1-Butyl-3-methylimidazolium pentafluorophenyl trifluoroborate as new type ionic liquids for MALDI analysis small molecules

Highlights

• Optimization of MALDI analysis small molecules: the choice of the best IL

• ∙The spectra for histidine containing an imidazolium fragment are much more intense

• ∙Homogeneity of the IL distribution in the depth and on the surface is shown

• ∙The size of the IL anion has no strong effect on the intensity of the spectra of analytes

Abstract

The use of matrix laser desorption/ionization mass spectrometry (MALDI) for the study of small molecules is still critical and limited by the efficiency of the matrices. In this work, for the first time, the effect of anion and cation in the structure of ionic liquids to obtain high-intensity MALDI spectra of amino acids is shown. Ten ionic liquids based on substituted imidazole were tested. Three amino acids (histidine, alanine and serine) were used as test compounds to assess the effectiveness of ionic liquids. It was found that the BMIMC₆F₅BF₃ ionic liquid, used for the first time as a MALDI matrix, showed the best characteristics in terms of ionization efficiency for the analytes under consideration. Mass spectrometric imaging was used to estimate the degree of homogeneity in the distribution of the analyte over the surface and in the volume of the matrix with ionic liquid. It has been shown that, without ionic liquid, approximately 40% of the total surface area with an extremely low amino acid content is observed on the matrix surface. Whereas in the presence of ionic liquid there are only 3% of such areas. The use of an ionic liquid allows the analyte to be concentrated in the surface layer of the matrix. At the same time, without IL, only when using a classical matrix, there is an uneven distribution of amino acid both in the surface layer and throughout the entire volume of the matrix.

Introduction

MALDI is a soft ionization method widely used in analytical chemistry, biology, biotechnology, biomedicine and organic chemistry [1,2]. A wide range of different matrices is used for the MALDI-MS [3]. It can be organic [4,5], inorganic and nanoparticles [6,7].

Co-crystallization of the analyte with these matrices leads to its non-uniform distribution over the target and the formation of so-called hot spots — areas with different matrix/analyte ratios. This leads to non-reproducible values of the signal intensities from the analyte at different points in the sample and at one point at successive pulses [8]. In addition, the matrix creates a high background in the mass spectrum, especially in the low-mass region [9]. This can lead to the loss of a useful analytical signal against the background of an intense matrix signal.

To solve the problems typical of classical matrices, Armstrong and Zhang were the first to use ionic liquids (ILs) as matrices in the analysis of a sample by the MALDI method [10]. The use of MALDI matrices based on IL made it possible to analyze small molecules, for example, amino acids and their short sequences [11]. It was found that the analyte in IL is distributed more uniformly than in other types of matrices. This leads to a more reproducible MALDI analysis compared to a similar analysis using classical matrices [12].

The properties of ILs depend on both the structure of the anion and the cation. Therefore, by varying the cation – anion combination, a wide range of ILs with various properties can be created. At the same time, it is known that properties such as hydrophobicity and thermal stability are largely determined by the structure of the anion [13].

The most commonly used cations for ILM are imidazolium, pyridinium, ammonium, phosphonium and pyrrolidinium. While the anions are usually used: halides, tetrafluoroborate, hexafluorophosphate, triflate, bis (perfluoromethylsulfonyl) imide, carboxylates, trifluoroacetate, hydrogen sulfate and nitrate [14].

The paper presents [15] a new platform for ILs with an aromatic anion based on pentafluorophenyltrifluoroborate salts [15]. It is expected that such anions will possess new properties due to more efficient delocalization of the negative charge and specific interactions of the aromatic ring [16]. There are scarce literature data on ILs with an aromatic anion [17]. There are no data on the use of ILs with such an anion as matrices for MALDI.

The aim of this work was to study the possibility of using a new IL for the analysis of small molecules having applied the example of model amino acids by the LDI method. The authors were the first to investigate the effect of the nature of the cation in the structure of an ionic liquid with a new aromatic anion –C₆F₅BF₃ used as a MALDI matrix on the efficiency of amino acid ionization. Ten ILs based on substituted imidazole with various anions and cations were studied as ILMs to study the line intensities in the MALDI spectra of test amino acids (histidine, alanine, and serine). To establish the efficiency of using IL with an aromatic anion based on pentafluorophenyltrifluoroborate salts, IL with the 1-Butyl-3-methylimidazolium cation and with the anions Cl-, Br-, BF₄-, PF₆-, C₆F₅BF₃- was considered. It is shown that the IL proposed by the authors has the best characteristics among the ten considered in this work.

For test amino acids, the distribution on the surface of the target after sample application and in the volume of a matrix containing IL with an aromatic anion based on pentafluorophenyltrifluoroborate salts and on the target surface not containing IL, but only amino acids with a classical matrix, has been studied. The method of mass spectrometric imaging of the surface was used to carry out the measurements, which made it possible to estimate the distribution of analyte both on the surface and in the depth of the IL film. New criteria for evaluating the suitability of an ionic liquid for MALDI analysis are proposed, such as the distribution of the analyte over the depth of the matrix - ionic liquid composition and the uniformity of the distribution of the analyte over the surface.