I
dilute mercury solutions would seem to involve considerable risk. "03 Solutions Containing C r ~ 0 ~(Figures ~ 7 and 8). In glass, the combination of 1%(v/v) H N 0 3 and 0.01% C r ~ 0 7 ~did - not prevent a rapid initial drop of Hg concentration (possibly due to adherence of hydrolyzed mercury salts to the walls), although little or no loss seems to have occurred after the first day. The combination of 5% (v/v) "03 and 0.01% Cr20i2- was quite successful, however. Under the latter conditions, no detectable losses occurred a t either the 0.1, 1.0, or 10.0 ng Hg/ml levels. Other tests under these conditions showed 10 ng/ml and 1 pg/ml Hg solutions to be equally stable for over five months. In polyethylene (Figure 8), it was necessary to increase the concentration of CrzO72- to 0.05% in order to achieve good stability. Quasi-Neutral Solutions Containing Cr20+- (Figure 9). To check on the possibility that the success of the 5% HN03-Crz072- combination might have been due to some property other than the high oxidation potential of the solution, tests were also run in a pH 6.0 solution containing 0.01% Cr2O72-. The behavior shown in Figure 9 is very similar to that shown by the 1% (v/v) "03CrzO72- curves of Figure 7, indicating that C r ~ 0 - i ~and/ or C r 0 & ion in itself cannot prevent a rapid initial decrease in Hg concentration at these levels (1 and 10 ng/ ml) in glass. No experiments of this type were run in polyethylene.
CONCLUSIONS Storage of Dilute Aqueous Hg Standards. Distilled water solutions containing 20.1 ng Hg/ml can be stored in glass without deterioratipn for as long as five months if the solution contains 5% (v/v) HNOs and 0.01% Cr2O+(added as either K2Cr20, or CrO3). Storage of such standards is safe in polyethylene containers for a t least 10 and 0.05% days if the solution contains 5% (v/v) "03 C r 2 0 ~ ~ The - . efficacy of this mixture is probably due to its ability to prevent the hydrolysis of dissolved mercury and to prevent its reduction to valencies lower than +2. Storage of Other Dilute Solutions of Mercury. Natural and waste waters almost always contain suspended and dissolved impurities which can adsorb, complex, and/ or reduce any mercury present. Treating the collecting vessel in advance with sufficient concentrated HNOs and KzCrz0, to make the final sample 5% (v/v) and 0.05%, respectively, in these reagents should help to maintain the mercury level in such solutions, but the treatment may not be able to provide protection against all types and levels of impurities. The precautions to be used in specific cases must be chosen to deal with the conditions existing. Received for review April 30, 1973. Accepted July 24, 1973. This research was supported by the NSF-RANN Program under NSF Interagency Agreement No. 40-23770, and performed a t the Oak Ridge National Laboratory, operated by Union Carbide Corporation under contract with the U.S. Atomic Energy Commission.
Solvent Extraction of Metal Chelates into Water-Immiscible Acetone Charles E. Matkovich' and Gary D. Christian* Department of Chemistry, University of Kentucky, Lexington, Ky. 40506 and Department of Chemistry, University of Washington, Seattle, Wash. 98195
Acetone is separated from aqueous solutions via saltingout with either saturated calcium chloride or 65 wt % sucrose. Several metal ammonium l-pyrrolidinecarbodithioate and dithizone chelates were successfully extracted from calcium chloride solutions into the acetone phase while metal oxine chelates were extracted from sucrose solutions. Solvent extraction curves are reported.
Acetone has been demonstrated to be the solvent of choice for obtaining optimal detection limits in flame spectrometry for a number of elements (1-3). If solvent extraction with this solvent were possible, maximum advantage could be taken of its beneficial effects. The separation of acetone from aqueous solutions for performing solvent extraction has recently been demonstrated, using Present address, Crompton & Knowles Corporation, Althouse Division, Reading, Pa. 19603. Present address, Department of Chemistry, University of Washington, Seattle, Wash. 98195. (1) J. W. Robinson, Anal. Chim. Acta, 23,479 (1960). (2) FEJ. Feldrnan, R . E. Bosshart, and G. D. Christian, Anal. Chem., 39, 1175 (1967).
(3) F. J . Feldrnan and G. D.Christian,Can. Spectrosc.,l4, 80 (1969).
