external standard method
The external standard method is the straightforward way of calibrating: you measure your standards separately from your sample, build a calibration curve from them, then measure the sample and read its concentration off that curve. The standards are "external" because they live in their own tubes, never mixed into the unknown — like checking your bathroom scale against known weights before you step on it.
In practice you prepare a series of standards of known concentration, run them through the instrument under the same conditions as the sample, and plot signal against concentration. The sample is then run the same way, and its signal is matched to the curve. Everything hinges on the standards and the sample being treated identically.
It matters because it is simple, fast, and ideal when many samples share a clean, well-understood matrix — you calibrate once and analyze a whole batch. Its weakness is exactly that separateness: if the sample's matrix changes the signal in ways the standards do not share, the answer will be biased, and a method like standard addition becomes safer.
A water lab measures nitrate in dozens of drinking-water samples each day. A single calibration curve from external nitrate standards is run each morning, and every clean sample that day is read against it.
Calibrate once with separate standards, then read many similar samples against the curve.
External standards assume the matrix does not bias the signal. When that assumption fails, internal standards correct for instrument fluctuations and standard addition corrects for matrix effects — both bring the calibration closer to the sample.