Detectors for Liquid Chromatography

Specialized Stationary Phases for Liquid Chromatography Chiral Stationary Phases for Liquid Chromatography Detectors for Liquid Chromatography Ultraviolet Detection of Chromophoric Groups Derivatizing Reagents for HPLC... 

Ni F, Thomas L, Cotton TM. 1989. Surface-enhanced resonance Raman spectroscopy as an ancillary high-performance liquid chromatography detector for nitrophenol compounds. Anal Chem 61 888-894. 

L. A. Allison and R. E. Shoup, Dual-electrode liquid chromatography detector for thiols and disulfides, Anal. Chem. 55, 8-12 (1983)... 

Zukowski, J. Tang, Y. Berthod, A. Armstrong, D.W. Investigation of a circular dichroism spectrophotometer as a liquid chromatography detector for enantiomers sensitivity, advantages, and limitations. Anal. Chim. Acta. 1992, 258, 83-92. 

A. MacDonald and T. A. Nieman, Flow Injection and Liquid Chromatography Detector for Amino Acids Based on a Postcolumn Reaction with Luminol. Anal. Chem., 57 (1985) 936. 

UV absorbance and fluorescence detection are only of moderate use as liquid chromatography detectors for organic compounds because most of these do not have very characteristic spectra and many do not even fluoresce. These indistinct spectra are marked by one or two broad bands. For a few classes, however, this is not the case. The polycycHc aromatic hydrocarbons (PAHs), for example, have spectra that contain several sharp bands in a distinct pattern for each PAH. For this class of compounds, these detectors are much more sensitive and give more information on the peak identities than any other type of detector. 

Fig. 3. Diagrams of electrochemical cells used in flow systems for thin film deposition by EC-ALE. A) First small thin layer flow cell (modeled after electrochemical liquid chromatography detectors). A gasket defined the area where the deposition was performed, and solutions were pumped in and out though the top plate. Reproduced by permission from ref. [ 110]. B) H-cell design where the samples were suspended in the solutions, and solutions were filled and drained from below. Reproduced by permission from ref. [111]. C) Larger thin layer flow cell. This is very similar to that shown in 3A, except that the deposition area is larger and laminar flow is easier to develop because of the solution inlet and outlet designs. In addition, the opposite wall of the cell is a piece of ITO, used as the auxiliary electrode. It is transparent so the deposit can be monitored visually, and it provides an excellent current distribution. The reference electrode is incorporated right in the cell, as well. Adapted from ref. [113],...

For amperometric techniques at a fixed potential (e.g., liquid chromatography detectors), the classical charging current is zero since dE/dt = 0. There is still a background current, however, due to surface redox processes or slow... 

Another recent development is the advent of pulse amperometry in which the potential is repeatedly pulsed between two (or more) values. The current at each potential or the difference between these two currents ( differential pulse amperometry ) can be used to advantage for a number of applications. Similar advantages can result from the simultaneous monitoring of two (or more) electrodes poised at different potentials. In the remainder of this chapter it will be shown how the basic concepts of amperometry can be applied to various liquid chromatography detectors. There is not one universal electrochemical detector for liquid chromatography, but, rather, a family of different devices that have advantages for particular applications. Electrochemical detection has also been employed with flow injection analysis (where there is no chromatographic separation), in capillary electrophoresis, and in continuous-flow sensors. 

In liquid chromatography, derivatization for detection enhancement is frequently needed, since no universal, sensitive, and simple-to-operate detector exists (the preparation of UV-absorbing derivatives is essential to obtain the sensitivity required for samples in the nanogram range). Since most fatty acids do not absorb UV radiation (at least not in the wavelength ranges of most commercial UV monitors), detection of quantities in the 1-ng range can be difficult. Tag-... 

Figure 1 is the ultraviolet spectrum of a 10 mcg/ml solution of vitamin D3 in methanol. The spectrum was obtained using a Cary Model 219 recording spectrophotometer (Varian Instrument Co., Palo Alto, CA). Vitamin D3 and related compounds have a characteristic UV absorption maximum at 265 nm and a minimum at 228 nm. The extinction coefficient at 265 nm is about 17,500 and 15,000 at 254 nm. An index of purity of vitamin D3 is a value of 1.8 for the ratio of the absorbance at 265 to that at 228 nm. The high absorbance at 254 nm enables one to use the most common and sensitive spectrophotometric detector used in high performance liquid chromatography (HPLC) for the analysis of vitamin D3 in multivitamin preparations, fortified milk, other food products, animal feed additives etc. 

In addition to the continuum sources just discussed, line sources are also important for use in the UV/visible region. Low-pressure mercury arc lamps are very common sources that are used in liquid chromatography detectors. The dominant line emitted by these sources is the 253.7-nm Hg line. Hollow-cathode lamps are also common line sources that are specifically used for atomic absorption spectroscopy, as discussed in Chapter 28. Lasers (see Feature 25-1) have also been used in molecular and atomic spectroscopy, both for single-wavelength and for scanning applications. Tunable dye lasers can be scanned over wavelength ranges of several hundred nanometers when more than one dye is used. 

After air sampling the sorbents are subsequently extracted (seldom thermally desorbed) and the pesticides were analyzed using gas chromatrography (GC) or high-pressure liquid chromatography (HPLC). For HPLC quantification is done by UV absorption at an appropriate wavelength, whereas for GC either an electron capture detector (LCD) or a mass spectrometer (MS) are used for quantification. Depending on the sampled air volume detection limits for HPLC methods are about 0.1-1 pg m for GC-ECD methods about 0.01-0.02 pg m and for MS methods about 0.001 pg m ... 

The routine use of the electrochemical detector is relatively new in liquid chromatography, but for a number of applications it has shown great utility. One of the areas where electrochemical detection has made progress is the analysis of indoles, catecholamines and their metabolites. In this area it competes, and in some instances competes favorably, with fluorescence detection. 

A variety of electroanalytical methods are used as detectors for liquid chromatography. Detectors based on conductometry, amperometry, coulometry, and polarography are commercially available. 

The use of spectrometers as detectors has become very prevalent in the past decade. More and more of the hyphenated techniques reviewed here have moved from the realm of unique devices found only in an academic or government research facility. Many are now available as offthe-shelf instrument packages readily available from several possible instrument companies. In some cases, these have become very commonplace detectors because they have been commercially available for a decade or more. These are the cases when a mass spectrometer or atomic emission spectrometer is used as a gas chromatography detector or when the mass spectrometer or UV absorbance spectrometer are used as a liquid chromatography detector. 

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