1839
Anal. Chem. 1982, 5 4 , 1839-1843 (24) Volgtman, E.; Wlnefordner, J. D. J . Liq. Chromafogr., In press. (25) SU, s. Y.; Jurgensen. A.; Bolton, D.; Winefordner, J. D., And. Lett. i o a i , 14, 1-6.
RECEIVED for review Nlovember 16,1981. Accepted June 15,
1982. This work was supported by Grant No. NIH-GM11373-17and by Grant No, DOE-AS05-78-EV06022-MODAOOE. Portions of this paper were presented at FACSS VIII, Philadelphia, PA, on Sept 23,1981.
Trace Metal Determination by Metastable Transfer Emission Spectroscopy Henry C. Na' and Thomas M. Nlemczyk" Department of Chemistv, Unlvers/?yof New Mexico, Albuquerque, New Mexico 8713 1
An emlsslon technlque based on excltatlon of atomic species by an energy transfer process from an active nitrogen plasma Is dlscussed. The malin excltation pathway appears to be a colllslonal energy transfer from the N2(A3&+) species In the actlve nltrogen plasma to the atomic specles of Interest. Aqueous solutions of trace metals are eiectrothermally dried and atomlzed from a tantalum boat. The actlve nitrogen Is produced In a mlcrovvave discharge and mlxed wlth the electrothermally produiced atomlc vapor In a flow cell. Detectlon llmlts for Ag, El, Cd, Cu, Mg, Pb, and TI are reported, and a h e a r dynamic range of 4 to 5 orders of magnltude Is seen In all cases. The iupper llmit to the llnear range Is related to the maxlmum conceiitratlon of the N2(b3Zu+)specles in the actlve nltrogen plasma. The technlque shows an lmmunlty to Interferences and the potentlal for multlelement analysis.
There exist many techniques that can be used for the cletermination of trace elements. None, however, possesses all the characteristics of an ideal trace element technique. Recently, a new technique has been investigated that shows a great deal of promise in fulfilling many of the requirements of the ideal technique. The technique involves the activation of nitrogen molecules in an electric discharge and the subsequent transfer of energy from the activated, Le., excited, nitrogen molecules to the atomic species of interest. Nitrogen so activated has been termed "active nitrogen" and has been the subject of many investigations. The first report on active nitrogen was made by Warburg in 1884 ( 2 ) . He observed a peach yellow glow when air was subjected to a discharge a t reduced pressures. The glow persisted for several seconds after the discharge was turned off and has since been referred to as the "nitrogen afterglow". In 1900,Llewis made the first systematic study of the nitrogen afterglow and confirmed its long lifetime and identified its band splectrum (2-5). He also found many metallic lines corresponding to electrode material and mercury lines due to the diffusion of mercury vapor into the afterglow region from the pumping system. The chemical activity of the afterglow was first emphasized by Strutt who suggested that recombination of nitrogen atoms might be responsible for the phenomenon (6). In 1935 Rayleigh reported that the lifetime of the afterglow was very much dependent upon the nature of the walls of the reaction chamber (7). Many ad-
ditional studies involving the properties of active nitrogen have appeared in the literature, and much of this work has been summarized by Wright and Winkler (8). The excitation of metallic species in an active nitrogen afterglow has been mentioned in or has been the subject of many reports. Most of these reports have focused on the nitrogen afterglow ~rori the mechanism of metal species excitation. The principal energy carriers in active nitrogen are either the metastable triplet state Nz(A32,+) molecules or the vibrationally excited ground-state Nz(X'Zg+). Many differing claims concerning the detailed excitation mechanism have appeared in the literature (e.g., ref 9-15);however, it is generally accepted that the Nz(A3Z,+) state is responsible for excitation of levels requiring more than 4.5 eV while the vibrationally excited ground state might participate in the excitation of lower energy levels. The potential for an analytically important technique is indicated in many studies where metal atom emission was discussed. For example, Meyer et al. observed emission from Hg a t the 253.7-nm line for concentrations of Hg as low as lo9 atoms cm-3 (16). Sutton and co-workers have shown a number of important applications and named the technique metastable transfer emission spectroscopy (MTES) (17-19). The applications have included the detection of metal vapors produced in a furnace (18) and the determination of P b in aqueous samples (19). The detection limit for P b in the aqueous samples was reported to be 0.2 ng. More recently Dodge and Allen have reported the detection of Hg and Zn using an active nitrogen excitation technique (20). The active nitrogen generation used in their system was a dielectric discharge, which they report produces a higher concentration of the metastable Nz(A38,+) and a lower concentration of nitrogen atoms. On the basis of the difference in the nitrogen excitation system they called the technique METAL (for metastable energy transfer for atomic luminescence). Detection limits of lo7 atoms/cm3 and los atoms/cm3 in the emission cell for Hg and Zn, respectively, as well as a linear dynamic range of 7 orders of magnitude for Zn determinations were reported. On the basis of the results of the previous work we have designed an experimental system to determine trace metal concentrations in aqueous solutions using an active nitrogen excitation system. The system employs a microwave discharge to produce the metastable Nz(A3Z,+),thus in accordance with the previous terminology we refer to the technique as MTES.
