Abstract A direct measuring method for the determina-tion of 15 inorganic components in wine by ICP-OES wasdeveloped. It was applied to 17 white wines from 6 Ger-man wine-growing regions. In these investigations 15 ele-ments (B, V, Mn, Zn, Fe, Al, Cu, Sr, Ba, Rb, Na, P, Ca,Mg and K) were involved. By using alcoholic calibrationsolutions the results of the direct measuring method arecomparable with those of the control methods. Typicalpatterns of elements obtained by the multicomponentanalyses can be evaluated by multivariate data analysis torecognize the origin of the wines.

1 Introduction

Several analytical methods can be used to determine themineral components in wine. In most methods extensivesample preparation is involved. Therefore, these methodsare time-consuming and produce relatively imprecise re-sults.

The ICP-OES is a multi-element method with verygood detection power and offers the right conditions forreliably and rapidly determining the analytes. Due to thehigh temperature of the plasma less matrix interference isobserved. Therefore the relatively complex organic matrixof the wine should have no real influence on the measure-ments and the analysis of the samples should be possiblewithout pretreatment [1, 2].

The aim of this study was to develop an analyticalmethod for wine samples and to guarantee the trueness ofthe measured values of the untreated samples. The resultsof standard addition and measurement of decomposedwine samples were compared with results taken from di-

rect analysis. Accurate and precise analytical results arerequired to make it possible to classify the wine accordingto wine-growing region by means of their typical elementpattern [37].

2 Experimental

2.1 Samples

The investigation included 17 white wines from 6 German wine-growing regions. All information regarding the analysed samplesis summarized in Table 1.

2.2 Apparatus

The ICP-spectrometers Maxim I (Fisons Instruments) and Plas-maquant 110 (Carl Zeiss Jena GmbH) were used.

The Maxim I is equipped with an axially-viewed plasmasource. The instruments optical system comprises an argon-rinsedechelle grating/prism spectrometer in double 0.5 m Czerny-Turnermounting. The available wavelength range is from 174 nm to 800nm. There are 67 lines of 51 elements. The dispersed radiation isfocused onto a mask with 67 exit slits, which are connected with alight-conducting fibre. The Maxim I has 11 thermally stabilizedphotomultipliers. Eight of these are joined with the light-conduct-ing fibres by means of a shutter mechanism. The signals they carryare sequentially registered. Three lines in VUV range are linkedtogether directly with a photomultiplier.

Unlike the Maxim I the Plasmaquant 110 has a verticallyaligned torch. It too has an echelle spectrometer, whose wave-length-range is from 193 nm to 852 nm. There are 138 lines of 74elements. Each of the 12 photomultipliers can observe 5 of the 138lines through lightconducting fibres linked to the focal plane. Theprinciple employed here allows the analyst to combine spectrallines as required, and the problem of analysis is optimally ad-dressed. The parameters of the analytical program and the wave-lengths used can be found in Tables 2 and 3.

2.3 Sample preparation

In order to be able to use aqueous calibration standards the organiccomponents of the wine, in particular the alcohol, must be re-moved from the samples. The wine samples were decomposed ac-cording to the following instructions: 50 mL volume of samplewith 30 mL of perhydrol are evaporated in a Kjeldahl flask for ox-idation of the organic components. 1 mL of conc. sulphuric acid is

G. Thiel K. Danzer

Direct analysis of mineral components in wine by inductively coupled plasma optical emission spectrometry (ICP-OES)

Fresenius J Anal Chem (1997) 357 :553557 Springer-Verlag 1997

Received: 30 May 1996 / Revised: 2 July 1996 / Accepted: 5 July 1996

ORIGINAL PAPER

Dedicated to Professor Dr. Rolf Borsdorf on the occasion of his 65th birthday

G. Thiel K. Danzer (Y)Friedrich-Schiller-Universitt Jena, Institut fr Anorganische und Analytische Chemie, Lessingstrasse 8, D-07743 Jena, Germany

added to the sample to ensure that the residue does not evaporateto dryness. The remaining residue is mixed with bidistilled waterup to its starting volume. These solutions are analyzed with theICP-OES [8, 9].

Using the direct measuring method the samples are analyzedwithout any pretreatment. However the calibration standard matrixmust be matched to the samples using ethanol, in order to com-pensate for intensity differences caused by viscosity influences ofthe organic components during sample nebulization. This results instandard solutions of analytes, which contain 12% or in the case ofmineral elements 1.2% ethanol. In both methods the calibrationrange is from 0 to 10 mg/L, for the mineral elements to 50 mg/Land for the more highly concentrated element potassium to 100mg/L.

