For the vast majority of uses, samples pass through an injection port at the start of the gas chromatography (GC) process. Therefore, it is very important to ensure that the proper injection port items are selected based on the application to be performed. Once the proper items are selected, a simple, routine preventative maintenance program will help prevent simple problems from turning into major problems. This article will highlight two recent calls to our Technical Service Chemists concerning GC performance problems, and the solutions that resolved them.
An analyst at a large environmental laboratory called our Technical Service group. She was running routine assays on two GCs, and noticed the chromatograms generated by one of the GCs contained tailing peaks (Figure 1). Additionally, analyte response seemed low. Believing the column had deteriorated, she inquired about the proper replacement column. To determine if the problem could be something other than the column, the Technical Service Chemist began to ask questions concerning the type of routine maintenance performed in the customer’s lab. It was discovered that the analyst was relatively new to her position and had not performed any maintenance on the GC inlet system. The Technical Service Chemist then explained that peak tailing and low response could be symptoms of a dirty inlet liner. At this point our chemist instructed the analyst to do an inspection of the inlet liner. Upon inspection the analyst found the inlet liner had a brown coating with what appeared be small fragments on the inside. It was determined the small fragments were bits of cored injection port septa, and that the brown coating was non-volatile residue that had accumulated over time. Both these phenomenon create adsorption sites that interact with the sample as it passes through the inlet liner.
Figure 1.Poor Chromatography Before Liner & Septum are Changed
The Technical Service Chemist suggested that the current septum and inlet liner should be changed. A molded Thermogreen® LB-2 septum was recommended due to its bleed-temperature-puncturability-optimized nature. Once injection port maintenance was completed, peak shape and overall chromatography improved (Figure 2). The analyst also learned that changing the septum daily, especially if the instrument is in heavy use, will save costly downtime, rework, and inaccurate results.
Figure 2.Normal Chromatography After Liner & Septum are Changed
A customer was working with a new method for volatile compounds. After installing a column he proceeded to set up a splitless injection method and obtained the results shown in Figure 3. He had a skewed solvent peak and lower analyte responses than shown in the sample chromatogram of the method. The customer confirmed he was using the correct solvent, initial temperature, hold time, and split vent time as indicated by the method. A call was placed to Supelco Technical Service. They questioned if the customer had changed the inlet liner and cleaned out the injection port. The customer acknowledged he had changed the inlet liner but had not cleaned out the injection port. The Supelco Technical Service Chemist asked the customer to do so, suspecting that the inlet liner may not be sealing correctly. This suggestion helped, but did not solve the problem. After further questioning it was discovered that the customer was using a split rather than a splitless inlet liner. This was the major cause of the solvent tailing. The split inlet liner did not allow efficient transfer of the sample onto the column. Plus, some of the sample was lost when the split vent opened.
Figure 3.Customer’s First Chromatogram – Low Response
For trace analysis that includes volatile components, it is recommended to use a < 2 mm I.D. inlet liner. The reduced volume of this diameter increases the linear velocity of the carrier gas through the liner. This produces a more rapid introduction of analytes onto the column in a narrow band. The improved focusing provides a better response, especially for lighter analytes. Following a suggestion to install a splitless liner, the customer obtained the chromatogram as shown in Figure 4.
Figure 4.Result of Installing the Correct Liner
The importance of proper product selection and preventative maintenance for the GC inlet are vital to the chromatographic process. A proactive approach and system awareness will reduce the risk of problems, saving both time and money.