A Step-by-Step Guide to Inlet Maintenance

May 01, 2018
Volume 36, Issue 5, pg 338

Troubleshooting investigations in chromatography often don't lead to a single causal factor, and the reason for problems or lack of robustness frequently is related to many small contributory factors. This situation is particularly true of the problems associated with sample introduction in capillary gas chromatography (GC). Good maintenance of the GC inlet can prevent a host of problems from compromising the quality of your results.

Septa should be of the right quality, size, and temperature rating for your instrument and application, especially where the inlet will be used above 350 °C. Septum purge gas flows should be checked (using a flowmeter) and adjusted periodically (3–5 mL/min is typical) to prevent noisy baselines or discrete interference peaks resulting from outgassing products.

Septa should be the right size and the torque used on the inlet sealing nut should be correct—overtightening will lead to split septa (see manufacturer's instructions). Septa can core if the wrong syringe type is used or the installation torque is incorrect. Ensure the autosampler syringe needle tip style is correct and undamaged.

Split and cored septa can allow sample vapor or carrier gas to leak from the inlet, which can result in quantitative inaccuracy, retention time irreproducibility, and instrument shutdown.

Septa should be changed for each major campaign of analyses.

Some instrument designs feature an O-ring (made from graphite or specialist plastic or rubber) to isolate the carrier and split flow from other instrument flow paths. These O-rings become deformed and brittle over time and cause problems with gas flows, which can result in poor quantitative accuracy and ultimately system shutdown. Regularly inspect the O-ring to check that it is donut shaped and not flattened and that it is fully flexible and not brittle. Consider changing the O-ring with each liner change.

The split line carries much of the dirt from your sample matrices to waste (atmosphere) and is not heated along its entire length, risking the deposition of higher boiling contaminants. Periodically, set the split flow to a high value (<250 mL/min), note how quickly the flow setpoint is achieved, and check the exhaust flow with a flowmeter. Any sluggishness in the flow–pressure ramp or inability to achieve the setpoint flow may indicate restrictions in the line, which will need to be removed and cleaned. Some manufacturers include a filter (normally charcoal) within the split exhaust flow, which should be changed periodically; six months to one year is a typical interval (make sure to check if this part is changed during preventative maintenance by your service provider). Split-line filter blockages can cause issues with quantitative reproducibility, broadened or tailing peaks, and can add noise to the chromatographic baseline.

The quality, design, and cleanliness of the liner used for capillary GC is of great importance in the qualitative and quantitative performance of your GC analysis. Liners and packing materials are typically constructed from quartz glass, which if not properly deactivated can cause peak shape issues (typically tailing) as well as quantitative problems because of analyte adsorption in the inlet. High-quality deactivated liners are available at a reasonable cost, and the liner should be seen as a consumable item that is regularly replaced to ensure good results and ensure that in-house cleaning and deactivation are not needed. It is important that the liner is deactivated (especially when dealing with more polar analytes), is of the right design (check with your manufacturer), and contains the right amount of deactivated packing material at the right position within the liner, which are all manufacturer and application dependent.

It is also important to note that liners can be installed incorrectly, so consult your manufacturer's literature to ensure that you are carrying out the installation correctly. Use plastic tweezers to install the liner and do not touch with bare or gloved fingers, because contaminating residues may be left on the inlet surface in both cases. The frequency of liner replacement depends on application type and cleanliness of samples. It is essential to establish the correct replacement schedule, and the practice of visually inspecting the liner to assess cleanliness is insufficient to properly assess its fitness for purpose.

Some inlets contain an inlet bottom seal and washers or spacers, which should also be regularly maintained. The deactivated metal surfaces will contact the sample vapors during every injection and therefore can contribute to peak shape and quantitative reproducibility issues. Deposition of sample contaminants and solvent vapors will compromise the deactivation of the seal over time, which is also seen as a consumable item. The frequency of seal changing should be established according to the application type and sample cleanliness. Seal changing during annual or biannual preventative maintenance operations should not be assumed to be sufficient.

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