boschetti&iacumin2005 rcms

RAPID COMMUNICATIONS IN MASS SPECTROMETRY

Rapid Commun. Mass Spectrom. 2005; 19: 30073014

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rcm.2161

Continuous-flow d18O measurements: new approach to

standardization, high-temperature thermodynamic and

sulfate analysis

Tiziano Boschetti* and Paola Iacumin*Dipartimento di Scienze della Terra, Universita` degli Studi di Parma, Parco Area delle Scienze 157/A, 43100 Parma, Italy

Received 26 July 2005; Revised 26 August 2005; Accepted 26 August 2005

The continuous-flow method for the determination of the O-isotope composition of solid samples

has significant advantages over off-line extraction methods, but the problem has arisen of the stan-

dardization of results that has only partially been resolved by the use of water standards. We pro-

pose a new approach to standardization that uses carbothermic reduction of calcium carbonate

standards catalyzed by high-purity AgCl. Analytical accuracy, precision, variance homogeneity

and long-term stability are proven. Preliminary data on the barium sulfate d18O analyses are

reported, with a closer look at the different results obtained by off-line and on-line methods on

intercomparison standards NBS-127 and MSS3. Copyright # 2005 John Wiley & Sons, Ltd.

Determination of d18O composition in organic and inorganiccompounds by thermal conversioncontinuous flow isoto-

pic ratio mass spectrometry (TCCFIRMS) is hindered by the

shortage of internationally distributed solid calibration stan-

dards. New standards are continually being prepared by

institutions such as IAEA and NIST, but the requirements

for more and more specificity and the sectionalism of scienti-

fic research have far exceeded the present capabilities to

produce appropriate standards, both in the case of the bulk

stable isotope analysis and in case of the compound-specific

isotope analysis.

Many laboratories use water standards for the 18O/16O

measurement normalization1 because water is an interna-

tionally distributed material and many samples are subject to

intercomparison exercises using an off-line validated

method.2 Use of water as standard for solid analysis in

TCCFIRMS is not free from problems due to: (a) evaporation

and isotopic fractionation of the small volume of water

standard (0.1mL); (b) calibration line too wide or with non-equispaced standards (non-homogeneous variance); and (c)

different reactivity of standards and investigated samples.

Water samples can be measured with helium dilution, but the

helium dilution offset cannot be compared with the absolute

solid sample isotopic results since they are not measured in

the same manner.3 Calcium carbonates have a validated off-

linemethod of analysis4 and are distributed internationally as

reference materials, but their use in the TCCFIRMS standar-

dization has been avoided because of the incomplete yield of

reaction.3 The usefulness of halide salt addition in the thermal

and carbothermal chemical analysis of many O-containing

compounds is also widely known.57 Most recently, non-

halogenated and halogenated catalysis were tested for the

continuous-flow isotope analysis of oxygen from carbonates

and silicates.8,9 In this study, we corroborate the positive

results of the catalytic carbothermic reduction of calcium

carbonate using AgCl and demonstrate its applicability to a

new standardization approach and to the determination of

the 18O/16O isotope ratio analysis of barium sulfate.

EXPERIMENTAL

The apparatus used in this study (Fig. 1) consists of a thermal

conversionelemental analyzer unit (TCEA, Thermo

Finnigan) composed of an outer reaction tube made of

Al2O3 ceramic (17 mm o.d., 14 mm i.d., length 470 mm) and

an inner glassy carbon tube (12 mm o.d., 7 mm i.d., length

355 mm) assembled following the instructions reported in

the ThermoFinnigan TCEA manual.10 A graphite crucible is

inserted in the hottest zone of the reaction furnace, and

heated to 140014208C. At these temperatures, carbon fromthe inner tube and glassy carbon grit, which partially fill the

