Anal. Chem. 1985, 5 7 , 1979-1983 (31) Alpert. N. L.; W. E.; Syzmanski, H. A. “Theory and Practice of Infrared Spectroscopy”, 2nd ed.;PienumlRosetta Press: New York, 1973; P 265. (32) Gurka, D. F.; Blllets, S.; Brasch, J.; Riggle, C. J.; Bourne, S., to be presented at the 1985 International Conference on Fourler and Computerized Infrared Spectroscopy in Ottawa, Canada, June 27, 1985.
RECEIVED for review March 7 , 1985. Accepted May 2, 1985.
1979
This paper has been reviewed in accordance with the US. Environmental protection A ~ peer and ~ administrative ~ ~ review policies and approved for presentation and publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. Part of this work was carried out on U.S.EPA Contract No. 68-033100.
Resonant Laser-Induced Ionization of Atoms in an Inductively Coupled Plasma Gregory C. Turk* and Robert L. Watters, Jr. Inorganic Analytical Research Division, Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
The use of tunable dye laser radlatlon to selectlvely lonlze atoms In an Inductively coupled plasma (ICP) has been Investigated. Laserinduced enhanced lonlzatbn was measured as an Increase In current between blased electrodes on either side of the laser lrradlated volume of the ICP. Interactlon between the radlo frequency plasma and the detectlon clrcultry requlred that an extended ICP torch be used and the electrodes placed 19 cm above the load COIL Resonant laser Induced lonlzatlon was detected for Fe, Mn, Na, and Cum Slngle photon, two-photon, and stepwlse modes of laser excitation were utlllred. Laser power dependence studles showed evldence of colllslonal lonlratlon of laser-exclted atoms In the ICP tall flame. Although analytical sensitivity was poor, this approach was qulte sensltlve for detecting transltlons to high-lying Rydberg levels.
We are reporting on what we believe to be the first observation in an argon inductively coupled plasma (ICP) of enhanced ionization of atoms induced by the absorption of visible and ultraviolet laser radiation. Our objective in this research was to learn if the method of laser-enhanced ionization (LEI) spectrometry (1-12), which normally utilizes a flame as an atomization source, could be applied to the ICP. If successful, the adaptation of LEI to the ICP could have two benefits. First, it may impove LEI spectrometry by providing an environment which has been demonstrated to be less susceptible to many of the chemical interferences encountered in flame-based spectroscopic methods. Second, this application may prove to be an effective mechanistic probe for elucidating excitation and collisional processes in the ICP. Laser-enhanced ionization spectrometry in flames is based upon the enhanced rate of collisional ionization of excited state atoms, produced by laser photoexcitation, relative to the rate from the ground state. For example, an atom in a 4-eV excited state populated by the absorption of ultraviolet light will be ionized at a rate approximately 8 orders of magnitude greater than that of the ground state atom in an air-acetylene flame. This enhanced ionization is detected as an increase in electrical current which flows between biased electrodes within the flame. The nonoptical nature of this detection process is unique among the methods of flame spectrometry. There are, however, several relevent differences between the chemical flame and the ICP which do not favor a highly sensitive LEI
measurement in the ICP. For example, collisional quenching rates are lower in the ICP than in the flame, resulting in high quantum yields for atomic fluorescence in the ICP (13,14). In LEI however, a collisional environment is required in order to ionize the laser excited atoms. On this basis, the ICP would be less suitable than the flame for LEI. One of the most important mechanisms of excitation and ionization in the ICP is generally understood to be reliant upon interactions between analyte atoms and excited argon atoms. Penning ionization, resulting from analyte collisions with Ar metastables at 11.55 eV and 11.72 eV (15) or excited argon atoms with energies from 14 to 15 eV (16),has been reported to be a major mechanism of analyte ionization. Since the excited argon collisional partner already has enough energy to ionize most analytes, laser photoexcitation cannot enhance the rate of ionization. This situation is much different than that in the air-acetylene flame, where the major collisional partner is a nitrogen molecule with a Boltzmann energy of only -0.2 eV and where 4 eV of laser photoexcitation dramatically enhances the rate of ionization. However, enhancement of collisional ionization is not the only possibility to consider. Photoionization of laser populated excited states, known as resonance ionization spectroscopy (RIS) ( l a ,can also lead to ionization enhancement. Both processes occur in the chemical flame, but collisional ionization of the excited atoms generally dominates (18). In this work we have made laser power dependence