Anal. Chem. 1981, 53, 1866-1872
1868
heterogeneous kinetic characterizations which was originally developed for irreversible systems has now been extended to provide access to the heterogeneous electron transfer kinetic parameters of redox systems which exhibit any degree of electrochemical reversibility. The impetus for the development of the method reported here derives from the need to quantitatively study heterogeneous electron transfer reactions of biological molecules as well as electrocatalytic processes at OTEs.
Crawley, C. D.;Hawkrldge, F. M. Blochem. Blophys. Res. Commun. 1981, 99, 516. Winograd, N.; Blount, H. N.; Kuwana, T. J. Phys. Chem. 1969, 73, 3456. Christie, J. H.; Lauer, G.; Osteryoung, R. A. J . Nectroanal. Chem. 1964, 7 , 60. Butler, J. A. V. Trans. Faraday SOC. 1924, 19, 729. Butler, J. A. V. Trans. Faraday Soc. 1924, 19, 734. Erdey-Gruz, T.; Volmer, M. 2.Phys. Chem., Abt. A 1930, 150, 203. Evans, J. F.; Blount, H. N. J. Am. Chem. SOC. 1978, 100, 4191. Seellg, P. F.; Blount, H. N. Anal. Chem. 1978, 48, 252. Kuwana. T.; Wlnograd, N. In “ElectroanalyticalChemistry”; Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 7. von Benken, W.; Kuwana, T. Anal. Chem. 1970, 42, 1114. HIII, R. In “Modern Methods of Plant Analysls”, Peach, K., Tracey, M. V., Eds.; Springer-Verlag: New York, 1956; Vol. 1, p 393. von Stackelberg, M.; Pllgram, M.; Toome, V. 2. Nektrochem. 1953, 57, 342. Hawkrldge, F. M.; Kuwana, T. Anal. Chem. 1973, 45, 1021. Kuwana, T.; Helneman, W. R. Acc. Chem. Res. 1976, 9 , 241 Kuwana, T. Ber. Bunsenges. Phys. Chem. 1973, 77, 856.
ACKNOWLEDGMENT The experimental expertise of E. F. Bowden, C. D. Crawley, and J. S. Sidwell is gratefully acknowledged.
LITERATURE CITED ( I ) Albertson, D.E.; Blount, H. N.; Hawkrldge, F. M. Anal. Chem. 1979,
51, 556. (2) Bowden, E. F.; Hawkridge, F. M.; Blount, H. N. Bloelectrochem. Bloenerg. 1980, 7 , 447. (3) Bowden, E. F.; Wang, M:; Balley, J. W.; Hawkridge, F. M.; Blount, H. N. J. Nectrochem. SOC.,in press. (4) Bowden, E. F.; Hawkridge, F. M.; Blount, H. N., submltted for publication in Adv. Chem. Ser. (5) Thomas, C. E.; Hawkrldge, F. M.; Blount, H. N. “Abstracts of Papers”, 182nd National Meetlng of the Amerlcan Chemlcal Society, Atlanta, GA, March 29 to April 3, 1981; Amerlcan Chemical Soclety: Washington, DC, 1961; ANYL 44.
RECENEDfor review May 4,1981. Accepted July 6,1981. This work was supported in part by the University of Delaware Institute of Neuroscience (NIH Biomedical VII), the North Atlantic Treaty Organization, the National Science Foundation (Grant No. PCM79-123481, and the National Institutes of Health (Grant No. GM27208-02).
Direct Determination of Manganese in Seawater with the L’vov Platform and Zeeman Background Correction in the Graphite Furnace G. R. Carnrlck, W. Slavln,” and B. C. Manning The Perkln-Elmer Corporatlon, Main A venue, Norwalk, Connectlcut 06856
Problems In the determinatlon of Mn In seawater were used as a model to study the graphite furnace system at steadystate temperature. Several factors had to be controlled carefully to obtaln rellable results agalnst simple standards that were Independent of sallnlty and variations In matrix composltlon. Use of the L’vov platform and integratlon of the absorbance signal reduced the sensltlvlty to matrix composltlon. Pyrolytlcally coated graphite reduced variations that depend upon the life of the tubes. The tubes appeared to fail by lntercalatlon of the Na or NaCl matrlx. The char temperature must not vary outslde the range of 1100-1300 O C . Zeeman background correction permitted use of larger seawater samples. The detectlon llmlt of the procedure using 20-pL samples was 0.1 pg/L ( 2 pg) Mn. By use of the Zeeman background corrector, less than 0.02 pg/L Mn was detected in seawater uslng a 75-pL sample. Seawater samples can be processed in less than 30 mln per sample.
