Measurements of emission intensity along the length of plasma which is nominally 8 cm in length indicate that the intensity was maximized with an optical arrangement which sampled the central region over a one-centimeter length. Analytical curves obtained under the conditions above were linear over three orders of magnitude as previously reported (5). The measurement precision, as estimated by five replicate determinations on standards, ranges from 5 % (relative standard deviation) down to 2 at nominal signal-to-noise ratios of 20 and 100, respectively. The detection limit, estimated as the amount required to produce a signal twice the standard deviation of the background, is 6 X lo-” gram. Quantitative measurements on 10-ml samples containing 0.01 ppb mercury (10-10 gram) have been made with a precision of =t10-12 %. Data indicative of the accuracy of the method are given in Table 11. Solutions of each sample were prepared as outlined above and analyzed by both the emission and the cold cell atomic absorption techniques. Sample volumes of 0.2 ml were used for the emission analysis while 5-10 ml was required for the absorption measurements. Duplicate results generally agreed within 10 for both techniques indicating that the comparative results are consistent within experimental error. It should be noted that the emission analyses were carried out on amounts of mercury ranging from 0.34 to 12 ng (distilled water and leaf B samples, respectively), while the absorption measurements covered the absolute range of 17 to 60 ng. This is indicative of the relative analysis capabilities of the two techniques. Moreover, because it is reasonable to anticipate that the accuracy and precision of the
I 1
analysis will be largely determined by the magnitude of the emission intensity, it might be inferred that the accuracy should be approximately i10 % or better at lower concentration levels than those in Table I1 when larger sample volumes are used, Emission spectrometric analyses of water samples collected from remote mountain lakes and streams lend credence to this supposition. Results obtained on analysis of 30 such samples ranged from 0.07 to 1.8 ng/ml. Duplicate emission measurements made on 10-ml sample volumes consistently agreed within *lo% or better. Analyses of small samples of human hair also produced duplicate results that agreed within that increment. On the basis of these results, it may be concluded that the absolute sensitivity of the microwave excitation technique is approximately two orders of magnitude better than those reported by April and Hume ( 4 ) and by the users of the atomic absorption method (1-3). The technique permits the analysis of a variety of sample types at unusually low concentrations and the analysis of small samples where sample size is limited. The analyses appear to be accurate to * l o % or better. By using the background correction system (IO), the interference problem from nitrogen oxides associated with the cold cell absorption method (12) does not present a problem. RECEIVED for review November 15,1971. Accepted February 16, 1972. Research supported by NSF Grant No. GP-21306. (12) W. J. Adrian, A t . Absorption Newslett., 10,96 (1971).
CORRESPONDENCE
I
Determination of Trace Lead in the Atmosphere by Furnace Atomic Absorption SIR: The use of nonflame methods for atomic absorption has become increasingly widespread in the last few years as more applications of nonflame techniques are being published every month in the literature. Among these the graphite tube atomic absorption furnace as designed by Woodriff et al. has demonstrated its sensitivity and precision for a number of elements (1-5). The very great sensitivity resulting from the application of this instrument suggests its usefulness for the determination of elements in particulates in air samples. There are, at present, no sampling methods which are readily applied to nonflame AA determinations. Most filtration methods that might be employed would require ( 1 ) R. Woodriff and G. Ramelow, Spectrocliirn. Acta, 24B, 665 ( 1968).
