Measuring Light At The Photodiode Array - Waters ACQUITY UPLC Operator's, Overview And Maintenance Manual

Measuring light at the photodiode array

The photodiode array detector measures the amount of light striking the
photodiode array to determine the absorbance of the sample in the flow cell.
The array consists of 512 photodiodes arranged in a row. Each photodiode acts
as a capacitor by holding a fixed amount of charge.
Light striking a photodiode discharges the diode. The magnitude of the
discharge depends on the amount of light striking the photodiode.
Photodiodes discharged by light
Light from grating
dispersed onto
diodes.
The detector measures the amount of current required to recharge each
photodiode. The current is proportional to the amount of light transmitted
through the flow cell over the interval specified by the diode exposure time.
Exposure time
The detector recharges each diode and reads the recharging current, one diode
at a time. The interval between two readings of an individual diode is the
exposure time. The detector requires less than 5 msec to sequentially read all
of the diodes in the array and process the data. The minimum exposure time is
5 msec. You can set exposure time from 5 to 500 msec.
Sample in flow cell
absorbs at specific
wavelengths.
Grating
Slit
Operating principles of the light-guiding flow cell
Deuterium lamp
Flow cell
Mirror
9