Waters ACQUITY UPC2, Overview And Maintenance Manual, Page: 19 / 84

Overview 15
Absorbance
The detector calculates the absorbance for each diode at the end of each
exposure time using the following equation:
where
S = obtained during sample analysis
D = obtained during the dark current test
R = obtained from the reference spectrum
n = diode number
Resolution
The data the detector report to the Empower or MassLynx database can be the
average of a number of data points. After calculating absorbance, the detector
averages absorbance values based on spectral resolution and sample rate.
Averaging spectral data based on resolution
Spectral resolution, or bandwidth, is the wavelength interval. in
nanometers, between data points in an acquired spectrum. The
detector’s lowest resolution setting is 1.2 nm. For example, in 3D-mode,
the detector averages three adjacent diodes for each reported
wavelength when you specify the spectral resolution as 3.6 nm. In 2D
mode, absorbance values are computed based on the bandwidth setting.
Averaging chromatographic data based on sample rate
Sample rate is the number of data points acquired per second. The
number of times the photodiodes are read during the sample rate
interval depends on the exposure time. For example, if exposure time is
25 msec, and sample rate is 20 Hz, then readings per data point are
calculated as follows:
The software then averages the readings and reports the average as a
single data point.
Absorbancen
Sn Dn –
 
Rn Dn –
 
-------------------------
log =
1 sec
20 samples
-------------------------- 1 exposure
25 msec
--------------------------

1000 msec
1 sec
--------------------------

2
exposures
sample
------------------------
=