reactor, primes the reduction of the analyzed compounds fol-

lowing the reaction

O-compounds Cs COg residualss;g;l 1

where subscripts s, g and l represent solid, gaseous and

liquid by-products, respectively. The space between the

tubes is continuously flushed with high purity He (5.5

Sapio, Monza, Milan, Italy) to prevent undesirable oxida-

tion and to carry the CO gas to a gas chromatography

(GC) column downstream of the reactor. We used two con-

figurations: high gas configuration, where the GC column

was held at 908C and the system flushed with He carriergas pressure at 1.0 bar; and low gas configuration, where

the GC column was held at 708C and the system flushedwith He gas at 0.85 bar. The combination of low tempera-

ture and gas pressure guarantees a good GC separation

between the 12C16O and 14N2 peaks when N-compounds

Copyright # 2005 John Wiley & Sons, Ltd.

*Correspondence to: T. Boschetti or P. Iacumin, Dipartimento diScienze della Terra, Universita` degli Studi di Parma, ParcoArea delle Scienze 157/A, 43100 Parma, Italy.E-mail: [email protected]; [email protected]

are analyzed. A trap, filled with 2/3 AscariteII1 20-30 mesh

(Thomas Scientific, Swedesboro, NJ, USA) plus 1/3 anhy-

drous Mg(ClO4)2 (Anhydrone1, CEInstrumentsThermo

Quest, Milan, Italy), is interposed between the reactor and

the GC column, thus cutting off acid gases (e.g. chlorine)

and water. The TCEA unit is coupled to a Delta Plus XP

spectrometer (ThermoFinnigan) through two open-splits

ConFloIII interface11 (Fig. 1). The mass spectrometer is oper-

ated in linear mode (extraction lens at 2.7 kV). To calculatethe yield of oxygen, variable amounts of benzoic acid were

used (Hekatech, Wedberg, Germany). Solid samples and

standards, weighed into silver capsules (3.2 4 mm,0.02 mL; Santis Analytical AG, Teufen, Switzerland), are cali-

brated against pure CO reference gas peaks (Messer 4.7;

1.5 bar at ConFloIII panel) in order to have comparable

area and signal (Table 1). Comparison with calibration,

intercomparison or laboratory standard materials16 is necessary

prior to every new batch of samples as signal variations are

possible due to d18O variation of the reference CO gas fromdifferent tanks (from 0.0 to ca. 5.5% V-SMOW), as is refo-cusing of the mass spectrometer after switching between

other units (e.g. CHN) and replacement of the ion source.

Table 1. Weighted amounts of some compounds analyzed by TCCFIMRS, proportionally calculated in comparison with the

relative area and signal (V) of standard CO reference gas peaks

Compound Melting pointa (8C) Min-max weightb (mg) Total Oc (%)

C7H6O2 benzoic acid 122 0.0740.154 26.202C12H22O11 sucrose 186 0.0490.080 51.415C6H10O5 cellulose 180290

d 0.0240.080 49.338KNO3 K nitrate 334 0.0470.081 47.474Ag3PO4 Ag phosphate 849 0.2090.329 15.289BaSO4 Ba sulfate 1580 0.1060.152 27.421CaCO3 Ca carbonate 8991339

e 0.0970.113 47.956

a Bale et al.;12 Bayley et al.;13 Kirk-Othmer.14b Resulting signals were compared with the reference peak of CO& 3.3V& 60000 area (1.5 bar at the ConFloIII unit).c Element atomic weight from Loss.15d Decomposition at 1808C, combustion at 2908C.e Decomposition at 8998C; melts at high pressure: 13398C at 100 bar.

Figure 1. Schematic diagram of the thermal combustion/elemental analyzer (TC/EA) continuous-flow (CF)

isotopic ratio mass spectrometer (IRMS) (modified from Gehre and Strauch9). Original Thermo Finnigan

configuration was modified with an as/an trap*: 2/3 AscariteII1 1/3 Anhydrone1 trap.

