Sequencing with large samples

Traditionally, LC and GC are used as separate steps in the sample analysis sequence, with collection in between, and then followed by transfer. A major limitation of off-line LC-GC is that only a small aliquot of the LC fraction is injected into the GC p. (e.g. 1 - 2 p.1 from 1 ml). Therefore, increasing attention is now given to the on-line combination of LC and GC. This involves the transfer of large volumes of eluent into capillary GC. In order to achieve this, the so-called on-column interface (retention gap) or a programmed temperature vaporizor (PTV) in front of the GC column are used. Nearly all on-line LC-GC applications involve normal-phase (NP) LC, because the introduction of relatively large volumes of apolar, relatively volatile mobile phases into the GC unit is easier than for aqueous solvents. On-line LC-GC does not only increase the sensitivity but also saves time and improves precision. 

Fig. 10 from our study of a large sample of stars in M5 (Cohen, Briley Stetson 2002) (not reproduced here due to limits of length) is typical of the GCs studied thus far in such detail. It shows a strong anti-correlation between C and N abundances, i.e. conversion of C into N, with strong-to-star variations in derived C and N abundances seen at all luminosities probed. (This sample contains mostly stars at the base of the RGB and just below the main sequence turnoff,... 

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