Techniques in Radiation Chemistry 1.1.1
“Joint innovative training and teaching/learning program in enhancing development and transfer knowledge of application of ionizing radiation in materials processing” (Project acronym: TL-IRMP) AGREEMENT NUMBER 2014-1-PL01-KA203-003611
Course module Total hours of lecture: 60 hours (6 ECTS) in URCA (05-15/09/2016) and 30 hours (3 ECTS) in KTU (03-07/10, 2016) learning programme Total hours of laboratory and exercises: 6 hours in URCA (09/09/2016) Total hours of laboratory and exercises: 6 hours in KTU (07/10/2016)
PROF.DR. DİLEK ŞOLPAN ÖZBAY
Hacettepe University Department of Chemistry 06800, Beytepe/Ankara-TURKEY
Training/learning course in URCA (Sept 05-07,2016), Reims-FRANCE TL-IRMP
This project has been funded with support from the European Commission. This publication reflects the views only of the author(s). Polish National Agency for the Erasmus+ Programme and the European Commission cannot be held responsible for any use which may be made of the information contained therein.
Date: Oct. 2017 OUTLINE: 1. Radiation chemistry of liquid systems 1.1.Techniques in radiation chemistry 1.1.1. Steady-state techniques 1.1.2.Pulse radiolysis
2.Radiation chemistry of water and aqueous solutions 2.1. Water radiolysis 2.2.Radiolysis mechanism 2.3.Reactions of intermediates 2.3.1.Characteristic of primary and secondary products
3.Organic solvents/solutions 3.1.Alkanes, alken and aromatic hydrocarbons 3.2.Other organic molecules 3.3.Irradiation of monomer
4.Radiation chemistry of organic solids 4.1.Primary and secondary effects 4.2.Radiation yield
5.Changes in physical and chemical properties 5.1.Natural and synthetic polymers 5.2.Polymer crosslinking, degradation, grafting, curing 5.3.Sterilization of medical devices and drugs
3 4 Prof.Dr.Dilek SOLPAN OZBAY Head of Physical Chemistry Division, Department of Chemistry, Hacettepe University, 06800, Beytepe/Ankara-TURKEY E-mail: [email protected]
RESEARCH INTERESTS -Radiation chemistry -Wood-polymer composites -The consolidation and conservation of historical wooden objects -Synthesis and characterization of homopolymers, copolymers and semi-interpenetrating polymer Networks -Hydrogels -Adsorption -The concentration and separation of some metals by using alloy membranes -Surface chemistry -Treatment of industrial wastewater -The determination of COD and BOD, DO values -The degradation and decoloration of textile dyes in wastewater by gamma-irradiation -Removal of some pollutants (such as textile dyes, pesticides…) by gamma-irradiation -The use of GC-MS-MS and IC in the determination of some degradation products and degradation mechanism -The investigation of synergetic effects of ozonation+gamma-irradiation, ozonation+UV for removing of some pollutants in water -The adsorption of heavy metal ions and some metals from water by using appropriate adsorbents -Grafting and antimicrobial polymers 5 Research in our laboratories is broadly organized around the following areas. A number of them are further broken down into subareas.
Polymer Chemistry · Synthesis, characterization and modification of smart polymers, hydrogels, membranes and adsorbents · Controlled/Living polymerization techniques · Synthesis of reactive and functional polymers · Nanostructuring of polymers for molecular imprinting · Conductive polymer blends and composites based on polyaniline · Solution and complexation behaviors of polysaccharides (chitosan, alginate, etc.)
Radiation Chemistry · Radiation synthesis of polymers, copolymers and hydrogels · Radiation induced crosslinking, degradation, grafting · Radiation induced degradation of water pollutants · Dosimetric responses of polymers, polymer blends 1. Radiation chemistry of liquid systems 1.1.Techniques in radiation chemistry 1.1.1. Steady-state techniques 1.1.2.Pulse radiolysis
AA Atom of any element: Z X can be symbolized. N: Neutron number, Z: Proton number and includes the place of the element in the periodic table. Mass number A = N + Z
131131 53 I proton number (atomic number) : Z= 53, Mass number A = N + Z = 131 neutron number: N = 131-53 = 78 7 Nuclide: A Nuclide is a particular nucleus characterized by a defined atomic number and mass number.
A Sodium nuclide. There are particular types of Nuclide. They are: Isotopes Isobars Isotones
A= Mass Number. Z=Atomic Number n=Number of charge (+ or -)
Isotopes: The atoms having same atomic number but different atomic mass number are called Isotope.
Isobars: Nuclides having the same mass number but having the different Proton/Atomic number are called Isobar.
Isotones: Atoms of different elements having different mass number and different atomic number but same neutron number are called Isotones. 8 9 Penetration of radioactive rays
Alpha particles may be completely stopped by a sheet of paper, beta particles by aluminium shielding. Gamma rays can only be reduced by much more substantial mass, such as a very thick layer of lead.
Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air, while gamma radiation travels many metres. All types of radiation become less intense the further the distance from the radioactive material, as the particles or rays become more spread out. The thicker the substance, the more the radiation is absorbed. The three types of radiation penetrate materials in different ways. 10 Radiation types Types of scattered radiation from the atomic nuclei of radioactive materials shown in the following diagram
Radiation
Ionizing radiation Non-ionizing radiation
Particle Wave Wave
Neutron Alpha Beta Gamma X UV-Vis IR Micro waves Radio waves
Radiation originating from atomic nucleus E>50 eV Radiation resulting from atomic orbit
Electromagnetic Radiation Spectrum
11 12 13 4. Radiation chemistry of liquid systems 4.1.Techniques in radiation chemistry 4.1.1. Steady-state techniques 4.1.2.Pulse radiolysis
•Nuclear chemistry (interests of the structures of the stable and unstable nuclei and nuclear reactions and the events related to them.)
