Field-Flow Fractionation (FFF; introduced here) separates samples of particles in a flow by applying an external field that causes distinct solutes to have different concentration distributions. The greater the difference in the distributions, the greater the separation of different solutes.

Traditionally, FFF strives to have just a single external force applied to the solutes — more than one external field has been assumed to (a) just increase the net forces when the fields align (termed additive-mode FFF by Bruce Gale) or (b) simply decrease the net forces when the fields compete. One would never apply opposing forces on purpose.

However, while trying to develop models for understanding hyperlayer-FFF (when unavoidable lift forces oppose the external field), I was surprised to find that two opposing fields can produce a dramatic drop in elution time of very specific solute sizes. Gary Slater and I termed this novel technique adverse-mode FFF”. By applying two strongly-opposing external forces, which scale differently with solute size, a sharp peak occurs in the speed of a solute for a given size. For solutes smaller than the peak position, the force that scales less strongly with size dominates. For larger solutes, the force that scales more strongly with size dominates. Exactly at the peak position, the two opposing forces balance. By increasing both field strengths together, the peak width can be narrowed and be made quite sharp.

All of this occurs for realizable field strengths. To measure an unknown solute size, adverse-mode FFF could following this recipe:

• An external field that scales directly with solute radius (such as flow-FFF, electrical-FFF or thermal-FFF) is applied to an eluting sample.

• A second field that scales with the solute volume (radius cubed; such as centrifugal-FFF) is applied to oppose the first field.

• By incrementally decreasing the strength of the second field from an initial large value, the peak position would shift to smaller values and an abrupt increase in the speed of the solute would occur when the peak location matches the solute’s size.

• The solute size can then be calculated.

• Having found the solute size, the accuracy can be improved by increasing both external field strengths together.