How XRF Works

TECHNOLOGY

Sample Analysis Via Energy Dispersive X-Ray Fluorescence (EDXRF)

Handheld Thermo Scientific NITON^® energy-dispersive x-ray fluorescence (EDXRF) analyzers, commonly known as XRF analyzers, are able to quickly and nondestructively determine the elemental composition of:

● Metal and precious metal samples
● Rocks and soil
● Slurries and liquid samples
● Painted surfaces, including wood, concrete, plaster, drywall and other building materials
● Dust collected on wipe samples
● Airborne heavy elements collected on filters

Thirty or more elements may be analyzed simultaneously by measuring the characteristic fluorescence x-rays emitted by a sample. NITON XRF analyzers can quantify elements ranging from magnesium (element 12) through uranium (element 92), measuring x-ray energies from 1.25 keV up to 100 keV. These instruments also measure the elastic (Raleigh) and inelastic (Compton) scatter x-rays emitted by the sample during each measurement to determine, among other things, the approximate density and percentage of the light elements in the sample.

XRF Excitation Model

How does EDXRF work? Each of the elements present in a sample produces a unique set of characteristic x-rays that is a "fingerprint" for that specific element. EDXRF analyzers determine the chemistry of a sample by measuring the spectrum of the characteristic x-rays emitted by the different elements in the sample when it is illuminated by x-rays. These x-rays are emitted either from a miniaturized x-ray tube, or from a small, sealed capsule of radioactive material.

A fluorescent x-ray is created when an x-ray of sufficient energy strikes an atom in the sample, dislodging an electron from one of the atom's inner orbital shells. The atom regains stability, filling the vacancy left in the inner orbital shell with an electron from one of the atom's higher energy orbital shells. The electron drops to the lower energy state by releasing a fluorescent x-ray, and the energy of this x-ray is equal to the specific difference in energy between two quantum states of the electron.

When a sample is measured using XRF, each element present in the sample emits its own unique fluorescent x-ray energy spectrum. By simultaneously measuring the fluorescent x-rays emitted by the different elements in the sample, handheld Thermo Scientific NITON XRF analyzers rapidly determine those elements present in the sample and their relative concentrations – in other words, the elemental chemistry of the sample. For samples with known ranges of chemical composition, such as common grades of metal alloys, a NITON analyzer also identifies most sample types by name, typically in seconds.

Light Element Analysis

Metal Sample Fluorescence

It is important to note that, except with special hardware, light elements cannot be measured directly with handheld XRF analyzers, simply because x-rays with energies below 2 eV – including the characteristic x-rays of all elements lighter than sulfur (element 16) – are largely absorbed in air within a short distance. For this reason, light element XRF analysis is best performed either with a helium gas purge or in a vacuum chamber in a laboratory environment. As the use of a vacuum with portable XRF is highly impractical (even minor punctures to the thin window used to seal the instrument from the environment will draw dust, debris and metal filings into the instrument), then an He purge unit is the most appropriate solution for light element analysis (Mg, Al, Si, P, S, Cl).

NITON Analyzer Sample Analysis Techniques

Handheld Thermo Scientific NITON XRF analyzers automatically compensate for many effects that would otherwise bias or distort sample analyses. These effects include:

● Geometric effects caused by the sample's shape, surface texture, thickness and density
● Spectral interferences
● Sample matrix effects including critical absorption of the characteristic x-rays of one element by other elements in the sample, and secondary and tertiary x-ray excitation of one or more elements by other elements in the sample.

By automatically adjusting for these effects, NITON XRF analyzers are able to determine the chemistries of samples of widely different sample compositions, typically in seconds, without any requirement for instrument users to input empirical, sample specific calibrations. In typical samples containing many elements, the elements may range in concentrations from high percent levels down to parts per million (ppm).

In sample matrices such as typical mining samples, metal and precious metal alloys, it is necessary to measure both lighter elements that emit lower energy x-rays (that are easily absorbed) as well as heavier elements that emit much higher energy x-rays (that penetrate comparatively long distances through the sample).

NITON Analyzers and X-ray Fluorescence

Compensation must be made for a variety of geometric effects. In these multi-element samples, it is also possible that one or more elements present act as critical absorbers. The effects of absorption, enhancement, and secondary fluorescence vary widely depending on the chemistry of the sample matrix, but in a sample with many elements in substantial concentrations, multiple absorptions, secondary and also tertiary x-ray fluorescence effects are typically present.

