In the Laboratory
Inductively Coupled Plasma–Atomic Emission Spectroscopy: Two Laboratory Activities for the Undergraduate Instrumental Analysis Course
W
Ara S. Kooser, Judith L. Jenkins, and Lawrence E. Welch* Department of Chemistry, Knox College, Galesburg, IL 61401; *
[email protected] Atomic emission spectroscopy (AES) is an essential topic within the undergraduate analytical chemistry curriculum. A 1991 study found that 43% of the schools teaching an instrumental analysis course featured an AES experiment, making it one of the most prevalent experimental techniques in this course (1). A number of AES laboratory experiments have also appeared in the chemical education literature over the years (2–4), with common themes being quantitative flame emission procedures for a particular element (5–7) or construction of an inexpensive spectrometer to measure emission signals in the undergraduate laboratory (1, 8, 9). The development and implementation of the inductively coupled plasma (ICP) source for AES has yielded many advantages over flame emission, most of which derive from the greater temperature of the plasma (10). Widespread industrial use and a drop in unit price now encourage consideration of ICP experiments for inclusion in the undergraduate curriculum. It would be a relatively simple proposition to adapt an AES experiment currently being done with a flame instrument for usage with ICP, and one could benefit from the advantages inherent with use of the plasma source. However, in contemplating how to introduce ICP–AES into our analytical curriculum, we wanted to find some laboratory activities that utilized the ICP in new ways. Ultimately, two different laboratory exercises were developed that fit this criterion. The first exercise involves an examination and comparison of spectral transitions originating from atomic species versus ionic species. In classic flame AES, it is understood that the role of the flame is to break down the molecular species into atoms, then to excite valence electrons to produce a high population of excited electronic states that will yield emission photons. However, the addition of too much energy will cause removal of the valence electron rather than excitation. It is possible to have excitation of one of the remaining electrons on the ion to produce an emission photon, but the spacings between quantum levels will have been altered by the electron removal, and the resulting ionic emission photon will differ in energy from its atomic emission counterpart. One can observe either the atomic or ionic emission, but they are competing processes from the same population of analyte atoms, so if both are present in significant amounts a smaller signal will be observed than if only one form were present. The argon ionization reaction that fuels the plasma also produces an electron, and in the central plasma region near the induction coils an extremely high electron number density results. The high electron density serves to suppress analyte ionization. As the distance above the induction coils increases, the electron number density drops precipitously. The temperature also drops, but it is still high 86
enough to cause ionization in the absence of suppressive effects. This gradient creates a plasma region where ionic emission predominates, because the excited ionic states are more populated than the atomic states. We have designed an exercise where two atomic lines and a single ionic line are isolated for study; the purpose of which is to probe the plasma for optimum positioning, and to examine the consequences of obtaining an emission signal in a location optimized for a different ionization state. The second exercise involves a determination of the temperature produced by the ICP, an important measure since the plasma temperature will affect its ability to populate excited states and atomize analytes. There are a number of different temperature definitions and measuring methods available (11–13). This experiment will focus on measurement of the most common of these, the excitation temperature (Texc), which describes the Boltzmann equilibrium relating populations at different quantum levels within the same ionization state of the analyte (11). The equation used to determine the excitation temperature can be derived from the Boltzmann equation for state populations, and is listed below as eq 1. The full derivation of this form, not a trivial exercise, is included in the lab documentation section of the supplemental material.W ln[(I )兾(gk A)] = ln C + (Ek兾kT )
(1)
Here I is emission intensity in watts, is the emission wavelength in m, the gk value is the statistical weight for the upper energy level of the emission transition, A is the Einstein A coefficient for spontaneous absorption in s1, C is a constant, Ek is the energy of the upper energy level in cm1, k is the Boltzmann constant in cm1 K1, and T is the temperature in Kelvin. Use of this equation for a series of well-characterized transitions will allow calculation of the excitation temperature (12). For our application, a series of Fe I (atomic Fe) lines were used to determine Texc at different plasma power levels (14). Materials and Methods Iron standards were diluted from a 1000 ppm stock solution made by dissolving 1.000 g of iron wire (99.95% pure from J.T. Baker, Phillipsburg, NJ) in 20.0 mL of 6 M HCl, followed by dilution with deionized H2O to a total volume of 1.00 L. All deionized water was polished with a Barnstead (Dubuque, IA) E-Pure system prior to use. Argon was obtained locally in gaseous form at 99.998% purity. The ICP instrument was a PS-1000 from Leeman Labs (Hudson, NH). The software was Revision 3.005, running under MS-DOS 6.22 on a 486 computer. The torch was a Fassel style, and
