Saturates aromatics response ratio

Another example12 is the separation of atmospheric hydrocarbons on an FID and a photoionization detector (PID), which is more sensitive for unsaturated hydrocarbons (Figure 6.7). The identities of the peaks are given in Table 1 along with the PID/FID response ratios. It can be seen that the ratios can be used as additional information in assigning peak identities. In fact, Figure 6.8 shows that the hydrocarbon type (saturates, olefins, or aromatics) can be assigned from the detector ratio in many cases. 

Figure 4. Effect of wire speed on aromatics/saturates response ratio. Sample, 10 fiL of vacuum gas oil solvent, n-heptane (1 mL/min) column, 1-ft fi-Porakl, mode, backflush operation for aromatics detector, Pye Unicom moving-wire detector.

Repeatability of the Saturates—Aromatics Separation. Results obtained with the dual detectors for ten individual injections of a 20-vol % solution of a vacuum gas oil in n-heptane are shown in Table IV. The precision (2(t) for the saturates and aromatics area response from the RI detector was 6%, relative. The precision for the aromatics/ saturates response ratio was 6.9%, relative. The retention volumes were much more reproducible and had 2(7 values of the order of 1-3%, relative. 

For the moving-wire detector, the precision (2o-) for the saturates response was dz5%, relative, while for the aromatics it was 8.6%, relative. The aromatic/saturate response ratio was 8%, relative. The precision for the retention volume was 1.4%, relative. 

Effect of Sample Size on Response. The response of the dual detectors was checked for linearity by analyzing the vacuum gas oil sample at various dilutions. Results are presented in Table I. The data indicate that the response for both detectors is essentially linear over a 25-fold range in concentration. Over this range, the ratio of the response for the aromatics with respect to that for the saturates is constant for both detectors to within several percent average deviation. Also, these data... 

Determination of Response Factors. Speciflc response factors were obtained by injecting known concentrations of saturates and aromatics fractions obtained by clay-gel chromatographic separation of various gas oil fractions as well as residua boiling above 510°C. All of the clay-gel saturates fractions showed the presence of some aromatic impurities (2-20%) by HPLC. This was particularly true of the saturates obtained from the 510°C residue samples. Also, the aromatics fractions showed the presence of some saturates (2-3%) by HPLC. The response factors for these saturates and aromatics fractions are listed in Table II. Based on the values shown in Table II, the response for the aromatics was about 1.7 times that for the saturates. The ratio of the response factors for the gas oil fractions differs from the ratio for the residuum samples by about 6%, relative. 

The results on the effect of temperature, contact time and methanol to toluene ratio on the isomer composition of xylenes on K2.5 salt are given in Table 3. It is seen that selectivity of p-xylene decreases with increase in the temperature whereas the selectivity of m-xylene increases, obviously, due to the isomerization. As contact time increases p-xylene selectivity increases. It is also found that the p-xylene selectivity increases with increasing methanol to toluene ratio. As methanol to toluene ratio increases the catalyst surface will be saturated with more of alkylating species which offers hindrance to the approach of the aromatic substrate and thereby resulting in the preferential alkylation at para position. In conclusion,it may be suggested that high Bronsted acidity is responsible for high para selectivity found in heteropolyoxometallates. 

The FIDs rely only on the amount of combustible material present, rather than light absorption or refraction characteristics of the solvent. This should make Aem respond more uniformly to mass over the particle size spectrum than RI or UV detectors. However, the literature indicates that nonuniformities are still a problem. Saturates and aromatics gave different response factors, possibly as a result of different carbon-hydrogen ratios in the materials. The differences in response were comparable to those in RI or UV detectors. These detectors are generally more expensive and more difficult to operate than RI or UV detectors, however. 

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