Large-volume direct injection

The sample is injected into the system via a valve injector, as schematically shown in Figure 1.2, A three-way valve is required, with two ports being connected to the sample loop. Sample loading is carried out at atmospheric pressure. After switching the injection valve, the sample is transported to the separator column by the mobile phase. Typical injection volumes are between 5 and 100 pL, but smaller and larger injection volumes are used for capillary-scale ion chromatography and large-volume direct injections, respectively. 

Cavalli, S., Polesello, S., and Saccani, G., Determination of acrylamide in drinking water by large-volume direct injection and ion-exclusion chromatography-mass spectrometry. Journal of Chromatography, 1039, 155-159, 2004. 

Berset J-D, Brenneisen R, Mathieu C (2010) Analysis of licit and iflicit drugs in waste, surface and lake water samples using large volume direct injection high performance liquid chromatography-electrospray tandem mass spectrometry (HPLC-MS/MS). Chemosphere 81(7) 859-866. doi 10.1016/j.chemosphere.2010.08.011... 

Teske, J., J. Efer, and W. Engewald, Large-Volume PTV Injection New Results on Direct Injection of Water Samples in GC Analyis, Chromatographia 1997 46(11/12) 580-586. 

Teske, J., Efer, J., and Engewald, W., Large volume PTV injection Comparison of direct water injection and in-vial extraction for GC analysis of triazines, Chromatographia, 47, 35,1998. 

Z. Yu and D. Westerlund, Direct injection of large volumes of plasma in a columnswitching system for the analysis of local anaesthetics , II. Determination of bupivacaine in human plasma with an alkyl-diol silica precolumn , ]. Chromatogr. A 725 149-155 (1996). 

J. Hermansson, A. Grahn and I. Hermansson, Direct injection of large volumes of plasma/semm of a new biocompatible exti action column for the determination of atenolol, propanolol and ibuprofen . Mechanisms for the improvement of clrromato-grapliic performance , J. Chromatogr. A 797 251-263 (1998). 

J. V. Sancho, C. Hidalgo and P. Hernandez, Direct determination of bromacil and diuron residues in environmental water samples by coupled-column liquid cliromatog-raphy and large-volume injection , 7. Chromatogr. 761 322-326 (1997). 

Use Scalable Heat Transfer. The feed flow rate scales as S and a cold feed stream removes heat from the reaction in direct proportion to the flow rate. If the energy needed to heat the feed from to Tout can absorb the reaction exotherm, the heat balance for the reactor can be scaled indefinitely. Cooling costs may be an issue, but there are large-volume industrial processes that have Tin —40°C and Tout 200°C. Obviously, cold feed to a PFR will not work since the reaction will not start at low temperatures. Injection of cold reactants at intermediate points along the reactor is a possibility. In the limiting case of many injections, this will degrade reactor performance toward that of a CSTR. See Section 3.3 on transpired-wall reactors. 

Direct sample injection and large-volume injection... 

There are basically three methods of liquid sampling in GC direct sampling, solid-phase extraction and liquid extraction. The traditional method of treating liquid samples prior to GC injection is liquid-liquid extraction (LLE), but several alternative methods, which reduce or eliminate the use of solvents, are preferred nowadays, such as static and dynamic headspace (DHS) for volatile compounds and supercritical fluid extraction (SFE) and solid-phase extraction (SPE) for semivolatiles. The method chosen depends on concentration and nature of the substances of interest that are present in the liquid. Direct sampling is used when the substances to be assayed are major components of the liquid. The other two extraction procedures are used when the pertinent solutes are present in very low concentration. Modem automated on-line SPE-GC-MS is configured either for at-column conditions or rapid large-volume injection (RLVI). 

SFE-GC-MS is particularly useful for (semi)volatile analysis of thermo-labile compounds, which degrade at the higher temperatures used for HS-GC-MS. Vreuls et al. [303] have reported in-vial liquid-liquid extraction with subsequent large-volume on-column injection into GC-MS for the determination of organics in water samples. Automated in-vial LLE-GC-MS requires no sample preparation steps such as filtration or solvent evaporation. On-line SPE-GC-MS has been reported [304], Smart et al. [305] used thermal extraction-gas chromatography-ion trap mass spectrometry (TE-GC-MS) for direct analysis of TLC spots. Scraped-off material was gradually heated, and the analytes were thermally extracted. This thermal desorption method is milder than laser desorption, and allows analysis without extensive decomposition. 

The sample can be introduced into either a flash vaporizer or directly onto the end of the column. The best technique depends on the application, the sample, the column type, and whether the column is heated isothermally or by temperature-programming. Instantaneous vaporization of the sample on injection is the usual method of ensuring a reproducible retention time and maintaining good efficiency of separation. This approach, however, is unsatisfactory for samples containing heat-sensitive compounds (commonly encountered in biomedical applications). Samples that are very dilute and require a large volume to be injected also cause problems. 

There are three injection techniques for introducing a sample into a GC equipped with a capillary column split injection, splitless injection, and on-column injection. Split injection is the most often used injection technique. When a certain amount of FAME sample (1 to 3 ll) is introduced into the GC injector that is normally set at a temperature much higher than the boiling point of the solvent, the solvent vaporizes instantly in the carrier gas and creates a large volume of gas that contains all of the injected FAME in it. The carrier gas that contains the FAME is then divided into two streams from the injector one is directed onto the column, and the second is vented to the atmosphere, clearing the sample out of the injection chamber momentarily. This way, only a limited amount of sample is introduced into the column, to avoid column overloading, and injection time is short, to avoid peak broadening. 

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