The color reaction chemistry used in this process (automated Murphy/Riley 1962) is appropriate for liquid samples in which ortho-P is present as the product of a digest or extract procedure.
Details of the method
A discussion of principles and practical considerations in continuous-flow colorimetry is available in the Colorimetry Overview document. The ortho-phosphate procedure is specified in replicable detail in [assay for Phosphorus-all forms].
Sample preparation and handling
Samples must be free of turbidity and particulate matter and must not contain any color that absorbes at the same wavelength as the analytical color chemistry tint. Liquid samples which cannot be analyzed immediately after collection must be preserved for shipment. The U.S. E.P.A. has published a table of acceptable preservation methods and holding times for many analytes. Detailed sample-handling instructions specific to SIEL are available here.