Other Detectors

Many other detectors have been used to monitor ion chromatography separations. Most of these detectors have been used only in special cases. Flame photometric detection [76, 77] has been used to detect alkaU, alkaline earth, and some rare earth metals. Atomic absorption (AA) detectors [78-80] have been used for arsenite, arsenate, monomethyl arsenate, dimethyl arsinate, and p-aminopheno-arsenate separations. Detectors of this type can be extremely sensitive, detecting arsenic down to 10 ng mL .  

Research into the development of environmental methods might consider the use of AA detection. Perhaps some of the more interesting detectors are those that use inductively coupled plasma (ICP) as an energy source and either atomic emission (AE) or mass spectrometry (MS) as the detector. ICP-AE and ICP-MS are well-developed analytical tools. One of the major advantages of these techniques is that a mixture of metals can be analyzed without the need for separation. Thus, workers who use these instruments normally do not think about their use as detectors. However, ICP-AE and ICP-MS cannot determine the oxidation or chemical state of a particular metal ion (Chapter 13). Some samples are quite important from a toxicological and environmental standpoint since the toxicity of a metal may depend on its oxidation state. For example, Cr(IIl) is not toxic (and even considered an essential element), but Cr(VI) is extremely toxic. An inductively coupled plasma atomic emission [81] detector was used to detect rare earth metals. 

Zemann, K. Mayrhofer, E. Schnell. and G. K. Bonn. Contactless conductivity detector for capillary electrophoresis. Book of Abstracts. 216 ACS National Meeting, Boston. August 23-27, ANYLO-75,1998. 

Haddad. Miniaturized movable contactless conductivity detection cell for capillary electrophoresis. Electrophoresis 24, 2144, 2003. 

Pietrzyk, Comparison of reversed stationary phases for the chromatographic separation of inorganic analytes using hydrophobic ion mobile phase additives,/. liq. Chromatogr., 7, 1935, 1984. 

Flammable liquid detectors should be located anywhere a mixture could exceed 25% of the lower flammable limit. 

A variety of other detectors have been used in HPLC. The most popular is the refractive index (RI) detector, although its use has declined during the nineties. The RI detectors are universal and nondestructive they operate by responding to a change in the refractive index of a solvent caused by the presence of a solute in the column effluent. Such detectors are usually used in preparative LC or gel-permeation chromatography, where sensitivity is less important. In addition, they are stable and easy to operate. However, RI detectors are not suited to gradient elution, and temperature control of the column effluent and detector are critical. 

Chemiluminescence is a very sensitive and selective technique. Reagent types, analytes, and detection limits have been summarized in a review by Imai.56 Chemiluminescence has been applied to the analysis of compounds that exhibit low UV absorbance, including metal ions, amino acids, fatty acids, and bile acids. Other detectors include detectors for radioactivity, nuclear magnetic resonance (NMR), and surface-enhanced Raman spectroscopy. Radioactivity detection is one of the most selective detectors, as only components that have been radiolabeled will be detected. The interface of NMR with HPLC and has been discussed in detail by Grenier-Loustalot et al.57 Surface-enhanced Raman spectroscopy is another technique that 

The chemiluminescence and bioluminescence detectors have much to benefit from FI techniques. Chemiluminescence and bioluminescence reactions are often rapid, and the duration of the signals extremely short therefore only the signals from slower reactions can be reproducibly monitored by batch procedures, whereas reactions in the millisecond range may be readily monitored in an FI system. Such detectors are often constructed in a spiral form to increase the light flux. 

The performance of fluorimetric detectors can also be enhanced using FI techniques. The flow-cells of such detectors used for HPLC may readily be adapted for use in FI systems. The reproducibility of fluorescence measurements are reponedly improved by better control of the reaction conditions in the FI systems, and selectivity may be enhanced by on-line removal of potential quenching interferents using FI separation techniques. 

In addition to the already mentioned GC detectors, there are many others - some being selective, vhile a few being more universal. In the following, a very short description of some of the detectors and their main applicability area is given. 

This is the classical ion chromatography detector and measures the eluate conductivity, which is proportional to ionic sample concentration (provided that the cell is suitably constructed). Its sensitivity decreases as the specific conductivity of the mobile phase increases. The active cell volume of 2 gl is very small. Good conductivity detectors have automatic temperature compensation (conductivity is highly temperature-dependent) and electronic background conductivity suppression. The linear range is not large. 

