Partial loop injection

Figure 4.6. (a) A diagram of a manual injector valve. A schematic of a Rheodyne 7125 injector valve during the LOAD (b) and INJECT (c) cycle during a partial-loop injection operation. 

In the external standard method,it is critical that the injections are precise. With modem instmments which employ autosamplers, adequate precision (typically <0.5%) can be achieved using full-loop injection. Poorer injection precision is normally associated with manual injections or with partial-loop injections by autosamplers. 

The injection system may be manual or automated, but both rely on the injection valve. An injection valve is designed to introduce precise amounts of sample into the sample stream with variation usually less than 0.5% volume difference from injection to injection. Figure 2.7 schematically represents a 6-port and 2-position device valve. In one position the sample is loaded and the other it is injected. In the load position, the sample from the syringe or autosampler vial is pushed into the injection loop. The loop may be partially filled (partial loop injection) or completely filled (fuU loop injection). Partial loop injection depends on the precision filling of the loop with small known amounts of material. If partial loop injection is used, the loop must not be filled to more than 50% of the total... 

Dionex Corporation (2009) Dionex Technical Note No. 81 Automatic Dilutions Using Chromeleon AutoDilution and the Partial Loop Injection Capability of the lCS-3000 AS Autosampler. Dionex Corporation (now part of Thermo Fisher Scientific), Sunnyvale, CA, USA. 

An injector valve operates in two modes— the fixed-loop mode or the partial-loop mode. In the fixed-loop mode, a sample is overfilled into the loop at 2-4 times the loop volume and the entire loop content is injected. In the partial-loop fill mode, a variable sample aliquot, measured precisely by a syringe at <50% of the loop volume, is injected. Note that the sample slug is introduced into the end of the sample loop and is back flushed onto the column to minimize band dispersion by the sample loop (Figure 9). Due to the emphasis on productivity, manual injectors are seldom used in the pharmaceutical laboratory except for preparative applications. 

Type 2 autosamplers behave in a very similar way to type I except partial loop volume can be filled and delivered. Additionally, this allows the injection of low volume with high precision compared to type I autosamplers. Both types of autosamplers suffer from carry over problems if appropriate wash cycle and wash solvents are not used between each sampling from the vials. 

Figure 1.7, Schematic representation of partial- and fuJl-loop injection methods.

The 7(M0 Sanvle Irriection Vatve. Here s s 6-port sample injection valve with a removable sample loop and 7000 psi pressure rating. Size, to ii to 2.0 ml The 7120 Syringe Load Sample Injector. Fil loops conventionally or In the partial loop variable vDlume mode with only Q5 fi sample loss. 

Full automation of separations by liquid chromatography requires automation of the sample introduction process. This includes automation of scheduled injections and automation of sample processing when required to isolate the analytes of interest from the sample in a form suitable for separation and detection. An example of the latter approach is on-line solid-phase extraction-liquid chromatography (SPE-LC) discussed in section 5.3.2. Automation of time scheduled injections increases accuracy and precision by removing human error [1,18,39]. Typical precision for manual valve injections is about 0.5% for complete-fill and 1-2% for partial-fill loop injections. Typical precision for automated injection is about 0.25% for complete-fill and 0.5% for partial-fill loop techniques. 

The sample is loaded at atmospheric pressure into an external or internal loop, or groove in the valve core and introduced into the mobile phase stream by a short rotation of the valve. The volume of sample injected is normally V2u ied by changing Ute volume of the sample loop or by partially filling a sample loop with a fraction of its nominal volume. External sample loops have volumes from about 5 microliters up to about 1 ml. 

Internal standards are used for many reasons in chemistry. Here we are using it to correct for differences in sampling volumes. It takes much practice for a person to accurately deliver the same size sample every time. It is nearly impossible for two people to accurately deliver the same sample each time if they are partially injecting a loop. If we add a known amount of internal standard to both our sample and our known standard mixture, we can calculate peak heights or areas relative to that of the internal standard. Variations in the injection size of the sample do not affect these relative areas. 

Sample is introduced into the sample loop in a variety of ways the vial may be pressurized to force the sample out, or a syringe may be used to draw the sample out. The syringe is usually controlled by a stepping motor so that different sample volumes can be injected reproducibly by partially filling the sample loop. Once the sample loop has been loaded, the valve is electrically actuated. Some autosamplers also include other features such as sample heating or cooling, and the ability to perform standard additions, thereby improving the precision of the analysis. 

The compressed gas is shifted in a high temperature shift converter and passed through a Rectisol CO 2 removal system. In this way, a clean stoichiometric gas is obtained, suitable for compression and injection into a standard ICI LP methanol loop. Byproduct tars are fed to a partial oxidation gasifier, adding to the synthesis gas supply. 

In the second type, the injector unit contains a device called a loop, which is a coiled tube of precise volume. The loop is loaded with sample, and once filled, its contents can be injected onto the column. It is always necessary to overfill such a loop to ensure complete loading. To overfill, the loop is loaded with sample until it overflows the excess sample spills out one of the injection valve vents. Although injectors differ in the amount of overfilling required, experience leads us to recommend overfilling by as much as 30%, since it is critical to load the injector loop completely. Partially filled loops will result in abnormal chromatographic profiles. There is waste with injectors of the type shown in Figure 4.1 IB, and if material is in short supply or expensive, the type of injector (Fig. 4.11A) might be preferred. 

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