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Application of strontium IR measured by MCICP-MS for food authentication
Article in Journal of Analytical Atomic Spectrometry · December 2013
Impact Factor: 3.47 · DOI: 10.1039/b307068a

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Application of strontium isotope abundance ratios measured by
MC-ICP-MS for food authentication
G. Fortunato,a K. Mumic,a S. Wunderli,a L. Pillonel,b J. O. Bossetb and G. GremaudcaSwiss Federal Laboratories for Materials Testing and Research EMPA, 9014 St.Gallen,
Switzerland. E-mail: giuseppino.fortunato@empa.ch; Fax: 1141 (0)71 274 77 88;
Tel: 1141 (0)71 274 76 77b
Swiss Federal Dairy Research Station FAM, 3003 Berne, Switzerlandc
Swiss Federal Office of Public Health BAG, 3003 Berne, Switzerland
Received 19th June 2003, Accepted 28th October 2003
First published as an Advance Article on the web 1st December 2003

Naturally occurring isotopes of such elements as strontium (Sr) have proved to be good tools for detecting
trends in the soil-vegetation system and the tracing of a variety of objects. Multiple-collector inductively
coupled plasma mass spectrometry (MC-ICP-MS) has been used for the precise determination of variations in
the isotopic composition of Sr. The method described has been applied to the establishment of the potential
and limits to determine the geographical origins of different Emmental-type cheese samples. The sample
preparation consists of (i) a freeze-drying procedure to remove water, (ii) an extraction step to eliminate thefat components and to obtain the cheese casein fraction, (iii) a thermal decomposition of the latter, and (iv) a
chromatographic matrix separation of the redissolved residue. The determination of the isotope abundance
ratios 88Sr/86Sr, 87Sr/86Sr and 84Sr/86Sr resulted in precisions of 0.002–0.01%. Simultaneously, the ion currents
for krypton (83Kr, 82Kr) and rubidium (85Rb) were measured to correct for interferences with the Sr isotopes
84, 86 and 87. These and further (argide) spectral interferences causing bias effects to the Sr isotope abundance
ratios have been investigated and an adequate computational correction procedure has been assessed. The
whole set of validation data has been used for the calculation of the combined standard measurement
uncertainty of the isotopic abundance ratio, resulting in a value of 0.016%. Comparison of the measured87
Sr/86Sr data with thermal ionisation mass spectrometric (TIMS) results, determined on the same cheesesamples, agreed within the stated measurement uncertainties, thus indicating that both the validation of thesample preparation procedures and the mass spectrometric measurements cause no evident bias effect withrespect to the Sr isotope abundance values. The 87Sr/86Sr isotope abundance ratios in cheese originating fromdifferent regions (alpine, pre-alpine, Bretagne, Finland, Canada, Australia) accorded to local geologicalproperties. No difference was found between ‘‘casein-bound’’ and ‘‘whole-cheese’’ Sr isotope abundance ratios
within the stated measurement uncertainties.

DOI: 10.1039/b307068a

Introduction
Strontium (Sr) has four naturally occurring stable isotopes:
88
Sr, 87Sr, 86Sr and 84Sr. Natural variations of the isotopic
composition of 87Sr/86Sr mainly arise from the natural b-decayof 87Rb to 87Sr (t½ ~ 4.88 6 1010 a). The Sr–Rb system has
been widely studied in relation to a range of applications, e.g.,
for both petrogenetic and environmental tracing as well as
geochronological purposes. The 87Sr/86Sr isotope abundance
ratio varies from place to place both on a global and local scale.
Relative natural abundance variations given by IUPAC fall ina range of 82.29–82.75% for 88Sr, 6.94–7.14% for 87Sr, 9.75–
9.99% for 86Sr and 0.55–0.58% for 84Sr, respectively.1 Some
exceptional specimens are known in which the element has an
isotopic composition outside the reported range. Sr behaves
almost identically to Ca in milk and dairy products due to its
similar chemical properties. The mass ratio of Sr/Ca is in therange of approximately 2.5 6 1024.
Food authenticity is one aspect which is receiving more
attention with the increasing mobility and low transport costsof our society. For the protection of consumers and producers
analytical methods have to be validated2–6 for the prevention of
frauds and mislabelling and to guarantee the origin of the
products. A number of different analytical techniques andparameters have been evaluated for the purpose of Emmentaler
cheese authentication, such as infrared spectroscopy, lightelement stable isotope mass spectrometry, major and traceelemental analysis, pH-value determinations, etc. It was shownthat one single method or one single parameter on its own isgenerally insufficient to achieve this task, and that results fromseveral methods should be combined using multivariate
statistical analysis in order to achieve a good differentiationof cheeses from various geographic origins.
The strontium isotopes represent one part of the local
geological conditions and may therefore be linked to the originof the raw cheese products. Values of Sr isotope abundance
ratios in terrestrial vegetation will be dominated by the Sr
isotopic composition of labile cations in the soil, which arethemselves influenced by bedrock, soil water properties andatmospheric inputs.7,8 The typical strontium mass content ofmobile strontium in soil and in solution ranges from 0.2 to20 mg kg21 (mg Sr leached per g of soil).9,10 For different typesof geological samples the overall mass contents of Sr range
from 1 up to 2000 mg kg21 and from 0.01 to 7620 mg L21 forhydrological samples (sea-water, rivers, rain …) and from 8 to2500 mg kg21 for biological samples (wood, roots …).
Biological processes, whether involved in plant or animal
metabolism, do not significantly fractionate strontium isotopes.11 Sillen et al.12 investigated the possible shift of theisotopic composition between labile Sr cations in the soil andplants in a well-defined geographic area. Interestingly,
differences of up to 0.5% in the composition of 87Sr/86Sr
have been found. It was suggested that this phenomenon could
be explained by the presence of significant amounts of dustThis journal is ß The Royal Society of Chemistry 2004

