INDEX detection limits

ChG-based SPR detection has recently been evaluated [94-97]. The SPR peak position is extremely sensitive to refractive index variation in the sensing medium caused by presence of molecules, and the technique works by monitoring the SPR dip shift using Kretschmann angular interrogation geometry. A refractive index detection limit of 3 x 10 refractive index units (RIUs) was experimentally demonstrated in the ChG-based SPR sensing system [95]. 

Deviation refractometers are the most commonly used. This version of the DRI measures the deflection in the location of a light beam on the surface of a photodiode by the difference in refractive index between the polymer solution and pure solvent. The Fresnel-type refractometers operate on the principle that the intensity of light reflected from a glass-liquid interface is dependent on the incident angle and the RI difference between the two phases. The deviation and Fresnel detectors typically have cell volumes of 5 to 10 pi, detection limits of about 5 x 10-6 refractive index units (RIU), and a range of 10 7 to 10 3 RIU.156 The deflection-type DRI is relatively insensitive to the buildup of contaminants on the sample cell and is therefore of special utility in laboratories that process large numbers of samples, such as industrial laboratories. 

Recently Chao and Gua9 employed polymer microrings with sharp asymmetrical resonance for measuring glucose solutions. They found a detection limit of 24 mg/dl corresponding to a 3 x 10 5 index change. 

In most biochemical sensors, detection limit is a very important parameter to evaluate sensing capability of a device, which denotes the smallest detectable amount or concentration of analytes. For microring sensors, detection limit is proportional to the variation of waveguide parameters affected by occurrence of analytes the index change of the surrounding medium, 8nc, in homogeneous... 

The detection limit of the microresonator-based refractive index sensing device is directly related to the g-factor of the resonator and the sensitivity of the resonant mode discussed above. The g-factor of a microtube resonator is determined by the total loss of a resonant mode, including radiation loss, absorption loss, and surface roughness scattering loss. The overall g-factor can be expressed as... 

Supersaturation of up to nearly 4 orders of magnitude is indicated relative to a log K= 4.9 which reflects freshly precipitated HFO. When elimination of all data points which are below the detection limits for Fe(lll) and for electrode measurements, values of Eh measured agree with Eh calculated from Fe(ll/lll) determinations and speciation calculations and the revised ferrihydrite saturation index diagram looks like fig. 3. 

Several kinds of detection systems have been applied to CE [1,2,43]. Based on their specificity, they can be divided into bulk property and specific property detectors [43]. Bulk-property detectors measure the difference in a physical property of a solute relative to the background. Examples of such detectors are conductivity, refractive index, indirect methods, etc. The specific-property detectors measure a physico-chemical property, which is inherent to the solutes, e.g. UV absorption, fluorescence emission, mass spectrum, electrochemical, etc. These detectors usually minimize background signals, have wider linear ranges and are more sensitive. In Table 17.3, a general overview is given of the detection methods that are employed in CE with their detection limits (absolute and relative). 

Although AS and AES can be detected at a low UV wavelength, sensitivity is lacking and a more suitable detection was achieved using indirect photometric detection, post-column colour formation reactions, or a pre-column derivatisation, suppressed conductivity detection and refractive index detection [1,42,43]. A comparison of detection limits for the determination of these anionic surfactants shows that photometric and conductivity detectors are better (picomole or nanogram range) than refractive index or fluorometry detectors by about a factor of 1000 [40],... 

For the cationic surfactants, the available HPLC detection methods involve direct UV (for cationics with chromophores, such as benzylalkyl-dimethyl ammonium salts) or for compounds that lack UV absorbance, indirect photometry in conjunction with a post-column addition of bromophenol blue or other anionic dye [49], refractive index [50,51], conductivity detection [47,52] and fluorescence combined with postcolumn addition of the ion-pair [53] were used. These modes of detection, limited to isocratic elution, are not totally satisfactory for the separation of quaternary compounds with a wide range of molecular weights. Thus, to overcome the limitation of other detection systems, the ELS detector has been introduced as a universal detector compatible with gradient elution [45]. 

Minimum reportable concentration. The lower concentration limit for a method is usually measured by determining the detection limit. This is basically an instrument signal to noise ratio, and it does not include calibration effects. At low concentrations the calibration process often has a major adverse effect on precision. Detection limits are useful for comparing the inherent sensitivity of methods, but they are not realistic indexes of measurable concentrations in routine analysis. 

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