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Introduction
Lamp Lifetime and Detector Noise
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Accela PDA Detector Hardware Manual
Thermo Scientific
Selecting an Appropriate Diode Array Scan Rate
When you configure the PDA detector, you have three options for the diode array scan rate:
20, 40, and 80 Hz. The diode array scan rate is the rate at which the PDA detector samples
the integrated intensity of the diodes.
The option you select affects the detector noise level, the useful lifetime of the deuterium
lamp, and the available sampling rates in the data system method.
As you increase the diode array scan rate, the sampling time per diode decreases. To achieve
the same integrated light intensity, you must increase the light throughput to the diode array
by opening the detector’s attenuators (
“Completing the Installation and Verifying Operation”
). As the lamp ages, it emits less and less light (see
). So again, to achieve
the same light throughput to the diode array, you must increase the attenuator aperture.
Eventually, the attenuator aperture reaches a physical limit. When you can no longer increase
the attenuator aperture, the integrated light intensity begins to decrease, and detector noise
level begins to increase. The attenuator aperture reaches this physical limit sooner at the
higher diode array scan rates.
The useful lifetime of the lamp depends on the acceptable noise level for your application. At
the 20 Hz diode array scan rate, you can compensate for the decreased light output from the
deuterium lamp by increasing the attenuator aperture throughout the lamp’s lifetime of
approximately 2000 hours.
To maximize the useful lifetime of the deuterium lamp, select a diode array scan rate that is
appropriate for your application:
• For standard chromatography applications, select the 20 Hz diode array scan rate. With
this selection, you can acquire up to 20 data points per second per chromatogram and
optimize integration for chromatographic peaks with baseline widths as narrow as
1 second.
• For most fast chromatography applications, use the default selection of 40 Hz. With this
selection, you can acquire up to 40 data points per second per chromatogram and
optimize integration for chromatographic peaks with baseline widths as narrow as
0.5 seconds.
• For fast chromatography applications that have chromatographic peaks with baseline
widths of less than 0.5 seconds, select the 80 Hz diode array scan rate.
IMPORTANT
For validated HPLC methods, record the appropriate configuration setting
for the diode array scan rate. The diode array scan rate affects the detector noise level.
Note
For information on creating methods to control the PDA detector, refer to the data
system Help.
