Micro-Extraction of Soluble Components from Small
Particles for Infrared Analysis
||Anna S. Teetsov, McCrone Associates, Westmont, IL
In a laboratory such as ours, very small particles are
received for identification every week. Light microscopy and infrared analysis
followed by SEM-EDX or Raman spectroscopy will normally suffice for
identification of the major components plus many of the minor ones in a
particle. However, organic components present at concentrations of less than 10%
can be difficult to identify by infrared and may not be observed. This paper
will discuss a simple and quick micro-extraction technique that can isolate and
concentrate small volumes of organics which may be present in particles.
Applications of this micro technique include separation and
identification of binding material in pigment particles in small paint chips
and isolation and identification of minute quantities of lubricant in wear
Ideally, as little as 0.1% of a 0.1 mm diameter particle can
be separated using this technique and analyzed by infrared spectroscopy.
The micro extractions are performed with polyethylene
micropipettes that can deliver nanoliter-scale drops of solvent precisely and
reliably, using capillary action, when the tip of the micropipette is touched
to a smooth surface. The size of the drops varies greatly with the tip diameter
and the amount of solvent in the micropipette. Figure 1 shows the dimensions
of the micropipette, the various tip sizes, the volume of the drops, and the
approximate spread of those drops on KBr or other polished surfaces at room
temp. Other micropipette characteristics have also been included in Figure 1.
Figure 1. Some interesting micropipette properties
colorful cap, which helps to locate the small pipette on the bench, must have
an opening so the pipette will dispense drops only by capillary action.
pipette is filled by capillary action as well.
B dispenses the correct volume for extracting soluble components.
same pipette is used for all solvents (except silicone). It is rinsed a few
times with the new solvent before it is used in the next extraction.
Micropipettes may be made from high density polyethylene
tubing or they may be made from low density 1-100 nL Eppendorf ™ Geloader Tips,
as described in The Particle Atlas1 and in The Microscope2.
Other Tools and Solvents
The micro solvent extraction is performed under a
stereomicroscope equipped with coaxial illumination, a transmitted light base
and up to 50X magnification.
In addition to the micropipettes, the following tools and supplies are needed:
Fine to medium tipped tungsten needles1,2,
polished KBr crystals, low E-glass slides, aluminum-coated slides or other
infrared-appropriate substrate, and a silicon wafer, ideally marked with a 1 mm
grid (to be used to check solvent cleanliness).
A set of frequently used organic solvents for extraction such
as amyl acetate and nonane should be available. Amyl acetate and nonane both
possess medium volatility and low to medium surface tension. Highly volatile
solvents may be difficult to work with, as they flash off too quickly. It
should be noted that some solvents contain non-volatile impurities which may
interfere with micro-analysis and should be pre-filtered, if this is the case.