1-4
2998 PDA Detector Optics Principles
at the photodiode and the concentration of the sample passing through the
flow cell. Beer’s law is expressed as
A
=
ε
lc
where
A
= dimensionless quantity measured in absorbance units
ε
= constant of proportionality known as the molar absorptivity
l
= path length in centimeters (1.0 cm in the detector’s normal flow cell)
c
= concentration in moles per liter
Beer’s law applies only to well-equilibrated dilute solutions. It assumes that
the refractive index of the sample remains constant, that the light is
monochromatic, and that no stray light reaches the detector element. As
concentration increases, the chemical and instrumental requirements of
Beer’s law are sometimes violated, resulting in a deviation from (absorbance
versus concentration) linearity. The absorbance of mobile phase can reduce
the linear range.
Absorbance as a function of concentration
Flow cell operating principles
The Waters TaperSlit™ flow cell used in the 2998 detector renders the
detector baseline essentially insensitive to changes in mobile phase refractive
index (RI). RI changes occur during gradient separations or result from
temperature or pump-induced pressure fluctuations.
To achieve RI immunity, a combination of a spherical mirror, a lens at the
entrance of the flow cell, and a taper to the internal bore of the flow cell
Concentration
Absorbance
Ideal
Actual
Linear range
for analyte
Summary of Contents for 2998
Page 12: ...xii Table of Contents...
Page 26: ...1 14 2998 PDA Detector Optics Principles...
Page 62: ...3 12 Maintaining the Detector...
Page 82: ...5 10 Spectral Contrast Theory...
Page 96: ...A 14 Safety Advisories...
