Injection packed column injector

The split-flow injector is very similar to the packed column injector except that only part of the carrier flow passes to the column, the rest exits to waste. By varying the exit flow-impedance, the split-ratio can be adjusted over a wide range. Without this type of split injection system, the small bore capillary columns would be virtually impossible to use. However, because of the waste of sample and the relatively small mass range obtainable from small bore columns, the large bore capillary column was introduced. 

Direct injection rehes on vaporization processes to introduce sample into the column. However, it lacks a purge activation function, a septum purge flow, or secondary cooling (Fig. 5). The direct injection mode is used with packed columns or wide-bore (0.53-mm-I.D.) capillary columns. Wide-bore columns are used as higher-efficiency replacements for packed columns without the need to extensively modify the packed-column injector. 

The process to convert a packed-column injector for use with wide-bore columns is relatively simple. The packed column is removed from the GC along with any injector (or detector) fittings attached to the base injector (or detector) body. A glass, direct-injection liner is placed into the packed... 

Kovat s retention index (p. 575) liquid-solid adsorption chromatography (p. 590) longitudinal diffusion (p. 560) loop injector (p. 584) mass spectrum (p. 571) mass transfer (p. 561) micellar electrokinetic capillary chromatography (p. 606) micelle (p. 606) mobile phase (p. 546) normal-phase chromatography (p. 580) on-column injection (p. 568) open tubular column (p. 564) packed column (p. 564) peak capacity (p. 554)... 

Injection generally occurs through a resealable rubber septum. The injector port is held at 150-250° depending on the volatility of the sample and direct injection of 0.1-10 /A of sample is made onto the head of the column. The amount of sample injected onto a packed column is ca 1-2 pg per component. Injection into packed... 

Direct vaporisation injection. For packed columns and megabore columns of 530 pm, which typically use a flow rate of 10 ml/min, direct vaporisation is a simple way to introduce the sample. All models of this type of injector are a variation of a simple assembly which uses a metal tube with a glass sleeve or insert. The glass insert is swept by the carrier gas and heated to the vaporisation temperature for the analytes undergoing chromatography. One end of the injector contains a septum made of silicone rubber that allows the syringe needle to pass through it into the system. The other end of the injector is connected to the head of the column (Fig. 2.4). The entire sample is injected into the column in a few seconds. 

Figure 2.4—Direct vaporisation injector used for packed columns. Typical septum injector with a microseal option (the Merlin microseal) that can be used for thousands of injections (reproduced by permission of Hewlett-Packard).

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

Figure 5 compares chromatograms of A8- and A9-THC-TMS and 11-hydroxy-A8-THC-TMS obtained on a glass capillary column and a 6ft. x 2mm I.D. packed column. The retention times of the cannabinoids are comparable, but the resolution and sensitivity achieved on the capillary column is far superior. The capacity of the capillary column is of course far less than that of the packed column. Nevertheless, by using a Grob-type splitless injector (7) we are able to inject several microliters of solution containing up to 50 ng of each cannabinoid without overloading the column. Use of a support-coated open tubular column should increase the column capacity, but at some sacrifice of resolution and sensitivity. 

Injectors Sample injection devices range from simple syringes to fully programmable automatic injectors. The amount of sample that can be injected into a capillary column without overloading is small compared with the amount that can be injected into a packed column, and may be less than the smallest amount that can be manipulated satisfactorily by syringe. Capillary columns are therefore used with injectors able to split samples into two fractions, a small one that enters the column and a large one that goes to waste (split injector). Such injectors may also be used in a splitless mode for analyses of trace or minor components. 

The development of modem capillary columns has led to improved resolution and has also simplified the process of running gcs considerably. The columns are normally made from fused silica capillary with an inside diameter of between 0.2 and 0.5mm, and are polymer coated. They have no packing, but instead the liquid stationary phase is bonded to the inside wall of the capillary, and this allows gas to flow very easily. Because of this the columns can be made much longer than packed columns (between 12 and 100m) and they are typically ten times as efficient. Capillary columns give extremely high sensitivity and only a very small quantity of material is required. For this reason the injector normally incorporates a splitter, so that only a small portion of the sample injected actually enters the column. 

The injection system in a GC analysis provides a means of introducing the sample onto the column. This is normally a simple injector design when packed columns are used. Capillary or open tubular columns require, however, more sophisticated design than do packed columns, because of their very low capacities and small internal diameters. A number of different injector designs have been recently developed. Unfortunately, no universal injector has so far been commercially available to handle all sample types. This article will discuss various approaches to sample injection. 

Perrier and Lear converted theophylline to its butyl derivative by means of tetrabutyl-anrnonium hydroxide and on-column alkylation in connection with a rapid extraction procedure. Samples of 100 yl plasma were extracted with chloroform-isopropanol (95 5) containing the internal standard, amobarbital. The solvent was evaporated and the residue dissolved in 1 ml toluene. With the aid of 10 yl of an aqueous solution of tetrabutylammonium hydroxide, theophylline and amobarbital were quantitatively extracted from the toluene, and by injection of an aliquot of this solution into the gas chromatograph, alkylation took place in the injector, and quantitation of theophylline in concentrations in 100 ul plasma was possible with a flame ionization detector, using a packed column with 3 OV-17 on Chromosorb G at 250°C. 

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