Guidance on measuring trace amounts

12th December 2017

Fig 1. Typical sample preparation methods
Fig 2. Sample preparation methods discussed in this article
Fig 3. Example of use of flat tyre dies

This paper introduces sample preparation methods useful for trace sample (small pieces, small amounts of powder, etc.) analysis such as that performed by the Rigaku ZSX Primus series of sequential wavelength dispersive X-ray fluorescence spectrometers. Fig. 1 shows typical sample preparation methods for X-ray fluorescence analysis.

Selection of sample preparation method
There are various sample preparation methods for trace amounts of sample depending on the sample size and amount. Fig. 2 shows the methods discussed here.

The choice of sample preparation method should take into consideration the need to recover the sample, the purpose of the analysis and the equipment specifications.

The method where the sample is embedded in a base material (sample embedded method) is popular when sample recovery is not necessary. Although this method makes it difficult or impossible to recover and reuse the sample, it is an easy sample preparation technique if pressing dies can be used. On the other hand, if sample recovery is necessary, accessories that will prevent spillage of a powdered sample, such as a thin film holder, trace sample cells, etc., will be required.

If only qualitative analysis is required, the sample holding method can be selected according to the sample size and amount. In the sample embedded method, if the sample powder area is smaller than the measuring area, a measurement without the sample (a blank measurement) must be performed to determine components in the diffraction pattern that arise from the base material.

A blank measurement is also necessary when the sample is a thin film.

Sample preparation method for trace sample amount
When sample recovery is necessary. Sample embedded method can be used for fragments of solids and for small amounts of powder.

The sample is embedded onto the base material by pressing lightly, and then pressed thoroughly to hold the sample. As this method embeds sample into the base material directly, recovery of the sample is very difficult or almost impossible. However, when the necessary tools for pressing samples are available, it is easy to make pellets by preparing only base material.

A material that has good formability and does not contain analyte elements is optimal as the base material.

An example of the sample embedded method with flat type dies is shown in Fig. 3. Pieces of broken glass and several hundred milligrams of cement powder are embedded in boric-acid.

When sample recovery is necessary. For analysis of a small piece of a foreign substance, the minute sample cell is suitable for holding a small piece of a foreign substance, such as broken pieces of metal. The accessory consists of a polyethylene cell covered with a thin film, a lid with a screw and an inside board

Analysis of a trace amount of powder
A sample cell for trace amount of powder sample is suitable for holding powder samples of up to several hundred milligrams. The sample can be easily recovered after analysis because no press forming is required. The accessory consists of the sample pan, a polyethylene cell and a base cap.

A sandwich type sample cell for trace amounts of sample is suitable for holding dust, flakes, etc. by placing them between two thin films (sandwich method). It is used mostly for qualitative analysis. The accessory consists of a polyethylene sample cell and an acrylic base plate.

Powder samples in the amount of several tens of milligrams, insufficient for minute or trace amount sample cells, can be measured using a combination of accessories. Both qualitative and semi-quantitative analysis can be performed.

Selection of thin film
Because thin films are frequently used to prevent spillage when measuring a trace amount of sample, considerations for the selection of film type are described.

Available thin films types are polyester, polypropylene, polyimide, etc. with thicknesses of around several µm–10 µm. Polypropylene, which has a high X-ray transmission rate and low levels of impurities, is normally suitable for the analysis of small pieces of foreign substance and small amounts of powder sample. However, different films may be chosen to take advantage of their respective properties. Considering the various film properties, polyester 6 µm and polypropylene 6 µm films are generally the most commonly used.

In many cases, polypropylene film is preferred due to its high X-ray transmission rate and low impurities. However, because its durability against X-ray irradiation is relatively low, the measurement time should be limited to below around 15 minutes when a high-power X-ray tube (4kW maximum)
is used. Lowering the X-ray tube power to around 2kW can significantly reduce damage to the film. Use of a primary X-ray beam filter is also very effective. If the analytes are mostly heavy elements, either polyester or polyimide film can be used.

It was shown that by using special accessories in combination with sample preparation equipment and unique preparation methods, challenges due to small sample amount can be overcome for routine analysis.

Satoshi Ikeda is with Rigaku.





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