When High Boilers Disappear (GC)

Were you ever trying to analyze high boiling point compounds (using GC) when they either disappeared, or their response severely decreased, while at the same time compound response for lower boiling point compounds looked fine? Well, it has happened to most of us at one time or another. Below I will provide several common reasons why this happens, and a few suggestions to help you fix the issue.

Several common causes of getting low sensitivity (response) for late eluting (high boiling point) compounds are:

1. The standard is too cold and/or the compounds have precipitated out of solution.

2. The compounds are not properly vaporizing in the injection port.

3. The column has contamination.

4. There is a cold spot somewhere in the system which is causing the compounds to condense.

5. High column bleed (baseline).

6. Purge valve (split) time is set too short during the splitless part of a split/splitless injection.

7. Missing insulation around the injection port and/or nut warmer cup not installed (see photo below).

 

To fix these issues:

1. Make sure the standard has been sonicated and is at room temperature.

2. Try another liner (possibly with wool if your current liner has none) and make sure the column is installed straight and far enough into the injection port (liner).

3. Trim the first foot (approximately 0.3 meter) off of the capillary column and try injecting the standard again.

4. Look for any instrument zones which may not be properly heated.

5. Install a new GC column, or one with lower bleed at higher temperatures.

6. Increase the purge valve time so that carrier gas sweeps the inlet liner volume at least 1.5 to 2 times. For example, if using a liner with an internal volume of 500µL and a carrier gas flow of 1mL/min, the purge valve (split) time should not be less than 45 seconds.

7. Install missing insulation and/or nut warmer cup (see photo below).

 

 

In addition to the suggestions above, if you are using a mass spec as your detector, you should review the spectra generated by the calibration gas (usually PFTBA) to make sure your instrument has enough sensitivity at the higher masses (like m/z 502). If not, try retuning the instrument. If that does not fix the issue, clean the source and try retuning after the instrument has stabilized.

One last comment; it can be difficult to chromatograph compounds with boiling points greater than approximately 400°C - 450°C using a split/splitless injection port. If your compounds fall into this boiling-point range, you may need an on-column injection port for adequate compound response. Please keep in mind that if your sample contains non-volatile compounds/matrix, the column can quickly become contaminated, which may lead to chromatography problems.

If you continue to have poor response for high-boiling point compounds after trying the suggestions listed above, send a chromatogram in a WORD document or pdf to technical service and include:

a) How you prepared your chemical standard.

b) The make/model & detector of your GC.

c) All of your instrument conditions.

d) The catalog number of the liner which you are using.

Once submitted, give us some time to review your data, and we will respond with specific suggestions. If the reason your high boilers disappeared isn’t listed above, I’ll add it to the list.