- About
-
The Global ETD Search service is a free service for researchers
to find electronic theses and dissertations. This service is
provided by the
Networked Digital Library of Theses and Dissertations.
Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
Search results
Showing 21 to 22 of 22 (0.128 seconds)| 21 |
Optimizacija metoda ekstrackcije i određivanja neonikotinoida tečnom hromatografijom u odabranim uzorcima / Optimization of extraction and determination of neonicotinoids using liquid chromatography in selected samplesJovanov Pavle 01 July 2014 (has links)
<p>Insekticidi novije generacije, neonikotinoidi, odlikuju se specifičnim načinom delovanja na nervni sistem insekata. Radi dobijanja što brže i kvalitetnije informacije o izloženosti životne sredine ovim insekticidima i količinama njihovih ostataka u hrani potrebno je raspolagati odgovarajućim instrumentalnim metodama za njihovo određivanje. Razvijene su i optimizovane analitičke metode zasnovane na tečnoj hromatografiji za određivanje sedam odabranih neonikotinoida (dinotefurana, nitenpirama, tiametoksama, klotianidina, imidakloprida, acetamiprida i tiakloprida) u medu i likeru od meda. Ispitivana je mogućnost određivanja klotianidina pomoću tečne hromatografije visoke efikasnosti sa detektrorom od niza dioda (HPLC-DAD) primenom kombinacije tečno-tečne i ekstrakcije na čvrstoj fazi iz uzoraka meda. Na osnovu preliminarnih rezultata može se zaključiti da korišćenje faznih-čvrsto kolona u kombinaciji sa tečno-tečnom ekstrakcijom dihlormetanom rezultira prihvatljivim prinosom klotianidina u uzorcima meda pri koncentraciji od oko 0,5 µg g<sup>-1 </sup>klotianidina. Radi dobijanja većih prinosa odabrana je disperzna tečno-tečna mikroekstrakcija (DLLME) kao tehnika pripreme uzoraka meda. Testirana je upotreba acetonitrila kao disperznog sredstva. Pored hloroforma, korišćen je i dihlormetan kao drugo ekstrakciono sredstvo, kako bi se uporedila efikasnost ekstrakcije. Zabeleženi su prinosi klotianidina od 69,7 i 68,3% u zavisnosti da li je korišćen hloroform, odnosno DHM kao rastvor za ekstrakciju. Može se zaključiti da je prinos ekstrakcije bio povoljniji pri odnosu 0,5 mL ACN i 2,0 mL DHM. Prinosi su se kretali od 68,4% do 92,1%, što je ukazalo da su parametri DLLME ekstrakcije optimalni. Kako bi se detaljnije ispitali ključni parametri DLLME tehnike, korišćena je metodologija površine odziva (RSM), kao i detekcija na osetljivijem kuplovanom masenom detektoru (MS/MS). Optimizovani su HPLC-MS/MS parametri kako bi se obezbedilo zadovoljavajuće hromatografsko razdvajanje i niske granice detekcije (GD, 0,5–1,0 μg kg<sup>-1</sup>) i određivanja (GO, 1,0–2,5 μg kg<sup>-1</sup>) ispitivanih neonikotinoida u medu. Upotrebom centralno kompozitnog dizajna konstruisani su kvadratni modeli ispitivanih faktora: zapremine ekstrakcionog (DHM, 1,0–3,0 mL) i disperznog (ACN, 0,0–1,0 mL) sredstva, izračunati statistički parametri i optimizovan proces DLLME upotrebom <em>Derringer</em>-ove funkcije poželjnih odgovora. Upotrebom MMC i SC krivih u opsegu GO–100,0 μg kg<sup>-1 </sup>ispitan je uticaj matriksa pri čemu zaključeno je da je najveći uticaj matriksa bio na odziv analitičkog signala nitenpirama, dinotefurana i klotianidina. Ispitani su prinosi odabranih neonikotinoida (R, 74,3–113,9%), kao i preciznost metode u uslovima ponovljivosti (RSD, 2,74– 11,8%) i intermedijerne reproduktivnosti (RSD, 6,64–16,2%). Brza (retenciona vremena 1,5–9,9 min) i osetljiva metoda, koja troši malu količinu rastvarača, primenjena je za ispitivanje 15 realnih uzoraka meda različitog cvetnog porekla. Rezultati su pokazali da ispitivani med nije sadržao ostatke ispitivanih neonikotinoida u koncentracijama iznad GD. Dalje istraživanje je bilo usmereno ka razvijanju i optimizaciji HPLC-DAD analitičke metode upotrebom DLLME i QuEChERS tehnika za pripremu uzoraka za određivanje 7 neonikotinoida u uzorcima meda. U ovom delu istraživanja optimizovani su i hromatografski parametri, upotrebom RSM sa Box-Behnken-ovim dizajnom i Derringer-ovom funkcijom poželjnih odgovora. Od ispitivanih neonikotinoida dinotefuran i imidakloprid su bili u najvećoj meri izloženi uticaju matriksa, bez obzira na proceduru pripreme uzoraka. Može se istaći da je uticaj matriksa na analitički signal dinotefurana bio izraženiji u slučaju MS/MS, apostrofirajući manju robusnost ove metode određivanja. Prinosi neonikotinoida su bili (R, 73,1–118,3%), preciznost u uslovima ponovljivosti (RSD, 3,28–10,40%) i intermedijerne reproduktivnosti (RSD, 6,45–17,70%), a granice detekcije (GD, 1,5–2,5 µg kg<sup>-1</sup>) i određivanja (GO, 5,0–10,0 µg kg<sup>-1</sup>). Metoda je primenjena za ispitivanje 7 neonikotinoida u 104 uzorkameda različitog cvetnog porekla sa teritorije Autonomne Pokrajine Vojvodine. Detektovano je prisustvo tiakloprida, imidakloprida i tiametoksama u količinama