Column, capillary purpose

For routine separations, there are about a dozen useful phases for capillary columns. The best general-purpose columns are the dimethylpolysiloxane (DB-1 or equivalent) and the 5% phenyl, 95% dimethylpolysiloxane (DB-5 or equivalent). These relatively nonpolar columns are recommended because they provide adequate resolution and are less prone to bleed than the more polar phases. If a DB-1, DB-5, or equivalent capillary column does not give the necessary resolution, try a more polar phase such as DB-23, CP-Sil88, or Carbowax 20M, providing the maximum operating temperature of the column is high enough for the sample of interest. See Appendix 3 for fused silica capillary columns from various suppliers. 

Principles and Characteristics As mentioned already (Section 3.5.2) solid-phase microextraction involves the use of a micro-fibre which is exposed to the analyte(s) for a prespecified time. GC-MS is an ideal detector after SPME extraction/injection for both qualitative and quantitative analysis. For SPME-GC analysis, the fibre is forced into the chromatography capillary injector, where the entire extraction is desorbed. A high linear flow-rate of the carrier gas along the fibre is essential to ensure complete desorption of the analytes. Because no solvent is injected, and the analytes are rapidly desorbed on to the column, minimum detection limits are improved and resolution is maintained. Online coupling of conventional fibre-based SPME coupled with GC is now becoming routine. Automated SPME takes the sample directly from bottle to gas chromatograph. Split/splitless, on-column and PTV injection are compatible with SPME. SPME can also be used very effectively for sample introduction to fast GC systems, provided that a dedicated injector is used for this purpose [69,70],... 

SFC-SFC is more suitable than LC-LC for quantitation purposes, in view of the lack of a suitable mass-sensitive, universal detector in LC. Group quantitation can be achieved by FID. The ideal SFC-SFC system would consist of a short (10-30 cm) packed-capillary primary column, interfaced to a long (5-10m) open-tubular column, but such a combination is difficult to realise, due to the different flow-rates required for each column type. Coupled SFC-SFC is often configured with a solute concentration device prior to valve switching on to the SFC. The main approaches to this concentration stage are the use of absorbent material or cryofocusing. Davies el at. [924] first introduced two-dimensional cSFC (cSFC-cSFC), and its use has been reported [925,926]. 

For the analysis of nonvolatile compounds, on-line coupled microcolumn SEC-PyGC has been described [979]. Alternatively, on-line p,SEC coupled to a conventional-size LC system can be used for separation and quantitative determination of compounds, in which volatility may not allow analysis via capillary GC [976]. An automated SEC-gradient HPLC flow system for polymer analysis has been developed [980]. The high sample loading capacity available in SEC makes it an attractive technique for intermediate sample cleanup [981] prior to a more sensitive RPLC technique. Hence, this intermediate step is especially interesting for experimental purposes whenever polymer matrix interference cannot be separated from the peak of interest. Coupling of SEC to RPLC is expected to benefit from the miniaturised approach in the first dimension (no broadening). Development of the first separation step in SEC-HPLC is usually quite short, unless problems are encountered with sample/column compatibility. 

It is of much interest to compare polymer monoliths with monolithic silica columns for practical purposes of column selection. Methacrylate-based polymer monoliths have been evaluated extensively in comparison with silica monoliths (Moravcova et al., 2004). The methacrylate-based capillary columns were prepared from butyl methacrylate, ethylene dimethacrylate, in a porogenic mixture of water, 1-propanol, and 1,4-butanediol, and compared with commercial silica particulate and monolithic columns (Chromolith Performance). 

Obviously, the main purpose for the introduction of CL detection coupled to CE separations is inherent to the development and improvement of sensitive and uncomplicated devices to achieve a decrease of the band broadening caused by turbulence at the column end, together with the attractive separation efficiency of CE setups. With this purpose in mind, Zhao et al. [83] designed a postcolumn reactor for CL detection in the capillary electrophoretic separation of isoluminol thiocarbamyl derivatives of amino acids, because, like other isothiocyanates, isoluminol isothiocyanate has potential applications in the protein-sequencing area. 

Since a comprehensive description of all monolithic materials would exceed the scope of this chapter and a number of other monolithic materials are also described elsewhere in this volume, this contribution will be restricted mainly to monoliths for chromatographic purposes and prepared by polymerization of monomer mixtures in non-aqueous solvents. Monolithic capillary columns for CEC are treated in another chapter and will not be presented in detail here. 

LOD and LOQ were measured to assess the sensitivity of the FID, ECD and TSD detectors for GC analysis of various nitroaromatic compounds. A parallel connection of the three detectors at the end of a single narrow-bore capillary column enabled direct comparison of the chromatograms. Structural effects on the response were evaluated and detection mechanisms were discussed. Recommendations were made for identification purposes and for analysis of environmental samples of nitro- and chloro-nitro-benzenes in a wide range of concentrations451. 

With the advent of capillary GC, [50-54] the need for separators and the concomitant risk of suppression of certain components vanished. Capillary columns are operated at flow rates in the order of 1 ml min and therefore can be directly interfaced to EI/CI ion sources. [48,49] Thus, a modem GC-MS interface basically consists of a heated (glass) line bridging the distance between GC oven and ion source. On the ion source block, an entrance port often opposite to the direct probe is reserved for that purpose (Chap. 5.2.1). The interface should be operated at the highest temperature employed in the actual GC separation or at the highest temperature the column can tolerate (200-300 °C). Keeping the transfer line at lower temperature causes condensation of eluting components to the end of the column. 

It follows that retention measurements on silica based stationary phases for the purpose of obtaining thermodynamic data is fraught with difficulties. Data from solutes of different molecular size cannot be compared or related to other Interacting variables ideally, thermodynamic measurements should be made on columns that contain stationary phases that exhibit no exclusion properties. However, the only column system that might meet this requirement is the capillary column which, unfortunately introduces other complications wmcn will be discussed later. 

Figure 2.6—Representation on the same scale of three types of GC column, a) Stainless steel column, 1 /8 in. inner diameter b) A 530 column of 0.53 mm inner diameter c) Capillary column of 0.25 mm inner diameter d) Details of a capillary column. On this scale, the thickness of the stationary phase can hardly be seen. The standard length is approximately 30 m. However, for analyses of high molecular weight materials or for screening purposes, the length is 15 m.

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). 

In gas chromatography, a guard column and a retention gap are each typically a 3- to 10-meter length of empty capillary in front of the capillary chromatography column. The capillary is silanized so that solutes are not retained by the bare silica wall. Physically, the guard column and the retention gap are identical, but they are employed for different purposes. 

---