Peak folding detection

To avoid LC conditions involving the use of IP agents—as reported above—an alternative route was reported by Di Corcia et al. [16] for the separation and detection of breakdown products generated out of commercial LAS in a degradation test. This method was based on the conversion of the carboxylic groups into methyl esters. After this modification, the addition of 0.2 M ammonium acetate to the mobile phase sufficed to give sharp peaks. In addition, a 10-fold enhancement of the sensitivity of the ESI-MS detector was obtained. 

Astheimer and Schwochau have applied the voltametry method to the determination of low technetium concentrations in the presence of molybdate and perrhenate ions. Using a Kemula electrode technetium is concentrated on a mercury drop from alkaline solution of 6.0 x 10 M KTcO by electrolysis at a potential of — 1.0 V vs. SCE. Anodic stripping in 1 M NaOH yields a characteristic stripping curve (Fig. 14). The height of the peak at —0.33 V is proportional to the concentration of technetium in the range of 10 to 3 x 10 M. Technetium can be detected with an accuracy of +4%. The determination of 0.5 ng of technetium in a 10 fold molar excess of ReO or MoO is possible. 

For an analytical LC injection, there is a shortfall of analyte in the active volume by between 50 and 100-fold, and therefore LC injections loadings must be increased by this order just to obtain a one-dimensional proton spectrum. The sensitivity problem is exacerbated by the fact that LC peaks are generally of larger volume than the active volume (Figure 6.34) and so not all the anal54e is detected [57]. 

Sample Concentration Experiments. A CLLE quality assurance blank was run by extracting 90 L of Milli-Q water with three CLLE samplers in a parallel configuration and concentrating the composited extract to 4 mL by Kudema-Danish evaporation. The 22,500-fold concentrate was analyzed by GC-flame ionization detection (GC-FID) and GC-MS. Thirty-two peaks were observed by using GC-FID analysis, but because of their low concentrations, only four contaminants were identified by GC-MS cyclohexene, 2-cyclohexen-1-one, n-butyl phthalate, and bis(2-ethylhexyl) phthalate. Cyclohexene is a solvent preservative that has been identified in commercial high-purity methylene chloride (16), and 2-cyclohexen-l-one is its air oxidation product. The phthalates are ubiquitous laboratory contaminants and have also been identified in commercial methylene chloride (17). 

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