from the two regions of scattered excitation radiation as a measure of the specimen cross section. Using this information, the correction for the change in the cross-section curve at the Fe K absorption edge, and for matrix enhancement effects could be calculated as reported elsewhere (12). However, for simplicity, these corrections are not included in this paper. ACKNOWLEDGMENT The authors thank Joe Jaklevic and Dick Pehl for their comments on the preparation of this paper. We are grateful to Fred Goulding and members of the Electronic Research and Development Group for developing and maintaining the x-ray system. We are indebted to David Gok for writing the basic computer program used and t o Karl Scheu for making the vacuum vapor deposited single element thin-film standards. LITERATURE CITED (1) (2) (3) (4)
F. Claisse, Norelco Rep., 4, 3, 95 (1957). C. L. Luke, Anal. Chem., 35, 56 (1963). H. J. Rose, I. Alder, and F. J. Flanagan, Appl. Spectrosc., 17, 81 (1963). R. Jenkins, “An Introduction to X-ray Spectrometry”, Heyden, London, 1974.
J. M. Jaklevic, F. S. Goulding, B. V. Jarrett, and J. M. Meng, in “Analytical Methods Applied to Air Pollution Measurements”, R. K. Stevens and W. F. Herget. Ed., Ann Arbor Science, Ann Arbor, Mich., 1974. R. D. Giauque, R. B. Garrett, and L. Y. Goda, in “X-ray Fluorescence Analysis of Environmental Samples”, T. G. Dzvbay, Ed., Ann Arbor Science, Ann Arbor, Mich., 1977. R. D. Gauque, F. S.Goukling, J. M. Jaklevic, and R. H. Pehl, Anal. Chem., 45, 671 (1973). W. H. McMaster, N. K. Del Grande, J. H. Mallett, and J. H. Hubbell, “Compilation of X-ray Cross Sections”, Lawrence Livermore Laboratory, Livermore, Calif., Report UCRL-50174, Section 11, Revision 1 (1969). F. J. Flanagan, Geochim. Cosmochim. Acta, 37, 1189 (1973). J. M. Ondov, W. H. Zoller, Llhan Olmez, N. K. Aras, G. E. Gordon, L. A. Rancitelli, K. H. Abel, R. H. Filby, K. R. Shah, and R. C. Ragaini, Anal. Chem., 47, 1102 (1975). W. Bambynek et al., Rev. Mod. Phys., 44, (4), 716 (1972). R. 0. Glauque, R. B. Garrett, and L. Y. Goda, Anal. Chem., 49, 62 (1977).
for review February 3, 1977. Accepted March 21, 1977. This work was done with support from the U.S. Energy Research and Development Administration. Any conclusions or opinions expressed in this report represent solely those of the author(s) and not necessarily those of the Lawrence Berkeley Laboratory nor of the U.S. Energy Research and Development Administration. RECEIVED
Atomic Emission Spectrometer/Spectrograph for the Determination of Barium in Microamounts of Diatom Ash Donald C. Bankston” and Nicholas S. Fisher Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
The development and routine application of a method for the determination of trace levels of barium in microsamples (5-10 mg) of diatom ash is described. Acid-dissolved llthlum metaborate fusion melts of ash samples are analyzed using a spectrometer/spectrograph equipped wlth a dc argon plasma jet excitation source and an echelle diffraction gratlng. Sample, standard, and blank solutiions are buffered by llthlum contributed by the flux, to a degree sufflclent to reduce matrix effects to acceptable levels. Previous barium determlnations by other analytical technlques, on seven interlaboratory reference materials, have been used to establish the accuracy of our resutts. The average relatlve standard deviation for the instrumental analyses was 0.07. Using recommended Instrument settings, moreover, the lowest concentration of barium visible in synthetic standard solutions lles just below 2 pg/L, which is equivalent to 2 pg/g In the ash.
Marine phytoplankton have been shown to concentrate and vertically transport barium in the world’s oceans, a finding which may have geochemical significance (1-5). To gain a better understanding of processes controlling biologically mediated barium transport, we have initiated a program to study the uptake of trace amounts of barium by several species of diatoms cultured under various controlled conditions. The only techniques which have been used successfully for the determination of barium in diatom ash are isotope dilution mass spectrometry (IDMS) ( 4 , 5) and conventional dc arc atomic emission spectrometry (AES) (6-8). Although IDMS
Table I. Table of Values Obtained from Synthetic Standard Barium Solutions x = IJg Y = Observed integrated Y ’ = Integrated light light intensity corrected intensity predicted by Ba/L solution for background and drift working curvea 2000 2059, 2008,1938,1983 1998 500 486,497, 511, 517 499 200 197, 204, 202 200 50 50 49, 49, 50 20 20 21, 18, 17, 18 5 3,1, 1 5 a The equation of the working curve is given as Y ’ = 0.999X - 0.318. affords an established microanalytical method, it involves a sample preparation that is elaborate, time-consuming, and potentially dangerous, inasmuch as it employs HF and HC104. Conventional AES utilizes comparatively simple and safe techniques for sample preparation, but is neither precise, accurate, nor sensitive enough to be used as a microanalytical method. However, the employment of an echelle grating mount and the replacement of the conventional dc arc by a dc argon plasma jet have given AES the capability to overcome these limitations, facilitating accurate and precise analyses of small amounts of material. In this paper, we describe a method for determining barium in diatom ash with an instrument which utilizes these AES modifications.
