Analytical Methods for Non-Traditional Isotopes (original) (raw)
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Isotope abundance ratio measurements by inductively coupled plasma-sector field mass spectrometry
Journal of Analytical Atomic Spectrometry, 2012
This tutorial reviews fundamental aspects of isotope abundance ratio measurement by inductively coupled plasma-sector field mass spectrometry (ICP-SFMS). After a synopsis of the scope of isotope abundance ratio measurement and a summary introduction to the factors affecting precision and accuracy, attention is turned to noise sources. Detailed theory behind Poisson or counting statistics and plasma flicker noise components is given, since much of the observed imprecision can be attributed to these sources. Using single collector instruments, ion beams from different isotopes are sampled in rapid sequence, and so ratioing of the signals will be subject to fluctuations derived from intensity variations, i.e., flicker noise. It is demonstrated that flicker noise can, under specified circumstances, become the limiting factor for the attainable precision. Furthermore, the practice of partitioning dwell times, ostensibly to optimize precision based on isotopic abundances and assumed Poisson statistics, is shown to be flawed and actually requires accounting for flicker noise. In addition to random uncertainty, various offset factors may contribute to systematic error in measured isotope abundance ratios. Two of these, namely mass scale shift and spectral interferences are ameliorated using ICP-SFMS. The former is eliminated when operating under conditions providing flat-topped peaks, such that the minor drift in mass calibration typical of the technique becomes inconsequential and the intensity remains the same. Isotope abundance ratio measurements are subject to three further important offset factors. First is abundance sensitivity, which quantifies the extent of peak tailing to neighboring masses and can present a considerable source of offset. Second is mass bias, resulting from the fact that all sector field devices exhibit increasing sensitivity with ion mass, and various empirical methods used to correct for this effect are compared and contrasted. Third is detector dead time, which affects mass spectrometers equipped with ion counting systems. Although a well-understood phenomenon, all current methods for determining the dead time on the basis of experimentally measured isotope abundance ratios are likely to yield biased estimates. Finally, the capabilities of ICP-SFMS for the determination of isotope abundance ratios are placed in perspective by making a brief comparison with other techniques.
Journal of the American Society for Mass Spectrometry, 1995
Departm ent of C hern is trv . Ind ia n a L ruver-it v. Bloomi n g ton, In d ia n a . USA Iso tope ratios and ab undance se nsitivitics have been de termined wi th an ind uctive ly co upled p las ma -time-of-fligh t mass spec tro me te r (lCP-TO FMS). Ab undance sensitivi ties are at least in the 10" ran ge for low abundance ion s that pr eced e high ab un dance ions . Three method s of d et ection for isotope-ratio me asurem en t have been co m pa red. The three systems inv ol ve ga ted d et ection foll o wed by ana log int egr a tion , ana log averaging , or ion co un ting . Ga ted ion coun ting offers excellent p recision-be tw een 0.64 and 1.000(' relative standard d ev iati on (RSD). Th ese va lues ap proach those p redicted from co unti ng sta tistics a nd are co mparable to th ose reported for othe r ind uct ively cou p led pla sm a -mass spec tro me try (ICP-M S) instruments. In add ition, a g rea ter n umber of accum u lated co un ts or lon ger ana lys is tim es wo uld affo rd p recision s of 0.17r w ith sta ble ga ting elec tronics. The accu racy of the coun ting me tho d is in the 1-lO r o;. ra nge if no correc tio n fo r mass bias is performed . Ho wever, th is io n co un ting me tho d suffers fro m a lim ited d yna mic ra nge d ue to pulse p ileup. Co ns tan t-frac tio n d iscr imination ga ted in teg ra tio n a nd commercia l bo xcar a verager tech n iques offer a br oad e r d ynamic ra nge beca use of thei r ana log na ture, b ut the attai na ble RSD va lues are limi ted by d rift in th e d et ection syste ms and by the methods employed to ca lcu la te an accurat e rati o. Overa ll, mass bias in the IC P-TO F:Y1S is more se ve re than p re vio u s work in ICP-MS due p rim ar ily to d et ect ion sys tem bias. {j It/II S"l A"lll~~Spcctrom 199!J , 6. 