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requiring hours to several days before results are available. During a plant turnaround, every minute of shut-down time is critical and much more rapid feedback is needed to make personal protective equipment decisions for workers. Por t a b le GC Systems. Portable gas chromatographs, usually with PID or FID detectors, provide selective benzene measurements down to 0.1 ppm, but have several drawbacks. They tend to be heavy and bulky, making them difficult to use in tight spaces and on ladders and catwalks. They tend to be relatively expensive and complex; therefore usually only the company industrial hygienist operates one. Sampling and analysis time is relatively long, similar to gas detection tubes, i.e., on the order of 5-10 minutes. Flame Ionization Detector s (FIDs). FIDs work by burning a gas in a hydrogen flame. In this environment, organic compounds produce ions, which are measured on an electrometer. The resulting current is proportional to the concentration of organic vapor. The FID is very sensitive and linear over many orders of magnitude, but because it requires hydrogen and a mechanically stable environment for the flame, the resulting instruments are complex. FIDs do respond to a broad range of organic compounds but are non-selective and therefore cannot measure benzene specifically unless connected to some type of separation system such as a gas chromatograph. FIDs are also relatively expensive, maintenance-intensive, and complex, which limits their use in most industries. Photoionization Detector s (PIDs) with filter tubes as an optimal monitor in g method. While PIDs are broadband detectors, they can give benzene-specific response by using a combination of a low-energy (9.8 eV) UV lamp and pre-filter tubes that remove organic interferences. This system provides rapid response down to 0.1 ppm in one minute with an accuracy of 10% in the 0.5-5 ppm range. Measurements of TWA-levels of benzene can be made in the presence of up to about 300 ppm of gasoline. The tubes also absorb moisture and thus benzene-specific measurements can even be made in steam vents. Compared to portable gas chromatographs, the PID-Filter systems are much smaller and lighter, considerably less expensive, easier to operate, much faster, and provide about the same accuracy and detection limits. The instantaneous feedback enables workers to make rapid personal protective equipment decisions and allows them to perform their job tasks with confidence that they are not being exposed to hazardous levels of benzene.