J. Anal. At. Spectrom., 2004, 19, 227–234

227

Table 1 Possible interferences for Sr isotopes and approximative mass resolving power m=Dm needed for the separation of the signals (10% valley
definition)
IsotopeType of interference

84

Isobaric
Molecular

84

Doubly charged

168

86

Sr1

Kr1
Ti36Ar1
44
Ca40Ar1
68
Zn16O1
48

Er21
Yb21

168

w10 000
w10 000
w10 000
w10 000

86

2000
2000

172

Kr1
Ti40Ar1
70
Ge16O170
Zn16O146

Yb21

components, differential weathering of highly radiogenic rocks
and an influence of whole soil constituents contributing toavailable soil Sr.
It is assumed that geological properties (e.g., Sr isotope
abundance ratios) are reflected directly in the cheeses when thecows are kept under a controlled dietary regime and are not fed
with industrially produced feeds or feeds from geographicallydistant sources. Only contamination and blending of the dairy
products with other Ca and Sr containing sources could
then provide a shift in the Sr isotope ratios and lead tomisinterpretation of results.3
MC-ICP-MS offers a way to establish accurate highprecision isotope ratios as long as the influencing parameters
are understood and under control. Typical internal precisions
for 87Sr/86Sr are in a range of 0.001–0.002%.13–15 Various
parameters may influence the determination of Sr isotope
abundance ratio by MC-ICP-MS. The main factors are
believed to be isobaric and molecular interferences such as
87
Rb1, 44Ca40Ar1, 48Ca40Ar1 and doubly charged rare earth
isotopes (167Er21, 168Yb21, 171Yb21 …) stemming from thesample, and 84Kr1 and 86Kr1 from the argon gas (Ar) used tooperate the inductively coupled plasma, by which both theprecision and the accuracy of the isotope abundance ratios canbe affected (Table 1).
This paper describes means of preventing or controlling theinfluences of either type of interference using chemical
separation methods or using measurement procedures withadequate computational correction of the contributions. The
main objectives of the current study are: (i) to determine the Sr
isotope ratios in cheese and thereafter to calculate and discuss
the measurement uncertainties; and (ii) to investigate the
distributions amongst the Sr isotope abundance values of thecheeses in order to discriminate between the geographical andgeological origin of the investigated foods as a tool for
confirming their authenticity and establishing their geographic
traceability.

Experimental
Sample selection
The ‘‘Emmental’’ type cheese samples were received from the‘‘Swiss Federal Dairy Research Station FAM’’ (Berne) andwere all designated with proven geographic origin from
Allgaeu (Germany), Bretagne and Savoie (France), Berne
and St.Gallen (Switzerland), Vorarlberg and Salzburg(Austria), Middle Finland, Australia and Canada (Table 2).
The cheeses were produced between the end of the year 2000
and the middle of 2002. The cheeses investigated were sampled
at ages between 2.5–4 months of ripening, which corresponds
to the age of that sold in stores. For the Canadian andAustralian samples no data are available concerning their dateof production and the manner of production itself.
The samples were supplied as slices from a whole block. The
first 2 cm from the rind were discarded.
228

87

Sr1

J. Anal. At. Spectrom., 2004, 19, 227–234

w10 000
w10 000
10 000
10 000
2000

88

Sr1

87

Rb1
V36Ar1
47 40
Ti Ar1
71
Ga16Ar1
69
Ga18Ar1
174
Yb21
174
Hf21
51

Sr1

w10 000
w10 000
w10 000
10 000
7000
2000
2000

—
53
Cr36Ar1
48 40
Ti Ar1
48
Ca40Ar1
72
Ge16O1
176
Hf21
176
Yb21
176
Lu21

—
w10 000
w10 000
w10 000
10 000
2000
2000
2000

Table 2 Origin of ‘‘Emmental’’ type cheese samples used for Sr
isotope abundance determination
RegionAbbreviation Location

Bretagne
BR1
Bretagne
BR3
Berne
BE1
Berne
BE3
St-Gall
SG1
St-Gall
SG3
Finland
FI1
Finland
FI104
SavoieSA1
SavoieSA3
French Jura TR1
Vorarlberg VO1
Vorarlberg VO3
Salzburg
SAL105
Allgäu
AL1
Allgäu
AL3
Australia
AUS1
CanadaCAN101

Altitude/ Date ofmproduction

Yffiniac
52
St Méen-le-Grand 100
Aarwangen475
Amsoldingen
644
Schänis750
Altnau450
Lapinlahti
124
Toholammi
108
Massingy
537
Avraissieux
540
Trevillier
800
Schwarzenburg
950
Lingenau
687
Unknown—
Unknown
760
Unknown
900
Unknown—
Unknown—

16.02.2001
16.02.2001
26.12.2000
26.12.2000
26.12.2000
26.12.2000
07.02.2001
22.08.2002
08.02.2001
08.02.2001
11.06.2002
02.02.2001
02.02.2001
15.10.2002
20.12.2000
20.12.2000——

