Detectors absorbance spectrometry

The main detectors used in CE are briefly described in Detection of Proteins. The most common detectors include absorbance, fluorescence, and on-line coupling with mass spectrometry (MS). 

Many applications for ion analysis use a UV detector with indirect detection, though other electrochemical, laser-induced fluorescence (LIE), or mass spectrometry detectors have been described. The main advantage of UV detection is its availability on commercial instruments and that both UV-absorbing and non-UV-absorbing analytes may be detected. Nowadays, electrochemical detectors are also available specific background electrolytes (BGEs) must be used and the detector has to be adapted to existing CE instruments. 

CZE = capillary zone electrophoresis EC = electrochemical detector GC = gas chromatography HCD = Hall conductivity detector HPLC = high performance liquid chromatography IDMS = isotope dilution mass spectrometry MS = mass spectrometry RSD = relative standard deviation SEE = supercritical fluid extraction SPE = solid phase extraction UV = ultraviolet absorbance detection... 

Although this section provides a brief description of most commonly nsed detectors for HPLC, most of the focus is on a few detection modes. Optical absorbance detectors remain the most widely nsed for HPLC, and are discnssed in some detail. We also focns on flnorescence, condnctivity, and electrochemical detection, as these methods were not widely nsed for HPLC in the past, bnt are especially well suited to micro- and nano-flow instrnments becanse of their high sensitivity in small sample volumes. Mass spectrometry has also come into wide and rontine nse in the last decade, but as it is the subject of another chapter, it will not be fnrther discnssed here. Miniaturization has been particularly important for capillary and chip-based electrophoresis, which often employs sub-nanoliter detection volnmes [36,37]. 

These vitamers are UV absorbers, but their detection is complicated by the low level present in foods and the low sensitivity of this detector. Other detectors, like flame atomic absorption spectrometry and inductively coupled plasma (1CP)-MS, may be applied, but without much increase in sensitivity. 

The contribution of flow analysis to improving the performance of atomic spectrometry is especially interesting in the field of standardisation. FIA can provide a faster and reliable method to relate the absorbance, emission or counts (at a specific mass number) to the concentration of the elements to be determined. In fact, flow analysis presents specific advantages to solving problems related to the sometimes short dynamic concentration ranges in atomic absorption spectrometry, by means of on-line dilution. The coupling of FI techniques to atomic spectrometric detectors also offers tremendous possibilities to carry out standard additions or internal standardisation. 

The beauty of absorbance and fluorescence modes as complimentary detectors for HPLC lies in their nondestructive nature. Fractions eluting from these detectors are readily available for confirmation by mass spectrometry. 

From a practical point of view, SFC may allow the use of many different detection principles, including both typical LC detectors (UV-absorbance, fluorescence) and typical GC detectors (flame ionization, mass spectrometry). Also, capillary SFC seems to be well within the posssibilities of current technology, while capillary LC is not. 

---