IC-MS and LC-MS Determination of Ionic Liquids, Counterions, and Impurities

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1 ICMS and LCMS Determination of Ionic Liquids, Counterions, and Impurities Leo Wang, Detlef Jensen, and William Schnute Dionex Corporation, Sunnyvale, CA, USA; Dionex Corporation, Olten, Switzerland INTRODUCTION Ionic liquid commonly refers to organic salts with relatively low melting points (below 00 ºC) that usually consist of an organic cation or anion and a counterion, in either organic or inorganic form. Ionic liquids exhibit unique characteristics such as extremely low vapor pressure, excellent thermal stability, electrical conductivity, and high polarity. The miscibility of ionic liquids with water or organic solvents varies with sidechain lengths on the cation and with choice of anion. A wide range of applications using ionic liquids have been reported in many areas such as catalysis, organic chemistry, electrochemistry, and separation science. Analytical methods for ionic liquid characterization are challenging due to the complexity of the cationic or anionic organic ions, counterions, and ionic impurities. Ion chromatography (IC), liquid chromatography (LC), and hydrophilic interaction liquid chromatography (HILIC) have been used for ionic liquid analysis, featuring ionexchange or reversedphase columns. 6 This study applies two analytical approaches to serve different application purposes. The LCMS method benefits from an Acclaim Trinity trimode column, able to separate cations, anions, and neutral species in a single run. This approach suits qualitative, confirmative, and semiquantitative applications. The ICMS method applies ionexchange separation mechanisms and provides detailed anionic profiles including anionic ionic liquids, counterions, and impurities. This second approach can be used for quality assurance, impurity analysis, and tracelevel residue analysis. INSTRUMENTATION Liquid Chromatography System: UltiMate 000 HPLC system Column: Acclaim Trinity P (. 00 mm, µm) Flow Rate: 00 µl/min Column Temp.: ºC Mobile Phases: A) CH CN; B) 00 mm NH OAc, ph.; C) DI H O Gradient: Time (min) A B C Detection: MSQ Plus mass spectrometer Ion Chromatography System: ICS000 ReagentFree Ion Chromatography (RFIC ) system Column: IonPac AS0 and AG0 ( mm) Flow Rate: 0. ml/min Column Temp.: ºC Mobile Phase: KOH gradient Gradient: Time (min) Hydroxide Concentration 0 mm 6 0 mm 60 mm 6 00 mm 0 00 mm 0. 0 mm Detection: st : Suppressed Conductivity nd : MSQ Plus mass spectrometer Interface Electrospray Ionization (ESI) Probe Temperature: 00 ºC Nebulizer Gas: Nitrogen at psi Needle Voltage:.0 kv See Tables A and B for further scan details.

2 Table A. Ionic Liquids, Counterions, and Impurities by LCMS Peak Analyte t R /min SIM Scan Event Polarity Cone Voltage Lidocaine. 0 positive Butylmethylimidazolium positive Ethylmethylimidazolium. 0 positive Sodium as [Na+6(CH CN)] positive 0 K positive 00 6 CH SO negative 9 7 BF negative 9 PF negative 0 9 Cl negative 90 0 Br negative 90 I negative 90 Tosylate negative 70 Docusate negative 0 Table B. Ionic Liquids, Counterions, and Impurities by ICMS Peak Analyte Formula t R /min SIM Scan Event Cone Voltage Fluoride [F+HF] Acetate CH COO Methanesulfonate CH SO Butanesulfonate CH (CH ) SO Chloride CI Trifluoroacetate CF COO Bromide Br Nitrate NO 9 Sulfate HSO 0 Tosylate CH C 6 H SO Tetraborate BF Triflate CF SO Phosphate H PO Iodide I Thiocyanate SCN Perchlorate CIO 7 Hexafluorophosphate PF RESULTS Choice of Analytical Methodology for Different Application Purposes Due to the variety of applications using ionic