Enhanced semiquantitative multi-analysis of traceelements in environmental samples usinginductively coupled plasma mass spectrometry

Maria Montes Bayon, J. Ignacio Garcıa Alonso* and Alfredo Sanz Medel

Department of Physical and Analytical Chemistry, Faculty of Chemistry, University of Oviedo, Julian Claveria, 8, 33006 Oviedo, Spain

A semiquantitative procedure for the determination of trace continuous function between Li and U and only those elementswith high ionisation potentials lie below their expected valuemetals in natural waters and sediments by ICP-MS was

developed. The method is based on the establishment of the for their given m/z ratio.10 The evaluation of the molar responsecurve using internal standard elements and the correction forinstrumental molar response curve versus mass using a multi-

elemental solution containing elements from Mg to Bi with less than quantitative ionisation degrees in the plasma mightbe the basis of more accurate semiquantitative proceduresdifferent ionisation potentials. The ionisation conditions in the

plasma (electron density, ne, and ionisation temperature, T

ion) in ICP-MS.

Semiquantitative procedures are offered by most ICP-MSwere determined by least-squares fitting, using the Sahaequation, by adjusting the molar response curve to a third- manufacturers and have been applied to the analysis of both

liquid and solid samples.9,11–13 Most of these procedures areorder polynomial. The effect of matrix constituents, such asNa+, K+, Ca2+, Mg2+, Cl− and SO

42−, on both the shape of based on the comparison of a ‘response table’ prepared by the

manufacturers with the actual count rates measured in thethe molar response curve and on the plasma ionisationconditions was evaluated. It was observed that the ionisation sample. The use of internal standards added to the sample is

mandatory to update the response tables to the actual sensi-degrees calculated in the plasma did not change significantly inthe presence of the matrix constituents. However, the shape of tivity of the instrument. However, very few studies have been

published on the effect of matrix elements on the shape andthe molar response curve was affected by high concentrationsof matrix components and this effect was corrected by adding magnitude of the response curves or their effect on ionisation

equilibria in the plasma.14 Beauchemin et al.14 observed aninternal standards to the samples. The semiquantitativeprocedure was applied to the analysis of local natural waters increase in signals in the presence of 0.01 Na, K and Cs,

little effect from 0.01 Ca and Mg and a decrease in signalsand the results were compared with those obtained by externalcalibration and standard additions. Validation was performed for the same concentration of B, Al and U. They did not

observe any shift in ionisation equilibria due to the easilyby analysis of riverine water (SLRS-3) and sediment (PACS-1and MESS-2) reference materials from the National Research ionised elements, but these effects were observed previously.15

Non-spectroscopic matrix effects in ICP-MS have beenCouncil of Canada with satisfactory results.ascribed mainly to defocusing of the ion beam due to space–Keywords: Inductively coupled plasma mass spectrometry;charge effects.16–19 It has been observed19 that matrix inter-

semiquantitative analysis; trace element; matrix eVect; naturalferences due to space–charge effects are mass-dependent18 and,

water; sediment.hence, difficult to correct in semiquantitative analysis whenthe number of internal standards used is limited to 1 or 2 as

The determination of trace elements in environmental solid recommended by most manufacturers.samples and natural non-saline waters is an area of particular This paper addresses the problem of semiquantitative traceinterest both for pollution control and drinking water quality element multi-analysis by ICP-MS taking into account themonitoring. Traditional methods involve the use of flame AAS, effect of typical matrix components in natural waters (Na+ ,electrothermal AAS, hydride generation AAS, ICP-AES and K+ , Ca2+ , Mg2+ , Cl− and SO42− ) on the shape of theanodic stripping voltammetry. In the last few years inductively response curve and on the ionisation equilibrium in the plasma.coupled plasma source mass spectrometry (ICP-MS) has been For this purpose, multi-elemental response curves are fitted torecognised as a powerful technique for the almost simultaneous a third-order polynomial by least squares using the Sahamulti-analysis of trace elements in environmental samples equation and the temperature-dependent electronic partitionbecause of its wide elemental coverage, excellent sensitivity, functions,20 the fitting parameters being electron density andspectral simplicity and capabilities for isotope ratio measure- ionisation temperature.ments.1–4 However, one of the main features of ICP-MS, nearuniform sensitivity across the Periodic Table, has not beenexplored in great detail, particularly in connection with itsmulti-elemental capability. This feature would allow stan- EXPERIMENTALdardless semiquantitative analyses for screening purposes.5–9

