Water purity investigation using proton induced X-ray emission

Nuclear Instruments and Methods 186 (1981) 599-604 North-Holland Publishing Company

599

WATER PURITY INVESTIGATION USING PROTON INDUCED X-RAY EMISSION Cheng-Ming FOU Department of Physics, University of Delaware, Newark, Delaware 197J 1, USA Received 31 January 1980 and in revised form 20 January 1981

An investigation into the possibility of using the highly sensitive proton induced X-ray emission analysis technique (PIXE) to carry out large scale monitoring of the trace elements in household tap water is presented. Quantitative result of lead contamination from samples examined is also presented as an example.

1. Introduction Analysis o f trace elements using the proton induced X-ray emission generally known as PIXE has been proven to be very effective in a wide variety of fields such as aerosol pollutants, geological samples, etc. [1]. This method had been compared to atomic absorption spectroscopy analysis (AAS) by several investigators under different conditions. Shiokawa et al. [2] had investigated soil and aerosol particles using PIXE as well as AAS and had found the agreement to be better than 10% for elements lighter than Fe and better than 50% for elements as heavy as Pb. All these trace elements were in the ppm to ppb range. Bearse et al. [3] examined blood zinc and found that the results from PIXE and AAS are within 10%. This method was also used to examine National Bureau of Standard's standard samples by Campbell et al. [4] and Zeisler et al. [5]. In their work published in the Journal of Analytical Chemistry, both groups reported the PIXE method is reliable and useful for microanalysis. PIXE was recently applied to the analysis of water by Akselsson and Johansson [6]. They added a complex-binding component to the water. By adsorbing the complexes on activated carbon, and collecting the carbon on a filter they prepared a sample suitable for PIXE in the form o f a pellet made of packed carbon powder. Contaminants in the ppb range were detectable. Another approach developed by Deconninck [7] involved passing the protons from the vacuum interior of an accelerator into the room atmosphere to b o m b a r d a drop or a jet stream o f water. This technique is often called in-air PIXE. Because no 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © North-Holland

preconcentration o f the contaminants was carried out the detection limit of this approach was in the ppm range. More recently, Rickey et al. [8] used the method o f vacuum filteration. By pumping at the water to be analyzed through a selected membrane which is permeable to water vapor only, the trace impurities in water are finally deposited on the membrane like a thin crust. It is then bombarded by protons under vacuum for PIXE analysis. To be able to detect trace elements down to ppb range a minimum o f 30 ml o f water is needed. Normally three days of pumping with a specially designed pump that can pump water vapor efficiently is required to prepare such a sample. Occasional rupture of the membrane makes the vacuum filtration a rather involved process. In all three approaches a high resolution nondispersive Si(Li) solid state detector was used to detect the characteristic X-rays from the impurities caused by the b o m b a r d m e n t of protons. Unlike the AAS technique, all trace elements are investigated simultaneously using the PIXE technique, and since this method is non-destructive the analysis can be repeated on the same sample over again. These advantages make the PIXE a very desirable technique for water samples analysis. Large scale water analysis would be possible if less time consuming or less complicated sample preparation can be established. The present investigation was carried out to see whether or not the combination o f the merits of the several approaches mentioned above, namely the preconcentration by adsorption in activated charcoal and the in-air PIXE analysis is a solution to this problem.

