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The equation for calculating absorbance is:
where:
A
= Absorbance at wavelength 1, referenced to wavelength 2
I
= Intensity of light transmitted through the reaction cuvette at wavelength 1
λ 1RC
I
= Intensity of light transmitted through the reaction cuvette at wavelength 2
λ 2RC
I
= Intensity of light transmitted through the dark cuvette at wavelength 1
λ 1
0
I
= Intensity of light transmitted through the dark cuvette at wavelength 2
λ 2
0
I
= Intensity of light transmitted through the open cuvette at wavelength 1
λ 1OC
I
= Intensity of light transmitted through the open cuvette at wavelength 2
λ 2OC
This is the basic equation to measure analyte concentrations in the rate methods and endpoint reac-
tions used in chemistries.
8.2.1
Rate Methods
The rate of the reaction is calculated from the difference between absorbances measured at certain
defined intervals on the linear portion of the reaction curve. Absorbances are taken throughout the
read time to confirm the linearity of the reaction curve. The absorbance rates are converted to analyte
concentrations by using the disc-specific calibration factors that are encoded on the bar code ring.
The equation for rate methods is:
where:
Δ A/ Δτ = Rate of change of absorbance
F
= Measurements taken at time F on linear portion of reaction curve
B
= Measurements taken at time B on linear portion of reaction curve
τ
= Time in minutes
I
= Other intensities as described above
λ XX
Operating Principles
8-3
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