Term
|
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 |
|
|
