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Ref. No. [UMCES] CBL 2017-050
ACT VS17-05
11
naphthylethylenediamine di hydrochloride to form a color azo dye. The absorbance measured at
540 nm is linearly proportional to the concentration of nitrate + nitrite in the sample. Nitrate
concentrations are obtained by subtracting nitrite values, which have been separately determined
without the cadmium reduction procedure.
All laboratory nutrient analyses were conducted on an Aquakem 250 auto-analyzer. For
phosphates, a statistically-determined method of detection limit for this instrument of 0.0007
mgP/L was established by prior laboratory studies for a wide range of salinities. An expected
working concentration range for this Verification and SOP was between 0.002 and 1.48 mgP/L.
The detection limits for nitrate and nitrite were similarly established at 0.0007 mgN/L and 0.0006
mgN/L respectively. The typical working concentration range for the nitrate method and SOP is
between 0.0049 – 5.6 mgN /L. The typical working concentration range for the nitrite method and
SOP is between 0.0042 – 0.28 mgN /L. The system contains an auto-dilutor to bring any higher
concentrations down to the established linear calibration range. A sample reagent blank is analyzed
in conjunction with every sample as part of the routine operation of the Aqaukem 250.
Approximately 40 samples per hour can be analyzed. All internal standards were verified and
calibrated using certified external nutrient standards (such as Spex Certi-Prep or NIST). In
addition, Field Trip Blanks and Field Sample Spike Additions (defined below) were conducted
once per week by ACT as part of established quality assurance/quality control (QA/QC) protocols.