Analyte linear velocity

Analyte retention time is the time that an analyte travels through the column, so the integral of the analyte linear velocity by time should be equal to the column length ... 

Figure 2-22. Analyte linear velocity in the column as function of time in linear gradient conditions.

This arrangement provides a thin film of liquid sample solution flowing down to a narrow orifice (0.007-cm diameter) through which argon flows at high linear velocity (volume flow is about 0.5-1 1/min). A fine aerosol is produced. This particular nebulizer is efficient for solutions having a high concentration of analyte constituents. 

Conventionally, analytical SEC columns have been produced with an internal diameter of 7.5 mm and column lengths of 300 and 600 mm. In recent years environmental and safety issues have led to concerns over the reduction of organic solvent consumption, which has resulted in the development of columns for organic SEC that are more solvent efficient (13). By reducing the internal diameter of the column, the volumetric flow rate must be reduced in order to maintain the same linear velocity through the column. This reduction is carried out in the ratio of the cross sectional areas (or internal diameters) of the two columns. Eor example, if a 7.5-mm i.d. column operates at 1.0 ml/min, then in order to maintain the same linear velocity through a 4.6-mm i.d. column the flow rate would be... 

Supercritical fluids possess favorable physical properties that result in good behavior for mass transfer of solutes in a column. Some important physical properties of liquids, gases, and supercritical fluids are compared in Table 4.1 [49]. It can be seen that solute diffusion coefficients are greater in a supercritical fluid than in a liquid phase. When compared to HPLC, higher analyte diffusivity leads to lower mass transfer resistance, which results in sharper peaks. Higher diffusivity also results in higher optimum linear velocities, since the optimum linear velocity for a packed column is proportional to the diffusion coefficient of the mobile phase for liquid-like fluids [50, 51]. 

H is the plate height (cm) u is linear velocity (cm/s) dp is particle diameter, and >ni is the diffusion coefficient of analyte (cm /s). By combining the relationships between retention time, U, and retention factor, k tt = to(l + k), the definition of dead time, to, to = L u where L is the length of the column, and H = LIN where N is chromatographic efficiency with Equations 9.2 and 9.3, a relationship (Equation 9.4) for retention time, tt, in terms of diffusion coefficient, efficiency, particle size, and reduced variables (h and v) and retention factor results. Equation 9.4 illustrates that mobile phases with large diffusion coefficients are preferred if short retention times are desired. 

Glycerin is used in Nasonex primarily as a humectant. For its quantification, both capillary gas chromatography method and HPLC methods may be selected. The GC is equipped with a flame-ionization defector, a 0.53 mm x 30 m fused silica analytical column coated with 3.0-p,mG43 stationary phase, and a 0.53 mm x 5 m silica guard column deactivated with phenylmethyl siloxane. The carrier gas was helium with a linear velocity of about 35 cm/s. The injection port and detector temperature was maintained at 240 and 260°C, respectively. The injection mode is splitless. The column temperature is programmed to be maintained at about 40°C for 20 min, then to increase to 250°C at a rate of 10°C/min and to hold at 250°C for 15 min. 

Analytical Methods. All of the sorbent extracts were analyzed by using GC-MS. The solutions were chromatographed on a 30-m X 0.32-mm i.d. Supelcowax 10 capillary column with a film thickness of 1 /um (Supelco). The Supelcowax 10 is a Carbowax PEG 20 M bonded-phase capillary column. The instrument conditions were as follows injector, 250 °C separator oven, 250 °C column over initial, 40 °C programmed to 250 °C at 6 °C/min linear velocity of helium carrier gas, 35 cm/s at 40 °C column head pressure, 6 lb/in.2 mass range, 33-333 amu scanned in 1-s intervals. Two-microliter aliquots were injected in the split mode at a 10-to-l split ratio. A typical chromatogram appears in Figure 1. 

At the constant linear velocity in Figure 24-6, increasing the thickness of the stationary phase increases retention time and sample capacity and increases the resolution of early-eluting peaks with a capacity factor of k < 5. (Capacity factor was defined in Equation 23-16). Thick films of stationary phase can shield analytes from the silica surface and reduce tailing (Figure 23-20) but can also increase bleed (decomposition and evaporation) of the stationary phase at elevated temperature. A thickness of 0.25 pm is standard, but thicker films are used for volatile analytes. 

Amperometric detectors can operate over a range of conversion efficiencies from nearly 0% to nearly 100%. From a mathematical point of view, a classical amperometric determination (conversion of analyte is negligible) is one where the current output is dependent on the cube root of the linear velocity across the electrode surface as described by Levich s hydrodynamic equations for laminar flow. Conversely, the current response for a cell with 100% conversion is directly proportional to the velocity of the flowing solution. While the mathematics describing intermediate cases is quite interesting, it is beyond the scope of this chapter. 

Linear velocity of the solvent through the cartridge will affect the recovery and bandwidth of the analyte. For example, a flow velocity of 0.3 ml/min gave a narrow band for riboflavin and a recovery of 100%. At the excessive velocity of 27 ml/min, decreased recoveries and band broadening were observed [34],... 

Isolation of material from an HPLC separation is greatly simplified when chromatographic conditions can be easily translated from the analytical scale to the preparative scale. If it is necessary to maintain equivalent analysis time, the linear velocity of the mobile phase must be kept the same. 

Jorgenson and Lukacs [8] have described a number of parameters, which may be helpful in characterizing CE separations. The linear velocity (v) of a given analyte in free-solution capillary electrophoresis may be represented by the following equation ... 

Direct current (DC) amperometry is used for the analysis of catecholamines, phenols, and anilines, which are easy to oxidize. A single potential is applied, and the current is measured. The current resulting from the oxidation or reduction of analyte molecules is dependent on many factors, including the concentration of the analyte, temperature, the surface area of the working electrode, and the linear velocity of the flowing stream over the surface of the working electrode. 

For unretained analyts the total linear velocity utot is ... 

The mobile phase can be allowed to flow under the force of gravity, a low pressure pump can be used, or compressed gas can be used to pressurize a solvent reservoir. The linear velocity should be about one-third of that used in analytical columns. The sample can be applied to the top of the column with a microsyringe or pipet using the stop-flow method, or an inexpensive, low-pressure valve can be used. The eluent is usually collected in separate tubes using an automated fraction collector. Inexpensive UV detectors with large solvent volumes are available, or flow cells can be fitted to conventional UV/visible instruments. 

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