anodic to that for silver deposition, returning to this initirtl potential (with subsequent silver stripping) after each pulse. Figure 5 is a derivative pulse polarograph current-potential curve for the same solution. The curves are quite reproducible and not affected by electrode rotation. The shape of the normal and derivative curves is in fair accord with what is theoretically expected, but rarely obcserved, for metal deposition a t solid e1el:trodes ( 3 ) . The limiting current for the normal mode current-voltage curve, in Figure 4, is in reasonable agreement with the current calculated from the Cotrell equation ( 2 ) . The current is measured 40 mseconds after the pulse is applied; this corresponds to the electrolysis time used in the calculations. Greater sensitivity may be obtained by making the current measurement at shortw times. I n the voltage swtlep mode it has proven possible to determine Pb(I1) in the presence of 500 to 1000 fold excess of Cu(I1) by initiating the cathodic sweep on a dropping mercury electrode from a potenial in the limiting current region for the Cu(I1) reduction, the Cu(I1) limiting current being unaffected by application of the sweep. By initiating a n anodic sweep from a
used to determine the Pb, with some slight increase in sensitivity. A detailed description of the instrument, capable of being used in both cathodic and anodic voltage scans in all modes of operation, will be published shortly ( 5 ) and intensive work on the application, performance, and limitations of the instrument is in progress.
LITERATURE CITED
( 1 ) Barker, G.
i VOLTS VS S C E
Figure 5. Derivative pulse polarogram for a 1.0 X 10-3M Ag(l) solution in 0.1M KNO, a t a platinum button electrode
potential cathodic to the Pb(I1) reduction, also on a dropping electrode, the peak current due to oxidation of the P b ( 0 ) from the electrode may also be
C., Gardner, A. W., At. Energy Res. Est. C/R 2297 August (1958); 2. Anal. Chem. 175. 79 (1960). ( 2 ) Delahay, P., “New Instrumental Methods in Electrochemistry,” p. 51, Interscience, New York, 1954. (3) Kolthoff, I. M., Lingane, J. J., “Polarography,” Tol. I, p. 203, Interscience, Kew York, 1952. (4) Schmidt, H., von Stackelburg, M., “Modern Polarographic Methods,” p. 70, Academic Press, S e w York, 1963. (5) Schlein, H., Parry, E., Osteryoung, R., in press.
E. P.PARRY R. A. OSTERYOUNG North American Aviation Science Center Canoga Park, Calif. RECEIVED for review March 16, 1964. Accepted April 9, 1964.
Study of IExperimental Parameters in Atomic Fluorescence FIame Spectrometry SIR: The basis of atomic fluorescence spectrometry was der,cribed by Winefordner and Vickers ( 6 ) , and the application of atomic fluorescence flame spectrometry to the analysis of zinc, cadmium, and mercury was discussed by Winefordner and Stas b ( 6 ) . This new method of spectral analysis was compared with other spectrometric methods in the two previous papers. I n this communication, greatly increased sensitivities of analysis are reported for zinc and mercury anc the atomic fluorescence of thallium i:, presented. The influence of flame type, excitation source type, solvent, sheath gas surrounding the flame gases, and inert gas added to the fuel gas on the atoinic fluorescenceparticularly the limit of detection-of zinc, cadmium, mercury, and thallium have been investigated. INSTRUMENTAL
The same instrumental setup previously described (6, 6) was used for all studies. However, 2,dditional equipment was necessary for part of the studies performed and will be described below. Most studies were performed using
