Peak compression in semi-preparative supercritical fluid chromatography

Major signal suppression from metal ion clusters in SFC/ESI-MS - Cause and effects

The widening application area of SFC-MS with polar analytes and water-containing samples facilitates the use of quick and simple sample preparation techniques such as "dilute and shoot" and protein precipitation. This has also introduced new polar interfering components such as alkali metal ions naturally abundant in e.g. blood plasma and urine, which have shown to be retained using screening conditions in SFC/ESI-TOF-MS and causing areas of major ion suppression. Analytes co-eluting with these clusters will have a decreased signal intensity, which might have a major effect on both quantification and identification. When investigating the composition of the alkali metal clusters using accurate mass and isotopic pattern, it could be concluded that they were previously not described in the literature. Using NaCl and KCl standards and different chromatographic conditions, varying e.g. column and modifier, the clusters proved to be formed from the alkali metal ions in combination with the alcohol modifier and make-up solvent. Their compositions were [(XOCH₃)_n + X]⁺, [(XOH)_n + X]⁺, [(X₂CO₃)_n + X]⁺ and [(XOOCOCH₃)_n + X]⁺ for X = Na⁺ or K⁺ in ESI+. In ESI-, the clusters depended more on modifier, with [(XCl)_n + Cl]^- and [(XOCH₃)_n + OCH₃]^- mainly formed in pure methanol and [(XOOCH)_n + OOCH]^- when 20 mM NH₄Fa was added. To prevent the formation of the clusters by avoiding methanol as modifier might be difficult, as this is a widely used modifier providing good solubility when analyzing polar compounds in SFC. A sample preparation with e.g. LLE would remove the alkali ions, however also introducing a time consuming and discriminating step into the method. Since the alkali metal ions were retained and affected by chromatographic adjustments as e.g. mobile phase modifications, a way to avoid them could therefore be chromatographic tuning, when analyzing samples containing them.

Packed column supercritical fluid chromatography of sodium stearyl fumarate aqueous suspension

A method for the determination of sodium stearyl fumarate aqueous suspension is described. This straightforward method is based on homogenisation of the sample, dilution of a known aliquot with methanol to a suitable clear solution and mixing with an internal standard; (S)-naproxen. Separation and quantification is performed by packed column supercritical fluid chromatography on a commercial tartaric acid network polymeric column (tertbutylbenzoyl) with UV-detection at 214 nm. The precision of the presented method upon repeated analysis of a 20 mg/ml suspension is 0.5% (n = 8), and the yield is near 100%. Less than 5 min is required for the chromatographic separation with a resolution of about 3 to the internal standard. With some modification of the chromatographic conditions water samples can also be analysed.

Packed column supercritical fluid chromatography of a peroxysome proliferator-activating receptor agonist drug

This paper describes packed column supercritical fluid chromatography (SFC) for the analysis of a peroxysome proliferator-activating gamma-receptor agonist that is a carboxylic acid. Evaluation of conditions for the separation of this candidate drug and related compounds in bulk substance is described. A Chiralcel OD column was used for this purpose due to its high selective retention of related substances and relative inertness, though the enantioselectivity was negligible, with methanol as polar modifier. A high enantioselectivity was obtained on Chiralpak AD and it was possible to determine the enantiomeric purity within 10 min on a 5 cm short column. Both the achiral and the chiral systems were run without acid additive in the mobile phase and the level of detection of impurities by area was about 0.1%. For the analysis of samples dissolved in water, without any isolation step, 2-propanol was used as modifier. Due to the column surface activity, evidently generated by injected water, citric acid 1 mM was included as additive in the 2-propanol in order to maintain symmetric and undistorted peak shape. The detection limit for the assay was 21 microg mL(-1) (50 nmol mL(-1)) for 5 microL injected (R.S.D. 6.4%, n = 8). A 5 cm short Chiralcel OD column was used. Determination of enantiomeric purity of the drug in aqueous samples required increased sensitivity. The sample was acidified and extracted into a small volume of 1-pentanol, out of which 25 microl was analyzed by SFC. The minor enantiomer at the 3% (w/w) level added could be confirmed. Its ratio remained constant during the procedure as measured relative to a reference solution in organic media.

Direct injection of aqueous samples in packed column supercritical fluid chromatography of isosorbide-5-mononitrate from drug release testing

A method for the analysis of aqueous samples of isosorbide-5-mononitrate (5-ISMN) is presented. It is based on packed column supercritical fluid chromatography (SFC) using 20% of 2-propanol in carbon dioxide as the mobile phase and a diol silica column as the stationary phase. Using the described conditions it is possible to quantitate 5-ISMN released from Imdur tablets in gastric media. The precision upon repeated injections was 2% (RSD) at the 20 microg/ml level (n=8), using peak height measurements, when the solution was circulated through the sample loop of the injector. Samples from drug release testing that had been analyzed with reversed phase LC were analyzed with the present method and the results agreed well. It is also possible to monitor the drug released in a dissolution-testing vessel through direct on-line continuous loading (recirculation) of the sample loop of the SFC instrument.