produced by various processes ( i.e. , cold vapor deposition , laser ablation , and arc discharge surface mediated growth ) , carbon nanotubes ( cnts ) contain metal impurities ( e.g. , fe , co , mo , ni , and y ) [ 15 ] which are introduced in the manufacturing process as catalysts for carbon structure catalytic growth or as contaminants [ 2 , 46 ] . it has been observed that even after rigorous purification those embedded impurities still persist in the cnt graphitic structure [ 2 , 3 , 69 ] . for example , an icp - ms study of commercially important cnts showed that impurity concentrations in three multiwall cnts ( mwcnts ) were between 4738 and 51867  g / g for ni and in two commercially relevant single - wall cnts ( swcnts ) between 1798 and 4217  g / g for co and 14721672  g / g mo . the currently accepted method for determination of cnts in workplace air is thermooptical analysis ( niosh method # 5040 ) for determination of elemental carbon . a drawback of this approach is that it does not distinguish between cnt and non - cnt sources of elemental carbon such as diesel exhaust [ 11 , 12 ] . the detection of cnt - bound metal impurities in environmental samples provides a means to distinguish cnt emissions from non - cnt background sources of elemental carbon [ 1215 ] . worker exposure to cnts can occur at any stage of the manufacturing process and during transport , storage , and handling of cnts , with inhalation being the major route of exposure [ 11 , 16 ] . different sampling methods have been suggested as possible strategies to collect samples for workplace monitoring of metal impurities in cnts such as filter cassettes , electrostatic precipitation , and surface wipes [ 13 , 15 ] . wipe sampling has emerged as a useful and cost - effective qualitative technique suitable for representative mapping of spatial variations of surface metal contamination within a workplace . the method provides an additional tool to detect workplace releases of cnts ( and potential exposures ) , evaluate housekeeping or clean - up procedures , and monitor the effectiveness of engineering controls . niosh   and oecd   provided guidance on collecting surface wipe samples in nanotechnology applications using premoistened ghostwipes towelettes , but neither included an analytical methodology for determining cnts in wipe samples . wipe sampling for cnt metal tracers is considered a qualitative indicator of the presence of cnts , not a quantitative exposure metric . as described by the oecd tiered approach , such qualitative assessments are useful for comparing particle concentrations at the emission source with background particle concentrations . even though wipe sampling may be considered qualitative , an analytical method is needed that includes an efficient digestion step and adequate detection limits to ensure reliable and reproducible metal determinations . the most commonly used analytical methods for determining metal impurities in cnts are neutron activation analysis ( naa ) [ 1 , 3 , 2022 ] , direct solid sampling electrothermal atomic absorption spectrometry ( dss - et aas ) , high resolution continuum source graphite furnace atomic absorption spectrometry ( hr cs gfaas ) , inductively coupled plasma optical emission spectrometry ( icp - oes ) [ 1 , 4 , 20 , 21 , 23 , 24 ] , and inductively coupled plasma mass spectrometry ( icp - ms ) [ 1 , 3 , 4 , 7 , 20 , 21 ] . out of all of these detection methods , icp - ms is a suitably sensitive approach for determination of particle - bound metals on wipe substrates , but an appropriate extraction method is needed for cnt - bound metals . the key challenge is the difficulty in quantitatively extracting cnt metal impurities during the digestion step [ 3 , 4 , 7 , 22 , 26 ] . a variety of sample pretreatments have been employed for determination of metals in cnt samples including wet digestion [ 1 , 3 , 23 ] , dry ashing combined with acid digestion [ 1 , 3 , 4 ] , carius tube digestion , microwave - assisted acid digestion [ 1 , 3 , 4 , 20 , 26 ] , and microwave - induced combustion [ 4 , 21 , 24 ] . grinberg and coworkers   reported incomplete recovery of metals from cnts with the single - step microwave digestion ( hno3 and h2o2 ) recommended by decker et al .   and proposed a multistep microwave - assisted procedure that involved the repeated use of strong acids as follows : hno3 combined with h2o2 ( 1x ) , hno3 alone ( 3x ) , evaporation near dryness , hno3 and hclo4 followed by hno3 addition and evaporation to dryness ( 4x ) , and finally dissolution and dilution in 1% hno3 . grinberg et al .   