liquids, different analytical methodologies are desired to meet different analytical goals. For example, pharmaceutical formulations using ionic liquids as active ingredients need analytical methods to determine those ingredients and counterions at high concentrations with high throughput; ideally, this means a single injection with simultaneous analysis. Other applications using highpurity ionic liquids or ionic liquids as synthetic solvents require analytical methods to determine tracelevels of impurities and the efficacy of the impurity removal process. An LCMS method was developed for highthroughput simultaneous determination of cationic, neutral, and anionic analytes. The Acclaim Trinity column was selected for the separation, because it features reversed phase, anionexchange, and cationexchange retention mechanisms. The ICMS method was developed to achieve low detection levels for anionic analytes, including anionic counterion and impurity profiles. Ion chromatography was the method of choice for quantification purposes due to the significantly improved sensitivity for anionic species observed with ICMS over the LCMS method. This observation can be explained by comparing the eluents entering the MS detector. For ICMS methods, the suppressed eluent is virtually only analytes in deionized water. This keeps the electrospray current at a very low level, enabling efficient ionization and detection of the analytes in the ESI process. In contrast, for the LCMS method, analytes are eluted in a mobile phase of higher ionic strength ( mm total acetate) where analytes are detected with much less efficacy. LCMS for Simultaneous Determination of Ionic Liquids, Counterions, and Impurities Previously reported LCMS methods used reversedphase columns to analyze organic cations or anions, and ionexchange columns to analyze inorganic counterions and impurities such as tetrafluoroborate (BF ), methanesulfonate (CH SO ), hexafluorophosphate (PF 6 ), chloride (Cl ), bromide (Br ), and iodide (I ). In this study, attempting to retain and resolve ionic liquids, counterions, and inorganic ions simultaneously, a trimode column was selected featuring reversedphase retention and cationic/anionicexchange retention mechanisms. Chromatographic behaviors of ionic organics were affected by mobile phase strength (including percentage of organic solvent in the mobile phase and the buffer concentration/ionic strength), buffer ph, and operating temperature. In this study, chromatography was optimized to simultaneously separate imidazolium and lidocaine cationic species, inorganic cations (sodium and potassium), inorganic anionic counterions (CH SO, BF, PF 6, Cl, Br, I ) and anionic species (tosylate and docusate). As shown in Figure, analytes were eluted in groups from a Trinity P analytical column in the following elution order: organic cations, inorganic cations, inorganic anions, and organic anions. ICMS and LCMS Determination of Ionic Liquids, Counterions, and Impurities

3 .E6 Total Ion Chromatogram Column: Acclaim Trinity P,. 00 mm, µm; Mobile Phase: A) Acetonitrile B) 00 mm ammonium acetate C) DI water; Gadient: % A, % B, 0% C from min to min, increase to 60% A, % B, % C from min to 0 min, increase to 90% A, % B, % C from 0 min to min, 90% A, % B, % C from min to min Flow Rate: 00 µl/min Inj. Volume: µl Detector: MSQ Plus Scales normalized to 00% SIM Chromatograms Peaks: m/z. Lidocaine. Butylmethylimidazolium 9. Ethylmethylimidazolium. Sodium (Na+6[CH CN]) Potassium 9 6. Methanesulfonate 9 7. Tetrafluoroborate 7. PF 6 9. Chloride 0. Bromide. Iodide 7. Tosylate 7. Docusate e Ion Chromatography System: ICS000 RFIC Column: IonPac AS0 and AG0 Flow Rate: 0. ml/min Inj. Volume: 0 µl Column Temp: 0 C Mobile Phase: Hydroxide gradient Time (OH ) 0 mm 0 0 mm 6 0 mm 60 mm 6 00 mm 0 00 mm 0. 