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Sample Preparation-Cups
Sample
Cups
There are a wide variety of sample cups available with many features. To best
compare them we will compare each feature, and show when it is beneficial. back
to top
Diameter
Cups come in a variety of diameters from 20 mm to 40 mm. For the most part
it comes down to individual user preference or the size of the hole provided
by the manufacturer but there are some important criteria for selecting
the diameter. Ideally the inner cup diameter should be larger than the
spot size on the sample. A quick way to determine spot size is to put a
drop of oil or water on a thin sheet of paper and analyze it for 10 minutes
at very high power. The x-rays heat the area they strike causing the liquid
to evaporate leaving a visible circle. Or just consult the instrument manufacturer.
The penetration depth of the x-rays for the elements of interest is also important.
For characteristic x-rays above 10 keV, penetration depths of more than 1 cm
are possible. With a 20 mm cup the x-rays may hit the cup wall. Larger diameter
cups, 40 mm, are recommended for higher atomic number elements in transparent
matrices to achieve the best performance. Alternatively the cup position must
be tightly controlled.
The third criterion is reproducibility of the film. The larger the diameter the greater the range of film heights at the center and the higher the probability that some cups come out wrinkled. For ease of assembly and best reproducibility 32 mm or smaller cups are often preferred.
A 6.3 mm cup is offered by Spex that is useful when only a limited volume of sample is available. back to top
Height
Height selection is also a function of penetration depth. It is important to
know the angle of the x-ray source when making this determination. If it
is at a 45 degree angle there is generally no benefit to having a cup that
is taller than it is wide. If the source points straight upward then a
deeper cup is beneficial when analyzing high energy x-rays from a x-ray
transparent sample. back to top
Single
Vs Double Open Ended
The obvious benefit to single open-ended cup is that it saves the step of putting
a cover on the cup, and they are usually less expensive. The problem with them
is that the amount of air that gets trapped varies causing the window film
to bulge different amounts each time. Even after venting the film will remain
stretched to a degree. A single open-ended cup will usually have poorer reproducibility.
This is exacerbated by the fact the fact that a bubble can form over the pin
hole causing the cup to pressurize and the film to bulge enough to affect the
results. In many cases this error is minor relative to the benefits, so they
are popular for analyzing high concentration elements in solution.
Double open-ended cups offer many other benefits. They offer a variety of choices for covering the sample. Even if a film cover is used, the pressure is distributed over both films causing less bulging. They are highly recommended for powdered samples since it allows the sample to be manually pressed. They are recommended when the highest degree of precision is required. back to top
Collar
Vs no Collar
Collars are intended to prevent wicking of the sample along the film and out
of the cup. They are recommended for all liquid applications. Tape can be used
instead of a collar. The Somar cup design uses a tall bottom retaining ring
designed to eliminate the need for a collar and it works provided that the
sample level is not above the top of the ring. back
to top
Cup
Covers
There is a lot of variation in cup covers. It is common to use no cover at
all for pressed powders, and some brave technicians will not cover solutions
or powders.
Window film is a common cover but it is labor intensive to install and causes both the top and bottom films to bulge slightly affecting reproducibility. It is recommended to put a pinhole it to equalize pressure. If the top film gets wet it is possible for a bubble to form across it holding in additional pressure. Cups with and without this bubble will read slightly different in some difficult applications.
A solid snap on cover is easier to install, but it flexes the bottom window as much as the single open ended cup and is prone to having problems with bubbles over the vent hole if it is allow to get wet. The solid covers area few cents more expense than a simple ring.
Solid vented covers are an excellent choice. They are easy to install, do not pressurize the cup and only cost a few cents more than a simple ring and the same as a solid cap. The holes should be large enough that even when wetted, bubbles do not form over the surface.
A variety of baffled cap designs are offered. Each is designed to slow the evaporation process while preventing the cup from becoming pressurized. Some are designed with the intent of causing the sample to reflux. These designs are slightly more expensive than a solid vented cap. Many operators have failed to find any benefit versus a solid vented cap, while others swear by them. They may be worth experimenting with when analyzing highly volatile samples. back to top
Inverted
Cups
Cups are offered that allow the samples to be inverted. One vender, Somar,
designed the cover so that it slightly depresses the Teflon window film covering
the sample. The cup is filled to the top and the Teflon film placed over it
leaving no air bubbles. The cups can then be inverted for use in instruments
with downward facing optics. The problem with this design is that the film
height changes from cup to cup, but many instruments with downward facing optics
have movable sample stages that can be used to position the sample reproducibly
enough for quantitative analysis. Other innovative designs have also been produced
for this purpose. back to top