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ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
W. Frech and A. Cedergren, Anal. Chim. Acta, 82, 83 (1976). W. Frech and A. Cedergren, Anal. Chim. Acta, 82, 93 (1976). J. Aggett and A. J. Sprott, Anal. Chim. Acta, 72, 49 (1974). K. Ohta and M. Suzuki. Talanta, 23, 560 (1976). F. J. M. J. Maessen and F. D. Posma, Anal. Chem., 46, 1439 (1974). F. D. Posma, H. C. Smit, and A. F. Rwze, Anal. Chem., 47, 2087 (1975). C. HendrikxJongeriusand L. de Gabn, Anal. Chim. Acta, 87, 259 (1976). E. H. Piepmeier and L. de Galan, Specfrochim. Acta, Part B , 3 1 , 163 (1976). (14) "Handbook of Chemistry and physics", 55th ed.,R. C. Weast, Ed., Chemical Rubber Co., Cleveland, Ohio, 1975. (15) B. V. L'vov, "Atomic Absorption Speckochemical Analysis", Adam Hilger, London, 1970, p 288. (16) E. Schroll, Z . Anal. Chem., 198, 40 (1963). (6) (7) (8) (9) (10) (1 1) (12) (13)
(17) V. P. Garnys and J. P. Matousek, Clin. Chem. ( Winston-Salem, N . C . ) , 21., 891 .~119751. ~, (18) E. J. Czobik and J. P. Matousek, Talanta, in press. (19) W. Frech and A. Cedergren, Anal. Chim. Acta. 88, 57 (1977). I
~
RECEIVED for review July 13, 1977. Accepted September 26, 1977. This work was presented in part at the 5th International Conference on Atomic Spectroscopy, Melbourne, Victoria, Australia, August 25-29,1975, and at the 3rd FACSS meeting and the 6th International Conference on Atomic Spectroscopy, Philadelphia, Pa., November 15-19, 1976.
Influence of Some Matrix Elements on the Determination of Copper and Manganese by Furnace Atomic Absorption Spectrometry Johanna Smeyers-Verbeke, Yvette Michotte, and Desire L. Massart Vrije Universiteit Brussel, Farmaceutisch Instituut, Paardenstraat 67, B- 1640 Sint-Genesius Rode, Belgium
An investigation of the effect of Mg(NO&, MgS04, MgCI2, Ca(N03),, CaCI,, and NaCl on the pulse characterization times of Cu and Mn carbon furnace atomic absorption signals is 7peak,T,, performed. The pulse characterization times 7Tdelay, and 7 2 were determined from the oscilloscope traces obtained by means of a memory oscilloscope. The results of this study Indicate that the time characterlstlcs of the carbon furnace atomic absorption signals of Cu and Mn in the presence of the different salts are not comparable. Consequently, it is probable that different atomization mechanisms should be considered to explain the different interference phenomena studied here. I t seems unlikely that the existing models, describing atomization processes in nonflame atomizers, could contribute much to the elucidation of the interference problem.
In a previous paper dealing with matrix effects in the determination of Cu and Mn using carbon furnace atomic absorption spectrometry ( I ) , a preliminary study of the peak form of the recorded absorption signals was carried out. I n the first instance, a comparison was made between the extent of some of the interferences by application of peak integration and peak height measurements. It was concluded that for Cu as well as for Mn, measurement by integration yields the same results as peak height measurements for those interferences which cause a suppression of the absorption signal. Positive interferences were shown to persist but to be reduced when the integration method is used. This implies that the peak becomes smaller and consequently this allows one to assume that the atomization of the analyte takes less time. This should mean that the curve form of the recorded absorption signals has changed. These findings were supported by registration of the absorption signals by means of a memory oscilloscope. The study of the peak form of atomic absorption signals generated during heating of flameless atomizers is predominantly limited to the investigation of metal solutions in pure water ( 2 , 3 ) .Few applications are found in the studies concerning matrix effects in flameless devices. T o our knowledge, only Ebdon and co-workers ( 4 ) give some oscilloscope traces in their interference study concerning Mn, 0003-2700/78/0350-0010$01 .OO/O
atomized with a carbon filament atom reservoir. The present article describes more detailed investigations into the influence of some matrix elements on the curve form of Cu and Mn carbon furnace atomic absorption signals. Especially interferences resulting in an increase of the absorption signals were investigated.
