Evaluation of Some Sample Support Films for Microsample X-Ray Analysis

Transcription

1 301 Evaluation of Some Sample Support Films for Microsample X-Ray Analysis A.W. Wilson, D.C. Turner, A.A. Robbins, Process Analytics, Orem, UT Abstract Reduction of matrix and support film background counts improves standard XRF detection limits to sub-ppb levels. Therefore, an ideal support film would contribute nothing to background, have no contamination, be chemically inert, strong, and temperature resistant. Support films must also repeatably position residues to facilitate automated analysis. This positioning has been accomplished using mechanical dimpling as well as by surface treating to make a hydrophilic spot in a hydrophobic field. A new film, AP2m, has demonstrated excellent temperature tolerance and chemical resistance. As an ultrathin film, it also provides excellent detection limits. Physical and chemical properties of APITM and AP2 films are compared with standard Mylar@, Prolene@, and Kapton@ films. Repeatability results demonstrate the residue locating abilities of dimpled and treated films. Introduction XRF detection limits can be improved by reducing background in three ways. First, the matrix can be removed by analyzing dried residues instead of liquids. Second, incident x-rays can be collimated to the residue size. Finally, an ultrathin sample support can be used to reduce x-ray scatter. Microsample X-Ray Analysis (MXA) combines all three of these techniques to reduce background and achieve ppb detection limits. Films form. The main contributor to background in the MXA technique is the support film itself. To improve the effectiveness of MXA, an ideal support film is sought. MXA has traditionally used films such as Prolene, Kapton, Mylar, and-more recently- API. These commercially available films are evaluated for important features such as background contribution, chemical p~rity,~ chemical resistance, strength, and temperature resistance. They are also evaluated for the ability to repeatably position residues and attainable minimum detection limits. Commercially available films are not acceptable for high-temperature sample preparation. AP2 is a new film being developed for analysis of high boiling point acids and organics. It is uniquely resistant to acids and bases at high temperatures. Its high surface energy repels all liquids more effectively than any other film studied. Thus, liquids accumulate and dry into a small spot on AP2 film. Chemical and Physical Properties. Desirable film characteristics are resistance to chemicals, high surface energy so that fluids accumulate into a small spot, strength to hold the sample intact for handling and analysis, and temperature-resistance (especially important for high boiling point acids). Table I lists physical properties for each film. 4 Nominal thickness is listed in pm and measured thickness is listed as pg/cm. I Fnrm1117l 1 (C,,H.,N,O,)- 75 I Rating scale: E=excellent, G=good, F=F air Mylar@ and Kapton are registered trademarks of DuPont, APITM and AP2TM are trademarks of MOXTEK, Inc., and Prolene@ is a registered trademark of Chemplex, Inc.

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Background Considerations Background Contribution. The main contributor to background in the MXA technique is scatter from the support film itself. Figure 1 plots the average integrated background from 5.2 to 18 kev of five films versus the mass thickness of each film. All films were analyzed under the conditions specified in Table II. As expected, background counts are directly correlated with the mass of the film. 800 a P 2 sm 3 & x 400 g $200 u ep 0 y=o.9727x Mass thickness (micrograms/cm ) Figure 1. Integrated background versus thickness. Table II. Analvsis Conditions Instrument Spectrace QuanX Tube Voltage 5bkV Tube Current 1.OO ma Tube Anode Rhodium Livetime Atmosnhere 500 set vacuum Chemical Purity. Chemical impurities in a support film may be confused with impurities in a sample being analyzed and should be minimal. Figure 2 displays background spectra of commercially available films showing their chemical impurities. To quantify the amount of impurities in each film, the contamination peaks for each element were integrated and compared to a calibration curve made fi-om peaks of known masses of each element. Table III lists the range of contamination in five samples of each film evaluated. r le+05 - le+04 % g Y h.z 2 2 ' le+03 le+02 le+ol Enew WV / Figure 2. Background spectra of various films.

4 303 Table IIL Range of Contamination Aluminum Silicon Phosphorus Calcium O-30 O Chromium Iron Copper o o-15 Zinc I I I I Contamination is Residue Locating Techniques Either costly instrument modifications or time-consuming, labor-intensive steps must be performed to analyze residues if they are not located in precise, repeatable positions. An experiment was performed to determine how residues are positioned when liquids dry on flat films. 50,MP aliquots of a 100 ppb multielement standard solution were deposited in nine sites on flat API, AP2, and Prolenefilms. These films were placed on a level surface and allowed to dry at 40 C. All 36 residues dried in random locations and none of the residues dried in position for automated analysis (see Fig. 3a). % illustration of a residue on flat Prolene. Its location is not in the center ofthe film, and automated MXA analysis would not be feasible (-7.5X) Illustration of a residue on flat Prolene. Ihe residue is in a small spot (-0.4 mm diameter). lfit were correctly located, MXA analysis would be possible (-50X) Figure 3a. Illustration of residue drying on a flat film.

