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X-ray Absorption Fine Structure

X-ray Absorption Fine Structure

XAFS spectroscopy is a powerful technique for determining element speciation, however, the potential limitations of the procedure must also be recognized. It depends on having a fully inclusive set of standard spectra that contribute to the spectra of the unknown samples. The omission of the spectrum of even a single standard that is significantly present in the unknown sample means that the analysis may provide an incorrect result.

 

If the spectroscopist is told that the unknown sample consists of some combination of compounds A, B, C, D, and E and he has the spectra for those five compounds, then the least-squares XANES analysis can be done rigorously and with some degree of accuracy. However, if some information is lacking, for example, either one of the standard spectra is not available, or it is not realized that other compounds might be present, then the analysis is no longer reliable and likely incorrect. If no information at all is available about the species that might be present in a given sample, then the analytical data becomes less evident, unless the spectra are highly individualistic (which is typically not the case for Pb)

 

Under ideal circumstances, least-squares fitting of XANES spectra should be able to quantify up to four or five species with reasonable precision. However, this depends on (i) having all the spectra of possible forms of the element in the database; (ii) there being no unrecognized forms in the sample; and (iii) that the standard spectra differ sufficiently from each other

 

Unfortunately, Lead is not always the most conducive element for analysis of XAFS data. The fine structure, which is what distinguishes one Pb form from another in XAFS spectra, tends to be relatively broad and weak and may not provide a strong basis for quantifying Pb2+ forms

There have been a numerous XAFS studies of Pb in soils and, although Pb XAFS spectra of the different soils tend to be quite similar, the interpretations of the speciation can differ significantly from study to study, in large part because of the different databases of Pb compounds available to the researchers. In many leading spectroscopist’s opinion, Pb speciation analysis of soils based solely on least-squares fitting of the Pb XANES region of a bulk sample may not yield reliable data. In such instances, the data can be improved if the quality of the data was at least good enough that both the XANES and EXAFS regions could be analyzed independently. A still better approach would be to combine Pb XAFS spectroscopy with a sequential extraction methodology to generate a set of subsamples (leached residues from the sequential extractions) that can be examined by XAFS spectroscopy. Such a combined approach should be more informative and reliable than the single XAFS analysis of the bulk soil.

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Services

X-ray Absorption Fine Structure

X-ray Absorption Fine Structure

Services

XAFS spectroscopy is a powerful technique for determining element speciation, however, the potential limitations of the procedure must also be recognized. It depends on having a fully inclusive set of standard spectra that contribute to the spectra of the unknown samples. The omission of the spectrum of even a single standard that is significantly present in the unknown sample means that the analysis may provide an incorrect result.

If the spectroscopist is told that the unknown sample consists of some combination of compounds A, B, C, D, and E and he has the spectra for those five compounds, then the least-squares XANES analysis can be done rigorously and with some degree of accuracy. However, if some information is lacking, for example, either one of the standard spectra is not available, or it is not realized that other compounds might be present, then the analysis is no longer reliable and likely incorrect. If no information at all is available about the species that might be present in a given sample, then the analytical data becomes less evident, unless the spectra are highly individualistic (which is typically not the case for Pb)

Under ideal circumstances, least-squares fitting of XANES spectra should be able to quantify up to four or five species with reasonable precision. However, this depends on (i) having all the spectra of possible forms of the element in the database; (ii) there being no unrecognized forms in the sample; and (iii) that the standard spectra differ sufficiently from each other

 

Unfortunately, Lead is not always the most conducive element for analysis of XAFS data. The fine structure, which is what distinguishes one Pb form from another in XAFS spectra, tends to be relatively broad and weak and may not provide a strong basis for quantifying Pb2+ forms in the absence of other data. This is particularly true for mixtures of possible Pb-O forms that may occur in soils, as the spectra tend to be rather similar, leading to potential errors.

There have been a numerous XAFS studies of Pb in soils and, although Pb XAFS spectra of the different soils tend to be quite similar, the interpretations of the speciation can differ significantly from study to study, in large part because of the different databases of Pb compounds available to the researchers. In many leading spectroscopist’s opinion, Pb speciation analysis of soils based solely on least-squares fitting of the Pb XANES region of a bulk sample may not yield reliable data. In such instances, the data can be improved if the quality of the data was at least good enough that both the XANES and EXAFS regions could be analyzed independently. A still better approach would be to combine Pb XAFS spectroscopy with a sequential extraction methodology to generate a set of subsamples (leached residues from the sequential extractions) that can be examined by XAFS spectroscopy. Such a combined approach should be more informative and reliable than the single XAFS analysis of the bulk soil.

X-ray Absorption Fine Structure

Services

X-ray Absorption Fine Structure

XAFS spectroscopy is a powerful technique for determining element speciation, however, the potential limitations of the procedure must also be recognized. It depends on having a fully inclusive set of standard spectra that contribute to the spectra of the unknown samples. The omission of the spectrum of even a single standard that is significantly present in the unknown sample means that the analysis may provide an incorrect result.

If the spectroscopist is told that the unknown sample consists of some combination of compounds A, B, C, D, and E and he has the spectra for those five compounds, then the least-squares XANES analysis can be done rigorously and with some degree of accuracy. However, if some information is lacking, for example, either one of the standard spectra is not available, or it is not realized that other compounds might be present, then the analysis is no longer reliable and likely incorrect. If no information at all is available about the species that might be present in a given sample, then the analytical data becomes less evident, unless the spectra are highly individualistic (which is typically not the case for Pb)

 

Under ideal circumstances, least-squares fitting of XANES spectra should be able to quantify up to four or five species with reasonable precision. However, this depends on (i) having all the spectra of possible forms of the element in the database; (ii) there being no unrecognized forms in the sample; and (iii) that the standard spectra differ sufficiently from each other

 

Unfortunately, Lead is not always the most conducive element for analysis of XAFS data. The fine structure, which is what distinguishes one Pb form from another in XAFS spectra, tends to be relatively broad and weak and may not provide a strong basis for quantifying Pb2+ forms in the absence of other data. This is particularly true for mixtures of possible Pb-O forms that may occur in soils, as the spectra tend to be rather similar, leading to potential errors.

 

There have been a numerous XAFS studies of Pb in soils and, although Pb XAFS spectra of the different soils tend to be quite similar, the interpretations of the speciation can differ significantly from study to study, in large part because of the different databases of Pb compounds available to the researchers. In many leading spectroscopist’s opinion, Pb speciation analysis of soils based solely on least-squares fitting of the Pb XANES region of a bulk sample may not yield reliable data. In such instances, the data can be improved if the quality of the data was at least good enough that both the XANES and EXAFS regions could be analyzed independently. A still better approach would be to combine Pb XAFS spectroscopy with a sequential extraction methodology to generate a set of subsamples (leached residues from the sequential extractions) that can be examined by XAFS spectroscopy. Such a combined approach should be more informative and reliable than the single XAFS analysis of the bulk soil.