Negative thermal ion mass spectrometry of oxygen in phosphates

Pergamon Geochimica et Cosmochimica Acta, Vol. 61, No. 1 I. pp. 2253-2263. 1997

Copyright 0 1997 Elsevier Science Ltd Printed in the USA. All rights reserved

0016-7037/97 $17.00 + .OO

PI1 SOO16-7037( 97)00057-4

Negative thermal ion mass spectrometry of oxygen in phosphates

C. Holmden,* D. A. Papanastassiou, and G. J. Wasserburg The Lunatic Asylum of the Charles Arms Laboratory, Division of Geological and Planetary Sciences.

California Institute of Technology, Mail Code 170-25, Pasadena, California 91 125, USA

(Received November 7, 1996; accepted in revised ,form January 28, 1997)

Abstract-A novel technique for the precise measurement of oxygen isotopes by negative thermal ion mass spectrometry (NTIMS) is presented. The technique is ideally suited to the analysis of oxygen isotopes in phosphates which form intense PO; ion beams. Since P is monoisotopic, the mass spectrum for PO, at 79, 80, and 81 corresponds to 160, “0, and l8O. Natural and synthetic phosphates are converted and loaded on the mass spectrometer filament as Ag,P04 precipitated directly from ammoniacal solution. To lower the work function of the filament, BaCl, is added in a 1: 1 molar ratio of P04:Ba. Using these procedures, Br- mass interference (at 79 and 81 amu) is eliminated for typical analyses. Experiments with ‘*O-enriched water show less than 1% O-exchange between sample PO, and adsorbed water, and there is no O-exchange with trace O2 present in the mass spectrometer source chamber. The ionization efficiency of PO, as PO; is >lO% compared to 0.01% for both conventional dual inlet Gas Isotope Ratio Mass Spectrometry (GIRMS) and secondary ion mass spectrometry (SIMS). Therefore, NTIMS offers exceptional sensitivity enabling routine and precise oxygen isotope analysis of submicrogram samples of PO, ( <2 1 nmoles equivalent CO* gas) without need for lengthy chemical pretreat- ment of the sample. Overall external precision is ? 1%0 (2g) for ‘*O/“O and ‘7O/‘6O with reproducibility of instrumental isotope fractionation (calculated from ‘8O/‘6O) of 20.5%~ amu-‘. Small phosphate samples including single mineral grains from meteorites, or apatite microfossils, can be analyzed by this technique. Cop&ight 0 1997 Ekevier Science Ltd

1. INTRODUCTION

We report on a Negative Thermal Ion Mass Spectrometry (NTIMS) technique for the isotopic analysis of all oxygen isotopes ( 160, 170, ‘*O) measured as POT. This contribution builds on earlier work of Heumann et al. ( 1989) and Wachs- mann and Heumann ( 1991)) who showed that ion beams of PO, and PO; can be obtained from phosphate compounds. Since P is monoisotopic (“P), the mass spectra of POX- and PO; reflect the isotopic composition of oxygen. We show that it is possible to measure oxygen isotope abundances in sedimentary, igneous, and biogenic phosphates by a modified direct loading technique, thus eliminating the need to convert phosphate oxygen to CO,, which is necessary for conventional dual inlet Gas Isotope Ratio Mass Spectrometry (GIRMS )

Although oxygen isotope analyses by GIRMS are highly precise (t0.2%~, 2a), relatively large volumes of sample gas (>O.l pmole) are required to achieve source pressures high enough to maintain viscous how through the capillary inlet and to compensate for the low Ionization Efficiency (IE) of the electron impact source (0.01%) (Brenna, 1994). We have obtained ionization efficiencies for PO, >lO% ( PO_: ions detected / PO, molecules loaded on the sample filament), readily permitting measurement of 70 ng of oxygen ( 100 ng Pod). The precision and accuracy of the oxygen isotope measurements depend on: ( 1) eliminating isobaric interferences with 79Br and “Br ~, (2) evaluating the potential for high temperature O-exchange between PO, oxygen

*Present address: Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada.

and extraneous oxygen on the filament or within the mass spectrometer source chamber, and (3) controlling the effects of instrumental isotope fractionation.

2. ANALYTICAL PROCEDURES

2. 1. Standard and Normal Solutions

We have used several apatite standards including two igneous apatites UMS- 1 (University of Michigan) and LA- 1 (Laramie Anor- thosite), and Florida rock phosphate (SRM- 12Oc), the last distrib- uted by the National Institute of Standards and Technology. These apatites were dissolved in 1.0 N HNO?. Calcium was removed by passing the solution through a cation exchange column containing Dowex 5OW-X8, 200-400 mesh. The pH of the resulting H3POJ solution was adjusted to 28.0 with clean NH,OH. and approximately 1.5 times the stoichiometric amount of AgNO, was added. In an open container, the slow evaporation of NH, (overnight) lowered the solution pH, causing large yellow crystals of Ag,PO, to precipitate (Firsching, 1961; cf. also O’Neil et al., 1994). This precipitate was rinsed four times in clean water. No attempt was made to eliminate or characterize potentially coprecipitating AgCI, AgBr, Ag,S, and AgZS04. In addition to the apatite standards, Johnson Matthey Ag,P04 and NaJP04 salts were used as normals. The Ag,P04 normal was prepared by dissolution in 10% NHIOH. The NaTPOd normal was used to prepare Ag3P0, by first dissolving NaiPOl in water, followed by conversion to Ag,PO,, as above. Solutions for loading a sample on the mass spectrometer filament were prepared by dissolving a known amount of AglPO, in 10% NH,OH.

