What exactly is an ion-pair reagent?
Why do some molecules “won’t stay / give terrible peak shapes” in reversed-phase HPLC?
Reversed-phase chromatography (C18/C8) excels at separating neutral analytes with clear hydrophobicity differences. But when a sample contains many charged or highly polar components (e.g., amine salts, quaternary ammonium salts, carboxylates, phosphates, nucleotides/oligonucleotides), these species are highly hydrophilic in the aqueous phase. As a result, they may show weak retention on a reversed-phase stationary phase, elute too early, fail to separate adequately, and/or exhibit tailing peaks.
At that point, there are two common approaches:
1. Switch the separation mode: ion exchange, HILIC, etc.
2. Stay with reversed-phase, but make charged analytes behave “a bit more hydrophobic”—this is the starting point of the ion-pair approach.
An ion pair is a pair of oppositely charged ions held together by Coulombic attraction (no covalent bond). Experimentally, it often behaves like a single entity. In other words: a positively charged analyte + a negatively charged counter-ion (or vice versa) → under appropriate conditions forms a dynamic ion pair / ion association, and can also create a charged adsorbed layer on the column surface, thereby changing both partitioning between stationary/mobile phases and electrostatic interactions.
What is an ion-pair reagent? Ion-pair chromatography works by adding an ion-pair reagent to the mobile phase so it can form an ion pair with the analyte. Separation mainly depends on two things:
1. how strongly and how extensively ion pairing occurs (degree/strength of association), and
2. how differently the ion pair partitions between mobile phase and stationary phase.
Because different analytes “pair more tightly or loosely,” and because the resulting ion pairs differ in whether they prefer the mobile phase or tend to stick to the stationary phase, their residence times on the column differ and they are separated. In essence, an ion-pair reagent is a counter-ion additive in the mobile phase.
Mechanisms of Ion-Pair Chromatography
1. “Put a hydrophobic coat on the analyte” (ion-pair formation / partitioning)
1. A charged analyte → very hydrophilic → does not like staying on C18.
2. After adding an oppositely charged ion-pair reagent → an ion pair forms → the overall entity becomes more hydrophobic → it is more willing to remain longer on the reversed-phase stationary phase.
2. “Lay down a charged thin film on the column surface” (dynamic ion exchange / adsorption)
1. Many ion-pair reagents are amphiphiles: one end is charged, the other is a hydrophobic hydrocarbon chain. In the system, the hydrophobic chain can adsorb to the C18 surface to some extent, while the charged head group faces the mobile phase. This creates a charged interfacial layer on the reversed-phase surface and introduces a degree of “ion-exchange-like” behavior: analytes with the opposite charge are attracted more strongly and retained longer, so both retention and selectivity change.
2. Note: The magnitude of this surface adsorption effect depends on alkyl chain length, reagent concentration, mobile-phase composition, and column chemistry. Therefore, method transfer often requires more thorough equilibration and washing.
Note: IPC retention/selectivity is often explained by a combination of two models—ion-pair formation in solution and adsorption of the ion-pair reagent onto the stationary phase to form a charged interfacial layer (dynamic ion exchange). Most real systems involve both; the dominant contribution simply varies.
Ion-Pair Reagent Classification
First, split into two major classes by “charge direction”
Analyte you want to retain | Which ion-pair reagent class to choose | Typical examples (illustrative) | One-sentence intuition |
Cationic / basic (amine salts, quaternary ammonium salts, positively charged drugs, etc.) | Anionic ion-pair reagents | alkyl sulfonates (C4–C12 homologs), anions of fluorinated strong acids (e.g., TFA/HFBA systems) | Use a negative counter-ion to pair with a positive analyte, increasing its RP retention |
Anionic / acidic (carboxylates, phosphates, nucleotides/oligonucleotides, etc.) | Cationic ion-pair reagents | tetraalkylammonium salts; alkylamines / amine salts (e.g., TEAA/TEA-HFIP systems) | Use a positive counter-ion to pair with a negative analyte, increasing retention and changing selectivity |
Product families classified by “application scenario”
Family | Charge direction | Most commonly matched analytes | Keywords | Common examples (illustrative) | LC-MS friendliness / reminders |
A. Alkyl sulfonates (commonly C5–C12) | Anionic | Cationic / basic (amine salts, quaternary ammonium salts, etc.) | “…sulfonate” + alkyl chain | heptane-/octane-/decane-sulfonate, etc. | Longer chains often give stronger retention but also stronger “memory / hard-to-wash” behavior—require more thorough equilibration and cleaning |
