Answer to Question #174707 in General Chemistry for Mitra

Question

About Molecular spectrophotometry 
 a) What is an absorbance spectrum and how do you measure one. Brief answer. 

b) You must determine the content of a solution with regard to the UV absorbing component Phenol. Briefly describe how the measurement should be done in practice. Focus on (I) selection of solvent (II) selection / handling of sample cuvettes (III) selection of wavelength and (IV) selection of intervals for the absorbance measurement, to minimize the measurement error. 

c) Diode array instruments have started to replace the traditional UV / VIS instruments in many laboratories. What is the main difference between these instruments? State an advantage and a disadvantage of diode array instruments compared to the conventional UV / VIS instrument. Motivate!

Answer 1

a) Absorbance spectroscopy is a molecular spectroscopy method that uses the wavelength dependent absorption characteristics of materials to identify and quantify specific substances. The absorbance of a solution increases as the attenuation of the optical beam increases.

b) Spectrophotometric measurements were made with a HP 8154 A spectrophotometer, using 1 cm quartz cells. Chromatographic determinations were carried in a Waters equipment, using a C18 column, a methanol:water (30:70 v/v) mobile phase and a spectrophotometric detector (252 nm).

c) 1. The UV/Vis detector can detect single wavelength at one time while the DAD can scan a wavelength range (i.e. 190-800nm). DAD is actually a UV/Vis detector it has an array of numerous photodidoes.

2. DAD will scan all the compounds which can respond in the given wavelength range (say 190-800 nm) there is no need of any adjustment. It will adjust by itself.

3. Both these detectors are equally easy to operate or interpret. But DAD is better than UV/Vis because it can scan whole 190-800 nm region, on the other hand, UV/Vis can scan a single wavelength only. so if you want to scan a wavelength range DAD is better option. But if you know the absorbance wavelength than you can detect that single wavelength by using UV/Vis detector.

In method development, DAD is a better choice because UV/Vis detector is very time-consuming. So for development a new method, I strongly recommend you to use DAD.