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Introduction The use of qNMR for purity measurement has been steadily growing in recent years. The assessment of the purity of calibration materials and.

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Presentation on theme: "Introduction The use of qNMR for purity measurement has been steadily growing in recent years. The assessment of the purity of calibration materials and."— Presentation transcript:

1 Introduction The use of qNMR for purity measurement has been steadily growing in recent years. The assessment of the purity of calibration materials and reference standards requires a detailed understanding of the uncertainties associated with the measurements on which the analysis is based. In addition, the measurement results should be traceable directly to SI units and it is here that qNMR offers advantages over chromatographic techniques. Direct purity determination using qNMR is normally achieved by adding an internal standard of known purity to the analyte. In cases where an internal standard is undesirable, purity by qNMR is achieved using external standards, however this method introduces additional sources of errors which need to be quantified before they can be applied to reference materials. The aim of this project is to optimize conditions on an Avance III 600 spectrometer equipped with a 5 mm PATXI 1H/D-13C/15N Z-gradients probe to give accurate purity assessment by external standards and to derive a full uncertainty budget from the results. Experiments to quantify three of these potential sources of uncertainty are described in this poster. Sample Depth Whilst the dimensions of the NMR tube are clearly vital to the externally standardised assay, it is often assumed that the NMR solution outside the coils has no impact on the spectrum. To a single NMR tube, 4cm depth of a stock solution of dioxane was added and 5 replicate analysis performed. Further stock solution was added to give a depth of 5cm and then 6cm with replicates performed in each case. A final set of replicates was performed with the 6cm deep sample displaced upwards by 3mm in the depth gauge. Figure 1 Figure 1 shows significant differences between sample heights with about 2% variation in the mean integrations between volumes. This equates to 0.1% per mm over the normal range of sample depth and thus the reproducibility of sample depth will impact on the uncertainty. Further experiments are required to determine how accurately this can be controlled in practise. Optimization and uncertainty budgets of externally standardised NMR purity assays Fahmina Fardus 1,2 John Warren 1 Adam Le Gresley 2 Jean-Marie Peron 2 1- LGC Ltd, Queens Rd, Teddington TW11 0LY 2- Kingston University, Penrhyn Rd, Kingston-Upon-Thames KT1 2EE Environmental Factors It is well known that NMR instruments are very sensitive to building vibrations which cause deleterious effects on the spectra. For quantitative work this poses a problem as the artefacts around the base of the resonances may interfere with the accuracy and reproducibility of the NMR measurements. The 600 MHz NMR facility at Kingston University sits on an anti vibration system with dampening equipment integrated into the magnet legs. The laboratory’s air conditioning unit cycles on and off during the day and so the impact of this on the uncertainty budget has been investigated. A series of 10 replicate experiments were performed during times with the air conditioning either exclusively on or exclusively off. Figure 4a shows the visual difference of the base of the resonance peaks when two spectra are overlaid, the red peak shows the air con off and the blue shows it on. Figure 4b shows the variation of the absolute integral plotted against number of replicates. As it can be seen the standard deviation of replicates is large compared to when the air con is off shown in red in Figure 4. Figure 4 a)b) Clearly the air conditioning is contributing a significant burden on the uncertainty budget with an increase in the relative standard deviation from 0.09% without air con to 0.48% with it on as shown in Table 2. Table 2 Conclusion The three factors discussed in this poster are all shown to impact on the uncertainty budget and highlight where steps can be taken to minimize their impact. These factors are just some of the experimental parameters being investigated in this study that will allow a comprehensive understanding to the uncertainties associated with qNMR and improve it’s methodology. Receiver Gain Compensation The receiver gain (RG) is a measure of the amplification of the analogue signal coming from the NMR probe but has been shown to deviate significantly from linearity 1. A series of experiments were run to calibrate a range of RG values and derive an uncertainty budget. Solutions at different concentrations ( mg/g) were prepared and run at RG series 2 n from 1 to 256 each sample was analysed 5 times at each receiver gain (excluding those that overloaded ADC). Figure 2 shows the integration results from the replicate runs normalised against the receiver gain. As anticipated, each receiver gain gave a consistent but distinct deviation from mean. Figure 2 Compensation factors for the series of receiver gains have been determined along with the associated uncertainties. Figure 3 compares mean normalised integrals, both before (red) and after (blue) calibration along with the standard deviations of the measurements. Figure 3 The expanded uncertainties were calculated for three cases: RG uncorrected – optimal but different RGs for reference and analyte – no compensation RG corrected - optimal but different RGs for reference and analyte - compensated Same RG - non optimal but identical RGs for reference and analyte Table 1 The expanded uncertainties for %w/w purity in are given at the 95% confidence level in Table 1. 1 Huaping Mo et al, Magn. Reson. Chem. 2010, 48, 235–238


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