AMBIENT Mass Spectrometry. Ion Sources in the Mass Spectrometer - Analyzed samples are ionized prior to the analysis in the mass spectrometer - A variety

AMBIENT Mass AMBIENT Mass SpectrometrySpectrometry

Ion Sources in the Mass Spectrometer

- Analyzed samples are ionized prior to the analysis in the mass spectrometer

- A variety of IONIZATION techniques are used for Mass spectrometry

- Electron Ionization (EI) & Chemical Ionization (CI) are suitable for gas-phase ionization

- EI and CI use is therefore limited to compounds sufficiently volatile and thermally stable

Ion Sources in the Mass Spectrometer

- A large number of compounds are thermally labile or do not have sufficient vapor pressure

- Therefore, ions of these compounds should be extracted directly from the condensed (liquid, solid) phase to the gas phase

Liquid and Solid Phase Direct Ion Sources

- Direct ion sources are divided to:

1. Liquid-phase ion sourcesAnalyte is in solution

2. Solid-state ion sourcesAnalyte is an involatile deposit

Liquid Phase Direct Ion Sources

- Analyte is in solution

- The solution is introduced by nebulization, as droplets into the mass spectrometer through vacuum pumping

- Electrospray (ESI), thermo spray and Atmospheric Pressure Chemical Ionization (APCI)

Solid Phase Direct Ion Sources- Analyte is an involatile deposit

- Analyte obtained by methods that introduce a matrix that can be either a solid or a viscous fluid

- The deposit is irradiated by energetic particles or photons that desorb and ionize ions near the surface of the deposit

- Such approaches are termed desorption/ionization methods (DI)

Desorption

Removal of a substance from a surface on which it is adsorbed

Solid Phase Direct Ion Sources

- The ions are extracted by electric field and focused towards the mass analyzer

condensed phase-to-gaseous ion transfer

- Examples: Matrix-assisted laser desorption ionization (MALDI), secondary ion mass spectrometry, field desorption, plasma desorption, fast atom bombardment ionization methods all use this strategy to produce ions

Generating Ions in Vacuum or in Air

- Vacuum conditions are a simple choice for creating and maintaining ions

- Vacuum environment is not absolutely necessary

- Ions can also be generated in air

AIR/Ambient Ionization

- A recent innovation of mass spectrometry

- The ability to record mass spectra on ordinary samples, in their native environment

- No sample preparation or pre-separation by creating ions outside the mass analyzer instrument


A few techniques developed recently that allow ions to be created under ambient conditions and then collected and analyzed by MS

AIR/Ambient IonizationAmbient ionization methods retain the characteristic advantages of MS:- Speed- Chemical specificity- Low detection limits- Applicability to complex mixtures through MS-MS


All the important characteristics of MS described before are used in ambient MS and do not require sample preparation


Applications in for example:

- Explosives on luggage- Drug metabolites in urine- Lipids in intact tissue- Active ingredients in pharmaceutical tablets

AIR/Ambient Ionization- A few techniques developed recently that allow ions to be created under ambient conditions and then collected and analyzed by MS

The main one is DESI

- A fine spray of charged droplets hits the surface of interest (ESI-like)

- It picks-up small molecules and large biomolecules and ionizes them

-De-solvated ions are delivered into the mass spectrometer

Desorption ElectroSpray Ionization (DESI)


e.g. PTFE

Cotte-Rodriguez et al., Anal. Chem. 2005


- DESI applies to both large and small molecules

- The charged micro droplets (used as projectiles) pick up proteins and other large biomolecules from the surface ionize them and transport them to the MS

- The process results in ESI like mass spectra of proteins


Gas phase solvent ions in the spray protonate or react with analyte small molecules on the surface


This results in the generation of ions from compounds with low desorption energies such as:1. Volatile and semi-volatile compounds (aromatic hydrocarbons)2. Low polarity molecules of small size (e.g. terpenes, lipids)3. Low molecular weight polar compounds (amino acids)


