Column, short, separation

Recently, multidimensional GC has been employed in enantioselective analysis by placing a chiral stationary phase such as a cyclodextrin in the second column. Typically, switching valves are used to heart-cut the appropriate portion of the separation from a non-chiral column into a chiral column. Heil et al. used a dual column system consisting of a non-chiral pre-column (30 m X 0.25 mm X 0.38 p.m, PS-268) and a chiral (30 m X 0.32 mm X 0.64 p.m, heptakis(2,3-di-(9-methyl-6-(9-tert-butyldimethylsilyl)-(3-cyclodextrin) (TBDM-CD) analytical column to separate derivatized urinary organic acids that are indicative of metabolic diseases such as short bowel syndrome, phenylketonuria, tyrosinaemia, and others. They used a FID following the pre-column and an ion trap mass-selective detector following the... 

It is usual to convert fatty acids to their methyl ester derivatives before separation by GLC, although it may be possible to analyse those with short chain lengths (two to eight carbon atoms) as the free fatty acids. Polar or non-polar stationary phases can be used and capillary (open-tubular) or SCOT columns will separate positional and geometric isomers. The cis isomers have shorter retention times than the corresponding trans isomers on a non-polar phase and visa versa on a polar phase. 

A fourth factor is the flow rate of the eluent (mobile phase). A too high rate decreases resolution because there is no time for molecules to diffuse into the pores of the matrix. In contrast, a very slow migration of solvent decreases the resolution by remixing the components by diffusion. The effect of diffusion is minimized if the chromatography is done at low temperature. If short separation times are necessary, pre-packed columns, elevated pressure, or HPLC columns are indispensable. 

Having found a prospective GC-column and completed initial runs with conventional separation times, the usual finding is that short separation times come with high temperatures and/or steep temperature gradients. If such separations at high temperatures cannot be run isothermally the cooling time to reach the start... 

Bouzas et al. [95] developed a quantitation method for the determination of drugs of abuse (opiates, amphetamine and derivatives, cocaine, methadone and metabolites) in serum by using online extraction coupled to LC-MS/MS. The online extraction procedure described consists of an extraction column and an analytical column, which were coupled online. A PPT procedure was performed with zinc sulfate an aliquot of 0.1 M zinc sulfate in methanol was added to the serum sample in a proportion of 2 1 (v/v) to serum. Analytes were extracted by a short pentafluorophenyl silica column and separated on a longer analytical column with the same stationary phase. Recoveries of all analytes were above 80 %. The proposed procedure by Matuszewski et al. [64] was used for the evaluation of matrix effect ME(%)=B/Ax 100, where ME is the matrix effect (suppression or enhancement) and B corresponds to peak areas for standards spiked after extraction into sample extracts and A to peak areas obtained in neat solution standards. Authors compared this method to offline SPE coupled with GC-MS and results showed that LC-MS/... 

In the assay described by Amici et al. (1994), a wide variety of pyrimidine and purine nucleoside 5 -monophosphates were separated from their nucleosides by chromatography on a Supelco LClg guard column (4.6 mm x 20 mm, 5 fim). The short column allows separations in less than a minute. The mobile phase was 0.1 M potassium phosphate buffer (pH 6.0) except in the case of adenosine and deoxyadenosine, when 5% methanol was also included. The flow rate was 2 mL/min. Compounds were detected by monitoring the effluent at 254 nm, although sensitivity could be improved in some cases by using a different wavelength. 

Podgornik, A. Bamt, M. Jaksa, S. Jancar, J. Strancar, A. Application of very short monolithic columns for separation of low and high molecular mass substances. J. Liq. Chromatogr. Relat. Technol. 2002, 25, 3099-3116. 

---