777
Anal. Chem. 1908, 58,777-780 (19) Welz, 8.; Melcher, M. Analyst (London) 1983, 708, 213-224. (20), Dedina. J.: Rubeska. I. Smcfrochim. Acta, Part 8 1980, 358, 119-1 28. (21) Jolly, W. L. J . Am. Chem. SOC. 1961, 83, 335-337. (22) Fazakas, J. Talanfa 1984, 37,573-577. (23) Kelth, L. H.; Crummett, w.; Deegan, J., Jr.; Libby, R. A,; Taylor, J. K.; Wentler, G. Anal. Chem. 1983, 55, 2210-2218. (24) Pierce, F. D.; Brown, H. R. Anal. Chem. 1977, 49, 1417-1422. (25) Dedlna, J. Anal. Chem. 1982, 5 4 , 2097-2102. (26) Craig, P. J.; Rapsomanlkis, S . J . Chem. Soc., Chem. Commun.
1982, 114.
.-
RECEIVED for review September 3,1985. Accepted December 2, 1985. This research was partially supported by Environmental Protection Agency Grant R 809416. Part of this work was presented at the 1985 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy in New Orleans, LA.
Factors Influencing the Determination of Molybdenum in Plant Samples by Electrothermal Atomic Absorption Spectrometry Michel Hoenig,* Yves Van Elsen, and Rent5 Van Cauter Institute for Chemical Research, Ministry of Agriculture, Museumlaan 5, B-1980 Tervuren, Belgium
The progresslve degradation of the pyrolytic graphite surface of atomlrers provides variable and mlsieadlng results of molybdenum peak-helght measurements. The changes in the peak shapes produce no analytical problems during the Ilfetime of the atomizer (-300 firlngs) when Integrated absorbance ( A .s slgnals) Is considered and the posslble base-line drms are controlled. Thls was demonstrated on plant samples minerallzed by simple digestion with a mixture of HNO, and H2OP The value of thls method was assessed by comparison with a standard dry oxldation method and by molybdenum determinations in Natlonai Bureau of Standards reference plant samples. The relative standard deviations ( n = 5) of the full analytical procedure do not exceed 7%.
Molybdenum is essential for plant nutrition. The molybdenum requirement by plants is species dependent, and its frequent determinations call for a rapid, reliable, and sensitive analytical method. Molybdenum can be determined in a nitrous oxide/acetylene flame by atomic absorption spectrometry (FAAS). At the 313.3-nm resonance line the characteristic concentration is about 0.2 pg-mL-I. Although the use of FAAS has gained some interest, the relatively poor analytical sensitivity of this method necessitates the use of various procedures to concentrate the analyte. Moreover, a number of interferences in the nitrous oxide/acetylene flame can alter the absorbance signal. These factors generally preclude the use of FAAS for the determination of molybdenum in plant matrices. In electrothermal atomic absorption spectrometry (EAAS) molybdenum is too refractory to be determined from the platform, and the determination is conducted from the wall of the new pyrolytically coated graphite tubes. The earlier graphite provides variable and misleading results with many matrices (I). Although molybdenum can be thermally pretreated up to about 1500 "C,without losses, the determination is not free from interferences, and the method of addition has to be frequently employed. The greatest inconvenience, however, was carbide formation reported mainly for older ET-atomizers with insufficient heating rates and with poor quality of tube coatings. This enhances the memory effects and reduces the absorbance. These effects can be further increased by additional carbon formation during the decomposition of biological material in the atomizer (2).
Table I. GTA Parameters for Molybdenum (20-rL Samples) char parameter
injection dry
temp, "C ramp, s hold, s Ar flow, L/min a
Max. heating rate.