The determination of Mn in seawater using graphite furnace atomic absorption spectrometry has been investigated by many workers (1-9). Seawater has been found to be difficult to analyze because of the matrix. If the matrix is atomized along with the analyte, the result is a large background signal which is often beyond the correcting capabilities of current instrumentation. The presence of large amounts of chlorides 0003-270018 110353- 1866$0 1.2510
has also been shown to provide interferences (10, II), usually making direct analysis difficult. To reduce the problems associated with the determination of Mn in seawater, most workers either have used matrix modification ( I , 4,5,9) or have extracted the metal from the seawater matrix (3,7). Few workers have been successful with the direct determination in seawater after volatilization of the matrix during the char program step (1,2, 6,8,12). Slavin and Manning (13) have shown that by using a furnace at steady-state temperature (the L’vov platform), the interference of a salt matrix on Pb, Cd, and T1 was greatly reduced as long as the background signal was within the limits that the deuterium arc background corrector could handle. The same authors (14,15)have recently shown that chloride interference upon the determination of Mn was very small when a furnace at steady-state temperature was used. A review of those papers which report a direct determination of Mn in seawater reveals a contradictory situation and a very difficult analysis. Probably the earliest effort to determine Mn directly in seawater using furnace atomic absorption was by Segar and Gonzalez ( 2 ) in 1972. They attributed the reduced sensitivity for Mn in a seawater matrix to covolatilization of some Mn with the salt matrix. More recent work suggests that this reduced sensitivity is a vapor-phase binding of a portion of the Mn by C1 (11, 14). In another early paper, Ediger et al. (16) used the HGA-2100 to detect less than 0.5 bg/L of Mn in seawater using direct injection. Their experiments established that the decrease 0 1981 Amerlcan Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
in signal for Mn (and Cu) in the presence of large amounts of NaCl during the atomization cycle was a chemical i n t e r ference, not covolatilization as had been proposed by earlier workers. Ediger et al. showed that it wag necessary to char away as much a8 possible of the seawater matrix to get maximum sensitivity for Mn and to be free of interference. Using the HGA-2100, Segar and Cantillo (6) developed a direct method for Mn (as well as Fe, Cu, and Cd) in seawater. Their detection limit for Pidn was about 0.3 pg/L, presumably in seawater. The conditions they recommend were very well chosen and their optimization procedures showed considerable insight into furnace interfferences, considering that the work; was published in 1975. Only ordinary graphite tubeci were available and they found that, as the tube aged, the analytictil signal fell linearly at a rate of 50% per 100 firings. Since variations in salinity produced relatively large changes in signal, the method of standard additions was required. McArthur (4) preferred to use “,NO3 matrix modification to determine Mn in seawater. Most of his paper involved the charring process. He fourtd considerable salinity dependence if charring was too rapid. There was considerable change in the salinity dependence iwith the age of the tubes. In spite of the success of the earlier workers, Kingston el, al. (7) were unable to determine Mn directly in seawater with the HGA-2100 furnace. They resorted to extraction on Chelex 100, followed by stripping into HNOp The “,NOS matrix modification technique was used by Montgomery and Peterson (9) for the determination of Mn (as well as Cu and Fe) in seawater using the HGA-2100 furnace. They showed that the pyrolytically coated tubes they used deteriorated very rapidly using the combination of ]“,NO3 and seawater. Manganese was determined in seawater (with Cu and Co) by Hydles (5) after adding 1% ascorbic acid to the sample. He used the HGA-2100 furnace and found significant loss of Mn from seawater between 600 and 900 “C. The direct furnace method of Sturgeon et al. (1) for Mn (and Fe and Zn) was very similar to the method of Segar and Cantillo (6,8). The Sturgeon detection limit was 0.2 pg/L for Mn in seawater, using 20-pL samples in the HGA,-2200 furnace and pyrolytically coated graphite tubes. They found a loss in sensitivity during the life of the tubes. They had to use the method of additions to accommodate small residual interferences. Klinkhammer (3)apparently used direct determination andl prepared standards by adding measured amounts of Mn to deep sea samples. However, the Klinkhammer paper focused upon an 8-hydroxyquinoline extraction into chloroform and his direct determination was used to confirm the extraction method. Both “dissolved” and “total” Mn are of environmental interest. Analysis of filtered samples or procedures which use chelation or ion exchange provide a measure of “dissolved” metal levels in seawater. In contrast, direct measurement of unfiltered samples provides “total” Mn levels. A recent summary of the literature (17) indicated that the base line level of total Mn in unpolluted deep sea samples was expected to be about 0.05 yg/L. Coastal and polluted waters range upward from that. Numerous trace metal measurements in coastal waters (12, 18) have reported levels ranging from 0.3 to 50 pg/L. We describe in this paper a direct method for the determination of Mn in seawater at levels above about 2 pg/L using a 20-& sample. For concentration levels lower than 2 pg/L we use the Zeeman/5000 (19,ZO). We discuss the control of the problems found by previous workers. We use ordinary pyrolytically coated graphite tubes and tubes that are coated by a new process that provided less variation in the analytical signal as the tube aged (21).
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Table I. Parameters for Determination of Mn in Seawater“ program step 3 4 5 2 (dry) (char) (atom.) (clean) (cool) 350 1200 2300 2700 20 1
temp,”C ramp, s 1 10 0 1 1 hold, s 45 70 5 6 20 int gas, 300 300 0 300 300 mL/min recorder 75 0 a Sample volume, 20 pL; wavelength, 279.5 nm; slit, 0.2 nm; integration time, 5 s.
