or
where aQ and aQ& represent the activities of the p-benzoquinone and hydroquinone components of quinhydrone. The other symbols have their usual meaning. In typical applications of the quinhydrone electrode the activities of the quinhydrone components are considered equal and the last term of Equation 2 is dropped. It is this final term which is the link between the absorbance or quinhydrone concentration decrease and the emf decay. Because the activities of hydrogen ion and chloride ion should remain essentially constant, the change in the measured potential with time at constant temperature can be written as
If we assume ideal behavior and Beer's law, and further assume that the decrease in the concentration ofp-benzoquinone is accompanied by a n increase in the concentration of hydroquinone (or a substituted hydroquinone), Equation 3 becomes
where AQ is the absorbance of p-benzoquinone. Equation 4 explicitly relates changes in the potential of the cell to changes in the absorbance ofp-benzoquinone. Plots of log AQ at 246 nm us. time gave good straight lines as did plots of emf us. time. Data from runs covering a range of acetate ion concentrations can be found in Table I. Column 4 is the slope of the absorbance plots while column 5 contains the slopes of the emf plots. Column 6 is the change in emf with time calculated from Equation 4 using column 4 absorbance data. The agreement is reasonable. It should be noted that the results obtained in this work do not preclude the use of the quinhydrone electrode. When a high degree of accuracy is desired and significant emf drift is encountered, an extrapolation back to zero time can and should be made. As an example, at 25 OC acetate ion gives a drift rate of 0.2 mV h r ' which results in an error of 0.004 pKa unit if one hour data is used without extrapolation or 0.002 pKa unit for 30 minute data should thermal equilibrium be established in that time. We do not expect that most organic anions would be as active as acetate ion. Our data on bimaleate, maleate, and on the anions of ethylene tetracarboxylic acid indicate a drift rate comparable to the water or neutral reaction.
RECEIVED for review September 11, 1969. Accepted October 9, 1969. The authors gratefully acknowledge the financial support of the American Cancer Society Grant No. P-272.
Determination of Vanadium in Natural Waters by Neutron Activation Analysis K. Daniel Linstedt' and Paul Kruger Civil Engineering Department, Stanford University, Stanford, Calv. VANADIUM HAS BEEN quantitatively determined by neutron activation analysis in a variety of sample matrices. For example, Brooksbank, Leddicotte, and Mahlman (I) determined the vanadium concentration in crude oil using a method for nondestructive comparator analysis. Kemp and Smales (2) applied activation analysis for determination of the vanadium concentration in rocks and meteorites. Their procedure included a sodium peroxide fusion followed by separation of vanadium from interfering radionuclides by a cupferronchloroform solvent extraction. Lukens, Heydorn, and Choy (3) developed a pre-irradiation concentration procedure for activation analysis for vanadium in blood. This selective concentration of vanadium permitted the post-irradiation radiation measurement to be made by purely instrumental methods. Grimanis, Pantazis, Papadopoulos, and Tsanos (4determined
vanadium and some other trace elements in several Greek Lakes. For the vanadium analyses, 100-ml samples were evaporated to provide pre-irradiation concentration. Following irradiation the vanadium was isolated by a 3.5-minute cupferron-chloroform solvent extraction prior to counting. In the present work, the occurrence and behavior of vanadium in several natural waters has been investigated as part of a study of the fate of this element in domestic supply waters. A generally applicable procedure has been developed for quantitative analyses of dissolved vanadium in natural waters. This procedure includes a rapid pre-irradiation concentration to enhance the sensitivity of analysis with low flux reactors, and a post-irradiation radio-chemical separation to reduce the interference of other radionuclides.
