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◊ Gas Chromatography
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X-ray Fluorescence
Field-portable, handheld device for simultaneously measuring a number of metals in various media. Energy dispersive X-ray fluorescence (EDXRF) is a method of detecting metals and other elements, such as arsenic and selenium, in soil and sediment. Some of the primary elements of environmental concern that EDXRF can identify are arsenic, barium, cadmium, chromium, copper, lead, mercury, selenium, silver, and zinc. Field-portable X-ray fluorescence (FPXRF) units that run on battery power and use a radioactive source were developed for use in analysis for lead-based paint and now are accepted as a stand-alone technique for lead analysis. In response to the growing need for field analysis of metals at hazardous waste sites, many of these FPXRF units have been adapted for use in the environmental field. The field-rugged units use analytical techniques that have been developed for analysis of numerous environmental contaminants in soils. They provide data in the field that can be used to identify and characterize contaminated sites and guide remedial work, among other applications. More recently, FPXRF analyzers have been used to detect metals in water. The water samples must be filtered and concentrated with an ion exchange membrane to achieve detection limits in the low parts per billion (ppb) range lower than applicable maximum contaminant levels (MCL). Many manufacturers of FPXRF units currently are conducting research to refine the procedures for preparation of water samples to make FPXRF analysis a practical field analytical technique for metals in water. FPXRF instruments were developed as an effective and nondestructive tool for measuring lead in paint and in house dust. Most homes constructed or painted before 1976 contain lead-based paint, which is one of the most common sources of lead ingested by children. In response, the U.S. Department of Housing and Urban Development (HUD) set guidelines for the inspection and abatement of contamination in public housing developments at which lead paint had been used. HUD considers any paint surface with a lead content greater than 1 milligram per square centimeter (mg/cm2) to be a lead-based paint surface. FPXRF units were designed to detect lead in paint at levels at or lower than that level. Air filters are used to measure concentrations of metals in household dust. When the volume of air that has passed through the air filter is measured, a conversion can be made to determine the concentrations of metals suspended as particulates in the air. Although the technique was developed for homes in which contamination with lead is suspected, it also has been applied in monitoring air emissions from industrial processes or from remediation processes conducted at a hazardous waste site. One of the advantages of EDXRF analysis is that it can be used not only to detect lead, but also to detect and measure many elements simultaneously. Generally, EDXRF units can detect and quantify elements from atomic number 16 (sulfur) through 92 (uranium). There are two types of EDXRF units: bench-top units that use an X-ray tube source and FPXRF analyzers that use a radioisotope as a source of X-rays. Instruments that use X-ray tubes as sources commonly are not used in the field because of the larger power requirements for the X-ray tube and the added weight of the instrument. Use of a radioactive source eliminates the need for a fixed power source for an X-ray tube, making the FPXRF unit truly portable. In FPXRF analysis, a process known as the photoelectric effect is used in analyzing samples. Fluorescent X-rays are produced by exposing a sample to an X-ray source that has an excitation energy similar to, but greater than, the binding energy of the inner-shell electrons of the elements in the sample. Some of the source X-rays will be scattered, but a portion will be absorbed by the elements in the sample. Because of their higher energy level, they will cause ejection of the inner-shell electrons. The electron vacancies that result will be filled by electrons cascading in from outer electron shells. However, since electrons in outer shells have higher energy states than the inner-shell electrons they are replacing, the outer shell electrons must give off energy as they cascade down. The energy is given off in the form of X-rays, and the phenomenon is referred to as X-ray fluorescence (click to view a schematic diagram of the X-ray fluorescence process). Because every element has a different electron shell configuration, each element emits a unique X-ray at a set energy level or wavelength that is characteristic of that element. The elements present in a sample can be identified by observing the energy level of the characteristic X-rays, while the intensity of the X-rays is proportional to the concentration and can be used to perform quantitative analysis. In other words, qualitative analysis is performed by observing the energy of the characteristic X-rays. A quantitative analysis is performed by measuring the intensity of the X-ray. The emissions of characteristic X-rays from three
electron shells commonly are involved in FPXRF analysis: the K, L, and M shells.
