There are two common calibration procedures: using a working curve, and the standard-addition method. Both of these methods require one or more standards of known composition to calibrate the measurement.
Instrumental methods are usually calibrated with standards that are prepared (or purchased) using a non-instrumental analysis. There are two direct analytical methods: gravimetry and coulometry. Titration is similar but requires preparation of a primary standard.
The chief advantage of the working curve method is that it is rapid: a single set of standards can be used for the measurement of multiple samples. The standard-addition method requires multiple measurements for each sample, but can reduce inaccuracies due to interferences and matrix effects.