Present address, Civil Engineering Department, University of Colorado, Boulder, Colo. 80302
Reagents. Ammoniacal Buffer Solution, pH 9.4. Dilute 100 ml of 4N ammonium hydroxide and 50 ml of 4N nitric acid to 1 liter with distilled water. Oxine-Chloroform Solution, 1%. Dissolve 10 grams of reagent-grade oxine in 1 liter of chloroform. Procedure. Samples of natural water are filtered through acid washed, hard finish, Whatman No. 42 filter paper to isolate the suspended particulate matter. The filtrate is concentrated prior to activation by passing 1-liter aliquots through a column of hydrogen-form Dowex 50W-X8 cation exchange resin. This type of resin has been shown to quan-
1
(1) W. A. Brooksbank, G. W. Leddicotte, and H. A. Mahlman, J. Phys. Chem., 57, 815 (1953). (2) D. M. Kemp and A. A. Smales, Anal. Chim. Acta., 23, 397 (1960). (3) H.R.Lukens, K. Heydorn, and T. Choy, Trans. Amer. Nuclear Soc., 8, 331 (1905). (4) A. P. Grimanis, G. Pantazis, C. Papadopoulos, and N. Tsanos, Third U.N. Intern. Conf Peaceful Uses A t . Energy, 854 (1964).
EXPERIMENTAL
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Table I. Standard Sample Analyses 1-fig vanadium samples Vanadium, fig Sample Added Found SD-10 1.0 1.3 SD-11 1.0 1.1 SD-12 1.0 1.0 SD-13 1.o 1.1 SD-14 1.0 0.9 SD-15 1 .o 1 .o SD-16 1.o 0.8 Mean 1.0 1.0 =t0.1 Table 11. Vanadium Concentrations in Natural Waters Mean vanadium Vanadium concentration range, concentration, Sampling site Pgil fidl 3.93 3.44-4.31 Sacramento River at Sacramento, Calif. 20.50 19.4C-21.60 Supply well in Fresno, Calif. 0.90 0.20-1.80 Green River at Flaming Gorge, Utah 3.40 1.50-5.20 Colorado River at Page, Ariz. 3.00 1.8O-4.10 Colorado River at Hoover Dam, Nev.-Ariz. 3.00 1.70-4.30 Colorado River at Parker Dam, Calif.Ark
San Joaquin River near Vernalis, Calif. Animas River at Cedar Hill, N.M. San Juan River, Shiprock, N.M. Colorado River near Loma, Colo. Colorado River at Yuma, Ariz.
6.75-7.01
6.87
0.20-0.50
0.30
0.50-49.20
7.50
1.9O-11.50
4.60
1.00-3.80
2.20
z
RESULTS AND DISCUSSION
titatively retain the quinquevalent vanadate species which is the form expected to occur in natural water systems (5-7). The indicated mechanism is a conversion of the anionic vanadates to the cationic pervanadyl ion, VOz+, or a reduction to the vanadyl ion, V02+, at the lower pH. Elution of the vanadium is achieved with 2N nitric acid. This procedure concentrates the vanadium in the water samples and reduces the dissolved solids in the vanadium fraction by about 5 0 z . The eluted samples are concentrated further by evaporation to volumes of 3 to 4 ml, and quantitatively transferred to polyethylene irradiation vials. Repeated analyses of vanadium standards and blanks have shown this pre-irradiation sample concentration procedure to be quantitative for quadrivalent and quinquevalent vanadium, and free of any detectable vanadium contamination from reagents or laboratory apparatus. Samples, standards, and reagent blanks are irradiated in the Stanford University IO-kW research reactor at a thermal neutron flux of about 10" n/cm2-sec for 5 minutes. The flux is monitored during each irradiation by attaching pure, thin vanadium foils to the polyethylene vial. ( 5 ) L. G. M. Baas-Becking, I. R. Kaplan, and D. Moore, J . Geol., 68, 243 (1960). ( 6 ) H. T. Evans, Jr., and R. M. Garrels, Geochim. Cosmochim. Acta., 15, 131 (1958). (7) H. Kakihana, Bull. Chem. SOC.Japan, 22, 242, (1949).
