GC Injectors

Figure 14.17 Schematic diagram of the on-line coupled LC-GC system VI, valve foi switcliing the LC column outlet to the GC injector V2, valve for switching the LC column to back-flush mode V3, LC injection valve RI, refractive index monitor detector UV, ulti avio-let monitor detector FID, flame-ionization detector.

A recent method, still in development, for determining total 4-nitrophenol in the urine of persons exposed to methyl parathion is based on solid phase microextraction (SPME) and GC/MS previously, the method has been used in the analysis of food and environmental samples (Guidotti et al. 1999). The method uses a solid phase microextraction fiber, is inserted into the urine sample that has been hydrolyzed with HCl at 50° C prior to mixing with distilled water and NaCl and then stirred (1,000 rpm). The fiber is left in the liquid for 30 minutes until a partitioning equilibrium is achieved, and then placed into the GC injector port to desorb. The method shows promise for use in determining exposures at low doses, as it is very sensitive. There is a need for additional development of this method, as the measurement of acetylcholinesterase, the enzyme inhibited by exposure to organophosphates such as methyl parathion, is not an effective indicator of low-dose exposures. 

GC-TEA Analysis. A Bendix model 2200 GC and Thermo Electron model 502 TEA were used. The GC injector temperature was 210 C. The TEA pyrolysis furnace was operated at 450 C and the cold trap was held at -150 C in isopentane slush. Oxygen flow to the ozonator was 20 cc/min and indicated pressure was 1.5 torr at a helium flow rate of 20 cc/min. TEA output was processed by a digital integrator (Spectra Physics System I). 

The concept of SPME was first introduced by Belardi and Pawliszyn in 1989. A fiber (usually fused silica) which has been coated on the outside with a suitable polymer sorbent (e.g., polydimethylsiloxane) is dipped into the headspace above the sample or directly into the liquid sample. The pesticides are partitioned from the sample into the sorbent and an equilibrium between the gas or liquid and the sorbent is established. The analytes are thermally desorbed in a GC injector or liquid desorbed in a liquid chromatography (LC) injector. The autosampler has to be specially modified for SPME but otherwise the technique is simple to use, rapid, inexpensive and solvent free. Optimization of the procedure will involve the correct choice of phase, extraction time, ionic strength of the extraction step, temperature and the time and temperature of the desorption step. According to the chemical characteristics of the pesticides determined, the extraction efficiency is often influenced by the sample matrix and pH. 

Numerous types of GC injectors have been manufactured over the past four decades. The most commonly used injection techniques have been reviewed and described by Grob, who correctly states that analysts must fully understand the techniques before they can make the most appropriate choice for their particular application(s). For most GC capillary column applications, the split/splitless, programmed-temperature vaporization (PTV) and on-column injectors remain the most popular. However, over the last few years, technology has progressed rapidly to provide injectors that allow more of the sample extract on to the GC column without overloading it. 

Desorption of an analyte from the SPME fibre depends on the boiling point of the analyte, the thickness of the coating on the fibre, and the temperature of the injection port. The fibre can immediately be used for a successive analysis. Some modifications of the GC injector or addition of a desorption module are required. It is possible to automate SPME for routine analysis of many compounds by either GC-MS or HPLC. A significant advantage of SPME over LLE is the absence of the solvent peak in SPME chromatograms. SPME eliminates the separate concentration step from the SPE and LLE methods because the analytes diffuse directly into the coating of the SPME device and are concentrated there. 

Grob K, Wagner C. 1993. Procedure for testing inertness of inserts and insert packing materials for GC injectors. J High Resolut Chromatogr 16 464-568. 

Nitrobenzene, 2,4-dinitrotoluene and 2,6-dinitrotoluene were determined in water by GC-EC or GC-CLD thermal energy analyzer (TEA) and by EI-MS, CI-MS and NICI-MS455, after solid-phase microextraction (SPME) with polydimethylsiloxane coated fiber. SPME is a technique to concentrate organic compounds dissolved in an aqueous matrix by adsorption on a solid stationary phase immobilized on a fused silica fiber. The analytes were thermally desorbed directly into the GC injector LOD was 9 pg/L for nitrobenzene and 15 pg/L for the dinitrotoluenes456. 

Hall et al. (1985) reported that no 1,2-diphenylhydrazine (less than pg/L) was detected in the Nanticoke River near the Chesapeake Bay. The analytical method involved liquid-liquid extraction, concentration, and. analysis by GC/MS. The Contract Laboratory Program statistical database (queried April 13, 1987) reported that 1 2-diphenylhydrazine has been detected n water at i of 357 hazardous waste sites at a concentration of (96 ppb (CLPSDB 1987), and has been reported at 7 of 117, sites. n the national Priority List database (ATSDR 1990) The U.S. EPA Contract laboratory Program uses GC methods to analyze the contaminants of interest. Since 1,2-diphenylhydrazine oxidize, to azobenzene in the GC injector port and both 1,2-diphenylhydrazine and azobenzene, have the same GC retention time and mass spectra, reports of 1,2-diphenylhydrazine from the Contract Laboratory Program may actually represent detections of 1,2-diphenylhydrazine, azobenzene, or both (see Chapter 6 for more details). 

The samples were placed in headspace vials. When saponification was performed, a few milliliters of NaOH solution were added to the sample. The vial was sealed with a headspace aluminum cap furnished with a Teflon-faced septum, immersed in a water bath maintained at 100 °C, and let equilibrate for 6 min before HSSPME. Afterward, the fiber was exposed to the headspace over the sample for 5-240 min, depending on the experiment. The sample was magnetically agitated during sampling. Once the exposition period was finished, the fiber was immediately inserted into the GC injector and the chromatographic analysis was carried out. Desorption time was set at 5 min. 

