Impurities identification with mass spectrometry

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

C. Eckers,N. Haskins and J. Langridge.The use of liquid chromatography combined with a quadrupole time-of-flight analyzer for the identification of trace impurities in drug substance. Rapid Communications in Mass Spectrometry, 1997,11(17), 1916-1922. 

The best method for getting rapid identification of trace impurities, which should ideally be carried out using chromatographic separation in conjunction with high resolution mass spectrometry so that elemental compositions can be determined... 

The technique of mass spectrometry (MS) is itself of no help in the detection of impurities, but in combination with gas chromatography (the GC-MS technique) it would be the method of choice for the identification of impurities. 

The more advanced instrumental methods of analysis, including GC, for the detection and identification of expls are presented (Ref 90) Pyrolysis of expls in tandem with GC/MS was used for the identification of contaminant expls in the environment (Ref 108). Isomer vapor impurities of TNT were characterized by GC-electron capture detector and mass spectrometry (Ref 61). Volatile impurities in TNT and Comp B were analyzed using a GC/MS the GC was equipped with electron capture and flame ionization detectors (Ref 79). The vapors evolved from mines, TNT, acetone, toluene, cyclohexanone and an organosilicon, were analyzed by GC/MS (Ref 78). Red water produced by the sellite purification of crude TNT was analyzed by GC/MS for potentially useful organic compds, 2,4-dinitrotoluene, 3- and 4-sulfonic acids (Ref 124). Various reports were surveyed to determine which methods, including GC/MS, are potential candidates for detection of traces of TNT vapors emitted from land mines factors influencing transportability of TNT vapors thru soil to soil/air interface are dis-... 

In drug preparations, sensitivity is not a particular problem as there is always sufficient of the active constituent present. Suitable extraction procedures should be carried out and each fraction should then be subjected to chromatographic analysis. The mass spectra, together with the appropriate retention parameters, may then be used for identification purposes. In many cases mass spectrometric analysis is undertaken as confirmation of an identification based on the physical characteristics such as size, colour, markings, etc., of the tablet or capsule. Mass spectrometry may be used not only to identify the major components but also the trace impurities. This information may be used for quality control purposes, and may enable the route of manufacture of certain drugs to be determined. 

As already emphasized, the process of identification of impurities and/or degradants begins early in drug development. Early brainstorming sessions should involve the analytical chemist, process chemist, formulator, and degradation chemist, as well as the mass spectrometry and NMR experts. It is imperative to involve all that are familiar with the project of interest. The... 

It is often difficult to determine the degree to which the chemistry proceeded on the entire library population and whether peaks in a mass spectrum are due to the product, side reactions, reagents, solvents, or impurities. Diversity Sciences developed mass-spectral methods to distinguish all components that are cleaved from a solid support and implemented the method into the analytical construct. While early studies demonstrated promising results for fragmentation methods with tandem mass spectrometry (MS/MS), stable isotopes were routinely implemented as signature peaks for the identification of compounds that are produced from solid-phase reactions [27]. 

Furthermore, LC-MS is used in a wide variety of other identification studies, e.g., in the identification of impurities and by-products in bulk chemicals used in industry, in the characterization of frequently applied fluorescent derivatives for LC in combination with fluorescence detection, for studying simulated biochemical processes via online electrochemical conversion units, and mass spectrometry via an LC-MS interface. 

If the objective is identification (qualitative analysis), it suffices to compare the spectrum of the analyte with that of a standard, both recorded in the same solvent and at an identical pH. This is not the main application of UV-Vis spectrophotometry as the best results in this context are provided by spectroscopic methods considered more effective for the study of the molecular structure of organic compounds (infrared, nuclear magnetic resonance, mass spectrometry, and X-ray diffraction). However, UV-Vis spectrophotometry is a source of relevant supplementary information that helps in the elucidation of molecular structures of drugs, impurities, metabolites, intermediate compounds of degradation, etc. 

A similar problem associated with synthetic peptides of chain length > 5 amino acid residues is the identification of side reactions. Those involve substitutions on imidazole-, phenol- and indole moieties of histidine, tyrosine, and tryptophane as well as conversions, transamidation, and cyclizations of aspartic and glutamic side chains. As long as structural variations are stable under the reaction conditions of an Edman degradation, they can be detected from the proper phenylthiohydantoines in combination with H-NMR- and mass spectrometry. Quantitative amino acid analyses of impure peptides after acidic total hydrolysis do not indicate those structural deviations between main product and contaminations. 

This paper presents a mass spectral study of impurities in illicit heroin. Identification of these impurities is of importance because of their potential contribution to pharmacological effects. The major impurities, 6-acetyl-morphine ( -c) and 6-acetylcodeine ( I-k), are predictable, owing to their origination from the naturally-occurring alkaloids-morphine ( -a) and codeine (r-j)(l) after reaction with acetic anhydride. In addition, the presence of a trace impurity (m/e 397, C22H23NO0, at less than 0.4%) was observed by gas chromatography-mass spectrometry (Fig. 2). 

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