Detectors cell volume detection

Figure 8.4 Effect of detector cell volume on separation efficiency (A) l-/xl cell (B) 8-ptl cell. Conditions column 250 mm X 1.5 mm I.D. Qg (5 fixn) mobile phase acetonitrile (90%)/ water (10%) flow rate, 100 /d/min injection volume, 1 /it detection, UV absorbance at 250 nm. Peaks 1, benzene 2, naphthalene 3, biphenyl 4, fluorene 5, anthracene. (Reprinted from Ref. 1 with permission.)...

The commonly used detector-cell volumes, though much smaller than required to minimize band broadening in conventional LC, render flow-cell FTIR detection compatible with miniaturized LC systems. In microbore-LC chromatographic peak volumes are several orders of magnitude smaller and peak concentrations are higher than in conventional LC. However, the sample capacity (both in mass and volume) of LC columns decreases in proportion to their cross-sectional area and the advantage of microbore-LC therefore is only partly fulfilled. 

The reduction of dimensions also reduces volumes which are accessible to the detector. Thus, detection principles related to geometric dimensions of the detector cell ai e not ideally suited for coupling to microsystems, whereas surface sensitive principles, such as electrochemical methods or optical methods utilizing the evanescent field of a waveguide, or methods which can be focussed on a small amount of liquid, such as electrochemiluminescence (ECE), ai e better suited. This is why electrochemiluminescence detectors ai e combined to microsystems. Moreover ECE has found wide applications in biochemistry because of its high sensitivity, relatively simplicity and feasibility under mild conditions. 

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. 

The detection of the migrating sample boundary in CE can be accomplished by UV, fluorescent, electrochemical, radiochemical, conductivity, and mass spectrometry (MS) means. The use of high-sensitivity detection systems is always a key issue in CE applications. The sensitivity of HPCE detectors may be at least 2 to 3 orders of magnitude better than that of HPLC detectors. Since the detection cell volume is very small, the concentration sensitivity... 

Hyphenation of chromatographic separation techniques (SFC, HPLC, SEC) with NMR spectroscopy as a universal detector is one of the most powerful and time-saving new methods for separation and structural elucidation of unknown compounds and molecular compositions of mixtures [171]. Most of the routinely used NMR flow-cells have detection volumes between 40... 

Consideration should be given to the flow rate of the sample through the detection cell. Shultz and co-workers have demonstrated the wide variability in reaction kinetics between ECL reactions, and hence the influence of flow rate on ECL intensity [60], For example, the rate constants (k) of the Ru(bpy)32+ ECL reactions of oxalate, tripropylamine, and proline were calculated to be 1.482, 0.071, and 0.011/s, respectively. Maximum ECL emission was obtained at low linear velocities for slow reactions ranging up to high linear velocities for fast reactions. That is, the flow rate and flow cell volume should be optimized such that the light-emitting species produced is still resident within the flow cell, in view of the light detector, when emission occurs. 

A spiral coil of Teflon, or a spiral groove on a stainless steel surface covered with quartz, is used as a flow cell in the CL detector. These cells are placed in front of a photomultiplier tube (PMT), which detects photons emitted by the CL reaction. The cell volumes are generally in the range of 60-120 J.L. 

Detection Inlet heat exchangers Flow cell volume and geometry MS ion sources Sprayers (e.g., in evaporative light scattering detectors) Data filtering effects in high speed applications... 

The detection cell must be designed with its volume small enough to prevent additional peak broadening in the detector in practice, this means the detection cell volume should be at least 10 times smaller than the volume of the first, most narrow, chromatographic peak. For nano-flow HPLC systems, in which the peaks can be sub-microliter, detection cells with a volume on the order of 100 uL or less are appropriate. For conventional HPLC systems, in which the peak volume is tens of microliters, there is no benefit to having a detection volume smaller than a few microliters, and indeed most conventional HPLC flow cells are around 5-12 pL. It is best to ensure that the detection cell is well matched to the sample and peak volumes, as making the detection volume too small will unnecessarily decrease the sensitivity of the detector. 

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