B. Perfluorocarboxylic acid anion additives (TFA/HFBA/PFPA…) | Anionic | Often used in peptide/protein/basic-analyte methods (peak shape/retention tuning) | “fluoro” + “acid” | TFA, HFBA, PFPA… | TFA and related additives often cause ESI-MS signal suppression / unstable spray; use with caution or switch to more MS-friendly systems |
C. Tetraalkylammonium salts (TMA⁺/TEA⁺/TPA⁺/TBA⁺) | Cationic | Anionic / acidic (carboxylates, phosphates, etc.) | “…ammonium” (quaternary ammonium) + four alkyl groups | tetrabutylammonium and related quaternary ammonium salts | More hydrophobic quats are often “stronger”; similarly watch equilibration/cleaning and column memory effects |
D. Volatile amines / amine salts (common in bioanalysis / LC-MS) | Cation source (amine protonated by acid) | Especially common for polyanionic systems (nucleotides/oligonucleotides, etc.) | “amine”; “…ammonium acetate” | triethylamine, tributylamine, TEAA, etc. | More volatile / easier-to-wash systems generally help LC-MS; amine/counter-ion concentration can strongly affect both retention and MS response |
E. Oligonucleotide-specific combinations | Scenario-specific formulations | Oligonucleotide IP-RP | formula-driven | TEAA; TEA + HFIP (variants like DIPEA + HFIP are also common) | IP-RP is a standard HPLC route for oligonucleotides; TEA/HFIP is frequently used in LC-MS application notes |
Compliance / management note: In some regions or institutions, PFOA and other PFAS/long-chain perfluoroacids may be restricted and/or require special waste collection and disposal. Follow local regulations and institutional policies.
Family B—Additional notes
These additives are commonly used in peptide/protein and basic-analyte methods (to improve peak shape and/or retention). They often do two things at once:
1. Acidification: lowers pH so basic analytes are more fully protonated, and suppresses secondary interactions related to silanol sites → tailing often decreases.
2. Counter-ion (anionic) action: TFA⁻/HFBA⁻ can associate with cationic analytes and can also form an adsorbed layer on the column → retention/selectivity increase, but column/system memory effects and ESI-MS suppression become more likely.
“Four Knobs” + Starting Ranges + Recommended Tuning Order
Whether ion pairing can truly “hold” an analyte is determined first by whether pH locks in the ionization state: amines must be sufficiently protonated, and acids sufficiently deprotonated (so the ionization distribution is stable and reproducible). If the charge state drifts during the run, simply increasing alkyl chain length or reagent concentration rarely yields stable retention and good peak shape; instead, it more easily introduces memory effects and requires long re-equilibration.
The Four Knobs
Knob 0 (Prerequisite) | pH / pKa / Degree of Ionization
1. Goal: Make the analyte’s ionization distribution stable and reproducible under the method conditions. For multi-site, multi-charge systems (e.g., oligonucleotides), the key is a stable distribution, not “forcing a single charge state.”
2. Reminder: Changing pH simultaneously changes both ion-pair formation (solution phase) and the dynamic adsorbed layer on the stationary phase—so pH is the strongest “global switch.”
Knob 1 | Counter-ion “Hydrophobic Strength” (Alkyl Chain Length / Structure)
1. Within a homologous series, a longer (more hydrophobic) chain typically gives stronger retention and larger selectivity changes, but it also more readily causes column/system memory and longer equilibration times.
Knob 2 | Counter-ion Concentration (the most direct “strength” knob)
1. In IP-RP/IPC, counter-ions are commonly used at the mM level. Higher concentration usually increases retention, but also slows equilibration and raises the risk of carryover/background and (for MS) ion suppression.
2. A common industry practice is to start at low mM and increase as needed. Many ion-pair recipes/examples are around ~5 mM (e.g., in the literature, 0.1% w/v sodium octanesulfonate is roughly 4–5 mM).
Knob 3 | Organic Percentage/Gradient + Re-equilibration (controls reproducibility)
1. The “slowness” of ion-pair methods often comes from the time required for the counter-ion to redistribute/regenerate on the column—especially after a gradient.
2. Rule of thumb: Every time you change the ion-pair reagent, its concentration, or the organic program, you must re-establish equilibrium → stable retention.
Starting Ranges for Three Common Scenarios
A. Amines / Cations
1. Chain-length starting point: Prefer C5–C8 first (easier to equilibrate; lower cleaning burden than C10–C12).
2. Concentration starting point: Start around ~1–5 mM; if insufficient, increase concentration or move to a longer chain.
B. Organic acids / Phosphorylated compounds / Anions
1. Starting point: Likewise, start at low mM (e.g., ~1–5 mM); if insufficient, increase concentration or switch to a more hydrophobic quaternary ammonium cation.
2. Gradient caution: Counter-ions such as TBA⁺ often redistribute slowly after gradients—this is a classic cause of long re-equilibration.
