10 7
20 10 11
11.2 9.0 74
37.3 6.5 50
32.8 13.0 90
23.4 4.5 49
35.8 9.2 18
Pd
11.0b 9.6
8.0
8.0
8.0
=The slit height is 12 m m ; slit width, 2000 pm; nominal spectral band width, 40 A; pre-amplifier, X1000. bThese elements used air as the oxidant while the other elements used oxygen. Not applicable because experimental problems led to the use of Perkin-Elmer muitielement nickel lamp. The current of the hollow cathode lamp was measured to be 180 m A during the actual pulse. This value is in addition to the background current.
b
C
d
I
, Initial Trigger
1
Signal Gate
with Shutter Open
I
!L! Figure 1. Block diagram of pulsed atomic fluorescence system
a, Demountable hollow cathode lamp; b, pulsing power supply; c, pulse generator; d , pulse delay unit; e, lens; f , flame; g, monochromator; h, photomultiplier tube: i, wide band amplifier; j , boxcar integrator; k , recorder; I, oscillloscope
f
Figure 2. Continuous-flow hollow cathode lamp
a, Helium outlet; b, helium inlet; c, stopcocks; d , quartz window (attached with epoxy); e, 7/25 J fitting; f . tungsten electrodes; g. 29/42 Tfitting; h, inlet for air cooling; i , outlet for air cooling; j , kovar seal; k , Teflon insulation; I , interchangeable hollow cathode with dimensions: outside length, 2.5 cm; outside diameter, 0.5 cm; hollow inside diameter, 0.25 cm; and hollow inside length, 2.0 cm. Note: The rest of the figure is drawn to scale
cathode lamp excitation source (see Figure 2). The interchangeable hollow cathode was shielded with Teflon (Du Pont) and was easily replaced via the standard taper joint. The lamp was pulsed with a power supply similar to that used by Dawson and Ellis (1). A Ditric (VlOS) total consumption atomizer burner with a stainless steel capillary (V105S) was used to aspirate the samples into the well regulated oxygen-hydrogen or air-hydrogen flame. A 0.35-meter monochromator (Heathkit EU-701-30) incorporating a 1P28 photomultiplier tube was used to convert the fluorescence radiation into an electrical signal. This signal was fed through a wide band amplifier into the boxcar integrator (10). The dual(10) W. G. Clark and A. L. Keriin, Rev. Sci. Instrum., 38, 1593 (1967)
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Time
(milliseconds)
Figure 3. Schematic of one complete pulse cycle
channel boxcar was controlled by the pulse generator. One pulse, used to control the reference channel, comes directly from the pulse generator, while the pulse controlling the signal gate is first passed into a pulse delay unit which positions the signal gate in coincidence with the fluorescence signal. A Tektronix oscilloscope (561A) was used as a monitoring device for convenient alignment of the reference and signal channels. The dc output of the boxcar integrator was fed directly into a Varian recorder (6-2000) for a permanent record. Reagents. Stock solutions (1000 ppm) of the metal cations were prepared from analytical reagent grade metals, oxides, or stable salts. The slightly acidic solutions were diluted with deionized, distilled water. Procedure. The excitation sources were optimized with respect to the helium pressure and flow rate within the demountable hollow cathode lamp. The continuous background current flowing within the lamp was optimized for maximum output intensity (1). This was accomplished by adjusting the background current level of the pulsing power supply. The high current pulses resulting from the triggering action of the pulse generator were superimposed on top of this low level continuous background current. The fluorescent signal was then optimized with respect to burner height, oxidant-hydrogen flow rates, and slit width. This optimization followed the criteria set forth by Parsons and Winefordner (11) for obtaining a maximum S/N. The choice of oxygen or air as the oxidant was based upon the limits of detection tabulated by Parsons and McElfresh (12). The sampling positions of both the reference and signal gates were then adjusted for maximum readout. The overall sampling (11) M. L. Parsons and J. D. Winefordner, Appl. Spectrosc., 21, 368 (1967). (12) M . L. Parsons and P. M . McElfresh, "Flame Spectroscopy: Atlas of
Spectral Lines,'' IFI/Plenum Publishing Corp., Washington, 1971.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
Table II. Comparison of Results Wavelength, Element
Nickel
Palladium
Copper
Silver Gold
Zinc Cadmium Mercury
A 2320.03 3524.54 3414.76 3461.65 3404.58 3609.55 3242.70 3516.94 3247.54 3273.96 2165.09 2178.94 2181.72 3382.89 3280.68 2675.95 2427.95 2138.56 2288.02 2536.52
Relative intensitya 10
Upper limit, PPm 100
Limit of detection, ppm
-
(ac
(4Y
(8)‘
1.
...
...
0.07
(74IC ...
3.
...
...
...
...
...
Oursb
( 75IC
0.07
7 4 3 10 6
1000+d
5 2 10 6
50
0.02
0.002
0.01
0.01
0.01
0.03
50
0.2
0.002
...
...
...
...
100
0.2
...
...
...
...
...
0.006 0.003 2.0
...
