the general public 
 and the environment can be exposed to several classes of hazardous compounds 
 containing metallic elements which occur naturally or are released by domestic 
 or industrial processes . the total concentration levels of pb , ni , as , and cd allowable in the pm10 fraction of ambient air ( particles with an aerodynamic diameter of 10 m or 
 less ) are now limited by european legislation [ 24 ] . 
 in order to enforce this legislation , to measure human and environmental 
 exposure , and to show compliance with limit and target values , the total 
 concentration levels of ambient metals , at multiple sites on nationwide air 
 quality monitoring networks , need to be measured . to this end , nationwide 
 networks for the measurement of a wide range of particulate - borne and gaseous 
 pollutants are now well established in many developed countries around the 
 world .   npl currently manages and 
 operates the uk heavy metals monitoring network ( the  network  ) on behalf of 
 the uk department for environment , food and rural affairs ( defra ) .   the  network  consists of 24 monitoring sites 
 around the uk collecting pm10 particulate matter which is then sent 
 back to npl for the analysis of the mass concentration of ni , as , cd , pb , ( as 
 required by european legislation ) and also hg , cr , cu , fe , mn , v , zn , and pt to 
 contribute to long - term uk data sets .   whilst the data quality objectives laid down 
 by the air quality legislation are not especially exacting  the maximum 
 allowable expanded uncertainty for pb determination is 25% , and for ni , as , and 
 cd is 40%it is still necessary 
 to ensure that these objectives are routinely and consistently met . additionally npl also sets a self - imposed maximum measurement uncertainty of 
 40% on the nonmandated metals . to determine the mass concentration of particulate - phase metals in ambient 
 air , particles are collected onto air filters which are then digested in acid 
 before being analysed by inductively coupled plasma - mass spectrometry ( icp - ms ) . the icp - ms is calibrated by a working curve method using matrix matched 
 solutions prepared from commercially available elemental solutions certified 
 for metal mass fraction , and compared with nist standard reference material 
 ( srms ) by npl to ensure consistency and accuracy . whilst npl includes a 
 component of uncertainty to take account of the possibility of the isotopic 
 composition of metals in ambient air varying within natural limits , for a 
 working curve method this assumes that the isotopic composition of the 
 calibration standards being used also falls within this assumed range . if it 
 does not , then an additional uncertainty contribution , or a correction factor , 
 may need to be applied . in order to validate this hypothesis , the isotopic 
 composition of the standards used for these routine analyses requires measurement . this work 
 presents the results of this analysis and the effect of these findings on the 
 uncertainty budget of the measurement . we also present the contributions to 
 overall uncertainty budget from the possible variations in the isotopic 
 compositions of the ambient air samples . particulate 
 samples were taken at all sites in the  network  using partisol 2000 
 instruments ( fitted with pm10 heads ) operating at a calibrated flow 
 rate , nominally of 1 mh , in accordance with european 
 standard method en 12341 . samples were taken for a period of one week 
 onto 47 mm diameter gn metricel membrane filters . the analysis for 
 particulate - phase metals took place using a perkinelmer elan drc ii icp - ms , 
 following npl 's ukas accredited procedure , which is fully compliant with the 
 requirements of european standard method en 14902 
 ( the eu  reference method  for the analysis of metals in ambient air ) . upon arrival at npl , the filters sampled with 
 particulate matter were cut accurately in half , and each portion digested at 
 temperatures up to 220c using a cem mars x microwave . the digestion mixtures 
 used were as follows.hg and pt : 5 ml 
 of nitric acid and 5 ml hydrochloric acid.all other metals : 
 8 ml of nitric acid and 2 ml hydrogen peroxide . hg and pt : 5 ml 
 of nitric acid and 5 ml hydrochloric acid . all other metals : 
 8 ml of nitric acid and 2 ml hydrogen peroxide . these digested solutions were then diluted with deionised water ( millipore , milli q , mass , usa ) prior to analysis . icp - ms 
 analysis took place as previously described   using at least four - matrix - matched gravimetrically 
 prepared calibration solutions 
 prepared from monoelemental standard solutions ( vwr , checked for total 
 elemental composition against the nist srm 3100 series ) . a detector dead time 
 correction was applied 
 and a full span dual detector linearity check was performed in order to 
 minimise any detector nonlinearity   since the concentration of different isotopes within 
 the samples may span several orders of magnitude . a quality control standard was repeatedly analysed ( after every two solutions ) , and the change in response 
 of the quality control standard was mathematically modelled to correct for the 
 long - term drift of the instrument . the short - term drift of the icp - ms was corrected by the use of an internal 
 standards mixture ( containing y , in , bi , sc , ga , and rh ) continuously added to 
 the all samples via a mixing block . each sample was analysed in triplicate , 
 each analysis consisting of five replicates . 
