Comparison of ethoxylated alcohols and polyethylene glycols by high-performance liquid chromatography and supercritical fluid chromatography using evaporative light-scattering detection

Towards improved characterisation of complex polyethylene glycol excipients using supercritical fluid chromatography-evaporative light scattering detection-mass spectrometry and comparison with size exclusion chromatography-triple detection array

The application of supercritical fluid chromatography (SFC) coupled to an evaporative light scattering detector (ELSD) and mass spectrometer (MS) was evaluated for the characterisation of three analogues of functionalised polyethylene glycol (PEG) 2000 (m-PEG-OH, m-PEG-cm and cm-PEG-cm (where m = OCH₃ and cm = OCH₂COOH)). These polymers are common excipients in drug product formulations for pharmaceuticals as they help provide the desired pharmacokinetic profile for successful drug delivery. A SFC-ELSD-MS method was developed which was selective to all three polymers, and allowed visualisation of these low UV chromophore materials. The method provided baseline resolution of the individual oligomers which allowed facile calculation of the polymer dispersity. A number of molecular weight characteristics were calculated, which showed the SFC-ELSD-MS methodology to be comparable with the current standard of analysis using size exclusion chromatography (SEC) with a triple detector array (TDA). The increased resolving power of SFC compared to SEC revealed a bimodal distribution of oligomers in the cm-PEG-cm 2000 polymer, which was not observed using SEC-TDA and exemplified SFC-ELSD as an orthogonal approach for polymer characterisation with the potential for much simpler, reduced sample and instrument preparation, calibration-less dispersity determination. When combined with SEC-TDA data, this combination allows a more complete characterisation of complex formulations excipients.

On the determination of underivatised fatty alcohol ethoxylates by electrospray ionisation–mass spectrometry

The oligomers of fatty alcohol ethoxylates (FAEs) exhibit large sensitivity differences in mass spectrometry with electrospray ionisation (ESI-MS) and atmospheric pressure chemical ionization (APCI). Standards of the oligomers from m=1 to 7 ethylene oxide units (EOs) and linear alkyl chains from n=10 to 18 carbon atoms were infused to examine the relative sensitivities or response factors in several media. The response factors of the [M+H]+ and [M+Na]+ peaks in 9:1 acetonitrile/water and methanol/water media containing acid buffers increased following irregular patterns when n and m increased. In methanol/water the response factors depended on the parity of m, being larger than the average trend for the oligomers with an even value of m with respect to those having an odd value. This was attributed to the presence of an uncompensated C-O-C or C-O-H dipole in the former oligomers. The advantages of using ESI over APCI and of measuring the [M+H]+ peaks in an acid methanol/water medium containing 0.1 M HCl are discussed. The advantages and limitations of using models of the response factors to evaluate oligomer concentrations with a reduced set of selected standards are examined. The determination of underivatised FAEs using acid media was made compatible with previous HPLC separation by implementing either a triconcentric nebulizer fed with an acid liquid sheath, or a capillary T-union inserted between the column outlet and the biconcentric nebulizer, and fed with an acid stream provided by a syringe pump.

Increasing UV detection sensitivity in the supercritical fluid chromatographic analysis of alcohol polyethers

Alcohol ethoxylates (AEOs) that contain a wide distribution of oligomers pose a challenge for ultraviolet (UV) absorbance detection due to the fact that the AEOs absorb strongly only in the range of commercial UV detectors between 190 and 200 nm. Most mobile phase components, with the exceptions of water and carbon dioxide, also absorb in this region. Ethoxylated hexadecanol and octadecanol were derivatized with disilazane-chlorosilane mixtures for the formation of phenyl containing silylethers. Derivatized samples were analyzed by supercritical fluid chromatography (SFC) coupled with both electrospray ionization mass spectrometry and UV absorbance detection. An increase in the number of phenyl groups incorporated into the derivatives increased the number of oligomers observed by UV detection. An increase in the number of oligomers detected increased the calculated average molar ethoxylate values. The average molar oligomer values calculated by SFC-UV for these alcohols were consistent with the nominal reported values.