Sample preparation
The sample preparation procedure for the determination of‘‘casein-bound’’ as well as ‘‘whole-cheese’’ Sr isotope abundance ratios are summarised in Fig. 1.
All operations, except the thermal decomposition and thefreeze-drying procedures, were carried out under clean room
conditions (class 100). All solutions were prepared using highpurity deionised 18 MV water (Elix-Millipore, USA). Analytical grade nitric acid (Fluka, Switzerland) was purified by subboiled distillation with an MLS duo PUR (Milestone,
Germany) system.
For each determination 1–2 g of cheese sample was weighedon an analytical balance Mettler AT201 (Mettler, Switzerland)and crushed into pieces of about 2 mm diameter after freezing
with liquid nitrogen. The freeze drying process was performed
on a Beta 1–16 (Christ, Germany) over 12 h at a condenser
temperature of 250 uC and at a pressure of 0.3 mbar. The
sample temperature was held at 10 uC. Typical water loss forweighed cheese was in the range of 25–35%. The extraction
was performed in three steps: first, twice with 15 mL of highpurity diethyl ether (Fluka, Switzerland) for fat extraction,then once with 15 mL hydrochloric solution adjusted to a
pH of 4 using a pH-meter (692 pH/ion meter, Metrohm,
Switzerland) to separate all non-casein bound Sr. The hydrochloric acid solution was prepared using concentrated acid
(Traceselect, Fluka, Switzerland) and high purity deionised
water. All operations were carried out in capped Teflon vials.
The thermal decomposition was performed in a tubular furnace(Nabertherm, Switzerland), first for 2 h at 200 uC then for
1 h at 650 uC in covered quartz vessels. The residue wasredissolved in 1.2 mL of 2% nitric acid. A strontium- and leadselective extraction chromatographic Sr.Spec2 (EiChrom,Table 3 Analytically relevant operating conditions for the VG
Axiom MC-ICP-MS
Rf-power
Plasma gas flow
Auxiliary gas flow
Nebuliser gas flow
Sampling cone material
Mass resolving power (10% valley) m/Dm
Dwell time
Points per replicate
Mass assignment to Faraday cup detectors

1250 W
14.0 L min21 Ar
0.58 L min21 Ar
1.0 mL min21 Ar
Pt
410
50 ms
40
Kr
83
L4
Sr/Kr
84
L2
Rb
85
L1
Sr/Kr
86
Ax
Sr/Rb
87
H1
Sr
88
H2
Aerosol drying unit
CETAC Aridus
Nebuliser type
TH 1 concentric
Typical liquid sample uptake rate
100 mL min21
Air temperature in clean hood for instrument (17 ¡ 1) uC

Fig. 1 Sample preparation and measurement scheme for Sr isotope
abundance ratio determination in cheese.

USA) resin was used.16–18 The separations were performed
using Teflon micro-columns (1 mL volume) loaded with 35 mgof the resin. The resin was first washed and activated by rinsing
with 1.5 mL of 8 M hydrochloric acid to precondition the resin
for surface strontium complexation. The sample solution was
carefully loaded onto the column and 0.4 mL of 2 M then
0.4 mL of 8 M nitric acid passed through. The strontium
fraction was desorbed by using 1.4 mL of water; the leadfractions could be desorbed using 1.4 mL of 8 M hydrochloric
acid. The total strontium and lead containing fractions werecollected, water and the hydrochloric acid removed and thedried sample redissolved in 1 mL of 2% nitric acid. The
Sr contents of the cheese samples were adjusted so thatthe intensity of 88Sr on Faraday cup H2 ranged between
3 6 108–5 6 108 counts s21 to obtain the optimum precision
for the Sr isotope abundance ratios.

multicollector system. Multicollector detector positions were
adjusted while a 500 mg kg21 Sr solution was aspirated.
Isotope ratios of the cheese samples were determined using
the following sequence: 500 mg kg21 Sr (NIST 987) – blanksolution-sample 1 – sample 1-500 mg kg21 Sr (NIST 987) –blank solution-sample 2 – sample 2.... Each measurement was
followed by a wash sequence with 2% nitric acid lasting 2 min,which guaranteed a lack of Sr carryover. The set-up of thismeasurement protocol permits both internal correction of theisotope ratios 87Sr/86Sr and 84Sr/86Sr with 88Sr/86Sr and astandard–sample–standard bracketing method using the isotopic reference material SRM 987 (NIST, USA).
Trace analytical determinations used for the draw up of thematrix separation procedure were performed by an external
three-point calibration using single element high purity
reference solutions (Johnson–Matthey, Germany). For thispurpose the mass spectrometer was operated in single collector
mode. Isobaric and molecular interferences were separatedusing various resolving power settings (m/Dm) of 10 000 for
39
K, 8000 for 88Sr, 4000 for 44Ca, 400 for 86Sr and 208Pb. The
mass resolving power settings are according to the 10% valley
definition.

