Time-weighted average sampling

According to Busch and Liedel (13), one can easily calculate an Upper Confidence Limit (UCL) at 95% confidence for a single 8 hour-time weighted average sample by the following equation ... 

Lestremau, R, Andersson, F.A., Desauziers, V., Fanlo, J.L. (2003). Evaluation of Solid-Phase Microextraction for Time-Weighted Average Sampling of Volatile Sulfur Compounds at ppb Concentrations. Anal. Chem., 75, 2626-2632. 

Khaled, A. and Pawliszyn, J., Time-weighted average sampling of volatile and semivolatile airborne organic compounds by the SPME device, J. Chromatogr. A, 892, 455-467, 2000. 

Static in-tube SPME time-weighted average sampling An integrated sampling is possible with a simple SPME system. This is particularly important in field measurements when analyte concentrations change with time and location. 

Figure 6 Use of SPME for in-tube time-weighted average sampling where C= concentration, Cs = concentration of analyte in the sample, Z= distance between needle opening and position of extraction phase, and f. = length of SPME needle (A) schematic, (B) adaptation of commercial SPME manual extraction holder.

Health and Safety. Remover formulas that are nonflammable may be used in any area that provides adequate ventilation. Most manufacturers recommend a use environment of 50—100 parts per million (ppm) time weighted average (TWA). The environment can be monitored with passive detection badges or by active air sampling and charcoal absorption tube analysis. The vapor of methylene chloride produces hydrogen chloride and phosgene gas when burned. Methylene chloride-type removers should not be used in the presence of an open flame or other heat sources such as kerosene heaters (8). 

As an example, several air samples collected during an 8-hour work shift for benzene (8-hour time-weighted average = 10 ppm) were 3 hours at 4 ppm, 1 hour at 15 ppm, 2 hours at 6 ppm and 2 hours at 9 ppm. The employee s time-weighted average exposure would be ... 

This knowledge is also important in order to determine if air tests conducted by OSHA compliance officers are valid. For example, if threshold limit value in the health standard is an 8-hour time-weighted average, the air sample should be obtained by sampling over the entire shift in the employee s breathing zone. It cannot be measured by a few short term samples, even if spaced over the full shift unless the worker is in a relatively fixed location with no variation in his work procedure or in the process. Such an event is generally the exception rather than the rule. 

An 8 hour time-weighted average limit (TWA) of 0.2 fiber per cubic centimeter (f jcc) of air based on an 8 hour time-weighted average (TWA) sampling period. This is the maximum level of airbome asbestos, on average, that any employee may be exposed to over an 8 hour period (normal work shift). 

Time Weighted Average Concentration Refers to concentrations of contaminants which have been weighted for the time duration of sample. A sufficient number of samples are needed to permit a time-weighted average concentration throughout a complete cycle of operations or throughout the work shift. 

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