THIN FILM SORBENTS FOR TXRF ANALYSIS

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1 Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42 9 Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42 9 NTRODUCTON THN FLM SORBENTS FOR TXRF ANALYSS A.P.Morovov, L.D.Danilin, V.V.Zhmailo, V.N.Funin, Yu.V.gnat ev, N.V. Pilipenko, M.G.Vasin, V.V.Nazarov, V.V.Chulkov Russian Federal Nuclear Center, Sarov (Arzamas-6), Russia. Total reflection x-ray fluorescence (TXRF) analysis is a method successfully used at present in analytical practice /l-3/. n this method a specimen in the form of an optically thin layer is applied to a smooth substrate and illuminated by an x-ray beam at a grazing angle less than critical for the substrate material. Under these conditions the primary x-ray beam is specularly reflected from the substrate surface, thus resulting in a lower background level of scattered radiation and significantly higher sensitivity of element analysis. Further increase in the sensitivity of analysis is possible by extraction and concentration of the elements of interest. However, this procedure is, as a rule, very cumbersome and complicated /4/. The research on synthesis and study of different chelate structures in the form of super-thin films for selective sorption of elements from solutions and applicability of such films in the TXRF method is carried out at Russian Federal Nuclear Center. n this case, investigation of a specimen of a complex compound may be reduced to analysis of elements sorbed by the film. The proposed method of concentration should simplify, in some cases, the procedure of specimen preparation, and it is convenient both for TXRF and ordinary XRF analysis. Preliminary results of film sorbent studies were presented at the previous Denver 97 conference /5/. Some results obtained in the course of further studies are described in this paper.. EQUPMENT The studies were carried out using a laboratory mock-up of the TXRF spectrometer with monochromatic specimen excitation /5/. ndustrial X-ray device DRON- with the tube BSV-9 served as a radiation source. Voltage at the tube is U = 50 kv, current = 20 ma. Tubes with the following anode materials were used: MO, W, Ag. Characteristic radiation of the specimen was registered with a Si(Li)- detector (manufactured by CANBERRA ). Erergy resolution of the detector is 55eV for quanta of the energy 5.9keV. The area of sensitive surface of Si crystal is 2 mm2. Specimen excitation by monochromatic radiation was used in the scheme. A monochromator consisting of two mirrors based on the multi-layer W-C structure, was used to cut a monochromatic line out of x-ray tube spectrum. 2. THE STUDES OF THN FLM SORBENTS Since traditional ion-exchange materials represent, as a rule, non-soluble polymer structures, the methods allowing to obtain poly-chelates in the form of solutions for subsequent manufacture of thin films selective to some metals (Hg, Au, metals of Pt group and some transitive elements) were developed. The experimental study showed that sorption of metals from solutions really takes place /5/. These results qualitatively agree with general considerations on element sorption by normal ion exchange materials. However, it was found

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the nternational Centre for Diffraction Data (CDD). This document is provided by CDD 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 CDD. Usage is restricted for the purposes of education and scientific research. DXC Website CDD Website -

3 Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol that ion sorption by thin films has its specific features. n particular, it turned out that elements sorbed by the film are concentrated in a surface layer and practically do not penetrate into the depth of the film. n this case it is natural to expect that sorption properties of film ionite, especially its sorption capacity, would worsen. This is the reason why the investigations towards the production of new types of sorption materials were undertaken. An idea to use filled matrices, both pure and preliminary modified by some low-molecular chelate, turned rather fruitful. Fine ground ion exchange resins - strong-base anionites, strong-acidic or weak-acidic, for example, phosphoric acid cationites were used as fillers. Condensation structures on the basis of phenol, formaldehyde, carbamide and oxiquinoline were also used as fillers. The advantages of such systems are evident, since they allow to increase and control exchange capacity by changing the amount of a filler. Moreover, more homogeneous volumetric distribution of sorbed ions should take place in such structures. Sorption of Hg, Cu, Co, Zn and a number of other elements in a wide range of concentrations and film exposure times in solutions was studied using filled structures. Fluorescence intensity of an element sorbed by a film as a function of angle was measured. The results of the measurements are shown below in Fig. l-5. Fig. shows the fluorescence of the film sorbents as a function of angle ca s.tl 800 ii c.- 8 ii 400 $ l?? ~ s 3 Grazing angle, mrad Fig.. The fluorescence of a microelement in the film sorbent as a function of grazing angle of the x-ray beam.. d - 2 pm heterogeneous film with Co as a microelement in the film sorbent, 2. d - pm homogeneous film with Hg as a microelement in the film sorbent. Fluorescence as a function of angle for the heterogeneous film (curve ) and for the homogeneous film sorbent synthesized earlier (curve 2) (Fig. ) are sufficiently different. n the angle range 9 < Qcrit the heterogeneous sorbent shows a long plateau, that testifies more homogeneous distribution of the sorbed element over the film volume. The function for drop specimens, in which admixture elements are a priori distributed uniformly over the mass of a sediment, has a similar shape.

