Thinlayer Chromatography with Flame Ionization Detection

Background

The fingerprinting of asphalt and its components can be made by TLC-FID much more rapidly and simply than by classical column chromatography and preparative TLC. TLC-FID involves separation of solvent-extractable organics on silica-coated quartz rods called Chromarods into saturates, aromatics and polar component classes combined with semiquantitative detection by automated FID.

Separation

The working principle of a commercially available instrument, the Iatroscan TH-10, designed to scan the adsorbent-coated Chromarods, is illustrated in Figure 9. A set of up to 10 coated rods is mounted on rod holders used for both chromatography and subsequent scanning. The samples to be analysed are dissolved in a solvent and applied near one end of the coated rods, which are then developed using suitable solvents, as in conventional TLC. Thereafter the developing solvent is removed by evaporation, and finally the coated rods are scanned in the FID device. The fractions resolved on each of the Chromarods are thereby succesively vaporized/pyrolysed and the ionizable carbon is detected at the collector electrode. The FID signals from each fraction are amplified and recorded as separate peaks.

In as much as the bitumens comprise a complex mixture of various substances which are impossible to isolate, a compositional analysis technique is generally used whereby a sample is separated into four fractions by performing multistage development with eluting solvents of varying polarities. The solvent elution sequence for asphalt separation is firstly, hexane for separating saturates; secondly, toluene for separating aromatics; and thirdly, dichloro-methane-methanol (95/5, v/v) for separating resins. The order of eluted components from the top of the rods is: saturates, aromatics, resins and asphaltenes.

Analysis

TLC-FID is a fast, multiple-analysis method which can be performed simultaneously on 10 samples. The total elapsed time for the analysis is less than 2 h (compared to about 2 days per sample with the SARA procedure). It is, however, destructive and does not yield fractions for further examination.

Linearity of response versus concentration is important in obtaining quantitative results. The sample size, the flow rate of hydrogen fed to the FID and the speed of scanning all have an effect on the linearity of response versus concentration, baseline stability of the FID signal and reproducibility of response values.

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