Chromatography Forum

I was recently asked to prepare standards of different concentration through dilution from the stock solution to construct calibration curves that will be used for quantitation of my samples.

I successfully prepared stock solution of the right concentration. However, after dilution of stock solution to lower concentrations, such as from 5000 ppb to 500, 300, 200, 100, and 10 ppb, I was unable to get the correct responses/concentrations after being analyzed using GC for my standards of lower concentrations.

What problems that I may have that leads to these inaccuracies?

Thanks!

What final volume are you using when preparing the solutions? Larger final volume allows more accurate measurements when possible.

Are you preparing each standard from the initial stock solution, or doing serial dilutions of each standard to the next? Example 100ul stock with 900ul solvent for the 500ppb then 20ul stock with 980ul solvent for the 100ppb or 100ul stock with 900ul solvent for 500ppb then 200ul of 500ppb with 800ul solvent for the 100ppb.

How are you defining the calibration curve fit? Are you expecting exactly a 5 fold difference in response between the 500ppb and 100ppb in a exactly linear fashion, or could your detector be non-linear and need a quadratic fit for the calibration? FID is normally linear over a large range of concentrations, Mass Spec and some other detectors may not have linear response over a large range of concentrations.