102
salting-out of the acetone for phase separation ( 4 ) . The best salting-out agents of the 79 studied were calcium chloride, magnesium chloride, and sucrose. The solvent extraction of radiotracer cobalt(I1) from saturated calcium chloride as the ammonium 1-pyrrolidinecarbodithioate (APCD) chelate was successfully performed. The present paper describes the solvent extraction of several metal APCD, dithizone, and oxine chelates with acetone using calcium chloride and sucrose as salting-out agents.
EXPERIMENTAL Reagent grade chemicals were used whenever possible. Saturated calcium chloride solutions were prepared at room temperature and were allowed to sit in the presence of solid calcium chloride with intermittent shaking for at least seven days before being used in an experiment. Dithizone was purified by recrystallization from chloroform. Saturated calcium chloride solutions were purified before performing dithizone extractions by adjusting the pH to the desired value and then adding an equal volume of a 50-ppm solution of dithizone in acetone. The mixture was shaken and the acetone phase discarded. The pre-extraction was repeated until no detectable impurities were extracted. This procedure also served to presaturate the calcium chloride solution with acetone. (4) C. E. Matkovich and G . D. Christian, Anal. Chem.. 45, 1915 (1973)
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
Solvent extraction equilibria with calcium chloride solutions and dithizone were studied as follows. A solution of the trace element a t the desired concentration (1-10 ppm) was prepared in 7 acetone containing 50 ppm dithizone. Equal portions ( 2 ml) of the 6 pH adjusted, pre-extracted aqueous calcium chloride solution and of the acetone solution were mixed in a centrifuge tube using a 5 r mechanical vortex mixer and then were allowed to sit for 15 mina 4 utes. The acetone phase was then separated and analyzed by flame spectrometry to determine the concentration of metal. A 3 blank was prepared by treating an equal aliquot of the pre-ex2 tracted calcium chloride with a 50-ppm solution of dithizone in l acetone. Aliquots of the standard metal solution in acetone were used for calibration, Extraction results were similar to those ob0 2 4 6 8 IO I2 14 16 18 20 22 tained by adding the trace metal initially to the aqueous phase. A similar procedure was used for APCD extractions except that VOLUME o f 01057 HCI,rnl 0.01 ml of freshly prepared 1% APCD solution and the correct Figure 1. Titration of aqueous calcium chloride solution amount of 1000-ppm stock metal solution were added to the Initial solution contained 30 ml of saturated calcium chlorlde saturated aqueous phase rather than the acetone phase.'The saturated calwith acetone, pius 20 ml of water cium chloride solutions were pre-extracted with APCD and acetone in a similar manner. Solvent extractions with oxine were performed using sucrose as the salting-out agent. A stock sucrose solution of 11.6rn was preTable I. pH of 100 ml of 60% Saturated Calcium Chloride pared. This was diluted 1:2 with a sample solution containing the before and after Preliminary Extractions sumetal of interest to give a concentration of j.8nz or 65 wt IC crose, which was arbitrarily chosen as the concentration for solApproximate volume of HCI vent extraction studies ( 4 ) . An extraction procedure similar to the (ml) and one described for dithizone extractions of calcium chloride solunormality PH" PH* tions was used. except solutions were allowed to sit for 30 minutes 2.25 (11.6N) 0.05 0.20 prior to separation of the phases. Atomic absorption spectrometry and flame emission spectrome0.45 0.50 1.05 try measurements were made with a n Instrumentation Laborato1.08 1.15 0.85 ry Model 153 Atomic Absorption/Flame Emission Spectropho1.55 1.50 0.55 tometer using either an air-acetylene or nitrous oxide-acetylene 0.85 (6N) 1.90 2.00 flame. pH measurements were made with an Instrumentation 0.75 2.62 2.80 Laboratory Model 245 Delta-Matic pH meter using a glass-satu0.70 3.05 3.15 rated calomel electrode pair with asbestos fiber junction.