NITON XRF analyzers compensate for all of these effects in order to determine the actual concentration of elements in multi-element samples from the modified fluorescence x-ray spectrum that these samples produce in the XRF analyzer. To do this, we employ multiple methods to determine the true composition of these complex samples from their x-ray spectra. These include:

● Fundamental Parameters (FP) analysis
● Compton Normalization (CN)
● Spectral matching (“fingerprint”) empirical calibrations
● User-definable empirical calibrations
● Various combinations of these techniques.

Fundamental Parameters (FP) Analysis

For measuring samples of unknown chemical composition in which concentrations of light and heavy elements may vary from ppm to high percent levels, Fundamental Parameters (FP) analysis is used to simultaneously compensate for a wide variety of geometric effects (including small and odd-shaped samples), plus x-ray absorption, and secondary and tertiary fluorescence effects. FP is the preferred analysis tool for mining, precious metals and all metal alloy testing applications. Using this powerful in-factory calibrated instrument, a NITON analyzer can then measure the full range of element concentrations in a wide variety of samples for years without any additional calibrations or user input of any kind.

Compton Normalization

Compton Normalization XRF techniques provide the best results for a wide range of environmental testing and some mining applications, particularly when it is necessary to measure sub-percent concentrations of heavy elements in samples composed mainly of light elements. In environmental testing projects, it is often highly desirable to be able to quickly measure low concentration levels of all of the eight Resource Conservation and Recovery Act (RCRA) heavy metals (Ag, As, Ba, Cd, Cr, Hg, Pb, Se) on site and in real time. Using Compton Normalization, NITON XRF analyzers can measure concentrations of many heavy metals.

Lead Paint Analysis

Heterogenous samples, such as Pb and non-Pb layers on painted materials, require the highly advanced lead paint algorithms that we have developed and patented.

Spectral Matching Empirical Calibrations

In the first empirical testing mode, the user "teaches" a sample to the instrument with a one-minute measurement, naming the sample at the same time. The sample's x-ray spectrum is then stored in a dedicated library on the analyzer, which holds thousands of these readings. When an unknown sample is measured in this mode, the new spectrum is compared to the taught spectra stored in the library via least-squares fit analyses. If the new sample spectrum meets the specific sample-matching criteria (defined by the user) for one of the stored sample spectra, the new sample is matched and identified by the given name of that stored sample. This Signature Match Mode is similar conceptually to doing fingerprint analysis.

In a second empirical testing mode, the user combines Signature Match Mode with chemistry from a known (e.g., lab certified) sample, storing this data in the instrument. When an unknown sample is measured in this mode, the sample spectrum is compared to the sample spectra stored in this library. In this mode, the unknown sample chemistry is calculated via extrapolation and interpolation from the stored chemistries of the named samples in the library, and the calculated chemistry is then compared to a stored grade look up table of chemical compositions. Empirical testing modes are well suited for measuring samples for which the chemical compositions are reasonably well known in advance.

User-definable Empirical Calibrations

While most applications are readily supported with the NITON analyzer’s robust calibration algorithms, there are occasional samples that require specialized processing to supply the most accurate results. For these few cases, the empirical testing mode provides users the ability to have their own application-specific empirical calibrations. These calibration equations may then be overlaid onto existing FP or CN calibration models, or used to create completely new modes to optimize analytical results for specific applications. Users can request custom linear regression equations, specifying the analyzed elements and elemental interferences for real-time processing on the XL3 during sample measurement. Empirical testing modes are well suited for measuring samples for which the chemical compositions are reasonably well known in advance.

NITON XRF Analyzers Make Their Users More Productive

Because of FP analysis and other advanced technology, in a variety of testing applications, NITON users require little if any specialized knowledge or laboratory training to work effectively in the field. In industries like mining and mineral exploration this means non-technical personnel can perform the work. In metal and precious metal analysis, the Thermo Scientific NITON XRF analyzer's standard FP algorithms are used in combination with a built in grade-identification library that enable the user to identify hundreds of metal and/or precious metal alloy grades in seconds. Costs are lower and users more productive with immediate on-site analysis – no more waiting for lab results

From the inside out, these revolutionary instruments were designed to make our customers more successful. Capitalizing on the explosive growth of consumer electronics, they incorporate 80 MHz real-time digital signal processing, and dual state-of-the-art embedded processors for computation, data storage, communication and other functions. The very best in technology has been engineered into the Thermo Scientific NITON XL3t and XL3p to provide users with high-performance today, scalability for tomorrow, and a robust foundation to develop future features and applications.