2.2. Sample Loading Procedure

We have performed extensive tests of sample loading techniques and have adopted the following procedure. A thin layer of colloidal Pt powder (Pt-black) is loaded as an aqueous slurry on a Pt filament (0.020” wide X 0.0012” thick) using a microsyringe fitted with polyethylene microtubing (PE-10); the colloid is dried with a small current through the filament. The deposit of Pt powder covers the width of the filament and is about 3 mm in length. Next, an aliquot

2253

2254 C. Holmden, D. A. Papanastassiou, and G. J. Wasserburg

of the ammoniacal Ag,P04 solution is loaded onto the Pt powder with a current of 0.2 A. The PO4 concentration is chosen so that a volume of 0.25 PL is dispensed. As the solution evaporates, Ag,P04 crystals precipitate within the area covered by the Pt powder. It is important to achieve a uniform load and not to allow the sample to spread outside the area covered by the Pt powder. To reduce the work function of the filament, Ba is loaded on top of the Ag,P04 as spectroscopically pure BaC12 (dissolved in water). Again, it is important to achieve a uniform BaCl* layer which covers the Ag,PO,, but does not contact other parts of the filament. Enough BaCl, is added to correspond to a 1:l molar ratio of Ba: P04. This mixture produces intense PO; ion beams and no accompanying Cl- or Br- ion beams. Use of excess BaCl, relative to PO, (Ba:PO, > 1S:l) results in significant Cl- ion beams, which can also be accompanied by Br- ion beams.

permit ionization of PO;, but apparently less satisfactory conditions for simultaneous emission of halogens. When emitters (Ba( N03)2, Ba( OH)*, or BaCl,) are loaded without the Ag,P04 sample, large Cl- and Br- ion beams form (-5 x lo-” A for 35C1- and -2.5 X lo-” A for 79Br-).

2.3. Mass Spectrometry

Using the above loading procedure, a PI607 ion beam is first observed at temperatures <776”C (lower detection limit of optical pyrometer in use). Data acquisition starts when the P 160; ion current is 5 X lo-” A. The PO; ion beam is either stable or increases its intensity throughout the 4-5 h run and, typically, we observe no halogen ion beams at data collection temperatures. At the beginning of data acquisition, we routinely measure PO;/PO; ratios of -500, decreasing to -250 by the end of the analysis. No other species are observable. Filament temperatures are nearly constant during each run, but vary from sample to sample (<776-860°C for the lpg loads). Smaller samples (l-10 ng) run at a higher temperature of -960°C. The PO, loading blank was determined using an emitter [Ba(OH),, Ba(N03)*, or BaC&] loaded along with Pt powder on a Pt filament. Although intense halogen beams formed in all cases, we observed no evidence for POj or PO; ion beams, indicating that the blank is negligible.

Oxygen isotopes are measured at masses 79 (3’P160y), 80 (3’P’701602), and 81 (31P’*O’602) in the sequence 79-81-81-80-80- 79 per cycle. The smaller abundance isotopes are measured each for 4 s, twice per cycle. Following measurement of the intense mass 79 signal, a sufficient time delay is provided to allow for decay of the electrometer signal before measurement of the small intensity mass 81 signal (Papanastassiou and Wasserburg, 1969). In addition, we measure mass 81 twice to check for a difference in sequential measurements (cf. Russell et al., 1978, and Niederer and Papanastassiou, 1984, for Ca). If the mass 79 signal has not decayed to a sufficiently low level before measurement of the mass 81 signal, the 81/79 ratio will appear too low, as the first background measurement at mass 80.75 will be too high. As expected, the second mass 81 measurement, within each cycle, yields 81/79 slightly higher than the first 81/79 by a few tenths of one permil. Although the second measurement is deemed to be more accurate, the sequential measurements were so similar that we chose to average them, increasing our precision with only a small systematic offset for 81/79. For 80/79 the sequential measurements were also averaged, but no systematic differences were observed between the first and second measurements.

Our early experiments utilized Ba(N03)* and Ba(OH)2 as emitters. However, we wanted to use an O-free emitter to limit the potential for high temperature O-exchange on the filament, and so we tried BaCl,. Also, we wanted to investigate the possibility that spectroscopic grade BaCIZ may help to convert trace bromide salts (possibly loaded along with the sample) to the chloride form on the filament and thus help to eliminate Br- emission. Surprisingly, no halogen ion beams were observed during a sample run using BaCl* despite loading 0.74 bg of Cl. To gain further insight into the chemistry occurring on the filament, several samples were prepared and the filament assemblies loaded into a vacuum chamber equipped with a viewing port. When Ag3P04 is loaded on a filament by itself, and the assembly is heated under vacuum, the yellow crystals slowly turn or- ange, red, and then melt at a current of 1.2 A ( -850°C). At this temperature, puddles of molten salt persist on the filament for some time. If the filament current is turned to zero, a yellow glass forms. Identical experiments with BaCl, show that it also melts at a current of - 1.2 A. When Ag,P04 and BaCl, are mixed together (following our standard loading procedure) melting occurs at a lower temperature ( -0.75 A; < SOO’C) and bubbles are observed to form in the melt, revealing the escape of gas. After degassing, the melt is slowly consumed by a reaction that sweeps across the sample forming a silver-gray solid.

3. RESULTS

3.1. Oxygen Isotopic Analysis of Microgram Quantities

Based on these experiments, a thin layer of colloidal Pt suspension (Pt-black) was loaded on the filament, prior to loading Ag3P04, to ensure uniform melting of the sample. Puddles of molten salt do not form with Pt powder on the filament. Rather, the load degasses and immediately reacts to form the silver-gray substance. Energy dispersive x-ray analysis shows that the silver-gray substance contains Ba, but no Cl or Ag. The phosphorus peak is hidden behind the prominent Pt peak. We speculate that the degassing observed in the stand-alone vacuum system is the volatilization of Cl2 (and Br2) which is removed by the vacuum system and most likely accounts for the absence of a Cl- (and Br-) ion beam. Silver most likely alloys with the Pt-black and Pt filament (Massalski, 1990). This appears to leave Ba3(P0,), on the filament. However, when we loaded Ba3(P04)* directly on the filament as a test, we obtained a lower ionization efficiency and halogen ion beams were present.

of PO,

3. I. I. Elimination of bromine mass interjerences. 3.1.2. Oxygen mass spectrum

Isobaric interferences from 79Br- and *‘Br- (79Br/8’Br = 1.03) must be eliminated for accurate isotope abundance measurements of oxygen as PO; by NTIMS. Attempts to eliminate halogen ion beams by purifying the reagents were not effective. Early experiments showed that the halogen ion beams could be controlled in part by manipulating the amount of emitter (Ba-compound) loaded. We chose, by experimentation, a 1: 1 molar ratio of Ba:P04 which provides adequate reduction of the filament electron work function to

The monoisotopic character of P allows the oxygen isotopes to be determined directly from measurements of molecular PO; A filament was prepared as described above. The mass spectra (Fig. 1) were acquired using the Faraday collector with measurements taken at 0.05 mass increments and with 8 s integrations of the electrometer signal ( 10 ‘I Ohm feedback resistor). Excellent resolution of all masses is obtained char- acterized by flat (2-3 Gauss) peak tops and low, stable back- grounds. The P’70’602 peak is shown in expanded scale (Fig.