Every organic molecule absorbs infrared light at one wavelength or another. When an IR detector is used, the mobile phase chosen must not be self-absorbent at the required wavelength. Hexane, dichloromethane and acetonitrile are suitable mobile phases for ester detection whereas ethyl acetate is not. The sensitivity is no greater than that of refractive index detectors. The most common wavelengths are given in Table 6.1. 

Fluorescence can also be induced with laserlight. The signal-to-noise ratio is very 

Coupling with spectroscopic techniques (HPLC-UV, HPLC-FTIR, HPLC-MS, HPLC-NMR), see Section 6.10. 

The scintillator required for this relatively weak radiation is either added as a liquid between the column and the detector or is contained as a solid in the cell. 

An ELSD reduces the HPLC eluent into a particle stream and meas-nres the scattered radiation. It offers nniversal detection for non-volatiles or semi-volatiles and has higher sensitivity than the RI detector (low ng) in addition to being compatible with gradient analysis.It is routinely nsed in combinatorial screening. Response factors are less variable than those of other detectors. An ELSD consists of a nebnlizer eqnipped with a constant temperature drift tube where a counter-current of heated air or nitrogen reduces the HPLC eluent into a fine stream of analyte particles. A laser or a polychromatic beam intersects the particle stream and the scattered radiation is amplified by a photomultiplier. Manufacturers include Alltech, Polymer Laboratories, Shimadzu, Waters, Sedere and ESA. 

A condnctivity detector measnres the electrical condnctivity of the HPLC elnent stream and is amenable to low-level determination (ppm-ppb levels) of ionic components snch as anions, metals, organic acids and sur-factants. It is the primary detection mode for ion chromatography. Mannfactnrers inclnde Dionex, Alltech, Shimadzn and Waters. 

LC/MS is the ultimate analytical technique, which combines the versatility of HPLC with the identification power of MS. The weak link in LC/MS has always been the interface which connects the liquid stream at atmospheric pressure to the high vacuum present inside the mass spectrometer. The development of several atmospheric pressure interfaces, electrospray and atmospheric pressure chemical ionization (APCI), has contributed to the tremendous success and popularity of LC/MS and LC/MS/MS in bioresearch, drug discovery, combinatorial analysis and pharmacokinetic assays. This topic is covered in more depth in a later chapter. 

The development of high-power NMR (800 MHz) spectrometers permits the simple operation of LC-NMR for metabolite analysis. 

Refractive index detection allows an extremely wide latitude in the selection of the eluent type, eluent pH and the ionic strength. In principle, refractive index detection can be substituted for conductance or UV absorption detection in many separations. However, in early work, refractive detection was found to be only moderately sensitive and was considered to be somewhat interference-prone [71]. Minimum detectable quantities for common anion such as chloride nitrate, or sulfate were reported to be in the 20 ng to 50 ng range (compared with 1 to 5 ng for direct conductance detection). 

The stability of R1 detection has improved dramatically and it should be considered seriously as an option for rugged, sensitive detection. This is due, at least partly, to control of the cell temperature, improved electronics and improved optical transducers. The new detectors are much less sensitive to variations in room temperature. 

The highest sensitivity levels can be reached only with careful control of the chromatographic temperature. Therefore column ovens should be considered when using RI. 

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Other Detectors Two additional detectors are similar in design to a flame ionization detector. In the flame photometric detector optical emission from phosphorus and sulfur provides a detector selective for compounds containing these elements. The thermionic detector responds to compounds containing nitrogen or phosphorus. 

Solutes that do not absorb UV/Vis radiation or undergo fluorescence can be detected by other detectors. Table 12.8 provides a list of detectors used in capillary electrophoresis along with some of their important characteristics. 

The detector. The function of the detector, which is situated at the exit of the separation column, is to sense and measure the small amounts of the separated components present in the carrier gas stream leaving the column. The output from the detector is fed to a recorder which produces a pen-trace called a chromatogram (Fig. 9.1fr). The choice of detector will depend on factors such as the concentration level to be measured and the nature of the separated components. The detectors most widely used in gas chromatography are the thermal conductivity, flame-ionisation and electron-capture detectors, and a brief description of these will be given. For more detailed descriptions of these and other detectors more specialised texts should be consulted.67 69... 