Results and discussion
Separation of the matrix

Mass spectrometry
All isotope ratio experiments as well as the trace analysis
measurements were carried out on a VG AXIOM MC-ICP-MS(Thermo-Elemental, Winsford, England). This is a double
focusing magnetic sector instrument equipped with an array of
10 variable Faraday collectors and a further fixed Faraday andphotomultiplier detector. The normal operating conditions
adopted for the mass spectrometer are summarised in Table 3.
The instrument is placed in a temperature (17 ¡ 1 uC)
stabilised clean hood (class 100). Before high precision ratio
measurement was started, the instrument was stabilized forabout 1.5 h under normal working conditions until thetemperature values within the mass spectrometer were constant(¡0.2 uC). Air temperature in the clean hood as well as inside
the instrument was monitored during the measurements. The
solutions were aspirated by a TH1 nebuliser and introduced
into an aerosol drying system (Aridus, CETAC, France) with
3 L min21 Ar and 20 mL min21 nitrogen (temperature 160 uC).
Ar gas with stated purity of 99.996% was used (Carbagas,
Switzerland). The liquid sample uptake rate was about
100 ml min21. The system was cleaned with 2% nitric acidfor 2 min before each measurement.
Once a week a gain calibration was performed for the

In most cases the influence from the matrix with respect to thecorrect determination of the isotope abundance ratios is
unknown. Therefore a matrix separation is an adequate
method for eliminating possible bias effects. The main
influences are caused by 87Rb1 on 87Sr, 44Ca40Ar1 on 84Sr
and 48Ca40Ar1 on 88Sr as the interferent elements are highly
concentrated in the cheese samples (Table 1). Krypton, an
impurity in the Ar gas used for the operation of the mass
spectrometer, cannot be eliminated but has to be screened on
mass 83Kr. High purity Ar gas has to be used.
The relatively low sensitivity of Faraday detectors demands
analyte contents, in our case, in the range of up to 300–
500 ng g21 strontium. Such contents permit us to obtain
relative standard deviations for the isotope amount ratios in arange of 2 6 1025 up to 1024. Furthermore, the removal of thematrix elements eliminates the danger of contamination of the
instrument inlet system and therefore prevents long wash out
times. It minimises long term contributions to signal driftcaused by deposition of solids originating from the matrix on
the surfaces of the instrument inlet system (nebuliser, torch,cones).
The validation of the matrix removal was performed either
by using digested cheese samples to test for the separation
J. Anal. At. Spectrom., 2004, 19, 227–234

229

Table 4 Analyte mass determination of different fractions collected for a matrix separation of digested cheese samplesa

Fraction 1
Fraction 2
Fraction 3
Fraction 4

Fraction 1
Fraction 2
Fraction 3
Fraction 4
mg kg21 cheeseaNa/mgc

K

Ca

Zn

Rb

Sr (m/z ~ 86)bSr (m/z ~ 88)b

Pb

2337
6.9
0.2
0.1

852
47.4
0.3
0.1

3719
135
6.5
0.3

34
1.0
0.1
0.1

0.55
0.02
0.00
0.00

0.72
0.09
6.34
0.10

0.78
0.07
6.47
0.11

v0.005v0.005v0.0050.03

Na (%)

K

Ca

Zn

Rb

Sr (m/z ~ 86)bSr (m/z ~ 88)b

Pb

99.7
0.3
0.0
0.0
2570

94.7
5.3
0.0
0.0
986

96.3
3.5
0.2
0.0
4231

96.6
2.9
0.2
0.3
38

96.3
3.2
0.2
0.2
0.6

9.9
1.2
87.5
1.4
7.9

10.4
1.0
87.1
1.5
8.1

—
—
—
101.0
0.02

Used sample mass for separation: 0.9 g cheese. b Measured isotope for mass determination. c Mass in the respective fraction.

efficiency of the involved analytes or by using the referencematerial SRM 987, including blank solutions, to check for massdependent isotopic fractionation on the resin.
Table 4 shows the separation of Sr21 for approximately 1 gof a cheese sample. Because the alkaline and alkaline earth
element ions have only a very small affinity for the chromatographic resin, they pass through the column with the sample
solution put onto the column and the fraction of 2 M nitric acid
(Table 4: fraction 1). Then 0.4 ml of 8 M nitric acid is eluted(Table 4: fraction2). The separation of Sr21 from Pb21 isensured by using water as the eluent for Sr21 (Table 4: fraction3). Finally, the lead ions can be desorbed using 1.4 mL of 8 M
hydrochloric acid (Table 4: fraction 4). The extraction yield of
Sr is about 70–90%. Possible Sr isotope fractionation on thechromatographic resin was investigated using SRM 987 (NIST,
USA). 2 g of a solution containing 500 mg kg21 of Sr SRM 987
was digested in the same manner as the cheese samples except
for the freeze-drying and the extraction process. Massdependent fractionation or a carry in of ubiquitous Sr would
have been detected in that way. Sr in the blank solutions did
not affect the result within the stated measurement uncertainties for the isotope abundance ratios. After separation andcomparison of the measured isotope abundance ratios,88
Sr/86Sr, 87Sr/86Sr and 84Sr/86Sr, with the values from nonseparated Sr SRM 987, no significant difference was detectable.
All isotope abundance values and their uncertainties overlap
with the data given in the certificate of the reference material
SRM 987.19
Correction of measured Sr isotope abundance ratios for mass
discrimination and evaluation of the measurement uncertainties
Two different types of correction for the mass discrimination
effect of the instrument were investigated: an internal and an
external correction. For internal correction, the mass discrimination was corrected using the isotope abundance ratio88
Sr/86Sr, assuming that the value is invariant in nature. For
external calibration a standard–sample–standard bracketing
method was used, whereby the isotope abundance ratio88
Sr/86Sr from the standard SRM 987 acted as the reference
value. As mathematical model equation an exponential law wasused with the mass discrimination per mass unit e:20


Rcert . 1e~ ln
(1)
Rmeas Dm88
e is consequently used to correct the measured isotope
abundance ratios from the analyte in the samples. The
corrected isotope abundance ratio can then be calculatedusing the model equation for the measurand R87:0

R87 ~ R87 6 exp(e 6 Dm87)(2)

