Grafit fırınlı atomik absorpsiyon spektrofotometresinde mangan tayininde çeşitli iyon karışımlarının girişim etkileri

Tez (Doktora)-- İTÜ Fen Bil. Enst., 1998.Thesis (Ph.D.) -- İstanbul Technical University, Institute of Science and Technology, 1998Grafit fırınlı atomik absorpsiyon spektrometresiyle (GFAAS) matriks varlığında yapılan analizlerde meydana gelen girişimler sonuçların doğruluğunu etkilemektedir. Şimdiye kadar yapılan çalışmalarda daima ortamda tek bir tuz (bir anyon ve bir katyon) bulunan örneklerin girişim mekanizmaları incelenmiştir. Ancak doğal örneklerde daima birden fazla anyon ve katyon bulunmaktadır. Bu çalışmada, GFAAS'de matriks olarak seçilen sodyum klorür, magnezyum klorür, sodyum sülfat, magnezyum sülfat, alüminyum sülfat, alüminyum klorürün ayrı ayrı ve kombine biçimleri uygulanarak tayin elementi olarak seçilen mangan üzerine bağımsız ve birleşik etkileri araştırılmış, buradan girişim mekanizmaları üzerine genellemelere varılmıştır. Girişim mekanizmalarını açıklamak için piroliz ve atomlaştırma sıcaklıkları ve bu sıcaklıklara erişim hızlarına bağlı olarak bağıl absorbans değerleri, zemin absorpsiyon (BG) ve atomik absorpsiyon (AA) sinyalleri irdelenmiştir. Bu çeşit deneylere ek olarak yoğun fazda oluşan esas bileşikler X-Ray Difraksiyonu ile saptanmıştır. Girişim etkileri sadece tayin duyarlılığının arttırılması yönünde analitik amaçlı değil, aynı zamanda kurutma, piroliz ve atomlaştırma basamaklarında oluşan moleküler bileşiklerin saptanması ve girişim mekanizmalarının aydınlatılması hedefine yönelik olarak incelenmiştir. Yapılan deneyler sonucu kuruma ve piroliz basamakları sırasında anyon ve katyonların ortamdaki miktarlarına, birbirlerine oranlarına ve fırın sıcaklığına bağlı olarak çeşitli bileşikler oluştuğu ve oluşan bileşiklerin bazılarının girişimleri arttırdığı bazılarınınsa tayin elementi üzerinde koruyucu etki yaptığı gözlenmiştir. Bastırıcı etkide, piroliz veya atomlaşma basamağının başlangıcında parçalanmadan fırından uzaklaşan matriks bileşenlerinin (sülfür, klorür vb.) tayin elementi ile yoğun fazda uçucu bir bileşik oluşturması, atomlaşma basamağında tayin elementinin hızlı bir şekilde oluşan matriks gazları ile birlikte sürüklenmesi (expulsion), tayin elementinin matriks mücrokristalleri içinde hapsolması (occlusion) ve bunların piroliz ya da atomlaşma basamağında parçalanmadan tayin elementini fırın dışına taşımaları, atomlaşma basamağında klorür, sülfür, oksijen gibi matriks parçalanma ürünleri ve tayin elementi arasında gaz fazı reaksiyonu, tayin elementi ile kısmen atomlaşmış matriks bileşenleri arasında yoğun fazda termal olarak kararlı bir bileşik oluşumu gibi mekanizmalar söz konusudur. Buna karşılık tayin elementinin matriks tarafından fiziksel olarak korunması, matriks bileşenlerinin atomlaşma öncesi ortamdan uzaklaşmasına ve tayin elementinin daha etkin bir şekilde atomlaşarak hassasiyetinin artmasına neden olmuştur. Although there are more than one anion and cation in real sample, the interference effects of only a single salt on the atomization of an element have been generally investigated in literature. Therefore it would be more realistic to investigate the interference of a mixture of more than one anion and cation. The thermal behaviour of any chosen salt in the tube and its gas and condensed phase interactions with analyte were discussed using different experimental techniques and approaches. For a given interferent and an analyte, the interference mechanisms obtained from different approaches are mostly in agreement. The mechanisms proposed can be summarized as (i) formation of a volatile compound of the analyte with sulphide, chloride etc. in the condensed phase removal from the furnace either during the pryrolysis or at beginning of atomization step without decomposing, (ii) expulsion of analyte together with rapidly expanding matrix in atomization step; (iii) occlusion of analyte in the matrix microcrystals and then being carried directly out of absorption volume without being atomized; (iv) gas phase reaction between the analyte atoms and matrix decomposition products e.g. chlorine, sulphur, oxygen etc. in the atomization step and (v) formation of a thermally stable compound in the condensed phase from the analyte and matrix constituents, causing low atomization. The occurence of those mechanisms depends on experimental conditions and the kind analyte and matrix. Some critical factors for a gas phase reaction are temperature of the gas-phase, the concentration of species which combine with analyte, having the same appearance time of metal and matrix atoms and the stability of any possible metal compound which may be formed in the gas-phase. Since thermal equilibrium is established at STPF (Standart Temperature Platform Furnace) conditions, the gas-phase reaction interference is substantially reduced, but it can not be completely eliminated. The kind of compound formed in the condensed