13


dimension column flow was 1 ml/min on 30


m x 0.25 mm x 0.25 µm Agilent HP5 phase and second dimension column flow was 32 ml/min on 5 m x 0.25 mm (phase thickness described below). Oven temperature program as single dimension. Microfluidic modulator settings as follows: Modulation delay 1.2 sec, modulation period 1.55 sec, sample time 1.45 sec. Detection carried out by flame ionisation detector conditions as in the single dimension.


Columns used: All columns were conditioned according to the manufacturer’s specifications.


Agilent J&W HP5 30 m x 0.25 mm x 0.25 µm (max temp 350°C) Agilent J&W DBwax 30 m


x 0.25 mm x 0.25 µm and 5 m x 0.25 mm x 0.15 µm (max temp 280°C) in single and dual dimensions respectively. Supelco columns


SLB-IL59 (max temp 300°C), SLB-IL61 (max temp 290°C), SLB-IL76 (max temp 270°C), SLB-IL82 (max temp 270°C), SLB-IL100 (max temp 230°C) and SLB-IL111 (max temp


270°C) were all 30 m x 0.25 mm x 0.20 µm when referred to in single dimension and 5 m x 0.25 mm x 0.25 µm in dual dimensions.


Sample preparation: Hydrocarbon standards were purchased from Sigma-Aldrich (Sigma- Aldrich Company Ltd, Gillingham, Dorset, UK). Diesel, soya bean methyl ester (SME), and lubricating oil samples were obtained from BP stocks. Diesel B20 samples were prepared by mixing by weight diesel and soya bean methyl ester so that 20% solution of SME in diesel was obtained. Methanol extracts of lubricating oil were prepared by taking a 1:1 mixture (by volume) of lubricating oil and methanol in a vial and shaking by hand and vortex, after the phases were allowed to separate the top methanol fraction was removed and analysed.


Results and Discussion Analysis in a single dimension:


As expected, increasing column polarity reduces retention time for non-polar hydrocarbons as shown in Figure 1. This occurs to such an extent that Hentriacontane (C31 n-alkane) elutes almost twice as quickly using an identical temperature program on SLB-IL111 when compared to a HP5 phase. This corresponds to a reduction in elution temperature of approximately 100°C. At a constant concentration of approximately 3 ng on column, the peak width at base of n- C31 increases and peak shape worsens (greater fronting) with column polarity so that on IL100 and IL111 the peak shape is poor. These changes are likely to be caused by the relative importance of different retention


Figure 1: Changes in Retention time and Peak width at base of C31 n-alkane with increasing column phase polarity


Figure 2: Elution of a lubricant product on various column phases.


mechanisms acting in each column phase. In the HP5 retention is prominently due to London dispersion forces (instantaneous dipole – induced dipole attractions) since there are no polar moieties to introduce other interactions. This mechanism appears to dominate in HP5 to IL61 also, though to a decreasing degree. From IL86 to IL111 the dominant retention mechanism changes so that dipole/dipole and electrostatic interactions are most important, retention decreases as the analyte does not dissolve in the liquid phase and peak width increases due to poor wetting of the surface. The peak width increase is most noticeable on the larger hydrocarbons. C11 is approximately Gaussian at the same concentration on IL111. Polar analytes of interest are present at concentrations in the order of 100-10000 times lower than the hydrocarbon matrix in our typical samples. Without


sample pre-treatment hydrocarbon concentrations will inevitably be large and the observed poor peak shape of the hydrocarbons at higher on-column loadings may inhibit analysis if compounds of interest elute in a similar region of the chromatogram as shown in Figure 2.


Figure 3: Elution of B20 biodiesel on various column phases showing peak position of n¬-Docosane – C22 (*) and palmitic methyl ester - 16:0 (#).


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