There are no compound peaks using my new packed/micropacked column. What now?10 Oct 2022
Set & Confirm the Carrier Gas Flow Rate
Sometimes we receive requests from customers stating that they purchased and installed a new packed or micropacked column and no compound peaks are detected when they inject their reference standard. They suspect the column is somehow defective, but are not sure how or why. They ask for help.
Please have the following information available when you contact us:
What is the carrier gas?
What was the flow rate (in mL/min) of the carrier gas exiting the column?
What head pressure (in psi) was needed to obtain this flow rate?
How exactly was this flow rate measured?
What type of flow meter was used (soap bubble, electronic, or other)?
Was the flow meter connected directly to the outlet of the column or to something else, like the detector?
What was the GC oven temperature when the flow rate was measured?
The reason we require this information in such detail is that in most cases, it is often discovered there is no carrier gas exiting the column and this is the cause of not detecting any peaks. Unfortunately, some customers simply set a head pressure, or rely on their instrument’s electronic pressure control settings, and do not actually measure the flow rate exiting the column. While setting a head pressure or EPC is usually sufficient for WCOT (Wall-Coated Open Tubular) columns, this is not the proper procedure when using packed or micropacked columns.
This necessary step (adjusting the carrier gas head pressure to obtain the proper carrier gas flow rate exiting the column) should also be performed if the column had been previously removed from an instrument and reinstalled at a later date.
One also needs to be careful using high head pressures because repeatedly exposing packed and micropacked columns to a high pressure and/or pressure surge/pulse can cause the packing to compress until the carrier gas flow decreases, or even stops completely. If the column had initially worked well and now there are not any compound peaks, especially if a gas sampling valve was used, it’s time to measure the carrier gas flow rate exiting the column to make sure it is still at the expected flow rate and the column is not plugged.
Additional Troubleshooting Steps
If the column’s carrier gas flow rate exiting the column was measured and found to be correct and there are still no compound peaks detected, what then? Usually technical service will ask if this same column (and/or same catalog number) had been successfully used in the past for this exact analysis. If this (or a similar column) has worked well in the past, why did the previous column need replaced? If the column had been replaced for the same issue, it is time to troubleshoot the instrument/set-up, not just the column.
If this is a new analysis for this column, how are you certain it will work well? Was a similar column used for a well-known published method (like an ASTM or EPA method), or was it used in some obscure method found while searching the internet? In either case, you may wish to contact the author(s) of the method, just to cover all areas/aspects of troubleshooting.
If the flow rate has been measured and is correct, and you know the column is appropriate for this analysis, ask yourself if the detector is sensitive enough to detect the on-column compound concentration(s). Different detectors have different levels of sensitivity depending upon the compounds. Even the same type of detector can have different sensitivity (think TCDs from several decades ago vs newer µTCDs).
If the carrier gas flow rate is fine, the column is appropriate for this analysis, and you are using the appropriate detector with enough sensitivity for the on-column compound levels, I suggest injecting a different reference standard, perhaps a straight-chain hydrocarbon marker standard (liquid or gas) just to make sure the instrument/detector is operating properly.
One last thought, could there be a leak at the injection port or gas sampling valve/connection, and/or loss of the sample/standard somewhere along the introduction pathway (before it even makes it into the GC column)? Sample/standard loss is common when injecting a gas sample with a Gas-Tight syringe when using a packed or micropacked column, especially for a column that requires a high carrier gas head pressure. The sample/standard can “blowback” through the hole in the injection port septa causing loss of some or even the entire injection volume. The higher the injection port carrier gas pressure, the more likely blowback will occur.
If there are no peaks detected when using a packed or micropacked column, follow the steps below to assist with determining the cause of the problem.
Confirm the carrier gas flow rate by attaching an accurate flow meter to the outlet of the packed or micropacked column.
Confirm that you are using the appropriate column for the analysis/compound list.
Confirm that the detector is appropriate. Confirm that it will detect the compounds of interest and that it will be sensitive enough.
Confirm the reference standard is not to blame by injecting a completely different standard.
Confirm that blowback, or a leak, is not to blame by checking the reference standard on a different instrument (and if possible, use a different column that requires a lower injection port head pressure).