The exponential law model gave the best overlap of the230

J. Anal. At. Spectrom., 2004, 19, 227–234

values and their uncertainties for the measured Sr isotope
abundance ratios 87Sr/86Sr and 84Sr/86Sr using the certified
reference material SRM 987 (NIST, USA). The estimatedcombined measurement uncertainties, a crucial need forcomparing results amongst different measurement techniquesand laboratories, were calculated from the external repeatabilities, the correction of the mass discrimination, thevariation of the blank values and from the influences frominterferent elements. Contributions from Faraday detector cup
efficiencies are not included in these calculations as the values
are at least one order of magnitude smaller than the values ofthe major uncertainties. Applying the uncertainty propagation
law21 without using second order covariancesn XLR87 2 . 2
u2c ðR87 Þ~
u ðx i Þ
(3)
Lxi
i~1
the expression for the uncertainty of the isotope ratios becomes
explicitly:uc ðR87 Þ~r
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

2
  
expðe.Dm87 Þ2 .u2 R087 z R087 .Dm88 . expðe.Dm88 Þ .u2 ðeÞ (4)(4)
0

The standard uncertainty u(R87 ) of the measured isotope
0
ratio R87 is estimated using the contributions of the isotope
ratio precision (uprec) of R87, the variations of the blank values(ubl) and the influences of the correction for the interference
from the isotope 87Rb (uRb). As an approximation, the values
were combined by adding the square root of the squared
uncertainties as independent contributions:  qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
u R087 ~ u2prec zu2bl zu2Rb(5)
By application of the uncertainty propagation law the
combined uncertainty of e is deduced using the fact that themass difference Dm of the two isotopes involved is considered
to be constant with negligible or zero uncertainty (u(Dm) w 0):sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ﬃ



2
Le 2 . 2Le.u2 ðRmeas Þu ðRcert ÞzuðeÞ~LRcertLRmeas(6)
sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ



2
1 2.
1
.uðRmeas Þ2uðRcert Þ2 z {
~
Rcert
Rmeas
Typical calculated measurement uncertainties uc(R87) of theanalyte in the samples not involved in chemical processing(digestion, matrix separation, ….) result in relative expanded
uncertainty values, e.g. for SRM 987 of 0.014% (coverage0factor k ~ 2). The contributions account for 12% from u(R87 )and 88% from u(e). For the results from cheese samples, wherea chemical procedure has to be carried out, uncertainty
contributions from the carry in of ubiquitous Sr into thesamples has to be considered additionally. Mass fractionation
effects during matrix separation can be neglected in this case as

Fig. 2 87Sr/86Sr isotope abundance values for SRM 987 doped withdifferent increasing amounts of Rb.

the validation of the separation procedure indicated nosignificant bias.
Correction of Rb and Kr signals: influence of Ca
The measurement of the 87Sr isotope is interfered by the 87Rb
isotope. The concentration of Rb in the Sr reference material(NIST 987) is small and the uncertainty contributions from the
Rb interference are much smaller than uncertainty contributions from, for example, the repeatability. The influence of
Rb was determined by doping a Sr reference solution withdefined amounts of a Rb solution to obtain concentrations of
0.0 mg L21, 0.5 mg L21, 1 mg L21, 5 mg L21 and 10 mg L21 of Rb
in 500 mg L21 Sr solution each (Fig. 2). The raw isotope
abundance ratio 85Rb/87Rb was determined in a solution
containing Rb only at a concentration of 10 mg L21. The
correction to the interference with 87Sr was performed point bypoint by subtracting the intensity from the value measured on
the Faraday cup H1 (mass 87) from the measured value on
cup L1 (mass 85) times the measured raw ratio of the single
element Rb.

I87 { R10 .I85
85
0
R87 ~
(7)
I86
After mathematical calculation to remove the bias effectcaused by 87Rb (eqn. 7), and the correction for the mass
discrimination, the value of R87 gives no evidence of ameasurable influence from Rb. This means that the bias
effect regarding the comparability of R87 for Rb separated andnon-separated samples can be guaranteed within the used Rb
amount contents. The isotope abundance ratio values overlap
within the stated measurement uncertainties. The fact of theFig. 3small variability of the 85Rb/87Rb value found in nature can beneglected. Firstly, the influence of the correction factor(measured 85Rb/87Rb) is small considering the subtraction of87
Rb from 87Sr and secondly the contents of Rb in theseparated Sr fraction of the cheese samples are smaller than
2 mg L21. This value makes a subtraction necessary to maintainthe required accuracy, but a change of the 85Rb/87Rb ratio of
the sample used for the calculation of the fraction on mass 87 of
1% affects the result of R87 only in the 2 6 1025 range.
Traces of Kr in the Ar gas used to operate the rf plasma
flame causes interferences with the signals for the isotopes 84Sr
and 86Sr. The signal of m/z ~ 83, corresponding to 83Kr, wasmonitored simultaneously during the Sr measurements. As the
Kr concentration in the Ar gas used was small, in general nocorrection was necessary. For higher amounts of Kr a reliablecorrection procedure is still lacking. It is suggested that this could
be due to a lack of information concerning the mass discrimination behaviour of Kr in the mass spectrometer inlet system.
The influence of 44Ca40Ar1 and 48Ca40Ar1 with respect to
the Sr isotope abundance ratios was investigated by means of
the addition of variable amounts of Ca to a mixture of
0.5 mg L21 SRM 987 standard solution and 5 mg L21 Rb to
obtain Ca concentrations of 1, 5, 10 and 100 mg L21. A serious
effect that leads to loss of close comparability of measurement
values and their uncertainties for different samples which haveundergone the same validated preparation procedure was notdetectable within the above Ca concentration range (Fig. 3).
However, the precision of the isotope abundance ratio R87
became larger by a factor of 3. This indicates that thebimolecular argide (44Ca40Ar1, 48Ca40Ar1) species are only
formed to a very small extent and that the stability of thespecies is slight under the instrumental conditions chosen. The
measured isotopic values indicate that a matrix separation forsamples containing less than 100 mg L21 Ca is only necessary
when precisions smaller than 0.003% (1 SD) have to beobtained. Nevertheless, the removal of a matrix with high Ca
content is advisable as possible long term signal drift andcontamination of cones can be avoided.
Sr in cheese samples, comparability to thermal ionisation mass
spectrometric (TIMS) results
Similar precision values were measured for the isotopic ratios
87
Sr/86Sr in the cheese samples and the reference material used
(SRM 987). The reproducibility was investigated by comparing
the isotope ratio results from independently prepared samples,for which the whole preparation process was applied to a
specimen several times (Table 5). The relative standard
deviation of the 87Sr/86Sr isotope ratio values, corrected

