LC-MS retention

Unknown 2. Furthermore, the match between the unknown and standard spectrum was 947 / 949 for the match and reverse match, respectively, when performing a NIST library search, which indicated a very good match between the spectra. Similarly, the LC-MS retention time also matched, with the m/z[M - H] = 146.0254 (4 ppm error) detected at 4.85 minutes. Thus, phthalimide was confirmed as the identity of Unknown 2. Similar to Unknown 1 however, phthalimide showed no activity in the Ames test for mutagenicity. Thus, although this was another successful example of CASE, the toxicity confirmation for the EDA remained incomplete. 

For most free amino acids and small peptides, a mixture of alcohol with water is a typical mobile phase composition in the reversed-phase mode for glycopeptide CSPs. For some bifunctional amino acids and most other compounds, however, aqueous buffer is usually necessary to enhance resolution. The types of buffers dictate the retention, efficiency and - to a lesser effect - selectivity of analytes. Tri-ethylammonium acetate and ammonium nitrate are the most effective buffer systems, while sodium citrate is also effective for the separation of profens on vancomycin CSP, and ammonium acetate is the most appropriate for LC/MS applications. 

Figure 3.26 Reconstructed ion chromatograms obtained from the ions of m/z 199, 163 and 285 for retention times between 3.95 and 6.65 min in the LC-MS analysis of a pesticide mixture. From applications literature published by Micromass UK Ltd, Manchester, UK, and reproduced with permission.

Applications With the current use of soft ionisation techniques in LC-MS, i.e. ESI and APCI, the application of MS/MS is almost obligatory for confirmatory purposes. However, an alternative mass-spectrometric strategy may be based on the use of oaToF-MS, which enables accurate mass determination at 5 ppm. This allows calculation of the elemental composition of an unknown analyte. In combination with retention time data, UV spectra and the isotope pattern in the mass spectrum, this should permit straightforward identification of unknown analytes. Hogenboom et al. [132] used such an approach for identification and confirmation of analytes by means of on-line SPE-LC-ESI-oaToFMS. Off-line SPE-LC-APCI-MS has been used to determine fluorescence whitening agents (FWAs) in surface waters of a Catalan industrialised area [138]. Similarly, Alonso et al. [139] used off-line SPE-LC-DAD-ISP-MS for the analysis of industrial textile waters. SPE functions here mainly as a preconcentration device. 

Figure 7.29 LC-PB-MS retention time-mass plot of a polymer additive extract. Courtesy of F. Ploeg, Waters/Micromass, Etten-Leur. Reproduced by permission of Waters Corporation...

Capillary HPLC-MS has been reported as a confirmatory tool for the analysis of synthetic dyes [585], but has not been considered as a general means for structural information (degradant identification, structural elucidation or unequivocal confirmation) positive identification of minor components (trace component MW, degradation products and by-products, structural information, thermolabile components) or identification of degradation components (MW even at 0.01 % level, simultaneous mass and retention time data, more specific and much higher resolution than PDA). Successful application of LC-MS for additive verification purposes relies heavily and depends greatly on the quality of a MS library. Meanwhile, MB, DLI, CF-FAB, and TSP interfaces belong to history [440]. 

Total theoretical peak capacity for the ID and 2D LC/MS analyses of the yeast ribosomal protein sample was calculated as 240 and 700, respectively. Individual separation peak capacities were calculated by dividing the total separation time by the average peak width at baseline, and the 2D peak capacity determined as the product of the peak capacity of the two dimensions. These theoretical calculations rely on optimal use of the two-dimensional separation space, which in turn is dependent upon the lack of correlation between the component retention times in the two separation modes. Thus, the maximum use of the theoretical peak capacity is not only dependent on the selection of chromatographic modes based on different physicochemical... 

Today, GC-MS (see Section 4.1.1) is a golden standard for detection and quantification of drugs and poisons volatile under GC conditions, whereas nonvolatile compounds require LC-MS (see Section 4.1.2). The GC-MS technique is much more popular for identification purposes than LC-MS, because of the easy availability of the reference mass spectra for many xenobiotics and their derivatives, either in printed or computer form. The most popular libraries are the NIST library, which contains the mass spectra of 130,000 compounds, the Wiley Registry of Mass Spectral Data, which contains 390,000 reference spectra, and the Pfleger-Maurer-Weber library, with 6,300 mass spectra and other data, such as chromatographic retention indexes. 

For the quantification of LAS and their metabolites in LC-MS, in particular the respective position of the phenyl ring at the alkane or the alkylcarboxylate chain influences the retention time in RP-HPLC significantly. This might also hamper the proper allotment and thus quantification of the individual species too. 

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