Introduction

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 particles.

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 Micropipette

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 A 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. The pipette is filled by capillary action as well. Tip B dispenses the correct volume for extracting soluble components. The 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 needles^1,2, 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.