EXPERIMENTAL Instrumentation. This technique utilizes a Spectrospan 11-B single channel optical emission spectrometer/spectrograph ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
1017
Table 11. Barium Concentrations (as pg Ba per g sample) in Selected Reference Materials Results of this Work Analytical instrument Matrix of standards Dc argon plasma jet emission spectrometer, with echelle grating from which Silica concentrations vredicted Relative sample size Macro Micro Solution 1 Solution 2 Solution 3 Solutions 1-3 incl. - Solution 4 Sample ~~~~~~~s X s n x s n x s n X s n sr X s NBS 9 1 Opal Glass 37.25 2.99 4 35.33 4 . 6 2 3 34.67 0.58 3 35.90 3.03 10 0.08 4 8 . 3 3 0.58 NBS 6 1 3 Synthetic Glass 33.33 1.15 3 32.00 3.46 3 n.d.a 32.67 2.42 6 0.07 n.d. USGS GSB Synthetic Glass 32.67 3.06 3 31.00 5.29 4 29.67 0.58 3 31.10 3.60 1 0 0.12 32.67 1.53 USGS GSC Synthetic Glass 31.33 2.31 3 37.33 2.31 3 32.67 1.53 3 33.78 3.27 9 0.10 36.67 0 . 5 8 USGS GSD Synthetic Glass 74.67 4.62 3 71.33 1.15 3 71.00 1.00 3 72.33 3.00 9 0.04 72.00 1.00 CRPG VS-N Synthetic Glass 844.00 10.44 3 834.67 8.74 3 869.50 13.40 4 851.40 18.93 10 0.02 824.33 3.21 IAEA S-5 Synthetic Glass n.d. n.d. 1947.25 27.10 4 n.d. 1803.75 33.95 Kieselguhr 93.00 0.00 3 n.d. 100.00 1.41 Baker Reagent n.d. n.d. '
(Spectrometrics,Inc., Andover, Mass.). The width of the entrance slit is set at 25 pm, and its height at 200 pm. The instrument has a focal length of 0.75 m. The analytical line is Ba" 455.40 nm, and, at this point in the spectrum, the wavelength dispersion of the spectrometer is 0.14 nm/mm. There are two parallel exit slits approximately 1 mm apart. These are congruent, each having a height set at 100 pm, a width set at 200 pm, and a spectral bandpass of 0.03 nm. From an exterior viewpoint, the analytical line is projected upon the righthand exit slit, and spectral background radiation having a wavelength of 455.27 nm passes through the lefthand slit. Just in front of this pair of exit slits is a rotating chopper that allows the passage of light through either slit, but not both simultaneously. The radiation detector is a Hamamatsu R106 photomultiplier tube. Each analog signal corresponding to an instantaneous background intensity is electronically subtracted from an associated analog signal corresponding to a momentary intensity of the analytical line, and the resulting differences are integrated over a time interval of 10.0 s. The source of excitation is a Y-type dc argon plasma jet equipped with a pair of thoriated tungsten electrodes between which current flows at 7.3 A. Argon gas flows through the plasma jet electrode assembly at a rate of 1.4 L/min (3 cfh) and into the nebulizer at a rate of 2.8 L/min (6 cfh). The rate of liquid sample uptake is set at approximately 0.7 mL/min and regulated by a peristaltic pump. The small periodic fluctuations in the rate of liquid delivery by this pump do not significantly affect results. Corroborative measurements by x-ray fluorescence spectrometry were made with a Philips PW 1410 X-ray Spectrometer. Analyses by conventional dc arc AES were made with a Jarrell-Ash 66-000 Atomcounter Spectrometer. Sample Preparation. The method of preparing samples of diatom ash is a modification of Ingamell's lithium metaborate fusion technique (9) for the aqueous dissolution of siliceous and other materials (IO). Each diatom culture is grown in 0.75-1 L of medium, harvested by filtration onto 1-pm Millipore filters (No. EAWP04700), washed, transferred to a Pyrex Petri dish, dried at 70 "C for about 15 h, and then ashed at 550 "C for the same length of time. (The melting point of Pyrex glass is never lower than 750 "C ( I I ) , and it does not soften at 550 "C.) With the Petri dish kept covered, essentially the entire yield of diatom ash is tapped loose from the dish. This ash is then weighed into a 2-mL plastic mixing vial (Spex Industries, Inc.). Slightly less than 10 mg of ash are typically produced, and enough powdered silicon dioxide (Johnson-Matthey, Specpure) is added to the vial to bring the total weight of its contents up to 10 mg. Seventy milligrams of LiBOz (Johnson-Matthey, Spectroflux 100A) are then added to the vial, which is shaken for 1 min on 1018