phase is related to many critera, too. One of them is the physical and chemical properties of compound e.g. solubilities of possible compounds between analyte element and matrix components determine the kind of metal compound precipitated during drying; the other criterion is the thermal XI behavior and stability of the compound e.g. any hydrated compound may be converted to a less volatile oxide upon thermal hydrolysis at elevated temperatures. In other words, the composition of the compound formed after drying step may change at elevated temperatures of pyrolysis step. Expulsion mechanism depends on the thermal expansion degree of the matrix, heating rate and closeness of appearance time of matrix constituents and analyte atom in the gase-phase. Occlusion mechanism depends on the thermal stability and size of matrix particles in which analyte is occluded. The main objective of all these kind of studies is to understand the origin of interferences in the analysis of an element by AAS. For this purpose almost in all the interference studies a single kind of salt of chloride, sulphate etc. has been chosen as the interferent and its interference mechanism was investigated. Actually in real samples, there are always many cations and anions. In this case, the situation is much more complex. During drying and pyrolysis, various compounds are formed depending on their stability, abundance and ratio of cations and anions and the furnace temperature. In this case, the analyte vaporizes from a complex condensed phase to again a complex gas-phase. In this study, interference effects of sodium chloride, magnesium chloride, sodium sulphate, magnesium sulphate, aluminium chloride and aluminium sulphate and their different combinations have been studied. In order to explain the interferences mechanisms, pyrolysis curves, background (BG) and atomic absorption (AA) signals for the interferent(s) were recorded. In addition to these kind of experiments, main compounds formed in the condensed phase were identified by X-Ray Diffraction. A Perkin- Elmer Zeeman Z/3030 atomic absorption spectrophotometer equipped with a HGA-600 graphite furnace, AS-60 autosampler and PR100 printer was used throughout this work. A manganese hollow cathode lamp was used as the spectral light source. The wavelength was set to 403.1 ran and slit width to 0.2nm. Pyrolytic graphite coated tubes with pyrolytic graphite platforms were used for the most of the experiments. For some of the experiments, special dual cavity platforms which have two cavities instead of one were used. Dual cavity platforms were inserted into pyrolytic graphite coated tubes having a slot instead of a hole for the convenient pipetting. Nitrogen was used as the purge gas. The gas flow was interrupted during atomization. Signal evaluation was performed by means of integrated absorbance values (peak area). Test solutions were prepared daily by necessary dilution of stock solutions using freshly distilled water. XRD experiments were performed by using SINTAG XDS 2000 model X-Ray diffractometer with a Cu a radiation source. The graphite furnace temperature program used for the experiments is given in the table below. Xll All data points given in this study are averages of several repetitions carried out on different days using different tubes and platforms. Table 1. Temperature program used for the determination of manganese Step Furnace Time/s Internal gas flow rate/(ml min"1) Read temperature/°C Ramp Hold - - - - 10 15 300 10 20 300 0 5 0 * 1 10 300 1 10 300 In the presence of sodium chloride, manganese never reaches its matrix free value. Sodium chloride has a depressive effect at all pyrolysis temperatures. The interferences can be attributed to (i) the gas-phase reaction and (ii) expulsion of analyte with matrix constituents in the atomization step. These two interferences appear only in the atomization stage and they are independent of the pyrolysis temperature. In the presence of magnesium chloride two opposing effects occur depending on the pyrolysis temperatures, (i) If low pyrolysis temperatures are applied, expulsion and gas-phase reaction in the atomization step are very likely, (ii) At elevated temperatures, magnesium chloride is hydrolyzed and is converted to magnesium oxide which does not cause expulsion or gas-phase reaction in the atomization step. Although some loss of manganese occurs owing to the volatilization of manganese chloride formed in the pyrolysis step, obviously, the magnesium oxide partially protects the analyte. However, it cannot be considered as a modifier because it is a source of interference. The effect of sodium sulphate on manganese can be explained by different mechanisms. From the results of the experiments (i) one of the possible mechanism may be the expulsion of analyte out of the furnace together with rapidly expanding matrix gases in the atomization step, (ii) Another possible process