The absorption problems for other detectors may be considered under three headings (1) attenuation along the beam path, (2) attenuation by the detector window, (3) absorption by the detecting medium. The results of absorption calculations (1.9) in Table 2 1 show the importance of these problems and suggest ways of dealing with them. 

Gas-flow proportional counter, 55 in aluminum analysis, 217 in comparison with other detectors, 65-67... 

The refractive index detector, in general, is a choice of last resort and is used for those applications where, for one reason or another, all other detectors are inappropriate or impractical. However, the detector has one particular area of application for which it is unique and that is in the separation and analysis of polymers. In general, for those polymers that contain more than six monomer units, the refractive index is directly proportional to the concentration of the polymer and is practically independent of the molecular weight. Thus, a quantitative analysis of a polymer mixture can be obtained by the simple normalization of the peak areas in the chromatogram, there being no need for the use of individual response factors. Some typical specifications for the refractive index detector are as follows ... 

Other detector imperfections - Detectors also exhibit a wide range of other features that make these devices less than perfect. A major attribute that describes performance is cosmetic quality. Due to defects in the material or fabrication errors, some pixels of an array can exhibit... 

Secondly, the intensity of response for a certain compound from one type of detector is not necessarily the same as that obtained from the other detector. This should not be unexpected, since the two detectors are measuring quite different properties of the analyte, in this case UV absorption at a particular wavelength and how readily it is ionized and fragmented under the conditions employed. These properties are urn-elated. 

It can also be readily seen that the analysis of data may well take a considerable amount of time, often significantly more time than the acquisition of the data itself, and that the use of profiles of specific masses may allow information not readily obvious from other detectors, and even the TIC trace, to be obtained. 

The application areas for LC-MS, as will be illustrated later, are diverse, encompassing both qualitative and quantitative determinations of both high-and low-molecular-weight materials, including synthetic polymers, biopolymers, environmental pollutants, pharmaceutical compounds (drugs and their metabolites) and natural products. In essence, it is used for any compounds which are found in complex matrices for which HPLC is the separation method of choice and where the mass spectrometer provides the necessary selectivity and sensitivity to provide quantitative information and/or it provides structural information that cannot be obtained by using other detectors. 

Fluorescence detectors, discussed in Chapter 1, are extremely sensitive picogram quantities of sample can sometimes be detected. However, most polymers (with the exception of certain proteins) are not fluorescent and thus these detectors are rarely used in GPC. Proteins, particularly those containing tryptophan, fluoresce intensely and are readily detected. Because both the IR and the fluorimetric detector are selective for certain functional groups, rather than being sensitive to analyte mass, there are many pitfalls in quantitation. These and other detectors have been reviewed.177178... 

The trend in modern SEC is to couple one or several other detectors (in particular molecular weight sensitive) to the concentration detector providing complementary... 

The experiments below use reverse phase chromatography with bonded silica columns and uv absorbance detection. If more extensive experimental facilities are available, some additional experiments are suggested. These are concerned with the preparation and evaluation of columns, and with the use of other detectors and modes of hplc. It should be possible to complete each experiment within a three hour practical period. 

Some commercial detectors come with built-in procedures and software that automatically corrects for a flat detector response of every pixel4. For other detectors the necessary correction has to be carried out by the user. 

Not only PMTs and other detectors such as avalanche photodiodes suffer from dead-time effects also the detection electronics may have significant dead-times. Typical dead-times of TCSPC electronics are in the range 125-350 ns. This may seriously impair the efficiency of detection at high count rates. The dead-time effects of the electronics in time-gated single photon detection are usually negligible. 

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. 

GC is coupled with many detectors for the analysis of pesticides in wastewater. At the present time the most popular is GC-MS, which will be discussed in more detail later in this section. The flame ionization detector (FID) is another nonselective detector that identifies compounds containing carbon but does not give specific information on chemical structure (but is often used for quantification because of the linear response and sensitivity). Other detectors are specific and only detect certain species or groups of pesticides. They include electron capture,nitrogen-phosphorus, thermionic specific, and flame photometric detectors. The electron capture detector (ECD) is very sensitive to chlorinated organic pesticides, such as the organochlorine compounds (OCs, DDT, dieldrin, etc.). It has a long history of use in many environmental methods,... 

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