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105 Lecture 3
Mass Spectometry
26
Chemistry
Undergraduate 1
05/04/2013

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Term
what is MS used for?
Definition

to determine the molecular masses of suitable compounds

 

it is often incorrectly called mass spectroscopy

Term
in MS what is produced from the sample which is then arranged according to their mass-to-charge (m/z) ratios?
Definition

a beam of ion is produced from the sample.

 

 

the final output is known as a mass spectrum. you can work out Mr, Molecular ion fragmentation , Relative isotope abundances

Term
who constructed the first MS?
Definition

Aston. he confirmed most elements have isotopes and won nobel prizel in 1922

 

in the 1980's, new soft methods of ionising samples developed an MS was no longer restricted to volatile compounds.

Term
what are the four basic components of a mass spectrometer? which condition do all components operate under?
Definition

four basic parts:-

  • sample inlet system
  • ion source
  • analyser
  • detector

all operate under condtions of high vacuum to ensure no ions collide

Term
what is the function of the ion source?
Definition

it produces molecular ions from sample

 

it accelerates ions so that they all leave the source with the same kinetic energy

 

Term
which part of a MS seperates ions based on their m/z ratio?
Definition

the analyser

 

 

the detector then records m/z and relative abundance of each ion type.

Term
on MS how do we calculate the kinetic energy?
Definition

KE = 2V

 

2 =charge on ion

V = coltage

Term

T/F

 

for the sample inlet system, small quantaties of sample are suffiecient to do MS?

Definition

True

 

< 0.1 mg for a routine sample

 

< 1 ng on some instruments

Term
how can liquid or gases be introduced to the sample inlet system?
Definition
by microlitre syringe and then volatilised within the heated inlet.
Term
what is used to introduce solids directly into the heated inlet?
Definition

a capillary

 

in addition, involatile samples can be introduced directly into a suitable ion source (eg MALDI technique)

 

specialised inlets are available to link MS to the output of a gas chromatograph (GC-MS) or other device for seperating mixtures.

Term
some applications of MS require the sample to be entirely atomised prior to analysis. what is this known as?
Definition

Atomic Mass Spectrometry

 

the sample is atomised and ionised by intense heating. An inductively coupled plasma torch is often used (>6000C)

 

the technique is called ICP-MS

 

uses of atomic mass spectometry include carbon dating and analysis of rocks

Term

the electron-impact EI method is the most widely used ionisation method for volatile samples with masses upto 1000 amu (dalton)

 

how are positive ions produced by EI?

Definition

Positive ions are produced by collision of gaseous sample molecules with a beam of high energy electrons (~ 70 eV)



M + e- ----> M+ + 2e-

Term
how come negative ions arent normally produced by EI collisions?
Definition

becase the incoming electrons (kinetic energy ~70 eV) have too much energy to be captured

 

multiple charged ions eg M2+ M3+ etc are NOT normally produced by electron impact ionisation Only uni-charged M+ ions.

 

Fragmentation of the parent M+ ion always occurs to some extent in EI

Term
what occurs to some extent to the parent ion in EI?
Definition
fragmentation occurs to the parent ion to some extent in EI
Term
name the ionisation method where sample needs not be volatile and is suitable for large bio-polymers in range 103-106 Da
Definition

matrix assisted laser desorbtion ionisation (MALDI)

 

here intense pulses from a laser strike a thin layer of the sample dispersed in a low m.w matrix eg. nicotinic acid

 

emission of positively charged sample ions occurs by a process that is not fully understood.

Term
in MALDI, at which length does the matrix efficiently absorb the laser light at?
Definition

266 nm

 

in MALDI, multiple charged ions (M2+ M3+) are often seen as minor peaks in a MALDI mass spectrum but there is very little ion fragmentation.

 

for high m.w samples (>2000 amu) 'Time of Flight' analysers must be used

Term

in which ionisation method, EI or MALDI would you see a lot of ion fragmentation?

 

when are 'Time of Flight' analysers used in MALDI?

Definition

EI, in MALDI there is very little ion fragmentation

 

'Time of Flight' analysers used in MALDI when samples have high m.w >2000 amu

Term
name the method of ionisation particularly suited to polar molecules with masses 103 to > 106 Da
Definition

Electro-Spray ionisation (ESI)

 

here, microlitres of the sample in solution are pumped through the nozzle. the intense electric field spreads the liquid into a fine spray of positively charged droplets. solvent is stripped from droplets by evaporation before the positively charged ions eneter the analyser

Term
what is often produced with large molecules using electro-spray ionization?
Definition

highly positive ions

 

 

fragmentation of molecular ions is not signficant in ESI

Term

once through the ionisation chamber, ions have been accelerated electrostatically and their kinetic energy/charge ratios are the same. which ions move fast and which ions move slow?

 

name the 2 fundamental approches to sorting ions in the mass analyser. which of these has lower resolution>

Definition

Light M+ ions - fast moving

Heavy M+ ions - slow moving

 

1. Use of magnetic or electric fields - most common but limited to ions with m/z ratio <2000 amu

 

2. Time of Flight measurements - used with MALDI and ESI sources, unlimited m/z but lower resolution

Term
how does the magnetic amalyser work?
Definition

ions deflected through circular arc ~90 degrees by magnetic field.

 

ions of low m/z deflected more strongly than those with larger m/z

 

ions of different m/z value are focused onto the detector by changing strength of the magnetic field

Term
name some related analysers using magnetic or electric fields to deflect the ions
Definition
electrostatic, quadrupole, ion-trap
Term
how does a a time-of-flight analyser work?
Definition

the ions all enter at the same time as they are produced in very short pulses by a laser.

 

the ions travel freely through a drift tube and their times of arrival at the detector are recorded. Flight times are typically in the micro-second range

Term
in the detector, what constitutes an electrical current and what amplifies and processes these signals?
Definition
positive ions striking the detector surface constitutes an electric current that is amplified and processed by the spectrometer's computer
Term
which ionisation mathod is used widely in organic applications?
Definition

EI is used because a feature of EI is the fragmentation of some or most of the parent M+ ions in the ionisation chamber.

 

this makes it possible to learn more about the structure of an organic molecule by inspecting what fragment ions are produced like a fingerprint.

 

however you cant always work out the exact structure without further evidence from NMR and IR

Term

some uses of MS

 

chemical identification - by fragmentation and accurate masses

 

forensic testing - presence of drugs, exlposives

 

enivironmental monitoring - pollutants, air quality

 

archaeology and geology - dating rocks and shit

Definition

medical research - some illnesses produce chemical species in abnormal quantities

 

biology - structural data on biopolymers. metabolic pathway determinations using isotopic tracers

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