Elemental composition Se 40.92%, F59.08%. The gas may be dissolved in nitric acid and dilute hydrofluoric acid and the solution appropriately diluted and analyzed for selenium (see Selenium). The hexafluoride may be decomposed with ammonia at 200°C and product selenium analyzed by AA, and gaseous products nitrogen and hydrogen fluoride diluted with helium and analyzed by GC-TCD or GC/MS. Alternatively, selenium hexafluoride diluted with helium is introduced onto the GC injector port and analyzed by GC/MS. Molecular ions have masses 194, 192, 196, and 190. 

A decongestant syrup was basified with ammonia and extracted into ethyl acetate, thus ensuring that the components extracted were in their free base forms rather than their salts, which is important for obtaining good chromatographic peak shape. Salts of bases will thermally dissociate in the GC injector port but this process can cause a loss of peak shape and decomposition. 

This method is based on the partitioning of compounds between a sample and a coated fibre immersed in it [16-18]. The volatiles and other compounds are first adsorbed onto the fibre immersed in a liquid sample, an extract, or in the headspace above a sample for a certain period of time. After adsorption is complete, the compounds are thermally desorbed into a GC injector block for further analysis. Particularly in food applications, headspace SPME is preferred to avoid possible contamination of the headspace system by non-volatile food components [16]. 

An alternative to the trap and special oven method is the use of small-diameter extraction tubes that can be introduced directly into a modified GC injector. Recovery is considered to be satisfactory when it attains 60%, although it is often quantitative with this device. This principle is also applied to badges used in industrial hygiene for monitoring pollution in the workplace or the environment (Fig. 20.3). In the latter application, air flows through the badge in a natural fashion to trap the analyte. 

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. 

SPME, a variant of SPE, involves the adsorption of organic pollutants from the matrix onto the solid-phase coating. The adsorbed analytes are then directly transferred to a gas chromatography (GC) injector using a modified syringe assembly or can be stored for a certain period of time.141... 

A very simple HF-MMLLE configuration has been employed by flame-sealing the two ends of the HFs. The HFs were then soaked in n-undecane for a period of time so as to allow them to fill with solvent this makes simple HF-MMLLE devices. In this way, a single HF was utilized for the MMLLE of eight polybrominated diphenylethers (PBDEs) in 100 mL samples of tap, river, and leachate water. The analysis was done by manual injection of 2 pL of the HF lumen content into a splitless GC injector followed by GC-MS analysis in selected ion monitoring (SIM) mode. Under optimal HF-MMLLE conditions, the extraction was exhaustive (E = 57-104%), giving very good enrichment (Ee = 2800-5200-fold), very low LOD (<1.1 ng I. ), and relative recoveries of 85-110%. Two PBDEs were detected and quantified in leachate water at concentrations of 3.5 ng I. for BDE 153 and 23 ng L 1 for BDE 183.96... 

The OPCW mobile laboratory includes a portable GC/MS. The system shown in Picture 5 including printer and helium connection kit is packed in five transport boxes. Because of the modular design of the instrument, it is shipped with two GC-ovens and two GC-injectors in order to allow the... 

Solid-Phase Microextraction Solid-phase microextraction (SPME) is a preconcentration technique based on the sorption of analytes present in a liquid phase or, more often, in a headspace gaseous phase, on a microbber coated with a chromatographic sorbent and incorporated in a microsyringe [15]. The analytes sorbed in the coating are transferred to a GC injector for thermal desorption. 

The direct coupling of SPME with 1CP-MS was described for the species-specibc determination of methyl-Hg [19]. A E>ber was inserted into a splitless-type GC injector, which was placed directly at the base of the torch. Immersion SPME was severely inBuenced by the matrix and led to a 70-fold decrease in sensitivity [19]. Analytical results showed good agreement between certified and measured values for the analysis of the NRCC seafood CRMs DORM-2 Dogbsh muscle and DOLT-2 Dogbsh liver [19]. 

Solid-phase microextraction (SPME) — is a procedure originally developed for sample preconcentration in gas chromatography (GC). In this procedure a small-diameter fused silica optical fiber, coated with a liquid polymer phase such as poly(dimethylsiloxane), is immersed in an aqueous sample solution. The -> analytes partition into the polymer phase and are then thermally desorbed in the GC injector on the column. The same polymer coating is used as a stationary phase of capillary GC columns. The extraction is a non-exhaustive liquid-liquid extraction with the convenience that the organic phase is attached to the fiber. This fiber is contained in a syringe, which protects it and simplifies introduction of the fiber into a GC injector. Both uncoated and coated fibers with films of different GC stationary phases can be used. SPME can be successfully applied to the analysis of volatile chlorinated organic compounds, such as chlorinated organic solvents and substituted benzenes as well as nonvolatile chlorinated biphenyls. 

Fig. 4.7.2. Schematic diagram of a Curie-point pyrolyzer (Fischer, Germany). Note the possible modifications of the wire tip (a, b, and c) for solid samples. Pyrolysis glass injector (/), ferromagnetic wire (2), carrier gas inlet (3), impulse cable from power generator (4), induction coil (5), aluminum box (6), adapter for GC injector (7), GC inlet (8), GC septum (9), GC oven (10)...

Fig. 4.7.4. Schematic diagram of Pyroprobe interface with coil probe connected to an GC injector ( on-line approach)...

---