Equilibrium headspace extraction

Static headspace extraction is also known as equilibrium headspace extraction or simply as headspace. It is one of the most common techniques for the quantitative and qualitative analysis of volatile organic compounds from a variety of matrices. This technique has been available for over 30 years [9], so the instrumentation is both mature and reliable. With the current availability of computer-controlled instrumentation, automated analysis with accurate control of all instrument parameters has become routine. The method of extraction is straightforward A sample, either solid or liquid, is placed in a headspace autosampler (HSAS) vial, typically 10 or 20 mL, and the volatile analytes diffuse into the headspace of the vial as shown in Figure 4.1. Once the concentration of the analyte in the headspace of the vial reaches equilibrium with the concentration in the sample matrix, a portion of the headspace is swept into a gas chromatograph for analysis. This can be done by either manual injection as shown in Figure 4.1 or by use of an autosampler. 

Exposure time can be arbitrarily chosen or the time to equilibrium can be used. Time to equilibrium can be determined by extracting for incremental time periods until a stable extracted concentration is achieved. Equilibration time is much greater for submersion extraction than for headspace extraction. It is imperative that the selected exposure time be used exactly and consistently for any measurements that are to be compared. 

SPME is a sample-preparation technique based on absorption that is useful for extraction and concentration of analytes either by submersion in a liquid phase or exposure to a gaseous phase (Belardi and Pawliszyn, 1989 Arthur et al., 1992). Following exposure of the fiber to the sample, absorbed analytes can be thermally desorbed in a conventional GC injection port. The fiber behaves as a liquid solvent that selectively extracts analytes, with more polar fibers having a greater affinity for polar analytes. Headspace extraction from equilibrium is based on partition coefficients of individual compounds between the food and headspace and between the headspace and the fiber coat-... 

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. 

Headspace Extraction Headspace (HS) extraction is a well-known method of sample preparation and is frequently used in many laboratories, especially in industrial applications. It involves a partitioning equilibrium between the gas phase and a sample (liquid or solid). In this technique, an aliquot of gas phase is sampled into GC. There are two types of analysis, static and d3Uiamic. In the static version, when the equilibrium is reached, the gas phase is injected into GC. In dynamic analysis, the volatiles are exhaustively extracted by the stream of gas. However, matrix effects result in decreased sensitivity for certain substances, especially polar and hydrophilic samples. A comprehensive book describing HS techniques was presented by Kolb [31]. 

HS-SPME-GC-MS showed that residual methyl methacrylate is released during thermal annealing of PMMA [40]. SPME fiber was carboxen/PDMS. The extraction time and temperature was 2 hours at 70 °C. A multiple headspace extraction method using a carboxen/PDMS fiber was developed for the quantitative determination of vinyl chloride monomer in PVC [41]. To reduce the equilibrium time, the PVC sample was finely ground before the extraction. Quantitative SPME methods have also been developed to determine vinyl chloride in liquid and solid samples [42] and to determine terephthalic acid and vinyl acetate monomers from aqueous solutions [43]. 

Liquid-liquid extraction has a counterpart for the determination of VOCs in the technique known as static or equilibrium headspace sampling. The technique is most often combined with the determinative step and is often referred to as static headspace gas chromatography, abbreviated HS-GC. The principles that underlie this technique will be outlined in this section. The decision to measure VOCs in the environment by either HS or purge and trap (P T or dynamic headspace) represents one of the ongoing controversies in the field of TEQA. This author has worked in two different environmental laboratories in which one used P T as the predominant technique to determine VOCs in the environment and the other used HS-GC. 

If the components of interest in a solid or liquid sample are volatile, a good way to analyze them is to examine the concentration of these analytes in the gas phase above the matrix (headspace) when in a closed container, either by taking a sample directly from the gas phase or trapping and concentrating the gas prior to analysis. This type of extraction techniques are known as headspace analysis (Smith, 2003) the analysis and subsequent separation of volatile substances is normally carried out by the technique of gas chromatography, which is a mature technology, reliable and supported by a large body of work. The sample can be in contact and in equilibrium with the extractant gas (static or equilibrium headspace), or volatile compormds can be extracted by a steady stream of inert gas (dynamic headspace). 

