Table 111. Comparison of Methods Sample Reference No. description 1. Experimental tea= (this paper) (13) 2. Ceylon tea leaf (12) 3. Japanese tea leaf. 4. Japanese green tea (this paper) (11) 5. Japanese green tea a Represents average of several sets of data.
Per cent EGCG
10.04 10.55 10.1 7.3 6.1
ECG 2.96 2.75 3.3 2.1 2.0
EGC 3.64 2.72 4.3 3.5 2.3
EC
CAT
1.20 0.63 1.6
0.24 0.35 ... 0.2
1.o
0.7
...
Total 18.90 16.90 19.2 14.1 11.1
cessful detection. The greater the difference between the measuring and reference gas flow rates, the greater will be the sensitivity of the GDB detector (21-23). Evaluation of Method. The extraction procedure was evaluated by performing a series of recovery experiments. Samples of fired green tea, previously analyzed for flavanols, were fortified with known amounts of epicatechin and epicatechin gallate. An average recovery of 98.3% was obtained as shown in Table 11.
The flavanol content of several fresh samples of experimentally grown tea was determined by this method. The results reported in Table I11 indicate that the method compares favorably with those reported in the literature. As expected, different flavanol concentrations are observed for green tea and fresh leaf samples. The first three samples were fresh leaf and should be compared separately from samples 4 and 5 which are fired green tea. The chromatograms of a typical two stage analysis of fresh green tea are shown in Figure 3.
(21) A. J. P. Martin and A. T. James, Biochem. J., 63, 138 (1963). (22) A. G. Nerheim, ANAL.CHEM., 35, 1640 (1963). (23) J. T. Walsh and D. M. Rosie, J . Gas Chromatogr., 5, 232 (1967).
RECEIVED for review August 22, 1968. Accepted November 6, 1968. Paper presented to the Division of Analytical Chemistry 153rd Meeting, ACS, Miami Beach, Fla., April 1967.
General Considerations Concerning Atmospheric Aerosol Monitoring with the Hydrogen Flame Ionization Detector R. W a y n e Ohline,' Edwin ThallY2and Ping H w a t O e y Department of Chemistry, New Mexico Institute of Mining and Technology, Socorro, N. M. 87801 The primary nonorganic constituent of the atmosphere giving a response with the hydrogen flame ionization detector is sodium. Consequently, it appears that continuous, nonattended operation of the detector for organic aerosol monitoring can be achieved if ion pulses arising from sodium-containing particles can be subtracted from the total charge delivered in the pulse mode. To understand the response characteristics for the detector to sodium salts, the rate of ion formation has been studied, and first-order rate constants for the ionization of sodium in five small hydrogen diffusion flames have been determined. Rate data obta-ined from these studies indicate that the detector can detect particles of sodium chloride 0.1 p in diameter.
THEPOTENTIALITIES of the hydrogen flame ionization method for the detection and sizing of aerosols have been realized for some time (1-3). However, an instrument which will monitor an actual polluted atmosphere has not yet been perfected. Our work indicates that sodium-containing aerosols are responsible for most of the charge delivered to the detector in
(1) R. W. Ohline, ANAL.CHEM., 37, 93 (1965). (2) H. Frostling and P. H. Lindren, J . Gas Chromatogr., 4, 243 (1966). (3) W. L. Crider and A. A. Strong, Rec. Sei. Instrum., 38, 1772 (1967). 302
ANALYTICAL CHEMISTRY
pulse form. Hence, if charge pulses arising from sodiumcontaining aerosols can be subtracted from the total charge delivered in pulse form, a detector useful for atmospheric organic aerosol monitoring may result. Our approach has been based largely on the work of Lawton and Weinberg (4,who demonstrated that useful rates of ionization data can be obtained in flames by measuring the charge yield under saturation conditions (i.e., where the potential on the collecting electrodes is not so small that ions recombine before collection, or so large that ion multiplication occurs). Their approach is clarified by examination of a current-potential curve for the flame ionization phenomena (Figure 1). In region a, recombination and escape of ions from the collecting region are competing with the ion-collecting ability of the field. In region b (the saturation region), increase in potential produces no charge yield increase. Hence, all ions are being collected and, therefore, one must conclude that a rate process is being observed. Lawton and Weinberg demonstrated that plots of the logAuthor to whom inquiries should be directed. Present address, Institute of Polymer Science, The University of Akron, Akron, Ohio. (4) J. Lawton and F. J. Weinberg, Proc. Roy. Soc., Ser. A , 277,
468 (1964).
resident time At =
~*(cm3) F cm3/sec.
