Detectors, characteristics

Table 9.1 gives a summary of some important detector characteristics. 

The pulse height can be computed if the capacitance, detector characteristics, and radiation are known. The capacitance is normally about 10 4 farads. The number of ionizing events may be calculated if the detector size and specific ionization, or range of the charged particle, are known. The only variable is the gas amplification factor that is dependent on applied voltage. 

VDU screen via suitable electronic amplifying circuitry where the data are presented in the form of an elution profile. Although there are a dozen or more types of detector available for gas chromatography, only those based on thermal conductivity, flame ionization, electron-capture and perhaps flame emission and electrolytic conductivity are widely used. The interfacing of gas chromatographs with infrared and mass spectrometers, so-called hyphenated techniques, is described on p. 114 etseq. Some detector characteristics are summarized in Table 4.11. 

Fluorescence detection can be up to four orders of magnitude more sensitive than UV absorbance, especially where laser induced excitation is used, mass detection limits being as low as 10-20—10 21 mole. Pre- and post-column derivatization methods are being developed to extend the applicability of fluorescence detection to non-fluorescent substances. Several types of electrochemical and mass spectrometric detector have also been designed. Detector characteristics are summarized in Table 4.21. 

Table 30.8, provides a comprehensive comparison of various typical detector characteristics invariably used in HPLC, such as response, concentration expressed in g ml 1 and the linear range. However, the linear range usually refers to the range over which the response is essentially linear. It is mostly expressed as the factor by which the lowest factor (i.e., Cn) should be multiplied in order to obtain the highest concentration. 

Table 30.2 Pattern of Typical Detector Characteristics in HPLC...

Concerning the requirements of the detector, it is important to stress that interfacing a detector with an FIA system yields transient signals. Therefore, desirable detector characteristics include fast response, small dead volume and low memory effects. FI methods have been developed for UV and visible absorption spectrophotometry, molecular luminescence and a variety of electrochemical techniques and also for the most used atomic spectrometric techniques. 

Detector Characteristics. The applied potential that produced the largest analytical signal was 0.56 V versus SCE. The decrease in analytical signal at potentials more positive than 0.56 V suggested a decrease in the active surface area of the electrode due to competitive solvent oxidation at the active sites. 

Ge(Li) Detector Characteristics. Resolution measurements for the 18-cm.8 Ge(Li) detector were made with the anticoincidence shield in the inoperative mode, with a normal operating bias of 1700 volts, and with a preamplifier designed in our Laboratory (3, 4), and operated in conjunction with a Tennelec TC-200 linear amplifier. Resolution at 1.33 M.e.v. was 2.62 k.e.v., FWHM (Figure 4). The electronic pulser resolution for the amplifier system at a slightly lower energy was 1.86 k.e.v., the total capacitance of the detector was 28 pF, the noise slope was 0.035 k.e.v./pF, and the leakage current at 1700 volts was 0.5 X 10"9 amp. 

The detector used to sense and quantify the effluent provides the specificity and sensitivity for the analytical procedure. Table 1 summarizes significant detector characteristics. 

The most important detector characteristic is the signal it produces, of course, and that topic is treated throughout this chapter. In this section we will define two other important characteristics, noise and time constant. 

The level of noise restricts the minimum signal that can be detected and attributed to an analyte, so it is important to keep it to a minimum. A detector characteristic that is often more meaningful than the noise is the ratio of the signal-to-noise, SIN. In most chromatographic work it is... 

The two detector characteristics just discussed, noise and time constant, are important detector specifications too. However, this section will focus on the detector signal and its relationship to quantitative analysis. There are slight but significant differences between the specifications for concentration and mass flow rate types of detectors that will necessitate some duplication in the discussion. Recall that the concentration detector signal is proportional to concentration (e.g., g/mL) and the mass flow rate detector to mass flow (e.g., g/sec). 

Another factor to be taken into account is the degree of over determination, or the ratio between the number of observations and the number of variable parameters in the least-squares problem. The number of observations depends on many factors, such as the X-ray wavelength, crystal quality and size, X-ray flux, temperature and experimental details like counting time, crystal alignment and detector characteristics. The number of parameters is likewise not fixed by the size of the asymmetric unit only and can be manipulated in many ways, like adding parameters to describe complicated modes of atomic displacements from their equilibrium positions. Estimated standard deviations on derived bond parameters are obtained from the least-squares covariance matrix as a measure of internal consistency. These quantities do not relate to the absolute values of bond lengths or angles since no physical factors feature in their derivation. 

Accurate performance criteria or specifications must be available to determine the suitability of a detector for a specific application. This is necessary, not only to compare its performance with alternatives supplied by other instrument manufactures, but also to determine the optimum chromatography system with which it must be used to achieve the maximum efficiency. The specifications should be presented in a standard form and in standard units, so that detectors that function on widely different principles can be compared. The major detector characteristics that fulfill these requirements together with the units in which they are measured are summarized in table 1. 

For reasons of expediency, the figures are taken exclusively from those at my disposal—i.e., from our own laboratory. No doubt several of the original articles found in the literature— and I have tried to cite most of them—contain better illustrations of the detector characteristics discussed. 

The instrument factor K includes conditions of primary sources, the geometrical arrangement of specimen respect to radiation and detection, and detector characteristics. The matrix factor refers to the interactions among the elements in the specimen. 

For a modern XRF equipped with a powerful computer system, the fundamental parameter method (FP method) is most widely used for quantitative analysis. The method determines the concentration of an element when its theoretical intensity matches its measured intensity. The fluorescence X-ray intensity of a given composition can be calculated using theoretical formulas with given specimen physical and instrumental parameters. The physical parameters include specimen density, thickness, X-ray absorption coefficients and fluorescence yield. The instrumental parameters include excitation voltage of the X-ray tube, optical geometry and detector characteristics. 

The spatial response characteristics are normally characterized by the point spread function (PSF) - the detector s signal following a delta function stimulus. Ideally the point spread is also a delta function. Experimentally this is seldom the case as detector characteristics generally give the signal a Gaussian spread. It is often the point spread function that is the main cause of the limited resolution in powder diffraction experiments. 

Stable set-up and accurate incident intensity monitoring. The data are normalized by incident intensity. It is important that the incident beam and detector characteristics do not change in an uncontrolled way during the experiment, or that this can be corrected, e.g. by monitoring the incident beam intensity as is done at synchrotron X-ray and spallation neutron measurements. 

---