EXPERIMENTAL SECTION Instrumentation. All tests were performed either on 15 Perkin-Elmer Model 5000 equipped with an HGA-500 furnacle or a Zeeman/5000 system. An AS-40 autosampler was used with the HGA-500. The experimental conditions are shown in Tabbe I. Pyrolytically coated graphite tubes and L’vov platforms (Perkin-Elmer part no. 0290-2310 and 0290-2311, respectively) were used. New pyrolytically coated tubes (Perkin-Elmer part no. B009-1504) were also tested (21). The Data System 10 was used to study peak shapes (22). When the Data System was used., the peak area and peak height data were calculated by a software program and printed out on a report. The base line was determined by averaging the value of the signal for 1s prior to initiation of the atomization voltage. An Ircon optical pyrometer was used to measure temperatune on the outer w d of the tube. Atomization and char temperatures were set by firing the HGA-500 and adjusting the set temperatune until the pyrometer indicated the desired temperature. Temperatures above 900 “C were measured by the pyrometer. Some of this work has been done with 50-pL aliquots of seawater. When a single 50-pL aliquot was dispensed onto thle platform, the dry and char times were increased to 90 and 140 s, respectively. The background signals indicated inadequatle matrix removal in this arrangement so we added multiple aliquots of 25 pL, drying (45 s) and charring (70 s) after each aliquot,. Materials. The methods development work utilized a coastal seawater sample given to us by the National Research Council, Canada. The sample was not acidified or filtered. It was stored in a polyethylene bottle. We acidified 200-mL aliquots to pH 1.6 in Ultrex grade HN03 (J. T. Baker Chemical Co.). The two Sandy Cove samples and the Bermuda sample werle also given to us by the :NationalResearch Council. They had an approximate salinity of 32%0and were collected from relatively unpolluted surface water. They were filtered through a nominal 0.45-pm membrane filter and acidified to pH 1.6 with Ultrex nitric acid. The Sandy Cove No. 8 sample is sample B from the paper by Sturgeon et al. (23). The Mn standards were prepared from a 1000 mg/L standard from Alfa Products, Ventron. A 50 mg/L intermediate standard was prepared in 1%HN03, Ultrex grade. The aliquots added to seawater for standardization generally contained 50 and 100 wg/L*Mn. Deionized water (Continental Water Systems) was distilled in a Corning “mega-pure” still. Procedure. The most serious problem encountered in determining very small amounts of Mn in seawater was the contamination of the sample by airborne particulates and the contamination of laboratory plasticware used in the analyses. Polyethylenebottles which were used to store standards, seawater, and distilled deionized water were cleaned by using a procedure recommended by Klinkhammer (3). A small amount of concentrated “OB was swirled in each bottle. Then the bottles were fiied with distilled deionized water and allowed to stand overnight. The bottles were then rinsed several times in deionized water followed by a single rinse in distilled deionized water. Sampb cups and pipette tips were soaked overnight in a Pyrex beaker in 20% HN03. After the cups and tips were rinsed several time13 in deionized water, followed by a single rinse in distilled deionized water, they were dried in an oven at 50 “C, and covered with
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
P a r a f i i for storage. The sample cups were rewashed and reused. The pipette tips were further rinsed several times with either seawater or distilled deionized water immediately before use. Standard polystyrene as well as polyethylene and Teflon cups were all washed in 20% "Os. The polyethylene cups gave lower blanks and more repeatable results than did the polystyrene or Teflon cups when distilled deionized water was used as a sample and were therefore used exclusively. Klinkhammer (3) also found that hard, linear polyethylene plasticware was suitable for this determination. We have done this work in a laboratory that does not have the clean-room facilities usually expected as required for seawater analysis (24). To detect occasional contamination of sample by airborne dust, we always delivered two separate aliquots of the sample into separate sampling cups. If the analyses differed by more than lo%, we assumed there was contamination and the sample was rerun. Graphite furnace analyses are occasionally subject to a random error resulting from a system malfunction. To detect these, we repeated the determination on each cup at least three times. A random result clearly variant from the others was rejected and the remainder were averaged. The standards were prepared by adding known amounb of Mn to a seawater sample having about 1pg/L of Mn. The additions provided several levels up to about 5 pg/L Mn. The slope of the working curve was found by subtracting from each addition the signal from the unspiked seawater, as if it were a blank. It is necessary that seawater samples be acidified to avoid loss of Mn to the walls of the polyethylene sample cups used on the A S 4 autosampler. In an experimenttypical of others, each 20-pL aliquot of the NRC sample was spiked with 100 pg (a total of about 6 pg/L) Mn. The sample was not acidified. In the interval between reanalysis, the concentration decreased about 0.4 pg/L per hour. In contrast, the acidified samples did not change in absorbance during the same intervals on the same sample table. It may be necessary to shake the sample prior to analysis. This is especially important for unfiltered samples, since some of the Mn is bound to particles which settle slowly.
