Liquid chromatography-mass spectrometry coupling and its application in pharmaceutical research

Porous layer open tubular nano liquid chromatography directly coupled to electron ionization mass spectrometry

A 25 µm i.d x 1.2 m length PS-DVB porous layer open tubular column (PLOT) was prepared and assessed in the configuration of a nano liquid chromatography coupled to an electron ionization mass spectrometry system (OT-nanoLC-EI-Ms), via the direct insertion of the column outlet into the ionization source. The developed system's operational parameters were comprehensively studied, and the setup performance was investigated employing both unidimensional and column switching configurations. As a result, the OT-nanoLC-EI-MS system demonstrated competitive applicability in separating non-amenable ESI compounds, such as polyaromatic hydrocarbons (PAHs) and non-amenable GC compounds such as thermolabile pesticides. Furthermore, with excellent chromatographic performance, the PLOT columns can work under more compatible EI-detection conditions - such as the elution with 100% organic solvent. For example, PAHs retention factors ranged between 1.5 and 2.2 for 100% MeCN mobile phase, and more than 33,000 plates per meter for naphthalene at 50 nL/min flow rate. In analyzing thermolabile pesticides, the column switching PLOT-nanoLC-EI-MS system provided LODs of 25 µg/L, demonstrating suitable intra e interday reproducibility (% RSD < 13%, n = 3), and possibilities the direct injection of raw samples with suitable robustness.

Mass spectrometry and mass-selective detection in chromatography

An overview of chromatography-mass spectrometry coupling is presented here, focussing mainly on possibilities offered by this detection technique. GC-MS and HPLC-MS are the two most often used variants, which have quite different characteristics. Various mass-spectrometric possibilities are briefly discussed: ionization techniques, determination of elemental formulas, structural information and the question of sensitivity and selectivity. Options for mass-spectrometric instrumentation and applications of tandem mass spectrometry are also mentioned.

The determination of a chlorinated benzofuran pharmaceutical intermediate by HPLC-MS with on-line derivatization

An HPLC-MS method for the analysis of 2-chloromethylbenzofuran, a pharmaceutical intermediate alkylating reagent employed in the preparation of a second generation HIV protease inhibitor, N-(2(R)-hydroxy-l(S)-indanyl)-2(R)-phenylmethyl-4(S)-hydroxy-5-(1-(4-(2- benzo[b]furanylmethyl)-2(S)-N'-9-t-butyl-carboxamido)-pipera zinyl))-pentaneamide, is described. Preliminary analysis of the protease inhibitor by HPLC-MS indicated that the quality of the drug substance was influenced by the composition of the chloromethylbenzofuran. Direct analysis of the chloromethylbenzofuran by LC-MS using atmospheric pressure ionization was unsuccessful, necessitating an alternative approach. The method described incorporated post-column derivatization of the chloromethyl benzofuran using a modification of the drug substance process chemistry yielding a derivative amenable to MS analysis using atmospheric pressure chemical ionization (APCI). This allowed measurement of an impurity in the chloromethylbenzofuran which was incorporated into the protease inhibitor.

Thermospray liquid chromatographic-mass spectrometric analysis of anti-AIDS nucleosides: quantification of 2',3'-dideoxycytidine in plasma samples

Thermospray liquid chromatography-mass spectrometry was investigated as a method for quantification of 2',3'-dideoxycytidine (DDC) from human plasma. A stable isotope analog of DDC ([15N2,2H2]DDC) was used as an internal standard. Selected ion monitoring of the protonated molecular ions for DDC and the internal standard was used to record mass chromatograms. The areas of the peaks in the mass chromatograms were used for quantification. The detection limit of DDC in this assay was 50 pg on-column. The calibration curve was linear over the desired range, 0.25-20 ng/ml. The major advantages of this assay over others are: no derivatization, high sensitivity, high specificity and short assay time.

