Headspace analysis calibration

Of the three general methods, the last seems to be the most practical. Theoretically, with high enough concentrations of hydrocarbons, the first method, the headspace analysis, should be both the most accurate and the easiest to calibrate. Operationally, it leaves much to be desired both because of the problems of sensitivity and those of the accommodation of the larger molecules in water. The second method, vacuum degassing, requires much more equipment than the third method and requires that large amounts of water vapor be removed before the sample is injected into the gas chromatograph. The last method is so much less complicated that even with its calibration problems it has been adopted almost universally. 

Accuracy The degree of accuracy needed is determined by the question being asked. In the Bulging Drum Problem, does it matter if the hydrogen gas was found to be 12.0% or 12.1% of the headspace Which calibration method—external standards, internal standards, or standard addition—is appropriate for your analysis ... 

In another investigation, ethylene oxide in polyvinylchloride was determined by dissolving 65 mg of sample in 1 ml of dimethylacetamide [189]. Headspace analysis was conducted on a glass column packed with Porapak T under isothermal conditions. The solvent was removed by back-flushing. An external standard was used for calibration. A vinylchloride monomer was also detected in this analysis (Figure 4.3). 

Partition coefficients can be determined by vapour-phase calibration (VPC) [54], by the phase-ratio variation (PRV) method [also known as the vapour-liquid equilibrium (VLE) method] [57] for many solvents in their aqueous solutions, and by VLE for ethanol in water. If two sample vials of different volume are both filled with the same sample, the partition coefficient, K, will be the same. In order to determine the solute s partition coefficient, K, each vial, at equilibrium, is subjected to headspace analysis in order to derive the slope of the linear equation (4.1). The concentrations of a solute in the two vials can be written as... 

Condition (a) above is not exclusive to headspace analysis in fact, it is a pre-requisite for quantitative analysis of any sample in gas chromatography. Essentially the same is true for condition (b). Condition (d) is primarily a design problem. Finally, constancy of p is assured by proper automation of the system (i.e. by exact repetition of the operational parameters) and by the fact that the calibration standard is carried through the same MHS steps as the sample itself Therefore, the greatest problem is posed by the need to ensure equilibrium between the two phases in the vial. 

Samples were contained in glass flasks of either the configuration of a serum bottle (100 ml) capped with a Mininert valve or of a specially-designed Erlenmeyer flask (125 ml) constructed with a screwtop and fitted with a sidearm and a Teflon stopcock. The latter flask was capped with a silicone septum held with an open-topped screw-cap, and allowed syringe-sampling as well as evacuation. Samples for headspace analysis were equilibrated at 30°C. Calibration curves were calculated as log (area) versus log (mass of methanethiol or dimethyl disulfide). 

Determination of solubility by headspace analysis offers several advantages over spectrophotometric techniques. First, because of the selectivity of chromatographic analysis, compound purity is not a critical factor second, absolute calibration of the gas chromatographic detector is not necessary if the response is linearly related with concentration over the range necessary for the measurements and finally, this method does not require the preparation of saturated solutions, since a partition coefficient, not a solubility, is actually measured. However, headspace methodology would probably not be applicable for determining PAH solubilities for three reasons. First, there is little data in the literature on the vapor pressures of PAHs. Second, the aqueous solubilities of most PAHs are too low to be measured by this procedure. Finally, adsorptive losses of PAHs to glass surfaces from the vapor phase would cause errors. 

Total vaporization of the analyte into the vapor phase can be realized for a limited number of samples and this provides the simplest case for quantification. As an example, a polymeric matrix containing residual water could be heated to 150°C to effectively drive all the water into the vapor. Headspace analysis of the vapor phase in conjunction with external calibration will give the concentration of water in the vapor phase. From this and knowledge of the vapor phase volume, the total amount of water present in the sample can be calculated. This is expressed in the trivial relationship ... 

There are two basic principal methods of headspace analysis in polymers. In one method a solution of the polymer is examined, in the other method solid polymer is examined directly. When working with polymer solutions, headspace equilibrium is more readily obtained than the solid approach and the calibration procedure is simplified. 

The more volatile monomers vinyl chloride, butadiene, and acrylonitrile can be determined by dissolution of the polymer and analysis of the equilibrated headspace above the polymer solution. By this method it was possible to determine vinyl chloride and bntadiene at the 0.05 ppm level and acrylonitrile down to 0.5 ppm. The injection of water into polymer solutions containing styrene and 2-ethylhexyl acrylate monomers prior to headspace analysis greatly enhanced the detection capability for these monomers making it possible to determine styrene down to 1 ppm and 2-ethylhexyl acrylate at 5 ppm. Incorporation of polymer into the calibration standards compensates for the effect which the polymer matrix has upon the equilibrium partitioning of the monomer between the solution and head space. The relative precision and error in the determination of these monomers near the quantitation limit was found to be less than 7%. 

