Appendix 4a Introduction to the Mass Spectrometer

The mass spectrometer is an instrument by which you can separate ionised/charged (+) particles of different mass and determine the amounts of each particle in a mixture.

The technique is called mass spectroscopy or mass spectrometry ('mass-spec' and 'MS' in shorthand!).

Mass spectrometry gives accurate information on the relative masses of isotopes and their relative abundance (proportions).

Mass spectrometry is an important method of analysis in chemistry and can be used to identify elements by their characteristic mass spectrum pattern - the technique is used in planetary space probes e.g. mass spectrometer instrumentation is incorporated in the Mars explorer vehicles.

The substance to be analysed is introduced/injected into a high vacuum (extremely low pressure) tube system (at K left diagram) where the particles are ionised by colliding with beam of high speed electrons (at Q in left diagram).

Note: If the sample is not already a gas, then substance must be vapourised, i.e. the material must be in the gaseous state to be analysed in a mass spectrometer.

The resulting (+) ions are accelerated down a tube (from + to - plates, P in left diagram) and then through a powerful magnetic field.

The charged or ionised particles are deflected by this powerful magnetic field (R in left diagram).

How much they are deflected depends on the particle mass and the speed of the particle and the strength of a magnetic field i.e. lighter particles of lower mass (and momentum) are deflected more than heavier particles of bigger mass (see right diagram below) for a given set of conditions.

By varying the strength of the magnetic field, it is possible to bring into focus onto an ion detector (N in left diagram) at the end of the tube (effectively an electrical event is detected), every possible mass in turn and a measure the strength of the ion current, which is a measure of how much of that ion has been formed from the sample under analysis.

A simplified diagram of a mass spectrometer tube system is shown below (left) with further explanation as to what is going on and an extra diagram to show the relative paths of light to heavy ions for a given strength of magnetic field.

KEY TO DIAGRAM and more detail of each component's function

K = sample injection point, it must be a gas, so a liquid/solid must be vaporised at the injection point.

IONISATION

Q = high voltage (high +/- p.d.) electron gun which fires a beam of high speed/energy electrons from a heated 'metal element' into the vaporised sample under analysis and causes ionization of the atoms (or molecules) forming positive ions (mainly monopositive in charge).

The collision of high KE electrons with atoms or molecules causes another electron to be knocked off the particle leaving a negative deficit i.e. a positively charged particle is formed e.g.

M_(g) + e^- ==> M⁺_(g) + 2e^-, usually written as just

M_(g) ==> M⁺_(g) + e^- (M might represent e.g. a metal atom or a molecule)

The ions formed should be written as [M]⁺, a notation that is handy if you are dealing with ionised molecule fragments with an overall single positive charge e.g. [CH₃]⁺ is seen in the mass spectrum of methane gas, CH₄.

The low pressure (~vacuum) is needed to prevent the ions from colliding with air particles which would stop them reaching the ion detector system.

ACCELERATION

P = are negative plates which accelerate the positive ions down the tube (there are positive plates at the start of the tube). A moving beam of charged particles creates a magnetic field around itself, and this 'ion beam' magnetic field interacts with the magnetic field at R.

DEFLECTION-SEPARATION

R = the magnetic field that causes deflection of ions, this is can be varied to change the extent of deflection for a given mass and to focus a beam of ions of particular mass down onto the detector. Hence, by programming the mass spectrometer to 'sweep' through all likely particle masses, in terms of the right hand diagram, you can increase the strength of the magnetic field to bring into focus onto the ion detector monopositive ions of increasing mass.

DETECTION

N = an ion detection system which essentially generates a tiny electrical current when the ions hit it. The strengths of the 'electronic' signals from the various ion peaks are sent to a computer for analysis, computation and display. They tell you the particle masses present and their relative abundance (see the mass spectrum diagram for the element strontium below).

MASS SPECTRUM

The resulting record of the ion peaks is called the mass spectrum or mass spectra. The highest peak is called the base peak and is often given the relative and arbitrary value of 100, particularly in the mass spectra of organic compounds.

