Headspace extraction quantification

Quantification for headspace extraction Quantification or calibration of asingle fiber is composed of nine GC runs. The solvent injection takes only the time required to prepare the solution. The solution in the headspace apparatus needs >30 min for equilibration and 5 min for extraction by SPME or with a gastight syringe. Quantification can be conducted before or after an experiment involving the SPME or when new compounds are evaluated. 

Because SPME extracts compounds selectively, the response to each compound must be calibrated for quantification. A specific compound can be quantified by using three GC peak area values from solvent injection, static headspace (gas-tight syringe), and SPME. The solvent injection is used to quantify the GC peak area response of a compound. This is used to quantify the amount of the compound in the headspace. The SPME response is then compared to the quantified static headspace extraction. These three stages are necessary because a known gas-phase concentration of most aroma compounds at low levels is not readily produced. A headspace of unknown concentration is thus produced and quantified with the solvent injection. Calibration must be conducted independently for each fiber and must include each compound to be quantified. 

Figure G1.6.2 Apparatus for quantification of headspace extraction in Support Protocol 1.

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. 

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. 

Figure 36 Extraction of coffee brew flavor components by means of headspace SPME. Quantification of guaiacol and skatole.

If quantitative extraction is not necessary, some sample conditions may be adjusted to increase volatility of selective compounds. It is important to recognize that the selective nature of these changes in sample conditions affects the relative volatility of compounds, and thus distorts the amount of each compound that volatilizes. Thus, quantification of enhanced extractions is not truly representative of the sample s relative headspace. 

SPME is based on the equilibrium between the analyte concentrations in the headspace and in the solid phase E>ber coating. Low extraction efficiencies are hence sufficient for quantification, but the amount of available analyte may be very small. Hence, there is an interest in the coupling of highly sensitive GC-ICP-MS with SPME. 

Solid phase micro-extraction (SPME) allows isolation and concentration of volatile components rapidly and easily without the use of a solvent. These techniques are independent of the form of the matrix liquids, solids and gases can be sampled quite readily. SPME is an equilibrium technique and accurate quantification requires that the extraction conditions be controlled carefully. Each chemical component will behave differently depending on its polarity, volatility, organic/water partition coefficient, volume of the sample and headspace, speed of agitation, pH of the solution and temperature of the sample (Harmon, 2002). The techniques involve the use of an inert fiber coated with an absorbant, which govern its properties. Volatile components are adsorbed onto a suitable SPME fiber (which are usually discriminative for a range of volatile components), desorbed in the injection chamber and separated by a suitable GC column. To use this method effectively, it is important to be familiar with the factors that influence recovery of the volatiles (Reineccius, 2002). 

TCA, was used as an internal standard. The extraction fiber of the SPME, coated with polymethylsiloxane, was exposed for 25 minutes in the headspace of the sample vial, and then injected into the injection port of the GC-MS by a Varian 8200 CX autosampler. Limit of quantification of this method was 5 ng/L. The method was linear from 5 to 250 ng/L with an overall coefficient of variation for replicate analyses of less than 13%. 

SPME is a patented sample preparation method for GC applications (32-36). The solvent-free technique was developed in 1989 by Janusz Pawliszyn (http. /Avww.science.uwaterloo.ca/ -janusz/spme.html) at the University of Waterloo in Ontario, Canada, and a manual device made by Supelco, Inc. has been available since 1993. In 1996, Varian Associates, Inc., constructed the first SPME autosampler. SPME involves exposing a fused silica fiber that has been coated with a non-volatile polymer to a sample or its headspace. The absorbed analytes are thermally desorbed in the injector of a gas chromatograph for separation and quantification. The fiber is mounted in a syringe-like holder which protects the fiber during storage and I netration of septa on the sample vial and in the GC injector. This device is operated like an ordinary GC syringe for sampling and injection. The extraction principle can be described as an equilibrium process in which the analyte partitions between the fiber and the aqueous phase. 

The search of adequate extraction techniques allowing the identification and quantification of wine volatile compounds has attracted the attention of many scientists. This has resulted in the availability of a wide range of analytical tools for the extraction of these compounds from wine. These methodologies are mainly based on the solubility of the compounds in organic solvents (liquid-liquid extraction LLE, simultaneous distillation liquid extraction SDE), on their volatility (static and dynamic headspace techniques), or based on their sorptive/adsorptive capacity on polymeric phases (solid phase extraction SPE, solid phase microextraction SPME, stir bar sorptive extraction SBSE). In addition, volatile compounds can be extracted by methods based on combinations of some of these properties (headspace solid phase microextraction HS-SPME, solid phase dynamic extraction SPDE). 

Application of an Automated Headspace Solid Phase Micro-Extraction for the GC-MS Detection and Quantification of Reductive Sulfur Compounds in Wines... 

The choice of quantification in headspace analysis therefore depends on the sample and the analyte. Procedures include total vaporization, the use of matrix-matched standards, multiple extractions, and standard additions. A method using an internal standard may be used provided knowledge of the equiH-brium constants for the standard and the analyte can be determined. 

A European collection of 50 recycled food contact PET samples was recently analysed by high-temperature (180°C, 1 h) SHS-GC-MS using three substances (limonene, benzophenone, methyl stearate 1-50 ppm) as external standards for quantification of the contaminants [976]. The results are shown in Fig. 2.45. Maximum contamination levels of 4 ppb were observed. The headspace method was verified by liquid extraction GC-MS. HS-GC-MS showed limits for the extraction of lesser volatiles for which liquid extraction should be applied. 

Analyte recoveries in P T experiments can vary widely due to matrix effects, purging efficiency, volatiUty, purge ceU design, choice of adsorbent, isolation temperature, and many other factors. Quantification with the various headspace techniques always requires method development in terms of extraction time and temperature in order to avoid degradation. With dynamic headspace (DHS) nearly 100% recovery of volatiles is possible provided headspace temperature is appropriate to remove most of the analyte in a reasonable time. Kolb et al. [32] have outlined the prospects of quantitation by means of headspace techniques. 

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