Superfund Research Program
Advanced Analytical Technique Developed for Detecting Metals in Biological and Environmental Samples
The identification and quantification of metals in environmental and biological samples are often difficult. Currently, many scientists use atomic absorption spectrophotometry or inductively coupled plasma-mass spectrometry to detect and quantify metals. Both of these techniques require destruction of the sample and are limited by the number of metals that can be analyzed at one time, as well as by sample size, matrix effects, and limits of detection.
While these well-known methods continue to be used, a lesser known technology without the above limitations is available for multi-elemental analysis of samples. This technology is known as Proton Induced X-ray Emission Spectroscopy (PIXE) and it is a powerful technique ideally suited for detecting part per million (ppm) levels of metals in biological samples for which the light elements (namely carbon, hydrogen, nitrogen, and oxygen) are predominant. Another strength is that a sample can be analyzed in its current matrix without destruction, enabling repeated analysis of the same sample.
The PIXE technique involves bombarding a solid sample with a high-energy proton beam. This results in the emission of x-rays whose energies are characteristic of the elements present in the sample and whose intensities are proportional to the concentrations of these elements. A silicon (lithium) detector and a multichannel analyzer are coupled with a computer program to identify and quantify the elements present in samples.
Researchers at the University of Arizona have optimized the instrumentation and sample preparation methods for PIXE analysis of biological tissues and environmental samples. Over the past few years the scientists have constructed a separate beam line in which the high-energy proton beam has been focused down to an area of a few square microns. This proton microprobe is unique because it can determine the location of toxic metal species in tissue, making PIXE a useful tool in certain types of toxicological studies.
For example, the PIXE technique can facilitate conducting interactive metal toxicity studies in which scientists are often interested in determining the preferred uptake of one metal ion in the presence of other metal ion species, as well as the effects that metal ions have on each other's accumulation. PIXE not only has the capability to detect the uptake of toxic metals by various organs, but it can also measure the uptake of toxic metals over a wide range of exposure. In a single analysis, the uptake of several metals by such tissue slices can be determined.
Another example of studies that can benefit from the PIXE technique includes investigations of the clearance of toxic metals in the body by a variety of chelating or complexing agents. The advantage of using the PIXE technique for this kind of study is that analysis for several metal ions can be performed on a single sample, thereby minimizing intersample variability. In addition, the effects of metal-metal interactions on uptake and clearance mechanisms can be readily addressed.
The PIXE technique is significant because it is one of the few methods available for the non-destructive multi-elemental analysis of small biological samples - up to 40 elements can be quantified simultaneously in samples weighing only a few milligrams. PIXE is not just limited to its current use in tissue samples - it is also readily adaptable to measuring metals in a variety of matrices such as soil, plants, and air filters.
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To learn more about this research, please refer to the following sources:
- Keith RL, Gandolfi A, McIntyre LC, Ashbaugh MD, Fernando Q. 1997. Use of the nuclear microprobe at the University of Arizona for the study of heavy metal deposition in rabbit renal tissue. Nucl Instrum Methods Phys Res A 130:358-361.
- Keith RL, Setiarahardjo I, Fernando Q, Aposhian HV, Gandolfi A. 1997. Utilization of renal slices to evaluate the efficacy of chelators to remove mercury from the kidney. Toxicology 116:67-75. PMID:9020508
- McIntyre LC, Leavitt JA, Ashbaugh MD, Borgardt J, Keith RL, Gandolfi A, Qiu L, Lott JR, Fernando Q. 1997. The nuclear microprobe at the University of Arizona. Nucl Instrum Methods Phys Res A 130(1-4):45-50. doi:10.1016/S0168-583X(97)00177-8
- Burton CA, Hatlelid K, Divine KK, Carter DE, Fernando Q, Brendel K, Gandolfi A. 1995. Glutathione effects on toxicity and uptake of mercuric chloride and sodium arsenite in rabbit renal cortical slices. Environ Health Perspect 103(Suppl.1):81-84. PMID:7621807
- Keith RL, McGuinness SJ, Gandolfi A, Lowe TP, Chen Q, Fernando Q. 1995. Interaction of metals during their uptake and accumulation in rabbit renal cortical slices. Environ Health Perspect 103(Suppl.1):77-80. PMID:7621806
- Lowe TP, Chen Q, Fernando Q, Keith RL, Gandolfi A. 1993. Elemental analysis of renal slices by proton-induced X-ray emission. Environ Health Perspect 101:302. PMID:8275986
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