Headspace oxygen measurements

Data from the stoppered culture tests was also used to determine the effect of declining 02 concentration on the rate of metabolism of the cells. The specific 02 consumption rate was determined to depend linearly upon the 02 concentration throughout the range of headspace concentrations measured (0-31 %). This seems to contradict the experience of other researchers [31-33, 50], who report saturation kinetics in plant cell cultures whenever 02 concentration exceeded 4-5% (gas phase). In the stoppered culture tests, at the same time 02 concentration declined, C02 and C2H4 concentrations increased. Other unidentified compounds may also have been produced. The declining rate of metabolism observed may have been caused by conditions other than declining oxygen. 

The headspace oxygen method is simple and reproducible and may be the best analytical method to evaluate the oxidative stability of fats and oils (14). Its application in measurement of lipid oxidation in food products other than fats and oils, however, is limited because protein oxidation also absorbs oxygen (15). 

There are many other methods for measuring lipid oxidation and quality by chemical means. Among the best-known procedures are the thiobarbituric acid (TEA) test, carbonyl value, and headspace oxygen analysis. These methods have been reviewed and discussed elsewhere (287, 307). 

Several temperature-catalyzed stability tests are used in evaluating the oxidative stability of oils and fats. The oldest method is the Schaal oven test (39). It is inexpensive but subjective, because it uses organoleptic and odor intensities in the procedure and still requires days to obtain the result. This approach has been standardized into a recommended practice (AOCS method Cg 5-97). In the active oxygen method (AOM) (39), the development of peroxide is measured with time. As the formation and decomposition of peroxides are dynamic processes, the results obtained by this method do not correlate well to the actual stability of the oils and fats observed under practical application conditions. Other methods that have been based on oxygen absorption are the gravimetric (59) and the headspace oxygen concentration measurement (60, 61). 

The Rancimat method has been used to measure the antioxidant activity of synthetic and natural antioxidants (2-/>2d) and has correlated well with oil stability measured by the Active Oxygen Method (27) and peroxide value measurement (28). Our study showed that using the Rancimat method to study the antiphotooxidative effect of carotenoids on the soybean oil was in agreement with the results using the headspace oxygen depletion method (16) and the peroxide value method (16,JT). 

Miller, M.E., Stuart, J.D. (2000) Measurement of aqueous Henry s law constants for oxygenates and aromatics found in gasolines by the static headspace method. Anal. Chem. 72, 622-625. 

Different types of equipment, depending on the resources available and the number of measurements required, can be used for determination of an OUR versus time curve. A rather simple and manually operated type was used by Bjerre et al. (1995). A still relatively inexpensive apparatus, simple to operate automatically, was designed by Tanaka and Hvitved-Jacobsen (1998). However, this type may introduce a minor error at especially low OUR values because of a potential release of oxygen into a headspace of nitrogen gas in the reactor. 

Direct measurement of oxygen uptake can be performed with O2 sensors in the headspace of a closed system and organic deposition by measuring the weight changes with a quartz microbalance. This method was applied to investigate the dependence of jet fuel autoxidation on temperature and the presence of antioxidants . 

Rancidity measurements are taken by determining the concentration of either the intermediate compounds, or the more stable end products. Peroxide values (PV), thiobarbituric acid (TBA) test, fatty acid analysis, GC volatile analysis, active oxygen method (AOM), and sensory analysis are just some of the methods currently used for this purpose. Peroxide values and TBA tests are two very common rancidity tests however, the actual point of rancidity is discretionary. Determinations based on intermediate compounds (PV) are limited because the same value can represent two different points on the rancidity curve, thus making interpretations difficult. For example, a low PV can represent a sample just starting to become rancid, as well as a sample that has developed an extreme rancid characteristic. The TBA test has similar limitations, in that TBA values are typically quadratic with increasing oxidation. Due to the stability of some of the end-products, headspace GC is a fast and reliable method for oxidation measurement. Headspace techniques include static, dynamic and solid-phase microextraction (SPME) methods. Hexanal, which is the end-product formed from the oxidation of Q-6 unsaturated fatty acids (linoleate), is often found to be a major compound in the volatile profile of food products, and is often chosen as an indicator of oxidation in meals, especially during the early oxidative changes (Shahidi, 1994). 

If the aim of excipient compatibility is to be predictive of the degradation expected with clinical or commercial dosage forms, then the packaging configuration should be representative of what will be used in that setting. Factors that could influence stability measurements include container headspace and permeability of oxygen and water vapor through the walls and cap (if a bottle) of the container. 

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. 

The following series of steps was performed for each stoppered culture test in order to obtain a plot of specific oxygen usage as a function of oxygen concentration. The 02 concentration at each time was converted to total millimoles N02 of 02 in the headspace. In each test, a suspension culture of A. annua was flushed with a gas mixture of known composition, then sealed and placed on a rotary shaker. The gas headspace was sampled at several hour intervals the measured concentrations from all the tests were used to calculate the 02 consumption and C02 production rates. 

Theoretically 3.3 mg of ascorbic acid will consume the oxygen in 1 cm of headspace air (19). Additional experiments were performed with measured amount of headspace air and ascorbic acid (Table XIV). These experiments were performed in 37-mL bottles with metal lids in a shaker at 150 rpm for 19 h. Six milligrams of ascorbic acid removed oxygen in the bottles with 1 cm of headspace and reduced the oxygen in the bottles with 2 cm of headspace. Ascorbic acid assays measured by absorption at 260 nm indicate a lack of material balance. Therefore, additional studies were performed in all glass bottles (Table XV). In... 

---