Continuum Sources - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Continuum Sources - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! I) CONTINUUM SOURCES 1) Thermal radiation (incandescence) – heated solid emits radiation close to the theoretical “Black Body” radiation i.e., perfect emitter, perfect absorber Behavior of Black Body - Total power ~ T4 therefore need constant temperature for stability when using incandescent sources - Spectral distribution follows Planck’s radiation law docsity.com Spectral Distribution Curves of a Tungsten (Black Body) Lamp At higher temp -> maximum shifts to shorter wavelengths. Low temp good for IR, but visible region requires high temp. UV vis IR docsity.com Visible Region sources are: a) Glass enclosed Tungsten (W) filament - normally operated at ~3000 oK with inert atmosphere to prevent oxidation. Useful from 350 nm to 2000 nm, below 350 nm glass envelope absorbs & emission weak b) Tungsten-Halogen lamps - can be operated as high as 3500 oK. More intense (high flux). Function of halogen is to form volatile tungsten- halide which redeposits W on filament, i.e., keeps filament from burning out. Requires quartz envelope to withstand high temps (which also transmits down to shorter wavelengths). Fingerprints are a problem – also car headlights docsity.com 2) Gas Discharge Lamps – two electrodes with a current between them in a gas filled tube. Excitation results from electrons moving through gas. Electrons collide with gas
excitation
emission At high pressure
“smearing” of energy levels
spectrum approaches continuum The higher the pressure, the greater the probability that any given molecule or atom will be perturbed by its neighbor at the moment of emission. docsity.com a) Hydrogen Lamp - most common source for UV absorption measurements H2 emission is from 180 nm to 370 nm limited by jacket Line spectrum from
100 watt Hydrogen Lamp at low pressure in Pyrex docsity.com d) High Pressure Mercury Lamp – can’t completely eliminate bands associated with particular electronic transitions even at very high pressures (e.g., 100 atm) docsity.com • For UV-vis absorption spectrophotometry usually use H2 for UV and tungsten for visible region (switching mid scan) • Sometimes use D2 instead of H2 • For fluorescence spectrophotometry use xenon arc lamp in scanning instruments • Can use He below 200 nm • Hg at low pressure is used in fixed wavelength (non scanning) fluorometers • Can use mixture of Hg and Xe docsity.com II) LINE SOURCES 1) Gas (Vapor) Discharge Lamps at low pressure (i.e., few torr) – minimize collisional interaction so get line spectrum - most common are Hg and Na - often used for λ calibration - Hg pen lamp - fluorescent lights are another example - also used UV detectors for HPLC 2) Hollow Cathode Lamps (HCL) – for AA 3) Electrodeless Discharge Lamps (EDL) - AA docsity.com Limitations of lasers 1) High cost in many cases 2) Wavelength range is somewhat limited 3) Many operate in pulsed mode – some are continuous wave (CW) Pulsed mode lasers are not always problematic as light sources, can use pulse frequency with gated detection docsity.com Wavelength Selection Three main approaches: 1) Block off unwanted radiation – optical filters 2) Disperse radiation & select desired band – monochromator 3) Modulate wavelengths at different frequencies - interferometer FILTERS 1) Absorption – colored glass, colored film, colored solutions – cheapest way docsity.com