C. Oligonucleotide IP-RP / LC-MS
1. TEAA: For oligonucleotide purification, a common approach is high concentration (~100 mM) to obtain sufficient retention and peak shape.
2. TEA + HFIP: A common starting recipe in method development is 15 mM TEA + 400 mM HFIP. Variants also exist that reduce HFIP by increasing the counter-ion’s “hydrophobicity.”
3. Note: These are typical starting conditions. Column chemistry/length/temperature and oligonucleotide length can substantially shift the optimal concentration and gradient.
Recommended Tuning Sequence
1. Decide the detector first (MS vs non-MS)
(1) MS (especially ESI-MS): Prioritize volatile, low-residue additives/ion-pair systems (e.g., TEA/TEAA/TEAB, TEA + HFIP). Use low concentrations and washable conditions whenever possible. If needed, consider a dedicated column/dedicated system to reduce cross-contamination.
(2) Non-MS: You may use stronger ion-pair reagents/more hydrophobic counter-ions to improve retention and peak shape, but you must treat memory effects (equilibration time, wash SOPs, interference with subsequent methods) as part of the method “cost” and include them in decisions and validation.
2. Lock the ionization state (pH/buffer): Make the target molecule stably charged first.
3. Choose the right family (correct charge direction): cationic analyte → anionic counter-ion; anionic analyte → cationic counter-ion.
4. Start with “low strength”: short chain / low hydrophobicity + low mM concentration for the first run to confirm you’re moving in the right direction.
5. Equilibrate sufficiently
(1) Practical tip: After each condition change, flush and equilibrate thoroughly before injecting; confirm retention-time stability with replicate injections.
6. Then increase “strength” (by priority)
(1) Increase concentration first (most straightforward).
(2) Then increase chain length/hydrophobicity (bigger effect, bigger side effects).
(3) Finally adjust %B/gradient/temperature (fine-tuning selectivity and efficiency).
IPC/IP-RP Troubleshooting Table (Symptom → Likely Cause → Priority Actions)
What you observe | Most common mechanistic reason | Priority checks / action order |
Retention-time drift (progressively later or earlier) | Counter-ion distribution on the column not yet stable; slow “regeneration/redistribution” after gradients | ① Extend re-equilibration / flush several more column volumes ② Run blanks/replicate injections until tR stabilizes ③ Reduce counter-ion hydrophobicity or concentration (reach equilibrium faster) |
Peak splitting / distorted peak shape (especially with gradients or after lot changes) | Inconsistent adsorbed layer and/or ionization state; sample solvent mismatched with initial mobile phase | ① Check sample solvent strength vs starting %B (dilute/change solvent if needed) ② Check pH/buffer capacity ③ Increase re-equilibration ④ Reduce ion-pair strength (concentration/chain length) |
Tailing persists (especially for basic compounds) | Unstable ionization; residual secondary interactions at silanol/metal sites; insufficient ion pairing | ① Lock pH first (ensure amines are fully protonated) ② Increase ion-pair reagent from low mM upward ③ Switch to a more hydrophobic homolog (C6 → C7 → C8 …) |
Baseline rise / ghost peaks / negative peaks (more obvious in UV) | UV background from additive/impurities; system residue or insufficient flushing | ① Run a blank gradient to locate the ghost-peak source ② Switch to higher-purity additives/solvents ③ Extend high-%B flush / replace tubing or frits if needed |
LC-MS signal drops sharply; spray unstable | Strong ion pairing (especially fluorinated acids) forms tight ion pairs with charged analytes, reducing ESI ionization efficiency | ① Lower additive concentration ② Switch to a more MS-friendly (volatile) approach ③ If necessary, use suppression-mitigation strategies (e.g., change acid system / post-treatment, etc.) |
Cannot wash it out; next method is affected | Typical column/system memory effect (strong adsorption of counter-ions) | ① Stronger and longer end-of-method flushing ② Use a dedicated column/system if needed ③ Prefer more volatile/easier-to-clean counter-ion systems when possible |
Retention/reproducibility suddenly worsens at low organic | At very low %B, C18 may undergo dewetting / phase collapse, misinterpreted as a “bad column” | ① Raise starting %B moderately ② Use columns more tolerant of low %B (polar-embedded / polar-modified RP) ③ Then reassess whether ion pairing is truly necessary |
