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The procedure for analyzing organic liquid samples using gas chromatography (gc). The experiment aims to identify the components in a mixture and determine their relative ratios by measuring their retention times and comparing them to those of standards. The document also covers the use of a gow-mac 350 gc with a thermal conductivity detector and electronic integrators for peak identification and quantification.
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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:
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-propanol, 2-methyl-1-propanol, 1-butanol, toluene, p-xylene, isopropylbenzene, and mesitylene.
Each group will now inject a pure sample of the above into the GC. Pull in 5- ul of air into the syringe, then draw 0.5-1 ul of the pure sample. After the appropriate amount of sample is in the syringe, pull back the plunger to draw in an additional 5- 10 uL of air. The air peak is carried along rapidly by the carrier gas and acts as a marker for calculating retention times. Each group will take turn to inject a different pure sample until the retention time of all 7 pure samples has been obtained.
Lab Report for “Analysis of a liquid organic mixture by GC”
Name: Section:
(A) The height of the peaks (B) The width of the peaks (C) The area under each peak (D) None of the above
(A) Carrier gas flow rate (B) Length of the GC column (C) Column temperature (D) Boiling point of the compound
Standards Retention Time of standards Corrected Retention Time 1-propanol 2-methyl-1-propanol 1-butanol toluene p-xylene isopropylbenzene mesitylene
First Fraction Retention Time Identity Relative % Compound 1 Compound 2
Second Fraction Retention Time Identity Compound 1 Compound 2