Transcript lecture 2 uv instrumentation

UV instrumentation
Abu Yousuf , PhD
Associate Professor
Department of Chemical Engineering & Polymer Science
Shahjalal University of Science & Technology
Sylhet-3114, Bangladesh
[email protected]
Component of spectrophotometer
Source
Monochromator
Sample compartment
Detector
Recorder
Radiation Sources
Continuum Sources:
-Hydrogen and Deuterium lamp –UV (high pressure
xenon or mercury gas –arc lamp)
–Tungsten filament lamp –VIS (350-2000nm)
- Tungsten halogen lamp
argon,
Line Sources:
–Source emitting discrete lines–mercury or sodium vapor
lamp for UV/VIS
–Hollow cathode and discharge lamps for atomic
absorption and fluorescence
Types of lamp
a) Tungsten filament lamp
Advantages- inexpensive, reliable and stable sources of visible radiation (350-8—nm),
longer life
Limitations- Evaporating tunhsten on the glass envelope reducing the output.
b) Tungsten halogen lamp
Is same as tungsten filament lamp but
the filament in a halogen fitted quartz
envelope.
Advantages- prevents tungsten evaporation
but more expensive than filament lamp.
Tungsten halogen lamp
Tungsten filament lamp
Deuterium lamp
A deuterium arc lamp (or simply deuterium lamp) is a low-pressure
gas-discharge light source often used in spectroscopy when a continuous spectrum
in the ultraviolet region is needed.
•Electrical excitation at low pressure (<0.5 torr) , low voltage
(~40V DC)
•Forms molecular excited state that undergoes dissociation and
Photoemission
•Provides continuum from ~160-380 nm
•it requires separate power supply
•It has shorter life time and expensive to replace
Lamp operational circuit
Line Sources in the UV and Vis
Hollow Cathode Lamp
– Cathode is coated with atom of interest
– Tube is filled with Ar or Ne
– High voltage ionizes gas, charged ions are accelerated
toward electrodes
• Produces sputtering of atoms (ground and
excited)
• Excited atoms emit light at atomic lines
– Design of HCL results in redepostion of metal atoms onto
electrodes - recycling
– Need to avoid excessively high potentials
• Line broadening (Doppler)
• Self-absorption
– Need separate lamp for each element
Wavelength Dispersion and Selection
Most instruments use a monochromator to separate light form the
source into discrete wavelength segments
• Components:
– Entrance slit
– Collimating/focusing device - mirror or lens, nonideal
– Dispersing device -filter, grating or prism
– Collimating/focusing device - mirror or lens
– Exit slit
A monochromator is an optical device that transmits a mechanically selectable narrow band of
wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input.
Collimate: To make parallel; line up.
Diffraction grating
Diffraction grating
Spectroscopy Slit
•Narrow slit have to be applied to improve quantification and
improve absorbance
•Slit below 0.14 mm is not affecting absorbance
•More important than narrow slit is reproducible slit settings
•Decreasing slit width causes reduction of radiation energy to
the power of second order
•Slit have to be set up to the narrowest value when good
resolution of spectrum is achieved.
Sample Considerations
Several possible fates for photon
– Reflection
– Scattering
– AbsorptionAbsorption
• Choose cell and sample composition carefully.
• “Match”
Detectors for UV-VIS
Photon Transducers: Convert photon energy to electrical signal
(current, voltage, etc.)
Detectors based on photoelectric effect:
• Detectors based on photoelectric effect:
Phototubes, Photomultiplier tubes
• Phototube:
– Incident photon causes
release of an electron
– Photocurrent α P
– Not best for low-light
scenarios
A photoemissive cell, commonly known as a
phototube, makes use of the photoelectric effect, the
phenomenon whereby light-sensitive surfaces give off
electrons when struck by light.
Detectors for UV-VIS
Photomultiplier:
– Ejected photoelectron
strikes dynode, secondary strikes dynode, secondary e- released
– Voltage accelerates e- to next dynode and so on
• big voltage divider
– Result is large charge packet hitting anode
• High Gain
Dynode an electrode onto which a beam of electrons can fall, causing the emission of a
greater number of electrons by secondary emission. They are used in photomultipliers to
amplify the signal.
Detectors for UV-VIS
• Semiconductor-based detectors
– Photodiodes, Photodiode arrays, CCD, CID
A photodiode is a type of photodetector capable of
converting light into either current or voltage,
depending upon the mode of operation
semiconductor detector
• Photodiodes and Photodiode Arrays:
-Reverse biased junction
– Photons produce e- - hole pairs → current
– Current α Plight
• less sensitive than PMTs but they are small
and robust
When a photon strikes a semiconductor, it can promote an electron from the valence
band (filled orbitals) to the conduction band (unfilled orbitals) creating an electron(-) hole(+) pair. The concentration of these electron-hole pairs is dependent on the amount
of light striking the semiconductor, making the semiconductor suitable as an optical
detector.
Detectors for UV-VIS
Forward Biased Junction:
Reverse Biased Junction:
When the battery is connected as shown, the positive terminal of the battery attracts
negative electrons away from the barrier. The negative terminal attracts holes away from the
barrier. The insulating barrier widens and no current flows. The junction is REVERSED
BIASED.
If the reverse voltage is made high enough, then the junction will break down and electron
current will flow from anode to cathode (under normal conditions, current flows from
cathode to anode, when forward biased).
– Photodiode Arrays: PDA
• Assembly of individual photodiodes on a chip
• Each diode can be addressed individually
• Experiment is set up so that monochromator disperses light across PDA,
With a small # of diodes per wave lengtht
• allow simultaneous collection of all wavelengths
A photodiode array (PDA) is a linear array of discrete photodiodes on an integrated circuit
(IC) chip. For spectroscopy it is placed at the image plane of a spectrometer to allow a
range of wavelengths to be detected simultaneously. In this regard it can be thought of as
an electronic version of photographic film. Array detectors are especially useful for
recording the full uv-vis absorption spectra of samples that are rapidly passing through a
sample flow cell, such as in an HPLC detector.
Instrument Assemblies
The wavelength selector is a filter and the detector is usually a photovoltaic
Single Beam Instruments
cell or a vacuum phototube. The source is a tungsten halogen lamp.
Two sources are used, a tungsten
halogen and a deuterium lamp where
a mechanism for source selection
should be available. The wavelength
selector is a grating or prism
onochromators and the detector is
usually a vacuum phototube or a
photomultiplier tube in higher cost
instruments.
Instrument Assemblies
•UV-Vis Double Beam Spectrophotometer (in space)
Two sources are existent in a double beam UV-Vis
configuration namely a tungsten halogen lamp and a
deuterium lamp and a lamp selection mechanism is present.
The beam from the source is split into two beams by a beam
splitter which is a semipermeable mirror. The detector is
usually a pair of photomultiplier tubes connected to a
difference amplifier and the wavelength selector is a grating
or prism onochromators
UV-Vis Double Beam Spectrophotometer (in time)
Two sources are existent in a double beam UV-Vis
configuration namely a tungsten halogen lamp and a
deuterium lamp and a lamp selection mechanism is
present. The beam from the source is split into two
beams by a chopper and the transmitted beam is passed
to the detector through a beam splitter; which is a
semipermeable mirror. The detector is usually a
photomultiplier tube and the wavelength selector is a
grating or prism onochromators.
Time separated double beam
Thanks to all