Table I. Precision a n d Results of Determination of Cu i n P l a n t Materials Flame' (1.8-9 sample) Plant materials
No. of samples
Mean, udg
Re1 std dev, %
HGA* (0.05-g sample)
No. of samples
Mean,
@IS
Wheat 3 2.22 2.6 6 2.18 Yellow lupin 4 4.65 1.3 5 4.65 Subterranean clover 4 5.30 2.0 5 5.26 Peas 4 6.39" 1. 7 6 6.46 100 Average mode. Calculated from mean of duplicate readings. 1.5-g samples.
H G A ~(0.01-g sample)
Re1 std dev, %
N o . of samples
Mean, uglg
Re1 std dev, M
3.1 2.2 1.6 3.3
6 5 5 6
2.29 4.91 5.37 6.41
4.2 3.8 2.6 1.5
and swirling with 9 ml of 3% HClOd; extract as before but use an ECKLI vortex mixer to mix vigorously after adding 1 ml of MIBK. Prepare the standards in test tubes so that they are extracted in the same way as the samples.
with a 95% confidence interval of f0.0036 pg Cu for a single determination. This variability in the blank may also account for the variability observed in recoveries of Cu added to three 0.01-gram samples. The means of duplicate recoveries, calculated from the grand mean of the flame and HGA analyses for each plant material, were 113, 106, and 98% for wheat, yellow lupin, and subterranean clover, respectively. These problems in accurately determining extremely small amounts of Cu could be overcome either by reducing the magnitude of the blank andlor by carrying a larger number of blanks through the whole procedure. However, the procedure described in this paper gives reasonable resuits with 0.01-gram samples and excellent results with 0.05-gram samples. The recommended procedure should be useful for analyzing Cu in small sections of plants. I t should also have application to the determination of Cu in other materials. Note added i n proof. Subsequent experience has shown that the variability in the blanks is due to occasional contamination in the test tubes. We have found it necessary to test each new test ,tube for Cu contamination by taking it through the whole digestion and extraction procedure. Those tubes which give an unacceptably high blank reading after two such treatments are rejected. We have also found that the transfer of the digested residue to a stoppered cylinder is not necessary. All procedures can be carried out in the same 20- X 150-mm test tube as follows. Digest as described: dissolve residue by warming
LITERATURE CITED (1) E. J. Underwood, "Trace Elements in Human and Animal Nutrition," 3rd ed. Academic Press, New York, N.Y., 1971, p 97. (2) 8. Sjollema, Biochem. Z.,267, 151 (1933). (3) J. E. Allan, "The Preparation of Agricuttural Samples for Analysis by Atomic Absorption Spectroscopy," Varian AercgraphNarian Techtron Publication, 1969, p 8. (4) J. S.Gladstones, J. F. Loneragan, and W. J. Simmons, Austral. J. Agr. Res., in press. ( 5 ) B. V. L'vov Spectrochim. Acta., 17, 761 (1961). (6) H. Massman, Spectrochim. Acta., Part& 23, 215 (1968). (7) W. B. Barnett and H. L. Kahn, Clin. Chem., 18, 923 (1972). (8) B. J. Stevens, Clin. Chem., 18, 1379 (1972). (9) P. Schramel, Anal. Chim. Acta., 67, 69 (1973). (10) H. J. M. Bowen, C.N.R.S. Int. Colloq. on Activation Analysis, C.E.N. Saclay, France, 2nd-6th Oct. 1972. (11) C. M. Johnson and A. Ulrich, "Analytical Methods for Use in Plant Analysis," Bull. Calif. Agric. Exp. Stn. No. 766 (1959). (12) C. S.Piper, "Soil and Plant Analysis," lnterscience Publishers Inc., New York, N.Y., 1947, p 274. (13) J. B. Willis, Aust. J. Dairy Techno/., 19, 70 (1964).
RECEIVEDfor review April 18, 1974. Accepted August 2, 1974. The authors gratefully acknowledge support for this project from a Rural Credit Grant of the Reserve Bank of Australia and from the Western Australian State Wheat Industry Research Committee.
