organic solvent and phenolic compound or resin. This information should be considered in future separations. Based on overall results, it is assumed that the factors affecting the adsorption and elution behavior of the weakly acidic phenolic components are as follows ; the dissociation of the phenols which varies with the pH of the medium, differential complexing of Cu(I1)-phenolic compound, the dielectric constant which can be changed with the addition of an organic solvent, the molecular adsorptivities of the phenolic compounds o n the resin, and the interaction between organic solvent and resin or phenols. Separation of Mixtures. The elution order of phenolic compounds in 0.05M CuCls-organic solvent media is: maminophenol .-,homopyrocatechol + pyrocatechol e guaiacol + salicylaldehyde + m-methoxyphenol + resorcinol + salicylic acid + p-aminosalicylic acid e phloroglucinol (Tables I11 and IV). The results obtained in this experiment show that it is possible to separate each phenolic component from the mixture. To determine the optimum condition for the separation, the range of elution volume must be considered, because of the overlapping of the elution. In the copper-containing eluent media, the following phenolic components can be separated from the mixture; 1. Pyrocatechol (or m-aminophenol) H salicylaldehyde t) salicylic acid. [0.8 x 18.5 cm, 0.05M CuC12-45z EtOH (pH 1.3), 0.5-0.6 ml/min] 2. Pyrocatechol (or homopyrocatechol, m-aminophenol) tt guaiacol (or salicylaldehyde) t--) m-methoxyphenol (or resorcinol) (--f phloroglucinol. [0.8 x 18.5 cm, 0.05MCuC12-30z MeOH (pH 1.3)] 3. m-Aminophenol t--f m-methoxyphenol ++ resorcinol. [0.8 X 52.5 cm, 0.05M CuC12-30% PrOH (pH 1.3), 0.71.O ml/min]
4. Guaiacol ++ resorcinol t-f salicylic acid. [0.8 x 18.5 cm, 0.05M CuC12-45z MeOH (pH 1.3), 0.5-0.6 ml/min] 5. m-Aminophenol ++ p-aminosalicylic acid. [0.8 x 18.5 cm, 0.05M CuC12-45% acetone (pH 1.3), 0.5-0.7 ml/min]. In eluents without Cu(II), several components can also be separated, but the tailing phenomenon should be borne in mind and considered. It is as follows;
-
6. m-Aminophenol homopyrocatechol (or pyrocatechol, salicylaldehyde, m-methoxyphenol, resorcinol) tf phloroglucinol. [0.8 x 18.5 cm, 0.1M KC1-45z MeOH (pH 131 7. Guaiacol tf m-methoxyphenol (or resorcinol). (0.8 X 18.5 cm, 30% MeOH-H20) 8. Salicylaldehyde t-f resorcinol. (0.8 X 18.5 cm, 45% MeOH-H20).
Consequently, it is emphasized that copper-containing eluents were very effective in separation work. Elution curves for the separation of the mixtures are illustrated in Figures 1, 2, and 3. Phloroglucinol which is adsorbed strongly on the resin can be eluted quickly, if there were a change of the eluent as soon as the other phenols are eluted (Figure 3). ACKNOWLEDGMENT
It is a pleasure to acknowledge the financial assistance of Tae Sun Park, the President of Yonsei University, and the technical assistance of J. H. Yu.
RECEIVED for review June 12, 1972. Accepted September 19,1972.
Analysis of Solution Samples by Microwave Induced Plasma Excitation F. E. Lichte and R. K. Skogerboe Department of Chemistry, Colorado State Unicersity , Fort Collins, Colo. 80521 REPORTSFROM VARIOUS LABORATORIES have dealt with the analytical applicability of the low power, microwave induced argon plasma as a spectrometric excitation source (1-8). Many of these reports have emphasized the fact that relatively small quantities of sample can be introduced into the plasma per unit time. Excessive introduction rates extinguish the (1) H. E. Taylor, J. H. Gibson, and R. K. Skogerboe, ANAL. CHEM., 42, 876 (1970). (2) Zbid., p 1569. ( 3 ) F. E. Lichte and R. K. Skogerboe, ibid., 44,1321 (1972). (4) Zbid., p 1480. (5) K. F. Fallgatter, V. Svoboda, and J. D. Winefordner, Appl. Spectrosc., 25, 347 (1971). (6) S. Murayarna, H. Matsuno, and M. Yamamoto, Spectrochim. Acta, 23B, 513 (1968). (7) M. Yarnarnoto and S. Murayarna, Spectrochim. Acta, 23A, 773 (1967). (8) H. Kawaguchi, M. Hasegawa, and A. Mizuike, Specrrochim. Acta, 27B, 205 (1972).
