Sunday, July 31, 2011

Rapid Headspace Analysis of Total VOC's by PID

The headspace method is based on Henry’s law which states that at equilibrium, the concentration in the gas phase is proportional to the concentration in the liquid phase times Henry’s law constant (KH):
C(liq) = C(gas) x KH
This means that “at equilibrium”, the liquid phase concentration can be determined by measurement of the concentration in the gas phase.

The PID Analyzers, Model 102 (Fig 1) is an easy to use photoionization based analyzer that has embedded software for headspace measurements of total VOC’s in soil, water, food, plastics, etc. The PID is very sensitive and has a 90% response time of 1 second. It is also an ideal tool for prescreening samples (to prevent sample overload) for the mass spectrometer or other analytical tool for determining the sample composition.
Fig. 1 Model 102 with multiple Heads

The headspace method is easy to perform, requires a minimum of equipment, and requires only that the sample and standards be at equilibrium and at the same temperature to obtain accurate results. A typical procedure involves weighing (or measuring) 1 g of soil into a 40 mL VOA sample vial, DI water is used to fill the vial. Then 15 cc of liquid is removed through the septum and the capped vial is placed in an oven at 60 C for 15 minutes, shaken and cooled to room temperature. Standards bracketing the samples should be run at the same time and under exactly the same conditions. The PID analyzers Model 102 should be in the headspace mode. Then take ten cc of gas is from the vessel via a gas syringe, connect it to the 102 via a tight piece of tubing and press ENT. The peak concentration will be displayed. The concentration can then be determined by comparison to a calibration curve generated from standards. Liquid samples can be handled in a similar manner. The total analysis time is < 15 seconds per sample. Fig. 2 Headspace for Water Sample

Fig.3 Headspace for Soil Sample

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