Analytes planar/nonplanar

Consider, for example, six different pairs of analytes, all containing an aromatic ring (bases, acids, isomers, etc.) which were separated under different conditions (methanol, methanol/water, mefhanol/buffer). The more symmetrical the hexagon, the more applicable is the corresponding stationary phase for the separation of aromatic substances in general. In addition, it is easy to decide which column is suitable, e.g., for the separation of quite strong aromatic acids (phthalic acid/terephthalic acid, Tere/Phthal ), which is best for weaker aromatic acids (3-hydroxy-/4 hydroxybenzoic acid, 3/4-OH ), and which is best for planar/nonplanar aromatic substances (triphenylene/o-terphenyl, Triph/ o-Ter ). 

Carbon fibers have not been studied as extensively as GC or graphite, and in most cases the fiber is pretreated. Thus most of the electrochemical properties of fibers are discussed in the next section, Preparation. A few general points are useful here, dealing with size and resistance. Since the majority of carbon fibers are 5-15 pm in diameter, they will exhibit nonplanar diffusion under most conditions, whether they are used as disks or as cylinders. For example, VDt for a typical analyte (D = 5 x 10 6 cm2/s) equals 2.2 pm at 10 ms. This is a significant fraction of a typical fiber diameter, so diffusion will become nonplanar even at short times. Thus any experiment lasting more than a few milliseconds will deviate from a response predicted for planar diffusion. Note that the deviation depends on whether the fiber end is used as a disk electrode or an exposed fiber is used as a cylinder, but quantitative theories have been presented for both cases [48]. 

PGC sorbents have even more highly homogeneous hydrophobic surfaces than GCB sorbents. PGCs are macroporous materials composed of flat, two-dimensional layers of carbon atoms arranged in graphitic structure. The flat, homogeneous surface of PGC arranged in layers of carbons with delocalized n electrons makes it uniquely capable of selective fractionation between planar and nonplanar analytes such as the polychlorinated biphenyls [92,94,95],... 

The degrees of planarity of the peropyrene-type PAHs appear to be determined by the balance between the loss in resonance energy when the molecules become nonplanar and the increases in energy due to decreased intramolecular steric strain and stronger solvation. The degree of planarity and the relative importance of these factors for each PAH structure can be estimated from the analytical behavior of that PAH. Conversely, these molecules can serve as excellent probes of the mechanisms of HPLC. 

A modified version of this criterion was developed by Mullins and Sekerka (1964). The authors attribute the incorporation of impurities essentially to the nonplanar growth of the crystalline layer and analytically describe this by superposing a planar layer with a sinusoidal disturbance. Afterwards, they determine the conditions under which this disturbance is damped and derive a criterion which generally guarantees such conditions during the process. The applicability of this stability criterion was proven for a variety of metallic and organic compounds. 

All these well-established concepts obviously are useful for the discussion of the planar cumuienes, such as ketenes, diazocompounds, thioketenes, and butatrienes. For nonplanar cumuienes (allenes, ketene imines, carbodiimides, pentatetraenes) the situation is not so simple, as the proper classification of electronic states depends essentially on the substitution patterns of the molecules (lc,24,25,73). The usual concepts based on VB arguments have led toclassiH-cations and analytical descriptions of allenic electronic systems (or orbitals) in terms of two perpendicular, isolated tt systems, especially in terms of two-center ethylenic tt systems (24). Sometimes, hyperconjugation of the eth-ylenic C=C tt system with the CH2 group orbitals of corresponding symmetry has been taken into consideration (24). 

Since the toxicity of a PCB mixture is principally associated with the presence of nonortho congeners, analytical procediues should be able to discriminate between the contents of planar and nonplanar congeners. This may be difficult because the ratio between ortho- and nonortho-PCBs is typically 100 in commercial mixtures as well as in environmental matrices, and peak overlapping between ortho- and nonortho-PCB isomers may occur. This makes the determination of nonortho-PCBs impossible without a preseparation step. ITPLC on a porous graphitic carbon (PGC) column with hexane as the eluent has been suggested for this purpose [41]. 

---