TFA — Trifluoroacetic Acid¶
Essential Reagent for Cleavage, Deprotection, and Analysis¶
Introduction¶
Trifluoroacetic acid (TFA) is the single most important reagent in peptide workup and analysis. In Fmoc SPPS, TFA serves dual critical roles: as the primary component of cleavage cocktails that release the peptide from the solid support and remove side-chain protecting groups, and as an ion-pairing agent in HPLC mobile phases that sharpens peptide peaks and improves resolution. With a pKa of 0.23, TFA is one of the strongest carboxylic acids, yet its moderate boiling point (72.4 °C) allows convenient removal by evaporation. Proper selection of TFA grade and understanding of its concentration effects are essential for reproducible cleavage yields and chromatographic performance.
Chemical and Physical Properties¶
| Property | Value |
|---|---|
| IUPAC name | Trifluoroacetic acid |
| CAS number | 76-05-1 |
| Molecular formula | CF₃COOH |
| Molecular weight | 114.02 g/mol |
| Boiling point | 72.4 °C (at 1 atm) |
| Melting point | −15.4 °C |
| Density | 1.48 g/mL (20 °C) |
| pKa | 0.23 |
| Dielectric constant (ε) | 8.55 (25 °C) |
| Viscosity | 0.81 cP (25 °C) |
| Vapour pressure | 105 mmHg (25 °C) |
| Refractive index | 1.285 (nD²⁰) |
Purity Grades¶
| Grade | Purity | Water Content | Typical Use | Price Index |
|---|---|---|---|---|
| Synthesis grade | ≥99.0% | ≤0.1% | Peptide cleavage | Low |
| Reagent grade | ≥99.5% | ≤0.05% | General laboratory | Low |
| HPLC grade | ≥99.8% | ≤0.01% | HPLC mobile phase | Moderate |
| LC-MS grade | ≥99.9% | ≤0.005% | LC-MS analysis | High |
| Biotech grade | ≥99.5% | ≤0.05% | GMP manufacturing | Moderate |
| Anhydrous grade | ≥99.9% | ≤0.005% | Water-sensitive couplings | High |
Selection guide: For routine peptide cleavage, synthesis grade is sufficient. For HPLC mobile phase preparation, use HPLC grade or higher. Do not substitute — lower grades contain visible-wavelength UV-absorbing impurities and higher water content that reduces cleavage efficiency.
Role in Cleavage Cocktails¶
TFA is the backbone of all standard cleavage cocktails. Its concentration in the cocktail determines whether the peptide is fully deprotected and released, or whether side-chain protecting groups are retained.
Fmoc SPPS Cleavage (Standard)¶
| Resin Type | TFA Concentration | Cleavage Time | Cocktail Components |
|---|---|---|---|
| Wang resin | 90–95% | 1.5–3 h | TFA/TIS/H₂O (95:2.5:2.5) |
| Rink Amide | 90–95% | 1–2 h | TFA/TIS/H₂O (95:2.5:2.5) |
| Rink Acid | 90–95% | 1–2 h | TFA/TIS/H₂O (95:2.5:2.5) |
| 2-Cl-Trt resin | 95% | 30 min | TFA/DCM (1:99) for protected peptide |
| Sieber resin | 1–5% | 5–15 min | TFA/DCM (1–5%) — retains side-chain PGs |
Concentration Recommendations by Cleavage Objective¶
| Objective | TFA % | Scavenger System | Time | Notes |
|---|---|---|---|---|
| Full deprotection + cleavage | 95% | TIS 2.5%, H₂O 2.5% | 1.5–3 h | Standard Reagent R |
| Full deprotection + Trp protection | 88% | Phenol 5%, H₂O 5%, TIS 2% | 2–4 h | Reagent B |
| Full deprotection + Trp + Arg(Pbf) | 82.5% | Phenol 5%, H₂O 5%, TIS 2.5%, EDT 5% | 2–4 h | Reagent K |
| Side-chain protected peptide | 1–5% | In DCM; no scavenger | 5–30 min | Sieber or 2-Cl-Trt only |
| Nⁿ-Boc removal (Boc SPPS) | 50% | In DCM | 20–30 min | Intermediate step |
Mechanism of TFA Action in Cleavage¶
TFA protonates the peptide-resin linkage (ester or amide bond), generating a labile O-acyl or N-acyl intermediate that cleaves to release the free peptide. Simultaneously, TFA protonates acid-labile protecting groups (tBu, Boc, Trt, Pbf), generating carbocations that must be trapped by scavengers. The water content in TFA (<0.05% in HPLC grade) affects the equilibrium: excess water slows the protonation equilibrium, while anhydrous TFA accelerates cleavage but may leave trace of benzyl-type protecting groups incompletely removed.
