Using high-performance ¹H NMR (HP-qNMR®) for the certification of organic reference materials under accreditation guidelines--describing the overall process with focus on homogeneity and stability assessment

Coaxial-Tube Quantitative Nuclear Magnetic Resonance for Various Solutions Using SI-Traceable Concentration References

Quantitative nuclear magnetic resonance (qNMR) is an accepted method for determining analyte concentrations using quantitative substances in one spectrum. Conventional qNMR is performed using a mixture of analytes and reference substances. In coaxial-tube NMR, two tubes are used as different solutions, similar to normal NMR spectra. Currently, coaxial tubes with various diameters are available; however, coaxial-tube qNMR is limited, and a general analytical protocol is yet to be proposed. In this study, we established an effective volume ratio (EVR) measurement method using the weight density and qNMR methods. Various analyte concentrations were determined using coaxial-tube qNMR and an SI-traceable reagent. The EVR required for the qNMR concentration calculation was determined using a coefficient of variation (CV) of <1% for an inner tube of ϕ 3 mm or less. The peak integral of each substance was correlated with the effective volume, depending on the abundance of the tube and matched 1H in the solution. The T1 relaxation times differed depending on the substructure, and the T1 values of the formate and OH groups varied for each tube set. Thus, each partial structural characteristic of the peak must be understood before qNMR is performed. The concentrations of various substances, including hygroscopic substances, were determined using coaxial-tube qNMR. Coaxial tubes eliminate the need to mix the analyte with the reference substance; thus, we can quantify the analyte without causing pH and structural changes caused by other mixtures and reuse the analyte for other test systems.

Accurate Determination of Trace Water in Organic Solution by Quantitative Nuclear Magnetic Resonance

A new method accompanied by a derived equation for accurate determination of trace water was developed by using quantitative 1H nuclear magnetic resonance (qNMR) spectroscopy. Given that the response for each chemically distinct moiety is uniformly proportional to the number of the corresponding resonant nuclei within the analyte, it is practicable to directly quantify the water content via its proton number using qNMR. In this study, three water standards with known water contents (e.g., 10.02, 1.006, and 0.103 mg/g), which were accurately determined by a well-established Coulometric Karl Fischer (CKF) titration method, were measured by using the developed qNMR method. An excellent agreement between the results from these two methods was obtained. Then, the water content of Sudan I was determined by high-field NMR (HF-NMR) spectroscopy, and the water contents of acetone and bioethanol were measured by low-field NMR (LF-NMR) spectroscopy. These results were compared with the water content measured by the CKF method to confirm the applicability of the established qNMR method. The developed method can eliminate the influences of environmental humidity and background water in the solvent; subsequently, the results calculated by the derived equation were comparable to the nominal values. Under the optimal conditions, the limit of quantitation of this method was as low as 6.7 μg. The recommended sample sizes for practical samples with various water contents (e.g., 10.02, 1.006, and 0.103 mg/g) were determined to be 5, 50, and 60 mg, respectively, which are much smaller than those required for the CKF method. The new method has a static and stable process without any side reactions, and the traceability to the SI unit can be directly achieved through the NMR internal standard. This method overcomes the limitations of the CKF method, especially for measuring methanol-insoluble substances.

Hygroscopic Tendencies of Substances Used as Calibrants for Quantitative Nuclear Magnetic Resonance Spectroscopy

Atmospheric moisture can contaminate calibrants for quantitative nuclear magnetic resonance (qNMR) spectroscopy and cause systematic errors in qNMR measurements. Therefore, coulometric Karl Fischer (CKF) titration was used to evaluate the hygroscopic tendencies of several organic compounds that are commonly used as calibrants for qNMR spectroscopy: benzoic acid, dimethyl sulfone, 1,3,5-trimethoxybenzene, acetanilide, dimethyl terephthalate, and 1,2,4,5-tetramethylbenzene. Samples were placed in a sealed humidity chamber at 100% relative humidity (RH) and a temperature of 295.4 ± 0.9 K. Over the course of months, portions of each sample were analyzed by CKF titration. All the compounds except dimethyl sulfone were resistant to changes in water content and thus are good choices for qNMR experiments. In contrast, dimethyl sulfone absorbed about 25 mass % of water over 5 weeks at 100% RH; such behavior could compromise qNMR experiments under certain conditions.

