Grafit fırınlı atomik absorpsiyon spektrofotometresinde girişim mekanizmalarının incelenmesi

Abstract

Grafit fırınlı atomik absorpsiyon spekrometresiyle (GFAAS) matriks varlığında yapılan analizlerde meydana gelen çeşitli girişimler, sonuçların doğruluğunu etkilemektedir. Bu çalışmada çinko ve kobalt elementlerinin GFAAS 'de tayini ve çeşitli klorürlü tuzların etkisi araştırılmıştır. Oluşan girişim mekanizmalarını aydınlatmak için analiz elementi ve matriksin ayrı oyuklardan buharlaşmasına ve böylece yoğun faz ve gaz fazı girişimlerinin ayrılmasına olanak sağlayan özel dizayn edilmiş çift oyuklu platform kullanılmıştır. Çeşitli deneysel şartlarda elde edilen ön-atomlaşma grafiklerinin yanı sıra ön-atomlaşma basamağı esnasında elde edilen atomik (AA) ve zemin absorpsiyon (BG) sinyalleri de girişim mekanizmaları hakkında yararlı bilgiler vermiştir. Nikel klorür, kobalt klorür, kalsiyum klorür, magnezyum klorür gibi kristal suyu içeren matrikslerin girişim mekanizmaları, atomlaşma öncesi uygulanan sıcaklığa bağlıdır. Eğer ön-atomlaşma sıcaklığı matriksin termal hidrolizle oksidine dönüşmesine neden olacak kadar yüksek ise analiz elementi genellikle uçucu bileşikleri halinde hidroliz sonucu oluşan HCİ (g) ile fırın dışına sürüklenmektedir. Termal hidrolizin olmadığı düşük ön-atomlaşma sıcaklıklarında, atomlaşma basamağında gaz fazı reaksiyonu ve/veya analiz elementinin hızla parçalanan ve genleşen matriks bileşenleri ile fırın dışına sürüklenmesi gözlenen başlıca girişim mekanizmasıdır. Sodyum klorür gibi hidrolize uğramayan bir tuzun varlığında gaz fazı ve sürüklenme mekanizmalarının yanı sıra özellikle analiz elementinin uçucu klorürlerine dönüşerek atomlaşma öncesi sıcaklıkla orantılı olarak fırını terk ettiği gözlenmiştir. Ayrıca analiz elementinin, sodyum klorür mikrokristalieri içinde hapsolması ve birlikte ön-atomlaşma ve atomlaşma basamağında fırından uzaklaşması diğer bir mümkün mekanizma olarak ortaya çıkmaktadır.

