US20140296485A1 - Anionic displacer molecules for hydrophobic displacement chromatography - Google Patents

Anionic displacer molecules for hydrophobic displacement chromatography Download PDF

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US20140296485A1
US20140296485A1 US14/349,092 US201214349092A US2014296485A1 US 20140296485 A1 US20140296485 A1 US 20140296485A1 US 201214349092 A US201214349092 A US 201214349092A US 2014296485 A1 US2014296485 A1 US 2014296485A1
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hydrophobic
displacer
general formula
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Barry L. Haymore
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Sachem Inc
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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07K—PEPTIDES
    • C07K1/00—General methods for the preparation of peptides, i.e. processes for the organic chemical preparation of peptides or proteins of any length
    • C07K1/14—Extraction; Separation; Purification
    • C07K1/16—Extraction; Separation; Purification by chromatography
    • C07K1/20—Partition-, reverse-phase or hydrophobic interaction chromatography
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D15/00—Separating processes involving the treatment of liquids with solid sorbents; Apparatus therefor
    • B01D15/08—Selective adsorption, e.g. chromatography
    • B01D15/26—Selective adsorption, e.g. chromatography characterised by the separation mechanism
    • B01D15/32—Bonded phase chromatography
    • B01D15/325—Reversed phase
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D15/00—Separating processes involving the treatment of liquids with solid sorbents; Apparatus therefor
    • B01D15/08—Selective adsorption, e.g. chromatography
    • B01D15/42—Selective adsorption, e.g. chromatography characterised by the development mode, e.g. by displacement or by elution
    • B01D15/422—Displacement mode

Definitions

  • DC Displacement chromatography
  • Displacement chromatography may be carried out using any one of four general chromatographic methods when suitable, high purity displacer molecules are available.
  • DC is used in (a) ion-exchange chromatography (cation-exchange, anion-exchange), (b) hydrophobic chromatography (reversed-phase, hydrophobic-interaction, hydrophobic charge-induction, thiophilic), (c) normal-phase chromatography including hydrophilic-interaction chromatography (HILIC) and (d) immobilized metal-ion affinity chromatography (IMAC).
  • Displacement chromatography is carried out by choosing (a) an applicable chromatographic method, (b) a suitable chromatography column with proper dimensions, (c) proper mobile phase conditions, (d) a suitable displacer molecule and (e) suitable operation protocols with properly configured LC equipment.
  • a suitable “weakly displacing mobile phase” carrier
  • the carrier may contain a pH buffering compound adjusted to a useful pH value.
  • Optimal displacement flow rates tend to be low, typically in the range of 35-105 cm/hr, though sometimes higher.
  • a suitable amount of the sample solution is loaded onto the column at the sample-loading flow rate.
  • the sample solution contains the material to be purified in the carrier along with the proper level of an ion-pairing agent or ion-pairing salt if the sample or displacer molecules are charged. Typical sample loadings are 50-80% of the operative breakthrough capacity.
  • a displacer mobile phase (displacer buffer), prepared from a suitable displacer compound at the proper concentration in the carrier solution, is pumped onto the column at the displacement flow rate until the displacer breakthrough is observed.
  • the purified sample comes off the column before the displacer breakthrough front. Fractions from the column are collected and separately analyzed for content and purity.
  • the displacer is removed from column using a “displacer removal solution”, and then the column is cleaned and regenerated to its original state for storage or for subsequent use.
  • displacement chromatography is easy to understand and easy to carry out.
  • a sample is “displaced” from the column by the displacer, rather than “eluted” from the column by the mobile phase.
  • a “displacement train” is obtained rather than an “elution chromatogram”.
  • the displacement train is composed of side-by-side “displacement bands” rather than solvent-separated “elution peaks” in a chromatogram.
  • a displacement band is large enough to saturate the stationary phase, a trapezoidal “saturating band” Is formed.
  • a small, triangular “non-saturating band” is formed.
  • the height of a saturating band is determined by the binding isotherm at the point of operation; the area of a trapezoid-band or a triangle-band is proportional to the amount of the component.
  • Hydrophobic chromatography depends almost exclusively on the unique solvation properties of water that result from the highly structured, self-associated, hydrogen-bonded liquid.
  • stationary phases uncharged C 18 column
  • binding is usually driven by entropy (+T ⁇ S), which often must overcome unfavorable enthalpy (+ ⁇ H).
  • T ⁇ S entropy
  • analyte-binding and displacer-binding often become stronger with increasing temperature.
  • Another useful feature of hydrophobic chromatography is the use of additives that modify both the structure and strength of the self-hydrogen-bonding of the aqueous-based solvent.
  • additives include: salts (NaCl, K 2 HPO 4 , (NH 4 ) 2 SO 4 ), organic solvents (MeCN, MeOH, EtOH) and polar organic molecules (urea, oligo-ethyleneglycol) in chromatography buffers.
  • Hydrophobic displacement chromatography can be carried out using chiral analytes, chiral displacers and chiral chromatography matrices. Under these conditions, an achiral displacer may be used, but a racemic mixture of a chiral displacer cannot be used. Racemic chiral analytes can also be purified using an achiral chromatography column and an achiral displacer. In this case, impurities, including diastereomers, are removed from the racemic compound of interest, but there is no chiral resolution of the enantiomers.
  • An anionic displacer may contain one or more chiral centers.
  • enantiomers are routinely resolved (separated) on a preparative scale.
  • a good, enantiomerically pure, chiral displacer can have performance advantages over a good achiral displacer when carrying out a displacement separation of enantiomers on a chiral stationary phase.
  • hydrophobic displacer molecules should possess a unique combination of chemical and physical properties in order for them to function efficiently. Some soluble, hydrophobic molecules can function as displacers, but only a limited few function well. Many of the molecules described in this document fulfill the necessary requirements for well-functioning displacers.
  • U.S. Pat. No. 6,239,262 describes various reversed phase liquid chromatographic systems using low molecular weight surface-active compounds as displacers.
  • U.S. Pat. No. 6,239,262 discloses an extremely wide range of possible charged moieties that may be coupled with hydrophobic moieties to form the disclosed surface active compounds used as displacers, but discloses that it is necessary to include a large proportion of organic solvent to mitigate the surface active properties of the disclosed displacers.
