In the new European Pharmacopoeia (EP 10), as well as in the 2019 update of USP 857, the control of photometric linearity is required.
For to check the photometric linearity, at least 3 absorption filters with different concentrations are required.
Hellma Analytics offers the following reference materials to check the photometric linearity of your spectrophotometer:
In this Excel tool, which is available for download, you can easily check the linearity of your spectrometer. Based on your input the correlation coefficient is automatically calculated and the the values are also shown graphically. If the measured values deviate too much from the linear trend line (R² <0.999) the R² field is automatically colored red.
Note for Linearity:
The correlation coefficient is a measure of the degree of linear correlation, if this is 1, all measured values are on a straight line and thus completely linear with each other. The coefficient of determination R² is the square of the correlation coefficient. The European Pharmacopoeia (EP 10) requires R² > 0,999.