Cutting Fused Silica Capillary Columns per LC-GC Application Note September 30 2016

Polymicro Technologies (a division of Molex) was the first company to mass produce fused silica capillary for the entire capillary gas chromatography market. That was thirty years ago and they are still the single largest manufacturer of the materials. (Polyimide coated, fused silica capillary for GC columns was an HP invention, but they reserved their production for themselves, alone.) Polymicro has made more GC capillary than anyone on the planet, by far: they are considered the experts and they are exceptionally good at what they do, of that there is no doubt.

I started my career at Polymicro in the late 1980: I know and respect their skills and I preferentially use their products in my own work. That being said, they can be a bit stubborn when confronted with new methods of doing things, even where those methods are supported by science and their own methods are not. It is not that their tried and true methods don’t work -- they definitely do --  but sometimes there is a better way, as it the case here.

An Application Note appeared in the latest LC-GC Magazine supplement, “The Application Notebook” (September, 2016), entitled “Cleaving Fused Silica Capillary Tubing” (authored by Joe Macomber at Polymicro). It’s a concise, one page Note of instructions for using Polymicro/Molex ‘Cleaving Stones’ in cutting chromatography columns and it is much better than any other set of instructions I’ve seen published before, but it could be a bit better. 

Side Note Gripe: The term ‘Cleaving Stone’ is somewhat misleading. I prefer to reserve the word ‘cleave’ for splitting apart things along crystal lattice lines or grain boundaries; the word ‘cut’ works just fine for amorphous glass capillary. The ‘cleaving stone’ is also not a stone. Stone is naturally occurring where synthetic alumina is far from natural. And while I am griping, isn’t ‘capillary tubing’ a bit redundant? We all know what fused silica capillary is, don’t we? It's a tube. Now, back to the meat of the matter…

The first problem that I have with Mr. Macomber’s treatise is that it does not describe the best tool for the job. I suppose if he had subtitled the Note “with a third rate tool” I’d have no problem with it because there are better tools than alumina wafers for cutting fused silica capillary. Another problem that I have with the Note is its seeming arbitrary selection of a 30 degree angle of attack for the wafer edge on the capillary. Why 30 degrees when 45 degrees makes a symmetrical defect?

Another minor problem I have is there being no instruction to clean the capillary prior to cutting it. When capillary fractures at “a sub-micron (sic) to micron size (sic) defect” it generates shock waves within the thin fused silica wall that dislodge most dust and debris that's adhered to the polyimide coating near the fracture site. This produces a cloud of dust that surrounds the newly opened column. I will post a video of this on YouTube, if or (hopefully) when I get a slow motion camera. The quasi good news regarding this omission is that it is virtually impossible to make one micron to submicron defect with a ‘cleaving stone’ using these instructions; the larger the defect, the smaller the fracture shock wave so that the debris cloud is fairly trivial when using the methods taught in the Note.

InnovaQuartz also sells ‘cleaving stones’. We call them 'standard wafers', ‘alumina wafers’ or ‘column cutting wafers’ or similar, and we have them imprinted with every element in the full periodic table of elements. They do have their place in the lab because they are low cost and can make decent column cuts. Caveat: it does require considerable practice to make extraordinary cuts with these crude tools and the technique for doing so differs from that described in the Note. I’d be remiss in my fiduciary duty (to my bottom line) if I also did not point out that the caption to Figure 1 (a depiction of the Molex ‘cleaving stone’) states, “The logo is not aligned with either edge type; one must feel the edge to determine which edge to use.” InnovaQuartz’ imprinting IS aligned with our wafers such that, when visible, the proper edge is to the column.

There is also a problem with the Note's Figure 2, depicting the ‘cleaving stone’ as it addresses the column at about 30 degrees: the waste segment of capillary – that piece beyond the score line – is too short to offer the firm grip that is required to apply sufficient stress across a submicron defect to initiate a fracture. But then, as briefly mentioned above, it is almost impossible to make a submicron defect if you “…draw the edge of the cleaving stone across the tubing…” as the Note instructs. Drawing the wafer’s edge across the capillary typically generates a trough in the fused silica OD (see my Photo 1, below). It tears and pulls the polyimide from the glass wall and produces glass shards and dust that will almost assuredly contaminate the column (also visible in Photo 1). If a glass shard contaminant is large and lodged in the inlet side of the column (Photo 2), the column will be destroyed once carrier gas flow is initiated. (The shard either blocks flow or it tumbles along the length of the column; imparting submicron to micron defects in the bore as it careens through, making the column ridiculously brittle.)

Photo 1: Trough cut in polyimide and fused silica by standard wafer drawn on column

Photo 2: Large glass shard in column

I should point out that Mr. Macomber never says that this methodology is appropriate for fused silica capillary columns, just “fused silica capillary tubing”. And while it is a virtual certainty that you will contaminate your column if you follow Joe's instructions, the amount of contamination is so small that you will probably never notice it (unless a large glass or polyimide shard is involved). My point is that slightly modifying the technique will essentially eliminate any potential for contamination and will produce much cleaner cuts, so why not modify the technique?

Submicron to micron defects MAY be produced with the crude, 90 degree edge of a ceramic wafer if the edge is pressed through the polyimide to the glass surface at 45 degrees, (as indicated by the white void in Photo 1),  but it takes a lot of practice to do so. (Micrometer is the proper term for 10-6 meters according international standards, but I gave up that quixotic lexicographic battle years ago.) If you want clean and orthogonal cuts on your columns, it is best to do as your father taught you, “Use the proper tool for the job.”

In selecting the proper tool, you have myriad choices and I can’t discuss them all in the space of a single blog post; I’ll bracket the field and describe the best and the cheapest. The best is a single crystal diamond edge cutter by Element Six – for example. These blades are expensive and are easily damaged in the learning curve. In expert hands they can make a few hundred cuts before they are scrap: that’s about a dollar a cut or attempted cut. I do use these blades myself, but only where I need absolutely perfect cuts, such as in preparing CE column blanks.

The tool that I use for routine GC column trimming is our own Sapphire™ brand ceramic wafer: the wafer with a diamond-honed 30 degree edge. It only costs about twelve dollars and can make hundreds of cuts at about a penny a cut. Our Sapphire wafer does not tear or delaminate the polyimide coating, and when used as directed, it does not generate any glass or polyimide shards. Instructions for using the Sapphire brand wafer, and a discussion of the science of fracture mechanics, are available under “IQ University”, on our website: www.innovaquartz.com.

 Thanks for reading,

@doctorsilica

#Sapphire wafer