Micro Powder X-ray Diffraction in the Laboratory
||Joseph R. Swider, Ph.D., McCrone Associates, Inc. Westmont, IL
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
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.
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).
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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).
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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.
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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.