Direct on-column injection

There are several types of capillary inlets in use today which can be divided into two categories direct on-column injection and split/splitless injection techniques. 

Many other special devices and procedures have been reported. For example, a special injector is commercially available to permit direct on-column injection for OT columns. One final variation is an injection port that allows the sample to be injected simultaneously on two columns. One recent example of its use is in the analysis of PCBs.4... 

S.Y. Yang and M.G. Khaledi, MLC Separation of Sulfonamides in Physiological Samples using Direct on-Column Injection, J. Chromatogr. A, 692 311 (1995). 

Sample introduction can be by direct on-column injection or isolated from the flash columns using cold spots. Either method can be optimized for maximum performance. 

As direct on-column injection can lead to a relatively rapid deterioration of the column through interactions of the liquid phase and the injection solvent or involatile impurities in the sample, an alternative has been devised in which a specially designed injection port takes over the function of the top part of the column. The sample is introduced into this inlet, which is cooled initially, and then its temperature is raised at a controlled rate so that the sample components are selectively vaporised. Injection systems of this type are relatively new and appear to be well suited to the analysis of lipids of high molecular weight, such as intact triacylglycerols. This is discussed further in Chapter 8. 

In practice, different types of injectors are used (see Table 2.18). The sample injection systems are classified as hot or cold according to their function. A separate section is dedicated to direct on-column injection. 

Cavagna, B., Pelagatti, S., Cadoppi, A. and Cavagnino, D. (1994) GC-ECD determination of decabromodiphenylether using direct on-column injection into capillary columns. Poster Thermo Eisher Scientific Milan, PittCon 2008, New Orleans. 

An observation of impurities in compounds analyzed by GLC must also take into account the possibility that an alteration could occur in the flash heater or on the column. To be positive that this has not happened, single peaks should be collected and rechromatographed to see if other components appear again. Corrective measures require different ehromatographic conditions, including different column tubing, different liquid phases, lower temperatures, and direct on-column injections. 

Ohm s law the statement that the current moving through a circuit is proportional to the applied potential and inversely proportional to the circuit s resistance (E = iR). (p. 463) on-column injection the direct injection of thermally unstable samples onto a capillary column, (p. 568) one-taUed significance test significance test in which the null hypothesis is rejected for values at only one end of the normal distribution, (p. 84)... 

E. Fogelqvist, M. Krysell and L-G. Danielsson, On-line liquid-liquid extr action in a segmented flow directly coupled to on-column injection into a gas cliromatograph , A a7 Chem. 58 1516-1520(1986). 

Cold on-column injection differs from the vaporization techniques discussed above in that the sample is introduced directly as a liquid into the column inlet, where it is subsequently vaporized. In this way, discrimination is virtually... 

The advent of high-resolution capillary gas chromatography (HR-CGC) with on-column injection has resulted in improved GC analysis of polymer additives [92-94]. The solution of the additive mixture is injected directly into the cold end of the capillary column by means of a cold injector. Thus, sample discrimination, the instantaneous evaporation of the sample solvent, is avoided. The nonvaporising, on-column injection combined with very high resolution of the capillary columns allows accurate separation, identification and quantification of additives of complex mixtures. With the solvent venting technique, the sample is introduced into the column without splitting and sample concentrations... 

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. 

For packed columns, 0.1-10 pi of a liquid sample or solution may be injected into a heated zone or flash vaporizer positioned just ahead of the column and constantly swept through with carrier gas (Figure 4.18(a)). The zone is heated some 20-50°C above the column temperature to ensure rapid volatilization of the sample. Alternatively, to minimize the risk of decomposing thermally sensitive compounds and to improve precision, samples can be deposited directly onto the top of the packed bed of the column (on-column injection). 

Isotopic analysis of amino acids containing natural abundance levels of 15N was performed by derivatization, GC separation, on-line combustion and direct analysis of the combustion products by isotope-ratio MS. The N2 gas showed RSD better than 0.1%c for samples larger than 400 pmol and better than 0.5%o for samples larger than 25 pmol. After on-column injection of 2 nmol of each amino acid and delivery of 20% of the combustion products to the mass spectrometer, accuracy was 0.04%e and RSD 0.23%o19. 

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). 

Like cold on-column injection the PTV has a low thermal mass, so can be heated and cooled quickly. However, unlike cold on-column the sample is injected into an injector liner and not direct onto the column. Unlike conventional split/spUtless injection, the aim is to make use of volatility selectivity of the system rather than to inhibit it. By selecting an appropriate starting temperature, the anal34es condense in the liner if desired the solvent may be flushed away through... 

Cold on-column injection. In this approach, the sample is injected directly on-column and its vaporisation occurs after the injection. This requires a very special microsyringe. The needle (steel or silicone) has a diameter of approximately 0.15 mm and it penetrates the column, or pre-column, kept at 4 °C before raising it to its normal operating temperature. This approach, which is very difficult to master without the aid of an autosampler, is useful for thermolabile compounds (applications in... 

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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