114
Following irradiation, vanadium is extracted from the aqueous samples with oxine-chloroform solvent. The separation procedure involves immediate transfer of the irradiated samples into an Erlenmeyer flask containing 8N sodium hydroxide and 330-day 49V radiotracer used for chemical yield determination. The pH is adjusted to 4 with dilute hydrochloric acid prior to extracting the vanadium into a 1 solution of oxine in chloroform. The vanadium is back extracted into a HN03-NH40H ammoniacal solution buffered to pH 9.4. The vanadium in the buffered phase is then washed with chloroform and transferred into a polyethylene counting vial. The elapsed time between the end of irradiation and commencement of counting averages 6 minutes. Sample 5 V activities are measured with a 3-in. X 3-in. NaI (Tl) well-crystal detector coupled to a 400-channel gamma ray spectrometer. In the time-sequence-store operational mode, the system stores integrated one-minute counted activities of the 1.43-MeV photopeak in successive channels. These photopeak activity data, recorded at the end of each counting sequence with a teletype printer, provide a sensitive decay curve. The 52V activity of the flux monitors is counted simultaneously with a second NaI (Tl) well crystal, and the photopeak is measured with a single-channel analyzer. The sample activities are normalized for the specific activities of the monitor foils. Following decay of the 3.77-minute 52V, the chemical yield is determined by liquid scintillation counting of the 49Vradiotracer. Quantitative analysis of the vanadium in each sample is made by the comparator method in which standard samples containing 10 pg of vanadium in demineralized water are irradiated for the same time period as the samples, and counted directly with no chemical separation.
Interferences. For vanadium analyses of natural water samples by neutron activation, significant interferences may be contributed by the gamma rays of 15h-24Na,2.3m-28Al, 37.3m-38C1, 5.0m-37S, 8.7m-49Ca, and 2.58hJ6Mn. The previously outlined analytical procedure is effective in eliminating these interferences through several of the process steps, The ion-exchange pre-irradiation concentration procedure eliminates the 38Cl,and % interferences by passing the chlorides and sulfates through the column. The highly selective post-irradiation extraction procedure removes nearly all interferences contributed by the cationic species prior to counting the samples. Any residual interfering radioactivity is corrected for through half-life analysis of the decay curves. In the majority of the natural water analyses reported, for which the vanadium concentration is about 3 pg/1, the z4Na background contributes only 3 to 5 z to the total activity at the normalization time of 5 minutes after the end of irradiation. Correction for this contribution is made by subtracting the long-lived component of the decay curve. Precision and Accuracy. A series of standard samples was analyzed to assess the precision and accuracy of the activation analysis procedure for vanadium. These samples were prepared by adding vanadium as the vanadate in the presence of 200 mg/l of NaCl in demineralized water. The NaCl provided 15h-24Na radioactivity interferences comparable to those encountered in river water samples. One-liter samples containing 1.O pg/l of added vanadium were treated identically to the natural water samples. Table I summarizes the results of analyses of seven standard samples. The analyses yielded a mean vanadium concentration of 1.0 pg/l with a standard deviation of i10%. This neutron activation analysis procedure was then applied to the analysis of 170 natural water samples. These sam-
ANALYTICAL CHEMISTRY, VOL. 42, NO, 1, JANUARY 1970
ples were collected at 11 different sites throughout the Sacramento, San Joaquin, and Colorado River Basins. For the sites listed in Table 11, samples from sites 1-6 were collected from penstock or pump intakes, and those from 7-1 1 represent grab samples from the water surface. The samples were collected in four-liter polyethylene bottles and stored for periods of up to one month in dark containers. Since the aim of this study was to observe the behavior of dissolved vanadium in river water and not that associated with suspended particulate matter, the samples were not acidified. A summary of the concentration data from these sites is presented in Table 11, with the range of concentrations observed at each site
as well as the mean concentration value for the samples analyzed. The vanadium concentrations in these natural waters ranged from a low value of 0.2 pg/l to a high value of 49.2 c(g/l. None of the samples analyzed had a concentration below the 0.1 pg/l sensitivity limit of the method for 1-liter samples.
