Laser ionization time-of-flight mass spectrometry for the evaluation of a local microenvironment in an emulsion†
This study verified that laser ionization time-of-flight mass spectrometry (LI-TOFMS) is applicable to the evaluation of a local microenvironment in an emulsion. Sodium dodecyl sulfate (SDS) and toluene were used as a disperser and as an extraction solvent, respectively, and were added to a model sample of an aqueous styrene solution. The emulsion sample was introduced into LI-TOFMS and a series of mass spectra were obtained. Then, the time courses of styrene and toluene were constructed by extracting peak areas of the corresponding species. As a result, the signal intensities of both styrene and toluene were sometimes increased suddenly, which could be recognized as spikes. Moreover, many spikes arising from both species simultaneously appeared. The maximum signal intensity of the styrene spike was 27-fold higher than the signal intensity of styrene obtained from an aqueous solution, while the signal intensity other than that from spikes was decreased by about one-third of that of an aqueous solution. These results suggest that the styrene in the aqueous solution was extracted into small toluene droplets. This method was also applied to an aqueous solution containing styrene as well as m-phenylenediamine as a hydrophilic compound. Consequently, the selective extraction of styrene to the solvent droplets was observable due to the difference in the partition coefficient. LI-TOFMS has several characteristic superiorities such as selectivity and robustness, and all the ions induced can be detected. That is, a number of constituents can be measured in an individual small droplet, which is irreplaceable in the direct evaluation of the local microenvironment in an emulsion sample. Therefore, LI-TOFMS can be useful in the measurement of time-dependent concentrations and/or condition changes such as with emulsion polymerization.