Ion-Pair Reagent Selection Guide and Representative Product Classification Tables (Tables 1–4: IPC Alkyl Sulfonates / Quaternary Ammonium Salts / Fluorinated Ion-Pairing Acids & Volatile Buffers / Supporting Toolbox)
Ion-Pair Reagent Product Tables | Quick Navigation (Define the task → pick the category → choose the strength)
Your task / symptom | What to check first | Typical examples | How to choose strength / how to use (common method-development logic) | When not recommended |
Basic / positively charged analytes (amines, quaternary ammonium species) elute too early on RP, tail, or show poor peak shape; you want to “hold them” | Anionic alkyl sulfonates for IPC (C3→C12 ladder) → Table 1 | sodium propanesulfonate / sodium 1-butane-/pentane-/hexane-/heptane-/octane-/nonane-/decane-/dodecane-sulfonate (incl. sodium 1-octanesulfonate UltraPure, sodium 1-heptanesulfonate, etc.) | Screen from weak to strong: C3/C4 → C5/C6 → C7/C8 → C9/C10 → C12. Need stronger retention → increase carbon chain length; peaks too broad / strong column memory → reduce chain length or reduce concentration. | Column memory / hard to wash: more obvious for C9–C12; system residue can affect subsequent methods. Be extra cautious for LC-MS (ion-pair reagents can contaminate/suppress). |
Acidic / negatively charged analytes (organic acids, phosphorylated compounds, anions) show poor retention or cannot be resolved on RP | Cationic ion-pair reagents (quaternary ammonium salts / quaternary ammonium bases) → Table 2 | TMACl, TEABr, TPA Br, TBAC, TBABr, TBAHS, TBAOH, tetrabutylammonium dihydrogen phosphate solution, tetrabutylammonium acetate | Start with “milder” TEA⁺/TPA⁺; if insufficient, move to TBA⁺ (more hydrophobic/stronger). If you need to adjust pH / increase anion fraction, consider TBAOH. Ready-to-use solutions (e.g., 0.5 M tetrabutylammonium dihydrogen phosphate) improve reproducibility. | Residue/column contamination: TBA⁺ and long-chain quaternary ammonium salts (especially TOAB, methyltrioctylammonium chloride) are more prone to “memory.” With strong base (TBAOH), confirm column pH tolerance and material compatibility. |
Peptides/proteins/amines: want better peak shape, less tailing, more retention—especially under acidic mobile phases | Fluorinated ion-pairing acids (fluorinated carboxylic/sulfonic acids) → Table 3 (primary); if “salt formation / counter-ion” assistance is involved → Table 4 (secondary; e.g., TFMSA) | TFA solution, heptafluorobutyric acid, nonafluoropentanoic acid, perfluorohexanoic acid, tridecafluoroheptanoic acid (LC-MS grade), PFOA; also TFMSA (superacid, more often for salt formation/counter-ion) | Screen from “lighter” to “stronger”: DFA / pentafluoropropionic acid → TFA → HFBA / longer-chain. More hydrophobic acids typically “hold” amines/peptides better and improve peak shape. | MS suppression and system memory: the more hydrophobic the fluorinated acid (e.g., PFOA, tridecafluoroheptanoic acid), the higher the risk of ESI suppression and memory effects. If LC-MS is the goal, prioritize a volatile route. |
Oligonucleotide HPLC purification or you want a more LC-MS compatible / easier-to-remove ion-pair system | Volatile ion-pair / buffers (TEAA/TEAB/TEA) → Table 3 | triethylammonium acetate (TEAA, 1 M), triethylammonium bicarbonate (TEAB, ~1.0 M), triethylamine | Prefer TEAA/TEAB (volatile, easier to desalt). For ready-to-use buffers, choose 1 M solutions. TEA can be used for salt preparation and fine-tuning pH/ionic strength. | Not the “strongest retention” route: if you must force strong retention, fluorinated acids or TBA⁺ can be stronger—but at the expense of MS and cleanability. “Volatile” ≠ “zero residue”; establish a wash SOP. |
You are doing ion-pair extraction / phase transfer / making salts more organic-phase friendly (not necessarily chromatography) | Phase-transfer / hydrophobic counter-ion toolbox → Table 2 (long-chain quats / PTC salts) + Table 4 (e.g., sodium tetraphenylborate) | benzyltrimethylammonium chloride solution, TEBAC, TOAB, methyltrioctylammonium chloride, sodium tetraphenylborate | Goal is “move ions into the organic phase”: choose more hydrophobic quats (TOAB / trioctyl-type), or use sodium tetraphenylborate to form hydrophobic ion pairs/precipitates. | Be cautious for chromatography: these are extremely hydrophobic and strongly adsorbing, easily causing column/system memory. For LC development, consider a dedicated system/column, or use only as a last resort. |
