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Many times in analysis, determinations
of specific ionic forms of elements are desired, particularly anions;
in water samples, measurements of nitrate, ammonium, or sulfate may
be of interest. In addition, extractions of soils, rocks or plant tissue
for some largely anionic elements such as N, P, Cl, S, and B are often
more conveniently determined by chemical methods based on measurement
of the anionic form of the element. Several instruments based on differnt
technologies can be used to measure ions in aqueous samples.
Ion
Chromatography
Ion chromatography is a separation technology that uses an anion exchange
column to separate anions moving through the column, which are then measured
by a detector system at the column outlet. At the Lab routine measurements
of nitrate (NO3-1), phosphate (PO4-3), sulfate (SO4-2), fluoride (F-1)
and chloride (Cl-1) can be made in aqueous systems such as stream and
pore waters. Detection limits for these constituents are roughly 1 ppm
(mg/L) in dilute solutions. Nitrite (NO2-1), and bromide (Br-1) analyses
can also be included in the sample run for an additional cost.
Ion chromatography is performed using a DIONEX DX500 modular chromatography
system with conductivity, electrochemical and absorbance detectors or
a DIONEX 4000i chromatograph with suppressed conductivity detection. Common
anions are separated using a IonPac AS4A column and detected by suppressed
conductivity. The SRS -II self regenerating suppressor (DX500 system)
enhances analyte sensitivity by providing superior suppression of the
background carbonate eluant conductivity resulting in significant improvement
in analyte detection limits. An older Dionex 4000i system is also available
for specialized analysis (see below).
Working calibration standards are prepared each day from dilution of a
1000 ppm mixed stock solution. A four point calibration is performed and
an independent check QC sample is analyzed every 10 samples. Because of
the long-term instability of nitrite in aqueous solution this ion is not
included in the routine analysis. However, if required, nitrite standards
can be made up on the day of analysis and this anion subsequently quantified
for an additional cost.
Colorimetry
For some analysis atomic spectroscopic (ie, ICP) or chromatographic
methods are not suitable due to the nature of the analyte or problems
with matrix interferences. Cyanide (CN-1) ) and ammonium (NH4+) in waters
or soilextracts or digests are examples of  such
analyses; colorimetric methods can be used very effectively to assay
for these and other species, often with little interference from matrix
salts or acids. In the Lab a Perstorp automated flow injection analysis
(FIA) system (shown at left) is available to perform colorimetric determinations
of these and other solutes in water or extracts. The unit automates
sampling, reagent addition, dilution, absorbance measurements and data
logging to file, and is capable of simultaneously determining up to
three analytes in a given sample set. Specific chemistries (color- developing
reagents) for sulfate, nitrate, ammonium, borate, phosphate and cyanide
can be used for single-element analysis at a reasonable per-sample cost.
These methods are particularly suited for ammonium in water, and for
the anionic compounds listed previously in extracts or digests (where
ion chromatography is ill-suited due to matrix interferences). Detection
limits are typically sub-ppm in solution. For P and B, ICP may be used
with slightly better detection.
Ionic
Speciation
Specific ionic species in solution can be determined if required using
ion chromatography as a separation technique; for instance, Fe(II) and
Fe(III) in waters or extracts can be determined by ion chromatography
with separation on IonPac CS5A column and absorbance detection following
post column derivitization. Specific measurement of Cr (VI) can be made
using an IonPac AS7 column with detection by UV/vis following postcolumn
derivitization. For other elements that show varying oxidation states
in solution (e.g., arsenic and selenium), methods can be devised for
separation of differing valence states on the IC, and eluent from the
IC injected into the ICP-MS for detection at sub-ppb levels. Additionally,
methods exist for determination of methylated forms of arsenic and mercury
based on column retention and either IPC-MS or cold-vapor absorption
detection (see "Metals" section for more details).
Other
Ion Analyses
Other dissolved ions are often determined potentiometrically; H+ (i.e.,
pH), and electrical conductivity (EC) are common potentiometric methods,
described in the Other Analyses section of this document. Other elements,
particularly the halogens (Cl, Br), can be determined using ion-specific
electrodes available at the Lab, and may be a good analytical choice
for large numbers of samples at higher (ppm+) levels requiring only
a single element analysis
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