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GC Inlet Maintenance - Split Vent Trap

Split/splitless inlets typically vaporize a sample dissolved in a suitable organic solvent under increased temperature.  The samples vapors are entrained into the carrier gas flow inside a liner in the inlet, and from there pass onto the column or out of the inlet via the split line (Figure 1). 

Split injection is conventionally used for analyses where the sample concentration is high and the user wishes to reduce the amount of analyte reaching the capillary column by performing an on-instrument dilution, whereas, splitless injection is used for trace analysis as the entire sample is transferred to the analytical column (Figure 2). 

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Figure 1: Split/splitless inlet (split mode).   Figure 2: Split/splitless inlet (splitless mode).
 

During both types of injection, sample solvent and analyte vapors are removed from the inlet via the split line.  Contained within the split line is the split vent trap - a consumable which is often forgotten about when troubleshooting and carrying out routine maintenance - but which can impact chromatographic results (Figure 3). 

The split vent trap is an impregnated carbon filter which serves two purposes.  Firstly, it protects carrier gas regulation components (for example, the back-pressure regulator which is housed after the split line) in the short term from performance-degrading perturbations because of injection, and in the long term from accumulation of sample residues that also can affect performance.1  Secondly, it traps and holds less-volatile sample components so that they are not released into the laboratory atmosphere.  An optional external split vent trap, installed after the back-pressure regulator, at the external split vent exit connection, can help ensure that toxic components do not leave the instrument; these can be useful if solvent exposure is also a concern.

Figure 3: Split vent trap.

Split vent traps can become blocked over time, resulting in an incorrect (low or fluctuating) or no split flow during injection or the inability to achieve high split ratios.  The tubing union on the split line exit is also susceptible to blockage as it is a restriction and a cold spot for high boiling materials to condense.  Symptoms of a blockage in the split line or split vent trap include poor quantitative reproducibility, an overloaded solvent peak with overloaded (fronting/broad) analyte peaks, peak tailing, poor accuracy, or an inability to achieve high split ratios.

To properly diagnose a problem with the split vent line or trap, manually check the split vent and septum purge flow rates using a digital flowmeter and make sure there are no leaks around the inlet seals and septum.  Leak checking should be done with an electronic leak sniffer and hydrogen or helium carrier gas.  Using a surfactant solution on a cool inlet will also find major leaks, but may then create a major headache because the solution can be drawn into the inlet by capillary action back through a leak; therefore, this is not an ideal way to check for leaks.

A blocked or restricted septum purge line is the result of a repeatedly overloaded inlet and the accompanying backflash of sample up into the septum area (Figure 4).  This problem is always accompanied by contamination of the split vent trap.  This problem is most common with splitless injection, however, low split ratios can also have the same effect.  To remedy this it may be necessary to replace the inlet nut assembly, and if so be sure to replace the split exit tubing and split vent trap as well.  

In extreme cases it may be necessary to remove the inlet entirely and solvent clean it.  If backflash is occurring repeatedly, ensure that the injection volume is optimized based on the solvent, inlet pressure and temperature, and the liner volume - the backflash calculator in CHROMacademy can be used to do this

Backflash calculator »

 

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Figure 4: Split/splitless inlet backflash.

If the septum purge flow is normal (e.g. 2–5 mL/min.), then the problem may be a routine build-up of contamination in the split vent trap.  The split vent trap should be replaced regularly as part of routine system maintenance.  The trap may last for up to a year if injections are primarily clean volatile samples with little to no high-boiling residues. Samples which contain high levels of involatile material, on the other hand, could necessitate trap replacement as often as monthly.  Replacing the split vent trap every six months is reasonable.

When the trap is being replaced ensure the incoming tubing and connections are free from any visible residue.  Residues indicate that the connecting tubing back to the inlet also needs replaced which may require a visit from your service engineer.  To avoid residue build-up, consider modifying sample preparation methods so that as little high-boiling material as possible is injected.  Following any inlet maintenance, be sure to leak check the new connections and verify the correct inlet flow rates.

 

References

  1. Hinshaw, J. V. LCGC North America 2015, 10, 758-763.

 

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Dr. Dawn Watson
 

This article was written by Dr. Dawn Watson.

Dawn received her PhD in synthetic inorganic chemistry from the University of Strathclyde, Glasgow. The focus of her PhD thesis was the synthesis and application of soft scorpionate ligands. As well as synthetic skills, this work relied on the use of a wide variety of analytical techniques, such as, NMR, mass spectrometry (MS), Raman spectroscopy, infrared spectroscopy (IR), UV-visible spectroscopy, electrochemistry, and thermogravimetric analysis.

Following her PhD she spent two years as a postdoctoral research fellow at Princeton University studying the reaction kinetics of small molecule oxidation by catalysts based on Cytochrome P450. In order to monitor these reactions stopped-flow kinetics, NMR, HPLC, GC-MS, and LC-MS techniques were utilized.

Prior to joining the Crawford Scientific and CHROMacademy technical team she worked for Gilson providing sales and support for the entire product range including, HPLC (both analytical and preparative), solid phase extraction, automated liquid handling, mass spec, pipettes, and laboratory consumables.

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