La importancia del significado del sustantivo

Inside the collision cell, the target ion fragments according to its kinetic energy and the potential held at the collision cell [134, 148]. After fragmentation, the collision cell will act as the ion source and the ions are accelerated into the second drift tube. A conventional MALDI-TOF/TOF-MS scheme is shown in Figure I.25.

Figure I.25 – Schematic representation of the tandem time-of-flight mass analyser.

I.5.2. SAMPLE PREPARATION: THE ROLE OF ELECTROMAGNETIC AND ACOUSTIC IRRADIATION

The potential of both microwave and ultrasonic irradiation to transform current methodologies, in which are involved biochemical reactions, is of great importance. In the drive towards cleaner, faster, low cost, high yields and high throughput methodologies, the introduction of such technologies presents an enormous potential. Numerous studies have reported the use of these alternative energy sources to catalyse a variety of reactions and chemical processes.

I.5.2.1 ULTRASONIC IRRADIATION I.5.2.1.1 ULTRASOUND

Ultrasound is defined based on the audible range of the human ear, which is able to detect sound frequencies between 20Hz to 20 KHz, considering a young, healthy person [179]. Therefore, ultrasound is a sound wave of a pitch above that frequency and it varies from 20 kHz to around 10MHz [180]. Figure I.26 presents the characteristic wavelengths and sound frequency ranges.

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Figure I.26 – Electromagnetic and acoustic spectra (Adapted from T.J. Mason and J.P Lorimer, Applied Sonochemistry: Uses of power ultrasound in chemistry and processing, Wiley-VCH, 2002).

Because ultrasound is a wave, it transmits energy just like an electromagnetic wave or radiation [181]. However, unlike an electromagnetic wave, it requires a medium to travel or propagate [181]. Sound waves, in which are included ultrasound waves, are transmitted through a homogeneous medium by vibrational motion of the medium molecules [179]. The sound source creates a mechanical disturbance or vibrational motion that is transmitted to the medium molecules that are consequently pushed to a distance from its original position [181]. Before returning to their original position each molecule transmits that force to the adjacent molecule, creating a chain reaction [181].

Due to the wide range of ultrasound frequencies and consequently wavelengths, the interaction between the ultrasonic wave and the propagation medium (matter) changes as a function of the wavelength. Accordingly, the applications of ultrasounds are conventionally divided in two broad areas, power and diagnostic [179].

High frequency ultrasound waves, around 2 MHz and 10 MHz, are normally used for medical diagnostic, since they do not affect the chemistry of the propagation medium, but instead are changed by that medium (e.g. scattered, reflected, refracted, attenuated or absorbed) [182]. The physical effect of the medium on the wave is of great importance, for instance, in imaging diagnostic systems, where the short wavelengths are vital to detect small areas of phase change.

Power or high energy ultrasound concerns the waves produced at lower frequencies, around 20 kHz and 100 kHz. At these frequencies a phenomenon called acoustic cavitation occurs within liquid systems and may produce both chemical and physical changes in the propagation medium [183]. The application of ultrasounds in chemistry deals mainly with this sonic spectrum and it is called sonochemistry [183]. Recently, high power equipment has been developed capable of generating cavitation phenomena at higher frequencies, 2MHz [179]. Following its successful use in chemistry, high energy ultrasound is now being used for therapeutic purposes (e.g. destruction of blood clots).

γ rays X rays UV Infrared Microwave Radio waves

55 I.5.2.1.2 SONOCHEMISTRY

As mentioned above, the high power of lower ultrasound frequencies in liquid media is concerned with the phenomenon of acoustic cavitation. The vibrational motion transmitted to the liquid medium induced by ultrasound waves causes alternately expansion and compression of the medium [179]. At sufficiently high power, the strength of the acoustic field may exceed the attractive forces of the particles in the expansion or rarefaction cycle and create cavities in the liquid medium [183, 184].