(2) R. Woodriff and R. Stone, Appl. Opt., 7, 1337 (1968). ( 3 ) R. Woodriff. R. W. Stone, and A. M. Held, Appl. Spectrosc., 22, 408 (1968). 141 R. Woodriff. B. R. Culver. and K. W. Olson, ibid... 24,. 250 _
I
(1970). ( 5 ) R. Woodriff and D. Skrader, ANAL.CHEM., 43, 1918 (1971).
either an ashing step or a dissolution of the filtrate (6, 7). For most other techniques this does not introduce substantial errors. Furnace techniques, on the other hand, are so sensitive that small volumes of air can be used and small amounts of the element of interest are determined. Under these circumstances, errors of greater relative magnitude are introduced with pretreatment. Since no pretreatment step is necessary using the following method, these errors are eliminated. EXPERIMENTAL
Materials and Equipment. Air samples are filtered through a graphite crucible of the type used for carrier distillation ASTM No. S-3. The dimensions are roughly 16 mm long (6) M. Katz, “Measurement of Air Pollutants,” World Health
Organization, Geneva, 1969. (7) W. Leithe, “The Analysis of Air Pollutants.” Ann ArborHumphrey Science Publishers, Ann Arbor, London, 1970. ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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6
Ii
P
5
7
8
9
A IO
I1
n
y--+.-J o m 2
3
v, 4
5
6
7
T I ME
Figure 3. Plot of variation of lead concentration on the Montana State University campus on June 8,1971
5-
Figure 1. Teflon crucible adaptor II
nom
I
2
3
4
TIME
Figure 4. Plot of variation of lead concentration at North Seventh Avenue and Main Street on June 1 1 , l W l
Figure 2. Standard curve for lead
by 6-mm. 0.d. with the center drilled out 4.75 mm to a depth of 7 mm. A grade of graphite designated Spectro XA-3 was fabricated at Poco Graphite Inc., P.O. Box 2121, Decatur, Texas 76234. Its density is 0.94 to 0.96 g/cc with impurities of less than 2 ppm. The best description one can give for this graphite is that it contains a network of large pores (induced or functional porosity) in a graphite matrix which contains smaller pores (the incidental porosity). The mean diameter of the former is 1.4 microns while that of the latter is 0.4 micron. The volume per cent porosity is 55.3z and the larger pores are interconnected to the extent of 9 6 x . This graphite was further characterized (8) and developed to be used as electrodes in diffusion fuel cells (8,9). The nature of (8) D. A. J. Swinkels and R. N. Seefurth, J. Electrochem. Soc., 115, 994 (1968). (9) M. W. Reed and R. J. Brodd, Carbon, 3, 241 (1965).
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
this material makes it ideal for filtration of particles of the types and sizes usually found in air samples (6). The adapter which was designed to hold the graphite crucible for filtering is shown in Figure 1. A few prototypes were machined of acrylic plastic but these were difficult to clean and exhibited pronounced memory effects. These problems were completely eliminated by machining the adapters out of 3/4inch diameter Teflon (Du Pont) bar stock. The Teflon holder could be soaked in a number of acids without fear and exhibited no memory effects. Another advantage that was found using Teflon was that the threads were self sealing so the O-ring could probably be eliminated. Procedure. A graphite crucible is placed in the Teflon adapter and 100 to 500 cc of air are drawn through it using a plastic disposable syringe attached to the side arm with a piece of surgical tubing. The unit was checked for possible leaks by immersing it under water and applying pressure to it. No air seepage was detected either at the Teflon threaded connections or at the surgical tubing connector. If one is in the field, the graphite crucible can be stored in a titanium holder in a desiccator until one is able to perform the determinations. Titanium is used since it can be soaked in dilute nitric acid for cleaning purposes. We have found that cups can be kept for a day but no attempt was made to store them for longer periods. The amount of lead in the crucible is then determined by inserting it into a graphite tube furnace with an internal temperature of 1800 "C and recording the absorbance, For these determinations, the technique for background absorbance correction described by Woodriff and Shrader ( 5 ) was used to determine if there were any interferences due to broadband absorption or scattering.