•Radiochemistry (interests about the chemistry of the material can be detected by the radiation they emitted. Issues with each of these two disciplines so that these are similar, nuclear chemistry and radiochemistry would be more appropriate to collect under the same name.)
•Radiation chemistry (implies the chemical effects of interactions of ionizing radiation with materials.)
a, b, g Radiation Source Object to be irradiated
The total energy lost per unit path length by the primary charged particle is obtained by the sum of integrating over all losses occuring in hard and soft collisions. The total energy lost per unit path is called the Linear Energy Transfer (LET).
14 Radiation chemistry may be defined as the study of the chemical effects produced in a system by the absorption of ionizing radiation. Included in this definition are the chemical effects produced by radiation from radiactive nuclei (a, b-, and g rays ), by high-energy charged particles (electrons, protons, deuterons, etc.), and by electromagnetic radiation of short wavelength (x- rays with a wavelength less than about 250°A, i.e., with an energy greater than about 50 electron volts.)
The chief difference between radiation chemistry and photochemistry lies in the energy of the radiation which initiates the reaction, the energy of the particles and photons concerned in radiation chemistry being very much greater than the energy of the photons causing photochemical reactions.
Thus in photochemistry each photon excites only one molecule and, by the use of monochromatic light, it is often possible to produce a single, well- defined, excited state in a particular component in the system.The excites species are distributed essentially uniformly in any plane at right angles to the direction of the beam of light.
In the radiation chemistry each photon or particle can ionize or excite a large number of molecules, which are distributed along its track.The high-energy photons and particles are not selective and may react with any molecule lying in their path, raising it to any one of its possible ionized or excited states. 15 TECHNIQUES IN RADIATION CHEMISTRY
Two main basically different experimental approaches are used to investigate radiation chemical reactions.
One of them is applied to detect, qualitatively or quantitatively determine intermediates and final products that are stable and do not change during the time of measurement. Such intermediates are ions or free radicals ‘frozen’ in solid matrices.
The other approach uses time dependent measurement in order to observe intermediates or the build up final products (pulse radiolysis).
16 Steady-state techniques The final products of radiolytic reactions are analyzed by the usual analytical technique,
like spectrophotometry, gas-chromatography, high-performance-liquid-chromatography (HPLC) and others.
In the radiolytic investigations the conversion is generally very low, being on the order of 0.0001-0.001, therefore sensitive analytical techniques are needed. A new generation of techniques,
for instance chromatographic and electrophoretic analytical instrumentation based on diode array technology now makes it possible to examine radiation chemical processes below 10 Gy dose (Barbara 1998).
17 In radiolytic reactions The main intermediates are
excited molecules, cations, free electrons, anions and radicals.
For the study of radical anions, cations and radicals formed in a solid matrix, e.g. in polymers, EPR spectroscopy is used since the 1950’s.
Anion and cation species can also be studied by UV spectroscopy. Absorption spectra of many organic radical anions and cations were measured in tetrahydrofuran or in halogenated hydrocarbon matrices (Shida 1988).
A widespread technique for studying the intermediates is the scavenger method. Yields of some of the product decrease, those of others increase or remain unchanged in the presence of scavengers, and new components appear among the products as a result of the scavenging process (Tabata 1991a). A general problem of the scavenger experiments is the practical impossibility of finding specific additives that react only with one type of intermediate being present during radiolysis. For instance substances generally used as radical scavengers may also react with electrons, since both radical and electron scavenging are related affinity of the substance. 18 A schematic illustration of a pulse radiolysis setup is shown on Figure. In the usual arrangements the measuring system is divided into two parts. The first part, including the light source for the sample illumination, the lens system, the shutter, preventing the heating and photolysis of the sample, light filters, and the measuring cell itself, is housed in the irradiation room.The measuring cell should be positioned in the beam so as to achieve an approximately homogeneous irradiation.
The other parts of the setup are outside the radiation shielding concrete wall and the Faraday cage protecting against electromegnetic interference. These include the monochromator, the photodetector, the oscilloscope/digitizer and the computer system for data acquisition and processing. Light is usually transmitted from the irradiation room to the measuring room through a hole in the shielding by means of mirrors and lenses. Less frequently light guide cable is used19 . 2.Radiation chemistry of water and aqueous solutions 2.1.Water radiolysis 2.2.Radiolysis mechanism 2.3.Reactions of intermediates 2.3.1.Characteristic of primary and secondary products
Historically, the evolution of gas from aqueous solutions containing radium salts was one of the earliest radiation-induced chemical reactions to be observed and then studied. Early work showed that a-particles decompose water into hydrogen and oxygen and that part of the oxygen remains in solution in the form of hydrogen peroxide. In aerated solutions the yield of hydrogen peroxide is increased by the presence of oxygen.
Water vapor is the simplest aqueous system. The more important ions formed in the mass spectrometer together with the reactions which have been postulated to explain their formation are determined.
In liquid water the ions and excited molecules will be formed closer together than in water vapor because of the greater density of the medium; the LET in the liquid is of the order of a thousand times greater than in the vapor. The greater density of the medium will also tend to restrict the primary species and the radicals derived from them to the particles tracks, where they will react among themselves to some extent before diffusion distributes them throughout the 20liquid. The total energy lost per unit path length by the primary charged particle is obtained by the sum of integrating over all losses occuring in hard and soft collisions. The total energy lost per unit path is called the Linear Energy Transfer (LET).
