Film thickness megabore columns

Fused-silica megabore column, DB-1, 15 m x 0.53-mm i.d., 1.5- rm film thickness, 100% dimethylpolysiloxane... 

Fused silica capillary columns give better separation than packed columns. Columns having inside diameters of 0.25, 0.32, and 0.53 mm and film thickness between 0.25 and 1 pm have found use in herbicides analysis. The stationary phase is generally made out of phenyl silicone, methyl silicone, and cyanopropyl phenyl silicone in varying compositions. Some common columns are DB-5, DB-1701, DB-608, SPB-5, SPB-608, SPB-1701, Rtx-5, AT-1701, HP-608, BP-608, or equivalent. Use helium as carrier gas flow rate 30 cm/s on narrowbore columns with 0.25 or 0.32 mm ID and 7 mL/min for megabore 0.53 ID columns. 

New developments in the gas chromatographic columns are of special interest to the flavor chemist. These include the availability of improved polar phases, columns of "non-standard dimensions (i.e. "microbore" and "megabore" columns), and columns with "super-thick" (i.e. 3 to 5 urn) films of stationary phase. 

Commercial columns are available with stationary phases that vary in thickness from 0.1 to 5 /rm. Film thickness primarily affects the retentive character and the capacity of a column, as discussed in Section 30E-6. Thick films are used with highly volatile analytes because such films retain solutes for a longer time, thus providing a greater time for separation to take place. Thin films are useful for separating species of low volatility in a reasonable length of time. For most applications with 0.25- or 0.32-mm columns, a film thickness of 0.25 xm is recommended. With megabore columns, 1- to 1.5- JLm films are often used. Today, columns with S-pm films are marketed. 

Product analysis of hydrogenation product was done using a Hewlett Packard 5890 gas chromatograph GC), equipped with a DB 1701 (5% crosslinked phenyl-methyl-silicone) megabore column (30 m long, 0.33 ID, 0.25 um film thickness) and a flame ionization detector. The temperature program was 60 °C for 2 min + 8 °/min to 230 °C for 15 min. The column flow rate was 1.5 cc/min helium and split vent flow rate of 60 cc/min helium. The injector and detector temperatures were 250 °C and 265 °C, respectively. Product and by-product identification was done by GC/MS analysis using chemical ionization techniques. 

Capillary columns are available from several manufacturers in a wide range of column internal diameters (O.l-l.Omm), column lengths (5-50 m), and stationary phase film thicknesses (0.1-5.0 pm). Generally, sample capacity increases but the efficiency decreases as the internal diameter or film thickness increases. The larger bore capillary columns with internal diameters between 0.53 and 1.00 mm are termed wide bore or megabore capillary columns and these have similar capacities, but greater efficiencies, than packed columns (see Table 2). 

Van Deemter plots were shown in Chapter 3, and they illustrate the effect of column flow rate on band broadening, H. There is an optimal flow rate for a minimum of band broadening. With packed columns, and also with thick film megabore columns, nitrogen is the carrier gas of choice since the van Deemter B term (longitudinal diffusion in the gas phase) dominates. Nitrogen being heavier than helium minimizes this B term and produces more efficiency. 

A one oc sample of the fuel gas to be analyzed is iq jected into a gas chromatograph where it is passed through a 60 meter, megabore, thick film, methyl silicone liquid phase, open tubular partitioning column, and separated into its individual constituents. 

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