Anal. Chem. 1984, 56:, 1923-1927
(5) Korn, G. A.; Korn, T. M. “Manual of Mathematics”; McQraw-Hill: New
(in these derivations, simple planar diffusion).
York, 1967;pp 215-216.
(6) Wagenknecht, J. H.; Goodln, R. D.; Klnlen, P. J.; Woodard, F. E. J . Nectrochem. Soc. in press. (7) Woodard, F. E.; Goodln, R. D.; Klnlen, P. J.; Wagenknecht, J. H. Anal.
Registry No. tert-Butyl-p-toluate, 98-51-1. LITERATURE CITED
Chem., in press. (8) Woodard, F. E.; Woodward, W. S.; Reilly, C. N. Anal. Chem. 1081, 53, 1251A. (9) Deming, S. N.; Morgan, S. L. Anal. Chem. 1973, 45, 278A.
(1) Nadjo, L.; SavQant, J. M. J . Nectroanal. Chem. 1973, 48, 113. (2) Imbeaux, J. C.; Savbnt, J. M. J . Electroanal. Chem. 1073, 4 4 , 169. (3) Woodard, F. E. In Monsanto Report MSL-3320,1963,available from
author upon request. (4) Savitzky, A.; Golay, M. J. E. Anal. Chem. 1964, 36, 1627.
1923
RECEIVED for review March 1,1984.Accepted May 1,1984.
Microsample Introduction into the Microwave-Induced Nitrogen Discharge at Atmospheric Pressure Using a Microarc Atomizer R. D. Deutsch and G. M. Hieftje* Department of Chemistry, Indiana University, Bloomington, Indiana 47405
A microarc atomizer has been evaluated for microsampie introduction into the recently developed microwave-induced nitrogen discharge at atmospheric pressure (MINDAP). The microarc, operating for the first tlme in a ntlrogen atmosphere, efficiently volatilizes discrete microvdumes of solution and the resulting vapor is swept into the MINDAP excitation source. The MINDAP system, unlike eariler mkrowave and rf plasmas coupled to the microarc, offers the economy of uslng nitrogen gas, possesses a high energy of excitation, and provMes good spatial and temporal stability. The combination microarcMINDAP system has been evaluated for detection limits, dynamic range, preclslon, and interelement Interferences. A comparison with other discrete sample-introduction multlelement analysis systems is made.
The recently developed microwave-induced nitrogen discharge at atmospheric pressure (MINDAP) was characterized originally with a dried-aerosol sample-introduction system (1-4). This new plasma exhibits high temperatures (2),low detection limits (3),and matrix interferences which can be overcome in a manner similar to that employed in flame spectrometry. The MINDAP system is investigated here as an analytical atomic emission source for microsampling analysis using a microarc atomizer. The microarc atomizer was developed in 1974 by Layman and Hieftje (5)as a device to convert discrete microquantities of liquid sample into atomic vapor for emission analysis in a microwave plasma. The microarc is a high-voltage, lowcurrent discharge that sequentially and efficiently desolvates, vaporizes, and atomizes sample volumes from 0.1 to 40 pL (5). This concept of separate atomization and excitation was discussed also by Falk et al. (6) who showed how it could enhance the sensitivity of other atomic emission measurements. The microarc has been successfullycombined with several plasma emission sources (5, 7-9). In conjunction with the inductively coupled plasma (ICP) (7,8),the microarc yielded lower detection limits than other microsampling techniques applied to the ICP (10-13). Argon and helium microwaveinduced plasmas (MIP), when coupled to the microarc (5,9), offer the sensitivity and freedom from interferences that has 0003-2700/84/0356-1923$01.50/0
been associated with the ICP. Yet their physical size, instrumentation, and operating requirements are more compact and economical. Elemental analysis using a microwave plasma as the excitation source has been limited to samples introduced as a vapor effluent from gas chromatography (14, 15), thermal atomizers (16,17), hydride generators ( I S , 19),and laser vaporization devices (20). Nebulizer systems have also been employed (21,22) but to a lesser extent and less successfully. Analyte is preferably introduced as a vapor because the MIP lacks the thermal energy needed to decompose the sample. Another limitation of the MIP is its small physical size which restricts the amount of sample that can be introduced before overloading occurs. The MINDAP system overcomes many of the inconveniences ordinarily associated with microwave plasmas: it readily accepts aerosol samples, possesses the high thermal energy needed for sample decomposition, and is relatively unaffected by high analyte concentrations. The MINDAP system has previously been evaluated for continuous solution analysis and is now the subject of investigation with discrete microvolume aliquots from the microarc sample-introduction technique. This is the first time that the microarc has been used in a molecular-gas atmosphere (Le., nitrogen). Its operating characteristics are therefore slightly different from those in an inert monatomic-gas atmosphere and a qualitative description of its behavior is included. The microarc-MINDAP combination yields picrogram to femtogram detection limits, a broad linear dynamic range, good precision (3-7% RSD), and essentially no interference either from sodium or phosphate.