Quantification of magic angle spinning dynamic nuclear polarization NMR spectra

Dynamic nuclear polarization (DNP) allows to dramatically enhance the sensitivity of magic angle spinning nuclear magnetic resonance (MAS NMR). DNP experiments usually rely on the detection of low-γ nuclei hyperpolarized from ¹H with the use of cross polarization (CP), which assures more efficient signal enhancement. However, CP is usually not quantitative. Here we determine the quantification performance of three different approaches used in MAS NMR, (conventional CP, variable contact time CP, and multiple-contact CP) under DNP conditions, and we show that absolute quantification in MAS DNP NMR is possible, with errors below 10%.

Application of Quantitative NMR Spectroscopy to the Quality Evaluation of Diclofenac Gargles as Hospital Preparations

Hospital preparations are frequently prepared in Japanese hospitals when ready-made formulations to meet patients' needs are unavailable. Although the quality of hospital preparations have to be ensured for efficacy and safety, such quality evaluation tends to be insufficient mainly due to lack of manpower and experimental environments in hospitals. In this paper, we investigated the applicability of quantitative (q)NMR spectroscopy to the quality control of diclofenac gargles as examples of hospital preparations, as it has various merits for the quantitative analysis of mixtures in solutions. Diclofenac gargles are composed of diclofenac, tranexamic acid, and lidocaine, and are used for the pain relief of stomatitis induced by cancer chemotherapy. Aliquots of the gargles, which were prepared five times, were mixed with dimethylsulfone as an internal standard, followed by qNMR measurements. Water signal suppression was achieved using a pulse program, water suppression enhanced through T₁ effects, because the pulse program was superior to other ones such as presaturation and one-dimensional nuclear Overhauser effect spectroscopy in terms of quantitativeness. Concentrations of the three medicinal ingredients were simultaneously determined based on the signals selected by considering the spectral separation and the quantitativeness. Consequently, the gargles were found to be prepared with constant quality, and were stable at room temperature for at least four weeks. qNMR is considered to be potentially useful for the quality control of various hospital preparations because of minimal sample pretreatments, lack of need of calibration curves, and its comprehensive detection abilities.

Quantitative NMR as a Versatile Tool for the Reference Material Preparation

The assessment of primary calibrator purity is critical for establishing traceability to the International System of Units (SI). Recently, quantitative nuclear magnetic resonance (qNMR) has been used as a purity determination method for reference material development, and many related measurement techniques have been designed to acquire accurate and reliable results. This review introduces the recent advances in these techniques (including multidimensional methods), focusing on the application of qNMR to reference material preparation.

Quantitative 1 H nuclear magnetic resonance (qHNMR) methods for accurate purity determination of glucosinolates isolated from Isatis indigotica roots

INTRODUCTION

Glucosinolates (1-5) are important secondary metabolites found in Isatis indigotica roots. Due to their high hydrophilic and ionic nature, purified glucosinolates often contain salt impurities and moisture. Accurate assessment of their purities is important for glucosinolates being utilised as chemical markers.

OBJECTIVE

To develop and validate quantitative proton (¹ H) nuclear magnetic resonance (qHNMR) methods for purity assessments of aliphatic and indole glucosinolates (1-5).

METHOD

Several NMR parameters such as pulse program, relaxation time, and delay time were optimised. Three qHNMR methods were developed using gluconapin (3), neoglucobrassicin (4), and sinigrin (5) for method validation and with maleic acid as internal standard.

RESULTS

The quantification was based on the integrated area ratios of an olefinic proton (H-4 for 1-3; H-6 for 4; and H-3 for 5) of the side chain from glucosinolates relative to the olefinic proton from the internal standard using deuterated water (D₂ O) as the solvent. The qHNMR methods were successfully applied for purity assessments of four aliphatic glucosinolates (1-3 and 5: progoitrin, epiprogoitrin, gluconapin, and sinigrin), and an indole glucosinolate (4: neoglucobrassicin).

CONCLUSION

The purity of glucosinolates isolated from I. indigotica and commercial sinigrin was accurately assessed using the developed qHNMR method. The qHNMR provides a reliable and superior means to determine the purity of glucosinolates.

