Length, column, resolution

The effect of flow rate on resolution by Toyopearl HW-55F and Toyopearl HW-55S columns has been studied using a bovine serum albumin sample. Eor both columns, resolution decreased with increasing flow rate (46). Resolution is increased, however, with decreasing particle size (47). Resolution is proportional to the square root of the column length, as theoretically expected, and indicates that longer columns can be packed as well as shorter columns. Therefore, for samples difficult to resolve, the solution may be to increase the column length. 

Resolution factors Column length ("f) Pore volume ( ) Column length ( ) Column length ( ) Void volume ( ) Column length ( ) Channel thickness ( i )... 

Resolution in GC is dependent on the stationary phase, the stationary phase thickness, column length, column diameter, column temperature, and the linear velocity of the carrier gas.1,2... 

Figure 8.39 Comparison of a straight and knitted open tubular reactor of equal length for preserving column resolution. (Reproduced with permission from ref. 6S6. Copyright Dr Alfred Huethig Publishers).

Early Results About the Influence of Column Length on Resolution 58... 

Other matters to consider in column choice are column length, column diameter, and particle size. Column efficiency (theoretical plate count) is determined by a ratio of column length to particle size. A shorter column with the same particle size may give a shorter run time but at a loss of resolution. A shorter column with a smaller particle size with a lower flow rate may give a similar resolution in a shorter time. Retention time reproducibility improves in systems where column temperature can be controlled, especially in cases where ambient room temperature varies. 

What length column is needed to separate at unit resolution components A and B where the distribution coefficients are Ka = 110, Kb = 120, and where the ratio 0 = VJVS = 20 The plate height H for both components is approximately 0.050 cm. 

Retention time and the peak efficiency depend on the carrier gas flow rate retention time is also directly proportional to column length, while resolution is proportional to the square root of the column length. For packed columns, the carrier gas flow rate is usually expressed in milliliters per minute at atmospheric pressure and room temperature. It is measured at the detector outlet with a soap film flow meter while the column is at operating temperature. Unless otherwise specified in the individual monograph, flow rates for packed columns are 60 to 75 mL/min for 4-mm id columns and 30 mL/min for 2-mm id columns. 

NC = no change i = a factor < f L = column length = column diameter N = eolumn plate number = resolution Fm = column hold-up oIunie tK = retention lime (proportional to the run time) F = flow rate of the mobile phase A/r = pressure drop across the eolumn B. B = gradient steepness parameter, Eq. (I. .30). 

Substances A and B have retention times of 16.40 and 17.63 min, respectively, on a 30.0-cm column. An unretained species passes through the column in 1.30 min. The peak widths (at base) for A and B are 1.11 and 1.21 min, respectively. Calculate (a) the column resolution, (b) the average number of plates in the column, (c) the plate height, (d) the length of column required to achieve a resolution of 1.5, and (e) the time required to elute substance B on the column that gives an R, value of 1.5. 

These columns can provide ultrahlgh resolution when several columns are combined In a row. Due to the fact that a linear relationship can be obtained between efficiency and column length, columns of a million theoretical plates or more can be packed (JO- Krejci ( ) have described an open tubular... 

The pressure required to pump the mobile phase through the column is the Aajor obstacle that prevents us from maximizing resolution and minimizing nalysis time simultaneously. It depends on flow rate, column length, column iiameter, particle size, and viscosity. The relationship is expressed by the Cozeny-Carman equation, which is discussed in detail in Section 2.2.5 ... 

To summarize the analysis of the performance of columns of different but constant particle size, resolution increases with the square root of. column length, but analysis time for maximum resolution increases in mrect proportion to the column length. How fast an analysis can be achieved pepends on the square of the column length. The resolution at the high-speed drops roughly proportional to column length. 

Efficiency, N, is a function of the carrier gas (i.e., helium vs. hydrogen vs. nitrogen, and column length and diameter. Doubling column length yields a 41% in resolution quadrupling of colunm length doubles resolution but at the expense of a fourfold increase in both time of analysis and cost. The question to be asked Is it worth it ... 

There are three factors which are dependent on the physical characteristics of the column, i.e. column length, column internal diameter and film thickness. Of these, column length is least important as it can be established that resolution is proportional only to the square root of column length. Thus to improve by a factor of two on the resolution attainable with a 25 m column, it would be necessary to move to one 100 m in length, and this would inevitably mean that the analysis time would be increased by a factor of four. 

If the capacity factor and a are known, then equation 12.21 can be used to calculate the number of theoretical plates needed to achieve a desired resolution (Table 12.1). For example, given a = 1.05 and kg = 2.0, a resolution of 1.25 requires approximately 24,800 theoretical plates. If the column only provides 12,400 plates, half of what is needed, then the separation is not possible. How can the number of theoretical plates be doubled The easiest way is to double the length of the column however, this also requires a doubling of the analysis time. A more desirable approach is to cut the height of a theoretical plate in half, providing the desired resolution without changing the analysis time. Even better, if H can be decreased by more than... 

The chromatogram in Problem 4 was obtained on a 2-m column with a column dead time of 50 s. How long a column is needed to achieve a resolution of 1.5 What height of a theoretical plate is needed to achieve a resolution of 1.5 without increasing the length of the column ... 

The separating power of a chromatographic process arises from the development of many theoretical plates to achieve adsorption equiUbrium within a column of moderate length. Even though the separation factor between two components may be small, any desired resolution may be achieved with sufficient theoretical plates. 

The value of Np required to achieve a desired resolution is determined by Eq. (16-168) or (16-171). Since N = L/HTU 2Np = 2L/HETP, Fig. 16-13 or Eq. (16-183) can be used to determine the range of the dimensionless velocity ReSc that maximizes Np for a given particle diameter and column length. 

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