Waters 2414, Overview And Maintenance Manual

Page: 21 / 130

June 11, 2015, 715004754 Rev. A
Page 21
1.2.2.3 Effect of refraction on the photodiode signal
The change in
φ
determines the shift (
Δ
x
)
of the light beam on the photodiode.
Because the 2414 RI 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 (
Δ
x
)
at the 2414 RI detector photodiode and
the change in RI of the solution is expressed as:
Δ
x = 2 Y (tan
θ
)
Δ
n
where:
Δ
x = Distance of the image shift at the photodiode
Y = Distance from the flow cell to the photodiode
θ
= Angle of incidence
Δ
n = Difference in RI between solvent and sample solution
The angle of incidence (
θ)
and the distance to the photodiode ( Y ) are fixed in the
refractometer, so the equation becomes:
Δ
x = C
Δ
n
where:
C = A constant representing the fixed values
By detecting how far the image shifts (
Δ
x
)
, the refractometer measures the difference
in RI (
Δ
n ) between the solvent-sample solution and the solvent alone.
The shift in the amount of the light beam striking each element of the dual-element
photodiode results in a change in the output voltage from the 2414 RI detector. The
integrator registers the changes in output voltage as peaks in your chromatogram.
1.2.3 Common RI detection problems
Changes in solution density caused by factors other than sample concentration are
the most common source of problems in RI detection. Changes in solution density can
be the effects of these causes:
• Environmental factors such as changes in temperature or pressure
• Inhomogeneities in the solution
1.2.3.1 Environmental factors
Even small changes in ambient temperature can cause baseline drift. Backpressure
pulses from a dripping waste tube can cause short-term baseline cycling. Refer to
Chapter 7 for more information.