William Kou Jonathan Mertz

Introduction The field of Mass Spectrometry using Ambient Ionization techniques has grown exponentially since 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

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