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
n 3
3 3 3 3 4 4
a Spex Wig-L-Bug. The resulting mixture is quantitatively transferred to a preignited graphite fluxing crucible (Ultra Carbon, Catalog No. 818004) and then fused at 950 "C for 14 min. At some time during this interval, about 7.5 mL of 4% HN03 (prepared by mixing one unit volume of Analytical Reagent grade, Mallinckrodt, 70% HN03 with 24 unit volumes of deionized distilled water) are pipetted into a 100-mL Teflon beaker containing a Teflon-coated magnetic stirring bar. This beaker is then placed atop a magnetic stir plate, which is activated just before the fusion interval ends. At that point, the molten glass bead is poured out of its crucible and into the beaker, which is then covered with a Teflon watchglass. If one or more beads of glass are retained in the crucible, these are allowed to solidify and can then easily be scraped or tapped loose and added to the contents of the Teflon beaker. After about 5 min of stirring, the glass is completely dissolved,and its solution is washed into a 10-mL volumetric flask, diluted with 4% HN03,mixed thoroughly, and then stored in a 1-02 (28.4 mL) Nalgene polypropylene screw-top stock bottle. Derivation of Some Diatom Ash Samples Analyzed for Barium. The procedure given above was incorporated into each of two experiments. The first of these involved testing barium uptake by the three diatom species Skeletonemu costutum (Clone Skel), Thulussiosira pseudonunu (Clone 13-1, Sargasso Sea isolate), and Frugiluriu pinrzutu (Clone 13-3, Sargasso Sea isolate). All of these clones were grown in 2-L flasks containing 800 mL of sterile f/2 culture medium (12) prepared from glass-fiber fiitered Woods Hole seawater. The barium concentration in autoclaved f/2 medium, as estimated by a method of standard additions using the spectrometer/spectrograph, was 10.0 pg/L. In a second experiment, designed to check the variation between different cultures of the same species, barium uptake was examined in three replicate 1-L cultures of 2'. pseudonunu (Clone 3H, estuarine isolate). All clones used in both experiments had been obtained from Dr. Robert R. L. Guillard of the Woods Hole Oceanographic Institution. Inocula for all cultures were taken from exponentially growing axenic stocks. Initial cell densities were set at 1 X lo4 per mL, and growth was monitored periodicallywith a Speirs-Levy eosinophil counter. Cultures were incubated at 18.0 f 0.5 "C and received 7500 lumens/m2 for 14 h per day. A t day 7, when the growth rates of the cells had decreased so that they were no longer rapidly dividing, they were suction-filtered out of the medium. Excess sea salts were then washed from the cells with two 5-mL rinses of pre-filtered 3.2% (w/v) NH,COOH. The outer edges of the filters containing no cells were then rinsed with 5 mL of deionized distilled water, and the filters were dried and ashed. Preliminary experiments had shown that this rinsing process effectivelyremoves excess salb from the filter but does not impart any barium to the system.
-
Table I1 (continued)
X
Results of this Work X-ray spectrometer Synthetic glass Macro Solution 5 - Pressed powder n X s s s' 4.26
0.12
31.33
2.42
0.08
31.33
5.61
0.18
51.40
3.71
5
31.50
2.51
0.08
57.00
2.55
82.67
2.94
0.04
110.60
1008.57
10.15
0.01
1028.40
2154.50
6.35
0.003
63.60
0.89
5
35.83
0.01
41
ng
ng
Isotope dilution (16 )
0.07
50
10
ng
Conventional AES (12)
5
0.04
70
10
ng
Conventional AES (12 )
2.61
5
0.02
100
10
11.46
5
0.01
Conventional AES (12 ) X-ray fluorescence (17 ) Conventional AES (17) Neutron Activation (18 ) Spark Source MS (18)
n.d.
2024.60
9.45
n.d. 90.67 4.18 0.05 n.d.: not determined. Interlaboratorv means.
a
sr
Results Obtained Previously Various Analytical technique(s) used n sr X Conventional AES (determined by authors) 4 0.30 b60.75
b913.22
5
ng 0.08
b1662.50
2
ng
ng ng ng: not given in the literature.
