of carbon, nitrogen, and sulfur is often an important characteristic
of environmental samples, and given the difficulty of analysis, specialized
instruments have been developed for their determination. For soils,
rocks, and waste materials, dry combustion methods which directly volatilize
gases from solid samples are the standard. For solutions, N can be measured
as NO3 or NH4 and S as SO4 by methods described under ion analysis.
Dissolved aqueous carbon (organic or inorganic) can be determined by
the DOC analyzer described below.
N, and S in Solids
The Leco 2000 analyzer in the Lab simultaneously determines C, N, and
S in solid samples within the approximate range of 0.05 to 5% by weight
of each element (varying with sample size used). The sample is weighed
into a tared ceramic boat and loaded into the autoloader where it combusts
in a furnace at 1350 oC. Approximately 1 g of combustion catalyst is added
to each sample to ensure complete combustion of the sample in the furnace.
Combustion gases are collected in a 4.5-liter ballast tank and then flow
to the detectors. C and S (as CO2 and SO2) are detected by infra-red absorption
measurement, and N2 by thermal conductivity detection.
The method is suitable for analysis of soils, sediments, sludges, and
plant tissue. Sample masses of up to 4 g can be analyzed (recommended
for soils where % CNS is relatively low) ensuring representative sample
analysis. For plant tissue sample weights of about 200 mg are generally
sufficient for CNS quantification, although at least 1 g of sample should
be submitted to provide enough sample mass for duplicate analysis. To
ensure homogeneity, it is recommended that samples be air-dried and finely
ground. Sample prep. can be performed at the Contaminant Analysis Laboratory
for a minimal charge. Analysis of a certified reference material is performed
every 10 samples as a check on instrument accuracy. A number of certified
reference materials are available, so that most sample matrices can be
matched fairly well.
Dissolved carbon concentrations are determined using a Shimadzu TOC-5050A
Total Organic Carbon Analyzer. The TOC-5050A has a wide linear range of
up to 400 ppm organic carbon and 5000 ppm inorganic carbon, with detection
limits around 300 ppb. For Total Carbon (TC), aliquots of aqueous sample
are automatically injected into a total carbon combustion tube (680oC),
is oxidized to CO2(g) which then flows in a carrier gas stream to a nondispersive
infrared (NDIR) detector where the CO2 content is measured. The output
is displayed as peaks and quantified by peak area using a calibration
curve constructed from 4 calibration standards. For Inorganic Carbon (IC),
aliquots of sample are injected into an IC reaction vessel (phosphoric
acid solution) where CO32-and HCO3- are converted to CO2 which then flows
directly to the NDIR detector. When both measurements are performed on
a single sample, Dissolved Organic Carbon (DOC) is quantified as the difference
between TC and IC concentrations as DOC = TC - IC.
Samples for aqueous C measurements should be filtered at at least 1 micron
(42 Whatman) to prevent clogging of the instrument; filtering may be done
at the Lab for an additional fee (see price list for details.