Analysis of a Liquid Organic Mixture by Gas Chromatography”
Introduction
In this experiment, liquid organic samples previously distilled will be analyzed on a
Gas Chromatograph (GC), which allows for the establishment of the identity of the
components in the mixture as well as the quantity of each component.
Objectives:
1. Learn how to operate a Gas Chromatograph.
2. Identify the components of a liquid sample by GC and determine their relative ratio.
Required Reading: Pavia Chapter 22
In order to analyze and quantify the components of liquid samples obtained from the
previous distillation, it will be necessary to find the retention times of each compound in the
mixture and compare them to that of standards (pure compounds). For example, if you
suspect the unknown may contain 1-propanol and toluene, then the retention times for 1-
propanol and toluene will need to be measured. This can be done by injecting a sample of
pure 1-propanol and pure toluene separately into the GC column and then noting the time
required for a peak to appear on the chromatogram (used to describe the graph of a GC run)
relative to the retention time of the air peak. You then analyze your liquid mixture and find
out the retention time for each component (assuming they are separated after going through
the GC column). Since the retention time should be a constant given a set of instrument
condition over a short period of time (let’s say within 2-3 hours), the identify of each
component can be assessed by matching the retention time of unknown to the standards. In
a strict sense, this method of identification is only possible if you know the unknown is from
a limited number of probably compounds (such as the list given at the end of the distillation
experiment from which your unknowns were made). If one has no such prior knowledge,
then another instrumental or chemical method may be required in order to positively identify
the unknown. To determine the quantity of each component in the mixture, it will be
necessary to integrate the area under each peak. Several methods of obtaining the area for
each peak are discussed in Pavia’s book. For our experiment this week, we will use a
GOW-MAC 350 GC connected to electronic integrators that keep track of each peak’s
retention time and its area.
Experimental Procedure:
Components of a gas chromatograph – There are two columns in our GOW-MAC 350
GC; column A has a polar stationary phase and column B has a non-polar stationary phase.
The oven may be heated to as high as 200 °C. The carrier gas used is helium. GOW-MAC
350 is equipped with a thermal conductivity detector (TCD) which gives signal based on the
amount of heat carried away by the sample. All the settings have been fine-tuned by your
instructor for each particular experiment, so there is no need for further adjustment on your
part.
1. Prior to an analysis make sure the instrument is turned on and equilibrated and
that the integrator’s light indicates that it is ready to plot signal.
