Direct injection techniques

Wahl and Deck were able to obtain an estimate of an assumed second-order rate coefficient ( 10 l.mole" .sec at 4°C) using a separation procedure based on the extraction of Fe(CN)e by a chloroform solution of Ph AsCl, in the presence of the ions Co(CN)g and Ru(CN)6, to reduce the exchange between the iron species in the two liquid phases. A similar estimate was obtained using a precipitation method in the presence of the carrier Ru(CN)6. A direct injection technique was used as short reaction times were necessary. Wahl has reviewed the large induced exchanges occurring in the chemical separation methods. The extraction procedure when the carriers Co(CN)6 and Ru(CN) are present provides the most satisfactory method of separation. ... 

Eichler and Wahl have attempted an isotopic study ( Os and Os) of the exchange reaction between Os(dipy)3 and Os(dipy)3 using a direct injection technique so that reaction times 7 x 10 sec were possible. With total osmium 10" M in aqueous sulphate media at 0 °C complete exchange was observed. The separation methods used were, (a) perchlorate precipitation (in presence of iron(II) carrier) and (6) extraction with p-toluenesulphonic acid in nitromethane, of the osmium(II) complex. A lower limit of 1 x 10 l.mole. sec was placed on the rate coefficient (0 °C, 3.0 M H2SO4). Dietrich and Wahl using the line broadening effect produced by Os(dipy)3 on the nmr spectrum of Os(dipy)3 have been able to propose a value of > 5x 10" l.mole . sec at 6 °C in D2O (0.14 M [Cr] and 5x10 M [D- ]). 

Table 7.31 Overview of direct injection techniques used in GC-MS analysis...

The dry combustion-direct injection technique provides many advantages over other methods, such as quick response and complete oxidation for determining the carbon content of water. Its primary shortcoming is the need for rapid discrete sample injection into a high-temperature combustion tube. When an aqueous sample is injected into the furnace, it is instantaneously vapourised at 900 °C and a 5000-fold volume increase can be expected. Such a sudden change in volume causes so-called system blank and limits the maximum volume of injectable water sample, which in turn limits the sensitivity [106,107]. 

Some applications also involved direct injection techniques in which BZDs are preferentially absorbed onto a precolumn and are back-flushed onto the analytical column using column switching techniques (Lauber et al., 1994 Iwase et al., 1994). 

Thermal stability and/or involatility of the main component can be a serious issue with direct injection techniques. At the very least, thermal degradants or involatiles can contaminate the injector and the head of the column, affecting the inertness of the system and causing deterioration in performance (both in recoveries due to adsorption and sensitivity and selectivity caused by peak tailing). 

Direct injection simply transfers the entire sample to the analytical column. Direct injection techniques include direct flash vaporization (hot direct injection) and cold on-column. Direct flash vaporization has become popular used with wide-bore capillary columns (> 530 /xm ID) having phase ratios less than 80 and high sample volume capacity and can easily accommodate injections of up to 10 /xl of sample. 

The most useful method for solvent residue analysis is GC. It can be performed by direct injection technique, or by headspace, solid phase microextraction (SPME), or single-drop microextraction (SOME) techniques [96]. GC has high selectivity, good specificity, is easy to perform, and involves simple sample preparation. Modem capillary GC allows separation of many compounds, together with their identification and quantification [96]. GC uses different detector systems, which are presented in Table 8.7. 

Two recently introduced direct injection techniques have been developed to deal with the special problems posed by biological samples. These techniques involve the use of restricted access media (RAM) [11,12] and turbulent flow chromatography [13] and are described later. 

Filtration is used as a sample-preparation method to remove particulates and debris that can potentially foul the LC lines, column frits, or mass spectrometer interface. Also, it is generally accepted that all workstations and pipetting systems can beneht from sample hltration because of the universal issues related to plasma clot formation that introduce pipetting challenges. Applications that use hltration include the removal of a mass of precipitated protein or of debris from raw plasma before use with any of the traditional sample-preparation techniques, as well as direct injection techniques (turbulent how... 

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. 

Fig. 7-16B the direct injection technique was tested. As can be seen from the chromatograms, the CO2 peak is very well separated from the inert gases and elutes as a symmetrical peak. CO2 elutes with an approximate capacity ratio of 1. Also methane (peak 3) is baseline separated from the inert gases. However, the CO peak co-elutes with the air peak. 

The application in Fig. 7-16 was performed on a 0.53 mm i.d. fused silica column. This is one of the most popular diameter columns because it is ideal for direct injection. Using the direct injection technique will always lead to loss some separation efficiency how much is lost depends on the type of compound analyzed and the actual retention on the column. In Fig. 7-16 (A and B) the separation of peak 1 and 2 is significantly better with split injection. For separations of very volatile compounds with a capacity ratio smellier then 0.2 it is always recommended to use the split injection (of course, this is vedid only for separation purposes). 

Graphite furnace techniques are about one order of magnitude more sensitive than direct injection techniques. Thus lead can be determined down to 50 pg/1 by direct AAS and down to 5 pg/1 using the graphite furnace modification of the technique. 

A Simple Approach to Gas Chromatographic Microanalysis of Alcohols in Blood and Urine by a Direct-Injection Technique... 

Machata (146) first used the direct injection technique to analyze ethanol in blood. He used GCFID with a packed column of polyethylene on kieselguhr to analyze... 

Direct-injection techniques into either GC or HPLC instruments bypass the extraction step and can offer a very rapid analytical process. In GC, solid-phase microextraction (SPE) can be used, while HPLC tends to require use of pre-columns that are backflushed with the use of column-switching valves. 

Direct injection techniques also may be used for this purpose. Many food products are liquids (either fat or water-based) which can be readily sampled by automated liquid sampling systems. Sensitivity also is often quite adequate. If one considers compounds present in a food at concentrations > 1 ppm (i.e. 1 pg/g), there would be > 1 T g/mg of those compounds in the food. A 20 pL injection would provide 20 ng to the gas chromatograph. That is very adequate for detection and accurate integration. The obvious problem here is sample decomposition in the heated injection port of the gas chromatograph, thereby producing artifacts. This problem has been addressed in various ways as is discussed in the literature and thus the approach can be iised for delivering aroma to the instruments for further analysis. 

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