EXPERIMENTAL Instrumentation. A double beam atomic absorption spectrometer, Perkin-Elmer Model 305, equipped with a deuterium background corrector and a graphite furnace Perkin-Elmer HGA 72 were used. The experimental conditions used are given in a previous paper (1). Registration of the absorption signals at the amplifier output was performed by means of a memory oscilloscope, Tektronix 7613. The time base of the oscilloscope was triggered at the beginning of the atomization cycle. In this way, the shift of absorption signals can be observed. From the oscilloscope traces, photographically recorded by means of a Polaroid camera, the pulse characterization times were determined. The integration unit used has been described in a previous paper ( 1 ) . Preparation of Solutions. Solutions of 50 pg/L Mn (as the chloride) containing, respectively, 500 mg/L Mg as Mg(NO&, MgS04,and MgC12;500 mg/L Ca as Ca(NO&; and solutions of 100 pg/L Cu (as the chloride) containing, respectively, 500 mg/L Mg as MgS04,500 mg/L Na as NaCl, and 500 mg/L Ca as CaC1, were prepared. Fifty-microliter samples were used for the investigations. RESULTS AND DISCUSSION In a simplified mathematical model for the transport of a sample through an analytical cell, L'vov ( 5 ) defines some characteristics of an analytical signal. The most important are: T~ t h e atomization time average time of residence of a n atom in t h e cell. I t T~ is defined as t h e time taken for t h e absorbance t o decrease from its maximum value, A,,, to a value e times smaller t h e length of time during which t h e signal is reT~ corded. On the basis of these parameters, equations are obtained for both methods of measuring absorption signals, the peak C 1977 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 50.
NO. 1. JANlJARY 1978
11
Table I. Pulse Characterization Times of the Mn Atomic Absorption Signal as a Function of the Destruction Temperature Destru ction tempera- A T , Tdelay, Tpeak, Ti, T2 > ture 1C ms ms ms ms 100 300 500 600 714 800 900 992 1102
2500 2300 2100 2000 1886 1800 1700 1608 1498
1150 950 730 595 490 410 330 270 240
2030 1820 1595 1490 1350 1290 1190 1150 1110
-____-
880 870 865 895 860 880 860 880 870
480 513
510
Oscilloscopic traces of (a) Mn (50 ppb) (br Mn (50 ppb) in the presence of MgSO, (500 ppm) Lertical scale: absorption. Horizontal scale. time (500 ms/scale unit) Figure 1.
515
method and the integration method. Sturgeon and co-workers investigated the practical usefulness of the two measurement methods for different elements atomized with the CRA 63 (2) and the HGA 74 (3). T o do this, apart from the characteristics specified by L'vov ( 5 ) ,the following characterization times are considered: Tdelay defined as t h e time elapsed from t h e s t a r t of t h e atomization cycle t o t h e first appearance of a signal 7peak defined as t h e time from t h e s t a r t of t h e atomization cycle to t h e peak maximum 71 t h e atomization time ( = T p e a k - 7delay) 72 t h e residence time, determined as given above. We used these different time characteristics to determine the influence of some matrix elements on the curve form of Cu and Mn atomic absorption signals. Characterization of the Mn Absorption Signal. The characterization times, Tdela,, Tpeak, T ~ and , T Z of the Mn absorption signal were determined a t constant drying and atomization temperatures (respectively 100 "C and 2600 "C) as a function of the charring temperature. This temperature was raised up to 1100 "C, the maximal destruction temperature applicable without any loss of the element. The results are given in Table I. A T represents the difference in temperature that is to be completed during the atomization cycle ( A T = 2600 "C - charring temperature). Increases in AT result in larger values of Tdelay and Tpeak whereas T~ and T 2 , within the experimental error, remain constant. This is caused of course by the fact that the larger the difference in temperature that is to be completed during the atomization, the longer it takes to reach the temperature a t which the atomization is possible. As a result Tdelay and ' consequently Tpeak increase. It can also be seen that the rate of increase of Td&y and peak becomes larger as AT increases, indicating that the rate of rise
