Anal. Chem. 1991, 63,772-775
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(3) Lawton, W. H.; Sylvestre, E. A. Technometrics 1971, 3, 617-633. (4) O'Haver, T. C.; Green, G. L. Anal. Chem. 1976,48, 312-318. (5) Gerow, D. D.; Rutan, S. C. Anal. Chlm. Acta 1988, 184, 53-64. (6) Tahboub, Y. R.; Pardue, H. L. Anal. Chem. 1985,57, 38-41. (7) Osten, D. W.: Kowalski, B. R. Anal. Chem. 1985,5 , 908-917. ( 8 ) Devaux, M. F.;Bertrand, D.; Robert, P.: Qannari, M. Appl. Spectrosc. 1988,42, 1020-1023. (9) . . Kvaiheim, 0. M.: Karstana. T. V. Chemom. Intell. Lab. Syst. 1989, 7, 39-51. (IO) O'Haver, T. C.; Begley, T. Anal. Chem. 1981,53, 1876-1878. (11) Cameron, D. G.; Moffatt, D. J. Appl. spectrosc. 1987,4 , 539-544. (12) Savitzky, A.; Golay, M. J. E. Anal. Chem. 1964,3 6 , 1627-1839.
(13) Manne, R. Chemom. Intell. Lab. Syst. 1987,2 , 187-197. (14) Karstang, T. V.; Eastgate, R. J. Chemom. Intell. Lab. Syst. 1987,2 , 209-219. (15) Kvalhelm. 0. M.; Karstang, T. V. Chemom. Intell. Lab. Syst. 1987, 2 , 235-237.
RECEIVED for review July 18,1990. Accepted January 3,1991. Norsk Hydro Ltd. is gratefully acknowledged for financial support to T.V.K.
Effect of Ascorbic Acid on Graphite Furnace Atomic Absorption Signals for Lead Shoji Imai* and Yasuhisa Hayashi Department of Chemistry, Joetsu University of Education, Joetsu, Niigata 943, J a p a n
Doublepeak atomic absorptlon dgnah for lead were observed in the presence of 1% (m/v) (mass/volume) ascorbic acld when the nonpyrolytlc coated graphlte (NPG) tube was used. The slgnal with double peaks could be separated Into two slgnais: The posltion of the first peak was In agreement wlth the posltlon of the slgnal In the absence of ascorblc acld udng the pyrolytic graphlte (PG) tube, and the posltlon of the second peak was In agreement wlth the posltlon of the signal In the absence of ascorbic acld uslng the NPG tube. A less porous and smooth surface was found by uslng scannlng electron microscopy on the NPG tube wail after pyrolyzing ascorbic acid. I t was consldered that pyroiyrlng ascorbic acid produced PG-coated sites on the NPG tube that lead to the appearance of the first peak.
INTRODUCTION Lead is one of the elements that has complex atomization mechanisms in the electrothermal graphite furnace. A signal with double peaks was observed in 1% (m/v) ascorbic acid by McLaren and Wheeler (I). They have also shown that the atomic absorption signal for lead is shifted to the lower appearance temperature (Tapp)in the presence of ascorbic acid and have suggested that signals with double peaks were caused by forming dimorphic forms of lead oxide (litharge and massicot). Salmon et al. (2) proposed that the appearance of signals with double peaks was caused by chemisorbed oxygen on the graphite. Regan and Warren (3) have reported that in the presence of 1% (m/v) ascorbic acid the temperature a t maximum absorption for lead can be reduced from 2370 to 1570 K without loss of peak height. Tominaga and Umezaki ( 4 ) have shown that the double peaks appeared in 0.05% (m/v) ascorbic acid, whereas 5% (m/v) ascorbic acid gives a single peak with a similar lowering of Tapp. Gilchrist et al. (5)have suggested that hydrogen and carbon monoxide released by the pyrolysis of ascorbic acid decrease the partial pressure of oxygen in the graphite furnace and thereby cause the equilibrium position of the reaction to shift to the right. Sturgeon and Berman (6) have proposed that pyrolyzing ascorbic acid forms active centers, which enhance the rate of scavenging of oxygen and lead to the lowering 0003-2700/9 110363-0772$02.50/0
Table I. Standard Atomization Conditions
stage
drying ashing atomizing* cleaning
temp"/OC 120 700
2500 3000
time/s ramp hold 30 20
0
0 0
2 3
5