0.01
0.02
0.007
0.008
... ...
0.1
0.05
...
...
...
... ...
0.02
10 8 10 4 10 10 10
10
50 1000+d
...
a These values represent relative intensities of these limes compared to the major line. Based upon the S / N ( R M S ) = 1 .OO for 1 6 measurements with a 99% confidence level (73). These values are taken from the literature and represent the limits of detection of comparable systems and in no way represent the best limits of detection obtainable. 1000 DDm was the maximum concentration investigated.
sequence was monitored by a n oscilloscope as illustrated in Figure 3. This monitor shows the reference gate, signal gate, and the signal in a single cycle mode. There was sufficient time delay between the initial triggering pulse and the fluorescence output signal of the photomultiplier detector t o allow the initial triggering pulse of the pulse generator to control the reference gate of the boxcar integrator. The adjustable pulse delay unit was then used to maximize the dc output of the boxcar integrator. The on-time for the integrator gates could not be varied, since the pulse delay unit put out a fixed pulse width. Table I shows the optimized instrumental parameters. Once the system was optimized for a n individual element, an analytical curve and a limit of detection were obtained. The limit of detection is defined as that concentration which gives a S / N (RMS) equal to 1.0 a t a 99% confidence level using 16 measurements ( 1 3 ) . Concentrations were checked near the limit of detection to ensure linearity of the working curves.
RESULTS AND DISCUSSION The analytical curves were linear from the limit of detection to the upper limit. The relative standard deviation was calculated for the useful range of these analytical curves and varied between 1 and 5%. The limits of detection, upper concentration limits, and the relative intensities of other lines are given in Table IT with a comparison of limits of detection obtained with comparable excitation systems. Platinum was not detected a t 1000 ppm and consequently was not further investigated. If a pulsed light source is used, it is possible t o produce intense radiational discharges, the intensity of which is proportional to the voltage applied. The limiting factors include the degree of ionization of the atomic species, the degree of line broadening, and the self absorption within the lamp, plus the stability of the metallic cathode. Some deterioration does occur and appears to result from sputtering of the metallic cathode. In some cases (Cu), this sputtering phenomena appeared to enhance the radiational output of the hollow cathode lamp. The volatility of certain elements (Zn, Cd, and Hg) resulted in occasional fogging of the quartz window. The output radiational level (13) M . L. Parsons, J. Chem. Educ., 46, 290 (1969). (14) R. M . Dagnall, G. ‘F. Kirkbright, T. S. West, and R. Wood, Anal. Chem., 43, 1765 (1971). (15) R. M . Dagnall, G. F. Kirkbright. T. S. West, and R. Wood, Analyst. (London),97, 245 ( 1 9 7 2 ) .
of the excitation source did decrease with time, resulting in a high relative standard deviation. However, the demountable hollow cathode lamp could be cleaned relatively easily. For mercury, the normal cathode was used as the anode while a mercury pool was used as the cathode. It is apparent that (at least for the case of gold) there were some unusual conditions inherent within the pulsed hollow cathode lamp system. For instance, the Au 2675.95-8, emission line from the excitation source was more intense than the 2427.95-8, line. In the nonpulsed techniques the reverse appears to be true (16). This is undoubtedly due to different excitation mechanisms within the hollow cathode systems. The d a t a for silver indicate the same trend (the S/N for the Ag 3382.89-8, line is greater than that for the 3280.68-8, line). This, however, is simply the result of increased flame noise (OH emission) in the area of the 3280.68-8, line. The limits of detection tabulated in Table I1 are not as good as the best reported; however, they are compatible with comparable excitation techniques. It can be shown that any two values for the limit of detection of a particular element may be considered identical if they agree within a factor of 3 (17). The detection limits obtained by this system were limited by the intensity of the excitation bource (applied power) and by the noise level within the system. The noise originated from the excitation source, the flame, and the electronic component of the system. Since there is a direct relationship between the measurement time and the charging time of the RC-integration circuit, one must apply practical measurement time limits. It is necessary to consider the amount of the sample, long term instrument stability, and practical analysis time. A measurement time of three minutes seemed to be a reasonable upper limit. It is the opinion of these authors that several improvements could be made on the system which would lower the limits of detection considerably. These would include changing from a turbulent total-consumption burner to a (16) J. Matousek and V Sychra, Anal. C h m . Acta. 49, 175 (1970). (17) M . L. Parsons and P. M. McElfresh, Appl. Spectrosc., 26, 472 (1972).
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 14, DECEMBER 1973
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laminar-chamber type burner, optimizing the hollow cathode geometry, increasing the pulse voltage applied to the lamp, and increasing the duty cycle of the pulsing system-ie., increasing the pulse rate.
ACKNOWLEDGMENT The authors are grateful to Hershel Butterfield for his expertise and help with the electronic circuitry. They fur-
ther acknowledge the helpful comments and suggestions of J. D. Winefordner. Received for review March 15, 1973. Accepted July 9, 1973. Part of this research was performed while on an NSF Tranineeship (J.W.) and was also partially funded by an A.S.U. Faculty Grant-in-Aid.
