Static headspace techniques

The term headspace analysis was coined in the early 1960s when the analysis of substances with odours and aromas in the headspace of tins of food was developed (Hachenberg and Schmidt, 1977 Kolb, 1980). The equilibrium of volatile substances between a sample matrix and the gas phase above it in a closed static system is the basis for static headspace gas chromatography (HSGC). The term headspace is often used without the word static, for example, headspace analyses, headspace sampler, and so on. 

An equilibrium is set up in the distribution of the substances being analysed between the sample and the gas phase. The concentration of the substances in the 

In static headspace analysis, the samples are taken from a closed static system (closed headspace bottle) after the thermodynamic equilibrium (partition) between the hquid/soUd matrix and the headspace above it has been established. 

For coupling of HSGC with MS, the internal standard procedure has proved particularly successful for quantitative analyses. Besides the headspace-specific effects, possible variations in the MS detection are also compensated for. The best possible precision is thus achieved for the whole procedure. The MHE procedure can be used in the same way. 

In static headspace analysis, the partition coefficient of the analytes is used to assess and plan the method (Kolb and Ettre, 2006). For the partition coefficient K of a volatile compound, the following equation is valid  

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SDBS concentrations were determined by HPLC as described previously. Dodecane concentrations in both the aqueous surfactant and squalane were determined by a GC/static Headspace technique. 

Quantitative Determination of Voiatiies in Poiymers and Quaiity Controi of Recycied Materiais by Static Headspace Techniques... 

Dilute with acetone static headspace technique GC/SCD 355 ug/L levels NR Nedjma and Maujean 1995... 

Nedjma M, Maujean A. 1995. Improved chromatographic analysis of volatile sulfur compounds by the static headspace technique on water-alcohol solutions and brandies with chemiluminescence detection. J Chromatogr A 704 495-502. 

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... 

ON WHAT BASIS CAN VOCs BE QUANTITATED USING STATIC HEADSPACE TECHNIQUES ... 

The difficulty that the EPA has had with the static headspace technique might be seen in the comment from Method 3810 (SW-846 3rd edition). This method was eliminated in the recently published final update (III) and replaced by Method 5021. The authors who wrote Method 3810 state (33) ... 

Several others techniques dealing with the injection problems have been developed. Among them the solid-phase microextraction method (SPME) and the full evaporation technique must be mentioned. According to Camarasu, the SPME technique seems to be very promising for RS determination in pharmaceuticals, with much better sensitivity than the static headspace technique. 

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... 

Current official GC methods are described in USP XXIII under chapter 467 Organic volatile impurities . Four methods (I, IV, V, VI) are mentioned. Methods I, V and VI are based on direct injection. They are suitable for water-soluble drugs and V for water insoluble drugs. Method IV describes the static headspace technique and is used for water soluble drugs. Method VI is very general and refers to the individual monograph which describes the chromatographic conditions (injection, column, conditions) which should be used. The main characteristics of these four methods are summarized in Table 16.2.2. 

The static and dynamic headspace techniques are the most common techniques for determination of volatile analytes from aqueous samples. In the latter, also called purge-and-trap, a gas is passed over the sample or through the sample as small bubbles and the volatile compounds in the sample are transported by the gas to a cryogenic or a sorbent trap, before subsequent GC separation. In the more common static headspace technique, the sample vial is thermostated (Figure 2.5) until... 

In addition, quantitative measurements are more reliable. The use of shaking devices in static headspace techniques can reduce RSDs to less than 2%. 

How then do the techniques differ For this, the terms recovery and sensitivity must be defined. For both methods, the recovery depends on the vapour pressure, the solubility and the temperature. The effects of temperature can be dealt with because it is easy to increase the vapour pressure of a compound by raising the temperature during the vaporization step. With the P T technique, the term percentage recovery is used. This is the amount of a compound which reaches the gas chromatograph for analysis relative to the amount which was originally present in the sample. If a sample contains 100 pg benzene and 90 pg reach the GC column, the percentage recovery is 90%. In the static headspace technique, a simple expression like this cannot be used because it is possible to use a large... 

Figure 2.27 Comparison of the purge and trap and static headspace techniques, (a) Purge and trap and (b) static headspace.

Comparable ratios are obtained in the analysis of a solid sample, for example, the analysis of residual solvents in a technical product. A run using the P T technique and 10 mL of sample at 150 °C gave a recovery of 63% for toluene. The sample contained 1.6 ppm, which corresponds to a quantity of 101 ng. The partition coefficient in the static headspace technique at 150 °C (for a sample of 1 g) is 95. The quantity of residual solvent in 19 mL of headspace is therefore 17 ng. For an injection of 0.5 mL, 0.4 ng are injected. The quantity injected is therefore smaller by a factor of 250 than that in the P T analysis. Furthermore, the reproducibility of this analysis was 7% for the P T technique and 32% for the static headspace analysis (RSD). 

Besides the sensitivity, there are other aspects which must be taken into account when comparing the P T and static headspace techniques. 

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). 

Analysis of Flavor Compounds from Microwave Popcorn Using Supercritical Fluid CO2 Followed by Dynamic/Static Headspace Techniques... 

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