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XRF: A powerful soil analysis tool

September 19, 2010
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Carpet and furniture cleaners want equipment and cleaning chemistry that works.

One of the problems today is with unsubstantiated claims.

With no fast, nondestructive reproducible method available to analyze soil, it''s no wonder that urban legends thrive.

Questions that concern us

These are some of the questions that have posed a challenge in the cleaning industry:

  • What is the composition of soil?
  • How much soil is left in the carpet after cleaning?
  • How effective are the cleaning chemicals or equipment in removing soil?
  • Does shampooing remove more soil than hot water extraction?
  • Are high-water flow systems more effective in removing soil than low-water flow systems?

Over time, various methods have been employed to evaluate soil removal; the soil removal process can range from visual, gravimetric (soil removal by weight), reflectance in which a photometer was employed after calibration with known soil samples.

Radio-labeled soils have also been utilized in evaluating detergency.

Comparisons to evaluate detergency have been made using both reflectance and radio-labeled soil methods.

Understand soil better

A soil is ordinarily composed in varying amounts of oil, wax, or grease; solid particles of diverse types; water-soluble or water dispersible materials such as sugar, salts, starches, etc.

For extremely light soiling, the darkening of the fabric is roughly proportional to the amount of soil.

As the degree of soiling increases, the amount of soil needed to give proportional darkening becomes enormously greater.

Real soils encountered in everyday use are many and complex and will vary with the locality, working conditions, age, and even economic status.

To run standardized tests one needs standard soil fairly representative of soils found in nature.

Over time, different researchers came up with different synthetic soil formulations, but they had shortcomings — the main one being the representative soils and reproducibility. (See "Where''s the dirt" in the side bar.)

Evaluating soil removal

To evaluate the cleaning process and have reproducible results a soil standard like the one mentioned in the sidebar must be employed.

A new method based on X-ray fluorescence for evaluating soil removal from carpets has recently been developed.

Scientists at the Professional Testing Laboratory of Dalton, GA and KeyMaster Technologies have developed a designer soil mimicking actual soil that emits fluorescence when subjected to X-rays.

The analyzer can read the fluorescence rays giving precisely the constituents of synthetic soil, and the amounts present (see Figure 1).

Under a licensing agreement between the National Aeronautics and Space Administration (NASA) and KeyMaster Technologies, a portable hand-held analyzer is being marketed to measure the amount of analyzer soil in the carpet.

What is X-ray Fluorescence analysis?

XRF (X-ray) spectrometry is an analytical method that determines the chemical composition of all kinds of materials.

The method is fast, accurate and non-destructive. X-ray spectrometry is a fairly simple technique that works by analyzing the emission spectrum generated when an atom is externally excited.

In XRF, X-rays are used to irradiate a sample and the elements present in the sample will then emit fluorescent X-ray radiation, with discreet energies characteristic to each element.

By measuring the energies of the radiation emitted by the sample, it is possible to determine which elements are present.

This method is called qualitative analysis. By measuring the intensities of the emitted energies, it is possible to determine how much of each element is present in the sample — known as quantitative analysis.

All matter contains elements and all elements are composed of atoms. Every atom contains an equal number of electrically charged protons and electrons and a number of uncharged neutrons.

Neutrons and positively charged protons are found in the central nucleus. The negatively charged electrons spin around the nucleus in discrete orbits.

The various electron orbits are called K, L, M, etc., where K is nearest to the nucleus.

How XRF works

X-rays are part of the electromagnetic spectrum (Figure 2).

Irradiating an atom with high-energy primary X-ray photons delivers sufficient energy for an electron to be ejected completely out of the atom.

The electrons of the inner shell K of the atom are ejected (Figure 3).

An outer shell L electron falls inward to fill the void created in the inner shell, and an X-ray characteristic of the atom''s elements is emitted (Figure 4).

The XRF analyzer''s detector gathers these emitted X-rays and the data are processed into an X-ray spectrum, which the instrument uses to measure the chemical composition of the sample.

In general, the X-ray spectral lines are proportional to the concentration of the elements in the sample, allowing quantitative chemical analyses.


X-ray fluorescence analysis has been used in thousands of applications since its introduction circa 1950.

The method is widely used for compositional control in large-scale industrial processing of metals and alloys, cements, and petroleum industry and inorganic chemicals.

It is an important method in materials characterization for research and technology providing chemical information without destroying the sample.

It is used in the analysis of pigments to establish authenticity of painting.

The method is also widely used in forensic problems where it is often combined with X-ray powder diffraction.

XRF spectrometry analysis is now one of the most respected techniques within the polymer and plastic industry for elemental analysis of catalysts, various pigments, flame-retardants, (UV)-stabilizers, and additives that are based on elements such as copper, zinc, iron, titanium, bromine, phosphorus and antimony.

In this industry in particular, XRF spectroscopy provides important advantages compared with other more time-consuming or even destructive analytical techniques.

The materials to be analyzed may be solids, powders, liquids, or films.

The crystalline state and the state of chemical bonding normally have no effect on the analysis.

All elements above the atomic number 11 from sodium to uranium can be routinely analyzed in a concentration range from 0.1 to 100 percent.

Lighter elements from boron to fluorine may also be detected. Special techniques are required for the analysis of elements with lower atomic numbers or of lower concentrations.

The counting times required for analysis range from a few seconds to several minutes per element, depending upon specimen characteristics and required accuracy, but may be much longer for trace analysis.

The results are in good agreement with wet chemical and other method of analysis.

Carbon, in its basic form, gives soiling the dark color. It is a light element, with an atomic number of 6 forms the majority of synthetic surfactants, oily soils, and organic matter.

The lower energy XRF emission from lighter elements means that they have less penetrating power and hence less sensitivity.

Carbon may be difficult to detect with the hand-held XRF analyzer.

Using X-ray fluorescence to determine soil removal from carpets

NASA modified and adapted the XRF method to quickly test alloys used in the space shuttle.

In collaboration with NASA, the Carpet and Rug Institute (CRI), KeyMaster Technologies and the Professional Testing Laboratory have adopted the method to evaluate different cleaning methods.

A study has been completed in which soil-like constituents were cleaned after dry vacuuming; where water was used as the cleaning agent and in some cases detergent based cleaning agents were used.

In the study, there was a clear correlation between the visual and the XRF, i.e. actual soil being removed.

Even though there was a direct correlation between actual and visual soil on the carpet, it cannot refute the fact that dark soil on a light colored carpet appears as more heavily soiled than on a dark carpet, whereas XRF may show the same amount of soil.

The CRI plans to use the XRF technology in testing of vacuum and extraction cleaners in its Seal of Approval program.

This is expensive equipment (in the tens of thousands) and testing to receive reproducible and scientifically valid readers the tests must be performed in a lab-controlled environment.

The synthetic test soil mixture must contain materials that fluoresce and are detectable by XRF.

XRF is a powerful analytical tool that is complementary to more commonly used methods.

Deployed as an analytical tool or in conjunction with other techniques, it offers a new tool in the arsenal of weapons available in the cleaning industry.

Only time will tell if this method of analysis will bring about revolutionary changes in the cleaning industry, or will just be an added dimension in analysis.

Aziz Ullah, Ph.D., MBA, is president of Fabpro Manufacturing, and is a leading formulator of top-quality carpet and upholstery cleaning products. He is a member of the American Chemical Society, senior member of the American Association of Textile Chemists and Colorists, and a member of The Textile Institute (UK). He can be reached at

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