RESULTS Ruggedness Testing. We have explored many of the variables that might introduce errors into the determination of Mn in seawater. In earlier experiments we charred with a 10-5 ramp to 1200 "C and held that temperature for 40 s. To be more sure that the matrix was more fully charred away we increased the char time to 2 min after the ramp. These experiments did not indicate a need for the longer char time, so we finally settled upon a char time of 70 s. We have not systematically optimized the char time. Our previous experience with interferences has supported the use of gas stop during the atomization step. Van den Broek et al. (25) and others have warned that in some cases the furnace is not really operated a t stopped flow when the settings indicate this condition. We tested whether gas was flowing during the atomization cycle by disconnecting the tube from the furnace and measuring the gas flow by displacement of a liquid from a graduated cylinder. Repeating the test several times, sometimes a small flow was detected, about 10 mL/min. We therefore tested the magnitude of error that would be introduced by this uncertainty in whether gas flow was fully stopped. Using the experimental conditions in Table I and the Data System, we measured a seawater sample. To each 20-pL aliquot of the seawater we added 100 pg of Mn, providing about 125 pg of Mn in each aliquot. The absorption profiles on the Data System 10 were indistinquishable when an argon flow of 30 mL/min was compared with zero flow. The average signal for three firings with zero flow was 0.133 abs-s. At 30 mL/min flow the average signal was 0.128 abs-s. The total spread of the points used for the calculations was about 0.01 abs-s. Thus, the effect of expected variation in the gas flow was probably less than 5%. In the same set of experiments we used a higher char temperature (1400 "C). The average of two repetitive firings was
a. I ~ T
,2800 WALL TEMPERATURE
->k
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~,
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;k,
_ _-"E 5 :$S E C >
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Atomization temperature. The temperature of the furnace wall is plotted along with the Mn absorbance trace which those conditions produced. In the left-hand panel, max power heating was used to 2700 O C . In the rlght-hand panel, max power was used to 2300 Flgure 1.
OC.
I
zot 8Ao
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1100
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CHAR TEMPERATURE
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Char temperature. The effect on the atomization signal is plotted as a functlon of different char temperatures for an aqueous solution Containing 2.5 pg/L Mn as well as for an Undiluted seawater sample containing about 1.5 pg/L Mn. The peak area signals were normalized. The background signal for the seawater sample for the same char temperatures is plotted against a scale on the right. Figure 2.
0.115 abs-s. Presumably some of the Mn was lost at the higher temperature, but even this variation produced a change of only 15%, quite consistent with the findings of Sturgeon (1) and with our data shown later. Atomization and Char Temperature Studies. It was important to determine the correct atomization temperature using the platform technique. Slavin and Manning (15) recommended a temperature not too much higher than the appearance temperature of the analyte metal, although the temperature is not critical. Figure 1 shows the temperature data superimposed on the Mn signal plot from the Data System 10. Even from the platform a t the 2700 "C atomization setting, Mn appeared while the temperature was rising. However, at the 2300 "C setting, the furnace had almost come to temperature before atomization started. Thus, atomization at 2300 "C was preferable. The large amounts of NaCl present in seawater are reportedly volatilized below 950 "C (26). As long as the metal to be determined has an appearance temperature high enough, most of the NaCl can be removed during the char cycle. The results of a char temperature study with the L'vov platform are shown in Figure 2. Manganese peak area signals for an aqueous Mn standard (2.5 pg/L) as well as for undiluted seawater (1.5 pg/L Mn) are shown. The background signals for 20 pL of undiluted seawater are shown, assuming a maximum absorbance of 2.4, although, because of photometric nonlinearity, the real value is probably much larger. The
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
temperatures on the abscissa were the optical pyrometer readings from the outside of the tube. The background signal began to drop precipitously at a temperature of about 1000 "C and was about 0.15 absorbance above 1080 "C. Since the major component of the background was NaC1, this correlates adequately with the literature (26). The same experiment was repeated im a new pyrolytically coated tube with results that were very similar. We believe that the plateau in the seawater curve at about 1200 "C was real since it appeared with both tubes. Sturgeon et al. (I)reported that the Mn loss from seawater was significant only abolve 1350 OC. Our results showed some loss of Mn at a temperature close to that where most of the matrix had been removed. There was a much more rapid loss of Mn from aqueous standards. Hydes (5) found smaller signals for Mn when seawater was charred at 1000 "C than at lower temperatures, indicating loss of A h from the seawater. Thus, we have shown that there is a narrow range of ternperature between about 1050 and 1300 "C where charring must take place. Even in that range, our d a h showed a small loss of Mn during charring. McArthur (4) reported that, at constant setting, the wear of the furnace tube caused the ternperature to rise by as much as 150 "C per 100 firings over the whole temperature range from 900to 2000 "C. In our previous work we had typically