Accurate elemental speciation by isotope dilution mass spectrometry with hyphenated techniques

Transcription

1 Accurate elemental speciation by isotope dilution mass spectrometry with hyphenated techniques Klaus G. Heumann Institute of Inorganic Chemistry and Analytical Chemistry Johannes Gutenberg-University Mainz, Germany

2 Important limitations of hyphenated techniques Accurate quantification is often not an easy task due to matrix effects, system instabilities etc. These techniques do not prevent and identify possible species transformations during the different analytical steps Lack of adequate validation methods

3 Validation of speciation analysis methods Difference between precision (statistical error) and accuracy (inaccurate values are related to systematical errors)

4 A series of letters to the Editor of the journal Analytical Chemistry were sent in 1999 Artifact formation of MeHg in sediments (March 1999) Concern over possible formation of MeHg artifacts during certain analytical procedures was first expressed at the 1996 Mercury as a Global Pollutant conference. Many laboratories now doubt that the CRMs they use are indeed properly certified. The findings on artifact formation are not sufficient to claim that MeHg results are overestimated. Methyl mercury revisited (September 1999) Recently, some of our European colleagues involved in the production and certification of environmental reference materials have declared that the potential for artifact MeHg formation during analysis is of little concern. We strongly disagree with this stance, because it undermines the scientist`s obligation to strive for the greatest accuracy possible. Who is correct? Can this be solved by letters to an analytical journal or should it be better solved by isotope dilution mass spectrometry?

5 The principle of isotope dilution mass spectrometry (IDMS) Sample Isotope diluted sample Spike Intensity Isotope 1 Isotope 2 Isotope 1 Isotope 2 Isotope 1 Isotope 2 N S = N Sp (h Sp 2 - R x h Sp1 ) / (R x h S 1 - h S2 ) R isotope ratio (only data to be measured) N S, Sp number of sample and spike atoms h 1,2 isotope abundance of reference and spike isotope

6 Experimental conditions for IDMS Only the isotope ratio of the isotope diluted sample must be measured and not an absolute amount of the analyte! Isotope ratio measurement is independent on: Matrix effects Signal drifts of the instrument Sample loss does not affect the result after the isotope dilution step has taken place (determination of recovery not necessary) However, detection methods with multi-element capability like ICP- MS have a high risk of wrong results by spectrometric interferences

7 HPLC/ICP-IDMS system for species-specific and species-unspecific spiking

8 The species-specific spiking mode Elemental species must be well defined by composition and structure, e.g. CrO 4 2-, IO 3 -, MeHg + Best spiking mode if isotope-labeled species is available (spiking prior to separation) Isotope-labeled species must usually be synthesized The isotopic composition is constant over the whole chromatographic peak (real-time determination) Isotope exchange between different species must be avoided

9 How complicated is synthesis of isotope-labeled species? Relatively simple for most inorganic species like iodate: HNO 3 /HClO 3 Na 129 I Na 129 IO 3 Not too complicated for organometallic species like MeHg + : 21 HgCl 2 + Me-Co Me 21 Hg + Me-Co = Methylcobalamin Usually too complicated or impossible for large biomolecules

10 Determination of inorganic iodine species in a mineral water sample determined by species-specific IC/ICP-IDMS Spike: 129 I - and 129 IO ng I/mL.3 ng I/mL Sum of species analysis 3.43 ng I/mL Total I by ICP-IDMS 3.44 ng I/mL

11 Commercially available spike compounds Institute for Reference Materials and Measurements (IRMM), Geel/Belgium: Isotopically labeled 22 HgMe + spike ISC Science, Gijón/Spain: 119 SnBu 3+ / 119 SnBu 2 2+ / 119 SnBu 3 + (mixed spike)

12 Chromatogram of a multi-species determination by species-specific GC/ICP-IDMS (mussel tissue, CRM 477) (N. Poperechna and K.G. Heumann) Intensity (cps) 2 Hg / 28 Pb 22 Hg / 26 Pb MeHg+ Me3Pb+ BuSn MeHg+ Me3Pb+ BuSn3+ Bu2Sn2+ Bu2Sn2+ Bu3Sn+ Bu3Sn+ Reference isotopes Spike isotopes Sn 12 Sn Retention time (min)

13 Results of multi-species determination by species-specific GC/ICP-IDMS in mussel tissue (CRM 477) and tuna fish (CRM 463) Species CRM 477 CRM 463 GC-ICP-IDMS Certified GC-ICP-IDMS Certified Me 3 Pb + (ng/g).34 ±.3.3 ±.2 * 4.1 ± ±.16 * MeHg + (µg/g).66 ± ± ±.15 BuSn 3+ (µg/g).93 ± ±.19 < Bu 2 Sn 2+ (µg/g).82 ±.3.78 ±.6 < Bu 3 Sn + (µg/g).85 ±.1.9 ±.8 < * Indicative value Excellent agreement between GC-ICP-IDMS and certified/indicative values

