Purpose: Provide step-by-step instructions for best practices in capillary column repair
Theory: GC capillary columns sometimes break on the column cage. While most of the causes of such failures are relatively benign, some may portent coming disaster (see #6 in “How to Properly Cut Capillary Columns”) and improper repair of a capillary column break can convert a benign failure into a catastrophic failure.
Benign sources of failure are inadvertent, local damage in handling, polyimide erosion or abrasion due to contact with the wire cage and the occasional, random flaw manifesting itself. Such breaks are, by definition, “improper column cuts” and such breaks typically generate dozens of glass fragments, many of which will find their way into the column bore and many of which can destroy a column if allowed to remain.
At the risk of appearing obvious, the goal of repairing a column is to retain use of the column while preserving as many theoretical plates as possible, so it is tempting to simply cut the column near the broken ends and insert a press fit union. Such repairs often do work, but in many cases another break will occur nearby within short order (typically toward the detector), caused by a glass chip scratching through the stationary phase as it is pushed down the column by carrier gas.
It is better practice to remove one coil of the column, half from each side of the break, by carefully cutting the column 180° from the break and then rejoining it with a union. On a typical 8 inch column, this practice will cost a bit over a half meter of column length but it insures against future loss of the entire column after the break.
Safety: Wear safety glasses and disposable gloves or finger cots while repairing your column. The bits of glass involved will rarely cut you, but a fragment in the eye really, really hurts. Rubber gloves or cots help you grip to the capillary securely, particularly where you’re averse to cutting large hunks off of your column.
Materials:
1) Press2Fit™ straight union (P/N: 3-2002) or angled, repair union (P/N: 3-2002A)
2) Safety glasses or goggles
3) Rubber or nitrile gloves or finger cots
4) Alcohol or other volatile solvent
5) Kimwipe or other low-lint lab wipe
6) Sapphire™ brand 30° honed edge wafer (P/N: 5-1001S1H), 30° diamond, sapphire or ruby blade (a standard scoring wafer can work, but will never give as good a result as an acute angle blade)
7) Tools required for removing the column and reinstalling it.
8) Polyimide Sealing Resin (P/N:3-2525S), optional
9) Toothpick or sharpened end of a wooden or polymer applicator stick, optional
Reference Documents: “How to Properly Cut Capillary Columns” in IQ University
Procedure:
Note: It is possible to perform a repair on a column without removing it from the instrument but your success rate will be far higher if the column is removed from the oven where access to the broken ends and loose coils of the column is facilitated.
1) Remove the column from the instrument.
2) Push the broken ends of the capillary column out of the coil on the cage until they are free at a point ~270° opposite of the break’s original location to allow grasping of the column to make clean cuts (Figure 1).
Figure 1 – Loosen column about the break in both directions
3) Follow the instructions for cutting capillary columns found in the document referenced above, placing the new cuts such that 2/3rds of the exposed capillary lengths are removed (~32 cm removed from each end) while ensuring the new cut ends will meet to within ± 2 cm when the original coil diameter is reconstituted (about 16 cm loose capillary from the point of tangency – Figure 2).
Figure 2 – Approximate length target for freshly cut column ends
4) If you are using a Press2Fit™ column repair union, you may simply insert the cut column ends into the union and reinstall the column. There is no requirement for polyimide resin or thermal cycling to produce a reliable seal; cold flow of the polyimide will accomplish this within minutes after insertion. Just be sure to orient the union on the column coil such that the angle minimizes stresses on the column at the union entry point and you are good to go (Figures 3 and 4). (It is not necessary to secure the repaired section within the cage restraints.)
Figure 3: Proper orientation of column repair union on column coil
Figure 4: Angled (column repair) union installed on coil
There is a valid reason for using polyimide resin if the union you are using is a straight union, however, or even if you instrument just vibrates a lot (Figure 5)….
Figure 5: Contact abrasion in column repair using standard (straight) union
Many instruments produces significant vibration during operation and even with Press2Fit’s chamfered openings, the curvature of the column coil causes the column to touch the union wall near the opening when installed on-coil. The application of a tiny amount of polyimide sealing resin is advisable for immobilizing the column within the union, and the best time to apply the resin is just before inserting the column end into the union.
Only a few millimeters of large bore columns will enter a press fit union before contacting the union wall and the polyimide resin will flow liberally once the temperature rises, so you need only apply a fraction of a drop of the resin at a point approximately 2-3mm from the end of the column just prior to insertion and the resin will flow to form a meniscus about the column circumference near the seal once heated. There will likely be plenty of bubbles and voids after curing the polyimide because “cure” requires driving off the solvent (n-Methyl-2-pyrrolidone in the case of IQ’s 3-2525S polymer) representing ~75% to 80% of the volume, then the polyamic acid resin forms the imide through ring closure and elimination of a mole of water per mole of rings closed. Regardless of the appearance of the polyimide, its purpose of immobilizing the column within the union is served by any contiguous point of contact between the column and the union wall.
Note on curing polyimide resin: The polyimide resin used to immobilize columns in connectors need not be pretty, and if you use any significant amount of resin, it surely will not be pretty: 20µl is plenty. But if you want to try to make your repair look as pleasing as you can, it helps to cure the polyamic acid very slowly; give the NMP and water plenty of time to escape without bubbling. This will take some time, so if your lab is busy, accept the bubbles.
NMP boils at just over 200°C so ramp from RT to 200°C at 4°C/min and hold there for 30 minutes to drive the solvent off. Then ramp to 300°C at 2.5°/min and hold there for 60 minutes to slowly imidize and evolve the water. If you use too much polyimide, this cure schedule will still give you bubbles so use half that amount, next time.
--doctorsilica #press2fit @doctorsilica