Injection of Samples into Flames and Plasmas by Production of Volatile Chlorides R. K. Skogerboe,' D. L. Dick, D. A. Pavlica, and F. E. Lichte2 Department of Chemistry, Colorado State University, Fort Collins, Colo. 8052 1
A variety of analytical methods and techniques have been developed to solve problems requiring determinations a t inordinately low concentrations or on unusually small amounts of material. In atomic spectrometry, one approach often used has been based on rapid delivery of the sample to the atomization and/or excitation media. Thus, the concentrations of analytical species in the media per unit time are enhanced in comparison to those observed for continuAuthor to whom requests for reprints should be addressed. Present address, Environmental Trace Substances Laboratory, University of Missouri, Colombia, Mo. 65201. 568
ous sample delivery systems and proportionate increases in the analytical signals observed are realized. Included in the systems that rely on this enhancement are those used for nonflame atomic absorption (1-5) and the rapid vaporization devices used in areas such as plasma emission spectrometry (6-9). Generally, these systems are limited in terms of the amount of sample that can be utilized, and the sample size used ultimately limits the ability to determine lower concentrations. The devices used also typically vaporize the entire sample matrix, and this can cause interference problems which are dependent on the matrix composition.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
An approach has been developed which offers promise for alleviating some of these problems. It is based on the fact t h a t many elements in a variety of compound forms may be readily and quantitatively converted t o halides which are volatile and easily dissociated in most energy media. T h e simple system developed for this conversion has been interfaced with both a flame atomic absorption unit and a microwave induced plasma system for evaluation. T h e results indicate that many elements can be rapidly and conveniently delivered to atomization and/or excitation media such t h a t low concentrations or small absolute amounts of the elements can be determined. Relatively large sample sizes ranging upwards to 2 or 3 ml can be utilized, and separation of analytical species from the major matrix constituents can often be realized. In view of the preliminary results reported herein, this approach should find utilization in the solution of diverse analytical problems.
To Burnet
U
m-
I O variac
I
II
Teflon
.-Nebulizer
Chamber
~-AsbeStos
a-Nylon
Figure 1. Schematic of the chloride generator and associated gas
manifold
EXPERIMENTAL Apparatus. A Varian-Techtron, Model AA-5, atomic absorption
unit equipped with an air-acetylene flame was used for the absorption measurements. The microwave induced plasma system has been previously described (6-8). For both instruments, the readout was displayed on 0.5-second full scale response, variable range, strip chart recorders. Appropriate instrument operating conditions were selected for each element. The chloride generation apparatus and the associated gas flow system is depicted in Figure 1 . The furnace section of the system was prepared by winding the 10-mm i.d. quartz tube with Nichrome wire and insulating with asbestos over the region shown. The furnace is heated to the required reaction temperature by connection to a Variac. The quartz sample cuvettes are 9-mm o.d., i mm i.d., approximately 5 cm in length, and hold 2-3 ml of solution. For use with the flame system, the unit was inserted into the nebulizer chamber through a Teflon plug which replaced the conventional nebulizer mounting plug. The blow-out plug in the rear of the nebulizer chamber was removed to permit access of the airhydrochloride acid tube which was arranged to deliver the gas mixture to the interior of the sample cuvette. To permit changing of sample cuvettes without turning off the flame, the simple Teflon plug valve depicted was installed. The system used for plasma excitation was analogous to this, except that the plug valve was not required. The gas flows were controlled with micrometer needle valves which were calibrated us. flow meters. Reagents. All materials used were ACS reagent grade or better. Standards were prepared by dissolution of metals, metal oxides, or metal carbonates in an appropriate acid and making t o volume with 3M "03. Procedure. Standard and sample solutions were delivered to the cuvettes with a syringe fitted with a Teflon tube to avoid contact with the needle. The solutions were dried at approximately 110 " C on a hot plate or under an infrared lamp. T o initiate a run, the air-HC1 stream was diverted from the furnace through the primary burner air inlet, the plug valve was closed to prevent flame blow-back, and the sample cuvette was mounted through the rear entry port. Following a 30-second preheating interval at a temperature of 850 O C during which the plug valve was opened, the airHCl stream was switched to enter the furnace and pass over the hot sample residue surface. Within a fraction of a second after this flow initiation, a transient absorption peak was observed for the analyte of interest. The process used for the excitation measurements was essentially the same. The plasma was operated on argon and the HCl was introduced during the sample conversion-vaporization period by diversion of the argon stream through a reservoir of concentrated acid. Following calibration of the system, samples were analyzed by the same techniques with periodic standardization checks to ensure freedom from drift and/or changes in reaction-vaporization conditions.
