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The UV-vis Spectrophotometer The UV Vis Spectrophotometer (or sometimes called the UV-Vis "spectrometer") is a simple piece of instrumentation, is very sensitive and reasonably cheap (Modern entry level devices can be purchased for around £3,000, ($5800 USD)). The workings of the device is shown below, detailing two types of double beam systems (double beam systems are very common).
Fig.1: A double beam double-detector system. Although it looks confusing, its a simple device, and is very similar to the IR Spectrophotometer, with A and B being bulbs (A is a deuterium bulb, for UV range radiation, and B is a tungsten bulb for the visible range). When a device scans a range of frequencies it simply uses one bulb for visible, then switches to the other bulb for the UV. The beam is focussed onto a mirror (C) that sends a beam through a slit (D) to make the beam narrow. The beam then hits the diffraction grating (E). This grating is a simple block that sends off different frequencies of light at different angles (like a prism). A motor rotates the grating, so that the frequency you want is refracted down to the rest of the system, so when a scan is performed the grating is simply rotated smoothly to diffract the separate wavelengths through the rest of the system one by one. The required wavelength of radiation is sharpened through another slit (F), and then past through a filter (G) which removes any wide angle diffractive radiation. The beam is then bounced off of a mirror through a chopper mirror (H), which works at outlined below (Fig.2) This splits the beam equally between the blank ("reference") cell (I) and the sample cell (J). The collected beam is focussed through a lens (K) and collected at detectors (photomultiplier tubes or PMT, one for each beam, L). The information is computed at the workstation M and the spectrum is formulated.
Fig.2: a chopper "semi"- mirror A simpler version of the above set up is commonly used as demonstrated below, whereby only one detector (a photomultiplier tube or PMT) is required:
Fig.3: a double beam-single detector array A and B are the bulbs passing a through the monochromator (the diffraction grating, C), then through a slit (D) into the chopper mirror (E). You can see that this method only needs one detector, as it repetitively sends the beam through both the reference cell (F) and the sample cell (G), which by a series of mirrors both are incident on the same PMT (H). The workstation (I) runs a timing algorithm to add up the reference and the sample responses separately, and deducts the reference from the sample to give the UV-vis spectrum. A single beam device works much the same way, with slits, mirrors and diffraction gratings just as a double beam device does, but does not have a chopper mirror or two cell slots. In this case, the reference (blank) spectrum for the solvent is taken separately before the sample. The workstation stores the reference spectrum, and then deducts it from the sample spectrum when the sample spectrum is taken, to give the sample UV-vis spectrum.
A Double Beam UV-Vis spectrophotometer with its sample compartment open. This system has a display so you can take absorbance readings off the actual device, but in our labs it is hooked up to a PC workstation, and printer so full scanning plots can be generated. We have several of these slightly dated devices, but due to excellent care from our technical team, are still in perfect working condition and are used routinely by undergrad lab classes and researchers alike.
Right: This is a pair of optically matched quartz cuvettes. They are exactly 10.00mm i.d. so the path length is exactly 1cm. As they are machined to be optically identical (as needed when using a reference and a sample cell simultaneously), they can range in price from £50 to £400 each (£100 to £800 per matched pair). |
Left:
This is the sample compartment, where you can see the two cell
slots. A false image beam is added so you can see where the UV
radiation passes.