A typical emission pattern, or emission spectrum, for a given element has several
peaks generated from the emission of X-rays from those shells. A FPXRF system has two basic components: the radioisotope source and the detector. The source irradiates the sample to produce characteristic X-rays, as described above. The detector measures both the energy of the characteristic X-rays that are emitted and their intensity to identify and quantify the elements present in the sample. The following sections describe each of the components in greater detail. Radioisotope Sources An X-ray source will excite characteristic X-rays from an element only if the source energy is greater than the binding energy, or absorption edge energy, of the electrons in a given electron shell. A given individual source can analyze only certain elements. Analysis is more sensitive for an element with an absorption edge energy similar to, but less than, the excitation energy of the source. For example, when using a cadmium-109 (C-109) source, FPXRF would exhibit more sensitivity to zirconium, which has a K shell energy of 15.7 kiloelectron volts (keV), than for chromium, which has a K shell energy of 5.41 keV. The radioisotope sources that are becoming standard in FPXRF units are Fe-55, Cd-109, and Am-241. Elements that those sources commonly analyze include:
Because individual sources by nature reliably analyze only a limited number of sources, FPXRF instruments that use more than one source have been developed, allowing them to analyze a greater number and range of elements. Typical arrangements of such multisource instruments include Cd-109 and Am- 241 or Fe-55, Cd-109, and Am-241. X-ray Tube Sources Miniature x-ray tube sources are now being employed by a number of vendors. The advantage of the x-ray tube sources is that it does not require licensing or special shipping, as do XRF units employing radioactive sources. These units usually have a low-power hot-filament cathode x-ray tube. The transmission anode operates at a high enough energy range (~35 keV) in order to simultaneously excite a large range of elements (k through u). Interferences and sensitivity problems associated with high energy sources are corrected using sophisticated software built into the XRF unit. Two basic types of detectors are used in FPXRF units: gas-filled and solid-state. Each detector has its advantages and limitations and is better suited to some applications than to others. Common solid-state detectors include Si(Li), HgI2, and silicon pin diode. Among those detectors, the Si(Li) is capable of the highest resolution but is quite temperature-sensitive and will register signal "noise" if not cooled sufficiently. The Si(Li) has a resolution of 170 electron volts (eV) if cooled to at least –90°C, either with liquid nitrogen or by thermoelectric cooling that uses the Peltier effect. The HgI2 detector can operate at a moderately subambient temperature and is cooled by use of the Peltier effect. It has a resolution of 270 to 300 eV. The silicon pin diode detector operates near ambient temperatures and is cooled only slightly by use of the Peltier effect. It has a resolution of 250 eV. Some elements produce peaks that are near each other in the spectrum, while very high concentrations of one element may produce a peak that overwhelms the peaks of other elements that are present at lower concentrations. The higher the resolution, the better the detector is able to separate characteristic peaks. The XRF operator must be careful to select an FPXRF unit that has sufficient resolution to satisfy the data quality needs of the project. The following link provides an illustration of this concept by providing the resolution differences among some common XRF detectors. Resolution is discussed in greater detail in a later section. The radioisotope source or sources are housed in a metal turret, with additional lead shielding inside the probe. To perform an analysis, a sample is positioned in front of the plastic film measurement window of the probe and measurement of the sample is initiated, usually by depressing a trigger or start button. Doing so exposes the sample to the source radiation. For units that use multiple sources, after the sample has been exposed to one source, the turret is rotated to expose it to the next source. The length of time the sample actually is exposed to each source is referred to as the count time. The sample is exposed to the radioactive source for a number of seconds. Fluorescent and backscattered X-rays from the sample reenter the analyzer through the window and are counted by the instrument's detector. X-rays emitted by the sample at each energy level are called "counts". The detector records the counts, measures the energy of each X-ray and builds a spectrum of analyte peaks on a multichannel analyzer (MCA). The unit’s software integrates the peaks to produce a readout of concentrations of analytes, and, usually, the standard deviation for each analyte. Numerous sample results and spectra can be stored for later viewing, downloading into a computer, or printing. Some units also allow the operator to recall previous results and even to view their spectra. At the completion of the exposure time, the instrument software statistically computes a concentration from the readings