InstrumentationIt has been observed that ICP-MS instrumental responsecurves follow a simple function of the mass of the measured The ICP-MS instrument employed was a Hewlett-Packard

Model 4500 (Yokogawa Analytical Systems, Tokyo, Japan)isotopes for elements which show quantitative ionisation inthe ICP. Molar response curves, as a function of the mass of fitted with a Meinhard nebulizer and a Peltier-cooled glass

Scott double pass spray chamber. Typical operating conditionsthe measured isotope, of ICP-MS instruments describe therelative degree to which ions are generated in the plasma, used to perform the semiquantitative method are summarised

in Table 1. These conditions were optimised daily prior to thetransmitted through the ion lenses and mass spectrometer anddetected.10 Usually, the response for heavier isotopes is greater analysis using a ‘test solution’ of 10 ng ml−1 of Li, Y, Ce and

Tl. The best semiquantitative results were obtained by maximis-than that for lighter isotopes, even for the same element, andthis causes the so-called mass discrimination effect on isotope ing the 89Y signal and keeping the signals for 7Li and 205Tl at

a similar level (in raw counts). Ce was used to check doublyratio measurements. However, molar response curves follow a

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acquisition conditions shown in Table 1. Several isotopes wereTable 1 Typical operating conditions

selected for polyisotopic elements in order to cover better thewhole mass range. The raw counts observed for each isotopeRf power 1300 Wwere transformed into molar response using the equation:Reflected power 0 W

Sampling depth 5.7 mmMolar response=

Carrier argon gas flow 1.27 l min−1Intermediate argon gas flow 1 l min−1 signal (counts)×atomic mass (mg mmol−1×100)

isotope abundance (mol.%)×concentration (mg l−1 ) (1)External argon gas flow 15 l min−1Spray chamber temperature 2 °CIons lens settings— The molar response has the units of counts m−1 of the

Extract 1 −221 V measured isotope and takes into account the differences inExtract 2 −106 V

atomic masses and isotopic compositions of the measuredEinzel 1,3 −144 Velements at a given mass concentration.Einzel 2 39.3 V

The molar response function versus mass was fitted to aOmega bias −48 VOmega (+) 6 V third-order polynomial using an Excel spreadsheet. The ionis-Omega (−) −7 V ation conditions in the plasma, viz., ionisation temperatureQP focus 8 V (Tion ) and electron density (ne), were determined by leastIon deflector 39 V

squares iteration in the spreadsheet using the Saha equationOxide level (CeO+/Ce−) <0.5%and the temperature-dependent electronic partition functionsDoubly charged level (Ce2+/Ce+ ) <1%published by de Galan et al.20 The best values found for TionDetection parameters—

Integration time 0.3 s per point and ne were used to determine the ionisation degrees of thePoints per u 3 elements to be measured in the sample.Repeats 3 The samples to be analysed were spiked with the internal

standards Be, Sc, Co, Y, In, Tb, Tl and Th at about 10 ng ml−1levels to construct the response curve of the sample by fitting

ionised and oxide levels. Typical values of such species observed to a third-order polynomial as described previously and thein our plasma are also given in Table 1. elements under study were determined by interpolation in the

response curve after ionisation corrections had been performed.A sample without internal standards was also measured toReagents and materialscorrect for the contribution of the signal at the masses of the

Natural element standard solutions containing 1000 mg ml−1 internal standards.of Mo, Ni, Zn, In, Cr, Cs, Tb, Sr, Sb, Mg, Cu, Pd, Sn, Tl, Pb,Bi and Se were obtained from Merck (Darmstadt, Germany) Digestion of sedimentsand stock solutions of Th, Co, Y, Mn, V, U and Sc from J.T.

