33
Anal. Chem. 1984, 56,33-37
Elemental Analysis of Estuarine Sediments by Lithium Metaborate Fusion and Direct Current Plasma Emission Spectrometry A. Y. Cantillo,*' S. A. Sinex,2and G.
R. Helz
Department of Chemistry, University of Maryland, College Park, Maryland 20742
The elemental analysis of sediments and related materials by itthium metaborate fudon and DC plasma emission spectrometry has been evaluated during the course of a major geochemical survey. Coeff icients of variation of replicate anaiyses for malor and trace elements were mostly in the 2-10% range, cornparable to reproducibility in routine atomic absorption work but poorer than reported in the best inductively coupled plasma emission work. Evaluation of systematic errors by use of geochemical standards indicated good agreement wlth the published results for Si, TI, V, Mn, NI, Cu, and Zn. The results for Ai, Cr, and Fe were slightly low, and those for Yb and Zr were substdntlaily low. Poor correlations with published data were obtained for La, Ce, and Zr. This method possesses many features that make it attractive for geochemical monitoring, but gnaiysts will have to exercise great care to avoid systematic errors.
Several groups interested in geological samples have described multichannel, inductively coupled plasma (ICP) emission spectrometric systems that are capable of high sample throughput and accuracy, using on-line corrections for spectral interferences (1-4). However, the instrumentation costs for these systems are large and therefore they are likely not to be economical in small laboratories. Commercial direct current plasma (DCP) emission systems are now available for a small fraction of the cost of multichannel ICP systems. They share some of the advantages of ICP, including often competitive detection limits (5). Single-channel versions of these systems are free from interelement interferences resulting from contamination of one channel by light from the slit of another (6). Also, because there is no photomultiplier crowding problem, single channel units permit greater freedom than multichannel systems in selecting analytical wavelengths to minimize spectral interferences. Obviously, single channel units do not have the same high sample throughput capacity that multichannel systems do. However in the analysis of rocks, soils, and sediments, this disadvantage can be partly offset by greater efficiency in sample preparation because of the ability of the DC plasma source to tolerate samples with a high dissolved solids content. Some ICP sources are intolerant of such samples (7), and previous workers who have analyzed geologic materials with ICP instruments have usually used acid digestion procedures in preference to faster fusion procedures (2-4). Because of a favorable earlier report (8)on the analysis of geological materials by a procedure involving lithium metaborate fusion followed by measurement with a single-channel DCP emission spectrometer, we adopted this approach in a Present address: Office of Oceanography and Marine Services, National Oceanic and Atmospheric Administration, Rockville, MD 20852
Present address: Department of Physical Sciences, Prince Georges Community College, Largo, MD 20772.
Table I. Geochemical Reference Standards: Descriptions and Sources std rock designation
description
source
ref a 27 34 19-23, 27-29 34 19-21, 23, 25, 27 26 35 35 19-21, 23, 25, 27, 28 27
BIR-1 BCSS-1 MAG-1 MESS-1 SDC-1
Icelandic basalt marine sediment marine mud marine sediment mica schist
USGS NRCC USGS NRC US GS
SL-1 SRM-1645 SRM-1646 QLO-1
lake sediment river sediment estuarine sediment quartz latite
IAEA NBSe NBS USGS
w-2
Centerville diabase USGS
More extensive compilations of data are also available (ref 32, 33, and others). U.S.Geological Survey, Reston, VA 22902. National Research Council, Chemistry Division, Montreal Road, Ottawa, Ontario K1A OR9, Canada. International Atomic Energy Agency, Laboratory Seibersdorf, Vienna, Austria. e U.S. National Bureau of Standards, Washington, DC 20234. a
large-scale study of major and minor elements in estuarine sediments (9-11). This paper contains an appraisal of our experiences with this method. EXPERIMENTAL SECTION Procedure. A modification of the lithium metaborate fusion method developed by Suhr and Ingamells (12,13) was used in the analysis. Approximately 0.2 g of sample was weighed to the nearest tenth of a milligram into a 7.5 cm3 drill point graphite crucible (Ultra Carbon Corp., Bay City, MI) containing 1.0 g of lithium metaborate. The sediment was placed in a small depression made in the borate flux to prevent incomplete fusion problems that are encountered when sediment is in direct contact with graphite. The sediment-flux mixture was fused in a muffle furnace at 950 50 "C for 15 min. The liquid borate bead was poured into a 150-mLTeflon FEP beaker containing 100 mL of 5% nitric acid (measured from a graduated cylinder). The beaker was then placed on a magnetic stirrer, and the contents were stirred for approximately 10 min. The resulting solution was transferred to an acid-washed 125-mL linear polyethylene bottle. Blanks containing only lithium metaborate were also fused in the manner described above. Reagents. Throughoutthe analytical work, deionized, distilled water was used. The lithium metaborate (LiB02)was Baker analyzed grade. The nitric acid used was analytical grade meeting American Chemical Society specifications. All linear polyethylene and Teflon FEP labware was washed in concentrated nitric acid and rinsed with deionized distilled water following a procedure similar to that recommended by Moody and Lindstrom (14). Stock standard solutions were prepared in a manner similar to that in Dean and Rains (15) or purchased from Alfa Inorganics (Danvers, MA). Working standards were prepared by dilution of the stock solutions with a solution of LiN03in 5 % nitric acid such that the lithium concentrations of both the standards and
*
0003-2700/84/0356-0033$01.50/00 1983 American Chemical Society
34
ANALYTICAL CHEMISTRY, VOL. 56, NO. 1, JANUARY 1984
-I_--__
____--_
Table 11. Emission Lines (nm) and Lowest Detectable Concentrations in Sediments (pg/g Except Where mg/g) emission line? nm
