Inverse capillary column

Ohm s law the statement that the current moving through a circuit is proportional to the applied potential and inversely proportional to the circuit s resistance (E = iR). (p. 463) on-column injection the direct injection of thermally unstable samples onto a capillary column, (p. 568) one-taUed significance test significance test in which the null hypothesis is rejected for values at only one end of the normal distribution, (p. 84)... 

It Is seen that, in a similar manner to the packed column, the optimum mobile phase velocity is directly proportional to the diffusiv ty of the solute in the mobile phase, However, in the capillary column the radius (r) replaces the particle diameter (dp) of the packed column and consequently, (u0pt) is inversely proportional to the column radius. 

It is interesting to note from equation (14) that when a column is run at its optimum velocity, the maximum efficiency attainable from a capillary column is directly proportional to the inlet pressure and the square of the radius and inversely proportional to the solvent viscosity and the diffusivity of the solute in the mobile phase. This means that the maximum efficiency attainable from a capillary column increases with the column radius. Consequently, very high efficiencies will be obtained from relatively large diameter columns. 

The CSPs prepared by the molecular imprint technique have also been used for chiral resolution by CEC [98-100]. Lin et al. [91] synthesized L-aromatic amino acid-imprinted polymers using azobisnitriles with either photoinitiators or thermal initiators at temperatures ranging from 4°C to 60° C. Methacrylic acid (MAA) was used as the functional monomer and ethylene glycol dimethacrylate (EDMA) was used as the cross-linker. The resulting polymers were ground and sieved to a particle size less than 10 pm, filled into the capillary columns, and used for enantiomeric separations of some amino acids at different temperatures. The relationships of separation factor and column temperatures were demonstrated to be linear between the logarithm of the separation factors and the inverse of the absolute temperature (Fig. 24). The authors also compared the obtained chiral resolution with the chiral resolution achieved by HPLC and reported the best resolution on CEC. The chromatograms of the chiral resolution of dl-... 

Barrales-Rienda, J. M. Gancedo, J. V., "Thermodynamic Studies on Poly[N-(n-octadecyl)-maleimide] (PMI-18)/ Solvent Systems by Inverse Gas Chromatography with Capillary Columns," Macromolecules, 21, 220 (1988). 

Pawlisch, C. A. Brie, J. R. Laurence, R. L., "Solute Diffusion in Polymers. 2. Fourier Estimation of Capillary Column Inverse Gas Chromatography Data," Macromolecules, 21, 1685 (1988). 

Flow rate To force the mobile fluid from inlet to outlet of a column, the inlet pressure must be higher than the outlet pressure. When the temperature is changed, the fluid viscosity changes, and then either the flow rate changes for the same inlet and outlet pressures or the pressure drop must be altered to maintain the same flow rate. The flow rate of a fluid through a packed or capillary column is inversely proportional to the viscosity. For example, from Darcy s law the flow of a gas through a packed column is given by ... 

Italian and Spanish ohve oil from the 1991-1992 crop year contained a very high level of 9,19-cyclolanosterol (>400 mg/kg), which was not found with the standard method for sterol analysis. Two isomers of this sterol were identified by GC/MS of the unsaponifiable fraction, and their levels were found to be inversely proportional to the levels of p-sitosterol in the oils. GC/MS of the unsaponifiable fraction with high-resolution GC capillary columns provides a relatively rapid means of checking product purity and the identity of individual components. Thus, triterpene diols were identifiable at m/z 203, ot-tocopherol at m/z 165, squalene at m/z 69, cholesterol at m/z 386, and brassicasterol, characteristic of canola oil and other Brassica oils, at m/z 398. 

Inverse gas chromatography has proven to be a particularly important technique for the investigation of polymers, with most studies making use of packed columns. IGC also has been recently extended to the investigation of fibers and of polymers coated on capillary columns. The preparation of each of these columns is very important to overall success. 

Solute Diffusion in Polymers by Capillary Column Inverse Gas Chromatography... 

Diffusivity data are available only for a limited number of polymer-solvent systems. This paper describes research that has led to the development of the use of capillary column inverse gas chromatography (IGC) for the measurement of diffusion coefficients of solute molecules in polymers at infinite dilution. The work has resulted in a precise, rapid technique for the diffusion measurements that circumvents the many problems attendant to classical sorption methods and packed column IGC methods. Initial results of the program appeared in two recent publications (1,2)- Some of the material introduced in those papers is discussed here to present background for... 

Following the ideas developed by Guillet and his co-workers, a method using Capillary Column Inverse Gas Chromatography (CCIGC) was developed (1.21 to measure diffusion coefficients in polymer-solvent systems at conditions approaching infinite dilution of the volatile component. 

Relative retentions..the a values..usually vary Inversely with column temperature, but are most strongly affected by the choice of liquid phase. In packed column chromatography, the choice of liquid phase Is usually the most effective route by which separation efficiency Is Influenced. In capillary GC, however, there Is normally such an abundance of theoretical plates that the choice of liquid phase Is a relatively unimportant parameter for many analyses. In some cases however. It does become desirable (or even necessary) to select a liquid phase in which the relative retentions of certain solutes Is larger. Until quite recently, this posed a real problem with the fused silica capillary column, because the more polar liquid phases, l.e. those In which relative retentions are usually greater, coated fused silica only reluctantly, and produced columns whose useful lives were quite limited. The development of stable bonded phase columns ( ) eventually overcame this difficulty (vide Infra). 

The optimum linear velocity for a capillary column depends on the pressure in the column because Wopt is proportional to the average diffusion coefficient, which varies inversely with pressure. Operation of a short wide-bore column at vacuum outlet conditions results in a significantly faster analysis than would occur if the same column was used under atmospheric outlet pressures. Mass spectrometry (MS) has made vacuum GC very easy to implement, since the mass spectrometer provides both detection and a source of vacuum. Vacuum GC can be achieved practically by incorporating a restriction at the inlet end of a wide-bore capillary column, and interfacing the terminal end of the column directly into the MS. The function of the restriction is to deliver an optimal helium flow for the mass spectrometer, and it can be as simple as a short section of 20 pm i.d. capillary (or a longer section of 100-150 pm i.d. capillary). An optimal carrier gas velocity of 90-100cms can be expected for a 10 m x 50 pm column with a restriction at the inlet, and a speed gain of a factor of 3-5 times can easily be obtained. 

From the 1980s, the chromatographic broadening technique has been substituted in the literature by inverse gas chromatography (IGC). The two main IGC methods for the measurement of diffusion coefficients in liquids are packed column IGCP and capillary column IGCP ... 

BAL Baltus, R.E., Alger, M.M., and Stanley, T.J., Solubility and diffusivity of cyclic oligomers in poly(dimethylsiloxane) using capillary column inverse gas chromatography, Macromolecules, 26, 5651, 1993. 

The column is the most important part of the system, as its function is to encourage repetitive partitioning of each solute molecule between the gas and the liquid or solid phase. Packed columns are commonly used with the polymer or blend to be analysed coated on to a non-reactive substrate (Chromasorb W) which is then packed into the stainless-steel GC column. More recently capillary columns have been employed. Here the polymer is dynamically or statically coated as a uniform film on to the walls of the fused silica capillary. Inverse gas chromatography is a very flexible analytical technique however, full commercialisation of the technique has yet to be realised. 

Pawlisch, C.A., Maoris, A., and Laurence, R.L. (1988) Solute diffusion in polymers. 2. Fourier estimation of capillary column inverse gas chromatography data. Macromolecules, 21 (6), 1685-1698. 

---