Determination of alcohol polyether average molar oligomer value/distribution via supercritical fluid chromatography coupled with UV and MS detection

Supercritical fluid chromatography (SFC) was used for the analysis of ethoxylated and propoxylated surfactants. Samples were derivatized to phenylated silyl ethers with a disilazane-chlorosilane mixture. Addition of a phenyl group to the surfactant allowed UV-absorbance detection of each oligomer. Acetonitrile and methanol were evaluated as mobile phase modifiers. Better peak shape was realized with methanol-modified CO2 on an octadecyl silica bonded phase than with acetonitrile-modified CO2. Peak assignments were made via SFC coupled with electrospray ionization-mass spectrometry (ESI-MS) in the positive ion mode. A sulfonamide-embedded alkyl stationary phase was also evaluated for separation of the derivatized samples. SFC-UV and SFC-ESI-MS data were jointly used for calculation of average molar oligomer values which were then compared to values calculated from 1H NMR data of non-derivatized samples. The derivatization or separation method using the sulfonamide embedded phase required no preliminary cleanup and yielded reproducible oligomer values that were consistent with those of the manufacturer's nominal values.

Separation of derivatized alcohol ethoxylates and propoxylates by low temperature packed column supercritical fluid chromatography using ultraviolet absorbance detection

Supercritical fluid chromatography (SFC) is capable of separating oligomers of alcohol ethoxylates (AEOs) and propoxylates (APOs) samples with pure carbon dioxide. The instrumental conditions, however, needed for separation necessitate both high temperature and high pressure. Derivatization of alcohol polyether samples with an UV absorbing agent has been achieved with a phenylated disilazane in hopes of employing a solvent-modified CO2 mobile phase in conjunction with both lower CO2 pressure and lower temperature for oligomer separation. A silylether containing a single phenyl group was formed via the derivatization of the hydroxyl termini of AEO and APO samples. The derivatized polyethers were detected at 215 nm with little or no interference from the mobile phase. Octadecylsilica (ODS) and a polar embedded alkyl bonded silica stationary phase were studied with the organic solvent-modified CO2 mobile phase. The combination of an ODS phase and the polar embedded phase, tandemly stacked, produced the best chromatographic separation of oligomeric species. Data from SFC-UV separations combined with peak assignments from SFC with electrospray ionization-mass spectrometric (ESI-MS) detection produced average molar oligomer values for each surfactant sample.

Separation of polyethylene glycol oligomers using inverse temperature programming in packed capillary liquid chromatography

Inverse temperature programming in packed capillary liquid chromatography coupled to evaporative light-scattering detection has been used to resolve native polyethylene glycol (PEG) oligomers. The model compound, PEG 1000, was separated on a 300 mm x 0.32 mm I.D. capillary column packed with 3 microm Hypersil ODS particles with acetonitrile-water (30:70, v/v) as mobile phase. The retention of the PEG oligomers increased with increasing temperature, different from what is commonly observed in liquid chromatography. The retention times of the oligomers were approximately doubled for each 25 degrees C increment of the column temperature in the temperature range 30-80 degrees C. The oligomers were almost unretained and co-eluted at a column temperature of 30 degrees C. At 80 degrees C a baseline separation of more than 22 peaks was obtained, but the last eluting peaks were severely broadened and all oligomers did not elute. When a negatively sloped temperature ramp from 80 to 25 degrees C at -1.5 degrees C/min was applied, the peak shapes were improved, additional peaks were detected and the analysis time was reduced by 48%. In the temperature programming mode, the intra-day precision of the retention times ranged from 0.5 to 5.8% (n=5).

