Comparative study of the linearity and sensitivity of electron capture and flame photometric detectors using a pesticide standard

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Abstract
The different types of 63Ni and 3H electron capture (ECD) and flame photometric detectors (FPD) were compared as part of the Canadian Check Sample Program on Pesticide Residue Analysis. Twenty-seven laboratories were supplied with chlorpyrifos standard solutions for the determination of linear range and minimum detectable amounts (MDA). Each laboratory selected its own operating conditions except for the FPD study. Results for the FPD were compared with standardized flame conditions designed to optimize the exponential factor in the S-mode. The MDA of the ECD (pulsed and direct-current) ranged from 0.09-0.17 pg/s for 63Ni and 3H sources, respectively. The corresponding MDA for the linearized ECD (pulse modulated or constant current) was 0.04 pg/s and a linear range of 104 or greater with N2 as the carried gas. Use of argone/methane and a pulse width of 0.1 .mu.s extended this range to 105 but the sensitivity was not as great as with N2 carrier gas. The practical limit of detector sensitivity was instrument-dependent, depending primarily on the noise level. In the FPD study, operating in the P-mode gave an average MDA of 5.97 pg/s and a linear range of 2 .times. 103. Similarly, the S-mode parameters were 81.1 pg/s and 8 .times. 102, respectively, with an average exponential factor of n 1.8. Attempts to apply optimized detected gas-flow conditions improved sensitivity and linearity in the S-mode only.