You need a background electrolyte / ionic strength (electrochemistry, non-aqueous systems, or “inert counter-ions” for some separations) | Supporting electrolyte / weakly coordinating counter-ion salts → Table 4 | sodium perchlorate (anhydrous), tetrabutylammonium perchlorate (electrochemical grade), potassium hexafluorophosphate | Mainly to increase conductivity/ionic strength and provide relatively weakly coordinating counter-ions (e.g., PF6⁻, ClO4⁻). Most common in electrochemical systems. | Safety & compliance: perchlorates carry safety risks (especially anhydrous / strong oxidizer / explosive hazard). Material compatibility and operating procedures must be explicit. |
You are using micelles/surfactants (electrophoresis, special selectivity, improved solubility/aggregation control) | Surfactant / micellar systems → Table 4 | SDS, CTAB, SDBS standard solution | A “micelle/interfacial selectivity” route: can change migration/retention/peak shape; more common in electrophoresis or special method exploration. | Not the same as classic IPC: more likely to introduce adsorption/memory and background interference; for LC, watch column/system contamination and cleaning. |
You need sample dissolution / anti-aggregation / consistent injection (especially peptides/polymers) | Sample-prep aids (used together with ion-pair systems) → Table 4 | HFIP | Used as a sample solvent/conditioning tool to improve solubility and conformational consistency; paired with ion-pair/RP systems to improve reproducibility. | Not a “counter-ion” itself: it is mainly a tool to put the sample into a controllable state; the method must still specify solvent fraction and evaporation/residue control. |
Table 1 | Anionic Ion-Pair Reagents: Alkyl Sulfonate Series (for “holding” cationic/basic compounds)
Product category | CAS No. | Aladdin Cat. No. | Name | Grade / purity | Key features or ion-pairing use |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 304672-01-3 | Sodium propanesulfonate monohydrate | Ion-pair chromatography grade, ≥99% | C3, lighter background: weaker retention boost; better when you only need mild peak-shape improvement / tailing control or want to reduce column memory | |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 14533-63-2 | Sodium 1-propanesulfonate [ion-pair chromatography reagent] | ≥98% (T) | C3 anionic IP reagent: lighter background, milder retention enhancement; suitable for basic samples needing only mild peak-shape correction | |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 2386-54-1 | Sodium 1-butanesulfonate | Ion-pair chromatography grade, ≥99% (T) | C4 alkyl sulfonate: mild ion-pairing; lighter background; useful when you do not want overly strong retention or prioritize MS compatibility / wash efficiency (still evaluate case-by-case) | |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 22767-49-3 | Sodium 1-pentanesulfonate | Ion-pair chromatography grade, ≥98% (T) | C5, gentler option: for mild peak-shape tuning or lower residue/memory; good low-strength control versus C6/C7/C8 | |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 2832-45-3 | Sodium 1-hexanesulfonate | Ion-pair chromatography grade, ≥98% | C6, common starting point: moderate retention enhancement and relatively easy to flush; frequently used as a first-choice method-development baseline for amines/quats | |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 22767-50-6 | Sodium 1-heptanesulfonate | Ion-pair chromatography grade, ≥98% (T) | C7: improves retention and peak shape for basic compounds; often the compromise choice when C6 is not enough but C8 is too strong | |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 207300-90-1 | Sodium 1-heptanesulfonate monohydrate | Ion-pair chromatography grade, ≥99% | C7: stronger than C6 yet easier to flush than C8; a common mid-level option during optimization | |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 5324-84-5 | Sodium 1-octanesulfonate | Ion-pair chromatography grade, ≥99% | C8, classic general-purpose: commonly improves retention and peak shape for basic drugs/amines; a frequent first-choice alongside C6/C7/C10 for strength comparison | |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 207596-29-0 | Sodium 1-octanesulfonate monohydrate | Ion-pair chromatography grade, UltraPureChrom™, ≥99% (T) | Classic anionic IPC reagent: forms ion pairs with amines/quats to boost RP retention and improve peak shape; C8 balances retention gain with manageable solubility/background | |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 35192-74-6 | Sodium 1-nonanesulfonate | ≥98% (T) | C9, stronger retention: for very polar cations/amine analytes that elute too early; higher risk of column memory—requires stricter washing and method “locking” | |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 13419-61-9 | Sodium 1-decanesulfonate | Ion-pair chromatography grade, ≥99% | C10, more hydrophobic/stronger: significant retention increase for cationic/amine analytes; useful when peaks elute too early or look poor, but cleaning cost and memory risk increase | |