EXPERIMENTAL SECTION Present address: PPG Industries, Inc., P.O.Box 4026, Corpus
Christi, TX 78408.
An overall block diagram of the experimental setup is shown in Figure 1. The atomic emission from the flow tube was
0003-2700/82/0354-1839$01.25/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1 1 , SEPTEMBER 1982
1840
I
i
MANIt VLU
I PMT
I I
n
I
ATOMIZER
SUPPLY
{- ’
I
COMPUTER
Flgure 1. Block diagram of the metastable transfer emission spectroscopy experimental system. TO PUMP 1
-
4
L
L
p
-
4
k-AJL2-
Flgure 2. A cross section drawing of the flow cell used in the experiments described here: (1) extension/emission chamber; (2) atomization/mixing chamber; (3) ring N, injector; (4)brass electrical feedthrough; (5)tantalum boat; (6)N, inlet chamber; (7) microwave cavity. monitored by using a 0.35-m monochromator (GCA McPherson Model EU-700). All data reported here were obtained using 600-bm slits which resulted in a spectral band-pass of 1.2 nm. No focusing optics were employed due to the diffuse nature of the emission source. The source radiation was modulated at 285 Hz using a mechanical chopper (PAR Model 192). The signal from the photomultiplier (RCA 1P28) was amplified (Keithly Model 427) and detected using a phase-sensitive detector (Keithly Model 821 and 822). The voltage pulse produced by the MTES experiment was then recorded on a storage oscilloscope (Tektronix Model 5119) for instant diagnostics and/or digitized and stored by an on-line computer. The digitization system consisted of a sample and hold amplifier (Datel Model SHM-IC-l), an analog to digital converter (Datel Model L12B281),and a computer (DEC PDP 8/e). Appropriate software was developed for averaging, peak height analysis, and peak area determination. Gas Handling System. The flow of N2 and Ar was controlled by metering valves and monitored with rotameters (Matheson Model 603). The pressure was measured with a manometer containing Octoil-S as the manometer fluid. The pumping system was a Welch Model 1402 which has a maximum pumping capacity of 160 L/min. Flow Cell. The heart of the system is a Pyrex flow cell, shown in Figure 2, where the atomization and emission processes take place. For experimental flexibility and cleaning convenience the flow cell is composed of three parts, an atomization chamber, an extension mixing chamber, and the nitrogen inlet and they are 25 mm, 25 mm, and 12 mm, respectively. The connections between the sections are standard O-ring connections. Atomizer. The atomizer consists of a tantalum boat (R. D. Mathis MES.005) supported by two brass rods. The rods are held in place and sealed to the glass side arms by Cajon Ultra Torr fittings. The brass rods serve as the electrical connection as well as heat sinks. The samples were introduced into the Ta boat using an Eppendorf micropipet through an injection port located directly above the Ta boat. The atomizer power supply was a modified carbon rod atomizer power supply (Varian Model CRA-90). The modifications included a variac in the primary to scale down the output power and incorporation of an interrupt
in the heating program after the drying cycle. The interrupt allows the flow cell to be pumped down and the active nitrogen source started before the atomization cycle is initiated. A maximum atomizer temperature of approximately 2200 “C was employed for all determinations reported here. Active Nitrogen. A McCarrol type N cavity was used to couple the output of the microwave power supply (Opthos Model MPG-4M) to the flowing nitrogen. The cavity was always tuned so that the reflected power was less than 2 W. The active nitrogen was introduced into the reaction chamber through a ring injector. Reagents. All chemicals used were reagent grade. Stock solutions and all dilutions were prepared with distilled, deionized water. The Ar and N2 used were “high purity” and used without further purification. Operating Procedure. A 5-rL aliquot was pipetted into a Ta boat for all results reported here. The sample was dried at atmospheric pressure using the “drying stage” of the atomizer power supply program. The system was then evacuated, the Ar and N2 flows were started, the microwave discharge was ignited, and the atomizer cycle of the power supply was initiated. The emission pulse was recorded and processed, and the results were printed out on the computer terminal.