of temperature of the atomization ic not constant. The shape of the absorption signal, however, i s not altered since 7 1 and T~ remain constant. Therefore once the temperature, at which the atomi7ation hecomes possible, is reached, the atomization proceeds with a constant rate for ever> measurement. This means that as a function of the charring temperature, oniy a shift of the absorption signals is observed. 'rime characteristics of the absorption signals of 26 and 50 ppb Mn atomized under identical conditions were compared. As was also shown by Sturgeon and co-workers for the atomization of Cu with the HGA 74 ( 3 ) ,identical pulse characterization times were obtained, indicating the independence of thp curve form of the amount of analyte vapoi ized. T h e same conclusions as mentioned ahove for Mn can be drawn for the Cu atomic absorption signal Effect of the Matrix on Pulse Characterization Times of Cu and Mn Atomic Absorption Signals. In a previous paper it was observed t h a t some oxy anion salts of Ca and Mg cause peak height enhancements of hoth the ahsorption signals of Cri and Mn: Ca(NO?)*, MniNOJ?, and R/[gS04 in the presence of Mn, MgS04 in the presence of Cu. One possible hypothesis was postulated a t that moment, h s e d on the atomization scheme proposed by Campbell and Ottoway (6) and by Fuller ( 7 ) . the interference results should be inter preted in terms of a faster reduction of the analy-tes, perhaps because of the presence of the interfering substance and therefore also a more rapid atomi7ation. If this hypothesis is correct, changes in the pulse characterization times of the Cu and Mn atomic absorption signal in the presence of these mixtures are to be observed. An example of the oscilloscopic tracls of Mn in the presence of MgS04 is given in Figure 1. 'The pulse characterization times of Cu and Mn and of both elements in the presence of the above inentioned matrices are given in Table T I . For the sake of completeness, pulse characteristics nf C'u and Mn in the prwenw of some matrices
Table 11. Matrix Effects on the Pulse Characterization Times of Cu and Mn Atomic Absorption Signal!; Influence on Peak Peak height area Tdelay, rpeak, 7 , : 2 Matrix (in %) (in 5%) ms ms ms mi; 50 ppb Mn
ppm Mg(MgS0,) PPm Mg(Mg(NO,)* + ppm Ca(Ca(NO,), + ppm Mg(MgC1,) 100 pph Cu + 500 ppm Mg(MgS0,) - 500 ppm Na(NaC1) + 500 imm Ca(CaC1,) 4
500 500 500 500
+ 25
+ 13
- 46
- 46
+ 26 + 24
+ 18 + 15
+ 47
+ 30
- 28 - 18
- 30
- 20
Temperature o f first atomization, - c:
240
1130
890
8CU
1200
610 350 270 800
1510 1160 1040 1700
900 81 0 770 900
690 830 660 800
1450 1400 1200 1670
400
1800
1400
1850
1200
400 400 400
1800 1800 3 800
1400 1400 1400
1400 I900 1900
1250 1200 1350
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
causing a suppression of the absorption signal are also given. By comparing the time characteristics of Mn in the presence of Ca(N0J2, Mg(NO&, and MgSO,, it becomes evident that no agreement between the four systems can be observed and that consequently a generalization of the effects is not possible. Indeed no systematic increase or decrease of Tdelay, T ~ or, 7 2 is noticed for the different systems investigated. Compared to the aqueous Mn solution, the presence of MgS0, causes a considerable increase in 7d&y while no appreciable change of 7delay is observed in the presence of Ca(N03)2. s o far as T~ is concerned, it must be noticed that only for the systems Mn/Mg(NO&2 and Mr1/Ca(N03)~is a smaller atomization time observed. Values of, respectively, 810 and 770 ms are obtained as compared to 890 ms for the pure Mn solution. Therefore a faster rate of atomization of Mn in the presence of Ca(N03)2and Mg(N03)2causes the interferences observed. T h e other systems mentioned all have T~ values which are completely comparable with the T~ values for the pure solutions so that the enhancement of the Cu and Mn absorption signal in the presence of MgSO, cannot be explained by an increase of the atomization rate of the elements. The