RECEIVED for review September 2, 1969. Accepted October 30,1969. Work carried out under U. S. Public Health Service Research Grant EF-00858. Presented at the 1968 International Conference, Modern Trends in Activation Analysis, Washington, D. C., October 1968.
Design, Construction, and Use of a Laser Fragmentation Source for Gas Chromatography Bohdan T. Guran, Robert J. O’Brien, and Don H. Anderson Eastman Kodak Company, Industrial Laboratory, Rochester, N . Y. 14650 PYROLYSIS TECHNIQUES have extended the use of gas chromatography (GC) in studies of materials with low volatility. Early in the development of GC, Davidson et al. ( 1 ) studied a number of polymers by collecting their pyrolysis products in a cold trap and subsequently transferring them onto a G C column for analysis. Later, more sophisticated thermal pyrolysis units were developed and coupled directly to gas chromatographs ( 2 , 3 ) . They allowed samples to be pyrolyzed directly in the carrier gas stream just before its entrance into the column. Such techniques usually employed hot wires, heated tubes and cups, or heated chambers. Many publications on these techniques are cited in recent review articles (4-6).
Although such thermal pyrolysis techniques allow the gas chromatographer to study nonvolatile compounds, they have several disadvantages which make resul?s difficult to interpret and reproduce. One of the major sources of error in these techniques is the difficulty of obtaining reproducible pyrolysis temperature of the sample and maintaining small temperature gradients in the pyrolysis chamber. Temperature differences at sample position between the carrier gas and the walls, as high as 100 OC have been reported (7). Other common sources of error are incomplete removal of solvents used in coating samples onto the filament wire, and catalytic effects of hot metallic surfaces on the decomposition of samples. A constant problem also is the formation of tars which accumulate in the pyrolysis chamber, in the tubing, and on the column packing. To overcome the problems associated with thermal degradation, Sternberg and Litle (8) developed a pyrolysis technique using high voltage electric discharge. It eliminated some (1) W. H. T. Davidson, A. L. Wragg, and S. Stanley, Chem. Ind., (London), 1954,1356. (2) E. A. Radell and H. C. Strutz, ANAL.CHEM., 31, 1890 (1959). (3) D. F. Nelson and P. L. Kirk, ibid., 34, 899 (1962). (4) R. W. McKinney, J. Gas Chromatogr. 2,432 (1964). ( 5 ) R. L. Levy, ibid., 5, 107 (1967). 40, 33R (1967). (6) R. S. Juvet and S. DalNogare, ANAL.CHEM., (7) R. A. Prosser, J. T. Stapler, and W . E. C.-Yelland, ibid., 39, 694 (1967). (8) J . C. Sternberg and R. L. Litle, ibid., 38, 321 (1966).
a4 \‘,$/I ’\
’
@6
Figure 1. Laser fragmentation source I-Laser rod 2-Flash tube 3-Mirror 4-Objective 5-Eyepiece 6-Fragmentation
cell
effects resulting from temperature variations but required long pyrolysis periods and large sampling volumes. The positioning of samples within the discharge cell was also critical in obtaining reproducible results. Many of the existing problems in pyrolysis could be eliminated if samples were fragmented more rapidly. This type of fragmentation should be obtained with laser radiation and has been reported by Wiley and Veeravagu (9). They successfully used a laser to fragment samples and analyzed the resulting fragments by gas chromatography. Recently, Folmer and Azarraga (10) reported using a ruby laser to obtain fragmentation of samples directly in the gas chromatographic stream. This paper describes the development of a neodymiumdoped glass laser as a fragmentation source for gas chromatography. The design, construction, and use of a sample fragmentation cell which allows on-line gas-chromatographic (9) R. H. Wiley and P. Veeravagu, J. Phys. Chem., 72,2417 (1968). (10) D. F. Folmer, Jr. and L. V. Azarraga, “Advances in Chromatography-1969,’’ A. Zlatkis, Ed., Preston Technical Abstracts Co., Evanston, Ill., 1969, pp 216-221.
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