RESULTS AND DISCUSSION Active Nitrogen Production. The active nitrogen was produced in a microwave cavity as discussed above. Although there is ongoing discussion concerning the mechanism of active nitrogen production, the low-pressure pathway proposed by Berkowitz et al. (21) is generally accepted as the major contributor to the concentration of excited-state nitrogen in the active nitrogen plasma. Their mechanism is as follows:
+ N(4S) + T B @ N2(5Zg+)+ T B N2(5Xg+) + T B Nz(B3n,) + T B
N(4S)
-
(1)
where T B is a third body. This mechanism may be oversimplified, but it does point out the features important in an analytical application. First, the A3Z,+ state of nitrogen is thought to be the main electronic energy donor in active nitrogen excitation as shown in reaction 4, where M is a metal
(4) atom. Reaction 3, the deexcitation of the B3H, state of nitrogen, is thought to be the primary reaction populating the A3&+ state of nitrogen. The radiation given off in this transition dominates the Lewis-Rayleigh afterglow and is responsible for a great deal of the observed background. The AB&+ state of nitrogen has an energy level of 140 kcal/mol and is thus capable of exciting many atomic species. The second feature in the mechanism of nitrogen metastable production is via a recombination of nitrogen atoms. The nitrogen atoms are produced in the microwave plasma and are the species responsible for the chemical reactivity of active nitrogen. This reactivity can be an aid in the atomization of molecular species. The nitrogen atoms may, however, limit the maximum N2(A3ZU+) concentration, for the N2(A3X,+) state is rapidly quenched to the ground state, Nz(XIZ,+),by collisions with nitrogen atoms (22-24). It is yet to be seen whether or not the presence of nitrogen atoms in the discharge is a help or a hindrance. Flow Cell Design. Many of the parameters of the flow cell design ultimately affect the analytical performance of the MTES instrument. The flow cell-atomizer design is shown in Figure 2. One limitation of this design is that the tantalum boat atomizer is in the line of sight of the optical system. For relatively volatile elements, e.g., Cd or Pb, this proves t o be no problem, but for more refractory elements the blackbody radiation produced by the T a boat is a major source of background. A flow cell designed such that the radiation from the T a boat was baffled was evaluated, but it did not yield N
ANALYTICAL CHEMISTRY, VOL. 54,NO. 11, SEPTEMBER 1982 * 1841 1
m
Table I. Effect of Mixiiig Chamber Length on the Determination of Cd sensitivity, ng-I peak peak height area
mixing chamber length, cm
___
10 20 30 40
130 131 149 190
cu
Cd
c-'
Pb
linear range, ng
68 13 83 92
0.10-500 0.10-500 0.06-500 0.03-500
G
k
m
6 1'8
b
1 0 1 2 14 16
8'
10 ' 1'2 14 ' 16
N, FLOW RATE (Arb. Units)
Figure 4. Relative response obtained for Cd, Cu, and Pb as a function of the Ar and N, flow rates. 0 0
0
3
-
0
Table 11. Detection Limits for Several Elements Experimentally Determined by the MTES Technique
0
3
a
n
element
flow cell pressure, torr
Ag Bi
1.1 1.0
Cd
2.2 1.9 2.4 1.3 1.5
cu
Mg Pb T1 Q+qb
wavelength, nm
detection limit, pg
328.1 306.8 326.1 324.1 383.8 368.3 351.9
2 300 30 20 80 40 5
1
20
60
80
100
120
MICROW,I\VE POWER OUTPUT (watt) Figure 3. Relationship between microwave power output and the emission signal intensity for 5.0 ng of Cd: (0)peak height; (A)peak area.
