A fluid analyte measurement system includes an extraction unit having a fluid flow path in intimate contact with an extraction solution flow path. The fluid flow path includes a fluid entry port and a fluid exit port and the extraction solution flow path includes an extraction solution flow entry port and an extraction solution flow exit port. The extraction unit extracts analytes from a fluid flow introduced within the fluid flow path through exposure of the fluid flow to an extraction solution introduced within the extraction solution flow path. A sample head vessel is coupled to the extraction solution flow exit port and at least one probe is coupled to the sample head vessel to generate signals indicative of the characteristics of the fluid flow analytes within the extraction solution flow. For example, this measurement system can be used to determine Na.sup.+ level within fuel oil. Circuitry is coupled to the probe(s), which circuitry is configured to measure the signals generated by the probe(s).