CUTTING AND CONNECTING COLUMNS by Stephen Griffin

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Slide Notes

CUTTING AND CONNECTING COLUMNS

Published on Dec 10, 2015

Leaks in gas chromatography can be extremely frustrating and time consuming to identify and rectify. Preventing leaks by employing best practices is a preferred strategy in all cases. Unfortunately, the vast majority of chromatographers have been taught incorrect techniques for cutting columns and for making quick connections in press fit type connectors.

Column trimming is not a art: it is a science called Fracture Mechanics. The number one practitioners of this science -- by orders of magnitude -- are telecom fiber optics installers and each and every one of them would be appalled to see how chromatographers have been told to cut capillary. There are those who claim that capillary is somehow different from fiber so the fiber cutting techniques don't work: BALDERDASH. This deck shows how capillary columns should be cut.

Even knowing how to cut columns is not always enough. Where Telco has semi-automatic and automatic tools for "cleaving" fiber, there is no such tool for the larger diameters of GC columns. Chromatographers have to learn to do it by hand. Accordingly, all column cuts will not be prefect so, when using a press fit, it is essential to select the one that is designed to seal on far less than perfect cuts.

Press fit connectors have a bad reputation throughout the GCverse, and it is a very well deserved reputation for the most part because manufacture of the devices that actually work, first time and every time, was halted a decade ago and cheap copies rapidly filled the void. The original is now back and better in every way, forming gas-tight seals on appallingly ragged column ends (does that sound like yours?).

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PRESENTATION OUTLINE

HOW to cut GC columns and guarantee no leaks

25 micron seal

A typical column seal in a press fit

We would all like to think we know how to cut a GC column well, but microscopy typically tells us this in not the case.

Most of the current press fit products are made by people who are not chromatographers and have never seen a GC instrument and have no real world experience with how their products are used. Heck, most of them don't even bother to do the math at all. I know; I got my start with the biggest one. (They didn't trust "long-hair college boys", nor did they believe science trumped "experience". The clash of cultures ended the relationship rather quickly.)

So what are my bonafides? We run an old HP5890 in my lab, my graduate research was in GC, I taught instrumental methods labs as a TA, I draw the best blank capillary in the world and I patented the laser forming method of making press fits (US Pat. No. 5,512,078).

This press fit, sold by a leading column manufacturer, is the best on the market, excluding ours. I'd say it was designed to seal on perfectly cut columns, but it was not designed by the manufacturer at all. Its a copy -- their interpretation of my design or the closest that they can get to my specifications. Either way, it barely seals at all.

125 micron seal

A much better seal

This is a seal that will not leak.

This column would leak in the press fit in the previous slide because the chip defect is bigger than the seal.

So what is the secret? Why do IQ Pres2Fit™ quick connects make such large seals? It is no secret at all. Its high school geometry and freshman mechanical engineering: angles and polymer compressive yield.

The coating on GC columns is a polyimide, usually HD Microsystems Pyralin PI2525. The coating ranges from 15 to 25 microns thick. Polyimide resists compression more than any other common polymer (~200 MPa): polyimide doesn't compress much at all.

The angle of contact is directly proportional to the seal size. Other press fits offer a 3 to 5 degree contact angle where IQ Pres2Fit™ quick connects offer contact angles of less than one degree. OUR SEAL IS 4 to 6 TIMES BIGGER.

The lower angle of IQ Pres2Fit™ also prevents the column from loading in the quick connect off of the connector axis...

How to cut your column?

You have been misled.

Alumina wafers are a commonly used tool for cutting columns: they're common give aways at Pittcon and everybody sells them with their logo. We sell them, too, with our logo (our yours) because you want them. These things are like a dull hatchet for heart surgery: to big and very brutal.

Good column cuts may be made with these cleaving stones, but it takes several tries for all but the most skilled and even the skilled are only about 50% successful. If you follow the instructions found on most websites, you'll hog out a V-groove of polyimide and glass that is too wide to seal in a conventional press fit. It'll seal in ours, but its still a bad idea.

See those glass chips in the column? They come from sawing across the capillary with the alumina edge and they are tiny sources of BIG problems in the future of this column. Carrier gas pushes these chips along the column bore. As they bound about inside, careening off of the column bore wall, they cut right through stationary phase and score the column form the inside.

The result? Brittle column: scrap unfit for a guard column.

Use Sharp tools for cutting --

-- 30 degrees is sharperWhen than 90 degrees

When you use a 90 degree edge like those provided by give away wafers, you displace 90 degrees of polyimide and if you saw away at the column like the big column companies tell you to, you displace 90 degrees of glass, too.

You could chose to use a diamond scribe -- and that is a fine choice -- but it comes at a fine price as well. The sharpest diamond crystal scribe is 30 degrees and they work beautifully.

IQ offers a more economical alternative: our Sapphire™ brand column scoring wafer. The Sapphire wafer offers a full 25 mm of 30 degree angle edge on an alumina wafer.

But don't you still displace 30 degrees of polyimide and glass?

Nope, its Zero if you orient and use the wafer properly.

proper ORIENTATION: Flat side to column, angle side to scrap.

Don't SAW, simply press down to nick the column

The Sapphire™ wafer is easier to orient to the column properly than alumina hatchets…I mean wafers. Just make sure the flat side is to the column and the angle side is to the scrap and gently press the sharp edge through the polyimide to just barely nick the column. 100% of the displaced polyimide will be thrown away with the scrap.

AND DON”T BEND THE COLUMN TO BREAK IT!

Fracture mechanics tells us that fractures propogate at right angles to the force.

Simply PULL THE CAPILLARY APART so the forces are normal to the desired cut plane.

The larger the nick in the glass, the less force is required to pull the separate the capillary BUT lower forces produce less flat cut faces. About a 1 to 3 micron defect is just about perfect. Practice on a scrap column section. Wear finger cots to improve your grip. Soon you'll be a pro.

Bending to break causes delamination (arrow) and non-planar cuts

Bending about the nick in the column will usually give you a less than planar cut face and often pulls the polyimide away from the glass. Neither result is optimal and either will leak in a standard press fit, but both will seal in IQ's Pres2Fit™ quick connects.

Even though this cut will seal in our press fits, it is not excuse to be sloppy because there is one more artifact of bending about the nick instead of pulling the capillary apart; huge chips are often produced.

The little chips in the earlier slide are bad news, but a big chip is a guarantee that your column will be turned into scrap once carrier gas flow through.

Let's be clear, I am not implying that you have been misled about press fits and column cutting for sinister reasons like selling more columns, you can decide for yourself.

I don't yet sell columns, I make components that extend column lifetimes --check out my fused quartz column caps. As a fellow chromatographer, I pledge that I'll resist the dark side and continue to teach ways to extend columns' useful lifetimes.

What do we find Below the cut on the prior slide?

A "column killer" chip.

You are a scientist. You trust the scientific method.

Why do you cut columns the way an alchemist would? It doesn't save any time and it certainly doesn't save you any money.

Fracture mechanics has been around for 100 years and was based on the study of glasses. If that's not enough, the methods taught herein are used in telecom fiber optic installation all over the world. Telecom technicians make 100,000 cuts to your one.

For more information or to discuss anything that you may not quite understand or take issue with, visit our website at innovaquartz.com or send me an e-mail at info@innovaquartz.com. Maybe we can talk about why columns sometimes crack....