Split Injections… The Good, The Bad and The Ugly

While I’m not necessarily a fan of the old spaghetti western film with Clint Eastwood, I thought it was an appropriate title for what I have seen with my work using a split injection for EPA Method 8270 extracts.  You may recall an earlier blog that I wrote about the benefits of a split injection for these dirty extracts.  So let’s start with the good.  Using a split injection can allow for an increased initial oven temperature (80°C) versus a splitless injection (45°C) because of the faster transfer of analytes through the injection port onto the head of the column.  This gives faster analysis times and much faster oven cool-down times, meaning more samples analyzed in a 12hr period.  I also noticed increased injection-to-injection repeatability and held a continuing calibration longer before instrument maintenance was needed.

This brings me to the bad…  Of course with a 10:1 split there is a compromise in sensitivity.  With an Agilent 5975 MSD I was able to get a nice linear calibration curve from 5 – 160 µg/mL (0.5 to 16 ng on-column).  Going down to 1 µg/mL was possible for most compounds with a few exceptions for those troublemaker phenols (2,4-dinitrophenol – I am talking about you!).  On a positive note, by decreasing the sample amount injected onto a 0.25mm ID x 0.25µm column, sample overloading is minimized. So maybe bad is a little harsh, but it goes with the theme.

Finally, let’s finish with the ugly, and I do mean ugly!  After injecting over 300 soil extracts from an underground storage tank site I performed maintenance on my split gas line.  When I took off the line that goes from the injection port to the EPC I couldn’t believe my eyes.  Not sure ugly cuts it here, I think my words were more like disgusting!  A simple rinse with some methylene chloride shined it right back up to looking new. Anyways, lesson learned, there are great benefits to using a split injection for Semivolatiles analysis if you can afford to lose some sensitivity, but make sure to do some routine maintenance on your split lines.

3 Responses to “Split Injections… The Good, The Bad and The Ugly”

  1. Jaap says:

    Great example Michelle! This is all stuff that leaves the hot injection port and accumulates on a cold spot. Of course much better then sending it all through the column. I am wondering how “EPC” settings react when a restriction builts up.. And downstream the split-line there is probably a filter also that may built-up restrictions. Are we still reading correct values for the flows?

    For the method: If we get sufficient sensitivity: what about diluting such samples? we may get back to a “splitless” type system, add some run-time, but we should be able to reduce this type of contamination.

  2. Dear Michelle –

    Yes, great example! As Jaap says, there is also a split vent acticated coal filter cartrigde in the end of the split vent brass tube (6890 and 7890 Aglient GC S/SL inlets), for protection of the EPC modules, which has to be changed as well. Very important.

    It is interesting to measure the weight of the split vent filter cartrigde before and after use ….. :-)

    With kind regards –
    Lars, Copenhagen

  3. Michelle says:

    Hi Jaap! Thanks for the comment. Since I had no expectation that the split line was so dirty I really didn’t evaluate the effects on flow control by the EPC. I did also change the split line filter which had some yellow color but it seems most of the material was deposited on the line itself. Often times when there is a heavily contaminated sample, dilution is necessary in order to achieve the correct amount of sample loading onto the column for a splitless injection. This adds an extra step and requires re-calibration. When the analyst is not aware that the sample has high levels of target analytes it can contaminate the system and require extensive maintenance to get back up and running again. With a split injection you get a better match for sample loading capacity, with an increase in sample throughput. Everything comes with a compromise, it just depends on which one you would rather live with.

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