On-column injection large volume

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. 

Applications If an extract needs further cleanup, it is possible to couple it with multidimensional chromatographic techniques such as LC-LC or LC-GC. The first chromatographic step can then be used for the on-line cleanup and concentration of the extract, and the second one for the final separation. Large-volume, on-column injection (LVI-COC) is particularly useful for coupled LC-GC in which 100-350 xL fractions of eluent from the NPLC cleanup separation step are transferred on-line to the GC column. For example, on-line removal of high-MW interfering material, such as polymers from a polymer/additive dissolution, can be achieved easily by using SEC before the fraction containing additives is transferred to the GC. 

AUTOMATED EXTRACTION AND ANALYSIS OE CHLORINATED COMPOUNDS BY LARGE VOLUME ON-COLUMN INJECTION GC/MSD... 

Termonia, A. and M. Termonia (1997). Full scan GC-MS quantitation of pesticides in spring water at the 10 ppt level using large volume on-column injection. J. High Resol. Chromatogr., 20 447 -50. 

Versini, G., Dalla Serra, A., Magni, P., Munari F. and Di Stefano, R. (1995) Optimization of a rapid method for wine aroma extraction and analysis in GC-FID and GC-NPD by large volume on column injection, in Proceed. 2nd National Congress on Food Chemistry, Giardini Naxos, Catania, La Grafica Editoriale, Messina pp. 187-198. 

Catalina, M. I., Dalliige, J., Vreuls, R. J. J., and Brinkman, U. A. Th., Determination of chlorophenoxy acid herbicides in water by in situ esterihcation followed by in-vial liquid-liquid extraction combined with large-volume on-column injection and gas chromatography-mass spectrometry, J. Chromatogr. A, 877, 153-166, 2000. 

Figure 3.9. Schematic diagram of an automated on-line solid-phase extraction-gas chromatography system. Large volume on-column injection with an early solvent vapor exit and retaining precolumn is used for sample transfer. (From ref. [117] Elsevier).

Vreuls, R.J.J. Romijn, E. and Brinkman, U.A. Th. In-vial liquid-liquid extraction with subsequent large-volume on-column injection into GC-MS for the determination of anilines in tap-, surface-, and wastewater. Journal of Microcolumn Separations 1998, 10 (7), 581-588. 

The organic samples can also be injected directly into a GC without any preconcentration using a large volume on column system injection. This was tested in the case of liquid-liquid extraction with pentane/dichloromethane (2 1 v/v), by comparing the peak areas obtained with both injection methods for different compound categories present in wines in different levels, such as isoamyl acetate, acetoin, n-hexanol, trans 3-hexenol, cis 3-hexenol, ethyl octanoate, linalool, diethyl succinate, hexanoic and octanoic acids, N(3-methylbutyl)-acetamide, monoethyl succinate, 4-vinylphenol and 4-vinylguaiacol... 

Cold on-column injection employs the direct introduction of the liquid sample into the oven-thermo-statted column inlet or retention gap (precolumn) without prior vaporization in a heated external chamber. The sample is subsequently vaporized from the liquid film formed in the column inlet or retention gap. Discrimination is virtually eliminated and quantification of components of different volatility is facilitated. On-column injection is easily automated using a wide bore retention gap connected to the separation column and is easily adapted to the introduction of large sample volumes. On-column injection is not suitable for the introduction of samples containing significant amounts of involatile matrix components that accumulate at the column inlet increasing its retentive power and activity. 

Several sample preparation techniques are performed inside the inlet system. Large-volume injection can be carried out by a number of methods including programmed temperature vaporisation (PTV). Automated SPE may be interfaced to GC using a PTV injector for large volume injection. SPE-PTV-GC with on-column injection is suited to analysis of thermola-bile compounds. 

Principles and Characteristics Although early published methods using SPE for sample preparation avoided use of GC because of the reported lack of cleanliness of the extraction device, SPE-GC is now a mature technique. Off-line SPE-GC is well documented [62,63] but less attractive, mainly in terms of analyte detectability (only an aliquot of the extract is injected into the chromatograph), precision, miniaturisation and automation, and solvent consumption. The interface of SPE with GC consists of a transfer capillary introduced into a retention gap via an on-column injector. Automated SPE may be interfaced to GC-MS using a PTV injector for large-volume injection [64]. LVI actually is the basic and critical step in any SPE-to-GC transfer of analytes. Suitable solvents for LVI-GC include pentane, hexane, methyl- and ethylacetate, and diethyl or methyl-f-butyl ether. Large-volume PTV permits injection of some 100 iL of sample extract, a 100-fold increase compared to conventional GC injection. Consequently, detection limits can be improved by a factor of 100, without... 

A retention gap is used to improve peak shapes under certain conditions. If you introduce a large volume of sample (>2 pL) by splitless or on-column injection (described in the next section), microdroplets of liquid solvent can persist inside the column for the first few meters. Solutes dissolved in the droplets are carried along with them and give rise to a series of ragged bands. The retention gap allows solvent to evaporate prior to entering the chromatography column. Use at least 1 m of retention gap per microliter of solvent. Even small volumes of solvent that have a very different polarity from the stationary phase can cause irregular solute peak shapes. The retention gap helps separate solvent from solute to improve peak shapes. 

On-column injection is best for quantitative analysis and for thermally sensitive compounds. It is strictly a low-resolution technique and cannot be used with columns whose inner diameter is less than 0.2 mm. It can handle dilute or concentrated solutions and relatively large or small volumes. Other column requirements are the same as in splitless injection. 

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. 

Compared with on-column injection into 320-/ m-ID columns, the advantage of the retention gap approach is that large sample volumes can be injected. The effects of particulates deposited on the precolumn is not severe. The approach can be automated and the length of the flooded zone is more controllable. The length of the flooded zone in the column inlet and the evaluation of different retention gaps have been studied [27]. A comprehensive review of the retention gap technique has been published [28]. 

K. Grob Jr., G. Karrer, et al., On-column injection of large sample volumes using the retention gap technique in capillary gas chromatography, J. Chromatogr., 534 129-155, (1985). 

K. Grab and B. Schilling, Maximum column temperature during on-column injections of large sample volumes in capillary gas chromatography, 7. Chromatogr., 299, 415-149 (1984). 

Injection ports for packed columns are aligned so that the sample can be deposited on a heated surface just before the column or directly on the end of the column. In the first instance, the injection port is heated above the boiling point of the sample in order to get rapid volatilization, but for on-column use, the injection port is kept at column temperature. On-column injection is usually preferred because there is less chance of decomposition and the sample is not exposed to a high injection port temperature. Remember also that a typical GC analyte will have a retention ratio of 0.25 or much less. This means that 75% or more of it is sorbed in the stationary phase, and that is where the on-column technique puts it—in the stationary phase. It is probably for this reason that on-column injection is so efficient. The use of large volumes of solvents will wash... 

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