The Analytical Approach
Trevor I. Martin Xerox Research Centre of Canada Mississauga, Ontario Canada L5L 1J9
Werner E. Haas Xerox Webster Research Centre Webster, N.Y. 14580
Edited by Jeanette G. Grasselli
ANALYSIS OF LIQUID CRYSTAL MIXTURES 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 prob lems involved in analysis of these electrooptically sensitive, unusual chemi cal mixtures? Gas chromatography (GC) or on-line gas chromatographymass spectrometry (GC/MS) have been used with some success in the past (1, 2), but we have found on-line liquid chromatography-mass spec trometry (LC/MS) to be the most use ful technique in the analysis of these liquid crystal mixtures. Before discussing in detail our ap proach to this analysis, however, it might be beneficial to review the na ture 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 op tical 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 elec tric field, undergo a transition from the helical state to an aligned homeotropic state. The practical application of this ef fect requires a room-temperature nematic liquid crystal having positive di electric anisotropy; chemical, electro chemical, and photochemical stability; high resistivity; low viscosity for en hanced response times; low birefrin gence to prevent undesirable optical effects; and absence of color. 0003-2700/81 /0351-593A$01.00/0 © 1981 American Chemical Society
In order to be useful in electrooptic applications, the material must also be in the liquid crystalline (mesomor phic) state, which only exists in a cer tain 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 tion of carefully balanced mixtures.
(a)
The mixtures may contain many dif ferent liquid crystal families—biphenyls, phenylcyclohexanes, cyclohexylcyclohexanes, Schiff bases, and others. The Analytical Approach
LC/MS seemed a perfect technique to identify the components in liquid crystal mixtures since individual com ponents of such mixtures are not un ambiguously identifiable by ultravio let-visible, infrared, or nuclear mag-
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CN|
CH2 CH 3
-CH 3 (CH 2 )n
(CH2)rr\ /
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η
Mol. Wt.
3 4 5
249 263 277
η
Mol. Wt.
5
279
m le 192 (b) CH 3 (CH 2 )n
C=N
Ο
-
~ C n + 1 H2n + 3 l + Η '
HO
C=N
m le 195
Figure 1 . Major MS f r a g m e n t a t i o n p a t t e r n s , (a) 4 - n - a l k y l - 4 ' - c y a n o b i p h e n y l s . (b) 4 n-alkoxy-4'-cyanobiphenyls ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981 · 593 A
netic resonance (NMR) spectrometry without prior separation, isolation, and purification. Nor could GC sepa rations for on-line GC/MS always be readily achieved, especially with the azoxy or Schiff base types of liquid crystals. The inherent sensitivity of the LC/MS technique seemed invalu able considering the small quantities of compounds contained in display de vices, typically a total of 8-10 mg. Also, it was desirable to develop an analytical procedure that would be ap plicable to future problems of this type. One such anticipated problem was the analysis of pleochroic dyes in the presence of liquid crystal materi als. Typically, these dyes may be high ly polar substituted aminoanthraquinones or trisazodyes, which are not suitable for GC separations because of low volatility at normal working pres sures or poor thermal stability at ele vated temperatures. However, LC/MS offers a unique and convenient solu tion.
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 as signment of molecular structures to unknowns depends on differences in the fragmentation patterns for differ ent classes of compounds. Next, the on-line LC/MS 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 acetonitrile/water as the mobile phase at flow rates of 1-2 mL/min. The columns selected for the analysis were packed with either Partisil 5 ODS or Ultrasphere ODS-5 μπι. Both columns were 25 cm X 4.6 mm id and possessed greater than 50 000 plates/m for the test com pounds. Column eluant was presented via a split device to the Finnigan LC/MS belt interface. Generally, the split ratio was adjusted to allow 0.10.5 mL/min to pass to the belt surface. The belt interface was connected to a Finnigan 4000 quadrupole mass spec trometer continuously scanned from m/e 45 to m/e 500 in the EI 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.
Very little information has been published concerning the mass spectra of liquid crystals or their fragmenta tion upon electron impact. To our knowledge, only two very recent pa pers have appeared (3,4), and they describe the mass spectra of only a few compounds. Since we had on hand in our labora tories a number of pure single compo nent liquid crystals, as well as a selec tion of commercial mixtures of 2-6 un known components, our analytical ap proach was as follows: The mass spectral fragmentation pathways were first obtained for sev eral typical members of various struc tural classes, using pure single compo nent liquid crystals whose structures were verified with additional spectro scopic techniques. Thus, a standard li-
14
mle 192 mle 194
L '
'
2
mle 195
6
mle 268 mle 270
When scanning this mass range, vir tually 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 pro grams. During acquisition of data files the mass spectrometer was under computer control, using the INCOS
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Scan Time
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6 800 40:00
1000 50:00
1200 60:00
Figure 2. RLC for commercial liquid crystal mixture with selected mass chromato grams for characteristic base peaks
2000 system interfaced with a Tektro nix terminal and cathode ray tube (CRT). For all the "synthetic" mix tures analyzed, satisfactory EI mass spectra were obtained for the compo nents eluting from the liquid chromatograph. Analysis of a Commercially Available Liquid Crystal Mixture
Next, several mixtures of "un knowns" obtained from various com mercial liquid crystal supply houses were run. For brevity, only one exam ple 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 sam ple was identified simply as a mixture of biphenyls and pyrimidines. An isocratic separation of this six-compo nent mixture was accomplished in 60 min on a Partisil 5-ODS column employing 65% acetonitrile/35% water as the mobile phase at a flow rate of 1.7 mL/min. The LC/MS run for this mixture is illustrated in Figure 2. The reconstructed liquid chromatogram (RLC) for this mixture is shown in ad dition to the mass chromatograms of several typical base peaks, ions m/e 192, 194, 195, 268, and 270. Three methods could then be used to unambiguously identify the indi vidual 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'-cyanobiphenyl. Second, a computer-assisted search for selected molecular ions in dicative of the individual homologous members of the various structural types of liquid crystals could be ini tiated. 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 com ponents. Third, when the first two methods were not successful, the mass spectrum of each component could be displayed on the CRT or printed, and interpreted using basic MS knowl edge. 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 was made that no branched alkyl groups were present. This assumption almost always holds true, since most liquid crystals useful for displays are long
ANALYTICAL CHEMISTRY, VOL. 53, NO. 4, APRIL 1981 · 595 A
Table I. Structures of Components Present in Commercial Liquid Crystal Mixture
Light
Component no.
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Nominal mol. wt.
Structure
Base peak
(1)
CHUCH,),—Ç > - ^ 3 — C N
251
194
(2)
CHa(CH 2 ) 4 —0—Ç^—Ç~y— CN
265
195
(3)
cHxcH^—£ >—(3—CN
279
195
(4)
C B f f l ) i - ^ V Q - CN
249
192
(5)
CI«CHU~Q-^J-Q-CN
313
270
(6)
CH
325
268
fluorescence
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