studies, and used different modes of laser excitation, as a means of distinguishing between photoionization (RIS) and collisional ionization (LEI) of the excited atoms. Another difference between the flame and the ICP is that ion fractions are already high in the ICP without supplementary laser excitation. This leaves fewer atoms available for laser-induced ionization. Laser-induced secondary ionization is, in principle, possible but not practical because of the high energy required to reach second ionization potentials. The electron density in the ICP (19-22) is several orders of magnitude greater than in the flame (23) and obviously makes the task of measuring small changes in ionization rates more difficult. In addition, radio frequency radiation from the ICP power source couples into the electrodes which are used to measure ionization. To keep the distance between the load coils of the ICP and the ionization probe electrodes as large as possible, an extended ICP torch was used in this work and ionization measurementswere made in the tail flame
This article not subject to US. Copyright. Published 1985 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
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14 1 bi #
Nd:YAG L o s e r
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Table I. Sensitivities for Laser-Induced Ionization in the ICP
“are
Atten. ,
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element
wavelength, nm
cu
324.8 296.7 279.8 589.0 589.0 f 568.8
Fe Mn Na Na
sensitivity, ionsl(rg1 mL) 2.4 X 7.2 X 1.8 x 8.4 X 1.2 x
loe lo6 107 lo6 108
+TJb - -P-l-o-t t-e-r - -
Figure 1.
--------Amplifier
Schematic diagram of instrumentation.
region above this extension. This region is well above the observation zone normally used for ICP optical emission and, therefore, is not as well characterized, although both temperature and gas velocity profiles have been investigated in extended torches (24-26).
EXPERIMENTAL SECTION A schematic diagram of the instrumentation used in this study is given in Figure 1. Laser excitation is provided by a dye laser (Molectron DL-16, Sunnyvale, CA) pumped by a NdYAG laser (Quanta Ray DCR-1, Mountain View, CA) operated at 10 Hz. Typical laser energies for this system are 1mJ per pulse at 600 nm and 0.3 mJ per pulse when frequency doubling is used. Laser bandwidth is specified by the manufacturer to be approximately 0.01 nm, and the pulse duration is about 6 ns. When the instrument is operated in the ultraviolet region, an automatic wavelength tracking system (Inrad 5-12, Northvale, NJ) is used for frequency doubling. Laser power studies were performed by passing the beam through a variable attenuator (Newport Research Corp., Model 935-5, Fountain Valley, CA) before the laser interacts with the ICP. Laser power is measured with a pyroelectric joulemeter (MolectronModel 53-05, Sunnyvale,CA) placed behind the ICP tail flame. The laser beam is partially focused at a position 19 cm above the load coil. The cross sectional area of the laser beam is approximately 1 mm2. The ICP is an argon plasma (Plasma-Therm, Model 2500, Kresson, NJ), operated at 27.12 MHz and 1250 W with automatic power control and impedance matching. For these studies the outer quartz tube of the conventional torch is extended 11 cm above the load coil. The outer argon gas flow is 14 L/min and the nebulizer argon flow is 0.7 L/min. No intermediate argon flow is used. The liquid uptake is regulated by a peristaltic pump to be 1 mL/min, into an adjustable concentric nebulizer (Instruments s.A., Metuchen, NJ). The load coil in the conventional ICP is oriented with the top of the coil connected to the matching network and the bottom of the coil connected to ground. This arrangement was not changed for these experiments. Ionization is detected with two stainless steel electrodes 2.3 cm high and 15 cm long, separated by 2.1 cm. The electrodes are placed on either side of the plasma tail flame and laser interaction region. A 1.1-kV potential is applied to one electrode with the other connected to ground through a 10-kQload resistor. The field from the biased electrodes causes arcing in the ICP tail flame unless the power is limited to 1250 W and the extended torch is used. Draft shields are used to minimize occasional arcing between the plasma and either electrode caused by room air turbulence. When a laser pulse causes an increase in ionization, the current conducted between the electrodes increases correspondingly,and a pulsed voltage signal develops across the load resistor. A 10 kHz to 1 MHz (-3 dB) filtering amplifier (Tektronix, Model AM502, Beaverton, OR) reduces the effect of 27-MHz radio frequency interference from the ICP and noise caused by low frequency fluctuation of conditions in the ICP tail flame. The resulting signal pulse has a rise time of about 400 ns and a decay time of about 3 jm Laser power dependence data were processed using a sample-and-hold analog to digital converter and a microcomputer.