the total consumption atomizer-burner with medium bore capillary ( 5 ) . For several of the studies the influence of a sheath of gas surrounding the flame was studied. For these investigations, the sheathed atomizer-burner previously described by Gilbert ( 2 ) was used. With a three-way valve foreign gasese.g., argon or nitrogen--could be added to the fuel gas (H2 or C2H2). For several studies, a chamber-type atomizer-Bunsen burner assembly was used (3). Several sources of excitation were used in these studies. For zinc and cadmium, Osram lamps with I-inch diameter holes cut in their outer soft glass envelopes to allow unobstructed pasiage of resonance radiation from the qu‘artz inner bulb were used. For mercury, several lamps were uqed in addition to the Hanovia lamp previously described ( 5 ) . A mercury Osram spectral lamp, type Hg-S, operated a t 1.1 amperes, a Philips mercury spectral lamp (The Ealing Corp., Cambridge 38, Mass.) operated a t 0.9 ampere, and a mercury electrodeless discharge tube were used. The mercury electrodeless discharge tube (Ophthos Instrument Co., Rockville, Xld.) was held at one end by a thermometer clamp in
the center of a 2450-mc. resonant cavity (Ophthos Instrument Co.) with a n observation port optically aligned with the flame cell. Power was supplied to the cavity by a coaxial cable from a 100-wat t microwave power generator (Model PGM-10 X 1, The Raytheon Co., Waltham 54, Mass.). All other lamps were placed in appropriate sockets and mounted as previously described ( 5 ) . For thallium, both the Osram spectral lamp operated at 1.0 ampere and the electrodeless discharge tube were used. Electrodeless lamps for gallium, indium, and selenium were also available (Ophthos Instrument Co.). The electrodeless discharge tubes were operated at only a percentage of full power (Hg and T1, 40%; Gal 60%; In, 50%; and Se, 60%). T o avoid overheating of the mercury and gallium lamps and to minimize self-reversal of source lines, these lamps were cooled to some extent by placing a small centrifugal blower below the resonant cavity and allowing a stream of air to circulate within the cavity. When the electrodeless tube or the Hanovia mercury vapor lamp was used, radiation was focused on the flame cell by a quartz lens with 5.0-mm. focal length. For the Osram and Philips VOL. 36, NO. 7, JUNE 1964
1367
lamps, greater sensitivities resulted when the source was placed as close as possible to the flame in the qame manner as previously described ( 5 ) . When the total consumption atomizer-burner was used, atomic fluorescence measurements of mercury and thallium were taken about I cm. above the luminous tip of the flame to avoid quenching in the case of mercury and to minimize thermal emission in the case of thallium. -111 other metals were studied in the flame about 1 cm. above the reaction zone. With the Bunsen burner flame, all measurements were taken about 2 cm. above the reaction zone. I n Table I the metals and the wavelengths used for atomic fluorescence measurements are given. In all cases excitation was performed using the appropriate unfiltered source. Also in Table I, the flame types and the fuel flow rates (oxygen flow rate was 2500 cc. per minute in all cases) are given. The procedure for esperimental measurement including the optimization of the monochromator slit width has been previously described ( 5 ) . The entrance
Table I. Wavelengths, Flame Types/ and Fuel Flow Rates for Various Metals Introduced into Flames Using Total Consumption Atomizer Burners for Atomic Fluorescence Measurements
Element Zn
Wavelength, A.
2139
Cd
2288
Hg
2537
TI
3776
Fuel
Flame type Hz/ilir Hz/Oz
flow rate,
c~./min.~ 8000 5000
CzH2/02
lo00
Hs/Air HdO2
8000 5000 1000
C2Hz/02
Oxygen flow rate was 2500 cc. per minute in all cases. 0
slit width when using the Bunsen burner was 0.50 mm. in all cases and when using the total consumption atomizer-burners was 0.25 mm., 0.20 mm., 0.40 mm., and 0.25 mm., respectively, for zinc, cadmium, mercury, and thallium. The exit slit was set at 0.56 of the value of the entrance slit for optimum spectral resolution.