21 Figure Water radiolysis: Formation of free radical species in water by means of ionizing22 radiation (Gehringer, 2003). Figure illustrates the effect of ionizing radiation on water which is known so result in the formation of molecular species and free radicals, as well as some ions.
For pollutant decomposition and microorganism inactivation, respectively just the free radical species are of interest. Pollutants and microorganisms are quite different targets for the attack of the free radical species.
The free radicals formed during water radiolysis are highly reactive, the OH. radicals are the most powerful oxidant known to occur in water, the same is valid for the - hydrated electron e aq as reductant. 23 The radiolytic events occur in three main stages taking place on different typical time scales: (1) The physical stage, which is achieved about 1 fs after the initial matter-ionizing radiation interaction, consists in energy deposition followed by fast relaxation processes. This leads to the formation of ionized water molecules (H2O+), excited water molecules (H2O*) and sub- excitations electrons (e−).
(2) During the physico-chemical stage (10−15–10−12 s), numerous processes occur, including ion- molecule reaction (a), dissociative relaxation (b), autoionization of excited states, thermalization of subexcitation electrons (solvation of electrons) (c), hole diffusion, etc.
(a) (b) (c)
(3) During the chemical stage (10−12–10−6 s), the species react in the tracks and then diffuse in solution. They can thus react with each other and also with surrounding molecules (in the solute). The track of the particles expands because of the diffusion of radicals and their subsequent chemical reactions. Recombination becomes unimportant after ca. 1 μs for low-LET radiation.
These three stages are summarized on the following Scheme. 24 25 Effect of N2O - . O2. together with OH radicals can initiate degradation of water pollutants. Nitrous oxide gas can be used to convert the hydrated electrons to hydroxyl radicals, although it is generally not practical for environmental applications. Hydroxyl radical scavengers such as t-butanol can be used to consume the hydroxyl radical for selectively studied hydrated electron. Hydrated electron is a powerful reducing agent and potentially could be used for treatment of halogenated compounds, but it is extremely reactivity towards molecular oxygen requiring that the solutions be deoxygenated prior to treatment, a costly prospect. - . By saturation the solution with N2O the e aq are transformed into OH radicals:
- . - 10 -! -1 e aq + N2O → OH + OH + N2 (k= 0.91 x10 L.mol s ) (r) -4 -1 - . . 1 kGy = 6.344 x10 mol dm (e aq + H + OH )
Therefore, it is clear that in solutions containing oxygen (pH: 6 to 8,5) involved . free radicals are: 46% OH and 54% O2. On the other hand in solutions . . saturated in N2O the acting radicals are: 90% OH and 10% 46% H
26 Effect of H2O2
. 10 -1 -1 - 7 -1 -1 The hydrogen peroxide reacts with H (k(20°C)= 2.4x10 M s ) and eaq k(20°C) = 1.5x10 M s as follows (Zona, 2003; Jankowska, 2004): - . - eaq + H2O2 → OH + OH (s)
. . H + H2O2 → OH + H2O (t)
In the presence of excess of hydrogen peroxide in solution it can scavenge the hydroxyl radicals with 10 -1 -1 rate constant k(20°C) = 3.1x10 M s ) (10), according to reactions: . . OH + H2O2 → H2O + HO2 (u) . 2HO2 → H2O2 + O2 (v)
The total effect of reactions (2.13) and (2.14) is the following process: . 2OH + H2O2 → 2H2O + O2 (w)
From the obtain results under corresponding experimental conditions one can estimate which type of free radicals is mostly involved in degradation of given substrate. It is interesting to point out that . . - when the primary radiolytic species (OH , H , e aq, H2, H202) are formed, a number of very fast reactions take place. When we deal with high dose rates, a very high concentration of primary species along electron traces is formed. They have less chance to diffuse away in order to react with pollutants because they are consumed within the trace by reactions between themselves. As a consequence of this fact, the degradation yield is low. This effect is strongly pronounced in the case of low pollutant concentration and implementation of higher dose rates (Getoff, 2002), (Borrely, 1998).
27 Factors Influencing Efficiency of Radiation-Induced Degradation of Water Pollutants
g
EB
Getoff, Radiat. Phys. Chem., 65 (2002) 437 28 - . . pH e aq H OH H2 H2O2
3-11 2.7x10-7 0.57x10-7 2.8x10-7 0.47x10-7 0.7x10-7
0.46 0 3.8x10-7 3.0x10-7 0.41x10-7 0.8x10-7
Radical and molecular product yields (in mol/J) in electron-irradiated water
Effect of O2 . . -1 -1 H + O2 → HO2 (k= 2.1 x1010 L.mol s ) . . HO2 + OH → O2 + H2O - - -1 -1 e aq + O2 → O2. (k= 1.9 x1010 L.mol s )
Effect Of N2O - . - 10 1 -1 e aq + N2O → OH + OH + N2 (k= 0.91 x10 L.mol- s )
Effect of H2O2 - . - eaq + H2O2 → OH + OH . . H + H2O2 → OH + H2O . . OH + H2O2 → H2O + HO2 (Excess) . 2HO2 → H2O2 + O2 (Excess) . 2OH + H2O2 → 2H2O + O2 (Total) 29 3.Organic solvents/solutions 3.1.Alkanes, alken and aromatic hydrocarbons 3.2.Other organic molecules 3.3.Irradiation of monomer
The yields of primary products (ions, electrons, and excited state) produced by exposure of an organic compound to ionizing radiation are essentially independent of whether it is in the gas, liquid, or solid state. However, the nature and yields of the final products are often dependent on the state. This is the result of the effects of density and temperature on the relative probabilities of competing reactions of the primary species and of the radicals which they produce. The density effects are of two types.
