Graphite Furnace Atomic Absorption Spectrometry
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Graphite furnace atomic absorption (GFAA) spectrometry is a highly sensitive spectroscopic technique that provides excellent detection limits for measuring concentrations of metals in aqueous and solid samples.
GFAA has been used primarily in the field for the analysis of metals in water. GFAA could be used to determine metals in soil, but the sample preparation for metals in soil is extensive and is not practical for field applications.
GFAA cannot be described as a truly field portable instrument. GFAA instruments are extremely sensitive and therefore, must be operated in a clean, climate controlled environment. This can be difficult but not impossible to achieve in a field environment. In addition, the 220-volt electrical power requirement often precludes remote operation. However, GFAA is an example of “taking the laboratory to the field.” Miniaturization of electronics has significantly reduced instrument size and weight, making it easier to use the instrument in a field laboratory.
In atomic absorption (AA) spectrometry, light of a specific wavelength is passed through the atomic vapor of an element of interest, and measurement is made of the attenuation of the intensity of the light as a result of absorption. Quantitative analysis by AA depends on: (1) accurate measurement of the intensity of the light and (2) the assumption that the radiation absorbed is proportional to atomic concentration.
Samples to be analyzed by AA must be vaporized or atomized, typically by using a flame or graphite furnace. The graphite furnace is an electrothermal atomizer system that can produce temperatures as high as 3,000°C. The heated graphite furnace provides the thermal energy to break chemical bonds within the sample and produce free ground-state atoms. Ground-state atoms then are capable of absorbing energy, in the form of light, and are elevated to an excited state. The amount of light energy absorbed increases as the concentration of the selected element increases.
GFAA has been used primarily for analysis of low concentrations of metals in samples of water. GFAA can be used to determine concentrations of metals in soil, but the sample preparation for metals in soil is somewhat extensive and may require the use of a mobile laboratory. The more sophisticated GFAAs have a number of lamps and therefore are capable of simultaneous and automatic determinations for more than one element.
Logistical needs include reagents for preparation and analysis of samples, matrix modifiers, a cooling system, and a 220-volt source of electricity. In addition, many analytical components of the GFAA system require significant space, which typically is provided by a mobile laboratory. A tabletop GFAA spectrometer and data processor are pictured above.
GFAA spectrometry instruments have the following basic features:
- a source of light (lamp) that emits resonance line radiation
- an atomization chamber (graphite tube) in which the sample is vaporized
- a monochromator for selecting only one of the characteristic wavelengths (visible or ultraviolet) of the element of interest
- a detector, generally a photomultiplier tube (light detectors that are useful in low-intensity applications), that measures the amount of absorption
- a signal processor-computer system (strip chart recorder, digital display, meter, or printer)
Click to view a schematic diagram of the basic components of a GFAA system.
Most currently available GFAAs are fully controlled from a personal computer that has Windows-compatible software. Aqueous samples should be acidified (typically with nitric acid) to a pH of 2.0 or less. Discoloration in a sample may indicate that metals are present in the sample. For example, a greenish color may indicate a high nickel content, or a bluish color may indicate a high copper content. A good rule to follow is to analyze clear samples first, and then analyze colored samples. It may be necessary to dilute highly colored samples before they are analyzed.
After the instrument has warmed up and been calibrated, a small aliquot (usually less than 100 microliters (µL) and typically 20 µL) is placed, either manually or through an automated sampler, into the opening in the graphite tube. Click to see a cross-sectional view of a graphite tube The sample is vaporized in the heated graphite tube; the amount of light energy absorbed in the vapor is proportional to atomic concentrations. Analysis of each sample takes from 1 to 5 minutes, and the results for a sample is the average of triplicate analysis.
Standard Operating Procedures (SOPs) are available for:
Graphite tubes must be changed after every 200 to 800 burns because they become pitted and produce data that are only poorly reproducible, and results in a loss of sensitivity.
The sample must be diluted if the absorbence is outside the calibration range. GFAA has a smaller linear concentration range than flame AA or inductively coupled plasma (ICP) spectrometry.
No data available
Performance specs include information on interferences, detection limits, calibration, sample preparation, quality control, and precision and accuracy.
The GFAA technique is subject to chemical, spectral, and ionization interferences. The composition of the sample matrix typically has the largest effect on the results of the analysis. Chemical interferences occur when the atoms are not completely free or in their ground state. Spectral interferences occur when atomic or molecular species other than the element being analyzed absorb energy at the wavelength of interest. Ionization interferences occur when the furnace causes complete removal of electrons from an atom, thereby lowering the concentration of ground-state atoms available to absorb light.
