Open-tubular columns capillary

Open tubular capillary (OTC) columns are analogues of the capillary columns used in gas chromatography and usually consist of glass columns with very low internal diameters (50 jum or less) with either a bonded organic layer, an adsorbent layer or a mechanically deposited liquid layer uniformly distributed over the column wall. In theory, for the performance of OTC columns to match that of conventional packed columns, the internal diameter must be in the range of 10-30 jam. 

The peaks eluted from OTC columns are of very low volume and this can create technical difficulties as the volumes of the flow cells in conventional detectors (5-10 jal) are much greater than the eluted peak volumes. If such detectors were used with OTC columns band dispersion could result, thereby negating the inherent advantages of these columns. Therefore, the flow cell volume should be in the range 0.01-1 /il. Similarly, there is a requirement in capillary systems for both a minimal dead volume and a reduction in the injection volume necessitating the development of speciahsed pre-concentration and sampling techniques. One further restriction of OTC columns is their small internal diameter which can result in sample overloading only quantities of less than 10 ng should be used. 

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J. V. Hinshaw, Jr and L. S. Ettre, Selectivity tuning of serially connected open-tubular (capillary) columns in gas chromatography. Part 1 fundamental relationships , Chromatographia 21 561-572 (1986). 

Several Interface designs are available for GC/MS and selection depends on the circumstances of the experiment [3-6,8,25,26]. Column flow rates of 1-2 ml/min (adjusted to STP) into the ion source are compatible with modem mass spectrometer vacuxim systems. This is also the optimum floi te range for open tubular capillary columns of conventional lensions. Coupling such columns to a modern mass spectromet er, therefore, presents... 

The selection of the column type is mainly determined by the composition of the sample. In general open-tubular (capillary) columns are preferred for low-density (gas-like) SFC, whereas packed columns are most useful for high-density (liquid-like) SFC. Open-tubular columns can provide a much larger number of theoretical plates than packed columns for the same pressure drop. Volumetric flow-rates are much higher in packed column SFC (pSFC) than in open-tubular column SFC (cSFC), which makes injection and flow control less problematic. 

Hunt et al. [354] used cSFC for the separation of extracts of poly(alkylene glycol) lubricants and sorbitan ester formulations. Doehl et al. [337] have compared the performance of cSFC-FID and pSFC-FID with both scC02 and scN20 in the analysis of the antiblocking agents oleamide and erucamide, the antistatic Armostat 400 and antioxidant Hostanox SE-10, none of which can be detected by UV absorption. By using open-tubular capillary columns, PAs as well as (un)substituted heavy carboxylic acids (> C ) can be eluted. 

Cross sections of (a) a packed column and (b) an open-tubular capillary column. 

What is meant by a GC open-tubular capillary column Why has the development of such a column been useful ... 

Contrast the packed column and the open-tubular capillary column in terms of design, diameter, length, how the stationary phase is held in place, ability to resolve complex mixtures, and amount of sample injected. 

An open-tubular capillary column is a very long (30 to 300 ft), narrow-diameter tube in which the stationary phase is held in place by adsorption on the inside wall. Such a column is useful because it allows the use of a very long column (for better resolution) with minimal gas pressure required. 

The packed column can be from 2 to 20 ft in length, typically has a diameter of V8 or V4 in., and has small particles, often coated with a thin layer of liquid stationary phase, packed in the tube. The open-tubular capillary column can be up to 300 ft in length, has an extraordinarily small diameter (capillary), and has the liquid stationary adsorbed on the inside surface of the tube. In terms of separation ability, the open-tubular capillary column is better because the mixture components contact more stationary phase (column is longer) while passing through the column. The amount injected for the open-tubular capillary column must be much less (0.1 mL maximum, as opposed to 20 mL for the V8-in. packed column) because the column diameter is much less and a greater volume would overload it. 

The Van Deemter equation can be applied to gas chromatography with a different emphasis on the relative importance of its terms. In fact, the interactions between an analyte and a stationary phase are much simpler in gas chromatography than those in liquid chromatography since the mobile phase does not modify the stationary phase in any way. The theoretical considerations are different for packed GC columns vs open tubular capillary columns. 

The features of open tubular capillary columns (Fig. 8.1a) and narrow inner diameter packed capillaries (Fig. 8.1b) are significantly different from those of the wider columns. The narrow packed capillaries are much more... 

Figure 8.1 Different types of microcolumns (a) open tubular capillary column (1-50 fim I.D.), (b) dry packed capillary column ( 200 jim I.D.), and (c) microbore column (0.5-1 mm I.D.), narrow-bore column (1-2 mm I.D.), and slurry packed capillary column ( 500 /xm I.D.). (Adapted from Ref. 2 with permission.)...

Figure 8.2 Schematic representation of open tubular capillary column preparation. (Adapted from Ref. 10 with permission.)...