koje su bile ispod MDK RS i EU. Analizirani su uzorci likera od meda - medice. Upoređivane su dve tehnike pripreme uzoraka, DLLME i QuEChERS i primenjeni optimizovani hromatografski uslovi i MS/MS parametri. U slučaju nitenpirama, dinotefurana i tiametoksama uticaj matriksa bio je najizraženiji. Metoda je validovana određivanjem prinosa neonikotinoida (R, 69,2–113,4%), preciznosti u uslovima ponovljivosti (RSD, 3,21–12,81%) i intermedijerne reproduktivnosti (RSD, 9,11–16,63%), kao i granice detekcije (GD, 0,5–2,5 µg kg<sup>-1</sup>) i određivanja (GO, 1,0–10,0 µg kg<sup>-1</sup>). Analizom 10 komercijalno dostupnih likera od meda otkriveno je prisustvo klotianidina i tiakloprida, evčzokinotš z na neophodnost daljeg kontrolisanja ovog proizvoda na prisustvo neonikotinoida. Ispitana je mogućnost uklanjanja odabranih neonikotinoida (dinotefurana, klotianidina i tiakloprida) iz vodene sredine (reke Dunav). Ispitivanje efikasnosti 6 različitih vrsta uklanjanja odabranih neonikotinoida (u prisustvu prirodne insolacije u laboratorijskim uslovima, sa dodatkom H2O2, sa dodatkom MWCNT, sa dodatkom MWCN+H <sub>2</sub>O<sub>2</sub>, sa dodatkom Fe-MWCNT, sa dodatkom Fe-MWCNT+H<sub>2</sub>O<sub>2</sub>) vršeno je upotrebom prethodno razvijene HPLC-MS/MS metode. Krive uklanjanja odabranih neonikotinoida, pokazale su da tokom 60 minuta pri prirodnoj insolaciji u laboratorijskim uslovima koncentracija smanjenje oko 25%. Analitički signal dinotefurana dobijen u prisustvu H<sub>2</sub>O<sub>2 </sub>pod istim uslovima ukazuje na uklanjanje ciljnog analita od oko 40%, tiakloprida od oko 70%, a klotianidina u potpunosti. Testirana je adsorpcija ciljnog analita na višezidnim ugljeničnim nanocevima (MWCNT). Ovim postupkom može da se ukloni oko 30% dinotefurana, oko 50% klotianidina i 60% tiakloprida. U kombinaciji sa H<sub>2</sub>O<sub>2 </sub>, MWCNT pokazuju bolju sposobnost uklanjanja za 15–50% u zavisnosti od ispitivanog neonikotinoida. Upotreba Fe-MWCNT i njihova kombinacija sa H<sub>2</sub>O<sub>2</sub> otvorila je mogućnost za dalja ispitivanja mehanizma uklanjanja. Ustanovljeno je nastajanje intermedijera kojima odgovaraju m/z od 117,5 i 140,6 u slučaju razgradnje dinotefurana u sistemima sa H<sub>2</sub>O<sub>2</sub>, MWCNT+H<sub>2</sub>O<sub>2</sub>, Fe-MWCNT+H<sub>2</sub>O<sub>2 </sub>i klotianidina u sistemu Fe-MWCNT+H<sub>2</sub>O<sub>2</sub>.</p> / <p>Neonicotinoid insecticides, as one of the fastest growing new generation of insecticides, have contributed to a significant reduction of toxicity for the environment; therefore, monitoring and determination of trace levels of the neonicotinoids in honey are necessary and demands highly efficient, selective and sensitive analytical techniques. The objective of the present work was to develop a rapid, sensitive, optimized and accurate analytical method based on liquid chromatography for determining seven neonicotinoid insecticides, dinotefuran, nitenpyram, thiamethoxam, clothianidin, imidacloprid, acetamiprid and thiacloprid in honey and honey liqueur samples. The possibility for determination of clothianidin in honey samples was investigated by HPLC with a diode array detector (HPLC-DAD). Based on preliminary results, it can be concluded that the use of a solid-phase column in combination with a liquid-liquid extraction with dichloromethane results in an acceptable recovery of clothianidin in the samples with a clothianidin concentration of about 0.5 µg g<sup>-1</sup>. After obtaining low recovery of clothianidin, dispersed liquid-liquid microextraction (DLLME) was selected as a technique for the preparation of honey samples.. The adequacy of acetonitrile as a dispersing agent was investigated. Besides the chloroform, a dichloromethane was used as a second extracting agent , in order to compare the relative efficiency of the extraction solvents. It can be concluded that the extraction recovery (68.4–92.1%) was more favorable with the use of 0.5 mL ACN and 2.0 mL DHM. Furthermore, LC-MS/MS parameters were optimized to unequivocally provide good chromatographic separation, low detection (LOD, 0.5–1.0 μg L<sup>−1</sup>) and quantification (LOQ, 1.0–2.5 μg L<sup>−1</sup>) limits for acetamiprid, clothianidin, thiamethoxam, imidacloprid, dinotefuran, thiacloprid and nitenpyram in honey samples. Using different <br />types (chloroform, dichloromethane) and volumes of extraction (1.0–3.0 mL) and dispersive (acetonitrile; 0.0–1.0 mL) solvent and by mathematical modeling it was possible to establish the optimal sample preparation procedure. Matrix-matched calibration and blank honey sample spiked in the <span style="font-size: 12px;">concentration range of LOQ–100.0 μg kg</span><sup><span style="font-size: 12px;">−1 </span></sup><span style="font-size: 12px;">were used to compensate the matrix effect and to fulfill the </span><span