600

CM. Fou / Water purity investigation using PIXE

2. Experiment With the large scale analysis of household tap water in mind, a commercially available filter * which is readily adaptable to the household faucet was used. The filter consists of a 5 mm thick paper filter shaped like a cup in which activated charcoal is packed. The filter cup is sealed in a clear plastic cartridge with inlet and outlet. The water runs through the paper filter first and then through the activated charcoal. Finally it passes through a fine mesh and emerges at the faucet. The procedure of having the filter right there at the receiving end offers the advantage of collecting impurities by accumulation; which is somewhat equivalent to sample preparation by preconcentration from a large quantity of water. It was anticipated that this procedure, by virtue of its ability to accumulate, elminates the need of adding a complexbinding compound to the water. Furthermore, since the filter comes in a replaceable cartridge form that is rugged, it can be sent by mail without special handling to a central location for analysis. This is another necessary requirement for a large scale program. After the plastic cartridge is cut open, the paper part and the activated charcoal part are separated for analysis. The paper part, which encounters the water first, trapped all particulates in the water and had turned brown. The activated charcoal was soaked with water. Since none of the two parts are suitable for proton bombardment under vacuum, the in-air PIXE technique is not only convenient but necessary. Analysis began immediately after opening or after the filters have been air dried showed no significant difference. Details of the University of Delaware-Bartol in-air PIXE system have been published elsewhere [9]. Briefly, a beam of protons passes through a 8/xm thin Kapton foil mounted over a pin-hole window at the end of the vacuum flight tube of the accelerator. (Kapton foil was chosen because of its tensile strength despite large doses of radiation.) The diameter of the pinhole for this investigation was 2 0 0 / l m . An even larger size pinhole was not needed because the counting rate is already high enough for the Si(Li) detector, besides, a larger area increases the chance of rupture. The sample to be analysed is placed a few mm from the window in room atmosphere such that the energy spread due to straggling in air is mini* Manufactured by Teledyne Water Pik, under the trade name of lnstapure. Detailed information about the filter paper is not available.

mized. A Si(Li) solid state X-ray detector of 170 eV resolution (at 5.9 keV calibration source energy) is also pushed to within 10 mm from the spot on the sample that is being bombarded by protons. Signals from the detector carrying energy information from the X-ray photons detected were electronically amplified, analysed and stored in a multichannel pulse height analyzer. Both the paper and the activated charcoal part of a new filter were examined first to determine their contribution to the X-ray spectra. Then the X-ray spectra from the used filter (paper and charcoal separately) were taken under exactly the same experimental conditions. Since the production cross-section of characteristic X-ray induced by proton bombardment is a smooth function of the atomic number of the target atom and the detection efficiency of the detector depends on the energy of the X-ray photon, numbers of characteristic X-ray photons recorded for the trace elements in a sample are not one-to-one corresponding to their concentrations in the same sample. One can deduce these concentrations from the data using the well tabulated tables and charts of excitation cross section vs. Z (atomic number) and efficiency vs. (photon energy) curves. But, to be more independent, an NBS (National Bureau of Standards) certified glassy material containing a number of trace elements was examined under identical conditions. The relative amount of trace elements in the filter can then be determined using the calibration curve derived from the comparison of the measured data with the certified analysis of the same NBS standard. The NBS standard is a homogeneous mixture of precisely measured traces of Si, Fe, Zn and Zr in some kind of resin. It is shaped into a solid bar 3 mm X 3 mm X 15 ram. Because of the comparable density of this resin with that of the activated charcoal or filter paper, only small correction applicable to a uniform thick sample was needed for the calibration curve. To obtain the absolute amounts of trace elements in the filter, one would have to know the total number o f protons that hit the sample, and the solid angle of the X-ray detector taking into account the size of the area the beam hits. However, since the relative abundance has already been determined all one needs to do is to determine the absolute total amount of one of the more abundant of the trace elements in the filter. For tile purpose of this investigation, manganese in the paper part of the used filter was determined because of its large neutron absorption

C.M. Fou / Water purity investigation using PIXE cross section by neutron activation analysis, using a 5 Ci P u - B e neutron howitzer. Part of the paper filter cup was weighed, crushed and packed alongside with a pouch of manganese powder in a plastic capsule. The capsule was then lowered into a neutron howitzer for neutron irradiation. The manganese powder in the pouch was weighed accurately on an electro-microbalance. When the capsule was removed from the howitzer, the pouch and the crushed paper filter cup were separated and the gamma photons following the decay of S6Mn in each of them were counted independently. Since they had been subjected to the same doses of neutron irradiation, the absolute amount of manganese in the paper filter M(Mn) was determined by: M(Mn) = My- . R F ' ' Mp M F, Rp ' where My = mass of the whole paper filter; M y ' = mass of the crushed part of paper filter; R F' = decay rate of the crushed part of paper filter after irradiation; Mp = mass of the Mn pouch; Rp = decay rate of the pouch after irradiation.