RESULTS AND DISCUSSION
I n Table I1 the limits of detection for zinc, cadmium, mercury, and thallium when excited by zinc and cadmium Osram lamps and mercury and thallium electrodeless lamps, respectively, are given. Limit of detection is used as previously defined (6). The influence of 30 and 70y0 methanol as a solvent and the type of flame and atomizer on the results can also be noted from the data in Table 11. The best limits of detection for mercury-Le., 0.1 p.p.m.--and for zinc-Le., 0.005 p.p.m.compare favorably with the lowest, limits listed in the literature by other spectrometric methods. The lowest limit of detection for cadmium-Le., 0.1 p.p.ni.-is somewhat poorer than that obtainable by atomic absorption flame spectrometry, and the 1.0-p.p.m. limit for thallium is considerably greater than the limit of detection obtained by normal thermal emission flame spectrometry. The results in Table 11, show that the flame type seems to have little influence on the results, which is to be expected for metals which have little tendency to ionize or form compounds in the flame gases. The influcnce of methanol on the zinc resuks i. to be expected according to the increased efficiency of atoniization with increase in methanol concentration (4). However, the conrtancy of the cadmium limit of detection with solvent composition cannot be esplained. Because of the observation of the atomic fluorcs-
Table 11. Limit of Detection in p.p.m. for the Atomic Fluorescence Flame Spectrometric Determination of Several Metals Introduced in Several Solvents into Several Flame Types
Flame type
CIHPIOP Hz/airb a
Solvent H20 30% CHIOH 70% CH30H HzO 30% CHIOH 707c CHaOH HzO
Zn 0 0 0 0 0 0 0 0
04
Elementa Cd Hg
T1
0 1
1 0
03
0 1
0 1 0 1
005 04
0 0 0 0
005 01 02
1 0
1 3 3 2
0 1 0 1
0 1 0 3
5 0
0 2
0 1
20
5 0
01 0 1 70 1 0 H20 S a t u r a l gas/air" Zinc and cadniirim excited bs Osram spectral lamps and mercury and thallium excited
by electrodeless discharge tubes * Total consumption atomizer-burner. c Chamber type atomizer-Bunsen burner
1368
ANALYTICAL CHEMISTRY
cence of mercury and thallium above the luminous tip of the flame, it would be espected that solvent composition should have considerably less effect. The higher detection limit for mercury in the natdral gas flame compared to measurements above the tip of the Hz/02,H2/air, or C2Hz/02flames is probably a result of the great quenching efficiency of excited mercury atoms by the flame gas products (6). Results for mercury above the luminous tip of the Bunsen burner flame were erratic and not usable. This was a result of the nonrigidity of the flame and its fluctuation caused by room air currents. Several other spectral lines for each metal were also measured. The 3076 L4, zinc line and the 3261 A. cadmium line were just detectable a t 1000 p.p.m, of , t h e respective metals. Xo other spectral lines of mercury were detectable even at' 1000 p.p.m. (the 1849 A. mercury line was not studied). The 5350 A. line of thalliuni gave essentially the same results as obtained in Table I1 for the 3776 -4. line. The 3530 A. line of thallium was only slightly detectable a t 1000 p.p.m. of thallium. The excitation of the mercury vapor by the Hanovia lamp gave considerably higher limits of detection-Le., 5 p.p.m. in the H2, O2 flame and 1 p.p.m. in the C?H2,'OPflame (5). The exitation of mercury in any flame by either Osram or Philips spectral lamps and the excitation of thalliuni in any flame by an Osram spectral lamp resulted in no detectable fluorescence even a t 1000 p.p.m. of mercury and thallium, respectively. This is probably a result of extreme self-reversal of the exciting lines. For each flame type, solvent, and experimental setup, the atomic absorption of the resonance radiation was also measured under the same experimental conditione used for the atomic fluorescence studies. In all cases the limit of detection by atomic fluorescence was about 50 times lower than by atomic absorption flame spectrometry. This is not surprising in view of the extremely small flames ( H 2 / 0 2 0.5 cm. in diameter and C2H2/02 1.0 em. in diameter) being used for atomic fluorescence measurements as compared to the flames normally used in atomic absorption flame spectrometry. In addition, the flame conditions are optimized for atomic fluorescence rather than atomic absorption measurements. Suitable changes in the flame cell will result in comparable limits of detection for the two methods as previously indicated. The addition of nitrogen or argon to the fuel gas and the use of nitrogen, argon, or oxygen in the sheath resulted in little change in the limits of detection and in the shapes and slopes of the analytical curves. I n most