First, the dose proximity of neighboring molecules in the solid favors reactivation rather than decomposition of excited molecules and favors prompt recombination in the parent cage of the fragments of any that do decompose.
Second, since the distance traveled by an energetic electron is depositing its energy is inversely proportional to the density of the medium, the tracks are shorter and the spur radii smaller in the solid than in the liquid (and in great contrast to the gas, where spur effects are negligible).
30 EFFECT OF THE PHYSICAL STATE ON THE RADIOLYSIS MECHANISM OG
GAS PHASE -The radiolytic intermediates (ions, excited molecules, radicals) diffuse rapidly out of the track and reach an homogeneous distribution in the whole system. As a consequence intra- track reactions are negligible. -Geminate recombinations and the influence of the polarity of the medium are much less important than in liquids. -Fragmentation reactions of excited molecules and ions acquire importance in the radiolysis mechanism because of the less effective collisional quenching (söndürme, su çıkışı ile soğuma). The relative abundance of lower molecular weight products increases -Effects arising from different L.E.T. of radiations (a, g, e- etc.) are far less important than in liquid -Radical coupling reactions often require the intervention of a third body to dissipate the excess of bond forming energy R. + R. + M R-R + M*
LIQUID AND SOLID PHASE -The radiolysis mechanism shows a variable contribution by the ionic and excitation component depending on the polarity of the medium -The efficiency of the collisional quenching decreases the role of fragmentation reactions -Intratrack reactions are important -The distribution and yield of radiolysis products is significantly influenced by the L.E.T. of radiations 31 The primary processes in the radiolysis of organic compounds can, in most cases, be represented by (1,2):
(excitation) A A*
(ionization) A A+ + e-
R.+ + S.
(ion dissociation) A+ M+ + N
(neutralization) A+ + e- A**
R. + S. (dissociation) A* and A** M + N where R. and S.are free radicals and M and N are molecular products. Excited molecule and ions are initially distributed along the track of the ionizing particle in small groups or spurs, which may overlap neighboring spurs as the active species (at this stage mainly free radicals) diffuse apart. Whether or not the expanding spurs overlap before these species have reacted depends upon the spacing of the spurs along the particle track and hence upon the LET of the ionizing particle. 32 Radiation chemistry of gaseous methane was often studied. Radiation decomposition results in the formation of hydrogen, ethane and ethylene as main products, longer chains and even polymer-like products also form in low yields (Table).
33 -Cyclohexane at room temperature – liquid alkane -In the molecule all C-C and C-H bods are identical, -For this reason the product distribution is simple.
-The main degradation products are cyclehexene and bicyclohexyl form.
-The yield of ring decomposition products in condensed phases is little. -The yields of ring decomposition products in gas phase irradiation is higher at low pressures.
-As intermediates of reactions in condensed phases were identified free electrons, cyclohexyl radical cations and cyclohexyl radicals.
-Unimolecular H2 elimination (look at the arrow) is a typical decomposition mode of the alkane excited molecules. The yield is ≈0.12mmolJ-1. -The cyclohexyl radical yield is 0.56mmolJ-1. It is more than twice the yield of scavengeable hydrogen atoms (G≈0.15mmolJ-1). -Therefore cyclohexyl radicals should be produced in reactions. -The cyclohexyl radicals in liquid phase disappear in self-termination reactions forming cyclohexene and bicyclohexyl.
+. -In the later works suggests the c-C6H13 cation as the intermediate that forms in an ion-molecule reaction.
34 In the radiolysis of liquid cyclohexane and a few other cyclic alkanes high mobility cations form. Their mobility is one order of magnitude higher than the molecular diffusion.
35 36 37 SCHEMATIC DESCRIPTION OF THE RADIOLYSIS OF ORGANIC FUNCTIONAL GROUPS
38 39 40 41 42 43 44 deamination decarboxylation
45 4.Radiation chemistry of organic solids 4.1.Primary and secondary effects 4.2.Radiation yield
Radiation chemistry of inorganic solids The radiation chemistry of inorganic solids may be regarded as a branch of solid state physics to which subject it has made substantial contribution (Grigoriev and Traktenberg 1996). The study of radiation effects in solids is of vital importance for nuclear technology, and also relevant to several other fields, e.g. Semiconductor technology. The irradiation effect in polymers will discussed in the following section.
Colouration of Gem Stones Gem and jewelry export constitutes one of major market in the Indian exports. Coloured diamonds and precious stones command a better price in the market. Use of high energy electron beam accelerators for enhancing the colour of natural diamonds has emerged as a novel technique that has been well accepted in the diamond industry as the radiation treated colour diamonds are much cheaper than the naturally occurring coloured diamonds [4]. The process is now regularly being carried out in India on a commercial basis. 46 47 The primary interactions of ionizing radiation with polymers include ionization, excitation, stabilization of electrons through the generation of hot electrons, ion neutralization, and free radicals. Free radicals are created either through scission of the main polymer chain or through the dissociation of the C-H side chain. The primary processes are shown Scheme 1.
The secondary reactions following the free radical generation include hydrogen abstraction, addition to double bond, recombination (crosslinking or branching), chain scission, oxidation and grafting, as shown in Scheme 2. Monomers can also be polymerized by radiation as shown in Scheme 3.
