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Murray & Associates STANDARD OPERATING PROCEDURE CONCERNING GROUNDWATER MONITORING SOP 7 The HNU Model 101 or a 580 B Photoionization Analyzer shall be used to measure the ' concentration of trace gases over a range of Iess than 1 ppm to 2,000 ppm by employing the principle of photoionization for detection The specific instrument used for investigations related to hydrocarbon contamination should be calibrated for direct readings in parts per million (ppm) ' volume/volume of benzene Both of these portable field analyzers consists of two components (1) probe which contains a fan for moving air into the sensor, an ultraviolet light (provides ionization energy), an ionization chamber and signal amplifier (2) readout assembly which ' contains a battery, ion chamber bias, meter readout and control panel Specifics of the detection principle/theory and functions of various components can be found in the manufactures instruction manual (for example HNU Systems, Inc ) ' To assure optimum performance, the photoionization analyzer should be calibrated with a standard gas mixture of known concentration from a pressurized container A daily procedure ' for calibration involves bringing the probe and readout in close proximity to the calibration gas, cracking the valve on the tank and checking the instrument reading This provides a useful spot check for the instrument A procedure conducted weekly for more accurate calibration of the instrument from a pressurized container is to connect one side of a "T" to the pressurized container of calibration gas, another side of the "T" to a rotameter and the third side of the "T" directly to the 8-inch extension to the photoionization probe (see Figure 2) Crack the valve of the pressurized container until a slight flow is indicated on the rotameter The instrument draws in the volume ' of sample required for detection, and the flow in the rotameter indicates an excess of sample Now adjust the span pot so that the instrument is reading the exact value of the calibration gas (If the instrument span setting is changed, the instrument should be turned back to the standby ' position and the electronic zero should be readjusted, if necessary)