  • the present invention in one embodiment, relates to a process for separating organic compounds from a mixture by reverse-phase displacement chromatography, comprising:
  • non-surface active hydrophobic anionic displacer molecule comprises a hydrophobic anion and a counterion, Cl, having the general formula A or B:
  • each CM or CM′ is an independent hydrophobic chemical moiety with a negative formal charge selected from: carboxylate (XVI), N-acyl- ⁇ -amino acid (XVII), sulfonate (XVIII), sulfate monoester (XIX), phosphate monoester (XX), phosphate diester (XXI), phosphonate monoester (XXII), phosphonate (XXIII), tetraaryl borate (XXIV), boronate (XXV), boronate ester (XXVI); wherein the chemical moieties (XVI)-(XXVI) have the following chemical structures:
  • CM and CM′ are independent charged chemical moieties having the same or opposite formal charge and are chemically attached to each other by a doubly connected chemical moiety, R*, which replaces one R 1 , R 2 (if present), R 3 (if present) or R 4 (if present) chemical moiety on CM and replaces one R 1 , R 2 (if present), R 3′ (if present) or R 4′ (if present) chemical moiety on CM′ (herein, an R group on CM′ is designated with a prime (′), e.g., R 1′ is the R 1 group on CM′);
  • R 1 , R 1′ , R 2 , R 2′ , R 3 , R 3′ , R 4 and R 4′ is a linear or branched chemical moiety independently defined by the formula
  • R* is a direct chemical bond or is a doubly connected, linear or branched chemical moiety defined by the formula
  • each R 5 and R 5′ independently are a linear or branched chemical moiety defined by the formula
  • each AR 1 independently is a doubly connected methylene moiety (—CX 1 X 2 —, from methane), a doubly connected phenylene moiety (—C 6 G 4 -, from benzene), a doubly connected naphthylene moiety (—C 10 G 6 -, from naphthalene) or a doubly connected biphenylene moiety (—C 12 G 8 -, from biphenyl);
  • AR 2 independently is hydrogen (—H), fluorine (—F), a phenyl group (—C 6 G 5 ), a naphthyl group (—C 10 G 7 ) or a biphenyl group (—C 12 G 9 );
  • each X, X 1 and X 2 is individually and independently —H, —F, —Cl or —OH;
  • any methylene moiety (—CX 1 X 2 —) within any —C x X 2x-2r — or within any —C u X 2u-2s — or within any —(CX 1 X 2 ) p — may be individually and independently replaced with an independent ether-oxygen atom, —O—, an independent thioether-sulfur atom, —S—, or an independent ketone-carbonyl group, —C(O)—, in such a manner that each ether-oxygen atom, each thioether-sulfur atom or each ketone-carbonyl group is bonded on each side to an aliphatic carbon atom or an aromatic carbon atom;
  • not more than two ether-oxygen atoms, not more than two thioether-sulfur atoms and not more than two ketone-carbonyl groups may be replaced into any —C x X 2x-2r — or into any —C u X 2u-2s —;
  • m x is the total number of methylene groups in each —C x X 2x-2r — that are replaced with ether-oxygen atoms, thioether-sulfur atoms and ketone-carbonyl groups
  • m u is the total number of methylene groups in each —C u X 2u-2s — that are replaced with ether-oxygen atoms, thioether-sulfur atoms and ketone-carbonyl groups
  • G is individually and independently any combination of —H, —F, —Cl, —CH 3 , —OH, —OCH 3 , —N(CH 3 ) 2 , —CF 3 , —CO 2 Me, —CO 2 NH 2 ; —CO 2 NHMe, —CO 2 NMe 2 ;
  • G* is individually and independently any combination of —F, —Cl, —R 2 , —OH, —OR 2 , —NR 2 R 3 , —CF 3 , —CO 2 Me, —CO 2 NH 2 ; —CO 2 NHMe, —CO 2 NMe 2 ;
  • Integer values r and s are the total number of contained, isolated cis/trans olefinic (alkene) groups plus the total number of contained simple monocyclic structures and fall in the ranges 0 ⁇ r ⁇ 2 and 0 ⁇ a ⁇ 2, the numeric quantity x+u ⁇ m x ⁇ m u falls in the range 0 ⁇ x+u ⁇ m x ⁇ m u ⁇ 11;
  • a group-hydrophobic-index for each R-chemical-moiety (n) is numerically equal to the sum of the number of aliphatic carbon atoms plus the number of olefinic carbon atoms plus the number of thioether-sulfur atoms plus the number of chlorine atoms plus one-fifth the number of fluorine atoms plus one-half the number of ether-oxygen atoms plus one-half the number of ketone-carbon atoms plus one-half the number of aromatic carbon atoms beyond the number six minus the number of hydroxyl-oxygen atoms beyond the number one;
  • an overall-hydrophobic-index (N) for each [CM] or [CM-R*—CM′] is numerically equal to the sum of the number of aliphatic carbon atoms plus the number of olefinic carbon atoms plus the number of thioether-sulfur atoms plus the number of chlorine atoms plus one-fifth the number of fluorine atoms plus one-half the number of ether-oxygen atoms plus one-half the number of ketone-carbon atoms plus one-half the number of aromatic carbon atoms beyond the number six minus the number of hydroxyl-oxygen atoms beyond the number one;
  • group-hydrophobic-indices ( 1 n and 1′ n) for R 1 and R 1′ fall in the range 4.0 ⁇ 1 n, 1′ n ⁇ 12.0
  • group-hydrophobic-indices ( 2 n, 2′ n, 3 n, 3′ n, 5 n, 5′ n and *n) for R 2 , R 2′ , R 3 , R 3′ , R 5 , R 5′ , R* when present, fall in the range 0.0 ⁇ 2 n, 2′ n, 3 n, 3′ n 5 n, 5′ n, *n ⁇ 12.0
  • group-hydrophobic-Indices ( 4 n and 4′ n) for R 4 and R 4′ when present, fall in the range 0.0 ⁇ 4 n, 4′ n ⁇ 5.0;
  • numeric value of the group-hydrophobic-index calculated for a cyclic chemical moiety is divided equally between the two respective R-chemical-moleties;
  • R 1 or R 1′ is identified as that R-chemical-moiety when only one such chemical moiety is attached to CM or CM′; wherein R 1 or R 1′ is identified as that R-chemical-moiety having the largest value of the group-hydrophobic-index when there are more than one such R-chemical-moieties attached to CM or CM′; wherein R 4 or R 4′ is identified as that R-chemical-moiety having the smallest value of the group-hydrophobic-index when there are more than three such chemical moieties attached to CM or CM′; and
  • Cl is a non-interfering, oppositely-charged counter-ion or mixture of such counter-ions, and the value of d is zero, a positive whole number or a positive fraction such that electroneutrality of the overall hydrophobic compound is maintained.
  • the aqueous composition comprising a non-surface active hydrophobic anionic displacer molecule is free of added salt other than a pH buffer.