ng
o.oo5
To ascertain whether the filters themselves were introducing significant amounts of barium into microsamples of diatom ash, the following procedure was implemented. A single Millipore filter was placed into each of two Petri dishes. Five such filters were placed into a third Petri dish. The contents of each dish were ashed at 550 "C for 15 h, transferred to a separate 10-mL volumetric flask, and dissolved in synthetic blank solution. Each ash solution was made up to volume with synthetic blank solution and then analyzed for barium. None of these solutions contained a detectable amount of barium. Synthetic Blank Solution. One gram of Johnson-Matthey Specpure SiOn and 7 g of Johnson-Matthey Spectroflux lOOA LiBOz are weighed into a 30-mL plastic mixing vial (Spex Industries, Inc) and shaken for 15 min on a Spex Mixer/Mill. Each of ten 0.8-g aliquots of this mixture is weighed into a preignited graphite fluxing crucible and fused at 950 OC for 14 min. The molten glass bead is then dissolved in 75 mL of 4 % HN03,and the resulting solution is poured into a 1-L volumetric flask. The pool of these ten replicate solutions is then made up to volume with 4% HN03, mixed well, and stored in a Nalgene polypropylene screw-top stock bottle. Synthetic Standard Solutions. Using BaClz. 2Hz0 (Baker and Adamson, Reagent Grade) as the primary source of barium, and the blank solution described above as the only solvent or diluent, a series of standard solutions is made, in which the respective barium concentrations are 2000,500,200,50,20,5, and 2 fig/L. These concentrations are very nearly equivalent to micrograms of barium per gram of dissolved ash. All standard solutions are stored in Nalgene polypropylene screw-top stock bottles. Instrumental Procedure. When being used to perform barium determinations, the spectrometer/spectrograph is used in its Present Mode, which is t o say that variable absolute light yields are integrated over a constant time interval. Analytical precision is typically such that, during a single aspiration of any solution, light integration need not be performed more than five times. Samples and all barium standards, except for the most concentrated, are aspirated in random order (13). The blank solution and the 2000 fig/L standard are run frequently enough to bracket aspirations of, at most, four other solutions in sequence. After every 11/2-2 h of continuous operation, the excitation source is turned off and its electrodes are cleaned and repositioned to control the accumulation of tungsten salts and ensure that the electrodes are not burned down too far to maintain the plasma discharge. Data Processing. Were it not for instrumental drift, preliminary data processing would be unnecessary, and concentrations
I
i
of barium in samples could be estimated directly from raw data, using a straightforward linear regression model (13). Indeed, this
sometimes proves to be the case. When instrumental drift does occur, however, it is manifested as either a shift in the mean blank count or a change in analytical sensitivity and is monitored by running the highest standard and a blank repeatedly, as described in the instrumental procedure above. A simple calculator program is used to correct raw counts first for baseline drift, and then for any change in sensitivity, before they are employed to predict concentrations of barium in samples, by linear regression. RESULTS AND DISCUSSION Preliminary Considerations. T o determine what the major elements in diatom ash are likely to be, two different macrosamples of this material were subjected to a qualitative multielement spectrographic analysis by using the photographic attachment of the spectrometer/spectrograph. One of these samples had been obtained from a culture of Chaetoceros curvisetus, and the other from a culture of T. pseudonana. A 0.1-g portion of each was blended with 0.7 g of lithium metaborate and fused at 950 "C for 14 min. Each of the resulting molten glass beads was dissolved in 7 5 mL of 4% "OB, and the solution so obtained diluted to 100 mL, thus yielding a sample solution with the same general relative composition as, but ten times the volume of, the kind of microsample solution described in the Experimental section above. Inspection of the photographs (5-9 exposures taken on Type 57 Polaroid Land Film) indicated that four of the most abundant elements in these two ash samples are silicon, magnesium, calcium, and iron. The most intense and sensitive emission line in the visible spectrum of barium is Ba" 455.40 nm. Moreover, both this wavelength and that at which its spectral background is monitored (455.27 nm) are isolated from significantly interfering emission lines. Ba" 455.40 nm is, therefore, used as the analytical line. Because varying amounts of Johnson-Matthey Specpure silicon dioxide are added to diatom ash samples as they undergo preparation for analysis, the possible contamination of this substance by barium was tested spectrometrically. It was found that the synthetic blank solution (described in the Experimental section), which contains SiOz at a concentration of 0.1% (w/v) and LBOz at 0.7% (w/v) in 4% HN03, yielded a mean integrated light intensity not significantly different ANALYTICAL CHEMISTRY, VOL. 49,
NO. 7, JUNE 1977
1019