EXPERIMENTAL SECTION Connection of the microarc atomizer to the MINDAP was straightforward and is detailed in Figure 1. Initially it was attempted to introduce the microarc-atomized sample through the central channel of the plasma torch. However, in this configuration the suspended MINDAP plasma was perturbed when the arc was struck. Consequently, in this investigation the microarc was connected to the side-on gas inlet of the torch (Figure 1).
Timing of the microarc and data collection were controlled by a laboratory computer in a manner similar to that described by Keilsohn, Deutsch, and Hieftje (7). A block diagram of the experimental setup is shown in Figure 2. The computer controls 0 1984 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
MINDAP dlscharge
TEM,
Viewing direction
hu
-
/
Cavlty
1 rj
MINDAP tarch
I
0
I I
T i m e (sec.)
Figure 3. Emission-time profile for 1 ng (1 pL of 1 pglmL) of copper at 324.7 nm: (A) analyte emlssion; (6) background emission.
t
N2
{-w
Flgure 1. Microarc-MINDAP operational configuration. Monochromator
uWAVE Power SUPP I Y
M i croar
Cons t an t Current Source Power
SUPP I Y
h r I
T 1 mer
2
struck, generated broad-band emission from the electrode material, The duration of the mc was empirically optimized by monitoring both signal and background emission traces, as described previously (7). For arc times beyond 1 s there was no detectable analyte emission; consequently, in later determinations the arc duration was 1s and data were collected for 2 s. This additional time enabled the computer to establish a base line, allowed for analyte transport to the plasma, and encompassed total decay of the emission signal. A typical emission-time profile accumulated under these operating conditions is displayed in Figure 3.
,-to-v
1
1
MINC 1 I
Computer
II
Figure 2. Block diagram of the instrumentation used to collect and analyze the transient emission signal from the microarc-MINDAP system: PMT, R928 photomultiplier tube; I-to4 Amp, Keithley 427 current amplifier; H.V.P.S., Kelthiey 244 high voltage power supply. the duration of the desolvation period, the ignition and duration of the arc, and the collection and processing of the emitted-radiation signals. The MINDAP plasma was sustained by using the conditions determined to be optimal for the nebulizer-based system (1). These conditions produced a suspended plasma configured vertically, with an applied microwave power of 250 W and reflected power of 20 W. The gas flow rates through the central channel of the torch and through the microarc assembly were 0.1 L/min and 1.75 L/min, respectively. Analyte emission was collected in a radial viewing configuration from the first 6 mm of the plasma tail flame. The microarc electrodes (30-gauge tungsten wire as the cathode and 24-gauge stainless steel syringe needle for the anode) were positioned to support a stable and reproducible discharge in the flowing nitrogen atmosphere; an interelectrode spacing of approximately 0.5 mm was employed. The current and voltage needed to sustain the microarc discharge in the flowing nitrogen atmosphere were 28 mA and 1800 VPp, which were higher than those required in the flowing argon systems (5, 7). Analyte desolvation was accomplished by o h m i d y heating the cathode from a constant current supply of 4 V and 2 A (typically 30 s for 1rL of solution) before initiating the burn. The appearance of the microarc discharge in the nitrogen atmosphere was similar to that noted in other gases. The stable arc emits a bluish plume, characteristic of the electrical breakdown of nitrogen. When first struck, the arc anchors to the tip of the hairpin-shaped cathode and then uniformly surrounds it. Because of the rather small separation between the microarc electrodes, care was required to dispense the microvolume sample onto only the cathode. In trials when the anode was contaminated by sample solution, the arc was difficult to initiate and, once