results as precise as those obtained with the model shown in Figure 2. It is most likely that the decrease in precision obtained when using the baffled flow cell was due to increased turbulence. In any case, the atomizer background was very reproducible and can be accurately subtracted from the analyte signal. The sample radiation is viewed by the optical system as the "plug" of metal vapor moves down the mixing tube toward the optical system. It was generally felt that the length of the tube would be an important variable as a longer tube would allow a longer viewing time, but the advantages might be offset by increased diffusion and depletion of the active nitrogen. Measuring from the active nitrogen introduction point, flow tube lengths of 10, 20,30, and 40 cm were evaluated. Cd samples were run, and the results are summarized in Table I. The best sensitivity and detection limit was obtained using the 40-cm tube. A longer tube could not be accommodated in the current MTES apparatus but should be evaluated. All results reported below were obtained by using the 40 cm length flow tube. The intensity of signal produced for a given amount of analyte was found to be very dependent on the condition of the flow tube walls. When the flow tubes were new they produced large signals; however, the magnitude of the signals was seen to decay after several hours of operation. Indeed, even the intensity of the Lewis-Rayleigh afterglow was seen to diminish. When this occurred the tube was removed, cleaned by acid washing, dried, and reinstalled. The performance would return t o the same level as a new tube, but would again degrade after a few hours. This problem was largely eliminated by soaking the flow tube in 1M phosphoric acid overnight before drying. After this treatment the tube would give reproducible high performance over periods of several weeks. Microwave Power Level. The magnitude of microwave power applied to the dlischarge affects the active nitrogen composition, hence the analyk signal intensity. Figure 3 shows
the relationship between the microwave power level and the signal intensity. Both peak height and peak area values imcrease with increasing power, but level off at high power levels. Operation on the plateau region frees the technique from variations due to small fluctuations in the microwave power supply output. All results reported below were obtained by using a microwave power level of 100 W. Gas Flow Rates. Initial experiments revealed that the experimental results were very dependent upon both the N2 and Ar gas flow rates and that the two flow rates were not independent. The optimum flow rates for Cd, Cu, P b were determined by using a two-dimensional simplex algorithm. The sensitivity of the measurement was used as the response function. Figure 4 shows the response surface as a function of the rotameter readings for the three elements. It should be noted that the rotameter readings are only relative numbers as the rotameter were not calibrated for use with Ar or calibrated for use a t low pressures. The ultimate flow cell pressure is dependent on the flow rates used for the determination. The flow cell pressure at the optimum flow rates for each element determined is reported in Table 11. A gas handling system that will allow the pressure to be varied independently of the gas flow rates is currently being evaluated. Atomization Conditions. The rate a t which the atomic vapor is introduced to the Ar flow will have a significant impact on the ultimate analytical utility of the technique. The rate of atomization has certainly been shown to be important in furnace atomic absorption spectroscopy. In the MTES technique it would be expected that the maximum sensitivity would result when the maximum instantaneous metal vapor concentration was obtained for a given amount of analyte introduced to the Ar flow. The maximum metal vapor concentration would be obtained for a given amount of analyte if the introduction occurs over the minimum time span, i.e., the heating rate of the T a boat is as rapid as possible. The experiment will be limited by the maximum analyte concentration allowable at any one point in the flow. This maximum is determined by the point where the concentration of the active nitrogen would be substantially reduced. The results of a heating rate experiment are shown in Figure 5. The results are as expected for the peak height mea-
ANALYTICAL CHEMISTRY, VOL. 54,
1842
I8
-
16
-
NO. 11, SEPTEMBER 1982 A U
-
14
-
A
t
5 a e
12-
!
