31


The samples were made up as follows: 20µL of stock solution were added to a headspace vial containing 1.98ml of diluent (DMAC,


[BMIM][PF6] and [BMIM][BF4]). Duplicates of each diluent were prepared. The approximate concentration of each residual solvent was 500 ppm relative to a test sample concentration of 20 mg/ml. For method validation, determination of system suitability was necessary and therefore 2 ml of each system suitability solution was transferred into 6 headspace vials and made up as


follows: 12µL of stock solution in 20ml of each diluent.


Instruments and Methods A capillary gas chromatograph Agilent 6890GC with a flame ionisation detector and Agilent Technologies Model 7694 headspace analyser was used for the solvent analysis. The column used was a 6% Cyanopropylphenyl; 94% dimethylsiloxane fused silica capillary column (dimensions: 25 m x 0.15 mm internal


diameter x 0.84 µm film thickness). The system was controlled via ChemStation and the data was processed using Empower.


The equilibration temperature was 85°C and equilibration time was 15 minutes with the sample shaker turned on. The transfer line and needle temperature were kept at 140 °C. The injector temperature was 180°C with an injection time of 2 minutes. Vial pressurisation was 30 seconds with 12 psi helium. The loop size was 1ml, with a fill time of 3 seconds and equilibration time of 18 seconds. The FID was kept at 250°C. The column temperature programme consisted of a starting temperature of 40°C for 2.5 minutes, rising to 50°C at 4.44°C per minute, then to 225°C at 80°C per minute. Finally the temperature was held at 225°C for 1.06 minutes. Split flow was measured manually and was 47 ml/min and the carrier gas flow was 1.2ml/minute.


Results and Discussion The chromatogram below shows the response towards the residual solvents studied when DMAC was used as the headspace gas chromatography diluent (Figure 2).


For all low boiling point residual solvents, the response increased when an ionic liquid rather than DMAC was used as the diluent (Figure 3). The change in residual solvent peak areas was used as a measure to quantify the difference in solvent response and hence determine the change in headspace sensitivity when ionic liquids were used as HS-GC solvents instead of DMAC.


5.00 6.00 7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 22.00 23.00 24.00 25.00 26.00


0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 Minutes Figure 2: Chromatogram of residual solvents in DMAC (system suitability solution)


35 30 25 20 15 10 5 0


4.50 5.00 5.50 6.00 6.50 7.00 7.50


[BMIM][BF4] [BMIM][PF6]


Figure 3: Relative responses of residual solvents in [BMIM][BF4] and [BMIM][PF6] compared to DMAC. Note that acetone, acetonitrile and dichloromethane were not included because of interference with the solvent peak of [BMIM][BF4]


0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 400.00 450.00 500.00 550.00 600.00 650.00 700.00 750.00 800.00 850.00 900.00 950.00 1000.00 1050.00


2.00 2.50 3.00 3.50


BMIM BF4 interfering peak


4.00


4.50


5.00 Minutes Figure 4: Overlay of chromatograms for the external standard solution in DMAC (green) [BMIM][PF6] (blue) and [BMIM][BF4] (black)


5.50


6.00


6.50


7.00


7.50


mV


mV


methanol


n-pentane ethanol diethyl ether acetone propan-2-ol acetonitrile DCM ethanol - 2.782 diethyl ether - 2.880 acetone - 3.240 t-butanol propan-2-ol - 3.397


acetonitrile - 3.618 DCM - 3.748


n-hexane t-butanol - 4.389 n-propanol ethyl acetate chloroform n-propanol - 5.334 ethyl acetate - 5.484 chloroform - 5.595 cyclohexane 1,4-dioxan 4-methyl-2-pentanone n-butyl acetate


cyclohexane - 6.096 1,4-dioxan - 6.274


toluene


4-methyl-2-pentanone - 6.549 toluene - 6.606


n-butyl acetate - 6.843 n-hexane - 4.865 n-pentane - 2.622 methanol - 2.079


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