Bibliography & Suggested Links

Below is a list of journal references concerning low-flow rate ESI-MS. This bibliography is by no means complete; if you have a citation that should be included, please e-mail it to us! The list is updated often to include "lab-on-a-chip" ESI references. If you have any please make a contribution. Please do not ask us for copies of any of these papers, as we can not fulfill such requests.

For a more general chromatography/mass spectrometry bibliography listing, visit Chem-Space Associates.

1. Nanospray: Flow rate 25-100 nL/min, no pumping

1.1 Picospray variant: Flow rate 0.1 nL/min to 20 nL/min, no pumping

2. Microspray: Flow rate 100-1 µL/min, pumped

2.1 Open tip micro-spray, no LC

2.2 Open tip micro-spray coupled with nanobore LC

2.3 Packed tip micro-spray

3. Capillary liquid chromatography as applied to MS

3.1 Coupling conventional ESI to nanobore LC (1 -10 µL/min, pumped)

4. CE-MS interfacing

4.1 Liquid make-up or sheath interface (1-10 µL/min, pumped)

4.2 Sheathless interface - 1 nl/min to 500 nL/min, EOF "pumped"

1. Nanospray: Flow rate approx. 25-100 nL/min, no pumping

M. Wilm and M. Mann "Electrospray and Taylor-Cone theory, Dole's beam of macromolecules at last?" Int. J. Mass Spectrom. Ion Proc. 1994, 136, 167-180.
Original Description of nano-electrospray, although it was called "micro-electrospray" in this paper. (This created much confusion because, at approx. the same time Emmett & Caprioli had named their ESI source as micro-electrospray, thus Wilm & Mann later came to call their implementation "nano-electrospray" or "nano-spray".) A theoretical description of ESI and the effects of flow rate on droplet size, signal etc. make up the bulk of the paper.

M. Wilm and M. Mann "Analytical Properties of the Nanoelectrospray Ion Source" Anal. Chem. 1996, 68, 1-8
A more detailed description of nano-spray, including information on needle fabrication. A very application oriented paper also describes a nano-spray compatible desalting method. As used here it is strictly an off-line method.

P. K. Blackburn and R. J. Anderegg "Characterization of Femtomole Levels of Proteins in Solution Using Rapid Proteolysis and Nanoelectrospray Ionization Mass Spectrometry" J. Am. Soc. Mass Spectrom. 1997, 8, 483-484.
Demonstrates utility of immobilized enzyme (Poroszyme Trypsin from PE Biosystems) combined with nano-spray to improve protein sample turn around. (At the 30-100 fmol level.)

1.1 "Picospray", fused-silica variant of nanospray: Flow rate approx. 0.1 to 20 nL/min, no pumping

G. A. Valaskovic et al. "Attomole-Sensitivity Electrospray Source for Large-Molecule Mass Spectrometry" Anal. Chem. 1995, 67, 3802-3805.
Non-pumped ESI (nano-spray) using tips fabricated from small bore fused silica tubing. Further improves the lower limit of flow to below 0.1 nl/min! Although off-line analysis is demonstrated, such tips are suitable for on-line coupling.

G. A. Valaskovic and F. W. McLafferty "Long-Lived Metallized Tips for Nanoliter Electrospray Mass Spectrometry" J. Am. Soc. Mass Spectrom. 1996, 7, 1270-1272.
The authors show the use of a stable oxide over-coating to improve the durability of sputtered metal coatings commonly used on fused-silica tips.

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2. Microspray

2.1 Micro-electrospray, open tip: Flow rate approx. 100 nL/min to 1 µL/min, pumped flow

D. C. Gale and R. D. Smith "Small Volume and Low Flow-rate Electrospray Ionization Mass Spectrometry of Aqueous Samples" RCMS 1993, 7, 1017-1021.
One of the earliest papers suggesting the benefits of reducing flow rate for ESI. At the time the trend (at least in commercial instrumentation) was to develop systems that operated at higher flow rates (ml/min). They utilized fused silica tips that were tapered by etching in HF.

M. S. Kriger, K. D. Cook, R. S. Ramsey "Durable Gold-Coated Fused Silica Capillaries for Use in Electrospray Mass Spectrometry" Anal. Chem. 1995, 67, 385-389.
The authors fabricated tapered tips by grinding fused silica tubing. They demonstrate the utility of silanization prior to gold coating to improve the adhesion of the gold film. They claim tip lifetimes in excess of 100 hours with this process and state such tips can withstand "multiple discharges". Operation in the sheathless interfacing with CE-MS was also investigated.

See also Emmett & Caprioli below.

2.2 Micro-electrospray coupled with nanobore LC: Flow Rate 100 nL/min to 5 µL/min, pumped flow, no sheath make-up

M. T. Davis, D. C. Stahl, K. M. Swiderek, T. D. Lee "Capillary Chromatography/Mass Spectrometry for Peptide and Protein Characterization" Methods: A Companion to Methods in Enzy. 1994, 6, 304-314.
Demonstration of off-line gradient formation in a loop valve for high performance nano-LC.

