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liquid chromatography column - Scott R.P.W.

Scott R.P.W. liquid chromatography column - John Wiley & Sons, 2001. - 144 p.
Download (direct link): liquidchromatographycolumntheory2001.djvu
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Katz, et al( 1) measured the efficiency of two different solutes (benzyl acetate and hexamethyl benzene) on a silica column 25 cm long and 9mm I D packed with Partisi 1 10 (actual mean particle diameter 8.5) employing six
different solvent mixtures. Measurements were made in triplicate and further replicate measurements made if the mean of the three differed by more than 3% from the extreme. They employed a specially designed chromatographic system with low extra column dead volume to ensure that the contribution of extra column dispersion to the values of (H) was less than 2%. The properties of the system are shown in table I
Table 1
Properties of the Mobile Phase and Solutes
Mobile Phase Benzyl Acetate Hexamethyl Benzene
ke Dn i (I0_5cm2/sec) Dm (10"5cm2/sec)
1 /4.6%(w/w) Et.Ac. 2.05 4.07 in n-pentane 3.61 351
2/4.9%(w/w) Et.Ac, 1.97 3.94 in n-hexarre 3.06 2.73
3/4.32%(w/w) Et.Ac. 2,04 4.06 in n-heptane 2.45 2.23
4/4.5%(w/w) Et.Ac. 2.01 4.01 in n-octane 2.01 1.71
5/4.4%(w/w) Et.Ac. 2.12 4.20 In n-nonane 1.65 1.35
6/4.8%(w/w) Et.Ac 2.01 4.01 in n-decane 1.46 1 17
Values for k' were obtained using the retention time of hexamethyl benzene as to. Values for k'e were obtained employing the retention time of the completely excluded solute polystyrene (t(e)0> (Mol. Wt.83,000). t(e)o was also employed for the measurement of the linear mobile phase velocity.
An example of the results they obtained for the solute benzyl acetate chromatographed with the solvent mixture 5.4%(w/w) ethyl acetate In n-hexane Is shown in figure (I). The curve through the points is the fitted curve to the Van Deemter equation and the contributions from the multipath

term, the longitudinal diffusion term and the total resistance to mass transfer, term derived from the curve fit, are included.
Figure 1 Graph of (H) versus (u) (Partisil 10) n-hexane +5.4%(w/w)Et. Acetate
Composite (H)
(H)
Multipath Term e Long.Diff.Term
Velocity
It is seen that an excellent fit is obtained with the Van Deemter equation with an Index of determination, for that particular fit, of 0.999885 However, the results from testing all the data to each of the dispersion equations need to be known in order to identify the equation that, overall, shows the best fit.
The value of (H) for each solute was determined in each solvent mixture over 10 different linear velocities that covered the normal practical range of velocities used in LC. Measurements at each velocity were taken in triplicate which resulted in a minimum of 180 values of (H) being taken for each solute. Each data set, from each solvent mixture, was fitted to each dispersion equation and the values for the respective constants (), (), (C), etc. calculated, together with the index of determination for each fitting, it ia seen that the data was sufficient in both quantity, and quality to be able to dentify the most appropriate dispersion equation with some confidence. The results obtained are shown in table 2.
1.5
Table 2
Experimental Values for the Dispersion Equation Coefficients Obtained by a Curve Fitting Procedure
n-pentane n-hexane n-heptane n-octane n-nonane n-decane 4.7%w/w 4.9%w/w 4.3%w/w 4.5w/w 4.4%w/w 4.85w/w Et.Acet. Et.Acet. Et.Acet. Et.Acet. Et.Acet. Et.Acet.
The Van Deemter equation H = A + - + Cu

0.00 II44 0 001210 0.001208 0.001237 0.000081 0.000065 0.000049 0.000045 0.003362 0 003661 0.004298 0.004786
A 0.001189 0.001144 0.000108 0.000091 0.002525 0.003008
The Giddings equation H =
A 0.001189 0.001144 0.000108 0.000091 0.002525 0.003008
1 +
' B
+ +


0.001 123 0.000086 003348
0001210 0.000065 0.003661
0 001407 0.000067 0.004001
0.001257
0.000053
0.004754
The Huber equation E + + + Du1/2

01455 0.001408 0 000986 0 001 196 0.000702 0 1612
0.000104 0.000092 0.000084 0 000065 0.000057 0.000056
0.003302 0.002728 0.002979 0 003622 0.002769 0.002310
D-0.00092 0.000331 0.000447 0.000046 0.001775 -0.06804
0 0 0 0 0 2.13100
The Knox equation h = - + Av'^ + Cv v
A 0.002509 0.002422 0.002390 0.002545 0.002608 0.002626
0.000123 0.000105 0.000096 0.000080 0.000064 0.000061
0.008720 0.001407 0.001754 0002003 0.002518 0002304
The Horvath equation =
1
+ + 02/3
1/3

0.001366 0001507 0.001013 0.000104 0.000092 0.000083 0.003572 0.002495 0.002744 D-0.00110 0.000541 0.000647
_0_0_
0.001 197 0.000825
0.000065 0.000056
0.003585 0.001948
0000080 0.002454
__
0.005583
0.000051
0.009169
-0.008577
97.30
1W
Examination of the results obtained by Katz et alas given in table 2 show rational fits between the experimental data and the equations of Van Deemter, Giddings and Knox. The fit of the data to both the Huber and Horvarth equations gave alternating positive and negative values for the D constant which is the coefficient of the term involving a fractional power of (u). Furthermore, for the Huber equation, the value of coefficient (E) is consistently zero and for the Horvath equation, is zero for four solvents mixtures out of six, with an extreme value of 97.3 for one solvent.
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