Measurement of 0 isotopes by NTIMS 2255

31p1603- (8 x lo-” A)

3 ’p ’8d80*-

I (5x lo-l3 A)

r

0 2

9 5

I

(I 3’p’7~6Q- (9x 70- ‘4 A) J __n J

79 80 Mass (amu)

81

Fig. 1. Oxygen mass spectrum for PO; obtained using the Faraday collector with 1 pg PO1 as Ag,PO, and 1.5 pg Ba as BaCl, , on a Pt filament prepared with Pt powder. The molar ratio of Ba:PO, was 1: 1. Since P is monoisotopic the molecular species 3’P’60;, 3’ P I7 0 160;, and 3’P ‘*O I601 correspond to 160, “0, and “0 (with a minor “P 1702 160- contribution to the 81 peak intensity). The scan was acquired digitally with measurements taken at 0.05 mass increments, for 8 s integration of the electrometer signal ( 10” Ohm resistor) at each mass. Peak flat tops are 3 Gauss wide. Note that the mass 80 and 81 peaks are equal to three times the natural abundances for “0 and “0 due to statistical weights in the production of PO,.

2). The background noise (~14 pV, lcr) is consistent with an integration interval of 8 s. The signal to noise ratio for P’70’602 is 657. Note that the intensities of ion beams at masses 80 and 81 are three times as large as the isotope abundance of “0 and j80, respectively, reflecting statistical weights in the production of PO;. Using the secondary electron multiplier in pulse counting mode, we have also identified lower abundance species at masses 82 (P’80’70’60) and 83 ( Pr802r60) in the correct relative abundances. Thus all ex-

G 0.010

>" 0.008

5 m 0.006

s 0.004 c -

0.002

petted PO; molecules are observed. A small correction to the mass 81 data for the low abundance P 1702’60 interference was applied ( -0.07%0). We also note that interference from S could show up at mass 80 (SO:-). We searched for this interference by loading BaSO, but did not detect it.

3.1.3. Oxygen exchange in the thermal ionization source

It is well established that there is no oxygen isotope exchange between liquid water and dissolved Pod”- even in

79.6 79.8 80.0 80.2 80.4 80.6

Mass (amu) Fig. 2. The mass 80 peak from Fig. 1 is expanded to show the peak shape and signal to noise ratio (= 657) for the low abundance “0 isotope. The background noise is 14 PV (lo) for the 8 s integration interval used.


hot solutions (90°C; Tudge, 1960). However, in the thermal ionization source chamber, filament temperatures are 800- 850°C and it is expected that O-exchange might be fast at these temperatures. Silver phosphate was chosen as a loading compound because it is nonhygroscopic, thermally stable, and any phosphate compound can be easily converted to this form. Barium chloride is also thermally stable and although it crystallizes with 2 waters of hydration (BaCl* 2H20) these waters are lost at temperatures <_5o”C.

The most likely source of extraneous 0 is from adsorbed water introduced during sample loading. Several experiments using isotopically enriched water and Ag,P04 prepared from the Na3P04 normal were undertaken to determine the extent of oxygen exchange between sample PO4 and extraneous 0 present either on the filament or adsorbed in the mass spectrometer source chamber (Table 1) . The enriched water was transferred to the filament as part of the BaCl, solution which was prepared in water with 6’*0 = 55,663%0 and 6”O = 46,495%0. For all experiments, the extent of 0 exchange was found to be limited to - 1%. The filament heat-up routine was varied to determine any relationship with O-exchange. In one experiment the sample was heated to 800°C in 40 min (A-62), and for another, over a period of 3 h (A-63). Sample A-64 was preheated overnight using a low current (0.25 A). No clear relationship between the different filament heat-up routines and percent O-exchange was observed. However, if the Na,P04 normal contained some condensed phosphates, the addition of HZ0 (with the BaC&) and heating on the filament could hydrolyze them and provide a mechanism for O-exchange. To test this possibility, the precursor Na3P04 solution was heated overnight in weak HN03 to hydrolyze possible condensed phosphates. Subsequently, the enriched water was added. This treatment showed the smallest amount of 0 exchange (0.7%) suggesting that minor amounts of condensed phosphates may have been present in the other experiments. Lastly, during one of the “O-enriched runs (A-63), air O2 was introduced into the mass spectrometer source chamber through a preci-

Table 1. Exchange exoeriments with “0 and I*0 enriched water.”

Sample Ratios PO

(POT) PO

(POT) %

exchangeh

Na3P04 normal A-62 A-63d A-63’ A-64’

50 587 t 2 500 2 3 1.0 50 819 ? 9 695 ? 8 1.4 40 836 2 17 708 k 16 1.4 30 5349 2 106 550 2 2 1.1

A-65g 30 407 k 7 355 t 6 0.7

a Enriched water isotopic composition: I60 = 88 4% “0 = 1.6%, ‘“0 = 10.0% [6”0 = 55,663 %O and 6”O = 46,493 d/b, (POT)].

b Calculated from 5170. ’ Ramp to 800°C in 40 minutes. d Ramp to 800°C in 3 hours. ’ 0, gas let into source. f Preheated sample for 12 hours at 0.25 A. 6”O for this sample

is high due to Br interference, not present for I70 (mass 80) from which the percentage exchange was calculated.

g Heated PO, solution in natural water for 12 hours to remove any condensed phosphates, then loaded with enriched water.

sion leak valve to determine whether isotope exchange would occur. No effect was observed.

In summary, O-exchange is less than 1% and negligible at the present level of external precision. If it is assumed that the main source of extraneous 0 is laboratory water with a S’“0 = -10%0, the shift imparted to PO4 oxygen is < -0.1%0. This effect can be further minimized using laboratory water with an isotopic composition similar to that of typical phosphates.

3.1.4. Isotopic analysis of Ipg standards and normals

After the initial experiments, we checked the internal precision and external reproducibility of POX- analyses for a relatively fixed sample size of lpg PO4 (2 10%). The results for five standards and normals are listed in Table 2 and plotted in Fig. 3a. All uncertainties are ?2a. The data follow a mass dependent fractionation trend, as expected. There are very few absolute determinations of the oxygen isotopes in natural samples. The two measurements reported by Nier ( 1950) on tank O2 and atmospheric O2 are the most widely cited for oxygen. The Nier (1950) data, however, plot off the NTIMS mass fractionation line (Fig. 3b) which reveals a small discrepancy between the Nier ( 1950) determinations and the measurements reported here. We also show oxygen isotope determinations by ion probe which are consistent with the NTIMS fractionation line.

