Development of a matrix-assisted laser desorption ionization mass spectrometric method for rapid process-monitoring of phthalocyanine compounds

doi:10.1016/j.aca.2012.04.038

Analytica Chimica Acta

Volume 736, 29 July 2012, Pages 69-77

https://doi.org/10.1016/j.aca.2012.04.038 Get rights and content

Abstract

Phthalocyanines (PCs), an important class of chemicals widely used in many industrial sectors, are macrocyclic compounds possessing a heteroaromatic π-electron system with optical properties influenced by chemical structures and impurities or by-products introduced during the synthesis process. Analytical tools allowing for rapid monitoring of the synthesis processes are of significance for the development of new PCs with improved performance in many application areas. In this work, we report a matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry (TOFMS) method for rapid and convenient monitoring of PC synthesis reactions. For this class of compounds, intact molecular ions could be detected by MALDI using retinoic acid as matrix. It was shown that relative quantification results of two PC compounds could be generated by MALDI MS. This method was applied to monitor the bromination reactions of nickel- and copper-containing PCs. It was demonstrated that, compared to the traditional UV–visible method, the MALDI MS method offers the advantage of higher sensitivity while providing chemical species and relative quantification information on the reactants and products, which are crucial to process monitoring.

Graphical abstract

Highlights

► MALDI TOFMS is developed as a rapid means of monitoring the process of phthalocyanine derivatives. ► All-trans retinoic acid is used as matrix to reduce ion fragmentation and background formation. ► Samples from the reaction process are taken for direct analysis by MALDI TOFMS without any cleanup. ► The method is demonstrated for monitoring the synthesis of brominated Ni- and Cu-phthalocyanine.

Introduction

Phthalocyanines are macrocyclic compounds possessing a heteroaromatic π-electron system with optical properties influenced by chemical structures and impurities or by-products introduced during the synthesis process. PCs and their derivatives have found diverse applications ranging from industrial colorants to catalysts, light-absorbing materials, optical information recording media for read/write compact disks, charge-generating materials, and a sensitizer for photodynamic cancer therapy [1]. The optical, chemical, and physical properties of PCs can be modulated by modifying their chemical structures, introducing differentially coordinated elements into the cavity of the macrocycle, or changing axial and peripheral ligands. Thus, rapid monitoring of the synthesis of PCs and their derivatives to provide compositional and structural information is critical in developing new PCs.

Traditionally, UV–visible spectrometry is used to monitor the reaction and product formation during the PC synthesis process. This method is useful to provide quantitative information on product formation, if the PC derivatives or products have different absorption wavelength from those of the reactants. However, it does not provide any structural or compositional information on the products. Electrospray ionization (ESI) MS and MALDI MS have been reported to be useful for qualitative characterization of PCs [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. In most studies, pure PCs and their derivatives were obtained by thin-layer chromatography or a silica-gel column purification prior to MS analysis [2], [8], [9], [10]. The purification steps were usually tedious and time-consuming. For process monitoring, minimum sample handling is always desirable. In addition, relative quantification of the reactants and products is needed to monitor the reaction process. While MALDI MS has been shown to be useful in process monitoring of synthetic and enzyme reactions of relatively large molecules that usually do not show much matrix interference [12], [13], it has not been widely used for monitoring chemical reactions of small molecules [14]. In this work, we report a MALDI MS method that can be used to monitor the reaction process of PCs, with no or little sample preparation for MS analysis. The performance of this method was demonstrated in the characterization of two model PC derivatives, brominated nickel-containing PC (NiPCBr_x) and brominated copper-containing PC (CuPCBr_x). This work illustrates that, by properly selecting the experimental conditions used for sample preparation and desorption ionization, MALDI MS can be a simple and convenient method for reaction monitoring of relatively low molecular weight molecules, such as PCs.

Section snippets

Chemicals and reagents

Ammonium chloride and acetonitrile were from J.T. Baker Chemicals (Phillipsburg, NJ, USA). The HPLC-grade 2,2-dimethyl-1-propanol was from TCI (Tokyo, Japan). 3-Nitrophthalonitrile, sodium hydride, N,N-dimethylformamide (DMF), n-hexane, ethyl acetate, 1,2-diazobicycle[5,4,0]undec-7-ene, nickel(II) dichloride, copper(II) dichloride, sinapinic acid (SA), copper(II) 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (PC 1) and copper(II) tetrakis(4-cumylphenoxy)phthalocyanine (PC 2) were from

Selection of MALDI matrix

Direct laser desorption ionization (LDI) can be used to desorb and ionize phthalocyanines. In fact, some metal-phathalocyanines have been proposed as matrices for analyzing other small molecules by MALDI MS [11]. PC derivatives were found to be ionized during LDI to form radical cations, not cations or protonated species, which is consistent with those reported by others [4]. However, we found that, for analyzing PC derivatives, fragment ions with the loss of side-chains were often observed.

Conclusions

In this study, we have developed and applied a rapid MALDI TOFMS method for monitoring the process of PC derivative synthesis. As examples, the production process of brominated metalphthalocyanine compounds typically used as compact disk dyes was followed by MALDI MS. These compounds were synthesized through cyclization, metallization, and bromination reaction. MALDI MS analysis of the reaction mixtures could be done directly with no sample pre-treatment. The MS method consumes a small amount

Acknowledgments

We are thankful to the National Science Council of the Republic of China, Taiwan [Contract No. NSC 95-2113-M-007-044-MY3 and Science Park Project No. 100A39 (GCRPD1A0011)] and the Ministry of Education of the Republic of China, Taiwan (Contract No. EMRPD1B0291) for the financial support. Additional funding was provided by the Natural Sciences and Engineering Research Council of Canada and the Canada Research Chairs program.

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¹: Part of the work was done at the University of Alberta while Y.T.C. was a visiting researcher.

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Development of a matrix-assisted laser desorption ionization mass spectrometric method for rapid process-monitoring of phthalocyanine compounds

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Chemicals and reagents

Selection of MALDI matrix

Conclusions

Acknowledgments

J. Chromatogr. A

Polyhedron

Polyhedron

J. Am. Soc. Mass Spectrom.

Eur. J. Org. Chem.

J. Am. Soc. Mass Spectrom.

Anal. Chim. Acta

J. Am. Soc. Mass Spectrom.

J. Am. Soc. Mass Spectrom.

J. Am. Soc. Mass Spectrom.

J. Am. Chem. Soc.

Forensic Sci. Int.

Tetrahedron Lett.