TRACE ELEMENTS IN BLOOD MEASURED BY PIXE

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1 2005 International Nuclear Atlantic Conference - INAC 2005 Santos, SP, Brazil, August 28 to September 2, 2005 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: TRACE ELEMENTS IN BLOOD MEASURED BY PIXE Suene Bernardes, Manfredo H. Tabacniks, Marcel D.L. Barbosa and Márcia A. Rizzutto Instituto de Física - Universidade de São Paulo Rua do Matão, travessa R 187, , São Paulo, SP suene@if.usp.br ABSTRACT The abstract is a very brief summary highlighting main accomplishments, what is new, and how it relates to the state-of-the-art. The quantitative measurement of trace elements in blood (and plasma) and its comparison with data from literature is shown. To optimize the analytical conditions and detection limits of blood analysis by PIXE (Particle Induced X-ray Emission), a study on several sample preparation methods was made. PIXE analysis was done using a 2.4 MeV proton beam and two Si(Li) detectors with respectively 50µm Be and 130 µm thick Mylar filters. Elementary concentrations were calculated relative to an internal gallium standard. A total of 13 trace elements were detected with concentrations ranging from 10 ng/g to 1 mg/g. 1. INTRODUCTION Blood is a suspension of cells (erythrocytes, leucocytes and platelets) in a complex fluid, the plasma, which is mainly constituted of water, minerals, vitamins, proteins, carbohydrates, and lipids. Serum differs from the plasma by the removal of coagulation factors. The minerals act as catalytic factors for many biochemical reactions and play an important role in the hormone production. In the plasma, the minerals can be chemically free or aggregated to proteins and other organic molecule [1]. The continuous development of increasingly more sensitive trace element methods, is shifting the research of trace elements in biological materials from simple toxicological contaminants to the better understanding of their function in the organisms. It is known that many trace elements are essential for life, but are also toxic, if in excess. The measurement of very small amounts of trace elements in diminutive organic samples is a research theme by itself, since sample preparation methods, data quantification and detection limits need to be tested and improved. The objective of this work was to measure the trace elements in sanguineous plasma by PIXE (Particle Induced X-ray Emission) and to compare the results with data from literature attempting to guide the answers of two basic questions: a) Which are the elements of biological interest? b) What is the necessary detection limit? PIXE is an elementary spectrometric method that relies on the detection of characteristics X-rays emitted by bombarding a sample with an energetic (~MeV) ion beam (H +, He +,..). PIXE is sensitive to all the elements in the periodic table above the aluminum, with a primary detection limit for solid samples of the order of 1 µg/g which can be extended to the ng/g level for liquid samples after a pre-concentration procedure [2].

2 2. EARLIER MEASUREMENTS The development of many element sensitive techniques and the systematic measurement of trace elements in materials and more specifically in biological materials can be tracked down to early 1950 [3, 4]. Initially, and probably aiming toxicological studies, many investigations dealt with the detection of poisonous contaminants at relatively high concentrations (like Pb, Hg, Cd etc.) [5]. In the search to understand the role of trace and ultra-trace elements in the environment and in the living bodies, the search for trace elements required decreasingly lower detection limits, while also reducing the amount of sample material needed. The multielementary and almost non-destructive physical spectrometric methods are specially suited for this kind of analysis. Starting around the 50ties, the X-ray Fluorescence analysis and the Neutron Activation Analysis, extended and complemented the data of the widespread monoelementary Atomic Absorption (and Emission) Spectrometry, a common equipment available in many chemical laboratories almost since the beginning of the 20th century. With the development and worldwide dissemination of the PIXE analysis in the 70ties [6], the search for trace elements in the environment and biology got a remarkable push. Thanks to its low sensitivity (in the ppm range), speediness and allowing yet smaller samples, when compared to the XRF, PIXE analysis grew in less than 10 years to a mature and powerful micro-analytical method [2]. Yet, there was still the need to lower the detection limits down to sub ng/g (ppb) level. The Inductive Coupled Plasma Atomic Emission (ICP-AES) and its successor, the ICP - Mass Spectrometer, developed in the 80ties [7, 8], achieved the ultimate trace element detection limits at a reasonable cost. The question nowadays changed from how many elements were detected to what is the necessary detection limit or which elements need to be detected. Figure 1. Compilation of published data on trace element concentration (ng/g) in whole blood. Squares ( ) refer to medians while ( ) indicate data from a unique source. Straight lines indicate the maximum-minim range of published data.

