Principles of Tofsims

TOF-SIMS is a SIMS technique that utilizes a time-of-flight mass analyzer to enhance its sensitivity and expand its range of application (Vickerman and Briggs 2001). TOF-SIMS provides a characterization of complex sample surfaces such as protein-adsorbed materials, and chemical mapping information, for example, the imaging of the distribution pattern of a particular protein.

SIMS refers to the mass spectrometry of ionized particles (secondary ions) emitted by a beam of primary ions bombarding a surface. SIMS provides a characterization of a target surface by means of mass spectra, depth profiles, and secondary ion images. Surface mass spectra allow the identification and quantification of all constituent elements, isotopes, and molecular species via: (1) the controlled desorption of atoms and molecular species, (2) the efficient ionization of these desorbed particles, and (3) the unambiguous identification of the generated ions by their charge:mass ratios.

A detailed understanding of the interactions between ions and material surfaces, such as the phenomenon of sputtering, for example, is crucial for an appropriate application of SIMS. A beam of primary ions bombards a surface, leading to interactions that cause the emission of a variety of types of secondary particles, including secondary electrons, photons, neutrons, and positive and negative secondary ions from the sample, some of which recoil, so that the primary ions are scattered or implanted. The sputtering yield depends on the energy of the primary ions, the particular elements, the measurement conditions, and the encasing atmosphere (Benninghoven et al. 1987; Bubert and Jenett 2002; Vickerman and Briggs 2001). The processes of secondary-ion generation include sputtering and ionization processes that have not yet been fully clarified, although several simulations have been carried out and hypotheses put forward (Benninghoven et al. 1987; Krantzman et al. 2001). The ions emitted from organic samples are analyzed in a mass spectrometer, resulting in positive or negative mass spectra consisting of the parent and fragment ion peaks characteristic of the surface. The major portion of the secondary particles are neutrons, while only a fraction of the ~ 10-6-10-1 of the total are positively or negatively charged, and are called secondary ions (Benninghoven et al. 1993). Typical secondary ions from organic samples on metallic substrates are Me+/-, [M + H]+, [M-H]-, [M + Sa] + and [M +Me]+,where Me isametal such as Ag, M is a molecule, H is hydrogen, and Sa is either Na or K.

SIMS currently employs two modes of measurement, dynamic and static. In the dynamic mode, primary ions with high current densities are used to bombard a sample surface, damaging the surface. Although the dynamic mode has an extremely high sensitivity, it is not applied to organic samples because the damage is too extensive to obtain useful chemical information. In the static mode samples are sputtered softly with low current densities to conserve chemical structures. In TOF-SIMS, pulsed primary ions with high current densities are used to obtain a high sensitivity and to prevent damage destroying the chemical structures.

TOF-SIMS is suitable for highly sensitive detection because TOF MS offers extremely high transmission in combination with parallel detection of all masses. Moreover, TOF-SIMS is able to prevent insult of a sample from charge-up using a low-energy (< 20 eV) electron flood gun, which is pulse-controlled with a primary ion beam. In addition, with TOF-SIMS, biomaterials, including proteins, are qualitatively and quantitatively measurable, and the distribution of particular molecules may also be ascertained.

All emitted secondary ions are accelerated to a given potential, V (222 keV) and all ions with the same electric charge have the same kinetic energy in the TOF mass analyzer, as shown in Fig. 1. The velocity of each secondary ion depends on its weight, as shown in Eq. 1:

where q is the charge of the secondary ion (C), E is a given voltage (V), m is the weight of the secondary ion (kg), and v is the velocity of the secondary ion (m/s).

The ions are allowed to drift through a path of a given length, L (m) for time t (s), before reaching the detector:

Therefore, ions having a mass-to-charge ratio, m:q, can be calculated using Eqs. 1 and 2:

Figure 2 shows the relationship between the time-of-flight and the mass of each secondary ion. The lighter ions reach the detector faster.

Fig. 1. Schematic of time-of-flight secondary ion mass spectrometry (TOF-SIMS; Cyclone type)


„ Flight L

V40 t>> * o Vlo m2 'q2 mi 'qi m4 / q4 m, / q.


Detector (Time-of-Flight MS)

Fig. 2. Principles of the time-of-fight mass spectrometer (TOF-MS)

Fig. 2. Principles of the time-of-fight mass spectrometer (TOF-MS)

To date, liquid metal ion sources (LMIS) with low melting-point temperatures have been used mainly as the primary ion source for TOF-SIMS, because, 69Ga+ ions (melting point 30°) in particular can be focused to a 50-nm minimum-diameter probe, while being pulsed at frequencies of up to 50 kHz and rastered at the same time.

Polyatomic ion sources such as Aux+ (the gold cluster ion), Bix+ (the bismuth cluster ion), and C6o+ (Postawa et al. 2003, 2004; Vickerman and Briggs 2001) have recently been developed and recognized to be useful for the measurement of biomaterials, especially proteins, because they produce a greater amount of the larger-fragment ions from proteins and polymers than the conventional primary ion sources. The production of larger-fragment ions from proteins is required for complicated samples that include several proteins. A further innovation of the ionization enhancement technique for SIMS will be required in order to obtain clear images of each individual protein.

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