Q and A on the details of UV-VIS Instruments
Discussing details of UV-VIS instruments, including the best cell type for UV work, classic double-beam configuration design, the lamp producing UV part of the source, grating types, operational aspects like detector biasing, amplification in PMT, and practical limitations affecting UV-VIS scans.
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Presentation Transcript
Q and A on the details of UV-VIS Instruments Which cell type is best for UV work ? quartz Name of classic double beam configuration design ? Which lamp produces the UV-part of the source in most UV-VIS instruments ? Czerny-Turner Deuterium lamp Source monochromator sample and reference- slits echellette ????? chopper PMT or photodiode detector ?????
Q and A on the details of UV-VIS Instruments (cont.) Name of old school grating (flat, high scatter) Echellette type grating Name of new school grating (low scatter, focusing) Concave holographic (sinusoidal or focusing) grating What does a UV instrument vary in d(sin in + sin out ) =n to scan ? angle in In the same equation above, what practical reason argues against varying out ? Want detector position to be fixed at out
Q and A on the details of UV-VIS Instruments (cont.) About what amplification (gain) occurs in a typical PMT ? ~100,000-1,000,000 (105 -106) Photodiodes in UV visible instruments are operated under ?????? bias. reverse 1) Linear response to light flux 2) Low capacitance=> fast response For what two reasons is the above biasing done ? Technical term for filtering out all but signals repeating at the chopper motor s turning rate (rpm) Phase locking
Q and A on the details of UV-VIS Instruments (cont.) What practical limitation sets the lower limit of a UV-VIS scan at 180 nm ? Quartz cells start to absorb at 180 nm and down What is the usual line count/mm of a UV-VIS monochromator s grating ? 1000-3000/mm What is the equivalent groove spacing in nm the above range of lines implies? Name two advantages of the holographic focusing grating design. 0.1 mm/(1000-3000) = 1000-330 nm 1) Allows higher intensity of light 2) Corrects spherical aberration issues connected with old style gratings (reduces focus problems)
Q and A on the details of UV-VIS Instruments (cont.) Wavelength range of most double beam instruments ? Gas phase uv-vis spectra are ??????? spectra? solution phase uv-vis spectra are ??????? spectra? If I is the output optical signal and Io is the incident optical signal, what is %T ? 180-1100 nm Line (discrete) spectra Broad band spectra %T= 100*I/Io What atomic level physical event creates UV-VIS absorption ? Electronic transitions between electron orbits
