20 15 10 5 0 0 500 1,000 1,5002,000 2,500 3,000 Molecular weight of fraction(amu) Figure 7.6 Molecular weight distribution of the pyridine-soluble part of coal-tar pitch(Boenigk et al., 1990). range of molecular weight from ~200 to >3,000 amu (Zander,1997).As an example, Fig.7.6 shows the molecular weight distribution determined by preparative size exclusion chromatography (SEC)of the pyridine-soluble fraction (95 vol%)of a coal-tar pitch (Boenigk et al.,1990).From results obtained by preparative SEC and other techniques, these authors conclude that a typical coal-tar pitch consists of~40 wt%of compounds with a molecular mass of s330 amu,~50 wt%of compounds with a molecular mass ~330-1,500 amu,while 10 wt%of the remaining pitch consists of high-molecular weight material (between ~1,500 and >3,500 amu). Coal-tar pitches are mainly composed of polycyclic aromatic hydrocarbons(PAH)and to a lesser extent of polycyclic aromatic compounds containing alkylsubstituents and/or het- eroatoms(O,S,and N).These compounds can be compiled into categories by fractionation by means of extrography (Granda et al.,1990).This technique is able to distinguish between pitches differing in origin (coal-tar or petroleum),their uses (binding or impreg- nating)and also the preparation process (thermal treatment,vacuum distillation,etc.). Thus impregnating and binder pitches mainly differ in the distribution of PAH and the amount of basic polycyclic aromatic compounds(Bermejo et al.,1994).A typical impregnating pitch has a higher content in both PAH of lower molecular weight and basic nitrogen compounds. There is also what is considered to be a third generation of pitches.Some of these are commercially available(Mitsubishi Gas Chemical and Mitsubishi Oil)and others are still under development.This third group includes the mesophase pitches(from organic precur- sors and from the pyrolysis of commercial pitches)and isotropic pitches of high carbon yield(from the direct treatment of tar,commercial pitches,and other tar distillation fractions such as anthracene oil).These will be discussed in Section 5 of this chapter. 2003 Taylor Francisrange of molecular weight from ~200 to 3,000 amu (Zander, 1997). As an example, Fig. 7.6 shows the molecular weight distribution determined by preparative size exclusion chromatography (SEC) of the pyridine-soluble fraction (95 vol %) of a coal-tar pitch (Boenigk et al., 1990). From results obtained by preparative SEC and other techniques, these authors conclude that a typical coal-tar pitch consists of ~40 wt % of compounds with a molecular mass of 330 amu, ~50 wt % of compounds with a molecular mass ~330–1,500 amu, while 10 wt % of the remaining pitch consists of high-molecular weight material (between ~1,500 and  3,500 amu). Coal-tar pitches are mainly composed of polycyclic aromatic hydrocarbons (PAH) and to a lesser extent of polycyclic aromatic compounds containing alkylsubstituents and/or het￾eroatoms (O, S, and N). These compounds can be compiled into categories by fractionation by means of extrography (Granda et al., 1990). This technique is able to distinguish between pitches differing in origin (coal-tar or petroleum), their uses (binding or impreg￾nating) and also the preparation process (thermal treatment, vacuum distillation, etc.). Thus impregnating and binder pitches mainly differ in the distribution of PAH and the amount of basic polycyclic aromatic compounds (Bermejo et al., 1994). A typical impregnating pitch has a higher content in both PAH of lower molecular weight and basic nitrogen compounds. There is also what is considered to be a third generation of pitches. Some of these are commercially available (Mitsubishi Gas Chemical and Mitsubishi Oil) and others are still under development. This third group includes the mesophase pitches (from organic precur￾sors and from the pyrolysis of commercial pitches) and isotropic pitches of high carbon yield (from the direct treatment of tar, commercial pitches, and other tar distillation fractions such as anthracene oil). These will be discussed in Section 5 of this chapter. 20 15 Weight fraction (wt %) 10 5 0 0 500 1,000 Molecular weight of fraction (amu) 1,500 2,000 2,500 3,000 Figure 7.6 Molecular weight distribution of the pyridine-soluble part of coal-tar pitch (Boenigk et al., 1990). © 2003 Taylor & Francis