inner gas flow/mL min-'
200 200 30 200
Values programmed in atomizer unit. If the optical temperature controller is used, the maximum current heating of 1400 K/s is carried out. of Tappof lead. They reported the Tappshift from 1045 to 955 K using the pyrolytic graphite coated graphite (PG) tube and from 1220 to 1010 K using the nonpyrolytic graphite (NPG) tube in 1% (m/v) ascorbic acid. They said that the active site provides active carbon for reduction. We found here, by scanning electron microscopy (SEM), that pyrolyzing ascorbic acid forms the pyrolytic graphite coating site on the NPG tube wall. In the present work, it was found that a lowering of Tapp for lead never appeared in 1% (m/v) ascorbic acid using the PG tube but did in 0.001 mol % / V ascorbic acid using NPG tube, and we discuss that the appearance of the double peaks was responsible for forming a pyrolytic graphite coating site on tube wall by pyrolyzing ascorbic acid. EXPERIMENTAL SECTION Apparatus. A Hitachi Model 2-8000 flame and graphite furnace atomic absorption spectrometer equipped with a Zeemann effect background corrector and an optical temperature controller system (Hitachi Model 180-0341) were used. Twenty microliters of sample solution was injected by an automatic sampler. The peak height and area were automatically printed out and displayed by using a Hitachi data processor. The analytical wavelength and slit width were 283.3 and 1.3 nm, respectively. An Oki personal computer if-800 Model 50 was used to record the absorbance signal profiles (20-ms interval). Output data from the optical temperature controller were acquired at 4-ms intervals by the personal computer and subsequentlystored on a diskette. The output data were calibrated by using the inner wall temperature monitored by a Chino Model IR-AH1S radiation thermometer and using an emissivity of one. For this thermometer, the wavelength is 960 nm and the accuracy is 0.5% to 1500 K, 1.0% to 2300 K, 2.0% to 3300 K. Reproducibility ( n = 10) of the ashing and atomizing temperatures for the NPG tube was 0.5% at 800 K and 0.3% at 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63,NO. 8,APRIL 15, 1991
TEMP.
TIME /
aec
Flgure 1. Variation of the atomic absorption signal of 100 1glL-l lead with the ascorbic acid concentration using the NPG tube. Ascorbic acid in % (mlV): (1) 0; (2) 0.001; (3)0.01; (4)0.1; (5) 1. Full scale of temperature: 273-3273 K. Atomizing stage: maximum current
heating.
Table 11. Appearance Temperature (Tapp) NPG tube ascorbic acid obsd
absence 1%"
ref 6
absence 1%"
(I
T,,,/K 1130 1030 1220 1010
PG tube T,,,/K 1510 1250
ascorbic acid
absence 1%'
absence 1%0
Tapp/K T,,,/K 1030 1020 1045 955
1240 1240
Mass/volume.
1419 K. A standard atomizer condition was shown in Table I. Each measurements was done 5 times. JEOL Model JMS T-100 and JCXA-733 scanning electron microscopes were used to examine the surfaces. The graphite furnaces were coated with 20 nm of gold by using a JEOL Model JFC-1100 ion sputter source. A Rigaku Model RAD-IIA X-ray diffractometer was used (Cu K a line) for the diffraction experiments. Reagents. An aliquot of a commerically available stock solution of Pb (1000 mg L-' in 1 mol L-' nitric acid solution, Wako Pure Chemical Industry Ltd.) was diluted with water to a suitable concentration before use as a standard solution. Lead monoxide of the massicot form of a commerically available reagent (Wake Pure Chemical Industry Ltd.) was used. Lead monoxide of the litharge forms was prepared from the massicot by heating for 4 days in boiling water. Assurance of the form was confirmed by powder X-ray diffraction. The other reagents used were of analytical reagent grade. Distilled and deionized water was purified by a Milli-&I1 system.
