the bottom of a y4-in. Teflon precolumn can be shaved thin and flared to allow it to slip over a 3’4-h. column. Received for review November 3, 1972. Accepted January 22, 1973. Trade names and company names are included as a matter of convenience to the reader, and such inclusion does not constitute any preferential endorsement by
the U. S. Department of Agriculture of products named over similar products available on the market. Contribution of the Agricultural Research Service, USDA, in cooperation with Colorado State University Experiment Station, Scientific Journal Series No. 1787. This research was supported in part by the Environmental Protection Agency.
Electron Spectroscopy (ESCA): Use for Trace Analysis David M. Hercules, Lawrence E. Cox, Stephen Onisick, Gary D. Nichols, and James C. Carver Department of Chemistry, University of Georgia, Athens, Ga. 30602
Electron spectroscopy (ESCA) has been adapted for use in trace metal analysis by using glass fiber mats with chelating groups on their surfaces. This represents a unique application of ESCA because to date, the technique has been used primarily for studying for atomic and molecular structure of gases and solids ( I , 2). ESCA has been particularly valuable in studying charge distributions in chemical bonds ( I , 3), and has considerable potential for the interpretive spectroscopy of organic and inorganic compounds (4).The use of ESCA for quantitative measurements has met with limited success except for determining relative numbers of the same atom in a molecule. Siegbahn et al. ( I ) , were able to establish elemental ratios in a variety of samples to *5-10%. Wagner has established elemental sensitivities for a number of elements using Na, F, or K as internal intensity standards (5). Workers in our laboratory (6) have developed a method for the analysis of molybdenum oxide mixtures that showed reasonable accuracy. Brinen and McC1ure (7) have used for trace analysis by electrodepositing metals on mercury coated platinum electrodes; however, they presented no estimate of detection limits. ESCA is primarily a surface technique, the photo-ejected electrons originating within the first ca. 20 A of the surface (8). ESCA has high intrinsic sensitivity because fractional monolayers on a surface have been measured; this indicates an inherent ability to measure 10-8-10-9 gram of a material or less. When applied to bulk analysis, ESCA is sensitive to concentrations of only ca. 0.1% even in the most favorable cases ( I ) . Although the use of high intensity X-ray sources and multichannel detector systerns could improve this figure by a t least two orders of magnitude, the limitation on ESCA would be for the use in the 10-100 pafis-per-million range or higher. Such senK. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin. J. Hedman, G. Johansson, T. Bergmark, S. Karlsson. i. Lindgren, and B. Lindberg, “ESCA Atomic Molecular and Solid State Structure ,~~ and WikStudies by Means of Electron ~ p e c t r o s c o p y Aimquist sells. Uppsala, Sweden, 1967. K. Siegbahn, C. Nordiing, G. Johansson, J. Hedman, P. Hedan, K. Hamrin, U. Gelius. T. Bergmark, L. Werme, R. Manne, and Y. Baer, to Free North Elsevier, Amsterdam, New York. 1969. D. A. Shirley, Ed., “Electron Spectroscopy,” North-Holland Publishing Co., Amsterdam, 1972. . progr,, 32, 183 S. H. Hercules and D. M. ~ e r c u i e ~ ,e c Chem, (1971). C. D.Wagner, Anal. Chem., 44, 1050 (1972). W. E. Swartz and D. M. Hercules, Anal. Chem., 43, 1774 (1971). J. S.Brinen and J. E. McClure, Anal. Lett., 5, 737 (1972). R. G. Steinhardt, J. Hudis, and M. L. Perlman, Phys. Rev. B., 5, 1016 (1972).
Figure 1. Diagram of holder for treating glass fiber disks with solutions for trace analysis. See text for details
sitivity is not sufficient to make ESCA attractive for use in geochemical, biomedical, or environmental applications where sensitivities in the parts-per-billion (ppb) range are required. In order to utilize the inherent sensitivity of ESCA for trace analysis, it is necessary to devise a way for the atoms Or ~ o l e c u l e sof interest to line UP on the Surface (in a monolayer Or less) SO they can be measured readily. Ion exchange Preconcentration has been used with X-ray fluorescence for measuring trace metals in solutions (9).Similarly thin layer chromatography (TLC) is widely used for the separation and preconcentration of organic materials for measurement by a wide variety of spectroscopic techniques. Unfortunately, diffusion within ion exchange resins and TLC piates limits the amount of (gadsorbed” material. directly on the surface. We have experienced difficulty in observing ESCA signals from TLC spots even in very favorable cases, What one needs is a “two-dimensional” ion exchanger or TLC plate in which diffusion into the bulk is limited, so that all “adsorbed” species can contribute to the ESCA signal. Use of silylizing agents for coating glass surfaces with organic functional groups is well known (IO). Silylizing (9) C. W. Blount, W. R. Morgan, and D. E. Leyden, Anal. Chim. Acta, 53,463 (1971). (10) M. L. Hair, ”Infrared Spectroscopy in Surface Chemistry,” Dekker, New York. 1967, Chap. 4.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