  • CM has a general formula XXIV or XXV:
  • R 1 is phenyl, 4-EtC 6 H 4 —, 4- n PrC 6 H 4 —, 4- n BuCeH 4 —, 4-MeOC 6 H 4 —, 4-FC 6 H 4 —, 4-MeC 6 H 4 —, 4-MeOCsH 4 —, 4-EtC 6 H 4 —, 4-ClC 6 H 4 —, or C 6 F 5 —; and each of R 2 , R 3 and R 4 independently are phenyl, 4-FC 6 H 4 —, 4-MeCeH 4 —, 4-MeOC 6 H 4 —, 4-EtOH 4 —, 4-ClCeH 4 — or CeF 5 —; and wherein in the general formula XXV, R 1 is 4-(4- n BuCeH 4 )CsH 4 — or 4-(4- n BuCeH 4 )-3-ClC 6 H 3 —.
  • CM has a general formula XVIII or XXIII:
  • Cl is a non-interfering inorganic cation or mixture of such non-interfering cations selected from the groups: alkali metal ions (Li + , Na + , K + , Rb + , Cs + ), alkaline earth metal ions (Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ ), divalent transition metal ions (Mn 2+ , Zn 2+ ) and NH 4 + ; wherein Cl is a non-interfering organic cation or mixture of such non-interfering cations selected from the groups: protonated primary amines (1+), protonated secondary amines (1+), protonated tertiary amines (1+), protonated diamines (2+), quaternary ammonium ions (1+), sulfonium ions (1+), sulfoxonium ions (1+), phosphonium ions (1+), bis-quaternary ammonium ions (2+) that may
  • FIG. 1A is a displacement trace for the purification of a crude synthetic peptide plotting time (x-axis) against relative absorbance units (y-axis) for the displacement chromatography process in accordance with an exemplary embodiment of the present invention.
  • FIG. 1B is a fraction analysis of the displacement data displayed in FIG. 1A plotting fraction number (x-axis) against concentration ( ⁇ g/mL) of each component in each fraction for the displacement chromatography process in accordance with an exemplary embodiment of the present invention.
  • FIG. 2A is a displacement trace for the purification of a crude synthetic oligonucleotide plotting time (x-axis) against relative absorbance units (y-axis) for the displacement chromatography process in accordance with an exemplary embodiment of the present invention.
  • non-surface-active with respect to a neutral non-surface-active anionic displacer compound employed in accordance with the present invention, means that the compound so described has a critical micelle concentration (“CMC”) greater than the concentration of the compound employed in a displacement chromatography process in accordance with the present invention.
  • CMC critical micelle concentration
  • the concentration of the non-surface-active displacer compound is less than about 80% of the CMC for that compound in water in the absence of organic solvent, salt or other agent that would affect the CMC.
  • the concentration of the non-surface-active displacer compound is less than about 60% of the CMC for that compound in water in the absence of organic solvent, salt or other agent that would affect the CMC.
  • the concentration of the non-surface-active displacer compound is less than about 50% of the CMC for that compound in water in the absence of organic solvent, salt or other agent that would affect the CMC.
  • the aqueous composition comprising a non-surface-active hydrophobic anionic displacer molecule employed in accordance with the present invention does not exhibit adverse surface-active characteristics due to one or a combination of two or more of (1) the anionic non-surface-active displacer compound is present at a concentration lower than its CMC; (2) the overall-hydrophobic-index (N) for each [CM] or [CM-R*—CM′] divided by the value of g falls in the range 10 ⁇ N/g ⁇ 24; (3) the group-hydrophobic-index ( 1 n) for each R 1 or R 1′ falls in the range 4 ⁇ 1 n ⁇ 12, the group-hydrophobic-index ( 2 n, 3 n, 5 n and *n) for each R 2 , R 2′ , R 3 , R 3′ , R 5 , R 5′ and R*, when present, falls in the range 0 ⁇ 2 n, 3 n, 5 n, *n ⁇ 12, and the group-hydrophobic
  • low organic solvent content generally refers to an organic solvent content in, e.g., an aqueous “carrier” composition comprising a non-surface-active anionic displacer compound in accordance with the present invention, of less than about 25% by volume.
  • the organic solvent content of the aqueous “carrier” composition contains less than about 20% by volume of any organic solvent.
  • the organic solvent content of the aqueous “carrier” composition contains less than about 15% by volume of any organic solvent.
  • the organic solvent content of the aqueous “carrier” composition contains less than about 10% by volume of any organic solvent.
  • the organic solvent content of the aqueous “carrier” composition contains less than about 5% by volume of any organic solvent.
  • the aqueous “carrier” composition contains no organic solvent.
  • the organic solvent is one or a mixture of two or more of methanol (CH 3 OH or MeOH), ethanol (C 2 H 5 OH or EtOH) or acetonitrile (CH 3 CN or MeCN).
  • the aqueous “carrier” composition contains a mixture of suitable organic solvents. In one embodiment, the aqueous “carrier” composition contains no organic solvent.
  • Hydrophobic displacement chromatography can be carried out using chiral analytes, chiral displacers and chiral chromatography matrices. Under these conditions, an achiral displacer may be used, but a racemic mixture of a chiral displacer cannot be used. Racemic chiral analytes can also be purified using an achiral chromatography column and an achiral displacer. In this case, impurities, including diastereomers, are removed from the racemic compound of interest, but there is no chiral resolution of the enantiomers.
  • An anionic displacer may contain one or more chiral centers; see for example, see chiral displacer compounds 689 and 698 (see Tables V-XIII below).
  • Displacers 613 and 639 represent displacers compounds that are achiral. With the proper choice of chiral chromatography matrix, mobile phase and achiral displacer, enantiomers are routinely resolved (separated) on a preparative scale. Depending on the specific circumstances, a good, enantiomerically pure, chiral displacer can have performance advantages over a good achiral displacer when carrying out a displacement separation of enantiomers on a chiral stationary phase
  • anionic hydrophobic displacers having the general formula A or B, have different useful pH ranges depending on the chemical nature of the charged moieties.
  • Anionic hydrophobic displacers that contain protonatable anionic groups should be operated at a pH of 1-2 units or more above the actual pKa values.
  • Tetraarylborates generally, these molecules are limited to the neutral pH range (6-8) owing to problems with hydrolytic instability outside that range. Some tetraarylborates have solubility limitations and can be used only with a narrow range of cationic ion-pairing agents.