RESULTS AND DISCUSSIONS The dependence of ascorbic acid concentration on lead atomic absorption profile in the NPG tube under the standard atomizer conditions is shown in Figure 1. The peaks at lower and higher temperature are defined as the first and the second peak, respectively. Although Tominaga and Umezaki (4) have reported that signals with double peaks appeared in 0.05% (m/v) ascorbic acid, signals with a small shoulder were observed in 0.001% (m/v) ascorbic acid, which exists a t the position of the first peak. Tappand T,, values for lead using the NPG tube are shown in Table I1 with those using the PG tube. For the standard solution, Tap,,and T,, were 1130 and 1510 K in the NPG tube, respectively. In the presence of 1% (m/v) ascorbic acid, 1020 and 1250 K were observed for the first peak in the NPG tube. Figure 1 also shows that Tappis independent of the concentration of ascorbic acid when it is presence. The lower shift of Tappmay result from the activation of the graphite tube surface, and an increase in ascorbic acid concentration may result in the development of a ratio of active surface area to total surface area. The appearance of double peaks indicates the existence of two types of atomization mechanism. Sturgeon and Berman (6) reported in the
773
NPG tube that Tappis 1220 K in the absence of 1% (m/v) ascorbic acid and 1010 K in the presence. The difference between their Tapp and our TW in the absence of ascorbic acid may result from the difference in properties of the two graphites. However, our Tappin the presence of ascorbic acid is in good agreement with their Tappbecause of a "leveling" effect on the reactivity of the graphite surface (6). The observed 110 K shift in Tappis half of the 210 K shift reported in ref 6. Tappand T,,, were defined, respectively, as temperatures a t which a signal larger than SIN = 3 appears and the maximum absorption occurs. The appearance of double peaks in 1% (m/v) ascorbic acid has been reported by McLaren and Wheeler (I). They suggested that the double peaks result from the formation of the massicot and litharge dimorphic forms of lead oxide on the GF surface. Conformity to their suggestion should lead to the distinct profile of atomic absorption for the massicot and the litharge forms. The massicot and the litharge forms each gave but a single peak signal with Tappof 1080 and 1090 K, respectively, where a small m o u n t of lead oxide was introduced into the tube by using a Pt wire. The signals for both lead oxides are identical, which suggests the signals with double peaks do not result from the formation of the dimorphic forms of lead oxide. Signals with double peaks were not observed in volatile organic matrices, 0.1% (m/v) urea and phthalic acid, but we observed double peaks in less volatile organic matrices, 0.1% (m/v) phenylalanine, saccharose, and starch. That is, when a large amount of residue, carbon, is produced by pyrolysis of organic material, double peaks appear. Iwamoto et al. (7) proposed that the formation of the first peak in the signals with double peaks for tin compounds in n-hexane is caused by reductions of tin oxide by the active carbon formed by pyrolysis of n-hexane. Sturgeon and Berman (6) proposed that thermal destruction of organic reagents produces carbon and carbon-containing products, or the graphite surface is activated, enhancing the rate of scavenging of oxygen and decreasing the appearance temperature. In the present work, since the background absorption due to the concentration of ascorbic acid was observed before lead atomic absorption, most of the amorphous carbon produced by pyrolysis of ascorbic acid would have been released from the tube wall before the atomization occurs. After 100 atomization cycles, we observed a metallic luster similar to that of the PG tube on the inner wall of the NPG tube. The tube wall was observed by SEM. Figure 2 shows micrographs of the wall of the NPG tube (Figure 2a), the PG tube (Figure 2b), and the NPG tube after 100 atomization cycles were carried out by using 1% (m/v) ascorbic acid as a sample solution (Figure 2c). A porous and notched graphite wall was visible on the NPG tube, and a less porous and smooth graphite wall was visible on the PG tube. After the pyrolysis of ascorbic acid in the NPG tube, the NPG tube wall is similar to that of the PG tube wall. In order to investigate the effect of coating the tube wall with carbon on the atomic absorption signal for lead, after pyrolysis of 1% (m/v) ascorbic acid solution by 0,20, and 100 atomization cycles, a lead standard solution was injected and its atomic absorption signal was measured. The results are shown in Figure 3. With no atomization cycle, signals with a single peak were observed with a Tappof 1130 K. After 20 cycles, signals with a shoulder were measureable with a Tapp of 1040 K, which is in agreement with that for lead in the presence of 1% (m/v) ascorbic acid using the NPG tube. After 100 cycles, signals with only one single peak were measureable. This result indicates that the appearance of the first peak depends on the surface modification, PG coating, of the tube wall by pyrolysis of ascorbic acid. In the presence of ascorbic
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0
ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
W
0
z
U
m
CY 0
m m U
0
TEHPERRTURE / K
Flgure 3. Effect of pyrolysis of ascorbic acid on the atomic absorption signal. The lead standard solution was measured after pyrolysis of 1 % (m/V) ascorbic acid solution by atomization cycle. (1) 0 cycle, 20 pg L-': (2) 20 cycles, 50 pg L-'; (3) 100 cycles, 50 pg L-'. Atomizing stage: maximum current heating.
c 0
a z 0 CY
m
m
a
0
873
1273
1673
2073
2473
2873
TEHPERRTURE / K
Figure 4. Atomic absorption signals of lead. (1) 100 pg L-' lead in 1 % (m/V) ascorbic acid, NPG tube; (2) 100 pg L-' lead in 1 mol % / V ascorbic acid, PG tube; (3) 50 pg L-' lead, NFG tube; (4) 50 pg L-' of lead, PG tube. Atomizing stage: normal mode of 3000 OC programmed.
