I NOTES
1
X-Ray Photoelectron Spectroscopy for Trace Metals Determination by Ion-Exchange Absorption from Solution Michael Ctuha, Jr., and William M. Riggs Instrument Products Division, E. 1. du Pont de Nemours & Company, Incorporated, Monrovia, Calif. 9 10 16
X-Ray photoelectron spectroscopy (ESCA) is one of several recently developed techniques for non-destructive surface elemental analysis. The method has been applied successfully in studies of catalysts, metals, alloys, plastics, textiles, and many other materials. ESCA has largely provided qualitative as well as semi-quantitative information over the past few years. Current efforts (1-3), have centered on improving quantitative aspects. In this paper, we report investigations of the quantitative potential for ESCA in conjunction with ion-exchange as a means of determining trace metals removed from solution in a thin surface film. Because of the shallow effective sampling depth with the use of convenESCA, in this case approximately 50 %., tional ion-exchange resins is unsatisfactory because most of the ions diffuse into the bulk beyond the escape depth for photoelectrons. A thin film approximating a two-dimensional ion-exchange surface is therefore desirable. Such surfaces can be obtained in several ways, including silylizing glass surfaces using reagents which have free organic functional groups after bonding to the glass. Such glass surfaces have been previously utilized in ESCA studies ( 3 ) .Alternatively, polymer surfaces can be treated with monomer vapors following activation in a plasma discharge to produce grafts of the monomer to the surface ( 4 ) .Investigation of the use of acrylic acid grafted polypropylene surfaces as two-dimensional ion-exchangers for scavenging trace metals from solution is described in this report.
ESCA Probe
m ' I
I
~~
11
RESULTS AND DISCUSSION Figure 2 shows the results of a %minute immersion in 50 ml of solution containing 10 ppm silver ion. The broad spectrum shows Ag 3d, 0 Is, and C Is as the predominant bands, with lesser peaks for AgSp, Ag4d, and Auger elec1838
~-
__
~
~
~
_ _
~~~-
~
~~-
/I
~~
~
A \ \
100 m I Beaker
Figure 1. Apparatus for ion-exchange on acrylic acid grafted poly-
propylene
i
A93d
I,
400
300
C Auger
EXPERIMENTAL Acrylic Acid Grafted Polypropylene. One-mil films of plasma grafted polypropylene were furnished by Surface Activation Corporation, Westbury, N.Y. The preparation and characterization of these films by various analytical methods including ESCA have been reported (4, 5 ) . Circular samples were cut from the film using a l/4-inch paper punch. No pretreatment of the film was necessary prior to ion-exchange. Ion-Exchange Apparatus and Procedure. The samples were attached to the end of a rod shaped probe using double sided adhesive tape. The probe was clamped over a beaker of the test solution with the film barely touching the surface of the liquid as shown in Figure 1. The solution was stirred continuously with a Teflon-covered magnetic stirring bar. After immersion for a predetermined time, the film was rinsed three times with deionized water, blotting each rinse with Kimwipe. ESCA. A Du Pont 650B Electron Spectrometer was used for recording the photoelectron spectra. A Mg anode was used at approximately 400 watts X-ray power. Broad (1000 eV) survey scans were run a t 10 eV/sec, 16 scans per sample, using a signal averaging multi-channel analyzer accessory. Single peak scans were obtained in the analog mode a t 0.05 eV/sec.
