Interfaces electrospray interface

For mixture.s the picture is different. Unless the mixture is to be examined by MS/MS methods, usually it will be necessary to separate it into its individual components. This separation is most often done by gas or liquid chromatography. In the latter, small quantities of emerging mixture components dissolved in elution solvent would be laborious to deal with if each component had to be first isolated by evaporation of solvent before its introduction into the mass spectrometer. In such circumstances, the direct introduction, removal of solvent, and ionization provided by electrospray is a boon and puts LC/MS on a level with GC/MS for mixture analysis. Further, GC is normally concerned with volatile, relatively low-molecular-weight compounds and is of little or no use for the many polar, water soluble, high-molecular-mass substances such as the peptides, proteins, carbohydrates, nucleotides, and similar substances found in biological systems. LC/MS with an electrospray interface is frequently used in biochemical research and medical analysis. 

The pump must provide stable flow rates from between 10 ttlmin and 2 mlmin with the LC-MS requirement dependent upon the interface being used and the diameter of the HPLC column. For example, the electrospray interface, when used with a microbore HPLC column, operates at the bottom end of this range, while with a conventional 4.6 mm column such an interface usually operates towards the top end of the range, as does the atmospheric-pressure chemical ionization (APCI) interface. The flow rate requirements of the different interfaces are discussed in the appropriate section of Chapter 4. 

Figure 2.2 Schematics of (a) in-line and (b) Z-spray electrospray interfaces. From applications literature published by Micromass UK Ltd, Manchester, UK, and reproduced with permission.

The advent of the electrospray interface has allowed the full potential of LC-MS to be achieved. It is now probably the most widely used LC-MS interface as it is applicable to a wide range of polar and thermally labile analytes of both low and high molecular weight and is compatible with a wide range of HPLC conditions. 

The electrospray interface, described in the previous section, enables mass spectra to be obtained from highly polar and ionic compounds. 

An involatile ion-pairing reagent would be deposited in the electrospray interface and lead to a reduction in performance. Some interfaces have been specifically designed to minimize this by removing the line-of-sight between the spray and the entrance to the mass spectrometer, and are thus more tolerant to involatile buffers. The performance of the interface will be improved by the use of volatile alternatives. 

A triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and selectivity in the quantitative determination of sulfonylurea herbicides. Ion trap mass spectrometers may also be used, but reduced sensitivity may be observed, in addition to more severe matrix suppression due to the increased need for sample concentration or to the space charge effect. Also, we have observed that two parent to daughter transitions cannot be obtained for some of the sulfonylurea compounds when ion traps are used in the MS/MS mode. Most electrospray LC/MS and LC/MS/MS analyses of sulfonylureas have been done in the positive ion mode with acidic HPLC mobile phases. The formation of (M - - H)+ ions in solution and in the gas phase under these conditions is favorable, and fragmentation or formation of undesirable adducts can easily be minimized. Owing to the acid-base nature of these molecules, negative ionization can also be used, with the formation of (M - H) ions at mobile phase pH values of approximately 5-7, but the sensitivity is often reduced as compared with the positive ion mode. 

The MS/MS response for each analyte must first be optimized on the specific instrument to be used. This is usually done by infusing a solution of the analyte into the HPLC mobile phase without a column present. The composition of the mobile phase should match that expected at the time of analyte elution within 25%. The instrument is first operated in the LC/MS mode, and the settings for the electrospray interface are... 

Perkin-Elmer Sciex API 3000 LC/MS/MS system Perkin-Elmer Sciex Turbo lonSpray electrospray interface... 

For HPLC/MS/MS analysis, a triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and speciflcity in the quantitative determination of oxime carbamates and their metabolites. This allows... 

HPLC, ConstaMetric 3500 MS and ConstaMetric 3200 MS Mass spectrometer, TSQ 7000 with atmospheric pressure ionization (API) electrospray interface Robotcoupe, Model RSI 25 Syringes, Luer lock, 10-mL... 

Davis, M. T., Stahl, D. C., Hefta, S. A., and Lee, T. D., A microscale electrospray interface for on-line, capillary liquid chromatography/tandem mass spectrometry of complex peptide mixtures, Anal. Chem., 67, 4549, 1995. 

Although the electrospray interface is a powerful tool, especially in the analysis of highly polar... 

The application of polymer monoliths in 2D separations, however, is very attractive in that polymer-based packing materials can provide a high performance, chemically stable stationary phase, and better recovery of biological molecules, namely proteins and peptides, even in comparison with C18 phases on silica particles with wide mesopores (Tanaka et al., 1990). Microchip fabrication for 2D HPLC has been disclosed in a recent patent, based on polymer monoliths (Corso et al., 2003). This separation system consists of stacked separation blocks, namely, the first block for ion exchange (strong cation exchange) and the second block for reversed-phase separation. This layered separation chip device also contains an electrospray interface microfabricated on chip (a polymer monolith/... 

Janini, G.M., Comads, T.P., Wilkens, K.L., Issaq, H.J., Veenstra, T.D. (2003). A sheathless nanoflow electrospray interface for on-line capillary electrophoresis mass spectrometry. Anal. Chem. 75, 1615-1619. 

The first reported case of timesharing for a mass spectrometer9 involved the design of an Ionspray interface with multiple sprayers to support the analysis of effluents from multiple columns. This approach led to the development of a multiplexed electrospray interface (MUX)10 using an LC/MS interface and multiple (identical) sprayers linked to a HPLC system and a spinning screen to allow the output of only a single sprayer to enter the MS (Figure 4.5). The injections of the HPLC systems... 

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