 for each element , the mass of each metal in solution ( and its uncertainty ) was then 
 determined by a method of generalised least squares using xlgenline ( an 
 npl - developed programme ) 
 to construct a calibration curve . the analysis of the isotopic ratios of the 
 calibration standards was performed by determining the blank - corrected 
 intensity at each appropriate m / z ratio . the isotopic composition of the calibration standards was measured by 
 analysing the separate monoelemental standard solutions used to make up the 
 calibration solutions . the mass fraction of the sum of all isotopes of metal 
 analyte in each standard solution was approximately 1 g / g . all isotopes of 
 the elements of interest were measured , not simply the ones used for 
 quantification . whilst no additional effort was made to determine additional 
 corrections for isotopes not usually used for quantification by the npl 
 procedure , additional type - b uncertainty components were included in the 
 uncertainty budget to account for unresolved inaccuracies owing to mass bias 
 and mass discrimination effects , based on conservative estimates from existing 
 literature data . given the low precision of the measurements , these factors are expected to have 
 a minimal contribution to the overall uncertainty . uncertainty contributions were 
 also added to account for residual dead time effects and detector nonlinearity . matrix effects were minimised by the matrix matching of all solutions prior to 
 analysis . total expanded measurement uncertainties for each analysis are 
 calculated using a full gum 
 approach and are expressed with a coverage factor of k = 2 representing 
 the 95% confidence interval . the working curve 
 method for the icp - ms analysis of metals in solution , with a low resolution , 
 high throughput instrument measuring at one m / z ratio per element relies 
 on the relative abundance of the isotopes under consideration being the same in 
 both the sample and the calibration solution . if this is not the case , a 
 multiplicative bias in the observed results for element x , x , 
 will be observed , and is given by 
 ( 1)x = na , cali  ni , samna , sami  ni , cal , 
 where na , cal is the amount of isotope a of element x 
 in the calibration standard , i ni , cal is the amount of all isotopes of element x in 
 the calibration standard , na , sam is the amount of isotope a of element x 
 in the sample , and i ni , sam is the amount of all isotopes of element x in 
 the sample . when the relative abundance of the isotope used for quantification in both the 
 sample and the calibration solution is the same , then x = 1 and no bias is observed . it is interesting to 
 note that the isotopic composition with respect to the other isotopes not used 
 for quantification has no effect on the measurement . ( as and mn are also 
 measured by the  network ,  but are monoisotopic 
 and therefore not considered as part of this treatment ) . rather than assess the 
 isotopic composition of each individual sample , the uncertainty budget 
 developed for this measurement includes a component of uncertainty to recognise 
 that the isotopic composition of the sample may fall anywhere within the range 
 of natural variations , or the representative isotopic composition , whichever is 
 the larger range . ( the actual range of isotopic 
 composition in environmental samples may be considerably narrower . ) 
 in practice , this assumption assigns x = 1 but imposes a relative uncertainty on this 
 value equal to the possible range of isotope abundances expected for the 
 isotopes used for quantification . relatively little detail exists in the 
 literature on the isotopic composition of metals in ambient air particulates . the vast majority of the work that has been published has been on pb isotopic 
 composition , where the greatest variation is expected . determination of isotope 
 ratios has been mostly used as a route to determining the origin of the pb 
 sampled , particularly with regard to specific industrial processes or 
 long - range pollutant transport . one study 
 has examined the pb isotopic composition in deposition in order to compare how 
 this changed before and after the closure of a local pb mine . others studies [ 20 , 21 ] 
 have used the changing pb isotope ratios in ambient particulate matter to 
 demonstrate the seasonal variation long - range transport of pollutants across 
 the asian continent . measured pb isotope ratios have also been used as a route 
 to determine the changing origins of pb emissions in an urban environment 
 during and after the phasing out of leaded petrol . cu and zn isotopes ratios have been analysed near a large zn refinery 
 as a means of determining the origin of metallic ores , and sr and nd isotope 
 ratios have been used in the discrimination of emissions from various 
 industrial sources and traffic emissions , respectively . 
