How to Properly Cut Capillary Columns July 21 2014

How to Properly Cut Capillary Columns

 

Purpose:     Cleanly cut column ends are important in CE for minimizing electric field distortions, but in GC a bad cut can cause leaks or even catastrophic column failure. This procedure describes the best practice in cutting fused silica capillary

 

Theory:           A.A. Griffith developed the first theoretical model for fractures during WWI, inaugurating the science of fracture mechanics. As luck would have it, Griffith studied glass fibers. Telecommunications fibers are made of fused silica -- just like GC capillary -- and more telecom fibers are cut each day, using techniques based in based in the science of fracture mechanics, than GC columns in a year.

 1)    Capillary cuts are controlled fractures within bulk amorphous materials, propagating from an induced defect -- the score line.

2)    The smaller the defect, the more force that is needed to initiate the fracture, but the flatter that fracture will be across the capillary face.

3)    Multiple score lines favor multiple fracture planes.

4)    Mode 1 cracks (opening mode, i.e. cuts) propagate normal to applied stress.

 Safety:            Wear safety glasses and disposable gloves or finger cots while cutting capillary. The bits of glass involved will rarely cut you, but a fragment in the eye really, really hurts. Rubber help you grip to the capillary securely, particularly where you’re averse to cutting large hunks off of your column.

 Materials:

 1)    Safety glasses or goggles

 2)    Rubber or nitrile gloves or finger cots

 3)    Alcohol or other volatile solvent

 4)    Kimwipe or other low-lint lab wipe

 5)   Sapphire™ brand 30° honed edge wafer, 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)

 6)    Column or practice capillary section

 Procedure:   

1)    Prepare your work area. If it is possible to do so, making the cut on a clean, hard surface will produce better and more reproducible results.

2)    Clean the column with an alcohol dampened lab wipe for at least a few centimeter on both sides of the intended cut locus.

3)    Regardless of what tool you may be using, the goal is to produce a tiny (submicron) flaw in the glass wall of the column where the flaw is orthogonal to the capillary axis. There are 10 to 30 microns of polyimide thickness between you and your target and if the cut is intended for press fit connection, this polyimide must remain as undisturbed as possible. Low angle (sharp) blades disturb less polyimide than high angle blades.

4)    Do not drag the blade across the column to make a score line as others instruct because doing so gouges out a huge valley of polyimide and glass (see 45 degree cut parallel to virtual (white void in photo) scoring wafer) and produces lots of tiny glass shards that are attracted by static forces into the column bore while resulting in multiple score lines that will compete for priority when propagating the fracture.

 

 

Result of dragging a scoring wafer (aka "cleaving stone") across capillary

It is far more preferable to use a sharp blade (30 degree acute angle) like a diamond fiber optics scoring blade or IQ's Sapphire brand, diamond hones scoring wafer and just barely penetrate the polyimide to induce a tiny flaw in the glass with steady pressure – no dragging –no glass shards are produced if you don’t saw away at the glass and your one flaw has a dominant terminus.

 5)    Do not fold over the column end to produce the fracture as others instruct. Fractures propagate orthogonal to the applied force. The flaw you just made starts the fracture in the desired direction – at right angles to the column axis – but a bending force competes with this directionality, pulling the fracture plane toward the column axis. Bending the capillary also stretches the polyimide around the break, causing it to detach from the glass: it usually tear and leaves either a flap of polyimide extending beyond the break or a section of bare glass (see photo where the flaw was tiny and produced with a Sapphire™ scoring wafer (represented by the white void) but the capillary was folded over to propagate the fracture).

 

 

 
  Result of folding the column to snap it: delamination of coating and non-planar cut

 

PULL the capillary apart. If it doesn’t open, you failed to produce a flaw in the glass: back up a centimeter or so and try again.

 Note the orientation of the Sapphire blade in the photo, where the flat side is to the column. This orientation disturbs the least polyimide while producing a minute flaw. Note also that no glass shards were produced. 

