Application of X-ray Fluorescence Elemental Analysis to Factory Level Paint Weight Quality Control. Measurements by non-laboratory personnel provide precise and rapid analytical data on the quantity and uniformity of applied coatings. XRF has proven to be an effective method for determining the weight of silicone coatings on paper and films, the loading of titanium dioxide on paper and the amount of silver on films.
XRF is a fast, non-destructive and comparative technique for the quantitative determination of elements in various matrices. XRF units are available in a variety of packages; however, unit types that are prevalent in the coatings industry are described in this chapter.
XRF benchtop analyzers utilize low levels of radioactive isotopes placed near the sample. Primary x-rays from the excitation source strike the sample, producing secondary x-ray fluorescence. These secondary x-rays have specific energies that are characteristic of the elements in the sample, independent of their chemical or physical state. These X-rays are detected in a gas-filled counter that outputs a series of pulses whose amplitude is proportional to the energy of the incident radiation. For example, the number of pulses from silicon x-rays is proportional to the silicon coating weight of the sample. Since the technique is non-destructive, samples can be reused at any time for further analysis.
To ensure optimal excitation, different applications may require alternating radioisotopes. For titanium dioxide in silicone coatings and paper, an iron-55 (Fe-55) source is used. Fe-55 x-rays are soft (low energy) and cannot penetrate deeply into the sample. For the silver on film, a more vibrant American light source was used.
High sensitivity can be achieved by placing the sample only a few millimeters from the excitation source. The closer the sample is to the light source, the higher the irradiation efficiency. For low-energy x-rays, such as silicon, which are easily absorbed by the atmosphere, a helium purge should be used. With good sample irradiation and helium purge where necessary, count times of 2 minutes are typical for most samples.
Calibration curves for different elements and materials can be stored directly in the instrument and recalled. Curves are established by measuring a set of known samples or standards of the same material. Since XRF is a comparative technique, subsequent analyzes depend only on the quality of the calibration standards provided.
The total cumulative intensity is actually a combination of the signal from the analyzed element and the signal from the background matrix. The background intensity will vary depending on the thickness or basis weight of the material. These differences can be determined by measuring uncoated or blank samples, which are automatically included in the calibration.
