Agriculture Reference
In-Depth Information
NMR sample tube (see Figure 8.7,
D
) and all absorptions are recorded rela-
tive to the absorption due to TMS.
8.13.
MASS SPECTROSCOPY
Mass spectroscopy as the name implies is a method by which the mass of a
molecule, that is, its molecular weight, of a pure compound is determined. A
sample is introduced into the instrument, ionized, and fragmented during the
ionization process. It and its fragments are accelerated down a path that can
be one of several types, separating ions and fragments on the basis of their
mass and charge, and finally the ions are detected and recorded. Because the
method depends on the ions moving unimpeded through the path, the inside
of the instrument must be under high vacuum at all times.
The ionization-fragmentation process can be accomplished by bombarding
the sample with electrons or with a chemical species. This process removes an
electron from the compound and fragments it, producing a positive species.
Accelerating plates at high negative voltage attract the particles that pass
through a hole or slit in the plate and move down the path. The path may be
a straight tube at the end of which is a detector. The time it takes an ion to
travel the length of the straight tube will depend on its mass and charge. This
is called a
time-of-flight
(TOF) mass spectrometer.
In a magnetic sector mass spectrometer the path of the ions is through a
curved tube with a magnet at the curved portion. The path of the charged
species will bend in the magnetic field depending on the strength of the mag-
netic field and the mass and charge of the ions. After passing the magnet, the
charged species will impinge on a detector and be recorded. When the strength
of the magnet is changed, the masses of the ions reaching the detector will
change and be recorded.
In a quadrupole mass spectrometer the ions pass into a path between four
rods attached to an electric circuit that can induce a range of frequencies in
the rods. Ions will resonate in the quadrupole until a certain frequency, which
depends on their mass and charge, is reached, and then the ions exit the
quadrupole and are measured. A diagram of a quadrupole mass spectrometer
is given in Chapter 9, Figure 9.5.
As with all the spectroscopic methods, this method is best suited to mea-
surement and elucidation of the characteristics of pure compounds. For this
reason, MS is often used as a detector for gas chromatographs. The MS of
choice for this use is the quadrupole mass spectrometer. For this reason it will
be discussed again in Chapter 9, Section 9.2.4.
Using mass spectrometry it is possible to determine the molecular weight
of the compound being analyzed. It is also possible to distinguish between iso-
topes of elements. Thus
14
N and
15
N can be separated and quantified using mass
spectrometry.
Much work has been done on the nitrogen cycle and fate of nitrogen com-
pounds in the environment using
15
N mass spectrometry. Fertilizer or other
