Non-specific detector

Interference occurs when compounds co-elute with the analytes and are not detected directly by a specific detector. The effect is to create negative peaks or an erratic response for the analyte. This problem can be identified by using a non-specific detector such as an ion trap MS detector, an MS in the electron impact ionization mode, or a flame ionization GC detector. 

If we were to use two detectors, one specific for only A and one specific for only B, the signals would appear, shifted In time, as shown In Figure 6a. The position of the Injection valve at the time these detector values were recorded Is also shown. Similarly, If we were to use on non-specific detector, the sum of the signals due to A and B would yield the detector signal labeled In Figure 6a as Combined Signal. 

A non-specific detector responds to all solutes present in the mobile phase and its catholic performance makes it a very useful and popular type of detector. Unfortunately, non-specific detectors in LC tend to be relatively insensitive. The FID is a nonspecific detector in that (with very few exceptions) it responds to all solutes that contain carbon. As an added advantage the FID also has a very high sensitivity. In LC the refractive index detector is probably the most non-specific detector but as already mentioned it also has the least sensitivity of the commonly used detectors. 

In contrast, the electrochemical detector responds only to substances that can be oxidized or reduced and thus, providing the mobile phase is free of such materials, it will only detect oxidizable or reducible substances when they are eluted. It follows that this detector is not only a solute property detector but is also a specific detector. The electrical conductivity detector is a non-specific detector and used widely in ion chromatography where it occupies a unique and almost exclusive position. In contrast, the electrochemical detector, in its... 

The tandem mass spectrometer also can operate as a relatively non specific detector by, for example, monitoring the neutral loss of 46 mass units (HCOOH) when using ammonium formate buffer ionization or 60 mass units (CH3COOH) when using acetate buffer ionization. In Figure 3, naphthol is detected in such a maimer by monitoring the 189 —> 143 neutral loss. 

The specificity or the selectivity guarantees that the method is really measuring the substance of interest. Interferences of all types affect the specificity of several methods e.g. chromatographic separations with non-specific detectors such as FID or ECD, etc. 

In this step of the validation it is investigated whether the matrix influences the signal of the detector. The study concerns either an extract or a digest this new matrix may influence parameters defined in the previous step. All the conclusions obtained in the first step have to be verified (calibration, linearity, chromatographic conditions and performance, internal standard etc.). For the determination of trace organic contaminants this step is of great importance as it has to assure that no interfering compounds remain because quantification is often performed with non specific detectors (e.g. ECD, FID,... 

The main criterion for the detector is that it gives a response specifically for tin, rather than the compound. This is because of the detection limits required for environmental analysis, which are not attainable with non-specific detectors. The most popular detection methods for organotin analysis are FPD using a tin selective filter, MS, MIP-AES, and ICP-MS. Recently, HPLC-MS methods based on atmospheric ionization including electrospray, ion spray, or chemical ionization have been used to detect the organotin species. These methods have the advantage that molecular information concerning the analyte is available, rather than just atomic information. 

Pore sizes of columns for SEC must match the sizes of dissolved macromolecules. The column also must reliably sort the analyzed macromolecules with lowest molar mass from the molecules of initial solvent and the low-molecular substances present in sample. The latter substances - that is various polymer additives, as well as humidity and gases, mainly oxygen dissolved in sample solution - are recognized with the non-specific detector such as differential refractometer, and the resulting peaks could inferfere with the peaks of sample. 

The detection problems. Due to preferential solvation of mactomolecules in mixed solvent (see section 11.2.4), the composition of the bulk solvent among polymer chains differs from the composition of the original sample solvent, which is mobile phase. In LC CC, macromolecules elute together or in the vicinity of their bulk solverrt. This prevents application of the non-specific detectors such as differential refractometers (see sections 11.6.1.4 and 11.7.3.2), which sensitively respond to changes in... 

The second alternative classification is to define detectors as specific and non-specific detectors. In this sense a specific detector would be exemplified by the fluorescence detector as it detects only those substances that fluoresce. An example of a non-specific detector would be the refractive index detector that detects all substances that have a refractive index different from that of the mobile phase. The classification of detectors as specific and non-specific is acceptable, but in this book detectors will be classified as bulk property detectors and solute property detectors as it more closely associates the detector with its basic method of measurement. 

The slow development of LC from the time of Tswett, to the late 1950 s was entirely due to the lack of high sensitivity on-line detectors. Since, the inception of effective LC detectors there has been a continuous synergistic interaction between column development and detector development which has resulted in the present highly sophisticated LC systems of today. There are a number of ways of classifying LC detectors, specific and non-specific detectors, mass and concentration sensitive detectors and finally bulk property and solute property detectors. The classification of detectors as bulk property and solute property detectors is recommended. Bulk property detectors respond to a change in some overall property of the eluent such as refractive index or dielectric constant whereas solute property detectors respond to some property that is unique to the solute alone. In practice solute property detectors are rarely ideal and many respond, at least weakly, to the same property of the mobile phase as well as the solute. 

Not all compounds detected Use of a non-specific detector (RI) Use of a different eluent... 

The past few years have seen the introduction of various new non-specific detectors attached to liquid columns. For example, a chain or wire is moved and drops of the effluent are deposited onto it. The volatile mobile phase solvent is then evaporated and the residue converted to gas form by pyrolysis standard GC detectors can then be used and their signals recorded. 

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