(by t-test, P < 0.05) from that produced by a 0.7% (w/v) solution of LiB02 alone in 4% “OB. Linearity of Calibration Curves. The synthetic standard solutions described in the Experimental section above were used to obtain a working curve for barium. The equation for this curve and the ordered pairs to which it was fitted are included in Table I. An analysis of variance (13) was used to evaluate the conformity of these pairs to the linear regression model, and it was found that they do not deviate significantly from linearity at P < 0.05. Sensitivity, Precision, and Accuracy. Under the analytical conditions employed, statistical tests indicated that it should be possible to detect barium at concentrations as low as 2 Fg/L in solution, which would closely correspond to 2 pg/g in the ash. The accuracy and overall precision of that segment of the technique which begins with the blending of sample and flux were estimated as follows. The calibration curve described above and another like it were used to measure the concentration of barium in each of two or three replicate macrosolutions of each of seven interlaboratory reference materials. (These macrosolutions were made to have the same general relative composition as, but ten times the volume of, the kind of microsolution described in the Experimental section.) The results of these measurements are given in Table 11. Values for solutions numbered 3 were estimated from the working curve described above; values for solutions numbered 1 or 2 were derived from the other calibration curve. Considering that the preferred mean concentration of barium in VS-N is the most thoroughly documented of the previously obtained results cited in Table 11, it is noteworthy that the overall mean barium concentration, obtained from solutions 1through 3 of this glass, can be shown by a Wilcoxon Rank Sum Test not to differ significantly from the preferred value ( P < 0.05). The working curve described in Table I was also used to estimate concentrations of barium in microsolutions (made as per Experimental section) of all interlaboratory reference materials except NBS 613. The results of these analyses are also included in Table 11,under the heading “Solution 4”,and serve to indicate (a) the accuracy of that portion of the microanalytical technique that begins with the mixing of sample and flux and (b) the extent of agreement between the micro- and macroanalytical adaptations of our determinative method. This agreement was closest for the USGS reference glasses, which are particularly well homogenized (14). Jaffrezic’s experimental results (15) indicate that microtechniques for performing trace element analyses of powdered mixtures are best conducted with homogenous powders. Among reference materials having major element compositions resembling those of diatom ash samples, only pulverized glasses like those mentioned in Table I1 possess the degree of homogeneity required of microtechniques. However, although their barium concentrations have been estimated repeatedly, these values have not been strictly standardized. We, therefore, re-determined these barium levels with x-ray fluorescence spectrometry, a macroanalytical method of established accuracy (16). We employed a series of powdered barium standards made in a synthetic glass matrix described in Table 111. The results (Table 11) show that x-ray fluorescence determinations of barium concentrations less than 200 ppm were close to previously obtained values. To test the degree of consistency between the x-ray fluorescence technique and our emission spectrometric method, 0.1 g of each of the standard powders described above was dissolved in the same way as the macrosamples of the reference glasses. The resulting solutions were used as calibration standards to predict barium concentrations in the 1020
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
Table 111. Composition of Synthetic Glass Matrix Chemical compoundsQ % (w/w) SiO, 65.37 7.56
Fez03
2.36 MgO 1.88 CaCO, 8.44 Na,CO, 8.90 4.40 KZCO, 1.09 TiO, a Johnson-Matthey “Specpure” chemicals ground with an agate mortar and pestle and sifted through a 200-mesh nylon sieve before being weighed into the matrix mixture. reference materials using our emission spectrometric method. These values are included in Table I1 under the heading “Solution 5”, and, except for the 5-5 measurement, each is significantly lower than the corresponding value obtained by x-ray spectrometry. The instrumental precision, described by the average relative standard deviation (SI) for replicate measurements on a single solution, was 0.07 with a range of 0.003 (for the highest barium levels) to 0.18 (for the lowest barium concentrations.) The precision of the entire microanalytical method, from the culturing of diatoms onward, was determined by simultaneously growing three separate T . pseudonunu (Clone 3H) cultures, harvesting and processing the cells, and analyzing the ash samples (see Experimental section); the overall precision of the method was found to be comparable to the instrumental precision. The results are shown in Table IV, which also presents the results from the other algal cultures, including the final cell density and pH of each culture, the ash weight of the culture and of each cell, and the concentrations of barium, copper, iron, and silicon dioxide in the ash. As shown in Table IV, less than 4% of the barium in the system was taken up by the cells; the barium concentration factor ranged from 1900 for T. pseudonuna, Clone 13-1, to 4800 for T . pseudonuna, Clone 3H. Nonspectral Matrix Effects. To ascertain whether the integrated absolute intensity of light emitted by barium a t the wavelength 455.40 nm is significantly altered by nonspectral interferences from certain major element constituents (Li and B from the flux;Si, Mg, Ca, and Na from diatom ash and/or added SiOz powder) of typical diatom ash sample solutions, the following experiment was performed. Seven matrix solutions were made, of which the actual or equivalent compositions are given in Table V. In each of these matrices, a series of three standard solutions was made, having the respective barium concentrations 5,50, and 500 pg/L. For the set of calibration standards in each matrix, the spectrometer was used to obtain a working curve. All seven such curves, which were derived under the same instrumental conditions, are described in Table VI. At the 0.05 level of significance, the following inferences can be drawn from these statistics: (a) The presence of lithium at a concentration of 1.4 X lo-’ M, in 4% HN03, causes an increase in the slope of the working curve over the value assumed by this statistic when the solution matrix consists only of 4% “OB. (b) The presence of boron at a concentration of 1.4 X 10-1 M, together with lithium at this same concentration, in 4% “OB, brings about a decrease in the slope of the calibration curve from the value assumed by this statistic when the solution matrix contains only 1.4 X lo-’ M lithium in 4% “03.
