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Definition: internal standard
An Internal Standard is a compound purposely added to both samples and standards at a known concentration in order to provide a basis for comparison in quantitation. Computationally, the only difference between "internal standardization" and "external standardization" is that the former works with area ratios and concentration ratios rather than areas and concentrations. Internal standardization can improve precision when the dominant sources of error are related to sample preparation or injection; because such errors affect both the internal standard and the analyte peak in the same way, they will tend to cancel out when the ratio of areas is calculated. Internal standardization can degrade precision when the dominant sources of error are related to separation, peak shape, or integration; because such errors affect the internal standard peak and the analyte peak differently, they will tend to combine when the ratio of areas is calculated.