Anal. Chem. 1990, 62,2501-2503
Other techniques that do not involve vaporization of the sample will become necessary as the molecular weight or complexity of a peroxide increases. Here, both PDMS and FAB using NBA appear to be good choices. Acquiring spectra in PDMS requires considerably more time than FAB, and mixture analysis is particularly suspect in a technique that provides ions from the surface layer. Although it requires the most sample, the PDMS technique should have consumed the least material since the actual sample ionized is negligible. However, the low molecular weight of these samples resulted in large losses by evaporation from the foil. Pinpointing the space where the PDMS dimer of artemisinin forms presents an interesting problem. It is unlikely to be due to a high concentration of this relatively volatile substance in the selvedge region since the ratio stays the same as the sample is depleted. Furthermore, dimer formation with much less volatile compounds such as high molecular weight peptides is not uncommon. While the source of the dimers may be the crystalline surface, the abundant dimer ion also observed in FAB (a solution technique) tends to argue that the phenomenon is a property of the compound itself. Jordan et al. (22) have noted that Rhodamine 6-G, which forms dimers in concentrated solution, also shows more dimer in PDMS when it is absorbed from such solutions. However, in this case the dye, besides being a preformed cation, was known to form a smooth monomolecular layer on the aluminized Mylar surface. No such assurance could be had with artemisinin, and the results are hard to compare.
LITERATURE CITED (1) Schwattz. H.; Schlebel, H. M. In Chemistry of the PeroxMes; Patai, S., Ed.; Wlley: Chlchester, U.K., 1983 pp 105-127.
2501
(2) bscetta, E.; Ounstone, F. D.; Scrimgeour, C. M. J . Chem. Soc., Perkin Trans. 1984, 1 , 2199-2205. (3) Belic, I.; Kastelic-Suhadok, T.; Kavcic, R.; Marsel, J.; Kramer, V.; Kralj, B. Tetrahedron 1978, 32, 3045-3049. (4) Fraser, R. T. M.; Paul, N. C.; Phillips, L. J . Chem. Soc. B 1970, 1278-1 280. (5) Ledaal, T. Tetrahedron Len. 1989, 4 1 , 3661-3664. (6) Bertrand, M.; Fllszbr, S.; Rousseau, Y. J . Org. Chem. 1988, 33, 1931-1934. (7) Kiayman, D. L. Science (Washington, D . C . ) 1985, 228, 1049-1055. (8) Luo, X . D . ; Yeh, H. J. C.; Brossi, A.; FHppen-Anderson, J. L.; Gilardl. R. Heterocycles 1985. 23, 881-887. (9) Lin, A. J.; Klayman, D. L.; Hoch, J. M.; Silverton, J. V.; George, C. F. J . Org. Chem. 1985, 50, 4504-4508. (10) Liu, J.-M.; Ni, M.-Y.; Fan, J.-F.; Tu, Y.-Y.; Wu, 2.-H.; Wu, Y.-L.; Chou, W.-S. Acta Chem. Sln. (Chin. Ed.) 1979, 37, 129. (11) Luo, X.D.; Yeh, H. J. C.; Brossi, A.; Fllppen-An&rson, J. L.; Oilardi, R. Hdv. Chim. Acta 1984, 67, 1515-1522. (12) Peraies. A.; Martinez-Rlpoll, M.; Fayos, J.: Savona, G.; Bruno, M.: Rcdriguez, B. J . Org. Chem. 1983. 48, 5318. (13) Lin, A. J.; Theoharides, A. D.; Klayman, D. L. Tetrahedron 1988, 42, 2181-2184. (14) Theoharides, A. D.; Smyth, M. H.; Ashmore, R. W.; Halverson, J. M.; Zhou, Z. M.; Rldder, W. E.; Lin. A. J. Anal. Chem. 1988, 6 0 , 115-1 20. (15) Van Driel, J. H.; Heerma, W.; Terlouw, J. K.; Haiim, H.; Schwarz, H. Org. Mass Spectrom. 1985, 20, 665-673. (16) Meili, J.; Selbi, J. Org. Mass Spectrom. 1984, 77, 581. (17) Sweetman, B. J.: Blak, I . A. Blamed. Envkon. Mass Spectrom. 1988. 17, 337. (18) Yang, Y.-M.; Sokolsoki, E. A.; Fales, H. M.; Panneil, L. K. Biomed. Environ. Mass Spectrom. 1088, 13, 489-492. (19) Yang, Y.-M.; Fales, H. M.; Panneii, L. K. Anal. Chem. 1085, 57, 1771- 1772. (20) Fohlman, A. J.; Peterson, P. A.; Roepstorff, P.; Hojrup, P.; Kamensky, I.; Save, G.; Hakansson, P.; Sundqvist, B. Biomed. Mass. Spectrom. 1985, 72, 380. (21) Saiehpour, M.; Hakansson, P.; Sundqvist, B. U. R.; Craig, A. G. Int. J . Mass Spectrom. Ion Rocesses 1987, 77, 173-186. (22) Jordan, E. A.; Macfarlane, R. D.; Martin, C. R.; McNeal, C. J. In!. J . Mass Spectrom. Ion Phys. 1983, 53, 345.
