Laboratory Instrument Refurbishment

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Do chromatographic columns have upper and lower temperature limits during use?

If a chromatographic column is used below the lower temperature limit, its performance will be unsatisfactory, resulting in broad and rounded peaks (e.g., loss of column efficiency), but the column itself will not be damaged. Using the column above the lower temperature limit will yield better chromatographic peak shapes.

The upper temperature limit for a chromatographic column is expressed, for example, as 325/350 degrees Celsius.

The lower temperature is the isothermal upper temperature limit. The column can be used indefinitely at this isothermal upper limit without significant damage to column bleed or lifespan. The higher temperature is the programmed temperature upper limit. Using the column at the programmed temperature upper limit for 10 to 15 minutes will not severely damage the column's lifespan or cause excessive column bleed. However, prolonged use or exceeding the programmed temperature upper limit will damage the stationary phase and the inertness of the fused silica column tubing, leading to a significantly shortened column lifespan or excessive column bleed.

Column Bleed

How to determine if it is column bleed?

True column bleed should originate solely from the column. The bleed observed by the user is actually the sum of all signals reaching the detector, including contributions from the following sources: inlet septum, inlet, and detector. However, it is possible to mistakenly attribute all these sources to column bleed.

The method to determine if bleed is normal is to check the detector. Disconnect and remove the column, then seal the detector inlet with a plug. Turn on the detector and record the signal value at 50 degrees Celsius. Heat the column oven to 320 degrees Celsius and record the signal value. For a clean detector, the FID signal should increase by 1 to 2 pA. If the signal increase exceeds 2 pA, consider cleaning the detector, makeup gas, and hydrogen gas lines immediately.

Once the detector signal returns to an acceptable level at 320 degrees Celsius, consider issues with the inlet. If visual inspection of the inlet liner shows obvious contamination, cool down the inlet, disassemble it, and clean the inlet cavity with solvent. After replacing the liner, reassemble the inlet and connect it directly to the detector using a "jumper tube" (1 to 3 meters of uncoated fused silica or stainless steel column tubing). Heat the inlet and the column oven to 320 degrees Celsius. Any increase in "bleed" signal at this point, with only the detector connected, originates from the front end of the instrument, such as the inlet septum, carrier gas lines, inline regulators, valves, or flow controllers.

Wrap a new septum with aluminum foil, smooth side down. If the signal from the "jumper tube" decreases, it indicates the need for a higher-quality septum. If the signal remains high, there may be deposits in the carrier gas lines, valves, or regulators, which need to be disassembled, cleaned, or replaced.

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