3.1.5. Precision and instrumental fractionation

Instrumental isotope fractionation is the most important factor influencing the precision of the oxygen isotope measurements. Isotope fractionation may vary within an individual mass spectrometric analysis or between repeat analyses of a sample. To minimize instrumental fractionation, we consistently loaded a fixed amount (within 10%) of Ag,PO, and BaCl, on the filament. For the mass spectrometric analyses, the temperature of the filament was increased to -8OO”C, incrementally, over the same period of time. Data acquisition was started when the intensity of the PI607 beam was 5 X lo-” A, which corresponds to a temperature between 776 and 850°C. We did not attempt to run at a prescribed temperature, because the ion emission is a sharply peaked function of temperature, and ion emission temperature can vary as a result of minor differences in loading procedures.

Four analyses of the LA-l standard are shown to demon- strate the fractionation trend with time (Fig. 4). We have chosen two analyses (A-100, A-102) which are atypical and show the steepest fractionation trends observed which cover a total range of 4%0 for 18O/‘6O (or 2%0 amu-‘). While analyses such as these (showing significant drift) would be repeated, even for these atypical analyses the data within the first hour of acquisition show a smaller range of 1%0 amu-‘. The other two analyses (A-101, A-103) are more typical of the bulk of the data showing a 1%0 amu-’ (within-run) total range of isotope fractionation. A typical analysis consists of 150 cycles (300 ratios) measured over 4-5 h. This observed range of isotope fractionation is very limited. By comparison, if the emission of thermal ions from the filament is

Measurement of 0 isotopes by NTIMS

Table 2. Oxygen isotope data by NTIMS for 1 microgram standards and normals.

PO;

2257

Run PO4 (!Jg) Ba (pg) Temp. (“C) 80/79 81179 6”O (POT) 6”O (POT)

A-61 A-66 A-69 A-70 A-71

A-73 A-74 A-88 A-89 A-91 A-92

A-94 A-95 A-96 A-97 A-98 A-105 A-106

A-100 A-101 A-102 A-103

A-118 A-120 A-121 A-122

1.1 1.1 1.3 1.3 1.3

1 1 1 0.9 0.9 1.2

1 1 1 1 1 1 1

1 I 1 1

1 1 1 1

1.6 800 0.0011510 2 05 1.6 776 0.0011498 * 09 2.0 849 0.0011510 ” 07 2.0 818 0.0011511 * 07 2.0 818 0.0011494 !I 10

Ag,PO, normal

1.6 1.6 1.5 1.5 1.5 1.5

850 0.0011538 2 07 0.0060836 2 26 778 0.0011539 2 08 0.0060846 -t 15 795 0.0011540 ? 04 0.0060864 2 16 793 0.0011538 ? 10 0.0060827 ” 12 8.57 0.0011536 +- 06 0.0060796 ? 23 864 0.0011542 2 07 0.0060871 2 23

SRM-120c, Florida rock phosphate standard

1.5 1.5 1.5 1.5 1.5 1.5 1.5

1.5 1.5 1.5 1.5

1.5 1.5 1.5 1.5

845 0.0011544 2 06 0.0060930 2 23 803 0.0011550 2 07 0.0060899 ? 16 833 0.0011555 t 07 0.0060998 t 16 834 0.0011540 t 08 0.0060896 2 14 861 0.0011529 + 06 0.0060833 2 10 844 0.0011544 + 05 0.0060970 2 34 810 0.0011541 + 09 0.0060991 2 30

Mean (20)

LA-l (Laramie anorthosite igneous apatite standard)

834 0.0011439 * 10 0.0059986 t 42 786 0.0011450 ? 10 0.0059963 2 13 830 0.0011450 2 08 0.0059996 2 29 839 0.0011447 2 08 0.0059980 t 18

Mean (20)

UMS-1 (University of Michigan igneous apatite standard)

827 0.0011438 2 07 0.0060110 r. 15 856 0.0011457 ? 07 0.0060077 2 14 818 0.0011444 2 04 0.0060067 t 17 820 0.0011447 ? 09 0.0060090 2 08

Na,PO, normal

0.0060545 -c 09 4.4 +- 0.4 6.5 t 0.1 0.0060464 2 11 3.3 * 0.7 5.1 t 0.2 0.0060602 ? 27 4.4 ?I 0.7 7.4 i 0.4 0.0060487 t 45 4.5 ? 0.6 5.5 2 0.7 0.0060514 2 32 3.0 2 0.9 6.0 ? 0.5

Mean (20) 3.9 + 1.4 6.1 ? 1.8

Mean (2~)

Mean (2~)

6.8 + 0.5 11.3 2 0.2 6.9 t 0.6 11.5 ? 0.2 7.0 t 0.3 11.8 ? 0.3 6.8 * 0.9 11.2 t 0.2 6.6 + 0.5 10.6 t 0.4 7.2 t 0.6 11.9 -c 0.4 6.9 ? 0.4 11.4 t 0.9

7.3 lr 0.5 12.9 2 0.4 7.9 z 0.7 12.4 ? 0.2 8.3 2 0.6 14.0 2 0.2 7.0 2 0.7 12.3 t 0.2 6.0 r 0.6 11.3 i 0.2 7.3 t 0.4 13.5 + 0.6 7.1 t 0.7 13.9 + 0.5 7.3 z 1.5 12.9 + 2.0

-1.8 ? 0.9 -2.8 -t 0.7 -0.87 t 0.8 -3.2 2 0.2 -0.87 2 0.7 -2.7 2 0.5 -1.1 2 0.7 -2.9 2 0.3 -1.2 t 0.9 -2.9 2 0.4

-1.9 2 0.6 -0.76 2 0.2 -0.26 2 0.6 -1.3 t- 0.2 -1.4 -f 0.4 -1.5 kO.3 -1.1 +- 0.7 -1.1 2 0.4 -1.2 2 1.4 -1.2 2 0.6

a Phosphate 0 data are reported as permil deviations from POT (PO-Three) which is defined as having the ‘8O/‘6O ratio of V-SMOW (0.0020052) measured by Baertschi (1976), and the corresponding ‘7O/‘6O ratio (0.0003820) determined from the NTIMS fractionation line (Fig. 3a). All uncertainties are 20. The 81/79 ratios were corrected by -0.07%0 for minor “P”02 160- interference.

equivalent to emission from a Knudsen cell, we would expect a kinetic isotope fractionation of - 6%0 amu-’ [i.e., a factor of (80/79)“*] between the emitted ions and the sample oxygen reservoir on the filament. Although we were careful to maintain uniform operating conditions from one analysis to another, the initial level of fractionation (i.e., the initial ‘8O/16O) varied from run to run for the same standard or normal by about 1%0 amu-‘. No clear relationship between instrumental isotope fractionation and filament temperature could be ascertained.