Hydrostibination of Alkynes: A Radical Mechanism*

The addition of Sb-H bonds to alkynes was reported recently as a new hydroelementation reaction that exclusively yields anti-Markovnikov Z-olefins from terminal acetylenes. We examine four possible mechanisms that are consistent with the observed stereochemical and regiochemical outcomes. A comprehensive analysis of solvent, substituent, isotope, additive, and temperature effects on hydrostibination reaction rates definitively refutes three ionic mechanisms involving closed-shell charged intermediates. Instead the data support a fourth pathway featuring open-shell neutral intermediates. Density-functional theory (DFT) calculations are consistent with this model, predicting an activation barrier that is in agreement with the experimental value (Eyring analysis) and a rate limiting step that is congruent with the experimental kinetic isotope effect. We therefore conclude that hydrostibination of arylacetylenes is initiated by the generation of stibinyl radicals, which then participate in a cycle featuring Sb^II and Sb^III intermediates to yield the observed Z-olefins as products. This mechanistic understanding will enable rational evolution of hydrostibination as a synthetic methodology.

Purity determination of pyributicarb by internal standard correction-high-performance liquid chromatography-quantitative nuclear magnetic resonance

An internal standard correction-high-performance liquid chromatography-quantitative nuclear magnetic resonance (ISC-HPLC-qNMR) procedure was established as a reliable quantitative method for complex organic compounds with low purity in order to solve the risk of qNMR inaccuracy because of insufficient resolution of impurity peaks from the selected quantitative peak. This method collects a small quantity of target analyte from low-purity organics by LC. After drying and re-dissolving in deuterated solvent containing internal standard, the solution was analyzed by ¹H NMR and HPLC. Another solution prepared by accurately weighing unpurified low-purity substance and internal standard was analyzed by HPLC. Based on the theoretical derivation derived from the Beer-Lambert law, using the ratio of the HPLC peak areas of two solutions as correction, the purity was then calculated without the same reference as target analyte. Compared to previous methods with similar selectivity and accuracy, it has advantages such as a less purified sample is required, time for lyophilization is reduced by half, and sample preparation is more controllable. The proposed method was verified by analysis of a suite of six commercially available, high-purity compounds, and the difference of results between it and direct qNMR was within 0.1%. The result of pyributicarb using ISC-HPLC-qNMR was 97.6% (U = 0.5%; k = 2), and the reference value was 97.61% (U = 0.22%; k = 2). The results demonstrate that the proposed method provides a new way for reference material producers to calibrate lower-purity organics and has the potential advantage of accurate quantification of lower-purity organics.

2 H SOLCOR: A novel tool for reducing volume variation as a source of error in external standard quantitative NMR

Tube to tube volume difference presents a challenge in obtaining correct external standard quantitative NMR (esqNMR) results. Deuterium (² H) NMR is easily observable, intrinsically quantitative, present in all samples, free of interfering signals, and insensitive to probe tune/match and sample saltiness. These properties make ² H peak integral an ideal parameter in esqNMR for correcting volume differences between the reference standard and analyte. We demonstrate a novel and practical technique abbreviated as "² H SOLCOR" (² H SOLvent CORrected), where the ² H peak integral from the solvent is used as a universal internal standard to correct volume variations in NMR tubes, thereby improving accuracy and precision of esqNMR method. Herein, this simple yet effective technique is described, and practical considerations for successful implementation are presented. ² H SOLCOR can be applied anywhere esqNMR is used, including where precious samples need to be accurately quantified for qualification as an authentic analytical standard.

Quantitative 1 H NMR spectroscopy (qNMR) in the early process development of a new quorum sensing inhibitor

2-methyl-5,6,7,8-tetrahydro-2H-chromen-4(3H)-one (called 6-oxo) is presented as a new AI-1 quorum sensing inhibitor for Vibrio harveyi. The development of a chemical process to afford traceable materials for new biological assays demands the development of analytical methods to ensure their purity and quality. This work describes the use of quantitative ¹ H nuclear magnetic resonance (NMR) spectroscopy (qNMR) to assess the purity of a sample of 6-oxo (99.88%) and a sample of its major process impurity (E)-1-(2-hydroxycyclohex-2-en-1-yl)but-2-en-1-one (called HCB; 98.28%). To explore the scope of the use of qNMR to quantify the amount of low-content components in samples related to the chemical process for 6-oxo synthesis, this work also determined the amount of 6-oxo in two HCB samples: (a) the high-purity HCB sample described above and (b) a crude HCB sample collected during the chemical process. Despite the complexity of the crude sample, the amount of 6-oxo was readily assessed and could help to estimate the extent to which 6-oxo was already formed during the HCB synthesis. This information can help the understanding of how the process parameters can be modified to improve the performance of the whole process, by controlling the reaction mechanisms working at each step of this chemical process. In this context, our results reinforce qNMR as a complementary analytical tool for the quantification of the main component found in a sample, contributing to the standardization of reference materials and thus allowing the development of analytical methods for process control and traceability of the samples used for biological assays.