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. 

Kawamura et al. [81] have surveyed nonylphenol by GC-MS (with quantification by GC-SIM-MS) in 207 samples of food contact plastics and baby toys. Crompton [43] has described the quantitative GC analysis of residual vinylchloride, butadiene, acrylonitrile, styrene and 2-ethyIhexylacrylate in polymers by solution headspace analysis. Considerably greater sensitivities and shorter analysis times were obtained using the headspace analysis methods than were possible by direct injection of polymer solutions into a GC. Similarly, various residual hydrocarbons (10 ppm of isobutane, n- and isopentane, iso- and neohexane) in expanded PS were determined by GC analysis of a solution of the sample with hydrocarbon internal standards accuracies of 5 to 10% were reported [82]. Residual n- and isopentane (0.001%) in expandable and expanded PS were also determined by a solvent-free procedure consisting of heating the polymer at 240°C in a sealed tube, followed by HS-GC calibration against known blends of n- and isopentane and n-undecane internal standard [82]. 

Often used quantitative calibration method in headspace analysis for matrix independent calibrations. 

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. 

Gas evolution from the hydrolysis of Grignard reagents can be used for the activity analysis. However, care must be taken in the calibration of standards, because the resulting gas will have some solubility in the solvent. Typically, the gas is analyzed by GC, taking the gas from the headspace of a closed system. The obvious limitation in this method is that only a selected amount of Grignard reagents (C4 or less) can be used, owing to the volatility of the hydrolysis products. 

SPME capillary gas chromatography (SPME-GC) can be used for the extraction of organometal compounds after these have been derivatized to a sufficiently volatile form (see also organotin speciation). A silica fiber coated with polydimethylsiloxane (PDMS) is brought into the (headspace) of the sample. After exposure, the fiber is inserted into the GC injection port and the compounds are thermally desorbed for subsequent analysis. This method has higher sensitivity compared to the injection of solvent on a capillary column (usually 1 fil) but requires the use of standard addition as a calibration method. After GC separation, analysis can be performed by furnace atomization plasma emission spectrometry (FARES)." ... 

Using the 500-ppm BTEX stock reference solution, prepare a series of calibration standards in which the BTEX is present in 10 mL DDI which is contained in a 22-mL HS vial with PTFE/silicone septa and aluminum crimp caps. Refer to the above calibration for reference as you prepare a series of working calibration standards for HS-GC analysis. Following the development of a calibration curve, inject the ICV (only one injection per vial is acceptable in HS GC), then inject the headspace above the aqueous samples. Following the development of a calibration curve, inject the ICV (only one injection per HS vial is valid) and the contaminated aqueous samples. 

In standard addition calibration, an additional known quantity of the analyte is added directly to the sample, following an initial analysis. In standard addition, the sample is divided into several equal portions, then add increasing levels of standard. In other words, the calibration curve is prepared with the sample thus all the points of the curve have the same composition, in this way matrix effects are eliminated. This typ)e of calibration is not often used in gas chromatography static headspace. 

Static headspace gas chromatography is a mature and reliable technique it is considered the technique of choice for the analysis of ethanol in biological samples, and is therefore jrresent in the vast majority of forensic laboratories around the world with the qualified personnel to operate it however, the applicability of this technique is not limited to this test and can be used for the analysis of various substances with minimal modifications, providing proper calibration and proper handling of matrix effects, excellent validation parameters, along with a clean injection. So, with this technique, various substances can be analyzed without the need of additional methods, and that would allow forensic laboratories to expand the number of cases they can take care of, with a minimal investment. 

It is possible to conduct calibration according to the admixture method in conjunction with this technique if the organic impurities can be recognized on the basis of the gas chromatogram. Equal quantities of water are measured into similarly shaped glass vessels so that the same size of gas space relative to the water volume is present in all calibration samples. Increasing quantities of the suspected substances are then measured into the water in correspondingly low concentrations, and headspace equilibrium is established, as in the analysis of the sample. In the case of fuel... 

Quantitative analyses can be performed either isothermally or with a temperature program. For analytes that are liquids at room temperature, assay calibration is by analysis of standard solutions prepared in analyte-free human blood. The same calibration solutions are used in the analysis of tissue digests. Analyte concentrations in the range 0.1-10 or 0.5-50 mg are usually adequate in acute poisoning. Portions of the standards are transferred to headspace vials for analysis as described above, and a... 

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