A 0.100 g portion of the sediment was transferred into aBaker (Phillipsburg, NJ, USA). The SLRS-3 Riverine Water(PTFE) digestion vessel and 3 ml of concentrated nitric acidReference Material for Trace Metals and the PACS-1 andand 0.5 ml of 30% v/v hydrogen peroxide were added. TheMESS-2 Sediment Reference Materials were obtained from thesample was digested in a microwave oven using the firstNational Research Council of Canada (NRCC) (Ottawa,heating programme shown in Table 2. After cooling 2 ml ofOntario, Canada).concentrated hydrofluoric acid were added and the sampleAll dilutions were performed by mass to 0.1 mg using awas digested using the second heating programme of Table 2.Precisa Model 180 balance (Orlikon, Zurich, Switzerland)This resulted in a clear digest. After cooling, the sample wasusing poly(propylene) containers. Dilutions were performed indiluted by mass to 50 g with ultrapure water. A second dilution1% v/v nitric acid prepared from concentrated 65%, nitric(1+9) was then performed and the elements Be, Sc, Co, Y, In,acid (suprapur, Merck) and filtered (0.22 mm) 18 MV ultrapureTb, Tl and Th were added as internal standards (10 ng g−1water, freshly obtained from a Milli-Q system (Millipore,final concentration). The semiquantitative procedure describedMolsheim, France).above was applied.Digestion of solid samples was performed using a Milestone

(Socisole, Italy) Model MLS1200 microwave digestor with anEM-45/A extractor module and an AC-100 open/close module. RESULTS AND DISCUSSIONMiddle pressure PTFE vessels were employed for the digestions

Semiquantitative method for drinking water analysisof the samples. The digestion programmes used for the sedi-ments are shown in Table 2. Response curves and ionisation corrections

The response (counts s−1 ) in ICP-MS for a certain isotope inSemiquantitative procedure for water analysis an unknown sample will depend on a certain number of

parameters such as: the concentration of the element in theA multi-elemental standard solution containing Mg, Al, Sc, V,sample, the isotopic abundance of the considered isotope, theMn, Co, Cu, Zn, Se, Sr, Y, Mo, Pd, Sn, In, Sb, Cs, Tl, Pb andatomic mass of the element and the efficiencies of nebulisation,Bi at about the 10 ng g−1 level was prepared and measured toatomisation and ionisation in the plasma and those ofevaluate the experimental molar response curve using theextraction, transport and detection in the mass spectrometer.

In most ICP-MS instruments, a plot of sensitivity versusmass yields a reasonably smooth response curve, once isotopic

Table 2 Microwave oven programmes for the digestion of theabundance and degree of ionisation have been taken into

sedimentsaccount.10 The response curve obtained, using eqn. (1), for astandard solution of 10 ng g−1 of Be, Sc, Co, Y, In, Tb, Tl andProgramme 1 Programme 2

Time/min Power/W Power/W Th is shown in Fig. 1, together with the response curve that1 240 was obtained for the same elements spiked into a natural water3 360 sample. As can be observed, both response curves show a4 600 600

continuous increase in sensitivity with mass up to Tl and Th10 Ventilation Ventilation

where the response reaches a plateau. Except for Be, all

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Fig. 1 Typical response curves obtained for the HP 4500 for astandard solution containing 10 ng g−1 of Be, Sc, Co, Y, In, Tb, Tland Th (#) and for the same elements spiked into a natural watersample ($).

elements included can be considered completely ionised in theplasma, so this response curve shows the relative transportand detection efficiency in the mass spectrometer for thedifferent elements of the Periodic Table for this particular Fig. 2 (a) Multi-elemental molar response of Mg, Al, Sc, V, Mn, Co,instrument and set of experimental conditions. As can also be Cu, Zn, Se, Sr, Y, Mo, Pd, Sn, In, Sb, Cs, Tl, Pb and Bi at 10 ng g−1

with no ionisation correction performed. (b) Final ionisation-correctedobserved in Fig. 1, the response curve obtained for the acidifiedresponse curve obtained for data shown in (a).natural water sample differs only slightly from that obtained

for the aqueous standard solution; this small difference couldbe due to matrix interferences depressing the signals, in this conditions in the plasma by least-squares fitting of the molarcase particularly at the high masses. It is clear that the use of response curve in the following way: firstly, tentative valuessuitable internal standards added to the sample could compen- are given for ne and Tion in the plasma and the ionisationsate for these matrix interferences when applying semiquantit- degree for all measured elements is calculated using eqns. (2)ative multi-analysis procedures. and (3). Secondly, the experimental molar response is divided

The solid line shown in Fig. 1 for both response curves by the ionisation degree thus calculated to obtain the ‘ionis-corresponds to a least-squares fit to a third-order polynomial ation-corrected’ molar response curve which is then fitted to aequation. It was observed that a third-order polynomial third-order polynomial by least squares. Finally, an iterationprovides the best fit for the response curve and can be used to procedure is started where the values of ne and Tion are modifiedinterpolate the response for different elements in semiquantit- in order to minimise the deviation between the polynomialative analysis. equation and the ionisation-corrected molar response curve.