M. T. Davis, D. C. Stahl, S. A. Hefta, T. D. Lee "A Microscale Electrospray Interface for On-line, Capillary Liquid Chromatography/Tandem Mass Spectrometry of Complex Protein Mixtures.
Detailed description of various coupling methods of tapered, fused-silica ESI needles to nano-bore LC columns.

M. T. Davis, D. C. Stahl, T. D. Lee "Low-flow High Performance Liquid Chromatography Solvent Delivery System Design for Tandem Capillary Liquid Chromatography-Mass Spectrometry" J. Am. Soc. Mass Spectrom. 19956, 571-577
A more detailed account of the specialized HPLC system optimized for low-flow ESI/MS interfacing.

M. T. Davis and T. D. Lee "Variable Flow Liquid Chromatography-Tandem Mass Spectrometry and the Comprehensive Analysis of Complex Protein Digest Mixtures" J. Am. Soc. Mass Spectrom. 1997, 8, 1059-1069.
Modification of the earlier described gradient LC system with the added functionality of variable flow rate. Thus they can do "peak parking" experiments where an LC peak of interest is "trapped" in the fused-silica ESI needle and the flow rate is dropped to (more less) nano-spray flow rates. Thus detailed MS/MS or MS/MS/MS is possible. This method combines the best features of the Wilm-Mann nano-spray source with the Emmett-Caprioli nano-LC ESI method.

2.3 Micro-electrospray with integral packed tip: Flow rate approx. 200 nL/min to 800 µL/min, pumped flow

M. R. Emmett and R. M. Caprioli "Micro-electrospray Mass Spectrometry: Ultra-High-Sensitivity Analysis of Peptides and Proteins" J. Am. Soc. Mass Spectrom. 1994, 5, 605-613.
Utilizes a fused-silica needle that is integrally packed with reverse phase media for desalting/concentration/separation. Exceptionally high sensitivity with zero post column loss. This method solves the primary limitation of Nano-spray, as well as all other variants of electrospray, by dramatically improving the concentration limit of detection.

P. E. Andren, M. R. Emmett, R. M. Caprioli "Micro-Electrospray: Zeptomole/Attomole per Microliter Sensitivity for Peptides" J. Am. Soc. Mass Spectrom. 1994, 5, 867-869.
The authors demonstrate even better concentration sensitivity (at the 500 zeptomole/µl level!). I believe that this is the highest true sensitivity demonstrated for ESI-MS.

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3. Capillary liquid chromatography as applied to mass spectrometry:

K. B. Tomer, M. A. Moseley, L. J. Deterding, C. E. Parker "Capillary Liquid Chromatography/Mass Spectrometry" Mass Spectrometry Review, 1994, 13, 431-457.
A key reference. Anyone wishing to pack his or her own nanobore columns should have a copy of this paper on hand. Contains a step-by-step column fabrication procedure.

3.1 Conventional ESI coupled with nanoscale-LC using a sheath flow: Total flow rate 1 - 10 µL/min, pumped flow

D. F. Hunt et al. "Characterization of Peptides Bound to Class I MHC Molecule HLA-A2.1 by Mass Spectrometry" Science, 1992, 255, 1261-1263.
Applications paper describing an important application of high-sensitivity nano-bore LC coupled with ESI-MS. This paper created quite considerable interest in LC-MS. As with any Science paper technical detail is a little thin. Their principle technical reference is the next paper.

D. F. Hunt et al., "Mass Spectrometric Methods for Protein and Peptide Sequence Analysis" in Techniques in Protein Chemistry II, J. J. Villafranca, Ed. (Academic Press, New York, 1991), pp. 441-454.
Principal technical citation for the above, well known, Science paper. Unfortunately this paper is also a little thin on important experimental details. Look elsewhere for technical information such as the excellent review by Tomer & Moseley.

C. E. Parker et al. "Application of Nanoscale Packed Capillary Liquid Chromatography (75 µm id) and Capillary Zone Electrophoresis/Electrospray Ionization Mass Spectrometry to the Analysis of Macrolide Antibiotics" J. Am. Soc. Mass Spectrom. 1992, 3, 563-574.

J. R. Perkins, C. E. Parker, K. B. Tomer "Nanoscale Separations Combined with Electrospray Ionization Mass Spectrometry: Sulfonamide Determination" J. Am. Soc. Mass Spectrom. 1992, 3, 139-149.

C. E. Parker, et al. "Nanoscale packed capillary liquid chromatography-electrospray ionization mass spectrometry: analysis of penicillins and cephems" J. Chromatogr. 1993, 616, 45-57.
Parker & Perkins published some of the earliest work in coupling nanobore LC to mass spectrometry. Lots of useful technical details, much of which is repeated in the previously cited review by Tomer et al.

