Oftentimes, when dealing with complex matrices that have a lot of non-volatile material, you may need to trim the front end of your column to prolong the column lifetime. I was recently working on a project where I wanted to see how short of a column I could use and still maintain resolution between some isobaric pairs of brominated diphenyl ethers (BDE 49 and BDE 71).
I started with a 15m x 0.25mm x 0.10µm Rtx-1614 column and GC instrument conditions that were optimized by Pro ezGC. I began trimming loops off of the front of the column and accurately measured the new column length by injecting air into my GC-MS to determine the holdup time. Then, I used the Agilent GC pressure/flow calculator to decrease the column length until the calculated holdup time matched my experimental holdup time. This new column length was used in the Agilent GC Method Translator software to calculate my new method. By doing this the analytes are eluting from the column at the same temperature, no matter what column length we use. This provides essentially identical chromatograms and maintains resolution (See figure below). I think I could have continued trimming my column, but I ran into two issues. First, my head pressure was reaching 0psi because I was operating under vacuum outlet conditions. Second, the oven ramp rate was getting so fast that my instrument couldn’t properly keep up. Really this just goes to show that you can trim your column in half (and the run time!), while still maintaining resolution, by properly translating the method.
In order to translate your methods remember the following things:
- Measure new column length using holdup time
- Input new column length and all original conditions into method translator (select translate only)
- Make sure to change ALL parameters necessary of new translated method (including your new column length) into GC software
- Check your instrument maximum heating rate to make sure your instrument can keep up to new method conditions.