  • Arylboronates For most displacer applications, the pKa values of ordinary arylboronic acids (pKa ⁇ 8.9) are too high to be useful in the neutral pH range. Even with electron-withdrawing groups such as meta —NO 2 (pKa ⁇ 7.4) or meta —Cl (pKa ⁇ 8.1), the pKa values are still high. However, the formation of boronate esters by the addition of certain polyols leads to compositions that have lower pKa values, by 2.0-2.5 pKa units in certain cases. The chemical nature of the polyol molecule also affects the hydrophobicity and solubility of the resulting boronate ester displacer.
  • Useful polyols include fructose, ribose, sorbitol, mannitol, cis-1,2-dihydroxycyclopentane and cis-3,4-dihydroxytetrahydrofuran which lead to displacers that are useful in the pH range 7.1-10.0.
  • a typical polyol concentration (mM) is about twice the concentration of the displacer or 25 mM greater than the concentration of the displacer, whichever is greater.
  • the displacer should bind to the column more strongly than all of the components of the sample or at least more strongly than all of the major components of interest.
  • a good rule of thumb is that no more than 1-4% of the sample mass should bind more strongly than the displacer.
  • An optimal displacer should bind to the stationary phase neither too strongly nor too weakly.
  • the proper binding strength depends on the analyte of interest and the associated binding isotherms. Usually, a range of displacers with a range of binding strengths is needed for a variety of different columns and analytes to be purified. If a displacer binds too strongly, poor performance is obtained such as lower resolution, lower analyte binding capacity, difficulty in displacer removal and longer cycle times. If a displacer binds too weakly, a poor displacement train may result with too much “tailing” of the displaced analytes underneath the displacer, or there may be only partial displacement or no displacement at all.
  • a convenient, rule of thumb method that helps in choosing displacers with the proper binding strength is to carry out simple gradient elution chromatography of potential displacers and analytes using columns and mobile phases similar to those that are to be used in the displacement experiment.
  • the displacer should elute 5-15 minutes later than the analytes of interest in a 60 minute gradient.
  • anionic displacer molecules supplemented with the proper counter ions and, when needed, small amounts of useful organic solvents provide a family of effective hydrophobic displacers with Langmuir-type binding behavior and useful ranges of binding strengths.
  • anionic hydrophobic displacer molecules have one extra requirement: an appropriate ion-pairing cation, Cl.
  • the cation significantly affects the binding isotherm of the displacer and the functioning and utility of the displacer.
  • the concentration of the cation is independently adjusted by adding appropriate amounts of Cl ⁇ /HCO 2 ⁇ salts of an ion-pairing cation.
  • the properties of an ion-pairing cation for a charged hydrophobic displacer strongly affects its displacement properties. A few cations are involved in ion-pairing in solution, and all cations are involved in ion-pairing in the adsorbed state on the hydrophobic matrix.
  • the same ion-pairing cation for displacer and analyte should be used for good chromatographic resolution.
  • Useful ion-pairing cations are usually singly charged. Owing to their higher solvation energies, divalent ions (Ca 2+ ) and trivalent ions (La 3+ ) are generally less useful but may be used in some specialized cases. Exceptions to the general rule are multiple, singly-charged moieties spaced apart in a single organic ion such as and Me 3 N + (CH 2 ) 4 N + Me 3 . These ions can also be useful as counter-ions for di-anionic displacers.
  • Ion-pairing cations with greater hydrophobic character tend to increase binding strength and also decrease solubility. Furthermore, when using hydrophobic displacers, resolution of DC may decrease if the cation itself is either too hydrophobic or too hydrophilic. Typically, intermediate hydrophobic/hydrophilic character of the cation gives best results, but this varies depending on the molecule being purified. The optimal ion-pairing cation for each purification should be determined experimentally. Volatile ion-pairing agents are conveniently removed under reduced pressure, while nonvolatile ones are readily removed by other means: diafiltration, precipitation or crystallization. Table I gives a partial list of useful monovalent ion-pairing cations.
  • Mixed ion-pairing cations often lead to loss of chromatographic resolution and are generally to be avoided.
  • hydrophobic chromatography or, more properly, “solvophobic chromatography”, where the principal solvent component is water, potential hydrophobic displacer molecules often have limited solubility. Hydrophobic molecules usually do not dissolve in water to any appreciable extent unless there are “hydrophillic groups” attached to the hydrophobic molecule, such as charged ionic groups, hydrophillic counter-ions, polar groups or groups that function as hydrogen bond donors or acceptors. Aromatic molecules interact with water in a unique fashion owing to the unique manner in which the pi-electrons act as weak hydrogen bond acceptors. Furthermore, aromatic molecules can engage in face-to-face pi-stacking in aqueous solution.
  • hydrophobic displacement chromatography One potential problem with hydrophobic displacement chromatography is the possible association of a hydrophobic displacer with a hydrophobic analyte in solution. This can lead to significant loss of resolution and contamination. Displacer-analyte association in the adsorbed state on the stationary phase also can occur but is less problematic with proper amounts of suitable ion-pairing agents present. A good method to deal with this problem is to use charged analytes and charged hydrophobic displacers with the same charge. These problems arise more frequently sometimes when neutral hydrophobic displacers and neutral hydrophobic analytes interact.
  • anionic hydrophobic molecules can self-associate, forming micelles and micelle-like, self-associated structures in solution. This situation can lead to loss of resolution in DC as well as unwanted foaming of displacer solutions.
  • the displacer in solution finds itself in various forms that are interrelated by various chemical equilibria.
  • micelles can act as carriers for hydrophobic analyte molecules causing them to exist in solution in various forms. This unwanted phenomenon is concentration dependent and is effectively inhibited by the addition of small amounts of a suitable organic solvent such as methanol, ethanol or acetonitrile.
  • a suitable organic solvent such as methanol, ethanol or acetonitrile.
  • anionic displacer molecules inhibit micelle formation and give better displacement results.
  • keeping the group hydrophobic indices below 12.0 for R-groups, R 1 -R 3 reduces the problem of unwanted detergency.
  • a displacer should have adequate purity.
  • the object of preparative chromatography is to remove the impurities from a component of interest. Contamination of the desired compound by the displacer itself is rarely a problem, but contamination by “early displacing” impurities in the displacer solution may be problematic in some cases depending on the amounts of the impurities and their binding properties. Thus, a good displacer should contain little or no early displacing impurities.
  • displacer In order to track the location and amounts of displacer throughout the DC experiment, to watch displacer breakthrough curves and to follow displacer removal during column regeneration procedures, it is useful to have a displacer with moderate ultraviolet absorption. High absorption is not needed nor is it preferred owing to the high concentrations of displacer and analyte. Generally, colorless displacers are preferred with a UV spectrum that has strategically located windows of low absorbance so that the analytes can be followed at some frequencies and the displacer monitored at other frequencies.
