Barebones Guide to the Varian GC-FID

IMPORTANT!

Only organic samples can be analyzed! If you have an aqueous sample, you must extract the organics you want to analyze using solvent (ethyl acetate, hexane, toluene, etc). Know your polarities to figure out the best solvent to use.

Know the boiling point or approximate boiling point! If your sample has a boiling point >350oC, GC may not be the best method for separation. Also, if you know the boiling points you can alter the temperatures to get the best resolution.

Using the GC-FID

The software uses a bar at the top of the screen to open the different parts of the software. It is referred to in order to open the method and data analysis software.

Click here to view a picture of this bar.

Method programming: Click on the rightmost icon of the bar. Select "Edit Method" to open the method editor. On the left of this screen are the parameters that can be changed. On the right is where the parameters are entered.

Click on "Column Oven" to edit temperature programming. To do an isothermal experiment (constant temperature) enter a temperature about 5o above your expected highest boiling point in the "temp" column, and the run time in the "hold" column. To run a gradient temperature experiment, put an initial temperature (such as 100o) in the "temp" column and a "hold" of 2-3 minutes. In the second row, enter your maximum temperature in the "temp" column and a ramping rate (10-20o usually) in the "rate" column. (What rate would you use if your boiling points are fairly close?). Put a "hold" of about 2 min. The Total Run Time is displayed in the "total" column of the second row.

Click on "Flow/Pressure" to edit pressure programming. NOTE: Do NOT use too high of a pressure or you can ruin the column - ASK before you change this! Just like the temperature programming, you can leave the pressure constant the whole experiment by entering only one row of information, or you can ramp it up utilizing a similar input screen as for the temperature programming.

Running your sample: Open the System Control by double-clicking on the leftmost icon in the bar. Open your method in the system control screen. Once the GC temperatures are stabilized, the "Ready" light on the instrument should turn orange, and the computer will also say "Ready" in green at the top. Clean the syringe out 3 times with ethyl acetate, then 3 times with your sample. Fill the syringe to 1 mL with sample, being careful to not get air bubbles. To inject, quickly put the needle all the way into the injection port and inject once you get it in all the way. The run will start itself.

Data Analysis: Click on the second button from the right in the icons at the top of the monitor and select View/Edit chromatogram. Open your file using File->New Chromatogram. Click on the file and click on Open File. To zoom, click and drag a box with the left mouse button. To change the y-axis, click on the scrollbar on the right of the chromatogram and drag up or down. Click on Results->Reintegrate Now to integrate. If you are missing peaks or too many peaks are picked, go to View->Integration Parameters and change the Initial Rejection Threshold.

To view the chromatogram and results, right-click on the chromatogram and select View Standard Report. This mode has the chromatogram and data such as retention times and peak areas. Click File->Print to print the standard report out. You can also print just the chromatogram after exiting standard report mode if you need.