found that this lengthy and aggressive procedure was necessary to ensure complete sample digestion and efficient recovery of residual catalysts ( co and ni ) and trace metal impurity content ( fe , mo , pb , and hg ) from the nrc swcnt-1 reference material investigated . iso / iec 13278   also recommended repeating the heating cycles with new reagent addition several times ( i.e. , three to six heating cycles depending on cnt structure ) when using microwave digestion to measure cnt metal impurities . yang et al .   emphasized the importance of the digestion step as they observed positive nonspectral interferences from undigested carbon residues remaining in the sample solution which they identified by comparing icp - ms and icp - oes results for the same swcnt digest . the purpose of this study is to identify an appropriate digestion approach for icp - ms determination of metal impurities in cnts collected using wipe samples . two approaches are evaluated with respect to the efficiency of the extractions : a microwave - assisted digestion method and an ultrasonic digestion method . although for this application 100% extraction efficiency is not required , the goal is to achieve reproducible and reliable recoveries that will enable the identification of cnt surface contamination . different cnt materials , including nist 2483 soot ( single - wall cnt ) , are digested with and without ghostwipes , as the digestion approach must be able to accommodate wipe samples which are quite bulky . the possibility that wipes may contain background contamination which can interfere with determination of the metals of interest is also considered . three swcnt products were obtained for this study : nist 2483 swcnt standard reference material ( raw soot ; national institute of standards and technology , gaithersburg , md , usa ) and two other swcnts test materials , aldrich - cnt ( sigma - aldrich co. , gillingham , uk ) and a research test sample provided by national research council ( test - cnt ) . the nist 2483-cnt is uniform and well - characterized carbon nanotube soot approx . 0.8  nm in diameter ( by tem ) produced by a chemical vapor deposition variant process . the crm was weighed straight from the bottle , as the certificate of analysis indicates that the crm does not require preparation prior to weighing . the aldrich - cnt test material was manufactured by arc discharge , with 4060  wt% carbon content and 3035% metal content . this cnt has average diameter of 1.7  0.2  nm , wall width of 0.4  0.1  nm , and bundled dimensions of 5 to 10  nm in diameter and 0.32 to 4.7  m in length . the test - cnt is a swcnt with an average diameter and wall width of 1.6  0.2  nm and 0.4  0.2  nm , respectively , narrower bundles ( diameter < 3  nm ) , and length between 0.07 and 5.2  m ( high proportion < 1  m in length ) . the test - cnt test material showed a higher proportion of amorphous material ( relative to nanotubes ) than that of aldrich - cnt . detailed characterization of test - cnt and aldrich - cnt test materials using sem and tem is provided in rasmussen et al . . nist 1633b srm ( national institute of standards and technology , gaithersburg , md , usa ) is a bituminous coal fly ash sieved through a nominal sieve opening of 90  m ( 170 mesh ) used for matrix comparisons and accuracy check . high - purity nitric and hydrofluoric acids ( seastar chemicals inc . , sidney , bc , canada ) and suprapur 30% aqueous solution of hydrogen peroxide were used for sample pretreatment . ultrapure milli - q water ( 18.2  m  cm ) was used for preparation of samples and calibration standards . high - purity multielement ( mes-1107 - 01 solutions a and b , 100  g / ml ) and individual standard stock solutions ( la and gd , 1000  g / ml ) were used to prepare the calibration standards ( delta scientific laboratory products ltd . , gw procedural blanks and sample ( nist 2483 ) replicates were spiked at levels varying between 4 g / l and 250  g / l , respectively , with icp - mscs high - purity standard solution ( 10  g / ml ) . individual high - purity standards solutions of ge , in , and re ( 1000 mg / l ) were used to prepare the internal standard solution . certified reference low and high level fortified waters for trace elements materials , tm-28.4 and tmda 54.5 , respectively , were used for quality control and were purchased from environment canada ( ottawa , on ) . ghostwipes ( gw ; environmental express , charleston , south carolina , sc 4250 , premoistened with deionized water , individually sealed packets ) were used in combination with nist 2483 swcnt and nist 1633b stms to assess the effect of gw on extraction efficiency . the present study used a 3.5  cm diameter circular disk of ghostwipes material as required by the wipe sampling device used for monitoring cnt releases . composed of polyvinyl alcohol polymer , ghostwipes dissolve completely during the analytical procedure allowing the sample material / analytes to disperse in the digestion media ( http://www.envexp.com/ ) . scp science digifilters 0.45  m ( teflon membrane ) for 50  ml tubes were used for filtration of diluted extracts . mettler toledo xp205 digital analytical balance equipped with a u - shaped antistatic electrode was used to achieve accurate weighing results for the aliquots ( 2  mg ) of tested cnts used in this study . as recommended [ 1 , 11 ] , appropriate personal protective equipment was worn and care was taken to avoid inadvertent generation of aerosols when handling the cnt test materials . an ethos touch control advanced microwave labstation ( milestone microwave laboratory systems ) equipped with ethos tc built - in atc-400-ce automatic temperature control was used for all microwave digestions . digiprep ms heating block ( scp science , qc , canada ) and branson ultrasonic bath ( model b8510dth ) were used in this study . for quantification of cnt metal impurities in digested samples , a nexion 300s dual - channel universal cell icp - ms system ( perkin elmer , canada ) coupled with a sc - fast autosampler ( elemental scientific , omaha , ne ) was operated in the standard and collision mode . the instrument was equipped with a high temperature apex - st pfa microflow nebulizer , cyclonic spray chamber with a pc3x chiller ( 2c ) , and triple cone interface ( nickel - platinum skimmer and sampler cone , and aluminum hypercone ) . the following conditions were used : plasma and auxiliary argon flow rates were 18 and 1.2  l / min , respectively . the nebulizer argon gas flow rate was 1.00  ml / min for the high temperature apex - st pfa microflow concentric nebulizer and the forward rf power was 1600  w. optimization was carried out daily with a normal tuning solution . the appropriate internal standards ( ge , in , and re ) and dilution factors were selected based on a preliminary semiquantitative analysis . perkin elmer optima 5300v icp - oes equipped with radial optical system ( 163 to 782  nm range ) was used at the wavelengths recommended by manufacturer for the elements of interest . the instrument was operated at 1400  w power and flow rates of 15  l / min , 0.2  l / min , 0.8  l / min , and 1.00  ml / min for plasma , auxiliary , nebulizer , and peristaltic pump , respectively . two digestion methods were evaluated during this study to compare their efficiency in extracting elemental impurities from cnts : a microwave digestion ( md ) method using nitric acid and hydrogen peroxide and an ultrasonic digestion ( ud ) method using nitric and hydrofluoric acids . for both digestion methods three to five aliquots of 2  mg of test material were used for analysis ( to mimic the particle mass on a surface wipe sample ) . spiked samples ( 20 and 250  ppb level ) and spiked procedural blanks ( 4 and 25  ppm level ) were analyzed along with the test samples . finally , nist 2483 swcnt and nist 1633b coal fly ash were combined with ghostwipes ( gw ) in order to compare the efficiency of investigated digestion methods in the presence of gw . recoveries were calculated for each method and their advantages and disadvantages were considered . the microwave digestion ( md ) method was preceded by a digiprep sample preparation / pretreatment step . during the digiprep step , the samples were placed in 50  ml teflon digitube vessels and were treated with diluted nitric acid ( 1  :  1 , 12  ml ) and allowed to sit at room temperature for 3045  min . this step allowed the gentle and complete dissolution of gw ; otherwise the generation of effervescence during heating may affect the sample and tube integrity . after effervescence subsided , the samples were treated successively at reflux ( 85c on digiprep ms heating block ) with diluted nitric ( 12  ml , 90  min ) , concentrated nitric acid ( 10  ml , 30  min reflux ) , and 30% hydrogen peroxide ( 5  ml , 45  min ) , and evaporated to 10  ml ( 90c ) . the resultant acid - peroxide digestion solutions were then transferred quantitatively to precleaned high - pressure microwave digestion vessels ( 100  ml teflon ) and 0.5  ml 30% hydrogen peroxide . the