Initial Detector Selection

MS/MS) is the standard detector for bioanalytical assays and drug discovery screening, its use for routine assays of drug substances and products is still limited due to its high cost and lower precision. Nevertheless, LC/MS/MS methods are increasingly used for ultra trace analysis or screening of complex samples. Other detection options include conductivity detection for ionic species and electrochemical detection for neuroactive species in biochemical research. 

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Here the sum runs over all possible initial states and the operator describes the interaction of the electrons and the radiation field with wave vector q and polarization A. In Eq. (1) it has been assumed that the detector selectively counts photo electrons with energy E, wave vector k, and spin polarization The corresponding final... 

Other important parameters in providing successful GC are the column packing, temperature conditions, and selection of a detector as specific to the analyte as possible. Maximum resolution of the halocar-bons is achieved with an 8-ft X 0.1-in. i.d. column of Carbopack-B coated with 1% SP-1000. The initial temperature of 45 °C is held for 3 min and then programmed at 8 °C/min to 220 °C. An organohalogen detector (OHD) is used. The aromatics are best resolved with a 6-ft X 0.085-in. i.d. column of Supelcoport coated with 5% SP-1200 plus 1.75 Bentone-34. They are measured with a photoionization detector. The temperature conditions are as follows 50 °C for 2 min then programmed at 6 °C/min to 90 °C. A 10-ft X 2-mm i.d. Porapak-QS (80-100 mesh) column at a temperature of 110 °C for 1.5 min and rapidly heated to 150 °C is now used for acrolein and acrylonitrile. This method employs a flame ionization detector (FID). 

In this manner, a nearly universal and very nonselective detector is created that is a compromise between widespread response and high selectivity. For example, the photoionization detector (PID) can detect part-per-billion levels of benzene but cannot detect methane. Conversely, the flame ionization detector (FID) can detect part-per-billion levels of methane but does not detect chlorinated compounds like CCl very effectively. By combining the filament and electrochemical sensor, all of these chemicals can be detected but only at part-per-million levels and above. Because most chemical vapors have toxic exposure limits above 1 ppm (a few such as hydrazines have limits below 1 ppm), this sensitivity is adequate for the initial applications. Several cases of electrochemical sensors being used at the sub-part-per-million level have been reported (3, 16). The filament and electrochemical sensor form the basic gas sensor required for detecting a wide variety of chemicals in air, but with little or no selectivity. The next step is to use an array of such sensors in a variety of ways (modes) to obtain the information required to perform the qualitative analysis of an unknown airborne chemical. 

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