How to Choose an XRF Analyzer

Nick Nugent

Introduction 

X-ray Fluorescence (XRF) is a versatile technique for elemental analysis, allowing the user to measure solids, powders, and liquids across a wide elemental range (typically B-Am). The wide range, ease of use, and stable calibrations make it perfect for Quality Assurance (QA)/Quality Control (QC) analysis, Research and Development, troubleshooting laboratories, and testing and inspection laboratories. This guide explores how to determine the best XRF analyzer for your specific needs.

 

What is XRF?

XRF is the characteristic radiation produced by an element when it is excited by an X-ray source with enough energy to expel an electron from its shell (K, L, M, etc.). Detectors are used to count the X-ray photons emitted from the sample. They are either an ionized gas chamber or a solid-state detector.

 

These detectors use either a scintillation/photomultipler combination or semiconductor properties to count the photons. Digital counters register the photons as counts per second (counts/s) hitting the detector, and the number of counts/s is proportional to the amount of each element in the sample. Calibration for this measurement is done using materials with known amounts of each element contained in the sample. These calibration standards, in most cases, can be bought from suppliers and are often referred to as Certified Reference Materials (CRM).

Unlike other elemental techniques, XRF cannot discriminate between oxidation states of an element. This means the total amount of an element in the sample is measured.

There are two other important things to consider with an XRF measurement:

1. XRF is a surface technique. Even the most powerful XRF analyzers will only sample a few microns (or a few cm for a light matrix and heavy element analyte) depth of the sample. That means homogeneity and surface preparation are important for solids to get an accurate result. Liquids that contain suspended solids that settle throughout the analysis will affect the results.

2. Since each element produces its unique characteristics of X-ray radiation, this radiation can sometimes induce secondary radiation or be absorbed by other elements in the sample. These factors can enhance or diminish the photon count and must be accounted for in the final measurement using corrective factors.

Determine the technology you need

The instrument you choose depends on your sample type, the number of elements, and the detection limits you need to measure. There are two main types of technologies, Energy Dispersive X-ray Fluorescence (EDXRF) and Wavelength Dispersive X-ray Fluorescence (WDXRF). Choosing between EDXRF and WDXRF will largely depend on factors like the complexity of your samples and the resolution required to meet your needs.

EDXRF

Energy Dispersive analyzers are typically low power, small (benchtop), and simple to use. Most systems make use of a polychromatic X-ray tube from a single anode material, and some beam filters for conditioning the excitation of the sample. X-rays are generally detected using a semiconductor material designed to distinguish between the different energies of incoming X-rays.

WDXRF

Wavelength Dispersive analyzers offer superior resolution for detecting specific elemental peaks. WDXRF instruments are best for applications requiring high precision or complex matrices. XOS uses MWDXRF(Monochromatic) technology, similar to WDXRF, that inserts an optic between the tube and the sample to monochromate the beam. This improves the excitation for each element and increases the signal to noise ratio. MWDXRF is used in the single element analyzers: Sindie (S), Clora (Cl), Phoebe (P), and Signal (Si).

Sample Preparation

Considering XRF is a surface analysis technique with a limited penetration depth, it is important to understand the resolution and accuracy of the sample you are preparing.

Solid Sample Analysis

Metals and alloys coming from a production facility are the primary type of solid samples analyzed. A representative sample, often called a coupon or “lollipop,” is produced from the batch for testing. When preparing the sample, the hardness of the metal/alloy and the nature of the crystalline structure need to be considered. Polishing just prior to analysis may be easiest but may introduce unwanted contamination onto the surface. Diamond polish is often used as carbon (C) is not typically measured with XRF. Softer metals might smear the surface during polishing creating inhomogeneity. In this case, an alternative like a lathe or a milling machine might need to be considered. In steel or aluminum production facilities, often the sample preparation and analysis techniques are coupled together using robot automation to maximize efficiency.

