A technician at a local waste disposal company was trying to develop a separatio

Question

A technician at a local waste disposal company was trying to develop a separation of substituted
aromatic compounds present in industrial sewage by normal phase adsorption chromatography. He first
tried separating chlorobenzene, nitrobenzene, toluene, and xylene on a Dupont Zorbax® silica column
using dichloromethane as the mobile phase and a fixed wavelength UV detector set at 254 nm. While
chlorobenzene and nitrobenzene were adequately resolved on the silica column, toluene and xylene were
found to coelute. Being an expert at liquid chromatography, he realized a less polar mobile phase solvent
would improve the separation of toluene and xylene. When carbon tetrachloride was used as the mobile
phase eluent, none of the solutes injected were detected by the UV detector. Puzzled, he analyzed the
effluent from the hplc system by gas chromatography and discovered that the four solutes had in fact
eluted from the column. After closely reading the labels of the two solvent bottles (see below), the
technician realized his mistake. What error of mobile phase selection did he make?

Explanation / Answer

Species:

chlorobenzene, nitrobenzene, toluene, and xylene

Stationary phase = Dupont Zorbax® silica

Mobile phase (1) = dichloromethane

Mobile phase (2) = carbon tetrachloride

Note that the wavelength we are using will only work for dichloromethane, since it is set for that specie, i.e. WL = 254 nm

For, carbon tetrachloride, the wavelength used (UV light) will not work, since the detetor will not "read" the signals

Since the chromatography is being runned anyways of detector, you will expect elution of all species (polar + nonpolar elutions)

Therefore, he should run again + use the correct wavelength set for CCl4

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