All the elements included in Fig. 1 were, except for Be, Fig. 2(b) shows the final ionisation-corrected response curvecompletely ionised in the plasma. Elements with high ionisation obtained for the data shown in Fig. 2(a). The best fit corre-potential will show molar responses well below the response sponded to ne=3.9×1014 e cm−3 and Tion=6529 K for thiscurve obtained for low ionisation potential elements. Fig. 2(a) particular measurement. All iterations were performed on anshows the response curve obtained for a standard solution of Excel 5.0 spreadsheet using the Solver application.Mg, Al, Sc, V, Mn, Co, Cu, Zn, Se, Sr, Y, Mo, Pd, Sn, In, Sb, Several pairs of values of ne and Tion could be a solution forCs, Tl, Pb and Bi at the 10 ng g−1 level. As can be appreciated, the Saha equation which will produce ionisation degrees whichsome of the elements, such as Zn or Se, lie below the third- could fit the response curve. However, there is always a pairorder polynomial ( least-squares fit, solid line) owing to their of values which gives the best fit between the experimentallower ionisation degrees in the plasma. Let us assume that the data and the polynomial regression. This is illustrated in Fig. 3ionisation degree of an element in the plasma (a) is given by where the square sum of the residuals for the polynomialthe Saha equation:20

Log[a/(a−1)]=3/2 log Tion−(5040/Tion )Eion+log (Zi/Za)

−log ne+15.684 (2)

where Eion is the ionisation potential of the element in eV, Tionis the ionisation temperature (K) and ne is the electron density(e cm−3 ). Zi and Za are the electronic partition functions forions and atoms, respectively. These partition functions can beobtained from ref. 20 as fifth-order polynomial equations formost of the elements of the Periodic Table. The temperature-dependent partition functions are expressed as:

Z(T )=a+b(T /103 )+c(T /103)2+d(T /103 )3+e(T /103)4

+f (T /103 )5 (3)

where the coefficients a, b, c, d, e and f are tabulated for arange of temperatures from 1500 to 7000 K.20 Fig. 3 Square sum of the residuals for the polynomial equation

If it is assumed that the temperatures which appear in eqns. against the electron density for a series of values of ionisationtemperature using the data presented in Fig. 2.(2) and (3) are the same, it is possible to calculate the ionisation

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equation is plotted against the electron density for a series of temperature does not seem to vary with the type or concen-tration of matrix element. There is a small variation from thevalues of ionisation temperature. As can be observed, for a

given temperature there is a value of electron density which data obtained on different days (each element was tested on adifferent day) but there are no significant differences. Fig. 5provides a minimum square sum of residuals, but there is a

temperature value which provides the best absolute polynomial shows the ionisation degrees calculated for Pb, Sb and Se forthe different concentrations of EIE tested. As can be observed,fit, in this case 6529 K. As was mentioned earlier, the partition

functions used in these calculations are tabulated for the Se showed an ionisation degree close to 10%, Sb near 40%and Pb around 90%; the ionisation degrees calculated weretemperature range 1500–7000 K, so the values obtained for

7500 K in Fig. 3 should be taken as an approximation. The not affected significantly by the presence of EIE up to about0.02 (i.e., 400 ppm Na). This rules out the existence ofoptimum combination of Tion and ne and hence, the ionisation

degrees of the elements in the plasma, will depend on the ionisation shifts in the plasma for the range of concentrationstested and supports the theory that the proportion of ions andplasma conditions; hence, these values must be evaluated each

day before performing the semiquantitative analysis. It was electrons contributed by the matrix will be small comparedwith the large number of ions contributed by argon andalso observed that small experimental variations in the

measurement of the response curve led to changes in the best nebulised water molecules in the plasma.18 Therefore, the shiftof analyte equilibrium towards atom formation, resultingfit values for ne and Tion but the ionisation degrees of the

elements were scarcely affected. in a suppression of analyte ion formation, is negligible withincreasing EIE concentration.

