| OCR Text |
Show for which the H T A S was designed to study tend to decompose with prolonged exposures at elevated temperatures, the instrument is fitted with a flow cell rather than a static cell found in similar commercial systems. Furthermore, an inert carrier gas (nitrogen) is used for sample transport to eliminate the possibility of decomposition through oxidation. By flowing a carrier gas laden with the sample of interest through the cell the length of exposure to elevated temperatures can be kept short (typically 1 second) to limit destruction of the sample. Operation of the HTAS begins with setting the temperature of the inlets, absorption cell and exhaust line. Next, the flow of carrier gas is set using two mass flow controllers which have been previously calibrated. The exact volumetric flow rate is measured using a bubble flow meter attached to the instrument's exhaust port. At this point a reference spectrum is obtained with the dedicated microcomputer workstation. Once a reference spectrum has been taken and stored, liquid sample is introduced into the heated sample inlet chamber at a fixed rate using a calibrated syringe pump. As the sample flows into the system, the operator will observe a shift in the spectra being displayed on the workstation. Equilibrium is considered to be established when the spectrum reach a fixed value (+/-3% full-scale). A sample spectrum is recorded at this point. This procedure is repeated for each wavelength region and temperature of interest. Molar extinction spectra are obtained from the recorded data by comparing the reference and sample spectra, and from the molar concentration calculated from the total flow rate of carrier gas and sample through the cell, and the cell temperature and pressure (which is taken to be ambient). The operation of the LS-PDU begins with setting the temperature of the inlets, reactor, exhaust line, and cold trap. Carrier gas flows are set using mass flow controllers, and the actual volumetric flow rate is measured with a bubble flow meter attached to the instrument's exhaust port. If a radiant exposure is to be made the illumination source is switched on and allowed to stabilize. Next, a measured amount of sample is introduced at a controlled rate using a calibrated syringe pump. After the sample introduction is complete, the system is run for an additional 5 minutes to ensure that all of the sample passes through the system and is residing in the cold trap. The illumination source is switched off, and the system is purged with helium in preparation to transfer the sample to the GC/MS/FID. After purging, the instrument's exhaust port is sealed and the system pressurized to the operating pressure of the GC/MS/FID. The GC/MS/FID's data systems are activated, and the cold trap is heated, releasing the trapped sample to the analytical system for analysis. The fraction of waste feed remaining following each exposure is calculated by comparing integrated chromatographic peak areas with those obtained from non-destructive tests (e.g., 100°C, purely thennal). The molar concentration of sample in the reactor is calculated from the total flow rate of carrier gas and sample through the system, and the reactor temperature and pressure (which is taken as ambient). 8 III-19 |
