土壤中痕量元素检测方案(ICP-MS)

APPLICATIONNOTE ICP-Mass Spectrometry Authors： Xiaoling Ma Guanyu Chen PerkinElmer， Inc. Shanghai Pudong， China Introduction The elemental analysis of soils is not only vitally important in theagriculture industry， but it is also important from an environmentalperspective. Toxic metals enter soil primarily through pollution， either directly (i.e. industrial waste or discharge) or indirectly (i.e. leaching from water， consumer pollution). These toxicmetals can be taken up directly by humans through the inhalation of windblown dusts， or they may enter thefood chain as a result of their uptake by edible plants and animals. In addition， these toxic metals can leach intogroundwaters and contaminate drinking water resources. Consequently， the consumption of these toxic metalsmay cause acute or chronically toxic responses in both humans and animals. Therefore， the identification of pollutants and determination of the level of toxic elements present in the soil areimportant aspects to be considered. In the past， the preferred method for leaching these elements from soil was using an extraction method with nitricand hydrochloric acids. However， this only partially extracts the metals of interest and does not provide an indicationof the total concentration in the soil， since some elements will not be leached and will remain in the crystal latticeof the soil material. For this reason， a four-step acid digestion using nitric acid (HNO；)， hydrochloric acid (HCI)，hydrofluoric acid (HF) and perchloric acid (HCIO) on a hotplate is often implemented to obtain a complete digestion. Over the years， the number of elements requiring measurement in soil studies has increased while their allowableconcentrations have decreased. In China， for instance， the concentrations of many elements， such as cadmium(Cd)， mercury (Hg)， arsenic (As)， lead (Pb)， chromium (Cr)， copper (Cu)， nickel (Ni)， zinc (Zn)， antimony (Sb)，beryllium (Be)， cobalt (Co) and vanadium (V)， are regulated in agricultural and/or development land. This hasfostered the demand for instrumentation with low detection capabilities. Historically，inductively coupled plasmaoptical emission spectroscopy (ICP-OES) was often used for these types of quantifications due to it being a multi-element technique capable of fast analyses. However， for some elements in the soil， the detection capabilities ofICP-OES are insufficient to provide their accurate quantification. As a result， inductively coupled plasma mass spectrometry (ICP-MS)is ideal as an analytical technique due to its multi-elementcapability， ability to measure at ultra-low concentrations， and itswide linear dynamic range. As with all analytical techniques，however， ICP-MS is not free from interferences. Plasma- and matrix-based polyatomic interferences are inherent to ICP-MS and need tobe accounted for through either the use of correction equations，collision or reaction chemistry.? In this application note， a four-acid digestion was used in order toobtain the complete sample dissolution of several soil samplesS.Thereafter， the samples were analyzed on PerkinElmer's NexlON@ICP-MS for 21 typically regulated elements in soil. Experimental Samples and Sample/Standard PreparationApproximately 0.250 g (accurate to 0.0001 g) of soil sample wasweighed into a 50 mL PTFE digestion beaker， followed by 0.5 mLultrapure water to wet the soil. Next， 5 mL HNO， (Ultrapure，68%， Suzhou JINGRUI Chemical Co. Ltd.， Suzhou， Jiangsu， China)was added， the vessel capped， and the sample allowed to soakovernight. The next day， the beaker was placed on a hot plate in afume hood and heated at 140°C until the volume was reduced toapproximately 3 mL. Next， 5 mL of HNO， 5 mL of HCl (Ultrapure，37%， Suzhou JINGRUI Chemical Co. Ltd.)， 2 mL of HF (Ultrapure，49%， Suzhou JINGRUI Chemical Co. Ltd.)