monitored the atomization temperature with an optical pyrometer. In this work we were forced to monitor the char temperature. We set the char temperature by correcting the setting until the optical pyrometer indicated the desired temperature. We changed the setting in the same way if the pyrometer monitor signal indicated that the temperature had drifted more than 100 "C. Since the char temperature was critical, we have monitored it during several life studies. Usually, but not always, the temperature remained within a spread of 50 "C during the 200- 400 firings life cycle of the tube. The Problem of a Seawater Blank. We have established that large changes in the matrix have only a small effect upon the recovery o f added Mn. This lends confidence to the assumption that the signal observed for Mn in seawater is entirely attributable to Mn. Still, there idi concern for whether some portion of that signal is some kind of blank. We have therefore conducted an experiment to expose a potentid blank. The instrument system we have used measures the double-beam signal using the Mn hollow cathode as source. The nonatomic absorption siignal is separately measured by using a continuum source (the deuterium arc). The difference in these two double-beam measurements is calculated each cycle s). Differences from theoretical performance olf either optical system might yield a small positive or negative net signal. This potential error can be exposed by using a narrow line source at a slightly different wavelength from that emitted by the Mn lamp. This alternative source, when corrected by subtracting the signal frlom the continuum lamp, should yield no signal in spit0 of the presence of the seawater sample. We used the Mg ion line at 279.6 nm, very close to the analytical Mn line at 279.5 nm. The Mg line was supplied by a multielement lamp containing Mg. We used the Data System 10 data handling system. There was no signal from the Mg ion line using solutions containing 0.1% and 3.5% NaCl or for seawater. Manganese Recovery. For the analysis of seawater, the use of simple standards was preferred over the method of additions. By use of a pyrolytically coated tube and a char temperature of 1085 O C , working curves were prepared by spiking distilled deionized water and seawater with known amounts of Mn. All results were corrected for the peak height absorbance obtained on unspiked samples of distilled deionized water and seawater. The slope of the working curve from
1889
1.0r
O.*
t SALINITY,
*/e
NACL
Flgure 3. Losses of Mn as a function of salinity, measured as % NaCI.
the distilled water standards was about 25% less than that from the seawater standards, indicating that aqueous standards were unacceptable for seawater analysis. Recovery experiments were conducted by spiking various dilutions of seawater with Mn. The dilutions ranged from pure seawater to distilled water and were calculated in perclent salinity, seawater being assumed to be 3.5% NaC1. The sample volume was 20 pL and the spike was such as to add 100 pg to each 20-plL sample. Each dilution was prepared in duplicate to detect contamination and the unspiked blank was also prepared in duplicate. Since there were six dilutions, there were thus 24 samples in all. Each sample was analyzed four times and the data were averaged. Each series therefore consisted of about 100firings. The results showed no variation in absorbance with salinity above about 0.05% NaC1. The salinity loss experiments were repeated with slightly different conditions on several occasions using different tubes and platforms, always with similar results. Figure 3 represents the data from the 50-pL variation of the method but there was virtually no difference in the several salinity loss plots. The loss of Mn for aqueous solutions was the same result found in the working curve experiment discussed above. In subsequent experiments we have shown that it is the IMg present in the seawater that reduces Mn loss at char temperatures above 1200 "C. The addition of about 0.1% R4g(NO& to samples of low Mg content removed the lomes shown in Figure 3. Preparation of Mn standards in 0.1% Mg(NCQ2 is probably preferable to making up the standards in Seawater. Detection Limit. Aliquots of distilled deionized water (containing 0.5% Ulltrex "OB) were spiked with decreasing amounts of Mn to determine the lowest amount of Mn dlistinguishable from base line noise. By use of parameters slightly different from those in Table I, a detection limit of about 1 pg was obtained for Mn. This detection limit is in agreement with that reported by Fernandez and Iannarone (27) for similar instrumentation. The detection limit for Mn in 20 pL of undiluted seawater was determined by measuring the standard deviation of a series of measurements for several seawater samples containing about 1pg/L Mn. The standard deviation corresponded to about 1pg which, using the usual 2u rule, provides a detection limit of about 0.1 pg/L in seawater. An estimate of the detection limit of the 50-pL method on the Model 5000 is provided by the statistical analysis of 15 repetitive determinations of the Bermuda sample whose concentrationis about 0.1 pg/L Mn. With data from the D&a System 10, the average peak height was about 0.018 absorbance with a coefficient of variation of about 8%, or about 0.01 pg/L. This correspondsto a detection limit of about 0 02 pg/L Mn (1pg) in the 50-pL seawater sample using the 2a rule. Figure 4 shows the 0.1 pg/L Bermuda sample and the 75-,uL method on the Zeernan/5000. It suggests a detection liniit better than 0.02 pg/L Mn in seawater. This is consistent with
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
I
0
3 0 TIME
3
CSEC>
Figure 4. Absorbance profiles on the Zeeman/5000 for the Bermuda sample which contained about 0.1 pg/L Mn. Three 25-pL aliquots were used for the profile on the right. The same program with an empty tube is shown on the left.
