hplc columns

[manar]

hi my question is why we are not using C20,C16,C12,C6 alkyl chains in stationary in column

[tom jupille]

For the same reason that we don't see many columns with 4.4mm or 4.2mm id. A small change doesn't have much effect.

[ConstantinS]

C16 reversed phases are wide spread in russia

[MG]

I think one of the Phenomenex "Synergi" phases are C12.

[JA]

why is it that 4.6 mm column diameters became commonplace?

[tom jupille]

"Which came first, the chicken or the egg?"

There was more variability early on. Waters columns used to be 3.9 mm; Merck columns were 4.0 mm, Whatman and DuPont columns were 4.6mm. My recollection on sourcing tubing in the late 70's was that 4.6mm with polished internal walls was less expensive than the narrower columns (used less steell, perhaps?). That meant that more newcomers to the column business used that size. Which meant that tubing manufacturers could realize economies of scale. Which meant that 4.6mm tubing became even less expensive. Which meant that more newcomers to the business used that size . . .

My point was that there is little or no functional effect from small changes in diameter (e.g., from 4.6 to 4.4). Once someone finds a size that works, economies of scale will tend to make others converge on that same size. The same argument holds for C18. The effect of chain length is more logarithmic than linear (or like a "geometric" rather than "arithmetic" series). A meaningful difference requires (approximately) halving or doubling. So you see the common alky columns (in the US, anyway ) being C18 - C8 - C4.

C18 was tried early on, and seemed to work OK.

Years ago, I heard a story which is probably apocryphal, but has a ring of plausibility about it. If any of you are old enough to remember the appearance of old Coca Cola bottles (circe 1950s or 1960s). The outside of the bottles weren't shiny and reflective, but had a dull matte finish to them. Supposedly, this resulted from Coca Cola treating the bottles with hot octadecyltrichlorosilane vapor in order to prevent the glass from chipping during bottling (in effect, they bonded a lubricating layer to the glass). When bonded-phase HPLC column packings were being developed, researchers looked for reagents which were commercially available in large quantities at reasonable prices. The ODS silanes (thanks to Coke!) fit the bill. The stuff worked, C18 got specified as the "L1" column by the USP, and the rest is history.

Note that this may be an "urban legend"(I have absolutely no hard evidence to support it) but it's a good story nonetheless.

[Uwe Neue]

I can confirm some of the history.

The standard initial column i.d. was around 4 mm. This was a rational decision based on extra-column effects of early instruments and a 30 cm (the "holy foot") column. It turned out that high-quality and reproducible column packing was not possible with the tubing that was initially available with a roughly 4 mm i.d.. Waters found a solution by finding early on a supplier that could deliver high-quality (i.e. good surface finish) tubing with 3.9 mm i.d., but it was a thin-walled tubing that could not be used at the high pressures needed. Thus they inserted this high-quality thin tubing into a tubing with a larger o.d. and welded these two pieces together. This was the origin of the original 3.9 mm i.d. Waters columns. Then, later on, the surface finish of the 4.6 mm tubing improved, and the rest is the history described by Tom. Also, Waters later on abandoned the double-wall design, after tubing of 3.9 mm i.d. with sufficient surface finish was commercially available. Waters still makes a lot of columns with a 3.9 mm i.d. but also with 4.6 mm i.d..

The availability of a commercial silane was indeed the trigger for the design of "C18" columns, but one also needs to think about the availability of raw materials for the designers of silanes. Also, there were some early investigations ito the influence of chain length on chromatography (Karl Karch, for example), but they did not point to anything significant except that C18 had more retention.

[ConstantinS]

Thanks for stories (the "urban legend" was the most thrilling)!

By the way, I was always wondering why C-around-18 phases provide the best packing ability (I don't know the exact term in English) about 80K, while C8 provides only about 60-80K, and C4 may be below 60K. I don't know anything about the packing ability of, for example, C30 phases. Is it higher or lower?
If it is lower, than why do C-around-18 phases provide the best packing ability?

[Uwe Neue]

Why do you think that this should be so, and how did you arrive at the conclusion that C18s are easier to pack? Our specs are are for the most part independent of the packing...