GFAAS ile bizmut tayininde girişim mekanizmalarının incelenmesi

Investigation of interference mechanisms on the determination of bismuth by GFAAS

Özet

Grafit fırınlı atomik absorpsiyon spektrometresinde (GFAAS) tayin elementinin ortamda bulunan matriks bileşenleri ile gaz ve/veya yoğun fazda etkileşmesi sonucu ortaya çıkan çeşitli girişim mekanizmalarının aydınlatılması genellikle çok zor ve yoruma açıktır. Bunun en önemli nedeni farklı girişimlerin genellikle benzer etkilere neden olması ve bunların birbirlerinden ayrılamamasıdır. Özellikle aynı anda birden fazla mekanizmanın oluşumu bu durumu daha da karmaşık hale getirmektedir. Bu mekanizmalar matriks ve tayin maddesinin cinsine, konsantrasyonuna ve grafit fırın programına bağlıdır. Bu çalışmada kullanılan ve özel olarak imal ettirilen çift oyuklu platformun iki ayrı oyuğuna tayin elementinin ve matriksin ayrı ayrı enjekte edilmesi sonucu bunların birbirleri ile yoğun fazda teması önlenmekte ve gözlenen girişimler sadece gaz fazı etkileşmeleri nedeniyle olmaktadır. Böylece çift oyuklu platform klasik tek oyuklu platformla teşhis edilemeyen gaz ve yoğun faz girişimlerinin birbirlerinden ayrılarak belirlenmesinde çok önemli rol oynamaktadır. Bu çalışmada bizmut elementinin GFAAS ile tayini sırasında nikel klorür (NiCl2) tuzunun girişim etkisi araştırılmıştır. Uygulanan piroliz sıcaklığına bağlı olarak analatın matriks ile uçucu bir bileşik oluşturması, matriks gazları ile birlikte sürüklenmesi, matriks parçalanma ürünleri ile gaz fazında reaksiyona girmesi gibi mekanizmaların varlığı ispatlanmıştır. Ayrıca çift oyuklu platformun kullanılmasıyla analat ile matriks parçalanma ürünleri arasında gaz/yoğun faz reaksiyonlarının varlığı gösterilmiştir. Bizmut ile ilgili olarak gözlenen girişim mekanizmaları Ni(NO3)2, Zr(NO3)4, Pd(NO3)2 ve kolloidal palladyum modifierleri kullanıldığında ortadan kalkmaktadır.

Abstract

In the presence of matrix, interferences occuring in graphite furnace atomic absorption spectrophotometer (GFAAS) causes errors. In graphite furnace, the interferences between analyte and matrix can be summarized as follows: (i) Formation of a volatile compound between analyte and matrix before the atomization step and its loss by vaporization in molecular form at the beginning of atomization step or during the pyrolysis step (ii) Expulsion of analyte together with rapidly expanding matrix gases in the atomization step (iii) Occlusion of analyte atoms in microcrystals of matrix in the condensed phase and their carrying out of the furnace without being atomized (iv) Gas phase reaction between analyte and matrix decomposition products in the atomization step (v) Formation of a thermally stable compound between analyte and matrix in the condensed phase The occurrence of those mechanisms depends on experimental conditions and the kind of analyte and matrix. Although the most of the interferences have been reduced using STPF (Stabilized Temperature Platform Furnace) conditions, appropriate modifier and advanced techniques, it can be completely removed. Therefore, elucidation of interference mechanisms as well as the use of optimum experimental conditions is equally important. In this study, the interference effects of nickel chloride( NiCl2) on the determination of bismuth by graphite furnace atomic absorption spectrometry (GFAAS) was investigated. For this purpose, by using a specially designed dual cavity platform (DCP), the condensed phase and gas phase interferences can be distinguished due to being allowed the analyte and matrix to be volatized from different cavities. Depending on applied pyrolysis temperature, the occurence of interference mechanisms such as the formation of volatile compound between analyte and matrix, expulsion with matrix decomposition products and gas phase reaction with matrix decomposition products were proved. In addition, the existence of gas/condensed phase reactions between analyte and matrix decomposition products was shown using o dual cavity platform. The effects of various experimental conditions such as pyrolysis temperature, pyrolysis time, atomization temperature, heating rate in the atomization step, matrix mass and atomization from wall or platform on sensitivity as well as atomization signals were investigated to explain the interference mechanisms. If the pyrolysis temperature was lower than 400 oC and analyte and interferent were mixed in the same cavity of the DCP, a 30 % temperature-independent depressive effect was observed. If analyte and interferent were mixed, in a temperature range between 400 oC and 600 oC, integrated absorbances increased about 20 % and then slightly reduced with temperature up to 1200 oC and finally suddenly drop to baseline at 1300 oC whereas integrated absorbances for matrix-free Bi begin to drop above 500 oC dramatically and continously, and become always lower than those in the presence of nickel chloride. If the analyte and interferent were separated, integrated absorbances for bismuth were reduced 10-15 % at 200 oC and remain almost the same up to 1000 oC and then drop to baseline at 1300 oC. Similar to the case for mixture of analyte and interferent, integrated absorbances were higher than those for matrix-free bismuth above 600 oC. The results show that the interference mechanism depends on the pyrolysis temperature. Different mechanisms become dominant under different experimental conditions. If the pyrolysis temperature is lower than 600 oC the interferences of NiCl2 on bismuth in the atomization step is due to gas-phase reaction. On the other hand, at temperatures higher than 600 oC, NiCl2.6H2O should be very effectively converted to a non-interfering form of (possibly NiO), which may be further converted to Ni upon reduction by carbon. In addition, at elevated temperatures, anhydrous NiCl2 should be removed from the furnace by volatilization. As a result, no reaction with Cl atoms in the gas-phase occurs in the atomization step as well as NiO or Ni does not cause any interference. When colloidal palladium were used as a modifier, the integrated absorbances of bismuth increased and its determination in the presence of nickel chloride was free from interference.It is found that colloidal palladium modified both the analyte and the matrices.