Band spreading, column

Column dispersity (band spreading) causes the measured molecular weight distribution to be broader than the true molecular weight distribution (Fig. 3.5). Because Zorbax PSM columns exhibit very low band-spreading characteristics, these columns have historically provided better molecular weight distribution accuracy than many gel-type columns. 

Elevated temperatures may be used with SynChropak GPC or CATSEC supports if necessary for solubilization, speed, or reduction of band spreading. Such conditions are not recommended for routine analyses, however, because column degradation is enhanced as temperatures are raised. 

Tne derivation of eq 10 was undertaken with the assumption that the sample effluent was monodisperse in molecular weight at any V. In reality this is never the case. First, if the frequency of branching varies with molecular weight, species of different molecular weight (but equivalent hydrodynamic volume) may elute at the same V (1 ). Second, molecular weight polydispersity at V results from assorted hardware mixing volumes and band-spreading of the SEC columns 0,11). 

Overall, the most effective factor in Equation 5.20 is the particle size. The smaller the particle size, the higher the column efficiency. Equations 5.13, 5.15, and 5.18 are depicted in Figure 5.6 against flow velocity as A, B, and C, respectively. The band spreading is thus influenced by Equation 5.15 at a low flow rate. The band spreading is influenced by Equations 5.18 and 5.19 at a high flow rate. For gas chromatography curve D is obtained. 

The magniiude of relative band spreading in the column is often measured by the height equivalent to a theoretical plate (HETP) which is ilriuilcil l>) H rtiul calculated rioiii (he o plcssiun tU)J... 

Equation (15) shows that H is proportional to the variance of the solute band at column outlet. If sever independent phenomena contribute to band spreading, the sum of their respective variances, af, determines the overaU band spreading measured by o-f as [Eq. (16)]... 

Three main independent contributions to band spreading inside the column have been identified (32) as longitudinal molecular diffusion, the unevenness of flow through the nonhomogeneous packing, and the resistances to mass transfer in the mobile and stationary phases. 

Sample introduction, contribution to band spreading, 48 duratioo of, 30 isoUnetic. 49 Samide loathng, 39 Sample size. 48,49.31,32.39 Sample, solubility of, 71 Sampling valve, for short column, 41 Saturator precotumn. 83 Schaeflin s add, 2 ... 

Perhaps more important is the observation that the kinetics of binding can contribute significantly to band spreading. If the results presented in that work are general, the kinetic plate height contribution sets a lower limit on the size of particles used in RPC at about 5 m and no practical gain in column efficiency would be made by using smaller particles. 

Equation 23-26 tells us that the standard deviation for diffusive band spreading is V2Dt. If solute has traveled a distance x at the linear flow rate ux (m/s), then the time it has been on the column is t = xlux. Therefore... 

The same relation holds for broadening in a detector that requires a time At for the sample to pass through. Sometimes on-column detection is possible, which eliminates band spreading in a detector. 

Example Band Spreading Before and After the Column... 

Figure 23-19 Band spreading from multiple flow paths. The smaller the stationary phase particles, the less serious this problem is. This process is absent in an open tubular column. [Adapted from H m. McNair and E. J. Bonelli, Basic Gas Chromatography (Palo Alto. CA Varlan Instrument Division. 1968).]...

Plate height is reduced in an open tubular column because band spreading by multiple flow paths (Figure 23-19) cannot occur. In the van Deemter curve for the packed column in Figure 23-15. the A term accounts for half of the plate height at the most efficient flow rate (minimum H) near 30 mL/min. If A were deleted, the number of plates on the column would be doubled. To obtain high performance from an open tubular column, the radius of the column must be small and the stationary phase must be as thin as possible to ensure rapid exchange of solute between mobile and stationary phases. 

A chromatographic band has a width, w, of 4.0 mL and a retention volume of 49 mL. What width is expected for a band with a retention volume of 127 mL Assume that the only band spreading occurs on the column itself. 

The volume of a chromatography system outside of the column from the point of injection to the point of detection is called the dead volume, or the extra-column volume. Excessive dead volume allows bands to broaden by diffusion or mixing. Use short, narrow tubing whenever possible, and be sure that connections are made with matched fittings to minimize dead volume and thereby minimize extra-column band spreading. 

High column efficiencies are possible due to no liquid phase contribution to band spreading. 

Nonsymmetrical band spreading may be attributed to nonlinearity of the equilibrium adsorption isotherm (i.e., deviation of isotherm from Henry s Law). This causes the sorbate to move through the column at different rates (rate dependent upon sorbate concentration). 

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