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For most analytes, ESI-MS operates primarily as a concentration
sensitive detector in which the signal response is a function of the
analyte concentration in the mobile phase. In conventional LC-MS,
analyses are performed with a packed column of 4.6 mm in diameter
operating with flow rates in excess of 0.5 mL/min. To handle these high
flow rates, most commercial ESI sources for mass spectrometers use
pneumatic assistance for aerosol generation. The direct coupling of a
conventional column to an ESI source has some disadvantages. At these
high flow rates, the mass spectrometer inlet samples typically less than
0.1% of the total aerosol produced. A great deal of the sample is
"wasted" because it does not contribute to ion current. To better
utilize sample, it is advisable to reduce the total volume (and hence
flow rate) of mobile phase exiting the column. The reduction in flow
rate has the effect of increasing analyte concentration and improving
the signal response, and hence sensitivity of the mass spectrometer.
Since nanospray methods permit direct operation of the electrospray
source at low flow rates (nL/min), the inside diameter of the column can
be reduced by orders of magnitude with virtually no analytical penalty.
A 75 µm ID nanobore column operating at 250 nL/min has a volume of some
3000 times less than a conventional 4.6 mm ID column. Theoretically,
one can obtain an analyte concentration factor of approx. 3000 fold, and
a much higher signal-to-noise response with the mass spectrometer.
Practically, the experimental number is lower, but sensitivity
improvements well in excess of 500 fold are commonly obtained. The
analytical challenge is then to get your sample into the smallest
possible volume for the highest possible signal response. The principle
analytical benefit is that samples with strict quantity limitations,
such as gel-separated proteins, are now readily analyzed. With 75 um ID
columns, limits of detection are typically in the sub-femtomole to
attomole range.
New Objective offers nanobore columns for the highest sensitivity LC/MS,
especially for proteomics. Choose from a standard nanobore column with
internal frit, our IntegraFrit™ column, or eliminate the headache of
coupling nanobore-LC to ESI/MS by trying a PicoFrit® column. Both
items are available in different bed lengths and packing materials.
References: McEwen, C. N.; Larsen, B. S. Electrospray Ionization on
Quadrupole and Magnetic-Sector Mass Spectrometers. Electrospray
Ionization Mass Spectrometry: Fundamentals Instrumentation and
Applications; Cole, R. B., Ed.; John Wiley and Sons, Inc.: New York, NY,
1997.
Tomer, K. B.; M. A. Moseley, L. J. Deterding, C. E. Parker. Capillary
Liquid Chromatography/Mass Spectrometry. Mass Spectrometry Review, 1994,
13, 431-457.
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