SDS-PAGE • Separation
SDS-PAGE electrophoresis: gel selection, running conditions and separation checks
Separate proteins reproducibly before transfer. Gel chemistry, running buffer and power conditions belong to a specific electrophoresis system; use the ladder, dye front, lane shape and run behavior to judge whether the separation is working.
Why this matters
Check separation quality before transfer
Western blot detection cannot restore resolution that was never achieved in the gel. This guide covers gel-system selection, loading, running conditions and the signs that a separation problem began before transfer.
Define the molecular-weight region that must be resolved
Record the target region and nearby bands that need to be separated, then choose the gel chemistry and percentage or gradient for that range.
1. Define the target region
Record the expected apparent molecular weight of the target and the nearest band or molecular-weight region that must be distinguished from it. Include known cleavage, isoforms or unusual migration when relevant.
A gel that is adequate for confirming presence may still be poor for resolving two nearby species.
2. Choose the gel chemistry before copying a percentage
Traditional Tris-glycine, Bis-Tris, Tris-acetate, Tricine and other systems do not have interchangeable running buffers or identical separation behavior. Select the system first and use its application guide for operating conditions.
3. Match pore range to the separation task
Lower-percentage polyacrylamide generally has larger pores and favors larger proteins; higher percentage provides smaller pores and stronger sieving of smaller proteins. A gradient gel is useful when a broad molecular-weight range must be retained on one gel.
4. Use system-specific power settings
Use the voltage/current guidance for the exact gel and electrophoresis cell. Bio-Rad notes that excessive voltage can reduce resolution and produce smiling or lane distortion.
Do not copy a voltage from another cassette size or buffer chemistry as a general SDS-PAGE setting.
Four factors that control SDS-PAGE separation
Use these factors to interpret the run and select conditions for the target region.
Stacking and resolving have different roles
The discontinuous Laemmli design concentrates samples into a narrow starting zone before size-dependent separation. Poor entry or a damaged stacking region can create broad lanes before proteins reach the resolving gel.
Gel percentage controls sieving
Polyacrylamide concentration changes pore size. Low-percentage gels have larger pores; higher-percentage gels have smaller pores. The useful range also depends on gel chemistry and the target region.
Gradient gels cover a broader range
A gradient creates progressively smaller pores through the gel and can retain useful separation across a wider molecular-weight span. For a narrow target region, a suitable fixed-percentage gel may provide a simpler separation.
Running buffer is part of the gel system
Running buffer controls the moving ion boundary and electrical behavior. A buffer that is not specified for the gel chemistry can alter migration and resolution even when the gel percentage looks appropriate.
SDS-PAGE run checklist
1. Confirm gel, buffer and apparatus compatibility
Do this
Record gel chemistry, percentage or gradient, cassette format, running buffer and the electrophoresis cell. Prepare the buffer exactly as specified for that system.
Check
The cassette seats correctly, wells are intact, inner and outer chambers are filled as required, and there is no leak that breaks the electrical path.
Stop and fix when
The gel chemistry and running buffer do not match, the cassette is damaged, or the chamber cannot hold the required buffer level.
2. Load samples consistently
Do this
Keep samples at a consistent final sample-buffer composition, include a molecular-weight ladder and load slowly without puncturing the wells.
Check
Samples settle at the bottom of each well without crossing into neighbors; the lysate is not unusually viscous and no precipitate is being forced into the well.
Stop and fix when
Sample spreads across wells, strings from the pipette because of viscosity, or precipitate or particulate material is obvious.
3. Monitor the dye front, current and lane entry
Do this
Use the system-recommended operating conditions and monitor the dye front, current or power behavior, lane entry and temperature-related distortion.
Check
The front moves evenly, the ladder and samples enter as expected, and the apparatus behaves within the manufacturer’s normal range.
Stop and fix when
The front is strongly curved, migration is unexpectedly slow or fast, current is abnormal, buffer heats excessively, or lanes begin to distort.
4. Check separation before transfer
Do this
At the planned endpoint, preserve the run record. When needed, stain a replicate or sacrificial gel, or otherwise check the molecular-weight region before changing transfer or antibody conditions.
Check
The ladder spans the required region and lane geometry is suitable for the downstream comparison.
Stop and fix when
The target region is compressed, small proteins have reached or passed the front, or the lanes are already smeared or bowed.
What to record from each SDS-PAGE run
Common run patterns and next actions
Use the ladder and lane pattern to distinguish a global run problem from a sample-specific problem, then address the corresponding cause first.
Observation
Smearing is strongest in the most heavily loaded lanes
Interpretation
Overload, viscosity or sample chemistry is more plausible than an antibody-specific problem.
Next action
Run a lower loading series and review salt, detergent, nucleic-acid viscosity and clarification while keeping downstream antibody conditions unchanged.
Avoid
Do not increase primary antibody or exposure to repair poor lane quality.
Observation
All lanes and the ladder smile or curve together
Interpretation
A global field, heating, buffer or apparatus problem is more likely than biology in individual samples.
Next action
Check the exact running buffer, chamber fill, system-recommended power setting and cooling or heat dissipation before repeating.
Avoid
Do not change gel percentage first when the whole gel shows the same geometric distortion.
Observation
Large proteins remain compressed near the top while smaller regions look acceptable
Interpretation
The selected pore range or gel chemistry may not provide enough mobility or resolution in the high-molecular-weight region.
Next action
Choose a system or gel range validated for larger proteins, such as a lower-percentage or appropriate gradient gel or a high-MW-oriented chemistry, then compare separation before transfer.
Avoid
Do not extend transfer time to fix separation that failed inside the gel.
Observation
Small proteins crowd the dye front or disappear from the gel
Interpretation
The gel may provide too little sieving in the low-molecular-weight region or the run continued beyond the useful endpoint.
Next action
Use a tighter or low-MW-oriented gel system and stop based on the required molecular-weight window.
Avoid
Do not diagnose membrane blow-through until you know the small protein remained in the gel long enough to be transferred.
Observation
Only selected sample lanes are distorted while the ladder remains clean
Interpretation
Sample-specific salt, detergent, viscosity, reduction or loading is more likely than an electrophoresis-cell problem.
Next action
Normalize final sample chemistry and compare a cleaned or diluted preparation with the original sample.
Avoid
Do not rebuild the entire running system before testing the sample-specific difference.
Related SDS-PAGE guides
Gel selection & preparation
Choose fixed versus gradient gels, precast versus hand-cast, and check gel quality before loading.
Open →
SDS-PAGE troubleshooting
Diagnose smears, distorted lanes, abnormal migration, heating and casting failures from the ladder and lane pattern.
Open →
Protein transfer
Move to transfer after the target region has been separated well enough to interpret.
Open →
Complete Western blot protocol
Return to the complete sample-to-image workflow.
Open →
Operating and methods references
These sources support the gel-selection and troubleshooting principles. Exact operating settings still come from the instructions for the gel chemistry and electrophoresis apparatus in use.
- Bio-Rad — Performing Protein Electrophoresis
SDS-PAGE setup, loading, system-specific running conditions and common distortions caused by excessive power or sample/buffer problems.
- Thermo Fisher — Overview of Electrophoresis
Explains SDS-PAGE, pore-size dependence on polyacrylamide percentage, discontinuous systems and gradient gels.
- Laemmli, Nature (1970)
Classic paper associated with the discontinuous SDS-polyacrylamide gel electrophoresis method.