Indirect refractive-index detection

Detectability may be a significant problem with homologous series of unsaturated compounds, particularly //-alkanes. For these compounds, refractive index detection or evaporative light-scattering, both of which are described elsewhere in the book, may be of use. Indirect photometry is a useful detection scheme for compounds that do not absorb in the UV. Acetone, methylethyl ketone, methyl propyl ketone, methyl isopropyl ketone, methyl isobutyl ketone, and acetophenone are added to an acetonitrile/water mobile phase, generating a negative vacancy peak when the nonchro-mophoric analyte emerges and a positive peak if the ketone is adsorbed and displaced.70 Dodecyl, tetradecyl, cetyl, and stearyl alcohols also have been derivatized with 2-(4-carboxyphenyl)-5,6-dimethylbenzimidazole and the derivatives separated on Zorbax ODS in a mobile phase of methanol and 2-propanol.71... 

Although AS and AES can be detected at a low UV wavelength, sensitivity is lacking and a more suitable detection was achieved using indirect photometric detection, post-column colour formation reactions, or a pre-column derivatisation, suppressed conductivity detection and refractive index detection [1,42,43]. A comparison of detection limits for the determination of these anionic surfactants shows that photometric and conductivity detectors are better (picomole or nanogram range) than refractive index or fluorometry detectors by about a factor of 1000 [40],... 

Recently, a comprehensive work was published that applied uniform experimental conditions, providing a basis for comparison and critical evaluation of the state of the art of detection (not quantitation) of AAs in their underivatized form. Based on the comparison of seven different detection systems under the same chromatographic conditions, the following conclusions could be drawn due to the intrinsic specificity of these detection systems, in order of listing, they ensured increasing selectivity associated with increasing sensitivity (characterized with the limit of detection (LOD) as follows refractive index detection LOD = 50 mg 1 indirect... 

For the analysis of aliphatic anionic surfactants by HPLC, other detection systems than UV or fluorescence detection have to be used because of the lack of chromophoric groups. Refractive index detection and conductivity detection provide a solution for this t5rpe of anionic smfactants but their detection limits are rather high and gradient elution is not usually possible. Another possibihty is the application of indirect photometric detection, which is based on the formation of ion pairs between UV-active cationic compoimds, such as N-methylpyridinium chloride, used as mobile phase additives and the anionic surfactants followed by UV detection [60]. Gradient elution with indirect photometric detection is possible in principle but the detection limits increase considerably [61]. 

Ion exchange chromatography analysis is typically based on use of a 25-cm Whatman Partisil-10 SCX strong cation exchange column and a mobile phase of methanol or acetonitrile/water. The mobile phase contains a salt, such as ammonium formate, and the pH may be adjusted by addition of acetic acid. Elution is in order of decreasing alkyl character, and retention time is increased by decreasing the concentration of the salt in the eluent. Most often, conductivity detection is not used, but rather direct or indirect UV absorbance or refractive index detection. 

This temperature rise can be detected directly (laser calorimetry and optical calorimetry), or indirectly by measuring the change in either the refractive index (thermal lensing, beam deflection or refraction and thermal grating) or the volume (photo- or optoacoustic methods). This review will focus primarily on photoacoustic methods because they have been the most widely used to obtain thermodynamic and kinetic information about reactive intermediates. Other calorimetric methods are discussed in more detail in a recent review.7... 

Several kinds of detection systems have been applied to CE [1,2,43]. Based on their specificity, they can be divided into bulk property and specific property detectors [43]. Bulk-property detectors measure the difference in a physical property of a solute relative to the background. Examples of such detectors are conductivity, refractive index, indirect methods, etc. The specific-property detectors measure a physico-chemical property, which is inherent to the solutes, e.g. UV absorption, fluorescence emission, mass spectrum, electrochemical, etc. These detectors usually minimize background signals, have wider linear ranges and are more sensitive. In Table 17.3, a general overview is given of the detection methods that are employed in CE with their detection limits (absolute and relative). 

For the cationic surfactants, the available HPLC detection methods involve direct UV (for cationics with chromophores, such as benzylalkyl-dimethyl ammonium salts) or for compounds that lack UV absorbance, indirect photometry in conjunction with a post-column addition of bromophenol blue or other anionic dye [49], refractive index [50,51], conductivity detection [47,52] and fluorescence combined with postcolumn addition of the ion-pair [53] were used. These modes of detection, limited to isocratic elution, are not totally satisfactory for the separation of quaternary compounds with a wide range of molecular weights. Thus, to overcome the limitation of other detection systems, the ELS detector has been introduced as a universal detector compatible with gradient elution [45]. 

Laser-based refractive index detector, Cuprammonium reagent,4-Aminobenzoic acid reagent, Indirect detection methods for cyclodex-trins, and sugar phosphates Reversible derivatization using 2-amino-pyridine ... 

Often it is required to detect compounds with no or only very weak chromophores such as sugars and amino acids. Refractive index detectors and mass sensitive detectors can be used but they are relatively insensitive in the context of biological sample concentrations. Indirect detection using a UV or fluorescent eluent can also be employed. However, the most common approach is the use of derivatisation. Derivatisation of some chemically reactive moiety on the analyte can be performed in two modes. In post-column derivatisation the sample is separated first and then reacted with a flowing stream of derivatising reagent being pumped into... 

As a general rule the mobile phase should not be detector-active, i.e. it should not have a property which is used for detection (exception indirect detection, see Section 6.9). Otherwise it is very possible that unwanted baseline effects and extra peaks will show up in the chromatogram. However, this recommendation cannot be followed in the case of bulk-property detectors such as the refractive index detector. 

Numerous research papers and reviews on carbohydrate separations by CE have been written for the past several years. Researches have successfully addressed problems, such as tremendous diversity and complexity of this class of compounds, polar and neutral nature of most carbohydrates, their low ultraviolet (UV) extinction coefficients, and lack of functional groups. In the previous edition of this book, Olechno and Nolan [16] published a comprehensive overview of the CE separation techniques, attempted and developed for intact and derivatized carbohydrates, charged and neutral, as well as detection approaches by UV, indirect fluorescence, electrochemical (e.g., amperometric) detection, refractive index, and laser-induced fluorescence (LIE). A variety of buffer systems were... 

Any of the methods of detection used in liquid chromatography can be used in IC, though some are more useful than others. If the eluent does not affect the detector the need for a suppressor disappears. Common means of detection in IC are ultraviolet (UV) absorption, including indirect absorption electrochemical, especially amperometric and pulsed amperometric and postcolumn derivatization. Detectors atomic absorption spectrometry, chemiluminescence, fluorescence, atomic spectroscopic, refractive index, electrochemical (besides conductivity) including amperometric, coulometric, potentiometric, polaro-graphic, pulsed amperometric, inductively coupled plasma emission spectrometry, ion-selective electrode, inductively coupled plasma mass spectrometry, bulk acoustic wave sensor, and evaporative light-scattering detection. 

Through the choice of stationary phase and eluent composition, the selectivity can be modulated, but the eluent must meet the requirements of the detection system. Although the conductivity detector is still the most popular, other types of detection can be applied for different analytes. These include electrochemical (amperometric, pulsed and integrated amperometric, potentiometric), photometric (UV-Vis, indirect photometric following post column derivatisation, chemiluminescence, refractive index), and fluorescence. 

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