plasma. This factor has consequently limited the general analytical utility and has precluded the direct analysis of solution samples unless the aqueous phase is largely removed prior to the plasma (5,8) or higher power in conjunction with specially designed coupling cavities is used (6-9). Another factor referred to in the literature which presents a problem involves the general difficulty in tuning the cavityplasma system to minimize the voltage-to-standing wave ratio ( I O ) . While such tuning is certainly not impossible, it is tedious and subject to significant variations from time to time. A design change in the Evensen cavity (11) which (9) H. Goto, K. Hirokawa, and M. Suzuki, Fresenius’ 2. Anal. Chem., 225,130 (1967). (10) J. M. Mansfield, M. P. Bratzel, Jr., M. D. Norgordon, D. N. Knapp, K. E. Zacha, and J. D. Winefordner, Spectrochim. Acta, 23B, 389 (1968). (11) F. C. Fehsenfeld, K. M. Evensen, and H. P. Broida, Rec. Sci. Imtrum., 36, 294 (1965). The cavity referred to is No. 5 in this
reference.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 2, FEBRUARY 1973
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M I C ROWAV E
POWER FINE TUNING S T U B
ARGON
I
t SAMPLE
I IMPEDANCE
/
AIR
COOLANT
-A' /
N
4
Figure 1. Diagram of cavity and tuning system
dramatically reduces the tuning problem and permits the emission spectrometric analysis of desolvated aqueous samples is reported herein. Detection limits which are highly competitive with several analytical techniques have been observed a n d potential interference effects are discussed. EXPERIMENTAL
Apparatus. The primary instrumentation utilized has been previously described (3). The desolvation system used was a slight modification of that described by Veillon and Margoshes (12) with the heated spray chamber operated at an optimum level of 160 "C. To permit the use of low gas flow rates consistent with optimal operation of the plasma, the nebulizer design reported by Valente and Schrenk (13) was utilized. The modified Evensen cavity is illustrated in Figure 1 and discussed below. Reagents. All reference solutions were prepared from A.R. grade metal salts or oxides. These were dissolved in nitric acid (doubly distilled in quartz) and diluted to appropriate volumes with distilled-deionized water. The final acid concentration for all solutions was maintained at 0.1M. Blanks were prepared in the same manner without the addition of the analytical element. Procedure. Sample solutions were analyzed by measuring the atomic emission from the plasma a t the respective wavelengths indicated below under conditions determined to be generally optimal for the elements studied. Because the transparency of the plasma containment tube changes significantly with time, a n end-on optical arrangement was utilized for all measurements. To facilitate the optical alignment and to maximize the analytical signals observed, the external optical arrangement provided for a magnification of 4 a t the entrance slit of the monochromator. RESULTS AND DISCUSSION
Cavity Modification. The changes in the cavity are shown in Figure 1. A primary improvement is obtained by running the plasma containing quartz tube axially through the cavity instead of using the more conventional transverse configuration. This change alone permits the ignition and maintenance of the plasma even when the argon support gas is saturated with water vapor. In the transverse coupling arrangement, the introduction of very small amounts of water vapor resulted in immediate quenching. The fact that the support
(12) C . Veillon and M. Margoshes, Spectrochim. Acta., 23B, 553 (1968). (13) S. E. Valente and W. G. Schrenk, Appl. Spectrosc., 24, 197 (1970). 400
