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ANALYTICAL CURRENTS Attomole peptide characterization Although several groups have successfully
noelectrospray ionization (nESI) interface.
a conductive paste and conveniently attached
coupled capillary electrochromatography
CEC/nESI-MS offers the enhanced sensitivity
to the end of the CEC column with a piece of
(CEC) and MS, most peptide work using CEC/
needed for peptide characterization and does
Teflon tubing.
MS has involved pressurized CEC, in which
not require the use of a sheath liquid or sheath
the flow is supported primarily by pressure
gas flow.
(as in HPLC) and voltage is used to alter se-
The CEC/nESI-MS system was used to separate mixtures of four synthetic peptides
The researchers were able to obtain a
at mid-attomole levels. CEC was run under
lectivity. Marjan Guc˘ek and co-workers at
stable spray without a sheath-type arrange-
ambient pressure, and low-electrolyte con-
the University of Ljubljana (Slovenia), TNO
ment by using a conductively coated fused-
centrations were used to prevent bubble for-
Pharma, and Leiden University (both in The
silica tapered tip, which serves as the point
mation. Voltages of 15–20 kV were found to
Netherlands) have now coupled conventional
for applying the electrospray voltage. A
be optimal; sensitivity decreased with higher
CEC, in which the flow is electrically driven,
small i.d. capillary was simply drawn to give
separation voltages. (Rapid Commun. Mass
to an ion trap mass spectrometer using a na-
a sharp tapered tip, which was coated with
Spectrom. 2000, 14, 1448–1454)
Nanojets From electrospray ionization MS to atomic absorption, liquid jets play an important role in analytical techniques. But what happens to these fluids when the jets are reduced to nanoscale dimensions for use with nanoscale devices or to transfer biomole5 ps cules? Uzi Landman and Michael Moseler of Georgia Institute of Technology present theoretical studies (large-scale atomistic molecular dynamic simulations) of a propane 20 ps nanojet and use it to predict the design features of a nanojet nozzle.
Out it comes. Simulations of a propane (blue) nanojet exiting a gold (yellow) nozzle. The exterior surface is heated to 230 K, and the pressure is 500 MPa. Evaporative cooling and steady-state flow start at ~1 ns. (Adapted with permission. Copyright 2000 American Association for the Advancement of Science.) 678 A
What the researchers conclude is that nanojets can form if the pressure is high enough. For example, a simulation with a 6-nm-diam gold nozzle, which had its outer surface heated to
10 ps
6 nm 9 nm
100 ps
A N A LY T I C A L C H E M I S T R Y / N O V E M B E R 1 , 2 0 0 0
50 ps
1000 ps
the boiling temperature of propane, yielded a steady-state jet with a velocity of 200 m/s when the injection pressure reached 500 MPa. In general, nozzle exit orifices with 2–6 nm diameters required pressures of 250–500 MPa to form the nanojet. Moreover, the jets form with gold nozzles that either have a nonwetting coating or are externally heated. The evolution of the jet after leaving the nozzle was also described. Initially, the jet breaks up into small fast-moving droplets and molecular clusters, but at steady-state conditions, droplets are formed with a narrow size distribution. According to the researchers’ analysis, the jet’s breakup into droplets is primarily the result of thermal fluctuations. (Science 2000, 289, 1165–1169)