RESULTS AND DISCUSSION Desolvation Technique. A factor that limited reproducibility and signal stability in initial microarc-MINDAP trials was uncertainty in the duration of the desolvation process. If desolvation were incomplete, initiating the arc resulted in intense broad-band emission from the MINDAP, caused presumably by tungsten oxide liberated from the sample electrode (23). On the other hand, waiting a sufficient period of time to ensure complete desolvation decreased the throughput of sample analysis. It was decided that the best way to overcome this difficulty would be to monitor the desolvation process. The voltage-derivative method developed by Layman and Hieftje (24) records the desolvation process electronically by monitoring changes in the microarc filament resistance as the solvent evaporates. Unfortunately this technique was not successful when incorporated into the microarc-MINDAP system. Erratic derivative signals were produced during the sample desolvation, caused probably by the high flow rate of gas and by the consequent instability in the solvent evaporation rate. Instead, it was found that desolvation could be monitored independently of gas flow and solvent evaporation rates by observing the emission of OH at 306.4 nm. During desolvation of an aqueous sample, water vapor is continuously swept into the MINDAP to yield strong emission a t 306.4 nm; a pronounced reduction in this signal indicates the end of desolvation. This spectroscopic monitor was found to be as sensitive and accurate as the voltage-derivative technique for determining the end of solvent evaporation but is more compatible with the flowing-nitrogen system. Compared to waiting a fixed length of time for desolvation, the spectroscopic technique increased sample throughput by as much as 50%. Analytical Figures-of-Merit. Detection Limits. Limits of detection for the microarc-MINDAP system were determined at the 95% confidence level for eight elements of varying excitation potential. The method used is similar to that previously described (7)and is based on a noise value equal to the standard deviation obtained from 30 background traces ( N = 30, a = 0.05, t = 2.045) (25). The values reported in Table I are the averages of a t least five separate determinations of the detection limit. For comparison, Table I
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
Table I. Detection Limits (ng/mL) for the MINDAP Obtained with Microarc and Nebulization/Desolvation Sample-Introduction Systems
element
2.92 3.80 1.61 1.84 4.33 2.09 3.04 5.17
1.0 0.13 0.4 0.2 0.5 0.072c 56 166
Table 11. Comparison of Detection Limits (pg) Offered by Several Discrete-Sample-Introduction/MultielementAnalysis Systems microarc microarc microarc ETA element MINDAP Ar-MIPO Conv-ICPb ICPc
CaU) Ca(I1)
0.05
CU(U
0.13 0.4 0.2 0.5
0.16
0.036 56 166
0.01 0.38 0.92
NaU) Pb(1) Zn(1)
I
I
I
I
I
1.2 4.4 5.4 0.22 12 0.29 82 120
*All volumes were 1 pL, unless otherwise indicated. Reference 3. 0.5 p L sample volume.