We consider that the first-order factor which yields a limited range in instrumental fractionation is the high ionization efficiency of PO; (> lo%, see below). This allows data to be

collected well before the PO, reservoir on the filament becomes significantly fractionated. A similar explanation was suggested for Mg+ ions, where the range in instrumental isotope fractionation is also extremely small [ 1.5%0 amu -’ , as compared to a possible kinetic isotope fractionation of -2% amu-‘, i.e., (25/24)“ ‘- 11, again due to the high ionization efficiency of Mg in a Si02-gel matrix (Lee et al., 1976).

3.2. Oxygen Isotopic Analysis of Nanogram Quantities of POa

The high ionization efficiency for PO4 as PO; by NTIMS, permits submicrogram samples of PO, to be analyzed. Data


0.000386

0.000385

0.000384

0.000383

0.000382

0.000381

0.000380

0.00199 0.00200 0.00201 0.00202 0.00203 0.00204

0.000388

0.000384

0.000380

0.000376

0.000372

0 Wash. U. ion Probe V Caltech ion probe 0 Nier (1950) 0 NTIMS

_ + -T-a&a; - - _

t Atmospheric 02

1 0.00198 0.00200 0.00202 0.00204 0.00206 0.00208

180p0 Fig. 3. (a) NTIMS data for three standards and two normals displayed on the three isotope plot for oxygen showing mass dependent fractionation. (b) NTIMS data are compared to other measurements of the absolute isotopic composition of oxygen including Nier ( 1950), for oxygen isotopes in Tank Oz and atmospheric O?. The Nier (1950) data fall off the NTIMS fractionation line indicating a small discrepancy between the two datasets. Oxygen isotope measurements by SIMS are also shown. The Caltech ion probe data on Burma spine1 (Gary Huss pers. commun.) are in excellent agreement with the NTIMS data. There is also excellent agreement between the NTIMS fractionation line and the grand mean of all SIMS oxygen isotope measurements of standards by the Washington University ion probe (Fahey et al., 1987). Using ‘XO/‘hO = 0.0020052 (Baertschi, 1976) for normalization of instrumental fractionation, Fahey et al. ( 1987), report “OlihO = 0.00038288. Earlier NTIMS measurements of oxygen isotopes by Wachsmann and Heumann ( 1991), using the molecular species PO, (masses 63, 64, 65)) rather than PO;, plot outside the frame of this graph with an apparent 38%0 excess of “0, relative to the NTIMS fractionation line. We checked the oxygen isotope composition of the PO; ion beam observing many peaks above and below the PO; mass range (63, 64, 65), suggesting that 0 compositions derived from PO; measurements are affected by isobaric interferences that are not yet determined.

for PO? samples ranging from 500 ng to 1 ng are listed in Table 3 and plotted in Fig. 5. The most significant effect of reducing the sample size is the difficulty of controlling interferences from Br which become problematical at about the 10 ng PO4 level. The smallest PO4 sample analyzed without Br interference was 100 ng. The 500, 250, and 100 ng samples were loaded in the same manner as the lpg samples with one exception; the area1 extent of the sample load on the filament was scaled down to a 0.5 mm diameter circle to ensure overlap between Ag,P04 and BaCl,, while maintaining the unity molar ratio for Ba:P04. This modifica- tion is important because it is the melting that occurs on the filament (at temperatures close to running conditions) that appears to promote the loss of halogens.

3.2.1. Bromine correction

At the 10 ng level and below, Br interferences are consid- erable. The most notable effect of Br emission is an increase in the 81/79 ratio and the inferred ‘sO/‘6O due to “Br- which contributes to the low intensity P “0 1602 peak. An almost equal amount of 79Br- contributes to the PI601 peak, but the effect is very small due to the more abundant P “07. Since the P I70 “j0; peak is free from Br interference, and the P “0.T peak is only marginally affected, the measured 80/79 ratio is very close to the true ‘7O/‘6O ratio. If bromine interference is present during the isotopic analysis of 0, this is easily determined on a three isotope plot, as ‘8O/‘6O (or b l8O) is shifted off the NTIMS isotope fractionation line


Laramie apatite standard (LA-l )

A A-102 0 A-101 o A-l 03

k 4

0 5b 1 do li0 2do 2io 360

Time (minutes)

Fig. 4. The four mass spectrometer runs of the LA- 1 igneous apatite standard are plotted to show the instrumental isotope fractionation as a function of time during a normal 4-5 h analysis. As expected, the 0 reservoir on the filament becomes depleted of the lighter IhO isotope with time, leading to a small, regular, increase in’70/‘60 and ‘“Ol’hO ratios during the run. The fractionation trends shown by A-l 00 and A-102 constitute the steepest trends observed for all standards and normals measured. Samples A-101 and A-103 display the range of total instrumental fractionation (1%0 amu-‘) typical of the majority of analyses.

towards more positive values along a nearly horizontal line with slope -0.01. The ‘“Br correction to the measured P”0 “0:/P I601 ratio is accurately determined from the measured P “0 160; /P I601 by back-extrapolation to the NTIMS fractionation line (Fig. 3a). This allows the precise determination of ‘7O/‘6O in the sample and the oxygen fractionation per amu in the sample, within the uncertainties permitted by the lower abundance of “0. This approach can

not be used for mass independent isotope fractionation effects in oxygen.

