A spray technique for preparing uniform large-area PIXE calibration standards from aqueous solutions

We describe a procedure for preparing reference standards that can be used to calibrate PIXE spectra of aerosol deposits collected on suitable backing films made, for example, of synthesised polymers (polypropylene, polycarbo nate etc.) or chemically modified biopolymers (e.g., cellulose actetate). A queous solutions of well defined concentration were prepared from metal sal ts. The solutions were spray-deposited on cellulose acetate backings using a nebuliser. Lateral uniformity of the deposited layer over areas ranging f rom several cm to several cm was accomplished by raster scanning the table on which the backing was mounted. By optimising the experimental parameters , deposition could be accomplished without any detectable wetting of delica te substrates like paper. The areal density of the deposited material was c alculated from the concentration of the solute, the volume of the consumed solution and the raster scanned area. The reference standards were analysed by PIXE using an external beam chamber backfilled with helium. The Si(Li) based detection system contained a thin funny filter so that X-ray spectra could be measured down to 1 keV. The proton energy on impact was 1.1 MeV. T his relatively low probe energy has the advantage of producing favourable e lemental X-ray peak heights relative to the bremsstrahlung-background, nota bly at energies between 2 and 7 keV. Under the same conditions sufficiently high X-ray yields were achieved for heavy elements like Zn and Pb. Using l arge area beams, bombardment-induced stress in the backing films could be k ept at a]ou level so that very reproducible background spectra were obtaine d. Hence a straightforward background subtraction routine could be employed . Sensitivities were determined for 23 elements ranging from Na to Pb. Adva ntages and problems associated with the different metal salts are discussed briefly. (C) 1999 Elsevier Science B.V. All rights reserved.