0 mm Detection: st: Suppressed conductivity nd: MSQ Plus Interface: Electrospray Ionization (ESI) Probe Temp.: 00 C Nebulizer Gas: Nitrogen at psi Needle Voltage:.0 kv See Table for details Injected Amount: 0 ng (0 µl) for conductivity ng (0 µl) for MS SIM See Table B for peak assignment. Suppressed Conductivity ppm for each analyte Response (µs) MS_SIM 00 ppb for each analyte Figure. LCMS for simultaneous analysis of ionic liquids, counterions, and impurities. Figure. Suppressed conductivity and MS SIM chromatograms for ionic liquids and anions. When used in a confirmative analysis mode, this LCMS method can detect subppb levels of major ionic liquid analytes (lidocaine, butyl methylimidazolium, ethylmethylimidazolium, PF 6, docusate), subppm levels of halogen impurities (Cl, Br, I ), and ppb levels of cationic counterions (sodium and potassium). ICMS Determination of Anionic Ionic Liquid Profiles As previously mentioned, the ICMS method provided better sensitivity for anionic species, and better suited the purpose of lowlevel quantification of anionic profiles. This ICMS method focused on the quantitative determination of anionic impurity profiles in ionic liquids, including not only halogen anions such as fluoride (F ), but also additional commonly encountered anions: acetate (CH COO ), butanesulfonate (CH (CH ) COO ), trifluoroacetate (TFA, CF COO ), nitrate (NO ), sulfate (detected as HSO ), triflate (CF SO ), phosphate (detected as H PO ), thiocyanate (SCN ), and perchlorate (ClO ). A multistep gradient (details described in experimental section) was applied to chromatographically separate all analytes, shown in Figure. Method Performance for Quantification Because this ICMS method was developed primarily for quantification purposes, method performance was evaluated with respect to calibration range, precision, and detection limits. The evaluation results are summarized in Table.

4 Table. Calibration, Range, and Method Detection Limits (MDL) Peak Analyte Calibration Range (ppb) Fitting r %RSD b MDL c Fluoride a Linear Acetate a Linear NC d NC d Methanesulfonate 000 Quadratic Butanesulfonate 000 Linear Chloride 000 Quadratic Trifluoroacetate 000 Cubic Bromide 000 Cubic Nitrate 000 Cubic Sulfate 000 Quadratic Tosylate 000 Quadratic Tetraborate 000 Cubic Triflate 000 Cubic Phosphate 000 Quadratic Iodide 000 Cubic Thiocyanate 000 Cubic Perchlorate 000 Cubic Hexafluorophosphate 000 Quadratic a Data obtained from previous experiments b %RSD calculated based on experiments with n > c MDL calculated by MDL = S t (99%, n > ) d NC not calculated Calibration curve and range were evaluated against a set of calibration standards from to 000 ppb at levels. Good coefficients of determination were achieved for each analyte with r > 0.99 from the lowest calibration standard with S/N > 0 to 000 ppb. Method detection limits (MDL) were statistically calculated by the equation MDL = S t 99%, where S is the standard deviation and t is the Student s t at 99% confidence interval (n > ). MDLs were achieved at low ppb levels for each analyte, ranging from.0 ppb (tosylate) to 6. ppb (sulfate). Chromatography System: UltiMate 000 HPLC System Column: Acclaim Trinity P (. 