Experimental ECD and NICI Procedures

The objective of the ECD and NIMS experiments is to measure the molar response of different compounds as a function of temperature. From these data the fundamental kinetic and thermodynamic properties of the reaction of thermal electrons with molecules and negative ions can be determined. The measurement is carried out in the same manner as the calibration of any detector. Known amounts of a compound are injected into the chromatograph and purified on a column, they then enter the detector. The response of the detector is normalized to the number of moles injected. When obtaining physical parameters, the detector temperature is changed and the procedure repeated. Since the molar response can vary by three to four orders of magnitude, the concentrations of the test molecule and the conditions in the detector at different temperatures must be taken into account. 

Once an ECD/GC or NIMS has been obtained, its suitability for the determination of fundamental properties can be examined by measuring the temperature dependence of the response for standard compounds. Although strictly speaking, it would only be necessary to measure the temperature dependence of three compounds—CCI4, acetophenone, and CH2C12—to obtain AD and AN, other compounds should also be studied. In addition, is valuable to determine the temperature 

If peaks are symmetrical, as is frequently the case with modern capillary columns, only the peak height must be converted. When recording the chromatograms on a strip chart recorder, the chart speed on the recorder should be increased to obtain an accurate measurement of the peak width. The area can then be obtained by triangulation. Other required data are the flow rate through the detector and the moles of sample in the detector. For samples that are split the measurement of the split ratio is also required to obtain the number of moles injected. The 

The [4 — 4]/4 max is the corrected height at the peak maximum and is dimensionless. The measured peak width at corrected half-height is converted to halftime by dividing by chart speed. The volumetric flow rate or total flow rate through the detector is measured with a bubble flow meter and is typically 50 to 150 mL/min. 

The moles injected are determined from the sample size, concentration, and split ratio and are typically in the range of 1.0 femtomole for a high-capturing compound such as CCI4 with a K of 1011 L/mole. The molar response of a negative-ion mass spectrometer can be determined in the same manner, but the moles into the mass spectrometer must be estimated. 

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