UVVis Absorption Spectroscopy Lecture 25 1 4 Multichannel

UV-Vis Absorption Spectroscopy Lecture 25 1

4. Multichannel Instruments Photodiode array detectors used (multichannel detector, can measure all wavelengths dispersed by grating simultaneously). Advantage: scan spectrum very quickly “snapshot” < 1 sec. Powerful tool for studies of transient intermediates in moderately fast reactions. Useful for kinetic studies. Useful for qualitative and quantitative determination of the components exiting from a liquid chromatographic column. • • • 2

Multi-channel Design 3

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A multichannel diode-array spectrophotometer, the Agilent Technologies 8453. 5

Location of Sample cell In all photometers and scanning spectrophotpmeters described above, the cell has been positioned after the monochromators. This is important to decrease the possibility of sample photodecomposition due to prolonged exposure to all frequencies coming from the source. However, the sample is positioned before the monochromator in multichannel instruments like a photodiode array spectrophotometer. This can be done without fear of photodecomposition since the sample exposure time is usually less than 1 s. Therefore, it is now clear that in UV-Vis where photodecomposition of samples can take place, the sample is placed after the monochromators in scanning instruments while positioning of the sample before the monochromators is advised in multichannel instruments. 6

Probe Type Instruments. 5 These are the same as conventional single beam instruments but the beam from the monochromators is guided through a bifurcated optical fiber to the sample container where absorption takes place. The attenuation in reflected beam at the specified wavelength is thus measured and related to concentration of analyte in the sample. A fiber optic cable can be referred to as a light pipe where light can be transmitted by the fiber without loss in intensity (when light hits the internal surface of the fiber at an angle larger than a critical angle). Therefore, fiber optics can be used to transmit light for very long distances without losses. A group of fibers can be combined together to form a fiber optic cable or bundle. A bifurcated fiber optic cable has three terminals where fibers from two separate cables are combined at one end to form the new configuration. 7

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Fiber optic probe 11

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6. Double Dispersing Instruments The instrument in this case has two gratings where the light beam leaving the first monochromators at a specified wavelength is directed to the second grating. This procedure results in better spectral resolution as well as decreased scattered radiation. However, double dispersing instruments are expensive and seem to offer limited advantages as compared to cost; especially in the UV-Vis region where exact wavelength may not be crucial. 14

Optical diagram of the Varian Cary 300 doubledispersing spectrophotome ter. A second monochromator is added immediately after the source. 15

Applications of UV-Vis Spectroscopy 16

The Molar Absorptivity We have seen earlier that validation of Beer’s law is dependent on the nature of the molar absorptivity. It was found that the molar absorptivity is influenced by: 1. The wavelength of radiation 2. The refractive index and is thus indirectly related to concentration 3. Electrostatic interactions taking place in solution; and thus electronic distribution 4. In rare cases, like methylene blue, the molar absorptivity is directly dependent on concentration 17

The molar absorptivity , however, is supposed to be constant for Beer’s law to be valid. The molar absorptivity is a measure of the ability of an analyte to absorb light at a specified wavelength. Therefore, the value of the molar absorptivity is crucial for the sensitivity of an analysis. The value of the molar absorptivity can usually range from zero (for a nonabsorbing species) to 105 for highly absorbing species). For quantitative analysis, a value of at least 103 is necessary for a reasonable quantitative analytical determination of an analyte. 18