I am working on flourescence sensing of heavy metals using quantum dots.Can any of you suggest how to find the detection limit. Whether it is calculated from formula or we can get from instrument?
It depends on the instrument. With the right equipment, a single molecule of a fluorescent substance can be detected. If you are using a spectrofluorimeter or a fluorescence plate reader, the detection limit can be ascertained by preparing a set of dilutions of the substance and measuring the fluorescence compared with a control containing only the solvent.
Adam B Shapiro is right. To be more precise, you measure a dilution series, plot it in a graph, perform a linear regression and calculate the standard deviation of the blanks. The detection limit of your substance with your instrument is the concentration at which the measurement value is 3 times the standard deviation of the blanks. You will find several ways to calculate this if you google around, but all of them are different versions of the same formula.