  • a useful displacer molecule should be chemically stable. It should be inert toward analyte molecules and chemically stable (non-reactive) toward water, common organic solvents, mild bases, mild acids and oxygen (air). It should be photo-stable and thermally stable under typical use and storage conditions and have a reasonable shelf life. It is greatly preferred that displacer molecules be visually colorless, yet have the requisite levels of UV absorbance. Dyes generally do not make useful displacers in DC. Useful displacer molecules also need to have low toxicity, not only to protect workers but to protect biological and drug samples that may come into contact with the displacer.
  • Porous particles with pore sizes of 80-100 ⁇ are suitable for traditional drugs and small peptides, pore sizes 120-150 ⁇ are suitable for medium and large oligopeptides and oligonucleotides and pore sizes 300-500 ⁇ are suitable for most proteins and DNA.
  • Non-porous particles can be used, but loading capacity will significantly decrease.
  • Initial evaluation is carried out using a good general purpose anionic displacer as the trimethylammonium salt with proper binding strength.
  • anionic analyte molecules require an anionic displacer.
  • anionic displacers can be used to purify anionic and neutral analytes. The displacer should bind to the column more strongly than the material to be purified, but the displacer should not bind too strongly.
  • Typical displacer concentrations are in the range 10-50 mM. Initially, displacer concentration is set at 10-15 mM. As needed, pH buffer and ion-pairing agent are added to the displacer solution.
  • the displacer solution and carrier solution should have identical compositions (including pH), except for the presence of displacer and the level of the ion-pairing salt.
  • Displacers 579, 609 and 634 are examples of good general-purpose anionic displacers. During method optimization, it may be helpful to increase displacer concentration up to 20-30 mM or higher.
  • Table I contains a list of useful, monovalent, ion-pairing cations (Cl) that are useful for hydrophobic chromatography. They are needed when the displacer or analyte is charged. For anionic analytes and displacers, binding isotherms strongly depend on the chemical properties of the counter-ion and its concentration. Those ion-pairing cations with moderate to moderately strong binding properties are usually the best to use. Start with trimethylammonium formate or phosphate during initial experiments. When the analyte requires an on-pairing agent. It usually dictates the choice of ion-pairing agent during the DC experiment. The ion-pairing agent for the analyte and the displacer should be the same.
  • E s is the excess factor for the sample
  • C s is the concentration of the sample (mM)
  • G s is the absolute value of the net charge of the sample at the operative pH.
  • the optimal value of E s is a parameter that needs to be determined experimentally.
  • the formula for calculating the suitable concentration of the ion-pairing agent in the displacer solution (C IPD , mM) is given by,
  • E d is the excess factor for the displacer
  • C d is the concentration of the displacer (mM)
  • G d is the absolute value of the net charge of the displacer at the operative pH.
  • the optimal value of E d is a parameter that needs to be determined experimentally. It is essential that at least a stoichiometric amount of the ion-pairing agent be present in the solutions (E s ⁇ 1.0 and E d ⁇ 1.0). In practice, it is our experience that E s should be in the range 1.1-10.0, more preferably in the range 1.2-6.0, more preferably yet in the range 1.5-4.5. Furthermore, it is our experience that E d should be in the range 1.1-10.0, more preferably in the range 1.2-4.0. Serious deterioration in chromatographic performance results when the ion-pairing concentrations are unoptimized or too low, that is E s ⁇ 1.0 and/or E d ⁇ 1.0.
  • Organic solvent content is an important parameter that needs to be optimized for each sample, column and displacer.
  • organic solvent should be less than about 15 volume %, more preferably less than about 10 volume %, more preferably yet about 5 volume %.
  • Octadecyl columns 2-3 volume % acetonitrile, 3-4 volume % ethanol or 4-5 volume % methanol is usually needed for optimal functioning of the matrix. Phenylhexyl and Octyl columns can usually tolerate the absence of organic solvent.
  • pH buffers are needed when there are ionizable protons in the sample, displacer, ion-pairing agent or on the stationary phase. Some analytes are only stable within certain pH ranges. For some analytes, chromatographic resolution is strongly pH-dependent. Anionic samples including anionic proteins (pH>pI) are purified using anionic displacers and anionic pH buffers. The cations associated with the anionic pH buffers should be the same as the ion-pairing cation but may be, in some cases, a cation that has significantly weaker ion-pairing properties.
  • N-methylmorpholine (7.4, NMM), triethanolamine (7.8, TEOA) and TRIS (8.1) can sometimes be used as pH buffers when trimethylammonium, triethylammonium or n-butylammonium is the ion-pairing cation.
  • Anionic compounds with mid-range pK a values can be useful pH-buffers: Carbonate (9.9), Borate (9.0), TABS (8.9), TAPS (8.4), TAPSO (7.6), methylphosphonic acid (7.6, MPA), MOPS (7.2), MOPSO (6.9), phosphoric acid (6.8), monomethylphosphoric acid (6.3), phosphorous acid (6.3), MES (6.2), 3,3-dimethylglutaric acid (5.9, DMG), succinic acid (5.2, SUC), acetic acid (4.6, HOAc).
  • a second “orthogonal” IP-RP DC step typically gives excellent purity ( ⁇ 99.5%) with excellent yield (90-95%).
  • DC experiments are carried out at relatively high loading, typically in the range 60-80% of maximum loading capacity.
  • the operative column loading capacity is not a fixed number; rather, it depends upon where on the binding isotherm the DC experiment operates.
  • the displacer buffer is then pumped onto the column.
  • the first front travels faster than the second and third fronts and limits the useable column capacity because the first front should exit the column before the displacement train (T 2 ) begins to exit.
  • the actual velocities of the fronts depend directly on the displacement flow rate.
  • the ratio, ⁇ , of the front velocities, Vel 1 /Vel 2 is given by the formula,
  • K m is the displacer binding capacity of the matrix (mg displacer per mL packed matrix) at displacer concentration of C d
  • C d is the displacer concentration in the displacer buffer (mg displacer per mL displacer buffer)
  • R is the ratio of the volume of the liquid in the column to the total volume of the column (mL liquid per mL bed volume).
  • Example 2 the respective ⁇ -value is 21.98, and the respective maximum usable capacity is 95.4%. Note that as C d increases, K m will also increase, but not as much if operating high on the nonlinear part of the isotherm. Thus, ⁇ will decrease and maximum % usable column capacity will decrease.
  • the column loading could be 105% of maximum based on the whole sample, but the column loading would be only 80% based on the amount of main product plus late-displacing impurities.
  • a sample is loaded onto the column through a sample injection valve using one of two methods.