(c) The presence of either silicon at a concentration of 1.665 lo-’ M, magnesium at a concentration of 6.2 X M, calcium at a concentration of 4.46 X M, or sodium a t a X
Table V. Actual or Equivalent Compositions of Seven Experimental Matrix Solutions Actual or equivalent matrix composition Matrix No. 1 4% HNO, 2 0.14 M Li in 4% HNO, 3 0.7% (w/v) LiBO, in 4% HNO, 4 0.1% (w/v) SiO, t 0.7% (wlv) LiBO, in 4% HNO, 5 0.025% (w/v) MgO + 0.7% (wlv) LiBO, in 4% HNO, 6 0.025% (w/v) CaO + 0.7% (w/v) LiBO, in 4% HNO, 7 0.025% (w/v) Na,O + 0.7% (w/v) LiBO, in 4% HNO,
M
E
U
M = i
d
M
a
+ M
ca 38
ri +
C Q M
E
2
&
'd m
32
Ll
E
W a?
c1 dl
ri
00
m
0
crl ri
W ri
ri
concentration of 8.07 X M, together with lithium and boron at a concentration of 1.4 X lo-' M, in 4% " 0 3 , causes a decrease in the slope of the working curve from the value assumed by this statistic when the solution matrix contains only lithium and boron a t a concentration of 1.4 X lo-' M, in 4% "03. Thus, the intensity of Ba" 455.40 nm is significantly altered by nonspectral interferences from lithium, boron, silicon, magnesium, and, to a lesser degree, calcium and sodium, occurring a t concentrations greater than or equal to those a t which they would usually be present in actual solutions of diatom ash samples. These interferences are manifested as variations among the slopes of the working curves obtained and may be caused by changes in electron pressure (17). The existence of these matrix effects raises the question of whether any significant departure of the composition of a diatom ash, from nearly pure silicon dioxide, would introduce interferences that could substantially affect the barium analysis of that ash. In this regard, the data of Table I1 indicate that the method of barium analysis described here has produced results which generally agree with corresponding values obtained by x-ray fluoresence spectrometry and other methods (14, 18-20) for the six interlaboratory reference materials IAEA S-5, NBS 613, CRPG VS-N, and the USGS synthetic glasses GSB, GSC, and GSD. The major compositions of these materials, as well as those of NBS 91 and Baker Reagent grade Kieselguhr, are given in Table VI1 (21). From the information in Tables I1 and VI1 it can be inferred that the technique of barium determination described here can justifiably be used on any sample of diatom ash, or mixture thereof with silicon dioxide, of which the major constituency is bracketed by pure silicon dioxide and the seven interlaboratory reference materials cited in Table VII. Time permitting, solutions of these materials can be employed as standards in a semiquantitative determination of the major composition of an ash sample using the dc argon plasma jet and echelle emission spectrometer as in the case of barium analysis. This approach was actually used to find the major constituencies of Kieselguhr and IAEA S-5. T o economize on time and sample solution, the photographic attachment of the spectrometer/spectrograph can be employed to perform crudely semiquantitative multielement spectrographic analyses of ash samples to determine whether their major compositions are bracketed by those of the synthetic blank solution and the appropriate reference materials. This sort of analysis requires that the same excitation parameters, exposure time, and film type be used for all samples and reference standards. Advantages. The source of excitation is a Y-type dc argon plasma jet, some features of which are described elsewhere (22, 23). Light emitted by singly ionized barium atoms is collected from a region of low spectral background, at a point almost just beneath the apex of the plasma where three streams of argon gas converge (Figure 1) and reaches temperatures estimated to lie between 5000-6000 K (23). Such ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
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Table VI. Statistics Relevant t o Calibration Curves Obtained from Sets of Working Standards for Barium in Different Solution Matrices Statistic Matrix 1 Matrix 2 Matrix 3 Matrix 4 Matrix 5 Matrix 6 Matrix 7 Y-intercept -1.185 5.141 6.119 7.337 1.204 6.095 1.833 -3.232 0.210 95% Confidence -1.680 -1.956 1.502 0.559 -2.575 to to to to to to to limits about 0.862 13.870 10.071 14.116 4.983 Y-intercept 5.623 10.736 1.702 Slope 1.530 1.497 0.265 1.642 1.598 1.599 1.517 1.480 0.258 1.626 95% Confidence 1.678 1.576 1.586 to to to to to to to limits about 1.727 0.272 1.658 1.513 1.621 slope 1.543 1.612 Table VII. The Major Weight Percentage Compositions of Some Reference Materials CRPG IAEA USGS Oxides NBS 91 NBS 613 VS-N S-5 GSB, GSC, GSD SiO, 67.53 72 55.57 67.75 64 6.01 2 13.44 2.37 14 -41203 0.081 0 4.14 0.57 7 Fez03 MgO 0.008 0 4.51 0.88 4 CaO 10.48 12 4.53 10.20 5 Na,O 8.48 14 5.95 12.08 4 3.25 0 8.12 0.68 4 K,O Total 95.839 100 96.26 94.53 102 conditions are necessary for a refractory element like barium