Head space measurement

At the date of this report, NECDF has 7 tons of >1 VSL carbon in storage on-site and is projected to generate 135 tons over the course of its operation.16 NECDF management intends to ship spent activated carbon for final treatment, using head-space measurements to determine that the material is acceptable for shipping.17... 

It is often necessary to remove all traces of solvent for further details see chap. 2.1.3. The content of solvent residue can be determined by gas chromatography, especially by head-space measurements with standardisation of the respective solvents. 

The focus of this chapter has been on the synthesis of new catalysts by parallel and combinatorial methods. Another aspect important to the development of new catalysts by these methods is the screening of these large libraries. We will not attempt to cover this topic comprehensively but do feel it is necessary to summarize some of the approaches that have been taken. Methods for screening libraries can be divided into both serial and parallel methods. Generally, the serial methods are adaptations of standard methods that allow for rapid individual analysis of each member of a library. Serial approaches for the analysis of libraries can be as simple as use of an auto sampler on a GC or HPLC system or as advanced as laser-induced resonance-enhanced multiphoton ionization of reaction products above the head-space of a catalyst (16) or microprobe sampling MS (63). The determination of en-antioselectivity in catalysis is a particular problem. Reetz et al. (64) reported the use of pseudoenantiomers and MS in the screening of enantioselective catalysis while Finn and co-workers (65) used diastereoselective derivatization followed by MS to measure ee. 

Wasik, S.P., Schwarz, F.P., Tewari. Y.B., and Miller, M.M. A head-space method for measuring activity coefficients, partition coefficients, and solubilities of hydrocarbons in saline solutions. J. Res. Nat. Bur. Stand., 89(3) 273-277, 1984. 

Previous reports 13] emphasized the importance of sample handling, and indeed because of the very volatile nature of the compounds measured in this type of analysis, sample collection deserves special consideration. In general, narrow mouth glass vials with a total volume in excess of 50 ml are acceptable. The bottles need not be rinsed or cleaned with organic solvents, but simply cleaned with detergent and water, rinsed with distilled water, air dried, and dried in a 105°C oven for one hour. The vials are carefully filled with sample to overflowing (zero head space) and a Teflon faced silicone rubber septum is placed Teflon face down on the water sample surface. The septa may be cleaned in the same manner as the vials, but should not be heated more than one hour because the silicone layer slowly degrades at 105°C. 

Several methods are available in the literature for the measurement of aliphatic amines in biological samples [28]. Problems with specificity and separation and cumbersome derivatisation and/or extraction procedures have limited the use of these techniques on a larger scale in clinical practice. The lack of a simple analytical method may have led to an underestimation of the incidence of the fish odour syndrome. For diagnosing the syndrome, an analytical technique should be used that is able to simultaneously and quantitatively measure TMA and its N-oxide in the complex matrix of human urine. Two such methods are currently available for this purpose proton nuclear magnetic resonance (NMR) spectroscopy and head-space gas analysis with gas chromatography or direct mass spectrometry (see below). 

Urine (5 ml) urine spiked with 0.2% (v/v) isopropylamine is placed in a screw-capped 15-ml vial [28]. Pelleted potassium hydroxide (3 g) is added before sealing the vial with an airtight polytetrafluoroethylene-lined septum cap. Potassium hydroxide raises the pH of the sample to ensure that the amines are present as volatile bases. The vial is heated in an aluminium block at 90 C for 20 min. While still in this block, 2 ml head-space gas is withdrawn through the septum with a disposable syringe and injected immediately on the gas chromatography column. The operating temperatures of the column, injector port and detector unit are 70 C isothermal, 150 C and 200 C, respectively, with nitrogen carrier gas at 60 ml/min. This allows quantification of TMA and other amines. TMA N-oxide is measured after quantitative reduction into TMA. For this, titanous chloride (30%, w/v 0.2 ml) is added to 2 ml urine in a screw-capped vial and incubated for 30 min at room temperature. The sample is then diluted ten-fold with distilled water and analysed as described above. The result represents the sum of TMA and TMA N-oxide present in the sample. 

