Detection frequency method

The detection frequency method uses a number of sniffers to quantify an odor in the GC effluent from a single concentration. The underlying assumption is that any random sample of sniffers will functionally express a range of sensitivities, so that some sniffers will detect an odor and others will not. The conclusion is that the fraction of a group that detects an odor is related to the group potency of the odor, a notion that can be supported by the large diversity in odor thresholds observed in humans. 

Figure G1.8.3 Sniffing chromatogram of eight volatile compounds in a reference mix obtained by the detection frequency method using eight assessors (see Alternate Protocol 1). Compounds 1, 100 ng 2-butanone 2, 20 ng diacetyl 3, 500 ng ethyl acetate 4, 100 ng 3-methyl-1-butanol 5, 20 ng ethyl butyrate 6,100 ng hexanal 7, 100 ng 2-heptanone 8, 500 ng a-pinene.

A further approach, the detection frequency method [74,75], uses the number of evaluators detecting an odor-active compound in the GC ef uent as a measure of its intensity. This GC-0 method is performed with a panel composed of numerous and untrained evaluators 8-10 assessors are a good agreement between low variation of the results and analysis time. It must be added that the results attained are not based on real intensities and are limited by the scale of measurement. An application of the detection frequency method was reported for the evaluation of leaf- and wood-derived essential oils of Brazilian rosewood (A. rosaeodora Ducke) essential oils by means of enantioselective-GC-0 (Es-GC-0) analyses [76]. 

The attained results may generally be well correlated with detection frequency method results and, to a lesser extent, with dilution methods. In the aforementioned research performed on the essential oils of Brazilian rosewood, this method was also used to give complementary information on the intensity of the linalool enantiomers [76]. 

Testing for flavor-impact chemicals in fermented pickles was compared using solid-phase extraction (SPE), purge-and-trap (P T) on Tenax, and SPME (5). Determination of which chemicals were the most significant flavor-impact chemicals was made by olfactometry experiments employing the detection frequency method (for details, see Chapter 12) and by recombination studies (i.e., adding suspect flavorants to an artificial salt brine solution to determine which potential flavorant and/or combination of flavorants most closely matched the flavor of the pickle brine). 

What can we do for reversible (non polarizable) electrodes In this case we use the fact that different processes occur at different timescales. Instead of the relatively slow change of the voltage in cyclic voltametry, AC potentials with varying frequencies are applied and the current is detected. The method is called impedance spectroscopy. Using impedance spectroscopy, even semiconducting [100] or insulating materials can be analysed by coating them onto metallic electrodes. 

There are several methods by which microwave resonance can be optically detected. The methods that are utilized in our laboratory, in order of increasing complexity of the attendant electronics, are (a) continuous-wave (cw) operation (b) AM or FM modulation of the micro-waves with lock-in detection at the modulation frequency (lock-in) (c) cut-off of the exciting light followed by... 

Gas chromatography-olfactometery (GC-O) provides a sensory profile of odor active compounds present in an aroma extract by sniffing the GC effluent. Several techniques have been developed to collect and process GC-O data and to estimate the sensory contribution of individual odor active compounds, including dilution analysis (29, 30), time intensity (31), and detection frequency (32) methods. GC-O has successfully been used to evaluate the odor active compounds of olive oil (33), soybean oil (34), and fish oil enriched mayonnaise (35). 

The TPPI-method or Marion/Wuthrich inethod [2.41] In the TPPI method the transmitter frequency (Otrans is positioned at the centre of the detected frequency range SW. Frequency discrimination is achieved by shifting the apparent sweepwidth from 1/2SW to 0 to SW. This apparent frequency shift is obtained by incrementing the phase of the selection pulse by 90° for each successive tl increments and sampling the data faster than normal by modifying the value of tl by the phc-factor (see section 3.3.1). The TPPI method guaranties both frequency discrimination and pure absorption lineshapes. 

Fignre 16.4 Various methods for FBG sensor conditioning as function of needed detection frequency and application type. 

The sum-frequency method is easier with respect to the optical arrangement but with more electronics involved. The pump beam h and the probe beam L2 are chopped at two different frequencies with the same chopper (Fig. 5.20). Both probe beams are detected by the same detector. The output signals are fed parallel into two lock-in detectors tuned to f and f - - f2, respectively. While the lock-in at f records the polarization spectrum of Li if Xi is tuned, the lock-in at (/i 4-/2) records the OODR spectrum induced by both lasers L and L2 at a fixed wavelength while X2 is tuned. 

GC-0 methods are commonly class ed in four categories dilution, time intensity, detection frequency, and posterior intensity methods. Dilution analysis, the most applied method, is based on successive dilutions of an aroma extract until no odor is perceived by the panelists. This procedure, usually performed by a reduced number of assessors, is mainly represented by combined hedonic aroma response method (CHARM) [67], developed by Acree et al., and aroma extraction dilution analysis (AEDA), rst presented by Ullrich and Grosch [68]. The former method has been applied to the investigation of two sweet orange oils from different varieties, one Florida Valencia... 

The choice of the GC-0 method is of extreme importance for the correct characterization of a matrix, since the application of different methods to an identical real sample can distinctly select and rank the odor-active compounds according to their odor potency and/or intensity. Commonly, detection frequency and posterior intensity methods result in similar odor intensity/concentration relationships, while dilution analysis investigate and attribute odor potencies. 

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