Paramagnetic Particles and Magnet properties


We use longer magnetic cylinders to further the distance from "North Pole" to "South Pole". This has the effect of getting the PMPs to travel at the highest rate possible given the innate strength of the magnetic material.

There are many parameters that effect the rate at which the PMPs travel to the magnetic posts.

Such factors include:

  • the size, strength and composition of the magnets
  • the makeup and size of the PMPs
  • the viscosity of the assay medium
  • "the inverse square law", that is the distance between the magnets and the PMPs

These Neodymium-Iron-Boron magnetics represent the state of the art and are economical and robust. We use these magnetic separators for our assays in this configuration as a first generation separation method.


Most automated immunoassay methods now make use of PMPs. We are developing manual methods to rival the automated methods but for now this "pull-aside" method using bottom magnets is the most popular.

This "pull-aside" method not only allows fluid to be aspirated without effecting the particles, but cellular debris or particulates that may contribute to increased non-specific binding fall to the center of the well and are aspirated during the wash cycle.

By far the most significant advancement in PMP technology involves the down-sizing of the particle to 300 to 1,000 nm (a micron) coupled to Streptavidin at costs that are almost insignificant when using more sensitive detection technologies like chemiluminescence.

The particle size of 300 to 1,000 nm is magic for most immunoassays.  Not only do you achieve "solid phase convenience at liquid phase kinetics" which translates into reduced incubation periods, but these smaller particles do not settle during incubation and the need for agitation during the assay is avoided.

If a particle size of 300 to 1,000 nm is magic for assay kinetics and they stay suspended during 15 or 30min incubation periods without the need for mixing, there is a point of diminishing returns.  PMPs as small as 10 nm are used for inclusion into cells and are used to move these cells to the magnets. However, the smaller the PMP the longer the transit time to the magnet.

Our sandwich and competitive immunoassay separate in less than 2 minutes.

So what about the size of the PMP pellet?  We initially found that we had to add "sterile MedFlies", sorry Jerry Brown, to see the spots. Very few active antibody or immunosorblent PMPs are actually needed for the assay. One strategy is to add magnetic particles without streptavidin to the active particles to make the PMP spot larger and more visible. I approached the vendors of the PMPs to offer color PMPs for visualization but no takers. In the end, we decided to trust in Science and go with just the PMPs needed for the assay, even if the spot was not observable.

Listed below are two of the better suppliers of PMPs with streptavidin.
There are at last count over a dozen other vendors, and the quality of the products vary considerably.

Dynal streptavidin coupled paramagnetic beads

Solulink Biosciences
 


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