These cavities are called cavitation bubbles. Once formed, the cavitation bubbles grow during successive cycles by a process called rectified diffusion [185]. When oscillating pressures are transmitted on the liquid medium, small amounts of gas from the medium diffuse into the bubble during the expansion half cycle and out of the bubble during the compression half cycle [185].

However, the amount of gas expelled during the compression cycle is less than the amount that enters the bubble, which originates the growth of the bubble until it reaches an equilibrium size [185]. Under proper conditions, the bubble implodes violently in the compression half cycle, producing an extraordinary phenomenon known as “hot-spot” that originates temperatures of about 5,000 K and pressures around 1000 atm [183, 186-188]. Moreover, the formation of radicals from the medium components has been reported, which may contribute to promote specific reaction mechanisms [184, 189, 190].

Regarding heterogeneous systems involving both immiscible liquid-liquid or liquid-solid mediums, in addition to the formation of the hot-spot phenomenon, other sonochemical effects take place [179, 191]. When a cavitation bubble is formed and collapses close to a phase interface, the collapse is not symmetrical due to the resistance to liquid flow provided by that interface [179]. The result of this bubble deformation is the formation of liquid micro-jets that propagate, primarily, towards the interface at velocities greater than 100 ms^-1 [186]. When these jets are formed near a liquid-liquid interface, they originate very fine emulsions that are more stable than those originated conventionally [184]. At a solid-liquid interface, the jets formed cause pitting and mechanical erosion of the solid surface that lead to disruption of the solid particles. This feature has great impact on the reaction catalysis, since it increases the surface area of the solid and facilitates the contact between the liquid medium and the solid particles [179]. This effect of asymmetric collapse is the reason why ultrasounds are extensively used for cleaning and extraction purposes.

I.5.2.1.3 FACTORS AFFECTING CAVITATION

Several factors affect the formation, growth and collapse of cavitation bubbles in liquid media and therefore determinate the success of ultrasounds to improve a particular reaction. Within these factors, the ultrasonic frequency, ultrasonic intensity, external temperature and pressure, solvent and nature of the dissolved gases are the most relevant [179, 184]. Other factor that has a great influence in the

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success of the cavitation effects within the reaction medium is the shape of the reaction vial, which may change according to the ultrasonic device used.

I.5.2.1.3.1 FREQUENCY

The ultrasonic frequency applied to the reaction medium is determinant for the cavitation process.

Increasing the ultrasonic frequency diminish the formation of cavitation bubbles [179]. The major explanation for this phenomenon regards the time of the expansion and compression cycles. At high frequencies, both cycles are very short to allow the bubble to grow to an adequate size and cause its violent collapse [179]. This is why most ultrasonic devices used in sonochemistry work between 20 kHz to 50 kHz [184].

I.5.2.1.3.2 INTENSITY

Increasing the ultrasonic intensity has an important influence on the cavitation phenomenon. The intensity affects both half cycles pressure and an increment will result in more violent bubble collapses [179]. This is the reason why some devices are now able to induce cavitation at higher frequencies, e.g. 1MHz. Despite the short cycle time at high frequencies that normally would require several cycles for the bubble to grow, with higher intensity, the pressure applied allows the bubble to grow more in each rarefaction half-cycle and most important, in the compression half cycle, the pressure applied is enough to promote the collapse of the bubble [179]. However, it is important to stress that this effect drops above a maximum value of intensity. One explanation to this cavitation drop effect might be the formation of very large bubbles in the rarefaction cycle and due to the short cycle time the pressure applied in the compression cycle is not enough to collapse the bubble [179]. Other explanation regards the formation of several cavitation bubbles close to the ultrasound source, acting like a wall that limits the cavitation effect inside the liquid medium [184].

I.5.2.1.3.3 TEMPERATURE AND PRESSURE

Temperature plays an important role in the cavitation process. Generally, higher temperature inside the liquid medium has a negative effect in cavitation, even though it allows an easier formation of cavitation bubbles [184]. This behaviour is linked with the increase of the vapour pressure inside the liquid and consequently inside the cavitation bubble, which results in a less violent bubble implosion [184].

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