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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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172
Mass Sensitivity
The mass sensitivity of a chromatographic system is that mass of solute (m) that will provide apeak with a height equivalent to twice the noise level.
The sensitivity of the detector (Xo) (or minimum detectable concentration) is defined as that concentration of solute that will provide a signal equivalent to twice the noise level. Now the concentration of solute at the peak maximum is approximately twice the average concentration of the solute in the peak volume. Thus, the minimum detectable mass will be that mass (m) that, when dissolved in a volume of mobile phase equivalent to the peak volume, will produce a concentration of Xp/2.
Now the peak volume can be taken as 4<jc,
Thus, m/4oc = Xp/2
or, m = 2ocXp ..................................................(25)
Now, from equation (9) oe = O.32oc
or, oc = oe/0.32
Substituting for (Oc) in equation (25),
m = 6.25oEXD .......................................... (26)
The importance of the extra column dispersion now becomes apparent, as equation (26) shows that the minimum detectable mass increases linearly with the extra column dispersion. It is also becomes obvious that it is of little use designing a detector for increased sensitivity (Xd) if this is achieved (as is often the case) at the expense of increased extra column dispersion (oe). Conversely, if the chromatographic system is designed to have very low extra column dispersion, a proportional reduction in the minimum detectable mass will be achieved even if the actual detector concentration sensitivity remains the same. It follows, that in the design of an optimized column for a particular analysis, the extra column dispersion will determine both the radius of the column and the mass sensitivity that will be available.
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References
(1) R. P. W. Scott and P. Kucera, JChromatogr. Sci.,9( 1971 )641
(2) M. Martin, C. Eon and G. Guiochon, J. Chromatogr., 108( 1975)229
(3) J. H. Knox and . T. Gilbert, J. Chromatogr., 186( 1979)405
(4) . E. Reese and R. P. W. Scott, J.Chromatogr. Sci, 18( 1980)479
(5) M. J. E. Golay, in" 6as Chromatography 1958",{. D. M. Desty)
Butterworths, London, (1958)36
(6) I. Halasz, H. 0. Gerlach, K. F. Gutlich and P. Walking, U. S. patent,
3,820,660, (1974)
(7) K. Hofmann and I. Halasz, J. Chromatogr. 173(1979)211
(8) K. Hofmann and I. Halasz, J. Chromatogr. 199( 1980)3
(9) R. Tijssen, Separ. Sci. Techno}., 13(1978)681
(10) E. D. Katz and R. P. W. Scott, J. Chromatogr. 268( 1983) 169
(11) R. P. W. Scott and C. F. Simpson, J. Chromatogr. Sci. 20 (1982)62
(12) J. G. Atwood and M. J. E. Golay, J. Chromatogr., 218( 1981 )97
(13) . E. Reese and R. P. W. Scott, J. Chromatogr. Sci., 18( 1980)479.
(14) E. Grushka and W. D. Cook, J. Chromatogr. Sci,9( 1971 )310.
(15) E. Katz, K. L. Ogan and R. P. W. Scott, J. Chromatogr.,289(1984)65
Chapter 11
LC COLUMN DESIGN-THE DESIGN PROTOCOL
All chromatographic analyses involve the use of a separation procedure that is associated with a large number of interacting variables, some of which are under the control of the chromatographer and some of which are not. The nature of the sample presented for analysis, the sample throughput of the analytical service and the cost effectiveness of the laboratory, all make their own individual demands on the chromatographic system, and, in particular, the design of the chromatographic column. These exigencies, (which, unfortunately, are sometimes in conflict with each other), result from the various and different requirements expected from the system and, for clarity, need to be incorporated into a design protocol From such a protocol a procedure can be developed that can identify the basic characteristics of the fully optimized column.
The Design Protocol contains three different sources of data which will be termed the column design Data Bases . The needs of the analyst will constitute the first data base which will be given the title Performance Criteria . The performance criteria must state explicitly, in numerical terms, the quality of the separation that is required in order to achieve an accurate analysis.
The apparatus employed for a given analysis will have operating specifications that are unique to the particular instrument that is available or that is selected These specifications will be determined by the design and method of manufacture of the instrument and will probably differ significantly from one instrument to another. They will control, and limit, the ultimate performance achieved by any column with which the instrument is used. However, it is likely that as a result of careful design by the manufacturer, the important instrument characteristics effecting column design will remain sensibly constant over the lifetime of the instrument. This will allow any column that is designed for optimum use with the instrument to also have a reasonable life span. The instrument
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specifications provide the second data base necessary for the design of the optimum column and this data base is given the term instrument constraints .It is important to realize, and it will become increasingly apparent during the development of the design procedure, that it is the instrument constraints that ultimately control and limit the optimum performance of the column.
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