Permeation devices are commercially available for this application. However, they are rather large and possess a footprint as large as the gas chromatograph. They also add substantial cost to any system, as much as 50% of the cost of the GC. We have designed a
small, all glass chamber as an integral part of the gas chromatograph.It is conveniently mounted in the space normally taken by a liquid autosampler. The chamber can contain multiple permeation tubes or wafers. Although the exact number depends upon the sulfur type and concentrations, four devices are easily placed in the chamber. The permeation tube chamber temperature is controlled by an injection port heated zone. Dilution gas (carrier gas) is controlled by an internal mass flow controller or a Programmable Pneumatic Control (PPC) zone.
A four port micro volume valve is used to pass either calibration gas from the permeation chamber or the actual sample to the gas sampling valve. A rotometer on the outfall of the gas sampling valve completes the sampling hardware. It is used to monitor correct dilution gas flow from the permeation chamber and to assure proper sample flow when the four port valve is in the sampling position.
The complete analyzer consists of a manifold sampling station consistent with the plant design and its mechanicals for moving carbon dioxide to storage, distribution or point of usage. The product to be tested is connected to the inlet piping of the gas chromatograph’s sampling system, which contains the four port selection valve. The highly modified gas chromatograph with integral permeation chamber, gas sampling valve, high-resolution methylsilicone capillary column, and sulfur specific chemiluminescence detector completes the analytical train. The detector signal is processed by an amplifier within the gas chromatograph and sent to a computer-based integrator. The integrator stores a factory method for operation of the system. All operational parameters and calibration details are provided with the system to minimize installation and training.
Calibration is accomplished by positioning the four port selection valve to allow gas to flow from the permeation chamber to the gas sampling valve. The calibrant is dimethysulfide for the standard system. Because the SCD® yields an equimolar response to all sulfur compounds, only one is necessary for calibration. Dimethylsulfide is used because it is less ordorous and toxic than others that might be present. It is also very likely to be present from certain manufacturing processes. After a sufficient purge of the sample loop, the calibrant is injected by pushing the “Start” button. The rest is automatic. An isothermal elution is held to separate hydrogen sulfide followed by a mild temperature ramp to elute all the sulfides of interest, typically through the carbon four mercaptans. The elution time is less than five minutes. If sulfur dioxide is required to be separated from carbonyl sulfide, a mild cryogenic start is needed. Since copious quantities of carbon dioxide are always present, this does not add to the cost of the analysis.
After the completion of the column elution, the integrator processes the recorded detector signals. Each sulfur peak exceeding the method threshold is compared to the calibration file for retention matches for peak identifications and to the intensity of the calibration component (dimethylsulfide). A report is then printed and the next cycle started.
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The detection limit for the SCD® is <1 pg sulfur per second. Practical detection limits are on the order of 5 ppb for gas or liquid samples when analyzed with this system. Large benefits have been realized from designing to minimum internal volumes, passivation of surfaces and the resulting signal to noise that such system improvements yield. Conclusion
A complete solution has been designed to permit beverage grade carbon dioxide to be accurately analyzed for sulfur compounds at low ppb levels. Sulfur speciation is accomplished by the use of high-resolution capillary chromatography with specificity from a chemiluminescence detector. Design criteria have dictated the analyzer to be easy to use and calibrate. The skill level at plant sites is usually nontechnical and often scalemen and truck drivers are trained to use this equipment. The reliability and uptime approaches that of process-type equipment with minimal maintenance and lower initial capital investment. The integration of the analytical equipment is complete when on-line sampling is employed.
Although this article described only the sulfur analysis requirement, many other impurities have been identified and must be measured as part of the total purity specification. The sulfur analyzers have been extended into a family of analyzers dedicated to beverage grade carbon dioxide analyses. Additional channels can be added to measure most organic impurities present. All are on-line and completely automated for ease of use.