Sulfur Specific Chemiluminescence Detector (SSCD)
The Sulfur Specific Chemiluminescence Detector (SSCD) is used for detection of various sulfur molecules when combined with a chromatographic column for separation of the components. The SSCD is specific to sulfur and produces equal molar responses for all sulfur compounds. Typical measurement limits are from 10 ppm to 5,000 ppm (low limit is 10% of full scale value).
Theory of Operation
In this detection process, sulfur monoxide is reacted with ozone to form an electronically excited species of sulfur dioxide that emits light when it relaxes to the ground energy state. The light emitted by this reaction is detected and amplified by a photomultiplier. The response from this reaction is linear over five orders of magnitude and demonstrates a selectivity to sulfur over carbon of 106 108 to one.
The figure below shows a schematic flow diagram of the major components of the SSCD detection system. A chromatograph injection valve injects the sample into a flow of air carrier gas that passes through a chromatographic column that separates the various sulfur components. From there, the sample passes to a dual chamber, flameless combustion chamber. Additional air is introduced just prior to entry into the first combustion chamber. Hydrogen is introduced into the second combustion chamber.
In the combustion chambers, the hydrocarbon matrix of the sample is oxidized primarily to carbon dioxide and water, while the sulfur components are oxidized to sulfur dioxide (SO2), water, carbon dioxide (CO2), and some sulfur monoxide (SO). Sample from the second combustion chamber is drawn to the reaction cell by a vacuum source. Ozone from an ozone generator is also routed to the reaction cell.
For the chemiluminescence reaction to take place in sufficient quantity for the required sensitivity, the reaction cell must be maintained at a very low vacuum. As reaction cell pressure is increased, relaxation of the excited sulfur dioxide to its ground state increasingly takes place through mechanisms of intermolecular collision rather than the chemiluminescence reaction. In addition, the SO formed in the combustion chamber is a short-lived free radical and must be transferred to the reaction cell quickly before competing reactions can occur. The low pressure of the reaction cell causes both a fast transfer time and a quenching of possible competing reactions. The primary reactions taking place in the system can be represented as follows:
RH + RS + O2 --> SO + SO2 + CO2 + H2O
SO + O3 --> SO2* --> SO2 + hv
At the low pressures of the reaction cell, carbon dioxide and water do not cause appreciable interference or intermolecular quenching of the signal, and water vapor condensation does not occur.
Although many sulfur compounds can chemiluminesce directly by reaction with ozone, the combustion steps in this process are necessary for several reasons. The direct chemiluminescence reactions exhibit a wide variation in molar response, probably due to different mechanisms of the attack of the ozone oxidant. In each case, the SO radical is probably the reactive intermediate that ultimately causes the chemiluminescence.
However, differing reaction kinetics and mechanisms of oxidation determine the various sensitivities observed. The chemiluminescence reaction is not affected by hydrocarbon or carbon dioxide quenching. Combustion of the sample in the flame serves to break down the large hydrocarbon molecules in order to maximize the sensitivity and selectivity of the detector. Combustion also serves to provide an equimolar response by breaking down the complex sulfur containing molecules into a common sulfur species prior to reaction.
Since the SSCD pre-combusts the sulfur containing compounds in the sample to SO and SO2 prior to the reaction with ozone, the chemiluminescence response is equimolar to all different types of sulfur compounds found in the sample. The burner used for combustion is a two-stage, or dual-chamber unit. Use of the dual-chamber combustion chamber has two advantages. Since combustion is more complete, hydrocarbon quenching and non-equimolar response are minimized. |
0 Items 