microwave digestion step was performed at 1000  w power in the following conditions : 20  min to reach 180c temperature , 10  min to increase from 180c to 220c , and 20  min at 220c . the digested samples were evaporated to nearly dryness ( 90c ) and the resultant residues were dissolved in 1% nitric acid , vortexed , and filtered through a 0.45  m filter to remove the possible undissolved residuum ( i.e. , undissolved cnts ) that may impact the icp - ms analysis . ( based on astm method e 1644 ) to accommodate the bulk of gw samples . preliminary work   showed that the digiprep method alone did not yield adequate recoveries for cnts in wipes and therefore it was necessary to add the microwave digestion step . the samples were placed in 15  ml digestion tubes with 4  ml nitric acid , 0.1  ml hydrofluoric acid , and 1.9  ml ultrapure water and allowed to stand at room temperature until gw dissolved completely and effervescence subsided ( 3045  min ) before proceeding to the next step . this step is most critical in the presence of gw because even with a small piece of gw ( 3.5  cm disc ) an exothermic reaction develops during the hot water bath stage . the capped digestion tubes were placed in the ultrasonic water bath ( 180  w , 69c ) for a total of 2  h ( 2  1  h ultrasonication with 1  min vortex in between ) . the solutions were then cooled down , diluted to 10  ml using ultrapure water , and centrifuged ( 3500  rpm , 10  min ) . a final filtration step ( 0.45  m ) was added to remove the possible undissolved residuum ( i.e. , undissolved cnts ) that may impact the icp - ms analysis . all sample digests were analyzed by icp - ms after appropriate dilution with 1% hno3 . the hno3-hf ud method is based on a method originally designed for ptfe filter - based aerosol samples by niu et al . . all sample digests were stored in the fridge in precleaned polyethylene screw - capped tubes and analyzed by icp - ms after appropriate dilution . the final digestion solutions were first screened for the presence of elements using icp - ms and isotopes such as b , al , ti , v , cr , mn , fe , co , ni , cu , as , se , sr , zn , y , mo , ba , la , gd , pb , and u were monitored . internal standard solutions of 25  ug / l ge , in , and re were used throughout the experiment for analytes in the mass ranges 87 , 95138 , and 207238 , respectively . analytical errors up to 10% for quality check standards were considered acceptable ( epa method 200.8 ) . spike recoveries of the study elements ( b , al , v , mn , co , ni , cu , mo , as , ba , la , and u ) were between 82% and 115% with md method ( except b 74% and cu 47% ; al spike not recovered ) and 86107% with ud method ( except mo 122% ) in the presence of gw ( spiked procedural blanks and nist 2483 sample ) . aluminum , boron , zinc , and copper spike recoveries were affected by the presence of gw that contain variable amounts of contaminant elements introduced during their manufacturing and/or packaging . therefore , five matrix blanks containing gw were included with each analytical batch and used for matrix blank corrections when samples were combined with gw . the lods were calculated according to the 1994 us - epa guideline for inductively coupled plasma mass spectrometry using 3 times standard deviation of 8 - 9 procedural blanks . student 's t - test and mann - whitney rank sum test were used for two sample comparisons . extraction efficiencies of two different methods , microwave digestion ( md ) and ultrasonic digestion ( ud ) , are compared for cnt impurities present at high concentrations ( > 0.5% ) in table 1 and for cnt impurities present in lower concentrations ( ppm range ) in table 2 without gw being present . nist 2483-cnt standard reference material ( srm ) , test - cnt , and aldrich - cnt were used to compare the extraction efficiency for the residual catalysts ( > 0.5% ) . nist 2483-cnt srm was used to compare the extraction efficiency for other trace metal / element impurities ( ppm range ) using the certified , informational , or reference values provided in the certificate of analysis . the catalyst residues in nist 2483-cnt are co and mo with certified mass fraction values of 9630  170  g / g and 34060  290  g / g , respectively , determined by instrumental neutron activation analysis ( inaa ) and cold neutron prompt gamma activation analysis ( pgaa ) . certified values were given also for ba , la , and gd present in smaller quantities in nist 2483-cnt , reference values for al , mn , and v , and informational values for the other metals ( b , as , and cu ) as shown in tables 2 and 3 . consequently , the efficiencies of the two investigated methods are compared using aliquots of nist 2483-cnt combined with gw ( table 3 ) . table 1 presents the high metal concentrations ( > 0.5% ) in nist 2483 , test - cnt , and aldrich - cnts obtained with both microwave and ultrasonic digestion methods . the results are presented as mean ( g / g ) and standard deviation of five independent determinations . as shown in table 1 for all investigated cnts , the md method had improved extraction efficiency for the high concentration metals ( co , ni , y , and mo ) when compared to the ud method . except for ni in test - cnt for which t - test indicated no significant difference ( p = 0.128 ) , for all other residual catalysts metals ( co , ni , y , and mo ) in all three cnts the difference between md and ud results was significant ( p < 0.05 ) . the greatest difference was observed for the nist 2483-cnt ( table 1 ) for which 25 times lower mo and co concentrations were extracted with ud method than with md method resulting in recoveries of 74  2.0% co and 82  2.5% mo with md method and just 16  1.7% co and 39  2.0% mo with ud method . although for test - cnt ( except for ni ) and aldrich - cnts the metal concentrations extracted with md method were significantly higher ( p < 0.05 ) than those obtained with ud method , the ranges were similar ( i.e. , test - cnt co : 17487  g / g md versus 14358  g / g ud ) when compared with results of nist 2483-cnt ( i.e. , co : 7100  g / g md versus 1521  g / g ud ) . overall , the results obtained with the ud method varied largely with the cnt analyzed , indicating that morphology and/or the way the residual catalyst metals were incorporated in the graphitic structure could influence the success of the sample pretreatment and consequently the recovery . this might be related to the manufacturing processes since the cnts are produced by various processes , that is , cold vapor deposition , laser ablation , and arc discharge surface mediated growth [ 3 , 4 ] , which influence the amounts and how strongly the residual catalyst metals are incorporated in the graphitic structure [ 7 , 22 ] . even though in case of ultrasonic digestion method the recoveries vary with the cnt investigated , this proves to be a quick and useful method for qualitative identification of high concentration metallic impurities in cnts . therefore , when comparing the two investigated methods , the md method appears to be the method of choice for quantification of high concentration metals from tested cnts due to improved extraction ( table 1 ) yielding high recovery of residual catalysts . the use of alternating oxidation and reflux steps with strong acids enhances the dissolution and extraction of metallic impurities from the cnt graphitic structure [ 3034 ] . 4.5  h for alternating oxidation and reflux steps plus evaporation and 4  h for microwave digestion and evaporation / filtration ) . in contrast , the ud method has a shorter sample - reagent contact time ( 2  h ultrasonication ) and does not include alternating oxidation and reflux steps . although the md method takes more time than the ud method ( and uses more reagents ) , the result is an increased extraction efficiency for high concentration metallic impurities in cnts . beside the catalyst residues present in high concentrations ( up to 3% ) , lower concentrations of other metals are present in cnts as a result of production procedures , postfabrication , and postpurification treatments . table 2 presents results obtained for lower concentration metal(loids)s in nist 2483-cnt with both md and ud methods . generally both methods extracted similar amounts for most of the lower concentration ( ppm range ) metal(loids)s investigated . even though slightly increased concentrations of al , v , mn , and ba were extracted with the ud method and of cu and la with the md method , in contrast , the md method extracted significantly higher ( p < 0.001 ) amounts of as , but significantly lower ( p < 0.05 ) amounts of b , gd , and u ( table 2 ) . figure 1 shows the percent recoveries obtained for high ( > 0.5% ) and low concentration ( ppm range ) metals in nist 2483-cnt srm obtained with ( a ) md and ( b ) ud methods with or without ghostwipes ( gw ) . percent recoveries were calculated using the certified / information / reference values available in the certificate of analysis for the investigated elements . generally the recoveries for low concentration metals ( i.e. , al , v , cu , ba , la , and