Detector Operation
Detector Stability and Response
The time required for system stabilization varies depending on the application, system cleanliness, presence of active sites and other factors. Useful results could be generated within 30 minutes of start-up, especially with a previously operated system. A longer stabilization time is likely to be required upon changing critical system components, such as the combustion tubes or the GC column. In addition, gas flow rates may drift initially as thermal equilibrium is reached due to changes in gas viscosity with temperature. Therefore, it is good practice to monitor gas flow rates and adjust them accordingly.
Even though a system may not be fully stabilized, sample injections can be made within minutes of instrument start-up. Whether the results are useful largely depends on application. Typically, an elevated baseline will initially be observed, which will diminish upon successive programmed runs.
After stabilization has been reached, the system should exhibit good short-term and long-term precision. Of course this also depends on the application and concentration of components being measured. As an example, analysis of thiophene in benzene at the 1 ppm sulfur level yielded 1.4% RSD (n=10) over about 2 hours and 3.6% RSD (n=42) over about 96 hours. As expected, carbon disulfide at a lower concentration of 90 ppb sulfur yielded 2.6% and 10.4% RSD, respectively.
Column Bleed
Accumulation of column bleed causes silicon dioxide to build up in the Burner. This silicon dioxide creates active sites that are detrimental to performance. In many cases, the choice of column can be optimized for a particular application. Column bleed can be minimized by the use of oxygen traps on the carrier gas, low-bleed columns, and lowest possible maximum oven temperature.
Coking
Contamination from some sample matrices can reduce sensitivity. Crude oils containing volatile metal complexes may contaminate ceramic tubes. The incomplete combustion of certain hydrocarbon-containing compounds leaves behind coke deposits on the tubes. Coke deposits may be removed from the Burner by reducing the hydrogen flow rate. The Dual Plasma Burner is much less susceptible to coke formation than other designs.
Hydrogen Poisoning
Hydrogen poisoning of the ceramic tubes occurs when there is no oxidizer flow through the ceramic tubes. The result is extremely reduced, or no response. Hydrogen poisoned tubes can not be reconditioned and should be discarded.