JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY AUGUST 199 1 VOL. 6 375 Electrothermal Atomic Absorption Spectrometry of Inorganic and Organic Arsenic Species Using Conventional and Fast Furnace Programmes Erik H. Larsen National Food Agency of Denmark MQrkh#j 5ygade 19 DK-2860 Soborg Denmark Analytical sensitivities for arsenate monomethylarsonate dimethylarsinate arsenobetaine arsenocholine and the tetramethylarsonium ion are determined by electrothermal atomic absorption spectrometry. Conventional and fast furnace programmes with and without a palladium-magnesium nitrate chemical modifier are studied. Special attention is paid to pre-atomization losses of the arsenic species tested. The conventional programme gives equal characteristic masses of about 16 pg of arsenic for all the species tested.The characteristic mass is defined as that mass of analyte which produces an integrated absorbance signal whose net area is equal to 0.0044 A s. The fast programme with the chemical modifier leads to slightly poorer sensitivities compared with the conventional programme. When using the fast programme without chemical modification substantial pre- atomization losses of the quaternary arsonium compounds in particular are seen at a pre-treatment temperature of only 200 "C. Keywords Electrothermal atomic absorption spectrometry; inorganic and organic arsenic species; fast furnace programme; pre-a tomiza tion loss; chemical modification Arsenic species present in biological material have been separated by high-performance liquid chromatography (HPLC) and detected on-line by inductively coupled plasma optical emission spectrometry (ICP-OES)' and by inductively coupled plasma mass spectrometry (ICP-MS).* Continuous hydride generation atomic absorption spectro- metry (AAS) has also been used as an on-line method of detection,' but only arsenic species that form volatile hydrides can be measured in this manner.A new hyphe- nated on-line HPLC-atomic absorption spectrometric method4 overcame this problem by pyrolysis of the sepa- rated quaternary arsonium compounds followed by gas- phase thermochemical hydride generation. As previously described,s off-line detection of arsenic species after separation by HPLC may be performed by electrothermal AAS (ETAAS). Although detection of the separated arsenic species using on-line techniques is ideal ETAAS is still advantageous because of the sensitivity and ease of operation.Furthermore the instrumentation is available in many laboratories. The fairly long time required for the analysis of a series of collected fractions may be inconvenient for practical analytical work. There- fore fast graphite furnace programmes6 appear attractive in order to shorten the time needed for analysis. However quantitative detection of arsenic by ETAAS is prone to errors owing to pre-atomization losses.' In order to produce precise and accurate analytical data arsenic measurements should be performed without pre-atomization losses. It is therefore of interest to study the conditions under which ETAAS gives equal and optimum sensitivities for all of the arsenic species in question.The purpose of this paper is to present the analytical sensitivities obtained by ETAAS for aqueous standard solutions of arsenate (AsV ) monomethylarsonate (MMA) dimethylarsinate (DMA) arsenobetaine (AsB) arsenocho- line (AsC) and the tetramethylarsonium (TMAs) ion using conventional and fast furnace methods. Experimental Chemicals Standard solutions each containing 1000 pg g-l of hydro- gen arsenate disodium salt MMA disodium salt DMA sodium salt AsB or AsC bromide in water were provided by the Commission of the European Communities Com- munity Bureau of Reference Brussels (Belgium). Tetra- methylarsonium iodide was donated by Dr. J.-s. Blais (Macdonald College Quebec Canada). A standard solution of this compound was prepared by dissolution in water and the concentration was determined accurately by ICP-OES and ICP-MS.Working solutions containing 50 ng ml-l of arsenic were prepared from each of the six standard solutions. A chemical modifier solution was prepared by mixing equal volumes of a 3000 pg ml-1 solution of palladium and a 2000 pg ml-l solution of magnesium nitrate in water. Palladium powder (22 mesh 99.998% purity) was purchased from Alfa Products (Karlsruhe Germany). Palladium metal (300 mg) was mixed with 4.5 ml of 65Oh nitric acid and left overnight followed by sonication and step-wise addition of 1-2 ml of 37% hydrochloric acid until dissolution was complete and finally made up to 100 ml with water. Instrumentation and Methods A Perkin-Elmer 3030 Zeeman AAS instrument with an AS- 60 autosampler and HGA-600 graphite furnace was used for recording the absorbance signals.Pyrolytic graphite coated graphite tubes and platforms in tubes were used throughout. Argon (300 ml min-l) was used as the purge gas except during atomization (gas stop). Arsenic absorbance was monitored at the 193.7 nm line (bandpass 0.7 nm) with an arsenic electrodeless discharge lamp source (8.5 W). The four graphite furnace programmes tested and other instrumental variables are given in Table 1. Method 1 is in accordance with the stabilized temperature platform fur- nace (STPF) concept,* while methods 2 3 and 4 are so- called fast programmes6 with 27-32 s furnace times. Methods 1 and 2 include the palladium-magnesium nitrate chemical modifier while methods 3 and 4 do not.Results and Discussion Optimization of Fast Furnace Programmes In the fast graphite furnace programme described by Slavin ef aL6 a drying stage at 700 "C for 1 s was followed by a pre- treatment step at 400 "C. They commented that the dry temperature might be too high for specific applications. In the present study a maximum drying temperature of 300 "C was used and also a temperature ramping from ambient temperature to 300 "C for 5 s was necessary in order to prevent sputtering of the analyte solution. During the376 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY AUGUST 1991 VOL. 6 Table 1 Graphite furnace settings for methods 1-4 Method Parameter Dry1 Dry2 1 Temperature/"C 100 130 