Scheme 1
Scheme 2
48 Scheme 3 5.Changes in physical and chemical properties 5.1.Natural and synthetic polymers 5.2.Polymer crosslinking, degradation, grafting, curing 5.3.Sterilization of medical devices The changes in the properties of the irradiated polymers in the solid state are in the following:
Chemical effects Physical effects -Crosslinking and chain-scission -Changes in mechanical properties -Gas formation -Colour change -Double bond formation -Changes in conductivity -Oxygen effect -Changes in cristallinity -Arrested radicals and effects after irradiation -Thermal transitions 49 -Effect of additive materials 50 51 Radiation processing of polymers
52 Figure 1 Figure 2
The relationship between the polymer molecular weight (MW) and the radiation dose, as shown in Figure 1.
G(X) and G(S) also depend on irradiation conditions, such as temperature and Atmosphere (Fig 2).
53 Crosslinking Chain scission (degradation)
Competitive reactions (occur simultaneously with predominance of one of them)
54 crosslinking chain scission
Promote changes in materials molecular structure
Physical effects: -Mechanical strength and Chemical effects: stability -Reactivity Flexibility -Oxidation & Peroxidation Torsional resistance Yang module Structural weakness -Thermal stability (Tdeg) Color degradation
55 1) -PMMA, Teflon, PIB (polyisobutylen) have ability to have chain scission.
-PE, PS, PVAc have ability to have crosslinking.
(CH2-CH) crosslinking PE (CH2-CH2) X
Y CH3 (CH2-C) chain scission PIB (CH2-C) X CH3
2) H2, NH3, gas formation during irradiation of PMMA, PAAm…….
3) CH2-CH From PVC, HCl(g) is released and CH=CH Cl
hn . 4) + H then + O2 . . O-O. C=O . + OCH2 56 H 5) In solid state, the radicals can be found during months
6) Commercial polymers contains generally some additives. It is important the effect of the additives to radiolysis. Antirad additives has aromatic cyclic structure and They reduce the degradation against radiation. Plastifiers have healing role for irradiated polymers.
Physical changes: 1)Mechanical properties of irradiated polymers depend on the ratio of chain scission and crosslinking and related to molecular weight.
2) Colour change can form when the double bond formed on irradiated polymers such as PMMA and PVC.
3) Conductivity change can increase when the double bond increased in irradiated polymers. 57 5.3. Sterilization of medical devices and drugs
Sterilization and Disinfection: Sterilization and disinfection are different terms.
-Sterilization is the process of free from all microorganisms present in or on the substance. After this process causing disease (pathogenic) and not all microorganisms are killed.
-Disinfection is the process to remove or to stop proliferation (çoğalma) of all microorganism and viruses in the any material or medium which may be harmfull to man, animal and vegetables. The disinfection is the inactivation of pathogenic microorganisms.
Although the degree of disinfection procedures and steps may be (according to the desired microorganism inactivation), there are no degrees of sterilization. A product or environment is sterile or non-sterile.
Dose of disinfection may vary according to the material to be applied or to the medium, allows the media to be disinfected and the number of microorganisms contained in the medium or material. The active substance used for disinfection is called as disinfectant. There are many different sterilization methods depending on the purpose of the sterilization and the material that will be sterilized.
The choice of the sterilization method alters depending on materials and devices for giving no harm.
These sterilization methods are mainly: -dry heat sterilization, -pressured vapor sterilization, -ethylene oxide (EtO) sterilization, -formaldehyde sterilization,
-gas plasma (H2O2 ) sterilization, -peracetic acid sterilization, -e-beam sterilization -gamma sterilization.
There is no single sterilization process for all the pharmaceuticals and medical devices. It is hard to assess a perfect sterilization method because every method has some advantages and disadvantages. For this reason, sterilization process should be selected according to the chemical and physical properties of the product.
It is clear that different sterilization processes are used in hospital and in industry applications.
59 Benefits of gamma irradiation include the following: -Depth of photon penetration allows for sterilization of materials of various density levels -Process does not require addition of heat or moisture -Well documented for its effectiveness as a sterilization process -Does not produce residuals as are of concern with Ethylene Oxide sterilization -Simple methods are available for documenting a high Sterility Assurance Level (SAL) such as 10-6 -Allows for terminal sterilization. Effects of gamma rays on living organisms Radiation effects on living organisms are mainly associated with the chemical changes but are also dependent on physical and physiological factors.
The physical parameters The physiological and environmental parameters Dose rate Temperature Dose distribution Moisture content Radiation quality Oxygen concentration
The action of radiation on living organisms can be divided into direct and indirect effects. Normally, the indirect effects occur as an important part of the total action of radiation on it. Figure shows that radiolytic products of water are mainly formed by indirect action on water molecules.
Several types of microorganism, mainly bacteria and, less frequently, moulds and yeasts, have been found on many medical devices and pharmaceuticals. Complete eradication of these microorganisms (sterilization) is essential to the safety of medical devices and pharmaceutical products. Radiation sterilization, as a physical cold process, has been widely used in many developed and developing countries for the sterilization of health care products. A minimum dose of 25 kGy was routinely applied for many medical devices, pharmaceutical products and biological tissues (Takehisa et al, 1998).
Fig. Effect of gamma rays on water molecules 5 MICROBIOLOGICAL ASPECTS OF RADIATION STERILIZATION
Direct and indirect effect of radiation Ionizing radiation can affect DNA either directly, by energy deposition in this target, or indirectly, by the interaction of radiation with other atoms or molecules in the cell or surrounding the cell (Fig). In particular, radiation interacts with water, leading to the formation of free radicals (hydrogen atoms H•, hydroxyl radical OH• and solvated electron e−) that can diffuse far enough to reach and damage DNA [8.5]. The OH• radical is most important; these radicals formed in the hydration layer around the DNA molecule are responsible for 90% of DNA damage. Consequently, in a living cell, the indirect effect is especially significant.