Anionic alkyl sulfonate (IPC: pairs with cationic/basic compounds) | 2386-53-0 | Sodium dodecanesulfonate | Ion-pair chromatography grade, ≥99% | C12, very strong: strongest combined ion-pair + hydrophobic enhancement; for extremely polar cations that are otherwise unretained, but behaves like a strongly adsorbing additive—rigorous washing required |
Table 2 | Cationic Ion-Pair Reagents: Quaternary Ammonium Salts / Quaternary Ammonium Bases / Highly Hydrophobic Quats (for “holding” anionic/acidic compounds)
Product category | CAS No. | Aladdin Cat. No. | Name | Grade / purity | Key features or ion-pairing use |
Cationic ion-pair reagent (quaternary ammonium) | 75-57-0 | Tetramethylammonium chloride | Ion-pair chromatography grade, ≥99% (AT) | More hydrophilic quat: weaker retention boost, but sometimes better for ionic strength / suppressing secondary interactions; useful as a control or low-hydrophobicity route | |
Cationic ion-pair reagent (quaternary ammonium) | 71-91-0 | Tetraethylammonium bromide | Ion-pair chromatography grade | TEA⁺-type quat: less hydrophobic than TBA⁺, giving milder retention enhancement; good for building a “counter-ion hydrophobicity ladder” | |
Cationic ion-pair reagent (quaternary ammonium) | 1941-30-6 | Tetrapropylammonium bromide | Ion-pair chromatography grade, ≥99% (AT) | Quaternary ammonium cation for pairing anionic analytes; hydrophobicity/sterics between TEA and TBA—useful for retention-strength screening | |
Cationic ion-pair reagent (quat salt / phase-transfer / RP-IPC) | 32503-27-8 | Tetrabutylammonium hydrogen sulfate (TBAHS) | Anhydrous, ≥97% | Typical TBA⁺ source: used to form ion pairs with anions to increase retention/extraction; also common in phase-transfer and electrolyte systems | |
Cationic ion-pair reagent (quaternary ammonium) | 1112-67-0 | Tetrabutylammonium chloride (TBAC) | Ion-pair chromatography grade, ≥99% | General-purpose TBA⁺ counter-ion for anions; chloride is a straightforward counter-anion—commonly used for method screening | |
Cationic ion-pair reagent (quaternary ammonium) | 1643-19-2 | Tetrabutylammonium bromide | Ion-pair chromatography grade, ≥99% | Same TBA⁺ family as TBAC; different halides can affect solubility/background and secondary interactions (useful as a TBAC comparison) | |
Cationic ion-pair reagent (quaternary ammonium base / strong base system) | 2052-49-5 | Tetrabutylammonium hydroxide (TBAOH) | Ion-pair chromatography grade, ~40% in H₂O | Provides TBA⁺ and also acts as a strong base for pH control; used in RP-IPC for anionic analytes (confirm column lifetime and materials compatibility under basic conditions) | |
Cationic ion-pair reagent (quat salt / RP-IPC buffer salt) | 5574-97-0 | Tetrabutylammonium dihydrogen phosphate | Ion-pair chromatography grade, 0.5 mol/L in water | TBA⁺ as cationic counter-ion to enhance RP retention of anions (e.g., organic acids/phosphorylated compounds); ready-to-use solution improves reproducibility and development efficiency | |
Cationic ion-pair reagent (quat salt / RP-IPC) | 10534-59-5 | Tetrabutylammonium acetate | Industrial grade, ≥90% (T) | TBA⁺ + acetate; used for RP retention enhancement or phase transfer. Acetate is relatively mild but not as MS-friendly/volatile as TEAA | |
Cationic ion-pair reagent (hydrophobic quat / phase-transfer) | 56-93-9 | B477142 | Benzyltrimethylammonium chloride solution | Industrial grade, ~60% in H₂O | BnMe₃N⁺: hydrophobic quat used in phase transfer/ion-pair extraction; can be explored in chromatography for special selectivity (watch residue) |
Cationic ion-pair reagent (hydrophobic quat / phase-transfer) | 56-37-1 | Triethylbenzylammonium chloride (TEBAC) | ≥98% | Hydrophobic quat for ion-pair extraction/phase transfer; can tune selectivity in separations (watch residue and cleaning burden) | |
Cationic ion-pair reagent (highly hydrophobic quat / extraction) | 14866-33-2 | Tetra-n-octylammonium bromide (TOAB) | ≥98% | Extremely hydrophobic quat: mainly for ion-pair extraction/phase transfer and forming organic-soluble ion pairs; if used in chromatography, often causes very strong retention and severe memory—cleaning/contamination must be assessed | |
Cationic ion-pair reagent (hydrophobic quat / phase-transfer) | 5137-55-3 | Methyltrioctylammonium chloride | ≥97% | Long-chain hydrophobic quat used for extraction/phase transfer and forming organic-phase soluble ion pairs; in chromatography, pay attention to strong adsorption and system memory |