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2005; 19: 30073014

3008 T. Boschetti and P. Iacumin

Aware of the lack of certified standards with an appro-

priate low 18O/16O ratio for CO reference gas standardiza-

tion, in this work we have always considered d18O(CO) 0%(V-SMOW) in every CO tank, so differences between known

and measured values of the standards after least-squares

standard calibration are fundamentally due to d18O varia-tions of CO from different tanks. In the case of substances

with high melting points, principally calcium carbonate and

barium sulfate, we have tested the effectiveness of some

analytical grade additives such as silver chloride (99.9%

AgCl; Strem Chemicals, Newburyport, MA, USA) and

hexamethylenetetramine (99% C6H12N4, A.C.S. reagent;Aldrich, St. Louis, MO, USA). These materials were preferred

to other catalysts due to the high yield of the previous

experiments,8 the relatively low causticity in comparison

with the fluorinated residual gas of fluoride salts (e.g. KF),

and the limited presence of other elements that could

invalidate the analysis by the formation of undesirable

residuals like oxides and carbides. Standards, samples and

additives were dried in a vacuum oven for 12 h at 1208C;afterwards, materials and silver capsules were stored in

vacuum driers before use.

Any CO2 by-product is avoided by the high temperature of

the reactor (>9808C) that displaces the Boudouard reaction:

CO2g Cs , 2COg 2completely toward CO.13

The theoretical evolutions of reactions reported through-

out the text were predicted with FACTSAGE-Web thermo-

dynamic data specific for high temperatures,12 assuming

solid activities and gases partial pressures equal to unity.

These assumptions are reasonable considering the high

purity of the samples and standards and the solids weight

calibration in order to bring a PCO of &1.01.5 bar. Themelting of silver capsules (960.88C) was not considered inthe modelling because the phase change Ag(s) to Ag(l) does

not involve a Gibbs free energy variation. In addition, the

formation of silver-containing residuals (e.g. silver carbide)

after carbothermic reduction is highly improbable.

RESULTS AND DISCUSSION

Calcium carbonate analysisMany authors have advised against the use of calcium carbo-

nate in TCCFIRMS standardization.1,3 In fact, at the operative

temperature of the reactor (from 133014508C), only two ofthree oxygen atoms react with C to form CO during the car-

bothermic reduction of the calcium carbonate:

CaCO3s Cs , CaOs 2COgGr1400C 198:3 kJ; Gr1600C 262:76 kJ

3Preliminary results on several calcium carbonate samples

from this study at standard configuration (furnace at 14008C,GC column at 908C, He carrier flow pressure at 1 bar) confirmthe yield of oxygen as CO at about 67%. Problems arising

from the incomplete yield of reaction are: (a) isotopic

fractionations17,18 and (b) memory isotopic effects related to

the presence of solid residual CaO(s) which is quite reactive

and can exchange with CO(g).19,20

Kornexl et al.8 observed that temperatures higher than

16008C are necessary for the quantitative release of oxygenfrom calcium carbonate. This temperature is unattainable for

a TCEA unit using an Al2O3 ceramic tube (classified as a

synthetic sintered mullite); other than, according to a recent

study,21 the higher the temperature, the higher the back-

ground on mass 28 (12C16O) due to diffusion of (light) oxygen

from the ceramic tube. In fact, in the range 133014508C, wehave found a significant linear correlation between tempera-

ture and the mass 28 signal (N 6; r 0.9944):mass 28 3:7549 0:1998T 4872 277

where T is the temperature in 8C and mass 28 is the signal inmV for the ion at m/z 28.

