Isotope dilution optimal

Figure 3.1 Optimization of spike addition for the isotope dilution technique y is the error amplification factor.

Example. Estimate the optimal amount of Rb spike to add into the sample in the above Rb isotope dilution example from Table 11.1. The amount of natural rock is still 0.03893 g. We also have a rough estimate of the sample concentration 45 ppm from a not-so-precise measurement method. 

Light-up, ion lens tuning, mass calibration, optimization of data acquisition parameters, determination of Cd in water by isotope dilution... 

Before performing the actual isotope dilution analysis it is important to optimize the instrument to ensure precise and accurate data. The following aspects must be considered. 

Knust U, Strowitzki T, Spiegelhalder B, Bartsch H, Owen RW (2007) Optimization of an isotope dilution gas chromatography/mass spectrometry method for the detection of endogenous estrogen metabolites in urine samples. Rapid Commun Mass Spectrom 21 2245-2254... 

Propagation of errors using isotope dilution ICP-MS has been considered to determine how to optimize the measurements [201]. Comparison of analysis results from external calibration versus isotope dilution can be used to assess the quality of external calibration results and the effectiveness of internal standards with external calibration [202,203]. Because isotope ratio precision depends on the total ion count rate, the use of high-efficiency sample introduction to generate larger signals can improve isotope ratio precision and, therefore, analysis precision [204]. 

Off-line dicarbamate solvent extraction and ICP-MS analysis [317] provided part-per-trillion detection limits Cd (0.2 ppt), Co (0.3 ppt), Cu (3 ppt), Fe (21 ppt), Ni (2 ppt), Pb (0.5 ppt), and Zn (2 ppt). Off-line matrix removal and preconcentration using cellulose-immobilized ethylenediaminetetraacetic acid (EDTA) have also been reported [318]. Transition metals and rare earth elements were preconcentrated and separated from the matrix using on-line ion chromatography with a NTA chelating resin [319]. Isotope-dilution-based concentration measurement has also been used after matrix separation with a Chelex ion-exchange resin [320]. The pH, flow rate, resin volume, elution volume, and time required for isotope equilibration were optimized. A controlled-pore glass immobilized iminodiacetate based automated on-line matrix separation system has also been described [321]. Recoveries for most metals were between 62% and 113%. 

C. Pin, B. Le Fevre, S. Joannon, Isotope dilution and plasma source mass spectrometry An optimal combination for high precision analysis of trace elements. Can. 

Quantification of aroma-impact components by isotope dilution assays (IDA) was introduced in food flavor research by Schieberle and Grosch (1987), when trying to take into account losses of analytes due to isolation procedures. The labeled compounds have to be synthesized, the suitable fragments have to be chosen, and calibration has to be effected. A quantitative determination of ppb levels of 3-damascenone (Section 5,D.38) in foods, particularly in roasted coffee (powder and brew), was developed by Sen et al. (1991a). Semmelroch et al. (1995) quantified the potent odorants in roasted coffee by IDA. Hawthorne et al. (1992) directly determined caffeine concentration in coffee beverages with reproducibility of about 5 % using solid-phase microextraction combined with IDA. Blank et al. (1999) applied this combined method to potent coffee odorants and found it to be a rapid and accurate quantification method. They also concluded that the efficiency of IDA could be improved by optimizing the MS conditions. 

CHOICE OF OPTIMAL CONDITIONS FOR ISOTOPE DILUTION BY SELECTING APPROPRIATE SAMPLE TO SPIKE MIXING RATIOS... 

REMARKS CONCERNING OPTIMAL ISOTOPE DILUTION WORK... 

Another successful project (coordinated by the University of Plymouth, Department of Environmental Sciences, United Kingdom) was aimed at developing a novel methodology for the determination of organotins employing isotope dilution high performance hquid chromatography inductively coupled plasma mass spectrometry (ID-HPLC-ICP-MS) [100]. The parameters for isotope dilution analysis have been studied and optimized and isotopically enriched Sn has been prepared for this purpose. The method was successfully used for the certification of TBT in CRMs 462 and 477. 

Hoelzl, R., Hoelzl, C., Kotz, L. and Fabry, L., The optimal amount of isotopic spike solution for ultratrace analysis by isotope-dilution mass spectrometry, Accred. Qual. Assur., 3, 185-188 (1998). 

In tests devised to determine the amount of racemization to be expected in peptide syntheses in which the carboxyl component is a di-or higher peptide, this method ranks below the racemization-resistant azide procedure but above almost all other standard methods. Using a very sensitive and accurate isotope dilution assay, 1 % racemization was observed in the formation of the Anderson test peptide (Z-Gly-Phe-Gly-OEt) and 7% racemization was observed in the formation of the Young test peptide (Bz-Lou-Gly-OEt) under optimized conditions. (The Young test was designed to exaggerate racemization problems, thus permitting more accurate studies of the effects of reaction condition variations.)... 

The same can be said about direct isotope dilution in solid materials, where careful ETV program optimization will be needed again in order to achieve a proper sample spike (normally added as a solution) isotopic equilibration. In this regard, the use of slurry sampling might help achieving complete and rapid isotopic equilibration, as reported by Li and Jiang [33] for the determination of Cr, Cd, and Pb in polystyrene (PS) and polyvinyl chloride (PVC) samples. 

Rychlik, M., Englert, K., Kapfer, S., and Kirchhoff, E., 2007. Folate contents of legumes determined by optimized enzyme treatment and stable isotope dilution assays. Journal of Food Composition and Analysis. 20 411-419. 

This will often require the isotopic dilution of P-y-ATP as purchased from NEN-Dupont or Amersham 3000 Ci/mmol ATP is generally 3-5 lM, whereas kinases may require 10- to 100-fold higher ATP concentration for a full activity An optimal resolution of phosphoamino acids can be achieved by two-dimensional electrophoresis (pH 1 9, then pH 3 5) The protocol is similar but in our hands, it did not prove to be required for the identification of the type of phosphorylated ammo acid... 

For optimization of the spike addition, it is necessary to know the approximate element concentration in the sample. As the precision of results is not significantly influenced over a wide range of analyte to spike mixing ratios, preanalysis via a semiquantitative method is typically sufficient. If the expected amount of analyte is very low, it can also be advantageous to work with a distinct excess of spike to make chemical isolation of the isotope-diluted sample easier. This means that conditions to the left of the minimum in Figures 8.5 and 8.6 are used. Under extreme conditions of analyte to spike ratio, not only the enhanced error multiplication factor has to be taken into account, but also the deteriorating... 

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