Will the equipment match the regulatory method?


Courtesy of Spectro Scientific

Anyone involved in field analysis of oil in water will con tinually get the question 'Does your equipment match the regulatory method?' Zj While the amount of oil in water is highly regulated, it can also be a challenging measurement. It is complicated by the fact that oil comes in many forms and the measurement is defined by the particular regulatory method. When EPA 1664 is the regulatory method, the 'oil' is anything that is extracted into hexane and remains after the hexane has been evaporated and shows up as weight. In regions where infrared analysis is the defining method, the 'oil' is extracted into the solvent and has carbon-hydrogen bonds that absorb infrared light at a specific frequency. Each method is looking at different properties of oil and can potentially give different results. Therefore, the answer to the commonly asked question of how one type of oil in water measurement compares with different regulatory methods is not always simple and straightforward, listed below are four factors that need to be considered.

1. Precision and bias for each method

There are acceptable errors for each method typically expressed in the precision and bias statement for the method. For example, EPA Method 1664 states in their 'Ongoing precision & recovery' (section 17.0) that for a 100 ppm sample the acceptable range is 78 -114 ppm. If the test includes the silica gel treatment (SGT) to remove the polar organics, the acceptable range is 64 -132 ppm. Therefore, if the result from a laboratory for a silica gel treated sample is 65 ppm and the alternate method result is 130 ppm, they are within the acceptable range.

2. Operator errors

As suggested previously, even the same method can give significantly different results. With any method where there is sample preparation, the human factor is added in. If a solvent/sample mix is only shaken for one minute rather than the required two minutes, the amount of oil extracted into the solvent will be significantly less. In some cases, it has been half the reading. Table 1 shows a comparison of a five way sample split analyzed on two InfraCal TOG/TPH Analyzers and at three laboratories. The results disprove the common misconception that the lab is always right.

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