Tech Tip: Using a press-fit capillary column connector

In modern gas chromatography the need to join two lengths of capillary column together in a leak-free, zero dead volume connection can arise in a variety of situations.  Whether just repairing a broken column, adding a guard column or splitting column effluent 2 or
3 ways, press-fit glass (quartz) connectors are one route to a quick, effective and low cost solution.
 

 

The connectors work by having a gently tapered inner profile at each connection point that grips the cut end of column. On heating the connection in the GC oven, the polyimide
coating of the column melts, bonding the column to the connector in a circular seal at this contact point to make the connection permanent.
 
In order for a leak-free connection to be made, it is essential that the capillary column has a perfect perpendicular cut at the end.  Any jagged spurs, cracks or chips around the column end will provide a site for leakage.  The cut may either be hand-prepared or cut using one of several custom-made tools designed for the purpose, but the end result must be the same: a smooth, perpendicular cut, free of jagged edges.
 

 

Hand-cutting using a ceramic wafer is the most common method of preparation, a few simple steps will ensure a clean perpendicular cut:
 
1.    Determine which edge of the ceramic wafer is to be used, one edge will be smooth and the other slightly serrated, the smooth edge is for scoring glass capillary columns.
2.    Wearing safety glasses, rest the end of the column across the balls of two fingers. With a single, long stroke from one end of the wafer to the other, draw the ceramic wafer perpendicularly across the column (i).  Hold the wafer at approx 45° to the column so that only the scoring edge rubs across the column.   The aim is to score the surface rather than cut through it, you are scratching off the polyimide to reveal the brittle fused silica, so the column should never break at this stage!
3.    Next, apply gentle pressure against the end of the column from the same side as the scratch, so that the column partially breaks and “hinges” backwards from the scratch (ii). If you completely break the column at this stage the edge on the opposite side will be jagged.
4.    Apply gentle pressure to the end of the column from the opposite “hinge” direction, so that the column bends forwards, breaking apart completely (iii & iv)
5.    Check the cut is clean and perpendicular using a magnifying lens, repeat if needed.
 

 

Connecting capillary columns together using the press-fit connector:
 
1.    Lower the carrier gas pressure through the column/connector to a minimum value that maintains a low flow through the column.
2.    Insert the trimmed end of each column firmly into the press-fit connector, wearing a latex glove will help get a good grip on the column so that you can push it in fully. You may be able to see a ring where the end of the column is sealing against the inside of the press-fit. If you remove it you should start with a new press-fit again, as the inside of the press-fit may have been scratched which would lead to a leak.
3.    Once both ends have been inserted, check each is leak-free. 
4.    Gradually increase the GC oven temperature to approximately 200°C (check this is within the column limits). This will soften the polyimide coating at the column ends, causing the polyimide to flow and then bake to seal against the connector.  If this step is done at a normal carrier gas pressure, sometimes the pressure combined with the softened polyimide pushes the column end from the connector causing a leak.
5.    Leave for approx 30 minutes; cool the oven and leak-check the connection again.  If leak-free, restore full operating pressure to the column and use.