Reproducible Injection Techniques

Cool on-column injection is used for trace analysis. Ah. of the sample is introduced without vaporization by inserting the needle of the syringe at a place where the column has been previously stripped of hquid phase. The injection temperature must be at or below the boiling point of the solvent carrying the sample. Injection must be rapid and no more than a very few, usuahy no more than two, microliters may be injected. Cool on-column injection is the most accurate and reproducible injection technique for capihary chromatography, but it is the most difficult to automate. 

Injection volumes are in the nanoliter range to avoid system overloading, since the total volume of the capillary is in the /rl range. Direct injection techniques have been developed to ensure efficient and reproducible injection. Techniques employed are electrokinetic injection (i.e., electromigration injection), hydrodynamic injection by pressure or vacuum, and hydrostatic injection by gravity. Organic acids are almost exclusively detected with indirect UV, whereas other analytes have been measured by direct or conductivity detection. 

Which chemist had the most reproducible injection technique ... 

Figure 3.3 Discriaination of n-alkanes depending on injection technique. A, filled needle B, hot needle C, cold on-coluan. A and B were obtained using a 1 40 split. Saaple n-alkanes 1 10,000 in hexane. (Reproduced with peraission froa ref. 24. Copyright Dr. Alfred Huethig Publishers). ...

The flow injection technique is based on three main principles sample injection, reproducible timing, and controlled dispersion [128]. The dispersion can be described as limited, medium, or large in a colorimetric system based on a reaction between the sample and a suitable reagent, a medium dispersion is preferred. Thus in the flow injection determination of nitrate, the reductor column should not excessively increase the dispersion. In a copperised cadmium reductor, more than 90% of the total nitrate is reduced within 1 - 2 s with minimum risk of further reduction of nitrite [167]. Consequently, the reductor can be made very small, which results in a minimal increase of dispersion. 

Gas chromatograph systems are composed of an inlet, carrier gas, a column within an oven, and a detector (O Figure 1-1). The inlet should assure that a representative sample reproducibly, and frequently automatically, reaches the column. This chapter will cover injection techniques appropriate for capillary columns. These include direct, split/splitless, programmed temperature vaporization, and cool on-column injection (Dybowski and Kaiser, 2002). 

The most reproducible, but most difficult injection technique to automate is cool on-column injection. The sample is injected directly onto a section of column that has been stripped of the stationary phase using a small-bore needle (Dybowski and Kaiser, 2002). 

Various methods ofachieving preconcentration have been applied, including Hquid -hquid extraction, precipitation, immobihzation and electrodeposition. Most of these have been adapted to a flow-injection format for which retention on an immobihzed reagent appears attractive. Sohd, sihca-based preconcentration media are easily handled [30-37], whereas resin-based materials tend to swell and may break up. Resins can be modified [38] by adsorption of a chelating agent to prevent this. Sohds are easily incorporated into flow-injection manifolds as small columns [33, 34, 36, 39, 40] 8-quinolinol immobilized on porous glass has often been used [33, 34, 36]. The flow-injection technique provides reproducible and easy sample handhng, and the manifolds are easily interfaced with flame atomic absorption spectrometers. 

With this injection technique a very rapid and distinctive start of the separation is obtained which is particularly important for the most volatile components. The most significant property of the inlet system is its ability to yield reproducible retention time values. 

Reproducibility of peak height is also quite dependent on the reproducibility of the sample injection. This is especially important on early and thus normally quite sharp, narrow peaks. On such early peaks the width of the peak is controlled more by the injection time rather than the chromatographic process. A fraction of a second increase in injection time can double the width of these peaks and reduce peak height 50%. The peaks most subject to error in peak height measurement from injection problems are those with retention volumes between one and two times the hold-up volume of the column. Peaks beyond five to ten times the hold-up volume are negligibly affected by injection technique. 

