MLSCI 466 - Lec 6 Mass Spectrometry

what technique is MS

microanalytical technique (pmoles/nmoles of sample)

what is MS

• tells us the structure/molecular weight of analyte
• shows chemical fingerprint of analyte (characteristic fragmentation pattern)
• destructive process

steps in MS

inlet/specimen delivery (e.g. GC or LC) --> ionization --> mass analyzer --> detector --> data receiving/storage

electron impact mechanism

1. heated filament produces electron beam from ion source
2. sample vapour enters inlet
3. magnets produce a field to guide electrons through source following a tight helical path
4. ionization produced as sample molecules and electrons collide
5. ions enter mass analyzer as they drift out of source and accelerate through strong electrostatic field

what is a quadrupole

a mass analyzer

components of quadrupole

4 parallel rods with opposie rods electrically connected and combined dc and rd voltage applied between the two pairs

electron multiplier detector

uses secondary electron emission to effect amplification

what is mass/charge ratio

• ratio of mass number (m) of a given particle to the number of (z) electrostatic charge units carried by the particle

• sometimes it is just the mass of the particle as most ions have only one electrostatic-charge

units for mass/charge ratio

m/z

what is a molecular ion

• results from ionization of analyte molecule
• intact molecule minus one electron
• precursor of all fragment ions

what is a base ion

most intense peak in mass spectrum i.e. highest number of ions

deconvolution

extracting one signal from a complex mixture involving treatment of noise, correction for baseline dift, and extraction of closely eluting peaks from anotehr

library match

compares your spectra to a reference database by doing a quality match (greater than or equal to 60%), looking at retention times (plus or minus 0.15 mins), and visual inspection (qualitative judgement)

problems with GC-MS

• compromised analysis of heat labile compounds such as oxazepam or methadone
• poor chromatography, extraction, or other factors related to the compound

how do we overcome issues with GC-MS

derivatization, hydrolysis, SIM vs full scan

full scan

monitors ALL ions

selected ion monitoring (SIM)

• only looks at some ions usually for quantitative purposes
• often includes derivatization step that depends on analyte functional groups

advantages of SIM

increased sensitivity

disadvantage of SIM

limited number of analytes detected

advantages of LC-MS

• suitable for volatile and nonvolatile solutes
• appropriate for ionic/polar analytes
• good sensitivity

disadvantages of LC-MS

• relatively low HPLC flow rates required
• must be able to form ions in solution
• ion suppression with high salt and under certain matrix conditions may occur

electrospray ionization theory

1. charge droplets formed
2. evaporation of charged droplets
3. ionic repulsion
4. ions enter mass analyzer

triple quadrupole (Tandem) MS

• a single m/z is selected by the quadrupole analyzer (Q1)
• fragmentation occurs in the collision cell (Q2)
• a single m/z is selected by the last quadrupole analyzer (Q3)

LC-MS steps

1. liquid sample is introduced to ion source

2. molecules of interest get converted to ions

3. under an electrical field and pressure gradient, ions move through the interface

4. ions can be transmitted, selected, or fragmented in the quadrupoles

5. ions are detected and the signal is communicated to the workstation -> spectra

TOF-MS principle

• ions are pulsed and accelerated into TOF analyzer
• determine m/z by the time it takes the analyte ions to fly through the flight path (smaller faster than heavier m/z)
• resolution increases with longer flight path (longer TOF tubes, reflector, faster acceleraiton)

components of TOF-MS/MS or QTOF

Q1 = filter Q2 = fragmentation TOF = separate ions

levamisole

anthelmintic/chemotherapeutic agent cutting agent associated with cocaine that causes acute, profound neutropenia

sample processing for levamisole GC/MS testing

urine specimen -> liquid/liquid extraction -> injection into GC/MS