If you have ever ordered a packed column, you are aware that mesh size of the solid support (packing) is one of the specifications you will need to know. But have you ever wondered what mesh size actually means?
To find the answer, let’s look at the products and process. The necessary products are ASTM Certified Standard Brass (or Stainless Steel) Test Sieves*, like shown below, and of course, the material to be screened.
* Depending upon the specific procedure, Compliance, Inspection or Calibration test sieves may be needed.
In order to obtain particles within specific a mesh size range, two screens are used in addition to a cover and a pan. The pan is placed on the bottom, then the top and bottom screens are placed on the pan. The top screen will have larger openings than the bottom screen.
Let’s assume in this example that the top screen is 80 mesh, which means that for every linear inch of screen, there are 80 little square openings. Let’s also assume the bottom screen is 100 mesh, meaning that there are 100 little square openings for each linear inch of screen. As you can guess, the openings in the 100 mesh screen are smaller than in the 80 mesh screen. If interested in reviewing additional sieve screen specifications, click here.
The bulk material is loaded on the top screen, the cover is placed on top, and the sieves are shaken (manually or mechanically) side-to-side while gently tapped. The particles which are small enough pass through the 80 mesh screen fall onto the 100 mesh screen, and if the particles are small enough, they also pass through the 100 mesh screen and are caught in the pan (these particles are referred to as “fines”). The particles which are caught on the 100 mesh screen are the desired size, and are referred to as 80/100 mesh size particles.
You will notice that many test sieves are often designated using the term microns (µm) instead of mesh. The relationship between these two terms can be found in this link.
So now that you know about particle mesh size, how does choosing a different range affect your analysis? Generally speaking, using smaller particles (100/120 mesh) rather than larger particles (80/100 mesh) of the same packing in the same dimension column will provide more separation power (theoretical plates), but carrier gas head pressure will need be higher (to maintain constant flow rates) and longer analysis times are common (especially for isothermal analysis).