Interface electrospray/mass spectrometer

The ionspray (ISP, or pneumatically assisted electrospray) LC-MS interface offers all the benefits of electrospray ionisation with the additional advantages of accommodating a wide liquid flow range (up to 1 rnl.rnin ) and improved ion current stability [536]. In most LC-MS applications, one aims at introducing the highest possible flow-rate to the interface. While early ESI interfaces show best performance at 5-l() iLrnin, ion-spray interfaces are optimised for flow-rates between 50 and 200 xLmin 1. A gradient capillary HPLC system (320 xm i.d., 3-5 xLmin 1) is ideally suited for direct coupling to an electrospray mass spectrometer [537]. In sample-limited cases, nano-ISP interfaces are applied which can efficiently be operated at sub-p,Lmin 1 flow-rates [538,539]. These flow-rates are directly compatible with micro- and capillary HPLC systems, and with other separation techniques (CE, CEC). 

With the work of Fenn and co-workers, liquid chromatography—electrospray interfaces for mass spectrometers were developed in 1984. Subsequently, the Pacific Northwest Laboratory began work in the area of CE—ESI—MS under the direction of Richard Smith and published the initial paper describing on-line CE—MS in 1987. Initial interface designs involved removing the polyimide at the end of the capillary in favor of a layer of silver for electrical contact. This interface was limited due to below optimum flow rates and limited lifetime of the metallized capillary. The introduction of the sheath flow design dramatically improved the CE—MS results. In lieu of being connected to a standard outlet buffer, the CE—MS interface used the outlet end of electrophoretic capillary connected directly to the electrospray mass spectrometer. 

Liu, C. C., Jong, R., and Covey, T. (2003). Coupling of a large-size capillary column with an electrospray mass spectrometer. A reliable and sensitive sheath flow capillary electrophoresis-mass spectrometry interface.. Chromatogr. A 1013, 9-18. 

Micro- and nano-HPLC systems (Fig. 15.11) rely on small-diameter and capillary columns packed with high-efficiency packing materials and very slow flow rates to produce concentrated solutions and sharp chromatography peaks to feed electrospray interfaces for mass spectrometers. 

Some reviews [5-7] have appeared on NCE-electrospray ionization-mass spectrometry (NCE-ESI-MS) discussing various factors responsible for detection. Recently, Zamfir [8] reviewed sheathless interfacing in NCE-ESI-MS in which the authors discussed several issues related to sheathless interfaces. Feustel et al. [9] attempted to couple mass spectrometry with microfluidic devices in 1994. Other developments in mass spectroscopy have been made by different workers. McGruer and Karger [10] successfully interfaced a microchip with an electrospray mass spectrometer and achieved detection limits lower than 6x 10-8 mole for myoglobin. Ramsey and Ramsey [11] developed electrospray from small channels etched on glass planar substrates and tested its successful application in an ion trap mass spectrometer for tetrabutylammonium iodide as model compound. Desai et al. [12] reported an electrospray microdevice with an integrated particle filter on silicon nitride. 

The main components of an LC-MS are the HPLC apparatus, an optional UV or photodiode array detector, the interface, the mass spectrometer and a computer system for data management and evaluation. The interface is the key component of the LC-MS system. All other components must be adapted to the particular interface that is used. Most commercially available systems work with thermospray, electrospray, or particle beam interfaces. Each interface has a distinct mode of action and its own operational parameters. 

The electrospray mass spectrometer (ESI-MS) was operated under positive ion mode and optimized collision energy level of 20%, scanned from m/z 100 to 700. ESI was conducted using a needle voltage of 3.5 kV. High-purity nitrogen (99.999%) was used as dry gas and flow rate at 9 L/Min, capillary temperature at 325 °C. Helium was used as Nebulizer at 45 psi. The ESI interface and mass spectrometer parameters were optimized to obtain maximum sensitivity. The... 

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. 

Dynamic FAB is an interface between a liquid chromatograph and a mass spectrometer and is, at the same time, an ion source. As an inlet/ion source, this technique fulfils a similar function to plasmaspray and electrospray, both of which are combined inlet/ion sources. 

LC can be combined with all kinds of mass spectrometers, but for practical reasons only quadrapolar, magnetic/electric-sector, and TOP instruments are in wide use. A variety of interfaces are used, including thermospray, plasmaspray, electrospray, dynamic fast-atom bombardment (FAB), particle beam, and moving belt. 

Another big advance in the appHcation of ms in biotechnology was the development of atmospheric pressure ionization (API) techniques. There are three variants of API sources, a heated nebulizer plus a corona discharge for ionization (APCl) (51), electrospray (ESI) (52), and ion spray (53). In the APCl interface, the Ic eluent is converted into droplets by pneumatic nebulization, and then a sheath gas sweeps the droplets through a heated tube that vaporizes the solvent and analyte. The corona discharge ionizes solvent molecules, which protonate the analyte. Ions transfer into the mass spectrometer through a transfer line which is cryopumped, to keep a reasonable source pressure. 