The standard addition method involves two standards. The ad-dition of the first standard should double the concentration of theanalytes in the sample and the other should triple it. The requiredconcentration of the standard solutions is estimated with the helpof the results taken from the calibrated methods. The analytes areadded to the samples to be treated in alternating concentrations.After adding the standard solution the original sample and thetreated samples have an equal final volume.

To determine the mineral elements, the sample solutions mustbe diluted independent of the applied method, in order to workwithin the linear calibration range. To determine the alkali metal

554

Table 1 Wine samples Producer/region Grape variety Vintage Abbreviationused

Winzervereinigung Freyburg/Unstrut e.G. Silvaner 1993 FSRiesling 1993 FRMller-Thurgau 1993 FM

Weingut Forschungsanstalt Geisenheim Weiburgunder 1993 GWRiesling 1993 GRSilvaner 1993 GS

Staatliches Weinbauinstitut Freiburg i.Br. Gewrztraminer 1993 BGVersuchs- und Lehrgut Blankenhornsberg Silvaner 1993 BS

Riesling 1993 BRStaatliches Weinbauinstitut Freiburg i.Br. Riesling 1993 DRVersuchsrebgut Durbach Kerner 1993 DK

Gewrztraminer 1993 DGChemisches Untersuchungsamt Mainz Mller-Thurgau 1994 NMStaatliche Lehr- und Versuchsanstalt Oppenheim Riesling 1994 NR(Gemarkung Nierstein)Chemisches Untersuchungsamt Mainz Silvaner 1994 OSStaatliche Lehr- und Versuchsanstalt Oppenheim Mller-Thurgau 1994 OM(Gemarkung Oppenheim) Riesling 1994 OR

Table 2 Operating conditions

a Only the trace elements ofdecomposition solutions weremeasured using 1 s integrationtimeb Variable parameter depend-ing on line-type

Parameter Maxim I Plasmaquant 110

Delay time 30 s transport time of sample 30 s transport time of sample10 s pre-integration time for signal 30 s stabilization time after powerstabilization regulation

Off-peak integration 1 3On-peak integration 3 3Integration time 3 s 3 saCoolant gas flow 13 L/min 12 L/minAuxiliary gas flow 1.2 L/min 1 L/minCarrier gas flow 0.48 L/min 200 kPaPower 1250 W 670 W bis 1210 WbPump speed 3 mL/min 1.6 mL/minNebulizer Meinhard nebulizer Cross-Flow nebulizer

Table 3 Analytes and wavelengths

Analyte Wavelength (nm)

Boron 249.6Vanadium 292.4, 309.3a, 310.2bManganese 257.6Zinc 206.2, 213.8Iron 259.9Aluminium 308.2, 396.1Copper 324.7Strontium 407.7a, 430.5bBarium 233.5, 455.3a, 493.4bRubidium 780.0Sodium 589.0Calcium 317.9Magnesium 279.0Phosphorus 214.9Potassium 766.4

a Only on Maxim Ib Only on Plasmaquant 110

contents, lanthanum is used as an ionization buffer. This step wasnecessary, because only arc lines can be used for determining theseelements. Their sensitivity at the high temperatures observed inplasma is relatively low; the alkali elements are strongly inclinedto ionization. Therefore the ICP-OES is not the best method for de-termining these elements.

When the wine is decomposed, orthoboric acid is lost. In orderto examine the measured values using the direct method, another in-dependent method must therefore be applied. A sensitive analyticalmethod already employed in the analysis of wine is the spectropho-tometric determination of orthoboric acid by chelate extraction with2-ethyl-1,3-hexanediol, and by using protonated curcumin [1012].In an acidic environment the orthoboric acid with curcumin forms ared substance, whose extinction is measured at 555 nm.

3 Results and discussion

3.1 Interpretation of the results

The ICP-OES was employed to determine 15 elements(B, V, Mn, Zn, Fe, Al, Cu, Sr, Ba, Rb, Na, P, Ca, Mgand K) in wine. The results for 17 wines are summa-rized in Table 4. Ten measurements were ascertainedfor the different methods. The minimum and maximumvalues have been given. In Figs. 1 to 3 the results for the

555T

able

4A

naly

tical

resu

lts (m

inimu

m an

d max

imum

value

in m

g/L)

Win

eB

VM

nZn

FeA

lC

uSr

Ba

Rb

Na

PC

aM

gK

FS2.

