Recently, we attended a technical presentation on the various analysis methods utilized for metals in the laboratory. It is common for us to collect soil and groundwater samples for metals analysis and have that analysis performed by the laboratory. It is always helpful and interesting to hear what exactly happens in the laboratory to help us understand how to be better consultants. We can learn quite a bit about both our field methods and what the data tells us when we are writing the report.
This specific presentation started off with a discussion on the metal speciation. Metal species are metals and semi-metals existing as hydroxides, organometallic compounds, biomolecules, and other forms. The determination of metal speciation, or what species of metal is present, aids in the determination and characterization of trace metals in air, soil, water, sediments, and tissues. These metals can be differentiated both chemically and physically, most commonly in an aqueous or soil solution.
Freezing samples is the most widely used and effective method of preservation, often with opaque containers for any light sensitive species. When selenium is suspected in aqueous solutions, the sample should be filtered as soon as possible as bacterial growth is common. Also be sure to only use temperature and filtration, not chemical preservatives or pH buffers when dealing with aqueous selenium, as these chemicals could alter the final reading. Handling arsenic is commonly aided with pH buffers however. For lower pH solutions with a high presence of iron, buffers from acetic acid or edetic acid (EDTA) are commonly used. For the broader ranges of pHs, however, hydrochloric acid (HCl) is most commonly used.
Ways to identify metal speciation include gas, liquid, and ion chromatography, and used in conjunction with atomic absorption (AA), atomic fluorescence spectrometry (AFS), and inductively coupled plasma mass spectrometry (ICPMS).
Gas Chromatography (GC)
Gas chromatography utilizes soil samples most commonly, and detects gaseous samples where volatiles are less common and easier to detect. AFS and ICPMS are the most common methods used to determine metal species in gas chromatography. Often a derivative step, the generation of hydride compounds, is needed to form volatile metals. The generation of hydride compounds helps to identify arsenic and selenium, converting different elements or species into volatile hydrides. However, hydride generation should be used without further treatment with UV or persulfate oxidations, as this can form arsines or methyl arsines. The EPA has regulated method 1632 that are suitable for most practical applications with arsenic and can be modified for selenium, and is easily adapted to show parts per trillion in their results. These methods aid in AA detection, and the pH is dependent on the isolation of the oxidation states of those species that are amenable to hydride generation or that are already volatile.
Liquid and Ion Chromatography (HPLC/IC)
These methods are most commonly used for arsenic-in-flame tests, but can rely on AFS or ICPMS for their methods. Since most arsenic and selenium are found in natural deposits, the most utilized options are ion exchange and ion pairing. Ion exchange involves separation of molecules with oppositely charged polarities, and allows for the separation of the present species by their charge. Ionic pairing uses the addition of an ionic surfactant which binds to the different species and ejects the accompanying amino acids, which are then used to identify the originating metal species.
Pros of GC
Pros of HPLC/IC
|· Faster run times
· Use of anions helps to minimize column issues
· Displays broad spectrum of arsenic and selenium species
· Can detect very low limits
· Able to use HCL and EDTA preservation
· Multiple techniques are EPA regulated
· Generally, more cost effective
|· Wider range of species to be detected
· Equipment is more commercially available, but more complex if not using hydride generation
· EPA 6800 is a recognized method
Cons of GC
Cons of HPLC/IC
|· Issues with hydride generations if there are high metal concentrations
· Manually intensive due to multiple analyses
· Limitation of species detected without further treatment
|· Longer run times
· Highly anionic species can interfere with columns, which dilutions are needed to minimize
· Eliminates ability to use HCl
· More complicated to operate, especially if attempting to detect lower limits
· Generally, more expensive, especially when used alongside ICPMS
Arsenic and selenium speciation are most commonly used for risk assessment and treatment options of affected populations on sample sites. ICPMS is the most common and cost-effective method used in industry, however, it is important to work with your local lab to determine the individual needs of each site, as ICPMS covers a common field of species encountered. Many labs also can quote various “packages” of detectable species, where only a few of the “packaged” species are expected to be encountered. To that end, if the provided sample has to be diluted, low levels will either be unavailable or not relevant.