Threshold Measurements

Basically, there are four major types of measures that are used in taste intensity measurements (a) threshold measures or estimates of the physical level at which the sensation of sweetness begins, (b) equal-sweetness matches between a sugar and other sweeteners, (c) category or rating scales, and (d) ratio scales. Each method has found its adherents and uses, and each possesses specific advantages and defects that indicate its use for one application, but contraindicate its use for another. These methods and their applications have been critically analyzed and reviewed, " " and it is, therefore, superfluous to deal with the topic here. 

Apparatus, developed in this laboratory for two types of thermochemical measurements—(a) gas-phase ion molecule equilibria and (b) collision-induced dissociation (CID) threshold measurements—will be described. For both purposes, a triple quadrupole mass spectrometer is used. It is only the front end modifications that provide the conditions for (a) or (b). 

In order to avoid such uncontrolled collisional activation, we chose to use apparatus which is closely related to the ion-source reaction chambers developed for thermal ion-molecule equilibria (see preceding Section A). In fact, sources like those shown in Figures 4 and 7 are well suited for providing thermalized ions however they provide somewhat low ion intensities, typically some 50,000 counts/s of a given major ion in a mass spectrum after mass analysis. However, such intensities are completely sufficient for CID threshold measurements and the source... 

C. Thermochemical Data from CID Threshold Measurements Treatment of Threshold Curves... 

The data obtained64 from CID threshold measurements are given in Table 11. The Af/ 9g and AH%9i values given in the table illustrate the magnitude of the kinetic... 

An established system for odour detection threshold measurements Guideline for olfactometric measurements in the Netherlands. Comparison with Western European guidelines... 

AN ESTABLISHED SYSTEM FOR ODOUR DETECTION THRESHOLD MEASUREMENTS... 

It is known that odour detection thresholds measured by different types of olfactometer can vary substantially. Not unreasonably, there are well-intentioned calls for standardisation of odour measurements , possibly as the basis for odour emission regulations, and so it might be useful to express some cautionary views. 

In threshold measuring situations small signals are presented and at times when the internal noise is low, the total impact of the noise plus signal may be lower than that of the strongest noise alone. In such a case the subject may say no to a presentation containing the signal (a miss) and yes to a presentation in which only the strong noise was present (a false alarm). 

Of these influences, the last three do not affect olfactometric measurements to a large extent. In supra-threshold measurements, using scaling methods, they may play a very important role, however. 

This paper describes some investigations into the physical operation of olfactometers, with reference to two commercially produced models, a Prosser and a TO4. It should be noted that the findings reported here are not necessarily typical of other copies of these olfactometers. The type of psychophysical procedure, eg suprathreshold or threshold measurements is not included in this paper. 

DRAVNIEKS, A. JAKE, F. 1980 Odour threshold measurements by dynamic olfactometry significant operational variables J.Air Poll, Control Assoc. 30 (12) 1284-1289. 

We should also briefly discuss the reliability of PC threshold measurements for determining ,. The problem here is that for each temperature the spectroscopic data (Atheoretical expression, such as that found in Eq. (45). Above T = 0, when phonons are involved, there is no well-accepted theory of the photon capture cross section to use for data fitting. In fact, even at T = 0, the theoretical situation is ambiguous because the exact form of the impurity potential is not known. Thus, unless the PC threshold data are very steep, the energies derived from them may be hard to determine with great accuracy. 

A cell of the Geiger type, with coaxial electrodes filled with Ar and the admixed vapor has also been tried by us.19 With the mass spectrometer, it could be directly proved that the first threshold measured by these methods, is actually due to a process of electron ejection, without any dissociation of the molecules. 

HPLC analysis of polycyclic aromatic hydrocarbons (PAH) in drinking water is one of the current and classical applications of fluorescence. In this case, the detector contains a fluorescence flow cell placed after the chromatographic column. This mode of detection is specifically adapted to obtain threshold measurements imposed by legislation. The same process allows the measurement of aflatoxins (Fig. 12.11) and many other organic compounds (such as adrenaline, quinine, steroids and vitamins). 

