ANALYSIS OF AQUEOUS METAL CONCENTRATION WITH XRF

The XRF method is widely used to measure the elemental composition of materials. Since this method is fast and non-destructive to the sample, it is the method of choice for field applications and industrial production for control of materials. Most XRF application focus on testing solid phase samples (e.g. soils or rocks). Because of relative poor detection limits, XRF analysis of aqueous samples from environmental sites has not found wide application. This study demonstrates that by coupling XRF technology with an ion exchange method can significantly lower the detection limits of many environmentally relevant metals in solution (such as Cu, Zn, and Ni). A Bruker AXS S-4 Pioneer wave length dispersive XRF in combination with Mylar bottom sample holders (Mylar thickness=3.5 mm) was used for this study. An ion exchange resin, Amberlite XRD-16 nonionic polymeric adsorbent, was used to concentrate Cu, Zn, and Ni from aqueous solutions. It was found that blank Amberlite samples caused very little XRF interference, but when metals were present, well defined peaks were detectable for each metal analyzed. A five point standard curve plus blank was prepared for each metal. The standard curve for copper, for example, was highly linear (R2=0.998) in the range from .4 ppb to 20 ppb. Similar correlation coefficients were obtained for zinc and nickel. The minimum detection limit (MDL), with 95% certainty, was calculated using the following equation:

MDL=(3 x C √Background) / (Peak √Time)

where C is the metal concentration, peak is the peak intensity, and Time is the peak acquisition time (sec). The MDL for copper, for example, was calculated at 19 ppb. For comparison, the current MCL for copper is 1.3 ppm. Because the XRF analysis is comparably fast and inexpensive, this new method may be of interest to those Geology departments that do not own other metal analytical equipment, such as AAS, but still need low detection limits.