For the sample preparation of your XRF analysis, you can use borates for fusion, additives for fusion and pre-weighted tablets for grinding and tabletting. The key is to create homogeneous samples and to avoid contmintion with trace impurities of your sample. Merck's Spectromelt® products can help you achieve that.
Non-wetting additives like lithium bromide and potassium iodide, for example, act as releasing agents that help to prevent the borate glasses from sticking to the platinum ware.
Spectromelt® fluxes for fusion sample preparation. Merck offers both lithium borates, Spectromelt® A as well as sodium borates, Spectromelt® B10.
The Spectromelt® C-series is especially developed as a grinding and tabletting aid. In automated sample preparation of powder samples speed of analysis is an important factor.