10-
A
Y
a
c In a C -c
8 A
+
6 E3
C
P
9
E
A
B
4 -
W
B
A
2 -
I
0
0
25
1
5.0
,
I
75
IO 0
Mass o f Cd (ng)
8
0
2.5
75
50
10 0
Figure 7. Working curves obtained for Cd when placed on the atomizer in different chemical forms (anion, peak height, peak area): SO,*-, 0, 0;NO3-, A, A: CI-, B, 0.
Mass o f Cd (ng) Figure 5. Working curves obtained for Cd as a function of atomizer heating rate (ramp rate ("CIS), peak height, peak area): 100, 0, 0; 200, A, A: 400,B, 0.
.22
I
t
0
0
n
0
400
600
0
50 -
0 0 n 0
O 0
n
A
n
Figure 8. Working curves obtained for Cd and Cu: (0) peak height; -
0 200
RAMP RATE (OWsec.) Figure 6. Relative response obtained for 5 ng of Cd as a function of atomizer ramp rate: (0) peak height; (A) peak area.
surements, i.e., increasing sensitivity with increasing ramp rate. As can be seen, however, there is almost no change in sensitivity when peak area is measured. This would indicate a constant photon per analyte atom ratio for the amount of analyte introduced and the heating rates used. The time duration of the signals produced in the MTES apparatus varies slightly with the gas flow rates, the sample mass, and the length of the flow tube and varies significantly with the atomizer heating rate. At a heating rate of 400 "C/s all metals produced a signal of 0.3 f 0.05 s full width a t half height. When a constant amount of analyte is introduced at varying ramp rates, the sensitivity of the peak height measurements no longer increase after a certain ramp rate. Figure 6 shows the results obtained for the introduction of 5 ng of Cd as the chloride. Note, again, that the signal stays constant using an area determination. The point at which the peak height flattens with increasing ramp rate is dependent upon the amount of analyte. The point shifts to lower ramp rates with increasing sample mass. This is presumably due to a local depletion of the active nitrogen concentration. In addition to the actual heating rate of the T a boat, one would expect that the form of the analyte on the surface would affect the analyte introduction rate. In order to determine the effect the form of the analyte might have, we atomized Cd as the chloride, the nitrate, and the sulfate. The heating program used eliminated the ashing cycle so that the maximum effect of the anion might be seen. The results of these
(A)peak area.
experiments are shown in Figure 7. The peak height measurements indicate that there is an effect due the chemical form. This is almost certainly due to a change in the rate of analyte atom introduction. Under the conditions used in this experiment it might be expected that some fraction of the analyte would be introduced to the Ar flow as a molecular species. The analyte atom formation rate thus is likely due to both the rate of vaporization from the surface and the rate of dissociation of any molecular species. The consistency of the peak area measurements indicates that any molecular species vaporized were atomized by the active nitrogen, something not unexpected based on previously obtained results (25). Analytical Results. Figure 8 shows the working curves obtained for Cu and Cd. For both metals the working curve is linear from the detection limit, 20 pg for Cu and 30 pg for Cd, to approximately 1 kg. Similar results are obtained for all metals tested to date. In all cases the calibration curves flatten at about 1 kg of analyte species. The upper limit of linearity is determined by the concentration of active nitrogen and thus should be related to the concentration of analyte atoms in the vapor phase and not on the identity of the analyte. Table I1 shows the detection limits obtained and wavelengths used for seven elements determined in the apparatus described here. The wavelengths reported in Table I1 are those that gave the best sensitivity. Note that the wavelengths are not necessarily the commonly used resonance lines. The excitation process, shown in reaction 4, is a collisional energy transfer and thus spin conservation must apply. An example is Cd where the 326.1-nm line gives a detection limit more than
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
