Rh as Permanent Chemical Modifier in Simultaneous Atomic Absorption Spectrometry : Interference Studies on As , Cd , Pb and Se Determination

Um estudo sistemático para verificar as interferências provocadas por Na, K, Ca, Mg, Cl, NO 3 , SO 4 2e PO 4 3nos processos de atomização simultânea de As, Cd, Pb e Se foi realizado utilizando um espectrômetro de absorção atômica com atomização eletrotérmica. A mistura de 250 μg W + 250 μg Rh foi termicamente depositada sobre a plataforma do tubo de grafite e utilizada como modificador químico permanente. O ajuste compromissado dos parâmetros do programa de aquecimento (Tp = 650 C e Ta = 2200 C) diminuiu a eficiência de decomposição e volatilização térmica de alguns interferentes durante a etapa de pirólise, tornando a atomização simultânea de As, Cd, Pb e Se mais suscetível à interferências. Os ânions Cl e SO 4 2causaram as interferências mais severas, enquanto NO 3 somente afetou a atomização dos elementos estudados quando presente em elevadas quantidades (> 27 μg). O ânion PO 4 3provocou uma forte interferência negativa sobre o Se. As interferências causadas pelos cátions Na, K, Mg, Ca afetaram principalmente o Cd, sendo que os cloretos dessas espécies provocaram interferências mais intensas do que os nitratos.


Introduction
Among the instrumental techniques available for trace and ultra-trace element determinations, atomic absorption spectrometry (AAS) occupies an outstanding position due to its high specificity, selectivity, and sensitivity, low spectral interference, and ease of operation.Despite of these attributes, its conventional mono-element operation mode restrains the analytical frequency and can be considered as the main drawback of this technique. 1This shortcoming is aggravated for electrothermal atomic absorption spectrometry (ETAAS) due to the long extent of the heating program atomizer, typically 1 to 3 min.
Multi-element atomic absorption researches have aroused interest since the first AAS stage in order to conceive a spectrometer able to determine several elements simultaneously or quasi simultaneously.3][4] Commercially introduced during the last decade, SIMAAS adjoined the multi-element capability for atomic absorption spectrometry, reducing time and costs associated with the analysis.Indeed, expressive advantages are obtained even when the spectrometer is operated in 2-element simultaneous mode: the sample and high purity reagent requirements, and the residue generation are almost reduced in 50 %, while the analytical frequency and the analytical results obtained with the same graphite tube are almost duplicated.
At the present, the most updated commercial instrumentation is a line-source simultaneous spectrometer, 1 equipped with transversely heated graphite atomizer with integrated platform, Zeeman-effect background corrector, and solid-state detector, making possible the operation under STPF conditions. 5This instrument allows simultaneous determinations up to six elements. 1IMAAS has been used for various multi-element determinations with acceptable performance  (Table 1), keeping the main features of ETAAS: high sensitivity, low detection limits, reduced sample requirements, and the possibility to carry out in situ sample decomposition inside the graphite furnace. Althugh the increase number of multi-element procedures, reports dealing with interference studies for SIMAAS are scare so far.12 The mixture palladium and magnesium nitrate has been widely used for multi-element determinations by SIMAAS.6,[8][9][10][11]14,15,20,23,25 It is claimed as universal chemical modifier due to the thermal stability improvement for 21 elements.27 Although this mixture seems to be the most suitable choice, other alternatives as rhodium-based chemical modifiers can also be explored for multi-element determinations.
The rhodium-based chemical modifiers have been successfully adopted for mono-element determinations by ETAAS.9][30][31] The results indicated its effectiveness for As, Cd, Cu, Pb and Se, and its performance is equal or superior than that verified for the universal chemical modifier.
The use of permanent chemical modifiers allows increase the graphite tube lifetime, eliminate volatile impurities during the thermal coating process, decrease the detection limits, reduce the total heating cycle time, and minimize the high purity chemical consumption. 28onsidering these favorable characteristics, the aim of this work is to investigate the performance of the permanent chemical modifier 250 µg W + 250 µg Rh to minimize interferences of Na + , K + , Ca 2+ , Mg 2+ , NO 3 -, Cl -, SO 4 2-and PO 4  3 on the simultaneous atomization of As, Cd, Pb and Se.Additionally, determinations of As, Cd, Pb and Se in non-potable water using this chemical modifier are proposed.

