Technical Report: Small Bore Columns

Small Bore Columns - Can they be a useful tool for your laboratory?
The availability

of HPLC columns with internal diameters of less than the 4.6 mm i.d. of the "standard" column have promised several important new opportunities for chromatographers:

Why doesn't everyone use small bore columns?
As usual with any good thing,

there are a few cautions to the user. Among them are:

How can you tell if you can profitably use these small bore columns? The first thing to consider is whether the HPLC system will allow the efficient use of a small bore column. The peak volume of each eluting band decreases significantly as the diameter of the column is decreased. Peaks with low k' values are especially critical, as they have the lowest peak volume in the chromatogram. (See table below.)

Column Dimensions (mm)	Internal Volume (mL)	Calculated Peak Volume (無)	Solvent Consumption (mL)	N	% Solvent Savings

4.6 x 250	2.50	550	18.9	19,000	0
4.6 x 150	1.50	430	11.4	11,000	40
3.0 x 250	1.06	240	8.0	19,000	58
3.0 x 150	0.64	188	4.8	11,000	74
2.1 x 150	0.30	94	2.4	11,000	87

NOTE: k' of last eluting peak is 6.6.

A well-designed standard HPLC system may tolerate column peak volumes as low as 110 microliters and still provide excellent efficiency. However, peak volumes below that size will require very careful design so that the capabilities of the small-bore columns may be fully utilized. In fact, columns below the 2.1 mm i.d. size require sophisticated HPLC hardware systems not commonly found in today's laboratories.

Which columns are appropriate for your application?
The columns

could be described in three categories:

Each of these categories has its merits and problems. The following discussion should allow you to decide what is best for your application.

Mid-Size
This column will perform near its optimum on most standard HPLC equipment. Therefore, you can expect to use up to 58% less solvent versus a comparable 4.6 mm i.d. column - with essentially no loss in efficiency, especially with retention of k' = 3 or greater.

Narrow-Bore
This column will not perform well with most existing equipment in HPLC labs.* It requires low volume injectors, detector cells, and connectors. Be sure to check the total extra column volume of your system to be sure it is less than 10 microliters. If it is more than 10 microliters, the performance of the column will suffer. Even as little as 50 microliters of extra column volume will cause a 45% loss in column efficiency. This column cannot be used with standard HPLC configurations, but with the appropriate configuration you can reduce solvent usage by 87% versus a comparable 4.6 mm i.d. column and appreciably increase the sensitivity of the method. Columns are available from reputable manufacturers for this size. If you plan to use a method where column reproducibility is important, be sure to buy your columns from a company where they are production items, and not "custom made," as the reproducibility of "one at a time" columns often is suspect.

Mini-Bore
As you might expect from reading the requirements above, mini-bore columns require the system extra-column volume to be exceedingly small - significantly below what is practical for most HPLC equipment. So do not try to use mini-bore columns unless you have invested in the proper equipment. Most column manufacturers have not yet made 1.0 mm columns available on a regular basis. The demand is very low and specialized in nature. You will have to be cautious in selecting equipment and column reproducibility most likely will not be good.