rr
0.2
I /
ZEEMAN/S00@
Figure 5. Absorbance profiles for various arrangements using the Sandy Cove No. 9 sample which contalned somewhat less than 1 pg/L Mn. With the conventional background corrector, the 20-pL method is compared with the signal from a 50-pL aliquot. Signals from the Zeeman/5000 instrument with the same sample are shown below. The 50-pL sample for the Zeeman analysis was added as two 25-pL aiiquots. HGA-500 furnace. The simplicity and ruggedness of the method are enhanced by using the L'vov platform, new pyrolytically coated graphite tubes, and area integration of the absorbance signal. Standards are prepared by adding known quantities of Mn to a seawater sample. The difficulties reported by earlier workers are removed or reduced by the combination of the above conditions. The constancy of the absorbance signal is better than previous workers report and that comes about partly because of the more stable conditions provided by the platform and partly by the better coated tubes and the use of area integration. We have noted that the tube failure was probably due to intercalation of sodium or some of the other constituents of the seawater. The intercalation did not occur until the pyrolytic layer was broken. Thus, the tube life could be extended if the pyrolytic layer were continuouslyrenewed, as suggested by Clyburn et al. (28). We have done this by adding to the automatic program in Table I a 15-9 step at 2300 "C during which Ar containing 5% methane flows at 50 mL/min. This temperature may not be optimum to coat the entire length of the tube but, since failure occurs in the center, it should continually recoat the portion of the tube that could be expected to fail first. Since most of the tubes we used survived 200-400 firing without recoating, we removed the recoat step from our routine program. However, the tubes were variable with respect to life. We would not expect this recoating technique to be necessary for the new pyrolytically coated tubes (21). For samples that range lower than 2 pg/L, the new Zeeman background correction system is extremely advantageous, providing reliable detection limits to at least 0.02 pg/L. Partly this improvement results from the ability to use larger samples (75 pL) without upset of the absorbance peaks. Partly it results from the fact that higher Mn lamp current can be used with the Zeeman system, thus providing better signal-to-noise ratios. This better signal-to-noise advantage of the Zeeman background corrector applies to those elements where, using conventional background correction, the brightness of the source must be limited by the brightness of the continuum lamp. Since only a single source is used for backgroundcorrected analyses using the Zeeman technique, the available signal-to-noise is limited only by the brightness of the analytical lamp. This advantage of the Zeeman technique will apply to those analyses performed in the spectral region between about 260
ANALYTICAL CHEMISTRY, VOL.
Y
a 60
a
L
‘0
50
1bO
150
2bo
I 250 300
# FIRINGS
Figure 6. The slgnal repeatability of a seawater sample during the life of the pyrolytically coated graphite tube. The iiverage of 5 firings is plotted every 50 firings. This tube failed after 310 firings.
and 400 nm. This includes such important determinations as Al(309.3 nm), Cu (324.’7 nm), Ag (328.1 nm), Cr (357.9 nm), Mo (313.3 nm), P b (283.3 nm), Ti (364.3 nm), T1 (276.8 nm), and V (318.4 nm). It is especially important for those of the above list where electrodeless discharge lamps (EDLs) are available. In the past, the full advantage of the EDLs for ultratrace analysis was limited by the need to backgroundcorrect with a less bright light source. Of the list above, EDLs are available for Al, Pb, Ti, and T1. Direct seawater analysis with the 20-pL method to limits which approach the aqueous detection limits of the graphite furnace should be available for those metals whose appearance temperatures are equal to or higher than Mn, in other words, for those metals which will permit a char step at 1200 “C. This probably includes Co, Cr, Cu, Fe, Mo, Ni, Si, Sn, Ti, and V in addition to the alkaline earth metals and many of the alkali metals. Signal Stability, Tuhe Life and Failure. We anticipated that the use of the pyrolytic platform and pyrolytically coated tubes would provide longer tube life for the seawater analysis than deposition of the sample on the wall. This is based on frequent reports in the literature that tube destruction appeared to be related to the effect of the acids and seawater components upon graphiite (4, 7,9). Also, Sturgeon (I) reported that, using his seawater procedure, there was a loss in sensitivity of 20% over the course of 100 firings. Such u decrease in sensitivity might be attributed to the erosion of the pyrolytic surface of the tube, followed by the soahing in of the matrix and therefore incomplete removal of the imatrix during charring. Many tubes were tested to failure and the variability of the signal was measured. One test used peak height data, a standard coated tube, an aqueous standard, and two seawater dilutions all of which contained about 2! pg/L Mn. Each of the three samples was prepared and fired in triplicate. Through 162 firings, there was no systematic change in the Mn signal. In another test, a seawater sample was fired repetitively in a pyrolytically coated graphite tube until the tube failed. Figure 6 shows the repeatability of the Mn peak area signal at intervals of 50 fiings. Each point on the graph represented an average of five firings. This particular tube failed after 310 firings of seawater. ‘I’ypically we find that the Mn signal is repeatable until close to the end of the tube’s life, to within the last 25 firings. By use of a slightly different protocol, a different, but similar tube gave results that wore very erratic. Not only was the precision of the analysis extremely poor but also all three of the aqueous samples showed an increase in peak height (a8 much as 100%) during a run of 99 firings. These data are probably similar to the observations of Montgomery and
