Desalting removes the salts, buffers and other polar contaminants from a peptide sample so that the peptides can be analysed or used cleanly. The most common route is reversed-phase cleanup on C18 resin: acidify the sample, bind the peptides, wash the salts away with aqueous acid, then elute the peptides in an organic solvent. This guide covers why it matters, the main methods, a working protocol, the losses to expect and how to handle the sample afterwards.
TL;DR
- Salts and detergents suppress peptide ionisation and add background noise to mass spectra, so desalting is a routine step before LC-MS.
- C18 reversed-phase cleanup (tips, spin columns, syringe columns) is the standard approach: peptides bind, salts wash through.
- Recovery is not complete. It typically exceeds 85% for standard peptides but can fall to around 35% for very hydrophilic ones.
- Choose the format by sample volume, binding capacity and downstream use, then keep handling clean: good tubes, few transfers, few freeze-thaw cycles.
- All BioForgeUK peptides are supplied for laboratory research only, and this guide is written for that setting.
Table of Contents
- Why peptide samples need desalting
- What desalting removes and what it leaves
- Peptide desalting methods compared
- How to choose a method
- Step-by-step C18 desalting protocol
- Recovery and limitations
- Handling desalted peptides
- Our perspective on desalting
- Research peptides from BioForgeUK
- FAQ
- Sources
Why peptide samples need desalting
Mass spectrometry measures ions, and anything that interferes with ion formation distorts the result. Excess salts and trace detergents interfere with peptide ionisation and add chemical noise or background to the spectra, as set out in a methods review on preparing proteins and peptides for mass spectrometry. The same review makes the broader point that careful sample handling and optimised steps are what give successful analysis and maximum sequence coverage.
The practical consequences are easy to recognise at the bench:
- weaker signal for the peptides you actually care about;
- a noisy baseline, with salt adducts spreading each peptide across several peaks;
- fouled columns and sources over time;
- poor reproducibility between runs.
Salt is not the only reason to clean a sample. Reduced peptides, for example, need to be moved out of their reducing agent quickly so that later sulfhydryl chemistry can proceed before they re-oxidise. If you are checking identity or purity data, it also helps to know what a clean sample should look like. Independent peptide purity lab results show how purity is reported across vendors, which is useful context when you are deciding how much cleanup your own samples need.
A related point: desalting is not the same as purification. Desalting removes small polar contaminants. It will not separate a target peptide from closely related peptide impurities. That job belongs to preparative chromatography, and our guide to HPLC versus UPLC explains how those techniques differ and what they mean for purity figures.
What desalting removes and what it leaves
The logic is simple. C18 resin retains non-polar solutes such as peptides, proteins and detergents, while salts and polar solutes such as DNA are not retained, according to the University of Washington proteomics protocols. Peptides stick to the hydrophobic surface under acidic, low-organic conditions; salts flow through in the load and wash.
Two things follow from this.
- Detergents are a problem, not a solution. Because they bind C18 along with the peptides, they can co-elute. Detergents interfere with LC and MS and are difficult to remove from digested protein samples, so the proteomics core guidance is to use them early in processing or avoid them where possible.
- Very hydrophilic peptides are at risk. If a peptide barely binds C18, it leaves with the salt. That is where most unexplained losses come from.
Peptide desalting methods compared
The formats differ in scale, speed and cost rather than in basic chemistry. Common offline formats include reversed-phase pipette tips, spin columns and syringe columns, while online cleanup uses a trap column or filter ahead of the analytical column, as described in a StageTip desalting procedure.
| Method | Best for | Strengths | Limits |
|---|---|---|---|
| C18 pipette tips or StageTips | Small volumes, individual samples | Fast, simple, low sample loss at small scale | Limited binding capacity |
| C18 spin columns | Moderate volumes, several samples in parallel | Handy with a benchtop centrifuge, higher capacity than tips | More consumable cost per sample |
| C18 syringe or cartridge columns | Larger amounts of peptide | High capacity | More dead volume, more solvent |
| HPLC desalting | Automated cleanup, precious samples | Reproducible gradient, can be run unattended | Instrument time, method set-up |
| Online trap column | Routine LC-MS workflows | No separate offline step | Salts and contaminants load onto the trap; needs maintenance |
| Gel filtration | Proteins and larger molecules, buffer exchange | Gentle, no organic solvent | Small, soluble peptides can be lost; dilutes sample |
C18 solid-phase extraction
This is the default for peptides. It is inexpensive, quick and works across a wide range of sample types. The cleanup format often depends on the binding capacity you need, so match the tip or column size to the amount of peptide, not the other way round.