 in all cases , the observed ranges of the isotopic compositions fell well within 
 the natural ranges predicted   and these natural ranges have been used to construct 
 the uncertainty budget presented in this paper . moreover , when the isotopic 
 composition of samples under consideration in this study has been measured 
 periodically , the abundance of the isotope used for quantitation has always 
 been well within these ranges as well . the major contributions to the overall measurement uncertainty for the 
 determination of the mass concentration of metal in ambient air are shown in 
 figure 1 . as can be seen , the 
 contribution from the possible variation in the isotopic composition of the 
 sample is strongly dependent on the element being determined . this contribution 
 is very significant for pb , significant for hg , and cd , less significant for 
 cu , zn and pt , and negligible for cr , ni , fe , and v.   this is highlighted in figure 2 which shows 
 the fraction of the overall measurement uncertainty contributed by uncertainty 
 in the isotopic composition of the sample being measured . as expected from the data in figure 1 , this 
 contribution is very significant for pb , notable for hg , cd and possibly cu , 
 and insignificant for all other elements . the summary uncertainty budget 
 presented in figures 1 and 2 ( and the measurement equation from which this has 
 been developed ) is only valid if the abundance of the isotope used for 
 quantification in the calibration standards also falls within this range 
 allowed for the samples .   if it does not , 
 then an additional uncertainty contribution , or a correction factor , may need 
 to be applied this is not something that may be taken for granted since the 
 calibration standards may often have been prepared from isotopically enriched 
 pure materials , and must be measured in order to determine whether an increase 
 in the uncertainty estimate for the overall determination was required . the 
 results of the determination of the isotopic composition of the calibration 
 standards used are shown in figures 3(a ) and 3(b ) which show that the majority 
 of isotopes demonstrated good agreement between the measured value and the 
 expected range of isotopic abundances . table 1 highlights the level of this 
 agreement for the isotopes used for quantification . where there is agreement with 
 the predicted range , we may assume that the uncertainty budget for the 
 measurement already covers the expected range of isotopic compositions for both 
 sample and calibration standard . where table 1 shows a lack of agreement , an 
 additional component of uncertainty equal to the discrepancy in agreement needs 
 to be added to justify the uncertainty statement . given the large overall 
 measurement uncertainties reported for the measurement of metal mass 
 concentration in ambient air , these small additional uncertainty components are 
 unlikely to increase the overall uncertainty of the measurement significantly . ( an additional component of uncertainty has been added , rather than adding a 
 term into the uncertainty budget to correct for bias , since in all cases the 
 standard error of the mean of the isotopic composition measurements is greater 
 than the observed disagreement in all cases , and thus a bias correction term is 
 not justified . ) perhaps not surprisingly , table 1 highlights that a disagreement 
 was found for the elements with the smallest predicted range of isotopic 
 compositions , where the accuracy of the isotopic measurement is more 
 critical . a brief inspection of figures 3(a ) and 3(b ) suggests that the bias between the measured isotopic abundance 
 and the centre of the expected range of isotopic abundances shows some tendency 
 to increase as the isotopic abundance decreases . since the isotopes used for quantification are generally ones with 
 high abundances , little attention is usually paid to the less abundant 
 isotopes , and therefore , there may well be more bias in these measurements 
 owing to unresolved interferences , unresolved detector nonlinearity , and 
 instrument instabilities which have a larger proportional effect on the 
 measurement results than for high abundance species . a summary of the main sources of 
 uncertainty encountered during the analysis of the mass concentration of metals 
 in ambient air as part of the operation of the uk heavy metals monitoring 
 network has been presented . it has been observed that the uncertainty 
 contribution from possible variations in the isotopic composition of the sample 
 depends on the element in question , but can be significant ( as in the cases of 
 pb , cd , and hg ) . the working curve method for the icp - ms analysis of metals in 
 solution , with a low resolution , high throughput instrument measuring at one m / z ratio per element relies on the relative abundance of the isotopes under 
 consideration being the same in both the sample and the calibration solution . calculation of the uncertainty in this analysis assumes that the isotopic 
 composition variation within the sample and calibration solution is limited to 
 a defined range . the results of the isotopic 
 analysis of these calibration standards have shown that the isotopic 
 composition of the calibration standards agrees with the expected range of 
 isotopic comparisons in the samples for all but four elements . in these cases , additional uncertainty components were required to be added to the uncertainty 
 budget to account for this bias , although the increase in the overall uncertainty 
 of the measurement was not significant . it is interesting to note that the 
 isotopic composition with respect to the other isotopes not under consideration 
 not used for quantification has no effect , in theory , on the measurement . the bias between the measured 
 isotopic abundance and the centre of the expected range of isotopic abundances 
 shows some tendency to increase as the isotopic abundance decreases . it has 
 been suggested that since the isotopes used for quantification are generally 
 ones with high abundance and little attention is paid to the lower abundance 
 isotopes , the bias in these measurements may well be due to unresolved 
 interferences , unresolved detector nonlinearity , and instrument instability 
 which have a larger proportional effect on the measurement results than for 
 high abundance species . in future , it may be 
 expeditious to determine more rigorously the isotopic composition of samples 
 collected across the  network  so that the uncertainty contribution from x for the elements where this is most 
 significant ( in particular pb ) may be reduced , thereby reducing the overall 
 measurement uncertainty .
the main sources of uncertainty encountered during the analysis of the mass concentration of metals in ambient air as part of the operation of the uk heavy metals monitoring network are presented . 
 it is observed that the uncertainty contribution from possible variation in the isotopic composition of the sample depends on the element in question , but can be significant ( e.g. , for pb , cd , and hg ) . the working curve method for the icp - ms analysis of metals in solution , with a low resolution , high throughput instrument measuring at one m / z ratio per element , relies on the relative abundance of the isotopes under consideration being the same in both the sample and the calibration solution . 
 calculation of the uncertainty in this analysis assumes that the isotopic composition variation within the sample and calibration solution is limited to a defined range . 
 therefore , in order to confirm the validity of this quantification methodology and its uncertainty budget , the isotopic composition of the calibration standards used for quantification has been determined . 
 the results of this analysis are presented here .