Table 3 | Fluorinated Ion-Pairing Acids + Volatile Buffers/Amines (Peptides/Amines, LC-MS, Oligonucleotide Routes—start here)
Product category | CAS No. | Aladdin Cat. No. | Name | Grade / purity | Key features or ion-pairing use |
Fluorinated ion-pairing acid (short chain, relatively “light”) | 381-73-7 | Difluoroacetic acid (DFA) | ≥98% | Shorter/lighter fluorinated carboxylic acid: can be explored as an alternative to TFA/HFBA, balancing ion-pairing capability with potentially better MS compatibility (must be verified experimentally) | |
Fluorinated ion-pairing acid (short-to-mid chain) | 422-64-0 | Pentafluoropropionic acid | ≥97% | Fluorinated carboxylic acid with ion-pairing strength between DFA and the TFA/HFBA family; useful for balancing peak shape, retention, and MS impact during screening | |
Fluorinated ion-pairing acid (peptides/amines; LC/LC-MS) | 76-05-1 | Trifluoroacetic acid solution (TFA) | Protein sequencing grade, 25% solution in water | Classic peptide/protein ion-pairing acid: improves peak shape, suppresses tailing, and increases retention; may suppress MS to some extent—often a trade-off between peak shape and MS | |
Fluorinated ion-pairing acid (peptides/amines; LC/LC-MS) | 375-22-4 | Heptafluorobutyric acid (HFBA) | Protein sequencing grade, ≥99% (GC) | More hydrophobic than TFA: stronger “ion pairing + retention boost” for basic/peptidic analytes and often improves peak shape further; higher risk of MS suppression | |
Fluorinated ion-pairing acid (peptides/amines; LC/LC-MS) | 2706-90-3 | N742543 | Nonafluoropentanoic acid | 0.5 mol/L aqueous solution | Fluorinated carboxylic acid with retention boost stronger than TFA but weaker than longer chains; commonly used for optimizing peak shape/retention for amines/basic analytes (ready-to-use solution improves reproducibility) |
Fluorinated ion-pairing acid (LC-MS ion pairing; mid chain) | 307-24-4 | Perfluorohexanoic acid | ≥98% | Mid-chain fluorinated carboxylic acid: stronger retention enhancement than TFA; MS suppression/memory risk between short-chain acids and PFOA—useful for “retention strength ladder” screening | |
Fluorinated ion-pairing acid (LC-MS ion pairing; method-specific) | 375-85-9 | Tridecafluoroheptanoic acid, high grade [ion-pair reagent for LC-MS] | ≥98% (T) | LC-MS-oriented fluorinated ion-pairing acid: strong retention enhancement for basic/cationic analytes; must evaluate MS suppression and system memory—best for difficult separations requiring a larger retention/selectivity shift | |
Fluorinated ion-pairing acid (LC-MS ion pairing; very hydrophobic, high MS suppression risk) | 335-67-1 | Perfluorooctanoic acid (PFOA) | Analytical standard, for environmental analysis | Very hydrophobic fluorinated carboxylic acid: very strong retention enhancement for amines/cations, but high MS suppression and memory risk; more often used as a standard/method reference and for compliance-related analyses | |
Volatile amine / amine-derived component (upstream for MS-friendly routes) | 121-44-8 | Triethylamine (TEA) | Protein sequencing grade, ≥99.5% (GC), ampoule | Forms TEA⁺ counter-ions, adjusts pH, and makes volatile salts with acids; used to prepare TEAA/TEAB volatile ion-pair systems or as an additive to modulate acidity/silanol interactions | |
Volatile ion-pair / buffer (common for LC-MS / oligonucleotides) | 15715-58-9 | Triethylammonium bicarbonate buffer (TEAB) | Volatile buffer, ~1.0 M in H₂O | TEAB: classic volatile buffer/ion-pair system for oligonucleotide purification and LC-MS compatible methods (easier to remove; lower residue) | |
Volatile ion-pair / buffer (common for oligonucleotide HPLC) | 5204-74-0 | T755551 | Triethylammonium acetate (TEAA), 1 M solution | Ready-to-use buffer for HPLC purification of chemically synthesized oligonucleotides; pH 7.0 | TEAA: a classic volatile ion-pair system for RP purification of oligonucleotides; balances retention with easier post-run removal (more desalting/MS-friendly than TBA-type systems) |
Table 4 | Other Counter-ions and Supporting “Toolbox” (Supporting electrolytes / salt formation / precipitation counter-ions / surfactants / sample prep / standards)
Product category | CAS No. | Aladdin Cat. No. | Name | Grade / purity | Key features or ion-pairing use |
Supporting electrolyte / counter-ion salt (ionic strength / electrochemistry / chromatographic background salt) | 7601-89-0 | S105271 | Sodium perchlorate, anhydrous (explosive hazard) | HPLC grade, ≥99% | Source of strongly weakly coordinating ClO4⁻: increases ionic strength; can serve as counter-ion/supporting electrolyte. Note explosive hazard and safety/compliance requirements. |