The death of a microorganism is a consequence of the ionizing action of the high energy radiation. Both prokaryotes (bacteria) and eukaryotes (moulds and yeasts) are capable of repairing many of the different DNA breaks (fractures). It is generally believed that microorganisms that are sensitive to radiation cannot repair doublestrand breaks, whereas radiation resistant species have some capability to do so. 4 Decimal reduction dose When a suspension of a microorganism is irradiated at incremental doses, the number of surviving cell forming colonies after each incremental dose may be used to construct a dose survival curve, as shown in Figure. The radiation resistance of a microorganism is measured by the so-called decimal reduction dose (D10 value), which is defined as the radiation dose (kGy) required to reduce the number of that microorganism by 10-fold (one log cycle) or required to kill 90% of the total number (Whitby & Gelda, 1979).
There are many factors affecting the resistance of microorganisms to ionizing radiation, thus influencing the shape of the survival curve. The most important factors are: a. Size and structural arrangement of DNA in the microbial cell; b. Compounds associated with the DNA in the cell, such as basic peptides, nucleoproteins, RNA, lipids, lipoproteins and metal ions. In different species of microorganisms, these substances may influence the indirect effects of radiation differently; c. Oxygen: The presence of oxygen during the irradiation process increases the lethal effect on microorganisms. Under completely anaerobic conditions, the D10 value of some vegetative bacteria increases by a factor of 2.5–4.7, in comparison with aerobic conditions; d. Water content: Microorganisms are most resistant when irradiated in dry conditions. This is mainly due to the low number or absence of free radicals formed from water molecules by radiation, and thus the level of indirect effect on DNA is low or absent; e. Temperature: Treatment at elevated temperature, generally in the sub-lethal range above 45°C, synergistically enhances the bactericidal effects of ionizing radiation on vegetative cells. Vegetative microorganisms are considerably more resistant to radiation at subfreezing temperatures than at ambient temperatures. This is attributed to a decrease in water activity at subfreezing temperatures. In the frozen state, moreover, the diffusion of radicals is very much restricted; f. Medium: The composition of the medium surrounding the microorganism plays an important role in the microbiological effects. D10 values for certain microorganisms can differ considerably in different media; g. Post-irradiation conditions: Microorganisms that survive irradiation treatment will probably be more sensitive to environmental conditions (temperature, pH, nutrients, inhibitors, etc.) than the untreated cells. 4Effect of temperature and additive on radiosensitivity of living organisms Temperature plays a major role in the radiosensitivity of microorganisms. As temperature decreases, water radicals become less mobile. As a general rule, microorganisms are less radiosensitive when irradiated at low temperatures (Thayer & Boyd, 2001). For example, whilst sensitivity of spores from Bacillus megaterium was constant between –268 and –148°C, an increase in temperature to 20°C led to a 40% increase in sensitivity. Effect of temperature was observed to be similar for oxic and anoxic spores (Helfinstine et al., 2005).
4 Poly(methyl methacrylate), PMMA, also is used in manufacturing of medical supplies that can be sterilized by gamma irradiation at dose of 25 kGy and used in absorbed dose measurements in intense radiation fields. In general, polymer radicals are responsible for changes in the physical properties of PMMA. In particular, gamma irradiation of PMMA causes main scission and hydrogen abstraction from an a-methyl or methylene group. scission results from a macroradical that itself is radiolysis product of a lateral bond as shown in the Figure (reaction I ) (Guillet, 1985). 5 Polystyrene Impact grades of polystyrene (PS) that have been copolymerized with minor amounts of butadiene are injection or blow moulded into trays or cases for medical products. PS is also formed into rigid medical devices, such as drainage monitoring units, shown in Fig.
Because of its cyclic backbone, PS neither cross-links nor degrades when exposed to sterilizing radiation. Care, however, must be taken not to overexpose this resin since some yellowing could result. 4 Polycarbonate (PC) is one of the most popular engineering resins in the medical device market. Bisphenol-A polycarbonate has been commercially available since the 1960s, and its use in medical devices dates from approximately that time.
The radiation-induced main chain scissions on PC occur in the carbonate groups, causing the evolution of carbon monoxide, carbon dioxide and hydrogen. The radiolysis of PC produces phenoxy and phenyl polymeric radicals that cause yellowness of the polymer. However, it has been reported in the literature that the crosslinking effect predominates at small doses, whereas at higher doses the main chain scission is more pronounced (Araujo et al, 1998). Polyurethane (PU) is widely used in various medical devices because of its biocompatibility, and has some reports concerning its physicochemical stability and biological safety. However, among substances which were produced by degradation of PU, it was reported that a carcinogen, 4,4’-methylenedianiline (MDA), was produced from PU sterilized by gamma irradiation.
On the other hand, a modified PU was produced and called thermosetting PU. In the case of thermosetting PU used in medical devices such as potting material in artificial dialysis devices, plasma separators, etc., the production of MDA upon sterilization showed a reverse tendency to non modified PU (Shintani, 1992). As shown in Figure, it was suggested that the mechanism of MDA production might be the cleavage at urethane linkage successive to the terminalamino group, by radiation or hydrolysis (Shintani, 1992).
MDA
4 Polyisoprene, especially in the form of natural rubber latex, is widely used in medical disposables, such as gloves and condoms, and found to be an effective barrier. Because of its unsaturation, natural rubber and many other elastomers will slightly crosslink when exposed to radiation sterilization conditions. Although isobutylene is well known to scission when exposed to radiation, a halogenated copolymer of isobutylene and isoprene, commonly brominated butyl rubber (BIIR), can be formulated to exhibit radiation response when used in the tyre industry.
