Trevor 1. Martin Xerox Research Cenwe of Canada Mlsslssauga. Ontario Canada L5L 1J9
Werner E. Haas Xerox Webster Research Centre Webster, N.Y. 14580
Edited by Jeanette G. Grasselli
ANALYSISOF LIQUIDCRYSTAL MIXTU. m-d It's fairly well-known in today's electronic age that the display device in your digital watch, calculator, desk clock, digital meter, and instrument panel contains liquid crystals. But have you ever considered the problems involved in analysis of these electrooptically sensitive, unusual chemical mixtures? Cas chromatography (CC) or on-line gas Chromatographymass spectrometry ( C C N S )have been used with some success in the past ( I , 2 ) ,but we have found on-line liquid chromatography-mass spectrometry (LC/MS) to be the most useful technique in the analysis of these liquid crystal mixtures. Before discussing in detail our approach to this analysis, however, i t might be beneficial to review the nature and types of liquid crystals. Liquid crystals are substances which have some of the properties of liquids (they flow, pour, and take the shape of their containers) and have some of the optical properties of solid crystals (such as birefringence and optical activity). Most of the liquid crystals used in display devices are of the twisted nematic (meaning threadlike) type. They are long, thin organic molecules which, under the influence of an electric field. undergo a transition from the helical state to an aligned homeotropic state. The practical application of this effect requires a room-temperature nematic liquid crystal having positive dielectric anisotropy; chemical, electrochemical, and photochemical stability; high resistivity; low viscosity for enhanced response times; low birefringence to prevent undesirable optical effects; and absence of color. 0003-27W/S1/0351-593AS01.OO/O @ 1981 AmsricanChernical Society
In order to be useful in eleclrooptic applications, the material must also be in the liquid crystalline (mesomorphic) state, which only exists in a certain temperature range. Above the upper limit of this temperature range the liquid crystal becomes an isotropic liquid and below it a crystalline solid. But all these demands cannot be satisfied by a single liquid crystal component. They require the formula. lion of carefully balanced mixtures.
The mixtures may contain many different liquid crystal families-biphenyls, pbenylcyclobexanes. cyclohexylcyclohexanes, Schiff bases, and others.
The Analfiical Approach L C N S seemed a perfect technique to identify the components in liquid
crystal mixtures since individual components of such mixtures are not unambiguously identifiable by ultravioIeGvisible. infrared. or nuclear maa-
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Figure 1. Major M S fragmentation patterns (a)4-n-alkyl-4'-cyanobiphenyls. (b) 4n-alkoxy-4'-cyanoblphenyls ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
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ANALYTICAL CHEMISTRY, VOL. 53. NO. 4. APRIL 1981
netic resonance (NMR) spectrometry without prior separation, isolation, and purification. Nor could GC separations for on-line G C N S always be readily achieved, especially with the azoxy or Schiff base types of liquid crystals. The inherent sensitivity of the L C N S technique seemed invaluable considering the small quantities of compounds contained in display devices, typically a total of a10 mg. Also, it was desirable to develop an analytical procedure that would be applicable to future problems of this type. One such anticipated problem was the analysis of pleochroic dyes in the presence of liquid crystal materials. Typically, these dyes may be highly polar substituted aminoanthraquinones or trisazodyes, which are not suitable for GC separations because of low volatility a t normal working press u e s or poor thermal stability at elevated temperatures. However, L C N S offers a unique and convenient sohtion. Very little information has been published concerning the mass spectra of liquid crystals or their fragmentation upon electron impact. To our knowledge, only two very recent papers have appeared ( 3 , 4 ) ,and they describe the mass spectra of only a few compounds. Since we had on hand in our laboratories a number of pure single component liquid crystals, as well as a selection of commercial mixtures of 2 4 unknown components, our analytical approach was as follows: The mass spectral fragmentation pathways were first obtained for several typical members of various struct u r d classes, using pure single component liquid crystals whose structures were verified with additional spectroscopic techniques. Thus, a standard li.
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bary of mass spectra was compiled and stored for comparison purposes. The major electron impact (EI) mass spectrometric fragmentation patterns for two representative classes of liquid crystal are shown in Figure 1.While not all-encompassing, the accurate assignment of molecular structures to unknowns depends on differences in the fragmentation patterns for different classes of compounds. Next, the on-line LCIMS procedure was checked using simple mixtures prepared from single components. During this phase of the program, the MS parameters were optimized. In all cases, satisfactory LC separations were obtained using acetonitrilelwater as the mobile phase a t flow rates of 1-2 mLlmin. The columns selected for the analysis were packed with either Partisil5 ODS or Ultrasphere ODs-5 pm. Both columns were 25 cm X 4.6 mm id and possessed greater than 50 000 plates/m for the test compounds. Column eluant was presented via a split device to the Finnigan L C N S belt interface. Generally, the split ratio was adjusted to allow 0.10.5 mllmin to pass to the belt surface. The belt interface was connected to a Finnigan 4000 quadrupole mass spectrometer continuously scanned from mle 45 to mle 500 in the E1 mode at 3 s per scan. Flash evaporation of solute from the belt into the source of the mass spectrometer was achieved by heating the belt to 300 “C. When scanning this mass range, virtually no interfering mass fragments arising from the mobile phase were observed. Backgrounds in general were very low with no change in level during complex gradient elution programs. During acquisition of data files the mass spectrometer was under computer control, using the INCOS
Flgure 2. RLC ..quid crystal mixture with selected mass chromatograms for characteristic base peaks
2000 system interfaced with a Tektronix terminal and cathode ray tube (CRT). For all the “synthetic” mixtures analyzed, satisfactory E1 mass spectra were obtained for the components eluting from the liquid chromatograph.
