Detectors, characteristics linear range

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. 

The detectors utilized for HPLC are designed to respond to the solute being eluted. HPLC detectors can be classified into two broad categories universal and selective. Selective detectors respond to some physicochemical property of the solute, while universal detectors respond to aU solutes independent of their physicochemical properties. The ideal detector would be highly universal and highly sensitive, have a wide linear range, and not be affected by change in temperature or mobile phase composition. Commercially available detectors possess some of these characteristics but not aU. 

The methods of quantitation and the criteria for precise and accurate determination for LC are similar to those used in GC, though there are a number of important differences. External standard calibration—i.e. where the detector response to a solution of known concentration is measured and then a calibration curve is constructed—is the recommended method for quantitation in LC. It is imperative that the linearity of detector response is confirmed over the concentration range of interest with standards prepared in a matrix similar to the sample. Table 6.2 details detector characteristics. The increased precision obtained compared to GC is attributable to the... 

Detector Minimum detectable amount (gs"Y Minimum detectable amount on-column Linear range Selectivity Other characteristics Main application... 

Detector Linearity. The concentration range over which the detector response is linear. Over its linear range the response factor of a detector (peak area units per weight of sample) is constant. The linear range is characteristic of the detector. 

These characteristics are illustrated in the mass spectrum shown in Fig. 5.3. Lanthanum at 1 mg 1 yields nearly 10 counts s , which is near the upper end of the linear range of the detector. Peaks are observed for La " " and LaO+, but they are small relative to La". The low abundance of LaO" shows that even refractory rare earth oxides are dissociated efficiently (but not H completely). Most elements yield much lower levels of M or MO than shown for La in Fig. 5.3. The two isotope peaks for Eu illustrate the isotopic capability of the technique. Finally, the total signal for the two isotopes of Eu is comparable to the signal for La" because these two elements are ionized to nearly the same extent in the ICP and because they are extracted and detected with similar efficiency. 

Detectors are usually conpued in terns of their operational characteristics defined by the nininvin detectable quantity of standards, the selectivity response ratio between standards of different conpositlon or structure, and the range of the linear portion of the detector-response calibration curve. These terns are wid. y used to neasure the perfomance of different chronatographic detectors and were fomally defined in section 1.8.1. 

Solute property detectors, such as spectroscopic andj electrochemical detectors, respond to a physical or chemical] property characteristic of the solute which, ideally, is] independent of the mobile phase. Althou this criterion is rarely met in practice, the signal discrimination is usually sufficient to permit operation with solvent changes (e.g., flow programming, gradient elution, etc.) and to provide high sensitivity with aj wide linear response range. Table 5.4. Solute-specific detectors complement ulk property detectors as they provide high ... 

The purpose of a detector is to monitor the carrier gas as it emerges from the column and respond to changes in its composition as solutes are eluted. Ideally a detector should have the following characteristics rapid response to the presence of a solute a wide range of linear response high sensitivity stability of operation. 

The ideal HPLC detector should have the same characteristics as those required for GC detectors, i.e. rapid and reproducible response to solutes, a wide range of linear response, high sensitivity and stability of operation. No truly universal HPLC detector has yet been developed but the two most widely applicable types are those based on the absorption of UV or visible radiation by the solute species and those which monitor refractive index differences between solutes dissolved in the mobile phase and the pure mobile phase. Other detectors which are more selective in their response rely on such solute properties as fluorescence, electrical conductivity, diffusion currents (amperometric) and radioactivity. The characteristics of the various types of detector are summarized in Table 4.14. 

First, the availability of high optical throughput in an FT-NlR analyzer means that lower-cost, more robust and also more linear DTGS detectors can be used routinely. These in turn allow (through the choice of suitable beamsplitter materials and source characteristics) a wider range of operation than the conventional... 

Atomic emission spectroscopy can be employed, generally with an inductively coupled plasma for thermal excitation. The sample is introduced into the plasma as a mist of ultrafine droplets, and the monochromator and detector are set to measure the intensity of an atomic emission line characteristic of the element. This technique is powerful, general, sensitive, linear, and able to measure over 70 elements, and, as a result, is widely used. Response is typically linear over four orders of magnitude in concentration with relative standard deviations of 1 to 3%. In low-salt aqueous solutions, detection limits range from 10 to 1000 nanomolar without preconcentration. Significant problems with saline samples remain, but use of Babington nebulizers alleviates these problems somewhat. 

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