Static headspace procedure

Due to the volatility of some of the compounds present in food, it is very important to utilize cryogenic cooling when the sample is introduced onto the GC column. This helps to prevent the loss of low-molecular weight volatiles and also tends to focus volatiles on the initial portion of the column, thus allowing for improved separation and quantification. The use of a film thickness of 1.0 mm will also aid in the retention of the aforementioned compounds. In the static headspace procedure, the 4-min pressurization step is also crucial, in that equal pressures between the sample vials and the GC must be attained to ensure reproducible sample injections. Forboth the static and SPME procedures, heating the samples for 30 min prior to injection is important to ensure proper equilibration between the sample and the head-space. 

The static headspace procedure is the simplest and can be applied to organic compovmds with high vapor pressure and low solubiHty in water. Only simple equipment is necessary and there are no great problems with impurities. However, the method is relatively insensitive because only a part of the vapor... 

One of the most elegant possibilities for instrumental sample preparation and sample transfer for GC-MS systems is the use of the headspace technique (Figure 2.9). Here all the frequently expensive steps, such as extraction of the sample, clean-up and concentration are dispensed with. Using the headspace technique, the volatile substances in the sample are separated from the matrix. The latter is not volatile under the conditions of the analysis. The tightly closed sample vessels, which, for example, are used for the static headspace procedure. 

Static headspace may also be carried out by substituting the heating step by a microwave treatment. In this procedure the material is immersed in a solvent that is transparent to microwaves relative to the sample in order to impart most, if not all, of the microwave energy to the sample [208]. Another configuration of MAP gas-phase extraction relates to dynamic headspace sampling. 

Schoenmakers et al. [72] analyzed two representative commercial rubbers by gas chromatography-mass spectrometry (GC-MS) and detected more than 100 different compounds. The rubbers, mixtures of isobutylene and isoprene, were analyzed after being cryogenically grinded and submitted to two different extraction procedures a Sohxlet extraction with a series of solvents and a static-headspace extraction, which entailed placing the sample in a 20-mL sealed vial in an oven at 110°C for 5,20, or 50 min. Although these are not the conditions to which pharmaceutical products are submitted, the results may give an idea of which compounds could be expected from these materials. Residual monomers, isobutylene in the dimeric or tetrameric form, and compounds derived from the scission of the polymeric chain were found in the extracts. Table 32 presents an overview of the nature of the compounds identified in the headspace and Soxhlet extracts of the polymers. While the liquid-phase extraction was able to extract less volatile compounds, the headspace technique was able to show the presence of compounds with low molecular mass... 

Monomers are either gaseous or relatively volatile liquids and so GC and GC-MS based techniques are used to determine them in both the rubber compound and the food simulant/food product. To simplify the analysis, a static headspace sampler is often used to isolate the monomer from the sample matrix an extraction procedure often presenting chromatographic problems with the extraction solvent obscuring the analyte. 

Gas chromatographic (GC) methods have been used for determining volatile oxidation products. Static headspace, dynamic headspace or direct injection methods are the three commonly used approaches. These methods were compared in an analysis of volatile compounds in an oxidized soybean oil. It was found that each method produced significantly different GC profiles (Frankel 1985). The dynamic headspace and direct injection methods gave similar results, but the static headspace is more sensitive to low molecular weight compounds. Lee and co-workers (1995) developed a dynamic headspace procedure for isolating and analyzing the volatiles from oxidized soybean oil, and equations were derived from theoretical considerations that allowed the actual concentration of each flavor component to be calculated. 

Neither soxhlet extraction nor steam distillation is designed to isolate volatiles from solids for subsequent determination. Slurrying the solids in water and then applying the PaT procedure has been reported A vacuum extractor with cryogenic concentration has been applied to both fish and sediment samples for determination of volatile priority pollutants PaT, LLE, and static headspace techniques have all been applied to the determination of volatiles in sludges from municipal waste treatment plants... 

In the publications mentioned above,it is possible for reader to find experimental procedures to conduct validation efficiently. Attention is drawn to the feet that, when using the static headspace technique in particular some other parameters have to be investigated, sueh... 

The different headspace sampling techniques can be classified into one-step procedures, such as static headspace, where an aliquot of the vapor phase is transferred in a closed container directly to the gas chromatograph, and two-step procedures, where the volatile analytes are transferred from the matrix of the headspace to a "trap" where they are released... 

Cleanup The concentration and/or isolation technique applied has a great influence on the cleanup procedure. HSA techniques such as static headspace and purge-and-trap with thermal desorption have the advantage that no further cleanup is necessary. These techniques are restricted to the analysis of volatile compounds. A high selectivity of the concentration and/or isolation technique decreases the necessity of a further cleanup procedure. By a proper selection of the extraction solvent or the sorbent, the determinands can be extracted and separated simultaneously from the majority of coextractives. Also, a derivatization procedure can increase the selectivity of the method. The following procedures can be used for cleanup. 

Cruwys, J.A. et al.. Development of a static headspace gas chromatographic procedure for the routine analysis of volatile fatty adds in wastewaters, /. Chromatogr. A, 945,195, 2002. 