Calcium-Phosphate Solute Volatilization Interference in Flame Spectrometry A Lateral Diffusion Interference Effect A. C. West,’ V. A. Fassel, and R. N. Kniseley Arnes Laboratory-USAEC
and Department of Chemistry, iowa State University, Arnes, Iowa 50010
One of the most studied of all “chemical” interferences in analytical flame spectrometry has been the suppression effect exerted by increasing concentrations of phosphate ion on calcium free-atom absorbance or emission. Although most investigators in the past have reported suppression effects, there have been scattered but repeated observations of enhancement effects as well. Among the earliest reports of the enhancements were those of Dippel et al. ( I ) and Rains e t al. ( 2 ) ; both groups of workers made their observations in flames formed by direct injection burners. Dippel and coworkers reported that increasing concentrations of phosphoric acid in the solution gave rise to a continuum emission, but this background was too weak to account for the observed enhancements. Rains ( 3 ) suggested that the enhancement was due to the suppression of CaO formation. Several investigators have reported that high temperature flames formed with either direct injection or premix burners caused the depression to be completely eliminated for P/Ca ratios up to a t least 100 ( 4 , 5 ) ;these results were entirely in accord with predictions based on the general mechanism that has been proposed for the interference. However, there have been several reports of small but significant enhancements of Ca absorption or emission by the presence of H3P04 when the nebulized sample was introduced into nitrous oxide-acetylene flames formed on linear slot burners (6-9). This enhancement was attributed initially to Ca ionization suppression by flame species containing P (6). Manning and Capacho-Delgado (7) later suggested that the enhancement was caused by a faster dissociation of solid particles because they could see no change in the Ca+ absorption with added H3P04. Wil1
On leave from Lawrence University, .4ppleton, Wis. 54911.
(1) W. A. Dippei, C. E. Bricker, and N. H. Furman. Anal. Chem., 26, 553 (1954). (2) T. C. Rains, H. E. Zittei, and M . Ferguson, Talanta, 10, 367 (1963). (3) T. C. Rains, in “Flame Emission and Atomic Absorption Spectroscopy,” J. A. Dean and T. C. Rains, Ed., Vol. 1, Marcel Dekker, New York, N.Y., 1969, p353. (4) V. A. Fassel and D. A. Becker, Anal. Chem., 41,1522 (1969). (5) V. G. Mossotti and M. Duggan. Appl. Opt., 7, 1325 (1968). (6) M. D. Amos and J. B. Willis, Spectrochim. Acta, 22, 1325 (1966). (7) D. C. Manning and L. Capacho-Delgado, Anal. Chim. Acta, 36, 312 (1966). (8) L. R. P. Butler and H. A. Fulton, Appl. Opt., 7, 2131 (1968). (9) S. R. Koirtyohann and E. E. Pickett, Anal. Chem., 40, 2068 (1968).
2420
lis subsequently subscribed to this point of view (IO). Our experimental results contradict all of these hypotheses. Koirtyohann and Pickett (9) showed that the horizontal distribution of analyte free-atoms in flames formed on slot burners may be significantly altered by mineral acid concomitants in the nebulized solution. We have confirmed the redistribution observed by Koirtyohann and Pickett and have called it a lateral diffusion interference (11). This interference gives rise to an increase in free atom population a t the center of the flame and a decrease in that population a t the edges of the flame. In our recent communication ( 1 1 ) , we showed that this interference effect appears to be the underlying cause of a wide range of enhancement effects induced by cationic concomitants. In the present paper, we show that the lateral diffusion interference effect satisfactorily accounts for the puzzling but real enhancements observed for the calcium-phosphate system.
EXPERIMENTAL The experimental facilities have been previously described (11). Spectrometer entrance and exit slits were 0.2 m m high and 0.1 mm wide. The Ca stock solution was prepared by dissolving reagent grade C a C 0 3 in a minimum of HCl. Ca concentration in the test solutions was 12.5 pg/ml throughout. The wavelength used was 4226.7 A. Gas flows used gave a flame with a primary reaction zone about 2 mm high and an interconal zone t h a t terminated a t 12-15 mm above the burner tip.
RESULTS AND DISCUSSION It should be emphasized a t the start that, although the data in this report were taken from flame emission measurements, identical results have been obtained for some of the same systems using atomic absorption. Figure 1 and Table I show that excess H3P04 does not enhance Ca emission intensity everywhere in the flame: all H3P04 concentrations used depress the emission low in the flame. However, a t 5 to 10 mm above the burner tip, where Ca emission is a t a maximum, enhancements are observed, and it is a t this height that interference effects are usually studied. If, on the other hand, elimination of the interfer(10) J. B. Wiilis, in “Analytical Flame Spectroscopy,” R. Mavrodineanu, Ed., Macmillan and Company, Ltd., London, 1970, p 557. (11) A. C . West, V . A. Fassel. and R. N. Kniseley, Anal. Chem., 45, 815 (1973).
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