The peak ionization signal and the output of the laser joulemeter were simultaneously recorded while manually varying the laser beam attenuator. Spectra were recorded with a gated integrator (Princeton Applied Research Model 165/162, Princeton, NJ) to average the ionization signal while scanning the dye laser wavelength. The integrator was set for an aperture duration of 2 ps and an input time constant of 10 fis.
RESULTS AND DISCUSSION Laser-induced ionization signals were observed for Cu, Fe, Mn, and Na. Excitation wavelengths and measured sensitivities are listed for these elements in Table I. The sensitivities are given as the number of ions produced per laser pulse per unit analyte concentration in micrograms per milliliter. The number of ions produced per laser pulse is determined from the area under the current vs. time signal pulse. These values are much lower than those typically observed in flame laser enhanced ionization (LEI) using identical instrumentation. In the case of Cu, a value of 2.4 X lo6 ions/(pg/mL) was measured in the ICP, as compared with 4.7 X lo9 ions/(pg/mL) measured in the air-acetylene flame. This reduction in sensitivity is largely a consequence of working above the extended ICP torch. Atomic emission of the Cu I line at 324.7 nm is a factor of 300 more intense at 15 mm above the load coil with a normal ICP torch than in the tail flame of the extended torch. This indicates that either the Cu atom density or the degree of atom excitation or both are reduced in the extended torch tail flame relative to the normal ICP observation region. Further evidence of cooling is given by the emission from Cu I1 at 224.7 nm, which is 14 000 times less in the extended torch tail flame. These observations are consistent with the lower temperatures predicted (24,25)and measured (26)well above the normal observation region in the ICP. Calculations by Barnes and Schleicher (24) suggest a temperature of between 2000 and 3000 K in this region. The level of root mean square blank noise measured in the ICP was about the same as that observed in the chemical flame using the same instrumentation. Consequently, detection limits were significantly worse in the ICP than those for the flame. In the case of Cu, with the lowest sensitivity listed in Table I, a limit of detection of 7 pg/mL was measured (3 times the root mean square blank noise using a 1-s boxcar averager time constant). Although we did not specifically measure detection limits for the other elements, these would be lower in proportion to their respective sensitivities listed in Table I. A measurement of 2.2-pA dc current carried by charged species in the ICP without LEI or photoionization is about &fold less than typical currents measured in the air-acetylene flame, indicating that electron densities are quite low. This current serves as the base line for the pulse current detected during laser excitation. The relationship between the laser ionization signal for 1000 pg/mL Mn and the voltage applied between the electrodes is plotted in Figure 2. Here the behavior is similar to that observed in flame LEI in that a minimum voltage is required to observe any LEI signal, and
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUQUST 1985
1981
* I " i
at-
* l
0
l
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300
i
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l 750
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l
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l
1200
l
1350
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'
~
Figure 2. Relative laser-induced ionization signal as a function of voltage applied across detectlon electrodes for 1000 pglmL of Mn aspirated into the ICP with the laser at 279.8 nm.
