I've been digging in the literature and finding that a good number of people have been using nonporous particles (C18) rather than porous. I was always of the understanding that porous was better due to larger surface area. Can anyone shed some light on this for me?
Thanks in advance

Oooohhh, that is a long story....

Basically,
Non-porous allow for smaller particels since the backpressure is less. In addition, a higher flow rate can be used. Mass transfer is thus improved, while capacity is less.

See LC/GC North America HPLC 2003 for review on recent supplier marketing. Some claims that these are equal or better than monolithic materials.

No Einar, I disagree... The backpressure of porous particles and non-porous particles is absolutely the same. Therefore the same flow rates can be used. Nothing higher, nothing smaller. The only thing that has an effect is the width of the particle size distribution, and even that is not a large effect.
The only reason for the existence of the non-porous particles in the smaller size range is the particle sizing technology, which becomes more difficult if the particles are lighter and smaller.
Therefore, the major disadvantage of non-porous particles is the fact that they have only a very small surface area. The major advantage is that they exist in particle sizes under 2 micron, but they are not unique any more....

Actually, in some cases (such as in the case of "perfusion" media with large pore size) the pressure for porous materials will actually be less than for non-porous materials at the same particle size but I agree with Uwe that the pressure should never be greater for porous materials except in the case of soft, deformable "gel" particles. Another point regarding non-porous materials is that for molecules which diffuse relatively slowly in solution (eg. proteins), non-porous media can provide significantly lower peak widths in gradient mode.