Gas Flows in Packed and Micro Packed Columns

We received a question from a customer who did not obtain the correct flow through his micro packed column.  It is often not clear what flow is used in packed columns.  General the optimization  of flow setting using packed columns, does not have a priority as packed columns are –per definition-  low efficiency systems and a few plates less or more by optimization does not make a big difference.  For this reason, we see a lot of users also use nitrogen as carrier gas.

What are the typical flow ranges that we use with such columns?

Formula 1. Flow in ideal system

In an ideal system, the flow in a tube is quadratic with the internal radius, see formula 1. Here F is the column flow in mL/min, R is column radius in cm, U is the linear gas velocity in cm/s. It means that when the column radius is reduced by a factor 2, the flow will roughly reduce by a factor 4 keeping the linear velocity constant. In capillary tubes, acc to Poiseuille, compressibility becomes a major issue and flow is dependent on the radius to the fourth power, which was used in a special application for vacuum GC.

Table 1 Generall flow guideline for different diameter packed columns using SilcoSmooth tubing

Flow indications for SilcoSmooth packed columns are listed in Table 1.

It is clear that especially for the micro-packed columns, the flows are relative small.  That also means that for avoiding peak broadening in some detection systems, one may need to use some make up gas or use a detector designed for capillary columns. Especially for using TCD this can be important.

For higher mesh sizes one can use similar flow rates, but the restriction increases rapidly and this results in much higher pressures. Also compressibility will become more important.