gd ) ranged between 75 and 100% with md and 76 and 104% with ud . both methods resulted in low recoveries of b ( 43% md and 62% ud ) and mn ( 35% md and 51% ud ) , whereas ud gave also lower recoveries for as ( 64% ud versus 100% md ) . with similar or increased recoveries ( i.e. , 85% al , 83% v , and 84% la ) for the low concentration ( ppm range ) metals investigated ( except as ) in nist 2483-cnt , the ud method is a short and practical method that may be used to detect and measure metals present in lower amount in cnts . moreover , since nist 2483-cnt is not certified for some elements of interest ( i.e. , ni ) and other metals present in higher concentrations were not well recovered with the ultrasonic digestion method ( e.g. , 16% co ) , another srm with carbonaceous matrix ( as recommended by decker et al . ) , nist 1633b , was analyzed using ud to assess the ability of the method to extract other metals , that is , ni and co when present in lower concentrations . this srm was recommended previously for quality assurance in the absence of specific reference materials for cnt analysis . it should be noted that elements in nist 1633b are generally in lower concentration range ( i.e. , ni : 120.6  1.8  g / g ; co : 50  g / g ) with the exception of al and fe ( 150.5  mg / g al and 77.8  mg / g fe ) . the certified values along with the results obtained for nist 1633b srm with ud method are presented in table s1 ( see table s1 in the supplementary material available online at http://dx.doi.org/10.1155/2016/3834292 ) . recoveries of 7891% were obtained with the ud method for most elements in nist 1633b ( with the exception of as , zn , and gd ; table s1 ) which were better than recoveries of 2969% achieved for the same srm using us - epa method 3051 . even though the ud method showed excellent recoveries for low concentration metals in nist 2483-cnt ( i.e. , al , v , cu , ba , la , and gd ) , recoveries were poor for the catalyst impurities ( i.e. , 16% co and 39% mo , resp . ) . moreover , the extraction efficiency of the ud method for the residual catalysts / high concentration metals varied with the cnts tested as shown in table 1 with better extraction efficiency for test - cnt and aldrich - cnt than for nist 2483-cnt . compared to the md method , the ud method results varied largely with the cnt analyzed , indicating that morphology and/or the way the residual catalyst metals were incorporated in the graphitic structure could influence the success of the sample pretreatment and consequently the recovery . this variability might be related to the manufacturing processes since the cnts are produced by various processes , that is , cold vapor deposition , laser ablation , and arc discharge surface mediated growth [ 3 , 4 ] , which influence the amounts and how strongly the residual catalyst metals are incorporated in the graphitic structure [ 7 , 22 ] . based on previous concerns about signal enhancement caused by nonspectral interferences [ 20 , 36 ] , the potential for signal enhancement was investigated for the md method . this was done by analyzing the digests of all three tested cnts ( nist-2483 , nrc , and aldrich ) with both icp - oes and icp - ms techniques and comparing the results ( figure 2 ) . the elements measured were co , ni , y , mo , and fe ( 5 replicates of each ) . t - test results showed that the co , ni , y , and mo results obtained with icp - ms were not significantly higher ( p > 0.05 ) than those obtained with icp - oes for all of the three cnts tested , except for ni in aldrich - cnt ( p = 0.031 ) .   figure 2(a ) presents the correlation of metal concentrations measured by icp - ms and icp - oes using all seven pairs of metal - concentration values from all cnts tested . the strong linear regression obtained ( slope = 0.9574 , r = 0.99992 , n = 7 ) indicates consistency between icp - ms and icp - oes results . also , different dilutions ( 10x and 100x ) of two spiked ( 200  ppb ) aldrich - cnt solutions obtained with md method were analyzed ( standard and/or collision mode ) and the results compared for co , ni , y , and mo ( figure 2(b ) ) . the results ( slope = 1.0002 , r = 0.999998 ) presented in figure 2(b ) showed no differences between the icp - ms results of the two dilutions . in summary , signal enhancement due to residual carbon does not occur with the md / icp - ms method and does not explain the higher recoveries observed with the md versus the ud method . wipe sampling is commonly used in occupational safety and health applications . among other sampling