HPLC desalting
When a core facility desalts by HPLC, the sample runs on a reversed-phase column with a defined gradient. One published HPLC desalting protocol uses an F5 UHPLC column and a specified mobile-phase gradient, preparing the peptides for mass spectrometry in a single automated run. It suits precious samples and labs that already have the instrument time.
Gel filtration
Gel filtration separates by size and is excellent for proteins, where buffer exchange is the aim. For short peptides it is a weaker choice: the gel-assisted approach is best suited to proteins, because small, more soluble peptides are likely to elute out of the gel matrix during washing, as noted on a proteomics facility page on protein desalting. Use it for a protein, not for a small peptide you cannot afford to lose.
How to choose a method
A comparative survey of sample-preparation protocols found that most of those compared performed similarly. The authors point to sample type, processing time, ease of use and consumable cost as the real deciding factors, in a 2022 comparison of preparation methods. In other words, there is rarely a single best method. Work through these questions:
- How much peptide do you have? Micrograms suit tips; milligrams call for columns or cartridges sized for that load.
- How hydrophilic is it? Highly polar or very short sequences bind poorly. Expect lower recovery and consider checking the flow-through.
- How many samples? For a dozen or more, spin formats or automated HPLC save time.
- What happens next? Electrospray and nanospray need clean, volatile solvents. A functional assay may tolerate different conditions, or need a specific buffer.
- Can you tolerate loss? If the peptide is scarce, favour the lowest-loss route and test it on a spare aliquot first.
If your next step is dissolving a lyophilised peptide, our peptide solubility guide covers choice of solvent by charge and sequence, and it is worth reading before you decide what the sample will sit in prior to cleanup.
Step-by-step C18 desalting protocol
The outline below follows the standard bind, wash and elute sequence used in published cleanup protocols. Always follow the manufacturer's instructions for the specific tip or column you use, because volumes and capacities differ.
1. Acidify the sample
Bring the peptide sample to a final concentration of 0.1% TFA before C18 cleanup, as the mass spectrometry preparation review describes. Acid protonates the peptides and helps them bind the hydrophobic resin.
2. Condition and equilibrate the resin
Wet the C18 with an organic solvent such as acetonitrile, then equilibrate with aqueous acid. Skipping conditioning is a common cause of poor binding.
3. Load the sample
Apply the acidified sample slowly so the peptides have time to bind. If recovery looks low, loading the flow-through back onto the sorbent for a second pass can sometimes improve capture of peptides that did not bind the first time, according to a step-by-step StageTip guide.
4. Wash
Wash the sorbent with aqueous acid to remove salts. This is the actual desalting step. Do not hurry it, and do not use a wash with enough organic content to start eluting your peptides.
5. Elute
Elute the peptides with an organic solvent mixture, for example acetonitrile with acetic acid, as the same guide outlines. For ESI, elution with formic or acetic acid in acetonitrile or methanol is advised in the Pierce C18 tip instructions.
6. Remove TFA if your ionisation source dislikes it
For ESI or nanospray, the same instructions advise rinsing the column with water to remove TFA, which may interfere with ionisation. Check this against your own instrument's requirements.
Recovery and limitations
Desalting always costs you something. Peptide recovery from C18 tips typically exceeds 85% for standard peptides but can fall to as low as 35% for very hydrophilic species, per the Pierce C18 tip protocol. Plan around that range, and do not assume a quantitative result.
The usual causes of poor recovery are:
- Weak binding. Very polar peptides pass straight through. Keep the load and wash fractions until you have confirmed your method works.
- Overloading. Exceeding the binding capacity of the tip or column sends peptide into the flow-through.
- Over-aggressive wash. Too much organic solvent in the wash strips peptide from the resin.