Journal of Chemical Education • Vol. 80 No. 1 January 2003 • JChemEd.chem.wisc.edu
In the Laboratory
the nebulizer a Hildebrand Grid model. Each emission reading was done in quintuplicate and corrected by subtraction of the signal from a blank, using a delay time of 30 s for sample uptake and a 30 s integration time for each reading. Results and Discussion Procedure Part I: Optimum Position The ICP should be ignited and the standard performance checks and tune-up procedures performed prior to taking any data. The 1000 ppm Fe solution was diluted 1:10 with deionized water prior to the experiment, and the resulting 100.0 ppm stock solution was provided to the students. The students were asked to prepare standards in the vicinity of 5, 10, 15, and 25 ppm Fe from this stock solution. They were also provided an unknown Fe solution, although they were informed that it actually contained 20.00 ppm of Fe. The plasma power was adjusted to 1.2 kW from the default value of 1.0 kW. The first spectral line selected for study was the 373.486 nm Fe I (atomic) transition, which will be abbreviated here as FeAtom1. While aspirating the 100.0 ppm Fe standard, the plasma was searched to determine the emission zone yielding optimum response for this line. This was a software operation on the PS-1000 called a Peak Both. The Cartesian coordinates of this optimum zone were recorded by the student after this operation (Srce X and Srce Y on the PS-1000). The four standard Fe solutions were then aspirated, and a calibration curve produced from the signal intensities. The plot should be highly linear in this calibration range. The optimization and calibration procedure was then repeated with the 259.940 nm Fe II (ionic) transition, which was abbreviated as FeIon, and a second atomic line at 371.993 nm, which was designated as FeAtom2. With the instrument still optimized for FeAtom2, the unknown solution was read with the FeAtom2 line. Next, the same unknown was read with the FeIon and FeAtom1 lines without further optimization. Following this, the Peak Both optimization was performed for FeAtom1, then the unknown solution was read using this line. With the instrument still optimized for FeAtom1, the unknown solution was then read using the FeIon line. Finally, the ICP was optimized via a Peak Both for the FeIon transition, followed by reading the unknown using the FeIon line and the FeAtom1 line at this new position. Part II: Temperature Determination The 100.0 ppm Fe standard was the only solution used in this section. The plasma power was set to 0.8 kW. The lines used in Part I were switched off, and nine new lines, detailed in Table 2 of the lab documentation section of the supplemental material,W were turned on. No calibration curves were needed for these lines. The spectrometer was then optimized via a Peak Both operation using the 373.486 nm line (the strongest line in the group and centrally located) and the 100.0 ppm Fe standard. The 100.0 ppm Fe solution was then aspirated and each of the nine lines was read and the intensities recorded. Following the completion of this operation, the nine lines were read again after the plasma power was adjusted to 1.0 and 1.2 kW, respectively. After each power change, a 5 min delay was enforced prior to data collection
to allow equilibration of the plasma, and the Peak Both optimization was repeated. For each of the power settings, the students were asked to prepare a Boltzmann Plot of ln(I 兾gkA) versus Ek and to solve for the excitation temperature knowing the relationship given above in eq 1. Discussion A set of student-generated data from Part I of the experiment is shown in Table 1, along with calculated Fe unknown concentrations. The data used to produce the calibration curves are given in Table 1 of the lab documentation section in the supplementary material.W It was quickly evident that the output was accurate when the unknown was read using the same line as that used for the Peak Both optimization, regardless of which line was chosen. Reading with the FeIon line when the instrument was optimized for the FeAtom1 line resulted in signals that were suppressed significantly below the actual value of 20.00 ppm for the unknown. The same can be observed for a reading at FeAtom1 when the instrument was optimized for the FeIon line. This observed suppression for unoptimized lines was proportional to concentration. If one were to optimize on FeIon and then read both the standards and the unknown at this position using the FeAtom1 line, a linear plot and an accurate result will then be obtained although the signals will be weaker. There is a natural tendency to attribute the depressed emission signal for the unoptimized lines to the fact that they are unoptimized rather than any difference in the ionization state. This can be addressed by examining the data produced utilizing the FeAtom2 line. When a Peak Both optimization is run on the FeAtom2 line, the unknown results using the unoptimized lines yield 3.525 ppm for FeIon and 16.31 ppm for FeAtom1. These results illustrate that when an optimization is run for an atomic line, one typically sees only a small variance for a different atomic line at the same position, but a huge suppression for an ionic line. This should suggest that this plasma region has a high electron number density, with ionic suppression being the expected outcome. Since atoms are then the most prevalent form of Fe at this position, one would not expect a great degree of suppression at the unoptimized FeAtom1, but certainly would expect to see a small FeIon signal, given the dearth of ionic Fe at this position.