As a first example we analyze a system consisting of a completely mixed wate volume in contact with a finite and completely mixed air space (Fig 21.9). The twc compartments can have a throughflow (of air or water, respectively). This particula setup could be anything from a large drinking-water cavern to a glass flask with head space filled with air to measure the Henry s law constant (Section 6.4). W< analyze the concentration of a volatile compound, such as tetrachloroethene o benzene, in both water and air, C,w and Cia. 

After 30 min equilibration at room temperature, the measurement run started. An air flow (room air filtered through active carbon) was conveyed over the sensors at a constant rate (lcm3/s) for 10 s to stabilize the baseline. An automatic syringe then suckled Asiago cheese head-space and conveyed it over the sensor surfaces for 60s. The sensors were exposed again to the reference air flow to eventually recover the baseline. The total cycle time for each measurement was 5 min. No sensor drift was experienced during the measurement period. Each sample was evaluated three times and the average of the results was used for subsequent statistical analysis (principal component analysis (PCA)). 

Fyhr et al. [201] reviewed several commercially available oxygen analyzers intended for the analysis of oxygen in the headspace of vials. However, preliminary validation revealed insufficient reproducibility and linearity. The authors developed headspace analysis systems. Sample volumes down to about 2.5 ml could be used without significant errors. Sample recovery was in the range 100-102%. It was necessary to measure the head-space pressure and volume in order to be able to present the assay in partial oxygen pressure or in millimoles of oxygen. Up to 40 vials per hour could be analyzed using this technique. 

Boyer and Probecker [191] determined organic solvents in several pharmaceutical forms using a Perkin-Elmer HS-6 headspace sampler. Typically, the samples were heated at 90°C for 10 min to establish equilibrium. Head-space samples were injected onto a Chromosorb 102 column. Ten injections of a mixed ethanol-acetone standard using methanol as the internal standard gave better precision than manual injections as measured by the relative standard deviation 1.63% and 2.48% for ethanol and acetone, respectively, using the sampler as compared to 4.77% and 3.93% by manual injection, respectively. Methods were reported for acetone and ethanol in dry forms such as tablets and microgranules, ethanol of crystallization in raw materials, and ethanol in syrups. Denaturants such as n-butanol and isopropanol in ethyl alcohol were determined using ethyl acetate as the internal standard. 

An illustration of the use of odor values as measures of performance in a specific application is given in Fig. 13.2. The ten odorants were all present at the same level (3.6%) in the model mixture that was incorporated in the fabric softener. However, their odor values in the head space over the fabric softener differ, from the most effective (Aldehyde C12 MNA) to the least effective (phenylethyl alcohol), by a factor of several thousand. The differences are even more pronounced in the wet, the dry, and the rewetted laundry. 

For instance, as is shown in Table 20.1, very different threshold values are obtained if the concentration of the substance has been measured in a solution the head space of which is smelled, or directly in the inspired air. In the case of solutions the specific nature of the solvent makes a great deal of difference (compare Table 20.2). In two solvent systems such as emulsions, the higher of the two thresholds prevails, since the odorant distributes itself between the two phases in such a way that it resides predominantly in the phase in which it is most soluble, which is usually the one in which its threshold is highest. Thresholds based on the concentration in air are the most meaningful values, since they are independent of the medium in which the odorant was disolved, but they are also the most laborious to measure, requiring an elaborate apparatus named olfactometer. 

This vapor screening methodology involves measuring agent concentrations in the head space of the drums after thermal equilibration at 70°F, which would provide a measure of the potential for exposure for an individual who might come into contact with the drum s atmosphere. The supporting documentation for the modification request used the EPA acute exposure guideline levels for GB, VX, and HD/HT to determine... 

For instance, the large solubility of dichloromethane in water causes a relatively low enrichment factor of ca. 80 (PDMS, 30 °C). As a result the detection limit for direct measurements of aqueous solutions with membrane covered IRE s is ca. 1.9 -10 mol/L. Using the head space variant with heated aqueous sample (90 °C) and cooled IRE (5 °C) the detection limit is improved by a factor of 30 to 6.2 10 mol/L. 

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