Detection Threshold selection

The plant sample in lane 6 is also positive for the transgene of interest. Because the band for the effect gene (middle band) is typically fainter than the band for the selectable marker gene (bottom band), it appears that for lane 6, the PCR product amplitication for the effect gene is below the assay detection threshold. Because the selectable marker is clearly present and the PCR amplitication worked, lane 6 can be interpreted as a positive result for the transgene of interest. 

The comparison of these two terms for faults of comparable intensity (he, a deviation of 10% from the true value) shows that for nominal values of Qgas the residual built from the estimation of [CO2] is much more sensitive to a fault of PCO2 than to a fault of Qgas- Two options are possible to manage this (i) select a low threshold to detect faults even for the less sensitive sensor with the risk to have a great number of false detections, (ii) select a threshold so that the most sensitive fault is correctly detected and include faults of the second sensor into this residual. 

For a compound to contribute to the aroma of a food, the compound must have odor activity and volatilize from the food into the head-space at a concentration above its detection threshold. Since aroma compounds are usually present in a headspace at levels too low to be detected by GC, headspace extraction also requires concentration. SPME headspace extraction lends itself to aroma analysis, since it selectively extracts and concentrates compounds in the headspace. Some other methods used for sample preparation for aroma analysis include purge-and-trap or porous polymer extraction, static headspace extraction, and solvent extraction. A comparison of these methods is summarized in Table Gl.6.2. 

Methoxypyrazines are compounds with very low detection thresholds which must be determined at very low levels. For these compounds, different selective isolation methods have been proposed (Allen et al. 1994 Sala et al. 2002). Some authors use a simple extraction (Kotseridis et al. 1999 Falcao et al. 2007) or an optimized headspace SPME procedure (Chapman et al. 2004 Prouteau et al. 2004) using in most cases isotopically-labelled internal standards to compensate for matrix effects. In spite of the claims of the authors, all these methods present some difficulties to accurately determine the compounds at the lowest levels at which they can be found. A recent report has presented an advanced method combining the preconcentration ability of headspace SPME with the selectivity of comprehensive GC (Ryan et al. 2005). 

The examples presented in this chapter demonstrate that a combination of various analytical approaches and the selection of suitable model systems can add valuable information to our knowledge about pathways and enzymes involved in the biosynthesis of chiral volatiles. Some of the techniques need further improvement, e.g. by use of radioactively labeled precursors the detection threshold of metabolites can be lowered significantly addition of precursors in concentrations comparable to those in natural plant or microbial systems would be possible. The investigation of the enantioselectivity of enzymes has to be emphasized, eventually not only enzymes commercially available or easily accessible in microorganisms but also those active in plant systems have to be studied. 

Although dietary lipids are mainly constituted of triglycerides (TG), long-chain fatty acids (LCFA more than 16 carbons) seem to be responsible for oral lipid perception. In a free-choice situation, rats have a weaker preference for TG and medium-chain fatty acids (8 to 14 carbons) than for LCFA (Tsuruta et al. 1999 Fukuwatari et al. 2003). This chemical selectivity is very tight, as LCFA derivatives, such as methyl LCFA, are not recognized (Tsuruta et al. 1999). The ability of rodents to detect LCFA specifically has also been confirmed with the conditioned taste aversion paradigm. It is noteworthy that both rats and mice can be conditioned to avoid specific LCFA (McCormack et al. 2006 Gaillard et al. 2008), with a submicromolar detection threshold (McCormack et al. 2006 Yoneda et al. 2007). 

Table 8.39 Detection threshold concentrations of selected salty Inorganic salts.

Table 8.41 Detection thresholds of selected acids in water according to various workers.

Diatomic chlorine (CI2) is a gas xmder ambient conditions, with a pungent, suffocating odor. Because its vapor density is twice that of air, chlorine forms a greenish-yellow cloud near the ground upon release into the environment (Haynes et al., 2013 O Neil et al., 2013). The CI2 odor detection threshold is approximately 0.2-0.4 ppm, with considerable variation among humans, although odor perception tends to decrease over time (NIOSH, 1976 ATSDR, 2010). Chlorine gas is slightly water-soluble (-4.4 g/L O Neil et al., 2013), but it reacts quickly with water to form hydrochloric acid (HCl) and hypochlorous acid (HOCl). Selected chemical and physical properhes of CI2 are listed in Table 24.1. 

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