4 Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42 2 Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42 2 The characteristics of sorption by heterogeneous film structures were studied using radioactive Co as a tag, that allowed to make measurements much faster. The structure vinylperchloride-polyethylenepolyamine with fine-disperse phosphor-acide cationite (PAC) in the form of the films of the mass from 3.0m3g to 6. Om3g and the surface area 2 cm2 was used. The kinetics of Co sorption from weak-acidic (ph from 5 to 6) solutions was investigated. The influence of time, medium ph, Co concentration in the solution, and temperature was studied. The obtained data show the following. Sorption of Co grows with ph, that is characteristic for weak-acidic cationites (Fig.2). For ph-5 near the maximum of sorption capacity of the film the equilibrium content of sorbed Co reaches - 30 %, the enrichment coefficient for Co being ,030 $ 0,025 & j_ 4 0,020 B 5 ([ 0,05 b 8 2 0,00 + 'is 5 0, ,000 O PH Fig. 2. Co sorption as a function of the solution ph (Microelement concentration in the solution Co= Om7 g/g, the solution volume V =30ml, the sorbent with PAC, the film thickness d=2pm, the exposure time t=l hour) n the temperature range between 20 and 70 C sorption of Co is increased approximately.6 times. The function is close to linear (Fig.3). The amount of the sorbed element as a function of its concentration in the solution for the fixed exposure time (t = hour) was studied. The measurements were carried out in the concentration range between CO = 0e9 g/g and CO = 0s3 g/g. The results are shown in Fig. 4. t is seen that at the beginning, for concentrations CO < 0m5 g/g sorption is close to linear. Then the measurements were extended in order to determine sorption as a function of exposure time for each value of concentration. Such measurements were carried out in the Co concentration range CO between 0m9g/g and 0m5g/g for five points with the step of one order of magnitude over concentration. The exposure time changed between 5 and 360 minutes. The measured values normalized at the unit concentration and unit sorbent area show that for exposure times t < 60 min sorption is close to linear (Fig.5). The normalized values of the mass of microelement extracted by the film differ about 2 times, while the solution concentrations differ lo4 times. Taking into account possible errors related to solution

5 Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol preparation, the technology of chelate synthesis, measurement results may be considered quite satisfactory. inaccuracies etc. the obtained $ temperature, C Fig. 3. Co sorption as a function of the solution temperature (the solution ph=4, the sorbent with PAC, the film thickness d = 2pm, the exposure time t=lhour) 0,Ol :ri~~kli+l ~~ :-:CLlln --ccl-c ::: ----CC--lei --- / --- LJ+ ---i--t+,++, l-rrllri F r T-Fi ~-- 7 T,, n - - -,-:-, -- T T rlt,t - ///l,/llll /l _:A / T 3 El : q = 3, rk l-i n z z zt z =, 3 3, E == * * ki*,* z z i - u - - :~22i~b- : x-- c c C-C~ - ;l_l_iliji_ = -- ElLl LJL mml_l_lll -:i_ljjl -_;_: - 0,00 L; -4-t = c!w o,ooo,, /!,,,,/,,~ lllll~,/l,/ ///,,, / llll//l, llllll 0,000 0,00 0,Ol ( concentration in solution, Om6 g/g. Fig. 4. The amount of the extracted microelement (Co) as a function of its concentration in the solution. (the sorbent with PAC, the film thickness d=4p.m, the exposure time t=lhour, the solution ph=4, the volume V =30 ml.)

