X-treme Monitoring in the Field

Advances in X-ray fluorescence instrumentation continue to push the technology toward better serving environmental consultants

• By Laura Stupi
• May 01, 2005

X-ray fluorescence (XRF) instrumentation has become an essential as a tool for expediting and improving site characterization for inorganic contaminants and corrective remediation. On site, it is useful for determining contamination boundaries, monitoring remediation efforts, pre-screening clearance samples of soil and sediment, and eliminating potential downtime from off-site lab testing. Short measurement times allow for a large number of samples to be measured and for the worker to complete a more accurate and detailed site description while still in the field.

The U.S. Environmental Protection Agency (EPA) is actively spearheading initiatives to encourage assessment of this type of technology. The most recent of these technology evaluations was held in January 2005. Portable XRF instruments were evaluated for measuring 13 different elements, including seven of the eight Resource Conservation and Recovery Act (RCRA) elements. EPA has established protocols for in situ testing with EPA Method 6200, focused on rapid analysis of samples in the field, and continues to encourage development of innovative technologies with its Environmental Technology Verification (ETV) program.

Recent innovations in miniaturized x-ray tube technology, Silicon Positive Intrinsic Negative (Si PIN) diode detectors, and data processing have improved detection limits on many of the toxic elements of interest to environmental consultants. Coupled with ease of transporting x-ray tube sources relative to their isotope-based counterparts, XRF analyzers are poised to become the must-have piece of equipment for environmental scientists.

X-ray Fluorescence
X-ray Fluorescence occurs when x- and gamma-rays from a source (either an x-ray tube or radioisotope) interact with the atoms of a sample resulting in the ejection of an inner shell electron. An outer shell electron falls inward to fill the void created in the inner shell, and an x-ray characteristic of the atom's element is emitted. 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 intensities of an element's x-ray spectral lines are proportional to the concentration of the elements in the sample, allowing quantitative chemical analysis.

X-ray Sources
Traditional hand-held XRF spectrometers use radioisotope excitation sources, such as cadmium (Cd)¹⁰⁹, americium (Am)²⁴¹, or iron (Fe)⁵⁵ (or some combination of the three). The recent introduction of miniature x-ray tubes has allowed their incorporation into portable XRF spectrometers as an alternative means of excitation. Tube-based technology offers the scientific community some promising advantages over traditional isotope systems. The higher x-ray flux of tubes provides faster results and better precision than radioisotope sources. X-ray tubes may be tuned for optimized analysis of specific targeted element suites. For example, an XRF system may run its tube at 40 kiloelectronvolt (keV) for the measurement of elements such as antimony (Sb) and cadmium (Cd), or at 15 keV for chromium (Cr) analysis. By changing the output voltage, improved excitation -- and thus sensitivity, can be obtained for targeted elements.

The miniaturized x-ray tube allows the user to travel more easily with the instrument, which is generally subjected to less strict regulations and licensing protocols than isotope based models. Unlike the isotope sources, there are no half-life limitations, and measurement times do not slow as the source ages. Additionally, users do not have to replace an isotope source after 12 to 18 months, resulting in lost productivity while the instrument is being serviced.

Although x-ray tubes may offer some advantages, a word of caution is necessary for those in the market for an XRF analyzer. An x-ray tube is an electronic device -- a miniature glass and ceramic capsule held under vacuum. A radioisotope source, on the other hand, is essentially a solid capsule of material encased in a tungsten housing. Isotope sources are inherently durable. As such, whenever the utmost in rugged reliability is needed from an XRF analyzer, isotope-sourced XRF systems must be considered. When choosing a source, it is important to establish what kind of instrument will best suit your needs and intended use.

Calibration
Early generations of portable XRF instruments required site-specific calibrations to correct for matrix-related errors. A series of samples indicative of the mineralogy of a site, loaded with varying concentrations of an element of interest e.g. lead (Pb), would be collected and analyzed by a standard laboratory method. The laboratory results would then be used to establish a calibration curve for the XRF instrument for that particular soil type. If multiple types of soil existed at a site, then multiple calibration curves were necessary.

Today's advanced XRF analyzers employ far more flexible and easy-to-use calibration methods. These methods avoid the need for site-specific standards and calibrations, and automatically compensate for differing mineralogies, densities, and sample characteristics. These techniques are based on Compton Normalization or Fundamental Parameter methods.

Compton Normalization uses Compton backscatter information to determine the degree of x-ray absorption in a sample and compensate for the absorption's causal factors in its chemistry calculations. Fundamental Parameters, typically utilized for heavy loading (greater than a few percentage points of the concentration of target elements), use an advanced mathematical algorithm to resolve inter-element effects and sample changes. Both methods eliminate the need for tedious study preparations and enable workers to obtain reliable and accurate results on site in very short time frames.

Detectors and Detection Limits
A key measure for most analytical instrumentation is the all-important limit of detection (LOD). Over the past 30 years of portable XRF analyzer evolution, LODs have continually improved with maturation of electronics, software, and calibrations. New XRF analyzers have detectors with resolutions under 200 electronvolt (eV) -- a major milestone in this industry -- and ever improving signal-to-background ratios resulting in unprecedented elemental sensitivities. Combine these improvements with flexible and x-ray tube tuning, and LODs of under 10 parts per million (ppm) for many target elements are now attainable.

Conclusion
Portable XRF instruments have long been used for preliminary investigations, process control during soil digging, excavation and transport, and in final clearance confirmations, all on site. The availability of accurate on-site analysis has made remediation times shorter and less expensive due to minimization of laboratory testing, idle worker and consultancy time waiting on results, and the elimination of having to back-correct any errors or mistakes should errant work proceed without receipt of monitoring results.

Advances in field-portable XRF continue to push the technology toward better serving environmental consultants. With the advent of miniaturized x-ray tubes, electronics enhancements, and new calibration methods, the era when large numbers of samples need to be sent for laboratory analysis is fast approaching its end. Even when lab and XRF costs are on par, the convenience and usefulness of on-site confirmation testing make portable XRF an essential tool for the successful consultant.

Reference
V. Thomsen and D. Schatzlein, Advances in Field Portable XRF, Spectroscopy, Volume 17, Number 7, pgs. 14-18

This article originally appeared in the 05/01/2005 issue of Environmental Protection.