Sample Preparation Equipment
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.
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.
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.
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.
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.
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.
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.