Complementary analysis of freeze-dried and frozen-hydrated plant tissue by electron-probe X-ray microanalysis: Spectral resolution and analysis of calcium.

Automated methods of peak determination including peak location, background determination and peak deconvolution are presented. Special attention is given to the problem of severely overlapped peaks, such as the primary calcium peak (Caka) and the secondary potassium peak (Kkβ). A method, based on the principle of electronic transitional probabilities, is presented for estimating Ca in the presence of a high background of K, and limitations of resolution are discussed. Minimally detectable concentrations of elements in prepared frozen-hydrated standards are estimated empirically by analysis of the relative variance. Minimally detectable Ca is estimated in both frozen-hydrated and freeze-dried tissue standards. Using our instrumentation and procedures P, S, Cl and K are detectable down to 24 mM, while Na and Mg are only detectable down to 38 mM in frozen-hydrated tissue. In the presence of 100 mM K, detection of calcium is possible down to 22 and 2 mM in frozen-hydrated and freeze-dried tissue, respectively. As an example, total Ca is shown not to conform to the radial gradient of K across the lettuce (Lactuca sativa L.) taproot.