Olsen (Sodium Bicarb) extracted samples contain high quantities of sodium bicarbonate which normally cause bubbles (foam) to be created when the sample comes in contact with the acidic reagents.  This will cause significant problems with accurate absorbance measurements in the flowcell.

Solution: By putting a FIAlab supplied OP pressure restrictor on the flowcell output line, and running the pump at 65%, the resulting overpressure prevents gas from coming out of solution. In this fashion, Olsen phosphate can then be run like other phosphate extractions (e.g., Bray).

Due to common “brown” coloring in samples, Olsen phosphate must usually be run using 880 nm for the primary and 925 nm for the reference wavelength.  It is recommended that a NIR LED light source be used instead of the LS1 Tungsten lamp. 

The sample loop should be 6 inches of .03” ID tubing.

Heat is set to 45 C.

For lower concentrations (commonly occurring in soil samples), the 10 cm flowcell is recommended.

It is critical that PEEK insert tube adaptors be used for the Carrier, R1, and R2 lines (between pump and LOV manifold) to prevent the pump tubing from popping off due to increase pressure.

The colorimetric molybdenum blue reaction is sensitive to changes in acidity, and works best in the pH 6-8 range.  The sample should not have a strong matrix absorbance at the selected detection wavelength.  Turbidity will cause high phosphate results and noisy data.  This can be a problem with acid digested samples.  High iron concentrations (>50 mg/L) will decrease the phosphate signal.  While the presence of As³⁺ can cause high phosphorus results.