What is XRF?
X-ray Fluorescence (XRF) is a commonly used scientific technique for the environmental monitoring of particulates in air and finding inorganic contaminants in soils and sediments. The words X-ray and fluorescence may be somewhat mysterious to a non-scientist but can be explained simply.
Let’s start with the word fluorescence. Most people know the term from a fluorescent light commonly found in school rooms, factories and stores. The word simply means to radiate light or some other energy. A typical fluorescent lamp uses electricity to excite argon and mercury atoms, generating white light radiation, while an incandescent lamp generates electrons as light. An XRF instrument employs X-rays to “excite” a material in order to characterize its composition by identifying the elements in a sample (qualitative analysis), or by reporting the amount of an element in the sample (quantitative analysis).
The science of X-ray spectroscopy
So, what about the word X-ray. We mentioned that X-rays are the excitation source in an XRF instrument, but that’s not its main purpose in our definition. Characteristic X-rays are a by-product of the excitation process. And for this, we need a bit of science. An X-ray is a subset of the electromagnetic spectrum, which spans everything from radio waves, to visible light, to X-rays and gamma rays. All materials are composed of atoms, which are represented as elements in a periodic table.
Atoms are composed of subatomic particles: neutrons, protons and electrons. Protons and electrons are electrically charged, while neutrons are “neutral.” The true workhorse of chemistry, electrons are bound to the atom by the proton charge of its nucleus. When an atom loses an electron, it becomes ionized and the charge of the atom typically attracts the nuclei of other atoms to form chemical bonds. Most elements, particularly metals, tend to bond with oxygen and become oxidized – such as rust on iron. They can bond with themselves to form an elemental material or find a highly reactive atom like fluorene or sodium and cause all sorts of havoc. Most elements behave nicely with other elements in commonly found compounds. Organic compounds bind with carbon atoms, while inorganic compounds generally do not.
Classifying elements by characteristic X-ray energies
How do we get and classify characteristic X-rays? Electrons orbit the nucleus in a series of shells, labeled K, L, M, N, etc. When an inner shell loses an electron from an irradiation source, becoming unbalanced, it pulls another electron from an outer shell in order to stabilize. This transition emits an energy level at a specific point in the X-ray range. Put a detector into your instrument to measure these characteristic X-ray energies and voilà, you know what elements are in your material. If you look at a periodic table for XRF, you’ll find a series of numbers that represent the energies of the characteristic X-ray for each transit from outer to inner shells that determine the element. These energy levels are described in kiloelectron volts (keV).
Resolving conflicting spectral peaks
If you look at the X-ray energies in the XRF periodic table, you notice that when you get past nickel, the numbers from the L shell are only a few hundredths of keV. Also, lighter elements with electrons lost from their innermost K shell may exhibit the same energy level as a heavier element losing an electron from their L shell. For example, barium-L (4.467 keV) is almost the same as titanium-K (4.508 keV). Geologists and agronomists looking at minerals in soils are likely to get a large number of closely reported L-line energies in their samples.
Energy-dispersive X-ray fluorescence spectrometry
An energy-dispersive XRF (EDXRF) instrument excites and detects all elements from beryllium to uranium, providing a spectrum of energies with energy peaks that characterize the material. If users have some knowledge about the material, they can adjust the amount of excitation energy to select a more specific spectral range or use a filter to screen out conflicting energies. The XRF software can also use spectrum fitting algorithms to assist in calibrating an instrument for greater precision.
Wavelength-Dispersive X-ray Fluorescence Spectrometry
But in some cases, overlapping energies simply cannot be resolved in an energy-dispersive instrument. For this, we need Bragg’s Law and specialized equipment. Bragg’s Law describes how X-rays travel through parallel planes of atoms in a crystal. After the material is excited, the resulting X-rays are aligned and sent through a diffracting crystal, which acts as a highly sensitive X-ray filter. A device called a goniometer steps through the range of crystal angles, limiting the measurement to very specific wavelengths. Whereas in EDXRF, a sample with barium and titanium would show one wide peak of energies around 4.5 keV, masking the presence of two elements, the WDXRF spectrometer shows two distinct peaks from each element.
Using XRF for Air Quality and Soil/Sediment Samples
X-ray fluorescence is a go-to technique for environmental scientists and regulators testing particulates in the air and identifying contaminants in soils and sediments. X-ray fluorescence is the preferred method to analyze samples related to air monitoring due to the non-destructive nature of the technique and its sensitivity. EDXRF only needs 20 minutes to identify up to 40 elements from filters that collect ambient particulate matter. Since filters are analyzed nondestructively, they can be used for further analyses by other techniques. Our Thermo Scientific ARL QUANT’X EDXRF Spectrometer provides minimum detection limits that are better in standard than the detection limits prescribed in the US EPA Compendium Method IO-3.3 for XRF analysis of particulate matter.
The composition of mineral-rich soil and sediment samples requires further precision. For example, the energy level for lead is 10.549 keV while arsenic is 10.532 keV and cannot be resolved with EDXRF. WDXRF is the commonly used technique for the analysis of soil and sediments due to its accuracy, precision, wide range of concentrations (ppm to 100%) and its ease of use. Automation can be used to handle high throughput of samples across the widest element range. Designed for high-throughput environmental laboratories, our Thermo Scientific ARL PERFORM’X WDXRF Spectrometer is able to process more than 60 samples per hour, offering rapid and precise analysis of up to 84 elements.
Watch our Reveal Webinar “Analytical Tools for Air and Soil Analysis” to learn more.
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