Liquid exclusion–adsorption chromatography, a new technique for isocratic separation of nonionic surfactants

A new technique of liquid chromatography, which allows baseline separation of fatty alcohol ethoxylates with up to 15-20 ethylene oxide units under isocratic conditions allows an accurate quantitative analysis of single hydrophobic chain surfactants. Using density and refractive index detection, the accurate weight fractions of the individual oligomers are obtained. Moreover, the contribution of preferential solvation can be determined. With refractive index detection alone, good accuracy can also be achieved.

Analysis of aliphatic alcohol ethoxylates in terms of alkyl and ethylene oxide chain lengths by reversed-phase liquid chromatography with evaporative light scattering detection

Aliphatic alcohol ethoxylates are nonionic surfactants which are incorporated in many industrial formulations as complex mixtures of alkyl homologs and ethylene oxide oligomers. Determination of both the homolog and oligomer distributions is required for product control. The proposed method consisted of three reversed-phase liquid chromatographic separation steps carried out on the same C18-bonded silica column. The first step was a preparative one: the sample mixture was fractionated according to the alkyl chain length without discrimination between ethylene oxide oligomers by using methanol-water eluent. The even homologs (EH) were collected together in a single fraction, the odd homologs (OH) in another. In the second and third steps, respectively, EH and OH fractions were separated according to the alkyl chain length and the number of ethylene oxide units simultaneously by changing the mobile phase composition to acetonitrile-water and by using evaporative light scattering detection.

Retention mechanism, isocratic and gradient-elution separation and characterization of (co)polymers in normal-phase and reversed-phase high-performance liquid chromatography

Synthetic (co)polymers or (co)oligomers with two (or more) repeating groups show not only molar mass distribution, but also composition and sequence distribution of the individual repeat units. To characterize such two- (or more-) dimensional distribution, liquid chromatography under "critical conditions" has been suggested, where the separation according to one type of repeating units is suppressed by balancing the adsorption and the size-exclusion effects. In present work it is shown that by combination of adequately selected separation conditions in normal-phase and in reversed-phase systems, the two-dimensional distribution mode can be adjusted to result in the separation following the distribution of any of the two repeat units in ethylene oxide-propylene oxide block (co)oligomers. Based on the retention mechanism suggested, prediction and optimization of the conditions for isocratic and gradient-elution separations of (co)oligomers is possible. HPLC-MS with atmospheric-pressure chemical ionization is a valuable tool for unambiguous identification of the individual (co)oligomers and their tracking in course of method development. Gradient elution can be used for the separation and characterization of block (co)oligomers of ethylene oxide (EO) and propylene oxide (PO) according to the number of the units in one block, while the separation according to the distribution of the units in the other block is suppressed. The effects of the arrangement of the individual EO and PO blocks in the block (co)oligomers (the sequence distribution) affects significantly the retention behavior and the selection of the optimum separation conditions.

Separation and characterization of octylphenol ethoxylate surfactants used by reversed-phase high-performance liquid chromatography on branched fluorinated silica gel columns

The separation and characterization of octylphenol ethoxylate surfactants were carried out by reversed-phase high-performance liquid chromatography on branched fluorinated silica gel columns. For Triton X-100, simultaneous separation of octylphenol ethoxylate oligomers, positional isomers of octylphenyl group and butylphenol ethoxylate oligomers was achieved. These oligomers were completely separated and identified by means of MS spectra. Ethoxylated oligomers are eluted in the sequence from small to large oligomers. Fifty-five oligomers of Triton X-405 could be separated by using gradient elution. To separate octylphenol ethoxylate surfactant, non-end-capped branched fluorinated silica gel columns were superior to end-capped columns. The relationship between ln k' and methanol concentration was linear, indicating that branched fluorinated silica gel columns were operating in the reversed-phase mode. As Van 't Hoff plots of capacity factor for all oligomers gave straight lines, the equilibrium of conformation for the ethylene oxide chain might lay to one side of either zigzag or meander conformers.