Due to the ease of reproducing injection volumes of gas with a gas-sampling valve and the difficulty of appyling the technique of internal standardization discussed below for gas samples, external standardization is the preferred approach to the analysis of gas samples. For these reasons considerable attention will be given to the preparation of gas standards and the problems associated with gas analysis. In many cases this touches also on the area of trace analysis since much of the gas analysis done today is the analysis of trace components in an air matrix. 

Consider a sudden loss of injection-to-injection reproducibility. With only a little experience, the analyst without an integrator will look to injection technique, a leak almost anywhere (syringe, septum, column fitting, external gas connection, etc.) or perhaps a contaminated column. Where one looks first depends on experience with the instrument and the analysis and... 

If the total salt content of a solution is high or if only small amounts of sample are available the injection technique may be used. If a piston burette is used for the injection, individual errors cannot be avoided, and reproducibility becomes somewhat poorer than with continuous aspiration. Also, the uncertainty of the volume measurement with small injection volumes becomes noticeable in the reproducibility. In Table 4 the data and statistical values of a simultaneous copper and nickel determination are shown. 

REPRODUCIBILITY OF THE INJECTION TECHNIQUE. SIMULTANEOUS MEASUREMENT OF A SOLUTION CONTAINING 2 jug Cuml-1 AND 5 jug Niml 1, INJECTED VOLUME 100 jul... 

Statistical data on the analysis of oxide products should be based on the same criteria as for iron and steel analysis. Reproducibility limits of about 1—2rel.% are valid in the case of direct analysis with A AS. The use of the injection technique leads to reproducibilities of 2—5rel.% for high salt content and 10—20rel.% for the graphite tube technique (low salt content) used as a trace method. 

Fig. 8.11 Two UviTox photoreactor modules connected in series. Each module contains a MP Hg lamp of PeV = 30 kW. The tangential injection technique (I) generates a rotating water column within the irradiation chamber of the photoreactor reproduced by permission of VitaTec UV-Sys-teme GmbH (Freigericht, Germany).

Flow injection analysis is a continuous flow method in which highly precise sample volumes are introduced into a stream using segmented or unsegmented flow. The method must be accurate, precise and reproducible before it can be considered as a useful technique and the following test proves that this technique does meet all the requirements. Tyson [3], carried out several studies involving flow injection techniques and atomic spectroscopy with considerable success. 

If you have someone with rough technique injecting samples who keeps bending plungers, or if you do not have an auto-injector and need to get reproducible injections. 

The attractive features of splitless injection techniques are that they allow the analysis of dilute samples without preconcentration (trace analysis) and the analysis of dirty samples, since the injector is easily dismantled for cleaning. Success with individual samples, however, depends on the selection of experimental variables of which the most important sample size, sample solvent, syringe position, sampling time, initial column temperature, injection temperature and carrier gas flow rate, often must be optimized by trial and error. These conditions, once established, are not necessarily transferable to another splitless injector of a different design. Also, the absolute accuracy of retention times in splitless injection is generally less than that found for split injection. For splitless injection the reproducibility of retention times depends not only on chromatographic interactions but also on the reproducibility of the sampling period and the evaporation time of the solvent in the column inlet, if solvent effects (section 3.5.6.2) are employed. The choice of solvent, volume injected and the constancy of thermal zones will all influence retention time precision beyond those for split injection. For quantitative analysis the precision of repeated sample injections is normally acceptable but the method is subject to numerous systematic errors that may... 

Reproducible injection for capillary electrophoresis on a microdevice/Lab-on-Chip is not easy to achieve. Different injection designs (e.g., T type, double-T type, and cross type) and different injection techniques have been applied (pinched injection, gated injection, double-L injection) within microfluidic devices. The volume and the concentration of the dispensed sample are the key parameters of this dispensing process, and they depend on the applied electrical field, the flow field, and the concentration field during the injection and separation processes. The problems and properties of the different modes are described below. 

The on-column injection techniques, in which the solvent is generally vaporized in a few meters of uncoated deactivated capillary (retention gap) and vented via an early vapor exit valve, are of good accuracy and reproducibility.A schematic diagram for on-column injection is shown in Fig. 1. The on-column large-volume injection... 

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