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Selective Sequential Extractions – What to Know

Soil in DepthAny time metals are an environmental contaminant of concern, the predictable approach is to look at the total metals concentrations and determine a path forward. Whether it be determining risk due to toxicity, selecting a remediation plan, or establishing a regulatory permit limit, the decisions that are made assuming that all molecular forms of a metal have the same physical and chemical properties can have a number of dire consequences. Elemental speciation should be the first choice to avoid such erroneous and potentially costly assumptions from being made. There are many methods for the speciation of metal in solid matrices, but often times the most informative and cost-effective approach is to use selective sequential extractions.

Selective sequential extractions (SSE) have a number of useful applications for nearly any industry trying to understand the nature of the metals that are present in environmental samples and to predict or model how they might move through the environment (mobility), be metabolized by surrounding organisms (bioavailability), and bioaccumulate or biomagnify up the food chain.

The basic premise of SSE is that a representative solid sample (soil, sediment, tissue, etc.) undergoes an initial extraction which is typically representative of what might be easily transported or dissociated from the solid matrix. This is the “mobile” fraction. Depending on the element, there may be species-specific information that can be elucidated from this fraction, as well as from subsequent fractions. Next, this same sample aliquot is extracted with a second solution that generally represents something that is more strongly bound than the first fraction but still relatively available to the surrounding environment depending on site conditions. Each subsequent extraction uses a “stronger” reagent solution, leading to the final extraction which is generally the residual fraction and represents strongly-bound and generally immobile metal complexes in the solid phase. It is very important to note is that if subsurface samples or any other type of sample that is in relatively anoxic or from reducing conditions are collected, the use of anoxic sample collection techniques and notification of the laboratory that glove boxes must be used to maintain anoxic conditions may be required. Otherwise, sample results will not be representative of site conditions.

There are limitations of SSE that are important for anyone requesting these methods to be aware of. The first is that “one size does not fit all.” The SSE methods that exist in the literature and in published methods are intended for a very specific element or set of elements. When any of these SSE methods are applied to elements they were not intended for or have not been studied in detail, decisions based on this data may be questionable. BAL can provide guidance with the selection of the most appropriate method(s). The second is that these are operationally-defined methods, which means that the results for each fraction are defined as what was extractable by that reagent solution for that particular solid sample. This can be different for each set of samples. The information that is present in the literature or method regarding what types of species are expected to be extracted in each fraction can vary somewhat, so that a particular species may be present in a previous sequential extraction or may come out in a later extraction. The strength of these methods is to provide a strong indication of which metals are highly mobile, which are bioavailable, and which are not. Brooks Applied Labs currently has selective sequential extraction procedures for the fractionation of Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Hg, Pb, Mo, Cd, and Tl.


In the News (Again) – Arsenic in Baby Food

BAL CBC Baby Food TestBrooks Applied Labs was recently mentioned in a Canadian Broadcasting Company news segment on their investigation into arsenic concentration of baby foods sold in Canada. BAL performed the testing which showed some baby foods contained inorganic arsenic above the US FDA guideline and the EU regulatory limit of 100 ppb. While minimizing exposure to this harmful element is beneficial to people of all ages, it is especially important for infants due to their smaller size and proportionally larger calorie requirements.  Our testing showed some baby foods contained inorganic arsenic above the US FDA guideline and the EU regulatory limit of 100 ppb.  Read more about the CBC Marketplace investigation here, or watch the full news program here.

Compared to other grains, rice is known to contain higher levels of the toxic and carcinogenic element arsenic.  But the arsenic concentrations and the proportion of the more harmful inorganic arsenic forms can vary widely in rice depending on a number of factors, including where the rice is grown, the growing conditions, and how it is processed after harvesting.  Quantifying the more toxic inorganic arsenic forms via speciation analysis, which Brooks Applied Labs has been performing for decades, is therefore necessary to assess arsenic-related risks due to rice consumption.


In Case You Missed It

Around the GlobeTeam Brooks Applied Labs has been incredibly busy the past several weeks, with conference presentations locally, nationally, and internationally!

Our scientists and technical specialists strive to develop new methods and provide educational opportunities to meet the needs of our current and future clients! Contact us to learn more about any of our services.



Selenium Se
Although selenium is a component of many minerals, it is only the seventieth most abundant element in the earth’s crust and occurs mostly in small crystals and in small quantities. It typically only occurs at elevated concentrations in the environment as a byproduct of anthropological activity. An essential micro-nutrient, selenium becomes toxic to most eukaryotic organisms at relatively low concentrations.

Selenium testing for total recoverable concentrations by conventional ICP-MS techniques is extremely prone to mass spectral interferences. The plasma gas (argon) and constituents of the sample matrix (calcium, carbon, chloride, sulfur, etc.) can easily combine to form polyatomic ions with the same mass-to-charge ratios as the various isotopes of selenium, resulting in false-positives and elevated detection limits.

The table below lists the different isotopes of selenium and the common interferences that can affect measurements at those isotopes.

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Selenium Related News

Se and Hg in Fish Collected Near Power Plants

Brooks Applied Labs’ data was featured prominently in a recent Environmental Toxicology and Chemistry journal article titled, “Selenium and Mercury in Freshwater Fish Muscle Tissues and Otoliths: A Comparative Analysis”.

Welcome to the Selenium Centre of Excellence

The Selenium Centre of Excellence (SCE) is a non-profit organization seeking to advance innovative scientific solutions to address the growing global concern of selenium for the betterment of its inhabitants, ecosystems, industry, and regulatory bodies.

The Importance of Selenium Speciation

The topic of selenium has garnered attention in various industries for decades now, but the challenge associated with removing selenium from wastewaters has not diminished. In order to understand the most effective form of treatment for this unusual element that is...

BAL is On the Road Again…

The SETAC North America 39th Annual Meeting will be held in Sacramento November 4-8 this year. Come learn more about the unexpected ways alligators can potentially cause you harm you when Elizabeth Madonick, BAL Technical Services Specialist, presents a poster on the...

Quantifying Trace Selenium in Seawater

Brooks Applied Labs (BAL) is now offering the most innovative method commercially available for the quantitation of selenium in saline waters at concentrations as low as 5 ng/L (parts-per-trillion). BAL’s novel method utilizes in-line matrix component separation and...

BAL Continues to Expand Selenium Speciation Capabilities

Efficient selenium treatment and understanding ecological risks associated with the contaminant requires detailed molecular information to make educated decisions. As with any element, the molecular form of selenium, or chemical species, dictates how the element will...

In Case You Missed It: BAL Presentations February – April

If you follow our newsletters, you probably have noticed that BAL staff have had a busy winter and spring attending and presenting at many conferences around the country and even overseas. In case you missed any of the announcements or would like a copy of the...


Lead Pb
Lead – Update with Lead info.

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Lead Related News

Filtered Beer & Wine can have Higher Arsenic and Lead Levels

There have been many research publications in recent years detailing the heavy metal levels in alcoholic beverages such as beer and wine, but few of these investigate how the processing of these beverages may influence the levels of toxic metals. The results of a new...

Arsenic and Lead in Fruit Juice

Everyone knows that too much of a good thing can be bad. But what happens when even a little of a good thing can be harmful?  Consumer reports tested different fruit juices for cadmium, lead, mercury, and inorganic arsenic. The results were startling.  Click here for...

Arsenic and Lead Bioaccessibility in Soils

Contamination from trace metals, such as arsenic (As) and lead (Pb) in soils, can lead to potential health risks from inhalation and ingestion of the soil or vegetables grown in the soil. While it is well known that different molecular forms of As and Pb have...