This process is used in many analytical laboratories and is very useful in the elemental analysis of solid, powdered and liquid samples. XRF and XRD require less sample preparation than many other analytical analysis methods. These systems accept liquid or solid discs; preparation of these discs may require the addition of binding aids (stearic acid etc.) or backings (H₃BO₃) if there is not enough sample present.

XRF/XRD is suited for robotic automation, robotics can generate repeatable sample preparation techniques as well as produce greater production outputs.

Robotic systems such as this one have much greater sample processing rates in comparison to the old way using human processing. Robots are not limited to set working hours, they do not experience much downtime, they have the ability to work 24 hours a day 7 days a week.

What is XRF/XRD?

It’s a non-destructive elemental analysis technique utilising the excitation and detection of characteristic X-Rays for each element. Able to analyse all elements from Be to transuranics. It is possible to analyse solid, powdered and liquid samples, as well as being able to analyse for halogens.

Sample Preparation

The importance of sample preparation is especially relevant today as XRF/XRD analysis plays a growing role in the daily activities of producers around the world. Fortunately, XRF/XRD analysis usually doesn't require extensive sample preparation work, and methods are normally inexpensive, easy to learn and easy to use. So even when the need for sample preparation is taken into account, XRF/XRD spectrometry is still easier and quicker than almost all other chemical analysis techniques.

XRF/XRD accepts liquids and solid discs. Particle size must be reduced by grinding to make samples homogeneous and representative of the bulk.

Material Methods

Glass: Homogeneous “Liquid”, no crystallisation

                        - Direct Measurement

Metals: Often very fine crystallised, approaching homogeneity

- Re-surfacing

Rock Material: Often consists of crystallites of different composition and size

- Grinding and pressing

                        - Fusion

Techniques

Grinding:

Use of a grinding aid may be performed to reduce time and improve homogeneity eg. Cellulose tetraborate.

Pressing:

Use of a binding aid may be performed to keep the sample together e.g. wax, stearic acid, however binders will reduce the analyte line intensity.

May require the use of a backing (H₃BO₃) if there is not enough sample present.

Fusion:

Sample to flux ratios are generally between 1+4 and 1+20 (sample+flux).

Types of flux:

• Borate fluxes of Na and Li (Na fluxes are hygroscopic[readily absorbs water from its surroundings])

• Poly-phosphates (for Boron analysis)

Furnace or fusion device:

• Induction heating
• Muffle furnace
• Gas burning

Requirements:

• Temperature and time
  • generally 1000-1200^oC [typically melting point + 200^oC]
  • Time can be influenced
• Additives
  • Efficiency of fusion and casting (LiF, B₂O₃, Li₂CO₃)
  • Non wetting (iodides, bromines, periodide’s, Chlorides of Li & NH₄)
  • Heavy absorbers (La, Ce, Ba oxides)
  • Internal standards
• Oxidising agent
  • Non oxidised samples require oxidation
    • Nitrates of Na, K, Sr (high temps)
    • Nitrates of Li & NH₄ (low temps)
    • Carbonates of Li & Na (low efficiency)
    • Mixtures
  • Many different recipes exist
• Cooling and Solidification
  • Cooling has an influence on crystallisation

Robotic Automation for XRF/XRD Sample Preparation

Robotic automation is being increasingly used in sample preparation laboratories in industry, the sample preparation involved for XRF analysis is ideal for robotic automation. Grinding and Pressing as well as fusion are all processes capable of being automated. Robots handle the conditions required for sample preparation easily and they will work non-stop 24hours a day. Robotics gives greater repeatability for sample preparation which is desirable and the corresponding analytical results are therefore more reproducible.

Technical Information on XRF

X-ray fluorescence (XRF) is the phenomenon where a material is exposed to X-rays of high energy, and as the X-ray (or photon) strikes an atom (or a molecule) in the sample, energy is absorbed by the atom. If the energy is high enough, a core electron is ejected out of its atomic orbital.

An electron from an outer shell then drops into the unoccupied orbital, to fill the hole left behind. This transition gives off an X-ray of fixed, characteristic energy that can be detected by a fluorescence detector. The energy needed to eject a core electron is characteristic of each element, and so is the energy emitted by the transition. The transition of an L shell electron dropping into the K shell is termed a Kα transition, while an M shell electron dropping into the K shell is a Kβ transition.

Typically the lightest element that can be analysed is beryllium (Z = 4), but due to instrumental limitations and low x-ray yields for the light elements, it is often difficult to quantify elements lighter than sodium (Z = 11).

There are two types of spectrometer:

• Wavelength Dispersive Spectrometers (WDX or WDS): the photons are separated by diffraction on a single crystal before being detected;
• Energy Dispersive Spectrometers (EDX or EDS): the detector allows the determination of the energy of the photon when it is detected; the EDX spectrometers are smaller (even portable), cheaper, the measurement is faster, but the resolution and the detection limit is far worse than the WDX spectrometers.