Unnatural Isotopic Composition of Lithium Reagents - Analytical


Isotopic analysis of 39 lithium reagents from several manufacturers indicates that seven were artificially depleted in 6Li significantly in excess of ...
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Anal. Chem. 1997, 69, 4076-4078

Technical Notes

Unnatural Isotopic Composition of Lithium Reagents H. P. Qi† and Tyler B. Coplen*

U.S. Geological Survey, 431 National Center, Reston, Virginia 20192 Q. Zh. Wang and Y. H. Wang

Institute of Salt Lakes, Chinese Sciences Academy, Xining, Qinghai 810008, China

Isotopic analysis of 39 lithium reagents from several manufacturers indicates that seven were artificially depleted in 6Li significantly in excess of the variation found in terrestrial materials. The atomic weight of lithium in analyzed reagents ranged from 6.939 to 6.996, and δ7Li, reported relative to L-SVEC lithium carbonate, ranged from -11 to +3013‰. This investigation indicates that 6Li-depleted reagents are now found on chemists’ shelves, and the labels of these 6Li-depleted reagents do not accurately reflect the atomic and (or) molecular weights of these reagents. In 1993, IUPAC issued the following statement: “Commercially available Li materials have atomic weights that range between 6.94 and 6.99; if a more accurate value is required, it must be determined for the specific material.” This statement has been found to be incorrect. In two of the 39 samples analyzed, the atomic weight of Li was in excess of 6.99. Lithium is composed of two isotopes, 6Li and 7Li. The fraction of 6Li in terrestrially occurring materials used to manufacture Li reagents ranges from 0.073 to 0.077.1 As early as 1968, Pauwels and others2 demonstrated that commercial lithium reagents showed undocumented depletion in 6Li that lowered the molecular weight of the reagents. In 1993, the Commission on Atomic Weights and Isotopic Abundances (CAWIA) of the International Union of Pure and Applied Chemistry (IUPAC) noted with concern the commercial dissemination of significant quantities of laboratory reagents that have been artificially depleted in 6Li, resulting in labels on containers of reagents with incorrect atomic weight values.3 To make chemists aware of this problem, the Commission treated Li specially in the 1993 (and succeeding) Table of Standard Atomic Weights of the Elements, which it updates biennially. To emphasize that 6Li-depleted materials are commonly available (especially in the United States and Europe), the value listed in the Table was enclosed in brackets and given the following footnote: “Commercially available Li materials have atomic weights that range between 6.94 and 6.99; if a more † Visiting from Chinese Sciences Academy. (1) Coplen, T. B.; et al., unpublished work. (2) Pauwels, J.; Lauer, K. F.; Duigou, Y. L.; De Bie`vre, P. J.; Derbus, G. H. Anal. Chim. Acta 1968, 43, 211-220. (3) IUPAC. Atomic Weights of the Elements 1993. Pure Appl. Chem. 1994, 66, 2423-2444.