Ramp/s 5 10 Hold/s 20 50 2 Temperature/"C 300 400 Ramp/s 5 1 Hold/s 3 10 3 Temperature/"C 300 400 Ramp/s 5 1 Holds 3 10 4 Temperature/"C 200 200 Ramp/s 5 1 Holds 3 15 Pre- treatment 1100 20 30 20 1 4 20 1 4 20 1 4 Chemical Sample volume/ Atomization Clean modifier/pl Pl 2200 2650 10 20 0 1 4 3 0 3 0 3 0 3 2300 - 10 10 10 2300 - - 10 2300 - - I c E 2 6 1 A' /- \ E \ 'n 1 1 1 1 2 3 4 Method 14 Fig.1 Characteristic mass in pg of As per 0.0044 A s for six arsenic compounds determined by using four different graphite furnace methods. A AsB; B AsC C TMAs; D DMA; E AsV; and F MMA. See Table 1 for details. (Graphic presentation does not imply a functional relationship between the individual methods) development of the fast programmes a small mirror was positioned in the light path which allowed observation of whether sputtering of the analyte solution injected onto the platform occurred. Characteristic Mass for Methods 1-4 The results expressed as characteristic mass i.e.the mass of arsenic that gives an integrated absorbance reading of 0.0044 As are shown in Fig. 1. An increasing value indicates a poorer sensitivity. Method 1 gives characteristic mass values for all six species of between 15.9 and 16.8 pg of arsenic which is close to the value for arsenate reported by other worker^.^ The conventional STPF furnace programme which in- cludes the palladium-magnesium nitrate modifier is there- fore efficient in stabilizing both the inorganic and organic arsenic species tested. The reported characteristic mass values were calculated from measurements of approxi- mately 500 pg of arsenic except in method I where approximately 1000 pg were injected.On increasing the amount of arsenic in the range 500-2000 pg the character- istic mass increased slightly. This indicates that for all six compounds a completely linear relationship between absor- bance and amount of arsenic measured does not exist. Method 2 leads to almost identical characteristic mass values for AsV MMA and DMA compared with method 1 while values of 19 pg of arsenic per 0.0044 A s are obtained for AsB AsC and TMAs. In spite of the use of the chemical modifier small pre-atomization losses of these three com- pounds do occur. Results from methods 3 and 4 (no chemical modifier used) demonstrate significant and varying pre-atomization losses from one arsenic compound to another. Losses are smaller for method 4 than for method 3 owing to a lower temperature (200 "C) in the drying stages of method 4.Even then large losses of particularly the cationic arsenic species are observed. For the detection of arsenic species after separation by HPLC Brinckman et aLl0 used a fast graphite furnace programme with a 200 "C pre-treatment temperature but without chemical modification or a platform in the furnace. They found that the sensitivity (peak area) for DMA injected directly into the furnace was only about half of that for MMA and AsV. The results from the similar method (method 4 of the present study) show a much smaller loss of DMA compared with their findings. This may be attributed to the use of a platform in the furnace. Woolson ef al." also used a fast graphite furnace pro- gramme without chemical modification.After drying of the sample solution their method included a 1200 "C ashing step for 7 s. They found the same relative response (peak area) for AsV and MMA. However no absolute measure of sensitivity is given and the results indicate that losses occur when no chemical modifier is used in conjunction with the high pre-treatment temperature. Interferences The interferences caused by loss of arsenic during the heating cycle of the graphite furnace have already been discussed. When determining arsenic in fractions eluted from an HPLC column the possibility of spectral interfer- ence from the sample matrix and from the mobile phase of the chromatographic system used must also be considered. In measurements made using ETAAS non-specific absor- bance from matrix constituents is commonly corrected for by a continuum source (deuterium lamp) or by use of the Zeeman effect. In principle all co-injected elements or molecules that absorb light from the continuum source in a structured pattern within the spectral bandpass used can cause overcompensation.Phosphate has been reported as such an interferentI2 in the 0.7 nm spectral bandpass around the 193.7 nm arsenic line. The sample clean-up and the chromatography itself significantly reduces the com- plexity of the sample matrix and therefore also reduces the presence of interferents originating from the sample matrix. In ion-exchange HPLC of arsenic species phosphate buffers have been used as the mobile phase,13 and the possible interference from phosphate in ETAAS must be paidJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY AUGUST special attention.However the phosphate interference is eliminated when using Zeeman-effect background correc- tion. l4 Conclusions In the present study equal sensitivities are obtained for the inorganic and all organic arsenic species by using the conventional STPF furnace programme. Therefore calibra- tion for all species is possible by standards prepared from only one calibrant e.g. arsenate. The conventional pro- gramme however is relatively slow in operation compared with the fast programmes which are about 2 min shorter in duration. The fast programme that includes the chemical modifier makes calibration of all species possible using two arsenic species as calibrants i.e. one for the quaternary arsonium compounds and one for the other species.The use of fast furnace programmes without chemical modification cannot be recommended because of substan- tial losses particularly of quaternary arsonium compounds. References 1 Morita M. and Shibata Y. Anal. Sci. 1987 3 575. 2 Shibata Y. and Morita M. Anal. Sci. 1989 5 107. 3 4 5 6 7 8 9 10 11 12 13 14 1991 VOL. 6 377 Haswell S. J. O’Neill P. and Bancroft K. C. C. Talanta 1985 32 69. Blais J.-S. Momplaisir G.-M. Marshall W. D. Anal. Chem. 1990,62 1161. Brinckman F. E. Blair W. R. Jewett K. L. and Iverson W. P. J. 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