Electrospray ionization INDEX

For detection of carbohydrates in principle, ultraviolet (UV), laser-induced fluorescence, refractive index, electrochemical, amperometric, and mass spec-trometric detection can be used. Mass spectrometry, with its various ionization methods, has traditionally been one of the key techniques for the structural determination of proteins and carbohydrates. Fast-atom bombardment (FAB) and electrospray ionization (ESI) are the two on-line ionization methods used for carbohydrate analysis. The ESI principle has truly revolutionized the modern mass spectrometry of biological molecules, due to its high sensitivity and ability to record large-molecule entities within a relatively smaU-mass scale. 

The most commonly used analytical technique for sugars is HPLC with a refractive index detector (RID). Although the HPLC-RID method is simple, the RID lacks sensitivity and selectivity. Therefore, UV and fluorescence detection is frequently used, coupled with pre- or postcolumn derivatization, for analysis with higher sensitivity. Liquid chromatography-mass spectrometry (LC-MS) using electrospray ionization also requires pre- or postcolumn derivatization. LC-MS using atmospheric pressure chemical ionization does... 

The logistical requirements of such an analysis are not stringent. Much of this work in the literature is transparent, since the details of such a sample manipulation are within the experimental section of a manuscript, and the work is not referenced or indexed as an application of TLC/MS. The references cited in this section are therefore more historical in nature, showing the development of the method through this logical first step in its development. Work of this sort continues with newer ionization methods, including electrospray ionization and APCI methods. As a caution, the flow of solvent that contains the sample extracted from a TLC separation should be passed through a fine particulate filter to remove the silica gel particles from the stream directed into the source of the mass spectrometer. 

Molecular weights and PDIs were measured by Waters gel permeation chromatography in THF relative to PMMA standards on systems equipped with Waters Model 515 HPLC pump and Waters Model 2414 Refractive Index Detector at 35°C with a flow rate of 1 ml/min. HRMS analyses were performed with Q-TOF YA263 high resolution (Waters Corporation) instruments by +ve mode electrospray ionization. 

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... 

---