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accurate value is required, it must be determined for the specific material.” Li laboratory reagents are commonly used as reference materials for Li isotopic measurements performed by inductively coupled plasma mass spectrometry (and by other methods, too). If a reagent selected by an analyst to serve as a calibration material has been artificially depleted in 6Li, then reported Li isotopic results of samples may be in error by significant amounts. Preparation of solutions using Li reagents that are unknowingly depleted in 6Li can lead to significant errors in solution concentration. This can be important in some chemical processes, such as in lithium niobate crystal growth. Precise knowledge and control of Li concentration is critical to ensure uniform lithium niobate crystals for integrated optical devices.4 In order to assess the magnitude of the problem of 6Li-depleted reagents residing on chemists’ shelves, 39 lithium reagents (from 11 vendors) commercially available in the United States were analyzed for their lithium isotopic composition. EXPERIMENTAL SECTION A VG-354 mass spectrometer at the Institute of Salt Lakes, Xining, Qinghai, China, was employed for Li isotope ratio measurements. It has an equivalent radius of 54 cm with a magnetic deflection of 90°. The rotating sample turret holds 16 filament inserts, and the resistance of the Faraday cup electrometer feedback resistor is 1 × 1011 Ω. The ion source employs double filaments. A Ta filament was loaded with the sample, and a Re filament provided thermal ions. Both filaments were degassed in vacuum prior to service. In the process of loading, 5 µg of B in the form of H3BO3 solution was placed on the Ta filament (in order to provide signal stability), through which a current of 1.2 A was passed to evaporate nearly all the water. A drop of sample solution containing 1-2 µg of Li was then added, and water was evaporated with the same current. Finally, the current was raised to 1.5 A for 3 min to ensure complete dryness. The filament loaded with sample was then inserted into the chamber of the ion source. The current of the ionizing filament was raised until the temperature reached 1450 °C. The sample filament was heated sufficiently to provide a 7Li ion beam of about 2.5 × 1011 A. The isotopic ratio, 6Li/7Li, was determined by scanning the m/z ) 6 and 7 peaks. For each sample loading, (4) Riley, J. E. Ferroelectrics 1987, 75, 59-62. S0003-2700(97)00466-6 CCC: $14.00

© 1997 American Chemical Society

Table 1. Li Isotopic Composition and Atomic Weight, Ar(Li), in 39 Reagents 6Li/7Li

no.

compound

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Li2CO3 Li2CO3 Li2CO3 LiCl LiCl LiCl LiCl LiCl LiCl LiF LiF LiOH Li2SO4 LiBr LiBr LiCl LiBO2 LiCl Li2CO3 Li2CO3 LiCl Li2SO4 Li2B4O7 Li2B4O7 Li2B4O7 LiOH LiNO3 Li2CO3 Li2CO3 Li2CO3 Li2CO3 Li2CO3 Li2CO3 LiNO3 LiCl LiF LiCl LiCl Li3PO4

a

vendor Baker Fisher Mallinckrodt Mallinckrodt Mallinckrodt Baker-Adamson Mallinckrodt Fisher Baker Fisher Fisher Fisher Mallinckrodt Fisher MCB Fisher Smith Fisher Certified Chemicals Baker Fisher Baker-Adamson AP&C Spex MCB Fisher Fisher Baker Baker Baker Baker Baker Baker Fisher Fisher K&K Aldrich Baker-Adamson Ventron

lot no. 801106 534080 DCH A060 DBM 780473 8152236 751742 712954 XNL-1 876458 R9147 734220 751100 1925 3242773 155 A111X294 7496 FS62 K1H10 745677 762479 3242789 8323325 8323320 OC1947 543489 8323323 792479 1966 714115 1121KK A080 121377

obsd

corra

fraction 7Li (in atom %)

δ7Li (in ‰ relative to L-SVEC)

Ar(Li)a

0.081 80 0.081 62 0.081 36 0.081 03 0.081 70 0.082 51 0.081 21 0.081 76 0.082 34 0.057 39 0.080 46 0.082 02 0.082 70 0.081 76 0.081 77 0.031 10 0.081 73 0.081 88 0.082 29 0.082 01 0.081 56 0.083 24 0.024 69 0.080 97 0.081 96 0.020 51 0.035 64 0.082 21 0.082 09 0.082 07 0.081 95 0.081 46 0.082 05 0.035 54 0.082 81 0.041 22 0.081 96 0.082 72 0.082 72

