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

Split 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, 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.