Analysis time, column diameter

An advantage of SFC-MS with small diameter capillary columns is that the flow rates are small enough that any Cl reagent gas may be used. Typical detection limits range from 0.1 to 10 pg depending upon the compound, analysis time, column configuration, and Cl reagent gas. 

The efficiency obtained from an open tubular column can be increased by reducing the column radius, which, in turn will allow the column length to be decreased and, thus, a shorter analysis time can be realized. However, the smaller diameter column will require more pressure to achieve the optimum velocity and thus the reduction of column diameter can only be continued until the maximum available inlet pressure is needed to achieve the optimum mobile phase velocity. 

The solvent consumption appears to be in conflict with the corresponding optimum flow rates. Substances with high (a) values have a very high optimum flow rate (over 11 per min. for (a=1.2) and the column diameter is over 6 mm which would indicate a very large solvent consumption. However, because the separation is simple, a very rapid separation is achieved with analysis times of less than a second. As a consequence, only a few ml of solvent is necessary to complete the analysis. The apparatus, however, must be designed with an exceedingly fast response and very special sample valves would be necessary. In contrast, a very... 

It is seen from Figure 15 that the analysis time ranges from about 10,000 seconds (a little less than 3 hr) to about 30 milliseconds. The latter, high speed separation, is achieved on a column about 2 mm long, 12 microns in diameter, operated at a gas velocity of about 800 cm/second. Such speed of elution for a multicomponent mixture is of the same order as that of a scanning mass spectrometer. 

Increasing the speed of analysis has always been an important goal for GC separations. All other parameters being equal, the time of GC separations can be decreased in a number of ways (1) shorten the column (2) increase the carrier gas flow rate (3) reduce the column film thickness (4) reduce the carrier gas viscosity (5) increase the column diameter and/or (6) heat the column more quickly. The trade-off for increased speed, however, is reduced sample capacity, higher detection limits, and/or worse separation efficiency. 

There have been very few method development processes proposed for 2DLC. One study (Schoenmakers et al., 2006) is titled A protocol for designing comprehensive two-dimensional liquid chromatography separation systems. This study advocates that one initially chooses the first-dimension maximum acceptable analysis time, the first-dimension maximum workable pressure drop, and the smallest first-dimension column diameter. The first two variables are then used to construct a Poppe plot (Poppe, 1997)—pronounced Pop-puh (Eksteen, 2007). 

Column diameter is an important parameter to consider in life science applications in which sample amounts are very limited and the components of interest may not be abundant. Researchers have reviewed micro HPLC instrumentation and its advantages.910 Nano LC-MS offers 1000- to 34,000-time reductions in the dilution of a sample molecular zone eluted from nano LC columns of 25 to 150 [Mi IDs in comparison to a 4.6 mm ID column. This represents a large enhancement of ion counts in comparison to counts obtained for the same amount of sample injected into a conventional 4.6 mm column. Solvent consumption for an analysis run or sample amount required for injection in a nano LC application may be reduced 1000 to 34,000 times compared to amounts required by an analytical column operated at a 1 mL/min flow rate. 

We thus are left with four main parameters the analysis time, the inlet pressure, the column length, and the particle diameter. The first two are measures for the cost" of each analysis in terms of time and pressure (cost of operation) whereas the choice of the other two determines the cost of the column (investment cost). In most applications, such as routine analyses, the latter will be small in comparison to the cost of operation and little attention is paid to the cost of the column, as long as it is possible to prepare it without additional research and investment. On ilie otlici hand, in rcseaich work t rien a lew separaliuns are petIbitiled... 

With increasing analysis time, S, 10, and 15 min, there is a marked decrease in the inlet pressure, 18.2,9.1, and 6.2 atm, respectively (see Table V). Both the necessary particle diameter, 5, 7, and 8.5 fim, respectively, and the column length, 8.5, 11.5, and 14 cm, respectively, increase with the analysis time. On the other hand, an increase in efficiency at constant analysis time is accompanied by a slight increase in column length, a... 

Larger diameter columns were also available for preparative chromatography. In later years, GPC analysis times were reduced and resolution was Improved by using shorter columns that were packed with smaller particle size material. A typical family of GPC columns that is available today contains 7pm particles... 

It is seen from equation (18) that the analysis time is proportional to the fourth power of the particle diameter and inversely proportional to the square of the diffusivity in the mobile phase. In a similar manner to column length, the analysis time is also directly proportional to the applied inlet pressure and inversely proportional to the mobile phase viscosity. 

Employing the conditions defined in the three data bases and the appropriate equations derived from the Plate and Rate Theories the physical properties of the column and column packing can be determined and the correct operating conditions identified. The precise column length and particle diameter that will achieve the necessary resolution and provide the analysis in the minimum time can be calculated. It should again be emphasized that, the specifications will be such, that for the specific separation carried out, on the phase system selected and the equipment available, the minimum analysis time will be absolute No other column is possible that will allow the analysis to be carried out in less time. 

The minimum analysis time is that achieved by employing the column of optimum length, packed with particles of optimum diameter and operated at the optimum velocity. Thus, the minimum analysis time, (t(min)), Is given by. 

Separation Ratio Particle Diameter (micron) Column Length (cm) Analysis Time Peak Capacity... 

It is seen that the optimum column radius for an open tubular column varies widely with inlet pressure arid the difficulty of the separation. Considering a separation of some difficulty, for example ( a = 1.02), it is seen that at an inlet pressure of lOOOp.s.f, the optimum column diameter would be about 4 micron whereas, at an inlet pressure of only 1 psi, it would be about 43 micron. The former would be quite difficult to coat with stationary phase and would demand detectors and injection systems of almost impossibly low dispersion A column of 43 micron in diameter, on the other hand, would be piactical from the point of view of both ease of coating and an acceptable system extra column dispersion. However, the lengths of such columns arid the resulting analysis times remains to be determined and may preclude their- use. 

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