Headspace sampling techniques quantitative analysis

Small solid seuaples can be analyzed directly by dynamic headspace sampling using a platinum coil and quartz crucible pyrolyzer and cold trap coupled to an open tubular column (341,369,379). This method has been used primarily for the analysis of mineral samples and of additives, catalysts and byproducts in finished polymers which yield unreliable results using conventional headspace techniques owing to the slow release of the volatiles to the headspace. At the higher temperatures (450-1000 C) available with the pyrolyzer the volatiles are more readily and completely removed from the sample providing for quantitative analysis. 

Dynamic headspace GC-MS involves heating a small amount of the solid polymer sample contained in a fused silica tube in a stream of inert gas. The volatile components evolved on heating the sample are swept away from the sample bulk and condensed, or focused on a cryogenic trap before being introduced onto the chromatographic column via rapid heating of the trap. The technique can be used qualitatively or quantitatively DHS-GC-MS is considered to be well suited towards routine quantitative analysis. 

In the direct extraction mode, the SPME partitioning resembles liquid-liquid extraction and, besides the type of fiber coating used, parameters important for optimal recovery are. e.g. agitation technique and pH. Eor headspace SPME, the most important parameters are the vial volume, the headspace-to-sample voliune ratio and the equilibrium temperature, i.e. similar parameters as in traditional HS [24,25]. Although the theoretical treatment of SPME relies on extraction under equihbrium conditions, it is not necessary to estabhsh full equihbrium to perform quantitative analysis. It was shown by Ai that quantitative data can be obtained from both two-and three-phase systems imder non-equilibrium conditions [26-28]. This ex-... 

Trace environmental quantitative analysis (TEQA) utilizes various determinative teehniques (Chapter 4) in combination with various sample prep techniques (Chapter 3). In this appendix, one specific trace analysis using static headspace sampling automatically coupled to capillary gas chromatography with element specific detection is described. LSQUARES is a computer program developed by the author in BASIC and is used in the quantitative analysis discussed below. The actual program written in GWBASIC is also listed after illustrating its use in TEQA. 

Static headsp>ace extraction, also known as equilibrium headsp>ace extraction, is one of the techniques used for qualitative and quantitative analysis of volatile substances in the forensic field, in this technique the sample is placed in a closed vial, the volatile analytes disseminate into the headspace of the vial (figure 1), once equilibrium is reached between the analyte concentration in the headspace and the analyte concentration in the sample, a portion of the headspace is taken and injected into the gas chromatograph this can be done manually or with an autosampler, this process will be usually carried out at a pressure and temperature above ambient conditions (Slack et al., 2003). 

Quantitative analysis using headspace techniques is dependent upon the partition of the analyte between the sample and vapor phases. The concentration of the analyte in the sample phase is usually... 

This simple technique permits the quantitative analysis of volatile compounds in various liquid, semi-liquid or solid foods, in biological fluids and tissues, and environmental contaminants in water, air and soils. The method is very sensitive to the equilibrium solute distribution between phases at the temperature selected for the analysis. Equilibration is greatly dependent on the solubility and viscosity of the samples. This method is particularly suited to highly volatile compounds because they have a favorable equilibrium between liquid (or solid) phase and its headspace, producing a higher concentration of volatile compounds in the headspace. 

Hammers, W.E. and Bosman, H.F.P.M. Quantitative analysis of a d5mamic headspace analysis technique for non-polar pollutants in aqueous samples at the ng/kg level. Journal of Chromatography 1986, 360, 425 32. 

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. 

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]. 

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. 

HS-SPME is a very useful tool in polymer analysis and can be applied for absolute and semi-quantitative determination of the volatile content in polymers, for degradation studies, in the assessment of polymer durabihty, for screening tests and for quality control of recycled materials. For quantitative determination of volatiles in polymers, SPME can be combined with multiple headspace extraction to remove the matrix effects. If the hnearity of the MHS-SPME plot has been verified, the number of extractions can be reduced to two, which considerably reduces the total analysis time. Advantages of MHS-SPME compared to MAE are its higher sensitivity, the small sample amount required, solvent free nature and if an autosampler is used a low demand of labor time. In addition, if the matrix effects are absent, the recovery will always be 100%. This is valuable compared to other techniques for extracting volatiles in polymers in which the recovery should be calculated from the extraction of spiked samples, which are very difficult to produce in the case of polymeric materials. 

Strong interactions between the polar matrix and polar analytes may lead to extremely long equilibrium times and errors in quantitation even when the MHS technique is used. In these cases, a displacer may be added to break the interactions between the matrix and analyte. Polar 2-cyclopentyl-cyclopentanone could be quantitatively determined in polar polyamide 6.6 by MHS-SPME if water was added as a displacer to break the hydrogen bonding between 2-cyclopentyl-cyclopentanone and polyamide. The addition of water also significantly reduced the equilibrium time. A correlation was found between the amount of 2-cyclopentyl-cyclopentanone emitted from polyamide 6.6 and the total amount of 2-cyclopentyl-cyclopentanone in the material. This correlation enables rapid assessment of the 2-cyclopentyl-cy-clopentanone content using headspace techniques under non-equilibrium conditions. The analysis time is significantly reduced if the polymer samples are milled to a powder prior to extraction. 

Several extraction techniques are treated and their applications to VOCs are referenced in the respective tables. As a general remark, the extraction of VOCs from the headspace usually affords shorter extraction times. The extraction of polar compounds is more difficult and several extraction techniques poorly extract these compounds. Water involves some problems in techniques like P T. The sample matrix should be considered in order to choose the adequate extraction technique. Recently, miniaturized and pol)mier-based extractions are interesting for the analytical chemist achieving lower quantitation limits, higher sensitivity, higher reproducibility and sample throughput, and lower analysis costs for some compoimds. 

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