Simultaneous distillation–extraction (SDE) using
the Likens–Nickerson
apparatus is a convenient technique used to isolate volatile organic
compounds (VOCs) from complex liquid matrices. The technique combines
steam distillation with solvent extraction. While analytical extractions
are normally followed by off-line separation/detection, it is advantageous
to couple extractions on-line with separation and detection systems
that are employed in the same analytical workflow. Here, we have coupled
the Likens–Nickerson apparatus on-line with a gas chromatograph
hyphenated with a mass spectrometer. For that purpose, we have devised
an automated liquid transfer setup comprising a peristaltic pump,
control unit, customized transfer vial with a drain port, and an autosampler
arm to deliver liquid extract aliquots at defined time points. The
on-line SDE-GC/MS system enables one to record real-time extraction
profiles. These profiles reveal extraction kinetics of various VOCs
present in the extracted samples. The data sets were fitted with the
first order kinetic equation to obtain numeric values characterizing
the extraction process (rate constants ranging from 0.21 to 0.01 min–1 for the ethyl esters from C6 to C19). A comparison of on-line and off-line results reveals that
the on-line system is more dependable, while the off-line analysis
leads to artifacts. To demonstrate the operation of the on-line SDE-GC/MS
system, we performed analyses of selected real samples (beer). The
real-time data sets revealed extraction kinetics for VOCs present
in these samples. The devised extraction-analysis system allows the
analysts to make an evidence-based decision on the extraction time
for different groups of analytes in order to maximize extraction yield
and minimize analyte losses.
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