RECEIVED April 6, 1990. Accepted August 17, 1990.
CORRESPONDENCE ~
~~
~
Is a Universal Model for Ion Formation during Mass Spectrometric Elemental Analysis Possible? Sir: Investigation of ion formation mechanisms is important for creation of the theory of mass spectrometric analysis methods. In the case of elemental analysis the results of these investigations must allow us to calculate relative sensitivity coefficients, i.e. correction factors for element content determination with respect to an element internal standard (I). The precision of these coefficient calculations may be a measure of the ion formation model’s reliability. Realization of the approximation is also of great practical importance because it gives the possibility of carrying out quantitative or semiquantitative (depending on the approximation precision) analysis without standards. A quasiequilibrium ion formation model for spark and laser ion sources was worked out at our laboratory (2). We processed a large number of our experimental results and practically all available literature data on spark source mass spectrometry and showed that our model approximates the RSC values more accurately than any of the empirical expressions published (3). The quasiequilibrial model works also well enough in secondary ion mass spectrometry (SIMS) (4). It was assumed that the model can have a wider application in other physical methods of analysis, mass spectrometric element analysis methods in particular (5). 0003-2700/90/0362-2501$02.50/0
Recently we have found corroboration of our idea in Analytical Chemistry, namely, the validity of the quasiequilibrium model in inductively coupled plasma (ICP) mass Spectrometry, both in its original nature (for liquid analysis) and for the analysis of solids with laser evaporation (6). However the published data need, in our opinion, a more accurate interpretation. To begin let us briefly review the essence of the quasiequilibrial model. A relative sensitivity coefficient for determination of an element (X) in reference to the internal standard (Int. st.) can be expressed as (7)
= txInt.at.q X Intat. (1) where 4 is the instrumental factor, connected with ion beam formation, mass analysis, and ion detection, and 3; or relative ion yield coefficient, is a physical factor, taking into account fractionating different elements in the ion formation processes. On the basis of the simple consideration, made in the frame of the quasiequilibrium model, we picked out two main atom discrimination sources of different elements in the ion formation processes. The material can reach plasma in the form of both free atoms and particulate matter (2),but we describe the process of substance evaporation and atomization as an RSCXInt.st.