53, NO. 12, OCTOBER 1981
181’1
Peterson (9) and reflect problems resulting from occasionisl inadequate pyrolytic coating of some batches of tubes. We tested more than 10 pyrolytically coated tubes to destruction using the seawater protocol of Table I. Only one failed in about 200 firings and many required more than 400 firings to cause failure. Each failed tube was examined t o determine the mode of failure. One of the new pyrolytically coated tubes was particularly interesting. It had failed after about 200 firings. The tube had fractured. When the two halves of the tube were examined, the failure appeared to be at the bottom of the tube. A pile of lose carbon had filled the space betwen the platform and the tube, although there was no loose carbon on thie sample surface of the platform. Thle tube surface was badly crumbled in that portion where graphite had collected. The platform itself looked new with no visible deterioration. There was no visible deterioration on the outside of the tube, except near the bottom. In an elegant paper in 1938, Ruff (29) discussed the reactions of solid graphite with various liquids and gases. He reported that Na and many metal halides were bound between the layers of covalently bound carbon,now called intercalatioi~. This binding caused the layers to expand and eventually rupture, turning the graphite into crumbled powder. Othsr papers in the carbon literature also describe this phenomenon. This tube had nevler been subjected to any corrosive materials, other than acidified seawater to which small quantities of Mn had been addled. Presumably, the tube was hotterit beneath the platform and the protective pyrolytic layer was stripped off thermally. This permitted the vapor-phase Na (or NaCl) to be intercalated, gradually penetrating th.e thickness of the tube and lifting larger areas of pyrolytiic graphite, finally causing failure of the tube. Sturgeon et al. ( I ) had also suggested intercalation as thle failure mode for graphite tubes used for seawater analysis. They speculated that the tubes failed by gradually intercalating compounds as the pyrolytic coating was stripped off the inner surface. “The intercalation compounds penetrate through the depth of‘the wall and blister the impermeablle layer of pyrolytic graphite on the exterior of the tube.” This failure explanation appears to be quite general and probably explains the improvement in tube lifetime afforded by pyrolytically coated tubes in many applications. ACKNOWLEDGMENT We thank R. E. Sturgeon of National Research Council for the samples and for clontinued advice and suggestions during this study. We thank F. J. Fernandez for his help with the Zeeman/5000 analyses in his laboratory. We thank Sabina Slavin, W. B. Barnett, R. D. Ediger, and Diane Lawrence for considerable help with the manuscript. Ringsdorff-Werke, in particular Hutsch, Kirch, and Ulsamer, was very cooperative in the work which led to improved pyrolytically coated grtiphite tubes. LITERATURE CITED Sturgeon, R. E.; Berman; S. S.; Desaulniers, A.; Russell, D. S. Anst/. Chem. 1979, 57, 2:364-2369. Segar, D. A,; Gonzalez, J. G. Anal. Chim. Acta 1972, 58, 7-14. Kllnkhammer, G. P. Anal. Chem. 1980, 52, 117-120, McArthur, J. M. Anal. Chim. Acta 1977, 93, 77-83. Hydes, D. J. Anal. Chem. 1980, 52, 959-963. Segar, D. A.; Cantlllo, A. Y. Adv. Chem. Ser. 1975, No. 147, 515. Kingston, H. M.; Barnes, I. L.; Brady, T. J.; Rains, T. C.; Champ, M. A. Anal. Chem. 1978, 50, 2064-2070. Segar, D. A.; Cantilln, A. Y. Anal. Chem. 1980, 52, 1766. Montgomery, J. R.; Peterson, G. N. Anal. Chlm. Acta 1980, 17,7, 397-40 1. Mannlng, D. C.: Slavin, W. Anal. Chem. 1978, 50, 1234-1238. L’vov, B. V. Spectrochim. Acta, Part B 1978, 338, 153-193. Segar, D. A.; Caniillo, A. Y. Spec. Symp.-Am. SOC. Limncrl. Oceanogr. 1978, 2 , 171-197. Slavin, W.; Mannlng, D. C. Anal. Chem. 197g. 57, 261-265. Mannlng, D. C.; Slavin, W. Anal. Chim. Acta 1980, 778, 301-3013. Slavin, W.; Mannlng, D. C. Spectrochlm. Acta, Part B 1980, 35B. 701-714.
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(16) Edlger, R. D.; Peterson, G. E.; Kerber, J. D. At. Absorpt. Newsl. 1974, 13, 61-64. (17) Slavin. W. At. Spectrosc. 1980, 1 , 66-71. (18) Yeats, P. A.; Bewers, J. M.; Walton, A. Mar. Pollut. Bull. 1978, 9 , 264-268. (19) Fernandez, F. J.; Myers, S. A.; Slavin, W. Anal. Chem. 1980, 52, 741-746. (20) Fernandez, F. J.; Bohler, W.; Beaty, M. M.; Barnett, W. 8. At. SpectrOSC. 1981, 2 , 73-80. (21) Slavin, W.; Mannlng D. C.; Carnrlck, G. R. Anal. Chem. 1981, 53, 1504- 1509. (22) Barnett, W. 6.; Cooksey, M. M. At. Absorpt. Newsl. 1979, 18, 61-65. (23) Sturgeon, R. E.; Berman, S. S.; Desaulniers. J. A. H.; Mykytiuk, A. P.;
McLaren, J. W.; Russell, D. S. Anal. Chem. 1980, 52, 1585-1588. (24) Tschopel, P.; Kotz, L.; Schulz, W.; Veber, M; Tolg; G. 2.Anal. Chem. 1980, 302, 1-14. (25) van den Broek. W. M. G. T.; de Galan, L.; Matousek, J. P.; Czoblk, E. J. Anal. Chlm. Acta 1978, 100, 121-138. (28) Nakahara, T.; Chakrabartl, C. L. Anal. Chlm. Acta 1979, 104, 99-111. (27) Fernandez, F.; Iannarone, J. At. AbsorptNewsl. 1978, 17, 117-120. (28) Clyburn, s. A.; Bartschmidt, B. R.; Velllon, C. Anal. Cbem. 1874, 46, 2201-2204. (29) Ruff, 0. Trans. Faraday Soc.1938, 34, 1022-1033.