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~
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30
I 40
I 50
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I 80
1 70
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Figure 3. Relative laser-induced ionization signal for 3000 pg/mL of Cu as a function of laser energy per pulse at 324.8 nm. 1
the signal is constant above a certain plateau voltage. This indicates that the electrodes collect the ionization signal efficiently and that the low sensitivity of the laser-induced ionization in the ICP is a consequence of low ion production. To gain some understanding of laser induced ionization mechanisms in the ICP, the dependence of signal level on laser power was studied. Of particular interest was whether the actual ionization was collisionally induced, as in LEI, or the result of photoionization, as in resonance ionization spectroscopy (RIS). The functionality of ionization signal and laser power can indicate the number of photons required by the total excitation-ionization process. This evidence can be useful in distinguishing between the LEI and RIS processes. As an example, the excitation wavelength for Fe of 269.69 nm populates an excited state 4.18 eV above the ground state. Since this state has an energy which is more than half the energy required to reach the 7.9 eV ionization potential, photoionization is possible by absorption of a second photon. As the laser power was varied between 0 and 270 pJ, a linear dependence of the signal was observed. One possible interpretation of this result is that ionization occurs after the absorption of only one photon and subsequent collision. However, the spectral irradiance of the laser beam within the ICP is high enough to expect optical saturation of the photoexcitation step. In this case, one would interpret the linear dependence of the Fe ionization signal on laser power as being due only to the increased rate of photoionization. Since the saturation of the photoexcitation step was not confirmed, the results are inconclusive. Linear dependence of signal on laser power was also observed for Mn, where again, the laser-populated excited state has more than half the energy required for ionization. For Cu, the situation is quite different. The 4p3I2level populated by laser excitation at 324.8 nm lies 3.82 eV above the ground state. This is less than half of the 7.73 eV ionization potential, and only a collisional ionization mechanism seems plausible. One would expect a linear dependence of signal on laser power if the optical step is not saturated and a flat or less than linear dependence in the case of saturation. However, as seen in Figure 3, the ionization signal increases with the square of laser power, indicating a two-photon absorption. A closer examination of the energy levels of Cu yields an explanation. Absorption of a second photon from the 4p energy level results in an excitation to a region of closely spaced levels near the ionization limit. The wavelength region near 325 nm is therefore replete with possible two-photon transitions, resonantly enhanced by the excitation to the 4p level. This is confirmed by the Cu ionization spectrum measured between 324 nm and 328 nm shown in Figure 4. In addition to the expected one photon transitions from the
1
10
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M
100
I
/
324
I
I
324.1
3 8
I
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326
I
326.5
327
Wavelength (nm)
I
'
Figure 4. Laser-induced ionization spectrum for Cu in the ICP. Solution concentration is 3000 pg/L. Time constant is 0.33 s. Line identification numbers refer to Table 11.
Table 11. Observed Copper Lines in the ICP line no.a
lower-upper levels, cm-l
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
0-61707 0-61666 30784-61605 0-61605 0-30784 0-61553 30535-61305 30784-61553 0-61475 0-61407 30784-61475 30535-61216 30784-61407 0-61216 30535-61078 0-61078 0-30535 30784-61305
Refer to Figure 4.
upper orbital wavelength, nm 16s 14d 15s 15s 4Ps 2 13d 13s 13d 14s 12d 14s Ild 12d lld 12s 12s 4~112 13s
324.02b 324.23' 324.36 324.56' 324.75 324.83b 324.90 324.91 325.24' 325.60' 325.73 325.85 326.45 326.620b 327.32 327.36* 327.40 327.55
Two-photon transition.
ground state to the 4p112 (324.4 nm) and 4~313(324.8 nm) levels, a number of other lines are also observed. We have made assignments of these lines based on calculations which assume Rydberg behavior for the d levels above principal quantum numer 11 and for the s levels above principal quantum number 7. These assignments are given in Table 11, which identifies two-photon transitions from the ground state to the l l d , 12d, 13d, 14d, 12s, 14s, 15s, and 16s states. The remaining observed lines correspond to transitions from
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
1962
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Laser Pulse Energy (pJ)
Flgure 5. Relative laser-induced ionization signal for 500 Mg/mL of Na as a function of laser pulse energy for single wavelength excitation at 589.0 nm.