'I I
Figure 2. Micrographs of inner wall of graphite tubes by SEM with 6000 X magnification. (a, top) NPG tube. (b, middle) PG tube. (c, bottom) NFG tube after 100 standard atomization cycles with maximum current heating using % (m/V) ascorbic acid.
acid, Gilchrist et al. (5)also discussed a lower shifted Tapp with a gas-phase eqilibrium model. This model suggests that the hydrogen and carbon monoxide produced by the pyrolysis of
ascorbic acid decrease the partial pressure of oxygen in the graphite furnace and thereby cause the equilibrium position of eq 1to shift to the right (5). This model is not substantiated by Figure 3, because after 20 or 100 cycles atomization were performed, ascorbic acid no longer resides in the graphite furnace. A PG tube has a less porous and smooth surface wall than a NPG tube. Atomic absorption for lead was measured by using the PG tube and is shown in Figure 4 with that using the NPG tube. The Tappand T,, values in the PG tube were 1030 and 1240 K in the standard solution and 1020 and 1250 K in the presence of 1% (m/v) ascorbic acid, respectively. The Tappand 7'- values for the first signal in the presence of 1% (m/v) ascorbic acid using the NPG tube is in agreement with the signal using the PG tube. This result supports the hypothesis that the formation of the PG-coated site on the NPG tube wall by pyrolysis of ascorbic acid results in the appearance of the first peak. The kinetic approach is one of the most useful methods of comparing atomization mechanisms (7-19).Recently, Holcombe (20) discussed the use of Arrhenius-type plots employing the leading edge of the absorbance signal using the data from computer simulations and concluded that extraction of the energetics of gas-phase dissociation processes has been likely in error. However, in practice, the method has been applied extensively to the atomization process and has provided valuable information. Therefore, in the present work, we try to use of the method proposed by Chung (8). A signal for the first peak in 1%(m/v) ascorbic acid using the NPG tube was in agreement with a signal in the absence of ascorbic acid using the NPG tube, and the second peak signal in it was in agreement with the signal in the absence of ascorbic acid
AMI.
ct".
1901, 63,775-781
using the NPG tube, mentioned above. The activation energies (E,) and Tap*for the standard solution were 192 f 5 kJ mol-' and 1030 K using the PG tube and 120 f 8 kJ mol-' and 1130 K using the NPG tube. Sturgeon et al. have reported two E, values of 192 kJ mol-' from 1040 to 1160 K and of 117 kJ mol-' above 1160 K, where the temperature of 1160 K is the bending point in the Arrhenius plot (9). Our values of E, and TaPp. are in agreement with Sturgeon et al.'s values. Chakrabarti et al. have also reported a E, value of 125 f 7 kJ mol-' in the temperature range from 1840 to 2220 K (16). Therefore, the kinetic approach supports the existence of two types of atomization mechanisms. The appearance of double peaks was also reported by Salmon et al. (2) in the PG tube using a sheath Ar gas containing 1% O2 when ascorbic acid is absent. They account for their appearance from the viewpoint of the activity change of the graphite surface when using chemisorbed oxygen: various sites with different activities exist on the graphite surface; as more O2is chemisorbed at higher O2concentration, the formation of a stable surface oxide renders the most active graphite sites inactive; at this, the mechanism for the production of lead vapor shifts to the less active site, which requires a higher temperature for the reaction; a combination of different release mechanisms arising from the reduction and vaporization at the graphite site with different activities would account for the formation of the double peaks. We share their opinion that the appearance of signals with double peaks results from the existence of the different active sites of the graphite tube surface. However, it is not known at this time whether active sites in our system are similar to those in their system. It is concluded that when ascorbic acid was pyrolyzed in the NPG tube, some parts of the inner wall of the NPG tube
775