02.

8n

.n.

0.9

1.2

0.3

0.5

1.4

1.9

1.1

1.8

0.17

0.2

40.

961

.28

0.06

0.0

91.

41.

610

13

29

3611

913

762

78

820

860

FR2.

83.

4n

.n.

0.8

1.0

0.7

0.9

2.3

3.0

1.0

1.5

0.06

0.1

00.

550

.74

0.04

0.0

71.

110

15

62

8075

93

67

8754

073

0FM

2.9

3.2

n.n

.0.

80.

90.

91.

12.

32.

50.

60.

90.

160

.20

0.57

0.6

80.

030

.05

1.2

69

59

7775

98

57

7678

087

0G

W3.

64.

00.

030.

91.

10.

70.

81.

51.

91.

42.

20.

060.

380

.48

0.11

0.1

81.

614

19

75

9392

112

68

8983

090

0G

R2.

73.

1n

.n.

0.6

0.8

0.7

0.9

1.2

1.5

1.2

1.7

0.02

0.0

30.

340

.45

0.08

0.1

11.

110

12

59

7568

87

70

9259

074

0G

S2.

32.

5n

.n.

0.5

0.7

0.5

0.6

1.2

1.7

1.3

2.1

0.03

0.0

50.

190

.30

0.07

0.1

11.

111

13

58

7065

75

60

7366

071

0B

G6.

06.

80.

060

.07

0.8

1.1

0.7

0.9

0.9

1.3

1.5

2.5

0.10

0.1

60.

210

.33

0.10

0.2

31.

511

13

119

145

67

8556

72

910

1090

BS

2.6

3.1

0.06

0.1

40.

91.

10.

70.

91.

11.

41.

42.

10.

110

.18

0.17

0.3

10.

080

.18

1.5

2.6

101

257

72

60

7162

78

590

790

BR

3.2

4.4

0.04

0.1

10.

60.

80.

91.

20.

50.

60.

81.

20.

060

.11

0.28

0.4

40.

080

.14

1.8

2.9

79

76

9274

91

62

7949

065

0D

R3.

44.

30.

061.

21.

51.

01.

20.

50.

61.

52.

40.

300

.46

0.24

0.3

80.

150

.26

1.8

89

252

286

162

200

821

0494

011

10D

K3.

44.

10.

031.

11.

30.

91.

20.

50.

71.

01.

50.

500

.71

0.12

0.1

90.

100

.18

1.0

810

155

174

58

8164

81

840

970

DG

3.5

4.7

0.05

0.0

70.

81.

10.

81.

10.

40.

61.

93.

00.

400

.63

0.15

0.2

60.

120

.31

1.3

111

426

129

061

74

68

8215

901

860

NM

4.3

4.5

n.n

.0.

81.

00.

40.

51.

01.

51.

11.

70.

050

.09

0.20

0.3

40.

070

.11

1.2

89

961

0293

108

63

7684

092

0N

R3.

94.

50.

010.

81.

11.

21.

52.

53.

11.

82.

50.

200

.32

0.46

0.5

90.

070

.10

1.4

182

587

112

104

128

831

0551

061

0O

S3.

94.

50.

021.

01.

20.

91.

22.

12.

41.

92.

60.

360

.50

0.44

0.5

60.

080

.11

1.4

111

471

88

102

135

69

8886

010

20O

M3.

84.

50.

020.

91.

11.

01.

23.

54.

22.

53.

40.

050

.10

0.48

0.6

10.

090

.15

1.0

121

593

111

77

9372

89

690

800

OR

4.9

5.8

0.01

1.0

1.2

0.9

1.2

1.7

2.2

1.3

1.9

0.04

0.1

10.

320

.43

0.05

0.0

91.