Matrix eVects on Tion

and ne

Matrix eVects on the response curvesConflicting reports exist on the influence of the sample matrixon the analytical signal in ICP-MS.14–19 Three main sources The mathematical calculations of the ionisation conditions in

the plasma do not impose a certain ‘shape’ on the responseof matrix effect are to be expected in ICP-MS: (a) those causedby changes in nebulisation efficiency which would affect all curves obtained. The only restriction is their fitting to a third-

order polynomial equation. Fig. 6 shows the polynomial equa-elements similarly, (b) those caused by changes in the ionisationconditions in the plasma which would affect mainly elements tions obtained for increasing concentrations of calcium nitrate

up to 400 ppm of Ca. As can be deduced from the graphs,of high ionisation potential and (c) those caused by changesin transport efficiency through the mass spectrometer which there is a small mass-dependent matrix effect which increases

with the Ca concentration. Similar results were obtained forwill affect elements of low and high masses differently. Thefirst and third sources of interference could be corrected by the other cations (added as nitrates) or anions (as the corre-

sponding acid). From the observed changes in the responsethe selection of suitable internal standards to be added to thesample but the second source could be a problem in the curves there seems to be a larger effect on heavier isotopes

than on lighter isotopes, which is in disagreement with resultsanalysis of samples containing high concentrations of easilyionised elements (EIE). obtained for other ICP-MS instruments19 and with the space–

Natural continental waters could present high concen-trations of EIE such as Na, Mg, K and Ca, and some anionssuch as Cl or SO42− . The effect of these elements and anionson the ionisation conditions in the plasma and, therefore, onthe ionisation degree of the elements to be determined wasstudied. The values of Tion and ne were calculated using themathematical treatment mentioned above for different concen-trations (0, 50, 100, 200 and 400 ppm for Na, Ca, Cl andSO42− and 0, 20, 40 and 80 ppm for Mg and K) of matrixelements added to the multi-elemental standard solution shownin Fig. 2. A solution of the matrix element in 1% nitric acidwas used as the blank to correct for possible elementalcontamination in the matrix solution. The results obtained forthe ‘best’ ionisation temperature versus the molar concentrationof the matrix element are shown in Fig. 4 for the EIE. As canbe observed, there seems to be a small increase in ionisation

Fig. 5 Variation of the ionisation degree of three of the elementstemperature from the data in the absence of matrix componentsunder study for different concentrations of EIE tested.

(four data points obtained on different days) but the ionisation

Fig. 6 Polynomial equations obtained for increasing concentrationsFig. 4 Effect of increasing amounts of concomitant elements on thecalculated ionisation temperature. of Ca (up to 400 ppm).

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charge theory of matrix interferences.18 By comparing Figs. 1 sample is illustrated in Fig. 1 and the final concentration resultsobtained for the elements Cr, Mn, Ni, Cu, Zn, As, Se, Cd, Sb,and 6 it can be concluded that this small change in the shape

of the mass response curve in the presence of matrix elements Hg, Pb and U in this sample are summarised in Table 4 andcompared with alternative quantification procedures. As cancan be corrected for by the selection of suitable internal

standards to be added to the sample. By combining the be seen, most of the results are in good agreement whencomparing all quantification procedures given the extremelydescribed ionisation corrections with corrections to the shape

of the molar response curve, the semiquantitative multi-analysis low concentrations of trace elements in this particular sample(except for Mn). If the results obtained by the proposed methodof trace elements in different sample types was developed

and applied. are compared with those given by the method provided by theinstrument manufacturer, no significant improvements can befound (compared with standard additions). Thus, on average,

Application to natural water analysisboth semiquantitative procedures can be considered adequatefor natural water analysis. The same similarities were observedThe semiquantitative method developed was applied to thein the analysis of other drinking waters.analysis of drinking water samples from Asturias, Spain. The