Combined in silico and 19F NMR analysis of 5-fluorouracil metabolism in yeast at low ATP conditions

The cytotoxic effect of 5-fluorouracil (5-FU) on yeast cells is thought to be mainly via a misincorporation of fluoropyrimidines into both RNA and DNA, not only DNA damage via inhibition of thymidylate synthase (TYMS) by fluorodeoxyuridine monophosphate (FdUMP). However, some studies on Saccharomyces cerevisiae show a drastic decrease in ATP concentration under oxidative stress, together with a decrease in concentration of other tri- and diphosphates. This raises a question if hydrolysis of 5-fluoro-2-deoxyuridine diphosphate (FdUDP) under oxidative stress could not lead to the presence of FdUMP and the activation of so-called 'thymine-less death' route. We attempted to answer this question with in silico modeling of 5-FU metabolic pathways, based on new experimental results, where the stages of intracellular metabolism of 5-FU in Saccharomyces cerevisiae were tracked by a combination of 19F and 31P NMR spectroscopic study. We have identified 5-FU, its nucleosides and nucleotides, and subsequent di- and/or triphosphates. Additionally, another wide 19F signal, assigned to fluorinated unstructured short RNA, has been also identified in the spectra. The concentration of individual metabolites was found to vary substantially within hours, however, the initial steady-state was preserved only for an hour, until the ATP concentration dropped by a half, which was monitored independently via 31P NMR spectra. After that, the catabolic process leading from triphosphates through monophosphates and nucleosides back to 5-FU was observed. These results imply careful design and interpretation of studies in 5-FU metabolism in yeast.

Molecular Design of Stable Sulfamide- and Sulfonamide-based Electrolytes for Aprotic Li-O2 Batteries

Electrolyte instability is one of the most challenging impediments to enabling Lithium-Oxygen (Li-O2) batteries for practical use. The use of physical organic chemistry principles to rationally design new molecular components may enable the discovery of electrolytes with stability profiles that cannot be achieved with existing formulations. Here, we report on the development of sulfamide- and sulfonamide-based small molecules that are liquids at room temperature, capable of dissolving reasonably high concentration of Li salts (e.g., LiTFSI), and are exceptionally stable under the harsh chemical and electrochemical conditions of aprotic Li-O2 batteries. In particular, N,N-dimethyl-trifluoromethanesulfonamide was found to be highly resistant to chemical degradation by peroxide and superoxide, stable against electrochemical oxidation up to 4.5 VLi, and stable for > 90 cycles in a Li-O2 cell when cycled at < 4.2 VLi. This study provides guiding principles for the development of next-generation electrolyte components based on sulfamides and sulfonamides.

Dynamics of the isoflavone metabolome of traditional preparations of Trifolium pratense L

ETHNOPHARMACOLOGICAL RELEVANCE

The flowering tops of Trifolium pratense L., popularly known as red clover, are used in ethnic Western and Traditional Chinese medicine, in a variety of preparations, including infusions, decoctions and tinctures. Red clover has been reported to be helpful for treatment of menopausal symptoms, premenstrual syndrome, mastalgia, high cholesterol, and other conditions.

AIMS OF THE STUDY

The aims were to compare the chemical dynamics between traditional preparations of infusions, decoctions, and tinctures, as well as to identify the chemical variability over time in a traditional red clover tincture. For this purpose, eight isoflavone aglycones as well as two glucosides, ononin and sissotrin, were used as marker compounds.

MATERIALS AND METHODS

Quantitative NMR (qHNMR), LC-MS-MS, and UHPLC-UV methods were used to identify and quantitate the major phenolic compounds found within each extract.

RESULTS

Infusions, decoctions and tinctures were shown to produce different chemical profiles. Biochanin A and formononetin were identified and quantified in infusion, decoction, and tinctures of red clover. Both infusion and decoction showed higher concentrations of isoflavonoid glucosides, such as ononin and sissotrin, than 45% ethanolic tinctures. Dynamic chemical variability ("dynamic residual complexity") of the red clover tincture was observed over time (one-month), with biochanin A and formononetin reaching peak concentrations at around six days.

CONCLUSIONS

Insight was gained into why different formulation methods (infusions, decoctions, and tinctures) are traditionally used to treat different health conditions. Moreover, the outcomes show that tinctures, taken over a period of time, are dynamic medicinal formulations that allow for time-controlled release of bioactive compounds.

