Laboratory
Analytical Methods
Analytical methods used by laboratories must be sensitive enough for accurate, reproducible, and complete results. The data must also be reasonable, and defensible in court.
We have 4 GC systems (including GCMS), 2 VOC concentrators, stereo microscope, Infrared absorption spectrometer, special desiccators, and analytical balances. We also have a NIST Traceable Dew Point meter (-5 to – 110 ºF, ). NIST traceable standards are used where available.
We use gas chromatography for nearly all volatile or fixed gas analytes such as: volatile non-halogenated hydrocarbons, halogenated hydrocarbons, O2, N2, A, CO2, CO, SO2, H2S, N2O, He, and H2 The detectors employed are: MS, ELCD, TCD, FID and HID. Special in-house methods are used for the highly reactive NO and NO2 .
The term Total Hydrocarbons is a misnomer; it actually means Total Volatile Organics. (In the strictest sense, it has come to mean gases or vapor which can be detected by a gas chromatograph using a flame ionization detector.) Regardless of which laboratory is used or how the sample is analyzed, this test can only determine hydrocarbons that are non-condensed during sampling, and remains so all the way through the analytical process. For example: Freon 113 is 100% volatile, gasoline is 99% volatile, diesel fuel is about 20% volatile, and oil mist is non-volatile. Therefore, if these were in the compressor air, the laboratory, one would see all the Freon, all the gasoline, some of the diesel fuel (that which remained vaporized) but none of the oil mist. Oil Mist coalesces rapidly and plates onto surfaces. The significance of this - the expectation of what Total Hydrocarbons really is - becomes especially significant in oxygen systems. The tests for it are the next paragraph. Hydrocarbons in oxygen piping is a special topic we will write about in the future.
Oil Mist + Particulate is determined by weighing the increase in weight of an analytical filter after passing a known volume of air through it. If properly designed - we use small pore depth filters - the method of sampling will be capable of capturing all the particulate and oil on the filter. The method does not discriminate between particulate and oil mist. We examine filters by gravimetry and microscopy. All suspicious filters are discussed in a supplemental report at no charge. In our lab, samples failing the Oil Mist + Particulate can be further analyzed to determine the actual oil content.
Oxygen Compatible Air is air that is safe to mix with pure oxygen. This means that oil deposits and certain types and sized of particulate must be absent. The reason for this is ignition. Once ignition starts in an elevated level of oxygen, the piping becomes the fuel, and it will burn until the oxygen content drops to ambient levels or there is nothing left to burn. Ignition has been well studied in numerous NASA contracts, and this has led to NASA's criteria for system cleaning specifications. This primarily comes down to particles larger than 100 microns and fibers longer than 300 microns. (Particles smaller than that are not a problem unless they stick together to form a larger mass which acts as a large particle.) We understand that some labs report particles down to 2 microns. This is not only not ridiculous but such reports are very suspicious.
Qualifying air as Oxygen compatible means :(1) sampling the air in a way that allows discovery of oil and particulate; (2) microscopic counting of solid particles and fibers larger than 100 microns; and (3) examining filters for fugitive oil by a method that is specific for condensed hydrocarbons. We have found the simple gravimetric methods (regardless of the balance sensitivity) are unreliable for measuring oil mist at trace levels, and have adopted methods developed by NAVSEA (Mil Std 1330D or better) as well as NASA criteria per the NASA safety manual. I n our lab, we have developed a technique to discover oil in an air sample at 0.005 mg/m3
We investigated several approaches for determining the moisture content of compressed air, and have unhappily reached the conclusion that remote sampling (sending an air sample to a lab for moisture determination) does not work Using the dew point meter, we were able to prepare standards which allowed us to evaluate methods as well as sample containers. The result led us to use on-site measuring of moisture with a color indicator tube. (See also Sampling for Moisture.)
Calibrations and raw data are reviewed for correctness and reasonableness before reporting. All suspicious or failing data are re-determined by a separate analyst. Some labs do not weigh filters that “look clean”. However we weigh every filter because not all oils have a strong color. Although we perform QA checks daily, we do not report them unless requested; they would just clutter the report, and confuse the reader. Our QA process follows the requirements of AIHA and ISO/IEC 17025.
© 2005 Analytical Chemists, Inc.