RECEIVED for review April 13, 1981. Accepted July 16, 1981.
Background Correction in Quantitat ive Mice1le-Enhanced Room-Temperature Phosphorescence via Selective Bimolecular Quenching L. J. Cline Love* and Marle Skrilec Department of Chemistry, Seton Hall University, South Orange, New Jersey 07079
A unlque background correction method for lnterferlng radiatlon present In mlcelle-stabilized room-temperature phosphorlmetry Is descrlbed. By use of reversible chemically induced quenching of the excited trlplet stales produced In fluld solutlon, both scattered llght and fluorescence occurrlng in the phosphorescence spectral reglon of interest can be determined and approprlate corrections applled. Appllcatlons of the method to a synthetlc sample of phenanthrene/B naphthaldehyde, a coal-derlved hydrogen donor recycle solvent, and an automatlc computerized spectrum correctlon for N-( 2-chloroethyl)carbarole are given.
Spectral interferences in phosphorimetrycan originate from several sources, the most commonly encountered ones being scattered and stray light, overlapping analyte or impurity fluorescence bands, and overlapping phosphorescence bands arising from other species present in solution. They can adversely affect the accuracy and sensitivity and are often the limiting factors in determining the usefulness of analytical methods based on phosphorimetry. Several techniques have been developed in low-temperature and solid-substrate room-temperature phosphorimetries to discriminate against unwanted radiation. Temporal and phase resolutions ( 1 , 2 ) discriminate against fluorescence and scattered light, which often are especially severe in low-temperature work, as well as shorter-lived phosphorescent components in the sample. Proper selection of excitation and emission wavelengths, and low-temperature narrowing of spectral bands can minimize interferences due to spectral band overlap ( 3 , 4 ) . Overlapping luminescence spectral bands from species with similar temporal characteristics could be distinguished by a variety of instrumental approaches including derivative spectroscopy and selective modulation (5-8). All of these methods require specialized, often expensive instrumentation. A t a minimum, some type of temporal discrimination has been essential in phosphorimetry, especially for compounds exhibiting small Franck-Condon shifts, because the fluorescence and scatter signals are often several orders-of-magnitudegreater than the 0003-2700/8 110353-1872$0 1.25/0
phosphorescencesignal, even in pure, single-componentsamples. For many compounds with large Stokes’ shifts, the micelle-stabilized room-temperaturephosphorescence (MS-RTP) technique reduces the need to correct for scattered light and fluorescence because clear, fluid solutions are used and highly effective spin-orbit coupling greatly diminished the fluorescence intensity in favor of the phosphorescencesignal (9, IO). Thus, a conventional fluorometer with no special time discrimination, temperature control, or modified sample compartment can be used in many applications. However, when interfering radiation is present in the phosphorescence region of interest, some type of correction is necessary. This paper describes a simple spectral correction technique for all interfering scatter and fluorescence, regardless of the severity of energy overlap with the phosphorescence band of interest as long as the MS-RTP signal/ background intensity ratio is reasonable. It requires no additional instrumentation or modifications of normal spectrofluorometers, but rather it relies on reversible chemically induced quenching of the excited triplet state. By use of the unique characteristics of MS-RTP, the sample solution can be used directly, in a sequential measurement, as the blank, resulting in improved sensitivity and selectivity. The basis of the technique is discussed, the limitations are described, and an application to the analysis of coal-derived hydrogen donor solvents for polycyclic aromatics is given.
EXPERIMENTAL SECTION Apparatus. Instrumental characteristics of a Farrand Mark
I spectrofluorometer used to obtain the uncorrected spectra of the standard and sample solutions are described elsewhere (IO). Uncorrected difference spectra of standard solutionswere obtained by using the computer-controlled spectrofluorometer described previously (11). Reagents. Thallium lauryl sulfate (TlLS)reagent was prepared and purified according to standard procedures (9). Sodium lauryl sulfate (NaLS) (BDH Biochemicals, Poole, England), specially purified for biochemical work, was used as received. Tripledistilled water was used throughout the study. The concentration of TlLS/NaLS surfactant solutions was kept constant at 0.10 M with a 30170% ratio of Tl/Na. 0 1981 American Chemlcal Society