0.081 66 ( 0.000 40 0.081 48 ( 0.000 23 0.081 22 ( 0.000 23 0.080 89 ( 0.000 18 0.081 56 ( 0.00 08 0.082 37 ( 0.000 24 0.081 07 ( 0.000 23 0.081 62 ( 0.000 24 0.082 20 ( 0.000 3 0.057 29 ( 0.000 66 0.080 32 ( 0.000 19 0.081 87 ( 0.000 44 0.082 56 ( 0.000 63 0.081 62 ( 0.000 18 0.081 62 ( 0.000 65 0.031 05 ( 0.000 08 0.081 58 ( 0.000 54 0.081 74 ( 0.001 6 0.082 14 ( 0.000 18 0.081 87 ( 0.000 28 0.081 41 ( 0.000 19 0.083 09 ( 0.000 57 0.024 64 ( 0.000 07 0.080 83 ( 0.000 18 0.081 82 ( 0.000 35 0.020 48 ( 0.000 11 0.035 57 ( 0.000 1 0.082 06 ( 0.000 27 0.081 95 ( 0.000 18 0.081 93 ( 0.000 18 0.081 80 ( 0.000 32 0.081 32 ( 0.000 19 0.081 91 ( 0.000 2 0.035 48 ( 0.000 12 0.082 67 ( 0.000 24 0.041 15 ( 0.000 18 0.081 82 ( 0.000 31 0.082 58 ( 0.000 18 0.082 58 ( 0.000 18

92.456 92.466 92.488 92.516 92.459 92.390 92.501 92.454 92.404 94.581 92.565 92.433 92.374 92.454 92.454 96.989 92.457 92.444 92.409 92.433 92.472 92.328 97.595 92.521 92.437 97.993 96.565 92.416 92.426 92.427 92.439 92.480 92.429 96.574 92.364 96.048 92.437 92.360 92.360

6 8 12 16 7 -3 13 7 -1 434 23 3 -5 7 7 1646 7 5 0 4 9 -11 2333 16 4 3013 1309 1 3 3 4 10 3 1316 -6 997 4 -5 -5

6.940 40 ( 0.000 34 6.940 60 ( 0.000 20 6.940 80 ( 0.000 20 6.941 10 ( 0.000 15 6.940 50 ( 0.000 68 6.939 80 ( 0.000 21 6.940 95 ( 0.000 20 6.940 48 ( 0.000 21 6.939 98 ( 0.000 26 6.961 77 ( 0.000 59 6.941 59 ( 0.000 16 6.940 26 ( 0.000 38 6.939 67 ( 0.000 54 6.940 48 ( 0.000 15 6.940 48 ( 0.000 56 6.985 86 ( 0.000 08 6.940 51 ( 0.000 46 6.940 40 ( 0.001 4 6.940 03 ( 0.000 15 6.940 26 ( 0.000 24 6.940 66 ( 0.000 16 6.939 22 ( 0.000 49 6.991 93 ( 0.000 07 6.941 15 ( 0.000 15 6.940 30 ( 0.000 30 6.995 92 ( 0.000 11 6.981 62 ( 0.000 09 6.940 10 ( 0.000 23 6.940 19 ( 0.000 15 6.940 21 ( 0.000 15 6.940 32 ( 0.000 27 6.940 73 ( 0.000 16 6.940 23 ( 0.000 17 6.981 71 ( 0.000 11 6.939 58 ( 0.000 20 6.976 44 ( 0.000 17 6.940 30 ( 0.000 27 6.939 54 ( 0.000 15 6.939 54 ( 0.000 15

Uncertainty given to 1 standard deviation.

there were 10 blocks of measurements, where each block consisted of 10 ratio measurements. A sample had two or five loadings. RESULTS AND DISCUSSION The 39 Li reagents analyzed in this study are listed in Table 1. A variety of chemical compounds (column 2) were analyzed from several vendors (column 3). The observed 6Li/7Li ratios (column 5) were corrected for mass spectrometric isotopic fractionation by using the 6Li/7Li certified reference material IRMM-016, which has a certified 6Li/7Li value of 0.081 37 ( 0.000 34 (2σ). The 6Li/7Li ratio of IRMM-016 was recently reanalyzed using an independently produced set of calibration solutions5,6 and was found to be 0.082 12 ( 0.000 28 (2σ). This new value was used for computing the fractionation correction. The 6Li/7Li value of the International Atomic Energy Agency Li2CO3 isotopic reference material L-SVEC (distributed by the National Institute of Standards and Technology as RM 8545) was also reanalyzed and is 0.082 15 ( 0.000 23 (2σ). This number is a calibrated measurement using the same solutions developed for IRMM-016 and is statistically identical to IRMM-016.6 The previous 6Li/7Li value for L-SVEC was 0.0832 ( 0.0002.7 Li isotopic ratios of samples are commonly expressed relative to L-SVEC in parts per thousand (‰) according

