Analysis time, column diameter reduction

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. 

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. 

The obvious advantages of a reduction of the column diameter in analytical enantioseparations include less consumption of packing materials and of high-purity organic solvents, less environmental problems, smaller sample size, and improved analytical characteristics (less dilution of sample, higher plate number and shorter analysis time) of the separation [120]. 

The primary benefit of reduced column diameter is reduced solvent consumption due to the reduced flow rate used with smaller diameter columns. A reduction of flow rate at constant column diameter also results in a reduced consumption of solvent per unit time. But since analysis time increases as well, the solvent consumption per analysis remains constant. It can be reduced only by reducing column diameter. 

The capillary format has proven to be ideal for separations requiring an applied electric field. The micrometer-dimensioned capillary cores facilitate the analysis of smaller, nanoliter-sized samples, which in turn means higher separation resolution and much shorter separation times. Reduction of column diameters means a commensurate reduction in detection volumes, however, making detection of low analyte concentrations even more challenging as numbers of analyte species available for detection are decreased. Sample preconcentration methods are thus required to compress more analytes into smaller volumes to improve detection limits. 

Recently, HSGC has been applied to the analysis of some environmental pollutants such as pesticides, polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). Using this technique, fast separations have been obtained using short capillary columns (<10m) with internal diameters of 50 pm. An important reduction in analysis time can be achieved so that separations by conventional capillary GC that are obtained in 20 min take no longer than 20 s using this technique. 

Increasing analysis speed for complex profiles without impairing resolution can only be realized by reduction of the internal diameter and the length of the capillary column. A lOmxO.l mm i.d. column offers the same resolution as a 25 m X 0.25 mm i.d. column. Because the column is 2.5 limes shorter, the analysis time is reduced drastically. Moreover, since the optimum carrier gas velocity is higher and the H-u curves are flatter for narrow bore columns, higher average carrier gas velocities can be used without loss of resolution. Presently, capillary columns with internal diameters in the order of 100 pm are in the picture for routine operation because state-of-the art cap-... 

An equally important trend is a reduction in column size and a concomitant increase in analytical speed. Short columns can reduce analysis time, and efficiency may be as high as 5000 theoretical plates since the particle size of the packing material is 3 /u.m. The analysis time is about 1 minute. A standard HPLC column has a diameter of 4.6 mm as compared to 2, 1, or even 0.5 mm for the new columns. The small size of these columns allows the use of more expensive solvents deuterated solvents might be used when it is necessary to analyze the collected samples by NMR. The use of ultramicrobore (50-/u.m diameter) columns allows feeding of the eluted peaks directly into a mass spectrometer of FTIR instrument. 

---