Application of Liquid Microjunction Surface Sampling Based Quantitative Mass Spectrometry Imaging Technique in Localization and Determination of Photoinitiators in Packaging Materials

KeyWord:liquid microjunction surface sampling  mass spectrometry imaging  photoinitiators  quantitation analysis  packaging materials

Abstract:

      To address the current issues of traditional methods for photoinitiators£¨PIs£© quantitation£¬which are time-consuming and have no spatial resolution£¬a quantitative mass spectrometry imaging method was developed£¬based on a liquid microjunction surface sampling£¨LMJSS£© technique which is a simple£¬flexible and low-cost ambient sampling method for mass spectrometry. In this technique£¬solid surface could be sampled in situ with liquid solvent in a confined area prior to ionization in MS£¬which makes it possible to achieve in situ analysis or mass spectrometry imaging£®Also£¬it is flexible to tune the sampling selectivity by solvent compositions or add standard in sampling solvent for quantitation in LMJSS technique£®In the previous study£¬the problem of in situ quantification on PIs in packaging paper was effectively solved by combing LMJSS technique with kinetic calibration method£®In this study£¬LMJSS technique was applied to the mass spectrometry imaging of PIs£®Results showed that higher sensitivities were obtained for PIs by directly connecting the flow probe of LMJSS system to the inlet of mass spectrometry£®Using this imaging method£¬the kinetic curve£¨signal-time curve£© of analytes could be obtained in each pixel£®According to the kinetic curves£¬it was proved that the extraction efficiencies of PIs in single pixel after 1 min LMJSS were all above 90%£¬making the quantitation by external standard calibration possible£®This new method was finally applied to the quantitative mass spectrometry imaging for PIs in packaging material. The absolute concentrations of PIs in each pixel could be obtained by scanning the sample surfaces with the LMJSS technique£¬and the quantitative results were compared with those by traditional PIs quantitation method using gas chromatography£­mass spectrometry£®From the results£¬it is found that more kinds of PIs could be detected by the new mass spectrometry imaging method than traditional method£¬and the quantitation results by two methods were mostly comparable£®Moreover£¬the imaging method could provide the spatio£­chemical information which could not be obtained by traditional method. For example£¬in a certain area of paper packaging material£¬more isopropylthioxanthone£¨ITX£©£¬4-benzoylbiphenyl£¨PBZ£©£¬2-ethylhexyl-4-dimethylamino benzoate£¨EHDBA£© and o-methyl-benzoyl benzoate£¨OMBB£© were found to distribute in regions of red inks£¬while benzophenone£¨BP£© and 1-hydroxycyclohexyl phenyl ketone£¨PI184£© mainly distributed in regions of green inks£®This specific spatial distribution of PIs makes them difficult to be detected by the traditional quantitative method£®The developed imaging method is of great significance for screening and localizing PIs in packaging materials£¬and for guiding and improving the coating formulation or process of UV curable ink£®It can also provide a technical support for the quality control of PIs in packaging materials£®