921! -927J I nd uctively co up led p las ma mass spec tro met ry (lCP-MS) w ith quadrupo le ma ss a nalyzers has been st udied an d tested extensive ly fo r iso tope-ratio an al ysis in a va riety of field s th at inc lud e ge ochemi ca l and environ me n tal app lica tions [I , 2]. Ho w ev e r. recen t w ork er s have pointed out th at its Widespread u se has been inh ibi ted by its lack of p recis ion [O.1 -1.0C:! relative sta nd a rd deviation ([~SD)] when compared to techniques such as th ermal ioniza tion mass spec tro rn etry (TIMS), which offers p recis ion levels bette r tha n O.005(i( RSD . The ma in di sad va ntage to TIMS is the lon g ana lys is tim es (u p to 1 day) requ ired to ac hieve the bette r precision . Also, to o bta in the h ig hes t precis ion W.I r;i·) on IC P-MS q uad ru pole instruments req ui res rapid peak-hoppi ng [ 1, -l L so the nu m ber o i iso to pes and e leme nts that ca n be measu red in a s ing ll' run of fixed tim e is limited . O bvio uslv. it wou ld be advantageous to increase p rec ision in ro ut ine ICI' -\ '1S; th e sensi tivity o f the me thod unable-, shorter ana lys is times a nd consequently higher sa m p le through p u t tha n a lte rna tive method s suc h as '["[1\.15 [!1]. T ypica l l y the RSD values obtai ned in ICP -:Y1S have been 2-3 time s high e r th an wou ld be e xpec ted from co un ting sta tistics A dd r ess reprin t rL' qU l'~t~to Dr. l"H \ \ 1. f l ll' Hit' , I) l ' p ,u t m CI1I l d C hern istrv . Ind ia n a U n i\ "l'rs il} , Bl p PIl1i n h t l )J1 , If' .,; -+ 7--+0 :;. (c ) 1995 Am e r ica n Snc il't\· for \ 1. 1.... Sp c'drcmwl n 1044 -0305 /95 /S 9.50 SSO I 1044-IJ:itJ5((I; )004 ,";·H [3]. Inst rument ins ta bility ha s been th e ma jor ca use of the poorer precision [3, 6, i].
Advances in isotope ratio mass spectrometry and required isotope reference materials
Mass spectrometry (Tokyo, Japan), 2013
The article gives a condensed version of the keynote lecture held at the International Mass Spectrometry Conference 2012 in Kyoto. Starting with some examples for isotope research the key requirements for metrologically valid procedures enabling traceable and comparable isotope data are discussed. Of course multi-collector mass spectrometers are required which offer sufficiently high isotope ratio precision for the intended research work. Following this, corrections for mass fractionation/discrimination, validation of the analytical procedure including chemical sample preparation and complete uncertainty budgets are the most important issues for obtaining a metrologically valid procedure for isotope ratio determination. Only the application of such metrologically valid procedures enables the generation of traceable and comparable isotope data. To realize this suitable isotope and/or δ-reference materials are required, which currently are not sufficiently available for most isotope s...
Journal of Analytical Atomic Spectrometry, 2004
In this work, several contributing factors to the observed mass bias in inductively coupled plasma mass spectrometry (ICP-MS) have been identified. Analyses of the isotopic compositions of B deposited on sampler and skimmer cones demonstrate enrichment of 10 B on the former and 11 B on the latter. Grounding the capacitive discharge system to enhance sensitivity also magnified the level of 11 B enrichment on the skimmer cone more than four-fold. This supersonic expansion of the ion beam behind the sampler is confirmed to be an important source of mass bias. Isotopic analyses of the Fe, Zn and Tl leached from used extraction lenses yielded a linear relationship between the levels of lighter isotope depletion and mass ratio. Although consistent with the space-charge effect, the fact that isotopically-heavy deposits were found demonstrates that the ion beam diverges into a relatively wide solid angle in the field-free region behind the skimmer. This severely impairs transmission of, in particular, the lighter isotopes. For a wide range of elements (Li, B, Fe, Ni, Cu, Sb, Ce, Hf and Re), the magnitude of the mass bias was found to be affected by the sample gas flow rate, as well as the distance between the sampler and the end of the torch, i.e., the sampling depth, employed in the Neptune multi-collector ICP-MS instrument. Mathematical analysis of the profiles of intensity variations as a function of these instrumental parameters revealed that the response peaks closer to the torch for the heavier isotopes of all studied elements. Owing to this spatial non-coincidence, tuning for maximum intensity on either isotope will result in sampling from a region where even slight plasma instabilities will be translated into substantial variations in mass bias. Therefore, in-plasma processes also contribute to the degree and temporal stability of mass bias. In light of these findings, recommendations for optimizing multi-collector ICP-MS with respect to obtaining the highest possible precision are presented.