The use of high temperatures for long periods reduces the

life of reactor tubes, the furnace and the thermocouple. In

addition, thermodynamic calculations reveal that at the temp-

eratures of 1400 and 16008C the Gibbs free energy of reaction(3) is lower than those of reactions (4) and (5), giving support

for the formation of calcium carbide or elemental calcium

(*note that calcium is liquid at 8428C and gaseous at 15038C):

CaCO3s 4Cs , CaC2s 3COgGr1400C 103:75 kJ; Gr1600C 212:75 kJ

4CaCO3s 2Cs , 3COg CaGr1400C 4:35 kJ; Gr1600C 108:73 kJ

5The non-total yield problems could be solved by using

catalysts.8,9 In this study, CO peaks were detected during the

blanks analysis of hexamethylenetetramine catalyst, prob-

ably due to the high hygroscopicity of amines and the

consequent hydration during the weighing routine and/or

the permanence of the samples in the autosampler before

analysis. It is also possible that the high nitrogen content of

this compound could pollute the oxygen isotope analysis due

to the mass interference generated by accumulation of N2 and

NO gas in the ion source of the mass spectrometer.

While on-line carbon reduction plus fluorination does not

give satisfactory results,22 catalytic carbothermic reduction

using AgCl as an additive is the recommended method for

the on-line analysis of carbonates.9 We obtained CO free

blanks using approximately 500mg of AgCl plus 300 mg ofglassy carbon powder (obtained from grinding of glassy

carbon grit used for reactor assembling) placed into the silver

capsule. This result is due to the low hygroscopicity and the

high purity of AgCl and of the mixture. Using these amounts,

the procedure was applied to calcium carbonate laboratory

standards, previously analyzed isotopically by the H3PO4decomposition method,4 and the results were used for

TCCFIRMS calibration at 14208C and in the low gas config-uration (Table 2). Remarkably, mean yields of 100% were

obtained. Most probably, the reactions involved are:

CaCO3s AgCls 4Cs , 3COg Agl CaC2s ClgGr1420C 24:57 kJ 6

CaCO3s 2AgCls 2Cs , 3COg CaCl2l AglGr1420C 438:13 kJ 7

High-temperature continuous-flow 18O measurements of sulfates 3009


Table 2. Analytical d18O (CO3) results of calcium carbonate laboratory standards. Two standards are commercial reagentsgrade (TCS2, CE-CO3) and two natural carbonates (TCS1, Carrara). Known and Measured columns report the IRMS values

measured on the CO2 (off-line acid decomposition method4) and on the CO (on-line AgCl catalyzed carbotermic reduction),

respectively. Equation of least-squares linear regression calculated on these data is reported

Code DescriptionWeight

(mg)Signal

(V at mass 28)CO

(rel. area) O-yield*Known**

(% vs. V-SMOW)Measured(% vs. CO) Mean Std.dev.

TCS1 CaCO3 Sardinian 0.122 4.3 157280 105 23.0 28.19 28.3 0.2stalagmite 0.155 5.2 186750 102 28.25

0.156 5.1 185143 102 28.54

TCS2 CaCO3 Merck ACS-ISO-F 0.144 5.6 202793 108 10.9 17.54 17.8 0.299% reagent grade 0.128 4.7 172545 103 17.87

0.148 5.5 200902 104 17.96

CE-CO3 CaCO3 Carlo Erba RPE 0.160 5.7 209184 102 3.3 10.16 10.6 0.499% reagent grade 0.170 6.7 245484 101 10.79

0.160 6.3 231538 102 10.910.138 3.9 143327 103 10.35

Carrara CaCO3 Carrara marble 0.121 4.6 167782 106 29.0 34.68 34.4 0.30.148 5.0 180347 103 34.120.141 4.3 152711 103 34.40

Least-squares regression parameters (ymeasured; xknown)Intercept: 7.5742 0.1768Slope: 0.9177 0.0095Pearsons correlation coefficient (r) 0.9994Standard error: 0.35

* In comparison with benzoic acid; yield excess due to weighting error.** d18O (vs. VSMOW) [1.03092 * d18O (vs. VPDB)] 30.92.49

Table 3. Analytical results and statistical tests relative to least-squares linear regression of international (IAEA-CH-6, NBS-18,