The need of column configurations and surface chemistries especially designed for CEC is now generally appreciated and novel approaches to improve the column technology for CEC—MS applications include the use of monolithic stationary phases [109,110], open-tubular capillary columns [86] and chip technology [111]. These configurations are currently under detailed investigation and the future will have to prove their applicability in routine analysis. 

In GC/MS, analytical packed columns are still often used, mainly because of their well-established characteristics, simple injection technique, high sample capacity, and the fact that most instrumentation has been designed for operation with such columns. However, the separation efficiency and transfer to the MS are in most cases inferior to open tubular (capillary) columns. 

Calculate the plate height contributed by sorption-desorption mass transfer (nonequilibrium) through a uniform liquid layer (configuration factor q = 2/3) of thickness 1.0 x 10 3 cm coated on the inside of an open tubular (capillary) column. The gas velocity v is 10 cm/s. The solute retention ratio is 0.10 and its diffusion coefficient Ds through the stationary liquid is 1.0 x 10 5 cm2/s. 

Column Open tubular capillary column of fused silica 30 m x 0.25 to 0.53 mm (id), or equivalent. 

Open tubular capillary columns (OT) offer the simplest means to carry out CEC experiments. They are capillaries bonded with a wall-supported stationary phase, which can be a coated polymer, bonded molecular monolayer, or a synthesized porous layer network. The stationary phase that is attached to the walls of the capillary is typically less than 25 pm, and charged functional groups are added such that the walls are charged to support EOF. In OT-CEC, the EOF mobility can be calculated using Eq. (1). 

There are a few references about designing a small chromatographic system, including all operational units, especially separation column, injection, and detection. Ericson et al. have used a short packed column (effective length 4.5 cm) on a chip [4], Dadoo and Zare [5] have demonstrated high-efficiency separation of five polycyclic aromatic hydrocarbons. Preparation of ultrashort packed and open-tubular capillary columns of lengths 1-2 cm have also been reported by Tsuda et al. [6,7],... 

Figure 1 Frontal zone profile of electroosmotic flow in open-tubular capillary column. A rectangular capillary (1 mm x 50 pm and 16.4 cm long) was used. Colored sample methanol solution of 0.1 mM Rohdamine 6G. Frontal zone profiles of 0, (white) and 02 (black) were successively taken. The period between two zones was 11.44 s. The distance between two frontal zones was 6.52 mm. Flow velocity of the center was 0.57 mm/s. The ratio of the flow velocities given by (flow velocity at half-radius)/(flow velocity at center) was 1.0027. The retarded speed of the flow velocity at the center compared to that at the corner was only 0.4%. Although the same scale was used for the X and Y axes, there are time intervals between 0, and 02 in (A). Figure (B) was obtained by the combination of 0, and 02 (overlapping two frontal zone profiles at the corner). Therefore the right and left profiles correspond to 0, and 02, respectively. Applied voltage, 1.59 kV current, 120 nA.

An open-tubular capillary column has the simplest design and is easy to use due to the small resistance to flow. Although use of open-tubular capillary columns... 

Figure 3 Progress of frontal zone under application of pulsed electric field. A pulsed electric field was applied in 2-s cycles (electric field was applied for half of the period, and was stopped for the other half). Frontal zone positions were measured from the digital picture on a CRT. The perpendicular line shows the direction of movement of the frontal zone, and 1 mm in length is equal to 167 pixels. The horizontal line shows the time, and the minimum time scale observed is equal to one-fifteenth. A round open-tubular capillary column 75 pm in diameter and 28 mm in length was used. Applied voltage, 1 50 V. Colored sample methanol solution of 1 mM Rhodamine 6G.

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).

This concept using open-tubular capillary columns in OT-LC and OT-EC (277] has later been extended by using capillary columns packed with Chirasil-Dex-coated macroporous silica particles in pLC and CEC. 

The gas chromatograph was equipped with a flame ionization detector. A 50-foot length of 0.020 inch i.d. stainless steel open tubular capillary column coated with Carbowax 1540 served as the main column. A freeze out trapping technique was used to concentrate the a-pinene before entering the main column. The pre-column trap consisted of an in-line capillary column, identical to the main one, inserted between the injector and inlet of the main column. The trap was located outside the oven and cooled with a dry ice-ethanol bath before injection of a 5 cc sample. A 80°C hot water bath was used to release the a-pinene. The operating conditions of the gas chromatograph were as follows ... 

The typical components of a gas chromatograph are shown in Fig. 32.1. A volatile liquid is injected through a septum into a heated port, which volatilizes the sample. A gaseous mobile phase carries the sample through the heated column, and the separated components are detected and recorded. Two types of columns are available packed and capillary. Open tubular capillary columns offer higher resolution, shorter analysis time and greater sensitivity than packed columns, but have lower capacity for the sample. 

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