PTV injector

A programmed-temperature vaporizer (PTV) has also been used as an interface for introducing the LC fraction to the GC unit (84,96) and to desorb the analytes retained in the SPE sorbent contained in the PTV liner. Water samples can then be injected directly in to the PTV injector. 

S. Muller, J. Efer and W. Engewald, Gas chr-omatograpliic water analysis by dkect injection of large sample volumes in an adsorbent-packed PTV injector , Chromatographia 38 694-700 (1994). 

Figure 3.7 Sphematlc Olagran of the Perkin-Eieer PTV injector. (Reproduced with pemission fron ref. 54. Copyright Dr. Alfred Huethig Publishers).

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... 

Modern GC instruments represent high resolution systems that are fully automated from sample injection to final data reduction. Utilization of new injection devices has provided the means to enhance the performance level significantly. Studies have shown, for example, that injection of the tranquilizer propio-nylpromazine and its sulfoxide into a hot injection port gave much poorer results than on-column injection at low temperature (44). In the latter case, however, nonvolatile sample components could enter the column. This disadvantage of classic sample injection can be eliminated through use of a programmed temperature vaporization (PTV) injector. 

When a PTV instead of a classic injector was utilized in the analysis of penicillin residues, the sensitivity and the precision of the analysis were markedly improved (45). With the cooled PTV injector, some microliters could be injected, and the split-splitless mode allowed solvent venting at low injector temperatures with open slit in a first step, and quantitative transfer of volatile or derivatized drugs by a freely selected linear heat-up rate between 2-12 C/s in the splitless mode in the second step. Sensitivity could be enhanced by multiple injections before heat-up. Nonvolatile components of a sample did not contaminate the chromatographic system, since they accumulated in the glass vaporization tube, which could be changed easily. 

J W, 0.23mm by 30m, 0.25/tm film. PTV injector with temperature ramp Mass Spec interface temperature. J W, 0.33mm by 30m... 

A programmed temperature-vaporization (PTV) injector (with a sorbent-packed liner) was used to preconcentrate and inject the sample. Thermal desorption was performed and the analytes were passed to a primary column (16 m X 0.32 mm i.d., film thickness 5 p.m, 100% methyl polysiloxane) and separated according to analyte vapour pressure. Selected heart-cuts were transferred to a second column (15 m X 0.53 mm i.d., Al203/Na2S04 layer, open tubular column with 10 (im stationary phase) where final separation was performed according to chemical functionality. 

GC column. The system enables volumes of up to 1.0 ml to be enriched in the packed liner in the PTV injector without breakthrough. By injecting 100 p.1, the detection limits for alachlor and metolachlor in SPETD-GC-MS/MS were 0.1-0.2 pig r1. 

The TDU equipment (commercially available from Gerstel GmbH, Miilheim an der Ruhr, Germany) is fully automated and connected on-line to a GC equipped with a programmable temperature vaporizer (PTV) injector for simultaneous cryotrapping of the analytes before injection. Another approach for analyte desorption is to place the stir bar in a small volume of a conventional HPLC liquid (or mobile phase) for HPLC analysis. The SBSE stir bars are trademarked as Twisters they can also be purchased from Gerstel. For more detailed information on SBSE technology, the reader is referred to two recent review articles.44 45... 

Juarez, M., de la Fuente, M.A., Fontecha, J. 1992. Improved gas chromatographic method for the determination of the individual free fatty acids in cheese using a capillary column and a PTV injector. Chromatographia 33, 351-355. 

In this separation, a 10-mL sample (large volume) containing a solution in tert-butyl methyl ether (tBME) of the pyrolysate of 1 mg cellulose obtained at 600° C was injected (off-line pyrolysis). The PTV injector was programmed at 20° C initial temperature for 2 min. and ramped with 10° C/min at 250° C and kept at this temperature for 1 min. Then the injector was further heated at 300° C. The split vent purge time was 2.5 min. The oven temperature for the first dimension separation was kept at 35° C for 2.5 min. then heated with 30° C/min. at 55° C and further heated with 3° C/min. to 240° C. The detector used in the first dimension was an MS system, which allowed the identification of a series of compounds from this chromatogram. The peak identification is given in Table 5.2.2. 

It has been shown that the PTV injector is a very useful technique for large-volume injection, especially for the analysis of a dirty sample. Because the vaporization of the solvent is carried out at a low temperature, nonvolatile matrix constituents remaining in the liner will not contaminate the GC column. However, the PTV injection technique is less suited when analyzing volatile compounds because only components with volatility significantly below that of the solvent are trapped in the cold liner, unless liners packed with a selective adsorbent is used [4]. 

The following phases were programmed concurrently using the Optic 2 PTV injector. 

Figure 2.6 PTV injector. To modify very rapidly temperatures, the chamber of the injector is surrounded by an heating element or cooled by the circulation of a cold gas...

Almost all MAE applications involve off-line procedures, and on-line systems have not been utilized in food and agricultural analyses. Only a few approaches involving an on-line system have been published, for example in the determination of organophosphorus compounds in air particulates (21). In this MAE-GC system, an interface based on solid-phase trapping was used. Methanol was as the extraction solvent, and before the solid-phase trap the extract was diluted with water to enable efficient trapping to a polymeric sorbent. The sorbent was then dried with a nitrogen flow and the trapped analytes were eluted with organic solvent to the GC equipped with a PTV injector. A similar system should be applicable to the analysis of other types of solid samples as well. 

In principle, SAE can be coupled on-line with both LC and GC. However, no SAE-LC application has been reported and only one SAE-GC application for the analysis of organophosphates in air particulates 27,28). In this application, the extract was transferred directly to the PTV injector of the GC. Direct transfer was possible by using a miniaturized extraction vessel, a very short extraction time (3 min) and a moderate flow rate (200 pl/min) in the extraction. 

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