1
0.60
RESULTS AND DISCUSSION Solvent Extractions from Saturated Calcium Chloride Solutions. The measured pH of an aqueous saturated calcium chloride solution which was saturated with acetone was 7.67 f 0.08. The absolute magnitude of the measured pH is uncertain a t these high salt concentrations. Critchfield and Johnson ( 5 ) have calculated the liquid junction potential for 3.75M sodium chloride to be 4.4 mV, which represents an error of only 0.07 p H unit. It is likely, therefore, that measured pH values in saturated calcium chloride are within a few tenths of a unit of the actual value. The acetone saturated solutions, however, contain about 27% (vol/vol) of acetone ( 4 ) , imparting a partially nonaqueous character to the solutions. Because of uncertainties in pH measurements, therefore, all measured values are considered nominal p H values. Thirty milliliters of the aqueous solution saturated with calcium chloride and acetone were diluted with 20 ml of water and titrated with 0.1N hydrochloric acid. The titration curve is shown in Figure 1. It appears from this curve that a weak base impurity was present in the solution. The presence of high concentrations of neutral salts is known to cause an enhancement in the breaks of titration curves of weak bases in either aqueous (6) or nonaqueous ( 7 ) solutions. The impurity did not appear to affect solvent extraction results. The pH of calcium chloride solutions was adjusted before the addition of acetone. Concentrated hydrochloric acid was used to prepare the very acidic solutions in order to minimize dilution of the calcium chloride. A comparison of the measured p H values before and after pre-extraction with acetone is given in Table I. The latter values were used in constructing solvent extraction curves. They were generally slightly lower in alkaline solution than the original pH values and slightly higher in acid solution. (5) F. E. Critchfield and J . B. Johnson, Anal. Chem., 31, 570 (1959). (6) F. E. Critchfield and J. B.Johnson, Anal. Chem., 30, 1247 (1958) (7) W.L. Schertz and G . D. Christian,Anal. Chem.. 44, 755 (1972).
3.50 4.00
0.40 0.30 1.65 1.40 0.95 0.60 0.40 0.35 0.25 0.15
3.70 4.15 4.60 5.10 5.60 6.10 6.50 7.00 7.38 7.90 8.40 8.90
4.50 5.00 5.50
(1N)
6.04 6.56 7.05 7.43 7.98 8.50 8.95
...
Before equilibration with dithizone in acetone. with dithizone in acetone. a
After equilibration
Table II. Stability of pH Adjusted Calcium Chloride Solutions PH p H after p H after initial 24 hours 48 hours 0.1 5 1 .oo 2.00 3.20 4.00 4.60
5.00 6.05 6.90 8.1 0 8.85
0.20
1 .oo 2.00 3.28 3.40 4.71 5.02 6.06 6.88
8.10 8.82
0.20 1 .oo 2.00 3.30 3.15 4.50
5.05 6.10 6.90 8.00 8.80
The stability of p H adjusted calcium chloride solutions was tested and the results are listed in Table 11. The pH adjusted solutions were not too stable in the p H range between 3 and 5 . This is the pH range corresponding to the end point observed in the titration of aqueous calcium chloride.
A N A L Y T I C A L CHEMISTRY, VOL. 46,
NO. 1 ,
JANUARY 1974
103
I
I
I
C
x 20 W
g
-
IO0
k
5
90 4ppm
80 0 70
Fe(ll1)
K
60
501
50 40
::[
40 30
NO DlTHlZONE
IO
2o
-
-
-
NO DlTHlZONE
--
IO 0
I
2
3 4
5 6
7
8 9 0
I
2
3 4
5 6
7 8 9 IO
0
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NO DlTHlZONE
. A. 1
1
1
1
'
1
'
1
1
l
,&
AAA.*
I
1
f
'
l
I
1
l
NOMINAL pH Figure 2.