style="font-size: 12px;">requirements of SANCO/12495/2011 for the accuracy (R 74.3–113.9%) and precision (expressed in </span><span style="font-size: 12px;">terms of repeatability (RSD 2.74–11.8%) and within-laboratory reproducibility (RSDs 6.64–16.2%)) of </span><span style="font-size: 12px;">the proposed method. The rapid (retention times 1.5–9.9 min), sensitive and low solvent consumption </span><span style="font-size: 12px;">procedure described in this work provides reliable, simultaneous, and quantitative method applicable for </span><span style="font-size: 12px;">the routine laboratory analysis of seven neonicotinoid residues in 15 real honey samples. Neonicotinoid </span><span style="font-size: 12px;">residues were not detected in any of the investigated samples. The objective of next study was to </span><span style="font-size: 12px;">develop and optimize HPLC-DAD analytical method with dispersive liquid-liquid microextraction </span><span style="font-size: 12px;">(DLLME) and QuEChERS sample preparation procedures for the simultaneously analysis of seven </span><span style="font-size: 12px;">neonicotinoids in honey samples. The liquid chromatographic conditions were optimized by response </span><span style="font-size: 12px;">surface methodology with <em>Box-Behnken</em> design and the global <em>Derringer</em>´s desirability. The optimized </span><span style="font-size: 12px;">method was validated to fulfill the requirements of SANCO/12495/2011 standard for both sample </span><span style="font-size: 12px;">pretreatment procedures providing results for accuracy (R, 73.1–118.3%), repeatability (RSD, 3.28–</span><span style="font-size: 12px;">10.40%) and within-laboratory reproducibility (RSD, 6.45–17.70%), limits of detection (LOD, 1.5–2.5 </span><span style="font-size: 12px;">gµ kg</span><sup><span style="font-size: 12px;">-1</span></sup><span style="font-size: 12px;">) and quantification (LOQ, 5.0–10.0 µg kg</span><sup><span style="font-size: 12px;">-1</span></sup><span style="font-size: 12px;">). For the first time, more than 100 honey samples </span><span style="font-size: 12px;">collected from all 7 counties of Autonomous Province of Vojvodina were analyzed. The presence of </span><span style="font-size: 12px;">thiacloprid, imidacloprid and thiametoxam was discovered in a small number of samples. The objective </span><span style="font-size: 12px;">of next study was to develop an optimized LC-MS/MS analytical method with DLLME and QuEChERS </span><span style="font-size: 12px;">procedures for analysis of 7 neonicotinoids in honey liqueur. The method was validated to fulfill the </span><span style="font-size: 12px;">requirements of SANCO/12495/2011 for both sample pretreatment procedures providing results for </span><span style="font-size: 12px;">accuracy (R, 69.2–113.4% for DLLME; 71.8–94.9% for QuEChERS), precision (RSD expressed in </span><span style="font-size: 12px;">terms of repeatability (3.21–10.20% for DLLME; 4.19–12.81% for QuEChERS) and within-laboratory </span><span style="font-size: 12px;">reproducibility (9.11–16.63% for DLLME; 11.32–16.40% for QuEChERS)), limits of detection (LOD, </span><span style="font-size: 12px;">0.5–1.5 gµ L</span><sup><span style="font-size: 12px;">-1 </span></sup><span style="font-size: 12px;">for DLLME; 1.0–2.5 gµ L</span><sup><span style="font-size: 12px;">-1 </span></sup><span style="font-size: 12px;">for QuEChERS) and quantification (LOQ, 1.0–5.0 gµ L</span><sup><span style="font-size: 12px;">-1 </span></sup><span style="font-size: 12px;">for </span><span style="font-size: 12px;">DLLME; 2.5–10.0 µg L</span><sup><span style="font-size: 12px;">-1 </span></sup><span style="font-size: 12px;">for QuEChERS). Analysis of real honey liqueur samples obtained from local </span><span style="font-size: 12px;">markets showed the presence of clothianidin or thiacloprid in four of the analyzed samples, therefore </span><span style="font-size: 12px;">implicating the necessity of ongoing control of this type of traditional product. Removal of selected </span><span style="font-size: 12px;">neonicitinoid insecticides - dinotefuran, clothianidin and thiacloprid using MWCNT and H</span><span style="font-size: 12px;"><sub>2</sub>O<sub>2 </sub></span><span style="font-size: 12px;">from </span><span style="font-size: 12px;">Danube water matrix was investigated. Efficiency of different systems for neonicotinoids removal </span><span style="font-size: 12px;">(under natural insolation in laboratory, with H</span><span style="font-size: 12px;"><sub>2</sub>O<sub>2</sub>, with MWCNT, with MWCNT+ H</span><span style="font-size: 12px;"><sub>2</sub>O<sub>2</sub>, with Fe-MWCNT, with Fe-MWCNT+H<sub>2</sub>O<sub>2</sub>) was evaluated with developed LC-MS/MS method. Analysis of </span><span style="font-size: 12px;">degradation rates revealed loss of 