00 mm, µm) Column Temp.: ºC Gradient: A) Acetonitrile; B) 00 mm Ammonium acetate; C) DI Water Time %A %B %C Flow Rate: 00 µl/min Injection: µl.e6.e.e.0e.e6 System: MSQ Plus Interface: Electrospray Ionization (ESI) Probe Temp.: 00 ºC Needle Voltage: 000 V Nebulizer Gas: Nitrogen at psi Samples A. Lidocaine, chloride salt, µm B. Docusate, sodium salt, µm C. Butylmethylimidazolium hexafluorophosphate, µm D. Ethylmethylimidazolium tosylate, µm E. Mixed standards of ionic liquids, counterions, and impurity ions, various concentrations from 0. µm to 000 µm See Table A for peak assigment Figure. LCMS analysis of commercial ionic liquids. The Total Ion Chromatogram (TIC) shown in Trace E is the sum of SIM scans. A B C D E Analysis of Commercial Ionic Liquids The two methods described here were used for the analysis of several commercial ionic liquids. Neat chemicals were dissolved in DI water at mg/ml, and injected directly for ICMS analysis. For LCMS analysis, the standard solutions were first diluted to µm and injected for analysis. Figure shows the LCMS chromatograms for four ionic liquids and counterions. Figure shows the conductivity and MS SIM traces of butylmethylimidazolium/pf 6 and detected anions by IC MS. Table summarizes the quantification results for the detected anions. ICMS and LCMS Determination of Ionic Liquids, Counterions, and Impurities

5 Response (µs).e.6e.0e Chloride Bromide.0E MS_SIM: 9 m/z MSA.6E MS_SIM: 97 m/z Sulfate Tosylate 0 ECD_ Figure. PF 6 and impurity anions by ICMS and conductivity. MS_SIM: m/z MS_SIM: 79 m/z MS_SIM: 7 m/z Table. Quantification of Anions in Commercial Ionic Liquids Analyte Lidocaine/HCI Butyl methyl imidazolium/pf6 66 Ethyl methylimidazolium/tosylate Acetate.99 ND ND Methanesulfonate ND BRL ND Chloride.. Bromide Nitrate BRL BRL BRL Sulfate BRL BRL.6 Tosylate BRL BRL Hexafluorophosphate BRL.0 CONCLUSION This study describes two approaches for analysis of ionic liquids: an LCMS method for ionic liquid quantification and confirmative analysis for counterions and impurities, and an ICMS method for low level quantification of anionic ionic liquids and anionic species, including counterions and impurities. Using the LCMS method, major ionic liquid analytes can be analyzed at subppb levels with the confirmation of major cation impurities: sodium and potassium. Using the ICMS method, quantification at lowppb levels was achieved for diluted solutions (lowppm level in original sample) with good coefficients of determinations (r > 0.99). Three commercially available ionic liquid samples were analyzed by this method, with the common impurities of chloride and bromide quantified at ppm levels. Acetate, sulfate, and hexafluorophosphate were also discovered in some samples. REFERENCES. Wilkes, J. S. Green Chem. 00,, 70.. Siddiqui, S. A.; Narkhede, U. C.; Palimkar, S. S.; Daniel, T.; Lahoti, R. J.; Srinivasan, K. V. Tetrahedron 00, 6, 96.. Shinde, S. S.; Lee, B. S.; Chi, D. Y. Org. Lett. 00, 0, 77.. Marisa, C. B.; Russell, G. E.; Richard, G. C. Chem. Phys. Chem. 00,, Huddleston, J. G.; Rogers, R. D. Chem. Comm. 99, Anderson, J. L.; Armstrong, D. W. Anal. Chem. 00, 7,. 7. Zhang, W.; He, L.; Gu, Y.; Liu, X.; Jiang, S. Anal. Lett. 00, 6, 7.. Qiu, H.; Jiang, S.; Liu, X.; Zhao, L. J. Chromatogr., A 006, 6, 60. ReagentFree, RFIC, and Trinity are trademarks and Acclaim, IonPac, and UltiMate are registered trademarks of Dionex Corporation. MSQ Plus is a trademark of Thermo Fisher Scientific. Dionex Corporation North America Europe Asia Pacific Titan Way P.O. Box 60 U.S./Canada (7) 9700 Austria () 66 Benelux () () 9 Denmark () France () Germany (9) Australia (6) 90 China () India (9) 76 7 Japan () 6 6 Korea () 6 0 Singapore (6) Sunnyvale, CA South America Ireland () Italy (9) Sweden (6) 7 0 Taiwan (6) 7 66 Brazil () 7 0 LPN 0 / Switzerland () United Kingdom () (0) Dionex Corporation