Solvent extraction curves of metal dithizonates
Salting-out agent, saturated calcium chloride; extracting agent, 50 ppm dithizone in acetone
The solvent extraction curves for metal dithizonates in saturated calcium chloride are shown in Figures 2 and 3. The curves with no dithizone were obtained for solutions containing the same concentration of metal but no chelating agent. The order of extractability of metal dithizonates with chloroform or carbon tetrachloride is given by Morrison and Freiser (8) as: copper(I1) > iron(I1) > cobalt-' (11) > nickel(I1) > zinc(I1) > manganese(I1). The present results follow the same general order, although the minimum nominal p H values for extraction are generally lower than the p H value in the former systems. Scandium, chromium, titanium, and vanadium dithizonates do not exist or are very unstable. Scandium(II1) has not been reported to form a stable complex with dithizone (8, 9). I t does, however, form a strong chloro complex and has been observed to extract from 8M HCL into tributylphosphate (8). Figure 2A indicates that appreciable extraction of scandium takes place a t all pH's studied in the absence of the chelating agent but that a slight enhancement occurs in the presence of the dithizone. This implies that the chloro complex of scandium is responsible for a large part of the observed scandium extraction into acetone but that a weak chelate with dithizone does exist. Chromium(II1) does not form a dithizonate complex (8, 9) and no extraction of chromium was observed in this study. Manganese(I1) forms a relatively unstable complex with dithizone which ext,racts in basic solutions (8, 9). Solutions of manganese dithizonates decompose by induced oxidation of the complex by the manganese. Reducing agents such as hydrazine or hydroxylamine will prevent this oxidation and are strongly recommended for manganese extractions ( 9 ) . Identical extraction curves were obtained for both 2 pprn and 5 pprn manganese (Figure 2 B ) . A similar extraction curve was obtained for 100 ppm manganese. The reproducible results of manganese extractions (8) G . H. Morrison and H. Freiser, "Solvent Extraction in Analytical Chemistry," John Wiley 8 Sons, New York, N.Y.. 1957. (9) J. Stary. "The Solvent Extraction of Metal Chelates," Pergamon Press, New York, N.Y., 1964.
104
Salting-out agent, saturated calcium chloride; extracting agent, 50 ppm dithizone in acetone
into acetone indicate that manganese dithizonates in acetone are less susceptible to oxidation than manganese dithizonates in chloroform or carbon tetrachloride. Iron(I1) has been observed to extract into carbon tetrachloride in the p H range 6 to 7 (6) and 7 to 9 ( 9 ) .In alkaline solution, it oxidizes to iron(II1) (9). The observed pH range of extraction in this study does not agree with the pH range of dithizonate extractions with chloroform or carbon tetrachloride. Iron(II1) is reported not to form a dithizonate (8, 9). However, it does form a very strong chloro complex (8) and can be extracted from 6-7N HCL into various organic solvents. The appreciable extraction of iron(II1) in this study in the absence of a chelating agent was probably due to formation of chloro ion-association complexes. It was surprising that iron(II1) extracted efficiently into acetone in the presence of dithizone. In basic medium, high concentrations of iron(II1) can oxidize dithizone or precipitate as iron hydroxide and interfere in the extraction of other metals (9). Cobalt(I1) is quantitatively extracted with dithizone into carbon tetrachloride at a pH of 5.5 t o 8.5 (9) and into chloroform a t a pH of 8 (9) or 7 to 9 ( 8 ) . It partially extracts from either HCl or calcium chloride solutions (8).A 1.70.N calcium chloride solution effects a 9.1% extraction of cobalt into 2-octanol (9). The calcium chloride concentration was much higher in the present study (about 5.8M) and may account for much of the extraction of cobalt in acid solution. Nickel(I1) can be completely extracted with dithizone into carbon tetrachloride a t a pH of 8 to 11 (9). Extraction a t nominal pH values similar to this was observed in this study. Nickel, like cobalt, forms a weak chloro complex in 4.5N HCl and 1.7N calcium chloride (8) and also, like cobalt, extracted a t lower pH's than expected for dithizone extraction. Copper(I1) quantitatively extracts as a primary dithizonate into carbon tetrachloride in very acidic solution (pH 1 to 4) (9). At a pH greater than 7, the secondary dithizonate forms and can be extracted. Copper could be effi-
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
Figure 4. Solvent extraction curves of metal-APCD chelates Salting-out agent, saturated calcium chloride; extracting agent, 0.005% APCD in the aqueous phase
