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10
Mar
Introduction: Phenol–Chloroform Extraction in Molecular Biology
Phenol–chloroform extraction has been part of molecular biology protocols for decades. Despite the availability of column kits, many laboratories still rely on organic extraction because it often delivers higher yields, especially for difficult samples such as tissues, lipid-rich samples, or low-input nucleic acid preparations.
The Challenge of Phase Separation
However, anyone who routinely performs phenol extraction knows that the phase separation step can be frustrating. After centrifugation, the aqueous phase containing DNA or RNA sits above the interphase and the organic solvent layer. Recovering this aqueous layer without disturbing the interphase requires patience and a steady hand. Even a small disturbance can introduce phenol contamination, which may interfere with downstream applications such as PCR, reverse transcription, or sequencing library preparation.
A Practical Lab Experience
In our lab, this step became particularly problematic during a project that required RNA extraction from tissue samples for downstream expression analysis. The samples were relatively viscous, and the interface between phases was not always well defined. To avoid contamination, we often left part of the aqueous phase behind during pipetting, which meant losing valuable RNA.
How Phase Separation Gel Tubes Work
We eventually began using phase separation tubes, such as PhaseShield™ Gel Tubes (200×2ml) (Cat.No. M2302-20-H), which contain a specialized gel that moves during centrifugation to form a barrier between the aqueous and organic phases. Once centrifuged, the gel sits tightly at the interface, trapping the organic solvent and interphase contaminants below it. The aqueous phase remains clearly separated above the barrier.
Workflow Improvements in Practice
The practical difference in the workflow was immediately noticeable. Instead of carefully pipetting only part of the aqueous layer, we were able to recover nearly the entire phase without worrying about contamination from the lower layers. This not only simplified the extraction process but also improved the consistency of RNA recovery across samples.
The benefit becomes even more apparent when working with limited material. In experiments where only small amounts of RNA or DNA are available, losing even a small fraction during extraction can significantly affect downstream analyses. Stabilizing the phase separation step helps maximize recovery and reduces variability between experiments.
Safety and Practical Advantages
Another advantage is improved laboratory safety. Phenol and chloroform are hazardous solvents, and minimizing contact with these phases is always preferable. The gel barrier reduces the need for precise manual handling of the interface, making the workflow safer and easier for both experienced researchers and new lab members.
Choosing Heavy vs Light Formulation
PhaseShield™ Gel Tubes are available in two density formulations, Heavy and Light, which can be selected depending on the density of the aqueous and organic phases in the extraction protocol. In practice, many RNA extraction workflows—including TRIzol-based protocols—tend to work best with the Heavy formulation.
Troubleshooting Phase Separation
Although phase separation tubes significantly simplify phenol–chloroform extraction workflows, improper centrifugation conditions, phase density differences, or incorrect gel selection may occasionally affect gel positioning. The table below summarizes common issues and recommended solutions.
| Problem | Possible Cause | Resolution |
|---|---|---|
| Phase Lock Gel is not phasing properly | Wrong Phase Lock Gel type (Heavy or Light) | Check the compatibility chart and choose the correct Phase Lock Gel type (Heavy or Light) |
| The centrifuge speed was not correct | Check the protocol to assure the centrifuge speed is correct | |
| Phase Lock Gel may have been frozen | Store Phase Lock Gel at room temperature (DO NOT FREEZE) | |
| Phase Lock Gel was not spun down prior to use | Spin Phase Lock Gel down prior to use | |
| Phase Lock Gel migrates above the aqueous phase | The aqueous phase is too dense | Pierce the Phase Lock Gel barrier with a pipette tip, add water or buffer to lower the density of the aqueous phase, mix and re-spin the tube |
| Phase Lock Gel remains at the bottom of the tube | The organic phase is not dense enough | Add chloroform to increase the density of the organic phase, mix and re-spin the tube |
| Phase Lock Gel is not phasing properly with samples in TRIzol Reagent or similar product | Aqueous phase is too dense | If the Phase Lock Gel barrier is intact and the aqueous layer can be removed completely by pipetting or decanting, proceed with the protocol |
| Add RNase-free water or buffer (up to 0.2 ml per 1 ml of lysis reagent) to lower the density of the aqueous phase, mix and re-spin the tube |
PhaseShield™ provides the same physical phase-separation principle used in Phase Lock Gel, while offering a cost-effective solution for routine phenol–chloroform extraction workflows.
Product: PhaseShield™ Gel Tubes (200×2ml)
Cat.No.: M2302-20-H
Conclusion
For laboratories that continue to use phenol–chloroform extraction because of its flexibility and high nucleic acid recovery, phase separation tubes can make one of the most error-prone steps in the protocol much more reliable. In our experience, they quickly become a small but very useful addition to routine nucleic acid purification workflows.
Biofargo team
