How to plan for dirty samples for both extraction and analysis

When juggling the responsibilities of working in a sample preparation lab as well as working as an analyst, it is very easy to get caught up in a never-ending cycle of samples. There is no situation “more frustrating” then when you have a bunch of wastewater samples that need to be extracted and analyzed ASAP and there is that one sample that is so much more challenging to extract than the others. After struggling all-day-long, you finally get the batch of rush samples set up to run on your gas chromatography (GC) system overnight only to come in the next morning to find that your mid and closing check standards are low and the data is effectively useless!

When trying to figure out the reason why I always ask myself the same two questions.

  1. When was the last time I did GC maintenance?
  2. What did the last customer sample look like that I ran previously?

Those two questions would help me figure out that either I hadn’t cleaned my GC in a long time (according to my GC maintenance logbook), or that the last batch of samples analyzed was very dirty. With this information in hand, it made it easier to get back on track and to get myself ahead of the problem on future samples because wasting time in a commercial lab is not something you can afford to do.

Benzoic acid, the phenols, and the anilines are a variety of different target compounds regulated in EPA methods 625.1 and 8270D that are subject to erratic chromatographic behavior, especially when the GC is contaminated with high boiling point material. DDT and endrin are another pair of challenging compounds (when it comes to contaminating a GC) and are two of the main indicators of a successful pesticide analysis for EPA method 608.3 and 8081. Wastewater and soil samples are notorious for being quite contaminated with non-target analytes that may be missed when you’re analyzing your samples. However, these non-targets can and will stick to your GC’s liner, seal, and column so it is important to be aware when running them. I always kept a list of customer samples to watch out for based on my experience so that I could dilute them to a more manageable level or know to clean my GC immediately after running them. Another indication that an extract is highly contaminated is if the solvent in the GC vial is either brown or black, which indicates that you should prepare in advance for that sample. Even if you are not juggling different responsibilities, you can start to put some precautions in place to help you handle challenging samples from start to finish. One way could include pre-diluting these difficult samples (as long as it’s appropriate to do so) so there is no chance of missing these samples and contaminating your instrument during a critical time.

So, what should you be doing to clean your GC to avoid getting poor chromatography for your challenging pesticide and semi-volatile compounds? I’d rather be safe than sorry and clean my entire injection port as well as clip my column slightly. Fresh septa, liner, O-ring, and a freshly cleaned gold seal will do wonders for your system and data, just make sure you let the injection port cool down enough or you may contaminate your system with fatty acids (when you burn your finger) as I did at one time. When it comes to clipping your column, I’d suggest cutting down about an inch and replacing the ferrule, however depending on the amount of contamination you’re seeing you may need to snip off a bit more of the column. Once that’s all done, you can turn the injection port and oven back on and bump the temperature up to right below the GC column’s upper-temperature limit and let it sit for at least an hour to get rid of any air, moisture, or contaminants that may have entered during the time maintenance was performed. With all that done you should be seeing better chromatography on those problem compounds in no time. This knowledge should help you be more prepared for dirty samples from the start of their extraction to their final analysis.

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