ciently extracted from very acid solution in the present study. An appreciable amount was extracted in the absence of dithizone. Zinc(I1) quantitatively extracts with dithizone into carbon tetrachloride a t p H of 6 to 9.5 and into chloroform at p H of 7 to 10 (9). In acidic solution only partial extraction takes place but the per cent extraction can be increased with a large excess of dithizone (8),The curve in Figure 20 agrees well with the literature data for other solvents. Irreproducible results were obtained in the extraction of titanium and vanadium with dithizone due to the unstable dithizone chelates of these metals or to calcium interference in the analysis. Calcium was observed to interfere in the atomic absorption analysis of both vanadium and titanium in the nitrous oxide-acetylene flame (10). Solvent extractions from calcium chloride solutions with oxine as the chelating agent proved unsuccessful due to the formation of the calcium oxinate. Extractions with oxine could be performed in sucrose solutions, however (see below). Extractions with APCD as the chelating agent were successful. The results are illustrated in Figure 4. The pH ranges for maximum extraction of most of the metalAPCD chelates studied in this experiment agree with the data presented for the same chelating agent using different organic solvents (9, 1 1 ) . Solvent extractions in the presence of large concentrations of salting-out agents are much more complicated than conventional systems without salting-out agents. The high chloride ion concentration favors the formation of metal chloro complexes. A general solvation form for the metal in these systems can be written as: M n L ( H 2 0 ,(ace) tone)H(HCl)r(chelating agent)D(chelating agent anion)s. (C1 -)F. The undissociated chelating agent and hydrochloric acid probably are not important in the complex. The chloride ion concentration was primarily from the salting-out agent and was, therefore, relatively independent of the pH. As the p H changes, the exact form of the most stable solvated metal species may change from chelate to chloro complexes. In acidic solutions, the dissociation of the chelating agent is repressed whereas the chloride ion concentration remains essentially the same. Thus, the metal has a greater tendency in acidic solutions to form a chloro complex. The chloro complex may or may not be soluble in the organic phase (as an ion association complex or via salting-out). The metal chelate, on the other hand, is not soluble in the aqueous phase. The formation of chloro complexes and ion pairs may account for some of the differences between the observed and literature extraction curves. (10) J. W. Husler and E.F. Cruft, Ana/. Chem.. 41, 1688 (1969). (11) G . D. Christian and F. J. Feldman, "Atomic Absorption Spectroscopy. Applications in Agriculture, Biology, and Medicine." Wiley-lnterscience, New York, N.Y., 1970.
~~~
Table I l l . Impurities in Calcium Chloride
Element
Ag Cd Cr
cu Mn
Mo NI
Zn
Concn in satd calcium chloride ppm
Solid CaC12u
Element
Wt
"a
20
a
Sr
05
Na
5
K
0 05 2 01
Mg
90
Fe Heavy metals as P b 0a
01 0 02 0 01 0 005 0 0005 0 0005
0 005
a Manufacturers assay
Some metal salts exhibit appreciable solubility in acetone-for example, manganese (2) and cobalt ( 3 ) .Cobalt chloride and copper chloride are soluble in acetone ( 2 2 ) and, hence, some of these metal species are found in the acetone phase in the absence of a chelating agent. Zinc chloride was experimentally observed to be soluble in acetone also. Most dithizonate chelates have different optimum pH ranges for carbon tetrachloride and chloroform extractions due to differences in solubility in the two solvents (8) and, therefore, changes in the optimum p H range for extraction into acetone as the solvent were expected and observed. Most of the extraction efficiencies in this study were less than 100%. This is primarily because equilibrium is only about 90% achieved after 15 minutes 14). By centrifuging samples or allowing them to sit for two hours, 100% extraction efficiency can be obtained in favorable cases ( 4 ) . In actual use of this solvent extraction system, the saturated calcium chloride solution would normally be diluted with an aqueous sample. Fortunately, the per cent recovery of acetone increases to a maximum at about threefourths saturated calcium chloride, and in this region the solvent recovery (and hence concentration of the extracted metal) is not greatly dependent on the calcium chloride concentration ( 4 ) . The per cent extraction of metal chelates should not differ greatly from those reported here. Impurities in Calcium Chloride. The trace metal content of saturated calcium chloride was determined by solvent extraction with APCD and acetone, followed by atomic absorption analysis. APCD was chosen as the chelating agent because of its ability to chelate with a large number of metals in acid solution (13, 1 4 ) . A 100-ml portion of saturated aqueous calcium chloride solution was (12) "Handbook of Chemistry and Physics," 50th ed., The Chemical Rubber Co., Cleveland, Ohio, 1969. (13) H . Malissa and E. Schoffman, Mikrochim. A c t a . 1955, 187. (14) H.Malissaand S. Gomiscek. Z. Anal. Chem., 169,401 (1959).
