Mass spectroscopy problem set -1


















a) What is the basic principle of mass spectrometry?

The basic principle of mass spectrometry (MS) is to generate ions from
 either inorganic or organic compounds by any suitable method, to
 separate these ions by their mass-to-charge ratio (m/z) and to detect
them qualitatively and quantitatively by their respective m/z and
abundance (Chap. 1.2.1).

b) Can you sketch the basic components of a mass spectrometer?

A mass spectrometer consists of an ion source, a mass analyzer
and adetector which are operated under high vacuum conditions.
A closer look at the front end of such a device might separate the
 steps of sample introduction, evaporation and successive
 ionization or desorption/ionization. More recent systems
will have some data system used to collect and process data from the detector .

scheme of any mass spectrometer

c) What does m/z mean?

The mass-to-charge ratio, m/z, (read "m over z") is dimensionless by
definition. It is best understood as calculated from the dimensionless
mass number, m, of a given ion, and the number of its elementary charges, z



Answer 1.2

a) How is a mass spectrum defined?

A mass spectrum is the two-dimensional representation of
signal intensity (ordinate) versus m/z (abscissa). The intensity
of a peak, as signals are usually called, directly reflects the
abundance of ionic species of that respective m/z ratio which
 have been created from the analyte within the ion source .

b) Assign the following terms to the EI mass spectrum below: base peak,

 molecular ion peak, and fragment ion peak.

EI spectrum
Spectrum by courtesy of NIST.

c) Can you identify the unknown compound from its spectrum?

Fragment ion peaks at m/z 12 and 16 point towards atomic ions
of 12 and 16 u, i.e., 12C+. and 16O+. ions. The ion at m/z 28
 corresponds to [M-16]+, and therefore indicates [M-O]+.
Thus, the molecular ion seems to consist of C and O exclusively.
 The mass difference between M+. and m/z 16 is 28 u and
should result from loss of a molecule of CO. Other neutral
 losses are not observed.
This spectrum belongs to carbon dioxide, CO2, 44 u.
The ion at m/z 22 is not a fragment ion, but is due to
the doubly charged molecular ion, CO22+.



Answer 1.3

a) What is the unit for atomic mass?

Since 1961 the unified atomic mass [u] is defined
 as 1/12 of the mass of one atom of nuclide 12C
which has been assigned to 12 u exactly by convention

b) Do you have an estimate of the magnitude of the 

masses mass spectrometry deals with? Calculate the 

molecular weight of some typical small molecule in

 units of kilograms.

1 u is obtained by dividing the mass of 1 mol 12C by
 the number of atoms (Avogadro's constant):
1 u = 0.001 kg/mol / 6.022 * 1023 = 1.666 * 10-27 kg
Thus, a molecule of CO2 has a mass of 7.330 * 10-26 kg,
 for example.
In general, molecular masses are in the range of 10-26-10-23 kg.

c) Can you tell m/z, Thomson, u and Dalton apart?

Some mass spectrometrists use the unit thomson [Th]
(to honor J. J. Thomson) instead of the dimensionless
 quantity m/z. Although the thomson is accepted by some
 journals, it is not a SI unit.
In particular mass spectrometrists in the biomedical field
of mass spectrometry tend to use the dalton [Da]
(to honor J. Dalton) instead of the unified atomic mass
 [u]. The dalton also is not a SI unit 

a) What is the unit for atomic mass?

Since 1961 the unified atomic mass [u] is defined as 1/12
 of the mass of one atom of nuclide 12C which has been
assigned to 12 u exactly by convention (Chap. 1.2.3).

b) Do you have an estimate of the magnitude of the 

masses mass spectrometry deals with? Calculate the

 molecular weight of some typical small molecule in units of kilograms.

1 u is obtained by dividing the mass of 1 mol 12C by the
number of atoms (Avogadro's constant):
1 u = 0.001 kg/mol / 6.022 * 1023 = 1.666 * 10-27 kg
Thus, a molecule of CO2 has a mass of 7.330 * 10-26 kg,
 for example.
In general, molecular masses are in the range of 10-26-10-23 kg.

c) Can you tell m/z, Thomson, u and Dalton apart?

Some mass spectrometrists use the unit thomson [Th]
 (to honor J. J. Thomson) instead of the dimensionless
quantity m/z. Although the thomson is accepted by
 some journals, it is not a SI unit.
In particular mass spectrometrists in the biomedical
 field of mass spectrometry tend to use the dalton [Da]
(to honor J. Dalton) instead of the unified atomic mass [u].
The dalton also is not a SI unit



Answer 1.4

a) What pressure units do you know?

Pascal (Pa, SI unit), bar (bar, SI conform), Torr (Torr)
and pounds per square inch (psi) are frequently employed.

b) Do you know their conversion factors?

Only the Pascal is an SI unit, but fortunately, the
 mbar is just a 100 times larger. The other non-SI units
 have disadvantageous conversion factors.

c) In what pressure range are mass spectrometers 

normally operated?

Mass spectrometers are usually operated in the high vacuum
 regime to ensure mean free paths significantly longer than the
 mass analyzer's dimensions.


a) How is sensitivity defined in mass spectrometry?

The sensitivity is the slope of a plot of analyte amount versus signal strength. In mass spectrometry, sensitivity is reported as ionic charge of a specifiedm/z reaching the detector per mass of analyte used. The sensitivity is given in units of C µg–1 for solids.
For gaseous analytes, it can be specified as the ratio of ion current to analyte partial pressure in units of A Pa–1.(Chap. 1.4.1)

b) What is the meaning of the term detection limit?

The limit of detection (LOD) or detection limit states the lowest amount of sample that can yield a signal just good enough for reliable detection. The LOD depends on the compound, its state (pure, in solution, component of a complex mixture), the type of instrument, and its modes of operation (ionization method and scan mode) (Chap. 1.4.2).

c) Do you know the meaning of the acronyms TIC and RIC?

The total ion current (TIC) can either be measured by a hardware TIC monitor before mass analysis, or it can be reconstructed by the data system from the spectra after mass analysis. The TIC represents a measure of the overall intensity of ion production or of mass spectral output as a function of time, respectively. The TIC obtained by means of data reduction, i.e., by mathematical construction from the mass spectra as successively acquired while the sample evaporates, is also termed total ion chromatogram (TIC). For this purpose, the sum of all ion intensities belonging to each of the spectra is plotted as a function of time or scan number, respectively.
The term reconstructed ion chromatogram (RIC) is used to describe the intensity of a given m/z or m/z range plotted as a function of time or scan number (Chap. 1.3).

d) What are the potential uses of TICs and RICs?

Plotting RICs is especially useful to identify a target compound of known m/zfrom complex TICs. RICs allow to extract at what time during a measurement the target compound is eluted. RICs can also be used to uncover the relationship of certain m/z values to different mass spectra obtained from the measurement of a single (impure) sample.







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