- THE MAGAZINE
- WEB EXCLUSIVES
National and international legislation aimed at reducing the health and environmental impacts of a variety of harmful substances in consumer goods and other manufactured products has been introduced around the world. Controls exist not only on the materials used in manufacturing but also on the procedures used for disposal of products at the end of their useful lives. Different standards apply in different countries to products as diverse as electrical and electronic products, toys and cosmetics. Certain notorious heavy elements, particularly lead, figure widely in the legislation, so elemental analysis may be the only way of demonstrating compliance.
X-ray fluorescence (XRF) spectrometry is not only suitable for the detection of these heavy elements, but in many applications has the added advantage of being nondestructive. This makes it particularly attractive for rapid screening tests. Certain handheld XRF spectrometers can provide laboratory quality screening results in a matter of seconds.
Analysis is not always necessary for a manufacturer or processor to demonstrate compliance to these rules; supplier documentation with a strong audit trail may be sufficient.
When such documentation is not available, or the history of the item is unknown, analysis will usually be needed.
There are many analytical methods available for the detection and quantification of heavy elements. Most of them, such as colorimetry, atomic absorption spectrometry (AAS), graphite furnace atomic absorption spectrometry (GFAAS) and inductively coupled plasma optical emission spectrometry (ICP-OES), require the sample to be in solution. Dissolution can be time-consuming and results in the destruction of the object under test. Furthermore, these methods cannot easily be used for field measurements, for example, to screen a consignment of goods on receipt.
XRF spectrometry is a well-established technique for the measurement of most elements in the Periodic Table, and has its best sensitivity for the heavy elements, including nonmetals such as chlorine and bromine, which are environmentally significant but difficult to measure with some of the other techniques. It also can, in many situations, be nondestructive, portable and capable of producing results in a few seconds. This makes it an ideal technique for screening products for heavy metals.
Small, handheld XRF spectrometers have been designed for this task, and employ the latest XRF technology to achieve the performance needed for meaningful product screening.
The technique works by irradiating the sample with a beam of X-rays. This induces fluorescence in the atoms in the sample, which is then re
Analytical PerformanceThe two components that define the fundamental performance of an EDXRF system are the X-ray source and the detector. The stability of the primary source of X-rays affects both the ultimate detection limit of the instrument and the precision of the analysis. Some early handheld EDXRF instruments used radioactive isotopes as the source of primary X-rays, but these have associated safety and stability problems.
In certain small, handheld XRF spectrometers, for example, a miniaturized low-power X-ray tube-a close relative to the ones used in high-performance laboratory analyzers-ensures exactly defined excitation and hence good precision. The detector used is an advanced silicon drift detector (SDD). Compared to the Si PIN diode detectors used in many other instruments, the SDD displays better resolution and can process information 10 times faster, giving faster analysis, producing reliable screening results, such as approximately 15 to 30 seconds.
Pass/FailXRF is without a doubt the quickest and most convenient method for detecting heavy elements in manufactured goods, and is recommended in a number of official methods, including IEC 62321 and ASTM F2617, both of which describe procedures for the measurement of lead, cadmium, chromium, mercury and bromine by XRF. There are, however, a number of factors that need to be considered when interpreting the results from XRF screening.
XRF measures elemental concentrations, whereas the legal limit may refer to a compound of the element. Typical examples are the brominated flame retardants polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) regulated by RoHS at 1,000 mg/kg. These contain up to approximately 70% of elemental bromine, so the pass/fail level for a screening test would be around 700 mg/kg bromine.
Speciation is a similar consideration. Hexavalent chromiumVI is extremely toxic, trivalent chromiumIII much less so. XRF measures the total element concentration, and does not discriminate between the two valence states. However, as a screening test, XRF can indicate the presence or absence of chromium–-clearly if the total chromium in the sample is below the chromiumVI limit, the sample also must be below the limit.