80
3
115 1500 1500 20 20 10 4.6c 1.0 3 3 0
atomize cool 2700
80
0 . 6 " ~ ~ 12 4b
0
3
Integration time. Base-line integration
step. The recently reported sensitivities for molybdenum determined by EAAS range from 6 to 10 pg for 0.0044 AU (3-6). The aim of the present work is to adress some problems related to the determination of molybdenum in plant matrices by EAAS. In addition to the problems ascribed to both graphite tube condition and signal processing, the choice of a suitable mineralization procedure is also an important factor in successful molybdenum analysis. The temperature usually accepted for dry ashing of plant material is about 450 "C; it is high enough to completely oxidize the organic matter while the volatilization losses are negligible. The losses due to incorporation in the insoluble residue, whose occurrence is by far the most frequent, are mainly attributed to the presence of silica in the plants and occasionally to reactions with the inner surface of unsuitable dishes. The hydrochloric or nitric acids normally used to leach the ashes do not dissolve the silica or the elements associated with it. The results obtained after calcination and acid leaching can be highly reproducible in each laboratory, but large scatter is observed when results obtained from various laboratories are compared. This incomplete extraction by simple acid leaching, often ignored, can be eliminated by more adequate dissolution of the ashes. For this purpose, only the C.I.I. method (7), which consists of recalcination of the insoluble residue and removal of the silica by hydrofluoric acid treatment, can ensure an acceptable recovery. Unfortunately, the application of the C.I.I. procedure is laborious and time-consuming,and the use of platinum dishes is imperative, which makes this method restrictive for routine analysis. The C.I.I. method remains, nevertheless, the best criterion to validate other procedures for mineralization of plant samples. EXPERIMENTAL SECTION Apparatus. All analytical work was performed on a Varian AA-1275 spectrometer with a GTA-95 graphite furnace including
0003-2700/86/0358-0777$01 O./ O. 0 1986 American Chemical Society -. S.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1980
778
I
i
Table 111. Recovery of Molybdenum from Reference Plant Materials molybdenum content, peg-' (dry matter) found after found after certified dry ashingb digestionC
SRM (NBS)"
1571, orchard leaves 0.30 f 0.10 0.340 f 0.025 0.360 f 0.018 1572, citrus leaves 0.17 i 0.09 0.240 f 0.020 0.235 f 0.015 a Standard Reference Material (National Bureau of Standards). *C.I.I. method, n = 3. 'HNO,-H,O,. n = 5.
I
0.20
I
0
l
10
j
20 pg K ADDED
Figure 1. Synergic effect of potassium and sulfate ions on molybdenum (200 pg) absorbance signal: 0 , new tube (10-35 firings): 0, aged
tube (100-125 firings). Absorbance-time profiles shown in the two insets illustrate that both the peak height and the peak area are affected by this synergism. Table 11. Recovery of Molybdenum Added (100 pg) to Plant Samples"
2 015
t-1
a
Y
s
P Lu
I Y
a
010
recovery, % f std dev HN03-H202digestion
plant species
C.I.I. method
lettuce spinach beet leaves
101.1 f 5.2 97.3 f 2.6
98.8 f 4.9
96.4 f 2.9 102.2 f 3.1 100.9 f 5.4
" n of subsamples = 3 and n of analysis per subsample = 3.
a programmable sample dispenser. Pyrolytically coated tubes (Varian part no. 63-100002)were used. Absorbance-time profiles were recorded with a Hewlett-Packard 82905 A printer. In all experimentsthe 313.3-nm molybdenum line was selected; a lamp current of 5 mA and a spectral band-pass of 0.5 nm were used. The atomizer parameters are summarized in Table I. The main matrix elements of plant solutions have been determined by ICP spectrometry (Instrumentation Laboratory, IL-200). Mineralization Procedure. Dried plant material (75 "C during 16 h) was ground with a Fritsch Pulverisette planetary ball mill equipped with agate bowls and balls. A test run performed with cellulose powder shows no contamination of the plant material during grinding. Samples (1.0 g) were placed into 250-mL Erlenmeyer flasks. After addition of 3 mL of 65% HN03 and 3 mL of H202(both Suprapure, Merck), the mixture was gently boiled under reflux for 1 h, cooled, and filtered into 50-mL volumetric flasks using medium-sizefilters. The solutions were diluted with deionized-distilled water.
RESULTS AND DISCUSSION Mineralization Procedure. In this work, analysis of molybdenum in plant material was first started by a short digestion with a mixture of H2S04,"OB, and HzOzcurrently used in our laboratory to assure total recovery of numerous elements (8). However, sulfuric acid together with potassium, one of the major elements of the plant matrix, gives rise to a depression of the molybdenum absorbance signal. This is apparent both for peak-height and peak-area measurements (Figure 1). Considering the possible interference of sulfate ions present in the samples, the mineralization procedure was modified, Wet oxidation of dried plant samples with only HN03 and
nos
'
100
200
'
FIRINGS
TUBE LIFETIME
Figure 2. Peak-absorbances in molybdenum measurements during the graphite tube lifetime: (a)200 pg of Mo, 6% "0,; (b) plant matrix; (c) piant matrix + 200 pg of Mo. The calculated amounts of moiybdenum in the same piant sample by the addition method were found to be 140 and 240 pg (0.7 and 1.2 pg-g-' dry matter) at age 1 and 2, respectlvely.