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3. Results and discussion The data obtained from a filter that had been used in the Washington, D.C. area for 45 d are shown in figs. 1 and 2 for the activated charcoal and the paper part of the filter. The X-ray spectrum from the unused filter are also shown for comparison. The spectra from the used and unused filters are normalized assuming that there was no accumulation of titanium from the water. This assumption is jusified by the observation that with this normalization the spectra show an increment for all elements originally present in the unused filter, except argon. Since there is about 1% of argon in the air, it is possible that after the filter has been used there are less voids in the filter to trap air, therefore the argon detected in the used filter is less than that in the unused filter. All other ways of normalizing the spectra from the used and the unused filters led to the unreasonable conclusion that trace elements originally in the unused filter had been carried away by the water that passed through it. It is perhaps also reasonable to assume that titanium is not likely to be a major contaminant in household tap water. Titanium was hardly measurable in most of the water samples by Rickey and his

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lead; (2) i m p u r i t i e s picked up b y the a c t i v a t e d charcoal are a l u m i n u m , sulfur, c h l o r i n e , c a l c i u m , chrom i u m , m a n g a n e s e , c o p p e r a n d lead. T h e a b s o l u t e a m o u n t o f all these trace e l e m e n t s in t h e p a p e r filter d e t e r m i n e d in the m a n n e r described in the last section t o g e t h e r w i t h t h e relative a b u n d a n c e o f t h e trace e l e m e n t s a c c u m u l a t e d in the charcoal are given in table 1. T h e a m o u n t o f lead a c c u m u l a t e d in the p a p e r filter was (3 -+ 1) mg. With an average o f 4 0 1 per day passing t h r o u g h this filter for d r i n k i n g and c o o k i n g p u r p o s e s , this m e a n s for 45 d or 1 8 0 0 1 o f water,

c o l l a b o r a t o r s investigated using v a c u u m f i l t r a t i o n t e c h n i q u e s [ 1 0 ] . A t a n y rate, the a m o u n t s determ i n e d here have to be c o n s i d e r e d as lower limits because there m i g h t be t i t a n i u m pick-up f r o m t h e water. Besides this n o r m a l i z a t i o n i n t r o d u c e s 10% u n c e r t a i n t y i n t o the a m o u n t d e t e r m i n e d since the a m o u n t of Ti in the u n u s e d filter varies f r o m u n i t to u n i t b y n o more t h a n 10%. T h e a p p a r e n t features f r o m the c o m p a r i s o n s h o w n in t h e t w o figures are (1) i m p u r i t i e s p i c k e d up b y t h e p a p e r filter are calcium, c o p p e r , zinc, m a n g a n e s e a n d

Table 1 Filter from Washington, D.C. area used on tap for 45 d. Amount of contaminants accumulated in the paper part of the filter. Contaminant

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Mn

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a) NAA = neutron activation analysts. b) Value obtained from NAA. Other absolute values deduced from this value for Mn and the relative concentrations determined in PIXE.

