Mass spectrometry

Mass spectrometry is among the most accurate techniques for the determination of molar masses. The procedure consists of ionizing the sample in the gas phase and then measuring the mass-to-charge number ratio (m/z) of all ions. Macromolecules present a challenge because it is difficult to produce gaseous ions of large species without fragmentation. However, two new techniques have emerged that circumvent this problem: matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization. We shall discuss MALDI-TOF mass spectrometry, so called because the MALDI technique is coupled to a time-of-flight (TOF) ion detector. Figure 19.1 shows a schematic view of a MALDI-TOF mass spectrometer. The macromolecule is first embedded in a solid matrix that often consists of an organic material such as trans-3-indoleacrylic acid and inorganic salts such as sodium chloride or silver trifluoroacetate. This sample is then irradiated with a pulsed laser, such as a nitrogen laser. The laser energy ejects electronically excited matrixions, cations, and neutral macromolecules, thus creating a dense gas plume above the sample surface. The macromolecule is ionized by collisions and complexation with small cations, such as H⁺, Na⁺, and Ag⁺. In the TOF spectrometer, the ions are accelerated over a short distance d by an electrical field of strength E and then travel through a drift region of length l. The time, t, required for an ion of mass m and charge number z to reach the detector at the end of the drift region is (see the Justification):

Where e is the fundamental charge. Because d,l, and E are fixed for a given experiment, the time of flight, t, of the ion is a direct measure of its m/z ratio, which is given by:

Fig. 19.1 A matrix-assisted laser desorption/ ionization time-of-flight (MALDI-TOF) mass spectrometer. A laser beam ejects macromolecules and ions from the solid matrix. The ionized macromolecules are accelerated by an electrical potential difference over a distance d and then travel through a drift region of length l. Ions with the smallest mass to charge ratio (m/z) reach the detector first.

Justification 19.1 The time of flight of an ion in a mass spectrometer Consider an ion of charge ze and mass m that is accelerated from rest by an electric field of strength E applied over a distance d. The kinetic energy, EK, of the ion is

EK = mv2 = zeEd

Where v is the speed of the ion. The drift region, l, and the time of flight, t, in the mass spectrometer are both sufficiently short that we can ignore acceleration and write v =l/t. Then substitution into this equation gives

Rearrangement of this equation gives eqn 19.6.

Figure 19.2 shows the MALDI-TOF mass spectrum of a polydisperse sample of poly(butylene adipate) (PBA, 1). The MALDI technique produces mostly singly charged molecular ions that are not fragmented. Therefore, the multiple peaks in the spectrum arise from polymers of different lengths, with the intensity of each peak being proportional to the abundance of each polymer in the sample. Values of Jn, Jw, and the heterogeneity index can be calculated from the data. It is also possible to use the mass spectrum to verify the structure of a polymer, as shown in the following example.

Fig. 19.2 MALDI-TOF spectrum of a sample of poly (butylene adipate) with Jn = 4525 g mol⁻¹ (Adapted from Mudiman et al., J. Chem. Educ., 74, 1288 (1997).)

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مواضيع ذات صلة

Units

Names of quantities

Working galvanic cells

Electrolysis

Experimental techniques

The rate laws

The electric potential at the interface

The structure of the interface

Catalytic activity at surfaces

Mechanisms of heterogeneous catalysis

Mobility on surfaces

The rate of desorption