2 research outputs found
Chemical transformation of aerosols from biogenic precursors
V doktorskem delu sem preučeval pomen singletnega kisika in hidrotrioksi radikala pri
pretvorbah organskih aerosolov. Kemijske pretvorbe s singletnim kisikom sem preučeval
eksperimentalno, pretvorbe s hidrotrioksi radikalom pa teoretično s semiempiričnimi
kvantnomehanskimi metodami in metodami, ki so osnovane na teoriji gostotnega funkcionala.
Kot modelno spojino sem izbral α-pinen, ki je količinsko najbolj pomemben monoterpen in eden
glavnih prekurzorjev sekundarnih organskih aerosolov.
Razvil sem enostaven postopek za oksidacijo terpenov v tekoči fazi s singletnim kisikom, ki sem
ga sintetiziral fotokemično z žarnice z volframovo nitko in acetonitrilno raztopino metilensko
modrega. Za oksidacijske produkte, štiri hidroperokside α-pinena, sem razvil ločbi s tekočinsko
kromatografjo (LC) in plinsko kromatografijo (GC), ter za zaznavo uporabil ultravijolični (UV),
plamensko ionizacijski (FID) in masnospektrometrični (MS) detektor. Za analizo nestabilnih
sistemov je primerna tudi kvantitativna nuklearna magnetna resonanca (NMR), ki kot absolutna
tehnika omogoča določitev absorpcijskih koeficientov, in sem jo v doktorskem delu uporabil za
identifikacijo hidroperoksidnih izomerov α-pinena. Za njihovo kvantifikacijo sem razvil GC-FID
metodo s predkolonskim sililiranjem in kvantifikacijo z odzivnimi faktorji na osnovi efektivnega
števila ogljikovih atomov. Z razvitim analiznim pristopom sem zaobšel težave z reaktivnostjo
analitov, njihovo temperaturno neobstojnost, neznane in nizke absorpcijske koeficiente in šibko
MS ionizacijo. Linearnost metode sem preveril med 1 in 90 mg/L, kjer so bili korelacijski
koeficienti nad 0,99. Metoda je ponovljiva z relativnimi standardnimi odmiki pod 5 %. Celoten
postopek, od sinteze hidroperoksidov do njihove kvantifikacije, je prenosljiv tudi na druge
terpenske hidroperokside.
Metodo sem uporabil za spremljanje staranja terpentina, pri katerem je masni delež raziskovanih
hidroperoksidov α-pinena narasel na 5 %. Pri radikalski oksidaciji in oksidaciji s singletnim
kisikom sem opazil razliko v deležu posameznih hidroperoksidov in jo uspel mehanistično
razložiti. Poleg tega mi je uspelo dokazati, da omenjeni hidroperoksidi nastajajo tudi pri
fotokemičnih reakcijah s PM10 delci. V raztopini hidroperoksidov sem z LC-MS in GC-MS opazil
dimerizacijo hidroperoksidnih derivatov α-pinena in jo uspel opisati s semiempirično
kvantnomehansko metodo PM6.
Do sedaj so bile določitve 1O2 v zraku zelo redke, v znanstvenem članku je bila objavljena le
določitev z α-terpinenom. Visoke koncentracije so lahko posledica reakcij z drugimi oksidanti, ki
reagirajo na podoben način kot 1O2, zato je za potrditev pravilnosti določenih koncentracij
potreben razvoj novih metod. Pri razvoju nove metode sem preizkusil tri različne kemijske pasti:
N,N–dimetil-4-nitrozoanilin, sečno kislino in furfurilni alkohol. Kot najprimernejši reagent se je
izkazal furfurilni alkohol, s katerim sem določil koncentracije singletnega kisika v raztopini med
2x10-12 in 4,5x10-12 mol/L. Pri meritvah v zraku so potrebne še nadaljnje izboljšave metode.
Pomembnost hidrotrioksi radikala (•OOOH) sem ovrednotil in silico. Dokazal sem, da •OOOH
ne nastaja pri reakciji α-pinena z radikalsko obliko ozona. Na dvojni vezi poteka adicija •OOOH,
saj je abstrakcija alilnega vodika termodinamsko in kinetično neugodna. Hidrotrioksi radikal je
manj reaktiven od hidroksilnega in hidroperoksi radikala, najbolj kritična pa je njegova majhna
stabilnost, zato verjetno ne igra pomembne vloge v atmosferski kemiji.In my PhD thesis, I investigated the importance of singlet oxygen and hydrotrioxy radicals in the
chemical transformation of aerosols. Chemical transformations involving singlet oxygen were
studied experimentally and transformations involving hydrotrioxy radicals were studied
theoretically, using semiempirical quantum mechanical methods and methods based on density
functional theory. Alfa-pinene was chosen as a model compound because it is very abundant in
the atmosphere and important for the formation of secondary organic aerosols.
I have developed a simple procedure for liquid phase oxidation with singlet oxygen using a light
bulb with tungsten filament and an acetonitrile solution of methylene blue. For the oxidation
products, four α-pinene hydroperoxides, I have developed a liquid chromatography and a gas
chromatography separation with an ultraviolet (UV), flame-ionization (FID), and mass
spectrometry (MS) detection. The quantitative NMR technique, as an absolute technique, allows
the determination of absorption coefficients, and was used to identify the hydroperoxide isomers
of α-pinene. The analytes had low absorption coefficients, weak MS ionization, and low thermal
stability. To overcome these limitations, I proposed a GC-FID approach involving pre-column
silylation and quantification via the effective carbon number approach. The linearity of the
method was confirmed between 1 and 90 mg/L, with correlation coefficients above 0.99. The
method is reproducible with relative standard deviations below 5%. The entire process, from the
synthesis of the hydroperoxides to their quantification, is transferable to other terpene
hydroperoxides.
The validated method was applied to study the aging of turpentine in which the mass fraction of
α-pinene hydroperoxides increased to 5%. It was found that the differences in abundance of each
hydroperoxide were due to radical oxidation or a singlet oxygen oxidation mechanism.
Furthermore, I was able to show that hydroperoxides are also formed by photochemical reactions
with PM10 particles. In solution, the dimerization of α-pinene hydroperoxides was observed by
LC-MS and GC-MS and the mechanism was explained by the PM6 semiempirical quantum
mechanical method.
So far, determinations of 1O2 in the air have been very rare. Only one determination using the
chemical trap α-terpinene was published in a scientific article. Since high concentrations may
result from reactions with oxidants that react in a similar manner to 1O2, the development of new
methods is needed to confirm the accuracy of determinations. In developing the new method, I
tested three 1O2 chemical traps: N, N-dimethyl-4-nitrosoaniline, uric acid, and furfuryl alcohol.
Furfuryl alcohol proved to be the most suitable reagent and was used to determine concentrations
of singlet oxygen in solution, ranging from 2x10-12 to 4.5x10-12 mol/L. Further improvements to
the method are required for air measurements.
The importance of the hydrotrioxy radical (•OOOH) was investigated in silico. It was shown that
•OOOH is not formed by the reaction of α-pinene with the radical form of ozone. The addition of
•OOOH occurs at the double bond because the allylic hydrogen abstraction is thermodynamically
and kinetically unfavorable. The hydrotrioxy radical is less reactive than the hydroxyl and
hydroperoxy radicals and has very low stability, so it is unlikely to play a significant role in
atmospheric chemistry