Multicapillary columns with diffusional bridging: the combined polydispersity and scatter problem.

Straight gas chromatography multicapillary columns (MCC) with 40 µm diameter capillaries (hereafter – 40 µm MCC) have been known for quite a long time and are well studied; they are used in portable gas analyzers. Some chromatographic characteristics of 25 µm MCCs, which appeared relatively recently, were also studied, while commercially available 60 and 80 µm MCCs are poorly studied. In this work the main analytical characteristics of 60 and 80 µm MCCs were determined and compared with the characteristics of 25 and 40 µm MCCs. It was shown that the maximum specific efficiency of the columns decreased with increasing column capillary diameter and is approximately 24.8, 18.2, 13.7 and 9.5 thousand theoretical plates (t.p.) per meter for 25, 40, 60 and 80 µm MCCs, respectively. It was established that the height equivalent to a theoretical plate of 60 and 80 μm MCCs was not varied significantly over a wide range of carrier gas velocities (nitrogen and helium), which allowed operating MCCs at high carrier gas flows essentially without loss of their efficiency. Moreover, for all MCCs the separation rate for peaks with a retention factor over 10 exceeded 600 t.p./s, and for peaks with a lower retention factor could be several thousand t.p./s, which is significantly higher than for conventional capillary and packed columns. It was established that it was possible to create very high carrier gas flows (up to 1000 cm3/min or more) for 60 µm MCCs and especially 80 µm MCCs at a relatively low pressure drop across the column. Therefore they can work as a part of chromatographic systems that require high carrier gas flow.

Short straight multicapillary columns (MCC) allow for fast gas chromatographic separation. However, their limited efficiency makes it difficult to use them for the analysis of multicomponent mixtures. Therefore, it is important to preliminarily evaluate the possibility of separation of target substances, based on the available information about their characteristics, in particular, by retention indices (RI). The paper provides an example of the choice of MCC and chromatographic conditions for the separation of explosives in stationary phases OV-5 and SE-54. To find the correspondence of the retention indices of the target substances with their retention factors k on the studied MCCs, the retention times of C10−C19 n-alkanes were experimentally determined at different column temperatures. Then the dependences of the calculated lnk on the quantity of carbon atoms m in n-alkanes CmH2m+2 were plotted and approximated by linear functions ln k = a + bm and their coefficients a and b were found for each temperature. From where, it was easy to calculate the retention factors of the target substances by their RI, assuming that RI = m·100, as well as to estimate the MCC efficiency required to separate neighboring peaks to obtain the desired level of separation. Based on the obtained equations, the explosives retention factors for MCCs at different temperatures were calculated and their required efficiency for separating adjacent peaks was estimated. The retention factors experimentally determined for some explosives on the MCCs at 140 °C turned out to be quite close to the theoretically calculated ones. In the future, the similar approach can be used to predict the possibility of gas chromatographic separation of mixtures of dangerous substances such as narcotic drugs.

Transient Taylor-Aris dispersion in N-capillary systems: Convergence properties of the band broadening in polydisperse multi-capillary columns with diffusional bridging

In the present work two new approaches to fast gas chromatography (GC) analysis are evaluated and compared to narrow-bore and conventional-bore columns. The first method uses packed capillary columns. These columns, 0.32 mm in diameter, are packed with spherical octadecylsilica particles with a diameter of 15 μm. A second type of column studied is the multicapillary column. In this column, an excellent sample capacity and a high speed of analysis are combined. The performance of all column types is considered. Given are H/u curves and two practical applications are described. Finally, micropacked capillary columns, multicapillary columns, and narrow-bore columns are compared to conventional standard-bore columns. This comparison includes the main parameters of interest such as sample capacity, detection limit, analysis time, pressure drop, and maximum obtainable plate number. Packed columns and multicapillary columns are very suitable for fast separations of relatively simple samples. ©1999 John Wiley & Sons, Inc. J Micro Sep 11: 155–162, 1999

Gas chromatography (GC) is a powerful analytical method to accurately and reliably identify the constituents of a complex mixture. There are numerous efforts to miniaturize GC system, in general, and separation columns, in particular, for rapid, dependable, and portable on-site analysis. This paper reports the development of two diverse self-patterned gold electrodeposition fabrication techniques for high-aspect-ratio microfluidic channels including multicapillary GC columns. First approach involves geometry-dependent tuned electroplating conditions to self-pattern gold along the vertical sidewalls without any deposition on horizontal top and bottom surfaces, while the second method provides highly conformal gold deposition inside the 3-D microchannels. Both reported approaches do not require a postdeposition patterning step while affording at the same time excellent bonding and stationary phase coating yields. The ability of thiol to self-assemble on gold surface is also utilized to form monolayer-protected gold (MPG) surfaces and is used as a stationary phase for micro GC. To evaluate the chromatographic performance of both schemes, 250-μm -deep 30-μm-wide 25-cm-long microfabricated multicapillary columns (μMCCs) with 16 channels are functionalized by self-assembly of octadecanethiol (C18H37 SH) to form the MPG surface. With about 7300-plate/m theoretical plates, these columns demonstrate the highest reported separation efficiency on 16-channel μMCCs and are capable of separating complex gas mixtures containing compounds with wide range of boiling points.