The equipment for static headspace extraction consists of a container, where equilibrium takes place, a device that heats the container at a constant temperature and an injection device, which transfers a portion of the headspace gas to the gas chromatograph. The container is a glass vial of between 5 ml and 25 ml capacity, which is sealed with a septum coated with polytetrafluoroethylene (PTFE) and an aluminum cap, using a crimp. Injection... 

The main issues in choosing an extraction mode and an agitation method involve the nature of the sample and matrix volatility and affinity to the matrix. From the point of view of fiber lifetime, headspace extraction is preferred, but samples must be at least somewhat volatile and not very strongly bound to the matrix. For samples with lower volatility, direct immersion is preferred, although fiber lifetime is a consideration if the matrix is especially dirty. The second consideration in choosing the extraction mode is equilibrium versus exhaustive extraction. In SPME, equilibrium extraction is much simpler to perform, so it is generally... 

The method described herein for the analysis of volatile organics is a modified procedure of that described by Schuberth (28). Aliquots of 10-20 mL of blood are obtained from the test subjects with the use of a Vacutainer and collected into 10-mL tubes containing 15 mg of ethylenediaminotetraacetic acid (EDTA) and 100 mg of NaF as anticoagulant and preservative, respectively. The samples are then stored at 4°C. Aliquots of 1.5 mL of blood are then added to a headspace vial containing 1.8 g of NaCl. Headspace extraction is done at a bath temperature of 50°C and an equilibrium time of 30 min. 

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. 

In SPME analytes can be adsorbed fl om a liquid sample, by immersion or headspace extraction, or from a solid sample, by headspace extraction using a polymer-coated fused silica fibre. Headspace solid-phase microextraction (HS-SPME) shortens the extraction time and facilitates analysis of solid samples, provided that the analytes are volatile. Pawliszyn et al. [992] have reported initial work on HS-SPME in 1993. These authors have evaluated theoretically and experimentally the equilibrium and kinetics of HS-SPME. 

Three phases are involved in headspace extraction, namely the condensed phase, its headspace and the SPME polymer. In the sampling process, the SPME fibre acts as a chemical pump , forcing compounds out of the headspace of a (liquid or) solid phase into a phase-coated fibre. For headspace sampling of volatiles the vapour phase should be in equilibrium with the sample. Sample/air/fibre partitioning of volatiles depends on many factors, including the nature of the sample matrix, presence of interfering compounds, sample and headspace volumes. 

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). 

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. 

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. 

Principles and Characteristics In boiling under reflux procedures a small amount of ground polymer (typically 3g) is placed in a headspace jar (typically 100 mL) and solvent (typically 30 mL) is added. After sealing, the jar is placed in an oven at a temperature where the solvent slowly refluxes. The solvent is, therefore, at the highest temperature possible without applying an external pressure. Consequently, reflux extractions tend to be much faster than Soxhlet extractions. Examples are Soxtec , Soxtherm , FEXTRA and intermittent extraction. Whilst, in theory, partitioning of the analyte between the polymer and solvent prevents complete extraction, this hardly ever constitutes a problem in practice. As the quantity of solvent is much larger than that of the polymer, and the partition coefficients usually favour the solvent, very low additive levels in the polymer result at equilibrium. Any solvent or solvent mixture can be used. 

In general, zero-headspace procedures are employed when the concentrations of volatiles in the soil are relatively low, and solvent extraction methods are used for more polluted soils. Irrespective of which procedure is used, quantitation of volatiles in soil is subject to serious errors if sufficient care is not taken with the sampling operation. Although direct purge-and-trap methods are frequently advocated for the determination of volatiles in samples collected by zero-headspace procedures, there are certain problems associated with this technique. Caution is advised since the procedure really collects only that fraction of the volatile that exists in a free form within the soil pore spaces or is at least in a facile equilibrium with this fraction. 

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