Dataset: Surface nanomolar Phosphate, Nitrate, and Nitrite concentrations from R/V Knorr cruise KN204-01 in the Subtropical northern Atlantic Ocean in 2011 (U.S. GEOTRACES NAT project)

Filtered (0.4 micromole) water samples from the Geotraces surface tow-fish and water from the upper 200 m water column (GEOTRACES Carousel/GoFlo system) were analyzed on an Astoria Pacific 300 series gas-segmented continuous flow nutrient auto-analyzer modified with 200 cm liquid waveguide capillary cells (LWCC; World Precision Instruments). Nitrate was reduced to nitrite using a Cd-column, then determined spectrophotometrically following formation of a highly colored dye, but slightly modified for use with a liquid core waveguide (see references). Nitrite is measured separately without the Cd column and then subtracted from the combined nitrate+nitrite sample. The molybdenum blue method modified for use with liquid core wave guide was used for phosphate (references below).

Recovery for phosphate measurements for LWCC relative to the MAGIC method is 100 ± 15 % for concentrations below 100 nmol/L. The relative standard deviation for nine measurements of 5 nmol/L phosphate was 1.6 %. The relative standard deviation for nine measurements of 60 nM phosphate was 0.9 %. LWCC detection limit (3 x σ of blank) for phosphate was 0.5 nmol/L, 1.5 nmol/L for nitrate and 0.6 nmol/L for nitrite.

Upper working limit for LWCC:
Phosphate 100 nmol/L
Nitrate 200 nmol/L
Nitrite 200 nmol/L