Open Tubular GC Columns

As the open tubular column is geometrically simple, the respective functions of (k ) have been explicitly developed and 

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In addition, the column diameter is usually about 360 (r=180 microns) and the film thickness of the stationary phase will be about 0.25 micron. 

It follows that, for all solutes eluted at a (k ) value of unity or more, the resistance to mass transfer in the stationary phase can be ignored and equations (28) and (29) reduce to 

for significant values of (k ) (unity or greater) the optimum mobile phase velocity is controlled primarily by the ratio of the solute diffusivity to the column radius and, secondly, by the thermodynamic properties of the distribution system. However, the minimum value of (H) (and, thus, the maximum column efficiency) is determined primarily by the column radius, secondly by the thermodynamic properties of the distribution system and is independent of solute diffusivity. It follows that for all types of columns, increasing the temperature increases the diffusivity of the solute in both phases and, thus, increases the optimum flow rate and reduces the analysis time. Temperature, however, will only affect (Hmin) insomuch as it affects the magnitude of (k ). 

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In the previous two chapters, equations were developed to provide the optimum column dimensions and operating conditions to achieve a particular separation in the minimum time for both packed columns and open tubular columns. In practice, the vast majority of LC separations are carried out on packed columns, whereas in GC, the greater part of all analyses are performed with open tubular columns. As a consequence, in this chapter the equations for packed LC columns will first be examined and the factors that have the major impact of each optimized parameter discussed. Subsequently open tubular GC columns will be considered in a similar manner. 

The optimum mobile phase velocity for an open tubular GC column is given in chapter 13, equation (14). Reiterating this equation,... 

M. G. Block, R. B. Callen and J. H. Stockinger, The analysis of hydrocar bon products obtained from methanol conversion to gasoline using open tubular GC columns and selective olefin absorption , ]. Chromatogr. Sci. 15 504-512 (1977). 

Another approach is similar to that used in for the preparation of polymer-layer open tubular GC columns (PLOT). Horvath s group prepared capillaries with a porous polymer layer as shown in Fig. 13 by in situ polymerization of vinylbenzylchloride and divinylbenzene [183]. The reaction of the N,N-di-methyldodecylamine with chloromethyl groups at the surface simultaneously afforded strong positively charged quaternary ammonium functionalities and attachment of C12 alkyl chains to the surface. The unreacted chloromethyl groups... 

Two principal GC-MS interfaces are available for open-tubular GC columns. The so-called direct interface provides the highest possible detector sensitivity, whereas the open-split interface offers the least possible interference with chromatographic separation. With the direct interface, the column exit is routed from the GC oven through a heated transfer line directly into the ionization chamber. As long as the vacuum-pumping system can remove the carrier gas and maintain a sufficiently low pressure, the MS detector will function. Also, little chance exists for adsorptive loses of solute because the analytes contact only the GC column. 

Gas Chromatography. The basic components of a gas chromatograph are a carrier gas system, a column, a column oven, a sample injector, and a detector. Very pure helium is the near-universal carrier gas for environmental and many other analyses. Open tubular GC columns are constructed of fused silica with low-bleed stationary phases of varying polarity chemically bonded to the silica surface. Columns are typically 30-75 m in length and have inside diameters (ID) in the range of about 0.25-0.75 mm. The column oven is capable of precise temperature control and temperature programming at variable rates for variable times. 

A range of open tubular GC columns were used. A 150 ft X 0.01 in. i.d. stainless steel wall-coated open tubular (WCOT) column (Perkin Elmer Corp., Norwalk, CT), a 50 ft X 0.02 in. i.d. support coated open tubular (SCOT) column (Perkin Elmer), and 33 ft and 100 ft X 0.03 in. i.d. porous layer open tubular (PLOT) columns. The latter were pre-... 

However, while the practical utility of open-tubular gc columns is now widely recognized, those fabricated in laboratories as well as available commercially have iintil very recently generally been of 0.2... 

Manufacmrers of open tubular GC columns often specify column radius Tc and stationary phase film thickness (df), which is usually much smaller. Simple geometry enables us to calculate the phase ratio ... 

The mid-1960s witnessed the marriage between gas chromatography (GC) and mass spectrometry (MS) (see the figure on page 234). Marcel J. E. Golay (1902-89) developed the idea of the capillary (or open-tubular) GC column in 1957. Capillary GC, capable of extremely... 

Multiple paths of solute molecules, the A term, present only in packed GC and HPLC columns and absent in open tubular GC columns. This term is also called eddy diffusion. 

In practice, the mobile-phase linear velocity is set slightly higher than optimum SO s to quickcn the chromatographic run time (i.e., the time between injection and separation, and then detection). Figure 4.5 is a plot of H versus u for a packed GC column and for an open tubular GC column. H is much lower for the open tubular column because multiple flow paths are eliminated. Note also that the curvature in the plot in Fig. 4.5 for the open tubular column is much less than that of the packed column. This... 

In the 50 years since its introduction, the use of GC by the petroleum industry has helped foster many breakthroughs in GC instrumentation. Open-tubular GC columns and the theory that describes them were first introduced by Golay and Ettre in the mid-1950s. The further development of open-tubular capillary columns was done by Desty of British Petroleum, and, with subsequent refinement, this technique is now the standard method for most GC applications. The use of GC for sample analysis was also quickly adopted by the pharmaceutical and food industries and is used for fundamental studies of reaction kinetics and physiochemical measurements. Today the use of GC for the analysis of complex samples such as serum proteins, natural products, essential oils, and environmental samples has become a routine with multidimensional separation techniques and multivariate chemometric analysis providing identificatimi and quantification of trace analytes from complex samples in the sub-ppb range. A GC system usually consists of the following elements (Fig. 1) ... 

After its introduction, GC developed at a phenomenal rate, growing from a simple research novelty to a highly sophisticated instrument. Moreover, the current-day requirements for high resolution and trace analysis are satis ed by modern column technology. In particular, inert, thermostable, and ef dent open-tubular columns are available, along with associated selective detectors and injection methods, which allow on-column injection of liquid and thermally labile samples. The development of robust fused-silica columns, characterized by superior performances to that of glass columns, brings open-tubular GC columns within the scope of almost every analytical laboratory. 

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