to be highly dissociated (24),and the great extent of barium dissociation attained by the plasma jet is largely responsible for the high sensitivities and low detection limits achieved by this method. Since the dc argon plasma jet is capable of processing sample solutions with relatively high total solute concentrations, individual analyte elements need not be extracted before being determined. This greatly simplifies the general requirements for sample preparation and specifically makes possible the analysis of barium directly in comparatively concentrated whole-sample solutions of siliceous materials. The system for the formation and introduction of aerosols into the emission zone of the plasma jet is illustrated in Figure 1and is efficient enough to allow for a relatively low rate of solution aspiration. This helps make possible the application of the spectrometer/spectrograph to a multielement microanalytical technique. The monochromator in the optical system of the spectrometer/spectrograph is an echelle diffraction grating (25). This provides high degrees of (a) optical resolution, without generating “ghost” lines, and (b) optical dispersion, without the use of focal lengths greater than 0.75 m. Ingamells’ metaborate fusion technique is a relatively safe, simple, and efficient means of putting whole samples of siliceous material into stable, aqueous solution, without significant loss of any major, minor, or trace elements from samples deficient in halides. A mixture of any of a wide variety of siliceous samples with LiBOz can be fused to yield a homogeneous glass with a high degree of solubility in 4% “OB. This makes it possible to dissolve diatom ash samples a t a concentration high enough to render their relative contents of barium determinable with an analytical instrument which has sufficient sensitivity, precision, and accuracy. Moreover, the contribution of lithium by LiB02,to the sample solutions, results in a substantial enhancement of Ba” 455.40 nm. The essentially complete recoverability of a LiBOz glass from its graphite fluxing crucible makes IngameW metaborate fusion technique adaptable to the dissolution of microsamples of siliceous material. Therefore, this method is attractive to the biologist, who has to provide only small quantities of material (510 mg) for accurate analyses. Moreover, the stability of the instrument and the simplicity of the sample handling suggest that the entire method is easily adaptable for work a t sea. 1022
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Baker Kieselguhr 93.00 2.88 0.86 0.40 0.20 2.35 0.40 100.09
TUBING TYGON
TAPERED G L b S S T U 0 E
Figure 1. Modified system for the formation and introduction of aerosols into the emission zone of the plasma jet
Inconveniences. Eliminating barium contamination is the main inconvenience involved in the analytical method presented here. It should first be established that the particular lots of LBO2,”OB, and SiOz employed do not contain detectable amounts of barium. All labware should be thoroughly cleaned, and, whenever possible, the handling of diatom or diatom ash samples for chemical analysis should be done beneath a dust cover, since laboratory dust may contain easily detectable trace amounts of barium. Applicability to O t h e r Types of Samples. Ingamells’ metaborate fusion technique is directly applicable to nearly all siliceous materials and materials rich in carbonates or phosphates (IO), including many geological samples. The spectrometer/spectrograph can be used to determine barium in any sample dissolved by Ingamells’ method. However, care should be taken to ensure that nonspectral matrix effects present in analytical samples are closely approximated in standard solutions. This can be done by using natural standards with major constituencies which bracket those of
the samples being analyzed, by making up standards in a synthetic matrix similar to the average major composition of the materials being analyzed, or by using either of the previous methods and buffering samples and standards by dissolving in them a fairly large amount of an element with a low ionization potential, such as potassium or lithium (16, 26). Applicability to Other Elements. Without any further sample preparation, the spectrometer/spectrograph was employed to estimate the major compositions of IAEA S-5 and Kieselguhr which are included in Table VII. The synthetic blank solution and the solutions of NBS-91, VS-N, NBS 613, and GSB were used as calibration standards in these determinations. This same approach was used to estimate concentrations of iron and copper in three samples of diatom ash. The results of these analyses are included in Table IV. A detailed consideration of the use of the dc argon plasma jet to perform multielement emission spectrometric analyses of siliceous materials of diverse major compositions will be presented elsewhere.