TFA in HPLC Analysis¶
| Parameter | Recommendation | Effect |
|---|---|---|
| Concentration | 0.05–0.1% (v/v) in water and acetonitrile | Optimum peak shape; below 0.05% broadens peaks |
| pH of 0.1% aqueous TFA | ~2.0 | Suppresses silanol ionisation on C18 |
| UV transparency at 214 nm | Excellent (A <0.002 AU) at 0.1% | Allows sensitive peptide detection |
| Volatility | Complete under vacuum | Compatible with LC-MS and fraction lyophilisation |
| Gradients | Compatible; TFA concentration kept constant | Maintain constant ion-pairing throughout run |
Note: TFA concentration must be identical in both mobile phases A and B to avoid baseline drift during gradient runs. A difference of even 0.02% produces a measurable UV absorbance drift at 214 nm.
Handling Hazards and Safety¶
| Hazard | H-Statement | Precaution |
|---|---|---|
| Corrosive | H314 — severe skin burns and eye damage | Acid-resistant gloves (neoprene or butyl rubber), face shield |
| Toxic by inhalation | H332 — harmful if inhaled | Fume hood mandatory; never work open-bench |
| Lachrymator | — | Causes profuse tearing at >1 ppm |
| Volatile | — | Store in vented cabinet; cap immediately after use |
| Water-reactive | — | Contact with water generates exotherm and HF |
| Tissue penetration | — | Absorbed through skin; wash immediately if splashed |
First aid: Skin contact — flush with copious water for ≥15 min, remove contaminated clothing. Eye contact — irrigate continuously for ≥20 min, seek immediate medical attention. Inhalation — move to fresh air; oxygen if available.
Waste Disposal¶
TFA waste requires special handling as it is not compatible with standard organic solvent waste streams:
| Waste Type | Disposal Method | Notes |
|---|---|---|
| Cleavage cocktail (spent) | Collect separately; neutralise with NaOH before aqueous disposal | TFA content high enough to acidify large volumes |
| TFA/water mixtures | Cannot be incinerated (generates HF) | Neutralise to pH 6–8, then aqueous disposal |
| Empty containers | Triple-rinse with water; dispose as corrosive waste | Residual vapour is highly corrosive |
| Spent rag/adsorbent | Seal in polyethylene bag; incinerator disposal | Avoid cellulose-based adsorbents (decompose) |
Never mix TFA waste with halogenated solvent waste streams containing DCM — TFA reacts with DCM under basic conditions to form explosive diazomethane-like intermediates.
Storage Recommendations¶
- Container: High-density polyethylene (HDPE) or PTFE — avoid glass for long-term storage (TFA etches glass, introducing silicate impurities)
- Temperature: Room temperature (15–25 °C) in a cool, well-ventilated area
- Shelf life: >3 years if moisture ingress is prevented. TFA itself is stable but absorbs water from air
- Purity monitoring: Check water content by Karl Fischer titration before use in moisture-sensitive cleavage applications
🔗 Related: Cleavage Process | Acetonitrile | Purification | Scavenger Selection Guide | Custom Peptide Synthesis — Quality Standards