Avrami et al (Ref 46) used an electrode beam app (Physics International Mod 738 Pulserad) to rapidly heat a slab of Al. The shock thus generated by rapid energy deposition was transmitted into pressed pellets of KDNBF (K dinitro-benzfuroxan). With shocks of about 0.2 microsec duration and a pellet thickness of 0.25 inch the initiation threshold measured was about 18 kbar... 

Fig. 22 Static light scattering close to the precipitation threshold. Measurements are performed at constant [Ca2+] by decreasing [NaPA] (route 1). The inert salt level is 0.1 M NaCl. Squared mean square radius of gyration is plotted vs. [NaPA], The symbols denote four different Ca2+ concentrations 3.25 mM ( ) 3.5 mM ( ) 3.75 mM (A) 4.0 mM ( ). Vertical lines indicate precipitation limits which appear as open circles in Fig. 23...

Ionization threshold measurements are also indicative of the proton transfer reaction. This has been seen for the phenol-B system (Jouvet et al. 1990 Solgadi et al. 1988). The ionization thresholds depend on the structure (AH - -B or A- - HB +) of the excited state (processes VI and VII in scheme 1). 

At first sight, it seems surprising to observe competitive reactions within the same complex. However, it must be noticed that in the ionization processes the internal energy distribution within the ions can be broad since the cluster s geometries in the Sj excited state and the ionic state can be very different. This can be seen by the ionization threshold measurements which do not exhibit clear onsets. Therefore, the presence of competitive processes can be explained by different barrier heights for the different channels. When the ions are prepared below one barrier and above the other one, only one product will be observed. Due to this broad internal energy distribution, on average, many channels can be detected. Coincidence detection of the zero kinetic electron and the product ions... 

The threshold measured for the reaction in Eq. (1), E0, corresponds to the highest energy along the reaction coordinate for dissociation, i.e., the activation barrier, which occurs at the transition state for the reaction. Such a transition state is termed tight if it corresponds to a specific molecular conformation see Fig. 2a. Interestingly, in the dissociation of many ions, there is no activation barrier for the reverse process (the association reaction), such that the activation energy for dissociation equals the overall reaction endothermicity for the process in Eq. (1) see Fig. 2b. In such a case, the measured threshold is equivalent to the bond dissociation energy (BDE) of AB+ (Eq. 2) ... 

In order to extend the data to higher n, a pulsed dye laser was frequency-doubled to give photons in the 4.5-6.5 eV range. No threshold measurements were possible as two-photon processes were always present even at the lowest laser fluences employed (< 100 nJ cm " )."... 

Product-oriented Threshold—measures the level at which Method of limits... 

Dibeler and Liston (, 4) have studied a series of cyanogen compounds and obtained their enthaplies of dissociation from photoionization threshold measurements. The values below are obtained from the halogen ion threshold measurements, except for HCN which is from the CN threshold measurements. The value below is calculated using the IP from CNCl measurements (3). 

Electron-impact threshold measurements for SF from SF have been reported by Hildenbrand (2) and Glemser et al. (4). The measured appearance potentials (AP) are in good agreement and indicate AP(SF. /SF ) = 12.67 0.1 eV. Combining this value with an... 

Two upper limit values for Dq CSF -F) can be obtained from threshold measurements by Harland and Thynne (6) on dissociative... 

Comparison of initiation-threshold measurements suggests that there is a minimum thickness, or a run-up distance, before detonation occurs and that this is independent of pulse-width for stresses up to 10 kbars. For long pulses (3.5 psec) in the gas gun, no evidence of detonation was detected for 1-mm-fhick samples. Detonation occurred with run-up distances in the range of 1-2 mm for impact stresses of 8.9 kbar. For stresses greater than 6.0 kbar, evidence of detonation was noted after a 2-mm run. In the thin-flyer-plate experiments at stresses of 8 kbar, dextrinated lead azide displayed a 2-usec initiation delay. Voreck and coworkers [8] determined that a similar minimum thickness of lead azide is required for complete detonation in an explosive train consisting of NOL-130, lead azide, and RDX (Figure 15). 

---