25% of the initial neonicotinoid concentration under natural insolation in </span><span style="font-size: 12px;">the laboratory conditions during 60 min. Addition of chemical agent H<sub>2</sub>O<sub>2 </sub>promoted loss of 40% of the </span><span style="font-size: 12px;">initial dinotefuran, 70% of thiacloprid concentration and total removal of clothianidin under same </span><span style="font-size: 12px;">conditions. With the addition of MWCNT concentration of dinotefuran, clothianidin and thiacloprid </span><span style="font-size: 12px;">decayed for 30, 50 and 60%, respectively. Iron modification of MWCNT in combination with H</span><span style="font-size: 12px;"><sub>2</sub>O<sub>2 </sub></span><span style="font-size: 12px;">increased the removal rate of selected neonicotinoid for 15–50%. Presence of intermediates was </span><span style="font-size: 12px;">discovered in systems of dinotefuran with H<sub>2</sub>O<sub>2</sub>, MWCNT+H</span><span style="font-size: 12px;"><sub>2</sub>O<sub>2</sub>, e-MWCNT+H<sub>2</sub>O<sub>2 </sub></span><span style="font-size: 12px;">and of </span><span style="font-size: 12px;">clothianidin in systems with Fe-MWCNT+H<sub>2</sub>O<sub>2 </sub></span><span style="font-size: 12px;">with m/z of 117,5 and 140,6. </span></p>
|
| 22 |
Voltametrijske metode zasnovane na primeni jednostavnih i savremenih elektroda/senzora na bazi ugljeničnih materijala za određivanje vodonik-peroksida u odabranim uzorcima / Voltammetric methods based on the application of simple and contemporary carbonaceousmaterials-based electrodes/sensors for the determination of hydrogen-peroxide in the selected samplesAnojčić Jasmina 22 November 2018 (has links)
<p>Cilj ove doktorske disertacije je bio razvoj brzih i pouzdanih voltametrijskih metoda zasnovanih na primeni jednostavnih i savremenih elektroda/senzora na bazi ugljeničnih materijala (ugljenične paste napravljene od grafitnog praha i parafinskog ulja i štampanih ugljeničnih elektroda) za određivanje H <sub>2</sub>O<sub>2 </sub>u odabranim složenim model i realnim uzorcima. U tu svrhu, ispitana je primenljivost različitih radnih elektroda. Amperometrijska metoda zasnovana na elektrodi od ugljenične paste (CPE) zapreminski modifikovane sa 5% (m:m) MnO2 je, pri optimizovanim uslovima i pri radnom potencijalu od 0,40 V u odnosu na zasićenu kalomelovu elektrodu (ZKE) u fosfatnom puferu pH 7,50 kao pomoćnom elektrolitu, omogućila kvantifikaciju H <sub>2</sub>O<sub>2</sub> u opsegu koncentracija od 1,4 do 65 µg mL -1 sa relativnom standardnom devijacijom (RSD) manjom od 10%. Ova metoda je primenjena za određivanje sadržaja H <sub>2</sub>O<sub>2 </sub>u uzorcima podzemne vode iz centralnog Banata (Pokrajina Vojvodina, Srbija) tretirane Fentonovim (Fe<sup> 2+</sup> i H <sub>2</sub>O<sub>2</sub> ) i Fentonu-sličnim (Fe <sup>3+</sup> i H <sub>2</sub>O<sub>2</sub> ) reagensima u cilju uklanjanja prirodnih organskih materija (POM) pri čemu su korišćene različite početne koncentracije gvožđa i različiti odnosi molarnih koncentracija gvožđa i H <sub>2</sub>O<sub>2</sub> . Utvrđeno je da oksidaciono stanje gvožđe (Fe<sup> 2+</sup> ili Fe <sup>3+</sup>) i molarni odnos jona Fe i H<sub> 2</sub>O<sub>2</sub> utiču na stepen potrošnje/razgradnje H<sub> 2</sub>O<sub>2</sub><br />u podzemnoj vodi sa visokim sadržajem POM. Takođe, u slučaju Fentonu-sličnog procesa, za sve početne koncentracije Fe <sup>3+</sup> i H <sub>2</sub>O<sub>2 ,</sub> signifikantna količina H <sub>2</sub>O<sub>2 </sub>ostaje neiskorišćena, što ukazuje na nižu efikasnost ovakvog sistema u poređenju sa Fentonovim procesom.Štampana ugljenična elektroda (SPCE) zapreminski modifikovana sa MnO 2 kao medijatorom je primenjena za određivanje sadržaja H<sub> 2</sub>O<sub>2</sub> u toku Fentonovog (Fe <sup>2+</sup> , H<sub>2</sub>O<sub>2</sub> ) i vidljivom svetlošću potpomognutog foto-Fentonovog (Fe<sup> 2+ </sup>, H <sub>2</sub>O<sub>2 </sub>, hν) procesa uklanjanja neonikotinoidnog insekticida acetamiprida (ACT). Pri optimizovanim uslovima (radni potencijal 0,40 V u odnosu na ZKE, fosfatni pufer pH 7,50 kao pomoćni elektrolit) amperometrijskog određivanja H <sub>2</sub>O<sub>2</sub> , postignuta je linearnost u opsegu koncentracija 0,01–1,24 mmol L-1(0,34– 42,2 µg mL -1<br />) i vrednost RSD nije prelazila 4,2%. U ispitivanimuzorcima (nakon odgovarajućeg<br />podešavanja pH vrednosti od 2,8 do 7,5 odmah nakon uzorkovanja radi stopiranja ili maksimalnog usporavanja procesa oksidacije, filtriranja, zamrzavanja i odmrzavanja neposredno pre merenja) sadržaj H <sub>2</sub>O<sub>2</sub> je određen metodom standardnog dodatka analiziranjem odgovarajućih amperometrijskih krivi. Paralelna HPLC-DAD merenja su vršena u cilju praćenja koncentracije/uklanjanja ACT. U slučaju foto-Fentonovog procesa (početne koncentracije 0,31; 2,0 i 3,0 mmol L -1 (70,0; 111,7 i 102,1 µg mL -1 ) za ACT, Fe<br /><sup>2+</sup> i H <sub>2</sub>O<sub>2</sub>, redom) nakon 10 min H <sub>2</sub>O<sub>2 </sub>je izreagovao, a može se smatrati da je ACT uklonjen nakon 5 min. U toku Fentonovog procesa ACT je uklonjen nakon 20 min tretmana i oko 10% početne koncentracije H <sub>2</sub>O<sub>2</sub> je ostalo u sistemu<br />neiskorišćeno.CPE je površinski modifikovane kompozitom na bazi nanočestica Pt (< 5 nm) i grafitizovanog ugljenika (Pt-C, 10% Pt na Vulkanu XC72) etodom nanošenja kapi. Nemodifikovana CPE i modifikovana (Pt-C/CPE) su okarakterisane primenom SEM/EDS i CV merenja. Pt-C/CPE je pokazala izuzetne elektrokatalitičke osobine u pogledu elektrohemijskeredoks reakcije H<br /><sub>2</sub>O<sub>2</sub> u poređenju sa nemodifikovanom CPE u fosfatnom puferu (0,1 mol L -1 ;<br />pH 7,50), a takođe i u acetatnom puferu (0,1 mol L -1 ; pH 4,50) kao pomoćnim elektrolitima. Prilikom amperometrijskog određivanja H <sub>2</sub>O<sub>2</sub> primenom Pt-C/CPE u model sistemima, zadovoljavajuća linearnost je postignuta u koncentracionom opseguH<sub>2</sub>O<sub>2</sub>od 0,15 do 1,45 µg mL -1 ,dok su vrednosti GO iznosile 0,06 µg mL -1 (pH 7,50, radni potencijal 0,20 V) i 0,10 µg mL -1 (pH<br />4,50, radni potencijal 0,50 V). Optimizovane analitičke metode su primenjene za određivanje sadržaja H <sub>2</sub>O<sub>2</sub> u komercijalno dostupnim proizvodima za ličnu negu: rastvoru za dezinfekciju (pH 7,50) i rastvoru za čišćenje kontaktnih sočiva (pH 4,50). Amperometrijski dobijeni rezultati su u dobrom slaganju sa rezultatima dobijenim primenom tradicionalne spektrofotometrijske metode bazirane na titanijum-sulfatu kao reagensu sa određenim koncentracijama 2,91% i 2,94% za<br />dezinfekcioni rastvor i 3,04% i 3,17% za rastvor zakontaktna sočiva, redom. RSD je bila manja od 2%. Postignuti rezultati su u dobrom slaganju sa sadržajem H<br /><sub>2</sub>O<sub>2 </sub>deklarisanim od strane proizvođača (3%) u oba ispitivana uzorka. Pt-C/CPE je takođe testirana za praćenje koncentracije H<sub>2</sub>O<sub>2</sub> u rastvoru za kontaktna sočiva u toku procesa njegove neutralizacije/razgradnje. Nakon 6 h procesa neutralizacije, 24,68 µg mL -1 je bila određena koncentracija H <sub>2</sub>O<sub>2 ,</sub> što je ispod dozvoljene<br />H<sub>2</sub>O<sub>2 </sub>koncentracije u rastvoru za kontaktna sočiva imajući u vidu granicu koja izaziva iritaciju oka. CPE je površinski modifikovana višezidnim ugljeničnim nanocevima (MWCNT) i kompozitima MnO 2 -MWCNT ili Pt-MWCNT metodom nanošenja kapi radi pripreme jednostavnih, osetljivih i pouzdanih voltametrijskih senzora za određivanje H <sub>2</sub>O<sub>2 </sub>u odabranom uzorku. Rezultati SEM/EDS analize kompozitnih materijala su potvrdili da su medijatori, čestice MnO 2 i Pt, nasumično raspoređeni na površini MWCNT i zastupljeni sa blizu 5% (m:m) u kompozitu izraženo<br />preko Mn i Pt. CV merenja su vršena sa pripremljenim radnim elektrodama u acetatnom (pH 4,50), fosfatnom (pH 7,50) i boratnom (pH 9,18) puferu kako bi se okarakterisalo osnovno elektrohemijsko ponašanje H <sub>2</sub>O<sub>2 </sub>i odabrali pogodni radni potencijali za amperometrijsko određivanje ovog ciljnog analita. Pt- WCNT/CPE je primenljiva za rad kako u fosfatnom puferu pH 7,50 tako i u acetatnom puferu pH 4,50 V kako pri negativnim tako i pri pozitivnim radnim<br />potencijalima, pri čemu su vrednosti RSD uglavnom ispod 2,5%. U slučaju MnO 2<br />-MWCNT/CPE, na potencijalu 0,30 V i višim vrednostima, oksidacioni signali H<br /><sub>2</sub>O<sub>2</sub> su signifikantni u blago alkalnoj sredini (pH 7,50), pri pH 4,50 ova elektroda pokazuje nezadovoljavajuće ponašanje, dok pri pH 9,18 ima prihvatljive performanse. Granice određivanja (GO) su bile u oblasti µg mL -1 . H <sub>2</sub>O<sub>2</sub> je određen u spajkovanom uzorku mleka metodom standardnog dodatka nakon odgovarajuće pripreme uzorka (pH podešavanje i centrifugiranje) i primenom optimizovane amperometrijske procedure (acetatni pufer pH 4,50, radni potencijal-0,75 V) koristeći Pt-MWCNT/CPE kao radnu elektrodu. RSD za tri ponovljena merenja je iznosila 2,5%, dok je prinos metode bio nešto veći od 71%.