MS Flashcards
In positive mode electrospray ionization (ESI), how do you avoid matrix adduction of acetonitrile with a neutral analyte?
That can sometimes happen, but can be avoided by adding 20 - 50 uL of potassium/sodium acetate or formic acid (lowers the pH)
How do you handle analytes containing lots of basic sites (e.g. amines)?
Can be handled in positive mode ESI. Lower the pH 0.1 to 1.0% acetic acid or formic acid is effective. Good solvents include ethanol, methanol and acetonitrile in H2O
How do you handle analytes containing lots of acidic sites?
Can be handled in negative mode ESI. Raise the pH 0.1 to 1.0% ammonium hydroxide. Good solvents include propanol, isopropanol and butanol in H2O
Which ionization source is used for GC
Electron (capture) ionization (EI) or electrochemical ionization (ECI)
When should electrochemical ionization (CI) source be chosen instead of electron impact ionnization (EI)
When analyzing compounds prone to fragmentation. CI is softer ionization than EI.
Why is the base peak (highest peak) often not the parent molecular ion in electron impact ionization (EI)
EI is a hard ionization source, which leaves low chance that the parent ion will be intact
Which ionization source is used in LC-MS
Electrospray inoization (ESI) turns liquid into aerosols from which ions are vaporized
APCI
APPI
What does a total ion chromatogram TIC show
A TIC shows everything that is recorded in a mass spec in a simplified way: shows time on x-axis and count on the y-axis. Count is just the total amount of ions hitting the detector
How can you display all recorded data from a mass spectrometer
In a matrix put time x-axis, and m/z on the y-axis with a color corresponding to the intensity
How do you calculate the chemical formula from a mass spec (alkane)
C = [M+1]/[M]
Describe the essential parts in an atmospheric pressure chemical ionization source. What are their roles?
(1)
A nebulizing gas flow creates a mist of droplets
(similar to ion spray)
(2)
Droplets are heated and evaporate
(3)
Neutral analytes are released into the gas Phase
(4)
A corona discharge releases electrons that ionize
nitrogen
N2 + e- –> N2+* 2e-
(5)
The charge is transferred to the solvent (in excess)
H20* + N2 + H20+*
H20+* + H20 H30+ + HO*
(6)
Which in turn transfers the charge to the analyte
(having a higher proton affinity than water)
+ M MH+ + H20
How does the N rule in electron ioninzation EI work?
An even number of N will have an odd m/z ratio
with an odd m/z ratio: N = 0 or 2
How does the N rule in electron ioninzation EI work?
In LC with ESI: What to do if the column flow rate is too high?
-Split off some flow
-Unaffected sensitivity since ESI is concentration-dependent
-Use an “ion spray”
In LC with ESI: What to do if the mobile phase pH yield gives neutral analytes?
-Tee-in additives that promote ionization (acid, base, Na etc.)
-Loss of sensitivity as ESI is concentration dependent