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 1 , J A N U A R Y 1974
105
'
01 ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' 0 I 2 3 4 5 6 7 8 9 1 0 1 2 3 4 5 6 7 8 9
NOMINAL
pH
Figure 5. Solvent extraction curves of metal oxinates Salting-out agent, 65 wt % sucrose: extracting agent, 50 pprn oxine
In
acetone
adjusted with acetic acid to the p H of maximum extraction of the desired metal. Three milliliters of freshly prepared 1% APCD solution were added and solvent extraction was performed with two 50-ml portions of acetone. The results are summarized in Table III. The concentrations were calculated assuming complete extraction of the impurities. Significant quantities of all the elements tested were present and would interfere in many analyses. However, purification of the calcium chloride solution is sufficiently and easily performed by pre-extraction. Other compounds potentially useful as salting-out agents (4) also contained impurities. For example, a saturated ACS reagent grade aluminum chloride solution was observed to contain the following concentrations of impurities: iron, 110 ppm; nickel, 1.5 ppm; cobalt, 0.005 ppm; copper, 0.3 ppm; and manganese, 0.2 ppm.
Solvent Extractions from Sucrose Solutions. Sucrose was chosen for further study as a salting-out agent because of its lack of transition metal impurities and the absence of a complexing cation. Extractions were performed using 65 wt % sucrose as the salting-out agent. The results are shown in Figure 5 . In all cases, the solvent extraction curves of metal oxinates using acetone were similar to those found using chloroform (9). Titanium(1V) is completely extracted by 0.1N oxine in chloroform in the pH range 2.5-9.0. Similarly, vanadium(V) is quantitatively extracted into chloroform at pH 2-6 (9). It is not extracted above pH 9. The extraction curve observed here for vanadium is very similar to the one reported by Talvitie (15). Iron(I1) is not extracted below a pH of 4 (9). In neutral and basic solutions, it is oxidized to iron(II1) and extracted as the iron(II1)-oxine chelate. Iron(II1) extracts with 0.01 to 0.1M oxine in chloroform in the pH range 2 to 10. Copper(I1) extracts into chloroform with 0.1M oxine in the pH range 2-12. Solvent extraction with sucrose as the salting-out agent is not as convenient as with calcium chloride solutions because of the high viscosity of the solutions. In addition, the sucrose concentration, pH, temperature. and volume have to be very carefully controlled (4). However, the use of sucrose as a salting-out agent does permit the use of chelating agents that would react with calcium. Applications. Since the atomic absorption measurement of manganese exhibits minimum detectability in acetone (2), the solvent extraction of the manganese-dithizone chelate into acetone is being evaluated for the determination of physiological levels of manganese in digested blood samples. Preliminary results are more precise than with other procedures and the method requires less sample. This study is being continued.
Received for review May 7, 1973. Accepted July 26, 1973. The work on which this report is based was supported in part by the Office of Water Resources Research, Department of the Interior, under provisions of Public Law 89379, as Project No. A-013-KY. 115) N . A . Talvitie, Ana/. Chem., 25, 604 (1953)
Stable Isotope Dilution Applied to the Determination of Zirconium in Geological and Lunar Samples Shin Tsuge, J. J. Leary, and T. L. lsenhour
Department of Chemistry, University of North Caroiina. Chapei H i / / . N . C . 27514 The method of metal labeled stable isotope dilution (MLSID) is developed in detail. This technique is then applied to the microdetermination of zirconium in standard geological and lunar samples as the volatile chelate tetrakis (1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionato) Zr(lV) [ Z r ( f ~ d ) ~by ] means of a mass spectrometer equipped with a voltage peak switching facility. Samples analyzed include a USGS standard ( A G V - l ) , a CAAS standard (SY-l),and five Apollo 14 and 15 samples (1431 0,135; 14321,188: 15021,96: 15301,78: and 106
15471,30). This method of zirconium determination requires only small samples (10-40 mg), can be performed using an ordinary electron impact mass spectrometer, and is essentially free from matrix interferences.
Zirconium determination in geological samples has always been notoriously difficult. Large variations in the reported zirconium concentration of lunar samples indicate the need to improve existing methods and/or develop new methods of analysis to determine this element.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