0 1990 American Chemical Society
2502
ANALYTICAL CHEMISTRY, VOL. 62, NO. 22, NOVEMBER 15, 1990
Table I. Atomization and Ionization Temperatures in the Analysis of Standard Samples by Means of ICP-MS with Laser Ablation
ablation temp according to
sample
no. of elem. determined
laser mode
atomization temp (quasiequilibrium model), K
steel
21
Q-switch
29 600
26 OOO
free-run
8 800
9OOO
Q-switch
13 600
16 500
free-run
4 900
5 900
10 400 9 900
Q-switch
8 600 3 400
9OOO 3 750
13 OOO 13 100
A1
cu
12 13
free-run
equilibrium evaporation of free atoms at some model temperature, which we call atomization temperature, T,. Processes of substance evaporation and atomization are described as an equilibrium process at some model htomization temperature T,. The joint action of both ionization during plasma heating and recombination during its expansion may be pictured as another equilibrium process at model temperature Ti.Then the relative ion yield coefficient can be calculated as
where Do is specific atomization energy of an element, i.e. the energy of the atom transition from the solid state to a monatomic gas and cp’ is the first ionization potential. If the instrumental factor has been found (e.g. through calibration by pure elements) and the contents of internal standards of these elements are known, model atomization and ionization temperatures can be obtained by using eqs 1 and 2. These temperatures can indicate the character of the ion formation processes. If model atomization temperature is much higher than the real value (about 8000 K when the power density on the solid surface is on the order of 109 W/sm2 (8)),this means that fractionating of elements during evaporation is less than that in the equilibrium process. In some experiments with a laser ion source the atomization temperature has reached 6 X los K (2). Atomization occurs as an explosive process and almost no fractionation takes place. The electron plasma temperature usually ranges between 5OOO and ZOO00 K, i.e. 5-20 eV (9). The model plasma temperature may surpass considerably these values, if plasma is strongly ionized and nonequilibrium recombination during expansion is significant. We observed extremely high Ti values (6 X lo4 to 6 X lo6 K) with laser ion source too (2, 3). Having found T,and Ti,one can determine the RSC values for all other elements by formulas 1 and 2 and provide an analysis without standards. The approximation error did not exceed 20% for all our experimental data on spark and laser source mass spectrometry (2, 3). The author of ref 6 has noted rightly that the quasiequilibrium approximation may be applied in ICP mass spectrometry with laser ablation of solids, where evaporation and ionization processes are separated spatially and temporarily. He performed calculations of solid surface heating by laser radiation and has obtained an expression for element fractionation in the evaporation process. It agrees completely with the first term of our formula 2. When comparing the experimental and calculated values, the author justly exhibits atom ionization potentials in his Table I. Nevertheless instead of representing the ionization process by means of the second term of formula 2, he selected a classical analytical procedure. He performed a calibration of the whole ICP mass spectrometer detector system by the use of standard solutions.
ref 6, K
‘
ionization temp (quasiequilibrium model), K 11600 11300
We processed all the data presented in ref 6 by means of the complete quasiequilibrium model formula. The device factor in formula 1was unknown, so we fried to calculate it by computing the results (concidentally with determining Ta and Ti). Discriminations in the mass analyzer depend only on the ion mass, and we represented it as a power function, X 7 bt.st.= ( m X / m I n t d a , or as an exponential one, TXInt.st = exp(a(mx - mht3). In both cases the coefficient “a” was close to zero and the approximation precision was almost the same as if no consideration of the RSC dependence on the ion mass was made. With all the errors due to the ion beam formation, mass analysis and registration were corrected by the author. That is why we assigned 7 = 1,and the {values are regarded as equal to RSC; i.e. we calculated the RSC values by means of formula 2. We found the coefficients T,and Tiby using the least-squares method. The results are tabulated in Table I. The approximation error was calculated by
