Packed-column inlets

Direct Sample passes directly from syringe to hot inlet where it vaporises. Packed column inlets Dilute samples Thermally stable Fair, some focusing required 0.1-2 100... 

Ferrules, Connectors, and Fittings PACKED-COLUMN INLET... 

FIGURE 9.1 Schematic of a packed-column inlet. Carrier gas flows into the inlet, around the column, and into the column head. The syringe pierces through a polymeric septum. The entire inlet is heated. (Figure courtesy of Prof. Harold M. McNair). 

The main advantage of the packed column inlet is that the entire sample that exits the syringe enters the column, making packed column injection highly reproducible. The pneumatics are also very simple and inexpensive. Method development is also very straightforward with only the inlet temperature as an easily adjustable variable. Further, packed columns and inlets typically operate at lower temperatures than capillary inlets, allowing the use of less expensive septa. 

The main disadvantages of packed-column inlets arise from contamination from nonvolatile sample components or septum pieces. Since the column is usually... 

Elution volume, exclusion chromatography Flow rate, column Gas/liquid volume ratio Inner column volume Interstitial (outer) volume Kovats retention indices Matrix volume Net retention volume Obstruction factor Packing uniformity factor Particle diameter Partition coefficient Partition ratio Peak asymmetry factor Peak resolution Plate height Plate number Porosity, column Pressure, column inlet Presure, column outlet Pressure drop... 

Inlets for syringe sampling are divided kito two main categories one for packed-column and the other for capiHary-column devices. Eor packed columns, all material kijected is carried by the mobile phase onto the column. The inlet is usually an open tube, but sometimes, albeit rarely, the inlet itself may be packed, eg, to assure that the first centimeters of the column do not become contaminated with degradation products or nonvolatile materials that may affect the efficacy of the column. 

The column may be packed or it may be an open tube but in this example, a packed column will be specifically considered. The column is considered to have a length (L) and inlet and outlet pressures and inlet and outlet velocities of (Pi), (Po) (ui) and (uo), respectively. The pressure and velocity at a distance (x) from the front of the column is (Px) and (ux), respectively. According to D Arcy s equation for fluid flow through a packed bed, at any point in the column. 

When packing of the porous materials is not sufficiently tight, repeated injection of a viscous solution leads to an increasing packing density, leaving a void space at the column inlet. When the void space was filled with extra porous materials, it was observed (1) that the resolution of HOPC decreased slightly compared with first-time use. When the column ceased to produce the void space, the separation performance became reproducible. 

Figtire 3.4 An example of a flash vaporization inlet for packed column gas chromatography. (Reproduced with permission from Hewlett-Packard Co.)... 

The simplest type of two-dimensional gas diromatography for heart cutting or trace enrichment using a packed precolumn and a capillary column is shown in Figure 8.16 [205]. Almost any modem gas chromatograph could be converted into a similar unit with the addition of a few auxiliary components. Preliminary separation t2dces place on the packed column, the effluent from which is directed either to a vent or to the capillary inlet by the Deans switch, nie effluent reaching the capillary inlet is split three ways. One portion passes to a detector used to monitor the preseparation, a second portion enters the cap Bry column and is... 

Mlcropacked column, which have internal diameters less than 1 am and a packing density similar to the columns described above (see section 1.7.1), are packed by special methods, since they are generally used in longer lengths and operated at higher inlet pressures than conventional packed columns [128,129]. The colled... 

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