collected from each energy level along the spectrum. Count times are not to be confused with the total analytical time, which includes all of the analytical functions, such as rotation of the source into position, and processing of the results by the instrument software, in addition to the count time of each source. Count times from 30 seconds per source to as long as 200 seconds per source can be employed, depending on the data quality needs of the project. As count times increase, the detector collects a larger number of X-rays from the sample, including more X-rays from elements that are present at comparatively lower concentrations. For that reason, the longer the count time, the lower the detection limits; typically, quadrupling the count time will cut the detection limit in half. For example, if a 50-second count time yields a detection limit of 100 parts per million (ppm) for a given element, increasing the count time to 200 seconds will lower the detection limit to approximately 50 PPM Using the instrument's software, the operator can select the appropriate count times. An FPXRF detector can be operated in the in situ or the intrusive mode. Count times of 30 to 60 seconds per source are common for in situ analysis, while count times for intrusive analysis may be as long as 200 seconds per source. The particular requirements of the job, such as the required detection limits or data sample precision, and the purpose of sampling--for field screening or for definitive analysis--will determine which mode is appropriate and what count times are needed. Descriptions of each mode follow.
While a clear distinction is made here between in situ analysis and fully intrusive analysis, sample analysis is in reality a continuum. Thorough homogenization will improve the precision and accuracy of the analysis dramatically; an "in situ prepared" sample can be collected, homogenized, and analyzed right next to the sample location (possibly right through a plastic bag used for homogenization). Drying the sample also may improve the results significantly, and, depending on the project’s data quality objectives, homogenization and drying may be all the preparation required for an intrusive analysis. Preparation of samples is discussed in greater detail in a later section. Target Analytes The target analytes are metals and other nonmetallic elements, such as arsenic and selenium. Performance specifications include information about interferences, detection limits, calibration, sample preparation, quality control, and precision and accuracy. Interferences There are a number of factors known as interferences that can affect the detection and quantification of elements in a sample. Some interferences can be inherent in the method of analysis, while others are the result of the instrument's setup, such as calibration methods. Other interferences may arise from outside sources, such as the sample matrix (for example, soils and sediment). Some factors can be prevented or minimized through careful preparation and sample design; others are natural effects that must be taken into consideration. To produce useful data, it is important that the analyst understand the interferences. Their effects and the procedures used to evaluate them are described below. Matrix Effects Moisture Effects Sampling Effects Chemistry Effects Detector
Resolution Effects An FPXRF operator must consider two types of detection limits: instrument detection limits (DL) and method detection limits (MDL). A DL is the absolute threshold concentration of a given element that a particular instrument can resolve, as determined by the standard deviation (SD) of an individual analytical result. DLs of 10 to 100 PPM are typical for soil samples, although DLs may be higher for elements like chromium and cadmium that have characteristic X-ray peaks far removed from the energy level of the sources typically used. MDLs depend on the analytical method (such as preparation and analysis times) and may be higher than DLs. The results of replicate measurements of a low-concentration sample can be used to generate an average site-specific MDL. The MDL is defined as three times the SD of the results for a replicate analysis of a low-concentration sample. With the exception of chromium which has a MDL as high as 900 milligram per kilogram (mg/kg) depending the instrument being used, the MDLs for most analytes are in the range of 40 to 200 mg/kg. Click to view a comparison of method detection limits for six commercial FPXRF instruments. FPXRF units are calibrated by any of several methods. The methods will vary according to the make of the unit and the use to which the data are to be put, such as for screening or for definitive analysis. Basically, there are two types of calibration, although there is some overlap between two. Fundamental Parameters Calibration Empirical Calibration Compton Normalization Sample Preparation Procedures for sample preparation for in situ and intrusive analysis vary considerably, since the two methods serve completely different purposes. Sample preparation for in situ analysis is fairly straightforward, while sample preparation for intrusive analysis can be fairly complicated, depending on the data quality required. In situ or “point-and-shoot” analysis requires little sample preparation.