In order to compare both semiquantitative methods further,final results were compared with those given by alternativeTable 5 shows the final results obtained for the analysis of thequantification procedures, such as standard additions andSLRS-3 Riverine Water using both methods and compares thecalibration with internal standards, and by the semiquantitativeresults with the certified values. The values obtained by thesoftware provided by the instrument manufacturer. In orderproposed method are clearly in closer agreement withto validate the procedure, the water reference material SLRS-3the reference values for Cd, Cr, Fe, Pb, Mo, U, V and Zn; onfrom the NRCC was also analysed and the results obtainedthe other hand, the results for As, Mn and Ni seem betterby both semiquantitative approaches were compared.using the instrument software. The results for Al, Sb and BaDrinking water samples were always filtered (0.45 mm),are similar for both methods.acidified with 1% nitric acid and the elements Be, Sc, Co, Y,

In conclusion, the method proposed here seems marginallyIn, Tb, Tl and Th added at 10 ng g−1 final concentration levelssuperior to the manufacturer’s software, both methods beingas internal standards. Similarly, another aliquot of the watersuitable for the determination of trace elements in naturalsample was only acidified and measured to act as an internalnon-saline waters at low ppb to ppt levels.standard ‘blank’.

In the first step, Tion and ne were calculated using the multi-elemental solution. Typical values for the calculated ionisationdegrees of the determined elements and internal standards are Application to the analysis of sedimentssummarised in Table 3. The response curve for one particular

The semiquantitative method was further validated by theanalysis of two reference sediment materials, PACS-1, an

Table 3 Calculated ionisation degree of the elements estuary sediment, and MESS-2, a marine sediment, both fromthe NRCC. Both sediments were digested as described underCalculated CalculatedExperimental and spiked with the internal standard elementsionisation ionisation

Element degree Element degree Be, Sc, Co, Y, In, Tb, Tl and Th at 10 ng g−1 concentrationsin the final solution. A non-spiked sediment was also measuredBe (IS)* 0.55 Se 0.10

Al 0.95 Sr 1 as an ‘internal standard blank’. The results obtained by theSc (IS) 0.98 Mo 0.93 two different semiquantitative procedures are summarised inV 0.96 Cd 0.58 Tables 6 and 7 and compared with the certified values. ForCr 0.95 In (IS) 0.97

PACS-1 (Table 6), which contains relatively high concen-Co 0.80 Sb 0.37

trations of trace metals there is good agreement between theMn 0.87 Ba 1proposed method and the certified values, except for As, SeFe 0.86 Tb (IS) (1)†

Ni 0.73 Hg 0.11 and Hg where our results are lower (As) or higher (Se, Hg)Cu 0.70 Tl (IS) 0.97 than the certified values. The reasons for this discrepancy areZn 0.41 Pb 0.90 being studied, noting that all these elements show low ionis-As 0.19 Th (IS) (1)†

ation degrees in the plasma. For MESS-2 (see Table 7), withY (IS) 0.94 U (1)†

lower trace element concentrations, the agreement between the* IS: Internal standard. † Partition functions were not available and

proposed method and the certified values can be considered100% ionisation was assumed.

satisfactory, again except for As, Se and Hg.

Table 4 Comparison of results obtained by the semiquantitative method from the software of the HP 4500 with those obtained by the proposedmethod for the analysis of a non-certified water sample, as quantified using standard additions and calibration with internal standards (IS)

Calibration Values of HPusing IS/ standard additions/ semiquantitative/ Proposed method/

Element ng g−1 ng g−1 ng g−1 ng g−1Cr 0.56 0.47 — 0.42Mn 150 187 190 165Ni 2.2 2.9 3.1 1.9Cu 2.0 2.2 3.7 2.5Zn 5.0 7.0 5.2 5.7As 0.2 0.23 0.28 0.14Se 0.95 1.3 2.1 1.3Cd — 0.04 0.063 0.04Sb 0.24 0.25 0.19 0.27Hg 0.16 0.27 0.18 0.18Pb 0.02 0.18 0.13 0.12U 0.86 0.81 0.93 0.86

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results for the heavier elements (Cd, Sn, Sb and Pb) are inTable 5 Comparison of results obtained by the semiquantitative

much closer agreement with the certified values.method from the software of the HP 4500 with those obtained by theproposed method for the analysis of a certified reference lake water(SLRS-3) CONCLUSIONS

