Chromatographic Performance of Capillary Columns

Zhang s group in China developed monolithic poly(styrene-co-divinylbenzene) CEC column in which EOF is supported by carboxyl groups of polymerized methacrylic acid units (Xiong etal. [51]). In a typical procedure, vinylized 75 mm i.d. capillaries were filled with a mixture of 5% styrene 21, 10% divinylbenzene 22, 5% methacrylic acid 1, and 80% toluene containing 1% azobisisobutyronitrile (in respect to monomers) and polymerized at 70°C for 24 h. The pore volume of 0.098 mL/g and mean pore size of 40 nm determined for this monolith appear to be rather small and do not correspond with the published SEM pictures that reveal existence of large pores, and the chromatographic performance of the columns in CEC mode. 

A poorly designed GC/MS interface can easily compromise the performance of both the capillary GC system and the mass spectrometer. The interface should provide an inert transfer surface, yield full sample transmission, and maintain the chromatographic performance of the column. 

A number of attempts to extend the usual temperature range of capillary columns in order to elute PAH above coronene have been successful. Grob (25) was able to chromatograph stable PAH-compounds from coronene up to rubrene (molecular weight 300 to 532) on a short DV-lDl column. Romanowski et al. (26) used a 15 meter SE-54 fused silica column to separate PAH in the molecular weight range from 300 up to 402, in combination with FID and MS detection. A recent review on the performance of capillary columns in PAH-analysis has been published by Lee and Wright (27). 

A gas chromatograph with a capillary column coupled to a mass spectrometer is an ideal analytical partnership. Effluent from the column has an elevated temperature and the molecules of interest are in a vapor state and ready to enter the ion source. This eliminates the need for desolvation that is required in high-performance liquid chromatography (HPLC)-MS. 

The increased ability of capillary GC to resolve organic species caused an unanticipated drawback for broad spectrum analysis. The amount of stationary phase on a capillary GC column is much less than on a packed column. This condition increases the likelihood of observing a decrease in chromatographic performance caused by the sample matrix. The altered stationary phase may cause reduced precision of retention times and peak areas. The changes in the chromatographic performance of the stationary phase are measured by the Grob general purpose test mix (9). 

Similarly, we polymerized the same mixture used for the preparation of capillary columns in glass vials and used the product for mercury intrusion porosimetry. Since we found that a strong correlation exists between the "dry" porous properties of the monoliths and their chromatographic performance, even dry porosity measurements may be used to tailor column performance. 

Capillary columns are used in the current high performance chromatographs. The functioning principle of this type of column is the same as was described for gas chromatography. There are three types of capillary columns used in liquid chromatography open tubular, partially packed, and tightly packed. The advantage of these columns is that they allow us to work with very small amounts of sample. 

Gas chromatographic/mass spectrometry (GC/MS) results were obtained at the Ohio State University Chemical Instrumentation Center using a Finnigan 4021 GC/MS instrument. Both electron impact and chemical ionization were performed on samples following separation of compounds by a gas chromatograph equipped with capillary columns containing 3% 0V-17 or 5% carbowax 20M. 

When on-column injection is used the end of the transfer capillary is inserted into the column inlet or retention gap where decompression of the supercritical fluid occurs. Carbon dioxide gas exits through the column and the seal made between the restrictor and septum (unless a closed injector is used). The analytes are focused by cold trapping in the stationary phase. The transfer line must be physically removed from the injector at the completion of the extraction to establish the normal carrier gas flow for the separation. Analyte transfer to the column is virtually quantitative but blockage of the restrictor is more conunon and involatile material accumulates in the injection zone eventually degrading chromatographic performance. The on-column interface is probably a better choice for trace analysis of relatively clean extracts with modest fluid flow rates than the split interface. When optimized both the on-column and split interfaces provide essentially identical peak shapes to those obtained using conventional solution injection. 

Since the introduction of GC, the basic parts of a gas chromatograph have been unchanged in function and purpose, even though the improvement has been occurring in design and materials. One area of dramatic advance in GC instrumentation was the introduction of the open tubular columns. Consequently, most GC analyses in practice are performed in capillary columns that show the separation with high efficiencies and high resolution. The... 

Lipsky, S.R. McMurray, W.J. Role of surface groups in affecting the chromatographic performance of certain types of fused-silica glass capillary columns H. Deactivation by esterification with alcohols and deactiva- 13. tion with specially prepared high-molecular-weight stationary phases. J. Chromatogr. 1983, 279, 59. 

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