Silica

The application of silica as adsorption material was established from the very earliest days of GSC. Later publications have shown the high separation and inertness of silica in PLOT columns but absolute retention and capacity were always the limiting factors in successfully applying silica as an adsorbent for these columns. The deposition of a layer of silica in a capillary column could not compete with the high selectivity and retention provided by alumina or the inertness and retention of porous polymers. Theoretically, silica should be a very interesting adsorbent because it has a very low catalytic activity.

The silica layer in a silica PLOT column is activated at 250°C and can be used for a wide variety of compounds such as Q to C8 hydrocarbons, volatile sulfur compounds and halogenated compounds in the C to C6 range. The hydrocarbon selectivity of a silica PLOT column is shown in Figure 11. Note the high separation efficiency of the Q-C3 compounds. Also cyclopropane elutes before propylene, making low level quantification in propylene possible.

Sulfur compounds are well separated from hydrocarbons and there is, therefore, no risk of quenching if a flame photometric detector is used. Silica adsorbents also produce very sharp peaks for sulfur compounds, making low level measurement possible. Determination of carbonyl sulfide and hydrogen sulfide in pure propylene is possible down to low ppb levels using sulfur selective detection as shown in Figure 12. Sulfur dioxide also elutes as a sharp peak.

Time (min)

Figure 12 Volatile sulfur impurities in propylene on a silica PLOT column. Experimental details: column, 30 mx 0.32 mm fused silica CP-SilicaPLOT, df=4 |im; oven, 50°Cp120°C, 10°Cmin_1; carrier gas, helium; injection, valve, 375 |L propy-lene; detection, sulfur chemiluminescence detection. Peaks: 1, carbonyl sulfide (34p.p.b.); 2, hydrogen sulfide (108 p.p.b.).

Figure 13 Chlorofluorocarbons on a silica PLOT column. Experimental details: column: 30mx0.32mm fused silica CP-Silica-PLOT, df=4 |im; injection, split; detection, FID. Chromatogram (A): oven, 35°C, 12minp150°C, 4°Cmin~1; carrier gas, helium, 2.5 mL min- Chromatogram (B): oven, 70°C, 12 min p 150°C, 4°C min~1; carrier gas, helium, 2.0 mL min~1. Peaks: 1, nitrous oxide; 2, CFC-23; 3, CFC-13B1; 4, CFC-32; 5, CFC-115; 6, CFC-12; 7, CFC-125; 8, CFC-143a; 9, CFC-22; 10, CFC-134a; 11, chloro-methane; 12, CFC-12B1; 13, CFC-114; 14, CFC 152a; 15, CFC-142b; 16, CFC-124; 17, bromomethane; 18, CFC-11; 19, dichloro-methane; 20, CFC-141b; 21, CFC-123; 22, trichloromethane.

Figure 13 Chlorofluorocarbons on a silica PLOT column. Experimental details: column: 30mx0.32mm fused silica CP-Silica-PLOT, df=4 |im; injection, split; detection, FID. Chromatogram (A): oven, 35°C, 12minp150°C, 4°Cmin~1; carrier gas, helium, 2.5 mL min- Chromatogram (B): oven, 70°C, 12 min p 150°C, 4°C min~1; carrier gas, helium, 2.0 mL min~1. Peaks: 1, nitrous oxide; 2, CFC-23; 3, CFC-13B1; 4, CFC-32; 5, CFC-115; 6, CFC-12; 7, CFC-125; 8, CFC-143a; 9, CFC-22; 10, CFC-134a; 11, chloro-methane; 12, CFC-12B1; 13, CFC-114; 14, CFC 152a; 15, CFC-142b; 16, CFC-124; 17, bromomethane; 18, CFC-11; 19, dichloro-methane; 20, CFC-141b; 21, CFC-123; 22, trichloromethane.

The selectivity of silica for separation of volatile halogenated hydrocarbons like the CFCs, is very specific. Unlike alumina, the CFCs do not decompose or react on silica, which makes their quantification possible at low levels (Figure 13). The low catalytic activity is also important for the analysis of penta-dienes. On alumina these compounds polymerize due to the catalytic activity of the surface, while on silica these compounds elute as symmetrical peaks.

One of the key benefits of deactivated silica as an adsorbent is a more reproducible retention behaviour in the presence of water. Although water is retained by silica, the water will elute from the column and the influence on retention is small. Because of this, samples containing moisture can be analysed at lower temperatures. This is especially important if isothermal or fast analysis conditions are required, as in a plant analyser.

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Solar Panel Basics

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