Preparation of DESIGNER extracts of red clover (Trifolium pratense L.) by centrifugal partition chromatography

Starting with an isoflavone-rich red clover extract (RCE), this study expands on the DESIGNER approach to Deplete and Enrich Select Ingredients to Generate Normalized Extract Resources using countercurrent separation (CCS) methodology. A hydrostatic CCS (also known as centrifugal partition chromatography, CPC) technique was used to enrich and deplete selected bioactive isoflavones of RCE extracts. In order to efficiently prepare large enough DESIGNER extracts from RCE for biological testing including in vivo assays, it was necessary to choose a balance between resolution and a loading capacity of at least 1 g per separation for the selected solvent system (SS). Adding 3 mL of DMSO to the sample containing equal amounts of upper and lower phases of hexanes-ethyl acetate-methanol-water (HEMWat 5.5/4.5/5/5, v/v) allowed 1 g of RCE to be dissolved in the sample without disrupting the chromatographic resolution of the target isoflavones. CPC experiments using other solubility modifiers, acetone and acetonitrile indicated that these modifiers increase solubility significantly, even better than DMSO, but the separation of target compounds was sufficiently disturbed to be unacceptable for producing the desired DESIGNER extracts. The preparation of DESIGNER extracts was achieved with two sequential CPC separations. The first produced a biochanin A enriched fraction (93.60% w/w) with only small amounts of other isoflavones: 2.30% w/w prunetin, 1.17% w/w formononetin, and 0.12% w/w irilone. Gravimetric investigations of this step demonstrated the high efficiency of CCS technology for full and unbiased sample recovery, confirmed experimentally to be 99.80%. A formononetin enriched fraction from this first separation was re-chromatographed on a more polar HEMWat (4/6/4/6, v/v) SS to produce a formononetin enriched DESIGNER fraction of 94.70% w/w purity. The presence of the minor (iso)flavonoids: 3.16% w/w pseudobaptigenin, 0.39% w/w kaempferol, and 0.31% w/w genistein was also monitored in these fractions. Chromatographic fractions, combined fractions, and DESIGNER extracts were analyzed with quantitative ¹H NMR (qHNMR) spectroscopy which provided purity information, quantitation, and structural identification of the components.

Simultaneous Quantification of Ellagitannins and Related Polyphenols in Geranium thunbergii Using Quantitative NMR

Compared to commonly employed liquid chromatography-based methods, quantitative nuclear magnetic resonance (qNMR) is a recently developed method for accurate quantification of natural compounds in extracts. The simultaneous quantification of ellagitannins and the related polyphenols of Geranium thunbergii were studied using qNMR after a short-term and long-term decoction. The qNMR fingerprint for quantifying ellagitannin was presented in this work. Geraniin was observed in the short-term decoction as a major component while corilagin was the major component of the long-term decoction. An aqueous acetone extract of G. thunbergii after long-term decoction was extracted with diethyl ether, ethyl acetate, and n-butanol. Corilagin was found as a major constituent in the ethyl acetate and n-butanol extracts. Furthermore, the contents of these polyphenols in G. thunbergii from six locations in Japan and three locations in China were quantified. The contents of geraniin and corilagin in G. thunbergii from Japan were higher than those from China. Our finding raised the possibility that qNMR can be effectively employed as a simple, accurate, and efficient method for quantification of ellagitannins in medicinal plants.

Qualitative and Quantitative NMR Approaches in Blood Serum Lipidomics

Nuclear magnetic resonance (NMR) spectroscopy in combination with chemometrics can be applied in the analysis of complex biological samples in many ways. For example, we can analyze lipids, elucidate their structures, determine their nutritional values, and determine their distribution in blood serum. As lipids are not soluble in water, they are transported in blood as lipid-rich self-assembled particles, divided into different density assemblies from high- to very-low-density lipoproteins (HDL to VLDL), or by combining with serum proteins, such as albumins (human serum albumins (HSA)). Therefore, serum lipids can be analyzed as they are using only a 1:1 (v/v) dilution with a buffer or deuterated water prior to analysis by applying ¹H NMR or ¹H NMR edited-by-diffusion techniques. Alternatively, lipids can be extracted from the serum using liquid partition equilibrium and then analyzed using liquid-state NMR techniques. Our chapter describes protocols that are used for extraction of blood serum lipids and their quantitative ¹H NMR (¹H qNMR) analysis in lipid extracts as well as ¹H NMR edited by diffusion for direct blood serum lipid analysis.

