47
for reactive silicate, 0.01 μmol.l
−1
for nitrite, and 0.05 μmol.l
−1
for nitrite. Nitrate
analysis through diazotization, as noted above, requires prior reduction to nitrite
which is usually accomplished by passing the sample through a column packed with
cadmium metal shavings coated with semi-colloidal copper. This column is
extremely sensitive to oxidation and must be maintained submerged in ammonium
chloride while not in use. Also repeated exposure to anoxic waters carrying sulfide
ion will inactivate the column. The phenol/hypochlorite method described for
ammonium analysis has been widely replaced with the more sensitive and less hazardous procedure of ortho-phthaldialdehyde OPA derivatization followed by fluorometric analysis (Holmes et al. 1999).
Many of these procedures have been automated and are now routinely performed
in the laboratory using dedicated instruments known as flow-injection analyzers.
These instruments propel aqueous samples and reagents with multichannel peristaltic pumps. Samples are automatically fed from carousel or linear array sample racks
to injection valves where a small-volume sample plug is inserted into the carrier
fluid flow. Method-specific capillary tubing manifolds allow injection of reagents to
the carrier stream. Following transit through mixing coils, the resulting colored
solutions are passed through flow-through colorimeters or fluorometers equipped
with appropriate color filters. Analytical standards, either purchased or prepared in
the laboratory at concentrations that bracket the expected sample concentrations,
are analyzed concurrently for calibration. Preparation of standards and reagents in
aqueous solution requires assurance of the absence of contamination with the target
analyte in the dilution water or in the reagents. Prior to use, raw tap water is routinely filtered, distilled, and then passed through ion exchange columns to remove
contaminants. Alternatively, for extremely exacting work where nutrient concentrations are very close to the method limit of detection, certified water may be purchased from chemical suppliers.
Autonomous Wet Chemistry Instruments
These technologies described above served as models for the current generation of
autonomous in situ instruments. Several such chemical-laboratories-in-a-can are
available on the market. Some manufacturers, responding to the need for conserving
reagents in autonomous instruments, have opted to switch from sample injection as
used in laboratory instruments to reagent injection since the instrument is after all
immersed in the sample. Instruments incorporating microfluidic devices are now
becoming available. In these miniaturized devices, fluid flow is along a network of
microchannels etched directly on a chip rather than having capillary tubing transporting the sample. Instrument reliability is improved with the elimination of loose
capillary tubing, end-fittings, and manifolds. Advantages of these low-volume
devices are reduced reagent use and analysis time, high sample throughput, and the
possibility for incorporating parallel analyses on multiple chips.
2.3 Electrochemical Sensors for Coastal Ocean Observing
for reactive silicate, 0.01 μmol.l
−1
for nitrite, and 0.05 μmol.l
−1
for nitrite. Nitrate
analysis through diazotization, as noted above, requires prior reduction to nitrite
which is usually accomplished by passing the sample through a column packed with
cadmium metal shavings coated with semi-colloidal copper. This column is
extremely sensitive to oxidation and must be maintained submerged in ammonium
chloride while not in use. Also repeated exposure to anoxic waters carrying sulfide
ion will inactivate the column. The phenol/hypochlorite method described for
ammonium analysis has been widely replaced with the more sensitive and less hazardous procedure of ortho-phthaldialdehyde OPA derivatization followed by fluorometric analysis (Holmes et al. 1999).
Many of these procedures have been automated and are now routinely performed
in the laboratory using dedicated instruments known as flow-injection analyzers.
These instruments propel aqueous samples and reagents with multichannel peristaltic pumps. Samples are automatically fed from carousel or linear array sample racks
to injection valves where a small-volume sample plug is inserted into the carrier
fluid flow. Method-specific capillary tubing manifolds allow injection of reagents to
the carrier stream. Following transit through mixing coils, the resulting colored
solutions are passed through flow-through colorimeters or fluorometers equipped
with appropriate color filters. Analytical standards, either purchased or prepared in
the laboratory at concentrations that bracket the expected sample concentrations,
are analyzed concurrently for calibration. Preparation of standards and reagents in
aqueous solution requires assurance of the absence of contamination with the target
analyte in the dilution water or in the reagents. Prior to use, raw tap water is routinely filtered, distilled, and then passed through ion exchange columns to remove
contaminants. Alternatively, for extremely exacting work where nutrient concentrations are very close to the method limit of detection, certified water may be purchased from chemical suppliers.
Autonomous Wet Chemistry Instruments
These technologies described above served as models for the current generation of
autonomous in situ instruments. Several such chemical-laboratories-in-a-can are
available on the market. Some manufacturers, responding to the need for conserving
reagents in autonomous instruments, have opted to switch from sample injection as
used in laboratory instruments to reagent injection since the instrument is after all
immersed in the sample. Instruments incorporating microfluidic devices are now
becoming available. In these miniaturized devices, fluid flow is along a network of
microchannels etched directly on a chip rather than having capillary tubing transporting the sample. Instrument reliability is improved with the elimination of loose
capillary tubing, end-fittings, and manifolds. Advantages of these low-volume
devices are reduced reagent use and analysis time, high sample throughput, and the
possibility for incorporating parallel analyses on multiple chips.
2.3 Electrochemical Sensors for Coastal Ocean Observing