  • the sample should be loaded under frontal chromatography conditions at the same point on the binding isotherm at which the DC experiment takes place.
  • the carrier is not passed through the column after the sample is loaded.
  • Method 1 A sample loading pump is used;
  • Method 2 An injection loop is used. Usually, only partial loop injection is used.
  • the sample in the loop should be driven out of the loop onto the column first by the carrier and then the displacer solution. Not more that 85-95% of the loop volume should be loaded onto the column so that sample diluted by carrier is not loaded.
  • the concentration of the load sample is an important operating parameter.
  • the optimal sample loading concentration (mg/mL) is the same as the output concentration of the purified product from the displacement experiment—the plateau region of the displacement train. Binding-isotherms, the column binding capacities and the output concentrations are initially unknown. Simply carry out the first displacement experiment with the sample solution loaded onto the column using initial estimates as shown below:
  • the loading sample solution is prepared at the concentration and amount described above. Enough excess solution is needed for overfilling the loop or filling the dead volume of a sample loading pump and delivery lines.
  • the pH, amount of pH buffer and amount of organic solvent are the same as the carier and displacer buffer. Dissolving the sample in the carrier changes its pH, so the pH of the sample solution will have to be re-adjusted after dissolution.
  • the amount of ion-pairing agent may be different.
  • the ion-pairing agent used in the sample solution must be the same one used in the displacer buffer. In this regard, the ion-pairing requirements of the sample dictate which ion-pairing agent is used in the sample solution and in the displacer solution.
  • the concentration of the concentration is the ion-pairing agent or ion-pairing salt is calculated. See “Concentration of ion-Pairing Agent” above.
  • composition and history of the sample should be known. If the sample contains a cation, its chemical nature and amount (concentration) should also be known. (a) Obviously, if no cation is present, then no adjustment is made in sample preparation. (b) If the cation in the sample is the same as the ion-pairing agent used in the DC, then the amount of added ion-pairing agent to the sample solution is reduced accordingly. (c) If the cation in the sample has significantly weaker ion-pairing properties than the ion-pairing cation used in the DC, then its presence is ignored. (d) If the cation in the sample has stronger ion-pairing properties than the ion-pairing cation used in the DC, then the cation should be exchanged or removed before proceeding.
  • Displacement chromatography gives excellent chromatographic resolution, especially with optimized protocols using a good C 18 -reversed-phase column.
  • the resolution is difficult to see because all of the bands come off the column together as back-to-back bands in the displacement train.
  • Many of the small impurity triangle-bands are less than 30 seconds wide ( ⁇ 100 ⁇ L).
  • an experiment with a displacer breakthrough time of 250 minutes and 80% sample loading the displacement train would be about 200 minutes wide, and more that 400 fractions would have to be taken so that chromatographic resolution is not lost during the fraction-collection process.
  • Analyzing 400 fractions is truly enlightening and interesting but also a daunting task. This is when online real-time fraction analysis would be useful. In practice, one may somewhat disregard resolution and collect only 100-130 larger fractions. Analysis of even this reduced number of fractions represents a substantial amount of work.
  • Example 3 analysis is even easier. Based on the method above, the beginning and ending of the main band of Interest is judged, a conservative pooling is made without any analysis and only one analysis is carried out on the final pool.
  • the displacer is removed using 5-10 column volumes of 95/5 (v/v) ethanol-water or 80/10/10 (v/v/v) acetonitrile- n propenol-water without any pH buffer or ion-pairing agent.
  • the object is to efficiently remove >99.9% or more of the displacer from the column in the shortest amount of time.
  • the flow rate is increased (100-400 cm/hr) in order to speed up the column regeneration process if the matrix will tolerate the increased back-pressure. Observing the displacer removal near the absorption maximum of the displacer (see displacer instructions) allows the regeneration process to be carefully monitored and optimized by UV detection.
  • Salts in aqueous solvents lead to solvents that are less hospitable to dissolved hydrophobic analytes and hydrophobic displacers resulting in stronger binding to hydrophobic chromatographic matrices.
  • This is the principle behind hydrophobic-interaction chromatography (HIC). So long as solubility of the analyte is sufficient in the salt solution, the addition of salt is a good way to modulate analyte binding and selectivity to a hydrophobic matrix.
  • HIC hydrophobic-interaction chromatography
  • analyte binding to a hydrophobic matrix is so weak that added salt is needed in order to obtain sufficient analyte binding.
  • Commonly used salt solutions include one of 0.5-2.5M (NH 4 ) 2 SO 4 , K 2 SO 4 , Na 2 SO 4 , NaCl and KCl. With the help of many different salts at various concentrations, HIC in displacement mode offers many options for useful chromatographic separations of proteins.
  • the protocol has line purging operations, a quick column regeneration and equilibration operations in order to make sure that the HPLC system and column are completely clean and properly equilibrated just before sample loading. These steps are simply precautionary and not always necessary.
  • the protocol includes the (a) a pre-equilibration sequence, (b) an equilibration sequence, (c) a sample loading sequence (d) a displacement sequence and (e) a regeneration sequence in a single protocol. In order to overcome problems with dead-volume in the system, all loading buffers, displacer buffers and sample solutions are purged through the system to waste just prior to pumping onto the column.
  • the sample solutions should be degassed so that gas bubbles do not form in them.
  • injection loops When injection loops are used, they need to be overfilled by about 10%. The overfill can be collected for further use.
  • Full loop injections should not be used, only partial loop injections. Experience dictates that only 85-95% of the loop volume can be used depending on the inner diameter of the loop tubing because the sample solution mixes with the driver solution and dilutes it.
  • the sample in the loop is driven onto the column by the loading buffer, but toward the end of the sample loading process, the driving solution is changed to the displacer buffer. This allows the displacer buffer to be purged through the system just prior to the displacer buffer itself being pumped directly onto the column. During the initial part of the regeneration process, slower flow rates are used Thus, problems with high backpressure rarely occur. Once most of the displacer has been removed, higher flow rates can be used.
  • sample protocol (Example 2) is shown below that has been optimized for purity without regard to time. It is important to carry out method optimization adapted for the specific physical properties and chromatographic properties of the sample of interest. Upon optimization, longer process times (600-800 min) often can be reduced to 200-300 minutes and in some cases reduced to 100-150 minutes.
  • Hydrophobic chromatography used in displacement mode has (a) high matrix productivity (gram of product per liter matrix over the lifetime of the matrix), (b) high volume productivity (gram of product per liter of column volume), (c) high solvent productivity (gram of product per liter of solvent used) yet (d) may have mediocre time productivity (gram of product per unit of time). Proper method optimization mitigates the time factor.