， and 1 mL of HCIOwereadded to the beaker， which was then covered with the PTFE capand heated to 180 ℃ on a hot plate for about two hours. Thebeaker was then uncovered， the condensate on the cap washedwith deionized water， and the washings added to the samplesolution. The temperature of the hot plate was adjusted to 160 ℃，and the heating of the sample continued in order to remove thesilicon in the form of SiF， which is highly volatile. In order toefficiently remove the silicon， the beaker should be gently shakenfrequently until white smoke is emitted and the solution becomesmore viscous. In cases where the solution remains cloudy andcontains visible black suspensions， the above digestion processshould be repeated until a sticky solution is seen. The remaining solution was dissolved with 2 mL HNO，， cooledto room temperature and quantitatively transferred into a 50 mLvolumetric flask by rinsing the beaker three times with an.appropriate amount of 1% nitric acid solution. The washingswere then added to the volumetric flask. The solution was madeup to the mark with deionized water， mixed well， and diluted fivetimes before measurement. Calibration standards were also prepared in a 1%(w/v) nitric acidsolution at the levels shown in Table 1. All calibrations and internalstandard solutions were prepared from the stock solutions shownin the "Consumables Used" table included at the end of thisapplication note. Internal standards were added on-line to all standards andsamples， eliminating the need for manual addition. Althougha variety of internal standards may be used， the three listed inTable 1 were found to be effective for all elements in that theywere absent in the sample， had a similar first ionization potential tothe analytes， and were reflective of the mass range under study. Table 2 shows the recommended isotopes for different elements inthis experiment. Table 1. Calibration Standards. Analytes Standard Standard IStandard Standard Standard 1 (ug/L) 2 (ug/L) 3 (ug/L) 4 (ug/L) 5 (ug/L) As， Cu， Cr， Ni， Pb， V， Zn， Sr 5 20 100 200 Ba， Mn 5 20 100 200 500 Be， Co，Th， U， Sn 0.5 2 10 20 Bi， Ag， Cd， Mo， Tl 0.05 0.2 1 2 Li 1 5 25 50 Table 2. Recommended Isotopes for Different Elements. Analyte Recommended Mass (amu) Li 7 Be 9 V 51 Cr 52 Mn 55 Co 59 Ni 60 Cu 63 Zn 66 As 75 Sr 88 Mo 95 Ag 107 Cd 111 Sn 118 Ba 135 TI 205 Pb 208 Bi 209 Th 232 U 238 In order to validate the accuracy of the technique， three soilcertified reference materials (CRMs) were analyzed： GSS-1， GSS-18，and GSS-21 (Institute of Geophysical and Chemical Exploration，Chinese Academy of Geological Sciences， Hebei Province， China).These three CRMs have different properties and origins and includea saline-alkali soil and grey calcareous earth where elementalconcentrations varied greatly between the different samplematrices. For example， the concentration between the different soilCRMs ranged from 529-1760 pg/g and 0.066-0.35 pg/g for Mnand Ag respectively. To increase productivity and simplify the analysis， all analyses werecarried out using helium (He) as a collision gas， which greatlyreduces the effects of polyatomic interferences and is suitable insituations where the matrix varies greatly and the interferences areunknown or are well-characterized. Specific NexION ICP-MSconditions are listed in Table 3. Table 3. Instrumental Parameters. Parameter Description/Value Nebulizer MEINHARD@ plus Glass Type C Spray Chamber Glass cyclonic Injector 2.0 mm ID quartz Nebulizer Flow Optimized for<2.5% oxides sweeps 20 Dwell Time 50 ms Replicates 3 Internal Standard Addition On-line Results and Discussion Accuracy of the methodology was established through the analysisof the three certified reference materials described earlier. Table 4shows the analyte concentration in these reference materials， withthe analyte recoveries appearing in Figure 1 showing that therecoveries of all analytes are within 10% of the certified values，thereby validating the accuracy of the method. Table 4. Analyte Concentrations in Reference Materials (units in ug/g). Element GSS-1 GSS-18 GSS-21 Li 35 32 28 Be 2.5 1.7 1.6 V 86 66 75 Cr 62 55 55 Mn 1760 529 700 Co 14.2 10.2 11.0 Ni 20.4 25 28 Cu 21 19.5 24 Zn 680 63 66 As 34 