Mg(U Mg(W
I
wavelength, excitation detection limits nm energy, eV microarc” nebulizerb 422.7 324.7 766.5 670.8 285.2 589.0 405.8 213.8
K(I) Li(1)
105,
1925
1.0
ETA AASd 1.0
0.06 3.9
0.81 0.045 0.4 3.6 280 0.12
100 20
0.5 0.4 1.0 0.02 0.4 0.4 0.7 1000
Microarc coupled to an argon-supported microwave-induced plasma (5). bMicroarc coupled to an inductively coupled plasma (ICP) (7). ‘Electrothermal atomization into an ICP (11). dCarbon furnace atomic-absorptionspectroscopy (26). includes detection limits obtained with the MINDAP coupled with a nebulizer sample-introduction system. The values in Table I are consistent with the general trend that elements with higher excitation potential have higher detection limits than those of lower potential. The results indicate also an improvement in detection limits when the analyte is introduced in a preatomized form by the microarc. Similar improvement has been reported (5, 7-9,24) when the microarc was used for sample introduction into other plasma systems. A comparison of detection limits offered by several discrete-sample-introduction/multielement-analysis systems is offered in Table 11. The microarc-MINDAP combination compares favorably with the far more elaborate and expensive systems. The increased sensitivity offered by the microarc over the nebulizer system is not surprising. The microarc concentrates the sample introduced into the plasma as a “plug” of analyte vapor much as a carbon furnace does in AAS. In addition, sample delivery to the plasma is nearly 100% efficient with the microarc whereas common nebulizers are a t best 5% efficient. One of the major advantages of using the microarcMINDAP is that it is mass sensitive rather than concentration sensitive. Therefore, preconcentration of sample solutions onto the microarc filament should improve detection limits even further. Finally, the separation of sample desolvation and atomization by the microarc promotes MINDAP stability and analyte excitation. When water vapor is introduced into the MINDAP, the plasma background and noise level increase, as shown previously (1,4). This shift in background level was noticed as early as 1917 by Strutt, who reported an increase in the NO band systems in the nitrogen afterglow upon the
Analyte Concentration C w g / m L )
Figure 4. Calibrationcurves for the microarc-MINDAP system. Curves for copper and lithium are superimposed: (A)i!n(I) 213.8 nm; (0)Cu(1) 324.7 nm; (6) Pb(1) 405.8 nm; (*) Li(1) 670.8 nm.
addition of water vapor (27). It has recently been suggested that the NH bands in the background spectrum are derived from the reaction of nitrogen with the dissociation products of water (28,29). Introduction of water vapor into the MINDAP might have the added consequence of quenchingthe production of excited nitrogen species necessary to sustain the nitrogen afterglow and which might be related to analyte excitation (30-32). Of the elements listed in Table I, lithium and sodium should not have been affected significantly by the influence of water vapor on background emission, since both lie in a low-background spectral region. Yet, the presence of water vapor hardly affected the detection limit for lithium, but reduced that for sodium by almost an order of magnitude. This change might be due to a transfer of energy to sodium from nitrogen in the first positive system of the Lewis-Rayleigh afterglow (33). Starr has demonstrated that vibrationally excited nitrogen molecules can transfer energy to the electronic levels of sodium by a collisional process (34). Working Curves. Figure 4 shows calibration curves for zinc (213.8 nm), copper (324.7 nm), lead (405.8 nm), and lithium (670.8 nm) with lines drawn to an order of magnitude above their respective detection limits. These curves have slopes of approximately unity over a concentration range of 3-5 orders of magnitude and are typical of most elements studied. The measurements at 3000 pg/mL were performed by using the preconcentrationtechnique (8);three 1-pL aliquota of 10oO pg/mL solution were dispensed onto the cathode before the arc was struck. Precision. The precision of the microarc-MINDAP combination was ascertained from five successive determinations of 10 ng of copper on each of five different days. The relative standard deviation on each day was between 3 and 7% whereas the reproducibility among days varied between 1and 4%. At least three factors appeared to affect the precision of the measurement: the electrode spacing and relative geometry, the position of the sample droplet on the cathode, and the surface characteristics of the sample electrode. These factors have been excellently characterized by Bystroff et al., who operated the microarc under different conditions than presented here (23). As a result, the qualitative discussion below will dwell on the effects the flowing nitrogen atmosphere has on these parameters. The electrode spacing affected the microarc characteristics greatly. If the electrodes were too close (1mm), the breakdown was not reproducible. The relative geometry of the electrodes influenced arc stability by controlling the position where the arc contacted the cathode. Greatest precision was found
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