Analysis of a Comrnerclally Available Liquid Crystal Mlxture Next, several mixtures of “unknowns” obtained from various commercial liquid crystal supply houses were run. For brevity, only one example will be discussed here. Although extensive data on electrical, optical, and physical specifications of the liq uid crystal mixture were supplied by the chemical manufacturer, the sample was identified simply as a mixture of biphenyls and pyrimidines. An isocratic separation of this six-component mixture was accomplished in 60 min on a Partisil5-ODS column employing 65% acetonitrile/35% water as the mobile phase a t a flow rate of 1.7 mL/min. The L C N S run for this mixture is illustrated in Figure 2. The reconstrueted liquid chromatogram (RLC) for this mixture is shown in addition to the mass chromatograms of several typical base peaks, ions mle 192,194,195,268, and 270. Three methods could then be used to unambiguously identify the individual components. First, a search of the existing library of mass spectra of authentic liquid crystal structures could be carried out. If the compound was in the library, a direct comparison of the two spectra for fit and purity could be made. This was done for component 4 (Figure 2), which was confirmed as 4-n-pentyl-4’-cyanobi. phenyl. Second, a computer-assisted search for selected molecular ions indicative of the individual homologous members of the various structural types of liquid crystals could be initiated. The net results of this selected technique could be displayed on the CRT of the INCOS terminal as a set of selected mass chromatograms. An examination of these selected mass chromatograms, along with those for the base peaks, often permits rapid identification of the individual components. Third, when the first two methods were not successful, the mass spectrum of each component could be displayed on the CRT or printed, and intemreted using- basic MS knowledge.With a combination of these three methods, it was possible to assign the structures depicted in Table I for the six components present in the liquid crystal mixture. The assumption WBB made that no branched alkyl groups were present. This assumption ahnost always holds true, since most liquid crystals useful for displays are long
ANALYTICAL CHEMISTRY. VOL. 53, NO. 4, APRIL 1981
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192
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odlike molecules. Occasionally, a mall quantity of a compound with a hiral branched alkyl group is deliberitely added to induce a uniform direcion of "twist" in the device. If this ype of component is suspected in a nixture, it must he isolated and the :%act nature of the branching conirmed with 'H NMR. 4naiysis of a Mixture isolated from h Commercial Display Devlce Finally, several commercial display ievices were broken apart, and the iquid crystal mixtures were extracted, :oncentrated, and subjected to analyiis by on-line LCiMS. The general
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procedure for extraction first involves removal of the reflecting polarizer from the cell surface. The adhesive used to bond the reflecting polarizer is removed by gentle swabbing with dichloromethane, The intact cell is placed in a Teflon beaker and carefully broken into small pieces with a stainless steel rod to expose the inner surfaces. Acetonitrile is added to the beaker, which is then heated to dissolve the liquid crystal mixture. The acetonitrile solution is decanted and the solvent is removed under reduced pressure to give the liquid crystal mixture (usually about 10 mg) as an oily film. The mixtures then are subjected
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P.O. Box 42 Urbana, Illinois USA Phone (21 7) 384-7730 Figure 3. RLC from LClMS run
cial display device CIRCLE 192 ON READER SERVICE CARD
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981
of liquid crystal mixture isolated from the commer-
The Universal POLAROGRAPHIC ANALYZER We don’t claim to have written the book on modern elearoehemical techniques, but we’ve certainly helped a lot of people use it. Since the late 60’s when polarographic methods were evolving into the highly practical and useful forms that we have today, EG&G Princeton Applied Research Corporation has supplied the scientific community with the highest quality instrumentation available. A prime example of this instrumentation is our Model 174A Polarographic Analyzer, perhaps the most popular instrument of its type. We have retained its original design for over ten years, and it’s easy to see why the Model 174A is still the industry standard. Today, thousands of industrial, government, and university laboratories throughout the world employ Model l74A Polarographic Analyzers in applications ranging from analyzing tap water for toxic heavy metals and anions to measuring the trace drug content of biological fluids, to determining the trace metal, vitamin, and stabilizer content of foods.
When combined with the Model 303 Static Mercury Drop Electrode, one obtains an extremely effective polarographic analysis system for the money. The Model l74A is flexible enough for detailed methods development, with provisions for differential pulse polarography and stripping voltammetry. normal pulse polarography, Tast polarography, linear potential sweep voltammetry. and phase sensitive ac polarography (requires 174/50 Interface and Lock-In Amplifier). However, the analyzer has been engineered to enable the operator to perform routine analytical work in a straightforward and timeefficient manner.
To find out more about how our electrochemical instrumentation can aid in your analyses, contact the Electrochemistry Applications Group. EGBrG PRINCETON APPLIED RESEARCH, P. 0.Box 2565, Princeton, NJ 08540; 609/452-21ll.
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ANALYTICAL CHEMISTRY. VOL. 53. NO. 4, APRIL I981
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rable 11. Identity of Components Present In Liquid Crystal Wlxture Isolated from Commercial Display Device
If vou are using chromatography electrophoresis, HPLC you should know BioRad.
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Every year we write a book on all of these techniques and the products we make that go with them. Like ion exchange resins and gels for gel chromatography; equipment and reagents for all types of electrophoresis; systems and columns for HPLC; plus materials for immunochemistry and affinity chromatography. The annual Bio-Rad technical catalog is always crammed with applications on a variety of separation techniques. For your free copy, call or write your nearest Bio-Rad office or circle the reader service number.
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