14 HPLC/ICP-IDMS system for species-specific and species-unspecific spiking

15 The species-unspecific spiking mode Necessary for all elemental species where exact composition and structure is not known, e.g. metal complexes of humic substances, proteins.. Addition of spike must be carried out after complete separation of species Spike may exist in any chemical form Signal intensity of the measured element must be independent on the species form (found for normal nebulizer systems like cross-flow, but not for ultrasonic nebulizer with membrane desolvator) The isotope ratio varies over the whole chromatographic peak (however, real-time concentrations are available)

16 Conversion of an isotope ratio chromatogram for a copper species into a mass flow chromatogram Isotope ratio chromatogram Mass flow chromatogram 65 Cu/ 63 Cu ratio spike isotope ratio Mass flow Cu [pg/s] Retention time [min] Retention time [min]

17 Distribution of heavy metals in SEC separated fractions of two different waste water samples from a sewage disposal plant by HPLC/ICP-IDMS In both waste water samples the distribution of different heavy metals is different Zn prefers a high molecular fraction, Cu interacts with most fractions and Mo only with a small fraction of medium molecular weight

18 For volatile elemental species GC/ICP-IDMS can be used in the species-specific and species-unspecific spiking mode

19 Example: Determination of monomethylmercury MeHg + by species-specific GC/ICP-IDMS Hg in environmental samples is present in different species MeHg + Me 2 Hg Biomethylation by algae and bacteria Hg Hg 2+ Air Soil and rocks

20 Hyphenated GC/atom spectrometric techniques for determination of MeHg + in water samples heating unit He + - valve capillary GC oven AFS AES atom fluorescence detector atom emission detector drying tube cold trap ICP-MS inductively coupled plasma MS Ethylation of mercury species by NaBEt 4 : alkylation and purging vessel MeHg + + NaBEt 4 MeEtHg Hg 2+ + NaBEt 4 Et 2 Hg

21 Gas chromatogram of a mercury species standard solution detected by AFS and ICP-MS Intensity [mv] Hg o MeHg + Hg 2+ GC/AFS: Quantification usually by external calibration Intensity [x 1 3 cps] Retention time [min] GC/ICP-MS: Additional information on isotope ratios Application of IDMS possible 21 Hg/ 22 Hg =.44 for all species

22 Characterization of a Me 21 Hg + spike solution by GC/ICP-MS Intensity (x 1 3 cps) Me 21 Hg Retention time (min) Spike isotope ratio 21 Hg/ 22 Hg = 6.53 (natural =.44) A species-pure monomethylmercury spike was obtained

23 MeHg + analysis in seawater by GC/ICP-IDMS using a Me 21 Hg + spike Intensity (x1 3 cps) Hg 21 Hg/ 22 Hg =1.65 MeHg + 21 Hg/ 22 Hg =4.45 Hg Hg/ 22 Hg = Retention time (min) Transformation of MeHg + into Hg o Transformation can only be identified by isotope-labelled species

24 Determination of MeHg + in a river water sample spiked with Me 21 Hg + prior to alkylation by NaBEt 4 and NaBPr 4 Intensity (1 3 cps) Hg o Ethylation 21 Hg/ 22 Hg = 1.69 MeHg + = 3.8±.1 pg/ml Hg Hg o Retention time (min) Propylation 21 Hg/ 22 Hg = 1.61 MeHg + = 3.6±.1 pg/ml Species transformation by ethylation but not by propylation However, in both cases identical results are obtained!

25 What can we learn from species-specific GC/ICP-IDMS of methylmercury? The use of isotope-labeled species identifies species transformations Even if species transformation takes place, accurate quantification is possible by speciesspecific GC/ICP-IDMS (if total mixture between sample and spike species has taken place prior to transformation) Species-specific ICP-IDMS can best be used for validation of analytical methods for elemental species!

26 Determination of MeHg + in environmental and biological samples by species-specific GC/ICP-IDMS CRM 58 (sediment) Sediment sample + Me 21 Hg + in diluted HNO 3 CRM 463 (tuna fish) Biolog. sample + Me 21 Hg + in TMAH Microwave extraction (5 min) Buffering at ph 4.8 and ethylation by NaBEt 4 In-situ extraction of MeEtHg by nonane (5 min) Certified value: (7.3 ± 3.4) ng g -1 Injection of nonane extract into GC 21 Hg/ 22 Hg isotope ratio measurement in separated mercury species (6 min) GC/ICP-IDMS: (72.6 ± 1.3) ng g -1 Certified value: (2.83 ±.15) µg g-1 GC/ICP-IDMS: (2.91 ±.7) µg g -1

27 Isotope exchange between different species must be avoided Is this precondition always fulfilled? Example: Determination of volatile halogenated hydrocarbons by GC/ICP-IDMS

28 Iodine isotope chromatogram by GC/ICP-MS of an 129 I-labeled ethyl iodide spike C 2 H 5 I 129 I 129 I/ 127 I = I-labeled iodide solution Addition of NaBEt 4 in MQ-water I Addition of acetate buffer Retention time (min) Extraction by nonane A species-pure ethyl iodide spike was obtained

29 Iodine isotope chromatogram by GC/ICP-MS of natural iodinated hydrocarbons spiked with 129 I-labeled ethyl iodide 6 Intensity (cps) 4 2 CH 3 I 127 I C 2 H 5 I 2-C 3 H 7 I 1-C 3 H 7 I CH 2 ClI CH 2 I I Retention time (min) Isotope exchange takes place with all iodinated species Species-specific IDMS not possible!

