Heart-cut

The main feedstock for catalytic reforming is heavy gasoline (80 to 180°C) available from primary distillation. If necessary, reforming also converts byproduct gasoline from processes such as visbreaking, coking, hydroconversion and heart cuts from catalytic cracking. 

After the cmde BTX is formed, by reforming in this case, a heart cut is sent to extraction. Actually, the xylenes and heavier components are often sent to downstream processes without extraction. The toluene produced is converted to ben2ene, a more valuable petrochemical, by mnning it through a hydrodealkylation unit. This catalytic unit operates at 540—810°C with an excess of hydrogen. Another option is to disproportionate toluene or toluene plus aromatics to a mixture of ben2ene and xylenes using a process such as UOP s Tatoray or Mobil s Selective Toluene Disproportionation Process (STDP) (36). 

More commonly, a fraction, based on chemical type, molecular weight or volatility, is heart-cut from the eluent of the primary column and introduced into a secondary column for more detailed analysis. If the same mobile phase is used in both dimensions, fractions may be diverted by means of pressure changes-an approach first used in 1968 in GC-GC by Deans (35), and applied by Davies et al. in SFC-SFC (36). If the mobile phases are different, valves are employed, and special... 

Efforts have been made, however, to extend the range or extent of samples that can be analysed by using a two-dimensional separation when used in heart-cut mode. This has been reported to include the use of numerous parallel micro-traps to essentially store the primary column eluent fractions ready for second-column separation, and the use of parallel second-dimension columns. 

Pigure 3.1 shows several potential on-line modes of two-dimensional GC operation. These couplings demonstrate HRGC-HRGC performed by using a single heart-cut from the primary to the secondary column, multiple heart-cuts, transferred to multiple intermediate traps, and heart-cuts transferred to a multiple parallel secondary column configuration. 

Figure 3.1 Two-dimensional gas clnomatography instmmental configurations (a) direct ti ansfer heart-cut configuration (b) multiple parallel ti ap configuration (c) multiple parallel column configuration.

Figure 3.4 Two-dimensional separation of dimethylnaphthalenes in crude oil using a 50 m methyl (95%)/phenyl (5%) polysiloxane primary column and a 50 m methyl (50%)/phenyl (25%)/cyanopropyl (25%) polysiloxane secondary column. The top trace indicates the primary separation monitor, while the following chromatograms indicate individual heart-cut secondary analysis. Reproduced from R.G. Schafer and J. Holtkemerr, Anal. Chim. Acta. 1992, 260, 107 (20).

Figure 3.5 Two-dimensional GC analysis of tobacco essential oil using non-polar primary and polar secondary separ-ations. The top tr-ace indicates the primary separ-ation, with the four resulting heart-cut cliromatograms shown below being obtained on the transfer of approximately 1-2 min fractions of primary eluent. Reproduced from B.M. Gordon et al. J. Chwmatogr. Sci. 1988, 26, 174 (23).

Figure 3.7 shows some early examples of this type of analysis (39), illustrating the GC determination of the stereoisomeric composition of lactones in (a) a fruit drink (where the ratio is racemic, and the lactone is added artificially) and (b) a yoghurt, where the non-racemic ratio indicates no adulteration. Technically, this separation was enabled on a short 10 m slightly polar primary column coupled to a chiral selective cyclodextrin secondary column. Both columns were independently temperature controlled and the transfer cut performed by using a Deans switch, with a backflush of the primary column following the heart-cut. 

Figure 3.7 (a) Chromatograms of (i) the dichloromethane extract of a fruit drink analysed with an apolar primary column, with the heart-cut... 

Figure 3.7 [continued) (b) Chromatograms of (iii) the dichloromethane extract of strawberry fruit yoghurt analysed with an apolar primary column, with the heart-cut regions indicated, and (iv) a non-racemic mixture of y-deca-(Cio) and 7-dodeca-Cj2 lactones isolated by heart-cut transfer, and separated by using a chiral selective modified cyclodextrin column. Reproduced from A. Mosandl, et al J. High Resol. Chromatogr. 1989, 12, 532 (39f. 