Polymer samples are typically small beads which can be measured in a liquid sample cup. They can also be melted and pressed into a disc which gives a more representative sample.

Liquid Analysis

To ensure accurate results when analyzing liquids, it is important to have enough liquid in the cup to achieve a critical depth as most will be light element matrices. Another consideration is whether there is a suspended or dissolved solid in the matrix. These solids may settle or precipitate during the sample preparation, waiting time, or during the measurement. These solids are representative of the sample but will alter the reported element concentrations as the solids settle. If you are unsure that the sample will settle, let it stand for some time before measuring. If the matrix is colorless and transparent, the settled solid might be clearly visible through the film window. The residue is part of the sample and should be analyzed to determine the total elemental sample concentration. Liquids can be measured in air, helium, nitrogen, or a vacuum to achieve the detection limits required.

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Powder Analysis

Powders may come from different areas, including chemical production, ores, minerals, or production processes. First, determine if the powder is fine enough to represent the total. For example, you may consider grinding the sample if it’s grainy or lumpy; however, grinding the sample with a mill can introduce trace amounts of the grinding vessel material in the sample. When choosing a tungsten carbide (WC) mill vessel as the grinding medium, tungsten should not be an analyte in the final result. Once a powder has reached the appropriate grain size, the next choice is whether to measure it loose in a liquid sample cup, pressed into an aluminum cup (with or without binder), or a pressed disc (used a lot in automation) and placed into the analyzer’s sample cup. The type of sample preparation you choose depends on the elements and detection limits, workflow of the lab, the number of samples, and level of automation.

Calibration Standards

As XRF is a comparative technique, the number of counts/s recorded need to be verified by samples containing a known quantity of that element. A newer XRF analyzer will give more count/s than an older analyzer due to aging of the tube and deterioration of other parts. Similarly, EDXRF analyzers with lower power tubes will also give fewer counts from a WDXRF system with a high-power tube. Additionally, measuring a standard under air, helium, nitrogen and vacuum path will give different results, mostly affecting lighter elements.

Things to consider when creating a calibration are:

1. Calibration Standards: Are there standards available with similar matrices as the samples and similar elemental concentrations as the critical elements? How many standards do you need to cover all elements at their respective concentrations in the sample?
2. Calibration Curve: Avoid relying solely on a two-point calibration (e.g., zero and a point just above the detection limit), as the calibration curve may not be linear. However, this approach might be necessary if no other options are available.
3. Concentration Range: If the sample’s concentration range varies largely, is it possible to measure on one calibration line? It may be possible if the matrix balance is water or hydrocarbon sample for example, but large swings in composition of a metal or mineral means it’s a different matrix.
4. Industry Standards: If the samples come from an industry known to have a lot of standardization (ASTM, ISO, DIN methods for example) then calibration standards are often widely available. In industries with fewer standards or more complex requirements, alternative methods may need to be explored.

Standard Addition

If matrix effects are interfering and no matrix matched standards are available, it is possible to spike the sample with a known quantity of the element of interest. By measuring the spiked sample versus the unknown sample, you can extrapolate the actual amount in the sample. This is mostly used for traces in the sample.

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Calibrating the Spectrometer

Assuming standards are available or can be created, the choice are:

• Create short range calibrations for each element, which will be more accurate but require more effort to perform and maintain.
• Create longer range calibrations, which cover more eventualities but are less accurate and require less work and maintenance.

Whichever calibration you choose, a graph with the counts/s versus the concentration of each element is produced, and a regression curve is calculated.

Take the Next Step

XOS is a leader in elemental analysis, offering solutions that improve public safety and efficiency in petroleum, renewables, plastic recycling, maritime, and other industries. XOS lab and process analyzers offer unrivaled precision at the push of a button. XOS offers XRF analyzers that measure sulfur, chlorine, phosphorus, silicon and a range of elements.

Not sure what technology or analyzer is best for you? Consult with one of our XOS experts to explore your needs.

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