![]() ![]() ![]() If the detector – such as a TCD – needs constant gas flow, then either turn it off or consider using a changeover manifold so there is no interruption of gas supply to the detector. If necessary, shut down the gas supply, but first, cool down the GC in order to avoid damage to columns and perhaps the detector. Most people use a central gas supply, cylinders and/or gas generators in the lab for their processes. If the contaminants are not in the gas source, where do they come from?ĭuring cylinder change out, there are many steps to perform and the process might take an hour or more. Think of it as drinking clean water through a dirty straw. Everything that the gas molecules come in contact with can be a source of contamination. The gas delivery system and all of its components could be the culprit in introducing contaminants into the gas stream. All three of these contaminants are present in the atmosphere so preventing them from mixing with the cylinder gas, regardless of the purity level, is critical. ![]() Hydrocarbons can also increase baseline noise, degrade analysis quantification and cause ghost peaks. Moisture can reduce column life, shift retention times and increase baseline noise levels. In some applications, it can also cause ghosts or unexpected peaks. column bleed, reduce column life and change retention times.These also have maximum levels defined by the column suppliers for the columns to work correctly. The three most common contaminates are moisture, oxygen and hydrocarbons (THC). Only those defined will be used to determine the gas purity if it is not on the list it is not deducted from the purity level. Purity, as defined by the supplier of the gas, is determined by subtracting the specified contaminant – and only those contaminants – from 100%. It is equally important to be aware of the impact the purities or impurities that have not been analyzed may have on your process. It is important to understand what levels of analyzed purities and impurities will affect your process. A proper specification will state what contaminants the gas was analyzed for and what their levels were. Even then you may find some gas suppliers do not certify their specifications and instead state what they think the purity is and what levels of impurities may be in the gas. is at the discretion of the supplier as to what it is called.īecause there is no uniformity between suppliers, the only way to know the purity is from the supplier’s certified specifications. A gas called Chromatographic, Research, six 9’s, etc. How a gas is named, qualified or certified is not to any standard in the U.S. What is more important is determining which gas will work and not affect the processes. The highest purity grade is not always the best. The first step in selecting the correct grade of a gas is to understand what contaminants will affect your processes and what the maximum level of acceptable contaminants is for your processes. Let’s explore the possible causes of these problems. All too often the common answer to problems with a chromatographic system is to buy a higher purity gas when in most cases this will not fix the issue. The cost of gas has a significant impact on operating expenses, so we expect the gas to perform consistently and provide consistent analytical results. People think these are the result of contaminants in the gas but, in most cases, it is the result of contaminants being introduced either during cylinder change out or from the gas delivery system itself. So what’s really contaminating your gas stream?Ĭhromatographers have many problems with their chromatographic systems after cylinder change out including elevated base line noise, ghost peaks, excessive column bleed and inconsistent analytical results. If you’re wondering where your gases’ impurities come from, the answer is not in the tank. By Frank Kandl & Reggie Bartram | Spring 2015 Issue ![]()
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