Ion Exchange Membrane
1
0 Auger
IWO
900
800
7OC
600
500
200
100
0
Binding Energy, ev
Flgure 2. Photoelectron spectra of silver ion absorbed in acrylic acid
grafted polypropylene trons. The 0 1s and C 1s signals are actually multiple peaks; an oxygen doublet and carboxyl and hydrocarbon C Is peaks correlating with the acrylic acid structure, Superimposed above the survey scan is an analog scan showing the Ag 3d doublet in greater detail. The most intense peak, Ag 3d5/2 a t approximately 370 eV was used in subsequent analyses. Figure 3 shows the results of tests comparing the rates of ion exchange absorption for lead and silver in separate experiments a t several concentrations. The shortest immersion time was 15 seconds. The variations reflect differences in the rates of reaction of the ions with the acrylic acid layer.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
7
50 -
p
100ppm A g t
lOppm P b t t
3
IO
5
Concentration, ppm
Figure 4. Variation in photoelectron intensity vs. concentrations of lead and silver ions c
0
cuep
~~
10
20
30
40
T i m e , min
Figure 3. Rates of exchange for lead and silver ions at various con. centrations
The fairly linear relationships observed between ion concentration (ppm) and photoelectron intensity (counts per sec) of the major peaks for lead and silver are shown in Figure 4. These measurements were performed on samples following 5-minute immersion times. Here again, separate experiments were conducted for each element. The data have not been adjusted for differences in relative atomic intensities of lead and silver which would bring the curves closer together. I t is of interest to determine the lowest amount detectable a t an extended immersion time. For this test, 50 ~1 (large drop) of solution containing 1 ppm Ag+ was placed on the film for 15 minutes. The drop was then transferred to a fresh membrane for an equal time. No residual silver was found in the latter. The first film containing a total of 50 nanograms of Agt gave a peak intensity which was 5 times the noise level with 5-minute data acquisition time. (For purposes of this discussion, noise will be considered the full amplitude of the noise envelope on the low Eb side of the peak. “RMS” noise would be approximately a factor of 2.5 lower.) Ten nanograms of silver would therefore give a peak height above background equal to the noise level, a t the same data acquisition time. Longer term signal averaging will, of course, enhance the detection limit. Ten nanograms of’ silver is approximately 5.6 X 1013 atoms. Knowing that the radius of a silver ion is about 1.2 8, (6), one calculates that a “monolayer” of silver ions corresponds to about 2.2 X l0l5 atoms/cm2. The sample area in this experiment is -0.28 cm2. Thus, the signal observed corresponds to 0.1 monolayer of silver ions. Since a pure silver sample in the instrument used under these scan conditions would produce a signal-to-noise ratio in excess of 500, 0.1 monolayer should produce signal strength about ten times t h a t observed. Thus, most of the silver absorbed in this experiment is fairly well shielded from observation in the ESCA experiment and optimization of the ion exchange surface should improve the detection limit by another order of magnitude. Calculations based on titration of the amount of acrylic acid grafted to the polypropylene indicate a thickness approximating 1000 A, about twenty times the escape depth for photoelectrons. Therefore it is
111 GZP
lob0
960
SO0
7010
600
500
400
300
200
100
0
Binding Energy. e V
Figure 5. Photoelectron spectra of a mixture of various ions absorbed in acrylic acid grafted polypropylene
not surprising that most of the silver ions have diffused to a depth in this layer beyond the ESCA observation depth. This suggests that much thinner acrylic acid grafts would give better results. Finally, a mixture containing 1 pprn each of Cu2+, Fe3+, Ba2+, Cd2+, Ca2+, Pb2+, and Hg2+ was tested utilizing a 5 minute immersion time. Silver was excluded from this test since most of the metal salts used were chlorides. Figure 5 shows a survey scan with analog scans for each element superimposed. Measurable peaks were found for all of the ions except Ba2+.A nitrogen peak ( N 1s) a t 400 eV was attributed to ammonia in the air coming from an Ozalid machine close to the laboratory. Lead showed the most intense peaks followed by mercury and calcium. The failure to observe Ba2+ may be due to competitive reactions among the ions present, coupled with the tendency of the nearby oxygen Auger lines to obscure the weak barium peaks. If detection of trace amounts of barium is important, use of an aluminum X-ray source will eliminate this interference. In view of the possible applications in water treatment and pollution it would appear that ESCA in conjunction with ion-exchange offers new possibilities for the determination of dissolved metals a t trace levels. One area in particular which may benefit is the study of corrosion and water-formed deposits as related to the anodic behavior of metals and alloys.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
1837
Additional studies are needed on the kinetics and competitive processes of ion-exchange for various ions on a given membrane. These studies would best be performed with specific applications in mind so that the solution pH, possible interferences, etc. can be realistically matched to the conditions of the system to be monitored.