Silicone rubber is widely used in medical applications, where sterilized is an essential requirement for all medical tools and devices that contact the body or bodily fluid and medical components must be sterilized frequently by gamma irradiation. Gamma radiation is known to induce changes in the molecular architecture of silicone rubber, resulting in an increase in molecular weight and a decrease in elasticity. Radicals are generated by chain scission and/or methyl or hydrogen abstraction (see Figure 12) and are subsequently terminated via oxidation reactions or coupled to form longer chain branches.
Hindered Amine Light Stabilizer (HALS) is among the more extensively used additives for protecting polymers against degradation by the combined effect of light, temperature, and atmospheric oxygen. The protection of the polymer from the light by these compounds takes place via a mechanism involving photo-oxidation of the amines to nitroxyl radicals (Lucarini et al, 1996). The scheme in Figure is generally accepted to explain the aspects of the chemistry mechanism of HALS action to inhibit polymer photo-oxidation. This scheme was used to guide a strategy to assess Tinuvin 622 action in radiolytic stabilization of PMMA (Aquino & Araujo, 2008). 5 RADIATION STERILIZATION OF DRUGS Radiation sterilization was applied in the food industry as a food preservation procedure and to eliminate microbiological contamination of herbal spices, then this method was applied in the pharmaceutical industry for sterilization of medical devices, disposable materials, implants and in the cosmetics industry [13.1–13.6]. In the 1980s, the process of radiation sterilization was also admitted for some drugs, including antibiotics, steroids and alkaloids, some raw plant products and herbal medicines, as well as veterinary drugs in the United Kingdom, Norway, India, Indonesia, Israel and Australia [13.9].
There are two main documents regulating the use of radiation sterilization presently in force: —The European standard (EN 522) on medical devices for the use of gamma rays and e-beams of energy ≤ 10 MeV (from accelerators) at a minimum dose of 25 kGy ensuring the sterility assurance level (SAL) of 10–6 [13.10]. —The international standard (ISO 11137) on medical instruments, devices and products, including drugs, vaccines and health care products, for the use of gamma rays, X rays and e-beams at different doses depending on the type and level of the microbiological contamination and the target level of sterility.
5 Radiosterilization of drugs in aqueous solutions may be achieved by the use of radioprotective excipients Aubert Maquille, Jean-Louis Habib Jiwan, Bernard Tilquin
Radiosterilization is a safe method recommended for the sterilization of thermosensitive solid drugs (EMEA, 1999; European Pharmacopoeia, 2005; USP, 2005 ). However, it is not considered for the sterilization of drugs in aqueous solutions (EMEA, 1999 ) as, compared to the solid state, a higher degradation is reported after irradiation of drug solutions (Jacobs, 1985; Boess andB¨ogl, 1996; Angelini et al., 1998 ). In irradiated aqueous solutions, the reactive species generated by the radiolysis of water (mainly • OH, • H and eaq− in deaerated water) react with the solute (indirect effect), giving rise to radiolytic products.
3 Drug: An irradiated molecule may be different than an unirradiated molecule, it is important to prove that the final product provides the promised benefits. Irradiation can affect not only the drug but also the excipients, containers, and closures. Even a minor change in charge, conformation, or solubility can have a dramatic effect on the intended use of the product. The drug development process takes considerable time and involves many steps (e.g. discovery, pre-Investigational New Drug [IND], IND, IND, and Phase I–III studies). The sterilization process needs to be evaluated early, even at the pre-IND stage to ensure the same formulation of product is used throughout all testing.
-Liquids are more difficult to irradiate than dry powder. Liquids may undergo pH changes if not buffered. They provide easy mobility to any reactive species created from the ionization process. Irradiation, as discussed earlier, creates ionization events or free radical formation. In a liquid state, these charged species can move freely throughout the solution and create more damage than in the limited- mobility dry or solid state. They also can be quite dense, resulting in wider dose distributions. -When free radicals are created, they do not remain free for long. The net effect may be scission to smaller polymers or cross-linkage to longer polymers, additions, and deletions that must be evaluated for safety, efficacy, potency and byproducts. -In general, proteins are less stable then many other compounds. -Aromatic rings (alternating double bonded structures) are more stable than aliphatic materials. -Heparin, steroids, antibiotics, and vitamins in dry form have been gamma irradiated successfully. RADIATION STERILIZATION OF DRUGS
The unquestionable advantages of radiation sterilization stimulating its widespread use include: —Reliability; —No residuals after radiation treatment in the material sterilized; —No harm to the natural environment; —Possibility of application in any form of packaging; —Possibility of application at any temperature, including below 0ºC, which permits sterilization of thermolabile drugs; —Possibility of application to drugs in any pharmaceutical formulations; —Possibility of application to reactive drugs, for example, those reacting with gases. Of course, no method is absolutely free of drawbacks and those of radiation sterilization are: —High cost (presently estimated as higher than that of any other method of sterilization); —Difficulty of processing (if the producers do not have their own source of radiation and have to transport the products to be sterilized); —Duration, when applying the method to bulk materials; —Possibility of drug damage due to an inaccurate determination of the sterilization dose or no validation of the sterilization process. 79 5METHODS OF ANALYSIS OF DRUGS SUBJECTED TO RADIATION STERILIZATION
The problem with radiation sterilization stems from the possibility that the ionizing radiation can not only destroy the microorganisms but also cause damage to the drug as a side effect.
This concern follows from insufficient knowledge of radioactivity and chemical changes that can take place in the chemical compounds subjected to ionizing radiation.
