International Sensitivity volume

The amperometric detector is currently the most widely used electrochemical detector, having the advantages of high sensitivity and very small internal cell volume. Three electrodes are used ... 

Micro-injections in micro-flow and nano-flow systems are done with injectors in which the external sample loop is replaced with the internal fixed volume within the injector body. HPLC-on-a-chip systems also build the column into the injector body. The internal path within the injector body is abladed with a laser, packed with micro-packing material, and this serves as the separating media. The injector inlet is connected to the pumping system and the outlet to the detector. Sample is loaded into an internal loop in the load position, then injected onto the chip HPLC by turning the injector. Obviously, in a system like this sample size is very limited and the detector is usually a highly sensitive mass spectrometer. 

Disadvantages of microbore columns are that the increase in sensitivity is partially negated by the smaller injection volume that can be accommodated (although volumes up to 10 xl can sometimes be injected onto 0.3 mm columns), internal dead volumes have to be minimized, and there is increased experimental difficulty. Nano/LC/ESI/MS is more applicable to the analysis of very small... 

Two physically reasonable but quite different models have been used to describe the internal motions of lipid molecules observed by neutron scattering. In the first the protons are assumed to undergo diffusion in a sphere [63]. The radius of the sphere is allowed to be different for different protons. Although the results do not seem to be sensitive to the details of the variation in the sphere radii, it is necessary to have a range of sphere volumes, with the largest volume for methylene groups near the ends of the hydrocarbon chains in the middle of the bilayer and the smallest for the methylenes at the tops of the chains, closest to the bilayer surface. This is consistent with the behavior of the carbon-deuterium order parameters,. S cd, measured by deuterium NMR ... 

Column internal diameter Volumetric flow rate Injection volume UV-deteaor cell volume Sensitivity improvement... 

Radioisotope-labeled nitrosamines have proven valuable in development of analytical methods and for demonstrating efficiency of recovery of nitrosamines from tobacco products and smoke (37-39). The very high specific activity required for low part-per-billion determinations has discouraged most analysts from using this approach. Unless a radiochromatographic detector with adequate sensitivity is available, samples must be counted independently of the final chromatographic determination, and one of the advantages of internal standardization, correction for variation in volume injected, is lost. 

Although cSFC shows relatively poor figures of merit (speed, sensitivity, detection dynamic range and sample capacity) as well as a limited application area, its applications tend to be unique. These include solutes that can be solvated with pure SCCO2 and quantified with FID. Linear density programs typical in cSFC are ideal for homologous series found in surfactants, many prepolymers, etc. Selectivity in cSFC, which can be achieved by mobile phase density and temperature programming, relies on selective interactions with the stationary phase. Quantitative analysis in cSFC may be rendered difficult by small injected volumes the use of internal standards is recommended. 

One type of ec detector (the coulometric detector) reacts all of the electroactive solute passing through it. This type has never become very popular (there is only one on the market at the moment). Another type (the amperometric detector) reacts a much smaller quantity of the solute, less than 1%. The currents observed with these detectors are very small (nanoamps), but such currents are not too difficult to measure and the detector has a high sensitivity, considerably higher than that of uv/visible absorbance detectors although not as good as fluorescence detectors. Noise equivalent concentrations of about 10, 0g cm-3 have been obtained in favourable cases. Another advantage of these detectors is that they can be made with a very small internal volume. 

By replacing conventional 3.5 or 5 jtm columns with sub-2-micron columns, gradient time can be reduced dramatically. The flow rate must be increased for optimal conditions as well but solvent consumption will be less than the amount used by the original method. To use the full power of these columns, an LC instrument must be thoroughly optimized toward lowest extra-column dispersion. The smaller the column (small ID and short length), the more sensitive the performance is to dispersion. With smaller internal diameter columns, the injection volumes and internal diameters of the capillaries should be reduced. 

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