Capillary columns column

Figure 14.4 Schematic diagram of the cliromatographic system used for the analysis of very low concentrations of sulfur compounds in ethene and propene CP, pressure regulator CF, flow regulator SL, sanrple loop R, restriction to replace column 2 VI, injection valve V2, tliree-way valve to direct the effluent of column 1 to either column 2 or the restriction column 1, non-polar- capillary column column 2, tliick-film capillary column SCD, sulfur chemiluminescence detector FID, flanre-ionization detector.

Figure 14.10 Schematic diagram of the aromatics analyser system BP, back-pressure regulator CF, flow controller CP, pressure controller Inj, splitless injector with septum purge V, tliree-way valve column I, polar capillary column column 2, non-polar capillary column R, restrictor FID I, and FID2, flame-ionization detectors.

Column type quartz-capillary column Column length 22 m (int. diameter 0.4 mm) ... 

Figure 7 CEC chromatogram with ultrashort packed capillary column. Column 75 pm in inner diameter and 15 mm in length pcked with IC-CATION-SW. Eluent 30% methanol aqueous solution containing 30 mM KH2P04 and 25 mM EGTA. Aplied voltage, -3.0 kV. Injection, -3.0 kV for 0.5 s. Samples (a) uracil (b) adenine (c) cytosine (d) dopamine (e) serotonin.

Figure 9 Separation with ultrashort open tubular capillary column. Column effective length 1.7 cm, whole length 4.7 cm, inner diameter 30 pm, stationary phase C22. Eluent mixture of 20 mM phosphate buffer (pH 7.0) and methanol (1 1). Applied voltage -2.1 kV. Current 2.4 pA. Detection UV 210 nm. Sample thiourea (1), naphthalene (2), diphenyl (3), and fluorene (4).

Fig. 1 Electrochromatogram of salsolinol enantiomers on a packed capillary column. Column ODS-C18,29 cm (23 cm effective length) X 75 /rm ID applied electric field strength ...

The column is perhaps the most important feature of a GC system. It contains the stationary phase and thus effects the separation of components in a mixture. The column may be made of glass or metal and typically 2-6 mm i.d. and 1-3 m in length when packed with stationary phase material or 0.2-0.7mm i.d. and lO-lOOm long if in the form of a capillary column. Columns are formed into a coil of between 4 and Sin (10-20cm) in diameter and specially designed end fittings are used to connect the columns to the injector and detector with minimum dead volumes. A packed column contains solid particles of uniform size coated with stationary phase in GLC or uses uncoated particles as the stationary phase in GSC (Figure 5.4). 

Gas chromatographic conditions are as follows Column OB-210 (30 m x 0.318 mm X 0.5 pm, fused silica capillary column), column temp 1 100X, time 1 > 5 min, rate = 20°/min temp 2 = 200 C, time 2 = 7 min, injector temp 250 C, detector temp 250°C, inlet P, 24 psi, retention times trans-stilbene 11.4 min, trans-stilbene oxide 11.9 min. 

Fig. 1 Electrochromatogram of salsolinol enantiomers on a packed capillary column. Column ODS-C18, 29 cm (23 cm effective length) x 75 jim I.D. applied electric field strength 250 V/cm mobile phase 20 mAf sodium phosphate buffer (pH 3.0) containing 12 vaM (i-cyclodextrin and 5 vaM sodium 1-heptanesulfonate. The pump was set at the constant pressure of 100 kg/cm. ...

The traditional GC separation with a reduced run time, i.e., fast GC, is now the cutting edge and can be achieved by the use of shorter capillary columns, columns with reduced internal diameter, thinner stationary-phase films, H2 as the carrier gas, higher carrier gas velocities, faster oven temperature programming rates, and combinations of these parameters. However, one must be careful to understand the impact of these changes on the chromatographic resolution. Despite the unavoidable decrease in efficiency, the overall analytical results are obtained with 95% reduction in analytical time, typically less than 2... 

In the first isolation of orobanchol, purified samples were analyzed by GC-MS after conversion to their TMS ethers [34]. Since decomposition of SLs and their TMS ethers occurs in the capillary column, column length should be shorter than 5m (DB-5). Most of natural SLs have been analyzed by GC-MS without derivatization, but sensitivity for hydroxy-SLs is rather low. Although detection limit of GC-MS ( ng) is more than 1,000 times higher than that of LC-MS/MS ( pg), fragment ions and fragmentation patterns obtained from GC-MS analysis provide more rich information on structural features of SLs than LC-MS/MS. In addition, SL isomers that cannot be differentiated by LC-MS/MS often have distinct retention times in GC-MS analysis. Unfortunately, crude samples cannot be analyzed by GC-MS, and extensive purification steps are required. 