Table 1. Experimental Data from Part I Unknowns Mean AES Signal
Calculated [Fe] (ppm)
Line Read
Line Optimized
FeAtom1
FeAtom1
670859
19.22
FeAtom1
FeAtom2
569263
16.31
FeAtom1
FeIon
99160
FeIon
FeIon
2761740
FeIon
FeAtom1
642775
4.474
FeIon
FeAtom2
508359
3.525
FeAtom2
FeAtom2
666868
2.822 19.43
20.69
NOTE: All unknown concentrations are actually 20.00 ppm.
JChemEd.chem.wisc.edu • Vol. 80 No. 1 January 2003 • Journal of Chemical Education
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In the Laboratory
with only a few exceptions, reflected the fact that the plasma temperature increased as the power was increased.
1.2 kW -18
1.0 kW
Hazards
0.8 kW -19
The iron solutions used in this experiment will be acidic because of added HCl, so should always be handled with gloves. Eye protection should be used throughout the experiment. Proper initial setup of the ICP instrument is necessary for safe operation, including ventilation of the plasma plume and ensuring that the operator is shielded from exposure to UV light and the intense heat.
y = -14.786 – 0.00017878x Std. Error: 0.00002464
-21
ln
lλ gk A
-20
-22
y = -13.449 – 0.00025761x Std. Error: 0.00005380 -23
-24 26000
Acknowledgments
y = -13.681 – 0.00023017x Std. Error: 0.00003483 28000
30000
32000
34000
36000
Ek Figure 1. Boltzmann plots with varying plasma power.
The complementary case, where the instrument was optimized to observe ionic emission, also displays the same type of behavior. Despite the suppression observed for lines of opposing ionization states to the optimized line, their signals are not suppressed anywhere close to a zero signal reading. Clearly, both ionization states are present in the plasma regardless of the position, with the equilibrium between them being pushed in one direction or the other dependent on the location. A set of student-generated output for Part II of the experiment is shown in Figure 1, with a tabulation of emission intensities given in Table 2 of the lab documentation section of the supplemental material.W The amount of scatter seen in these plots is typical. As can be seen from eq 1, the slope of these plots should yield 1兾kT. Solving for the excitation temperatures from Figure 1 yielded Texc values of 5600, 6300, and 8000 K as the plasma power was increased from 0.8 to 1.0 to 1.2 kW. Given these line fits, precision is not high (confirmed by the standard error for the slope shown on each graph) and the uncertainty of the Texc measure is several hundred degrees in all cases. The values are somewhat high but not unreasonably so when compared with Texc findings from ICP research groups (11, 14). Despite the uncertainty, the student data,
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We would like to thank Jan Lentz, Tom Moses, Mary Armon, and Gary Francois for their assistance during the preparation of this manuscript, and would like to thank David Mead, Jr. and Autumn Anderson for their role in acquisition of the instrument. W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Smith, G. E.; Sanford, C. L.; Jones, B. T. J. Chem. Educ. 1995, 72, 438–440. 2. Salin, E. D. J. Chem. Educ. 1984, 61, 70–72. 3. Hughes, E.; Dugas, C. D.; Hodges, S.; Tracey, S. L. J. Chem. Educ. 1991, 68, A286. 4. Jackman, D. C. J. Chem. Educ. 1985, 62, 161–2. 5. Coetzee, C. J. J. Chem. Educ. 1972, 49, 33. 6. Tripp, T. B.; Neadeau, J. L. J. Chem. Educ. 1974, 51, 130. 7. Goodney, D. E. J. Chem. Educ. 1982, 59, 875–6. 8. Mebane, R. C.; Rybolt, T. R. J. Chem. Educ. 1992, 69, 401–2. 9. Shields, G. C.; Kash, M. M. J. Chem. Educ. 1992, 69, 329–331. 10. Greenfield, S. J. Chem. Educ. 2000, 77, 584–91. 11. Inductively Coupled Plasma Spectrometry and its Applications; Hill, S. A., Ed.; CRC Press: Boca Raton, FL, 1999; p 35–70. 12. Inductively Coupled Plasmas in Analytical Atomic Spectroscopy, 2nd ed.; Montaser, A.; Golightly, D. W., Eds.; VCH: New York, 1992; pp 375–376. 13. Alder, J. F.; Bobelka, R. M.; Kirkbright, G. F. Spectrochim. Acta 1980, 35, 163–175. 14. Wiese, W. L. Spectrochim. Acta 1991, 46, 831–41.
Journal of Chemical Education • Vol. 80 No. 1 January 2003 • JChemEd.chem.wisc.edu