Both manual procedures and automated devices for gas sample injection have been developed manual injection using gas-tight syringes [93], sample injection using a loop in a headspace autosampler with electropneumatic systems [2], and automated injections by means of an autosampler equipped with gas-tight syringes and by means of a static-headspace autosampler equipped with a trap to preconcentrate and focus the VOCs [94-96]. 

The headspace analysis procedure is simple the food is sealed in a container, then brought to the desired temperature and left for a while to establish an equilibrium between volatiles bound to the food matrix and those present in the vapor phase. A given volume of the headspace is withdrawn with a gas syringe and then injected into a gas chromatograph equipped with a suitable separation column (static headspace analysis). Since the water content and an excessively large volume of the sample substantially reduce the separation efficiency of gas chromatography, only the major volatile compounds are indicated by the detector. The static headspace analysis makes an important contribution when the positions of the aroma sub-... 

The static headspace method is therefore an indirect analysis procedure, requiring special care in performing quantitative determinations. The position of the equilibrium depends on the analysis parameters (e.g. temperature) and also on the sample matrix itself. The matrix dependence of the procedure can be counteracted in various ways. The matrix can be standardized, for example, by addition of Na2S04 or NajCOj. Other possibilities include the standard addition method, internal standardization or the multiple headspace extraction procedure (MHE) as published by (Kolb and Ettre, 1991 Zhu et al., 2005) (Figure 2.11). 

The theoretically achievable or effectively necessary sensitivity is not the only factor deciding the choice of procedure. The specific interactions between the analytes and the matrix, the nature of the sample with for instance the potential of foaming, the performance of the detectors available and the legally required detection limits play a more important role. US EPA methods are mainly based on P T use while European methods usually apply static headspace methods. 

The static headspace technique is very simple and quick. The procedure is well documented in the literature, and for many applications the sensitivity is more than adequate, so that its use is usually favoured over that of the P8dT technique. There are areas of application where good results are obtained with the static headspace technique which cannot be improved upon by the P8dT method. These include the forensic determination of alcohol in blood, of free fatty acids in cell cultures, of ethanol in fermentation units or drinks and residual water in polymers. This also applies to studies on the determination of ionization constants of acids and bases and the investigation of gas phase equilibria. 

The elution of the organic compounds collected involves extraction by a solvent (displacement) or thermal desorption. Pentane, CS2 and benzyl alcohol are generally used as extraction solvents. CS2 is very suitable for activated charcoal, but cannot be used with polymeric materials, such as Tenax or Amberlite XAD, because decomposition occurs. As a result of displacement with solvents, the sample is extensively diluted, which can lead to problems with the detection limits on mass spectrometric detection. With solvents additional contamination can occur. The extracts are usually applied as solutions. The readily automated static headspace technique can also be used for sample injection. This procedure has also proved to be effective for desorption using polar solvents, such as benzyl alcohol or ethylene glycol monophenyl ether (1% solution in water, Krebs, 1991). 

Multiple headspace extraction, a quantitation procedure used in static headspace involving multiple extraction and measurement from a single sample. The vial pressure is released between the... 

In the case of the model mouth, a general decrease is also observed in the amount of each compound released from the sample on dilution with saliva (Fig. 3). However, the decreases in amount of each compound released were not proportional to the decreases in the aroma compound concentration of the samples on dilution with increasing volumes of saliva. The amounts of 2-heptanone released from samples containing 20%, 40%, and 60% saliva expressed as a percentage of the amount released from the 100% emulsion sample were 66%, 58%, and 46%, respectively. The decreased effect of the values on the release of the aroma compounds observed in the model mouth may be partially explained by the differences in the operating procedures between the model mouth and the RAS. In the case of the RAS, the initial conditions before dilution of the headspace above the sample with nitrogen and collection of the aroma compounds onto Tenax were very close to those used in static headspace analysis unless initially flushed before sampling. In the model mouth, the sample flask caimot be sealed before collection of the aroma compounds thus equilibrium conditions in the flask are never achieved before headspace dilution. 

All three procedures make use of a FID detector and a static headspace injection system. The differences are in the choice of columns ... 

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). 

Hon et al. [34] describe a simple piece of equipment for the determination of down to 80 pg/1 of mercury by AAS using a static cold vapour procedure. In this method [35], the sample was digested with the sulfuric acid, a measured portion pipetted into the reduction vessel, and the vessel immediately capped. The reductant, comprising 1% stannous chloride, was introduced. The evolved elemental mercury in the headspace was then introduced into the absorption cell by water displacement. Maximum sensitivity is obtained when the volume of the displaced air is equal to the internal volume of the absorption cell, and the mercury solution is 9 M in sulfuric acid. The peak absorbance at 253.7 nm exhibited a marked decline for hydrochloric acid concentrations above 1.5 M and for nitric acid concentrations above 3 M. The calibration graph obtained for mercury(II) in 9M sulfuric acid is linear from 0 to 17ng/ml, and the sensitivity is 0.08 ng/ml. A windowless absorption cell can also be used with a narrower linear calibration range. 

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