2 orders of magnitude bellow the normally used 228.8-nm line. It is clear from the data presented here that the MTES technique shows significant promise as an analytical technique. The low detection limits achieved are due to several factors. The energy transfer process is very efficient. This is mostly due to the fact that the large number of vibronic levels of the ground and metastable state nitrogen molecules allow for near resonant conditions in the energy transfer process. Also, the optical arrangement allows an atom to go through the excitation/emission cycle many times while in the observation zone. Finally, in conitrast to high-temperature emission techniques, the background produced by the active nitrogen is very low. These factlors will almost assuredly allow lower detection limits than tlhose reported here. The data presented in Figure 4 clearly show that the technique has multielernent capabilities. Although different optimum conditions were found for each element determined, a set of compromise conditions can be found that would not result in a significant decrease in the sensitivity for any one element. The only limitation would be that the total sample mass cannot exceed the level where the active nitrogen population is significantly decreased and nonlinearity results. Although not extensively investigated, the data presented here indicate that the tlechnique might show an immunity to interferences. Spectral interferences should be minimum due to the low temperature and low background of the active nitrogen plasma. Many interferences have been reported for atomic absorption studies where electrothermal atomization is used. The reactive nature of the active nitrogen plasma eliminates many of these problems, for the atomization need not be complete on the electrothermal atomizer surface. The chemical reactivity of the active nitrogen plasma might be due to the presence of nitrogen atoms. As discussed above, nitrogen atoms are quenlchers for the A3&+ state of nitrogen, so are quite possibly one of the limitations in establishing the maximum concentration of N2(A32,+) in the nitrogen flow. If an increase in the N2(A3B,+)concentration is achieved at the expense of the nitrogen atom population, increased in-
1843
terference might result, and the technique would likely suffer when any real sample analysis was attempted. This is an area that clearly needs further investigation. Finally, operation at reduced pressure is a disadvantage. Speed and convenience of analysis will both benefit if the technique can be made to operate at atmospheric pressure. Further research in this area is proceeding.
LITERATURE CITED Warburg, E. Arch. Sci. Phys. Net. 1884, 72,504. Lewis, E. P. Ann. Phys. (Leiprig) 1900, 2, 459. Lewis, E. P. Astrophys. J. 1900, 72,8. Lewls, E. P. Phys. Rev. 1904, 78, 125-128. Lewis, E. P. Asfrophys. J. 1904, 20,49. Strutt, R. J. Proc. R. SOC. London, Ser. A 1911, 85,219-229. Lord Rayleigh Proc. R . SOC. London, Sei-. A 1935, 757,567-584. Wright, A. N.; Winkler, C. A. “Active Nitrogen”; Academic Press: New York, 1968. Gann R. G.; Kaufman, F.; Biondl, M. A. Chem. Phys. Lett. 1972, 76, 380-384. Sadowskl, C. M.; Schiff, H. I.; Chow, G. K. J. Photochem. 1972, 7 , 23-38. Duthler, C. J.; Brolda, H. P. “Chemiluminescence and Bioluminescence”; Lee, J., Hercules, D. M., Comeir, M. J., Eds.; Plenum: New York. 1973. Yamashlta, T. Sci. Llght (Tokyo) 1972, 27,44-52. Haug, R.; Rappenecker, G.; Schmidt, C. Chem. Phys. 1974, 5, 255-264. Duthler, C. J.; Broida, H P. J. Chem. Phys. 1973, 5 9 , 167-174. Young, R. A.; St. John, G. A. J. Chem. Phys. 1988, 48, 2572-2574. Meyer, J. A.; Setser, D. W.; Clark, W. G., J. Phys. Chem. 1972, 76, 1-9. Capelle, G. A.; Sutton, D. 0.Appl. Phys. Lett. 1977, 30,407-409. Capelle, G. A.; Sutton, D. G. Rev. Sci. Instrum. 1978, 49, 1124-1 129. Melzer, J. E.; Jordon, J. L.; Sutton, D. .G. Anal. Chem. 1980, 52, 348-349. Dodge, W. B., HI;Allen, R. 0. Anal. Chem. 1981, 53, 1279-1288. Berkowitz, J.; Chupka, W. A.; Kistiakowsky, G. B. J. Chem. Phys. 1956, 25, 457. Baker, R. R.; Jacob, A.; Winkler, C. A. Can. J. Chem. 1971, 49, 1671-1676. Thrush, B. A.; Wlld, A. H. J. Chem. SOC.,Faraday Trans. 2 1972, 68, 2023-2030. Shemansky, D. E.; Carlton, N. P. J. Chem. Phys. 1969, 57, 682-700. Erspamer, J. P., Niemczyk, T. M. Anal. Chem. 1982, 5 4 , 538-540.
RECEIVED for review April 26,1982.
Accepted June 14,1982.
