GENERAL DESCRIPTION
The IM method works on the principle that particles of
a given size and charge behave in a predictable manner
when carried in a laminar flow of air and subjected to
an electric field. A schematic of the system, starting
with an electrospray generator and ending with a condensation
particle counter, is shown in Fig. 1. In the IM
method for lipoprotein fractionation, the lipoproteins
in a volatile solution are introduced into a flow of air
(1.6 L/min) containing approximately 5% CO2 by
means of an electrospray. In the electrospray chamber,
the desolvated and highly charged lipoprotein particles
are nearly neutralized by ionized air, introduced by a
polonium -particle emitter present in the chamber.
The proportion of singly charged particles emerging
from the electrospray chamber can be calculated using
Fuch charge distribution (19 ). Fuch charge distribution
describes the probability that a submicron-diameter
particle of any given diameter will have exactly N
charges. The particles are then carried in the airflow to
the differential mobility analyzer (DMA). As the particles
enter the top of the DMA, they are confined in a
thin flow stream by a laminar concurrent flow of air
called a sheath flow. The particle-free sheath flow recirculates
through the DMA at a 20 L/min flow rate. As the
particles are carried through the DMA, an electric potential
across the sheath flow causes the particles to
drift toward a collection slit. Ramping the applied po
tential causes particles of different diameters to pass
through the slit, thus allowing lipoprotein particles between
30 and 542 Å to be sampled; the size range can be
extended to include larger particles if desired. At any
given electrical potential, particles of predictable size
pass through the collection slit and enter a separate air
stream (1.6 L/min) that carries them to a particle
counter. The particle counter first enlarges the particles
by condensing a vapor onto each particle and then detects
the droplets, now several microns in diameter, via
light scatter. Knowledge of the electrical potential applied
to the DMA, the dimensions of the DMA, and the
flow rate of air passing through the DMA permits accurate
calculation of particle diameter and the number
of particles in a discrete size range. Particle diameter is
determined from the following equation