Selecting The Appropriate Sampling Rate - Waters ACQUITY Refractive Index Detector Overview And Maintenance Manual

The detector's optics bench directs light as follows:
• Light from the source lamp is focused by the focusing lens through the
aperture and collimating lens, forming a beam.
• The light beam passes through the sample and reference sides of the
flow cell to the mirror.
• The light beam is reflected back through both sides of the flow cell and
the collimating lens to the dual-element photodiode.
The difference in the amount of light striking the elements of the photodiode
(because of sample refraction) results in a deflection from the baseline on the
chromatogram.

Selecting the appropriate sampling rate

A sufficient number of points must fall across a peak to define its shape. Thus,
at very low sampling rates, the resolution between peaks is lost. Empower
software uses the index of the data point closest to the end time, minus the
index of the data point closest to the start time, to calculate the Points Across
Peak value for each integrated peak in the chromatogram.
The Points Across Peak value appears in the Peaks table, at the bottom of
Tip:
the Review Main window. If the Points Across Peak field is not visible,
right-click anywhere in the table, and then click Table Properties. Click the
Columns tab, and then scroll down to find the Points Across Peak field. Clear
the check box, and then click OK.
To maintain acceptable accuracy and precision, specify between 20 and 25
points across the narrowest peak. Increasing the sampling rate tends to
increase baseline noise. For that reason, choose a sampling rate based on the
signal-to-noise ratio (S/N) consistent with the precision requirements of the
assay. For well behaved systems, the relative standard deviation (RSD), in
percent, equals 50/(S/N). Thus for a signal-to-noise ratio of 100:1, the RSD
would be approximately 0.5%.

Selecting the appropriate sampling rate

October 13, 2014, 715003547 Rev. C 33