methods ( e.g. , filter cassettes , electrostatic precipitation ) surface wipes have been suggested as a strategy to collect samples ( including cnt ) for workplace monitoring [ 17 , 3739 ] . ghostwipes ( gw ) meet the us regulatory standard for sampling pb in house dust and were used for quantification of additional metal(loid)s ( i.e. , as , cd , cr , cu , and ni ) to provide information on typical background loadings for these metals in urban canadian homes . although gw have the advantage of dissolving completely during acid digestion , their presence in the sample matrix adds to the analytical challenges encountered in the quantification of cnt metal bound impurities , since wipes are bulky and may contain impurities . the md method includes the digiprep sample pretreatment step that controls the dissolution of the bulky gw matrix ( especially useful where the study design calls for one or two 15  15  cm gw per digestion tube ) . the present study was designed for 3.5  cm discs of gw matrix , which permitted the use of smaller test tubes ( 15  ml ) required by the ultrasonic method . the efficiencies of the two investigated methods were compared using aliquots of nist 2483-cnt combined with gw ( table 3 ) . nist 1633b srm was also analyzed with both methods with / without gw ( table s1 , supplementary material ) . results obtained in the presence of gw ( table 3 ) are significantly lower ( p < 0.05 ) for co and mo in nist 2483-cnt with the ud method than with the md method , similar to results observed without gw ( table 1 ) . regarding lower concentration ( ppm range ) elements , the md method extracted similar concentrations ( p > 0.05 ) for most elements whether gw were present or not ( except for b and as , p < 0.05 ) . in contrast , with the ud method recoveries were significantly lower ( p < 0.05 ) for almost all elements in the presence of gw than without gw ( except b , al , as , and gd ; p > 0.05 ) suggesting that the ud method is more sensitive to interferences from gw . problems are caused by high gw blanks for certain elements ( al , ti , ni , mn , zn , and fe ) , which result in nondetects after blank correction , particularly when present in cnts at lower concentrations . this is consistent with a previous study   which concluded that gw blanks were so elevated for zn that residential zn loadings could not be reported . al , ti , and ni were recovered only with the ud method , and mn , fe , and zn were not recovered with either of the two methods ( table 3 ) . high blank values with poor recovery and/or nondetects after blank correction were also observed for certain elements ( e.g. , al , ti , zn , fe , and mn ) in our preliminary work with nist 2483-cnt combined with whole gw   caused by uneven distribution of background contamination of these elements in the wipes used . ( however , high blanks for certain elements would not necessarily be problematic if those particular metals are not target elements . ) other factors may include interferences from organic carbon derived from gws or uncertainty arising from the small sample mass used in this study ( 2  mg versus 2540  mg recommended in the certificate of analysis ) . figure 1 shows the same information as tables 2 and 3 , expressed as percentage recoveries ( observed / certified value ) for certified elements in nist 2483 . combining the results into these histograms clarifies the overall trends : the md method ( a ) yields acceptable recoveries for most elements with or without the presence of gw , but the ud method ( b ) yields poor recoveries for the catalyst elements and is generally more sensitive to the presence of gw . the ud method does result in acceptable recoveries ( 7598% ) for certain elements ( v , la , and gd ) with or without the presence of gw . despite generally lower recoveries , the cost - effective ud method may be considered acceptable for semiquantitative purposes such as identifying cnt releases in the workplace environment using wipe samples . different responses of the cnts tested in the present study indicate that the cnt matrix may influence the success of the sample pretreatment and consequently the recovery . thus , different matrices are likely to respond differently to a given digestion method , as observed for the ud method which gave excellent results for co in nist 1633b ( table s1 , supplementary material ) but poor recovery for co in nist 2483 ( table 1 ) . poor recoveries for the ud method ( figure 1(b ) ) are not likely due to readsorption of the extracted metals on residual cnts , due to the strong acid digestion ( ph < 2 throughout ) . chen et al .   