When air bubbles enter a liquid chromatography during an experiment, many laboratory workers will surely frown – not only do they increase baseline noise and produce annoying ghost peaks, but more importantly, they directly affect the accuracy and reproducibility of quantitative results, potentially ruining all the hard work.
But don’t panic. There is a systematic way to deal with bubbles. Remember the principle “first prevent, then diagnose, finally treat”, and you will be able to handle the problem easily.
I. Prevention First: Eliminate Bubbles at the Source
The best way to deal with bubbles is to keep them from appearing at all. This requires good operating habits, with the following key points:
Don’t skip mobile phase degassing
Why is degassing so important? When the mobile phase is taken from a cool storage area into the laboratory, or when it is heated during use, dissolved air can come out of solution and form bubbles in the pump or detector.
What to do? Most modern instruments have an online degasser – keep it turned on. Before use, it is best to sonicate the prepared mobile phase for 10‑20 minutes, especially aqueous phases or solutions with a high proportion of buffer salts. For extremely demanding analyses, helium sparging is a reliable alternative.
Pay attention to solvent handling
When preparing the mobile phase, avoid uneven mixing. For example, a large proportion difference between the organic and aqueous phases (e.g., 90% water with 10% organic) is particularly prone to bubble formation. It is recommended to add the aqueous phase slowly to the organic phase, or use a magnetic stirrer to aid mixing. Also, try to use HPLC‑grade solvents – they are purer, contain fewer impurities, and are less likely to generate bubbles.
Be thorough during system start‑up and priming
Before starting the instrument, especially after changing the mobile phase, always use the “Purge” function at a relatively high flow rate (e.g., 5 mL/min) to expel bubbles from the pump and online degasser. At the same time, use a syringe to draw liquid from the purge valve outlet until no bubbles are visible. If the method starts with a high initial flow rate, do not apply it directly. Instead, set a gradient that ramps from a low flow rate (e.g., 0.2 mL/min) up to the target flow rate, giving the system time to adapt to the pressure.
II. Diagnosis: Where Are the Bubbles?
If you observe pressure fluctuations, baseline noise, or unstable retention times, do not rush to take action. First, determine where the bubbles are located.
Bubbles in the pump: The pressure will fluctuate widely, sometimes even dropping to zero.
Bubbles in the detector: The baseline will show regular, sharp spiky noise peaks.
Bubbles in the column: Peak shapes will deteriorate, peaks may split, and retention times will drift.
III. Treatment: Step‑by‑Step Bubble Removal
Once you have identified the bubble location, follow these steps methodically – don’t panic.
Step 1: Stop injection and detection immediately
Stop data acquisition and turn off the detector lamp (e.g., DAD/UV lamp) – deuterium lamps are expensive and need to be protected.
Step 2: Back‑flush the column (recommended)
This method is effective and causes little damage to the column. First, disconnect the column outlet from the detector and connect it to the injector waste port or directly to a waste bottle. Then reduce the pump flow rate (e.g., 0.2‑0.5 mL/min) and back‑flush the column with mobile phase for 1‑2 minutes. The column pressure will help push bubbles out of the end that connects to the pump – that end has a larger‑pore frit and is less likely to clog.
A reminder: before back‑flushing, confirm that the column can be used in reverse (most modern columns can). After back‑flushing, reinstall the column in the correct direction.
Step 3: Flush the whole system at a higher flow rate
After reinstalling the column, first disconnect the detector and place the detector inlet into a waste bottle. Flush the system at a higher flow rate (e.g., 2‑3 mL/min, but do not exceed the column pressure limit) for 5‑10 minutes to push any remaining bubbles into the waste.
Step 4: Re‑equilibrate the system
Reconnect the detector and equilibrate the system at the initial flow rate specified by the analytical method. Wait until the baseline is stable and the pressure is steady before resuming your experiments.
IV. Special Case: Bubbles Stuck in the Pump
If bubbles become trapped in the pump check valves, the pump may not function properly. In this case, use the instrument’s “Purge” function or open the purge valve and flush the pump at a high flow rate. You can also use a syringe at the purge valve outlet to aspirate repeatedly, helping to expel the bubbles. Sometimes gently tapping the check valves can also dislodge trapped bubbles.
Final reminders:
Prevention is always better than cure – follow proper degassing procedures every time.
When a problem occurs, first locate the bubble position accurately.
During bubble removal, avoid excessively high flow rates or pressure to prevent damaging the column or instrument.
If none of these methods works, there may be another issue such as a system leak – don’t force it; contact an instrument engineer promptly.
