Packed capillary columns optimization

ESI Interface for CapiUary-LC and Nano-LC Columns Currently, the applications of capillary and nano-LC are on the upswing especially for many biochemical studies, where the sample amounts and volumes are both limited. For such samples, packed capillary columns of 50 to 300 xm i.d. are the ideal solutions. As pointed out above, the combined use of small-i.d. columns with an ES ion source has the advantage of optimal detection sensitivity because of its concentration-dependent response. Because these columns operate in the flow range nanoUters to microliters per minute, an ideal LC/MS system is realized when these columns are connected directly to nanospray or microspray sources [42,43]. The coupling of these columns to a conventional ES ion source can also be accomplished if an additional sheath liquid is added to increase the flow to a range that is acceptable by the source. 

A multidimensional system using capillary SEC-GC-MS was used for the rapid identification of various polymer additives, including antioxidants, plasticizers, lubricants, flame retardants, waxes and UV stabilizers (12). This technique could be used for additives having broad functionalities and wide volatility ranges. The determination of the additives in polymers was carried out without performing any extensive manual sample pretreatment. In the first step, microcolumn SEC excludes the polymer matrix from the smaller-molecular-size additives. There is a minimal introduction of the polymer into the capillary GC column. Optimization of the pore sizes of the SEC packings was used to enhance the resolution between the polymer and its additives, and smaller pore sizes could be used to exclude more of the polymer... 

The principal methods of interfacing SFC with ICP-MS have been discussed by Carey and Caruso [94]. Where packed SFC columns are used, the SFC restrictor is connected to a heated cross flow nebulizer and the nebulizer gas flow carries the sample to the plasma. For the more commonly used capillary columns, the SFC restrictor is passed through a heated transfer line that is connected directly to the torch of the ICP-MS. For optimal resolution of peaks, the restrictor should be positioned so that it is level with the injector of the ICP torch. This position may be varied slightly (Fig. 10.15). Heat is applied where the transfer line and torch connect to prevent freezing of the mobile phase when it decompresses after exiting the restrictor. To transport the analyte to the plasma efficiently, a gas flow of approximately 0.8-1.0 mL/min is used. This gas flow may also be heated to improve peak resolution. 

P Molander,T. E. Gundersen, C. Haas,T. Greibrokk, R. Blomhoff, and E. Lundanes, Determination of retenoids by packed-capillary chromatography with large-volume on-column focusing and temperature optimization, J. Chromatogr. A 847 (1999), 59-68. 

Figure 8 Affinity chromatography for the analysis of the methanol extract of Artemisia capillaris under optimized conditions. SCO, scoparone CAP, capillarisin. Experimental conditions the column used was of 150 mm x 4.6 mm ID packed with HSA immobilized on silica (7 pm), column temperature was 35 °C, flow rate was 0.8ml min-1, and UV detection wavelength was set to 238 nm. Initially, 10 min isocratic elution with mobile phase of 1.5% acetonitrile in 10 mmol I-1 phosphate buffer (pH 6.0) then, 5 min linear gradient elution from 1.5 to 12% acetonitrile in 10 mmol I-1 phosphate buffer (pH 6.0) with the elution of the latter mobile phase kept for an additional 45 min and finally, another 45 min linear gradient elution from 12% acetonitrile in 10 mmol I-1 phosphate buffer (pH 6.0) to 15% acetonitrile in 10 mmol I-1 phosphate buffer (pH 7.4). Reproduced from H. L. Wang H. F. Zou J. Y. Ni L. Kong S. Gao B. C. Guo, J. Chromatogr. A 2000, 870, 501-510.

However, final optimizations of these separations have not been realized or possible. Some workers have utilized pressurized flow to solve the problems of obtaining reasonable EOF without silanol-analyte interaction however, this does not solve the problem. It just forces the analyte to elute and approaches electro-HPLC, rather than true CEC. There are real differences between electro-HPLC, PEC, and CEC that need to be recognized. There does not, in general, seem to have been any serious attempt to utilize any chemometric software approaches in CEC/PEC for biopolymer separation optimizations or rationale for doing so. At this time, packings are simply used because they were on the shelf in a laboratory or commercially available and not necessarily because they were really the best for protein-peptide separations in PEC/CEC. There remains a need for research-oriented column choices from commercial vendors to avoid the need to pack capillaries in-house with commercial HPLC supports. 

Two column types are in use, the packed and the capillary column. The selection is often dictated by the nature and complexity of the sample. The present trend is to replace PCGC by CGC whenever possible, because data obtained with the latter technique are much more reliable. The fundamental difference between the two methods is reflected in the resolution equation, the key equation for separation optimization ... 

In the case of capillary columns, finding appropriate solutions for sample introduction is much more challenging than with packed columns. Since packed columns have dimensions similar to normal HPLC, optimal sample introduction can be achieved with ordinary sample loop injectors (see Section 12.2.4.2). 

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