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agents are a class of organosilicon compounds which bond readily to glass surfaces and impart chemical properties to the glass surface, depending upon the nature of the functional groups attached. A silylized glass surface is a monomolecular layer of an organic coating and can serve as a basis for a “two-dimensional” ion exchanger for ESCA analysis. By coating the surface with a chelating group, we have achieved specific scavenging of metals from solutions. In the experiments reported here, we have used glass surfaces coated with the dithiocarbamate group to analyze for trace quantities of heavy metals. This type of surface is produced by treating glass fiber with an amino functional silylizing reagent and subsequently with carbon disulfide and sodium hydroxide to form the dithiocarbamate:
3::
glass
OH
+
+
CS,
+
(CH30)3SiCH2CH2CH2NHCHzCHzNH2
NaOH
-P
-J:;
O-SiCH2CH2CH2NHCH2CH2NH2 1
2
2
group in 2 is capable ,of reacting
with heavy metals as a chelating agent Chelating glass surfaces were prepared from heat-treated Owens Corning Fiberglas manufactured from E-glass. Samples were cleaned by refluxing in 2-propanol for 30 minutes prior to the application of the silylizing reagent. The glass fiber was removed from the hot alcohol and allowed to dry for several minutes. It was placed in a 4% solution of Dow-Corning 2-6020 silylizing reagent which was prepared according to the manufacturer’s directions (11). After five minutes the sample was removed and allowed to dry thoroughly in a desiccator over calcium sulfate. Conversion to the dithiocarbamate was effected by refluxing the silylized glass fiber for 4 hours in absolute ethanol, 1M i n NaOH and 1.5M in CS2. The glass was then washed several times in ethanol, dried and stored in a desiccator. Measurement of the N(ls)/S(Bp) intensity ratio by ESCA on the glass fiber samples, compared to similar measurements on sodium diethyldithiocarbamate, indicated that only one nitrogen of the organic functional group had reacted. Reaction of trace metals in water with treated glass fiber disks was accomplished using the apparatus shown in Figure 1. This allows a solution containing metals to flow through a glass fiber disk to react with the chelating groups on the surface. Such an arrangement is preferable to stirring in solution because less time is required to react the metals with the glass fiber, and because the glass fiber disks tend to disintegrate on stirring unless they are carefully supported. The apparatus of Figure 1 was constructed from 2 pieces of Teflon (Du Pont) that fit together tightly and are sealed by an “0” ring. The small(11) Dow-Corning Bulletin 03-037, August, 1968, Chem. Prod. Div., Dow-Corning Corp., Midland, Mich., 48640.
1974
1
I
\
I
155 0
150 0
145
0
,
140 0
A
135 0
Figure 2. ESCA spectrum of lead obtained from a 20-ppm s o b tion using glass fiber disks Solution used: 100 ml. treated glass fiber support as described in text. Number of scans: 2400. The peak at the higher binding energy corresponds to the Si 2s level from the Fibergias, while the doublet at the lower binding energy corresponds to the Pb 4f level. The expected theoretical intensity ratio for the 4f712 to 41512 peaks is not found experimentally because of interfering peaks which arise from the Si 2s level which was excited by secondary X-ray (Mg K a 3 . 4 )
--c
SiCH2CHzCHzNHCHzCH2 NHC ‘S-Na’
The R-CH2- NHCHs ‘S-Na’
-
BINDING ENERGY ( e V )
glass
1
500 r
er piece (clear area) fits into the larger piece (hatched area). The glass fiber disk sits in a depression between the two pieces. The path of flow for the solution is indicated by the dotted lines. Sample volumes of 100 ml were used for trace metal solutions and the time to flow through the apparatus was 20-25 minutes, After reaction with the solution, the glass fiber disks were washed with deionized water, dried, and stored over calcium sulfate. Solutions of lead nitrate, calcium chloride, mercuric nitrate, and thallium(1) nitrate, all lO-sM, were prepared using deionized water and stored in polyethylene bottles to prevent loss. The ESCA spectra were obtained using an AEI ESlOO spectrometer equipped with a Varian average transient computer (CAT). Both direct analog and CAT readouts were used. Detection limits of ca. 10 ppb were observed for lead, calcium, thallium, and mercury. An ESCA spectrum obtained from a glass fiber disk treated with 100 ml of a 20 ppb lead solution is shown in Figure 2. The ultimate detection limit for trace metals will be determined by the stability of the dithiocarbamate complex, amount of available sample, and instrumental parameters. For the copper dithiocarbamate complex, K , = 1015 (E), and the metals studied here have Kf’s of similar magnitude or larger. This means there will be strong binding between the glass surface and the metals even at sub parts-per-billion levels. The amount of metal required for monolayer coverage is ca. 20-50 nanograms based on an ESCA probe area of 0.25 cm2. Considering that 100 ml of a 20-ppb lead solution contains 2 micrograms of lead, one is not approaching the theoretical limit of detection. The spectrum of Figure 2 was obtained using the Varian CAT; however, an analog signal from one scan of a 20-ppb lead solution could be detected. Considering the results showed in Figure 2, the factors discussed above and the fact that no optimization of the chemical treatment of glass fiber disks was attempted, one may extrapolate that the ESCA glass fiber disk technique should be useful for heavy metals in the parts-per-trillion range. (12) L. G. Sillen and A. E. Marteil, “Stability Constants of Metal-ion Complexes,” Chem. SOC. Spec. Pub/., 17, The Chemical Society, London, 1964.