previously showed that the adsorption of metals on the surface of swcnts at ph less than 2 was negligible . wipe sampling for cnt metal tracers has emerged as a useful and cost - effective qualitative technique suitable for representative identification of cnt surface contamination in the workplace ( and potential exposures ) and routine monitoring of control measures . even though wipe sampling may be considered qualitative , an analytical method is needed that includes an efficient digestion step and adequate detection limits to ensure reliable and reproducible metal determinations . for the purpose of wipe sample analysis , the microwave digestion ( md ) method yielded acceptable recoveries for catalyst elements at high concentrations ( table 1 ) as well as for elemental impurities at low concentrations ( tables 2 and 3 ) in the tested cnt reference materials . the md method involved prolonged contact time between the reagents and acid , with alternating oxidation and acid reflux steps , which permitted the elemental impurities to be efficiently extracted from the cnts . moreover , no signal enhancement due to residual carbon compounds was observed when icp - ms was used in conjunction with the md method . performance of the md extraction was not affected by the presence of ghostwipes ( gw ) in the digest matrix , with the caveat that elevated and variable gw blanks for certain elements ( such as al and zn ) must be considered . an advantage of the md method is that it eliminates the use of hf , which is avoided in many labs due to safety concerns . it is concluded that the md method is appropriate for quantification of metallic tracers in wipe samples used to identify the presence of cnt in work environments . in contrast , the ultrasonic digestion method showed incomplete extraction of catalyst metals present at high concentrations ( table 1 ) , but near - total extraction efficiencies for elemental impurities present at low concentrations ( tables 2 and 3 ) in the tested cnts . however , for nist 1633b reference material , the ud method showed equivalent or better recoveries than those reported for us - epa method 3051 , and since the ud method involves fewer steps than the md method , it may serve as a cost - effective semiquantitative approach for screening purposes . for example , the ud method was useful for identifying the presence of cnt metal impurities ( surface wipe samples ) in a preliminary mapping survey of cnt releases in a manufacturing plant . the ud method appeared to be more sensitive to interferences from gw than the md method . also , compared to the md method , the extraction efficiency of the ud method varied to a greater degree with different carbonaceous matrices ( various cnts and coal fly ash ) . nonetheless , cnt extraction efficiency is a parameter that can be evaluated using certified reference materials for cnts , unlike other sources of laboratory and sampling uncertainty which are not as easily constrained ( e.g. , weighing errors , spatial and temporal variability ) .
residual metal impurities in carbon nanotubes ( cnts ) provide a means to distinguish cnt from non - cnt sources of elemental carbon in environmental samples . 
 a practical and cost - effective analytical approach is needed to support routine surface monitoring of cnt metal tracers using wipe sampling . 
 wipe sampling for cnt metal tracers is considered a qualitative indicator of the presence of cnts , not a quantitative exposure metric . in this study , two digestion approaches ( microwave - assisted nitric acid / h2o2 digestion and ultrasonic nitric / hf acid digestion ) in conjunction with inductively coupled plasma mass spectrometry ( icp - ms ) determination were evaluated for their ability to extract metal impurities from cnt particles captured on wipe substrates . 
 aliquots of different carbon nanotubes ( including nist 2483 single - wall cnt ) with and without ghostwipes ( astm e-1792 compliant ) were used to compare the performance of the digestion methods . 
 the microwave digestion method accommodated the bulky wipe sample and also eliminated potential icp - ms signal interferences related to incomplete digestion . 
 although quantitative recoveries requiring lengthy multistep digestion protocols may be necessary in other applications , the near - total recoveries achieved in the present study for cnt catalyst elements were adequate for identifying surface contamination of cnts in the workplace using wipe sampling .