Energy dispersive X-ray fluorescence (ED-XRF) instruments perform nondestructive analysis of a wide range of elements with accuracy and precision. They are relatively inexpensive and are an excellent complement to other laboratory tools, providing rapid analysis that requires limited sample preparation. Based on these benefits, benchtop XRF has become an important tool in analytical laboratories since the development of ED-XRF in the 1960s. In recent years, however, many organizational laboratories have begun looking to a new development in X-ray fluorescence technology to solve testing needs—the portable XRF system. Handheld X-ray fluorescence allows organizations to “take the laboratory with them,” performing a wide range of elemental analysis regardless of a sample’s size, shape, or location.
X-ray fluorescence technology is based on the fact that when sufficient energy light waves (X-ray or photon) are absorbed by an atom, the inner shell electrons are excited to an outer shell or are removed completely. The empty inner shell that remains is “filled” by electrons from an outer shell of the atom. When generated in this process, the difference in energies between the two shells involved is excess energy that is emitted as radiation (fluorescence), which is constant for each element. Energy dispersive X-ray fluorescence instruments are able to identify the element involved by determining the energy emitted by a particular sample.
The two major components in the ED-XRF process are the X-ray tube and the detector tube. The X-ray tube generates the light waves (X-rays) that excite the electrons, and the detector tube collects the excess energy created from the electron stimulation. While benchtop units have minimal limitations as far as component size, one of the major obstacles in manufacturing portable XRF instrumentation is developing high-performance, durable units that are also lightweight and comfortable to use.
In a benchtop system, a high-powered X-ray tube is utilized that is generally run at 50–60 kV and up to 1000 μamps. These tubes range in size from 6 to 12 in. in length by 3 to 6 in. in height and may weigh up to 20 lb. Because of size and weight constraints, portable XRF systems commonly use a low-powered microtube that runs at a tube voltage of 5–40 kV and 5–200 μamps. This lower-powered tube creates limitations in elemental range, but the small size and packaging make a lightweight component that is versatile in application.
The detector tubes in benchtop models vary in technology and size, the largest being almost 16 in. long. A solid-state microdetector is utilized in the portable XRF system, The Pocket (Skyray Instrument Inc., Seaford, DE). This microdetector provides a lightweight, compact option that can still offer wide detection range and high accuracy.
Additional obstacles in portable XRF development include powering the system and data analysis/storage. Benchtop instruments are powered by standard 110-V ac outlets. However, a power cord would limit the portability of handheld XRF systems; this was resolved with rechargeable battery packs that have life spans of 5–8 hr. Data storage and analysis are accomplished through removable PDA systems. Most utilize Bluetooth® technology and allow for on-the-spot data analysis, which is easily transferred to a desktop computer for long-term storage and analysis.
Because XRF is still a relatively new technology, portable XRF manufacturers are constantly working to improve the instruments. One of the lightest handheld units available is The Pocket, which weighs less than 4 lb. However, Skyray engineers are developing lighter-weight components and body materials to achieve the goal of an even lighter handheld unit. Also on the agenda are developments of performance-enhancing features that include additional filters and a camera system to provide sample alignment assurance.
While design innovations, mobility, and low cost have been major factors in the popularity of portable XRF, the development of various hazardous substance regulations has fueled the sector’s growth. These regulations, including Restrictions on Hazardous Substances (RoHS) and Waste from Electrical and Electronic Equipment (WEEE), limit the amount of hazardous metals (lead, cadmium, etc.) found in a wide range of product lines. This has created a demand for accurate screening tools by organizations that would otherwise not have a need for XRF technology in their laboratories.
Portable XRF technology does not currently compete with benchtop XRF in accuracy or detection limit, but it is an excellent pass/fail screening tool for hazardous substances. Handheld units are able to read below the levels currently required by various regulations, measured in parts-per-million (ppm). Organizations that decide to take in-house control of compliance testing often turn to the lowest-cost option, and portable units offer the affordability required, reducing the cost per sample. Additionally, many independent laboratories provide hazardous substance analysis, and some offer compliance verification at the client’s site with portable XRF. These laboratories often have more accurate methods (inductively coupled plasma [ICP], benchtop XRF, etc.), but the portable units provide a convenient method for on-site pass/fail screening.
In comparing technologies, tests were performed to demonstrate the repeatability of portable and benchtop instruments using the XRF Pocket (portable XRF) and EDX2800 (benchtop model) (both from Skyray Instrument Inc.). The handheld results were obtained utilizing a device that would hold the gun rather than have an operator hold it. This approach eliminates human error and creates an optimum setting, since an operator may not always keep the gun extremely still during the measurement period.
Repeatability is important to XRF units and is an excellent representation of an instrument’s quality. When an instrument performs with high repeatability, a user is able to run multiple tests and can expect that results from the final tests are as accurate as the first test. This is referenced by the standard deviation (SD) of results— the lower the better.
Table 1 - Comparison of results of portable and desktop XRF*
In one comparison, a reference standard containing arsenic (As), bromine (Br), lead (Pb), and cadmium (Cd) was used. Eleven measurements (per instrument) were run on the sample with results shown in ppm. Table 1 shows the summarized results for this first sample test and includes the minimum, maximum, and mean measurements with standard deviation for each element and the standard deviation difference between units. As seen in the table, the lead results were the closest match in standard deviation between the instruments, with a difference in standard deviation of only 0.5975. The standard deviations for the arsenic, bromine, and cadmium were higher for the portable unit by 2.9952, 2.5084, and 3.6690, respectively. Keeping in mind that a benchtop instrument should be the more accurate of the technologies, this shows that portable XRFs are an excellent tool for pass/fail screening of lead.
Figure 1 - Handheld XRF offers on-the-spot analysis of steel and other metals.
A second comparison of the technologies was run using a piece of stainless steel and identifying the amounts of various metals present. Alloy verification and identification of unknown metals are common uses of portable XRF systems (Figure 1). This test was run using the same XRF instruments, but results are in millimeters (mm) of thickness. Both units performed very well, with the highest standard deviation from all six elements being 0.19 over the course of 11 readings. However, the portable unit slightly outperformed the benchtop, providing a lower standard deviation for four of the six metals in the sample. This level of repeatability, plus the benefits of mobility and no restrictions in sample size, explains the popularity of portable XRF technology in the recycling industry.
In addition to hazardous substance detection and alloy identification-related needs, handheld instruments offer an extremely versatile “out of laboratory” tool. Portable XRF provides accuracy in geological, pharmaceutical, and metallurgical applications; thus it has broad appeal to research, industrial, and quality assurance laboratories.
Portable XRF is a major innovation to an already important analytical technology. Handheld models will continue to be developed and improved in the future, offering greater elemental range and analytical capabilities for organizations that want to get outside of the laboratory.
Mr. Henseler is Marketing Manager, Skyray XRF, P.O. Box 1558, Seaford, DE 19973, U.S.A.; tel.: 716-204-2388; e-mail: [email protected].