RESULTS AND DISCUSSION The halide conversion approach used in the present study has previously been used as a means of delivering nonvolatile species t o a dc discharge for emission spectrometric measurements (10). T h e generalized reaction in-
volving the conversion of a metal oxide, carbonate, or salt t o the volatile chloride is: MnYm(s)+ excess HCl,,,
A
MCl,,,,
+
H,Y
While such reactions may not result in complete conversion in static reaction systems, the continuous removal of the metal chloride by vaporization into the gas stream of the present system should result in the total conversion of the analytical species which undergo the reaction. Since the present system can be operated at temperatures up to 1000 "C, it is projected that it is possible t o produce and vaporize all thermally stable metal chlorides with boiling points in this range. A survey of the literature indicates that more than 30 elements have the required properties. T h e results obtained on eight elements in the present study support this possibility. T h e chloride generator system depicted in Figure 1 was designed with simplicity and economy in mind. T h e unit can be readily installed in the AAS burner in a few minutes and is convenient to use. Optimization experiments were carried out to determine conditions which would result in rapid conversion and delivery of the analytical species t o the flame or the plasma. These indicated that temperatures in excess of 1000 "C resulted in the greatest delivery rate and therefore the highest analytical signal per unit concentration. Similarly, higher flow rates of argon-HC1 or air-HC1 sweeping over the sample surface produced greater analytical signals. Combinations of high temperatures and high flow rates were obtained which resulted in total sample delivery in periods of less than 0.5 second. These delivery rates approached the limiting response times of the readout systems and were not as reproducible as desired. For the flame system, a compromise reaction temperature of 850-900 "C and an air-HC1 stream flow rate of 600 ml/min produced transient absorption signals of 1-5 seconds' duration depending on the element in question. These signals could typically be reproduced t o f10% or better. Thus, these conditions were used throughout the investigation. T h e remaining air required to sustain a stoichiometric flame was introduced through the primary air inlet of the burner. For the plasma system, the same temperature was utilized with an argonHCl flow of 200 ml/min. T h e primary flow of argon used t o sustain the plasma was a n additional 2000 ml/min. For both systems, preheating of the sample prior to introduction of the HCl resulted in increased analytical signals. Extended preheating, however, resulted in vaporization of some analytical species without HCl present. A 20- to 30-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
569
Table I. Detection Limits Obtained via the Chloride Generation Approach Detection limits Element
Bi
0
5
IO
15
20
25
30
35
40
NANOGRAMS METAL
Analytical wavelength, nm
w/mla
Sanograms
223.1 228.8
0.0005 Cd l b 0.0005* 1 0.0005 Ge 265.2 5 0,003 Mo 313.3 5b 0.003b Pb 217.0 Zb 0.001b 405.8 2 0.001 Sn 4 0.002 T1 276.8 0.1 0.00005 Zn 213.9 1 0.0005 a Use of a 2-ml sample volume assumed. Values obtained with flame atomic absorption. All others obtained with the plasma emission system. 1
Figure 2. Example atomic absorption analytical curves (A)Pb 217.0 nm. (0) Mo 313.3 nm, ( 0 )Cd 228.8 nm
second preheat interval avoided this problem and produced results that were both more sensitive and reproducible than those observed without preheating. Example analytical curves for the atomic absorption system are presented in Figure 2. These demonstrate that lead, cadmium, and molybdenum could be determined a t the nanogram level. Curves obtained in both absorption and emission were typically linear over the range examined, but these were generally restricted to 1 to 2 decades. A summary of the detection limits obtained is given in Table I. These demonstrate that the approach offers a sensitive means of determination which compares favorably with other techniques. To obtain preliminary data regarding the general applicability of this approach, a series of natural water and animal tissue digests were analyzed by direct flame and by chloride generator atomic absorption. The dry tissues were dissolved in a nitric/perchloric acid mixture to obtain a solution nominally 2-3% in dissolved solids. The comparative results are summarized in Table 11. While the comparisons are limited, they imply that interference effects are not particularly prominent for samples of the type investigated. Further studies will be necessary to demonstrate this on a more comprehensive basis. Based on these initial investigations, the following potential advantages of the chloride generation approach for sample delivery may be advanced. The system permits the utilization of larger samples and could be redesigned to extend this capability. The analytical species are delivered to the atomization/excitation media in gaseous forms which are readily dissociated and without solid materials present which may cause interference problems, e.g., light scattering in atomic absorption. The inexpensive system provides sensitivity which is comparable to that achieved with nonflame atomization systems and thus precludes the necessity of acquiring such systems. I t permits the sensitive determination of elements such as molybdenum which are difficult to determine a t low concentrations with the more conven-
570
Table 11. Comparative Atomic Absorption Analysis Results Lead concentration, ug/ml Sample
Bone Lung Brain Water-1 Water-2 Water -3 Redistilled
Direct flame
0.3
