Detector cell

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. 

The primary reference method used for measuring carbon monoxide in the United States is based on nondispersive infrared (NDIR) photometry (1, 2). The principle involved is the preferential absorption of infrared radiation by carbon monoxide. Figure 14-1 is a schematic representation of an NDIR analyzer. The analyzer has a hot filament source of infrared radiation, a chopper, a sample cell, reference cell, and a detector. The reference cell is filled with a non-infrared-absorbing gas, and the sample cell is continuously flushed with ambient air containing an unknown amount of CO. The detector cell is divided into two compartments by a flexible membrane, with each compartment filled with CO. Movement of the membrane causes a change in electrical capacitance in a control circuit whose signal is processed and fed to a recorder. 

Equilibrium is assumed to occur in the detector cell, so the introduction of the mass of solute (dm) will result in a change in concentration of solute in the mobile phase and adsorbent of (dXm) and (dXs), respectively, where (Xs) is the concentration of... 

The twenty curves shown in Figure 24 are graphs of (0) versus (v) together with the integral of (0) versus (v) for different values of (Ca) and (( )). They are all normalized to the same peak height. The curves include the practical range of heat loss factors that might be expected from an heat of adsorption detector cell. 

Under conditions where the capacity of the detector cell is less than the plate capacity of the column (Casignal dominates. Conversely,... 

The Golay equation [9] for open tubular columns has been discussed in the previous chapter. It differs from the other equations by the absence of a multi-path term that can only be present in packed columns. The Golay equation can also be used to examine the dispersion that takes place in connecting tubes, detector cells and other sources of extra-column dispersion. Extra-column dispersion will be considered in another chapter but the use of the Golay equation for this purpose will be briefly considered here. Reiterating the Golay equation from the previous chapter. 

The peaks shown were obtained using a low dispersion UV detector (cell volume, 1.4 pi) in conjunction with a sample valve with a 1 pi internal loop. All tubes were of... 

Small particle size resins provide higher resolution, as demonstrated in Fig. 4.41. Low molecular weight polystyrene standards are better separated on a GIOOOHxl column packed with 5 /u,m resin than a GlOOOHg column packed with 10 /Ltm resin when compared in the same analysis time. Therefore, smaller particle size resins generally attain a better required resolution in a shorter time. In this context, SuperH columns are best, and Hhr and Hxl columns are second best. Most analyses have been carried out on these three series of H type columns. However, the performance of columns packed with smaller particle size resins is susceptible to some experimental conditions such as the sample concentration of solution, injection volume, and detector cell volume. They must be kept as low as possible to obtain the maximum resolution. Chain scissions of polymer molecules are also easier to occur in columns packed with smaller particle size resins. The flow rate should be kept low in order to prevent this problem, particularly in the analyses of high molecular weight polymers. 

The contribution of the equipment between injection unit and detector cell should be negligable in relation to the column for a sufficient column characterization short connections with narrow capillaries and zero dead volume unions are the precondition for reliable plate numbers. Every end fitting of a column causes additional band broadening. In the past a column type was offered that could be directly combined without any capillary links unfortunately, it has disappeared from the market. 

The detector cells normally are connected by a capillary. For the interpretation of the detector signals the volume of this connection must be known data of the following detectors have to be shifted for the delay toward the first cell. Usually this dead volume cannot be measured immediately, as it has to be determined indirectly by test analyses, e.g.,... 

These combined HDF and GPC separations require the use of detectors such as static light scattering or viscometers to help sort out the convoluted elution profiles seen in those type of experiments. It should also be remembered in these situations that the typical refractive index or ultraviolet detector responses may not be representative of the actual mass fraction of insolubles eluting from the column because of the significant light scattering that can occur with those large particles in the detector cell. 

It must be pointed out that (Vm) refers to the volume of mobile phase in the column and not the total volume of mobile phase between the injection valve and the detector (Vo)- In practice, the dead volume (Vo) will include all the extra column volumes (Ve) involved in the sample valve, connecting tube detector cell etc. 

The electrical conductivity detector is probably the second most commonly used in LC. By its nature, it can only detect those substances that ionize and, consequently, is used frequently in the analysis of inorganic acids, bases and salts. It has also found particular use in the detection of those ionic materials that are frequently required in environmental studies and in biotechnology applications. The detection system is the simplest of all the detectors and consists only of two electrodes situated in a suitable detector cell. An example of an electrical conductivity detector sensing cell is shown in figure 13. 

In using the metal lamps, the monochromator setting on the Schoeffel 970 was adjusted to give a maximum signal when a 10 H solution of compound I was in the detector cell. 

In all cases, low volume detector cells ( 8 pi) ere employed in order to limit extra band broadening. 

Equation (2) is true in the case where there is no instrumental spreading, that is, where all particles present in the detector cell are of exactly the same diameter. Because of axial dispersion, and skewing caused by entrapment or adsorption, corrections for instrumental spreading are required. In this case x v) is described by ... 

The recorder response F(v) is related to the absorbance of the detector cell contents A (v) by... 

When the more general Mie scattering theory is applied the approach adopted in deriving the previous fomulae cannot he used. One is, however, able to derive an analytical expression for the moments of the size distribution within the detector cell. They are given as ... 

This information together with l(o,v) which represents the number of particles in the detector cell at retention volume v, leads to the evaluation of overall sample diameter averages. 

Because of chromatographic dispersion, the sample fraction in the detector cell is polydisperse. The weight-average and number-average molecular weights of the polydisperse fraction are calculated as... 

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