what is difference of uv and pda
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Answer Posted / fathy
There are three types of UV detectors, fixed wavelength,
variable wavelength and diode array. The fixed types use a
lamp which emits at a certain wavelength. Mercury vapor
lamps are probably the most common and emit intense light at
253.7 nm. Because of the intensity of the radiation, fixed
wavelength detectors can be up to 20 times more sensitive
than variable wavelength detectors. Compounds containing
carbonyl groups, multiple double bonds, or aromatic rings
can be detected at this wavelength.
The variable types use a deuterium or similar lamp that
produces a broad spectrum of wavelengths that are separated
by a diffraction grating. A diffraction grating consists of
a large number (15,000 – 30,000 per inch) of very fine
groves etched into a high polished surface. The grating
works like a prism but generally the resolving power is
higher for gratings than for prisms. The light from the
grating is reflected to a barrier containing a tiny slit.
The instrument is adjusted so that only the wavelength of
interest passes through the slit. The selected wavelength
is passed through the sample. Some of the light is absorbed
by the sample and the amount passing through the sample is
measured and is proportional to the concentration of the
absorbing compound.
Fixed and variable spectrophotometers select a single
wavelength of light to pass through the sample. On the
other hand, the photodiode array detector passes a wide
spectrum of light through the sample and then the light is
separated into individual wavelengths after passing through
the sample. The spectrum of light is directed to an array of
photosensitive diodes. Each diode can measure a different
wavelength which allows for the monitoring of many
wavelengths at once. Most typically, only one or two
wavelengths are monitored during a chromatographic run.
Monitoring two peaks instead of one can provide information
on the purity of the peak, or it can be used to quantify a
peak when an interfering peak is present.
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