NBS-19) and laboratory standards (CE-CO3, TCS1, TCS2). Linearity (Fishers test calculated value is greater than the tabulated

values) and variance homogeneity (Hartley and Cochrans test calculated values are smaller than the tabulated values) are

confirmed. Least-squares regression line described in this table is parallel to least-squares regression line of Table 2, as

confirmed statistically by Students t-test on the slopes

CE-CO3 IAEA-CH-6 TCS1 TCS2 NBS-19 NBS-18

Known values (% vs.V-SMOW) 3.3 36.4 23.0 10.9 28.6 7.2

7.62 38.56 25.77 14.61 31.44 10.76Measured (% vs. CO) 7.35 37.74 25.71 14.86 30.41 11.11

7.14 38.12 26.01 14.49 30.40 11.46 S(sum)

Std.dev. 0.24 0.41 0.16 0.19 0.60 0.35 1.95Variance 0.057 0.168 0.025 0.036 0.360 0.124 0.77Mean 7.37 38.14 25.83 14.65 30.75 11.11 127.85Sy 22.11 114.41 77.50 43.95 92.26 33.33 383.56Sy2 163.07 4363.86 2002.13 644.03 2837.71 370.54 10381.34Least-squares regression parameters (ymeasured; xknown)Intercept: 4.4354 0.1417Slope: 0.9254 0.0065Pearsons correlation coefficient (r) 0.9996Standard error: 0.33

ANOVA Deviance Degree of freedom (DF) Variance

Total 2208.020 17Between (B) 2206.480 5 441.296Within (W) 1.53997 12 0.12833

Variancetests

DF(num/den)

Calculatedvalues

Tabulated values(significance level)(0.05) (0.01)

Fisher test (B/W) F(5/12) 3438.745 3.11 5.06Hartley test F(6/2) 14.3003 19.5 99.3Cochran test 0.46691 0.61 0.72



The accuracy, precision, linearity and variance homo-

geneity were checked by the analysis of three laboratory

standards and three IAEA internationally distributed

materials:16 NBS-19 calcium carbonate calibration standard

and two intercomparison standards, NBS-18 calcium carbo-

nate and IAEA-CH-6 sucrose (Table 3). For the sucrose

standard, internationally agreed d18O measures are not yetavailable, but a value of 36.4% (V-SMOW) is commonlyreported.1,23 The high standard deviation on NBS-19 is

probably due to the large grain size of this reference material

(a poor feature of the material when related to the small

amount weighted for the TCCFIRMS analysis). The linearity

and variance homogeneity of regression were evaluated and

confirmed statistically (Fishers, Hartleys and Cochrans

tests; Table 3). In comparison with the least-squares

regression line data of Table 2, the difference between the

intercepts (i.e. measured d18O value) is mainly due to thevariation of the d18O composition of the CO reference gas;the parallelism between the two regression lines was

confirmed statistically by a Students t-test at high prob-

ability values (P> 0.1). Long-term stability is less than the

calibration error (0.3%) and was evaluated by continuousanalysis throughout 12 h of keratinic samples having d18Ovalues lower than the lowest one of the calibration standards

(3.3%). The deviations from the two calibrations performedat the start and the end of the data acquisition are shown

in Fig. 2.

Barium sulfate analysisAs with calcium carbonate, the use of barium sulfate stan-

dards in TCCFIRMS oxygen isotope analyses was also

recently advised due to the memory effects related to oxygen

scavenging by Ba2 ions resulting from carbothermic reduc-tion of BaSO4.