  • Ion-Pairing Agent Trimethylammonium (Me 3 NH + );
  • Fraction Size 520 ⁇ L; 5 mL formic acid added to each fraction, pH reduced to 3.5; samples immediately frozen ( ⁇ 20° C.) until analysis or pooling.
  • Amount of Me 3 NH + in sample 4.7 times stoichiometric amount. Good results are obtained with reasonable loading (42.9 g/L), excellent purity and excellent yield (99.3% purity 90% yield) using a small “analytical-type” column in one step from crude peptide.
  • This example is designed to show the purification of a crude synthetic anionic peptide at neutral pH (7.1) using an anionic displacer with a cationic ion-pairing agent. Rather than using a primary or secondary amine, a protonated tertiary amine was chosen as IP agent in order to prevent any possibility of trans-amidation reactions.
  • UV photodiode array detector after column flow cell: 0.5 mm pathlength, 9 ⁇ L volume
  • conductivity detector flow cell: 170 ⁇ L volume
  • flow cell is removed when fractions are being collected for analysis.
  • cleaned column is briefly purged with A-buffer to remove column storage buffer. See Example 3 for description of other details.
  • C-Buffer 10% (v/v) 1-propanol, 10% (v/v) DI water in acetonitrile.
  • Ion-Pairing Agent n Butylammonium ( n BuNH 3 + )
  • Fraction Analysis Fraction analysis is not carried out. Purified fractions are conservatively pooled based on the shape of the displacement trace.
  • Sample Injection 25 ⁇ L of ⁇ 1 mM sample solution in A buffer UV detection: 208-220 nm depending on compounds to be analyzed Flow Rate: 1.0 mL/min.
  • a buffer 5% CH 3 OH (v/v) in HPLC-grade distilled water with 15 mM phosphoric acid (HPLC-grade) and 25 mM trimethylamine (HPLC-grade); pH of buffer before methanol addition is 7.10 +/ ⁇ 0.05.
  • B buffer 5% H 2 O (v/v) in HPLC-grade CH 3 OH with 15 mM phosphoric acid (HPLC grade) and 25 mM trimethylamine (HPLC-grade).
  • B buffer 5% H 2 O (v/v) in HPLC-grade CH 3 OH with 15 mM methylphosphonic acid (HPLC-grade), 25 mM trimethylamine (HPLC-grade) and 35 mM 35 mM cis-3,4-dihydroxytetrahydrofuran.
  • Survey Gradient Method 100% A 0-2 min 100% A to 100% B 2-62 min 100% B 62-70 min
  • the gaseous HCl is removed by using a trap containing KOH pellets prior to using a low temperature trap to retain the solvent and other volatiles. Finally, the product is placed under high vacuum (0.01 torr) for 24 hours at ambient temperature in order to complete the removal of volatiles.
  • This preparation of 4- n hexylbenzenesulfonic acid (free acid) is also used in subsequent preparations (product a).
  • the product is dissolved in a minimum amount of dry n-pentane ( ⁇ 1.5 L) and then used as a pentane solution (product b); the concentration of the sulfonic acid in pentane is estimated by HPLC.
  • Hexylbenzenesulfonic acid (free acid) is a unique molecule being appreciably soluble in virtually all solvents: n-pentane, cyclopentane, methylene chloride, 1,2-dichloroethane, diethyl ether, tbutyl methyl ether, acetonitrile, toluene, dioxane, isopropanol, absolute ethanol, methanol and water.
  • the HCl addition is terminated after 18-20 g HCl is added.
  • the stirring mixture is carefully warmed to about 50° C. and maintained at that temperature for about 6 hours.
  • the solvent and excess HCl are removed under vacuum (rotary evaporator).
  • 300 mL 2-propanol is added to the mixture and again the solvent is removed under vacuum.
  • the residue is taken up in 150 mL diethyl ether, the solution is filtered and the solvent and other volatiles are removed under vacuum using a rotary evaporator.
  • the product is placed under high vacuum (0.01 torr) for 24 hours at ambient temperature in order to complete the removal of all volatiles.
  • the crystals are washed with cold, anhydrous solvent: dichloroethane/pentane (50/50) and then n-pentane. Residual pentane is removed under vacuum. When warmed to room temperature, solid monohydrates melt and then resolidify again when cooled to ⁇ 20° C. In contrast, the monohydrates of the intermediate alkylsulfonic acids ( n pentyl, n hexyl, n heptyl) do not crystallize from dichloroethane at ⁇ 20° C.
  • the solid is recovered by filtration through sintered glass and washed twice with absolute ethanol and washed twice again using acetonitrile.
  • the solid product is dried by sucking dry air through it overnight. Recrystallization is accomplished by dissolving the product in 4 ⁇ its weight of hot ethanol-water (50/50 w/w) followed by quick filtering through sintered glass into a clean flask. If needed, small amounts of absolute ethanol are added to the filtered solution in order to make up for that which lost during vacuum filtration.
  • the filtered solution is heated to about 65-70° C., and then an equal amount (4 ⁇ ) of warm (65-70° C.) absolute ethanol is rapidly added with stirring.
  • the warm mixture is allowed to stand at room temperature for about 4 hours during which time crystals (white platelets) of the desired product begin to form.
  • the crystallization mixture is then allowed to stand at ⁇ 20° C. for about 4 hours to complete the crystallization.
  • the cold mixture is rapidly vacuum-filtered using a cold fritted glass filter, and then the mixture is allowed to warm up to ambient temperature.
  • the product is washed once using small amounts of acetonitrile and then dried by passing dry air through the filter. This procedure produces about 82 g (57% yield based on amount of reacted n hexylbenzene) of a white crystalline product with a HPLC purity of >99.8% (HPLC).
  • n pentyl, n heptyl, n octyl, n nonyl, n decyl and n undecyl are similarly prepared: n pentyl, n heptyl, n octyl, n nonyl, n decyl and n undecyl.
  • alkylammonium salts of alkylbenzenesulfonic acids may be similarly prepared. All are soluble in water and simple alcohols. Some are insoluble in diethyl ether and n-pentane (such as trimethylammonium and tetramethylammonium salts) while others are soluble in diethyl ether yet insoluble in n-pentane (such as n butylammonium and n hexylammonium salts).
  • the white solid is dissolved 2-propanol at ambient temperature, filtered and then recrystallized from 2-propanoVdiethyl ether according to Example 20, Method A above. This procedure yields about 84 g (84% yield) of a white, crystalline product with HPLC purity of >99%.