For intrusive analysis, the sample first must be collected and then prepared for analysis in a sample cup. Some or all of the following steps are necessary, depending on the data quality needed:
Quality Control Ensuring that the data generated by FPXRF analysis are of a known quality is vital to ensuring the usefulness of those data, regardless of their purpose. Quality control (QC) measures take several forms and can be performed in the field, during sample analysis, and after sample data have been collected. The amount and type of QC necessary will depend on the project's data quality objectives. A much higher degree of QC is necessary to produce defensible, definitive data, but analytical results from intrusive analysis have been demonstrated to compare favorably with results obtained through traditional laboratory methods, given that sample preparation has been thorough and QC adequate. By nature, results obtained in situ are of lower quality because of the lack of sample preparation, but, with the use of proper QC, in situ data can be corrected. A typical QC program would include the following measures:
Each of the measures identified above is discussed in detail below. Energy calibration check samples are used to test FP calibrations. A check sample consists of a pure element, such as iron, lead, or copper, and is analyzed to determine whether the characteristic X-ray lines are shifting, which would indicate drift in the detector. The check also serves as a gain check in the event that ambient temperatures are fluctuating significantly (more than 10 to 20° F). The energy calibration check should be run at a frequency consistent with the manufacturer's recommendations. Generally, the check would be performed at the beginning of each working day, after the batteries have been changed or the instrument shut off, at the end of each working day, and at any other time at which the instrument operator believes that drift is occurring during analysis. Two types of blanks can be used during FPXRF analysis. The first is an instrument blank, which is used to verify that there is no contamination in the spectrometer or on the probe window. The instrument blank can be silicon dioxide, a Teflon block, or a quartz block. The instrument blank should be analyzed a minimum of once daily, preferably once for every 20 samples, and should not contain any target analytes at levels higher than the MDL. The second type of blank is a method blank. The method blank is used to monitor sampling and analysis methods for laboratory-induced contaminants or interferences. The method blank can be “clean” silica sand or lithium carbonate that undergoes the same sample preparation procedures as the environmental samples. The method blank should be analyzed with the same frequency as the instrument blank and should not contain any target analytes at levels higher than the MDL. Calibration verification check samples are used to check the accuracy of the instrument and assess the stability and consistency of the analysis of the target analytes. Accuracy is a measure of the instrument's ability to measure the “true” concentration of an element in a sample. The check sample can be an SSCS or an SRM, such as the National Institute of Standards and Technology (NIST) SRMs, that contains the target analytes, preferably at concentrations near any action levels for the site. The check sample should be run at the beginning and the end of each day or for every 20 environmental samples. The percent difference (%D) between the true value and the measured value should be less than 20 percent. Instrument precision refers to an instrument's ability to produce the same result for a number of measurements of the same sample. The precision of FPXRF measurements is monitored by performing several analyses of samples that contain low, medium, and high concentrations of target analytes. It is especially important to know the precision of the instrument in measuring concentrations that are similar to action levels, because precision is dependent on analyte concentrations of analytes: as the concentration increases, the precision increases. A minimum of one precision sample should be run per day by conducting from 7 to 10 replicate measurements of the sample. The precision is assessed by calculating a relative standard deviation (RSD) of the replicate measurements for the analyte. The RSD values should be less than 20 percent for most analytes, except chromium, for which the value should be less than 30 percent. Click to see a comparison of instrument precision. Click to view the percent recovery by the FPXRF instrument for a number of metals in performance evaluations and standard reference materials. Click to view the performance of an FPXRF instrument during the analysis of commercial performance evaluation (PE) samples. PE samples are commercially available standards containing certified concentrations of various target analytes. Confirmatory samples are collected from the same sample material that is analyzed on site, but are sent to an off-site laboratory for formal analysis. The results of the on-site analysis are compared with the results of