Detector cells, types

FIGURE 9.2 Different detector cell types used to increase the path length for optical detection, (a) Transcapillary illumination, (b) axial-capillary illumination, (c) Z-cell capillary, (d) bubble cell capillary, and (e) rectangular capillary trans-illumination. 

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. 

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. 

If a nonstandard flow cell snch as a semi-micro flow cell with a shorter path length is nsed, a label on the outside of the detector signifying the flow cell type and path length is recommended. 

Once a series of samples is placed on the carousel, the analysis proceeds automatically with a cycle of approximately 3 minutes (including complete wash-out of the previous samples). When a representative sample flUs the two detector cells, the flow is stopped and the measurements are collected from the instruments. When stable readings are obtained, they are compared with the data on file for the sample type and then presented to the analyst for acceptance. Measurements for colour are also possible with suitable changes in the design. Figure 7.2 shows the communication hnes required and the protocols necessary to consolidate and operate the system. Such a system has been in operation on a routine basis for several years in a major fragrance and flavour company in the UK. 

In general, it can be said that, often of necessity, the detector cell may be relative large with a low aspect ratio and thus, would theoretically produce serious band dispersion. In practice the predicted dispersion is reduced by deign of the inlet and outlet tubes, as discussed above, to ensure maximum secondary flow in the cell and thus, minimize dispersion. The success of the procedure to reduce detector cell dispersion depends on the type of detector and the principle of detection. For example.it is far easier to design a low dispersion electrical conductivity cell than a low dispersion UV absorption cell. 

The detector cell was a three-electrode system consisting of a flow-through nickel working electrode, a saturated calomel reference electrode (SCE), and a stainless steel outlet tubing counter electrode. The tubular-type electrode cell housing was constructed of molded Teflon, which was machined to provide the channels and to accommodate the fittings. The working electrode area was... 

Disposable pTAS will be ideal for medical use [14]. However, the high fabrication cost of sophisticated pTAS including micropumps and microvalves is a real problem. One of the basic components of medical pTAS taking this into account is illustrated in Fig. 2. A detector cell consists of micro sensors and a 3-way microvalve is placed at the sample inlet. Flow is controlled by a suction pump and an injection pump connected to the detector cell. The calibration solution flow is also controlled by an individual pump and a 3-way valve. In this system, only sample flow reaches the detector cell. The upper parts of the system are free from contamination and corrosion so that they can be reused many times, while the detector cell has to be disposed of. To realize this system, a 3-way microvalve which can handle whole blood is indispensable. A separable channel type microvalve whose channel part is disposable while actuator part is reusable is useful for the 3-way microvalve of the detector cell [15]. Mechanically fixed stack structures including disposable parts are useful in many medical pTAS. 

The first approach uses a suppressor device which is located between the analytical column and the detector cell. This device chemically removes the mobile-phase buffer counterions, thus reducing the background conductivity. This type of detector increases postcolumn dead volume and puts... 

Concentration Versus Mass Flow Rate. The most common type of chromatographic detector is one that produces a signal that is proportional to the concentration (mass/volume) of analyte. Typical examples are given in Table 1. The concentration to which the detector responds is the concentration of an analyte in the detector cell, and therefore the volume of the detector cell is important. Generally small cell volumes are desirable,... 

The different forms of dispersion profiles that are obtained from various types of connecting tubes used in LC are shown in figure 9. These dispersion curves 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 the same length and a flow rate of 2 ml/min was employed. The peaks were recorded on a high speed... 

Two kinds of conductivity detector are distinguished contact detectors and contactless detectors. Both types were originally developed for isotachophoresis in 0.2-0.5-mm-inner diameter (i.d.) PTFE tubes. Contactless detectors are based on the measurement of high-frequency cell resistance and, as such, inversely proportional to the conductivity. The advantage is that electrodes do not make contact with the buffer solution and are, therefore, outside the electric field. As these types of detectors are difficult to miniaturize down to the usual 50-75-jU.m capillar inner diameter, their actual application in capillary electrophoresis (CE) is limited. 

Potentiometric electrodes of all types In flow-injection analysis (FIA) glass, ion-selective, amperometric electrodes, etc., can all theoretically be used in a detector cell to quantify some chemical substance. 

Relevant parameters such as the characteristics of the flow cell, type of detector and data acquisition and treatment are briefly presented here. For a further discussion of developments in instrumentation related to the detection unit, overview articles [81—83] are recommended. 

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