Contamination of Injection System Split-Vent-Lines: A Maintenance item not to Underestimate

2011-jaap-pasfoto4Split injections are widely used because its easy and the amount allowed onto the column can be adjusted with the split-vent flow. Split injections are therefore applicable for measurement of components that have % levels, down to ppm levels.

When doing split injections, the majority of sample is vented via the splitter.

Especially when there is a severe matrix present, a lot of sample matrix will be split off also.

Normally after the splitter, there is a charcoal filter that is placed there to trap the compounds that are split, however, the split-vent-line has also a lower temperature, which means that matrix may already accumulate in the split line connections with the injector body.

Michelle Misselwitz already pointed out contamination of inlets doing soil extracts from an underground storage tank, see http://blog.restek.com/?p=2769, and saw split lines being very dirty, they even clog up changing split ratio.

Fig. 1 Problem of humps in the chromatogram: contamination of injector caused also non reproducible area counts between runs

I similar experience was brought to our attention 2 weeks ago. Customer analyzed mono and di-glycerides in oils and fats.  Area counts seem to systematically drop and chromatography looked bad, see fig. 1.  In the chromatogram “humps” were formed in the same area as the glycerides were eluting.  There seem to be non-reproducible splitting and  memory effects.  After disconnecting and flushing this through with solvent a lot of “gunk” and debris was cleared from the split line. Once the line was air dried and replaced consequent analysis was predictably much better and split flow and column flow could be maintained at set points.

The new chromatogram is shown in fig. 2, with the blue line.

Fig. 2 After cleaning the split-vent line(which was VERY dirty), results improved

The clogging of the split line was causing the non-reproducibility  and the residues in the split line somehow were also back flashing into the liner causing the humps.

 

If  a cleaner sample cannot be generated, we may try to inject less. For example the amount entering a column doing a 1 ul injection with a split of 1:60, is similar when we inject 0.25 ul and a split of 1:15.

If  smaller volumes cannot be injected, another way is to dilute the sample 4 times and use 4x lower split ratio. Also here the same absolute amount will be injected on to the column, but contamination of liner, split-vent line and column will be 4x less.

 

Other then that: its good to take a look at the condition of the split-vent-line as a systematical maintenance item.

 

Specific thanks to Stephen Botfield from Britannia Food Ingredients, UK, for sharing his expertise.

8 Responses to “Contamination of Injection System Split-Vent-Lines: A Maintenance item not to Underestimate”

  1. Jason Hoisington says:

    It’s important to note that even with a splitless injection you are eventually splitting the flow out the split vent line, so there is potential for a build up of non-volatile residue to occur, although probably at a slower rate than with a split injection. I’ve seen a reduction in tailing of active compounds by cleaning or replacing split vent lines in splitless injections and it can be surprising how quickly it can build up if your sample extracts are particularly nasty. Also, if you are doing any type of backflushing to reduce carryover all of that is flushed back through the split vent line as well, adding to any residue that would be deposited during the injection.

    Jason

  2. Good point Jason. Although the intention for a splitless injection is, to transfer the full sample amount to the column. However, after the split injection time has passed, the split vent line is opened again and any residue that is left in the liner, will also evaporate and will go into the split vent line, where is will deposit. Of course, part of this materials will also go to the column, contaminating the inlet.

  3. […] During splitless injection if split line is not opened after injection time. In splitless injection the injection port must be flushed immediate after the injection time. Sometimes split lines can be blocked due to contamination, see: http://blog.restek.com/?p=5454 […]

  4. Tim Mason says:

    I have replaced several vent lines in my day. When the vent line is removed for replacement, it is a good idea to throughly clean the injector.

  5. Joel Shuman says:

    Excellent points Jaap. If I recall correctly, we had to change the charcoal filter and do a solvent flush at least once. Not surprisingly, this was after doing a number of
    difficult matrix introduction (DMI) using our ATAS Optic3 injector.

    –Joel

  6. Eric Ayres says:

    It is interesting how things outside of what is considered the sample flow path can affect your analysis. I had a vendor who changed the way the made the injection port o-rings, which caused my pesticides analysis to exhibit “degradation” peaks for Endrin. That vendor swore up and down that there was no way that could happen. Funny thing is, they make the instruments!

  7. Andreas Landin says:

    Could someone please share a thought on if this would affect splitting efficiency solely, or if dirt from the lines also could reach inlet and eventually columns? In case something reaches the column, what would the mechanism be? None of the exhaust flows change direction, do they?

  8. The problem is that deposited dirt will have a chance to back-diffuse. Theoretically flows go one way, but practically the inlet is not always under positive flow. For example, when you do maintenance, there is no flow and when inlet is hot, back diffusion will contaminate. I can also imagine that during Splitless times this problem will also pops up.
    And even back diffusion through a positive flow is possible.. as the data on the chromatograms also showed..

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