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1 The ACQUITY Refractive Index Detector
quartz walls of the flow cell, the solvent in the reference side of the flow cell,
and the solution in the sample side of the flow cell.
Of these refractive media, only the solution in the sample side of the flow cell
changes over the course of a run. As a result, the external angle of deflection of
the reference does not change until a change in the RI of the sample causes
the light beam's refraction from its zero position.
The relationship between the external angle of deflection and the RI of the
sample solution is expressed as
Δn ≅ φ/tanθ
where
Δn = Difference in RI between the solvent and the solvent-sample
solution
φ = External angle of deflection (in radians)
θ = Angle of incidence (in radians)
Effect of refraction on the photodiode signal
The change in the external angle of deflection determines the shift (Δx) of the
light beam on the photodiode. Because the detector uses a dual-pass optics
bench assembly, the light beam passes through the flow cell twice before
reaching the photodiode, doubling the image shift.
The relationship between the image shift (Δx) at the refractive index detector
photodiode and the change in RI of the solution is expressed as
Δx = 2Y(tanθ) Δn
where
Δx = Distance of the image shift at the photodiode
Y = Distance from the flow cell to the photodiode
θ = Angle of incidence
Δn = Difference in RI between solvent and sample solution
The angle of incidence and the distance to the photodiode are fixed in the
refractometer, so the equation becomes
Δx = C Δn
where
C = A constant representing the fixed values
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October 13, 2014, 715003547 Rev. C
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