Influence of Ca doping for 87Sr/86Sr isotope abundance ratios in SRM 987 (NIST, USA).
J. Anal. At. Spectrom., 2004, 19, 227–234

231

Table 5 ‘‘Casein’’-Sr isotope abundance ratios for ‘‘Emmental’’ type cheese samples

Berne BE3a
Savoie SA3
Vorarlberg VO3
St. Gallen SG3
Finland Fl 104
Allgaeu AL3
Bretagne BR3
Australia AUS1
Canada CAN101
a
d

Nb

87

Sr/86Src mean

RSDd (%)

87

Sr/86Sre mean

RSD (%)

4
7
4
2
2
4
6
2
1

0.707 88
0.708 650.708 120.708 38
0.709 270.708 33
0.707 55
0.709 500.707 74

0.003
0.003
0.005
0.001
0.012
0.008
0.001
0.002
—

0.707 87
0.708 630.707 76
0.708 27—0.708 140.707 54
—
—

0.005
0.003
0.008
0.008
—
0.004
0.011
—
—

87

Sr/86Src mean

3
4
3
3
3
3
3
2
2

0.708 76
0.708 270.708 05
0.708 390.712 930.707 89
0.709 75
0.708 60
0.708 18

RSDd (%)
0.003
0.005
0.002
0.004
0.010
0.001
0.001
0.007
0.001

Numbers denote different sampling sites. b N, number of independent sample preparations. c Internally corrected for mass discrimination.
Relative standard deviation of results from independent prepared samples. e Externally corrected for mass discrimination.

internally using the 88Sr/86Sr isotope ratio, ranged from 0.001
to 0.012%. The standard deviation of the mean for the 87Sr/86Sr
results can be used as the standard uncertainty of an overall
parameter for the influence of sample preparation. The
expression for the calculation of uc(R87) has to be expandedwith the measured standard uncertainty u(Rcheese).
uðR87,cheese Þ~rﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
(8)

(8)
2
  
ðexpðe.DmÞÞ2 .u2 R0 z R0 .Dm. expðe.DmÞ .u2 ðeÞ zu2 ðRcheese Þ87

87

The uncertainties for the isotope abundance ratios R87, cheese
are in a range of 0.03 ¡ 0.005% (Table 6). The contributions of
the influence parameters to the combined standard uncertainty0
of R87, cheese are approximately 9% from u(R87 ), 65% from u(e)and 26% from u(Rcheese). The results and their uncertainties
guarantee a full comparability with different methods andlaboratories. The values for the same cheese samples measured
by thermal ionisation mass spectrometry (TIMS) overlap
within the stated uncertainties (Table 7). The absolutedifferences for the 87Sr/86Sr isotope abundance values range
between 4 6 1025 and 2.8 6 1024.
The external standardisation method was used to verify theassumption of constant R88 values for the cheese samples.
These were used for internal correction of mass discrimination.
Isotopic fractionation of Sr caused by geochemical processes(e.g., weathering of soils) or environmental influences (atmospheric transport of Sr or Rb) have to be considered.22,23 Using
the standard–sample–standard
bracketing approach, constantcert
K values RRmeasfrom the reference material are used. Most of
Table 6 Uncertainty budget for R87,cheese

uprec
ubl
uRb
uR87
ue
ucheeseuc(R87)aaBerne BE1a
Savoie SA1
Vorarlberg VO1
St. Gallen SG1
Finland Fl1
Allgaeu AL1
Bretagne BR1
Trevillier TR1
Salzburg SAL1

Nb

Estimate

Standard
uncertainty

Uncertainty
relative

0.708 530.708 530.708 530.708 53
20.0210.708 530.708 530.000 01
0.000 030.000 020.000 030.00010.000 06
0.000 11

0.002
0.002
0.002
0.005
0.382
0.008
0.016

Contribution
to uc (R87) (%)

8.9
65.4
25.7

Coverage factor k ~ 1.