the excited 4p1I2 and 4p3i2levels to a variety of high lying s and d levels. The laser power dependence study was done with the wavelength tuned to the peak of the large spectral feature a t -324.8 nm, which is actually an overlap of four transitions. The peak of this spectral feature actually corresponds best with the two-photon transition to the 13d level at 324.83 nm rather than with the single photon transition at 324.754 nm. Calibration of the wavelength axis of Figure 4 was done by addition of a single correction factor to the wavelength indicated by the grating drive of the dye laser, with the correction factor arrived a t by maximizing the overlap between the measured and calculated spectra. It is ironic that although poor analytical sensitivity is exhibited, the observation of weak two-photon transitions is a demonstration of high “spectroscopic” sensitivity. The laser induced ionization of Na is another case for which the laser populated level (3p at 2.10 eV) has less than half the energy needed to reach the ionization potential of 5.14 eV. However, the complication of two-photon transitions to high-lying Rydberg levels is not observed, and collisional ionization from the 3p excited state is the most reasonable mechanism. This conclusion is consistent with fluorescence quantum efficiency measurements reported by Uchida et al. (13),who found unity quantum efficiency for Na at an observation zone 10 mm above the load coil but decreased quantum efficiency (increased collisional quenching) at increased observation heights. It is likely that the entrainment of air causes quenching due to the enhanced collisional activity of molecular species. The laser power dependence of signal, as shown in Figure 5, exhibits saturation at high laser powers. This behavior is consistent with the single photon absorption-collisional ionization mechanism. It is interesting to consider whether the saturation seen in Figure 5 is due to optical saturation or to the ionization of all analyte atoms within the laser beam (27,28).This question was answered by using stepwise excitation (29,30). A second laser tuned to 568.82 nm was directed into the ICP in temporal and spatial coincidence with the fiit. The second laser further excites the sodium atoms from the 3p level populated by the first laser to the 4d level at an energy of 4.28 eV. The effect is to increase the collisional ionization rate by diminishingthe energy gap to the ionization limit and also to populate a level from which absorption of photons of either wavelength results in photoionization. As indicated in Table I, a 14-fold enhancement of the signal resulted, thus indicating that the saturation of signal amplitude seen in Figure 5 is optical in nature. Laser power dependence studies of either laser in the stepwise excitation of Na again showed saturation of signal. The question of whether this is caused by optical saturation or total ionization remains unanswered at this time, and thus
the question of whether further improvement in signal strength is achievable. We have demonstrated that selective ionization can be induced by laser excitation in the ICP tail flame and detected. The sensitivity of the measurement is not suitable for analytical work but should be sufficient for diagnostic studies. A two-photon ionization spectrum for Cu has been recorded, and evidence of a mechanism of collisional ionization of excited state Na atoms has been observed. The lack of sensitivity is most likely the result of low atom population in the tail flame. If the experiment could be modified to allow work in the analytical emission zone, the signal would be expected to improve linearly with the higher analyte concentration, but background noise would also be much higher, increasing with the square root of electron population. Reversing the load coil such that the grounded end is on the top has been suggested by Date (31). This modification is used in ICP mass spectrometryto allow the plasma to be probed by the sampling orifice of the mass spectrometer.