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gas stream is subjected to the maximum microwave field a t the cavity center over a greater lineal distance offers the most plausible explanation for the improvement observed with the axial arrangement (11). A second modification is directed a t improving the cavity tuning to increase the microwave coupling efficiency and t o prolong the life of the magnetron. This tuning system also increases long term stability of the plasma and reduces the necessity of changing the adjustment for different analyses. Tuning is accomplished by matching the Q factors of the magnetron/coaxial cable and the plasma systems and is determined by minimization of the voltage-to-standing wave ratio. Although several of the tuning modifications previously reported (14, 15) have been examined, the approach depicted in Figure 1 was adopted because of the improvements obtained. In effect, the impedance matching device consisting of 20-gauge copper wire was attached to the fine tuning stub, insulated with glass tubing to prevent shorting to the cavity wall, and extended out from the cavity at nominally 45" to its axis. Experimental observations indicate that the standing wave ratio is minimized when the length of this wire is one-half wavelength--i.e., 6.1 cm for a 2450-MHz discharge. Addition of this device normally produces a standing wave ratio of unity for the argon stream but somewhat higher levels when sample solutions are introduced. By attaching a slider coupled to ground and moving it along the extended wire, the ratio can readily be reduced to unity when sample is introduced. A system tuned in this convenient manner maintains stability over very long periods and is usable for a wide variety of sample types and sample introduction rates. The combination of the axial configuration and the tuning modification have made it possible to sustain the plasma during the analysis of solutions. Analysis of Solutions. To optimize conditions for the analysis of solutions, the support gas flow rate, the microwave power level, and the temperature of the heated spray chamber (12) were systematically examined. Selection of the optimal gas flow rate was primarily dictated by the characteristics of the nebulizer used (13). At flows below 500 ml/min, nebulization was inefficient while succeedingly higher levels resulted in a gradual reduction in the emission intensity observed. At the same time, higher flows rapidly diminished a memory effect attributed to deposition of the analytical species on the walls of the plasma containment tube. Thus, a compromise flow of 900 ml/min was utilized throughout the study. At this flow, the pneumatic nebulization rate for aqueous solutions was 1.5 ml/min. In all cases, an increase in the input power produced a proportionate increase in the net line intensity of the spectral emission without a concomitant increase in the variation of the background signal. The power was consequently set a t 100 watts which is nearly the maximum output level of the generator utilized. Increasing emission intensity was obtained by increasing the temperature of the heated spray chamber. The effect was maximized when the temperature of the exit gases with solvent nebulization reached 140 "C. At chamber temperatures above 200 "C, a decrease in the emission intensities was observed due to a buildup in the internal pressure of the spray chamber and a concomitant reduction in the nebulization rate. All measurements were subsequently made at a temperature of 160 "C which was adequate for the total vaporization of (14) B. McCarrol, Rec. Sei. Instrum., 41, 279 (1970). (15) R. M. Dagnall, R. Pribil, and T. S. West, Analyst (Londotz),93, 281 (1968).
NO. 2, FEBRUARY 1973
0
h
Table I. Comparison of Detection Limits for Various Analytical Techniques Detection limits, pg/ml Previous Atomic Flame microwave absorption Wavelength, emission Radiofrequency plasma Element A" (22) plasma (23) (5, 6, 8,9, 18-21) (22) As 1937 0.2 ... 0.1 4 B 6 ... 0.03 0.03 2498 Cd 0.005 2288 2 0.03 0.02 co 0.005 2407 0.05 0.003 ... 0.5 2536 ... 0.02 Hg 0.03 Pb 4058 0.2 0: 008 1 0.5 Se 1960 ... ... ... V 0.02 4379 0.01 0.006 0.1 Zn 0.002 ... 0.009 0.02 2139 Wavelength used in present work. Limited by blank from quartz containment tube.
