# UV-Vis Spectrophotometers¶

Like most spectroscopy, UV-Vis respects the beer lambert law and its limitations, Including:

• Linearity breaks down when a solution is too concentrated
• As intermolecular distances decrease, the charge distribution of the molecule changes and so does $$\varepsilon$$
• Linearity can break down when a reaction is taking place
• Something as simple as protonation/deprotonation of the analyte with the solvent can drastically change the linearity of the sample.
• As discussed in Monochromator, it is practically impossible to select an individual wavelength of light from a broad source, so any
• While increasing the path length can negate a lower concentration solution, it can also increase scattering, causing a loss of linearity to occur

## The UV-Vis Spectrophotometer¶

There are four types of UV-Vis spectrophotometer:

#### Single-beam¶

• These are the most basic, 0% T is read with the shutter closed 100% with the shutter open and no sample
• The blank spectrum must be recorded first and is subtracted from the measure spectrum
• Can be slow to operate, requires more intervention

#### Double-beam in space¶

• The monochromatic beam is split with a beam splitter and one beam passes through the sample while the other passes through the blank
• Two photodetectors are utilised and the difference between the two is compared

#### Double-beam in time¶

• The double-beam in space has the disadvantage of using two different photodetectors, each with their own response curves and tolerances. It also assumes that the beam splitter will allow exactly 50% of the light in either direction
• The double-beam in time physically changes the beam to pass through the blank, then the sample with a certain periodicity using a rotating shutter
• The photodetector knows this periodicity and will take intermittent readings for the blank and sample individually

#### Multi-channel¶

• Multi-channel instruments are Photodiode Array based and will measure the entire spectrum at once with whatever accuracy is provided by the array utilised

The lamps usually consists of two types of bulbs

#### Tungsten¶

Provides full coverage of the visible band (350 - 800 nm)

#### $$\ce{H2}$$ or $$\ce{D2}$$¶

Used for the UV range (160 - 350 nm)

An alternative for high intensity spectroscopy is to use a xenon lamp (200 - 1000 nm), however these are expensive and short-lived

## The sample holder¶

• Must be completely transparent to the wavelengths being measured, as such quartz is used rather than glass
• They can come in a variety of path lengths to accommodate various concentrations of solutions

## Photometers vs Spectrophotometers¶

• Photometers can only measure light at a single wavelength using filters, however are cheap and have a high energy throughput
• Spectrophotometers use a monochromator to scan through a range of wavelengths making them more versatile, and more expensive. Complex optics give them a lower S/N

## Modern Instruments¶

#### UV Photometers¶

• Use a mercury lamp and filter specifically for the 254 mm emission reading
• Useful for identification of organic molecules, since they absorb around this region

#### Probe type photometers¶

• Use fibre optics to transmit and receive the signal
• Removes the need for a sample cell
• Can be used for identification in solid-phase chemistry