LC-mass spectrometry

AHLs can be tentatively identified by comparison of the unknown with synthetic AHL standards after Thin Layer Chromatography (TLC) in which the plates are overlaid with agar containing one of the AHL biosensors described above [37,39,44,45]. However, for the unequivocal identification of AHLs the use of more powerful methods such as LC-mass spectrometry, nuclear magnetic resonance and infrared spectroscopy as described below are required. 

Gaudry and Ploux (180) summarized recent HPLC, TLC, GC, column chromatography, paper chromatography, and LC-mass spectrometry techniques for biotin in biological and pharmaceutical samples. Russell (44) reviewed a recent HPLC determination for biotin in royal jelly. 

Table 3.8 Maximum Allowable Flow Rates for LC-Mass Spectrometry Interfaces8...

In spite of the fast development in its early days (late 1960s and early 1970s), the miniaturization of HPLC followed a slow progress until recently, with the development of LC-mass spectrometry (MS) using electrospray-type interfaces. 

Is the method specific and stability indicating as shown by analysis of samples subjected to stressed stability studies (pH, light, heat, oxidation) Normally, specificity is determined through peak purity using ultraviolet (UV) diode array or liquid chromatography (LC)/mass spectrometry (MS) analysis. In methods for analysis of drug products, placebo formulations (and stressed placebos) must yield blank chromatographic baselines. The development... 

GC-mass spectrometry (GC-MS) is most frequently and effectively used to identify the essential oil constituents by using database libraries of both retention indices and mass spectral fragmentation patterns. LC-mass spectrometry is less frequently used for the identification of the essential oil constituents due to increased experimental complexity. One of the recent technological developments is the combined use of GC-MS and FTIR spectrometries which can provide additional real time information for molecular identification without the need for macroscopic separation of mixtures [55,61-67]. 

Liquid chromatography (LC) has already been described and is an excellent separation technique for compounds that are nonvolatile, thermally unstable and relatively polar in nature. The usual detectors for LC are based on refractive index, conductivity, amperometry, light scattering, UV and fluorescence, all of which have been discussed in Section 3.2. However, sometimes it is desirable to have a more powerful detector attached to an LC instrument and, as such, the following combinations are possible LC-infrared spectrometry, LC-atomic spectrometry, LC-inductively coupled plasma-mass spectrometry, LC-mass spectrometry, LC-UV-mass spectrometry, LC-nuclear magnetic resonance and even LC-nuclear magnetic resonance-mass spectrometry. 

In 2006, Rolando et al. (27A99) developed a new methodology based on liqnid chromatography (LC), mass spectrometry (LC-MS and LC-MS/MS) for the identification and qnantihcation of free-radical-spin-trap adducts. The improved method involved the use of a nano-LC fitted with a 75-pmcolumn for separation that allowed for shorter analysis time and higher sensitivity. The main advantage of... 

The main advantages of monolithic columns are the superior separation performance and low flow resistance. In addition due to their continuous nature, frits are not required to retain the stationary phase. The production process of monolithic columns is more flexible than that of packed columns e.g., photo-polymerization can be applied to prepare monolithic structures or add selectivity locally. Both polymer- and silica-based monolithic capillary columns have been used for highly efficient separations in LC-mass spectrometry (MS) applications for proteomic research [24,25]. 

Because of some of the problems with bioassays and immunoassays, liquid chromatography (LC)-based techniques are increasingly applied as an alternative. While modern LC-based assays have a comparable sensitivity to immunoassays, they oftentimes are characterized by a higher selectivity [18, 19]. Muller et ah, for example, used LC/mass spectrometry with matrix-assisted laser desorption ionization in ex vivo pharmacokinetic studies in combination with enzyme inhibition experiments to investigate the complex metabolism of dynorphin Al-13, a peptide with opioid activity, up to the fifth metabolite generation [20, 21]. 

Current trends in AA analysis identify the best conditions for the separation of enantiomers and the development of LC-mass spectrometry (LC-MS, LC-MS/MS), detailed elsewhere. 

Hyphenation refers to the online combination of a separation technique and a spectroscopic detection method that provides structural information on the analytes concerned. Liquid chromatography (LC), mass spectrometry (MS), and gas chromatography (GC) are the most popular hyphenated techniques in use today. The choice of detection is important to the overall scheme of LC make up and is contingent upon criteria such as the noise, sensitivity, and linearity. Of the two basic categories of detectors, viz., solute and bulk property detectors, UV detection belongs to the former category. 

When two compounds with very close, or even super-imposable, retention times and identical UV spectra are found in a chromatogram, it is necessary to use more sophisticated detectors that can yield much more structural information without requiring the isolation of the compounds. LC-mass spectrometry (LC-MS) and LC-nuclear magnetic resonance (LC-NMR) are often used, as it will be presented later. 

Recent work includes extraction studies of aqueous extractants from acrylate coatings and adhesives using liquid chromatography LC)-mass spectrometry MS)/LC-MS-MS detection methods [21], extractant studies on elastomeric materials using MS techniques [22], and studies on rubbers using LC-atmospheric pressure chemical ionisation MS [23]. 

The role of advances in chromatographic techniques has been a step point in the development of phytochemistry [67]. Because of the complexity of crude herbal extracts, various online hyphenated techniques have been developed for the analysis of the complex mixtures. These techniques include liquid chromatography (LC), mass spectrometry (MS), LC nuclear magnetic resonance (NMR), and LC-NMR-MS [68]. They facilitate the structure determination of unknown constituents in crude extracts. For example, they are of great applicability in the analysis of flavonoids and other phenolic compounds [69, 70]. 

Because purity determined by liquid chromatography (LC) or LC-mass spectrometry (LC-MS) with ultraviolet (UV) or evaporative light scattering detector (ELSD) detection represents only relative purity, the presence of impurities such as inorganics, materials with poor chromophores and/or good evaporative properties, and materials that tend to be retained by a guard column cannot be detected. Therefore, we measure the absolute purity of six representative compounds in each library at both optimization and production stages. 

---