Give an overview of the 5 ambient ionisation techniques
- DESI –> tissue and surface imaging
- DART –> Rapid screening of samples
- Paper Spray –> biofluid and point-of-care tests
- SAWN –> soft analysis of solutions
- ELI –> Clean, Sustainable ionisation method
Give the definition of ambient ionisation
Ionisation realised at atmopsheric pressure directly at the MS inlet
Explain the advantages of Ambient ionisation
Allows for:
- Minimal modifications to the instrument
- Open air analysis
- No Sample pretreatment
- Direct surface analysis
- Using ‘real life’ object
- MS Imaging without vacuum or matrix
Explain the advantages over MALDI and ESI
- Can probe odd shaped samples
- Don’t need liquid or gas phase
- High throughpout: low cost
- No sample pretreatment
Describe the 3 general methods of ion generation used in Ambient MS
Spraying (DESI)
Electric Field (DART)
Gas/Heat or laser-assisted (DAPPI)
Describe DESI
Desorption Electrospray ionisation: “Spray-based ionisation using charged microdroplets to desorb and ionise molecules directly from a surface” Surface has samples & can move. Forms multiply charged ions
Describe the droplet pick-up model in DESI
- Highly charged droplets form a thin layer on the surface of the sample: analytes dissolve
- Bombardment of more dorplets generates secondary charged droplets containing the sample compounds towards the inlet.
Define DART
Direct Analysis in Real Time: “Plasma-based ionisation where metastable gas species create reagent ions that charge analytes in open air without contact or solvent’
Advantage: sample does not have to be altered because no high voltage or pressure
Describe the ion generation process in DART
- Inert gas collides with electrons between electrodes
- Collisions produce electronic and vibronic excited molecules –> Penning ionisation –> forms radical cation
- Plasma moves separates neutral from ions
- Metastable neutrals create reactive ions in the air which then ionise the sample
Define Paper Spray
“Capillary wicking and high voltage generate ions directly from paper loaded with a small liquid sample’ : apply voltage to paper with small liquid sample
Define SAWN
Surface Acoustic Wave Nebulisation: “Acoustic waves nebulise a liquid film into charged droplets for soft, solvent-free ionisation”
Dependent on solvent & frequency.
Biggest advantages: speed & ionisation directly from liquid & soft.
Describe the use of SAWN in Art history & conservation
Analytical challenges:
- Complexity of the samples
- Lack of knowledge about degradation
- Need: non-invasive. Microsampling
SAWN-MS: faster & less sample
Sampling –> vortex in solvent –> SAWN-MS
Describe ELI
Electroless Ionisation: ‘Self-charging liquid-gas interface creates ions without applied voltage or spray’
Liquid sprayed with froce through nozzle with ‘charge transfer layer’
1. Prepare sample & choose ELI nozzle (+ or -)
2. Fill fyringe with sample and place in holder
3. Spray towards MS-inlet