Interferences mechanisms in graphite furnace atomic absorption spectrometry (GFAA.S) have been extensively investigated and the literature is full of reports on different theories. Since chloride is present as a major anion in many matrices as well as one of the most powerful interferents, it has been widely dealt in numerous papers. Interferences mechanisms of chlorides or other salts depend on the material of the atomizer, heating rate, pretreatment and atomization temperatures the chemical form and the amount of the interferent and analyte and their thermochemical properties. Therefore, it is not always a contradiction that some different authors may have reported different interference mechanisms for the same matrix and analyte. In order to explain the chloride matrix interferences, several mechanisms have been proposed: (i) loss of analyte as its volatile compound during the pretreatment step, (ii) occlusion of the analyte in the agglomerates or microcrystals of the matrix which are thrown from the furnace without decomposing during atomization or maybe pretreatment step, (iii) expulsion of the analyte from the furnace as a result of expulsion of gaseous products released by the decomposition of matrix during atomization, (iv) interferences caused by the formation of stable gaseous molecular chloride species during atomization. In this study, interferences of the sodium chloride, cobalt chloride, magnesium chloride, calcium chloride and nickel chloride matrices with the Vll determination of zinc and cobalt have been investigated. In order to explain the interferences mechanisms, preteratment curves obtained with a specially designed dual cavity platform which differentiates the gas-phase and condensed phase interferences, background (BG) and atomic absorption (AA) signals for the interferent, for large amount of analyte and for their mixed and separated solutions recorded during pretreatment step, time resolved absorbance (ABS.) signals taken during atomization step and influence of many other experimental parameters tried were all interpreted in accordance with thermochemical properties of the analyte and interferent. Zinc and cobalt were chosen as the analyte elements because they have different volatility as well as there exist few studies in the literature on the effect of the interferents on the both elements. In addition, much of those investigations were performed with old instruments where peak height was used together with slow response heating rates and therefore not reliable. A Perkin-Elmer Model Zeeman/3030 atomic absorption spectrometer equipped with an HGA-600 graphite furnace and PR-100 printer were used throughout this work. Test solutions were prepared daily by necessary dilution of stock solutions with freshly distilled- deionized water. Argon (99,99% pure, BOS) was used as the purge gas. The operating parameters of graphite furnace are given Table 1. All the data points given in this study are averages of several repetitions on different days using different tubes and platforms. Because of the modifications in the tube and platform design the pretreatment curves as well as the characteristic mass data obtained here cannot be compared with those of a conventional atomizer. With stabilized temperature platform furnace (STPF) concept, gas-phase reactions seems to be negligible or completely unlikely. In many cases, the interference mechanisms could not be separated clearly viii from each other. This is, most probably due to the fact that for any given condition, more than one interference occur at the same time which makes the situation more complex. Table 1. Time-temperature determination of zinc and cobalt. program for the Step Furnace Time(s) Internal Gas Temperature ( °C) Ramp(s) Hold(s) Flow (ml/min) 300 300 300 0a 0b 300 300 a,b Atomization conditions for zinc and cobalt, respectively. In the presence of sodium chloride, losses of zinc and cobalt can partly be attributed to the occlusion of the analytes in microcrystals of the interferent and some of these are thrown from the furnace without decomposing early in the atomization stop or during in the pretreatment step. In addition, expulsion of the both analytes together with the violently expanding matrix gases seems to be a likely interference mechanisms as well. When a hydrated chloride is used as the interferent, thermal hydrolysis play an important role in the interference mechanisms. For low pretreatment temperatures, hydrated chlorides are decomposed mainly in the atomization step and in this case, a gas-phase reaction between chlorine and analyte atoms and/or expulsion of the analyte together with rapidly expanding matrix gases become dominant. On the other hand, if the pretreatment temperature is high enough to cause thermal hydrolysis of the chloride salt, then ix HC1 (g) is formed and the matrix is converted to its oxide. The analyte might, in this case, be co- volatilized together with rapidly expanding HCl(g) during the pretreatment step. No considerable gas-phase interferences (including expulsion) has been found for the influence of cobalt chloride on zinc. The dominant interference mechanisms is that zinc chloride is formed in the condensed phase during pretreatment step and carried out of tube with hydrogen chloride gas generated in large amounts by thermal hydrolysis of cobalt chloride. When analyte and hydrated chloride salt were pipetted into the separate cavities of a dual cavity platform, the chloride of the analyte element may be formed during the pretreatment step upon a gas- phase/condensed phase reaction between HCl (g) generated in one cavity by hydrolysis of the salt and the analyte in the other cavity. In the presence of nickel chloride cobalt is lost by gas-phase reaction and/or expulsion mechanisms. The effect of nickel chloride on zinc is similar to that of cobalt chloride. At low pretreatment temperatures, thermal hydrolysis does not occur and in this case expulsion and/or gas-phase reaction should be the sources of interferences. On the other hand, at high pretreatment temperatures co-volatilization of zinc chloride together with HCl(g) is dominantly responsible for the losses of zinc. Condensed and gas-phase reactions again seem to be likely for separated solutions as well. The effect of magnesium chloride on cobalt is quite different from other salts. When low pretreatment temperatures are applied, expulsion and/or gas-phase reaction cause some losses whereas at high pretreatment temperatures magnesium chloride is converted to its oxide which protect the analyte from the attact of HCl(g) and delay the atomization acting as a modifier. Actually, it is known that magnesium nitrate, which is also converted to its oxide, is used as a modifier for cobalt as well as for many elements. However, magnesium chloride can not prevent the formation and loss of zinc chloride during high pretreatment temperatures. In the presence of calcium chloride, the losses of zinc at low pretreatment temperatures may be attributed to gas-phase reaction and/or expulsion mechanisms whereas formation of volatile zinc chloride and its vaporization is the main cause of temperature dependent signal depression at high pretreatment temperatures. In contrast to other salts, calcium chloride increases the integrated absorbance for cobalt. The low volatility matrix decreases the contact of the analyte with the platform and delay its atomization by acting as a modifier or a second platform. Since cobalt atomizes more efficiently at high temperatures, the delay of atomization results in greater absorbances. The other interference mechanisms, even if they occur, can not overcome the increasing effect. On the other hand, at high pretreatment temperatures the absorbances for cobalt decrease because of the vaporization of some cobalt chloride. It was shown that thermal hydrolysis of calcium chloride is not completed even at high pretreatment temperatures.