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. 

Utility of pulsed ultrafiltration-mass spectrometry, where microsomal fractions are entrapped in stirred ultrafiltration chambers and the output of the chamber is introduced directly into an electrospray mass spectrometer shows particular promise [37]. 

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. 

Inject the sample at a rate of 5 pL/min using a Harvard pump into the injection port of a precalibrated electrospray mass spectrometer. 

After those first attempts to establish analytical applications of electrospray, it took more than ten years for the first bona fide electrospray mass spectrometer to emerge [14]. Yamashita and Fenn published the first electrospray MS experiment in a 1984 paper which was appropriately part of an issue of the Journal of Physical Chemistry dedicated to John Bennett Fenn [15]. They electrosprayed solvents into a bath gas to form a dispersion of ions that was expanded into vacuum in a small supersonic free jet. A portion of the jet was then passed through a skimmer into a vacuum chamber containing a quadrupole mass filter. With this setup, a variety of protonated solvent clusters as well as solvent-ion clusters (Na+, Li+) could be de-... 

Fig. 11. The experiment of indirectly detecting an intact viral particle after its transmission through an electrospray mass spectrometer. The viral particles were observed under an electron microscope after collection since the massive particles exceeded the working range of the electron multiplier normally used with this mass spectrometer...

N-terminal sequence analyses were performed using ABI 477/120A or 473A protein sequencers. For mass spectrometric analyses, aliquots of the collected peak fractions were also concentrated in a Savant SpeedVac to 20 pmol/pL and infused using a Harvard syringe pump into a PE SCIEX API III electrospray mass spectrometer (ESI-MS) operating in the positive mode with an orifice potential of 80 volts. 

Mass spectra of individual peptide samples were analyzed in an electrospray mass spectrometer constructed at The Rockefeller University and described elsewhere (7). The peptide samples were dissolved in a mixture of water, methanol and acetic acid (20 19 1) to a concentration of 10 pM and sprayed at a voltage of 3-4 kV. 

The solution of [15N]r-metHuLeptin (0.3 mg/ml) in PBS (0.1 M sodium phosphate, 0.1 M sodium chloride, pH 7.2) was incubated with endoproteinase Asp-N at an enzyme-to-substrate ratio of 1 75 (w/w) at 25 °C for 5 h. The digestion was terminated by adding 5 )tL of 5% TFA to the reaction. Peptides were separated by a Vydac C4 reverse-phase analytical column (4.6x250 mm) using a Hewlett Packard HPLC (Model 1090), which is on-line connected to a PE-Sciex API-100 electrospray mass spectrometer. The column was initially equilibrated with 95% mobile phase A (0.1%TFA) and 5% mobile phase B (90% acetonitrile in 0.1% TFA). A linear gradient from 10 to 50% mobUe phase B was run over a period of 85 minutes at a flow rate of 0.5 raL/min. The splitting of the flow (9 1) was achieved post UV cell, allowing 50 iL/min of the eluent to be analyzed by the electrospray mass spectrometer. 

Soft ionization techniques such as electrospray ionization and matrix assisted laser desorption are now routinely used to determine the mass of large hydrophilic polymers like proteins (27). However, as is usual for the ionization process, the presence of sails and detergents, which is common for biological samples, can affect the process significantly. The use of the on-line capillary reversed-phase HPLC in combination of the electrospray mass spectrometer (LC/MS) has made it possible to analyze such samples directly (10,16, 28). When GAP-43 isolated from the membrane fractions of bovine brain was analyzed, a single major peak with a minor peak corresponding to a phosphorylated species was observed (Fig. la). To study the posttranslational modifications in detail, the protein was digested with specific proteases such as lysyl... 

Under acidic conditions, an analyte protein of unknown molecular weight is known to possess multiple positive charge, between +5 and +8. A mixture containing this protein and others were subjected to capillary electrophoresis in 10-mM trifluoroacetic acid, and the eluate from the capillary was fed directly into the ionization source of an electrospray mass spectrometer. As the protein eluted from the capillary, the mlz range of the mass spectrometric detector was scanned, and peaks were observed at mlz values of 938, 1071, 1250, and 1500. What is the molecular weight of the protein ... 

The problem, of course, comes from the implicit assumption that the gel matrix has no specific interactions with the soluble polymer, and that the relationship between effective volume and molecular weight is the same for the polysaccharide of interest and the standards. A recent development has been to place instruments which measure molecular weight at the exit of a GPC column, so that the column is used only for fractionation, and a full molecular weight distribution of a polydisperse polymer can be obtained. Viscometers and light-scattering monitors can be so employed, as can on-line electrospray mass spectrometers. The last technique is particularly powerful, since the masses determined by the mass spectrometer are absolute. 

Luftmann, H. (2004) A simple device for the extraction of TLC spots direct coupling with an electrospray mass spectrometer. Analytical and Bioanalytical Chemistry,... 

Liquid chromatography-mass spectrometry (LC-MS) is an extremely power tool for the analysis of peptides, providing not only information on the purity of the product but also coMormation of structures. The s)rstem typically consists of a microbore HPLC system coupled to an electrospray mass spectrometer. Using such a system the composition of a crude peptide mixture can be quickly determined and by-products identified, enabling synthetic protocols to be rapidly optimized. 

FIGURE 3 HPLC chromatogram of the tryptic peptides produced from recombinant human tissue plasminogen activator (Mr 60 kDa) with UV monitoring (A) or total ion current (TIC) as measured by the electrospray mass spectrometer (B). Reprinted, with permission, from Ling et al. (1991). Each tryptic peptide is identified by its order in the sequence from 1 to 51. Glycopeptide masses observed are listed in Table I. 

FIGURE 4 HPLC chromatograms of the peptides produced from bovine fetuin by trypsin and Asp-N as measured by the electrospray mass spectrometer (A) Total ion current (TIC) (B) reconstructed ion chromatogram (RIC) for ion 204. Glycopeptides containing residues Asn 81, Asn 138, Asn 158, Ser 253, and Thr 262/Ser 264 were observed in the LC/MS analysis. Reprinted, with permission, from Carr et al. (1993). 

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