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBER 1973
Quantitative results using our technique can be achieved by adding another metal as an internal standard and removing both metals from solution with the treated fiber glass disk. For example, cadmium can be added as an internal standard for lead when using the dithiocarbamate glass. Another convenient internal standard is the use of silicon lines from the glass. In a variety of studies involving glass fiber we have found these to be useful intensity references. An advantage of the ESCA-glass fiber disk method is that conventional chelate chemistry can be used t o control selectivity; chelates such as dithiocarbamate can be used to scavenge a single metal or several metals of a group. Application of various chelating func-
tional groups to glass fiber surfaces can be accomplished by conventional synthetic organic techniques once the initial silylizing function has been attached. ESCA sensitivities are comparable for most elements ( 5 ) . Therefore, combining the scope of conventional chelate chemistry, the ease of synthesis of chelating functional groups and the wide applicability of ESCA as a measuring device, the ESCA-glass fiber disk technique appears to be one of very general utility. Received for review March 30, 1973. Accepted June 6, 1973. This work was supported by the National Science Foundation under Grant GP-32484.
I CORRESPONDENCE Conductance of Quaternary Ammonium Salt Dispersions in Polymeric Films Sir: The recently developed coated wire ion selective electrodes (1-3) for inorganic and organic anions and cations, consisting of a film of a polymeric composition incorporating the exchange material, which is directly coated on a metallic conductor, without the conventional internal reference solution, pose some very puzzling questions concerning their underlying functioning mechanism. Why are super-Nernstian slopes sometimes observed ( I ) ? Why are the selectivities sometimes greater than those of the corresponding barrel type liquid membrane electrodes (1-3)? How is a well-defined, reversible potential established in such a system? In elucidating the mode of behavior of the new electrodes, one important question that arises is the nature and extent of electrical conductivity of the polymeric films. This study is addressed t o examining this question. EXPERIMENTAL Materials. A number of quaternary ammonium salts were prepared from Aliquat 3365 (methyltricaprylylammonium chloride) (General Mills) and a series of tetraalkylammonium halides, from butyl to heptyl, (Eastman Organic) by repeated shaking of the material as received with 1.OM aqueous solutions of the appropriate salt. Specimens of unplasticized polymers were dissolved in minimal amounts of appropriate solvents; polyethylene in formic acid, polystyrene in CHCls, Nylon 66 (Du Pont) in CsHs, and poly(methylmethacrylate) in methylacetate. Epoxy resin, of the ordinary household type, was mixed with an equal amount of curing agent (diethylene triamine). Quaternary ammonium salt-polymer solutions were mixed in a 1:6 weight ratio and cast either in thin disks or as ca. 2.5-mm beads in which were embedded two fine (No. 30) platinum wire electrodes spaced ca. 0.17 mm apart. Samples thus prepared were dried in uacuo for 12 hr and then placed in a dry argon atmosphere for storage and measurement. (1) R. W. Cattrall and H. Freiser,AnaL Chem., 43, 1905 (1971). (2) H. J. James, G. D. Carmack, and H. Freiser, Anal. Chem., 44, 856 (1973). (3) 6.M. Kneebone and H . Freiser, Anal. Chem., 45,499 (1973). '
Electrical Conductivity Measurements. Direct current conductivity was determined by applying a potential across the samples with a 0-600 V power supply and measuring the current using a Keithley Model 160 multimeter in the nA mode. The input resistance of the measuring circuit was ca. 1012 by standardization against known resistors. Absolute conductivities of the materials tested were determined according to ASTM Procedure 257-61 ( 4 ) using the disks with steel and mercury electrodes. Samples with the embedded wire electrodes, enclosed in argonfilled capillaries, were used in the examination of the temperature dependence of conductivity. These were thermostated in an insulated water bath with both current and temperature measurements made at regular intervals. Approximately 5 minutes was necessary for thermal equilibrium t o be reached at each new temperature. Upon application of voltage across the sample, the current was observed to decrease with time. Instead of using an extrapolation to zero time, the value after 10 seconds was used. This procedure gave results that were reproducible to better than 3%. If the imposed voltage was removed from the sample for a few minutes, the identical current-time relationship was observed.
DISCUSSION OF RESULTS The most surprising feature of the conductivity behavior of the polymer dispersed salts is the unusually high temperature coefficient. From the plots shown in Figure 1, some of these materials compare favorably with commercially available semiconductor thermistors. Indeed the temperature dependence of the conductivity follows the operational semiconductor expression cr = cro exp(--E,/")
(1)
As may be seen from Table I, listing the values of E, calculated for a series of Aliquat salts in various polymeric matrices using Equation 1, the activation energies vary from 0.6 to almost 3 electron volts (or 15-65 Kcal/mol). This is in sharp contrast to the behavior of cellulose acetate films impregnated with alkali metal salts which was (4) American Society for Testing and Materials, Philadelphia, Pa., Method D 257-61.
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