Gas chromatography (GC)

-1–

Gas chromatography (GC) Identification and quantification of the alkane concentration in commercially lamp oil

Task: -

Calibrate the GC using mixtures of alkane reference samples.

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Learn how Kovàts indexes can be used for identification

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Identify and quantify the alkanes in commercially lamp oil.

Materials: Alkane standard solutions “A, B and C” (100mg/L, 50mg/L, 20mg/L), diluted lamp oil solution (500mg/L) “D”

Procedure:

The samples “A, B and C” to be analyzed is a mixture of C10, C12, C14 and C15 alkanes in hexane at varying concentrations, typically called standard solution. A standard should be analysed at least three times to monitor the reproducibility of the injection and/or the detection. Since we want quantify substances the analysis equipment must be carefully calibrated beforehand. GC#2 is prepared for analysis by loading method “Praktikum210710.M” in the corresponding software Chemstation. The underlying oven program starts at 50°C (hold time 2 min) with a rate of 15°C/min to the final temperature of 180°C (hold time 18 min). The amount of 1µl of sample „A“ is injected in the column inlet and the "Start" button of the GC panel is pressed immediately. This will be repeated two times for “B” and “C”. Be careful with the syringe during injection, the plunger is very thin and can break very fast. The analysis can be stopped via the software if all four alkane peaks are visible in the chromatogram. The analysis of lamp oil must not be stopped earlier to avoid column contamination through high-boiling substances in the lamp oil. After calibration of the system the lamp oil ”D” is injected and peaks are identified and quantified according to the method below.

Identification: Calculate the mean retention times (RT) for every single alkane. Check also the area concentration relation for every alkane and plot it in a diagram (area vs c). Is this relation linear (plot R2)? How large is your residual standard deviation (RSD%) of the area (calculate it for 100mg/L)? Using this data the following curves are to be plotted: a) Kovàts Index (C-number * 100) vs. log(RT) Lab course chromatography SS 2010 - GC

-2– Kovàts Index

Boiling point [°C]a

C8

800

125.6

C9

900

150.8

C10

1000

174.1

C11

1100

195.9

C12

1200

216.3

C13

1300

235.4

C14

1400

253.5

C15

1500

270.6

C16

1600

286.8

Alkane

a

CRC Handbook of Chemistry and Physics, 78th edition

For plot a) the linear regression and the correlation coefficient R² are to be calculated (via Excel). How good is the linear fit with your data? Can you estimate the RT to be expected by using a C16 alkane? Identify the alkanes in the lamp oil sample with the help of this plot. Quantify the amount of alkanes in one liter lamp oil with the help of the area values of the standard.

Lab course chromatography SS 2010 - GC