Powder pressing method is simple, fast, and low-cost. It is suitable for trace analysis of medium and heavy elements as well as the determination of main, secondary, and trace elements in the relatively consistent category with no high accuracy requirements. As one of the commonly used sample preparation methods for XRF analysis, it is particularly suitable for the analysis of large quantities of solid powder samples. It is to put into a special mold the powder sample that has been dried or roasted, ground and mixed evenly, and then samples for XRF analysis are produced with pressure of a pellet press. In order to preserve well and prevent any tablet edge damage, the powder pressing method with the metal ring (or plastic ring) has been used since the 1980s, but gradually replaced by the pressing method with lining and bottom material which has high efficiency but low risk of contamination. At present, the commonly used lining and bottom materials are boric acid and low-density polyethylene(PE). The following is a practical comparison of these two materials provided by Yang Yong, the engineer who has many years of maintenance experience for pre-treatment equipment for XRF analysis.

1. Usage method

The molds and methods for boric acid and low-density polyethylene are exactly the same, and they can be interchanged. But low-density polyethylene can be directly pressed into cups(see the figure below) for direct use in sample preparation, improving sample production efficiency.

2. Application comparison

Table 1

Since the physical properties of boric acid and low-density polyethylene show great unlikeliness (see Table 1), the difference during production is obvious:

(1) Low pressure polyethylene has good chemical stability to be prefabricated into cups. Besides, using a funnel to add samples can be deleted in the process of sample preparation, shortening production time and improving production efficiency. As to boric acid, it is volatile and water soluble with short-term preservation.

(2) The granularity of low pressure polyethylene is larger than that of boric acid, resulting less risk of molds contamination during use.

(3) Boric acid is volatile so that long-term use under the influence of thermal effects(X-ray exposure to the sample) will lead to a decrease of fluorescence vacuum and reduce the service life of the optical tube; yet the use of low-pressure polyethylene reduces the pollution risk of the instrument.

(4) The fly-edge phenomenon occurs in the outermost ring of the sample after the low-pressure polyethylene powder is completed(the fly-edge is removed before the measurement); Rigid boric acid does not.

(5) The rigidity of boric acid causes busting out easily when making high-pressure samples, while polyethylene does not.

3. Summary

In summary, these two materials in the sample preparation by powder pressing method for XRF analysis have the following advantages and disadvantages:

Table 2

After comparison, we can conclude that low density polyethylene with obvious advantages as a reagent for lining and bottom can completely replace boric acid.