24 In agreement with the literature,8 our preli-

minary analysis on barium sulfate samples by TCEA in

standard configuration, at 14008C and without additives,gives yields of 9497%. Tests on five barium sulfate samples

at 14208C and at a variable height of the graphite crucible inthe reactor give a mean yield of 75%. From these yield data,

we infer that two competitive reactions control the carbother-

mic reduction of barium sulfate:

BaSO4s 4Cs , BaSs 4COgGr1400C 614:60 kJ; Gr1420C 628:21 kJ

8

BaSO4s 3Cs Ags , BaOs 3COg AgSgGr1400C 215:70 kJ; Gr1420C 229:34 kJ

9Based on these reactions, oxygen isotopic fractionation and

memory effects of BaO residuals should be expected during

analyses.In this study, we have investigated the use of AgClC

additives and TCEA low gas configuration on BaSO4

Figure 2. Long-term stability test on 54 keratinic samples (13 h of analysis). The calculated

d18O values (0.3% vs. V-SMOW) of the samples plotted on the dashed line represent thebest fit of the two standard calibrations (d18Ofirst calibration/d

18Osecond calibration 1).



intercomparison standards and we have obtained an average

yield of 100%, probably following the reaction:

BaSO4s AgCls 4Cs , 4COg Agl BaSs ClgGr1420C 539:19 kJ 10

Table 4 shows some results of the effectiveness of the ana-

lyses applied to two barium sulfate standards and using cal-

cium carbonate as calibration standards. Certified values

were obtained by other authors using an off-line sulfate reduc-

tion: MSS-3 (10.5% vs. V-SMOW, Okayama University stan-dard)25 and NBS-127 (9.3% vs. V-SMOW, seawater sulfatefrom Monterey Bay, California, distributed from IAEA and

NIST with the codes NBS-127 and RM8557, respectively).16

Our NBS-127 d18O value of 8.5% is comparable with thosefrom other TCCFIRMS analyses, but not with the results

obtained by the off-line reduction technique. In Table 5, the

published and communicated d18O(SO4) values of NBS-127from on-line and off-line techniques are summarized, the

difference being statistically significant according to

Students t-test.

The true oxygen isotope composition of seawater sulfate is

an unresolved problem. In the past decades, the proposed

d18O values obtained by off-line methods were 8.6%,31,32

8.7%,33 9.39.6%,3436 and 9.9%.37 Curiously, the lowestvalues agree with the present on-line NBS-127 determina-

tions. Off-line carbothermic reduction of BaSO4 has been

carried out by heating the BaSO4-graphite mixture with

conventional furnaces,38 electrical resistance,25,39,40 and

radio-frequency induction.34,4144 Reaction temperatures of

the various methods were variable from 850 to 12008C. In thereduction, CO and CO2 form via the reaction:

BaSO4s 3Cs !8501200C

BaSs 2COg CO2g 11

The gas products CO2CO are expelled from the reactionchamber to a vacuum line where CO2 is collected in a liquid

nitrogen trap and the remaining CO is subjected to a high-

voltage electrical discharge from 1 up to 6 kV for COCO2conversion (inverse of reaction (2)) within a quartz or Pyrex

tube. Explanations of the differences related to d18O of theseawater sulfate were:

1. isotope effects during conversion of CO into CO2;

2. oxygen isotopic exchange between CO and quartz/Pyrex

walls in the conversion tube (particularly if the tube is not

cooled);

3. isotopic alteration during long storage of the seawater

samples;

4. contamination of the BaSO4 precipitated by impurities like

organic materials, carbonate and phosphate.

Table 5. Summary of the published and communicated d18O(SO42) values of NBS-127 from on-line and off-line techniques

(differences being statistically significant according to Students t-test)

d18O(SO4) seawater sulfate standard (NBS-127, RM 8557)

On-line method Off-line method

Hall et al.27 9.2 0.2Kornexl et al.8 8.7 0.2 Gonfiantini et al.16 9.3 0.3Bohlke et al.26 8.6 0.2 Bottcher et al.28 9.5This work 8.5 0.2 S. Halas (pers. comm.)a 9.69 0.06

Palmer et al.29 8.9 0.2

Mean 8.6 Mean 9.3

Median 8.6 Median 9.3Std.dev. 0.1 Std.dev. 0.3

aAt the time of writing, this value was corrected to 9.91% (Peryt et al.30).

