An Overview of XRF Basics
3. Sample Preparation Techniques
for XRF Analysis
3.3 Preparation of Liquid Samples
Provided that the liquid sample to be analyzed is single phase and relatively involatile, it represents an ideal form for presentation to the X-ray spectrometer. A special sample cup (liquid sample holder) and helium path instrument must be used for measurement. The liquid phase is particularly convenient since it offers a very simple means for the preparation of standards and most matrix interferences can be successfully overcome by introducing the sample into a liquid solution. Although the majority of matrix interferences can be removed by the solution technique, the process of dealing with a liquid rather than a solid can itself present special problems which, in turn, can limit the usefulness of the technique.
For example, the introduction of a substance into a solution inevitably means dilution and this, combined with the need for a support window in the sample cell, plus the extra background arising from scatter by the low atomic number matrix, invariably leads to a loss of sensitivity, particularly for longer wavelengths (greater than 2.5 Å). Problems can also arise from variations in the thickness and/or composition of the sample support film. The most commonly used types of film are 4 to 6 mm Spectrolene and 2.5 to 6.5 mm Mylar.
The process of introducing a sample into a solution can be tedious and difficulties sometimes arise where a substance tends to precipitate during analysis. This itself may be due to the limited solubility of the compound or to the photochemical action of the X-rays causing decomposition. In addition, systematic variations in intensity can frequently be traced to the formation of air bubbles on the cell windows following the local heating of the sample. Despite these problems, the liquid solution technique represents a very versatile method of sample handling in that it can remove nearly all matrix effects to the extent that accuracies obtainable with solution methods approach very closely the ultimate precision of any particular X-ray spectrometer.
Where the concentration of an element in a sample is too low to allow analysis by one of the methods already described, work-up techniques must be used in order to bring the concentration within the detection range of the spectrometer. Concentration methods can be employed where sufficiently large quantities of sample are available. For example, gases, air or water that are contaminated with solid particles can be treated very simply by drawing the gases, air or water through a filter disk followed by direct analysis of the disk in a vacuum environment. Concentration can sometimes be effected simply by evaporating the solution straight onto confined spot filter paper.