Reaction zone V “
Figure 2. Idealized flame reaction zone A p p I i e d Pot e ni i a I
Figure 1. Ion current as a function of applied potential for ionization in a flame arithm of ion yield cs. the reciprocal of the absolute flame temperature for the ionization of hydrocarbons in hydrogenair flames yielded straight lines from which activation energies for the ionization process could be obtained. However, they did not extend their work to the calculation of rate constants for ion formation. It is believed that the rate constant itself represents the best parameter for ionization detector description, since a knowledge of the flame geometry and flow profiles then allows prediction of ultimate sensitivity. We have measured the charge yield of sodium in five small hydrogen diffusion flames, and calculated the first-order rate constants for the reaction Nao(g) + Na+(g) e- for temperatures believed to be representative of these flames. The method of evaluating the rate constant for first-order kinetics is clarified by considering the flame reaction zone as idealized in Figure 2 along with the first-order rate equation :
+
dn --
dt
= kn
Fuel carries the ionizing material into the reaction zone, where each atom exists for At sec. A certain fraction of these will ionize and be collected on the collecting electrode. Hence, dn is known from the charge delivered to the collecting electrodes; and the number of atoms, n, is known from the amount of salt introduced into the flame. For the case where dn is small compared with n, the rate constant can be calculated directly from the rate expression in differential form. In terms of measured variables, dnjn is given simply by the ratio of charge yield per gram of sodium to the charge yield if all the sodium ionized, or g/4200, where g is the coulombic yield (coulombs per gram sodium) and 4200 is the number of coulombs which would be obtained per gram if all the sodium ionized. It follows that the rate constant is given by:
EXPERIMENTAL
Charge Yield Measurements. Figure 3 shows the essentials of the apparatus used in determining charge yield. A wire mesh cylinder 1 in. long and 1 in. in diameter was charged to -600 V relative to the burner tip. The burner was machined from aluminum with a 1/L6-in, diameter tip and 0.0135 in. diameter bore. The capacitor used for collecting the charge transferred at the flame was of high quality construction (Astron Metalite AS 1196 400 WVDC). To obtain the charge yield per unit mass of sodium salt, a small quantity of solution (0.1 to 1.0 p1, 0.025 to 0.10 g saltil) was introduced into the base of the flame on a 0.005-in. diameter platinum wire. The quantity of salt introduced never exceeded 0.1 pg. The charge delivered to the capacitor caused a potential change across the capacitor terminals which was monitored by a Heathkit potentiometric recorder. The potential on the capacitor was never allowed to exceed a few hundred millivolts. Hence, the potential drop across the collecting region remained essentially 600 V. Although more sophisticated circuitry employing an electrometer was available for these determinations, this was found to be unnecessary, as tests of this apparatus showed that accurate charge yield measurements were obtained. Some variation was noted when different personnel in our laboratory attempted these measurements. However, after some practice, a relative standard deviation of 8 % was attained. In addition, it should be noted that the range of available potentials using this technique is not large, since at potentials above 1200 V, ion current resulting from the presence of the hot platinum becomes appreciable. This current is presumably due to removal of thermionic electrons from the immediate vicinity of the platinum. Consequently, it is necessary that the total ion yield remain low (less than lo-’ coulomb), since larger charge yields require larger potential gradients to maintain the saturation condition ( 4 ) . It follows that the use of this method to obtain the ionization rate constants for metals with very fast ionization rates may be attended with difficulty.
600 Volts
Shorting Switch
where F is the hydrogen flow rate measured at room temperature (293 OK), P’* is the reaction zone volume, and T* is the absolute flame reaction zone temperature. Equation 2 is valid only when the ionization rate is a maximum in the region of maximum temperature (Le., when the ionization is a thermal phenomenon). For the alkali metals, a thermal mechanism of ionization in hydrogen-nitrogen-oxygen flames has been confirmed (5). ( 5 ) D. E. Jensen and P. J. Padley, “Eleventh Symposium on Combustion,” The Combustion Institute, Pittsburgh, Pa., 1967, p 351.
I
Pot en t i o m e t r i c Recorder
I
Figure 3. Schematic of apparatus used in determining charge yield VOL. 41, NO. 2 , F E B R U A R Y 1969
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