Photoionization detector characteristics

The near-ir spectmm of ethylene oxide shows two peaks between 1600—1700 nm, which are characteristic of an epoxide. Near-ir analyzers have been used for verification of ethylene oxide ia railcars. Photoionization detectors are used for the deterrnination of ethylene oxide ia air (229—232). These analyzers are extremely sensitive (lower limits of detection are - 0.1 ppm) and can compute 8-h time-weighted averages (TWAg). 

At the temperatures and pressures generally used in gas chromatography the common carrier gases employed behave as perfect insulators. In the absence of conduction by the gas molecules themselves, the increased conductivity due to the presence of very few charged species is easily measured, providing the low sample detection limits characteristic of ionization based detectors [259]. Examples of ionization detectors in current use include the flame ionization detector (FID), thermionic ionization detector (TID), photoionization detector (PID), the electron-capture detector (ECD), and the helium ionization detector (HID). Each detector employs a different method of ion production, but in all cases the quantitative basis of detector operation corresponds to the fluctuations of an ion current in the presence of organic vapors. 

Benzene may be analyzed by several tech-niqnes, which include GC, MS, IR spectroscopy, and hydrocarbon analyzer. Its analysis at the ppb level in wastewater, groundwater, and potable water may be performed by a GC equipped with a photoionization detector and using the purge and trap or thermal desorption techniques. GC-FID may be employed to analyze benzene at ppm range. GC/MS is an excellent confirmatory test. Benzene may be identified by its primary characteristic ion with an m/z ratio of... 

SP-2401" and 3% SP-2250. ° Detectors used by EPA standards procedures, include photoionization (PID)," electron capture (ECD)," Eourier transform infrared spectrometry (PTIR), " and mass spectrometry detectors (MSD)." ° Method 8061 employs an ECD, so identification of the phthalate esters should be supported by al least one additional qualitative technique. This method also describes the use of an additional column (14% cyanopropyl phenyl polysiloxane) and dual ECD analysis, which fulfills the above mentioned requirement. Among MSDs, most of the procedures employ electron impact (El) ionization, but chemical ionization (CI) ° is also employed. In all MSD methods, except 1625, quantitative analysis is performed using internal standard techniques with a single characteristic m/z- Method 1625 is an isotope dilution procedure. The use of a FTIR detector (method 8410) allows the identification of specific isomers that are not differentiated using GC-MSD. 

Analysis of toluene may be performed by GC using photoionization or flame ionization detectors or by GC/MS. The analysis of wastewaters, potable waters, soils, and hazardous wastes may be done by the same EPA Methods as those used for benzene (see Section 26.2). In GC/MS analysis the primary characteristic ion for toluene is m/z 91. Air analysis may be done by charcoal adsorption, followed by desorption with carbon disulfide and injecting into GC-FID (see Section 26.2 and NIOSH Methods 1500 and 1501). 

Xylenes in drinking water, wastewaters, soils, and hazardous wastes may be analyzed by EPA analytical procedures (Methods 501, 602, 524, 624, 1624, 8020, and 8240) (U.S. EPA 1992 1997). These methods involve concentration of the analytes by purging and trapping over suitable adsorbent columns before their GC or GC/MS analyses. The primary characteristic ion for GC/MS identification (by electron-impact ionization) is 106, which characterizes ethylbenzene as well. Photoionization and flame ionization detectors are, in general, suitable for ppb- andppm-level GC analysis, respectively. Air analysis may be done by NIOSH Method 1501 (see Section 26.2). 

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