hn1 hn2 optical l selector source sample detector signal processor Instrument components: UV-VIS hn1 hn2 optical source l selector sample detector signal processor

a liquid sample is usually contained Instrument components: UV-VIS sample containers a liquid sample is usually contained in a cell called a cuvet that has a flat material fused silica glass  only Vis quartz

Common cuvets for visible and ultraviolet spectroscopy. Instrument components: UV-VIS Common cuvets for visible and ultraviolet spectroscopy.

Wavelength selectors 1. Filter 1.1 Absorption filter Instrument components: UV-VIS Wavelength selectors 1. Filter 1.1 Absorption filter 1.2 Interference filter 2. Monochromators

“a wavelength selector that uses either Instrument components: UV-VIS Filter “a wavelength selector that uses either absorption, or constructive and destructive interference to control range of selected wavelengths” the simplest method for isolating a narrow band of radiation

work by selectively absorbing radiation from a narrow region Instrument components: UV-VIS Absorption filters work by selectively absorbing radiation from a narrow region Interference filters use constructive and destructive interference to isolate a narrow range of wavelengths

effective bandwidths, range 30-250 nm Instrument components: UV-VIS Absorption filter coloured glass effective bandwidths, range 30-250 nm

Colour of visible light Instrument components: UV-VIS Colour of visible light Wavelength of maximum Absorption (nm) Colour absorbed Colour observed 380-420 violet green-yellow 420-440 violet-blue yellow 440-470 blue orange 500-520 green purple 520-550 yellow-green violet 550-580 yellow violet-blue 580-620 orange blue 620-680 red blue-green 680-780 purple green

Instrument components: UV-VIS

Two filters can produce narrower band.

Interference filters consist of a transparent dielectric (CaF2 or MgF2 that occupies the space between two semitransparent metallic films

Monochromators: components entrance slit lens or mirror to produce a parallel beam dispersion elements (prism or grating) to disperse the radiation into its components wavelength focusing lens or mirror exit slit

Prism: (many older instruments) shaped like a bar with a triangular cross section disperse incident radiation according to wavelength (see figure) the degree to which light is bent depends on the refractive index of light passing (in a glass prism) and form glass to air the refractive index varies with wavelength; hence different wavelength can be separated

Diffraction Grating: (most modern instrument)

it is coated with aluminum to make it reflective on top of the aluminum is a thin protective layer of silica (SiO2) to prevent the metal surface from oxidizing, which would reduce its reflectivity when the light is reflected from the grating, each groove behaves as a source of radiation when adjacent light rays are in phase, they reinforce one another when they are not in phase, they partially or completely cancel one another

Path of light diffracted by a grating

Resolution required to separate two lines of different wavelength the ability to disperse radiation is called resolving power Ex: In order to observe an absorption band at 599.9 nm without interference from an absorption band at 600.1 nm, we must be able to resolve, or separate, the two bands.  resolving power (R) =?

R = nN Resolution of grating The resolving power of a grating is given by R = nN where n = the order N = the total number of grooves in the grating

Ex: Suppose that we can obtain a grating with 500 line/cm. How large a grating would be required to separate the sodium D line at 589.5 and 589.0 nm in first order?

But the grating contains 500 lines/cm Soln. R = nN ; n = 1 R = N = 1178.5 But the grating contains 500 lines/cm Thus, long of grating = 1178.5/500 = 2.357 cm

Ex: How many lines per centimeter must be cut on a grating 3 cm long to resolve the same sodium D lines? The required resolution is 1178.5 nN = 1178.5 If n = 1, N = 1178.5 lines Thus, N/cm = 1178.5/3 = 392.83 = 393 lines/cm or 1179 lines on the entire grating

Detectors (UV-VIS) (lecture notes) Photo cell

Photomultiplier tube

Photodiode Array (PDA)

Spectrophotometers