pratica espectometria

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CHM2922 – GROUP ACTIVITY 1: INTRODUCTION TO QUANTITATIVE
EXPERIMENTATION (GC)
MEASURING THE ALCOHOL CONTENT OF WINE BY GAS CHROMATOGRAPHY
OBJECTIVES:
1. To use a Gas Chromatograph to determine the amount of alcohol in a wine sample*
.
2. To investigate the basis of Gas Chromatography and understand how a GC instrument works*
.
3. To report the collected experimental data scientifically.
*
Group activity
INTRODUCTION:
Analytical Chemistry usually involves operating a scientific instrument to analyse a sample
qualitatively or quantitatively. In this prac exercise, the sample is a wine extract and you are being
asked to determine its alcohol content. As an analytical chemist you should be able to i) operate the
instrument, ii) understand the basics of its operation, and iii) report the experimental data clearly
and meaningfully. Each of these tasks requires an understanding of the other two in order to be
performed satisfactorily. Completing the tasks in this exercise will help you gain some of the skills
needed to perform more in depth qualitative or quantitative analytical analyses over the course of
the semester.
You should already have been divided into groups. Parts I and II can be performed in either order.
Part I involves using the instrument. Every student should have a go at injecting a sample.
Demonstrators are available to help. Part II involves investigating some Group Research and
Discussion questions, with guidance from your demonstrators. Part III requires you to complete
and submit a report using your Group’s collected data. This report is to be completed individually
and handed in before you leave. It will form your individual assessment for the exercise.
PART I USING THE GAS CHROMATOGRAPH
PROCEDURE:
Make an injection of 1 µL of pentane to determine tM, (you will investigate what tM means in Part
II). Ensure that the syringe is quickly inserted into the septum, and injected very rapidly. Press the

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START button on the GC at the same time that you perform the injection. Press the STOP button
when the peak has printed on the integrator chart. Make sure the peak has been integrated on the
chart recorder and a retention time (tR) in minutes has been recorded.
Rinse the syringe with ultrapure water after every set of injections, and rinse the syringe with three
lots of new sample before injecting another sample.
Make 1 µL injections of pure ethanol, and make sure that a retention time, tR has been recorded. As
you will find in Part II, unknown peaks in the sample chromatograms can be identified using these
tR values.
Make triplicate injections of wine sample (the ‘unknown’ ethanol) and quality control (QC) samples
provided. Ensure the peak areas for the ethanol peaks have been recorded.
PART II INVESTIGATING GAS CHROMATOGRAPHY AND THE GC INSTRUMENT
In Part I you used the Gas Chromatograph instrument to measure alcohol in a sample of wine. In
Part II you will spend about 45 minutes researching and discussing the principles of Gas
Chromatography, how the instrument works, and what results need to be reported and why.
RESOURCES: You can use reference books (see the Reference section below), the web (you will
probably need to look up more than Wikipedia, although it may be a good place to start), and the
lab supervisor and demonstrators (who may clarify questions but won’t necessarily provide all the
answers).
PROCEDURE: Discuss the following questions with your demonstrator as a group, then break up to
research one or other question (in pairs, threes or groups depending on numbers). After about 45
minutes get back together and report and discuss your answers with the other group members and
your demonstrator.
QUESTIONS:
Question 1. A Gas Chromatograph separates complex mixtures of compounds (in this case a wine
sample) into individual compounds (here it is ethanol which is of interest) allowing them to be
identified and measured.
Investigate:
Principles of the procedure: How does the Gas Chomatograph function to separate compounds?
In particular, what is a capillary column and how does it work to separate a mixture’s components?
What physical properties of the compounds are used as a basis for the separation? What is a
stationary phase and a mobile phase? Does it matter whether the samples for analysis are solids,

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liquids or gases? How is the sample introduced into the instrument, and once introduced where and
how does it pass through the instrument? What is being detected when a peak appears on the
printout? How is it being detected? Errors: You carried out multiple injections to help evaluate
errors. How are errors reported? What aspects of the procedure that you have been investigating
might contribute to these errors and how?
Question 2. When you perform an experiment you must be able to interpret, analyse and report the
collected data in a way which tells how successful the experiment has been. In this experiment you
have injected a mixture (wine) into a Gas Chromatograph which - we hope - has separated it into its
individual compounds (ethanol is what we are most interested in). The compounds should appear as
single peaks on a chromatogram with areas corresponding to the amount of compound present.
Investigate:
Quantifying the procedure: What is the tM (retention time for unretained material) and tR
(retention time of the solute), and why are they important? How can they be measured from the
chromatograms you have collected? What is the capacity factor, k', and how can it be measured
using your chromatograms? Finally, what is the definition of resolution, R, and how can you
determine it from your collected chromatograms? Errors: You carried out multiple injections to
help evaluate errors. How are errors reported? How do errors in the parameters you have been
investigating affect the confidence you have your identification and quantification of alcohol in
wine? What does injecting pentane and the Quality Control (QC) tell you about your analysis?
PART III ANALYSING AND REPORTING RESULTS
Once you have performed an experiment (Part I), understand the basis of how the instrument
operates and which data to analyse and how (Part II), you must be able to provide an easily
understandable report which contains the relevant information and results. A proforma has been
provided to help you do this (see the Report section below). You should report the following:
Elution parameters
Record the following information in table form: tM, tR and t'R for ethanol and acetone respectively.
Calculate k' for both ethanol and acetone. Are these within acceptable limits? Using the tR values
and estimated peak widths for the acetone and ethanol peaks to determine the resolution, R.

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Ethanol quantification-calibration method.
In a table, record the peak areas for triplicate runs of the unknown sample and the QC. Use the
appropriate calibration data (available on Moodle) to determine the concentration of ethanol in each
sample. Report the average mean ethanol concentration (%v/v), standard deviation and the % RSD
for each sample.
REPORT: For this write up, use the ‘Prac 1 Urforma’ provided, downloadable from Moodle.
This urforma is a shortened version of the proforma for writing up a full practical report. The
urforma contains additional information and prompts to help you write up the prac.
You should write up your report and hand it in before you leave the laboratory. This will ensure that
you have feedback regarding your prac writing by the following week for when you start writing
full prac reports.
REFERENCES
Background Reading: Skoog, D.A., Holler, F.J., and Crouch, S.R. (2007). 'Principles of
Instrumental Analysis' (Thomson, CA) 1039 pp
Method : Williams S. (Ed.) (1984). ‘Official Methods of Analysis of the Association of Official
Analytical Chemists (AOAC), Inc.