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Fig. 3. Repeated runs on the filter with expanded spectra to verify the lead peaks. The top most spectrum was obtained from the proton bombardment of a lead sheet, showing the three L X-ray lines from lead. The middle spectrum is from the charcoal part of the filter and the bottom spectrum from the paper part of the filter. All three spectra were obtained under the same experimental condition. Because the Ka X-ray from As coincides with the Lc~ X-ray from Pb, therefore the observation of the L/j X-ray from the Pb is necessary and sufficient for the positive identification of lead in the spectra. L7 X-ray from Pb is too weak to be observed in the spectra. there was at least 2 parts per billion by weight of lead in that tap water. The filtration can not be expected to be 100% effective because lead was also picked up in the activated charcoal which was behind the paper filter. Although the presence of lead is visible in the charcoal spectrum (fig. 2) no absolute amount was determined because the amount of Mn in the charcoal was not enough for neutron activation analysis using the available howitzer. Nevertheless, the actual amount of lead in that tap water must be higher. To verify that this is, in fact, a characteristic L X-ray from lead instead of a K X-ray from arsenic, which happens to have the same energy, expanded spectra from both parts of the used filter were taken again and compared to the spectrum from a lead sheet taken under exactly the same condition. All three spectra are shown in fig. 3. The presence of all the L X-ray lines of lead clearly demonstrates that there was accumulation of lead in the filter. While the present investigation had shown the possibility of large scale analysis of household tap water using the above mentioned procedure and technique, there are several places where improvements can be made after systematic research is carried out. They are: 1) The adsorption characteristics of activated charcoal for various pollutants commonly found in tap water must be studied such that one can deduce from

the amount adsorbed in the charcoal the actual amount that is present in the water. 2) The filtration capability of the paper filter for particulates of different sizes must be carefully examined to allow reliable estimation of the amount of insoluble impurities in tap water. 3) Absolute quantitative measurement of all the trace elements in the unused filter (such as titanium which was used as normalization reference because of its virtually nonexistence in water) must be determined such that in the future absolute quantities of trace elements picked up by the filter can be deduced from the relative abundance measured without having to resort to other microanalysis such as the neutron activation analysis described above. Even before these systematic improvements are made, if one is interested in the monitoring of the household tap water on a large scale, routine basis, the above described technique of on-site accumulation using a filter combined with in-air PIXE analysis without the need of sample preparation offers the convenience of handling; the sensitivity, reproducibility and accuracy of detection, which are needed to detect any change from what is normal. The author acknowledges the pleasant collaboration with the Bartol Research group on the development of the in-air PIXE system, and the partial sup-

604 port of the University toward the purchase g u i d a n c e received f r o m early stage o f t h e in-air appreciated.

C.M. Fou / Water purity investigation using PIXE o f Delaware R e s e a r c h Office o f t h e Si(Li) d e t e c t o r . The Dr. L. G r o d z i n s o f MIT in the P I X E s y s t e m is also gratefully

References [1] S.A.E. Johansson and T.B. Johansson, Nucl. Instr. and Meth. 137 (1976) 473. [2] T. Shiokawa, T.C. Chu, V.R. Navarrete, H. Kaji, G. Izawa, K. Ishii, S. Morita and H. Tawara, Nucl. Instr. and Meth. 142 (1977) 199. {3] R.C. Bearse, D.A. Close, J.J. Malanity and C.J. Umbarger, Anal. Chem. 46 (1975) 499.

[4] J.L. Campbell, B.H. Orr, A.W. Herman, L.A. McNelles, J.A. Thomson and W.B. Cook, Anal. Chem. 47 (1975) 1542. [5] R. Zeisler, J.B. Cross and E.A. Schweikert, Anal. Chem. 49 (1976) 2124. [6] K.R. Akselsson and S.A.E. Johansson, IEEE Trans. Nucl. Sci. NS-26 (1979) 1358. [7] G. Deconninck, Nucl. Instr. and Meth. 142 (1977) 275. [8] F.A. Rickey, P.C. Simms and K.A. Mueller, X-ray fluorescence analysis of environmental samples ed., T.G. Dzubay (Ann Arbor Science Publ. Inc., 1977) pp. 135 143. [9] C.M. Fou, V.K. Rasmussen, C.P. Swann and D. Van Patter, IEEE Trans. Nucl. Sci. NS-26 (1979) 1378. [10] P.C. Simms and F.A. Rickey, Report to EPA No. 600/ 178o058 (September 1978).