Don’t run out of atoms.
Figure 1. The protagonist in this figure has insufficient iron atoms for his proposed procedure.
How many atoms do you have?
How many atoms could we possibly need?
This depends on three things. The concentration of the analyte, the precision of the measurement which is desired, and the detection efficiency of the instrument. Since the presence or absence of individual ions of interest in an analytical volume is a poisson process, the minimum number of atoms needed in the least abundant species measured is the square of the inverse of the desired precision. So 1% precision requires 10,000 of the least abundant atom. A permil requires a million, and 0.1 permil requires 108.