Freeze drying: a novel method for preparation of solid analytical tungsten and molybdenum standards

The lecture lesson starts with a brief review of freeze-drying history, its scientific basics, and numerous applications. Freeze drying is also a new effective method to prepare solid analytical standards, which is superior to the conventional methods blending and spiking. The freezing process in liquid nitrogen is completed in fractions of a second. Hence, the original homogeneous distribution of the dopants is preserved and the subsequent sublimation step minimizes any disturbance. Freeze-dried standards exhibit superior lateral distribution and uniform depth distribution. Tungsten standards obtained by freeze-drying aqueous solutions of doped highly soluble ammonium metatungstate showed an outstanding degree of homogeneity. Various tungsten standards doped with 10–28 elements ranging from 10 to 200 ppm each were successfully used as controls for a number of analytical techniques including glow discharge mass spectrometry and DC ARC optical emission spectroscopy. Molybdenum standards prepared by freeze-drying aqueous solutions of doped slightly soluble ammonium paramolybdate, revealed, however, some degree of segregation. Compared to standards prepared by the conventional methods, the freeze-dried standards were still superior. By introducing the starting mixture into liquid nitrogen as a mist, the homogeneous distribution of the dopants was significantly improved.