R. R. Deans, A new teclmique for heart cutting in gas cliromatography , Chmmato-gmphial 18-22(1968). 

Figure 5.4 Schematic diagrams of a heait-cut valve configuration system. Reprinted from Journal of Chromatography, 602, S. R. Villasenor, Matrix elimination in ion cliromatography by heart-cut column switching techniques , pp. 155-161, copyright 1992, with permission from Elsevier Science.

Step 2) Introduce heart-cut to the analytical column and detector. At the predetermined time interval, which was previously calculated by eluting analyte standards without the analytical column, i.e. the onset of the heart-cut, valve B is closed to divert the precolumn effluent to the analytical column. 

Step 4) Precolumn clean-up not shown in Figure 5.4. After the heart-cut analytes have been transferred to the analytical column, a step-gradient programme is used to flush the precolumn of the more strongly retained compounds. An additional pump configuration makes precolumn clean-up possible while the analysis is running. 

S. R. Villasenoi, Matrix elimination in ion chromatography by heart-cut column switcliing techniques , 7. Chromatogr. 602 155-161 (1992). 

Figure 10.3 Gas cliromatograms of a cold-pressed lemon oil obtained (a) with an SE-52 column in the stand-by position and (b) with the same column showing the five heart-cuts (c) shows the GC-GC chiral chromatogram of the ti ansfeired components. The asterisks in (b) indicate electric spikes coming from the valve switcliing. The conditions were as follows SE-52 pre-column, 30 m, 0.32 mm i.d., 0.40 - 0.45 p.m film tliickness cairier gas He, 90 KPa (stand-by position) and 170 KPa (cut position) oven temperature, 45 °C (6 min)-240 °C at 2 °C/min diethyl-tert-butyl-/3-cyclodextrin column, 25 m X 0.25 mm i.d., 0.25 p.m film thickness cairier gas He, 110 KPa (stand-by position) and 5 KPa (cut position) oven temperature, 45 °C (6 min), rising to 90 °C (10 min) at 2 °C/min, and then to 230 °C at 2 °C/min. Reprinted from Journal of High Resolution Chromatography, 22, L. Mondello et al, Multidimensional capillary GC-GC for the analysis of real complex samples. Part IV. Enantiomeric distribution of monoterpene hydrocarbons and monoterpene alcohols of lemon oils , pp. 350-356, 1999, with permission from Wiley-VCH.

Another way to improve the analysis of complex matrices can be the combination of a multidimensional system with information-rich spectral detection (31). The analysis of eucalyptus and cascarilla bark essential oils has been carried out with an MDGC instrument, coupling a fast second chromatograph with a matrix isolation infrared spectrometer. Eluents from the first column were heart-cut and transferred to a cryogenically cooled trap. The trap is then heated to re-inject the components into an analytical column of different selectivity for separation and subsequent detection. The problem of the mismatch between the speed of fast separation and the... 

Multidimensional HPLC offers very high separation power when compared to monodimensional LC analysis. Thus, it can be applied to the analysis of very complex mixtures. Applications of on-line MD-HPLC have been developed, using various techniques such as heart-cut, on-column concentration or trace enrichment applications in which liquid phases on both columns are miscible and compatible are frequently reported, but the on-line coupling of columns with incompatible mobile phases have also been studied. 

In biomedical analysis, LC-LC has been used most extensively and successfully in the heart-cut mode for the analysis of drugs and related compounds in matrices Such as plasma, serum or urine. Table 11.1 gives an overview of analytes in biological matrices which have been determined by heart-cut LC-LC systems. A typical example of such an approach is the work of Eklund et al. (16) who determined the free concentration of sameridine, an anaesthetic and analgesic drug, in blood plasma... 

Considering the numerous applications, heart-cut LC-LC has convincingly proven its value. Nevertheless, in LC-LC specific method development is generally needed for each analyte. Moreover, heart-cut procedures require accurate timing and, therefore, the performance of the first analytical column in particular should be highly stable to thus yield reproducible retention times. This often means that in LC-LC some kind of sample preparation remains necessary (see Table 11.1) in order to protect the first column from proteins and particulate matter, and to guarantee its lifetime. 

LC is not only a powerful analytical method as such, but it also allows effective sample preparation for GC. The fractions of interest (heart-cuts) are collected and introduced into the GC. The GC column can then be used to separate the fractions of different polarity on the basis of volatility differences. The separation efficiency and selectivity of LC is needed to isolate the compounds of interest from a complex matrix. 

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