6 Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol E A? E 2i t ii o^ UN tie a, E i=f my 3 x-x-- TT./ -,- + + &, ,- + -,-,- 7--riri T - - r ~, T ~/-,- / /ll,,,,,,,., -,- + + k,m i ~/~ + ~!-,-,- _ /!- _! _ / t A - C=lOOO rig/g / 0 C=lOO rig/g -~ -,- //, lllll 0 C=lO nglg / 3-,/ / l/l- -,/!,,,, X C=lng/g ---t---l --/- l~,-t+t,~ ---- T--+~~,~+~,~,~,- o,o, l/l,,,j --- ~~ ; / ll/ /, / / j l!l~,,~ time, min Fig.5. The kinetics of sorption in the solutions of different concentrations. (the sorbent with PAC, the film thickness d=4p.m, the solution ph=4, the volume V =30 ml) Quantitatively the rate of the microelement mass storage in the film may be described as: dm - = k CO, where CO- is the microelement concentration in the solution, k - the dt proportionality coefficient. The values of k, found from the measurements are given in the table. Microelement concentration in solution, K, g/min wk average Note for illustration that under these conditions a film extracts x 0. 0m9g/cm2 of a microelement from a solution with the concentration Oe9 g/g within 5 minutes, that is quite a measurable amount for the TXRF method. For exposure times t 2 hour the process of a substance storage in a film becomes significantly non-linear. And this is not explained by the sorbent saturation, since the absolute amount of the substance concentrated in a film within these times is relatively low and is significantly less than the maximum mass limited by sorption capacity of the film. This may point to the change of the processes governing sorption kinetics. The studies with solutions of complex chemical compound were carried out in order to estimate the sorption selectivity. The effect of selectivity of a microelement extraction from a solution is qualitatively illustrated in Fig. 6 (a, b).

7 Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol t 7 50 ki c 0 G : 00 3 t /, i Fe Cb CLl ~ - Hs ~ -.--L 3r / / / / Sr~ ~ i.i; :J 0 /, /, /, j 8-i-t~7-rerri / i i --r-l-l-7--w O Energy, kev Fig.Ga. The fluorescence spectrum of a drop of solution (V=O.OS ml). (the chemical analysis of the solution: V, Cr, Mn=20 pg/ml, Fe, Ni, Cu, Hg=2 pg/ml, Ga=5 p.g/ml, 50% of sea water, ph=l, the measurement time 30 minutes.) E j.!j 00 0 $ 80 a, ! ~-,---+--, / ; Hs~(L~) ~ ~ ~ ~, ;,-,..--J--.-A ,--, Ml+ i ~ i / ~ t&---i- ~, t-+--f (Hg(@) ~ ~ ~ ~ 20 i t Energy, kev Fig. 6b. The fluorescence spectrum from the film sorbent kept in the solution (the solution: V, Cr, Mn=20 pg/ml, Fe, Ni, Cu, Hg=2 yglml, Ga=5 p.g/ml, 50% sea water, ph=l, the sorbent exposure t = 60 min, the measurement time 60 minutes).

8 Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-nternational Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol An initial solution was prepared on the basis of the sea water with the addition of the known amount of some elements: V, Cr, Mn, Fe Ni and other. A drop of thus prepared solution of the volume V = 50 ~ was applied at a substrate, dried and studied in the TXRF geometry. The fluorescence spectrum of this specimen is shown in Fig. 6a. The heterogeneous film sorbent vinylperchloride-polyethylenepolyamine with fine-disperse chelate based on carbamideformaldehyde resin and oxine designed for extraction of Hg was kept in contact with such solution for 60 minutes. After that it was analyzed under the same conditions as a drop specimen of the initial solution. The fluorescence spectrum of the film is shown in Fig, 6b. t is seen from the comparison of Fig. 6a and 6b that the process of sorption has a selective nature, and the sensitivity of determination of Hg is much higher with the use of a heterogeneous film sorbent. The coefficient of the film enrichment with Hg is in this case = 00. CONCLUSON Some characteristics of thin film sorbents on the basis of heterogenic structures were studied. A sorption material in such structures is introduced in an ultra-disperse form as a filler into an organic mass, from which thin (-p.m) films are formed. The advantages of such systems are evident, since they allow to significantly increase and control exchange capacity by changing the amount of a filler. The preliminary results show that selective sorption from solutions of a complex chemical analysis really takes place. For short exposure times (t < 60 min) sorption is close to linear in a wide range of concentrations CO between 0m9 g/g and 0m5 g/g. The mass of a sorbed microelement is approximately proportional to the microelement concentration and the time of the sorbent contact with the solution. t is planned to go on with the investigation of quantitative characteristics of sorption of microelements from solutions by thin film sorbents. ACKNOWLEDGEMENTS The authors would like to thank Prof. P.Wobrauschek for discussions and useful remarks. This work was carried out under the STC contract # REFERENCES [l] Y.Yoneda, T.Horiuchi Rev. Sci. ns@., 97, vol. 42(7), p [2] H.Aiginger, P.Wobrauschek Nucl. nstr. and Meth., 974, vol. 4, p. 57. [3] JKnoth, H.Schwenke, Z.Fresenius Anal. Chem., 978, vol. 29, p [4] A.Prange Spectrochimica Acta 44B, 989, p.437 [5] A.P.Morovov, L.D.Danilin, V.V.Zhmailo, Yu.V.gnat ev, V.V.Nazarov, M.G.Vasin, V.V.Chulkov, V.N.Funin. Adv. X-Ray Anal. 998, 4