Universal detector response

The FID has wide applicability, being a very nearly universal detector for gas chromatography of organic compounds, and this, coupled with its high sensitivity, stability, fast response and wide linear response range ( — 107), has made it the most popular detector in current use.70... 

The first classification is based on the nature of the detector response. Table 4.7 ranks several chromatographic detectors as specific and nonspecific. A nonspecific or universal detector responds to all solutes present in the mobile phase and this performance makes it a... 

Bulk property detectors function by measuring some bulk physical property of the mobile phase, e.g., thermal conductivity or refractive index. As a bulk property is being measured, the detector responses are very susceptible to changes in the mobile phase composition or temperature these devices cannot be used for gradient elution in LC. They are also very sensitive to the operating conditions of the chromatograph (pressure, flow-rate) [31]. Detectors such as TCD, while approaching universality in detection, suffer from limited sensitivity and inability to characterise eluate species. 

ELSD Quasi-universal No dependence on eluent conditions Droplet size control Moderate sensitivity (low ng) Compound-dependent and non-linear detector response [31,51-53]... 

The selectivity universal detectors exhibit a response to every sample (except the mobile phase), whatever the chemical species. Selective detectors respond to samples that exhibit a physical or chemical property (UV absorbance, for example). In this mode the mobile phase should not interfere. 

The assumption for Eq. (1) is that the ELSD peak area is directly proportional to mass. For compounds of widely varying structures, charges, or vapor pressure, or for varying mobile phase compositions (e.g., gradient HPLC), the ELSD response can vary markedly (16-22). Thus, Eq. (1) is limited in its applicability. Similarly, mass spectral (MS) detectors are universal, but the response per unit weight depends greatly on the ionization type (e.g., electrospray, fast-atom bombardment, etc.) and on the ionization efficiency of the analyte. Refractive index is another universal detector, but it too suffers from variability in response depending on the mobile phase... 

Individual compound identification in all GC methods with the exception of GC/MS relies on the compound retention time and the response from a selective or non-selective detector. There is always a degree of uncertainty in a compound s identity and quantity, particularly when non-selective detectors are used or when the sample matrix contains interfering chemicals. To reduce this uncertainty, confirmation with a second column or a second detector is necessary. Analyses conducted with universal detectors (mass spectrometer or diode array) do not require confirmation, as they provide highly reliable compound identification. 

The detector converts a change in the column effluent into an electrical signal that is recorded by the data system. Detectors are classified as selective or universal depending on the property measured. Selective (solute property) detectors, such as fluorescence detectors, measure a physical or chemical property that is characteristic of the solute(s) in the mixture only those components which possess that characteristic will be detected. Universal (bulk property) detectors measure a physical property of the eluent. Thus, with refractive index (RI) detectors, for example, all the solutes which possess a refractive index different from that of the eluent will be detected. Selective detectors tend to be more sensitive than universal detectors, and they are much more widely used. Universal detectors are more commonly used in preparative chromatography, where a universal response is desired and sample size is large. 

If a solute of interest does not contain a chromophore, it may be detected by indirect UV detection. Indirect detection is a technically simple and sensitive method for the detection of compounds with little inherent detector response. Indirect UV detection is a nondestructive technique, in which the analyte is characterized in native form. Indirect detection is a universal detection mode, with few requirements as to the exact nature of the analyte. The properties of indirect detection have been reviewed by Yeung.22 Indirect detection is particularly attractive for the analysis of biological compounds. Optical systems are the same for direct or indirect detection the only difference is that, in indirect detection, the mobile phase, rather than the analyte, contains a UV chromophore. 

In general, the RI detector is not very sensitive, and it cannot be used in gradient elution. Both positive and negative peaks can appear, depending on their refractive indices relative to the mobile phase, and this is considered somewhat disadvantageous. However, it is universal in response, which accounts for its popularity, especially in SEC. 

Some detectors are not compatible with gradient elution, such as the electrochemical detector or the refractometric detector. The latter one is a universal detector, which gives a response for almost all sample compounds, but also for the mobile phase components. The only universal detector that can be used for gradient elution is the evaporative light-scattering (ELS) detector, but it is approximately two orders of magnitude less... 

The application of dual detection [UV and refractive index (RI)] to the SEC analysis of polystyrene-poly(methyl methacrylate) (PS-PMMA) has already been studied in this laboratory (2). Both MWD and CCD were determined using a methodology outlined by Runyon et al. (3). This approach relies on SEC column calibration with narrow polydis-persity standards for each of the homopolymers as well as a measure of the detector response factors for each homopolymer to produce a copolymer MWD. In the case of PS and PMMA this is feasible, but in other block copolymer systems the availability of suitable molecular weight standards may be more limited. In addition, this procedure does rely on true SEC and is not valid for block copolymers for which the universal calibration does not hold true for both blocks in a given solvent system. 

In contrast to component-specific detectors, such as ultraviolet (UV) absorbance and fluorescence, conductivity detection is a universal detection method. This means that a bulk property (conductivity) of the buffer solution is continuously measured. A migrating ionic component locally changes the conductivity and this change is monitored. As such, conductivity detection is universally sensitive because, in principle, all migrating ionic compounds show detector response, although not to the same extent. 

Detectors may be classified on the basis of selectivity. A universal detector responds to all compounds in the mobile phase except carrier gas. A selective detector responds only to a related group of substances, and a specific detector responds to a single chemical compound. Most common GC detectors fall into the selective designation. Examples include flame ionization detector (FID), ECD, flame photometric detector (FPD), and thermoionic ionization detector. The common GC detector that has a truly universal response is the thermal conductivity detector (TCD). Mass spectrometer is another commercial detector with either universal or quasi-universal response capabilities. 

The identification of the chemical forms of an element has become an important and challenging research area in environmental and biomedical studies. Two complementary techniques are necessary for trace element speciation. One provides an efficient and reliable separation procedure, and the other provides adequate detection and quantitation [4]. In its various analytical manifestations, chromatography is a powerful tool for the separation of a vast variety of chemical species. Some popular chromatographic detectors, such flame ionization (FID) and thermal conductivity (TCD) detectors are bulk-property detectors, responding to changes produced by eluates in a characteristic mobile-phase physical property [5]. These detectors are effectively universal, but they provide little specific information about the nature of the separated chemical species. Atomic spectroscopy offers the possibility of selectively detecting a wide rang of metals and nonmetals. The use of detectors responsive only to selected elements in a multicomponent mixture drastically reduces the constraints placed on the separation step, as only those components in the mixture which contain the element of interest will be detected... 

In conclusion, an ELSD with SFC provides a sensitive analytical tool for qualitative and quantitative analysis of solutes. Detection depends only on the solute being less volatile than the least volatile mobile-phase component. Detection is independent of the basicity or presence of a chromophore for a given solute. The detector response is a logarithmic function of the mass of the solute. The SFC-ELSD combination should be considered whenever a universal high-throughput analysis is needed. 

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