ACKNOWLEDGMENT The authors are grateful to Arthur Bradley, Surface Activation Corporation, Westbury, N.Y., for the acrylic acid graft samples and for helpful suggestions.
LITERATURE CITED (1)R . S. Swingle. Anal. Chem., 47, 21 (1975). (2)J. S.Brinen and J. E. McClure, J. Electron Spectrosc. Relat. Phenom.. 4, 243 (1974). (3)G.D. Nichols, D. M. Hercules, R. C. Peck, and D. J. Vaughn, Appl. Spectrosc., 28,219 (1974). (4)A. Bradley and J. D. Fales, Chem. Techno/., April 1971,p 232. (5)A. Bradley and M. Czuha, Anal. Chem., 47, 1838 (1975). (6) F. A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry." 3rd ed., Interscience, New York, 1972,p 52.
RECEIVEDfor review February 18, 1975. Accepted May 8, 1975.
Analytical Methods for Surface Grafts Arthur Bradley Surface Activation Corp., 1150 Shames Drive, Westbury, N. Y. 11590
Michael Czuha, Jr. DuPont lnstrument Products Division, Monrovia, Calif. 9 10 16
Surface modification of plastics, textile fibers, and other materials to improve serviceability may involve exposure to ionizing radiation, ultraviolet light, or corona discharge. Under suitable conditions, such excitation can result in free radical sites to initiate graft copolymerization. A radical change in surface properties of these materials is observed without modification of bulk behavior. Thus, various organic compounds have been grafted to wool fiber ( I ) , cellulose (2), and polyester ( 3 ) to enhance shrink-resistance, moisture retention, or launderability. Plasma-initiated polyacrylic acid grafts provide polymer films and sheets with a new dimension of surface polarity ( 4 , 5 ) . Potential applications include improved wettability for battery separators and better adhesion properties and printability for packaging films. The effects are more pronounced and more permanent than those obtained by exposure to corona (air) discharge, although the technology and apparatus bear some formal similarity and the cost of treatment is comparably small. This paper describes various techniques used in characterizing the surfaces of grafts on polypropylene, polyethylene, and polyester films.
EXPERIMENTAL AcryIic Acid Grafted Films. Samples used in this program were prepared by the gas discharge initiated surface grafting method previously described ( 5 ) . Free radical sites were created on 1-mil polypropylene film (Dow Chemical Co, Midland, Mich.) by exposure to argon plasma in the range 0.5 to 4 Torr and quenched with acrylic acid vapor close to its equilibrium vapor pressure, about 4 Torr a t room temperature. Barium Ash Determination. To 500 ml of an aqueous solution of barium chloride, saturated a t room temperature, was added 10 g of barium hydroxide. Grafted film specimens were immersed in this reagent for a t least 30 min a t 60-70°, then removed and given three successive 1-min rinses in fresh deionized water to remove excess barium salts. Dried specimens were ashed a t the Schwarzkopf Microanalytical Laboratory, Woodside, New York City. Conversions from % ash to mequiv acid/g were made on the assumption that the ash was BaO and that 1.53% ash corresponded to 0.100 mequiv/g of barium and 0.200 mequiv/g of acidic hydrogen. Titration. Weighed samples of film between 0.1 and 0.2 g were cut into slivers and placed in 125-m1 Erlenmeyer flasks. A 50-ml portion of 0.05N aqueous sodium chloride (technical grade) was added to each, followed by 1.00 ml of 0.1N NaOH. The flasks were stoppered and allowed to stand for a t least 4 hr, with frequent 1838
I
A
ISCA
IO'
