Packed/Micropacked Column Troubleshooting – Part 1 – Missing Peaks

Unlike decades ago when packed columns were found in most analytical laboratories, there is an entire new generation of analytical chemists who may have no experience with packed columns.  A column which contains particles is very different than a WCOT (Wall-Coated Open Tubular) column (also known as liquid phase capillary column). These differences may cause an analyst frustration because troubleshooting packed/micropacked column issues is sometimes very different than troubleshooting liquid phase capillary columns.  Hopefully this blog post series will provide you, the analyst, troubleshooting steps to help resolve several of the most common issues we hear about in technical service.  I also hope it provides enough guidance to help you determine if a specific packed/micropacked column is worth troubleshooting, or if the column simply needs replaced with a new one.

If you need help with ordering or installing & conditioning a packed/micropacked column from Restek, this post series is not for you, but the links below should help.

• Packed Column information for the beginner (restek.com)
• Looking for a quote for a GC custom packed column? (restek.com)
• What to do when your packed/micropacked column is no longer available (restek.com)
• Packed Columns Instruction Sheet (restek.com)
• Micropacked Columns Instruction Sheet (restek.com)

Normally I would begin this series by writing a post on troubleshooting a packed/micropacked column when no peaks are detected, but I already have a post for this topic.  An informative video is also available.

• There are no compound peaks using my new packed/micropacked column. What now? (restek.com)
• GC Troubleshooting—No Peaks (restek.com)

Another topic I will not discuss in this series is troubleshooting baseline issues.  As with the topic on missing peaks, I have already written a post (includes capillary columns). 

• Troubleshooting GC Column Baseline Issues (restek.com)

Finally, the topic of unexpected/unwanted “ghost” peaks has been previously discussed (includes capillary columns).  Once again, a video is also available.

• Are your ghost peaks coming from the GC column, or something else? (restek.com)
• GC Troubleshooting—Carryover and Ghost Peaks (restek.com)

As a result, I will begin with a discussion on the topic of one or more missing peaks, but not all.  In other words, the suggestions below are for when your instrument is able to detect some peaks, but not others.  There are several common reasons this may occur.

1. The detector is not sensitive enough to detect the on-column compound concentration.
2. The compound(s) have condensed out somewhere along the sample flow path.
3. The compounds are reacting with something or breaking down.
4. The compounds are being absorbed.
5. Faulty injection.
6. Over conditioning the column.

Let’s discuss each of these individually.

1. Have you performed this analysis in the past on this exact instrument and was the sensitivity more than sufficient, or was it barely sufficient? Have you verified the detector is still as sensitive now as it was then?  What happens if a higher on-column concentration is injected? You may want to try installing the column into a different GC and/or detector (if possible).
2. If the later eluting compounds have decreased in area counts and the early eluting compound area counts have remained the same, condensation may be the issue. Check for cold spots.
3. Are the compounds too reactive to analyze without using a specialized instrument (such as trying to analyze nitric oxide (NO) without using a dedicated NOx detector). Are the compounds prone to breaking down with the introduction of heat (thermal degradation).
4. Certain compounds, such as hydrogen sulfide, are often problematic to analyze because they may be absorbed anywhere along the entire sample flow path if not completely inert. To troubleshoot, you may want to consider installing a new column (or better yet, for troubleshooting purposes only, install a new capillary column like a Rxi-1ms, which is incredibly inert) to determine if the packed/micropacked column is to blame, or if the problem is somewhere else along the sample pathway.
5. Often all peaks will either disappear or show a low response if the injection is faulty (such as with a fully or partially plugged syringe needle), but if there is a leak at the injection port or somewhere along the sample pathway, compound discrimination may occur and affect certain compounds (such as lighter, early eluting compounds) more than others.
6. Sometimes over-conditioning a column can produce detrimental effects, such as increasing surface activity or causing the loss of theoretical plates (which will often affect separation of closely eluting compounds, or cause retention times to shift early). Only condition a new column (or bake-out a used column) long enough to achieve a stable baseline, and certainly not overnight unless you know that the column will not suffer any negative effects.  Follow the recommendations on the manufacturer’s instruction sheet.

I hope you have found this helpful.  Keep watching for future installments.