HzOz resulted in a total recovery of molybdenum, despite the fact that this method does not yield a "complete" tissue digestion since the silica is not destroyed. This rapid procedure proposed here has been compared with the standard C.I.I. method on plant samples using known additions of molybdenum before mineralization. Table I1 shows that the results obtained with both methods are very close and indicate that the recovery is total. The accuracy and precision obtained with these methods for Standard Reference Materials (SRM) 1571 and 1572,the only reference plant materials with certified values for molybdenum, are shown in Table 111. In all cases the values determined fell in the range of certified limits. Furnace Determinations. According to Neumann and Munshower (6) the lifetime of pyrolytically coated tubes (Varian CRA-90) was reduced to approximately 60 firings for molybdenum determination. Similar results were obtained by Steiner and Ryan (9) who found in a modified CRA-90 that HC104and the high temperature used destroyed the pyrolytic coating after about 80 firings. A study of the atomization of molybdenum in various atomizers including carbon rods and commerciallycoated graphite tubes showed a lifetime of about 180 firings (4).The effect of the voltage setting on the mo-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4,APRIL 1986
lybdenum signal using the carbon rod atomizer (CRA-63,tube version) during the atomization step was studied by Barbooti and Jasim (5). They observed that with increasing voltage the molybdenum signal becomes sharper and higher with no significant change of integrated absorbance. In that study a better sensitivity was obtained by the peak-height method. Figure 2 demonstrates the dependence of peak-height measurements on the state of the tube coating. In a singleelement HN03 solution, the peak-height values decrease strongly after about 100 firings (curve a). Similarly, many workers reported previously a gradual decrease in the sensitivity of AA determinations of molybdenum when a furnace coated with pyrolytic graphite was used. This suggests a progressive destruction of the tube coating by repeated exposure to high temperatures and strong acids, resulting in increased porosity of the graphite surface (10, 11). In plant matrix, on the other hand, the signal is progressively enhanced (Figure 2b). A similar enhancement is observed if the same plant matrix is spiked with molybdenum: the enhancement concerns surprisingly only the part of molybdenum initially present in the sample (curve c). These anomalous results are not due to high uncorrected background signals generated by the matrix elements (the background absorbance controlled during the entire lifetime of the atomizer does not exceed 0.2 AU for 20 p L of plant samples). With an increased number of firings, the absorbance-time profiles of molybdenum in plant matrix show that the analyte signal becomes higher and sharper without change in the peak area. This observation indicates the modification of the atomization mechanism. As one can suppose from the fact mentioned above, the number of active sites on the graphite available for reaction with molybdenum strongly affects the measured absorbance signals. The availability of active sites should be influenced by the changing surface properties of graphite during the lifetime of the tube as well as by the presence of other species in the atomizer. The species might change the surface characteristics by interaction with graphite, or a reaction similar to molybenum, or creation of other available sites, or formation of interlamellar compounds with graphite that favor molybdenum atomization. In practical work, it is difficult to distinguish clearly between different mechanisms controlling the number of available sites, because these mechanisms are interrelated. Slavin et al. (1,3) presented evidence for the advantages of integrated absorbance signals using the Perkin-Elmer HGA-type furnaces. Since the rate a t which the analyte is vaporized is often controlled by the matrix, the peak height may vary with the matrix, in contrast to the integrated absorbance, which remains constant. On the other hand, the detection limits obtained by peak-height measurements are generally lower, and the determinations near the blank levels appear to be more reliable because they are less affected by the base-line alteration (12). The integrated absorbance measurements permit one to overcome the problems related to the changes in atomization rate of molybdenum with changing tube conditions (Figure 3). The effects of tube lifetime on the molybdenum signals are similar for samples in simple HN03 medium (curve a), in plant matrix (curve b), and in plant matrix spiked with molybdenum (curve c). In this situation, the progressive slow decrease of molybdenum signal during the tube lifetime can be easily corrected by a periodic one-point recalibration. The integration method is not yet entirely devoid of shortcomings. Small uncorrected changes in the base line are of minor importance in peak-height measurements. They may, however, affect the results when integrated absorbance is used due to the summation of the displacement throughout the integration period. The “base-line value” can vary not only
779
I
3 0.15 U Y
W
0 2
4
m C
u)
m 0.10 U
a
0 W L
l
100
Figure 3.