30 81 Br and 79 Br chromatograms of a 79 Br-labeled ethyl bromide spike Br Intensity (cps) B r chromatogram 79 Br Intensity (cps) Br chromatogram Retention time (min) Species-pure 79 Br-labeled C 2 H 5 Br spike could be synthesized by ethyl tosylate: NH 4 79 Br + C 2 H 5 Ts C 2 H 5 79 Br + NH 4 Ts

31 81 Br and 79 Br chromatograms of natural brominated hydrocarbons spiked with 79 Br-labeled ethyl bromide Br Intensity (cps) Br chromatogram C 2 H 5 Br CH 2 BrCl C 4 H 9 Br CHBr 3 79 Br Intensity (cps) C 2 H 5 Br 79 Br chromatogram CH 2 BrCl C 4 H 9 Br CHBr Retention time (min) No isotope exchange occurs between C 2 H 5 79 Br spike and other brominated hydrocarbons: Species-specific IDMS is possible! Bond strength: C - I < C - Br < C - Cl < C - F

32 Existing experience with the application of species-specific determination of elemental species by ICP-IDMS Element Isotope Alkylated compounds Inorganic compounds Reference Br 79 Br C 2 H 5 Br Br -, BrO 3 - Heumann Cr 53 Cr CrO 2-4 Kingston, Heumann Hg 21 Hg CH 3 Hg + Donard, Frech, Heumann I 129 I CH 3 I, C 3 H 7 I I -, IO 3 - Heumann Pb 26 Pb (CH 3 ) 3 Pb + Ebdon, Heumann Se 82 Se (CH 3 ) 2 Se, (CH 3 ) 2 Se 2, SeO 2-3, SeO 2-4 Heumann (CH 3 ) 3 Se + Se-methionine Garcia Alonso and Sanz-Medel Sn 117 Sn (C 4 H 9 ) 3 Sn +, (C 4 H 9 ) 2 Sn 2+ Garcia Alonso and Sanz-Medel Tl 23 Tl (CH 3 ) 2 Tl + Heumann

33 Is species-unspecific determination of elemental species necessary by GC/ICP-IDMS even if volatile compounds are usually well defined? If different species of the same element are to be determined species-unspecific spiking will avoid synthesis of all spike compounds

34 Schematic figure of the species-specific and speciesunspecific isotope dilution technique for sulfur speciation (J. Heilmann and K.G. Heumann) Sample + isotope-labeled analyte for species-specific spiking technique Unspiked sample for species-unspecific spiking technique GC-Injector GC capillary ICP-MS Continuous addition of a species-unspecific spike by gas cylinder or permeation tube Units used in the speciesunspecific spiking technique

35 Isotope chromatogram of species-unspecific GC/ICP-IDMS for sulfur species determination in a standard solution using 34 S-enriched dimethyldisulfide (J. Heilmann and K.G. Heumann) S CH 3 14 S S 34 S/ 32 S S S CH 3 CH For quantitative analysis spike flow must be calibrated Retention time [min]

36 Determination of MeHg + and inorganic Hg 2+ in biological materials by species-unspecific ETV/ICP-IDMS (I. Gelaude, F. Vanhaecke and R. Dams) Ar Water bath ICP-MS permeation tube 2 Hg (spike) Fractionated evaporation of MeHg + and inorg. Hg by a twostep ETV temperature program Isotope dilution of all Hg species by isotope-enriched 2 Hg Permeation tube guarantees continuous spike flow into the time-delayed analytes evaporated by ETV

37 Determination of MeHg + and inorganic Hg 2+ in the certified reference material TORT-2 (lobster) by species-unspecific ETV/ICP-IDMS (I. Gelaude, F. Vanhaecke and R. Dams) Method Concentration (µg Hg/g) MeHg + Inorg. Hg Total Hg ETV/ICP-IDMS Certified.16 ±.2.12 ±.6.28 ±.5.15 ± ±.6

38 Summary Elemental speciation can be carried out by species-specific and species-unspecific ICP-IDMS applying coupling with HPLC, GC and CE Online coupling of ICP-MS with GC and HPLC is technically easy but more difficult for CE Isotope-labeled species must normally be synthesized for speciesspecific IDMS Species-specific IDMS is an ideal tool for method validation Hyphenated techniques in combination with IDMS are the only possibility to obtain real-time concentrations of species However, spectrometric interferences in ICP-MS can always affect the results Because of a lack of alternatives GC/ICP-IDMS and HPLC/ICP-IDMS are the most powerful methods for quantitative elemental speciation and therefore also suitable for routine analysis

39 Many thanks for your attention and.... I wish you all successful investigations in your work on elemental speciation and many interesting information on this topic during the following seminars!