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In JoVE (1)
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Articles by Nathan W. Lloyd in JoVE
JoVE General
फल वाष्पशील इलेक्ट्रॉनिक नाक विश्लेषण का उपयोग
1Department of Plant Sciences, University of California, Davis, 2Department of Chemical Engineering and Material Science, University of California, Davis, 3Department of Viticulture and Enology, University of California, Davis
फल में अस्थिर यौगिक के विश्लेषण के लिए एक तेजी से विधि वर्णित है. अस्थिर नमूने के homogenate के headspace में मौजूद यौगिकों तेजी से और अलग हो रहे हैं अल्ट्रा तेज गैस क्रोमेटोग्राफी (जीसी) एक सतह ध्वनिक लहर (देखा) सेंसर के साथ युग्मित के साथ पकड़ा है. डेटा को संभालने और विश्लेषण के लिए एक प्रक्रिया पर भी चर्चा की है.
Other articles by Nathan W. Lloyd on PubMed
Measuring Local Equilibrium Flavor Distributions in SDS Solution Using Headspace Solid-phase Microextraction
Solid-phase microextraction (SPME) sampling of the headspace above an aqueous micellar solution of sodium dodecyl sulfate (SDS) was shown to be effective for quantifying the equilibrium partitioning of limonene solute between water and SDS micellar aggregates. Concentrations in the headspace were determined from the amount absorbed by the SPME fiber during 1 min extractions, with the quantity on the fiber determined using gas chromatography/mass spectrometry (GC/MS). Headspace concentrations as a function of surfactant concentration were fit to a mass balance to yield the partition coefficient and critical micelle concentration. When the total limonene in the system was low enough that it could be completely dissolved by water in the absence of micelles, a constant value for the partition coefficient of 1700 M(-1) was obtained, independent of the limonene concentration. However, at higher total limonene concentrations, the partition coefficient became a function of the amount of limonene in the micelles, as confirmed by separate experiments in which either limonene or SDS concentration was varied. The observed increase in partition coefficient with increasing limonene likely signals a shift from micelles to swollen micelles and ultimately to microemulsion droplets. The effect of SDS concentration on the aqueous solubility limit of limonene could also be observed in HS-SPME experiments where either SDS or limonene was varied.
Measuring Gas-liquid Partition Coefficients of Aroma Compounds by Solid Phase Microextraction, Sampling Either Headspace or Liquid
Hydrophobic compounds are important odorants and nutrients in foods and beverages, as well as environmental contaminants and pharmaceuticals. Factors influencing their partitioning within multi-component systems and/or from the bulk liquid phase to the air are critical for understanding aroma quality and nutrient bioavailability. The equilibrium partitioning of hydrophobic analytes between air and water was analyzed using solid phase microextraction (SPME) in the headspace (HS-SPME) and via direct immersion in the liquid (DI-SPME). The compounds studied serve as models for hydrophobic aroma compounds covering a range of air-water partition coefficients that extends over four orders of magnitude. By varying the total amount of analyte as well as the ratio of vapor to liquid in the closed, static system, the partition coefficient, K(vl), can be determined without the need for an external calibration, eliminating many potential systematic errors. K(vl) determination using DI-SPME in this manner has not been demonstrated before. There was good agreement between results determined by DI-SPME and by HS-SPME over the wide range of partitioning behavior studied. This shows that these two methods are capable of providing accurate, complementary measurements. Precision in K(vl) determination depends strongly on K(vl) magnitude and the ratio of the air and liquid phases.
Diffusion of Sodium Dodecyl Sulfate Micelles in Agarose Gels
The gradient diffusion of ionic sodium dodecyl sulfate micelles in agarose gel was investigated at moderate concentrations above the CMC. Of particular interest were the effects of micelle, gel, and sodium chloride concentration on the micelle diffusivity. Holographic interferometry was used to measure the gradient diffusion coefficient at three sodium chloride concentrations (0, 0.03, 0.10 M), three gel concentrations (0, 1, 2 wt%), and several surfactant concentrations. Time-resolved fluorescence quenching was used to measure aggregation numbers both in solution and gel. The micelle diffusivity increased linearly with surfactant concentration at the two larger sodium chloride concentrations and all gel concentrations. In general, the strength of this effect increased with decreasing sodium chloride concentration and increased with gel concentration. This behavior is evidence of decreasing micelle-micelle electrostatic interactions with increasing sodium chloride concentrations, and increasing excluded volume effects and hydrodynamic screening with increasing gel concentration, respectively. The only exception was at 0.1M sodium chloride and 2 wt% agarose, which showed a slight reduction in the slope compared to 1 wt% agarose. It was found that the concentration effect is quite strong for charged solutes: at a NaCl concentration of 0.03 M in a 2% agarose gel, in a solution with 3% SDS micelles by volume, the micelle diffusion coefficient is doubled relative to its value in the same gel at infinite dilution. The extrapolated, infinite-dilution diffusion coefficients and the rate at which the micelle diffusivity increased with surfactant concentration were compared with predictions of previously published theories in which the micelles are treated as charged, colloidal spheres and the gel as a Brinkman medium. The experimental data and theoretical predictions were in good agreement.
