Packed LC Columns

Consider first the equation for the optimum particle diameter. Reiterating equation (18), 

It is first necessary to identify the relative magnitudes of the resistance to mass transfer in the mobile phase, 

A liquid mobile phase is far denser than a gas and, therefore, carries more momentum. Thus, in its progress through the interstices of the packing, violent eddies are formed in the inter-particular spaces which provides rapid solute transfer and, in effect, greatly increases the effective diffusivity. Thus, the resistance to mass transfer in that mobile phase which is situated in the interstices of the column is virtually zero. However, assuming the particles of packing are porous (i.e., silica based) the particles of packing will be filled with the mobile phase and so there will 

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the resistance to mass transfer term for the static mobile phase will be 

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Equation (1) can be used in a general way to determine the variance resulting from the different dispersion processes that occur in an LC column. However, although the application of equation (1) to physical chemical processes may be simple, there is often a problem in identifying the average step and, sometimes, the total number of steps associated with the particular process being considered. To illustrate the use of the Random Walk model, equation (1) will be first applied to the problem of radial dispersion that occurs when a sample is placed on a packed LC column in the manner of Horne et al. [3]. 

In a packed column, however, the situation is quite different and more complicated. Only point contact is made between particles and, consequently, the film of stationary phase is largely discontinuous. It follows that, as solute transfer between particles can only take place at the points of contact, diffusion will be severely impeded. In practice the throttling effect of the limited contact area between particles renders the dispersion due to diffusion in the stationary phase insignificant. This is true even in packed LC columns where the solute diffusivity in both phases are of the same order of magnitude. The negligible effect of dispersion due to diffusion in the stationary phase is also supported by experimental evidence which will be included later in the chapter. 

Effect of Mobile Phase Compressibility on the HETP Equation for a Packed LC Column... 

D a) is the D Arcy s constant, which for a well-packed LC column takes a value of about 35 when the pressure is measured in p.s.i. 

In the previous two chapters, equations were developed to provide the optimum column dimensions and operating conditions to achieve a particular separation in the minimum time for both packed columns and open tubular columns. In practice, the vast majority of LC separations are carried out on packed columns, whereas in GC, the greater part of all analyses are performed with open tubular columns. As a consequence, in this chapter the equations for packed LC columns will first be examined and the factors that have the major impact of each optimized parameter discussed. Subsequently open tubular GC columns will be considered in a similar manner. 

If a local concentration of solute is placed at the midpoint of a tube filled with either a liquid or a gas, the solute will slowly diffuse to either end of the tube. It will first produce a Gaussian distribution with a maximum concentration at the center and finally, when the solute reaches the end of the tube, end effects occur and the solute will continue to diffuse until there is a constant concentration throughout the length of the tube. This diffusion effect occurs in the mobile phase of a packed LC column but the end effects are never realized. The diffusion process is depicted in figure 2. 

Kapila et al. [501] used an on-line SFE-LC to determine chlorinated phenols in wood chips over the concentration range 1-500 mg/kg. Following the extraction, the sample was loaded into a sample loop of the HPLC and chromatographed using a conventional packed LC column and UV detector. 

By adjusting the composition of the mobile phase, a wide variety of solutes of differing polarity can be separated easily. Even closely related molecules can be resolved on efficient HPLC columns. Mobile phases may need to be complex—depending on the nature of the sample components—and it may be necessary to use several solvents and additives such as acids, bases, salts, or surfactants. The stationary phase can be changed to aid in achieving a separation, but this necessitates owning an assortment of packed LC columns, which is expensive. In addition, the diversity of mobile phases that can be prepared is far greater than the variety of commercially available stationary phases. 

One of the first UV detectors suitable for use with packed LC columns was described in 1966 by Horvath and Lipsky [4] and about a year afterward Kirkland [5] described a miniaturized version of the detector in fact, sensor design has changed little since that time. The cell was 10 mm long and 1 mm ID. having a total volume of about 7.5 pi. Kirkland claimed a noise level of 0.0002 absorbance units, and an upper limit of linear dynamic range of 1.2 absorbance units, which was equivalent to a concentration of about 10 g/ml. This gives a... 

The first conductivity detector was developed by Martin and Randall as long ago as 1951 [1]. Improved cell designs have been described by Harlan [2], Sjoberg [3] and more stable and sensitive electronic circuits have been Avinzonis and Fritz [4] and Berger [5]. Scott et al. [6] inserted electrodes in the wall of a column to monitor the progressive band dispersion along a packed LC column. Keller [7] described a bipolar electrical conductivity detector and Kornilova et al. [8] describe a electric conductivity sensor for use in LC having a volume of 0.1 pi. 

In supercritical fluid chromatography (SFC), a supercritical fluid, usually carbon dioxide, is used. An SFC column resembles a GC column or a packed LC column. The operating temperature is lower than that in GC, which makes it more suitable for the analysis of thermally labile compounds. Another advantage is that water is not present and this is particularly valuable for the analysis of phosphatic antioxidants. In addition, high molecular mass antioxidants can be analyzed with SFC. SFC and FTIR have been used for the analysis of light stabilizers (UV absorbers) and antioxidants [43]. The eluted compounds were deposited on a KBr window after they had passed through a capillary restrictor. The hmit of detection was aroimd 100 ng. 

In SFE-LC, the most common interfrce is based on solid-phase trapping 2.15,44,58-63,66-68,70-80), although a few other types of interfrices such as impactor interfrce 69) open-tubular trapping 64) and the sample loop interface (65) have been developed as well. Direct trapping into a conventional packed LC column is not possible because of the high back pressure that analytical columns create. Because of the back pressure, the fluid cannot be efficiently decompressed and thus it will retain (partially) its solvation properties and efficient trapping will not be achieved, especially if modifiers are used in the... 

When a monolithic column is used for SFE-LC (Figure 2 A), die coupling is much more simple (75). Monolithic columns have much lower low backpressure than packed LC columns, and CO2 is efficiently depressurized in diem. In addition, monolithic columns can tolerate complete drying of the packing material without problems with the performance. In die on-line system, the extract is directly trapped in die beginning of the column, and after the extraction is completed, the LC analysis can be started. 

The discriminator comprises the analytical column (which is housed in an oven) and a restrictor which immediately follows it. The column is usually a capillary GC column, but packed LC columns can also be used. Once the sample is injected into the supercritical stream it is carried into the analytical column. Different types of stationary phases are available with varying compositions and polarities. The ovens used in SFC are generally conventional GC or LC ovens. 

It is seen that in packed LC columns the maximum efficiency possible is directly proportional to the particle diameter and the quality of the packing, and taking (y) to be 0.5 as suggested by Giddings ... 

Whereas packing LC-columns is a common technique, packing particles into CE-capillaries requires more efforts. First of all, the down-scaled slurry method is sensitive itself, although average particle sizes are small compared to the LC stationary... 

A chromatogram of a two-component mixture on a 2,s-cm packed LC column is shown in the figure below. The flow rate was 0.40 mL/min. 

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