How to determine Detection Limit for Instrument, especially AAS?

Detectection limit is applicable for all instrument. It is mean variance of the blank for the expected analyte. Example : As you requested take AAS, IN Flame technic select  the wave length for the expected analyte and run the blank for10 time and calculate signal variance at this wavelength, i.e minimum available background of the instrument , then multiply with a factor. example3 times, then it becomes the minimum possible which is your detection limit of the instrument.