A Quantitative Analysis of Ethanol in Beer using Gas Liquid Chromatography – flame ionisation detector

method with an Internal Standard

Intro

Gas chromatography is an instrumental method for the separation and identification of

chemical compounds (Daniel C. Harris 2007). Chromatography involves a sample (or

sample extract) being dissolved in a mobile phase (which may be a gas, a liquid or a

supercritical fluid). The mobile phase is then forced through an immobile, immiscible

stationary phase (F. James Holler 2014). The phases are chosen such that components of

the sample have differing solubilities in each phase. A component that is quite soluble in

the stationary phase will take longer to travel through it than a component that is not

very soluble in the stationary phase but very soluble in the mobile phase. As a result of

these differences in mobilities, sample components will become separated from each

other as they travel through the stationary phase (Daniel C. Harris 2007)

Gas liquid chromatography may be used for quantitative analysis as well as for

qualitative work; the larger the amount of a particular component in a mixture, the

greater the response of the detector and hence the larger the peak. The peak area gives a

measure of the amount of a particular component in a mixture (F. James Holler 2014).

For quantitative analysis a calibration curve is obtained relating peak area or peak

height to concentration of the substance in the sample. This is done just before the

analysis so that operating conditions are as identical as possible. In the burner the

eluent is mixed with hydrogen & air, which are ignited electrically. Compounds eluted

from column burn and form ions in the flame.

Aims

To introduce the use of GC in analytical chemistry. To demonstrate sample preparation

for GC. To introduce compound identification and quantification by GC. To demonstrate

the use on an internal standard in GC.

Method

Calibration: pipette equal quantities of the first standard (2% ethanol) and the internal

standard (propanol) into a test tube or volumetric flask and mix thoroughly. Fill a GC

vial with the solution from before and cap it, Repeat these steps with the remaining

ethanol standard such as 4%, 6%, 8% and 10% ethanol. Place a small sample of each of

the three beers in the ultrasonic bath to remove any carbon dioxide for 15 minutes.

Filter the beer samples to remove any sediment. Pipette equal quantities of the first beer

sample and the internal standard like what we did with the ethanol, mix carefully and

place in a GC vial as the same as before. Repeat with all remaining beer samples. Prepare

a blank with distilled water and cap in a GC vial.

Results

Chromatogram of 2% aqueous ethanol:

Chromatogram of the of first beer (sample 1)

A graph to show a table and calibration graph for the peak areas of the known peak

areas vs. the concentrations of ethanol and plotted for the concentration of the first beer

that was tested which gave me a 1.965 mg/ml by using the value of its peak area which I

retrieved from the chromatogram. Using the peak area of the unknown beer I can work

out its cocnetration using the graph which is hilghlighted in the red at the bottom

showing you that the concentration of the beer is 1.96 mg/ml.

A calibration graph and table to show the correlation of peak height to the concentration

of ethanol in a sample of beer. Here the concentration of the unknown beer is 2.8 mg/ml

using its peak height.

Discussion

Advantages: Flame ionization detectors are used very widely in gas chromatography

because of a number of advantages (Cottyn, Bernard 1989). Cost: Flame ionization

detectors are relatively inexpensive to acquire and operate. Low maintenance

requirements: Apart from cleaning or replacing the FID jet, these detectors require no

maintenance. Rugged construction: FIDs are relatively resistant to misuse.

Linearity and detection ranges: FIDs can measure organic substance concentration at

very low and very high levels, having a linear response of 10^6. High sensitivity, large

linear response, low noise, easy to use, good stability and reproducibility, gives a stable

baseline as it is not significantly affected by temperature or carrier gas fluctuation, has

very little or no response to water, CO2 and carrier gas impurities & hence gives zero

signal when no sample is present (Cottyn, Bernard 1989). The column has a high

polarity column, Wide range of working temperature (260ºC), Equivalent to USP Phase

G16, Ideal for separating alcohols. 

Disadvantages: Flame ionization detectors cannot detect inorganic substances. In some

systems, CO and CO2 can be detected in the FID using a methanizer, which is a bed of Ni

catalyst that reduces CO and CO2 to methane, which can be in turn detected by the FID.

Another important disadvantage is that the FID flame oxidizes all compounds that pass

through it; all hydrocarbons and oxygenates are oxidized to carbon dioxide and water

and other heteroatoms are oxidized according to thermodynamics (Cottyn, Bernard

1989).. For this reason, FIDs tend to be the last in a detector train and also cannot be

used for preparatory work. No structural information, destructive

Conclusion

I found that the peak height chromatogram did not come out as clear as the peak area as

the r-squared value is much closer to one in fact the r-squared value was 0.988 which is

only 0.022 away from perfect ration which I have never achieved before. This shows a

good laboratory quality management, the flame ionisation method is very limited when

compared to GC-MS as this method has parameters to what in can test.

References

Cottyn, Bernard G., and Charles V. Boucque. "Rapid method for the gas-

chromatographic determination of volatile fatty acids in rumen fluid." Journal of

Agricultural and Food Chemistry 16.1 (1968): 105-107.

Daniel C. Harris (2007) Quantitative Chemistry Analysis , 7th edition edn., United

States of America: W. H. Freeman and Company.

Dirk W Lachenmeier, Rolf Godelmann, Markus Steiner, Bob Ansay, Jürgen Weigel

and Gunther Krieg (2010) 'Rapid and mobile determination of alcoholic strength

in wine, beer and spirits using a flow-through infrared sensor', chemistry central

journal, 4(5), pp. 52-64.

F. James Holler, Stanley R. Crouch (2014) Skoog and West's fundamentals of

analytical chemistry , 9th edn., United Kingdom: Mary Finch.

Ortega, Catalina, et al. "Fast analysis of important wine volatile compounds:

Development and validation of a new method based on gas chromatographic–flame

ionisation detection analysis of dichloromethane microextracts." Journal of

Chromatography A 923.1 (2001): 205-214.