GC capillary tubing – things you may not know

The other day a customer asked me about our GC capillary tubing specifications, and was surprised to learn that the outside diameter (OD) of both 0.25 mm and 0.32 mm internal diameter (ID) MXT tubing is the same (0.41 mm), while our 0.28 mm ID MXT tubing has an OD of 0.56 mm. He was not the first person surprised with those facts.  For reference, the table below lists tubing ID, OD and OD +/- ranges.




Another common question we get asked in tech service is which product we recommend for cutting GC capillary tubing.  I personally suggest a ceramic wafer (pictured below is catalog number 20116).  What you may not know is that these wafers have four smooth edges for cutting fused silica tubing (using a single, gentle scoring motion) and four serrated edges for cutting MXT tubing (using a firmer, sawing motion).  So how can you tell which edges are smooth and which are serrated?  A good magnifying glass is helpful.


One last common question concerns the color of fused silica tubing.  Keep in mind that fused silica is actually a clear glass; the color you observe is caused by the polyimide coating (which gives the tubing its flexibility).  Colors can range from a bright yellow to black.   Don’t be concerned if your columns vary in color, performance is not affected.

FS Column

I hope you have found this post helpful.  Thanks for reading.


4 Responses to “GC capillary tubing – things you may not know”

  1. Yuki says:

    Hi Alan,

    I have a question regarding the last section of this blog(i.e. the color of polyimide coating). Why are some WCOT columns yellow??

    I thought Restek bakes columns at max isothermal temp for at least 12 hours before shipping new columns, rendering polyimide coatings the black color, and that’s why PLOT columns are yellow and most WCOT columns are black.

    I was using Rtx-PCB 60m 0.25mm 0.25um and it was yellow, too as if it had not been baked, though the max temp was higher than 300. Isn’t this column supposed to look black if it had been baked before shipping like other WCOT columns?

  2. Alan Sensue says:

    Hello Yuki: Thank you for reading my post and for your reply. I had a brief conversation with the product line manager after I read your comments/questions, and she made the statement that the final color of the column “Depends on the bonding temperature, time, and the dye lot of the polyimide.” In summary, depending on the particular lot# of fused-silica tubing used, the initial color and the final color can vary quite dramatically from other or previously manufactured columns, even those with the same catalog number. However, since all Restek columns are manufactured under very strict SOPs, and every column is tested after it is manufactured, you do not need to concern yourself with its final color. I hope you found this helpful. Regards, Alan

  3. Hei-Yin Chow says:

    Hello Alan,

    I’m curious as to why the outer diameter of the MXT tubing is the same for the 0.15mm, 0.25mm, and 0.32mm.
    This would result in different wall thickness for the metal column, is there any particular reason for this?

  4. Alan Sensue says:

    Hello: Thank you for reading my post. Please keep in mind that we do not manufacture this tubing at Restek. In addition, there may be different suppliers of different sized tubing (this information is proprietary). We purchase our tubing based on what is available, quality, price, and a few other factors. You are correct, if tubing has the same outside diameter (OD) but different inside diameters (ID), the wall-thickness will be different. Our primary tubing choice is for a specific ID, followed by OD. I hope this answers your question. Alan

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