Water static headspace method

Stuart et al. [127] studied the analysis of volatile organic compounds using an automated static headspace method. Recoveries decreased in the following order water, pure sand, sandy soil, clay and topsoil. A full evaporation technique that uses little or no aqueous phase and higher equilibration temperature gave the most reproducible analyte recoveries. 

Fig. 8F.1 Ethyl hexanoate (a) and isoamyl alcohol (b) partition coefficients [K (mol/mol)/(mol/mol)] at 25 °C in water, in 10 vol.% ethanol, and in 20 vol.% ethanol, with three static headspace methods PRV, VPC, and LC-SH (reprinted with permission from Athes et al. (2004) J Agile Food Chem 52 2021-2027. Copyright (2004) American Chemical Society)...

Table 4.4 Substance list, LODs are given for the static headspace method in water.

Cummins, T.M., Robbins, G.A., Henebry, B.J., Goad, C.R., Gilbert, E.J., Miller, M.E., and Stuart, J.D. A water extraction, static headspace sampling, gas chromatographic method to determine MTBE in heating oil and diesel fuel. Environ. Sci. Technol, 35(6) 1202-1208, 2001. 

A static headspace is frequently used for the determination of VOCs in complex matrices such as food [32, 33], urine [34], blood [35], and swimming pool water [36]. This method is also routinely used in the analysis of residual solvents in the pharmaceutical industry [37]. Currently, there are various types of automatic HS systems (Fig. 14.5) [38]. 

Soil spiked with trichloroethylene and toluene was analysed using a gas chromatograph equipped with a PT concentrator that was found to be replaceable by a headspace unit in order to simplify the overall assembly. The headspace analysis of soil samples was found to be restricted by incomplete desorption of the contaminants in soil-water mixtures this shortcoming, however, was effectively overcome by the addition of methanol. Henry s law constants for volatile organics in methanol must previously be determined if the method is to be applied to soils [142]. A comparison of the performance of static and dynamic (PT) headspace modes in the determination of nine VOCs in five different soils revealed poor PT recoveries in relation to those of static headspace (which ranged from 68 to 88%) the latter, however, required longer development times [143],... 

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 headspace injector has been selected and flic method developed for water soluble products using water as dissolution medium or using N,N-dimethylformamide for water-insoluble products. IfN,N dimethylacetamide and/or N,N-dimethylformamide are suspected in the drug under investigation 1,3-dimethyl 2-imidazolinone (DMI) is used as dissolving medium. 

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... 

For the analysis of trace quantities of analytes, or where an exhaustive extraction of the analytes is required, purge and trap, or dynamic headspace extraction methods, are preferred over static headspace extraction methods. Purge and trap has been used for both solid and liquid samples, which include environmental [water (45-47) and soil], biological (47,48), industrial, pharmaceutical, and agricultural samples. Like SHE, purge and trap relies on the volatility of the analytes... 

Validation of a SPME method for target analytes should be performed using standard reference materials with similar matrix, when available. Another possible and frequently used way is validation of a SPME method against well-accepted extraction techniques, such as purge-and trap [13,25,46] or static head-space [46]. Several interlaboratory studies demonstrated that SPME is a reliable technique for the quantitative analysis of volatile organic compounds [46] and pesticides in water samples [47 48]. We have validated our SPME-GC-MS method for the determination of nitrous oxide in urine by means of the comparison with static headspace [33]. 

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. 

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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