results of this investigation can be related to the temperatures of first atomization of the elements in the presence of the matrices considered. These temperatures are also given in Table I1 and were determined as specified in a previous paper (I). The increased Tdelay values for the systems M n / M g S 0 4 and M ~ I / M ~ ( N are O ~ a) ~ direct consequence of the higher temperatures of first atomization of Mn in the presence of these matrices. T h e agreement between the temperatures of first atomization of Mn and Mn in the presence of Ca(NOJZ is reflected in the Tdelay values for both systems which are completely comparable: 240 ms for the pure Mn solution, 270 ms for the Mn/Ca(NO& systems. Also the system Cu/MgS04 has a Tdelay value which is comparable to the Tdelay value of the pure element, as a direct consequence of the identical first atomization temperatures of both systems. Several hypotheses, describing atomization processes in nonf!ame atomizers, have been proposed. Campbell and Ottawajr (6) and Aggett and Sprott (8)give a thermodynamic approach for the production of atoms based on a reduction of the metal oxide by the graphite. Since the thermodynamic approach does not explain the fact that several of the metals form thermodynamically stable carbides at the temperature at which the reduction of analyte oxides becomes thermodynamically feasible, Fuller (7) proposed a kinetic approach to the atomization process that moreover allows one to predict absorbance peak shapes.
Recently also Sturgeon and co-workers (9) proposed a mechanism of atom formation for a number of elements using a combined thermodynamic and kinetic approach but further experimental work is required to substantiate many of the conclusions drawn. Despite the fact that an equilibrium in the atomizers cannot be obtained as a result of, among others, the rapid rate of rise of temperature causing a thermal gradient and because of the presence of an Ar stream evacuating the metal vapor, all of the models proposed are based on the assumption that an equilibrium has been attained. No general agreement exists concerning the proposed atomization mechanisms of the pure elements so that it seems unlikely that the described models could contribute to a large extent to the elucidation of the interference problem. The results of our study indicate that the matrix can cause a decrease of T~ or T~ and an increase of T d e h y but that these phenomena are not related in a simple way since the occurrence of one of these does not involve the occurrence of the others. Since the existing models agree only approximately for atomization from pure solution, it seems likely that they will not be adequate for explaining interference effects in general. It seems probable that the interference of each kind of matrix on each atomized species should be regarded as a separate phenomenon. The present article must be seen as a warning for analysts interested in carbon furnace analysis. The complex nature of the processes taking place in the furnace must show analysts that attention is to be paid when new samples are to be analyzed. Nevertheless analysis can be carried out with success if one is aware of the interference phenomena that can occur by analyzing an unknown sample.
LITERATURE CITED J. Smeyers-Verbeke, Y . Michotte, P. Van den Winkel, and D. L. Massart, Anal. Chem., 48, 125 (1976). R. E. Sturgeon, C. L. Chakrabarti, I. S.Maines, and P. C. Bertels, Anal. Chem., 47, 1240 (1975). R. E. Sturgeon, C. L. Chakrabarti, and P. C. Bertels, Anal. Chem., 47, 1250 (1975). L. Ebdon, G. F. Kirkbright, and T. S. West, Anal. Chim. Acta, 5 8 , 39 (1972). B. V. Lvov, "Atomic Absorption Spectrochemical Analysis", Adam Hilger Ltd., London, 1970. W. C. Campbell and J. M. Ottaway, Talanta, 21, 837 (1974). C. W. Fuller, Analyst(London), 99, 739 (1974). J. Agett and A. J. Sprott, Anal. Chlm. Acta, 72, 49 (1974). R. E. Sturgeon, C. L. Chakrabarti, and C. H. Langford, Anal. Chem.. 48, 1972 (1976).
RECEIVED for review July 18, 1977. Accepted October 5 , 1977. Work supported by the Fonds voor Geneeskundig Wetenschappelijk Onderzoek (F.G.W.O.).
