Calibration of LA-ICP-MS for trace element analysis of phosphate materials

The calibration of a LA-ICP-MS system was investigated for calcium phosphate materials. Synthetic phosphate pressed powder pellets were successfully prepared and spiked with up to 1000 [mu]/g of 12 elements (Mg, Al, Co, Ni, Cu, Zn, Cr, As, Cd, Ba, Pb, U). Calibration lines with good correlation factors, >0.96 for all elements were produced. Precision on the synthetic pellets were about 8-9% RSD, comparable to the 6-8% RSD achieved with NIST 610 glass. Nine natural reference materials were also investigated as pressed powder compacts. Their natural elemental content was used to produce good calibration lines (R[sup]2 >0.96 for Cr, Ni, Zn, Sr, Cd, La, U). Using the standard additions (SA) method, ~x3 and x5 the natural content were spiked into the materials. Results showed that the majority of elements gave linear response between natural content and added spike. However, the difference between the natural content and the calculated value by SA varied significantly between reference materials for the same element, and within the same reference material for different elements. Thus, when comparing all nine reference materials, a range of sensitivities were observed. Scanning Electron Microscopy images were obtained for the pellets from the area close to the ablation tracks to assess the physical properties. The images showed loose material around the track being removed. Chemical 'aging' for some pellets was observed such that counts per unit concentration decreased with time. The pellets also became amorphous over time. Moreover, one sample (BCR-33), rich in sulphur, showed re-crystallisation features on the surface of the pellet. A number of features are thought to contribute towards the variable elemental responses in some materials including melting of the material during the ablation process, different size particulates being removed from the sample and the formation of discrete grains of specific chemistry.