Table 4. Some results of the effectiveness of the analyses applied to two barium sulfate standards and using calcium carbonate

as calibration standards

Weight (mg) O-yield (%)Signal

(V at mass 28)Measured

(% vs. CO)Calculated

(% vs. V-SMOW)Off-line value

(% vs. V-SMOW) dcalc doff-lineNBS-127 0.164 104 3.6 15.5 8.6 9.3 0.7

0.122 90 3.3 15.3 8.5 0.80.175 107 4.0 15.4 8.5 0.80.159 102 3.7 15.2 8.3 1.0

Mean 101 8.5 0.8Std.dev. 8 0.2

MSS-3 0.175 110 4.0 16.4 9.7 10.5 0.80.162 102 3.4 16.9 10.1 0.40.128 101 2.7 17.1 10.4 0.10.126 91 2.2 16.8 10.0 0.5

Mean 101 10.1 0.4Std.dev. 8 0.3



Only the possible role of process 2 was experimentally

assessed.25,39 Process 1 is subject to a kinetic fractionation:17,25

kineticCO-CO2 k1k2 1:02 12

where k1 and k2 are, respectively, the rates constants of the

C16O and C18O molecules in the forward reaction. Thus, if a

yield less than 100% of CO2 is obtained from BaSO4 reduction

(and if the yield of CO2 from CO is from 90 to 98 percent),25,34

sequential electrical decomposition of CO leads to an enrich-

ment in 18O in the final CO2 gas.

To a first approximation, the CO electrical decomposition

is analogous to a Rayleigh distillation in a closed system

because there is a complete mixing of the product CO2; then:

pt rt;0 1000 1 fr

1 fr 1000 13

Supposing that drt,0 8.6% (seawater SO42) and calculat-ing the product composition (dpt) at fr 0.1 to 0.0 (fraction ofCO unconverted), the oxygen composition of the total CO2from BaSO4 reduction should be between 8.9 and 11.3%(Table 6). Fractionation connected to CO glow discharge

processes has been studied recently at voltages between 3 and

9 kV, at 50 Hz,45 confirming the enrichment of 18O in CO2 and

indicating the presence of a brown-black deposit on the

reactor wall connected to the formation of a mixture of

polymeric forms of malonic anhydride C3O2 (also noted

by some authors during the setting of the method for

BaSO4 processing; G. Cortecci and A. Longinelli, personal

communication).

Effects 3 and 4 can be rendered negligible provided that

pure BaSO4 is analyzed.46 Other problems could derive from

defective mixing of BaSO4 with graphite, because the CO

generated could diffuse and react with uncoated BaSO4particles:47

BaSO4 4CO , BaS 4CO2 14CO2 could diffuse back into carbon to generate more CO

according to the Boudouard reaction, but at the temperature

of the method the rate of reaction (14) is higher than the

reaction rate of the Boudouard reaction. This problem,

however, was minimized by mixing BaSO4 with a large

excess of graphite.23

CONCLUSIONS

Continuous-flow determination of d18O composition inorganic/inorganic materials is an already established

technique, reducing sample size and times of analyses in

comparison with an off-line technique. Our method of stan-

dardization extends these advantages by working with only

solid standards. Our method can analyze samples in the

range 36.4 d18O 3.3 (% vs. V-SMOW), but long-term sta-bility was experimentally confirmed up to 6%. Eventually,a carbonate standard with a value of d18O&20% could besynthesized using Kim and ONeils equilibration method48

with a water sample of d18O&50% (e.g. nival precipita-tions or Artic ice). Considering the small amount of sample

required for on-line analyses, solid standards with homoge-

neous small grains are recommended. Our determinations on

different standard matrices (calcium carbonate and barium

sulfate) were successfully performed using AgCl which (i)

obtains the complete reduction of the compounds and (ii)

acts on the kinetic of reactions primed by the gaseous chlor-

ine. Differences between off-line and on-line reduction techni-

ques in the d18O determination of barium sulfate (0.6%) arerelated to some analytical problems in the former method,