200 FlRlNQS TUBE LIFETIME
Integrated absorbances in molybdenum measurements
during the graphite tube lifetime: (a)200 pg of Mo, 6 % “0,; (b) plant matrix; (c) plant matrix 200 pg of Mo. The calculated amount of molybdenum by the addition method is the same during the entire tube lifetime: 160 pg (0.8 pg-g-‘ dry matter).
+
because of the progressive tube alteration, but it may also change at random, regardless of the sample nature. To avoid this, an additional control step was programmed just before the atomization step. The duration of this reading is equal to the actual integration time and the obtained base-line value is, if necessary, substracted from the atomization absorbance value (see Table I). A similar processing is used in the recent Perkin-Elmer spectrometers: this procedure is then called “Baseline Offset Compensation” or BOC ( 3 ) . The BOC approach improves significantly the accuracy of integrated peak absorbance signals (13). In the peak-area determinations of approximately 300 vegetable samples, with molybdenum levels ranging from 0.1 to 4 pgg-l, the mean value of the slopes of standard working curves was 0.00044 A.s/pg (range, 0.000 35-0.00055 A.s/pg). Due to this slight variation of working curve slopes, the application of standard addition method is recommended to ensure the best accuracy of the results. The full analytical procedure including the mineralization provided relative standard deviations from 3 to 7% depending on the magnitude of measured concentrations. The detection limits (2a) in complex samples seemed to be lower than 5 pg. By use of the experimental conditions summarized in Table I, the sensitivity varies from 8 to 13 pg/0.0044 Aes.
CONCLUSIONS Finally, the results obtained in this study suggests that EAAS is a very convenient and sensitive technique for plant molybdenum determination. No major analytical problems are encountered when the integrated absorbance is used and possible base-line drifts are controlled. The relatively long lifetime of the graphite tube guarantees numerous molybdenum determinations: the tested tubes provided a lifetime in excess of 300 firings. In addition, no memory effects were observed during our work. The fast atomizer heating rates and the improved quality of present tube pyrocoatings can
780
Anal. Chem. 1986, 58,700-705
ensure a quantitative volatilization of molybdenum during the atomization step. Furthermore, the proposed digestion method of plant material with HNO, and HzOzis simple, rapid, and effective for the mineralization of total molybdenum and is thus suitable for routine analysis. Registry No. Mo, 7439-98-7;K, 7440-09-7;SO4-, 14808-79-8; "03, 7697-37-2;HZOz, 7722-84-1.
LITERATURE CITED (1) Slavin, W.; Manning, D. C.; Carnrick, G. R. Anal. Chem. 1981, 5 3 , 1504- 1509. (2) Welz, 6. "Atomic Absorption Spectroscopy", Verlag Chemie: Weinheim, Fed. Rep. Ger., 1976; p 160. (3) Slavin, W.; Carnrick, G. R.; Manning, D. C.; Pruszkowska, E. Atom. Spectrosc. 1983, 4 , 69-83.
(4) Wan Ngah, W. S.; Sarkissian, L.L.; Tyson, J. F. Anal. Proc. (London) 1983, 20, 597-599.