6)    INSPECT THE CUT END plus about a centimeter or two of the column with 20X minimum magnification (30X to 40X is preferred -- IQ sells a 30X loupe). Even proper technique will occasionally produce a few glass shards. Large glass shards can be pushed along the entire length of your column by carrier gas flow and as they careen off the wall, they induce flaws suitable for fracture propagation: you column becomes brittle as an egg shell (see large shard in 0.53mm column, found 11mm inside of a column).

 

 
 Large, dangerous shard in 0.53mm column

                

If you see shards in the column, move a few centimeters in and cut the column again. You’ll never succeed at shaking shards out of a column.

If the cut end is perfectly flat and at right angles to the column axis, has no chips or cracks in the glass and the polyimide is intact, the column will seal in any press fit, even poor copies. If the cut end is less than perfect, cut the column again or use IQ’s Press2Fit™ connectors: Connectors Designed for a Less Than Perfect World.

 

(See note on cracks: even Press2Fit can’t seal columns with axial cracks.)

 

 
Similar imperfect cuts installed in press fit obtained from Restek versus IQ's Press2Fit™

 

Additional Information:

 If you press too hard with a sharpened blade of any kind -- ruby, diamond, sapphire – the blade will chip and render that portion useless for future cuts. It is far simpler to apply a reproducible force when the column section is on a flat, hard surface than it is if you have to hold the column section. IQ’s Sapphire™ brand ceramic wafers are low cost and offer 25mm of edge length so even if you chip them a lot, you should get dozens of good cuts before having to discard the wafer.

 It is not necessary to use polyimide to seal a capillary into a press fit fitting. Much as “pipe thread tape” is useless on Swagelok® fitting threads because the seal is formed elsewhere, polyimide resin can’t fix a bad seal in a press fit. Polyimide resin is only useful where press fit installation is such that significant vibration or other motions could potentially dislodge the capillary from the connector.

If you do use polyimide resin, use only semiconductor grade (polyamic acid resin like HD Microsystems Pyralin PI2525 -- our P/N: 3-2525 -- the stuff used to make the capillary). Wire grade polyimide resin (sold to you by others) is not filtered for particulates so contaminants within it can score your column. You can tell wire grades from semiconductor grades by the MSDS: semiconductor grades will only have polyamic acid (roughly 15% to 25% w/w) dissolved in n-methyl-2-pyrrolidone (NMP) where wire grades use cheaper solvents.

 

Notes on cracks:

 There are two types of cracks that occur on columns when cutting: circumferential and axial. Circumferential cracks are produced by hesitation in scoring the column – multiple flaws with slightly differing orientations that fracture almost simultaneously cause circumferential cracks to form.

 

In the photo above, the angled flaw was dominant over the properly oriented flaw so the capillary opened at an angle, but the correctly oriented flaw still fractured, albeit incompletely. You would probably reject this cut anyway (although without the crack, it would seal in IQ’s Press2Fit) but with a second fracture plane in play, it is simply unacceptable: recut the column.

 

 

Axial cracks result when the capillary was produced (drawn) at too high a speed or at too cold a temperature, leaving high residual stresses in the glass. The stress can be so high that it is impossible to cut the capillary without getting axial cracks. We see this far less than we did 15 years ago, but it still happens. Return the column to the manufacturer because there is no connector on Earth that can form a gas-tight seal on this stuff.

All capillary has some residual stress because it is cooled from ~1800C to near ambient temperature within in seconds. Capillary manufacturers are not the same company as column manufacturers (with one exception); it's made by fiber optics companies because the equipment needed is virtually identical to that needed to make fiber. Capillary is also a low profit product so companies sometimes cut corners to maintain a profit margin. China recently entered the capillary market so there could be a bit of a battle on the horizon and that may adversely affect quality all around. But the opposite is also possible. Cross your fingers....

 #press2fit @doctorsilica