are coated with PG carbon. Lead analyte contacts two types of tube surfaces with different activities; the PG-coated site provides the first peak and the NPG site provides the second peak. ACKNOWLEDGMENT We gratefully acknowledge Humio Miyakoshi (Shin-Etsu Chemical Industry Co. Ltd.) for his assistance in obtaining the scanning electron micrographs. LITERATURE CITED (1) McLaren, J. W.; Wheeler, R. C. Analyst 1977, 702. 542. (2) Salmon, S. G.; Davis, R. H., Jr.; Holcombe, J. A. Anal. Chem. W81, 5 3 , 324. (3) Regan, J. T. D.; Warren, J. At. Absorpt. Newsl. 1978, 17, 89. (4) Tominaga, M.; Umezakl. Y. Anal. Chim Acta 1982, 139, 279. (5) Gllchrlst, G. F. R.; Chakrabartl, C. L.; Byrne, J. P. J. Anal. At. Spectrosc. 1980, 4 , 533. (6) Sturgeon, R. E.; Berman. S. S. Anal. Chem. 1985, 57, 1268. (7) Iwamoto, E.; Miyaraki, N.; Ohkubo, S.; Kumamaru, T. J. Anal. At. spectrosc. 1989, 4 , 433. (8) Chung. C. H. Anal. Chem. 1984, 56, 2714. (9) Sturgeon, R. E.; Chakrabarti, C. L.; Langford, C. H. 1978, 48, 1792. (10) Akman, S.; Genc, 0.;Ozdural, A. R.; Balkis, T. Spectrochlm. Acta 1980, 358, 373. (11) Frech, W.; Zhou. N. G.; Lundberg. E. Spectrochim. Acta 1982. 378, 691. (12) Smets, B. Spectfochim. Acta 1980, 358, 33. (13) Suzuki, M.; Ohta, K.; Yamakita, T.; Katsuno, T. Spectrochhn. Acta lS81, 368, 679. (14) Fuller, C. W. Analyst, 1974. 9 9 , 739. (15) L'vov, B. V. Specbpchim. Acta 1978. 338, 153. (16) Chakrabarti, C. L.; Wan, C. C.; Teskey, R. J.; Chang, S. 6.; Hamed, H. A.; Bertels, P. C. Spectrochim. Acta 1981, 368, 427. (17) Suruki. M.; Ohta, K.; Isobe, K. Anal. Chim. Acta 1985, 773, 321. (18) Frech, F.; Lundberg, E.; Cedergren, A. frog. Anal. At. Spectrosc. 1985, 8, 257. (19) Bass, D. A.; Holcombe, J. A. Anal. Chem. 1987, 5 9 , 374. (20) Holcombe. J. A. Spectrochim. Acta 1989, 448, 975.
RECEIVED for review July 31,1990. Accepted January 18,1991.
Double- Injection Flow Injection Analysis Using Multivariate Calibration for Multicomponent Analysis David A. Whitman,' Mary Beth Seasholtz, Gary D. Christian,* Jarda Ruzicka, and Bruce
R.Kowalski
Center for Process Analytical Chemistry, Department of Chemistry, University of Washington, Seattle, Washington 98195
A flow Injection analysls (FIA) system Is presented In whlch the reagent and sample are simultaneously Injected for multicomponent analysls. A sknple eight-port valve ls described to perform the double lnjectlon. The method Is Illustrated by the determlnatlon of nickel and Iron In a model platlng bath solution. First-order callbratlon methods lncludlng classical least squares (CLS), prlndpal components regresslon (PCR), and partial least squares (PLS) are used to analyze the complex tlme proflles that thls method provldes. Thls study shows that very moderate sampllng rates and small callbratlon data sets can be used wlth multlvarlate callbratlon technlques.
INTRODUCTION We have previously shown time-based selectivity in flow injection analysis (FIA) by injecting a large volume of sample Present address: Department of Laboratory Medicine and Pathology, University of Minnesota, U M H C 198, 420 Delaware St, Minneapolis, MN 55455. 0003-2700/91/0363-0775$02.50/0
(Figure la) into a singleline FIA manifold (I). In that method, selectivity is achieved by measuring an unreacted analyte in a portion of the sample zone in which no reagent overlap occurs; i.e., the dispersion coefficient is unity (at T3a in Figure la); a second analyte is measured where reagent overlap occurs (e.g., a t T4a). We present here a method in which time selectivity is obtained through simultaneous injection of both sample and reagent in a technique commonly called zone penetration (2). The method has the advantage of using sample and reagent volumes that are in the microliter range, thus being as small as those used in conventional FIA. Zone penetration is a variation of the merging zones method first described by Bergamin et al. (3)and also by Mindegaard ( 4 ) . Both groups developed the technique as a method to reduce reagent consumption and provided a conventional single peak response. In zone penetration, sample and reagent are simultaneously injected by using a single-line manifold whereas merging zones uses a two-line manifold. Figure l b shows a schematic diagram of the zone penetration principle. If the sample occupies the leading injection loop and the reagent in the second, the resulting sample/reagent overlap 0 1991 American Chemical Society