Supporting electrolyte / counter-ion salt (electrochemistry / common non-aqueous salt) | 1923-70-2 | Tetrabutylammonium perchlorate | Electrochemical grade | Classic non-aqueous supporting electrolyte (TBA⁺/ClO4⁻). In an ion-pair context, it is also a “hydrophobic quat + weakly coordinating anion” combination (clean background, good conductivity). | |
Supporting electrolyte / counter-ion salt (weakly coordinating anion) | 17084-13-8 | Potassium hexafluorophosphate | ≥99.98% metals basis | PF6⁻ is a common weakly coordinating anion: used as a relatively “inert counter-ion/electrolyte background,” common in non-aqueous/electrochemical/ionic-liquid contexts. | |
Hydrophobic anion / precipitation-type counter-ion (BPh4−) | 143-66-8 | Sodium tetraphenylborate | AR, ≥99% | Typical hydrophobic weakly coordinating anion (BPh4⁻): forms hydrophobic ion pairs with quats/amine salts to enable organic extraction or precipitation (“make ions more organic-phase friendly”). | |
Sulfonic acid strong acid / counter-ion (salt formation / ion-pair extraction / acidification) | 75-75-2 | Methanesulfonic acid | Suitable for synthesis | Strong acid often used to form mesylate counter-ions (improving salt stability/crystallinity/solubility); can also convert amines into controlled counter-ion forms to alter retention/partitioning. | |
Sulfonic acid strong acid / counter-ion (salt formation / acidification) | 104-15-4 | p-Toluenesulfonic acid | ≥98% | Common strong acid and counter-ion (TsO⁻): used for amine salt formation, salt screening, and catalysis; counter-ions can strongly affect solubility/crystallinity/partitioning. | |
Sulfonic acid strong acid / counter-ion (salt formation / chiral environment) | 3144-16-9 | (+)-10-Camphorsulfonic acid | Moligand™, ≥99% | Chiral/rigid sulfonate counter-ion: used to form camphorsulfonate salts for improved crystallization/resolution and salt screening; can also influence partitioning/retention as an organic counter-ion. | |
Fluorinated ion-pairing acid (superacid / triflate counter-ion) | 1493-13-6 | Trifluoromethanesulfonic acid (TFMSA) | ≥99.5% | Source of triflate (OTf⁻): very strong acid, weakly coordinating anion; used to prepare OTf salts to modify solubility/reactivity; can serve as an extremely strong counter-ion in some separations (watch corrosion/safety). | |
Sulfonate / aromatic counter-ion (counter-ion and ionic-strength control) | 515-42-4 | Sodium benzenesulfonate | ≥97% | Aromatic sulfonate counter-ion: can build sulfonate counter-ion environments and tune ionic strength/partitioning; useful as a comparison to alkyl sulfonates (difference lies in hydrophobic architecture and possible π-interactions). | |
Surfactant / micellar system (can form ion pairs / improve peak shape) | 151-21-3 | Sodium dodecyl sulfate (SDS) | For electrophoresis, anionic | Strong anionic surfactant: used in micellar/electrophoresis systems; can enhance interactions via micelles/ion pairing to improve migration/peak shape for cationic samples (more micellar than classic IPC). | |
Surfactant / micellar system (can form ion pairs / improve peak shape) | 57-09-0 | H108986 | Cetyltrimethylammonium bromide (CTAB) | Ion-pair chromatography grade, ≥99% | Cationic surfactant: forms ion pairs/micelles with anions and can create special selectivity; used when strong hydrophobic cationic counter-ions are needed (watch adsorption/memory). |
Surfactant / micellar system (can form ion pairs / improve peak shape) | 25155-30-0 | Sodium dodecylbenzenesulfonate standard solution (SDBS) | Analytical standard, 1000 μg/mL in water | Anionic surfactant/aromatic sulfonate: used in micellar systems and for enhancing ion-pair/hydrophobic interactions; here as a standard solution—primarily for quantitative/benchmark purposes. | |
Fluorinated solvent / sample-prep aid (not a typical IPC reagent, but often paired with ion-pair methods) | 920-66-1 | Hexafluoroisopropanol (HFIP) | For GC derivatization, ≥99.8% | Fluorinated alcohol solvent: widely used to dissolve peptides/polymers, reduce aggregation, and improve injection consistency; not a counter-ion itself, but often used as a sample-prep/solvent optimization tool in ion-pair/RP workflows. |
Note: The above are representative Aladdin catalog numbers. For additional specifications, please refer to the full list at the end of the document or search by CAS/name on the Aladdin website.
Aladdin: https://www.aladdinsci.com/