The routine external calibration is convenient but often not accurate, especially for MALDI mass spectrometers of early models.8 The internal calibration could be accurate but has the following drawbacks.8 A prior knowledge of the sample quantity is needed to determine how much standard molecule should be added to the sample for the internal calibration. If the standard molecule quantities are insufficient, the standard MS peaks may not be identified for the calibration. If too many standard molecules are added, the sample MS peaks will be suppressed. The standard molecule MS peaks may overlap with the sample MS peaks. Unexpected or unknown molecules in the standard sample also add interfering MS peaks to the spectrum. The internal calibration can also be performed using matrix or enzyme autolysis MS peaks. However, these peaks are suppressed when the sample signal is strong.