Structural analysis of non-volatile compounds by liquid chromatography/mass spectrometry and liquid chromatography/tandem mass spectrometry: thermal isomerization of benzylpenicillin in a Plasmaspray interface

Isomerization and degradation of benzylpenicillin ((2S,5R,6R)-3,3-dimethyl-7-oxo-6-(2-phenylacetamido)-4-thia-1- azabicyclo[3.2.0]heptane-2-carboxylic acid) were studied using a combination of Plasmaspray (PSP) liquid chromatography/tandem mass spectrometry (LC/MS/MS) and liquid secondary ion tandem mass spectrometry (LSI MS/MS). Benzylpenicillin was isomerized to benzylpenicillenic acid (3-mercapto-N-[[5-oxo-2-(phenylmethyl)-4(5H)- oxazolylidene]methyl]valine) in the PSP interface/ion source. The isomerization was inferred from the probe temperature dependence of PSP LC tandem mass spectra and discrepancies in the daughter ions between PSP LC and LSI tandem mass spectra. High temperature at the PSP interface was responsible for the isomerization, since the difference between PSP LC and LSI tandem mass spectra became smaller as the probe temperature was lowered. It was also found that benzylpenicillin was decomposed to benzylpenilloic acid (5,5-dimethyl-2-[(phenylacetamido)methyl]thiazolidine-4-carboxylic acid), N-(phenylacetyl)glycine, N-(phenylacetyl)glycinal and 3-mercaptovaline in the PSP interface/ion source. The degradation products formed in the interface/ion source were identical to those formed in acidic solution. The results show that degradation of penicillins can be investigated by PSP LC/MS and PSP LC/MS/MS.

Simultaneous determination of beclomethasone, beclomethasone monopropionate and beclomethasone dipropionate in biological fluids using a particle beam interface for combining liquid chromatography with negative-ion chemical ionization mass spectrometry

A new simple and sensitive assay has been developed for the simultaneous quantitative measurement of beclomethasone dipropionate and its hydrolysis products in human plasma and urine. Beclomethasone 17.21-dipropionate, beclomethasone 17-monopropionate, beclomethasone and the internal standard, dexamethasone 21-acetate, were measured by combined liquid chromatography and negative-ion chemical ionization mass spectrometry with methane as the reagent gas. A particle beam interface from Hewlett Packard was used. Under mild operating conditions, abundant and stable characteristic high-mass ions were generated in the ion source of the mass spectrometer by a resonance electron-capture mechanism. The fast extraction procedure requires 1 ml of plasma or urine, and the quantification limit of the method is 1 ng ml-1 for the three tested compounds.

Approach to the analysis of diuretics and masking agents by high-performance liquid chromatography-mass spectrometry in doping control

The application of thermospray and plasmaspray high-performance liquid chromatography-mass spectrometry to the analysis of diuretics and probenecid has been investigated. The latter method gave better ionization efficiency than the former, and its response was optimized by altering the solvent composition: best results were obtained with water-methanol-acetonitrile-trifluoroacetic acid. Using different proportions of these solvents, three isocratic systems were developed to separate the compounds under study. The principal characteristic of plasmaspray positive-ion mass spectra was a protonated molecular ion and very little fragmentation was evident. In the negative ionization mode, the plasmaspray method gave mass spectra showing more fragmentation, which resulted in additional structural information. The ability of trifluoroacetic acid to form negative cluster ions precluded its use as a mobile phase component. The minimum detectable amounts determined by the analysis in the positive-ion mode was compound-dependent, but generally ca. 10-150 ng. In many cases the compounds could be detected in urine extracts.

Liquid chromatographic-mass spectrometric studies on rifamycin antibiotics

The use of a direct liquid introduction type liquid chromatographic-mass spectrometric interface to study highly thermally labile rifamycin antibiotics is described. Using negative ionization, abundant molecular ions were observed, and the spectra, also contained structurally significant fragments. Variation of the high-performance liquid chromatographic parameters did not change the spectra, thus making it easy to change chromatographic conditions. In quantitative studies, a surprising correlation was found, indicating that the mass spectrometric signal was proportional to the square of the sample concentration.