C
2501
cn
2001
s
1501
I
1001
I 10
I 100
I
1000
1 B000
S o d i u m Concentration (wg/mL)
Figure 5. Sodium interference on 10 ng (1 pL of 10 pg/mL) of calcium at 422.7 nm. 300
-0
250
1
I
1
I
1
-
-
0 v) A
200 u
H v
s
e,
-
-
n
-
0
1
1501
Importantly, all the volume dispensed onto the electrode is analyzed, unlike the situation with nebulizer systems which are seldom greater than 5 % efficient. Because the microarc-MINDAP combination responds to sample mass and exhibits minimal matrix interferences, calibration curves can be prepared by using only a single solution standard. A broad linear dynamic range of 3-5 orders of magnitude is typical for these curves. Unlike the argon (5) and helium (9) MIPS, the microarc-MINDAP system can handle sample masses up to a t least 3 pg. This discrete sample-introduction system exhibits detection limits which are comparable to those produced by other microwave-inducedplasmas (5,9),inductively coupled plasmas (7, 8),and flameless atomic absorption (26) techniques. The system is virtually free from matrix (phosphate and sodium) interferences, unlike the ICP-microarc combination and nebulizer-MINDAP systems. The overall precision ranges between 3 and 7%. The operational features of this new combination make it convenient to use. The sample introduction and emission systems are inexpensive to construct and operate because of their low power requirements and the use of nitrogen for the plasma and sample-deliverygas. Both the microarc and detection electronics can be easily automated (5, 7)for accurate and precise measurements. Moreover, analysis time is short-approximately 1min from dispensing the sample until the computer finishes calculating and writing the results on a storage disk. Although inherent sensitivity and economy make this new combination attractive, it is by nature a discrete-sampling technique. Continuous sample introduction is therefore impossible and signal averaging is time-consuming. Fortunately, the MINDAP is dominated by low-frequency noise (4) in which time constanta longer than a second do not appreciably improve precision. Because the MINDAP is used in an emission mode, simultaneous multielement detection is possible even for discrete sample analysis. Other minor drawbacks presently associated with this system could be minimized. The precision and sample throughput rate could be improved further by measuring automatically the mass of the analyte dispensed onto the cathode using the described OH-band monitoring technique. The microarc-MINDAP combination could be improved also by optimizing the applied power and nitrogen flow rate for greatest signal-to-background ratio.
F 0.I 1 I0 100
1°8.01
PO4 / Ca M o l a r Ratio
Figure 6. Phosphate interference on 10 ng (1 pL of 10 pg/mL) of calcium at 422.7 nm.
empirically when the electrodes were aligned so the arc was directed at the tip of the hairpin-shaped cathode (cf. Figure 1). The position of the sample droplet on the cathode determined both precision and sample vaporization efficiency. The more centralized the droplet was at the tip, the more complete became the vaporization step. When a determination was attempted before the solvent was completely evaporated, two effects occurred: broad-band emission from the electrode material and difficulty of the sample aliquot adhering to the cathode. The droplet would bead-up on the wire and be blown off by the flowing nitrogen gas. Before the sample would adhere reproducibly to the cathode, between five and ten 1-s strikes of the arc were necessary. An investigation of the electrode surface might be able to help to explain these effects. Interferences. Two classical interferences on calcium emission were examined the ionization effect caused by the addition of sodium (Figure 5 ) and the vaporization effect from the addition of phosphate (Figure 6). The absence of interferences is evident from the constant calcium atom emission intensity as the interferent concentration is increased. The ionization interference is essentially absent even at a concomitant solution concentation of 1%. Similarly, vaporization effects are not present even at a phosphate/calcium molar ratio of 60. In comparison to the nebulizer system, which was severely influenced by the solution matrix (3),the microarc exhibited no interference even in the absence of releasing agenta. These results are consistent with previous work using the microarc (5) and are attributable to ita ability to separate and efficiently perform the processes involved in decomposing the sample into its atomic constituents. CONCLUSION The microarc-MINDAP system is an economical and sensitive spectroscopic tool for microvolume sample analysis. Sample volumes between 0.1 and 10 p L can be easily analyzed.
ACKNOWLEDGMENT The authors wish to thank J. P. Keilsohn and J. W. Olesik for their helpful discussions. Registry No. Ca, 7440-70-2;Cu, 7440-50-8; K, 7440-09-7;Li, 7439-93-2; Mg, 7439-95-4; Na, 7440-23-5; Pb, 7439-92-1; Zn, 7440-66-6; N2, 7727-37-9; phosphate, 14265-44-2. LITERATURE CITED (1) Deutsch, R. D.; Hieftje, G. M. Appl. Spectfosc., in press. (2) Deutsch, R. D.; Hieftje, G. M. Appl. Spectfosc., in press. (3) Deutsch, R. D.; Keilsohn, J. P.; HieftJe, G. M. Appl. Spectrosc., In
press.