RESULTS AND DISCUSSION Since no such interferences were present in air samples, future work will be performed with a single channel chopped source and a lock-in amplifier with a probable increase in sensitivity. The Glan-Taylor polarizers used in this system absorb in the 217 nm region, so the 283.3 nm line had to be used. This resulted in decreased sensitivity. For the standard curve shown in Figure 2, lead was added to the graphite crucibles as lead nitrate solution. One of the advantages of this type of furnace is the fact that chemical matrix effects are minimized and within experimental error the response for a given quantity of lead does not vary regardless of its chemical nature . Plots of the variation of the concentration of lead with time can be seen in Figures 3 and 4. Figure 3 represents samples taken on the Montana State University campus on June 8, 1971, which was during the week of final examinations, therefore traffic was not expected to be very heavy. The winds were out of the west at about 5 miles per hour and the temperature was about 75 OF. It can be seen from the figure that the levels of lead were rather low in the morning, averaging 0.1 microgram per cubic meter, but were generally higher in the afternoon with sharp increases at 3 : 05,4: 30, 5 : 00, and 5:30. These levels are generally consistent with the traffic flows that were observed. Figure 4 was taken at North Seventh Avenue and Main Street in Bozeman. Traffic from the interstate highway as well as traffic going to Yellowstone National Park must pass this point. The samples were taken on June 1 1 , 1971, and it was sunny, calm, and dry all day.
Again the levels of lead are consistent with observed traffic flows. The samples were taken during noon hour rush and there are noticeable increases in the lead concentrations at 12 : 00 and at 12 : 50. The sensitivity of this technique, based on a scale deflection of lx, is approximately 5 x gram per sample. Preliminary experiments with a standardization technique have resulted in a coefficient of variation of 1 . 2 z . This method of sampling should prove useful for most particulate impurities found in the air, within the limits of the sensitivity of the instrument and the size of the particles. The extreme sensitivity of the method of analysis combined with the sampling technique should make it valuable for spot checks as well as for determining the background levels in places where the concentrations would not be expected to be very high. ACKNOWLEDGMENT
The authors are indebted to Wayne Fagan of Poco Graphite Inc. for his technical advice,
RAYWGQDRIFF JEROME F. LECH Department of Chemistry Montana State University Bozeman, Mont. 59715 RECEIVED for review January 25, 1972. Accepted March 28, 1972. Work supported by Grant No. GP-28055 from the National Science Foundation.
Infrared Spectra of Stoichiometric Oxides of Vanadium SIR: Research at present in progress in this laboratory on the activity of vanadium-oxide-based catalysts has led to the recording of the infrared spectra of the oxides of vanadium, Vz03, V2O4, and V205. The spectra were recorded between 1100 and 300 cm-1 (which corresponds to the spectral zone in which the compounds exhibit absorption bands) directly on finely ground, pure powders, using a Perkin-Elmer 457 spectrograph. V Z O and ~ V204 were supplied by Schuchardt, V205 by Carlo Erba (RS). The spectra recorded for the three oxides are shown in Figure 1, and the positions of the bands and estimated peak intensities are summarized in Table I alongside the wavenumber of absorption maxima and intensities deduced, for the same compounds, from the figures of the well-known paper of Frederickson and Hausen ( I ) on the spectra of vanadium compounds. According to the data of Table I, particularly in the case of vzo3 and V204,the spectra obtained by us are quite different from those reported in the cited work ( I ) . In view of the fact that the data supplied by Frederickson and Hausen ( I ) have recently been used by many authors
Table I. Positions of Absorptions Maxima and Relative Intensities for V203,v204, and VzOs,in cm-' Present work v203
995 vs
Frederickson and Hausen' 980 w 860 vw 825 vw 800 vw
775 w 540 s VzOa 980 vs 890 vw 850 vw 810 vw
710 vw
vzoj
530 m 1022 vs 980 w 840 vs 665 610 vs 515
1020 w 980 w 950 w 880 s 850 m 725 m 1020 vs 830 vs
475 w (1) Leo D. Frederickson, Jr., and D. M. Hausen, ANAL.CHEM., 35,
a
See Ref. (I).
818 (1963). ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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