No thanks! I would like to know more about CHROMacademy

 Over 3000 E-Learning topics / 5000 Articles & Applications
 

7 Must Know Points to Understand Atmospheric Pressure Chemical Ionization

CHROMacademy delivers key knowledge quickly in bite-size modules, and to prove this here is our 7 point guide to understanding Atmospheric Pressure Chemical Ionization.

 

1) Atmospheric Pressure Chemical Ionization (APCI) uses gas-phase ion-molecule reactions at atmospheric pressure.  It relies on the formation of a plasma of ions comprised, mainly, from the HPLC mobile phase components. 

Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player




 

2) In APCI the eluent is introduced into the interface using a capillary of similar design to the ESI source.  However, no potential is applied to the capillary, instead the liquid emerges from the capillary surrounded by a flow of inert nebulizing gas into a heated, vaporizing region.

nebulizing gas

 


 

3) The combination of nebulizing gas and heat forms an aerosol with the sprayed eluent that begins to rapidly evaporate

Aerosol droplet
Aerosol droplet consisting of analyte and mobile phase molecules

 
 

CHROMacademy Lite Membership is FREE and it only takes two minutes to register.

With a Lite Membership you are given access to:
  • this month's webcast & tutorial
  • selected eLearning modules
  • featured CHROMacademy Content
  • the CHROMacademy forum

Register here »

Test drive CHROMacademy Check out
more great content available FREE to our
Lite members »

 

You may also like...

10 Must Know Points to Understand Electrospray Ionisation »

Optimizing LC-MS and LC-MS-Ms
Webcat & Tutorial »

Mass Analyzers eLearning Module »

 
ask the expert
 


4) A pin is placed within or at the end of the heated region that has a high potential
(5 – 10 kV) applied to it and produces an electrical discharge (1-5 μA) which ionizes molecules within the aerosol.  A combination of molecular collisions and charge transfer processed cause an ionized gas plasma to be formed, primarily from the eluent molecules that are in vast excess.

aerosol

If nitrogen is the nebulizing and auxiliary gas with atmospheric water vapor present in the source then the following primary and secondary reaction cascade will occur in the corona discharge region.

APCI cascade reaction

                                         APCI Cascade Reaction

 

5) The cascade reactions which occur in the corona discharge region result in the production of water cluster ions which predominate in the mass spectrum.  Declustering of analyte ions may be achieved using one, or a combination, of the following approaches:

  • Using a counter current gas flow at the nozzle plate, also known as a curtain gas
  • Using a heated transfer capillary between the API region and the nozzle-skimmer region
  • Using a drift voltage between the nozzle and skimmer plates to promote intermolecular collisions between the ion clusters and background gas molecules (collision induced dissociation CID)

 

 

Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player

 

 

Collisional area6) Sample molecules that elute in the gas phase into this plasma may be ionized via transfer of protons to give either positive or negative ions depending on the proton affinity of the analyte species relative to the solvent gas plasma molecules.


Negative Ion Mode

If the solvent (S) has a larger proton affinity than the analyte (M), then a proton is abstracted from the analyte by the solvent.

Also anions (A-) can be attached

The formation of ions by associative electron capture

Dissociative electron capture occurs in the presence of good leaving groups (X) like halogens

 

Positive Ion Mode

If theanalyte (M) has a larger proton affinity than the solvent (S) then the analyte will take a proton from the protonated solvent

If both species present similar proton affinities adducts are formed

Charge exchange or electron transfer occurs with reagent gases which do not contain available hydrogen atoms. 
The analyte (M) has a lower proton affinity than the solvent (S)


 

7) A less well documented mechanism for APCI is the triboelectric effect.  As the mobile phase and analyte species exit the nebulizer the shear forces generated by the nebulizing gas tear the liquid stream into droplets.  The friction generated by this process generates an electric charge (the triboelectric effect) at the liquid-gas interface.  This can cause ion formation in some non-volatile analytes.  Triboelectric APCI does not depend upon the electrons generated by the corona discharge electrode and ions may be generated (with their associated mass spectrum signals) when the corona discharge is very low or turned off.  Triboelectric APCI is most common for analyte molecules that are moderately polar and/or non-volatile.

 
nebulizing gas
 

CHROMacademy Forum

loading data
loading data
loading data
loading data
loading data

group  subsCHROMacademy can deliver to corporate clients on a multi-user subscription basis.
Served up from secure servers to the corporate intranet or individual desktops.

  • Microsite - your own learning site powered by CHROMacademy
  • Your Landing Pages -with your logo and branding
  • Customized Assessments - Based on content agreed upon Certificate of Completion
  • Certification Programs - Offer your learners a goal to strive towards
  • LMS : Connect - Our Learning Management System is S.C.O.R.M. compliant and will connect to your system
  • Engagement Package - E-newsletter stimulation program derived from your content and ours
  • Full archive of Essential Guide webcasts & tutorials
  • 1000’s of eLearning topics - HPLC / GC / Sample Prep / Mass Spec
  • Ask the Expert - our experts will answer your chromatography questions within 24 hrs.
  • Assessments - test your knowledge
  • Application notes & LCGC articles
  • Troubleshooting and virtual lab tools

Request a quote

 

 Home | About UsContact Us | SubscribeTerms and Conditions | Advertise | Privacy Policy 

loading data

loading data

loading data

 

loading data


loading data