210

13

56

6884

105

67

8748

065

0

Fig.1 Comparison of the results of the applied methods for boron; spectrophotometric method, directly measured on Plas-

maquant 110, directly measured on Maxim I, standard ad-dition method on Plasmaquant 110

Fig.2 Comparison of the results of the applied methods for phos-phorus; measured after decomposition on Plasmaquant 110,

measured after decomposition on Maxim I, directly mea-sured on Plasmaquant 110, directly measured on Maxim I,

standard addition method on Plasmaquant 110

Boron

Phosphorus

Samples

Samples

elements boron, phosphorus and potassium are givengraphically as examples. They have been selected tocover all magnitudes of the evaluated contents. Measur-ing the decomposed wines provides a wide variabilityand altogether lower values caused by losses during ox-idation and evaporation in the Kjeldahl flask, viscosityinfluences by the sulphuric acid during nebulization andcontamination by the instruments and chemicals. Theseeffects have also been observed in other investigations[13]. When using the direct method, apparent differ-ences in intensity, caused by the influence of the or-ganic components in the sample nebulization and mea-surement process, could be compensated by addingethanol, the main organic component of wine, to thecalibration standards.

In some cases, especially when measuring trace ele-ments with contents below 10 mg/L, the standard additionmethod provides noticeably higher values than the othermethods. However, previous investigations gave no clear

information about the reasons for this observation orwhether these results are reliable. The analysis resultswere evaluated by investigating whether the methods usedachieved the same results within the scope of measure-ment accuracy. One way of displaying the relation be-tween the results of the methods employed is linear re-gression. The mathematical equation corresponding to thebias function y = b x + a can be compared with the idealfunction y = x. In this way systematic errors can be rec-ognized by calculating the confidence interval of the re-gression coefficients a and b.

Table 5 shows the regression coefficients a and b aswell as their confidence intervals as results of the compar-isons between direct measurement values and those of thedecomposed samples and between direct and standard ad-dition measurement values. It can be seen from this tablethat there are significant differences in the results of themethods compared first for 11 analytical lines. Comparingthe direct measurement values with results of standard ad-dition there are even significant differences for 14 analyt-ical lines. Particularly great differences were observed, inthe first case for barium, and in the second for copper.These elements appear in very low quantities in all winesso that the resulting errors in measurement cause such dis-crepancies. Furthermore, the evaluation of measurementsof alkali elements gives unsatisfactory results. As alreadystated, the ICP-OES is fairly unsuitable to determine theseanalytes, because the high temperature of the plasma re-duces the sensitivity of the arc lines for these easily ion-ized elements.

With some elements the differences, although signifi-cant, can be considered less so. In a concentration of 1 mg/Land lower, the differences are tolerable from roughly 15%. Furthermore, the direct method is applicable, be-cause its results occur mainly between those of the com-parative methods. Therefore the significant differencescould also be traced back to systematic errors in the com-parative methods.

556

Fig.3 Comparison of the results of the applied methods for potas-sium; measured after decomposition on Plasmaquant 110,

measured after decomposition on Maxim I, directly mea-sured on Plasmaquant 110, directly measured on Maxim I,

standard addition method on Plasmaquant 110

Table 5 Validation of the di-rect method by means of re-gression coefficients

b(D;A) a(D;A) b(D;S) a(D;S)

B 0.963 0.106 0.152 0.427 0.993 0.053 0.255 0.264V 0.930 0.428 0.056 0.050 1.141 0.074 0.008 0.007Mn 0.872 0.075 0.033 0.072 0.996 0.080 0.134 0.077Zn 206 1.024 0.053 0.039 0.045 1.184 0.051 0.012 0.043Zn 213 0.947 0.063 0.020 0.045 1.166 0.049 0.003 0.043Fe 0.943 0.031 0.089 0.057 1.099 0.031 0.034 0.056Al 308 0.897 0.051 0.003 0.082 1.238 0.130 0.115 0.211Al 396 0.868 0.051 0.047 0.092 1.143 0.075 0.107 0.135Cu 0.890 0.278 0.022 0.007 1.286 0.047 0.024 0.012Sr 0.913 0.047 0.030 0.023 1.394 0.105 0.191 0.052Ba 0.421 0.082 0.024 0.012 1.158 0.038 0.009 0.006Rb 0.780 0.113 0.645 0.127 1.035 0.392 0.096 0.439Na 0.536 0.055 6.115 0.683 0.450 0.068 5.654 0.846P 0.972 0.028 11.766 3.107 0.881 0.016 5.688 2.224Ca 0.771 0.043 6.512 4.664 0.858 0.043 11.058 4.711Mg 0.775 0.082 3.775 6.908 0.874 0.097 7.630 8.213K 1.192 0.076 147.349 64.942 0.759 0.070 220.259 59.915

