CHEM 425 GAS CHROMATOGRAPHIC ANALYSIS

Background

Chromatographic techniques are used to perform qualitative and quantitative analysis on complex mixtures. A sample is injected into the chromatograph and is carried through the column by a mobile phase. The different components in the mixture travel through the column at different rates, based on their chemical and physical properties. The time required for a given component to travel through the column to the detector is called its retention time. One of the properties that determines retention time for gas chromatography is volatility; the less volatile the component, the longer its retention time.

As each component exits (or elutes) from the column it enters a detector and produces a signal peak. The peak area is proportional to the concentration or mass of that component in the sample. Qualitatively, the identity of unknown components can be determined by comparing their retention times with known standards. Quantitatively, the concentration of a given component can be determined by measuring the peak area.

In this lab, you will analyze a mixture of n-alkanes, determine the reproducibility of both the retention time and the relative peak areas, and determine the identity and % volume of solute in an unknown sample.

Procedures

a. Standard Mixtures. The instructor will have individual solutes from a homologous series of straight chain hydrocarbons (n-alkanes). Prepare a series of 4 standard solutions of the solutes (.05, .10, .15, and 20% by volume of each solute) using isooctane as the solvent and dodecane as the internal standard. It is essential that each standard solution contains the same volume % of the internal standard (0.10%?). You will use these solutions to prepare calibration curves (area counts vs volume % of solute) for each of the three solutes in the standard solutions.

The chromatograph will be setup up by the instructor/TA. To inject a 0.5 µL sample into the chromatograph, pull the plunger on the 10 µL syringe back to 3, dip the needle into the sample and pull the plunger back to 3.5. Withdraw the needle and pull the plunger back to 7. You should be able to see a small amount of liquid inside the syringe. Record the actual volume collected for injection. Inject the sample into the column as directed by the TA. Be careful to apply pressure axially (straight down the barrel) to the syringe plunger to avoid bending the plunger. When the chromatogram is complete you will see peaks for each component in the sample. Identify each of the components based on retention times (i.e. volatility). From the chromatogram, obtain peak area counts for each of the components to use in generating the calibration curves.

b. Statistics and Calibration Curve. Repeat the injection procedure for each of the standard solutions at least three times. Record the retention times for each run. Calculate the average retention time and absolute standard deviation for the internal standard and each solute.

The chromatogram print-out will report the peak area counts for the internal standard and solute peaks. Divide the solute peak area count by the peak area count for the internal standard to obtain the normalized peak area for that solute concentration. Record the average peak area and standard deviation for each solute. Plot the normalized solute peak area vs. volume% of solute.

c. Unknown. Obtain a volume of unknown from your instructor and analyze it following the procedures outlined above. Collect several chromatograms. Based on comparison of retention times, identify the unknown component. Calculate the peak area of the unknown component for each of the chromatograms collected. Based on your plot, report the volume% of the unknown component in the original sample.