ARTICLE pubs.acs.org/ac
Tetraalkylphosphonium-Based Ionic Liquids for a Single-Step Dye Extraction/MALDI MS Analysis Platform Katherine S. Lovejoy,† Geraldine M. Purdy,† Srinivas Iyer,‡ Timothy C. Sanchez,‡ Al Robertson,§ Andrew T. Koppisch,*,‡ and Rico E. Del Sesto*,† †
Materials Physics and Applications-Material Chemistry Division (MPA-MC), Los Alamos National Laboratory, Mail Stop J514, Los Alamos, New Mexico 87545, United States ‡ Bioscience Division, Los Alamos National Laboratory, Mail Stop M888, Los Alamos, New Mexico 87545, United States § Cytec Canada Inc., 9061 Garner Road, Niagara Falls, Ontario, Canada L2E6S5
bS Supporting Information ABSTRACT: Room temperature ionic liquids, or RTILs, based on tetraalkylphosphonium (PR4þ) cations were used as the basis of a platform that enables separation of dyes from textiles, extraction of dyes from aqueous solution, and identification of the dyes by MALDI-MS in a single experimental step for forensic purposes. Ionic liquids were formed with PR4þ cations and ferulate (FA), R-cyano-4-hydroxycinnamate (CHCA), and 2,5-dihydroxybenzoate (DHB) anions. The use of tetraalkylphosphoniumbased ionic liquids in MALDI-MS allowed detection of small molecule dyes without addition of a traditional solid MALDI matrix.
I
onic liquids (ILs) are salts that are molten at or near room temperature with unique properties that distinguish them from either aqueous or organic solvents. Completely composed of ions, ILs are versatile solvents and functional materials that have found utility in a range of applications such as transition metal catalysis,1 liquid-in-glass thermometers,2 and dye sensitized solar cells.3,4 Frequently used IL cations include pyrrolidinium, ammonium, imidazolium, and phosphonium ions. Depending upon the anion, the aprotic phosphonium ILs can be more thermally stable at high temperatures than ammonium salts5,6 or less reactive.7,8 They resist oxidation and reduction over a wide voltage range and can be designed for miscibility or immiscibility with water. Ionic liquids have been used in various analytical processes, and the development of these applications, including separations, mass spectrometry, spectroscopy, and electrochemistry, have recently been reviewed.9 Our interest in this area is in the creation of an analytical method that allows nondestructive study of fiber-associated dyes while minimizing procedural steps, cleanup requirements, toxicity, and cost to the end user. The use of ionic liquids in sample preparation and their potential value for combining solvation, dissolution, and analysis has been broadly reviewed.10 Fiber type and dye composition varies greatly among textiles, which promotes forensic analysis because these characteristics can be used to identify the origin of fibers and confirm the similarity of one fiber sample to another. In approaching the problem of developing a platform for onestep fiber analysis, we identified three general stages in our r 2011 American Chemical Society
desired process, each with issues that had been previously addressed, to greater or lesser extent, in the literature. First, we required the ability to separate dyes from textile fibers, including wool, and solubilize them. The separation of dyes from wool can require high temperatures, due to the stability of the helical keratin, and has been accomplished with alkylimidazolium ILs,11 although the thermal stability of tetraalkylphosphoniums suggests potential applicability. Second, we required extraction of hydrophilic dyes from aqueous solution. Phosphonium-based ILs have been used in liquid-liquid extractions for some time,9,12 including for extraction of sulfur from fuels13 and lactic acid from aqueous solution,14 and are known to extract analytes from aqueous solution, including dyes15 and carbohydrates.16 Imidazolium-based ionic liquids also extract dyes from aqueous solutions,9,17-21 and efficiency of the process, in the case of anionic dyes, depends on pH and increases with increasing temperature and increasing alkyl chain length of the cation.17 The third general stage in the process design requires identification of the extracted dyes. Extensive work has shown that many ILs are matrixes that promote the ionization of analytes for MALDI-time of flight (TOF) mass spectrometry (MALDIMS).22-24 The performance of quaternary phosphonium ionic liquids in mass spectrometry has been investigated only for liquid injection field desorption/ionization25 and electrospray ionization.26 Except for use of phosphonium salts as additives in Received: November 9, 2010 Accepted: January 11, 2011 Published: March 16, 2011 2921
dx.doi.org/10.1021/ac102944w | Anal. Chem. 2011, 83, 2921–2930
Analytical Chemistry 2-component matrix systems,27 little is known about PR4-based ionic liquids in the context of MALDI-MS, so an investigation of their properties will provide additional data in this area of study. The application of ILs as MALDI matrixes intrinsically addresses a disadvantage of commonly used solid matrixes, which form heterogeneous spots on the MALDI plate and result in poor spot-to-spot reproducibility. IL-based samples do not evaporate to inhomogeneous crystalline spots, which improves data reproducibility, reduces the need for many laser shots at a single sample, and improves the possibility of obtaining quantitative MALDI-MS data.22,28,29 Additionally, ionic liquids solubilize a wide range of nearly insoluble analytes, including dendrimers, high molecular weight polymers, and fibrous proteins, and can stabilize and suppress aggregation of delicate enzyme samples.30,31 One challenge for MALDI-MS is the analysis of analytes with low molecular weights32 (