3 A survey in 10 of the most accessible journals and publications supplied data for elementary concentration data for 68 elements in whole blood. The data were compiled in simple scatterplots of their median values for plasma and serum in Figure 1, ordered according to their atomic number. The maximum-minimum ranges, when available, were also plotted as straight lines. Concentration data range over 11 orders of magnitude and include samples from different healthy populations, most measured by PIXE and ICP [9-21]. 3. MATERIAL AND METHODS To verify the actual PIXE detection limits and the agreement with published data, three plasma samples were analyzed by PIXE. The samples were micro-pipetted on Nuclepore (10 µm thick Makrofol ) filters stretched on 25mm diameter PVC rings. The filter surface was prepared by pipetting 20µl of a plasma solution spiked with gallium solution, as an internal standard resulting in 50 µg/ml of Ga in the final solution. The picture of a sample ready for PIXE analysis is shown in Figure 2. PIXE analysis was done using a 2.04 MeV proton beam collimated to a diameter of 5mm. X-rays were measured with two Si(Li) detectors, respectively with a 50µm Be filter and a 150µm Mylar filter, tailored to record respectively the low energy and the high energy X-ray spectra. The samples were irradiated during 10 minutes with a beam current of 20 na. The complete PIXE setup at LAMFI [22] is described elsewhere [23]. The x-ray spectra were analyzed using the well-known non-linear multiple gaussian fitting program AXIL [24]. Quantitative calibration of the PIXE setup was made using a set of mono-elementary evaporated thin film standards (mostly from Micromatter Co.) and a semi-empirical fit. Figure 2. Pipetted sample on polymer film ready for analysis. 4. RESULTS Fourteen elements were positively identified in the PIXE spectra. Data were converted to mass concentration using the internal Ga standard concentration in Equation 1 I R C Z = CGa (1) SR

4 where I R represents the relative-to-gallium X-ray intensity of element with atomic number Z, C Z is its measured concentration, (µg ml -1 ), C Ga represents the Gallium internal standard concentration (50 µg/ml ) and S R is the relative-to-gallium yield of the PIXE system. The measured average concentrations are reproduced in Table 1. The quoted uncertainties are the standard deviation of the averaged values. Table 1. Elementary mass concentrations in sanguineous plasma measured by PIXE Element Average concentration (µg/ml) Si 2.2 ± 2.2 P 49.1 ± 2.9 S (0.63 ± 0.16)10 3 Cl (6.0 ± 1.3)10 3 K 372 ± 35 Ca 77 ± 15 Ti 0.16 ± 0.14 Fe 1.4 ± 0.9 Co ± Ni ± Cu 1.11 ± 0.37 Zn 1.55 ± 0.63 Se 0.19 ± DISCUSSION Several materials were tested as a substrate for the pipetted blood samples: thin polymer films like Mylar, Nuclepore, Kimfol and Kapton, graphite discs and analytical ashless paper filters. Blood was noticed not to adhere on Mylar or Kimfol and regular graphite presented too much contamination. Kapton and Nuclepore thin films, which showed good adhesion to blood, were used in the present study. Paper filters presented good adhesion and high sample load but showed low uniformity of the sample mass across the filter. Measured elementary concentrations were compared to whole blood and serum values from literature in Figure 3. The results are in good agreement with the literature, except for phosphorous whose value is below expectation and nickel with results above expectation. It may be the consequence of sample preparation or may just be a regional effect. It must be stressed out that the measured trace elements in plasma are being compared to literature data of whole blood and serum measurements. The results are expected to compare to serum values, since serum differs from plasma by the absence of coagulation factors. The difference

5 between whole blood and serum (or plasma) is evident in the iron concentrations, which is partially removed with the erythrocytes, leucocytes and platelets. The need for better detection limits is quite visible in Figure 3 where no data are lower than 1 ppm. Actually, inspecting Figure 1, it is found that around 18 more elements were expected to be measured, if the PIXE detection limit for liquid samples could be set to its nominal detection limit at about 1 ppb. Figure 1. Water-filled borehole using conventional discrete ordinates formulation for the transport equation. Elementary concentrations in serum (S) and whole blood (WB) compared to the PIXE data measured (M) in this work. The error bars indicate one standard deviation. 6. CONCLUSIONS PIXE analysis of plasma samples deposited on paper filter seems to be a promising technique for measuring trace elements in blood above the 10 ppb wgt concentration. Besides the need to succeed reaching the nominal PIXE detection limits, a further 10-fold reduction of the detection limits is needed to cover most of the possible important trace elements in blood serum, which would make PIXE analysis a very competitive technique in bio-trace-elements research. This can be achieved by pre-concentrating the liquid samples and by reducing the film thickness of the substrates. ACKNOWLEDGMENTS The authors like to acknowledge M.R. Antonio and M.V.S. Lima for help running the accelerator, N. Added for helpful discussions and CNPq for financial support.

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