A. General
Information
The
Alpkem Flow Solution IV with autosampler is equipped with
nitrate/nitrite, ammonia/TKN, and orthophosphate cartridges allowing for
a variety of measurements of different forms of nitrogen and phosphorus
in solution. The WinFlow 4.01 data collection software is run on a HP
Vectra VL7 computer.
B.
Theory of Segmented Flow Analysis (SFA)
Samples are aspirated from a sample cup through a
sample probe. The sample is pumped into the analytical cartridge where
segmentation (nitrogen bubbles) and mixing with reagents are precisely
controlled. As the sample progresses through the system, it may be
subjected to specific conditions necessary to create the desired
chemical reaction. Segmentation serves to minimize sample dispersion
and carryover.
The chemical reaction usually results in the
development of a colored product. This colored product is pumped
through a flowcell in the detector. Light of a selected wavelength is
continuously passed though the flowcell and the amount of light absorbed
by the colored product generates a signal proportional to analyte
concentration.
The detector signal is collected by the computer,
processed and displayed on the screen as a peak. Determination of
analyte concentration is based upon comparison of the peak height or
peak area (color intensity) of the sample with that of known calibration
standards.
C. Theory of Operation: NO3-N & NO2-N, NH4-N, PO4-P
Nitrate is reduced quantitatively to nitrite by a
cadmium column. The nitrite formed, in addition to any nitrite
originally present in the sample, is diazotized with sulfanilamide and
subsequently coupled with N-(1-naphthyl)ethylenediamine dihydrochloride.
The resulting colored azo dye is colorimetrically detected at 540 nm.
Nitrite singly may be measured by performing the same analysis without
the cadmium column.
Ammonium reacts with salicylate and hypochlorite in
a buffered alkaline solution in the presence of sodium nitroferricyanide
(pH 12.8-13) to form the salicylic acid analog of indophenol blue. The
blue-green color produced is measured at 660 nm.
Orthophosphate reacts with molybdenum VI and
antimony III in an acid medium to form an antimonyphosphomolybdate
complex. This complex is subsequently reduced with ascorbic acid to
produce a blue color which is measured at 660 nm.
NOTE: We are
currently not offering PO4-P analysis on this instrument.