spray droplets without significant reduction in the nebulization rate. The memory effect referred to above and mentioned as a serious problem by Winefordner et ai. ( 5 ) was observed to have a maximum duration of one minute except for boron and selenium. This problem was eliminated by using a bypass system whereby the argon could be periodically passed through a reservoir of HC1. Introduction of gaseous HC1 into the plasma between samples results in a rapid cleanup except for selenium, in which case the continuous presence of HC1 was required. As indicated by Veillon and Margoshes ( / 6 ) , the memory effect within the desolvation system itself is greatly reduced through the use of acid solutions. The elements studied were chosen as representative of several different spectral property groups. Detection limits were defined as the concentrations required to produce a signal t times the standard deviation of the background where t is the t statistic at the 95 % confidence level for the n = 5 to 6 measurements used in computing the background variation. Table I presents the limits determined together with results reported for other similar analytical techniques. It may be noted that the analytical curves were linear over three orders of magnitude when plotted on rectilinear coordinates and that the measurement precision, as estimated by the relative standard deviation for 5-7 repeat measurements, was consistently between 1 and 8 %. To fully assess the analytical potential of the present system, studies of potential interference effects are required. While such studies are in progress and will be reported in depth a t a later date, the following preliminary observations can be made. With the exception of the intense O H band emission, spectral interferences originating from excitation of molecular fragment species (1-5) have been negated through the use of the vibrating refractor plate system described by Snelleman et ai. (17). Interference effects due to the presence of sodium reported by several investigators (6, 16, 18-21) are not prominent for the present system-i.e., emission intensities were not significantly affected even when the sodium concentration of (16) C. Veillon and M. Margoshes, Spectrochim. Acta, 23B, 503 (1969). (17) W. Snelleman, T. C. Rains, K. W. Yee, H. D. Cook, and 0. Menis, ANAL.CHEM.,42, 394 (1970). (18) G. Pforr and K. Langner, 2. Chem., 5, 115 (1965). (19) G. Pforr and V. Kapicha, Collect. Czech. Chem. Commun., 31, 4710 (1966). (20) S. Murayama, Spectrochim. Acta, 25B, 553 (1969). (21) I. Kleinmann and V. Svoboda, ANAL.CHEM.,41, 1029 (1969).
Present work 0.03 0.01* 0.0004 0.06 0.003 0.005 0.04 0.08 0.0006
the nebulized solution exceeded that of the analyte by a factor of 1000. The classical suppression of calcium emission due to the presence of phosphate is characteristic also of the microwave plasma but is eliminated by the addition of lanthanum. Enhancement of the measured signal due to the addition of organic solvents also occurs. The latter is attributed to both a n increase in the nebulization efficiency and changes in the energy characteristics of the plasma. The fact that, the microwave plasma serves as a n excitation source offering unusually good detection capabilities is a general indicator of its analytical potential. The comparative results presented in Table I indicate that the detection limits observed herein are enhanced by a factor of ten or more over those reported in previous microwave plasma (5,6,8,9,18-2/), flame emission (22), and atomic absorption (22) studies. F o r the elements studied herein, the microwave and the high power, radiofrequency (23) plasma results appear to be essentially equivalent in terms of sensitivity. A notable difference in the capabilities of the latter two systems involves the ability to determine elements having a tendency to form stable monoxides--i.e., monoxides with dissociation potentials above 7 eV. While such elements can be excited in the microwave plasma, the detection limits realized with the present system are poorer than those achieved with the radiofrequency system (23). Experimental evidence indicates that power levels above the 120-watt maximum for the generator available for the present work will be required to maximize the excitation of elements in this class. A generator capable of such power delivery is currently under construction. I n view of these observations, it may be inferred that the microwave plasma emission technique offers a useful and sensitive means for the solution of many analysis problems. To cite one example, those elements such as As, B, Hg, Pb, and Se not readily determined at low concentration levels by direct atomic absorption analyses can be determined at concentrations characteristic of many environmental analysis problems. The system is convenient to use, relatively inexpensive, and is likely to enjoy widespread utilization for the solution of a variety of analytical problems. RECEIVED for review July 28, 1972. Accepted October 5, 1972. Research supported by N.S.F. Grant Number GP21 306.
(22) E. E. Pickett and S. R. Koirtyohann, ibid., (14), p 28A. (23) G. W. Dickinson and V. A. Fassel, ibid., p 1021.
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