(6) 2. A. Vinogradova and V. V. Kovalski’iy, Dokl. Akad. Nauk SSSR, Engl. transi. earth sci. sect., 147, 217 (1962). (7) J. P. Riley and I. Roth, J. Mar. Bioi. Assoc. U.K., 51, 63 (1971). (8) G. Thompson, D. C. Bankston, and L. Surprenant, unpublished work, 1972. (9) C. 0. Ingamells, Anal. Chim. Acta, 52, 323 (1970). (10) N. H. Suhr and C. 0. Ingamells, Anal. Chem. 38, 730 (1966). (1 1) G. J. Shugar, R. A. Shwglar, and L. Bauman, “Chemical Technicians‘ Ready Reference Handbook’ , McGraw-HiiI, New York, 1973, p 18. (12) R. R. L. Guillard and J. H. Ryther, Can. J. Mlcrobiol., 8, 229 (1962). (13) E. L. Crow, F. A. Davis, and M. W. Maxfield, “Statistics Manual”, Dover Publications, New York, 1960. (14) A. T. Myers, R. G. Havens, and W. W. Nlles, in “Developments in Applied Spectroscopy”, Vol. 8, E. L. Grove, Ed., Plenum Press, New York, 1970, p 132. (15) H. Jaffrezic, Talanta, 23, 497 (1976). (16) J. P. Wiilis, H. W. Fesq, E. J. D. Kable, and G. W. Berg, Can. Spectrosc., 14, 3 (1969). (17) D. W. Golightly and J. L. Harris, Appl. Spectrosc. 29, 233 (1975). (18) U S . Department of Commerce, Washington, D.C., “National Bureau of Standards Certificate of Analysis, Standard Reference Materials 612 (3-mm Thick Wafers), 613 (1-mm Thick Wafers), Trace Elements in a Glass Matrix (1972). (19) H. de la Roche and K. Govindaraju, Analusis, 2, 59 (1973). (20) J. Heinonen, InternationalAtomic Energy Agency, Laboratory Selbersdorf, Vienna, Austria, personal communication, 1974. (21) US. Department of Commerce, Washlngton, D.C., “Bureau of Standards Certificate of Analyses of Standard Sample No. 91, Opal Glass” (1931). (22) S. Greenfiekl, H. McD. McGsachin, and P. B. Smith, Talanta, 22, 1 (1975). (23) R. K. Skcgerbce, I. T. Urasa, and G. N. Coleman, Appl. Spectrosc., 30, 500 (1976). (24) B. V. L‘vov, “Atomic Absorption Spectrochemical Analysis”, translated by J. H. Dixon, American Elsevier Publishing Co., New York, 1970, pp 128-132. (25) P.N. Keliher and C. C. Wohlers, Anal. Chem., 48, 333A (1976). (26) P. W. J. M. Boumans, “Spectrochemical Excitation”, Plenum Press, New York, 1966, p 163.
ACKNOWLEDGMENT We thank V. T. Bowen and R. R. L. Guillard for helpful suggestions, M. H. Bothner, C. C. Woo, and R. Fabro for technical assistance, and P. B. Brewer, F. L. Sayles, and G. Thompson for critically reviewing the manuscript. LITERATURE CITED H. J. M. Bowen, J. Mar. Biol. Assoc. U.K., 35, 451 (1956). K. Bostrom, 0. Joensuu, and I. Brohm, Chem. Geol., 14, 255 (1974). Y. Chung, ,Earth Planet. Sci. Lett., 23, 125 (1974). A. C. Ng. M.S. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1975. (5) L. H. Ghan, J. M. Edmond, R. F. Stallard, W. S. Broecker, Y. C. Chung, R. F. Weiss, and T. L. Ku, Earth Planet. Sci. Left., 32, 258 (1976).
RECEIVED for review January 21, 1977. Accepted March 28, 1977. This research was funded by ERDA Contract E(111)-3563.00. Contribution No. 3912 from the Woods Hole Oceanographic Institution.
Gas Stripping, Sorption, and Thermal Desorption Procedures for Preconcentrating Volatile Polar Water-Soluble Organics from Water Samples for Analysis by Gas Chromatography P. P. K. Kuo, E. S. K. Chian,* F. B. DeWalle, and J. H. Kim Environmental Engineering, Civil Engineering Department, University of Illinois, Urbana, Illinois 6 180 1
The efficiencies of (1) the removal of organic compounds from aqueous solutlon by gas stripping, (2) the adsorption of the strlpped organics on TenaxGC adsorbent, and (3) the thermal desorption of the organics from the adsorbent were detennined uslng volatile polar water-soluble organics as the prlnclpal model compounds. Good mixing between the stripping gas and the liquid phase being stripped, as provided by the Bellar and Llchtenberg stripping apparatus, results In the effective removal of organics. Strlpping the compounds at an elevated temperature Is suggested. Breakthrough and displacement of lower alcohols occurs on the Tenax-GC under the experimental conditlons. The adsorption and the desorption efflclencies are better than 75% and 80%, respectlvely, for the majority of the compounds studied. The procedures for gas strlpping, sorption, and thermal desorption are dlscussed to achleve a reliable analytlcal method for determlnlng volatile polar water-soluble organics In water by gas strlpplng/gas chromatographlc analysis. The detection limits are at the ppb level or lower for most of the compounds studied.
Recent concern about the presence of possible carcinogenic
compounds in drinking water has led to increased interest in developing techniques for identifying and determining the concentrations of volatile organic compounds in water samples. The techniques reported by Bellar and Lichtenberg and others (1-3) for determining volatile organics in water has been extensively studied (1,4-6). In essence, the process involves gas stripping of the sample and adsorption of the organic compounds on a sorbent medium such as Tenax-GC followed by thermal desorption and analysis by gas chromatography (GC) or gas chromatography/mass spectrometry (GC/MS). Previous studies have focused on volatile organics which are not highly soluble in water, such as aliphatic and aromatic hydrocarbons, the higher alcohols, and the higher ketones (1, 4-9). This study examines the effectiveness of the technique for determining volatile polar organics (VPOs) such as the lower alcohols, ketones, aldehydes, and ethers, which are water soluble. Some of the VPOs are potentially toxic (10) or are precursors of many other toxic compounds formed as a result of chlorine disinfection processes (11). They have been widely found in municipal water supplies, such as that of New Orleans (3). Besides the gas stripping/gas chromatographic method, ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
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