D.;Hleftje, G, M. Appl. Spectrosc., In press. ( 5 ) Layman, L.; Hieftje. 0.M. Anal. Chem. 1971, 4 7 , 194-202. (6) Faik, H,; Hoffmann, E.; Jaekei, I.; Ludke, Ch. Spectrochlm. Acta, Part 6 1979, 346, 333-339. (7) Keilsohn. J. P.; Deutsch, R. D.; Hleftje, G. M. Appl. Spectrosc. 1983, 37, 101-105. (8) Deutsch, R. D.; Hieftje, 0. M. “Mlcroarc Mlcrosampilng for a Mini ICP”; I X Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, PA, 1982; Paper 179. (9) Zander, A. T.; Hieftje, G. M. Anal. Chem. 1078, 50, 1257-1260. IO) Nixon, D. E.; Fassel. V. A.; Knisely, R. N. Anal. Chem. 1974, 4 6 , 2 10-2 13. 11) Gunn, A. M.; Millard, D. L.; Klrkbright, G. F. An8kSt (London) 1978, 103, 1066-1073. 12) Dahiqulst, R. L.; Knoll, J. W.; Hoyt, R. E. 26th Pittsburgh Conference, 1975; Paper 341. 13) Ark, A.; Broekaert, J. A. C.; Leis, F. Spectrochlm. Acta, P a r t 6 1981, 366, 251-260. 14) Beenakker, C. I. M. Spectmhlm. Acta, Part 6 1978, 326,173-187. (4) Deutsch, R.
Anal. Chem. 1984, 56, 1927-1930 (15) Qulmby, 8. D.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1978, 50, 2112-21 18. (16) Frlcke, F. L.; Rose, 0.; Caruso, J. A. Talanta 1975, 23, 317-320. (17) Runnels, J. H.; Gibson, J. H. Anal. Chem. 1987, 39, 1398-1405. (18) Robbins, W. 8.; Caruso, J. A.; Fricke, F. L. Analyst (London) 1979, 104, 35-40. (19) Barett, P.; Copeiand, T. R. “Applications of Plasma Emission Spectroscopy”, 139, Barnes, R. M., Ed.; Heyden: London, 1979. (20) Ishizuka, T.; Uwamino, Y. Alia/. Chem. 1980, 52, 125-129. (21) Lichte, F. L.; Skogerboe, R. K. Anal. Chem. 1973. 45, 399-401. (22) Beenakker, C. 1. M.; Bosman, B.; Boumans, P. W. J. M. Spectrochim. Acta, Part B 1978, 338, 373-381. Layman, L. R.; Hieftle, G. M. Appl. Spectrosc. 1979, (23) . . Bvstroff, R. I.; 33, 230-240. (24) Layman, L.; Hieftje, G. M. Anal. Chem. 1974, 4 6 , 322-323. (25) Winefordner, J. D.; Vickers, T. J. Anal. Chem. 1984, 36, 1939-1946. (26) Instrumentation Laboratories, Publication AID 91, Wiimington, MA.
1927
(27) Strutt, R. J. Proc. R . SOC.London, Ser. A 1917, 93, 254-267. (28) Goudmand, P.; Pannetier, 0.; Dessaux, 0.; Marsigny, L. C.R. Hebd. Seances Acad. Sci. 1983, 256, 422-424. (29) Pannetier, G.; Goudmand, P.; Dessaux, 0.; Tavernier, N. J. Chlm. P h y ~ 1984, . 61, 395-406. (30) Lewis, E. P. Phys. Rev. 1904, 18, 124-128. (31) Strutt, R. J. R o c . R . SOC. London, Ser. A 1911, 85, 219-229. (32) Strutt, R. J. R o c . R . SOC.London, Ser. A 1913, 88, 539-549. (33) Wright, A. N.; Winkier, C. A. “Active Nitrogen”; Academic Press: New York, 1968. (34) Starr, W. L. J. Chem. Phys. 1965, 43, 73-75.
RECEIVED for review March 1,1984. Accepted April 23,1984. This work was supported by the Office of Naval Research and by the National Science Foundation.