Like any analytical technique, XRF can be subject to interferences from the sample matrix. One element present in a high concentration can cause errors in the measurement of another, minor element. An example might be the presence of high concentrations of bromine- or antimony-based flame retardant additives in plastics.
The fluorescence intensity itself also can be affected by the matrix as shown in these spectra of polyvinyl chloride (PVC) and polyethylene (PE) samples, each with the same lead content of 500 mg/kg spell out. This effect can be taken into account in the calibration procedures used.
If the sample is homogeneous, the result of the XRF analysis will normally be highly reliable. If not, caution has to be exercised in interpreting the results.
Being highly energetic, X-rays can penetrate a small distance into the sample. This means that if the sample is covered by a thin layer of paint or some other coating, the result will be a composite value of the concentrations in the paint and in what lies beneath.
By the same token, a thick layer of paint will give a higher reading than a thin layer, even if the concentration of the measured element is uniform throughout the paint. The same would apply to different thicknesses of plastic, as shown by these spectra of PE samples with 1-, 2- and 9-millimeter thicknesses, each with a lead content of 500 mg/kg. Some handheld XRF spectrometers have built-in thickness correction to overcome this problem. Sometimes it may be acceptable to mechanically homogenize the sample, for example, by milling, as a precursor to screening measurements.
The X-ray beam has a finite cross section. If the toxic material is concentrated in a very small area of the object that is smaller than the X-ray beam, once again an average result will be obtained. A small component on a printed circuit board might be such an example.
While XRF is recommended for screening measurements, the standard also recognizes that only methods that eliminate sample effects can be used as verification tests for the final confirmation of compliance. These are invariably solution techniques such as ICP-OES spectrometry.
It has been mentioned before that in the case of RoHS, analysis of finished products may not be needed if it can be demonstrated that fully compliant materials have been used in their manufacture. A high sensitivity, laboratory EDXRF instrument is suitable for screening bulk materials.
Repeatability, Precision and the Pass/Fail DecisionAs mentioned previously, when configured for RoHS screening, a handheld XRF analyzer can compare results with stored limit values and display results as above limit, below limit or inconclusive. Results that are well below or well above the limit do not present a problem. However, when the measured value is close to the limit value, any uncertainty in the measurement makes the decision more difficult.
The uncertainty of an analytical measurement is normally expressed using well-known statistical conventions. The precision of the measurement is expressed in terms of the standard deviation (SD, sigma, σ or repeatability) of a number of measurements and indicates the spread of the data about the average-or mean- result.
The smaller the SD, the better is the precision of the measurement. Two sigma is twice the standard deviation and indicates that 95% of readings will fall within this range.
Clearly any concentration reported that is of the same magnitude as the measurement error is going to be unreliable, which leads to the concept of limit of detection (LOD), the lowest concentration that can be reliably measured. This is conventionally three sigma.
For cadmium, the RoHS limiting value is 100 mg/kg. If the repeatability of the analytical instrument for this measurement is 5 mg/kg, it follows that if the analyzed value is smaller than 55 mg/kg, the limiting value has probably been met and the instrument will report below limit. If the analyzed value is higher than 145 mg/kg, the sample is above limit.
In the range between these values, the instrument reports an inconclusive result and more investigation will be needed. Clearly the smaller the value of sigma, for example, the lower the LOD, the narrower this gray area will be. The low LODs achievable with a handheld XRF spectrometer make it particularly efficient as a screening tool by reducing the gray area to a minimum and therefore the number of complicated and time-consuming laboratory analyses that will be needed.
Other Solutions for Compliance TestingWhile handheld XRF is the ideal tool for rapid product screening, certain types of samples may need a different approach.
The nature and variety of the samples encountered when testing products for compliance with new environmental standards like RoHS present analysts with many challenges. A handheld XRF spectrometer can handle a wide range of elemental screening tests with minimum uncertainty, reducing the need for complex and time-consuming laboratory analyses, but when laboratory analysis does become necessary, a range of handheld XRF spectrometers and ICP-OES instruments are available and capable of fully meeting those challenges. NDT