Introduction

X-ray diffraction (XRD) has been the indispensable tool for identifying crystal phases.  The most common use of the X-ray diffraction phenomenon is in powder diffraction instruments where several randomly oriented crystals in a small amount of powder are rotated in an X-ray beam.  Rotations in several directions expose planes in the sample’s crystal lattice multiple times and produce distinct, detectable diffraction events particular to the sample.

Typical powder instruments use sealed tubes to generate X-rays.  While these tubes are low-cost and easy to maintain, their X-ray flux can only analyze samples 100’s of micrometers in size in a reasonable timeframe because of the low probability of a constructive diffraction event being detected. For samples in the 10 micrometer size range, the long time needed (10’s of hours) to produce enough diffraction events to create a usable XRD pattern would not be practical. More X-rays are needed to increase the diffraction probability.  Another option is to analyze the particle at a synchrotron source, also not cost-effective and most likely inconvenient.

These limits of particle analysis have been overcome at McCrone Associates, Inc. with the implementation of a Rigaku MicroMaxx-007 rotating anode source combined with the RAPID-SPIDER X-ray detector.  The instrument has successfully analyzed particles as “micro-powder” samples down to 6 micrometers in size.

Sample Preparation

A focused X-ray beam requires that a sample be positioned precisely in its path.  Powder diffraction requires that the sample must have rotation in at least one direction; using more than one rotation direction simultaneously is advantageous to allow as many crystallographic planes to be exposed as possible.  The apparatus holding the sample must not be crystalline itself and it should not be a highly absorbing material; both qualities would interfere with the sample pattern.

Typical powder diffraction instruments mount particles on glass fibers too large for particles in the 10 micrometer size range. At McCrone Associates, our glass fibers are manufactured in-house by our cleanroom staff. The tip of a glass rod is heated to melting and pulled to form a thinner fiber approximately 5-10 micrometers in size (Figure 1).

click image to enlarge (59K) Figure 1

An example of a glass fiber pulled to approximately 5-10µm in diameter to load particles for micro-XRD. 

The particle to be analyzed is located in a stereomicroscope and adhered to the glass fiber with a small amount of soluble gum.  Although these materials-glass fibers and soluble gum-seem simple, they are remarkably robust. Standards mounted on fibers over three years ago using this method are still in use today.

Although the majority of our samples are micro-size, two other major types have been analyzed. Particle samples or a group of particles that total a larger size (~50 to 100’s of microns) can be adhered to a MiTeGen® mount, manufactured by MiTeGen, LLC.  These mounts are made of a polyimide polymer which has low X-ray absorbance and scatter (Figure 2).

click image to enlarge (72K) Figure 2: Top portion of a MiTeGen mount.

For many powder samples, the amount is in the microgram range, too little for a typical theta-2theta scanning instrument.  We utilize 0.1mm inner diameter S-glass capillaries from the Charles Supper Company for limited powder samples (Figure 3). A small amount is placed in the funnel end of the capillary and the sample is vibrated or tapped into the narrow portion.

click image to enlarge (32K) Figure 3: Sealed end of a 0.1mm diameter S-glass capillary filled with powder.

Although these capillaries have low X-ray absorbance, there is still some scatter; an empty capillary image is usually subtracted from the sample image to remove the scattered data.