The use of high-speed multicapillary column in comprehensive two-dimensional gas chromatography with flow modulation

Impact of mobile phase composition on the performance of porous polymeric monoliths in the elution of small molecules

Computational fluid dynamics simulations of the band broadening in an idealized, and highly ordered 3-D model of silica monoliths are reported. As this high degree of order induces only a minimal eddy-dispersion, the band broadening is very sensitive to the intra-skeleton diffusivity and retention factor of the analytes. The simulations hence provide a maximal view on how the C(m)- and C(s)-contributions to the band broadening depend on the intra-skeleton diffusivity and retention factor. By comparing two model-structures with different external porosities, some interesting qualitative insights on the effect of the through-pore diameter are obtained as well. Because of the precisely known intra-skeleton diffusivity, the obtained plate height data also provide an ideal test case for the general plate height expression of chromatography. Writing out this model, identifying the geometrical parameters, and determining their value using a parameter fitting algorithm, a set of parameter values can be found which allows to accurately predict the retention factor dependency of the band broadening over a very broad range of mobile phase velocities. Remaining modeling problems are the apparent intra-skeleton diffusion dependency of the shape factor describing the intra-skeleton mass transfer, and the absence of mathematical expressions to predict the model shape factor values.

We present a series of numerically calculated plate height and flow resistance data obtained for an idealized chromatographic support mimic with variable bed porosity (0.3 </= epsilon </= 0.9), yielding a unique insight into how the main chromatographic performance parameters can be expected to vary with the external bed porosity, unbiased by any differences in molecular diffusivity or retention factor. The influence of pore heterogeneity effects is considered as well. It is found that the product h(min)nu(opt) depends only very weakly on epsilon and on the degree of pore heterogeneity. It is also found that the minimal separation impedance E(min) decreases monotonically with epsilon. This shows that the minimal plate height increase that can be expected for large porosity systems is always more than compensated by their reduced flow resistance, in agreement with the current observations in real silica monolith columns. Using the computed plate height data in an optimization analysis, it is found that large porosity supports can always potentially yield shorter analysis times or larger plate numbers than small porosity supports but need submicrometer feature sizes to actually achieve this. Assuming a lower limit on the producible or useable structural feature sizes, it is found that small N separations can best be performed with a small porosity packing, whereas large N separations require a large porosity packing if the column length (L) is left free. A plot yielding epsilon(opt) as a function of the required plate number has been established, showing that roughly epsilon(opt) approximately log(N) in both the ordered and the disordered support cases. It is also shown that the maximal increase in peak capacity ever to be expected from the use of high porosity supports is a factor of 2 (if the mobile-phase viscosity can be kept constant), potentially to be increased by a factor of 1.5 by increasing the homogeneity of the packing.

Sense and nonsense of miniaturized LC-MS/MS for bioanalysis.

An explicit expression for the retention factor and velocity dependency of the mobile zone mass transfer band broadening in packed spheres beds used in liquid chromatography

Chromatographic properties PLOT multicapillary columns

Multiscale simulation of liquid chromatography: Impact of retention on the plate height in packed beds.

Building upon the micromachined column idea proposed by the group of Regnier in 1998, we report on the first high-resolution reversed-phase separations in micromachined pillar array columns under pressure-driven LC conditions. A three component mixture could be separated in 3 s using arrays of nonporous silicon pillars with a diameter of approximately 4.3 microm and an external porosity of 55%. Under slightly retained component conditions (retention factor k' = 0.65-1.2), plate heights of about H = 4 microm were obtained at a mobile phase velocity around u = 0.5 mm/s. In reduced terms, such plate heights are as low as hmin = 1. Also, since the flow resistance of the column is much smaller than in a packed column (mainly because of the higher external porosity of the pillar array), the separation impedance of the array was as small as E = 150, i.e., of the same order as the best currently existing monolithic columns. At pH = 3, yielding very low retention factors (k' = 0.13 and 0.23), plate heights as low as H = 2 microm were realized, yielding a separation of the three component mixture with an efficiency of N = 4000-5000 plates over a column length of 1 cm. At higher retention factors, significantly larger plate heights were obtained. More experimental work is needed to investigate this more in depth. The study is completed with a discussion of the performance limits of the pillar array column concept in the frame of the current state-of-the-art in microfabrication precision.

Capillary number theory is very important for chemical flooding enhanced oil recovery. The difference between microscopic capillary number and the microscopic one is easy to confuse. After decades of development, great progress has been made in capillary number theory and it has important but sometimes incorrect application in EOR. The capillary number theory was based on capillary tube bundles and Darcy’s law hypothesis, and this should always be kept in mind when used in chemical flooding EOR. The flow in low permeability porous media often shows obvious non-Darcy effects, which is beyond Darcy’s law. Experiments data from ASP flooding and SP flooding showed that remaining oil saturation was not always decreasing as capillary number kept on increasing. Relative permeability was proved function of capillary number; its rate dependence was affected by capillary end effects. The mobility control should be given priority rather than lowering IFT. The displacement efficiency was not increased as displacement velocity increased as expected in heavy oil chemical flooding. Largest capillary number does not always make highest recovery in chemical flooding in heterogeneous reservoir. Misuse of CDC in EOR included the ignorance of mobility ratio, Darcy linear flow hypothesis, difference between microscopic capillary number and the microscopic one, and heterogeneity caused flow regime alteration. Displacement of continuous oil or remobilization of discontinuous oil was quite different.