conditions all ions can be stored for a prolonged time period. Following individual
ions can be forced by changing the field conditions to leave the ion-trap for a
subsequent detection. These mass spectrometers are also very sensitive and exhibit
higher mass resolution.
Technical improvement of this basic principle is achieved by the ion cyclotron
resonance mass spectrometry, that needs Fourier-transformation (FT-ICR MS), and
the so-called Orbitrap. Both are devices with very high sensitivity and high mass
resolution.
All different types of mass spectrometer exhibit their own advantages and
disadvantages and are therefore techniques commonly used in different applications
and laboratories. A very rough differentiation can be done taking the three basic
parameters sensitivity, mass resolution and costs into account (see Table 4.3).
As one can see, mass resolution is an important parameter in mass spectrometry.
In simple word, it is the accuracy in mass detection and can be illustrated as signal
width of the individual masses detected. Considering a given signal width, two
neighbored signals need to have a minimum distance to become resolved as illustrated in Fig. 4.28. Or with other words, if the signals are nearby and have a broad
width, they appear as one superimposed but not well separated signal. Formally, the
resolution of a mass spectrometer is given relative to the measured masses. A low
resolution of around 1000 means that a mass is accurate by 1/1000 of the measured
value. For a mass of 500 Da this implies an accuracy of 0.5 Da or, simply spoken, a
second signal with a difference of 0.5 Da can be detected just separately. High
resolution mass spectrometry allows mass resolution of 5000 up to 1,000,000.
The higher the mass resolution, the better the separation of similar but not
identical masses. This issue becomes interesting by a closer look on the atomic
masses. Ignoring decimals leads to the so-called nominal masses, e.g. 12 Da for
carbon, 1 Da for hydrogen or 16 Da for oxygen. If we have a look on fragments and
molecular ions in mass spectra that means e.g. 15 Da for a methyl group or 32 Da for
methanol. However, atom masses are not nominal, but have variances as exemplified
for some elements in Table 4.4.
These variances have implication for the exact masses of fragment and molecular
ions measured in mass spectrometry. The exact mass of a methyl group is not 15 Da
but 15.023475 Da. Further examples of nominal vs. exact masses are given for the
molecular masses of some selected pollutants in Table 4.5.
If a mass spectrometer is able to measure masses very accurately, it enables to
resolve or deconvolute signals with the same nominal masses but different molecular
compositions. This is the main clue of exact mass spectrometry: to obtain information on the elemental composition of measured masses (fragments or molecular ions,
Table 4.3 Brief comparison
of most common mass
spectrometers
MS type
Mass resolution
Sensitivity
Costs
Sector field
High
Low
High
Quadrupole
Low
Low
Low
Ion-trap
Low–high
High
Low–high
Time-of-flight
High
High
High
68
4 Instrumental Analysis
ions can be forced by changing the field conditions to leave the ion-trap for a
subsequent detection. These mass spectrometers are also very sensitive and exhibit
higher mass resolution.
Technical improvement of this basic principle is achieved by the ion cyclotron
resonance mass spectrometry, that needs Fourier-transformation (FT-ICR MS), and
the so-called Orbitrap. Both are devices with very high sensitivity and high mass
resolution.
All different types of mass spectrometer exhibit their own advantages and
disadvantages and are therefore techniques commonly used in different applications
and laboratories. A very rough differentiation can be done taking the three basic
parameters sensitivity, mass resolution and costs into account (see Table 4.3).
As one can see, mass resolution is an important parameter in mass spectrometry.
In simple word, it is the accuracy in mass detection and can be illustrated as signal
width of the individual masses detected. Considering a given signal width, two
neighbored signals need to have a minimum distance to become resolved as illustrated in Fig. 4.28. Or with other words, if the signals are nearby and have a broad
width, they appear as one superimposed but not well separated signal. Formally, the
resolution of a mass spectrometer is given relative to the measured masses. A low
resolution of around 1000 means that a mass is accurate by 1/1000 of the measured
value. For a mass of 500 Da this implies an accuracy of 0.5 Da or, simply spoken, a
second signal with a difference of 0.5 Da can be detected just separately. High
resolution mass spectrometry allows mass resolution of 5000 up to 1,000,000.
The higher the mass resolution, the better the separation of similar but not
identical masses. This issue becomes interesting by a closer look on the atomic
masses. Ignoring decimals leads to the so-called nominal masses, e.g. 12 Da for
carbon, 1 Da for hydrogen or 16 Da for oxygen. If we have a look on fragments and
molecular ions in mass spectra that means e.g. 15 Da for a methyl group or 32 Da for
methanol. However, atom masses are not nominal, but have variances as exemplified
for some elements in Table 4.4.
These variances have implication for the exact masses of fragment and molecular
ions measured in mass spectrometry. The exact mass of a methyl group is not 15 Da
but 15.023475 Da. Further examples of nominal vs. exact masses are given for the
molecular masses of some selected pollutants in Table 4.5.
If a mass spectrometer is able to measure masses very accurately, it enables to
resolve or deconvolute signals with the same nominal masses but different molecular
compositions. This is the main clue of exact mass spectrometry: to obtain information on the elemental composition of measured masses (fragments or molecular ions,
Table 4.3 Brief comparison
of most common mass
spectrometers
MS type
Mass resolution
Sensitivity
Costs
Sector field
High
Low
High
Quadrupole
Low
Low
Low
Ion-trap
Low–high
High
Low–high
Time-of-flight
High
High
High
68
4 Instrumental Analysis