Injector volume

Figure 1.18. Types of chromatograms, (a) Differential chromatogram (b) integral chromatogram (c) peak resolution. O, injection point OX, injector volume OY, detector volume OA, holdup volume, VM OB, total retention volume, VR AB, adjusted retention volume,...

Injector volume. The volume of carrier gas (mobile phase) required to fill the injection port of the chromatograph. 

Sample Introduction. The small internal diameters of SFC capillary columns place stringent requirements on saoiple introduction in order to avoid band-broadening due to too large of an injector volume. These requirements have been met using a 0.2- jL internal volume high pressure valve operated in a splitting mode (12). Split ratios of 3 1 to 5 1 are usual. Recently, valves designed specifically for use with capillaries have become available (Valeo Instruments Co., Houston, Texas). These valves offer internal volumes as low as 60-nL and show promise for use in capillary SFC. 

Reduce the sample injector volumes <20pL for isocratic analysis... 

Sample solvent effects on gas chromatographic analyses involve a very different set of considerations. For capillary GC, the effect of the injection solvent has been shown to affect injection precision caused by the expansion volume of the vaporized injection solvent. If the solvent evaporates too fast to a volume larger than the injector volume, then the rapid pressure increase in the injector can cause the sample to leak out through the septum, leading to poor injection precision (11,12). Different solvents have different expansion volumes and expansion rates, therefore proper solvent selection can overcome this problem. Other solutions are smaller injection volume, slower injection speed, or lower injector temperature (11). 

The typical GC syringe injection volume of 0.5-2 pL of liquid solvent typically expands to 0.5-1.5mL vapor volume, depending on injector temperature. This is a volume increase on the order of lOOOx. A capillary GC column is operated at a much lower flow rate than a packed column (typically on the order of 1 mL/min instead of 20-40 mL/min). The handling of the volume of vaporized solvent and analyte(s) in capillary GC required a more complex control of gas flows in and out of the injector volume. At the typical capillary column flow rates, it may require a minute or more for all this vapor volume to be transferred to the column. In order that the analytes (if not the solvent) should not spread out after entering the column, the column oven must be maintained at a temperature much lower than that used to vaporize the sample in the injector space. This requires that a separate injector port volume be independently heated to a different and higher temperature than the initial oven temperature. 

The aqueous phase may be injected if the sample is sufficiently concentrated. A number of problems may be encountered under these circumstances. When water is converted to steam, the volume increases dramatically 1 il of water becomes more than 1000 jtl of steam. This is larger than the injector volume of many current gas chromatographs, and the steam may degrade the performance of the system. Polar gas chromatography liquid phases such as Carbowax and PEG will degrade in the presence of steam unless they are bonded to the column. 

The necessity of carrying out injection at high pressure and the atomization into fine droplets using an injector imposes very precise volatility characteristics for the diesel fuel. French and European specifications have established two criteria for minimum and maximum volatility therefore, the distilled fraction in volume % should be ... 

Steam is injected into a reservoir to reduce oil viscosity and make it flow more easily. This technique is used in reservoirs containing high viscosity crudes where conventional methods only yield very low recoveries. Steam can be injected in a cyclic process in which the same well is used for injection and production, and the steam is allowed to soak prior to back production (sometimes known as Huff and Puff). Alternatively steam is injected to create a steam flood, sweeping oil from injectors to producers much as in a conventional waterflood. In such cases it is still found beneficial to increase the residence (or relaxation) time of the steam to heat treat a greater volume of reservoir. 

Injector The sample, typically 5-200 )J,L, is placed in the carrier stream by injection. Although syringe injections through a rubber septum are used, a more common means of injection is the rotary, or loop, injector used in ITPLC and shown in Figure 12.28 of Chapter 12. This type of injector provides reproducible injection volumes and is easily adaptable to automation, a feature that is particularly important when high sampling rates are desired. 

Together with the techniques described above, other techniques using hot injectors for the transfer of large-volumes in capillary gas chromatography have been developed. Transfer of large-volume solvents in a programmed temperature vaporizing... 

J. Staniewski and J. A. Rijks, Potential and limitations of differently designed pro-grammed-temperature injector liners for lar ge volume sample intr oduction in capillary GC, 7. High Resolut. Chromatogr. 16 182-187 (1993). 

S. Muller, J. Efer and W. Engewald, Gas chr-omatograpliic water analysis by dkect injection of large sample volumes in an adsorbent-packed PTV injector , Chromatographia 38 694-700 (1994). 

Although the problems associated with septum injectors can be eliminated by using stop-flow septumless injection, currently the most widely used devices in commercial chromatographs are the microvolume sampling valves (Fig. 8.3) which enable samples to be introduced reproducibly into pressurised columns without significant interruption of the mobile phase flow. The sample is loaded at atmospheric pressure into an external loop in the valve and introduced into the mobile phase by an appropriate rotation of the valve. The volume of sample introduced, ranging from 2 piL to over 100 /iL, may be varied by changing... 

The injector temperature should be determined by the nature of the sample and the volume injected, not by the column temperature. When analyzing biological or high-boiling samples, clean the injector body with methanol or other suitable solvent once per week. Install a clean packed injector liner and a new septum, preferably near the end of a workday. Program the column to its maximum temperature, then cool the column and run a test mixture to check the system using standard conditions. 

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