cases, argon resulted in a slight decrease in limit of detection as would be eq)ected from the discussion presented by Alkemade ( I ) . S o analytical curves will be presented in this communication because they are all similar t o the ones previously given ( 5 ) . Essentially the same accuracy of analybis and signalto-noise ratio as preLiously measured ( 5 ) were again found in t,hcse studies. S o detectable fluo1,escence was observed for the 2874, 4033, and 4072 .I. lines of gallium, for the 2560, 3040, 3256, 4102, and 451 1 A. lines of indium, and for the 2040 and 2591 A. lines of selenium when esciteti by their respectilre electrodeless discharge lamps. This was primarily a result of the estremely low intensity of the spe-tral lines emitted
by the respective electrodeless lamps. These results are certainly not indicative of the method. At the present time additional studies, particularly with respect to sources of excitation, methods of sample atomization, flame type, and amplifier type, are being conducted. These studies should result in increased sensitivities of analysis of the metals already studied and the e\tension of atomic fluorescence flame spectrometry to other metals. LITERATURE CITED
(1) Alkemade, C. T. J., International Conference on Spectroscopy, College Park, Md., June 1962. (2) Gilbert, P. T., Jr., Pittsburgh Conference on Analytical Chemistry and Ap-
plied Spectroscopy, Pittsburgh, Pa., February 1961. ( 3 ) Mavrodineanu, It., Boiteux, H., “L’Analyse Spectrale Quantitative Par La Flamrne,” Masson et Cie, Paris, 1954. (4) Winefordner, J. D., >Tansfield, C. T., Vickers, T. J., ANAL.CHEM.35, 1607 (1963i. -, ( 5 ) Winefordner, J. I)., Staab, It. A , , Ibid., 3 6 , 165 (1964). (6) Winefordner, J. I ) . , 1-ickers, T. J., Ibid., 36, 161 (1964). J. 11. WINEFORI)NER Department of Chemistry University of Florida Gainesville, Fla. R. A. STAAR Ivorydale Technical Center Proiter & Gamble Co. Cincinnati, Ohio \ - -
RECEIVEDfor review March 20, 1964. Accepted April 14, 1964. This communication taken in part from the Ph.1). thesis of R. A. Staab.
Gas Chromatographic Response to Plug-Shaped Sample Inputs: Calculation of Plate Height from Response Curve SIR: Recently, the desire has been expressed for a met’hod whereby plate heights can be computed from gas chromatographic response curves when the sample input profile is in the form of a plug ( 3 ) . In previous work ( I , 4 ) , we discussed the calculat,ion of response profiles for various sample input profiles in linear chromatographic systenxi.e., no solute-solute interactions. If it is assumed that the response, S ( t ) , to an ideal impulse can be adequately expressed as a Gaussim,
S(t)
plate height can be computed as for a Gaussian response. When the plug width becomes sufficient’ly large, the peak is flat on top-approaching the response to a step- and the plate height can be computed as for a step response ( I , 4 ) . However, for intermediate plug widths, neither method yields the correct value for the plate height of the column. Figure 2 illustrates four of the more convenient parameters that can be used to calculate plate height in the case of a
=
(2~)-~”% ex],[-~
(t -
tR)’/2U2]
(1)
where t R is the retention time and u is the standard deviation of the peak. Then the response, R(t),to a plug input of unit, height is
R(t) = (2x)-“2
S
(t -
(t -
tR),’U
tE
- s)/u
e--’*%y
(2)
where s is the width of the sample plug. Time, t , is taken as zero at, the beginning of sample input, and t equals s a t the end of the plug input. Equation 2 is equivalent to Equation 29 in our earlier paper (4) and is similar in form to Equation 9 of Peniston, Agar, and McCarthy (2). Thli shape of this Gaussianed-plug response is illustrated in Figure 1 for various relative plug widths. When the plug width becomes sufficiently small, the peak approaches the response to an impulse and the
( t - tR)/U
Figure 1 . Chromatographic response to plug-shaped sample inputs ( I )
Gaussianed-plug
re:l)onse.
Methods
a, b, and c all give the correct value for
the true plate height, H , when the input is an ideal impulse. Method d gives the correct value for true plate height when the input is an ideal step. However, when applied to the response to plug inputs of intermediate widths, these four methods yield four different numerical values for the apparent Idate H,*, height,, H * . Values of /lo*, and H,* have been comlnited for various plug widths by standard mathi3matical analysis of Equation 2. I3ecause the equations do not appear in rlosed form and iterative numerical computations are necessary, no details will be presented here. The results of these calculations are givcn in Figures 3 and 4 in a form which permits their direc,t use in converting an apparent plate height, H*, into the true plate height, H, nhen the plug width, s, and one of the measures of peak width- At,,, Atli2, or Ati-or the tramition time, 7 , are experimentally known. In Figure 3, the i,rxlative value of the apparent plate height is plotted as a function of the ratio of sample plug width to peak width for each of the three different ways of measuring the peak width of the response curve. Thus, the true plate height, H ,can be calculated from an experimental apparent plate height-- H0*, or H,*-in the following n a y . First, the value of s / A t is determined from the experimental values of the input plug VOL. 36, N O . 7, JUNE 1964
1369