<br />Kompozitni materijali koji se sastoje od MWCNT i čestica na bazi Pd (Pd-MWCNT) ili Pt (Pt-MWCNT) su primenjeni za pripremu zapreminski modifikovanih SPCE (Pd-MWCNT-SPCE i Pt-MWCNT-SPCE) i površinski modifikovane SPCE (Pd-MWCNT/SPCE). Ove elektrode, kao i nemodifikovana SPCE i MWCNT-SPCE, su okarakterisane primenom CV i amperometrije u fosfatnom puferu pH 7,50 radi određivanja H <sub>2</sub>O<sub>2</sub> . Pd-MWCNT-SPCE i Pd-MWCNT/SPCE su se pokazale pogodnim za određivanje H <sub>2</sub>O<sub>2</sub> na radnim potencijalima između -0,50 i 0,50 V, a PtMWCNT-SPCE na ispitivanim radnim potencijalima od -0,80 do 0,70 V. Ove elektrode su zatim modifikovane enzimom glukoza oksidazom (GOx) metodom nanošenja kapi rastvora GOx i Nafion ® -a na njihovu površinu, pri čemu je optimizovana količina nanetog biofilma. GOx/PdMWCNT-SPCE je pokazala bolje analitičke performanse za određivanje glukoze u poređenju sa GOx/Pd- WCNT/SPCE. Kao optimalan radni potencijal GOx/Pd-MWCNT-SPCE je odabrana<br />vrednost potencijala -0,40 V u odnosu na ZKE, sa zadovoljavajućom linearnošću u ispitivanom opsegu koncentracija glukoze od 0,16 do 0,97 mmol L -1 (od 29,1 do 174 µg mL -1), dok je GO iznosila 0,14 mmol L -1 (25 µg mL<br />-1 ). Optimizovana metoda zasnovana na GOx/Pd-MWCNT-SPCE je uspešno primenjena za određivanje glukoze u uzorku livadskog meda. Dobijeni rezultati su u dobroj saglasnosti sa onima dobijenim primenom komercijalno dostupnog aparata za merenje glukoze. Pogodan radni potencijal za GOx/Pt-MWCNT-SPCE je bio -0,50 V u odnosu na ZKE, pri čemu je zadovoljavajuća linearnost postignuta u ispitivanom opsegu koncentracija glukoze od 65,8 do 260,6 µg mL -1 , sa GO 35 µg mL -1 . Optimizovana metoda zasnovana na GOx/Pt-MWCNT-SPCE je uspešno primenjena za određivanje glukoze u u uzorku belog grožđa i uzorku tableta (Traubenzucker-bonbons), pri čemu su dobijeni rezultati u dobroj saglasnosti sa rezultatima dobijenim primenom Accu-Chek aparata.<br />Na osnovu dobijenih rezultata može se zaključiti da su razvijene analitičke metode pre svega jednostavne, pouzdane i pogodne za dobijanje brzih informacija o sadržaju H <sub>2</sub>O<sub>2</sub> u različitim tipovima uzoraka. Svakako odabir pogodne radne elektrode, kao i optimizacija eksperimentalnih uslova su ključni faktori za uspešno određivanje H <sub>2</sub>O<sub>2</sub></p> / null / <p>The aim of this doctoral dissertation was the development of fast and reliable voltammetric methods based on the application of simple and contemporary electrodes/sensors based on carbonaceous materials (carbon paste made of graphite powder and paraffin oil and screen printed carbon electrodes) for the determination of H <span id="cke_bm_571S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_571E" style="display: none;"> </span>O<sub>2</sub> in the selected complex model and real samples.For this purpose, applicability of different working electrodes was investigated.The amperometric method based on carbon paste electrode (CPE) bulk- modified with 5% (m:m) MnO 2 , under optimized conditions, with a working potential of 0.40 V vs. the saturated calomel elect<span id="cke_bm_572E" style="display: none;"> </span>rode (SCE) and a phosphate buffer solution (pH 7.50) as supporting electrolyte, enabled the quantitation of H <span id="cke_bm_577S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_577E" style="display: none;"> </span>O<sub>2</sub>in the concentration interval from 1.4 to 65 µg mL −1 with a relative standard deviation (RSD) of less than 10%. This meth<span id="cke_bm_578E" style="display: none;"> </span>od was applied for the determination of the H<span id="cke_bm_583S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_583E" style="display: none;"> </span>O<sub>2 </sub>consumption in samples of groundwater fro<span id="cke_bm_584E" style="display: none;"> </span>m the Central Banat region (Province of Vojvodina, Serbia) treated by the Fenton (Fe <sup>2+ </sup>and H <sub>2</sub>O<sub>2</sub> ) and Fenton- like (Fe <sup>3+</sup>and H <sub>2</sub>O<sub>2</sub> ) reagents to remove natural organic matter (NOM) at differentinitial concentrations of iron species, and of their molar ratios to the initial concentration of H<sub>2</sub>O<sub>2</sub> . It was found that the form of Fe (Fe <sup>2+</sup> or Fe <sup>3+</sup> )and the molar ratio to H <sub>2</sub>O<sub>2</sub>influenced the degree of the H<sub> 2</sub>O<sub>2</sub> decomposition in the groundwater with high NOM content. Besides, in the case of the Fenton-like process, for all initial doses of Fe <sup>3+</sup> and H<span id="cke_bm_604S" style="display: none;"> </span><sub> 2</sub><span id="cke_bm_604E" style="display: none;"> </span>O<sub>2</sub>, a sign<span id="cke_bm_605E" style="display: none;"> </span>ificant amount of H<span id="cke_bm_610S" style="display: none;"> </span><sub> 2</sub><span id="cke_bm_610E" style="display: none;"> </span>O<sub>2</sub> remained unused, whi<span id="cke_bm_611E" style="display: none;"> </span>ch also indicates a lower efficiency of such system compared to the Fenton process. Screen printed carbon electrode (SPCE) bulk-modified with MnO <sub> 2</sub> as a mediator was applied for amperometric determination of the H <span id="cke_bm_617S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_617E" style="display: none;"> </span>O<sub>2</sub> content during the Fenton (Fe <sup>2+</sup> , H <span id="cke_bm_624S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_624E" style="display: none;"> </span>O<sub>2 </sub>) and <span id="cke_bm_625E" style="display: