with use of PDA detector, we can measure the area or height
of particular peak at different wavelengths ranging from
200 to 800 nm by injecting the solution at once. Where as
in uv detector we can measure the area or height of the
peak only at two different wavelengths. But that
wavelengths also to be selected before injecting the
Including above answers, the main difference between PDA &
UV detector is that we can calculate peak purity in PDA
detector only which helps us to confirm that the only
single impurity comes at perticular retention time.
in ultra violet detector, you cn only analyse the compound
at particular wavelength you selected but in photo diode-
array detector, you can select multiple of wavelength due
to presence of polychromator (wavelength between 190-380nm)
and hence peak purity identification is also carried out
due to this property.
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
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.
In acid base titration, after complet consumption of HCl by
NaOH, addition of next drop of NaOH will give pink colour
with phenolpthalein indicator. Why this pink colour appear?
Why not blue, black, green or anything elese?