CAPO4 is the phosphate concentration that the instrument reports as its measurement. CAPO4 is the calculated concentration of an ambient sample using the CAS slope, where CAS is the manufacturer's scale factor, which is provided by Sea-Bird. nnVAPO4 is used in a QC process to monitor potential drift or spot pump failures or other mechanical problems. VAPO4 is the calculated concentration of an ambient sample using the VAS slope, where VAS is a variable slope calculated in-situ using the on board standard. VAS is only calculated during the Cal Spike frequency runs, used on subsequent samples between the Cal Spike. So, VAS and VAPO4 are not always good values and can affect whole groups of samples between the Cal Spike runs.nnThe calibration for CAPO4 does not include a variable ambient sample of unknown quantity as VAPO4 does.