4. DISCUSSION

Measurement of oxygen isotopes has long been the exclusive domain of dual inlet GIRMS utilizing CO1 or O2 gas ionized by electron impact. We have shown that oxygen isotopes can also be measured by thermal ionization methods using molecular PO;. NTIMS offers much higher ionization efficiency ( > 10%) and, therefore, greater sensitivity compared to GIRMS (IE = 0.01%). Sample sizes for dual inlet GIRMS must exceed the viscous flow limit, which is l-2 Imoles of CO? for a standard (bellows) run, and - 100 nmole of CO* if a microvolume cryogenic reservoir is avail- able (Brenna, 1994). Our standard 1 bg load of PO4 for NTIMS corresponds to only 2 1 nmoles of COZ, with analyses down to 2 nmole ( 100 ng PO,) possible without Br interference.

In practice, conventional phosphate oxygen isotope analysis uses much larger PO, samples than the minimum required by GIRMS because of the difficulty of quantitatively con- verting PO, oxygen to COZ. The conventional approach in- volves first purifying the apatite by dissolution and reprecipi- tation as BiPO, or Ag,P04, followed by high temperature fluorination with BrF, in a Ni reaction vessel to form OZ. The 0, is then passed over hot carbon and converted to CO, for mass spectrometric analysis. Yields must be close to 100% to avoid isotopic fractionation. Using BiPOl, Luz et al. ( 1984) reported an external precision ? 1%0 for S”O (2g) using 10 mg samples of apatite. With NTIMS we achieve comparable precision with 1 O4 times smaller samples. More recently, Crowson et al. (1991) have reported an external precision of ?O.l%a for 6’*0 (2a), on 2-30 mg apatite samples converted to Ag,P04, and reacted in a miniaturized, phosphate-dedicated fluorination line. Although precision is improved by a factor of 10, sample requirements remain high. Other GIRMS techniques for preparing CO2 from phosphate oxygen avoid the use of BrF5. O’Neil et al. ( 1994)

Table 3. NTIMS Oxygen isotope data for submicrogram quantities of standard phosphate.

PO; b”0 (POT) PO4 Ba Temp. “0 6’xO

Run (ng) (ng) (“C) IE” ( 10’ ions/s) 80179 81/79 (measured) (corrected)h (POT)

Ag,PO, normal

A-80 500 725 859 SO.2% 5.3 0.0011535 + 07 0.0060826 k 24 6.5 k 0.5 6.5 -c 0.5 11.1 f 0.4 A-82 250 363 834 90.4% 5.5 0.0011555 + 04 0.0060994 t 11 8.2 k 0.4 8.2 2 0.4 13.9 2 0.2 A-l 14 100 145 883 *0.8% 5.5 0.0011546 2 06 0.0060936 t 37 7.5 ? 0.5 7.5 -t 0.5 13.0 2 0.6 A-83 IO 15 966 > I .7% 3.4 0.0011632 2 16 0.00922 2 61 15.0 t 1.4 18.2 t 1.2 A-109 IO I5 977 1.1% 1.1 0.0011601 2 62 0.006163 ? 22 12.3 k 5.4 12.3 i 5.3 A-l 10 IO 270 962 >lO% 2.3 0.0011630 t 18 0.006301 -c 40 14.8 2 1.5 14.9 t 1.5 A-115 10 15 977 >7.0% 4.1 0.0011613 k 16 0.010 2 3 13.4 k 1.4 17.4 t 1.7

Mean (20) 13.9 ? 2.5 15.7 2 5.3 A-l I2 1 730 994 16% 1.0 0.0011650 2 65 0.00672 k 12 16.6 k 5.6 17.1 2 5.5 A-113 I 730 988 8% 1.1 0.0011669 2 59 0.00730 ~fr 44 18.2 5 5.0 19.4 2 5.3

Mean (2~) 17.4 2 2.3 18.3 z 3.3

a loniration Efficiency (IE) = PO; ions collected/PO, molecules loaded. IE is given as a lower limit in cases when the run was stopped before sample exhaustion.

’ 81/79 used to correct for Br- interference, except for the first three analyses for which 6”O are also reported.


Table 4. Comparison of 6”O for oxygen standards between NTIMS and GIRMS.

Standard

UMS-1 SRM-120~

LA-I

61x0 (%o POT) (NTIMS)

-1.2 t 0.6 12.9 t 2.0

-2.9 f 0.4

6’xO (%o V-SMOW) GIRMS

(GIRMS) Technique

12.1 +- 0.4” decomposition 21.9 t- 0.2” decomposition 21.3 f O.lh fluorination

7.27 2 0.14 fluorination

a Measured at the University of Michigan using the thermal decomposition technique of O’Neil et al. (1994).

h Measured at North Carolina State University by Crowson et al. (1991) using the BrFc fluorination technique of Clayton and Mayeda (1963).

’ Measured at the University of Alberta by Farquhar et al. (1993) using the BrFs fluorination technique of Clayton and Mayeda (1963).

presented a thermal decomposition technique in which phosphate converted to Ag,P04 is reacted with C (in a closed- system) releasing 25% of the sample 0 for analysis. Stuart- Williams and Schwartz (1995) react Ag3P0, at 550°C with BrZ gas releasing 17% of the sample oxygen for analysis. Unlike quantitative fluorination, analytical precision is dependent on reproducing the isotopic fractionation that results from incomplete recovery of the sample phosphate oxygen. Oxygen isotope data must be corrected using an empirically determined correction factor before they can be compared to data obtained by conventional fluorination.

The NTIMS technique offers great sensitivity and ease of sample preparation since PO, (as Ag,P04) is loaded directly for analysis: no conversion to CO> or 0, is necessary. Laser- assisted fluorination offers simplified sample preparation, and lower O2 blanks compared to conventional resistance heated bomb fluorination (Sharp, 1990), but reduction of sample size is still limited by the viscous flow requirement. Even specialized, Continuous Flow Isotope Ratio Monitor- ing Mass Spectrometry (CF-IRMS), utilizing a helium car- rier stream to overcome the viscous flow limit (Merritt and Hayes, 1994), must contend with the problematic aspects of gas preparation from natural samples.

The sample size reported here for reliable NTIMS measurements approaches that of Secondary Ion Mass Spectrom- etry (SIMS) which has the lowest sample requirements (Gi- letti and Shimizu, 1989). An analysis spot 20 pm in diameter and 2 pm deep, uses 2 ng of apatite, containing 3 X 10” atoms of oxygen. Over a period of 1 h, -3.6 x IO” ions can be counted (counting limit of 10h cps in pulse counting mode) yielding an effective, combined IE and ion transmis- sion of about O.Ol%, compared to > 10% for NTIMS. Se- quential point analyses of the same mineral grain during the same run have yielded internal precisions of -+ 1.5%0 (20) for “O/ I60 (Giletti and Shimizu, 1989)) but reproducibility is poor for samples analyzed on different days (2-4%0, 2~) due to the large, and variable, instrumental fractionation.