Therefore, safe application of radiation sterilization needs to be preceded by showing that ionizing radiation does not change the content and physicochemical properties of specific drugs and thus does not change their pharmacological activity.
This procedure requires determination of the ‘safe dose’ of radiation ensuring the desired effect; that is a dose lethal to all microorganisms but not disturbing the therapeutic effect of the drug [13.17–13.19]. Changes (before and after irradiation) that can appear in the drugs subjected to radiation sterilization can be detected by the following methods:
—Spectrophotometric methods: • UV–VIS and spectrophotometry of derivatives; • IR, IR–Raman, FTIR, near infrared spectroscopy (NIRS); • Mass spectrometry, in particular, with electron impact (IEMS) and chemical ionization (CIMS); • Nuclear magnetic resonance (NMR); • Electron spin resonance (ESR, EPR). —Chromatographic methods: • Thin layer chromatography (TLC); • Paper chromatography (PC); • Gas chromatography (GSC, GLC); • Liquid chromatography (LC, HPLC); • Capillary electrophoresis (CE). —Thermal analysis methods: • Thermogravimetry (TG); • Differential thermal analysis (DTA); • Differential scanning calorimetry (DSC). —Crystallographic methods; —Diffraction methods to be applied to crystalline (X ray diffractometry) and amorphous substances (X ray powder diffractometry): • Rheological methods — measurements of viscosity by different methods; • Polarimetric methods — optical rotation measurements; • Electrochemical methods (polarographic, potentiometric); • Coupled techniques, for example, GC–MS, HPLC–MS, HPLC–IR–MS, GC–IR, TLC–UV–IR, DTA–GC–MS, HPLC–MS/MS. The study of radiation sterilized drugs using analytical chemistry methods should be followed by microbiological examination and biological tests in vitro and in vivo. As follows from the parameters describing the process of radiolysis in time (Tables), the free unpaired electrons appearing as a result of their ejection from the drug molecules in water solutions can react with water molecules much faster than in the solid state, leading to the formation of free radicals H• and • OH and, later, H2 O2 molecules, initiating oxidation reactions. Almost all drugs decompose faster in dilute solutions than in the concentrated ones [40]. This phenomenon is best illustrated by the example of saccharide solutions, for example, glucose and other drugs [21] (see Table).
The advantages of using radiation sterilization instead of the thermal one are more pronounced for the drugs in solid state. Table in the left clearly shows that the per cent decomposition of, for example, papaverine hydrochloride is much smaller when it is sterilized by radiation, although for caffeine or phenobarbital, the differences are not so dramatic.
Ionizing radiation initiates in drugs not only the oxidation reactions as mentioned, but also some other reactions, including radiolytic dissociation leading to the breaking up of different types of bonds, hydrolysis, deamination, deacetylation, decarboxylation, polymerization and isomerization [46]. At present, the best, although not yet completely, known is the sterilization of antibiotics, especially derivatives of synthetic and semi-synthetic penicillins, cephalosporins and some other types of antibiotics. Penicillins The water solutions of penicillins are not suitable for the radiation sterilization [52]. The reason is the radiolysis of water with the formation of free radical • OH inducing break-up of the β –lactam ring leading to the formation of benzylpenilloic and benzylpenicilloic acids [13.50]. The sensitivity of water solutions of different penicillins to radiolysis is different. For instance, ampicillin and amoxycillin undergo 90% decomposition on irradiation with 0.5 kGy, while cloxacillin needs irradiation at 5 kGy to decompose [13.38]. Some penicillins, for example, benzylopenicyllin or phenoxymethylpenicyllin acid, in the solid phase undergo small decomposition on irradiation (Fig.).
On irradiation with high doses (700–800 kGy), penicillins can undergo radiolytic decomposition with the formation of simple gas products, such as CO and CO2 and, in small amounts, also H2 and CH4 [13.47] (Table 13.6). The irradiation effect on penicillins has also been studied by the ESR method. The results have confirmed the appearance of free radicals and permitted their structures to be proposed (Fig.). Other antibiotics
The radiation stability of antibiotics other than penicillins and cefalosporins has not been so thoroughly studied.
The effect of radiation on only one compound — chloramphenicol — has been comprehensively studied, including isolation and identification of the products of radiolysis and determination of the mechanism of decomposition (Table).
Only for chloramphenicol, two main radiolysis products do not show toxicity while the products formed in the amount of about 10% on radiolytic decomposition of gentamycin sulphate cause an increase in the preparation toxicity [38]. 5 Sulphonamides
Sulphonamides, known to undergo decomposition during thermal sterilization, were one of the first groups of drugs whose potential for radiation sterilization was investigated.
Sodium sulphacetamide was studied after irradiation at 10 kGy and 15 kGy dose in water solution and in the solid phase.
The decomposition in the solid phase and in solution takes place according to the same mechanism, giving the same radiolysis products, and only the rate of decomposition in the solid phase was a few times lower (Table in the next page, Fig.). 5 Steroids Steroids in the solid phase are a group of drugs exceptionally resistant to gamma irradiation because of their high resistance in solid phase to gamma irradiation and e-beam irradiation. They are nondecomposition or very low decomposition (within 1%) upon irradiation at doses of 25–50 kGy [92].
Steroids can be sterilized by radiation. Upon irradiation of the solid phase steroids with higher doses (100–200 kGy), it is possible to detect and identify some products of decomposition [38] and suggest the mechanism of this process (Fig.).
Two major types of radiolytic degradation schemes were found: —Loss of the corticosteroid side chain at D ring to produce C-17 ketone; —Conversion of C-11 alcohol to C-11 ketone [90] (Fig. 13.9 and Table in the next page).