Apparatus
Measurements were carried out by using a SIMAA-6000 electrothermal atomic absorption spectrometer with a longitudinal Zeeman-effect background correction system, Echelle optical arrangement, solid state detector and standard THGA tube with pyrolytic coated integrated platform (Perkin-Elmer, Norwalk, CT, USA).
Argon 99.999% v/v (Air Liquide Brasil S/A, São Paulo, SP, Brazil) was used as protective and purge gas.
Arsenic, Cd, Pb and Se were simultaneously determined in non-potable water samples with high saline content from Quality Technologies Pty Ltd (Mount Isa, Australia) using 250 µg W + 250 µg Rh as permanent chemical modifier.The samples were diluted 1:10 or 1:20 with 0.1% v/v HNO 3 for the multi-element determinations.Addition and recovery tests were performed to verify the reliability of the obtained results for As, Cd, Pb and Se determination.

Procedure
All glassware, polypropylene flasks and bottles (Nalge Company, Rochester, USA) were cleaned with detergent solution, soaking in 10% v/v HNO 3 for 24 h, rinsed with Milli-Q water and stored into a closed polypropylene container.
Solution manipulations were conducted in a class-100 laminar flow bench (Veco, Campinas, Brazil) to avoid airborne contamination.Reference solutions containing 2.5 µg L -1 of Cd 2+ + 50 µg L -1 of As 3+ , Pb 2+ and Se 4+ in 0.1% v/v HNO 3 , with and without the concomitant species, were used for the heating program optimization and for the interference studies.These solutions were prepared directly in the autosampler cups by dilution (1:1) of the stock reference solution, containing 5.0 µg L -1 of Cd 2+ + 100 µg L -1 of As 3+ , Pb 2+ and Se 4+ in 0.1% v/v HNO 3 , with blank or interfering solutions.
The permanent chemical modifier 250 µg W + 250 µg Rh was obtained according to the thermal coating process described in the literature. 28The heating program was optimized with pyrolysis and atomization temperature curves in absence and presence of the mixture 250 µg W + 250 µg Rh as chemical modifier.The instrumental adjustments and the heating program for the graphite tube are showed in Table 2.
The influence of increasing amounts of Na + , K + , Mg 2+ , Ca 2+ , Cl -, NO 3 -, SO 4 2-and PO 4 3-in the analytes' absorbance signals was verified.The concomitant amounts (cations and anions) added to the analytical reference solutions were based on cation concentration, ranging from 50 to 5000 mg L -1 .The interference studies of cations were executed using NO 3 -and Cl -salts, while for anions were used only Na + salts.
Experimental measurements were made with at least three replicates and based on integrated absorbance throughout this work.

Results and Discussion
When ETAAS is used for mono-element determinations, all experimental and instrumental parameters are optimized for only one analyte.Consequently, the best optimized pyrolysis and atomization temperatures are used in the heating program, minimizing condensed-and gas-phase interference. 3On the other hand, the adoption of compromised conditions for multi-element determinations by SIMAAS are mandatory.The heating program temperatures and chemical modifier must be carefully selected to achieve the best atomization efficiency for all the analytes.In general, the most volatile analyte determines the pyrolysis temperature while the least volatile analyte determines the atomization temperature.Despite of these adverse temperatures, the use of a transversely heated atomizer with integrated platform offers optimum conditions for both volatile and non-volatile elements. 3Add to that, STPF conditions adoption and the use of an appropriate chemical modifier have guaranteed similar performance of SIMAAS in comparison to monoelement ETAAS.This fact reinforces that the success of any atomic absorption spectrometer for simultaneous determination depends on using STPF conditions.

Simultaneous heating program optimization
The characteristic masses (m o ) and the best pyrolysis and atomization temperatures for As, Cd, Pb and Se obtained by SIMAAS in absence and presence of the 250 µg W + 250 µg Rh are presented in Table 3 and compared with mono-element ETAAS data indicated in the literature. 29,30It can be observed an agreement between the results obtained by SIMAAS with those obtained by mono-element ETAAS (Table 3).The higher pyrolysis temperatures achieved for Cd, Pb and Se using SIMAAS were probably due to the higher Rh mass deposited on the graphite tube surface (250 µg).However, this data are in discordance with previous work, which indicates that masses higher than 200 µg are not recommended due to the sensitivity decrease (about 15% for each analyte). 28he higher characteristic masses for Cd, Pb and Se in comparison with mono-element ETAAS (Table 3), can be related to the compromised conditions adopted for the simultaneous determination.The necessity to consider compromised adjustments imposes 650 o C as pyrolysis temperature, due to the low thermal stability achieved for Cd in presence of W+Rh.
The atomization temperatures for all analytes were also increased in presence of the W+Rh permanent modifier (Table 3), for Cd (1400 o C) and Pb (1600 o C) were lower than those verified for As (2200 o C) and Se (2000 o C).The atomization temperature selection for the simultaneous heating program (2200 o C) was based on the As thermal behavior.This is also the maximum temperature recommended for the heating program in order to avoid loss of deposited Rh by vaporization. 28The adoption of higher temperatures for the atomization and/or cleaning steps impairs the W+Rh coating lifetime.