A serial dilution technique may be used to help verify the absence of chemical and spectral interference. In cases in which interference is suspected, samples should be treated in one or more of the following ways: (1) samples should be diluted and reanalyzed successively to determine whether the interference can be eliminated, (2) matrix modifiers should be added, or (3) the sample should be analyzed by the method of standard additions. It is common practice to add matrix modifiers to all samples to compensate for potential chemical and spectral interferences.
Numerous metals can be analyzed by GFAA, as long as their atoms can be vaporized in the graphite furnace. Such elements include aluminum, arsenic, barium, boron, cadmium, calcium, chromium, cobalt, copper, iron, lead, lithium, magnesium, manganese, mercury, molybdenum, nickel, potassium, selenium, silicon, silver, sodium, titanium, terbium, vanadium, ytterbium, and zinc. Click to view a table showing the typical detection limits for some of the metals listed above. Detection limits can be very low with GFAA analyses (below most of the maximum contaminant levels [MCL] for drinking water established by the U.S. Environmental Protection Agency (EPA) and usually are 10 to 100 times lower than the detection limits for analyses by flame AA or ICP for the same element.
Continued calibration of the instrument is a component of the overall quality control plan and should be performed by analyzing one mid-concentration standard after every 10 analyses. The relative percent difference (RPD) between the initial calibration and the continuing calibration should be less than 15 percent.
Samples are analyzed in triplicate, but it only takes about 5 minutes for sample analysis. Water samples should be acidified with nitric acid to a pH of less than 2. If dissolved metals analysis is required, the water samples should be filtered through a 0.45 micrometer (m) filter. If the water samples are very turbid, they should be centrifuged prior to analysis or allowed to settle. To alleviate interferences, matrix modifiers should be added. The water samples have a holding time of 6 months after they are preserved. In the field, most water samples are analyzed within a few hours after collection.
Method blanks are analyzed with each batch of 20 samples analyzed. Method blanks monitor laboratory-induced contaminants or interferences. A method blank must not contain any analyte in a concentration higher than the practical quantitation limit.
Matrix spike (MS) and matrix spike duplicate (MSD) samples are analyzed to evaluate the efficiency of the sample preparation, matrix effect, and the precision of the analysis. MS-MSDs are prepared with each batch of 20 samples. The advisory control limit range for spike recovery is 50 to 150 percent. The advisory control limit for RPD in water samples is 25 percent.
Laboratory control samples (LCS) are used to evaluate the accuracy of the analysis. The LCSs are obtained from outside sources and contain known amounts of metals. The values obtained by analysis of the LCSs are compared with the known true values. The supplier of the LCSs usually provides control limits. The results obtained should fall within the published range of acceptance values. When no control limits are provided, a range of 50 to 150 percent should be used.
Contamination of samples can be a major source of analytical error because of the extremely low detection limits achieved with GFAA spectrometry. The work area used for sample preparation must be kept clean. That requirement is particularly important in a mobile field laboratory, where it is easy for airborne dust to contaminate the analytical equipment and glassware.
Other standard analytical practices include:
- Use of precleaned glassware or washing of the glassware in acid
- Use of trace-metal-grade pipette tips
- Use of trace-metal-grade distilled, deionized water and nitric acid
- Prevention of accumulation of dust in the autosampler cups
The advantages of GFAA spectrometry include:
- Greater sensitivity and detection limits than other methods
- Direct analysis of some types of liquid samples
- Low spectral interference
- Very small sample size
The limitations of GFAA spectrometry include:
- Longer analysis time than flame AA or ICP analysis
- Limited dynamic range
- High matrix interference
- No true field-portability, with a mobile laboratory setup usually required
- 220-volt power source required
GFAA costs vary significantly. Instrument design and accessories affect instrument prices. Manufacturers listed below should be contacted directly for cost information.
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.
Innovative Technologies Evaluation (SITE) Measuring and Monitoring Program
The SITE Demonstration Program encourages the development and implementation of innovative treatment technologies for (1) remediation of hazardous waste sites and (2) monitoring and measurement. In the SITE Demonstration Program, the technology is field-tested on hazardous waste materials. Engineering and cost data on the innovative technologies are gathered so that potential users can assess the technology's applicability to a particular site. Data collected during the field demonstration are used to assess the performance of the technology, the potential need for pre- and post-treatment processing of the waste, applicable types of wastes and waste matrices, potential operating problems, and approximate capital and operating costs.
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EPA's Environmental Technology Verification (ETV) Program verifies the performance of innovative technologies. ETV was created to substantially accelerate the entrance of new environmental technologies into the domestic and international marketplaces. ETV verifies commercialized, private sector technologies. After the technology has been tested, the companies will receive a verification report that they can use in marketing their products. The results of the testing also are available on the Internet.
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EPA's California Environmental Technology Certification 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.
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