Although NTIMS offers extremely high sensitivity for oxygen isotope analysis of phosphates, dual inlet GIRMS is inherently more precise because GIRMS measures differences in isotopic abundance ratios between sample and reference gases, not absolute ratios (McKinney et al., 1950).

Isotope ratio differences are more precisely measured because a variety of instrumental fractionation effects cancel when sample and reference gases are measured during the same mass spectrometer run. Isotope ratio differences are reported as delta (5) values,

6”‘.“‘0 = (R, / R2 - I) x 1000, R, =

’ “,“‘I ‘hO,umplr and R2 = (‘8~‘7’/ ‘hO,tiindard

which are permil (%‘cc) differences of the sample isotopic composition relative to an internationally recognized standard such as PDB (Pee Dee Bele.) or SMOW (Standard Mean Ocean Water; Craig, 1957).

Thermal ionization does not lend itself to this style of measurement. Isotope abundance data are measured and reported as isotope ratios. To achieve reproducible measurements the instrumental fractionation must be minimized and reproduced from run to run. Therefore, we adopted a fixed filament heat-up and measurement routine for all standards and normals (with the exception of the NaiPOj normal for which the heat-up routine was not strictly monitored and samples were loaded without Pt powder). The 20 reproducibility for standards and normals (Table 1) varied from ?0.4%0 to t2.0%0 for “O/“O, and +-0.4%~ to ?1.5%0 for ‘70/‘h0. The least reproducible standard was SRM-120c, which is Florida rock phosphate and the least pure of the phosphate standards analyzed. Overall, we consider the reproducibility of our NTIMS oxygen isotope analyses to be about +- 1%0 (20 ) for both ‘“0/‘60 and “O/“O, (and 6 I80 and 0 “O), corresponding to the control and reproducibility of instrumental isotope fractionation to 20.5%~ amu ’ (2a).

To compare absolute NTIMS data to relative GIRMS data, it is necessary to know the absolute ‘80/‘h0 and “O/“O of SMOW (S’*O-0, 5 “O=O), which are not known very

0 40 80 120 160 200 240

S’s0 (%, POT)

Fig. 5. Results of analyses of submicrogram samples of PO4 using the Ag,P04 normal. Reducing the sample size changes the level of isotope fractionation (i.e., the point on the NTIMS fractionation line where a sample falls). The expected trend with decreasing sample size is along the fractionation line towards increasing b “0 and 6 “0. This trend is generally observed. At 10 ng and below, Br- interferences (masses 79 and 81) cause 8 1179 to be high but have only a minor effect on 80/79. The small interference from ‘“Br- on the measured 80/79 ratio (“O/“O) can be precisely corrected using the relatively larger shift in the 81/79 ratio due mostly to "'Br-


precisely. The original SMOW composition was defined by Craig ( 1961 ) relative to NBS-l water. The NBS-l water composition was derived by manipulation of Nier’s Tank O2 datum (Nier, 1950), and ?J’~O measurements of air by GIRMS relative to the PDB standard (Craig, 1957). From these data, the absolute ‘XO/‘hO and ‘70/‘“0 ratios for a number of standards were deduced (Craig, 1957). For SMOW, Craig ( 1961) reported an ‘“O/‘hO ratio of 0.0019934 -+ 0.0000025 (with a corresponding “O/‘“O = 0.0003722). These absolute oxygen isotope ratios for SMOW differ from our NTIMS measurements, in that the SMOW composition is based on Nier ( 19SO), and our oxygen isotope measurements differ from the Nier ( 1950) pion- eering measurements (see Fig. 3b). We have tested for non- linearity in the Lunatic III feedback resistors by using both 10 ’ ’ and 10 “I Ohm resistors. No differences in the measured oxygen isotope compositions were detected.

A V-SMOW (Vienna-SMOW) standard was created in 1966. It was designed to have the same oxygen isotopic composition as the originally defined SMOW. Baertschi (1976) determined ‘80/‘60v_sMoW (but not “O/IhO) ob- taining 0.0020052 +-0.0000005: about -6%~ different from the “O/‘“O of SMOW deduced by Craig ( 1961) using data in Nier ( 1950). Based on the small discrepancies between Nier ( 1950) and our NTIMS data, and since we have not actually measured the isotopic composition of V-SMOW by NTIMS. our NTIMS delta values will be referred to as permil deviations from POT (PO-Three), rather than from V- SMOW. We define the POT composition to have the same ‘80/‘“0 as V-SMOW (0.0020052) and with a corresponding “O/‘“O of 0.0003820 determined from the NTIMS fractionation line. We consider this composition to be precise and self-consistent, within an isotope fractionation factor. It should represent the most reliable isotope abundances for oxygen. Removing the discrepancy attributed to “0, we can plot blpoT, values obtained by NTIMS against the 6,v.s,o,, values obtained by GIRMS. The NTIMS data are to first approximation consistent with the GIRMS terrestrial fractionation line with slope 0.52 (Fig. 6). If we use only our data, we calculate a slope of 0.587 with uncertainties of about lo%, due to the relatively limited range of observed fractionation. Based on this. we do not believe there is a serious discrepancy in the fractionation line slope defined by NTIMS and GIRMS.