(5) Barbootl, M. M.; Jaslm, F. Talanta 1981, 28, 359-364. (6) Neumann, D. R.; Munshower, F. F. Anal. Chlm. Acta 1981, 723, 325-328. (7) C . I . I . (ComitE Inter-Instltuts d'Etude des Techniques Analytiques du Diagnostic Follaire); Mathodes de raf6rence pour la dhrmination des 6l6ments min6raux dans les v6g6taux; IIi6me Coli. Eur. Med. Contr6ie Alim. Plant. Cult.: Sevilla, 1968. (6) Hoenig, M:; De Borger, R. Spectrochim. Acta Part B 1983, 388, 873-880. (9) Steiner, J. W.; Ryan, K. M. Analyst (London) 1984, 109, 581-583. (IO) Sturgeon, R. E.; Chakrabarti, C. L. Anal. Chem. 1977, 49, 90-97. (11) Sneddon, J.; Fuavao, V. A. Anal. Chim. Acta 1985, 767, 317-324. (12) Hoenig, M.; Scokart, P. 0. Anal. Lett. 1984, 77, 1947-1962. (13) Barnett, W. B.; Bohier, W.; Carnrick, G. R.; Slavin, W. Spectrochim. Acta, Part B in press.
RECEIVED for review July 1, 1985. Accepted October 3,1985.
Determination of Copper and Zinc in Human Head Hair by Inductively Coupled Plasma Atomic Emission Spectrometry with a Direct Sample Insertion Device C. V. Monasterios, A. M. Jones,l and E. D. Salin* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6
The analysls of slngle strands of halr wlthout dlgestion Is demonstrated uslng a direct sample lnsertlon devlce and Inductlvely coupled plasma atomlc emission spectrometry. Copper and rlnc are determlned In three donors and the resuits compare well with those of conventlonal methods. The detection limits for various elements in a Img halr sample are estimated to range from 0.2 to 10 ppm in the solld form.
Human hair offers a number of important advantages as a biopsy material (1-3). The sample can be readily obtained without trauma to the patient, is easily collected, and requires no special equipment or storage procedures. Hair does not readily deteriorate and can be stored until it is convenient to perform an analysis. Its trace element composition reflects the trace metal composition of the medium from which it was formed and so serves as a historic record of body metabolism and exposure levels (4). Trace element concentrations in hair are related to those in blood serum and urine but are usualy at least 10 times higher ( I ) . Hair analysis can provde valuable information in case of systematic intoxication by trace metals whether induced by deliberate means or by the environment (5-8). Nutritional status (9)can be monitored by hair analysis, and the diagnosis of elementally related diseases (10) is also possible. Human hair is naturally a solid sample. After conversion to a liquid form by digestion a number of procedures have been used to analyze hair. All of these methods may suffer from contaminationfrom reagents, solvents, and the container as well as loss of volatile elements by evaporation (11). In most cases the sample is significantly diluted, thereby degrading the trace detection capability of the method. Solid hair 'Present address: Department of Chemistry, University of British Columbia, Vancouver, BC, Canada.
sampling methods of analysis are highly convenient. They usually require smaller amounts of sample, and solid samples are easy to handle, store, and transport. Total anaysis time is often shorter since no chemical pretreatment of the sample is required. Unfortunately, most solid analysis methods have their own limitations. Sample inhomogeneity and matrix effects may cause errors in analysis, and the preparation of standards can be inconvenient. Flame atomic absorpotion (FAA) is the most widely used technique for hair analysis after conversion to liquid format (4,12-15). Inductively coupled plasma (ICP) atomic emission spectrometry (AES) has become popular because of its multielement capability, high sensitivity, wide dynamic range, and relative freedom from matrix effects (16-18). A number of recent studies have utilized ICP-AES for multielement analysis of hair samples (19-22). A microwave plasma configured for AES has also been used to analyze small samples that had been vaporized from a tungsten filament (23). Alder and co-workers have used nonflame (furnace) atomic absorption (NAA) for the direct analysis of solid hair samples by placing hair segments or powder directly into the furnace followed by ashing and atomization (24-29). They studied 13 elements in 1-cm sections of hair. Their detection limits in 100 wg of hair ranged from 0.03 to 10 ppm for Mn and Zn, respectively. It was our intention on the initiation of this project to exploit a sample introduction technique for ICP-AES called the direct sample insertion device (DSID) first reported by Salin and Horlick (30) and shortly thereafter by Ohls and Sommer (31). This device has since been used extensively by Kirkbright and co-workers (32-35) and our laboratory (3, 36-38). In summary, this device allows a probe to be inserted into the central core of the plasma along the axis normally used for the aerosol central channel. The probe has been either a graphite electrode (30-32,38) or a wire loop (36,37). It was our expectation that human hair would provide a
0003-2700/86/0358-0780$01.50/00 1986 American Chemical Society