Use of Organic Solvents for Inductively Coupled Plasma Analyses P. Barrett* and E. Pruszkowska Perkin-Elmer Corporation, Spectroscopy Division, 901 Ethan Allen Highway, Ridgefield, Connecticut 06877
The effect of important parameters in ICP, such as the incident power, nebulizer gas flow, and diameter of Inlector tube, on bgckground and analyte emission was investigated. By use of the C line, C,, CN bands and several atom and ion lines, changes In their intensity as a function of the parameters were investigated. Optimum condltions were chosen and several oil samples were analyzed. Results together with recommended values are presented.
effects of rf power, nebulizer parameters and the nature of the solvent on the detection limits and on spectral interferences were also discussed. The purpose of our study was to examine how parameters such as an incident power, nebulizer flow, and diameter of injector tube affect analyte and background emission using organic solvents. The main objective was good plasma stability for a variety of solvents without sacrificing analytical performance. A set of optimum conditions was chosen. Several elements were determined in oil samples using the optimized conditions.
The inductively coupled plasma (ICP) has become an important and widely used technique for multielement analyses. Its application has extended to a wide variety of matrices, including organic samples. Determinations of trace metals in oils have been reported (1-5). Not only metallic elements but also nonmetals such as S and P have been determined in oils (6,7). The analysis is simple and fast because oil samples diluted with an organic solvent are introduced directly into the ICP. Various solvents, including xylene, MIBK, kerosene, chloroform, methanol and others, have been used as diluents (8,9). Several workers have reported difficulties in sustaining a stable plasma, especially when volatile solvents were used ( 3 , 4 , 8 , 9 ) .With organic solvents, the ICP has also been used as a detector after separation by liquid chromatography (10, 11) and after extraction (12). In a study by Boorn and Browner (8) the quantitative effects of 30 common organic solvents on analytical signals obtained from a low power (1.75 kW) argon ICP were studied. The tolerance of an ICP discharge for organic solvents was discussed in terms of the “limiting aspiration rate”. The limiting aspiration rate for a particular solvent was defined as an uptake rate permitting stable plasma operation with no appreciable carbon deposition on the inner torch surfaces for a period of 1h. A reasonable correlation was found between limiting aspiration rates and evaporation factors for a number of solvents. In general, the ICP has decreasing stability as evaporation rates of solvents increase. This indicates that solvent vapor loading is the major factor influencing plasma stability with organic solvent introduction. Moreover, the
EXPERIMENTAL SECTION A Perkin-Elmer Model ICP/5500 inductively coupled plasma emission spectrometer equipped with the Model 3600 data system and a PR-100 printer was used. The instrument utilizes a 27.12-MHz72.5-kW generator and automatching network. A demountable torch, a dual-tube spray chamber, and a cross-flow nebulizer were used for all experiments. The spray chamber and nebulizer/end cap assembly are made by injection molding using Ryton. A Model 056 strip-chart recorder was used for monitoring analyte peaks and plasma background. A Rabbit peristaltic pump (Rainin Instrument Co.) introduced sample solution into the nebulizer. The nebulizer argon flow was monitored by a precision flowmeter with a needle valve instead of the gas pressure gauge supplied with the instrument. Standard solutions of elements were prepared from Conostan metalloorganic standards (Ponca City, OK) diluted with appropriate solvents. A Conostan S-21blended standard of 21 elements or single-element Conostan standards were used. Organic solvents used as diluents were reagent grade except for kerosene which was technical grade. The oil samples analyzed in this study were the following NBS Standard Reference Materials: 1634 and 1634a Trace Elements in Fuel Oil; 1084 and 1085 Wear Metals in Lubricating Oil; 1621a, 1622a, 1634a,and 1634 Sulfur in Residual and Distillate Fuel Oil.
0003-2700/84/0356-1927$01.50/0
RESULTS AND DISCUSSION A series of experiments were performed to show the effect of changing parameters on several atom and ion line intensities as well as on some carbon atom and band intensities. The investigation of carbon species allowed us to examine the efficiency of organic matrix destruction. Measurement of atom 0 1984 American Chemical Society