ACKNOWLEDGMENT The authors thank J. C. Travis, J. R. DeVoe, M. S. Epstein, and W. C. Martin for helpful discussions and F. C. Ruegg for technical assistance. Registry No. Cu, 7440-50-8; Fe,7439-89-6; Mn, 7439-96-5;Na, 7440-23-5. LITERATURE CITED (1) Travis, J. C.; Turk, 0. C.; DeVoe, J. R.; Schenck, P. K.; Van Dijk, C. A. Prog. Anal. At. Spectrosc. 1984, 7, 199-241. (2) Travis, J. C.; Turk, G. C.; Green, R. B. Anal. Chem. 1982, 54, 1006A-1018A. (3) Turk, 0. C.; Travls, J. C.; DeVoe, J. R. J. Phys., Colloq. 1983, C7, 301-309. (4) Green, R. 6.; Keller, R. A.; Schenck, P. K.; Luther, G. G.; Travis, J. C. J. Am. Chem. Soc. 1078, 98, 1517-1518. (5) Turk, G. C.; Travls, J. C.; DeVoe, J. R.; O’Haver, T. C. Anal. Chem. 1979, 51, 1890-1896. (6) Travls, J. C.; Schenck, P. K.; Turk, 0. C.; Mallard, W. G. Anal. Chem. 1979, 51, 1516-1520. (7) Axner, 0.; Bergllnd, T.; Heully, J. L.; Lingren, I.; Rubinsztein-Dunlop, H. J. Appl. PhyS. 1984, 55, 3215-3225. 18) Van Dllk. Ca. A.: Aikemade. C. Th. J. Combust. Flame 1980, 38, 37-49.. ’ Berthoud, T.: LiDinskv. . . J.; Camus, P.; Stehle, J. L. Anal. Chem. 1983, 55, 959-962. Messman, J. D.; Schmidt, N. E.;Parli, J. D.; Green, R. B. Appl. Spectrosc. l98S, 39, 504-507. Chaplygln, V. I.; Zorov, N. 6.; Kuzyakov, Yu. Ya. Talanta 1983, 30, F, -n- s--s-a- R- . Axner, 0.; Bergllnd, T.; Reully, J. L.; Lingren, I.; Rubinsztein-Dunlop, H. J . PhyS., COllOq. 1983, C7, 311-317. Uchida, H.; Koslnskl, M. A.; Omenetto, N.; Winefordner, J. D. Spectrochlm. Acta, Part 8 1984, 398, 63-68. Montaser, A.; Fassei, V. A. Anal. Chem. 1978, 48, 1490-1499. Blades, M. W.; Horllck, 0. Spectrochlm. Acta, Part 8 1981, 368, 861-880. Boumans, P. W. J. M. Spectrochim. Acta, Part 8 1982, 378.75-82. Young, J. P.; Hurst, G. S.; Kramer, S. D.; Payne, M. G. Anal. Chem. 1979, 51, 1050A-1060A. Curran, F. M.; Lin, K. C.; Lerol. 0. E.; Hunt, P. M.; Crouch, S. R. Anal. Chem. 1983, 55, 2382-2387. Kainlcky, D. J.; Fassel, V. A,; Kniseiey, R. N. Appl. Spectrosc. 1977, 31, 137-150. Uchida, H.; Tanabe, K.; NoJlri, Y.; Haraguchl. H.; Fuwa. K. SpecPochim. Acta, P a r t 8 1981, 368, 711-718. Montaser, A,; Fassei, V. A.; Larsen, 0. Appl. Specfrosc. 1981, 35. 385-389. (22) Blades, M. W. Spectrochlm. Acta, Part 8 1982, 378,869-879. (23) Alkemade, C. Th. J. I n “Flame Emlssion and Atomic Absorption Spectrometry”; Dean, J. A., Rains, T. C., Eds.; Marcel Dekker: New York, 1971; Vol. I, Chapter 4, p 128. 124), Barnes. R. M.: Schlelcher. R. G. SDectrochlm. Acta, Part 8 1975, 308, 109- 134. (25) Barnes, R. M.; Nikdel, S. Appl. Spectrosc. 1978, 30, 310-320. (26) Barnes, R. M.; Schlelcher. R. G. Spectrochlm. Acta, Part 8 1981, 368,81-101. (27) Schenck, P. K.; Travis, J. C.; Turk, 0. C.; O’Haver, T. C. J. Phys. Chem. 1981. 85, 2547-2557. (28) Schenck. P. K.; Travis, J. C.; Turk, G. C. J. Phys., Colloq. 1983, C7, 75-84. (29) Turk, G. C.: DeVoe, J. R.; Travis, J. C. Anal. Chem. 1982, 54, 643-645. (30) Gonchacov, A. S.; Zorov, N. 6.; Kuzyakov, Yu. Ya.; Matveev, 0. I . Anal. Lett. 1970, 12, 1037-1041.