the 87Sr/86Sr values of externally corrected isotope ratios agreewith the values of the internally corrected ratios (Table 5). A
significant difference was only found for the sample originatingfrom Vorarlberg. The repeatabilities are larger by a factor of
about two, which can be attributed to instrumental drift as thesimultaneous measurement of all isotopes involved is notpossible. The relative standard deviation of the corrected87
Sr/86Sr values (reference material SRM 987), determined over
1 month and used for the standard–sample–standard bracketing method, has a value of 0.007%. This partly explains thegenerally higher standard deviation of externally corrected Sr
isotope abundance values with respect to internally correctedratio results giving values of 0.002%. The comparison of
externally and internally corrected 87Sr/86Sr isotope abundancevalues shows that 88Sr/86Sr isotope ratios are constant within
the stated and required uncertainties and can be used forinternal correction of instrumental mass discrimination.
The question of whether a casein separation is necessary or
not was investigated by comparing the results of both sample
preparation procedures (Fig. 1). Possible intake of Sr during
the cheese preparation process (use of brine) may influence theresults of the origin investigation. The 87Sr/86Sr isotope
abundance ratio values of ‘‘casein’’ and ‘‘whole’’ Sr in cheesegive no evidence of a measurable influence. The isotope
abundance ratio values of Sr in samples from both preparation
procedures range within the stated measurement uncertainties(internally corrected for mass discrimination).
Authentication
According to the investigations of Rossmann et al.3 and Oda
et al.24 on butter and brown rice as well as origin studies onwoods,25 cements,26 bones and ivories,27 the measured 87Sr/86Sr
isotope ratios for cheese samples show a similar behaviour withrespect to the geological subsoil. The values reflect the mineralcomposition and the age of the rocks they have interacted with.
Sr in mafic rock type terrain, or better Sr in its bioavailable soil
moisture, has 87Sr/86Sr isotope abundance values ranging from0.700–0.710: Sr in sedimentary carbonate rich rocks has evenhigher values of 0.710–0.750. Geochronologically, cheesesamples from older crystalline areas (samples FI 1, FI 104,
AUS 100) showed the highest 87Sr/86Sr values, whereas samples

Table 7 Comparison of Sr isotope abundance values for MC-ICP-MS to TIMS
87

Sr/86Sra
mean

Uncertainty
uc (R87)

Deviation fromTIMS value

Berne 3
Savoie 3
Vorarlberg 3
St. Gallen 3

0.707 88
0.708 650.708 120.708 380.000 280.000 280.000 290.000 28

20.000 08
20.000 08
20.000 09
20.000 04

Allgäu 3
Bretagne 3

0.708 33
0.707 55

0.000 290.000 28

20.000 09
20.000 07a232Internally corrected for mass discrimination.
J. Anal. At. Spectrom., 2004, 19, 227–234

Berne 1
Savoie 1
Vorarlberg 1
St. Gallen 1
Finland 1
Allgäu 1
Bretagne 1

87

Sr/86Sra
mean

Uncertainty
uc (R87)

Deviation fromTIMS value

0.708 76
0.708 270.708 05
0.708 390.712 930.707 89
0.709 750.000 280.000 290.000 280.000 280.000 290.000 280.000 28

20.000 05
20.000 13
0.000 05
20.000 06
20.000 16
20.000 28
20.000 11

Fig. 4 87Sr/86Sr isotope abundance values and origins for different
cheese samples.