Figure 3.15 Chromatogram of a comprehensive Grob mixture on a 15 m x 0.32 mm i.d. Carbowax 20M capillary column. Column conditions 75 to 150°C at 1.7°C/min 28 cm/s He. Designation of solutes appears in Table 3.7. (From ref. 104.)...

Used in virtually all organic chemistry analytical laboratories, gas chromatography has a powerful separation capacity. Using distillation as an analogy, the number of theoretical plates would vary from 100 for packed columns to 10 for 100-meter capillary columns as shown in Figure 2.1. 

The column is swept continuously by a carrier gas such as helium, hydrogen, nitrogen or argon. The sample is injected into the head of the column where it is vaporized and picked up by the carrier gas. In packed columns, the injected volume is on the order of a microliter, whereas in a capillary column a flow divider (split) is installed at the head of the column and only a tiny fraction of the volume injected, about one per cent, is carried into the column. The different components migrate through the length of the column by a continuous succession of equilibria between the stationary and mobile phases. The components are held up by their attraction for the stationary phase and their vaporization temperatures. 

Interest in this method has decreased since advances made in gas chromatography using high-resolution capillary columns (see article 3.3.3.) now enable complete identification by individual chemical component with equipment less expensive than mass spectrometry. 

The hydrocarbons are separated in another column and analyzed by a flame ionization detector, FID. As an example, Figure 3.13 shows the separation obtained for a propane analyzed according to the ISO 7941 standard. Note that certain separations are incomplete as in the case of ethane-ethylene. A better separation could be obtained using an alumina capillary column, for instance. 

The resolution of capillary columns enables the separation of all principal components of a straight-run gasoline. The most frequently used stationary phases are silicone-based, giving an order of hydrocarbon elution times close to the order of increasing boiling point. 

Gas chromatography is not an identification method the components must be identified after their separation by capillary column. This is done by coupling to the column a mass spectrometer by which the components can be identified with the aid of spectra libraries. However tbe analysis takes a long time (a gasoline contains aboutTwo hundred components) so it is not practical to repeat it regularly. Furthermore, analysts have developed te hpiques for identifying... 

One of the most important advances in column construction has been the development of open tubular, or capillary columns that contain no packing material (dp = 0). Instead, the interior wall of a capillary column is coated with a thin film of the stationary phase. The absence of packing material means that the mobile phase... 

Another approach to improving resolution is to use thin films of stationary phase. Capillary columns used in gas chromatography and the bonded phases commonly used in HPLC provide a significant decrease in plate height due to the reduction of the Hs term in equation 12.27. 

In gas chromatography (GC) the sample, which may be a gas or liquid, is injected into a stream of an inert gaseous mobile phase (often called the carrier gas). The sample is carried through a packed or capillary column where the sample s components separate based on their ability to distribute themselves between the mobile and stationary phases. A schematic diagram of a typical gas chromatograph is shown in Figure 12.16. 

The most common mobile phases for GC are He, Ar, and N2, which have the advantage of being chemically inert toward both the sample and the stationary phase. The choice of which carrier gas to use is often determined by the instrument s detector. With packed columns the mobile-phase velocity is usually within the range of 25-150 mF/min, whereas flow rates for capillary columns are 1-25 mF/min. Actual flow rates are determined with a flow meter placed at the column outlet. 

A chromatographic column provides a location for physically retaining the stationary phase. The column s construction also influences the amount of sample that can be handled, the efficiency of the separation, the number of analytes that can be easily separated, and the amount of time required for the separation. Both packed and capillary columns are used in gas chromatography. 

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1-10 )J,L are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000-10,000 theoretical plates. Assuming Wiax/Wiin is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of... 

A capillary column that does not contain a particulate packing material. 

Capillary Columns Capillary, or open tubular columns are constructed from fused silica coated with a protective polymer. Columns may be up to 100 m in length with an internal diameter of approximately 150-300 )J,m (Figure 12.17). Larger bore columns of 530 )J,m, called megabore columns, also are available. 

Capillary columns are of two principal types. Wall-coated open tuhular columns (WCOT) contain a thin layer of stationary phase, typically 0.25 pm thick, coated on the capillary s inner wall. In support-coated open tuhular columns (SCOT), a thin layer of a solid support, such as a diatomaceous earth, coated with a liquid stationary phase is attached to the capillary s inner wall. 

An important problem with all liquid stationary phases is their tendency to bleed from the column. The temperature limits listed in Table 12.2 are those that minimize the loss of stationary phase. When operated above these limits, a column s useful lifetime is significantly shortened. Capillary columns with bonded or... 

A technique for injecting samples onto a capillary column in which only a small portion of the sample enters the column. 

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