Sources of Uncertainty in Isotope Ratio Measurements by Inductively Coupled Plasma Mass Spectrometry
Analytical Chemistry, 2001
A model is presented describing the effects of dead time and mass bias correction factor uncertainties, flicker noise, and counting statistics on isotope ratio measurement precision using inductively coupled plasma mass spectrometry (ICPMS) with a single collector. Noise spectral analysis is exploited to enable estimation of the flicker noise parameters. For the instrument used, the flicker noise component exhibited a fairly weak frequency (f) dependence (∝ f-0.33(0.12), but was directly proportional to the total number of counts, Q. As white noise, determined by counting statistics, is given by Q 0.5 , the isotope ratio measurement uncertainties will actually cease to improve when Q exceeds a certain threshold. This would suggest that flicker noise could become the limiting factor for the precision with which isotope ratios can be determined by ICPMS. However, under most experimental conditions, uncertainties associated with mass discrimination and dead time correction factors are decisive. For ratios up to ∼22 (115 In/ 113 In), optimum major isotope count rates are generally below 0.3 MHz, for which precision in the mass discrimination factor is limiting. The model derived could be used as a starting point for determining optimum conditions and understanding the limitations of single-collector ICPMS for precise isotope ratio measurements.
Measurement of light stable isotope ratios by SIMS
International Journal of Mass Spectrometry, 1998
Mass bias occurring during analysis of the light stable isotopes of oxygen, carbon, and sulfur in geological materials by secondary ionization mass spectrometry has been investigated. The effects of instrumental parameters (primary ion beam, secondary ion energy, and polarity) were evaluated by measuring sulfur isotope ratios in conductive sulfide minerals. The role of analyte chemical composition (matrix effect) on mass bias was investigated in sulfides (sulfur), silicates and oxides (oxygen), and carbonates (oxygen and carbon). For oxygen and carbon, various correlations between mass bias and matrix parameters have been identified. The application of several empirical models for prediction of oxygen isotopic mass bias indicates that for silicates, depending on mineral composition, bias can be predicted with an accuracy that is typically within two times that of the precision. However, extension of these models to other matrices has proved problematic, indicating that additional factors are important. (Int J Mass Spectrom 178 (1998) 81-112)
International Journal of Mass Spectrometry, 2001
Multiple-collector inductively coupled plasma mass spectrometry has been used for the precise measurement of variations in the isotopic composition of Mg in a range of materials. The contributions of C™C, C™N, and Mg™H molecular species to the mass spectrum in the Mg mass region are minimised. Variations in sample 26 Mg/ 24 Mg and 25 Mg/ 24 Mg ratios are expressed as ␦ 26 Mg and ␦ 25 Mg units, which are deviations in parts per 10 3 from the same ratio in the SRM 980 Mg standard. The long-term repeatability of the 26 Mg/ 24 Mg and 25 Mg/ 24 Mg ratios of a sample Mg solution relative to the SRM 980 Mg isotope standard are 0.12‰ and 0.06‰, respectively, at 95% confidence. The addition of Na, Al, and Ca in a solution of Mg having a known isotopic composition induces 0.2‰-1‰ increase of ␦ 26 Mg. This chemical bias is a result of a mass-dependent process and is observed to be greater with Ca than Na. Isobaric interference from doubly charged 48 Ca ions on mass 24 is observed to be significant when [Ca]/[Mg] Ն 0.5. The results obtained on nine terrestrial material show a variation of Mg-isotopes of 4‰ in ␦ 26 Mg. When plotted in a three-isotope diagram, all the data fall on a single mass fractionation line. The excess of 26 Mg has been determined by the deviation from that mass-dependent relationship, and its long-term repeatability is 0.06‰ at 95% confidence. (Int J Mass Spectrom 208 (2001) 89 -98)
International Journal of Mass Spectrometry, 1998
The combination of a glow discharge ionization source with a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer provides several advantages for elemental analysis; among these is an isotope ratio measured at the highest possible mass resolving power. In this report, lead ratios (206 Pb/ 208 Pb, 207 Pb/ 208 Pb, and 207 Pb/ 206 Pb) in a sample composed of 10% lead oxide, 10% mercuric oxide, and 80% silver powder (w/w) were observed to be precise to better than Ϯ3% relative standard deviation (RSD at 1) in a commercial FTICR instrument that used a conventional elongated cell. Upon incorporation of a cell whose excitation voltages approximated those in an ideal cell of infinite length, the precision improved to better than Ϯ0.4% RSD. The isotopic bias between measured and known values was also evaluated. Using a cathode composed of 5% National Institute of Standards and Technology (NIST) SRM 987 SrCO 3 in silver powder, biases that averaged less than 2.5% were detected using the cell of improved design. These results compared favorably with those obtained using a commercial magnetic sector glow discharge mass spectrometer, although it is still unclear how glow discharge mass spectrometry biases vary in general.