Figure 9.12 Schematic diagram of the silica sheath electrospray needle used to interface capillary zone electi ophoresis with a mass spectrometer.

It was also the first of a number of interfaces, with the others being electrospray and atmospheric-pressure chemical ionization, in which ionization is effected directly from solution within the interface itself, i.e. the mass spectrometer was not nsed to prodnce ions from the analyte simply to separate them according to their m/z ratios. 

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. 

The method for chloroacetanilide soil metabolites in water determines concentrations of ethanesulfonic acid (ESA) and oxanilic acid (OXA) metabolites of alachlor, acetochlor, and metolachlor in surface water and groundwater samples by direct aqueous injection LC/MS/MS. After injection, compounds are separated by reversed-phase HPLC and introduced into the mass spectrometer with a TurboIonSpray atmospheric pressure ionization (API) interface. Using direct aqueous injection without prior SPE and/or concentration minimizes losses and greatly simplifies the analytical procedure. Standard addition experiments can be used to check for matrix effects. With multiple-reaction monitoring in the negative electrospray ionization mode, LC/MS/MS provides superior specificity and sensitivity compared with conventional liquid chromatography/mass spectrometry (LC/MS) or liquid chromatography/ultraviolet detection (LC/UV), and the need for a confirmatory method is eliminated. In summary,... 

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. 

As with GC/MS, LC/MS offers the possibility of unequivocal confirmation of analyte identity and accurate quantiation. Similarly, both quadrupole and ion-trap instruments are commercially available. However, the responses of different analytes are extremely dependent on the type of interface used to remove the mobile phase and to introduce the target analytes into the mass spectrometer. For pesticide residue analyses, the most popular interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Both negative and positive ionization can be used as applicable to produce characteristically abundant ions. 

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... 

Confirmatory system Agilent Series 1100 liquid chromatograph Eclipse XDB Cig HPLC columu, 150 x 4.0-mm i.d., 3.5- im particle size MicroMass Quattro II triple-quadrupole mass spectrometer using an electrospray ionization (ESI) interface... 

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... 

The mass spectra of mixtures are often too complex to be interpreted unambiguously, thus favouring the separation of the components of mixtures before examination by mass spectrometry. Nevertheless, direct polymer/additive mixture analysis has been reported [22,23], which is greatly aided by tandem MS. Coupling of mass spectrometry and a flowing liquid stream involves vaporisation and solvent stripping before introduction of the solute into an ion source for gas-phase ionisation (Section 1.33.2). Widespread LC-MS interfaces are thermospray (TSP), continuous-flow fast atom bombardment (CF-FAB), electrospray (ESP), etc. Also, supercritical fluids have been linked to mass spectrometry (SFE-MS, SFC-MS). A mass spectrometer may have more than one inlet (total inlet systems). 

ESI-MS is the most successful method of coupling a condensed phase separation technique to a mass spectrometer. Because the input to ESI is a liquid, electrospray serves as an interface between the mass spectrometer and liquid chromatographic techniques, including SEC and CE (capillary electrophoresis). In LC-MS the flow-rate should lie in the range recommended for the HPLC pump and the mass spectrometer (typically 0.001 -l.OmLmin-1). Recent advances in (nano)electrospray technology include the development of the use of very low solvent flow-rates (30 to 1000nLmin-1) [130,131],... 

Figure 4. Ion source and reaction chamber for ion-molecule equilibria. Solution to be electrosprayed flows through elestrospray capillary ESC at 1 -2 pL/min. Spray and ions enter pressure reduction capillary PRC and emerge into forechamber FCH maintained at 10 torr by pump PL. Ions in gas jet, which exits PRC, drift towards interface plate IN under influence of drift field imposed between FCH and IN. Ions enter the reaction chamber RCH through an orifice in IN and can react with reagents in the reagent gas mixture RG. This flows into RCH and out of RCH to FCH where it is pumped away. Ions leaking out of RCH through orifice OR are detected with a mass spectrometer. To reduce the inflow of solvent vapors into the pressure reduction capillary PRC, a stream of dry air is directed through the pipe Al, at 60 L/min, and pure N2 is directed at SG into the annular space at the entrance of the pressure reduction capillary, PRC. From Klassen, J. S. Blades, A. T. Kebarle, P. J. Phys. Chem. 1995, 99, 1509, with permission.

Detection of the effluent in a 2D system is carried out at the end of the second dimension s column. UVand LIF are the most widely used and the simplest methods of detection for CE separations because they are performed on-column. MS detection, unlike UV and LIF, is carried out on the effluent as it exits the CE column. The direct coupling of CE with mass spectrometry has shown great potential in proteomic research (Janini et al., 2004). The method of choice for detection of peptides is MS-electrospray ionization (ESI). However, ESI requires a special interface between the CE column and the mass spectrometer that has proven not to be a simple matter (Issaq et al., 2004). 

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