Download (direct link):
PRINT "Column Efficiency" N
PRINT “Optimum Flow-Rate" 0
PRINT "Maximum Sample Volume” V "ml"
PRINT "Maximum Extra Column Dispersion” S "ml"
In a similar manner to the program for optimization of a packed column, the program written in table (2) is in a very simple form that can be translated easily to the basic language used by other computers. The output will be sent directly to the monitor screen in the output window. However, by replacing the PRINT statements by LPRINT statements the output will be sent to a printer If the clipboard is defined a file, and the result print statements are replaced by write statements, then the results can be sent to the clipboard and pasted into word processing text. An example of an output from the program is shown in Table (3).
OPEN TUBULAR COLUMN DESIGN FOR LC
BASIC CHROMATOGRAPHIC DATA
Separation Ratio of the Critical Pair i.OI
Capacity Ratio of the Last Eluted Peak 5
Diffusivity of Solute in Mobile Phase 0 000035cm2/sec
Viscosity of Mobile Phase 0.023 Poises
Column inlet Pressure (ps i > 1 ps.i
Optimum Column Radius 4 318E-03, cm
Optimum Column Length 2056 cm
Minimum Analysis Time 577893,sec
Column Efficiency 313601
Optimum Flow-Rate 1.25IE-06 ml/sec
Maximum Sample Volume 1.00E-03, ml
Maximum Extra Column Dispersion 2.84E-04, ml
It is seen from Table (3) that an open tubular column with an I 0 of about 86 micron and 20 meters long, operating at an inlet pressure of only 1 p s.i. can complete the very difficult separation in about a week A very long analysis time, perhaps, but not much longer than would be required by an optimized packed column and in this case the separation is carried out with a simple tube to coat and with no high pressure pump required.
The Open Tubular Column in LC
The properties of open tubular columns shown in figures (1) to (6) indicate that the areas where such columns would have practical use is very restricted. At pressures in excess of 10 ps.i., and whatever the nature of the separation, whether simple or difficult, the optimum column diameters are so small that they would be exceedingly difficult to fabricate or coat with stationary phase. The maximum sample volumes and extra column dispersion that could be tolerated would also be well below that physically possible at this time. At relatively iow pressures that is at pressures less than 10 p.s.i. the diameter of the optimum column is large enough to fabricate and coat with stationary phase providing the separations required are difficult, i.e. the separation ratio of the critical pair must be less than 1.03. However, even under these conditions the sample volume will be extremely small, the extra column dispersion restricted to an almost impossibly low limit and the analysis time would be very iong Nevertheless, open tubular columns used for very difficult separations
might be preferable for biochemical separations, if made of suitably inactive materials. Capillary columns could be easily made bio-compatible. Furthermore, the fact that open tubular columns can be operated at very low inlet pressures and thus, do not require a pump make them economically attractive. Nevertheless, unless the separation is extremely difficult, and there are special conditions required that arise from the nature of the sample, open tubular columns, operated by pressure induced flow, should be avoided if possible.
Examining equations (11), (20) and (21) that algebraically describe the magnitudes of the optimum column radius, maximum sample volume and maximum detector dispersion, it is seen, that it is the magnitude of diffusivity of the solute in the mobile phase, that is responsible for the impracticality of open tubular columns. The success of open tubular columns in GC is due solely to the relatively high solute diffusivity in a gases. In LC the problem of low solute diffusivity is not so severe in packed columns due to the aided diffusion between the particle resulting from secondary flow, in fact the resistance to mass transfer in the mobile phase in a packed column is largely located within the porous particles not between them Where the flow of mobile phase is driven by electro-osmosis there is no parabolic velocity profile to cause band dispersion and thus,capillary electrophloresis can be carried out very successfully.
To render the open tubular column suitable for normal LC analysis it is necessary to increase the diffusivity of the solute by supplying external energy in a suitable form. The possibility of supersonic agitation of the liquid core of the tubular column might well be worth investigation. The column could be situated as flat spiral between two plates one fixed, and the other attached to a piezoelectric driver. In this way it might be possible to generate sonic waves across the diameter of the tube and thus, artificially increase the diffusivity in a rather a dramatic manner, if such a system could me made to transfer sonic energy to the liquid core of the column, it could make open tubular columns a very attractive alternative to packed columns. Another approach would be to use low dispersion tubing for LC columns such as serpentine tubing (6). Such tubing, as has already been discussed, introduces strong secondary flow and thus achieves rapid radial mixing. To date, the author is unaware of serpentine tubing being used for LC columns but they have been used as low dispersion connecting tubes and for heat transfer tubes very successfully.