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1983
Anal. Chem. 1985, 57,1983-1986 (31) Date, A,, Brltish Geological Survey, London, personal communication, September, 1984.
RECEIVED for review February 12,1985. Accepted May 3,1985. Certain commercial instruments are identified in this paper
to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the equipment identified are necessarily the best available for the purpose.
Background Correction Errors Originating from Nonsimultaneous Sampling for Graphite Furnace Atomic Absorption Spectrometry James M. Harnly and James A. Holcombe*'
US.Department
of Agriculture, Nutrient Composition Laboratory, Beltsuille, Maryland 20705
Slgnlflcant errors In computed background-corrected absorbances arlse from nonsimuitaneous measurement of the sample and reference signals. Processing the data In groups of three (two reference measurements bracketing the sample measurement) Instead of In pairs (a reference and sample measurement) slgnlflcantly reduces the absorbance errors. Error reduction In excess of 50-fold can be reallzed depending on the shape of the background profile. Integration of the corrected absorbance signal over the ilfetime of the background signal results in a canceliatlon of all errors.
Accurate background correction is essential for analytical determinations by graphite furnace atomic absorption spectrometry. There are currently four methods (commercial and prototype) being used: (a) the two-source method (i.e., a hollow cathode lamp and an H 2 or D2continuum source, ref I), (b) the Zeeman-effect method (2-4), (c) the wavelengthmodulation continuum source method (5),and (d) the pulsed hollow cathode approach (6). All of these methods are similar in that two measurements are required to compute the background corrected absorbance; a sample measurement which reflects the combined analytical and the backgroundequivalent absorbance and a reference measurement which reflects only the background-equivalent absorbance (or the combined reduced sensitivity analytical and backgroundequivalent absorbance). In addition, each method employs only a single detector necessitating repetitive sequential measurements of the two signals. Since the two signals are not measured simultaneously,a rapidly changing background can lead to errors in the background corrected absorbances. This paper will evaluate those errors arising from the finite time interval between the measurement of the sample and reference signals. Errors will not be considered which are caused by structured background (i.e., molecular absorption), nonuniform distribution of the analyte (7) or background within the furnace, or nonlinear electronics used in gathering and processing the data.
THEORY Two approaches to the calculation of background corrected absorbances will be considered which are independent of the four background correction methods. As previously stated, 'Present address: Department of Chemistry,University of Texas at Austin, Austin, TX 78712.
all the background correction methods make repeated, sequential measurements of the sample and reference signals. The sample signal is measured at the analytical wavelength and reflects the analyte plus background absorbance, A l f b . The background signal is measured either on or off the analytical wavelength and reflects the background alone, Ab, These absorbances can be used to compute the background corrected absorbance in two ways. The asymmetric mode uses a single pair of measurements where AI,ASYM represents the background-corrected atomic absorbance. The bracketing mode averages two Ab measurements made before and after Al+b to give a background corrected absorbance
-
A1,BRAC
=
Al+b
-
-
I;1 Abz
(2)
As shown in eq 1and 2, the asymmetric and bracketing modes can be different treatments of the same raw data, e.g., data from alternate sampling of Al+b and Ab can be used in either eq 1 or 2. If the time interval between measurements is not uniform, then
where At, and At2 are the time intervals in seconds between the sample (Al+b)and background (Abland Ab) measurements, respectively. It can be seen that eq 2 is a special case (Atl = At2) of the general expression, eq 3. Currently, the bracketing mode of background correction is employed by two prototype instruments, the wavelengthmodulated continuum source spectrometer (5) and a longitudinal Zeeman spectrometer (8), and one commercial instrument, the Perkin-Elmer Zeeman/30-30. All other commercial and prototype instruments, to the best of the authors' knowledge, use the asymmetric mode.
ERROR FUNCTIONS It can be shown that the errors arising from the asymmetric and bracketing modes of background correction calculation are proportional to the first and second derivatives, respectively, of the background absorbance function. By definition, the first derivative of an absorbance signal, A , which varies with time ( A = f ( t ) ) is given by
0003-2700/85/0357-1983$01.50/00 1985 American Chemical Society
