GC compound responses lower than expected? Maybe this will help.

We occasionally receive calls in tech service from a customer who has been experiencing lower than expected compound responses when analyzing a chemical standard.  In many cases we hear “It worked fine the last time we did it”.  So what could have happened since the last time you successfully did the analysis?  Several of the most common situations (and their potential fixes) are listed below.   The suggestions provided in this post assume you have successfully done this analysis previously, and only recently have experienced lower compound responses/sensitivity.

If this is a new analysis for your laboratory, I suggest reviewing this link first GC compounds – poor peak shapes and missing peaks

 

A.   When only one or a few compounds throughout the chromatogram have a lower response.

 If the compounds are reactive (like many pesticides), system activity may be the cause. Perform routine GC maintenance like trimming (or replacing) the column, replacing the injection port liner, and replace any other consumable which may contact the sample.   Confirm the chemical standard is OK by analyzing it on different instrument.  You may also want to try analyzing a new chemical standard which has a different lot#, or one from a different manufacturer.

Inject a higher concentration of the problem compound(s) to “prime” the instrument, but don’t forget to run a solvent blank afterward to make sure you do not have carryover.  If you suspect that your injection port may be the source of activity, replace your current inlet liner with a Uniliner (just don’t forget that Uniliners are designed to be used in the splitless mode).

Sometimes problematic compounds are listed in the analytical method, along with suggestions for improving their response.  If someone in your lab is familiar with the method, make sure you ask them for troubleshooting advice.

 If the problematic compounds are not reactive and activity isn’t the issue, you may be experiencing compound discrimination in your injection port or contamination somewhere in your system.  If doing split injections, try using splitless mode; but first, dilute the standard so that the on-column concentration is the same as when performing a split injection.  If already using splitless mode, try a different liner style, preferably one with a restriction at the bottom (like a gooseneck liner) to make sure the compounds are efficiently funneled into the column.  If no improvement is observed, the column may have contamination in it; try baking it for 30 minutes at its maximum isothermal temperature (just remember to trim the inlet side of the column first so you don’t drive any contamination deeper into the column).  Finally, make sure you have stored your column properly when it is not in use.  

 

B.  If you notice that only the early eluting compounds have a lower response, then you should focus on the split ratio, instrument leaks, and/or any standard preparation/dilution which may cause loss of these compounds.

 Just like listed above, if using split mode, analyze the standard in splitless mode (remember to dilute the standard so that the on-column concentration is the same as when performing a split injection).   If response improves, then you need to optimize your split ratio or try a different style injection port liner.

Leaks at the injection port can also discriminate among the lower molecular weight compounds.  Use an electronic leak detector to sniff around the injection port, especially where the column connects to the injection port and around the injection port septa.  

If analyzing volatile compounds (especially those which are gases at room temperature), make sure that the chemical standard is cold before you open it.  Work quickly to minimize heat transfer from your hands and from the surrounding room.  Only dilute the stock solution into a very cold solvent (never into a solvent which is at room temperature).  Some customers have told me that they even refrigerate their syringe or measuring glassware.  Store your standard solutions in vials which have very little, or no headspace.

 

 

C.  If only later eluting compounds have a lower response, first review this post When High Boilers Disappear (GC).

If analyzing semi-volatile compounds, make sure that the chemical standard is at least at room temperature, and has been gently sonicated for at least five minutes.  If dilutions are needed of the stock standard, make sure the solvent is at least at room temperature.  If the solution is cold, you risk higher boiling point compounds precipitating out of solution.  If you suspect this may have occurred, please review the three paragraphs below for suggestions.

Never open an ampule which has precipitates floating inside it. Before opening any ampule you should hold it up to a light and if there are any “floaties” visible you need to get this solid into solution before opening the ampule.  Otherwise, it will likely not be possible to transfer the solution properly.  If you do see "floaties", you may want to try the following.

I used two beakers, one should be small enough where the unopened ampule can be placed inside and upright without tipping over. That beaker is then filled with warm water until it is high enough to cover the internal contents of the ampule, or at least halfway up the ampule. Then that beaker is put inside a second larger beaker which is also filled with warm water (about half-way up the smaller beaker), then these beakers are placed into a sonicator which is filled with warm water. Sonicate for 15 minutes on the low setting, dry the outside of the ampule, hold it up to the light and make sure everything is back in solution. If not, repeat but with slightly warmer water.

So how warm should the water be?  That depends on the solvent inside the ampule.  I would keep it lower than the boiling point of the solvent, but above room temperature.

 

D.  When all compounds have a lower response.

If all the compounds have a low response, then instrument sensitivity may be the cause.  Start by trimming and reinstalling the column into both the injection port and detector.  Then, confirm all gas flows (especially for the detector) are at the recommend values.  Next, verify the analytical method hasn’t been accidently changed.  Finally, make sure the autosampler is working properly and the syringe needle is not plugged.  Analyze your standard on another instrument if possible.

In addition, try another lot # of standard, or even another manufacturer’s standard, on the same instrument, back-to-back runs.  If the standard had been diluted from a stock solution, prepare another running standard to confirm no dilution mistakes were made.

Finally, make sure the detector is functioning properly.  You may want to check the troubleshooting section of your instrument manual and/or contact your instrument manufacturer.

 

Hopefully this post will help you isolate the issue.  Thanks for reading.