5 304 Residue Illustration of a residue on dimpled Prolene. The residue is located within the analysis site (-7.5X) Illustration of a residue on dimpled Prolene. The residue is in a small spot (-0.4 mm diameter). If it were correctly located, MXA analysis would be possible (-50X) Illustration of a residue on treated API. The Illustration of a residue on treated AP 1 residue is located in the center of the frame (-50X). The residue (dark ring) is located and is good for MXA (-7.5X) within the treated area (light circle). Figure 3b. Illustrations of residues on dimpled and treated films. Residue positioning has been successfully demonstrated using two methods. ahquots of a 100 ppb multielement standard solution were deposited in nine sites on dimpled Prolene and treated APl films. These films were placed on a level surface and allowed to dry at 40 C. The residues dried into a central analysis location in each case (see Fig. 3b). Residue positioning techniques include mechanical dimpling-a patented 5 technique that places small 6-mm and 2-mm concavities in the surface of the film (see Fig. 4), and surface treating-creating a 1.5- mm hydrophilic spot in the center of a hydrophobic field (patent pending-see Fig. 5).

6 305,.&.2+: zz-et,- :, i, :._:jyg..ig;f.; v-t&.,,,,-,i. ~ i :.,.~;.:..i: liquid (\, ~~~~.:.:;~~;;;l~~~,,/ --- +( _ -_--.~:~~~~~~~~ dimples -7 zy- 7,. residue,/.._ Ai / jg f dimples, 77 Figure 4. Cross section of a dimpled film. treated area ;-: treated area liquid _I_;&;.;;;.. ;;;;i..:l^:,:.::i:ii i.:/..i : ::.;e,~~...:.~:;... i. :,,,, -:,,;y i*:y..:.. ;,..::::.. ~,,.~~;,r:~::~, ~:,: ;/;;:;:,:;&,,:::;: &;;,.;::..,:,:,:,.,.i:i _.-::-Ll::..J:.. -_A-2-..II..::.. LF,,..:.,,li... i,,$,,,:,..;,:;...>&zz i., ^....;.,.,...._ 9._.-~.~::i i ii: residue ;z,.. Figure 5. Cross section of a treated film. f Residue Locating Repeatability. A test was performed to measure repeatability of the two residue locating techniques. A 50 ppb multielement solution was deposited on 36 sites each of dimpled Prolene and treated APl films. These solutions were dried at approximately 42 C. MXA analysis was then performed on each residue according to the analysis conditions in Table II. MXA analysis quantifies concentration of elements in a liquid by taking a spectrum of a residue, deconvoluting the spectrum into peak counts and background counts for certain elements, comparing peak counts to a known calibration curve for each element, and giving a concentration based on this calibration curve. Table IV shows relative standard deviations of the 36 analysis results for dimpled Prolene and treated APl. The percent relative standard deviation on APl treated films was <looh for most elements.* An F test was performed to determine if the differences in standard deviations between centering techniques was significant. At a 95% confidence interval, significant differences in standard deviations did exist between the two residue centering techniques for K, Ti, Fe, and Ni. In all cases where significant differences existed in standard deviations, APl was the preferred film (e.g., the standard deviation in titanium concentration was lower in APl than in Prolene). Therefore, treated API positions more repeatably than dimpled Prolene. Table IV. Statistical Comparison of Residue Locating Techniques Dimpled Prolene Treated APl Element RsR+ Element RSD+ Phosphorus 38% I Phosnhoru Potassium 33% Potassium Titanium 18% Titanium Iron 14% Iron Nickel Copper Arsenic 14% 12% 11% Nickel Copper Arsenic 7% 7% 8% 9% 1 Rubidium I 10% 1 Rubidium +Relative standard deviations of MXA readings of a 50 ppb solution deposited on 36 sites of each film. Conclusions Minimum detection limits (MDL) for a variety of elements were calculated by drying a fixed volume

7 306 of a multielement solution with a known concentration of each element. The mass of each element in the residue was 2.5 ng. Equation 1 shows how minimum detection limits were calculated: j,,fdl =3 massd@ peak where bkg is the number of background counts and peak is the number of peak counts for each element. Table V gives minimum detection limits for 17 elements on the five films evaluated. API film provides the lowest detection limits of all films tested. (1) Rb In evaluating films minimum detection limits, background, contamination, residue locating capability, and chemical compatibility were considered. Given these parameters, Table VI lists recommended films for each class of chemical. Table VI. Recommended Films for Chemicals Chemical Group Recommended Film(s) Acids IS, HCl API, Prolene H,SO,, HJ O, AP2 m03 API Bases NaOH, NH40H APl, Prolene Solvents (MeOH, Acetone) AP2, AP1* * A 1: 1 dilution is required to use solvents on AP1.

8 307 References 1. C. Meltzer and B-S. King, Advances in X-Ray Analysis, vol. 34, A. Nielson, D.C. Turner, A. Wilson, D.C. Wheny, and R. Wong, Advances in X-Ray Analysis, vol. 39, in press. 3. S. Davies, X-Ray Spectrometry, vol. 26, N. Downs et al., A Comparison of APl TM with Other Thin-Film Substrates Used for PlXE Analysis, in press. 5. US Patent #5,544,218. Acknowledgments The authors express appreciation to Spectrace Instruments for use of the QuanX system.