from regions where rocks are high in carbonate, basaltic and/or
young crystalline showed the lowest values (Fig. 4).
The isotopic data for the samples originating from Finland
have an absolute difference of about 5 permil. FI 1 comes fromthe area of central Finland (gneissic bedrock) and FI 104 fromthe western coast in middle Finland (mica schist bedrock). The
difference in the Sr isotope abundance can be due to different
causes: bedrock composition and its weathering behavior or
influences of precipitation.9,10 Sr isotope values from the upper
part of the soil located near a marine environment can be
influenced by precipitation of aerosols composed of sea salts,
depending on the prevailing meteorogical conditions. Thus, Sr
isotope ratios of the upper part of the soil have values which
represent the mixing of both soil and sea-water values. The
whole set of the influences described underpins the difference inthe observed isotopic values.
The samples originating from the sedimentary Swiss molassebasin (SG 1, SG 2, VO 1, VO 3, BE 1, SA 1) showed similar
isotope 87Sr/86Sr values within a range of 0.780–0.788. The
relatively large scatter of the values from cheese samples
originating from the same region (e.g., BE1, BE3) can be
explained by locally derived geological distinctions (BE1originates from the Swiss molasse basin, BE3 from the Prealps
located between the molasse basin and the alpine helveticum).
The comparison of the Sr isotope composition of basalts fromthe Prealps (Gets nappe)28 with BE3 shows overlap of thevalues although the definitive soil composition or its bioavailable soil moisture of the cheese production sites was not taken
into account. The 87Sr/86Sr ratios for the basalts are in therange of 0.7071 up to 0.7085, whereas for the cheese sample avalue of 0.707 88 was found.
From the statistical point of view there are two main
cornerstones to include: first the measurement uncertainty ofthe sample Sr abundance ratio values to differentiate the resultsfrom each other, and second the variations of the values fromthe soils where the strontium uptake took place. Fig. 4 shows
evidence that the differentiation of samples, for example fromthe Swiss Molasse Basin (SG 1, SG 2, VO 1, VO 3, BE 1, SA 1),
is not possible in the form of a general assignment and thus
other parameters from the food samples have to be taken into
account. On the other hand, samples from Finland, Australia
and partially from Bretagne are distinguishable. This observation is comparable to origin studies of brown rice, where adifferentiation between products from Australia and products
from Japan, China and Vietnam was possible.24 The Australianrice gave the highest strontium isotope values (R87 from 0.715
to 0.717), followed by Chinese and Vietnamese rice (R87 ~0.710 to 0.711) and Japanese values (R87 ~ 0.706 to 0.709).
The scatter of the investigated isotope values implies a moregeneral approach to the origin determination of food products.
A more detailed Sr isotope geological mapping of the soils has
to be generated to trace back the values directly to theirgeographical origin. This is one of the main promising
attractions of the Sr isotope ratio method, as a direct‘‘geographical’’ property is used for the origin assessment.
The comparison with light element isotope ratio procedures
allows the assumption that more stable Sr isotope values withrespect to time dependent factors (climatic changes) will be
generated.
Furthermore, complementary analytical methods and parameters (infrared spectroscopy, trace elemental contents,
volatile compounds, isotope abundance ratios for otherelements, ….)2 have to be applied additionally to obtain
more detailed information concerning the production process,to indicate the origin and to establish the traceability of foods.
Using light element stable isotope ratio analysis for olive oilsamples from different south European regions, and applying
statistical procedures, partially good discrimination behaviour
was observed.29 It was concluded that the mathematicalclustering of results from samples originating from the same
region gives evidence to a differentiation due to climatic areas.
The combination of light element stable isotope ratio (N, C, O)
and free amino acid ratio results by multivariate data
treatments showed the possibility for food authentication incasein cheese samples from Sardinia, Sicily and Apulia.30 These
results make it obvious that combining different methods seems
to be the most promising procedure for origin determinations
at the moment as various properties of the samples are taken
into account. It indicates that the presented results from theinvestigated cheese samples by the strontium isotope ratio
method have to be compared with other methods andprocedures to obtain qualitative and quantitative values ofthe discrimination behaviour.
Cheese is a living system continuously undergoing microbial,
enzymatic and chemical modifications and therefore theripening time—which can be different from dairy to dairy—
may play an important role in terms of the stability of values tobe measured. It is suggested that Sr abundance isotope ratios
may overcome this problems as they are not influenced by timedependent factors or any custodial handling. The method
described offers a way of obtaining very stable instrumental
responses and thus comparable results with defined andcomparable measurement uncertainties. Using additional
measured parameters relating to the origin of the investigatedfoods, discriminant analysis becomes much easier to interpretas the effects from the measurement procedure itself and realdifferences of the values found in nature are attainable.
As it is assumed that the strontium isotope ratio values
found in the cheese samples reflect the geological properties of
their place of production, the traceability of the dairy products
can only be guaranteed if a strontium variation in the soil isobservably larger than the stated measurement uncertainties.
As for many other techniques under investigation in the field offood authentication, the results can be changed or blurred, e.g.,
if supplementary forage or milk from very different locations
has been used in quantities which may have different isotopic
ratios.
With respect to accurate isotope analysis, other elements will
be considered, such as boron and magnesium. These elements
are candidates with known variability in nature and will give
more specific information with respect to the soil properties,
therefore enabling analysts to make more accurate distinctions
between origins in a more accurate way.
J. Anal. At. Spectrom., 2004, 19, 227–234

23388 
Rmeas, observed amount ratio 86 Srof the reference material
Sr
Dm mass difference of measured isotope pair or peak width
at 10% peak height
e mass discrimination correction factor per mass unit
I 87 intensity of mass 87 measured on detector cup H1
I 86 intensity of mass 86 measured on detector cup Ax
I 85 intensity of mass 85 measured on detector cup L1

Conclusions
Sr isotope abundance ratios have been used to assess theassignment of cheese products according to their geographic
origin.
A correct and appropriate sample preparation procedureand high precision isotope ratio determinations provide isotope
data with small combined measurement uncertainties to enabledifferentiation of the origin, providing precise geological Srdata are available (soil and/or soil moisture). The examinationof the influencing parameters with respect to possible bias
effects and an appropriate determination of the measurement
uncertainties permit comparability of the measured values.
It becomes obvious that the Sr isotope ratio data can be used
to assign the origin of the cheese in cases where there is a large
enough difference in the Sr isotope ratios of the milk
production site, that is to say different geology and age ofrocks and soils.
Concerning the stability of the Sr isotope abundance ratioswith time (summer versus winter production) the investigations
have to be extended, to obtain results from the same locations.
For a more detailed discrimination two further approaches
are available: the use of conventional methods of analysis, suchas trace elemental analysis and infrared spectroscopy, or amore differentiated isotopic Sr mapping of soils and theirbioavailable moisture. Such approaches may produce promising additional results.

Acknowledgements
We thank our collaborators, P. Horn and S. Hölzl from theLudwig-Maximilians-University in Munich/Germany, for thesubmission of the TIMS isotopic values and the helpful
suggestions made during the investigations. We would like tothank G. Urbach from the CSIRO Division of Food Processingin Victoria/Australia for the reviewing made during the
preparation of the manuscript.

Appendix
Symbols
R corrected isotope abundance ratio
R’ measured87  isotope abundance ratio
isotope abundance ratio, corrected for mass
R87 86 Sr
Sr
discrimination
87 
0R85 , measured isotope amount ratio 85 Rb
Rb
Rcheese isotope amount ratio of cheese samples
88
Dm88 difference of mass Sr and the mass of 86Sr
Dm87 difference of mass 87Sr and the mass of 86Sr
uX standard measurement uncertainty of X
uc combined measurement uncertainty
uc (R8787, cheese), combined standard measurement uncertaintyof 86 Sr
isotope abundance ratios from cheese samplesSrs standard deviation
88 
Rcert certified isotope amount ratio 86 Srof the referenceSrmaterial

234

J. Anal. At. Spectrom., 2004, 19, 227–234

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22
23
24
25
26
27
28
29
30

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