10.7 9.7 Sr 155 242 205 Mo 1.4 0.61 0.68 Ag 0.35 0.066 0.073 Cd 4.3 0.15 0.139 Sn 6.1 2.4 2.4 Ba 590 459 510 TI 1.0 0.55 0.51 Pb 98 20 17 Bi 1.2 0.25 0.25 Th 11.6 9.9 8.4 U 3.3 2.3 2.0 Figure 1. Analyte recoveries in reference materials. With the accuracy of the methodology established， the stability wasverified over a five-hour continuous analysis of soil samples. GSS-1was analyzed before and after five hours. Figure 2 demonstratesthe outstanding stability of the NexlON ICP-MS system， despitethis challenging and complex matrix. It is important to note thatthis data was acquired without recalibration or excessive rinsingbetween samples and therefore mimics a typical run in a commerciallab. Over five hours， GSS-1 was analyzed once every hour， and theRSD% for GSS-1 were well within 4% (Figure 2). Figure 2 alsoshows that the recoveries of all analytes in GSS-1 are within 10%of the certified values. During the same analytical run， the internal standard recoveriesrelative to the calibration blank were also monitored (Figure 3).Here， internal standard recoveries were all within 15% of theoriginal value in the calibration blank and well within the limits of30%， as specified in U.S. EPA Method 6020B， which is used as areference，4 further validating the robustness of the methodology.This exceptional stability and robustness are the direct result ofNexlON's instrumental design， including the solid-state， free-runningRF generator specifically designed for ICP-MS. This unique generatorallows rapid impedance matching with changing matrices to delivera plasma source with uncompromised performance， robustness andlong-term reliability.5 The wide aperture cones of the Triple ConeInterface， in combination with the Quadrupole lon Deflector， ensurethat there is less sample deposition on the cones while also havingno lenses to clean or maintain， which results in less maintenance，outstanding stability and high matrix tolerance. Figure 3. Internal standard stability over five hours of soil analysis. Consumables Used Component Description Part Number Sample Uptake Tubing Green/Orange(0.38 mmid)， Flared， PVC， Package of 12 N8145197 (MP2 Peristaltic Pump) Spray Chamber Gray/Gray Santoprene N8145160 (MP2 Drain Tubing (1.30mm id)， Package of 12 Peristaltic Pump) As=1000 mg/L N9300180 Ba=1000 mg/L N9300181 Cu= 1000 mg/L N9300183 Cr=1000 mg/L N9300173 Mn=1000 mg/L N9303783 Ni=1000 mg/L N9300177 Pb=1000 mg/L N9300175 V=1000 mg/L N9303808 Zn= 1000 mg/L N9300178 Single Element Standard Be=1000 mg/L N9300172 Co=1000 mg/L N9303766 Ag=1000 mg/L N9300171 Cd=1000 mg/L N9300176 Mo= 1000 mg/L N9303784 TI=1000 mg/L N9300170 Th=1000 mg/L N9303842 U=1000 mg/L N9303844 Sn=1000 mg/L N9303801 Li= 1000 mg/L N9303781 Bi=1000 mg/L N9303761 Sr=1000 mg/L N9303802 Ge=1000 mg/L N9303774 Internal Standard Rh=1000 mg/L N9303794 Re=1000 mg/L N9303793 This work demonstrates the ability of PerkinElmer's NexlONICP-MS to easily analyze 21 elements in soil over an extendedperiod of time. Outstanding accuracy and stability are achievedand are a function of the distinctive features of the NexIONICP-MS， such as： ·Triple Cone Interface and Quadrupole Ion Deflector， whichdeliver outstanding matrix tolerance and stability ●UUnique solid-state free-running RF generator designed specifically for ICP-MS applications， which provides accurateimpedance matching to quickly adjust to changing plasma loadsWith accuracy and stability guaranteed， the NexION ICP-MS deliversa complete solution to addressing the challenges in soil analysis. 1. Falciani R. et al， J. Anal， At. Spectrom.， 15：561-565，2000. 2. Melaku S. et al， Analytica Chimica Acta.， 543： 117-123，2005. 3. Guo W. et al， Science of the Total Environment， 409：2981-2986，2011. 4. U.S. EPA Method 6020B， October 2012. 5. Badiei H. et al， "Advantages of a Novel Plasma Generatorfor the NexlON 1000/2000/5000 ICP-MS"， PerkinElmerTechnical Note， 2017. PerkinElmerFor the Better Copyright ◎ PerkinElmer， Inc. All rights reserved. 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