none;"> </span>visible light-assisted<span id="cke_bm_618E" style="display: none;"> </span>, photo-Fenton (Fe <sup>2+ </sup> , H <span id="cke_bm_631S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_631E" style="display: none;"> </span>O<sub>2</sub> , hν)<span id="cke_bm_632E" style="display: none;"> </span> based removal of neonicotinoid insecticide acetamiprid (ACT). Under optimized conditions (working potential of 0.40 V vs. SCE, phosphate buffer pH 7.50 as supporting electrolyte) amperometric determination of H <span id="cke_bm_637S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_637E" style="display: none;"> </span>O<sub>2 </sub>showed a linear dynamic range from 0.01 to 1.24 mmol L -1 (from 0.34 to 42.2 µg mL -1) and the RSD did not exceed 4.2%. In the investigated samples (after appropriate pH adjustment from 2.8 to 7.5 instantly after the sampling<span id="cke_bm_638E" style="display: none;"> </span> to stop or maximum decelerate the oxidation processes, filtering, and storage of the deep- frozen sample with defrosting immediately before the measurements) the H <span id="cke_bm_643S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_643E" style="display: none;"> </span>O<sub>2</sub> contents were determined by the standard addition method by analyzing the corresponding amperometric curves. Parallel HPLC-DAD measurements were performed to monito<span id="cke_bm_644E" style="display: none;"> </span>r the concentration/removal of ACT. In the case of the photo- Fenton process (initial concentrations: 0.31; 2.0 and 3.0 mmol L -1 (70.0; 111.7 and 102.1 µg mL -1 ) of ACT, Fe<sup> 2+</sup> and H <span id="cke_bm_650S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_650E" style="display: none;"> </span>O<sub>2</sub>, respec<span id="cke_bm_651E" style="display: none;"> </span>tively) after 10 min of irradiation H <span id="cke_bm_656S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_656E" style="display: none;"> </span>O<sub>2</sub> was consumed and it can be consi<span id="cke_bm_657E" style="display: none;"> </span>dered that ACT was removed after 5 min. During the Fenton process ACT was removed after 20 min of treatment and around 10% of the initial concentration of the H 2O2 remained still unused.<br />CPE was surface modified with a composite of Pt nanoparticles (< 5 nm) on graphitized carbon (Pt-C, 10% Pt on Vulcan XC72) by simply dropcoating method. The unmodified CPE and the modified one (Pt-C/CPE) were characterized by EM/EDS and CV measurements. The PtC/CPE showed remarkable electrocatalytic propertiestoward the electrochemical redox reaction of H<span id="cke_bm_662S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_662E" style="display: none;"> </span>O<sub>2</sub> compared to modified CPE in phosphate buffer (0.1 mol L -1 ; pH 7.50), as well in acetatebuffer (0.1 mol L -1 ; pH .50) supporting electrolytes. Amperometry of H2O2 in the concentration range from 0.15 to 1.45 µg mL -1 with the Pt-C/CPE showed acceptable linearity, while the obtained values of LOQs were 0.06 µg mL -1 (pH 7.50, working potential 0.20 V) and 0.10 µg mL -1 (pH 4<span id="cke_bm_663E" style="display: none;"> </span>.50, working potential 0.50 V). The proposed analytical methods were applied to the determination of the H 2O2 content in commercially available personal care products; i.e., disinfection (pH 7.50) and contact lens cleaning solutions (pH 4.50). The obtained amperometric results are in good agreement with those measured by traditional titanium sulfatereagent based spectrophotometric method with determined concentrations as 2.91% and 2.94% for the disinfection product, and 3.04% and 3.17% for the contact lens solution, respectively. RSD was lower than 2%. The obtained results are in a good agreement with the amounts of the H<span id="cke_bm_668S" style="display: none;"> </span><sub> 2</sub><span id="cke_bm_668E" style="display: none;"> </span>O<sub>2 </sub>declared by producers (3%) in the both investigated samples. The Pt-C/CPE was also tested for monitoring of the H<span id="cke_bm_674S" style="display: none;"> </span><sub> 2</sub><span id="cke_bm_674E" style="display: none;"> </span>O<sub>2</sub> residual concentration in contact lens solution during its neutralization/decomposition rocess. At 6 h of<span id="cke_bm_675E" style="display: none;"> </span> neutralization treatment 24.68 µg mL -1 of the H <span id="cke_bm_680S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_680E" style="display: none;"> </span>O<sub>2 </sub>was determined which is almost half of the allowedH2<span id="cke_bm_681E" style="display: none;"> </span>O2 concentration in the case of the contact lens solution concerningthe limit of eye irritation. CPE was surface modified with multiwalled carbon nanotubes (MWCNT) and with composites of MnO<sub> 2 </sub>-MWCNT