The development of a pretreatment system to assist surface plasmon sensor-based measurement of arsenic in water is described. The system proposed addresses important issues, regarding the reliable in situ detection of arsenic in water. This system uses a primary filter made of nonactivated cotton fibers for particulate matter and chemical retention agents without modifying the arsenic concentration in the water sample. A secondary filter was designed for retention of mercury, lead, and other heavy metals without alteration of the arsenic concentration in the collected water samples to be sensed. This filter was made with amino-functionalized carbon nanotubes. The results of the operational assessment of this filter show a retention efficiency of 98% for suspended solids, 96% for mercury ions, and 2% for arsenic, a remarkable improvement toward the accurate detection and quantification of arsenic in contaminated waters.