where RSCxe,, and RSCxd are experimental and calculated by formula 2 corresponding RSC values and n is the number of elements. The error ranges between 0.26 and 0.38. As the table indicates, the model temperature of atomization calculated by means of the quasiequilibrium model is close to “ablation temperature” calculated by the author of ref 6. This confirms the validity of our consideration. Indeed, the real temperature of the surface cannot be this high. That is why the values obtained should not be called “ablation temperature”. They are model temperatures, indicating a high evaporation velocity, making the process deviate from equilibrium. For the case of the Q-switch regime the deviation is, of course, greater than in the free running regime, when the velocity of evaporation is lower. The ionization temperature in all cases under consideration was constant within the limits of experimental and calculation errors. This is natural enough because the conditions of the torch operation were kept stable. Nevertheless there are some differences between Tivalues for different matrices. They cannot be explained by some RSC errors or some model imperfection because these temperatures differ slightly for different laser regimes for the same matrix though atomization temperatures differ by several times. The change in Tican be connected with both unreproducibility of conditions in the torch with sample changing and possible influence of the matrix on the ionization process. If the last reason is the main one, the calibration performed by the author of ref 6 cannot consider all the errors appearing in the ionization process. Variations in Ti values during analysis from 10000 to 13000 K may lead to great errors if they are not taken into account. The mean value of the ionization temperature is close to the real electron temperature of the plasma. It is not overestimated as in the case of spark and laser source mass spectrometry. This effect can be connected, perhaps, with
ANALYTICAL CHEMISTRY, VOL. 62, NO. 22, NOVEMBER 15, 1990
the peculiarities of the ionization process in the ICP. Note, that the calculated values of model atomization temperature are the results of ICP mass spectrometer calibration by standard solutions, i.e. the usual ICP technique. Consequently, we proved simultaneously the possibility of application of the quasiequilibrium model to ICP mass spectrometry. Nevertheless the conclusion drawn is based on the indirect data, because the relative sensitivity coefficients for the analysis of liquids are not presented in ref 6. That is why we had to reduce the plasma action to ionization only. However it is known that influence of the matrix and of the chemical form of analytes in ICP analysis is appreciable (IO). Consequently, atomization energy can influence the discrimination of different elements in inductively coupled plasma. To solve this problem, one must compute data obtained in ICP-MS analysis of standard solutions. It is of great interest to consider the possibility of the quasiequilibrium model application to glow discharge mass spectrometry. From the viewpoint of this model glow discharge as a method of action on a substance is more similar to ICP with laser ablation than to spark or laser plasmas. A substance is sputtered in the glow discharge mainly by ions and ionized mainly by plasma electrons. The atomization and ionization processes in this method are divided as well. The progress achieved by using the quasiequilibrium model in spark and laser source mass spectrometry, then in secondary ion mass spectrometry, and, at last, in ICP with laser ablation makes us optimistic about the possibility of the quasiequilibrium model application for the description of ion formation processes in practically all mass spectrometric methods of element analysis. We call on researchers working with different methods of mass spectrometric element analysis to collaborate with us to create the theory and improve the accuracy of the analysis results.
2503
Registry No. Al, 7429-90-5;Cu, 7440-50-8;steel, 12597-69-2. LITERATURE CITED (1) Trace analysis by mass spectrometry; Ahearn, Arthur J., Ed.; Academic Press: New York and London, 1972; 460 p. (2) Ramendik, G. I.; Manzon, B. M.; Tjurin, D. A,; Benjaev, N. E.; Komle va, A. A. Talanta 1987. 34, 61-67. (3) Tjurin, D. A.; Ramendik, G. I.; Chernoglazova, G. I . Zh. Anal. Khlm. 1989, 4 4 , 2157 (in Russian). (4) Ramendik, G. I. fresenius’ 2.Anal. Chem., in press. ( 5 ) Ramendik, G. I. fresenius’ 2.Anal. Chem. 1989, 334, 613. (6) Hager, J. W. Anal. Chem. 1989, 67. 1243. (7) Ramendik, G. I.; Manqpn, 8. M.; Tjurin. D. A. Zh. Anal. K h h . 1989, 4 4 , 996 (in Russian). . (8) Mesyats, G. A.; Proskurovskiy, D. I. Impulsnyi Electricheskiy Raztyad v Vakuume; Nauka: Novosibirsk, 1984; p 177. (9) Derzhiev, V. I.; Ramendik, G. I. Sov. phvs.-Tech. phvs. (Engl. Transl.) 1978, 23, 291; translated from Zh. Tech. fiz. 1978, 48, 312. (10) Hieftie, G. M. fresenius’ 2.Anal. Chem., 1989, 334, 607. Corresponding author.
G. I. Ramendik* Kurnakov Institute of General & Inorganic Chemistry USSR Academy of Sciences Lenin District, 31, B-71, Moscow USSR D. A. Tjurin Yu. I. Babikov Vernadsky Institute of Geochemistry & Analytical Chemistry USSR Academy of Sciences Kosygin str., 19 117975 GSP-1, MOSCOW USSR RECEIVED for review January 5, 1990. Revised manuscript received May 24, 1990. Accepted June 26, 1990.