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4. CE-MS Interfacing

4.1 CE-MS Interfaces, liquid make-up or sheath: Flow rate 1-10 µL/min total flow rate

R. D. Smith, C. J. Barinaga, H. R. Udseth "Improved Electrospray Ionization Interface for Capillary Zone Electrophoresis-Mass Spectrometry" Anal. Chem. 1988, 60, 1948-1952.
Due to the technical difficulties with the sheathless interface (mostly due to an inefficient tip shape), the authors combined CE with a coaxial liquid sheath flow, increasing flow to 5 µl/min, for improved stability. Much better stability but you pay a price in lower analytical sensitivity. In some ways this method was a step backwards for CE interfacing. This is witnessed in many subsequent papers from the Smith group in where they returned to the sheathless interface to improve sensitivity.

E. D. Lee, W. Mück, T. R. Covey, J. D. Henion "on-line capillary zone electrophoresis-ion spray tandem mass spectrometry for the determination of dynorphins" J. Chromatogr. 1988, 458, 313.
CE/MS interface using a "T junction" to make up additional flow rate (as opposed to the coaxial sheath arrangement). Electrical contact is also applied through the make-up liquid.

F. Garcia and J. D. Henion "Gel-filled Capillary Electrophoresis/Mass Spectrometry Using a Liquid Junction-Ion Spray Interface" Anal. Chem. 1992, 64, 985-990.
One of the few literature references concerning interfacing capillary gel electrophoresis with mass spectrometry. The interface arrangement is substantially the same as the original CE-junction paper above.

M. A. Moseley et al. "Optimization of Capillary Zone Electrophoresis/Electrospray Ionization Parameters for the Mass Spectrometry and tandem Mass Spectrometry Analysis of Peptides" J. Am. Soc. Mass Spectrom. 1992, 3, 289-300.
Well written paper with lots of good technical detail. Utilizes coaxial sheath interface combined with amino coated CE columns for high performance, high sensitivity analysis.

4.2 CE-MS interface, sheathless: < 1 nL/min to > 500 nL/min flow rate, electroosmotic pumping

R. D. Smith, J. A. Olivares, N. T. Nguyen, H. R. Udseth "Capillary Zone Electrophoresis-Mass Spectrometry Using an Electrospray Ionization Interface" 1988, 60, 436-441.
Detailed technical account of the early methods in use in the Smith laboratory for the implementation of a sheathless interface. Nice paper w/explicit technical details. They use etched fused silica tips with a metal coating. Note this kind of tip shape is not optimal for ESI stability.

G. A. Valaskovic, N. L. Kelleher, F. W. McLafferty "Attomole Protein Characterization by Capillary Electrophoresis-Mass Spectrometry" Science, 1996, 372, 1199-1202
Demonstration of coupling "pico-spray" with CE for ultra-high performance sheathless coupling with small bore (5 µm ID) CE columns. When coupled to Fourier Transform MS a sub-attomole limit of detection for a 29 kDA protein was demonstrated.

G. A. Valaskovic and F. W. McLafferty "Sampling Error in Small-bore Sheathless Capillary Electrophoresis/Electrospray-ionization Mass Spectrometry" RCMS, 1996, 10, 825-828.
The authors report a previously unknown systematic sampling error generated by solvent evaporation that can develop in a sheathless CE-MS interface. A simple system for eliminating this error by immersing the ESI end of the column inside a liquid drop is demonstrated.

J. C. Severs A. C. Harms, R. D. Smith " A New High-performance Interface for Capillary Electrophoresis/Electrospray Ionization Mass Spectrometry" RCMS 1996, 10, 1175-1178.
A novel type of sheathless interface that uses a tiny piece of micro-dialysis tubing for electrical contact. Think of it as an in-union contact without the union. It also provides a unique opportunity to alter post-column chemistry.

J. F. Kelly, L. Ramaley, P. Thibault "Capillary Zone Electrophoresis-Electrospray Mass Spectrometry at Submicroliter Flow Rates: Practical Considerations and Analytical Performance" Anal. Chem. 1997, 69, 51-60.
The authors utilized tapered fused silica tips combined with and "in-union" contact mechanism to eliminate the conductive coating and improve performance of the sheathless interface.

K. B. Bateman, R. L. White, P. Thibault "Disposable Emitters for On-line Capillary Zone Electrophoresis/Nanoelectrospray Mass Spectrometry" RCMS, 1997, 11, 307-315
The authors investigate a number of different methods for coupling tapered fused-silica needles to CE columns to improve the reliability of the sheathless interface.

P. Cao and M. Moini "A Novel Sheathless Interface for Capillary Electrophoresis/Electrospray Ionization Mass Spectrometry Using an In-capillary Electrode" J. Am. Soc. Mass Spectrom. 1997, 8, 561-564.
A variation on improving sheathless interfaces. Electrical contact is established by inserting a small platinum wire inside a small hole fabricated near the end of the capillary column. Not as versatile as the dialysis approach of Severs et al., but perhaps somewhat easier to fabricate; seems to have very robust performance.

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