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Trace Total Mercury Analysis
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Arsenic
speciation analysis of soil and sediment can provide insightful
information associated with risk assessment, fate and transport, and
chemical equilibria within the substrate. The unparalleled degree of
variability associated with soil and sediment matrices makes it one of
the most technically difficult materials to extract representative data
for arsenic speciation analysis. Applied Speciation and Consulting, LLC
(ASC) understands the complexity of soil and sediment systems and
applies appropriate quality control measures and analytical method
necessary for quantification.
The sorption of various arsenic species onto Fe, Mn, and Al have been
well documented throughout literature.
Due to the ubiquitous nature of Fe, Mn and Al, their concentrations and
molecular structures can vary significantly impacting interactions with
the different arsenic species. The pH and Eh of the substrate must also
be taken into consideration as those parameters drive the equilibrium of
the As, Fe, Mn, and Al species as well as their degree of protonation.
The complexity of soil and sediment matrices cannot be understated;
therefore, understanding the interactions and experience involved with
different extraction methods becomes of paramount importance when
quantifying arsenic species.
Extraction procedures must meet three qualifiers: (1) the extracting
solution must solubilize all forms of arsenic, (2) the conditions of the
extraction must not induce reduction of native arsenate to arsenite, and
(3) the method must not cause oxidation of native arsenite contained in
the sample to arsenate. Due to their chemical properties arsenite and
arsenate are the most efficiently adsorbed by the sample matrix.
Many arsenic extraction procedures apply ligand exchange to reduce
readsorption which is a function of the extraction solution. Often these
procedures involve the application of phosphate (PO4-3) at different pH
values. The similar molecular structures and chemical properties of
phosphate and arsenate allow for competitive binding resulting in higher
extraction efficiencies and a reduction in readsorption.
Multiple extraction procedures are applied as the extraction
efficiencies for the different arsenic species are pH dependant. The
ability to alter the extraction solutions to optimize the specificity
for target arsenic species is limited; therefore, speciation must be
performed on every extract. The extraction efficiencies can often be
correlated with the different components of the soil or sediment. An
excellent example is the different sorption efficiencies for arsenite
and arsenate onto goethite and amorphous Fe. Low recoveries of arsenite
using phosphate at pH ~3 may suggest that goethite is not the major Fe
species; rather, amorphous Fe which may have resulted from deposition of
precipitated Fe is the controlling factor. This would suggest that the
matrix may act as a viable sink for arsenite inferring the arsenic
contaminated site may have less impact on the local aquifer than
initially expected. Unfortunately, assumptions are often made toward the
specificity of the extraction solutions which is represented by only
performing a total arsenic analysis of the extract.
Other extraction procedures may facilitate hydroxide, sulfate, chloride,
or other anionic molecules. While these options may result in consistent
high extraction efficiencies, meeting the first qualifier as previously
mentioned, they often result in speciation conversion which cannot be
accounted for unless arsenic speciation analysis is performed on the
extracts. This leads to the importance of meeting the three main
qualifiers for identifying viable extraction procedures.
In addition to speciating the sample extracts, equilibrium shifts must
also be monitored in the form of matrix spikes and laboratory control
samples for every target species. Due to the variability inherent with
soil and sediment matrices the application of a certified reference
material would not be representative of sound scientific practices;
therefore, the best representation of the method efficiency are matrix
spikes. Although matrix spikes are artificial representations of the
extraction efficiencies they do identify issues with readsorption and
species conversion. Other standard quality control measures (e.g.
preparation blanks, laboratory control samples, matrix duplicates, etc.)
must also be employed to identify accuracy and precision associated with
the applied methods. The limitations of some analytical methods for
arsenic speciation may hinder the applicability of the different quality
control parameters; therefore, the importance of analytical method
selection also contributes to the confidence in generated results.
Applied
Speciation and Consulting applies ion chromatography inductively coupled
plasma - mass spectrometry (IC-ICP-MS), the most robust and widely
accepted method for speciation analysis of arsenic and other metalloids.
Sample extracts can be introduced directly into an ion exchange column
which separates the different metalloid species according to their
interactions with the column packing material. The eluting species are
then introduced into an ICP-MS serving as a very sensitive elemental
detector. In addition to matching retention times of known standards for
species identification ancillary elements can also be monitored for
confirmational purposes. Correlating molar ratios of arsenic and other
elements can also facilitate the identification of new arsenic species
which may assist in identifying transitional species for modeling
purposes.
Our experience allows us to choose different ion exchange columns and
eluants for different matrices to provide better resolution of target
species. More importantly, for higher profile cases, secondary
confirmation using different columns and eluants can provide
indisputable data when necessary.
Our scientists have tremendous experience with trace analysis. If you
have any questions or would like a quotation, please feel free to email
us at info@appliedspeciation.com or call
(425) 483-3300 .
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