Potassium

Samples

3.2 Evaluation and interpretation of the results by multivariate statistics

In this study a methodical basis is established to investi-gate the wines, which by means of their element patternscould give information as to their origins. The evaluationof the results by multivariate data analysis provided initialindications as to the distinction of origin. To achieve this,pattern recognition methods such as discriminant analysiswere used. To differentiate between the wines from the 6different wine-growing regions, up to 5 discriminant func-tions can be calculated. The two with the greatest dis-crimination power have been used in Fig. 4. The winesfrom Durbach are proven to differ significantly from therest. The wines from the Saale-Unstrut region and fromBlankenhornsberg are also clearly distinguishable. Thesamples from Geisenheim, Nierstein and Oppenheimwere not fully separated by these two discriminant func-tions. Phosphorus, iron, strontium and copper are the mostefficient elements in group separation. These results,however, should only be considered preliminary. Due tothe small data set available, they are not fully representa-tive. Moreover, the discrepancies with enological findingsshow that it is not clear from the size of this data set,whether the elements considered significant by the classi-fication methods give indications of the structure of thepopulation, that is to say of the whole region of origin, oronly of the random samples taken. It follows then that intaking a greater number of wine samples one can discoverwhich element contents are reproducibly influenced bythe wine growing and the wine making process and whichare not influenced at all. This knowledge would make itpossible to judge more effectively the origins of the wines.This point will be investigated further.

4 Conclusion

The investigation undertaken has shown that direct analy-sis of mineral and trace elements in wine using ICP-OESis possible in spite of some problems. This is proven par-ticularly by the short time necessary for sample pretreat-ment and the high level of measurement precision. Onerequirement, however, is that calibration be achieved withstandard solutions containing ethanol. By doing this, in-tensity differences caused by viscosity influence of the or-ganic components during sample nebulization can becompensated. In some cases it is possible to explain sys-tematic differences between the direct method under eval-uation and the established reference methods. The appli-cability of the direct method becomes clear in that the re-sults achieved in this way are mostly between those of thecomparative methods.

Acknowledgements The authors would like to thank Prof. Dr. H.Eschnauer for his assistance in obtaining the samples. We espe-cially thank Winzervereinigung Freyburg/Unstrut e.G., For-schungsanstalt Geisenheim, Staatliches Weinbauinstitut Freiburg i.Br., and Chemisches Untersuchungsamt Mainz, who supported ourwork by supplying the wine samples. We are also grateful to Dr.C.-J. Ntzold, Dr. V. Jngel and K. Weniger from the firm CarlZeiss Jena GmbH for making it possible for us to use the ICP-spectrometer Plasmaquant 110.

References

1. Interesse F-S, Lamparelli F, Alloggio V (1984) Z Lebensm Un-ters Forsch 178 :272278

2. Interesse F-S, DAvella G, Alloggio V, Lamparelli F (1985) ZLebensm Unters Forsch 181 :470474

3. Siegmund H, Bchmann K (1977) Lebensm Unters Forsch 164 :17

4. Siegmund H, Bchmann K (1978) Lebensm Unters Forsch 166 :298303

5. Jakob L, Eschnauer H (1990) Offprint from Deutsches Wein-bau-Jahrbuch 41 :215222

6.Borszeki J, Koltay L, Inczedy J, Gegus E (1983) Z LebensmUnters Forsch 177 :1518

7. Larrechi M-S, Rius F-X (1987) Z Lebensm Unters Forsch 185 :181184

8.Eschnauer H, Jakob L, Meierer H, Neeb R (1989) MikrochimActa [Wien] III : 291298

9.Eschnauer H, Meierer H, Neeb R (1985) In: Sansoni B (ed) In-strumentelle Multielementanalyse. VCH, Weinheim

10.Koch OG, Koch-Dedic GA (1964) Handbuch der Spuren-analyse. Springer, Berlin Gttingen Heidelberg

11.Fukui S et al. (1983) Eisei Kagaku 29(5) :323328; cited fromChem Abstr 1984 :50107

12.Mair JW, Day HG (1972) Anal Chem 44(12) :2015201713.Heil M (1995) Dissertation, Neustadt/Weinstrasse Kaisers-

lautern

557

Fig. 4 Representation of wine groups in the discriminant plane; pFreyburg/Unstrut, k Geisenheim, -- Blankenhornsberg, G Durbach,M Nierstein, + Oppenheim