Online analytical determination modes

The column-switching system was applied to the determination of STR and DIHS in pork and bovine muscle and kidney. Perchloric acid was used to precipitate proteins and extract analytes from the tissue. The clear supernatant was further cleaned up by an offline SPE on a cation-exchange SPE column. After the washing of the cartridge with water, the analytes were eluted with phosphate buffer (pH 8.0) and diluted with HSA, perchloric acid, and water. The enrichment was achieved in the online mode. After loading the sample, the enrichment precolumn was flushed with HSA at pH 3.3 for 5 min. Using the ratio of MeCN to aqueous component of 83 17... 

Analytical methods for the determination of one antidepressant and/or its metabolite(s) were usually performed in isocratic mode, with total run times from seconds to a few minutes. However, as previously mentioned, multianalyte procedures are preferable, particularly if the method is intended for clinical or forensic analysis. Gradient separation was usually applied when the most common antidepressants were included in the methodology however, total chromatographic run times varied widely, from 5 to 40 min [57, 76], depending on column length, extraction technique (offline vs. online techniques), biological matrix or the specific application of the method. 

There are two common ways to operate SFE, in online mode or offline mode. In the online mode, the outlet of the SFE instrument is directly hnked to an analytical instrument. Direct coupling with different chromatographic techniques allows simultaneous analyte preparation, separation and determination. The main drawback of the online extraction method is the limited sample size. In the offline mode, the extracted analytes are trapped and later, the extracted analytes can be analyzed by means of different chromatographic techniques. The offline collection system is chosen according to the extracted analyte characteristics. The use of online methods reduces possible errors and sources of contamination. Studies on trapping methods have been performed to optimize the corresponding parameters. ... 

Brossa et al. developed an automated SPE-GC-MS method for the determination of endocrine disrupting compounds including six phthalate esters. The interface device was a programmed temperature vaporizer (PTV), whose liner was packed with Tenax. The samples were spiked with 50% of methanol and 15 ml of this mixture were preconcentrated. Before elution, the precolumn was dried with nitrogen. The analytes were desorbed in the backflush mode with three ethyl acetate fractions of 100 /rl and online transferred to the GC system. The performance of the method was tested with several environmental water samples. The recoveries achieved were satisfactory and the detection limits were between 1 to 36 ng/1. 

Multidimensional chromatography is one of the best options to prevent unwanted coelutions of analytes with other matrix components present in a variety of complex samples, and also to incorporate automated online sample preparation in the analytical process. Nowadays, well-developed methodologies already exist using multidimensional modes as LC-LC or SPE-LC in combination with spectroscopic or electrochemical detection for the selective and sensitive determination of many analytes, mainly in the biomedical, pharmaceutical, environmental, and food fields. 

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