Presentation on theme: "UV instrumentation Abu Yousuf, PhD Associate Professor Department of Chemical Engineering & Polymer Science Shahjalal University of Science & Technology."— Presentation transcript:
UV instrumentation Abu Yousuf, PhD Associate Professor Department of Chemical Engineering & Polymer Science Shahjalal University of Science & Technology Sylhet-3114, Bangladesh
Continuum Sources: -Hydrogen and Deuterium lamp –UV (high pressure argon, xenon or mercury gas –arc lamp) –Tungsten filament lamp –VIS ( nm) - Tungsten halogen lamp Line Sources: –Source emitting discrete lines–mercury or sodium vapor lamp for UV/VIS –Hollow cathode and discharge lamps for atomic absorption and fluorescence Radiation Sources
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 ~ 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 Collimate: To make parallel; line up. 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.
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. Spectroscopy Slit
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: 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.
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 Detectors for UV-VIS 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.
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 Photodiodes and Photodiode Arrays: -Reverse biased junction – Photons produce e- - hole pairs → current – Current α P light 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. semiconductor 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). Detectors for UV-VIS
– 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.spectroscopyspectrometeruv-vis absorption spectraHPLC
Single Beam Instruments The wavelength selector is a filter and the detector is usually a photovoltaic cell or a vacuum phototube. The source is a tungsten halogen lamp. Instrument Assemblies 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.
Time separated double beam Instrument Assemblies UV-Vis Double Beam Spectrophotometer (in space) 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 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 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.