Anion interference
The effect of Cl -, NO 3 -, SO 4 2-and PO 4 3-on the simultaneous atomization of As, Cd, Pb and Se are showed in Figure 1.The results were expressed as absorbance ratios calculated between absorbance signals with and without the anions.
The interference effect of Cl -in ETAAS has been largely   investigated. 3The use of chemical modifier is important to avoid loss of volatile chlorides during pyrolysis step.In this work, the pronounced effect observed from 0.8 µg Cl - on Cd absorbance signal can be attributed to the volatilization of cadmium chloride during the pyrolysis step at 650 o C (Figure 1a).Nevertheless, Cd loss was not observed when lower pyrolysis temperatures (< 500 o C) were used.Probably, the formation and volatilization of CdCl 2 occurred before the interaction between Cd and Rh on the graphite tube surface.It can be concluded that Cl -ions, even in low amounts, damaged the thermal stabilization obtained for Cd in presence of the W+Rh as permanent chemical modifier.Similar behavior was observed for low masses of NO 3 -(1.3mg) and SO 42-(1.0mg), which also reduced the Cd absorbance signal (Figures 1b-1c).The increase of Cl -, NO 3 -, SO 4 2-masses did not cause any expressive change in the Cd absorbance ratios.On the contrary, the initial decrease of the Cd ratio was not observed in presence of PO 43-(Figure 1d), probably due to the increase in the thermal stabilization of this element.The synergistic effect between W+Rh and PO 4 3-was efficient to stabilize Cd at 650 o C. Cadmium and Pb form very stable oxyphosphorous compounds 3 and, for this reason, NH 4 H 2 PO 4 is usually used as chemical modifier for these elements.
The effect of SO 4 2-on As and Se was more pronounced than on Pb atomization (Figure 1c).For the latter, the effect of Na 2 SO 4 is aggravated in strong acid solutions. 324][35] Arsenic exhibited strong signal suppression in presence of the SO 42-(Figure 1c).This effects can be attributed to the interactions between As and Na 2 SO 4 and the resulting decomposition products, particularly sulfide species. 36elenium suffered more strong interference in presence of PO 4  3-(Figure 1d).Additionally, As was the most affected analyte by NO 3 -(Figure 1b).It can be supposed that the low pyrolysis temperature adopted for the simultaneous heating program (650 o C), impaired the adequate formation of the atomic precursors for As.

Cation interference
The performance of 250 µg W + 250 µg Rh to overcome the interference caused by Na + , K + , Mg 2+ and Ca 2+ on the simultaneous atomization of As, Cd, Pb and Se atomization are showed in Table 4.The results were expressed as absorbance ratios calculated between the absorbance signal with and without the cations.
In general, chloride salts caused more pronounced interference than nitrate salts.Cadmium was the most affected analyte, mainly in presence of alkaline ions (Table 4).The interference for all analytes was negligible only in

Table 3 .a
Characteristic masses (m 0 ), pyrolysis (T p ) and atomization (T a ) temperatures for 25 pg Cd + 500 pg As, Pb and Se in 0.1% v/v HNO 3 in absence and presence of 250 µg W + 250 µg Rh as perma-Results for the multi-element determination of As, Cd, Pb and Se by SIMAAS; b Results for the mono-element detemination As, Cd, Pb and Se(29,30).

Table 1 .
Multi-element analytical procedures developed by SIMAAS

Table 2 .
Spectrometer settings and heating program parameters for the multi-element detection of As, Cd, Pb and Se a Optimized condition for As, Cd, Pb and Se in presence of 250 µg W + 250 µg Rh as permanent chemical modifier; Total program time: 69s; Injection temperature: 20°C.

Table 4 .
Absorbance ratios for 25 pg Cd + 500 pg As, Pb and Se in presence of Na + , K + , Mg 2+ , Ca 2+ as chloride and nitrate salts

Table 5 .
Analysis and recovery test results for the simultaneous determination of As, Cd, Pb and Se in non-potable water samples