William Kou
Jonathan Mertz
Introduction
• The field of Mass Spectrometry using Ambient
Ionization techniques has grown exponentially
since 2004.
• The direct analysis approach that these
techniques provide allow the collection of
samples in their native state with little to no
preparation.
• These techniques are revolutionizing the field of
mass spectrometry and achieving things that
were thought to be out of reach.
How It works
• The two main techniques used in ambient
ionization are known as DESI and DART
• These techniques desorb and ionize the analytes
so they can be analyzed by the Mass Spectrometer
Desorption
• The extraction of an absorbate from an
absorbent.
• Usually involves a change in state.
Ionization
• Acquisition of charge by neutral analyte
molecules
DESI – Desorption ElectroSpray
Ionization
• A spray of charged droplets impacts a
condensed phase sample on a substrate and
creates a thin liquid film
• The analyte is then desorbed from the
substrate
• An additional splash of the solvent creates
microdroblets which are then drawn into
the inlet of the mass spectrometer.
• As the solvent evaporates away, the analyte
is ionized.
Advantages
• Rapid results
• Gentle
• Sensitive
• Requires little to no sample
preparation
• Simple, and has potential
for portable devices
• Signal to noise ratio is much higher in DESI
(100:1) than LC-ESI (30:1)
• It can detect some compounds that other
techniques such as liquid chromatography
can not detect.
• DESI can be used to examine aqueous
solutions on suitable surfaces.
Mass Spectrum using DESI
DART- (Direct Analysis in Real Time)
• Shoots a beam of charged metastable gas, such as
helium or nitrogen
• This beam then collides with the sample, desorbing
and charging the sample upon impact
• The charged particle is then drawn into the Mass
Spectrometer via vacuum
Advantages
• Little to no sample
preparation
• Non-destructive
• Continuous sample
collection
• Rapid Results
Excellent signal to noise ratio
Practical Use
• Identification of traces left behind by explosives,
toxic industrial compounds, chemical warfare
agents, illicit drugs, some foods, inks,
fingerprints and skin.
- This can provide valid evidence to convict
someone in court.
• Can be used by pharmaceutical companies to
rapidly analyze compounds in their natural form
• Able to analyze illicit drugs within biological
fluids
- this is valuable because most other methods
require sample extraction before analysis
• Identifying contaminants in foods such as
melamine, which can make the product appear
to have a higher protein content.
• DESI can be used to produce images of latent
fingerprints
• Analysis of ink can be performed by DART,
picking up differences in inks.
• An application for this could be to verify that
documents have not been changed
Conclusion
Further growth in the field and applications of
these techniques is ongoing. The success in the
examples given show that these techniques are
promising and worth further investigation and
testing.
Disadvantages of these techniques
• There is a certain amount of “noise” present in
the spectrum
- to remedy this problem you could easily use
tandem MS or MS/MS
• These two techniques do not readily ionize every
compound you may come across
- there are techniques which were developed
from DESI and DART (listed in the article) that
can account for these compounds.
- The use of different dopants also effects the
detection of desired analytes
- There are also more techniques and dopants
being developed and discovered
• http://www.youtube.com/watch?v=SJk9OCNm
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