Compositional Analysis of Merck E7 Liquid Crystal Intermediates Using UltraPerformance Convergence Chromatography (UPC² ) with PDA Detection

This application note illustrates the efficient, cost-effective compositional analysis of Merck E7 liquid crystal intermediate compounds using the ACQUITY UPC² System with PDA detection.

Benefits

Greater than 20-fold increase in sample throughput, and greater than a 300-fold reduction in the volume of toxic solvents used.

Liquid crystals combine the physical and optical properties of both liquids and solids. They flow and pour like liquids, but have some of the optical properties of solids, such as birefringence. They also react predictably to an electric current, which enables the control of light passage. Due to these properties, liquid crystals are used in many items with electronic displays, for example: watches, calculators, mobile phones, desktop monitors, and TVs.

The E7 nematic liquid crystals mixture contains cyanobiphenyl and cyanoterphenol components, at a specific composition, which possess relatively high birefringence and positive dielectric anisotropy. Due to these properties, it is widely used in polymer dispersed liquid crystals.^1,2,3 The specific composition is critical to ensure physical properties and characteristic of the liquid crystal. Even small changes can have pronounced effects on factors such as the nematic to isotropic transition (T_NI), and glass transition (T_G) temperatures.

For the compositional analysis typically a chromatographic technique would be used for the analysis of liquid crystal intermediate compounds, for example: HPLC with UV detection,⁴ HPLC with MS detection,⁵ and GC with MS detection.⁶

Convergence Chromatography (CC) is a normal phase separation technique that uses carbon dioxide as the primary mobile phase, with the use of a co-solvent such as methanol. Waters UltraPerformance Convergence Chromatography (UPC²) builds upon the potential of CC while using proven and robust Waters UPLC technology.

Many liquid crystal intermediate compounds are not very stable at high temperatures, have low volatility, and have similar UV spectra. Therefore, utilizing the separation powers of UPC² with CO₂ as the mobile phase is an ideal alternative to both HPLC and GC analysis.

This application note describes the compositional analysis of Merck’s E7 liquid crystal mixture utilizing UPC² with PDA detection offering: robustness, selectivity and sensitivity, with reduced run times, and associated savings in the cost and disposal of toxic solvents.

Sample description

The Merck E7 liquid crystal intermediate compounds were purchased from Sigma Aldrich (their structures are shown in Table 2). Individual stock solutions were prepared to a concentration of 5 mg/mL, dissolved in either heptane/ethanol (9:1) or methanol. Serial dilutions of the stock solutions were carried out in heptane/isopropanol (9:1) in order to prepare mixed standards, or in methanol to prepare infusion MS tuning standards.

Instrument control, data acquisition, and results processing

Empower 3 Software was used to control the ACQUITY UPC² and the ACQUITY UPC² PDA Detector, as well as for data acquisition.

The UPC² conditions were optimized for the analysis of the selected liquid crystal intermediate compounds. Retention times and UV optimum absorbances were established by analyzing single component standards (see Table 3).

The analysis of the four Merck E7 liquid crystal intermediate compounds was achieved using the ACQUITY UPC² System with the ACQUITY UPC² PDA Detector.

UPC² and PDA conditions were optimized, considering different UPC² columns, co-solvents, column temperatures, mobile phase temperatures, and system CO₂ back pressures. The elution of all compounds was achieved in a 3-minute run, offering a 20 to 90 fold time saving when compared to HPLC.^4,5

Mixed calibration standards, 0.01 to 0.40 mg/mL, were prepared and analyzed for all the compounds considered. The calibration curve results generated by Empower Software for 4-cyano-4'-n-puntyl-biphenyl, are shown in Figure 1.

The UV chromatograms for each liquid crystal intermediate compound in a mixed 0.1 mg/mL calibration standard, are shown in Figure 2, and the associated UV spectra, are shown in Figure 3.

Compositional analysis

The ratio of the individual liquid crystal intermediate compounds in the required mix is paramount to achieving optimum optical quality, performance, and lifetime of the electronic device. Therefore the ability to quantify the ratio is critical to ensure optimum efficiency of the liquid crystal.

In order to demonstrate compositional analysis, a mix of the four compounds in the E7 was prepared, one containing the correct ratio and one at an incorrect ratio; both were then analyzed using the developed UPC² conditions with PDA detection. The resulting UV chromatograms acquired are shown in Figure 4, and the achieved results are provided in Table 4.

Utilizing the custom calculation function of Empower 3, results can be quickly reported, highlighting preset pass/fail criteria. This removes the need for manual calculations and in doing so, eliminates potential human calculation errors. Generated compositional custom QC reports are shown in Figure 5.

By utilizing the efficiency of ACQUITY UPC2 with PDA detection, a cost-effective, sensitive, and selective compositional analysis of E7 liquid crystal mixtures can be achieved.

The ratio of the individual liquid crystal intermediate compounds in the required mix is paramount to achieving optimum optical quality, performance, and lifetime of the electronic device. Therefore the ability to quantify the ratios is critical to ensuring optimum efficiency of the liquid crystal.

Many business and analytical benefits are provided when compared to HPLC for the analysis of E7 liquid crystal mixtures, with typically greater than a 20 fold increase in sample throughput and a 300 fold reduction in the volume of toxic solvent required.

Using the custom calculation functions available in Empower 3, results can quickly by reported, highlighting preset pass/fail criteria. By removing the need for manual calculations helps to eliminate potential human calculation errors.

1. Mouquinho A, Petrova K, Barros M T, and Sotomayor J. New Polymer Networks for PDLC Films Application, New Polymers for Special Applications, Chapter 5: 139-164. (2012) [cited 2013 August 6]. Available from: http:// www.intechopen.com/books/new-polymers-for-special-applications/ new-polymer-networks-for-pdlc-films-application.
2. Bedjaoui L, Gogibus N, Ewen B et al. Preferential salvation of the eutectic mixture of liquid crystal E7 in a polysiloxane. Polymer. (2004) 45: 6555-6560.
3. Mucha M. Polymer as an important component of blends and composites with liquid crystals. Progress in Polymer Science. (2003) 28: 837-873.
4. Brás A, Henriques S, Casimiro T, et al. Characterization of a Nematic Mixture by Reversed-Phase HPLC and UV Spectroscopy: Application to Phase Behavior Studies in Liquid Crystal-CO2 Systems. Electronic-Liquid Crystal Communications. March 28, 2005. [cited 2013 May 20]. Available from: http://www.e-lc.org/tmp/M.___Dion%EDsio_2005_03_21_07_41_31.pdf .
5. Martin T, Hass W. Analysis of Liquid Crystal Mixtures. Analytical Chemistry. (1981) 53(4): 593-602.
6. Laclercq P, van den Bogaert H. Mass Spectra of Liquid Crystals. Organic Mass Spectrometry. (1991) 26: 276-278.