In the MS:

- Gas phase ions characterized by m/z ratio- By MS/MS spectra

- Soft ionization (minimum fragmentation), intact molecular ions



- Quantitative accuracy is limited by matrix effect (vary with analyte)

- Internal standard can make it better but it cannot be used for solid samples

- Detection limits are very low (10-15, femtomole)

- Peak stability can be more than 20% RSD


Takats et al., Journal of Mass Spec. 2005





Dependence of signal intensity on surface temperature and surface potential (melittin is a small peptide)



Dependence of signal intensity on SPRAY IMPACT ANGLE, SPRAY VOLTAGE, GAS INLET PRESSURE & FLOW RATE


Signal intensity vs. the amount of peptides deposited

Pure peptides5 orders of magnitude

Peptides mixture (tryptic digest), much less dynamic range and ion

suppression effect

Dynamic Range of DESI


Detection of Different Metabolites with DESI

Reactive DESI- Adding reagents to the DESIO spray to cause interface reactions of the reagents with the condensed phase analytes

- Thus, adding chemical selectivity

- Examples:1. Redox reactions of metal complexes2. Covalent modifications of particular functional groups3. Formation of non-covalent complexes

Applications/examples of DESI

Example of a DESI Application

- DESI of a dry urine spot (10µl) on a paper- No sample preparation, just deposition on surface- Spray with Methanol/water/acetic acid- Rate of one analysis per second


Human Urine sample


Detection of Explosives with DESI

- RDX explosive- 1ng RDX on mm2

-Desorbed from an insulating tanned leather surface

-Negative ion DESI -Acetonitile/methanol/TFA spray solvent

Detection of a Chemical Warfare Agent Simulant with DESI

- The analyte- DMMP- Positive ion DESI- Nitrile gloves that were exposed for less than 1 sec. to DMMP vapor and then washed and dried

Detection of Dried E.Coli Culture with DESI

DESI mass spectrum of a dried sample of E.coli cells

In the future: bacteria on skin, food stuffs and biological fluids will be measured by these type of methods

Spatial Analysis of Native Surfaces with DESI (Plant or animal tissues)

- Seed section of Conium- Positive ion DESI- M/Z/ 126 corresponded to the protonated alkaloid Coniceine

Spatial Analysis of Native Surfaces with DESI (Plant or animal tissues)Distribution of the alkaloid Coniceine across a stem cross section

Spatial Analysis of Native Surfaces with DESI (Plant or animal tissues)Tomato fruit skin analysis

Spatial Analysis of Native Surfaces with DESI (Plant or animal tissues)Tomato fruit skin analysis

In Vivo Sampling of Living Tissues Surfaces with DESI

- Person taking 10mg of over-the-counter the antihistamine Loratadine

- 40 min after taking the tablet, molecule became detected in Saliva or skin

- Surface concentration remained above the detection limit for another 50 min.

In Vivo Sampling of Living Tissues Surfaces with DESIDirect Tissue Profiling of Liver Tumor Tissue

Cooks et al., Science 2006

In Vivo Sampling of Living Tissues Surfaces with DESI

DESI-MS/MS Data for Main Ingredients in Tablets

Chen et al., Anal. Chem. 2005

HIGH THROUGHPUT DESI-MS/MS Data (Claritin Tablets)

Chen et al., Anal. Chem. 2005Speed of 0.76 sample/s

DESI FINGERPRINT

DESI of Flower Tissue

Detected essential oils, carotenoids and antioxidants

Hibiscus flower

Other Ambient MS MethodsDART: Direct Analysis in Real Time (different projectiles)

DAPCI: Desorption Atmospheric Pressure Chemical Ionization (different projectiles)

ELDI: Electrospray-assisted Laser Desorption/Ionization (uses laser for desorption)

The End

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AMBIENT Mass Spectrometry. Ion Sources in the Mass Spectrometer - Analyzed samples are ionized prior to the analysis in the mass spectrometer - A variety