or Pt-MWCNT by drop coating method to prepare simply, sensitive and reliable volta<span id="cke_bm_669E" style="display: none;"> </span>mmetric sensors for the determination of H <span id="cke_bm_687S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_687E" style="display: none;"> </span>O<sub>2 </sub>in selected sample. The results of the SEM/<span id="cke_bm_688E" style="display: none;"> </span>EDS analysis of composite materials have confirmed that the mediators, MnO <sub>2</sub> and Pt articles, are randomly distributed on the surface of MWCNT and represent nearly 5% (m:m) of the composite expressed as Mn and Pt. CV measurements were performed with prepared electrodes in acetate (pH 4.50), phosphate (pH 7.50) and borate (pH 9.18) buffers to characterize the basic electrochemical behavior of H <span id="cke_bm_694S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_694E" style="display: none;"> </span>O<sub>2</sub> and to select the working potentials suitable for amperometric determination of this target analyte. The Pt-MWCNT/CPE performs well in phosphate buffer pH .50 and acetate buffer solution pH 4.50 in the negative as well as in the positive polarization range with RSD mainly lower than 2.5%. In case of MnO<sub> 2 </sub>-MWCNT/CPE at <span id="cke_bm_695E" style="display: none;"> </span>0.30 V and above the H<span id="cke_bm_701S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_701E" style="display: none;"> </span>O<sub>2 </sub>oxidation signal is rem<span id="cke_bm_702E" style="display: none;"> </span>arkable in slightly alkaline media (pH 7.50), at pH 4.50 this electrode showed poor behavior and at pH 9.18 offered acceptable performance. LOQs were in the µg mL -1 concentration range. H<span id="cke_bm_707S" style="display: none;"> </span><sub>2</sub><span id="cke_bm_707E" style="display: none;"> </span>O<sub>2 </sub>was determined in a spiked milk sample by standard addition method after appropriate sample preparation (pH adjustment and centrifugation) and using optimized amperometric p<span id="cke_bm_708E" style="display: none;"> </span>rocedure (acetate buffer pH 4.50, working potential -0.75 V) by Pt-MWCNT/CPE as a working electrode. RSD for three repeated measurements was 2.5%, while the recovery of the method was a bit higher than 71%. The composite materials consisting of MWCNT and Pd (Pd-MWCNT) or Pt containing particles (Pt-WCNT) were applied to the preparation of bulk- modified SPCEs (Pd-MWCNTSPCE and Pt-MWCNT-SPCE) and surface modifiedSPCE (Pd- MWCNT/SPCE). These electrodes, as well as unmodified SPCE and MWCNT-SPCE, were characterized by CV and amperometry in phosphate buffer solution of pH 7.50 for the H<span id="cke_bm_713S" style="display: none;"> </span><sub> 2</sub><span id="cke_bm_713E" style="display: none;"> </span>O<sub>2 </sub>determination. Pd-MWCNT-SPCE and PdMWCNT/SPCEare convenient for the etermination of H 2O2 at working potentials from -0.50 to 0.50 V, and Pt-MWCNT-SPCE at investigated working potentials in the range from -0.80 to 0.70 V. These electrodes were then modified with glucose oxidase (GOx) by drop coating a solution of GOxand Nafion ® on their surface, whereby the applied amount of biococktail was optimized. GOx/PdMWCNT-SPCE showed better analytical performance for glucose determination in comparison with GOx/Pd-MWCNT/SPCE. The optimal working potential for GOx/Pd-MWCNT- SPCE was -0.40 V vs. SCE and <span id="cke_bm_714E" style="display: none;"> </span>satisfactory linearity was obtained in the investigated glucose concentration range from 0.16 to 0.97 mmol L -1 (from 29.1 to 174 µg mL -1 ), hile the LOQ was 0.14 mmol L -1 (25 µg mL -1 ). The optimized method based on GOx/Pd-MWCNT-SPCE was successfully applied to the determination of glucose in multifloral honey sample. The results are in a good agreement with those obtained by commercially available equipment for determination of glucose. Optimal working potential for GOx/Pt-MWCNT-SPCE was -0.50 V vs. SCE, and the satisfactory linearity was obtained in the investigated concentration range ofglucose from 65.8 to 260.6 µg mL -1 , with LOQ of 35 µg mL -1 . The optimized method based on GOx/Pt- MWCNT-SPCE was successfully applied for determination of glucose in white grape and glucose tablets (Traubenzucker-bonbons) samples, whereby the obtained results were in a good agreement with the results obtained by Accu-Chek device. Based on the results, the developed analytical methods are first of all simple, reliable and suitable for obtaining fast information about the content of H <sub>2</sub>O<sub>2</sub> in different types of samples. Certainly the selection of a suitable working electrode, as well as the optimization of experimental conditions are key factors for the successful determination of H<sub>2</sub>O<sub>2 </sub>.</p>
|
Page generated in 0.2734 seconds