MASS SPECTRA

For elements you get a series of signals or ion peaks for each isotope present and the ratio of peak heights gives you the relative proportion of each isotope in the element so that you can calculate the relative atomic mass of an element. This 'simple' spectra of mononuclear ions like Sr⁺ is only true for non-molecular elements like metals (see mass spectrum of strontium diagram below) or noble gases, but for molecular elements like nitrogen or the halogens things are not so simple (see chlorine example below).

For larger e.g. organic molecules, things can be very complex indeed, as molecules fragment and many different ions can be formed BUT you can get the relative molecular mass of a molecule by identifying what is called the molecular ion peak, that is, when one electron is knocked of the molecule but the molecule retains its full molecular structure.

e.g. benzoic acid (M_r = 122) gives a molecular ion peak of m/e = 122, due to [C₆H₅COOH]⁺

but you also get fragments such as [C₆H₅]⁺ with an m/e of 77 as the molecule breaks up on further electron impact.

CHLORINE EXAMPLE

Chlorine is a good example of a molecular element whose mass spectra can be a bit tricky when first encountered ...

Chlorine consists of two principal isotopes, chlorine-37 (25% is ³⁷Cl) and chlorine-35 (75% is ³⁵Cl).

BUT, chlorine consists of Cl₂ diatomic molecules, which, on ionisation, can split into chlorine atoms.

The result is a series of 5 different mass peaks from the various isotopic atomic or molecular ion possibilities.

1. [³⁷Cl³⁷Cl]⁺ or [³⁷Cl₂]⁺ m/z = 74

2. [³⁷Cl³⁵Cl]⁺ m/z = 72 (note that you must show the two isotopes separately)

3. [³⁵Cl³⁵Cl]⁺ or [³⁵Cl₂]⁺ m/z=70

4. [³⁷Cl]⁺ m/z=37

5. [³⁵Cl]⁺ m/z=35

m/z means the relative mass of the ion over its charge, which for our purposes the charge is taken +1 (little z) and the mass (little m) is the relative atomic/formula mass of the particle. m/z sometimes quoted as m/e. You should write the structure of the ion in square brackets and put the charge on the outside of them in the top right - this is an important and universally accepted notation in mass spectrometry.

Examples of the calculation of the relative atomic mass of an element using % of isotopes is given in Part 1 of GCSE-AS (basic) calculations, an example of calculating relative atomic mass from a mass spectrum is given below for the metallic element strontium.

The mass spectra of organic compounds can be very complex as the molecules fragment under electron bombardment, but the resulting mass spectra can used to identify compounds from their 'finger-print' pattern of ion peaks of different mass and particular proportions for a given set of experimental conditions.

The largest m/z value gives the molecular mass of a molecule, i.e. the ion of largest mass, prior to fragmentation, is formed when the original whole and neutral molecule, loses one electron e.g. for ethane it would be due to the formation of [C₂H₆]⁺, m/z = 30 and is called the molecular ion peak.

STRONTIUM EXAMPLE

A 'simple' element spectrum to interpret AND a subsequent relative atomic mass calculation based on the mass spectrum of the element strontium

The relative atomic mass of an element, A_r, is the weighted average mass of the isotopes present, compared to 1/12th of the relative mass of the carbon-12 isotope. [ ¹²C is given the relative mass value of 12.0000 ]

Quite often the highest m/e peak is arbitrarily given the relative value of 100, as in this case and referred to as the base peak, but the peak lines might well indicate % abundance of isotopes. The diagram of abundances is sometimes called a stick diagram.

Relative peak height = relative abundance as measured from the ion current detector signal.

The mass spectrum shows strontium consists of four isotopes, ⁸⁴Sr (peak height = 0.68), ⁸⁶Sr (peak height = 12.0),⁸⁷Sr (peak height = 8.47) and ⁸⁸Sr (peak height = 100.0)

The sum of the heights = 0.68 + 12.0 + 8.47 + 100.0 = 121.15

So we can now calculate the weighted average mass of ALL the isotopes.

