5. Quality Control
47
5.3.4 Instrument Linearity with Liquid Samples
The linearity for the instrument and application at the wavelength used can be
checked by using a dilution series of a solution.
The result depends on the purity of the dye used and the meniscus of the liquid in
the wells.
As a reference, a dilution series of Orange G solution for measurements at
492 nm can be used.
The dilution series should be within the range of 0.1 to 3.0 OD. To reach about
3 OD it is recommended to use 125 mg.l
-1
Orange G (Sigma, Cat. No. O7252).
For other wavelengths, different solutions must be used.
200 µl of each dilution are then pipetted into the microplate, a minimum of at least
two samples should be used for each dilution to reduce the errors caused by
pipetting.
The microplate is then measured and a linear regression of OD against
concentration is drawn from the average of the measured OD values.
Determine the unweighted residual square value R
2
of the regression line.
Typical residual square values for a standard application are equal or better than
R
2
= 0.998.
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