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GC Analysis of Organic Liquid Mixtures: Identifying Components and Quantification, Lab Reports of Chemistry

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.

Typology: Lab Reports

2020/2021

Uploaded on 05/12/2021

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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:
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.
pf3
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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:

  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.
  1. Make sure the syringe is clean. This can be done by flushing the syringe with a sample of acetone, moving the plunger up and down several times to force out any residual acetone. Then flush the syringe 3-4 times with your sample.
  2. Next the retention time of the standard will be established. Since your unknowns were made from the following compounds, each of these compounds will need to be injected, one at a time, into the GC and the retention time calculated.

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.

  1. Introduce the sample-air mixture onto the chromatography column by pushing only 2/3rd^ of the needle into the injection port (through a rubber septum) and quickly, but smoothly, injecting the sample into the instrument. During this operation hold your thumb on the plunger since the positive helium pressure in the instrument may push the plunger and the sample out of the syringe.
  2. Immediately push the start button on the integrator.
  3. While the chromatogram is being traced, wash the syringe with acetone 4- times, being sure to dry the syringe by moving the plunger up and down. Place the syringe back on the bench, ready for the next person to use.
  4. Label the chromatogram with the conditions used and the name and amount of the sample analyzed.
  5. On the chromatogram, the integrator will print retention time on each peak (in the unit of minutes). Wait until all peaks have emerged, then push stop button. The integrator will print a summary of all peaks monitored. For the standards, you would only need the corrected retention time of each pure compound (retention time of compound minus retention time of the air peak which is the first peak, assuming that enough air has been injected)
  6. After the retention times of all standards have been measured, you are ready to analyze your unknown. Follow the same procedure as above when injecting your unknown sample (the distillate from the previous experiment).
  7. The integrator will print a summary result for your unknown. In this case, you would want to calculate the corrected retention time (that of each peak minus the retention time of air) of each component and compare them to the standards. A match in retention time allows you to determine their identity. To quantify each component, take the area of that component and divide that by the total area of all components that are of interest to you. (i.e. excluding the area of air, and solvent such as acetone or ether) This gives the relative percentage of that component, and the sum of the relative % of all interested components should total 100%.

Lab Report for “Analysis of a liquid organic mixture by GC”

Name: Section:

  1. Which of the following variables will double in quantity if twice as much sample was injected into the GC operating under the identical conditions? (Circle one)

(A) The height of the peaks (B) The width of the peaks (C) The area under each peak (D) None of the above

  1. What is the purpose of injecting air into the GC along with the sample?
  2. The retention time of a compound on a GC chromatogram depends upon (circle all that are true)

(A) Carrier gas flow rate (B) Length of the GC column (C) Column temperature (D) Boiling point of the compound

  1. Complete the following tables:

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

  1. Are the identity assignments of GC consistent with that of b.p.? Discuss the success of your distillation in terms of its separation quality based on the relative % above. (use back of this page)