For NTIMS each sample has a well defined instrumental fractionation range of only 1 .O%r amu -’ ; however, the actual fractionation value measured by NTIMS can be substantially different from that measured by GIRMS. In particular, based on the high ionization efficiency for PO, as PO, of > IO%, we would expect the lighter oxygen isotopes to be enhanced in the ion beam relative to the heavier isotopes throughout the duration of the run, as is observed. Figure 7 shows the relationship between b’xOcIRMs (on the abscissa) and b “ONriMs (on the ordinate) for analyses of standards determined by the thermal decomposition technique, including SRM- 120~ and UMS- 1 performed at the University of Mich- igan. and the BrF, fluorination technique, including SRM- 120~ performed at North Carolina State University (Crow- son et al., 19Y 1 ). and LA- I performed at the University of

Alberta (Farquhar et al., 1993). The data on LA- 1 and SRM- 120~ are essentially consistent with a line of slope unity, as would be expected if there is a uniform offset between measurements by the different techniques. The agreement for UMS-1 is less satisfactory, corresponding to a shift as large as 3%0 from the slope unity line. The best fit line with slope of unity shows an intercept of - IO?&, which is an estimate of the offset in 6 I80 between the two methods. For a more detailed comparison of data for the oxygen standards it is important to consider possible differences between GIRMS measurements of oxygen extracted from phosphate using BrFs and then converted to COZ, with measurements of oxygen obtained by thermal decomposition of phosphate in the presence of carbon ( O’Neil et al.. 1994). Based on the discussion of the data by O’Neil et al. ( 1994), it appears that measurements on COZ obtained by the phosphate decomposition technique show a bias in 5 “0 of up to 2%~ (cf. their Table 3 and Fig. 2), relative to measurements using the BrFs extraction technique. Therefore, in Fig. 7 we have also shown the data obtained by phosphate thermal decomposition reduced by 2%0 in S”O. The agreement between all the data when this correction is applied shows some definite improvement. However, it is also clear that additional measurements of oxygen on standards are required by all techniques before it is possible to unambiguously define the calibration between them.

For the NTIMS technique, the larger range in instrumental fractionation observed for the SRM-120~ standard may be due to the impurities in this standard. We made no effort to determine the purity of the Ag,PO, salts prepared from any of the apatite standards. Such an effort will be undertaken in future analyses. It has also been reported that the preparation of AglPO, by slow NH, volatilization can induce isotope effects of up to 2%0 between first formed crystals and later crystals (Stuart-Williams and Schwartz. 1995, see also O’Neil et al., 1994). This effect is large enough to represent another source of variation in isotope analyses of standards between laboratories and analysis techniques that use the Ag,P04 salt. In our own work, Ag,POJ was precipitated quantitatively, but only a fraction of the nonhomogenized precipitate was used to prepare the standard solution,

Due to the high ionization efficiency of POJ and the simple loading technique, negative ion mass spectrometry is ideally suited to the measurement of oxygen isotope abundances in small samples of phosphate including single apatite mineral grains, apatite microfossils, or detailed microsampling of larger apatite mineral substrates. The paleoenvironmental re- cord of conodonts (tooth-like microfossils of an extinct eel- like animal) have scarcely been exploited due to their ex- ceedingly small size ( 100 pm scale) and weight (5 -20 pg). NTIMS offers the high sensitivity needed for phosphate oxygen isotope analysis of individual conodonts for paleoceanographic and paleoclimatic investigations in Paleozoic marine sediments, where carbonate oxygen isotope systematics are more likely to be disturbed. In addition, the techniques presented here are directly applicable to the analyses of 0 in individual phosphate crystals from meteorites.

5. CONCLUSION

Oxygen isotope analysis has long been the exclusive domain of GIRMS utilizing CO, as the analyte gas. Building


-5 0 5 10 15 20 25 30 35 40

PO Fig. 6. Isotope fractionation for both GIRMS and NTIMS are compared on the same graph using the V-SMOW and POT compositions, respectively. The data for both measurement techniques are consistent with each other once the small discrepancy assigned to ‘7O/‘6O is adjusted (cf. Fig. 3b). The GIRMS data are from Clayton and Mayeda ( 1983) and Robert et al. ( 1992). All GIRMS data were obtained using the conventional BrF, technique.

For GIRMS I&&. Thermal decomposition

8 measured estimated correction

6fF5 fluorination V

5 10 15 20 25

6’*0 (%o V-SMOVV)

Fig. 7. Comparison of 6”O for the PO4 standards measured by NTIMS and GIRMS. Standards analyzed by NTIMS and GIRMS are consistent with a slope 1 line. GIRMS data were obtained by the conventional fluorination technique (LA-l in Farquhar et al., 1993; SRM-12Oc in Crowson et al., 1991), and the thermal decomposition technique of O’Neil et al. (1994). O’Neil et al. (1994) report a positive 6’*0 offset of O.l-2%0 between these GIRMS techniques. Although the offset appears to be systematic in the positive direction, no correction factor was prescribed in O’Neil et al. ( 1994). We have shown the analyses performed by thermal decomposition with an estimated correction of 2%0 in the 6”O value, relative to 6’*0 values obtained by fluorination. The y-intercept of -10%0 is an estimate of the relative S’*O offset between NTIMS and GIRMS analyses.

on the work of Heumann et al. ( 1989) and Wachsmann and Heumann ( 199 1) , we have developed a thermal ionization technique for oxygen isotope abundance determinations using PO, ions. Compared to conventional, dual inlet GIRMS, NTIMS offers a lo4 improvement in sensitivity and the ad- vantage of a modified direct loading technique for oxygen isotope analysis of submicrogram samples of natural phosphate and achieves an external precision that is only two to five times worse than the precision typically reported by GIRMS analyses of phosphates ( kO.2 to +- 1 .O%O 20). With a small trade-off in precision, specialized equipment is not needed to analyze oxygen isotopes in submicrogram samples of phosphate.

With NTIMS, the scale of inquiry for oxygen isotope analyses of phosphates is now similar to that for isotopic analysis of Sr, Nd, and Pb, routinely analyzed by thermal ionization methods. Coupling these radiogenic isotope sys- tems with oxygen isotope analyses in phosphate minerals will be possible, as will the analysis of individual conodont microfossils for paleoceanographic and paleoclimatic stud- ies. NTIMS measurement of oxygen is ideally suited to the analysis of phosphates but may not be limited to this single application, provided PO, compounds can be formed from other oxygen bearing minerals without oxygen isotope fractionation.

Acknowledgments-The senior author thanks Robert A. Creaser for sharing his fascination with negative ion mass spectrometry and for early discussions on the mass spectrometry of phosphate. James O’Neil and James Farquhar are thanked for supplying their phos-


phate standards, UMS-1 and LA1 1, respectively. Constructive re- views by J. R. O’Neil and S. M. Savin are appreciated. Henry Ngo and Lindsey Hedges are thanked for laboratory support. Appreciation is also extended to Mukul Sharma for many enjoyable conversations relating to the progress of this work. C.H. acknowledges NSERC of Canada for post-doctoral scholarship support. This work was sup- ported by NASA Grant NAGW-3337 and is Division contribution 5688 (942)

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Negative thermal ion mass spectrometry of oxygen in phosphates