Table 6. Calculated isotopic composition of total CO2 resulting from BaSO4 carbon reduction off-line method considering

fractionation effects due to the disproportionation of CO to CO2. The CO2 values were calculated as [(8.6 y) (dpt x)]. Boldcontoured values are the most probable isotopic values considering the molar ratio of x (CO2) and y (CO2) obtainable by the

method. In the calculations, we have assumed no significant difference between the first formed gases from BaSO4 carbon

reduction,25 so drt,0 d(CO) d(CO2) 8.6%

x = 1-y 1-y 1-y 1-y 1-y 1-y 1-y 1-y 1-y % of CO2 from BaSO4y = (0.9*(1-fr)) (0.8*(1-fr)) (0.7*(1-fr)) (0.6*(1-fr)) (0.5*(1-fr)) (0.4*(1-fr)) (0.3*(1-fr)) (0.2*(1-fr)) (0.1*(1-fr)) % of CO2 from CO

fr pt

0.10 13.64 12.69 12.23 11.78 11.32 10.87 10.42 9.96 9.51 9.050.09 13.29 12.44 12.01 11.59 11.16 10.73 10.31 9.88 9.45 9.030.08 12.92 12.18 11.78 11.38 10.98 10.59 10.19 9.79 9.39 9.000.07 12.53 11.89 11.53 11.16 10.79 10.43 10.06 9.70 9.33 8.970.06 12.12 11.58 11.25 10.92 10.59 10.26 9.92 9.59 9.26 8.930.05 11.69 11.24 10.95 10.65 10.36 10.07 9.77 9.48 9.19 8.890.04 11.22 10.86 10.61 10.36 10.11 9.86 9.61 9.35 9.10 8.850.03 10.71 10.44 10.24 10.03 9.83 9.62 9.42 9.21 9.01 8.800.02 10.15 9.97 9.81 9.66 9.51 9.36 9.21 9.06 8.90 8.750.01 9.50 9.40 9.31 9.22 9.13 9.04 8.95 8.87 8.78 8.690.00 8.60 8.60 8.60 8.60 8.60 8.60 8.60 8.60 8.60 8.60

dptmean isotopic composition of the CO2 product reservoir at the time t following Rayleigh distillation under closed system conditions (see textfor details); fr remaining fraction of CO reactant reservoir at the time t.



such as isotopic exchange of the gases with the quartz cham-

ber, a kinetic fractionaction factor connected with the incom-

plete reaction yields and possible memory effect caused by

polymers of malonic anhydride. In the light of this and con-

sidering the higher temperature and the single step of the

reduction reaction to produce the measuring gas in the

TCCFIRMS method, we think that the on-line high-tempera-

ture carbothermic reduction using the AgCl additive is

the best way to produce 18O/16O ratios of barium sulfate

samples.

AcknowledgementsWe are grateful to Gianni Cortecci (Institute of Geosciences

and Earth Resources, CNR, Pisa, Italy) who improved the

manuscript. G.C. also provided us with MSS3 standard.

Many thanks are due to also Antonio Longinelli, Lorenzo

Toscani and Giampiero Venturelli (University of Parma,

Italy) for useful comments and suggestions. CaCO3 off-line

analyses were performed by Maura Pellegrini and Enrico

Selmo at Earth Sciences Department, University of Parma.

We are indebted to Prof. Stan Halas (Mass Spectrometry

Lab., Institute of Physics, UMCS, Lublin, Poland) for permis-

sion to publish his results on the NBS-127 standard. Finally,

special thanks are extended to Guido Giazzi and Roberto

Manzoni (Thermo Electron, Rodano, Milan, Italy) for support

and technical assistance on the TCEA unit. The valuable com-

ments of Prof. John J. Monaghan (University of Edinburgh)

and of an anonymous referee helped to improve the clarity

of the manuscript.

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