Chromatography columns, characteristics

The measurement of effluent volume is not very reliable because of the effect of the geometry and packing characteristics of any column. It is often more useful to use a reduced parameter, such as V/V0, which is not so dependent upon column characteristics and is comparable with the calculation for RF values in thin-layer chromatography. 

Ogan, K. and Katz, E., Retention characteristics of several bonded-phase liquid-chromatography columns for some polycyclic aromatic hydrocarbons, J. Chromatogn, 188, 115, 1980. 

In a gel chromatography column packed with particles of average radius 22 pm at an interstitial velocity of 1.2 cm min- of the mobile phase, two peaks show poor separation characteristics, that is, =0.85. 

Eluate from a chromatography column can be passed through a plasma to atomize and ionize its components and measure selected elements by atomic emission spectroscopy or mass spectrometry. An atomic emission detector directs eluate through a helium plasma in a microwave cavity. Every element of the periodic table produces characteristic emission that can be detected by a photodiode array polychromator (Figure 20-14). Sensitivity for sulfur can be 10 times better than the sensitivity of a flame photometric detector. 

The porous polymer stationary phases which for many years have been available in packed gas chromatography columns has only recently become available as a coated capillary [24]. These cross-linked porous polymer columns are produced by copolymerizing styrene and divinylbenzene. The pore size and surface are varied by altering the amount of divinylbenzene added to the polymer. These PLOT capillary columns exhibit the same separative characteristics as Poropak Q packed columns. 

Because the electroosmotic flow affects the amount of time a solute resides in the capillary, both the separation efficiency and resolution are related to the direction and flow of the EOF. The EOF flow profile, as shown in Figure 4.7, is comparatively pluglike. Unlike the laminar flow that is characteristic of pressure-driven fluids,5 the EOF has minimal effect on resistance to mass transfer. As a result, the plate count in a capillary is far larger than that of a chromatography column of comparable length. 

In contrast to GC, liquid chromatography hyphenated with mass spectrometry (LC-MS) does not require a derivatization step before sample analysis. Separation of metabolite from sample matrix is achieved using chromatography columns with various stationary phases of different physicochemical characteristics. LC-MS is more often used than GC-MS because it is more suitable for unstable compounds, compounds difficult to derivatize, and nonvolatile compounds [6, 7]. Therefore, a wider range of metabolites with various physicochemical properties can be determined using LC-MS. Moreover, the sample pretreatment procedure is much simpler, which can have a great impact on minimization of analytical variability. 

In addition to different column characteristics, one more factor must be taken into consideration in gas chromatography, namely the sample capacity. This is defined as the maximum permissible sample size that can be injected into a column without more than 10% loss of efficiency, and it is expressed as... 

When interfaced to a liquid or a gas chromatograph, the mass spectrometer functions as a powerfiil detector, providing structural information in real time on individual analytes as they elute from a chromatography column. Depending on the operating characteristics of the mass spectrometer and the analyte peak width, several mass spectral scans can be acquired across the peak. The sum of aH ions produced is displayed as a function of time to yield a total ion chromatogram. The mass spectrometer is considered to be a universal detector because all compounds have mass... 

The confusion that a combination of these different definitions can generate, together with the difficulties encoimtered in the determination of some of the column characteristics involved (particularly the internal and the external porosities) makes useful a careful consideration of these issues. Given the stage of sophistication that the modeling of chromatography has now reached, it is not possible to tolerate errors, confusions, or approximations in the definitions nor in the estimations of the critical parameters related to the porosities, the velocities, and the equilibrium constants, nor to accept that more errors be made in the estimation of these parameters than those that are always involved in any measurement process. 

The product exhibits a single peak (retention time 5.7 minutes) on a 4-m. gas chromatography column packed with silicone fluid QF on Chromosorb P and heated to 191°. This material has the following spectral characteristics i.r. (CCI4) 1675 (conjugated C==0) and 1623 cm. (conjugated C=C) ... 

In pressure-driven flow Darcy s law provides the defining relationship between the column pressure drop and column characteristic properties. For gas chromatography, the mobile phase velocity at the column outlet is given by Eq. (1.12)... 

This method was developed by Stefan Huber (Karlsruhe, Germany) and consists of three size exclusion chromatography columns which divide the organic carbon into several fractions as a function of size, but also hydrophobic and ionogenic characteristics. A sample of up to 3 mL is injected into the instrument and filtered in-line with a 0.45 )um filter. The deposit on the filter is backwashed after 5 minutes and directly analysed with the TOC analyser to determine the particulate organic carbon content (POC). 

A different film selection approach relies on the differences between partition coefficients of the pertinent components of the sample matrix into film materials chosen from gas-liquid chromatography column substrates based on their retention characteristics (41,42). Vapor/polymer partition coefficients typically have great selectivity only between classes of chemical compounds, and only moderate selectivity within a given class, so this approach requires careful choice of the film if the sample matrix contains chemically similar materials. A chemometric approach suiy offer some relief in this situation ... 

What characteristic makes a photodiode array spectrophotometer suitable for measuring the spectrum of a compound as it emerges from a chromatography column and a dispersive spectrophotometer not suitable What is the disadvantage of the photodiode array spectrophotometer ... 

Broske [69] investigated the correlation between the method of pretreatment of the fused-silica surface and the retention characteristics of components with different polarity. Broske used in his work the so-called dual column method (see, for instance, ref. [70]). This was accomplished by connecting a section of fused-silica tubing downstream to a standard 25 mx 0.32 mm capillary column. A direct correlation was found between the retention characteristics on uncoated tubing and the inertness measured on the chromatography column obtained. 

---