which can affect the sensitivity in a similar way, is a gas-phase reaction in the atomization step between manganese atoms and decomposition products of sodium sulphate. As long as the matrix does not vaporize and is not converted to another chemical form at elevated temperatures, the influences of expulsion and gas phase reaction interferences do not change with the pyrolysis temperature. Therefore, temperature independent sensitivity loss for manganese in sodium sulphate should be originated from expulsion or gas-phase reaction interferences or both of them. In the presence of magnesium sulphate, the atomization of manganese is considerably delayed. Magnesium sulphate is converted to magnesium oxide with the generation of In the presence of magnesium sulphate, the atomization of manganese is considerably delayed. Magnesium sulphate is converted to magnesium oxide with the generation of SO3. Magnesium oxide or magnesium sulphate should effectively imbed the analyte delaying its vaporization. At low pyrolysis temperatures, magnesium sulphate is not completely converted to magnesium oxide. Therefore it is decomposed in the atomization step causing a negligible gas-phase reaction between analyte and decomposition products or expulsion of analyte out of furnace. The depressive effect of aluminium chloride on manganese decreases at elevated pyrolysis temperatures. Aluminium chloride may be converted to a form which does not cause any interference with increasing temperature. In this case, it can be proposed that at elevated temperatures aluminium chloride is converted to aluminium oxide which does not cause expulsion or gas phase reaction in the atomization step and delays the atomization of manganese by having a protecting effect for it. Aluminium sulphate has a deppressive effect on manganese at low pyrolysis temperatures. This effect completely disappears when pyrolysis temperature is above 1200°C. Aluminium sulphate is converted to its oxide as aluminium chloride. Since aluminium sulphate is not completely converted to its oxide, at low pyrolysis temperatures, it directly decomposes in the atomization step. Consequently, a gas- phase reaction is possible between manganese and decomposition products of the matrix as well as expulsion of manganese out of the furnace with the decomposition products. If sodium, magnesium and chloride ions are present simultaneously interferences are reduced at elevated pyrolysis temperatures. Obviously, the magnesium oxide generated by the hydrolysis of magnesium chloride acts as a modifier and partially protects the analyte, but the losses can not be completely eliminated because of the interference due to sodium chloride and the volatilization of manganese chloride. In the presence of sodium, magnesium, sulphate and chloride ions, although simple combination of major cations and anions, i.e. magnesium chloride, sodium sulphate, magnesium sulphate and sodium chloride (all or at least some of them depending on their comparative solubilities) are expected to be the most likely matrix compounds in the condensed phase during drying and pyrolysis steps, some other compounds including spinels, carbides, nitrides etc. containing Mn, Mg, Na, S, O, CI, C may form which is verified from XRD analysis of the sample platform after thermally treating at 300°C and 900°C. In the presence of these ions, the appearance time of manganese is much higher than those in the presence of only sodium chloride, sodium sulphate or magnesium chloride but almost coincides with that in the presence of magnesium sulphate for all pyrolysis temperatures which are the most likely compounds expected in the condensed phase. It can be concluded that magnesium sulphate or its decomposition products of magnesium oxide should be mainly responsible from the delay of manganese signals. Due to the delay of manganese atomization, the combination reaction in the gas phase between analyte atoms and chlorine should be less likely. Moreover, expulsion interference by other compounds is made less probable by MgO. XIV Obviously, magnesium sulphate prevents any interference due to sodium chloride and other matrix concomitants at the concentration range of ion mixture studied. In the presence of aluminium, magnesium, chloride and sulphate ions, the manganese signals are similar to that in the presence of magnesium sulphate. In this case, the other possible compound should be aluminium chloride in the presence of these ions. In addition, different combination of these ions may form depending on the pyrolysis temperature. The reason of the depressive effect at low pyrolysis temperatures may be the expulsion effect caused by hydrated compounds which decompose rapidly in the atomization step. At elevated temperatures, since the interfering species ( such as CI, S) are removed by thermal hydrolysis or decomposing, the interferences are lessened or completely disappear. DoktoraPh.D