lowest detectable concn in sediments
lowest detectable concn in sediments
emission line, nm
A1 Si Ti V Cr Mn Fe co
396.2 Ga 417.2 2 288.2 Y 371.0 2 334.9 Zr 339.2 6 347.9 Nb 408.0 357.9 Mo 319.8 403.1 La 394.9 5 373.7 Ce 418.7 10 340.5 1 353.2 DY Ni 341.5 Yb 369.4 1 cu 324.8 W 400.9 Zn 213.9 T1 351.9 a Wavelengths are reported only to 0.1 nm because of the physical limitations of setting the monochromator. Composition on a dry weight basis that will produce a solution by the described procedure with a concentration that will produce a signal twice the average standard deviation after subtraction of the blank,
-
-
~
the sample solutions matched. The geochemical standards used and their sources are listed in Table I. Apparatus. Analysis of the borate solutions was done by direct current argon plasma emission spectrometry with a Spectraspan IV Echelle spectrometer equipped with the Spectrajet I11 DC plasma source (Spectrametrices, Andover, MA). Conditions for analysis were as recommended by the manufacturer, and the smallest available slits were used. The emission wavelengths used are listed in Table 11. The spectral band-pass in the worst case was 0.02 nm. Tables of emission lines were checked for possible interferencesbefore selecting analytical wavelengths (IO). Analysis of the sediment-borate solutions commenced as soon as possible after fusion. Concentrations were calculated from three 5-s integrated emission signals. The detection limit will be defined as the concentration of an element in 0.2g of sample that, after being fused with 1 g of LBOz and dissolved in 100 mL of aqueous solution, gives an emission signal, less blank, equal to twice the standard deviation of the background (16). Detection limits per unit weight of solid sample are listed in Table 11. These detection limits are specific for our procedure. Doubling the sample weight or halving the final volume will halve the detection limits assuming no change in standard deviation. Lowering the flux to sample weight ratios, however, can result in recovery problems for some elements. The graphite crucibles lose up to 10% of their initial mass during the fusion process and their surfaces are thus continuously degraded. Part of the mass of graphite lost during firing is trapped in the molten borate bead and is transferred to the nitric acidborate solution. This results in the presence of black particles in the solution which are insoluble in acid, nonmagnetic,and range in size from fractions of a millimeter to millimeters. The formation of these particles showed no correlation with sample characteristics or fusion time. Fusion temperatures above 1100 "C appeared to decrease the formation of particles. Comparison between two fusions of the same sample, one in which particles appeared and one in which particles did not, showed no significant differences in the concentrations of the elements determined (IO). RESULTS AND DISCUSSION Development and Testing. The lithium enhancement of the emission signal previously reported in the literature (17) was observed in this work. The concentration of Li+ was kept constant for both the standard and sample solution to minimize this effect. The calibration curves of all the elements analyzed were found to be linear up to 1000 bg/L, except for those of Mn, Al, Si, Ti, and Fe which were linear up to 200 pg/L, 200,800,10,and 200 mg/L, respectively. The effect of variations in the time of fusion was investigated by using two samples of sediment from the Chesapeake Bay and the NBS river sediment standard (SRM-1645).Four samples of each of the above were fused for 15,30,45,and 60 min, and the resulting molten borate bead dissolved and was analyzed as previously described. No significant differences due to variations in the time of fusion were noted (IO).
I _ _
2t Zn
-
0
20
;i' 5
v
i ' w w e +, v * ~ a
0
0 50
0
60
40
200
600
400
't
6: '" ":" r;
100
150
0
200
400
600
CONCENTRATI ON (pg /g ) Flgure 1. Comparison of results obtained within 1 month of fusion and during reanalysis of the same solutions 2 to 10 months later (pg/g dry weight). Sediment-borate solutions were stored at room temperature before reanalysis.
To determine whether the ACS reagent grade nitric acid used to dissolve the molten borate bead contributes significantly to the blanks, blanks were prepared with ultrapure nitric acid (G. Frederick Smith Chemical Co., Columbus, OH). All the values were comparable except in the case of Mn and La, which were higher in the ACS reagent grade nitric acid blank, and Ni and Zn, which were higher in the ultrapure acid blank. It was concluded that use of ultrapure acid offered no particular advantage over reagent grade acid. The possibility of contaminating samples during fusion was investigated by comparing blanks prepared from fused LiBOz with blanks prepared by simply dissolving LiBOz reagent. Differences were negligible, indicating that this type of contamination was not a problem for the elements under study. The stability of the solutions was tested by reanalysis several months after original preparation, of a randomly selected set of 30 solutions prepared from a suite of more than 300 sediment samples from the Chesapeake Bay. Typical results are shown in Figure 1. The newly determined concentrations were within 10% or better of the original values. The largest differences were observed for Co and Cu (not shown in Figure 1). In all cases, the reproducibility of the measurements was worse in solutions with concentrations close to the lower limit of detection. No evidence for systematic loss with time beyond the 10% range was observable of any for the elements discussed in this paper. Precision and Accuracy. The precision of the analytical method was evaluated by performing replicate analysis (n = 6) of the USGS marine mud (MAG-l), the IAEA lake sediment (SL-l), the NBS estuarine sediment (SRM-1646)
ANALYTICAL CHEMISTRY, VOL. 56, NO. 1, JANUARY 1984
Table 111. Comparison of Replicate Analysis of the USGS Marine Mud (MAG-1) (gg/g Dry Weight Except Where Percent) analyzed analyzed simultaneously over 10 months ( n = 6)a ( n= 15)b A1 (%) Si (%) Ti (%) V Cr Mn Fe (%) co Ni
cu
Zn Ga Y Zr La Ce DY Yb
7.6 (
