U.S. Geological Survey Open-File Report 02-371
by S.A. Jablonski, E.L. Mecray, J.M. Munson, and D.S. Blackwood
|Total and Organic Carbon Analysis - CHN|
|Inorganic Carbon Analysis - Coulometer|
|Biogenic Silica Analysis|
|Radiochemistry (Gamma Analysis)|
|Acid Washing Procedure|
|Sediment Grinding Procedures|
|Geochemistry - Core Sectioning Procedure|
|Job Hazard Analysis|
|Material Safety Data Sheets (non-USGS link http://hazard.com/msds/ )|
The Woods Hole Field Center (WHFC) has the capability to measure organic,
inorganic, and total carbon contact using Coulometer and CHN Analyzers.
Total and organic carbon percentages are determined using the Perkin
Elmer 2400 Series II CHN Analyzer (Figure 1). From these analyses, one
can determine the percent inorganic carbon of a sediment sample by subtracting
the percent organic carbon from the percent total carbon:
The organic carbon fraction in sediments is important because it serves
as a binding site for contaminant metals. The abundance of organic carbon
controls many diagenic processes. Normalizing contaminant data by the
% organic carbon allows for the distinction of specific anthropogenic
CHN Analyzer procedures - The CHN Analyzer uses a combustion method to convert the sample elements to simple gases (CO2, H2O, and N2). The dried and ground sediment sample is first oxidized using classical reagents like Silver Vanadate, Silver Tungstate, and EA-1000, which is mixture of chrome and nickel oxides. Products produced in the combustion zone include CO2, H2O, and N2. Elements such as halogens and sulfur are removed by scrubbing agents in the combustion zone. The resulting gases are homogenized and controlled to exact conditions of pressure, temperature, and volume. The homogenized gases are allowed to de-pressurize through a column where they are separated in a stepwise steady-state manner and quantified as a function of their thermal conductivities (Perkin Elmer Instruction Manual).
In order to measure %organic carbon in sediment samples, one must first acidify the samples to remove all inorganic matter. The inorganic material is removed by adding sulfurous acid to the sediment so the inorganics will turn to gas and leave the sample. Using this method, only the organic material is analyzed by the CHN Analyzer (Verardo, David J. et. al).Coulometer procedures - The Coulometer provides an accurate and absolute determination of the concentration of carbon dioxide (CO2) evolved from an acidification process. The coulometer cell is filled with a cathode and an anode solution (proprietary through UIC) with a colorimetric indicator. A platinum cathode and a silver anode are positioned in the cell and the assembly is positioned between a light source and a photodetector in the coulometer. When a gas stream passes through the solution, CO2 is quantitatively absorbed, reacting with the elements in the cathode/anode solutions to form a titratable acid. This acid causes the color indicator to fade. Photodetection monitors the change in the solution's color as percent transmittance (%T). As the %T increases, the titration current is automatically activated to stoichiometrically generate a base at a rate proportional to the %T. When the solution returns to its original color (original %T), the current stops (UIC Coulometrics Instruction Manual).
Equipment used for these procedures includes the instruments described above (Perkin Elmer 2400 Series II CHN Analyzer and a UIC Coulometrics Coulometer with an Acidification Unit), as well as a ball-mill grinder used to grind the dried sediment sample to a fine powder, ovens used to insure the sample is dry, desiccators used to cool and store samples in a moisture-free environment, and the chemicals involved in the analysis process.
This laboratory is equipped to handle wet sediment samples (cores and grabs) as they are returned from field collection. Samples are collected from marine and coastal areas and returned to the laboratory for analysis. Cores are sectioned and sub-sampled using titanium tools to minimize contamination (Figure 3). Samples from cores and grabs are freeze-dried (Figure 4) and sent to contract laboratories for various analyses including toxicity tests, mercury concentrations, foraminifera identification, pollen counting, nitrogen isotope analysis, and organic and inorganic contaminant concentrations.
Core sectioning procedures - A fully illustrated tutorial on the methods used at the Woods Hole Field Center is presented in this publication.
Freeze Drying procedures
- A fully illustrated tutorial on the methods used at the Woods Hole
Field Center is presented in this publication.
This laboratory is equipped to determine gamma decay emissions from dried sediment samples. This method allows for the determination of the sample's age and the sedimentation rates in sediment cores. Two types of gamma detectors are used: well-type (Figure 5) and planar-type (Figure 6). These high purity germanium (HPGe) detectors are in constant use and are located in a shared laboratory on the Woods Hole Oceanographic campus.
Radioisotopes, such as Pb-210 and Cs-137, can be measured by gamma emission and used in conjunction with a numerical model to estimate sedimentation rates. These measurements are important in modeling chronological changes such as heavy metal pollution. The WHFC conducts studies in the New York Bight (NYB) and in Long Island Sound (LIS) because they are in close proximity to high-density population centers and sewage dumping. Anthropogenic sources have added metals, carbon, bacteria, and organic contaminants to the sea floor. Although these pollutants have been dispersed over time, they are still present in the sedimentary record and can be measured. Cs-137 is a fallout isotope produced from bomb tests in the 1950's and 1960's and since it is not a naturally occurring isotope, it is known that the onset date for this isotope is 1954 and this information can be used to date the sediment cores collected from those areas (Santschi et al, 1999).
Instrumentation for these procedures requires both a well-type and a planar-type
HPGe detector and a freeze dryer unit.
The WHFC has the capability to digest sediment samples to be further analyzed by Inductively Coupled Plasma - Emission Spectrometry (ICP-ES) for biogenic silica. Biogenic silica fluxes indicate ecosystem productivity over time. The WHFC does not, at this time, have the instrumentation to analyze the biogenic silica extracts, so contract laboratories are used.
Biogenic silica digestion procedures - A fully illustrated tutorial on the methods used at the Woods Hole Field Center is provided in this publication. The method used is adapted from Mortlock, R.A. et. al (1989).