- Incomplete elution. Hydrophobic peptides may need a stronger organic mix or a second elution.
- Adsorption to plastic. Peptides stick to tube walls, especially at low concentration.
Because recovery varies by sequence, quantitative work should compare like with like: the same method, the same format, the same operator where possible.
Handling desalted peptides
A clean sample can be re-contaminated in minutes. Use high-quality receiver tubes to prevent contamination and limit loss through non-specific adsorption to plastic, and minimise both sample transfers and freeze-thaw cycles, as the same Pierce guidance recommends.
Some practical habits:
- Label tubes before you start so that you are not handling open tubes longer than necessary.
- Go to the mass spectrometer promptly where you can, rather than storing dilute solutions.
- If you must store, aliquot once and freeze, rather than thawing a stock repeatedly.
- Dry down eluates only if your next step requires it, and note the volume you resuspend in.
For longer-term storage, our guides to peptide stability, lyophilised peptide storage and peptide storage containers cover the conditions that protect samples, and our peptide handling guidelines for UK research labs set out day-to-day practice. If you are working from a freeze-dried vial, the reconstitution guide covers getting it into solution before any cleanup.
Our perspective on desalting
We supply research peptides, and a lot of the questions we hear are really about what happens after the vial arrives. Our view is that desalting should be treated as part of the experiment design, not a chore bolted on at the end. Decide what the sample must be free of, run the method on a spare aliquot, and keep the flow-through until you have proof that nothing valuable is leaving with it.
We also think purity figures and cleanup are often confused. A high HPLC purity figure on a certificate describes the peptide relative to related peptide impurities. It says nothing about what your own buffer, detergent or reagent carry-over will do to your mass spectra, and desalting is how you deal with those. Reading the certificate properly is the other half of the job, which is why we wrote a guide to reading a peptide certificate of analysis. If endotoxin matters for your cell work, see our note on endotoxin testing too.
Most importantly, keep it simple. Pick one validated method, document it and use it consistently, so differences between samples reflect biology and chemistry rather than cleanup.
— Neil
Research peptides from BioForgeUK
If you are starting from the supply side, we provide precision research peptides with 99%+ HPLC purity, backed by quality controls, verification processes and Certificates of Analysis, with a secure checkout. Every product is supplied strictly for laboratory and scientific research, not for human or veterinary use.
You can read how we define that boundary on our research use only page, or browse the range from the BioForgeUK home page.
FAQ
Do I always need to desalt peptides before mass spectrometry?
Not always, but usually. Salts and detergents suppress ionisation and add background noise, so any sample prepared in buffers, or digested from protein, normally needs cleanup. A peptide dissolved only in clean, volatile solvent may need less. Test a small aliquot if you are unsure.
Which method is best for small peptides?
C18 reversed-phase cleanup is the usual choice, in tip, spin column or syringe format depending on how much sample you have. Gel filtration suits proteins better, because small, soluble peptides can be lost in the gel. Method choice is often decided by sample type, time, ease of use and consumable cost.
How much peptide will I lose?
Recovery typically exceeds 85% for standard peptides and can drop to around 35% for very hydrophilic ones. Overloading, harsh washes and adsorption to tube walls add to the loss. Keep the flow-through and wash fractions while you validate your method.
Why is TFA a concern for electrospray?
TFA helps peptides bind C18 but can interfere with ionisation in ESI and nanospray. Protocols advise rinsing with water to remove it, and eluting with formic or acetic acid in acetonitrile or methanol. Check your instrument's recommendations before you choose.
Is desalting the same as purification?
No. Desalting removes salts and other polar contaminants. Separating a target peptide from closely related peptide impurities needs chromatographic purification, and purity is reported separately, for example by HPLC.
Sources
- Preparation of Proteins and Peptides for Mass Spectrometry Analysis
- Peptide fractionation and clean-up protocols, University of Washington Proteomics Resource
- StageTip C18 desalting procedure
- Peptide desalting by HPLC
- A comparative survey of sample preparation methods, 2022
- How to desalt peptides for LC-MS using C18 StageTips
- Desalting peptide mixtures with Pierce C18 tips
- Sample preparation, Proteomics and Metabolomics Core
- Peptide purity lab results