MHE Multiple headspace extraction

The four most common approaches to quantitative HSGC calibration are classical external standard, internal standard, standard addition, and multiple headspace extraction (MHE). The choice of technique depends on the type of sample being analyzed. 

Multiple headspace extraction (MHE) is used to find the total peak area of an analyte in an exhaustive headspace extraction, which allows the analyst to determine the total amount of analyte present in the sample. This technique, along with the mathematical models behind it, was originally presented by McAuliffe [17] and Suzuki et al. [18]. Kolb and Ettre have an in-depth presentation of the mathematics of MHE in their book [15], and the reader is encouraged to reference that work for further information on the mathematical model. 

Kolb [203] describes a stepwise gas-extraction procedure called multiple headspace extraction (MHE). Using this method, Kolb found that the determination can be performed with only two extractions. The volume of the sample was compensated for by adding a similar volume of an inert material such as glass beads. Ethylene oxide in surgical silk sutures was determined by this procedure. The extrapolated total area (four steps) was nearly identical to the total area value obtained using the two-step MHE process, 184 versus 183, respectively. 

Headspace-GC-MS analysis is useful for the determination of volatile compounds in samples that are difficult to analyze by conventional chromatographic means, e.g., when the matrix is too complex or contains substances that seriously interfere with the analysis or even damage the column. Peak area for equilibrium headspace gas chromatography depends on, e.g., sample volume and the partition coefficient of the compound of interest between the gas phase and matrix. The need to include the partition coefficient and thus the sample matrix into the calibration procedure causes serious problems with certain sample types, for which no calibration sample can be prepared. These problems can, however, be handled with multiple headspace extraction (MHE) [118]. Headspace-GC-MS has been used for studying the volatile organic compounds in polymers [119]. The degradation products of starch/polyethylene blends [120] and PHB [121] have also been identified. 

In 1977 Kolb and Pospisil proposed a method for the quantitative analysis of volatiles in solid samples [48] by using headspace extraction and gas chromatographic detection. The method, termed discontinuous gas extraction, is based on stepwise gas extraction, followed by a subsequent analysis of the extracted volatiles. The method theoretically calculates the total amount of analyte in a soUd sample after a few successive extractions and makes the quantitation of volatile analytes in soUd matrices possible. The proposed method was validated by measuring the styrene content in polystyrene by discontinuous gas extraction and by a procedure proposed by Rohrschneider in which the polystyrene is dissolved in dimethyl formamide (DMF) [49]. The two methods were in good agreement, which supported the validity of the discontinuous gas extraction. Kolb and Pospisil later elaborated the theoretical treatment of discontinuous gas extraction and in 1981 the method was re-named as multiple headspace extraction (MHE) [50]. 

Another version of this static headspace chromatography is what has been called by Kolb multiple headspace extraction (MHE) chromatography. This is a multi-step injection... 

Multiple headspace extraction (MHE) determines the total peak area for an analyte in an exhaustive headspace extraction, so the analyst can calculate the total amount of analyte in the sample. 

One of the weaknesses of HSGC is that quantitation is not as straightforward as in classical GC. Calibration of a HSGC system relies either on the availability of the matrix void of the analyte or on the composition of the matrix being known so that it can be simulated by individual ingredients. If neither option works, for instance if the matrix is not available and cannot be simulated, the so-called standard addition method has to be used. Even in situations where a standard addition method may lead to uncertainties, the multiple headspace extraction (MHE) technique allows a reliable quantification of trace constituents. 

In order to obtain quantitative results by HS-GC, the system must be calibrated. Absolute quantitation is not possible. Quantification can be done by the conventional external calibration method with liquids containing the analytes concerned in known concentrations or by means of standard addition. Pausch et al. [958] have developed an internal standard method for solid headspace analysis of residuals in polymers in order to overcome the limitations of external standardisation cfr. Chp. 4.2.2 of ref. [213a]). Use of an internal standard works quite well, as shown in case of the determination of residual hydrocarbon solvent in poly(acrylic acid) using the solid HS-GC-FID approach [959]. In the comparison made by Lattimer et al. [959] the concentrations determined by solid HS-GC exceeded those from either solution GC or extraction UV methods. Solid HS-GC-FID allows sub-ppm detection. For quantitative analysis, both in equilibrium and non-equilibrium conditions, cfr. ref. [960]. Multiple headspace extraction (MHE) has the advantage that by extracting the whole amount of the analyte, any effect of the sample matrix is eliminated the technique is normally used only for method development and validation. 

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). 

Maggio, A, Milana, M.R. et al. (1991) Multiple headspace extraction capillary gas chromatography (MHE-CGC) for the quantitative determination of volatiles in contaminated soils, in 13th International Symposium on Capillary Chromatography, Riva del Garda, May 1991 (ed. P. Sandra), Huethig Verlag, pp. 394-405. 

Petersen, M.A. (2008) Quantification of volatiles in cheese using Multiple Headspace Extraction (MHE). Presentation 2008, University of Kopenhagen, Department of Food Science, Quality and Technology. 

Vulpius, T. and Baltensperger, B. (2009) Multiple Headspace Extraction (MHE) With Syringe Sampling, MSC ApS and CTC Scientific Poster. 

Volatile analysis with static and dynamic headspace, multiple headspace extraction (MHE), as well as thermal desorption... 

Another version of this static headspace chromatography is what has been called by Kolb multiple headspace extraction (MHE) chromatography. This is a multistep injection technique which was alluded to in the Suzuki publication and more openly developed by McAuliffe. The principle of this method is the following. After the first extraction has been made and the aliquot injected, the gas phase is removed by ventilating the vial and re-establishing the thermodynamic equilibrium. The equilibrium between the analyte in the solid or liquid phase and the gas phase will be displaced each time. After n extractions the analyte content in the liquid or solid phase becomes negligible. It is flien sufficient to sum the peak areas obtained for each extraction (which decrease exponentially) and, from an external calibration curve, determine the amount of RS in the substance. 

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