Certified HP Proposed An enhanced semiquantitative method for ICP-MS is pro-value/ semiquantitative/ method/ posed. In the development of this semiquantitative method the

Element ng g−1 ng g−1 ng g−1matrix effects observed from EIE on the ionisation temperature

Al 31 26 37 and electron density in the plasma were very small. However,V 0.3 0.37 0.30

there was a decrease in the signal of the analytes in theCr 0.3 0.73 0.30

presence of matrix elements which could be compensated byFe 100 140 107the experimental establishment of the response curve directlyMn 3.9 3.8 4.8

Ni 0.83 1.1 2.0 in the sample.Zn 1.04 3.3 1.58 The proposed semiquantitative method provides multi-As 0.72 0.87 0.26 elemental data of near ‘quantitative’ quality for both waterMo 0.19 0.36 0.16

and sediment samples. The determination of the ionisationCd 0.013 0.14 0.019

conditions in the plasma, its use to correct for ionisationSb 0.12 0.16 0.09degrees and the correction of mass-dependent matrix inter-Ba 13.4 12 11

Pb 0.068 1.9 0.26 ferences, by the use of internal standards added to the sample,U 0.045 0.37 0.043 provides a fast screening method of great applicability in

routine environmental analysis laboratories.In comparison with commercial ‘semiquant’ software, the

proposed method seems superior when analysing complexTable 6 Comparison of the manufacturer’s and proposed semiquantit-

samples such as sediments but is only marginally superior forative methods for a certified sediment (PACS-1)drinking water samples.

HP Proposed Certifiedsemiquantitative/ method/ value/ We thank Hewlett-Packard for the loan of the HP 4500 and

Element mg kg−1 mg kg−1 mg kg−1 Silvia Fernandez Torre for the digestion of the sedimentV 73 102 127 samples. This work was funded under Project PA-MAS95-02Cr 65 85 113 (FICYT, Asturias, Spain).Mn 290 502 470Ni 27 38 44Cu 214 350 452 REFERENCESZn 495 740 842

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Argentine, M. D., and Barnes, R. M., Microchem. J., 1997, 56, 352.semiquantitative/ method/ value/10 Garcıa Alonso, J. I., Thoby-Schultzendorff, D., Giovanonne, B.,Element mg kg−1 mg kg−1 mg kg−1

Koch, L., and Weismann, H. J. Anal. At. Spectrom., 1993, 8, 673.V 160 230 252 11 Denoyer, E. R., J. Anal. At. Spectrom., 1992, 7, 1187.Cr 64 95 106 12 Cromwell, E. F., and Arrowsmith, P., Anal. Chem., 1995, 67, 131.Mn 245 358 365 13 Pearce, N. J. G., Perkins, W. T., and Fuge, R., J. Anal. At.Ni 40 36 49.3 Spectrom., 1992, 7, 595.Cu 31 33 39.3 14 Beauchemin, D., McLaren, J. W., and Berman, S. S., Spectrochim.Zn 109 150 172 Acta, Part B., 1987, 42, 467.As 17.8 11 20 15 Olivares, J. A., and Houk, S. R., Anal. Chem., 1986, 58, 20.Se 5.3 2.5 0.72 16 Tan, S. H., and Horlick, G., J. Anal. At. Spectrom., 1987, 2, 745.Sr 123 153 125 17 Evans, E. H., and Caruso, J. A., Spectrochim. Acta, Part B, 1992,Mo 0.2 3.3 2.85 47, 1001.Cd 0.26 0.29 0.24 18 Evans, E. H., and Giglio, J. J., J. Anal. At. Spectrom., 1993, 8, 1.Sn 3 2.4 2.27 19 Garcia Alonso, J. I., Sena, F., Arbore, Ph., Betti, M., and Koch, L.,Sb 1 1.3 1.09 J. Anal. At. Spectrom., 1995, 10, 381.Hg 1.38 0.8 0.092 20 de Galan, L., Smith, R., and Winefordner, J. D., Spectrochim.Pb 20 21 21.9 Acta, Part B, 1968, 23, 521.

Paper 7/07496DWhen the certified values are compared with the results Received October 17, 1997

obtained by the instrument ‘semiquant’ software, there is a Accepted January 2, 1998clear disagreement for the low mass transition elements inboth sediment samples. For the semiquantitative software the

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