to the equation and these values are listed in column 8. We report

δ7Li (in ‰) )

[

[7Li/6Li]sample

[7Li/6Li]L-SVEC

]

- 1 × 1000

δ7Li values rather than δ6Li values as recently recommended by IUPAC.8 Column 9 lists the atomic weight, Ar(Li), of each sample. Figure 1 shows these data along with other atomic weight and isotopic composition information. Liu and others9 also analyzed Li-bearing laboratory reagents. Their three most enriched (in 7Li) samples (LiCl) are shown in Figure 1. The zero on the δ7Li scale is defined by L-SVEC. (5) Qi, H. P.; Taylor, P. D. P.; Handrickx, F.; Verbruggen, A.; De Bie`vre, P. The gravimetric preparation of synthetic mixtures of lithium isotopes. Submitted to Int. J. Mass Spectrom. Ion Processes. (6) Qi, H. P.; Taylor, P. D. P.; De Bie`vre, P. Calibrated isotopic composition and atomic weight of a natural Li isotopic reference material. Submitted to Int. J. Mass Spectrom. Ion Processes. (7) Flesch, G. D.; Anderson, A. R.; Svec, H. J. Int. J. Mass Spectrom. Ion Processes 1973, 12, 256-272. (8) Coplen, T. B. Pure Appl. Chem. 1996, 68, 2339-2359. (9) Liu, W. G.; Xiao, Y. K.; Zhou, Y. M. Chem. Bull. (in Chinese) 1996, 5, 20-30.

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Figure 1. Isotopic composition and atomic weight of Li reagents.

The IUPAC standard atomic weight (Figure 1), 6.941 ( 0.002,8 is intended to apply to lithium of natural terrestrial origin. The IUPAC representative isotopic composition (Figure 1), 92.5 ( 0.2 atom % 7Li,10 is intended to span the range of the chemicals and (or) materials most commonly encountered in the laboratory. The range of naturally occurring materials (Figure 1) is near that of Ar(Li) and the IUPAC representative isotopic composition.1 The samples in this study (Figure 1) lie considerably outside the ranges for naturally occurring materials, the IUPAC representative isotopic composition, and IUPAC standard atomic weight, in accord with artificial depletion of lithium in the 6Li isotope. The range in fraction of 7Li is from 92.328 to 97.993 atom %. The corresponding ranges in δ7Li are from -11 to 3013‰ and in Ar(Li) from 6.9392 to 6.9959. Thus, it is possible that a laboratory chemist might unsuspectingly use a reagent with 6Li/7Li of onefourth that expected. This is mind-boggling. (10) IUPAC. Isotopic Compositions of the Elements 1989. Pure Appl. Chem. 1991, 63, 991-1002.

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Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

Two samples yielded Ar(Li) values greater than 6.99, samples 23 and 26 in Table 1. To our knowledge, such high δ7Li values have not been documented before. Thus, CAWIA may want to consider revising the wording on the footnote for Li in the Table of Standard Atomic Weights to indicate that the range in atomic weights of commercially available materials is between 6.94 and 7.00, instead of 6.94 and 6.99. ACKNOWLEDGMENT We thank Prof. K. G. Heumann, Dr. T. D. Bullen, and two anonymous reviewers for constructive reviews of this paper.

Received for review May 7, 1997. 1997.X

Accepted July 11,

AC9704669 X

Abstract published in Advance ACS Abstracts, August 15, 1997.