Therefore A_r = {(0.68 x 84) + (12.0 x 86) + (8.47 x 87) + (100.0 x 88)}/121.15 =  87.7

The book value is 87.62, BUT this calculation does NOT take into account the very accurate relative atomic masses based on the carbon-12 scale, it merely uses the mass numbers, which are always integer.

ISOTOPIC MASSES - definition and uses

Very accurate isotopic masses are usually a tiny fraction different from a whole number but provide invaluable information.

Modern mass spectrometers are exceedingly accurate and very sophisticated instruments and can measure mass to at least 4 decimal places. They can readily distinguish between N₂, CO and C₂H₄ molecules, all with an integer M_r of 28.

The very accurate molecular ion masses are [N₂]⁺ = 28.0061, [CO]⁺ = 27.9949 and [C₂H₄]⁺ = 28.0313

A very accurate mass spectrometer (for high resolution mass spectroscopy) can even differentiate between organic molecules of the same integer molecular mass.

e.g. for the molecular mass 103, some possible, however unlikely, molecular formulae could theoretically be

C₅HN₃ = 103.0170,  C₃H₅NO₃ = 103.0269,  C₂H₅N₃O₂ = 103.0382,  C₇H₅N = 103.0427,  CH₅N₅O = 103.0494

Appendix 4b The Time of Flight Mass Spectrometer (for advanced level students only!)

Ion mass separation using a time-of-flight mass spectrometer - a more modern instrument

• In a time-of-flight mass spectrometer the ions are formed in a similar manner by electron bombardment, and the resulting ions accelerated between electrically charged plates.

• Again, the sample must be a gas or vapourised and is bombarded with an electron beam or laser beam to knock off electrons to produce positive ions.

• However, the method of separation due to different m/e (mass/charge) values is then dependent on how long it takes the ion to travel in the drift region' i.e. the region NOT under the influence of an accelerating electric field.

• The ions are accelerated in the same way between positive to negative plates in an electric field of fixed strength i.e. constant potential difference.

• The smaller the mass of the ionised particle (ionized atom, fragment or whole molecule) the shorter the time of flight in the drift region where no electric field operates.

• This is because for a given accelerating potential difference, a lighter particle is accelerated more to a higher speed than a heavier ion, so the 'time of flight' down the tube is shorter.

• Therefore the ions are distinguished by different flight times NOT by different masses being brought into focus with a magnetic field as described in section 4a BUT the separation by time of flight is still determined by the m/e (m/z) value of the ion.

• The general principles of the separation are required knowledge but the mathematics is NOT needed by A level students, but if you are interested, a simplified summary is given below

  • t = K_inst√(m/q)

  • t = time of flight, m = mass of ion, q = charge on ion,

  • K_inst = a proportionality constant based on the instrument settings and characteristics e.g. the electric field strength, length of analysing tube etc.

  • Therefore t is proportional to the square root of the mass of the ion for particles carrying the same charge - the bigger the mass the longer the 'flight time'.

  • The first equation is derived partly from the extra mathematics outlined below.

  • KE = qV, the kinetic energy imparted to the ion is given by its charge x the potential difference of the accelerating electric field.

  • The acceleration, for a fixed electric field, results in an ion having the same kinetic energy (KE) as any other ion of the same charge q but the velocity v of the ion depends on the m/e (m/z) value.

  • v = d/t (or t = d/v), where v = velocity of accelerated particle in the drift region, d = length of tube in the drift region. (or t = d/v)

  • KE = ¹/₂mv², so the bigger m, the smaller is v in the drift region and hence the basis of detecting ions of different mass by different 'flight times'.

  • The diagram makes the method look simple, but far from it, the instrument works in a pulsed manner i.e. pulsed electric field, and some pretty sophisticated electronics are used to analyse the signals from the detector and the software calculates the mass of the ion based on the drift flight time.