Transcript Instrument
I Instrumentation for UV and visible absorption Lamps • • • • • • Generally need a continuous source Tunable laser would be ideal (not available) Choice depends on wavelength region Visible – Tungsten UV – H2 or Deuterium (~160 -350nm) Visible – Tungsten (~ 350 – 2500 nm) Deuterium (arc) lamp • Low power discharge (100w) through low pressure (~10 torr) of deuterium. • D2 + Ee →D2* → D’ + D’’ + h • As the two atomic species can have a variety of kinetic energies, so the light emitted will be a continuum. Deuterium lamps Tungsten Filament Lamp • Visible and Near Infrared • Filament temperature 2870 K • Stable because of good voltage control Quartz/halogen lamps • Iodine is added • Higher operating temperature (~3500 K) allows higher energy output but requires quartz envelope (melts at higher temp than glass) • W + I2 →WI2 (volatile) • When they hit the hot filament they decompose and release W • Increases lamp life Ruby laser Some atoms emit photons which stimulate further emission Light from flash tube excites ruby atoms Leaves through half-silvered mirror Optical materials • Need light to be able to pass through sample holder, etc. • Visible – glass –strong, cheap • Usually cuts off ~ 360 nm • UV – quartz • Below 200 nm, O2 absorbs – so purge with dry nitrogen (gets you to 160 nm) • lower =vacuum UV Useful transmission rangea for optical materials Material Range fused silica 170 nm - 3.6 μm glass 360 nm - 2.5 μm sodium chloride 200 nm - 15 μm potassium bromide 230 nm - 25 μm potassium chloride 200 nm - 18 μm thallium bromide-thallium iodide 500 nm - 35 μm cesium iodide 230 nm - 50 μm calcium fluoride 125 nm - 9 μm barium fluoride 130 nm - 12 μm lithium fluoride 104 nm - 7 μm sodium fluoride 195 nm - 10.5 μm cadmium fluoride 200 nm - 10 μm lead fluoride 290 nm - 11.6 μm lanthanum fluoride 400 nm - 9 μm magnesium fluoride 110 nm - 7.5 μm aLimits are taken as wavelengths where percent transmittance falls to 60 percent for a 1-cm thickness. Absorption filters • • • • Just in visible region Coloured glass or dye between plates Cheap Cut-off or band-pass Interference Filters • Two transparent plates coated wth partially reflecting metal films • Separated by dielectric materialCaF2 or,MgF2 ( thickness t) • Exiting beams can have travelled extra distances = multiples of 2t • If 2t =n /, constructive interference will occur – orders of that of light will pass through the filter • Smaller bandpass than absorption filters Transmission Gratings • Light interference • Diffraction or reflection Reflection Gratings • Holographic gratings: • 2 collimated beams of light are used to produce interference fringes in a photosensitive material on flat glass. • The light-exposed material is washed away and the grooves are coated with a reflective layer, eg Al Grating normal Monochromatic Beam at incident Angle i CD = extra distance travelled n = CD – AB = d(sini + sinr) CD = dsini AB = -dsinr Grating Characteristics • Resolution: nN wavelength n diffractio n order N Number of grooves illuminate d (typically1200 1400 / mm) The more grooves, the better the resolution Dispersion: n d cos d spacing between adjacent grooves angles in radians n the diffraction order Dispersion is better if the spacing between grooves is smaller Monochromator • Grating and slits • Usually other mirrors as well Slit width The slit width is defined by the bandwidth of radiation it allows through. Resolution of closely spaced bands is achieved at the expense of decreased S/N. Slits should be as wide as possible, but small compared to width of absorbance band Unwanted orders of light • Need a filter to remove these • Always have filter as well as a grating Errors – Stray radiation Po A log P Po Ps As log P Ps As A • Low A – P similar to Po • High A – P is small - low S/N ratio • For most modern instruments, once above a certain concentration, the error is mostly in the cell positioning