the off-site analysis to determine whether they are comparable within the acceptable range. The acceptable range is determined by the analytical method, if applicable, or by the user. The purpose of a confirmatory sample is to judge the accuracy of the data obtained by analysis on site and to allow corrections, if necessary. One confirmatory sample usually is submitted for every 10 to 20 samples analyzed on site, depending on the nature of the job. Advantages Most instruments weigh less than 30 pounds and can be operated using battery power for 8 to 10 hours. A sample can be analyzed in less than five minutes. Throughput is a measure of the maximum rate of analysis that realistically can be obtained when using an instrument. That measure includes not only analytical time, but all sample preparation, QC, and data processing necessary to produce useable results. Throughput usually is expressed in samples per hour or samples per day. A throughput of 50 to 100 samples a day typically can be achieved for intrusive analysis, and as many as 150 samples per day can be analyzed in situ. Analyses of as many as 35 elements can be performed simultaneously in a single analysis. The sample is not destroyed during preparation or analysis; therefore, it is possible to perform replicate analyses on a sample and send the same sample for confirmatory analysis, so that comparability studies can be performed. The sample also can be archived for later use as a soil standard. Because no solvents or acids are used for sample extraction, no waste is generated; disposal costs therefore are eliminated. Operators usually can be trained in one or two days. The software is menu-driven. No data manipulation is required. Instruments are marketed for use by general scientists. Little or no sample preparation is required; therefore, sample throughput is enhanced and time and money are saved. Detection limits for chromium are 200 mg/kg or higher. Action levels for some elements, such as arsenic or cadmium, may be lower than the detection limits of XRF. Concentrations of elements in different types of soil or matrices might change, causing interferences--for example, between arsenic and lead. Site-specific calibration standards can compensate for some of those effects. It is difficult to obtain soil standards. One of the best sources is SRMs from NIST. Those standards cost from $200 to $500 each. A specific license is required to operate some FPXRF instruments. The total cost of attending a radiation safety course, obtaining the necessary paperwork, and paying the fee for the license can range from $500 to $1,000. The Cd-109 source should be replaced every two years. The cost of replacement is approximately $4,000 to $5,000. Any instrument that has a Si(Li) detector will require liquid nitrogen and a dewar (aluminum container) to hold the liquid nitrogen. This requirement adds the time and cost of obtaining and handling liquid nitrogen to cool an instrument with a Si(Li) detector before analysis can be performed. XRF costs vary significantly. Instrument design and accessories affect instrument prices. Manufacturers listed below should be contacted directly for cost information. Comparing Field Portable X-Ray Fluorescence (XRF) To Laboratory Analysis Of Heavy Metals In Soil Sample Handling Strategies for Accurate Lead-in-Soil Measurements in the Field and Laboratory
Verification/Evaluation Reports Verification of the performance of site characterization and field analytical technologies is conducted through a variety of programs. Evaluation and verification reports from EPA's Superfund Innovative Technologies Evaluation (SITE) Measuring and Monitoring Program, EPA's Environmental Technology Verification Program (ETV) program, along with links to certification statements from California EPA's (CalEPA) California Environmental Technology Certification Program, are provided below. Superfund
Innovative Technologies Evaluation (SITE) Measuring and Monitoring Program See ETV reports below EPA's
Environmental Technology Verification (ETV) Program
CalEPA's environmental technology certification program is a voluntary program that provides participating technology developers, manufacturers, and vendors an independent, recognized third-party evaluation of the performance of new and mature environmental technologies. Developers and manufacturers define quantitative performance claims for their technologies and provide supporting documentation; CalEPA reviews that information and, when necessary, conducts additional testing to verify the claims. The technologies, equipment, and products that are proven to work as claimed are given official state certification. The certification program is voluntary and self-supporting. Companies participating in the program pay the costs of the evaluation and certification of their technologies. Technologies that have been certified through this program are listed below. Links are provided to the web sites that provide the Certified Environmental Technology Transfer Advisory and Certification Notice for the technologies. No reports available for this technology
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