  • the solvent is removed from the filtrate under vacuum (rotary evaporator), 300 mL additional 2-propanol is added and then also removed under vacuum. More 2-propenol is added so that the total volume of the mixture is about 270 mL, and then 730 mL peroxide-free, inhibitor-free diethyl ether is added and mixed. This mixture is cooled to ⁇ 20° C. overnight in order to complete crystallization of the product.
  • the cold mixture is quickly filtered through sintered glass, washed with diethyl ether, dried by passing dry nitrogen through the filter-cake, further dried in a vacuum oven (50° C., 25 torr) for about 6 hours and stored in a desiccator over P 2 O 5 .
  • the white solid is stored in a sealed container that is kept in a desiccator over P 2 O 5 .
  • This procedure produces about 128 g (79% yield based on amount of reacted propylbiphenyl) of a hygroscopic, white crystalline solid that is 98-99% pure by HPLC.
  • the monohydrates of alkylbiphenyl sulfonic acids crystallize nicely as white solids from dichloroethane at room temperature.
  • the mixture is filtered using sintered glass under a dry N 2 atmosphere, washed with twice with diethyl ether and dried by passing dry N 2 through the filter cake.
  • the product is recrystallized a second time from warm absolute ethanol and washed/dried as before. Finally the product is dried in a vacuum oven (45° C., 25 torr) for about 6 hours.
  • the white solid is stored in a sealed container that is kept in a desiccator over P 2 O 5 . This procedure produces about 65 g (92% yield) of a white crystalline solid that is >99.5% pure by HPLC. Na + , NH 4 + and other alkylammonium salts are similarly prepared.
  • the aqueous sulfite solution is added all at once to the reaction flask, and the reaction mixture is heated at 76° C. for about 20 hours with vigorous mechanical stirring.
  • the reaction mixture is allowed to stand overnight at 15-20° C. in order to allow the mixture to fully separate into two layers.
  • the layers were separated, and the upper layer (product plus acetonitrile) is dried with anhydrous magnesium sulfate.
  • the solvent is then removed under vacuum, and a white crystalline product forms.
  • the mixture is filtered using a sintered glass filter, washed several times with peroxide-free diethyl ether and dried on the filter by passing dry N 2 through the filter cake.
  • the product is further dried in a vacuum oven (50° C., 15 torr) overnight. This procedure yields about 23.9 g (86% yield) of a white crystalline solid that is >99% pure by HPLC.
  • Two more additions of NaOH solution followed by octanoyl chloride with stirring are carried out: 8.96 g 50% NaOH, 9.16 g octanoyl chloride, stirring for 1 hour at 10-15° C., 7.52 g 50% NaOH, 7.59 g octanoyl chloride, stirring for three hours at room temperature.
  • the mixture is filtered through sintered glass, the filtrate is cooled to 10-15° C., and conc.
  • aqueous HCl (reagent, 37%) is added dropwise until the pH is about 1 (pH paper).
  • a clear oily layer or sticky solid forms as the crude product comes out of solution.
  • 250 mL peroxide-free diethyl ether is added with stirring in order to take up the product in ether.
  • the layers are separated, the upper ether layer is dried over anhydrous magnesium sulfate which is subsequently removed by filtration, and the ether is removed under vacuum leaving behind a colorless oil or sticky white crystalline mass.
  • This material is taken up in a minimum amount of slightly warm (30° C.) diethyl ether, and then allowed to crystallize at ⁇ 20° C. overnight.
  • the cold mixture is quickly filtered using sintered glass, then washed with n-pentane and finally dried under vacuum (0.01 torr) at ambient temperature to yield a pure white crystalline solid ( ⁇ 42 g, 81% yield).
  • a dry, filtered diethyl ether solution of L-N-octanoyl-2-phenylglycine, free acid (5 mL ether per g) is placed in a suitable Erlenmeyer flask and magnetically stirred at ambient temperature.
  • Gaseous trimethylamine (>99%) is slowly bubbled into the stirring ether solution while maintaining the temperature below 30° C. Immediately, white crystals of the trimethylammonium salt crystallize from solution.
  • the supernatant liquid is sufficiently basic (pH>8, damp pH paper), the addition of the amine is terminated, and the mixture is allowed to stand at 5° C. for two hours.
  • the cold mixture is quickly filtered through sintered glass, washed with ether and then dried under vacuum producing a near quantitative yield (>95%) of the trimethylammonium salt.
  • the product is recrystallized once from absolute ethanol/diethyl ether yielding an HPLC purity of >99.5%. No octanoic acid or its salt is detected.
  • the white solid is stored in a sealed container that is kept at room temperature in a desiccator over P 2 O 5 .
  • the warm solution is briefly heated to 70° C. and then allowed to cool to room temperature for about 6 hours in order to begin crystallization. Then 110 mL peroxide-free, inhibitor-free diethyl ether is added, the mixture is stirred and then allowed to stand overnight at ⁇ 20° C.
  • the di-ammonium salt is filtered using sintered glass, washed twice with diethyl ether, washed once with HPLC-grade n-pentane and dried by sucking dry nitrogen through the filter cake. The recrystallization process is repeated a second time by dissolving the white crystalline product in an appropriate of hot denatured ethanol.
  • the product is further dried in a vacuum oven (45° C., 25 torr) for about 6 hours and then stored in a desiccator over P 2 O 5 .
  • This procedure yields about 43 g (88% yield) of a stable, solvent-free, mildly hygroscopic, white crystalline salt that is pure enough to add to HPLC buffers in the range of 10-100 mM.
  • HPLC-grade amines and acids for use as HPLC buffers are difficult to obtain except for a very limited range of reagents that are commonly used such as triethylamine and phosphoric acid. For this reason, it is useful to obtain stable, high-purity ammonium salts of buffering anions for HPLC uses.
  • the ammonium salts are chemically more stable than the corresponding free amines, and they are easier to handle than pure amines themselves.
  • This general procedure with minor modifications is used to make purified di-ammonium salts of methanephosphonic acid, phosphoric acid and phosphorous acid. Good quality, HPLC-grade phosphoric acid, good quality, freshly recrystallized, solid phosphorous acid or methanephosphonic acid are used.
  • Liquid amines are distilled prior to use in order to remove many of the impurities.
  • High purity (>99%) gaseous amines, Me 3 N, Me 2 NH, MeNH 2 , EtNH 2 are delivered in the gaseous form from pressurized cylinders.
  • aqueous solutions of these amines are not pure enough to be used.
  • High purity, quaternary ammonium hydroxide solutions in water are also used.
  • Either of two solvents, HPLC-grade methanol or HPLC-grade denatured ethanol are used for crystallization at a usage rate of 2-20 mL solvent per gram of salt.
  • a wide range of crystalline di-ammonium salts are made using this method.

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