Use of Hybrid Capillary Tube Apparatus on 400 MHz NMR for Quantitation of Crucial Low-Quantity Metabolites Using aSICCO Signal

Metabolites of new chemical entities can influence safety and efficacy of a molecule and often times need to be quantified in preclinical studies. However, synthetic standards of metabolites are very rarely available in early discovery. Alternate approaches such as biosynthesis need to be explored to generate these metabolites. Assessing the quantity and purity of these small amounts of metabolites with a nondestructive analytical procedure becomes crucial. Quantitative NMR becomes the method of choice for these samples. Recent advances in high-field NMR (>500 MHz) with the use of cryoprobe technology have helped to improve sensitivity for analysis of small microgram quantity of such samples. However, this type of NMR instrumentation is not routinely available in all laboratories. To analyze microgram quantities of metabolites on a routine basis with lower-resolution 400 MHz NMR instrument fitted with a broad band fluorine observe room temperature probe, a novel hybrid capillary tube setup was developed. To quantitate the metabolite in the sample, an artificial signal insertion for calculation of concentration observed (aSICCO) method that introduces an internally calibrated mathematical signal was used after acquiring the NMR spectrum. The linearity of aSICCO signal was established using ibuprofen as a model analyte. The limit of quantification of this procedure was 0.8 mM with 10 K scans that could be improved further with the increase in the number of scans. This procedure was used to quantify three metabolites-phenytoin from fosphenytoin, dextrophan from dextromethorphan, and 4-OH-diclofenac from diclofenac-and is suitable for minibiosynthesis of metabolites from in vitro systems.

Separating Thermodynamics from Kinetics-A New Understanding of the Transketolase Reaction

Transketolase catalyzes asymmetric C−C bond formation of two highly polar compounds. Over the last 30 years, the reaction has unanimously been described in literature as irreversible because of the concomitant release of CO₂ if using lithium hydroxypyruvate (LiHPA) as a substrate. Following the reaction over a longer period of time however, we have now found it to be initially kinetically controlled. Contrary to previous suggestions, for the non‐natural conversion of synthetically more interesting apolar substrates, the complete change of active‐site polarity is therefore not necessary. From docking studies it was revealed that water and hydrogen‐bond networks are essential for substrate binding, thus allowing aliphatic aldehydes to be converted in the charged active site of transketolase.

Improving the Performance of High-Precision qNMR Measurements by a Double Integration Procedure in Practical Cases

Quantitative (1)H NMR (qNMR) is a widely applied technique for compound concentration and purity determinations. The NMR spectrum will display signals from all species in the sample, and this is generally a strength of the method. The key spectral determination is the full and accurate determination of one or more signal areas. Accurate peak integration can be an issue when unrelated peaks resonate in an important integral region. We describe a "hybrid" approach to signal integration that provides an accurate estimation of signal area, removing the component(s) that may arise from unrelated peaks. This is achieved by using the most accurate integration method for the region and removing unwanted contributions. The key to this performing well, and in almost all cases, is the use of areas from deconvolved peaks. We describe this process and show that it can be very successfully applied to cases where the highest precision is required and for more common cases of NMR-based quantitation.

Internal Referencing for ¹³C Position-Specific Isotope Analysis Measured by NMR Spectrometry

The intramolecular (13)C composition of a molecule retains evidence relevant to its (bio)synthetic history and can provide valuable information in numerous fields ranging from biochemistry to environmental sciences. Isotope ratio monitoring by (13)C NMR spectrometry (irm-(13)C NMR) is a generic method that offers the potential to conduct (13)C position-specific isotope analysis with a precision better than 1‰. Until now, determining absolute values also required measurement of the global (or bulk) (13)C composition (δ(13)Cg) by mass spectrometry. In a radical new approach, it is shown that an internal isotopic chemical reference for irm-(13)C NMR can be used instead. The strategy uses (1)H NMR to quantify both the number of moles of the reference and of the studied compound present in the NMR tube. Thus, the sample preparation protocol is greatly simplified, bypassing the previous requirement for precise purity and mass determination. The key to accurate results is suppressing the effect of radiation damping in (1)H NMR which produces signal distortion and alters quantification. The methodology, applied to vanillin with dimethylsulfone as an internal standard, has an equivalent accuracy (<1‰) to that of the conventional approach. Hence, it was possible to clearly identify vanillin from different origins based on the (13)C isotopic profiles.

Nuclear magnetic resonance using electronic referencing: method validation and evaluation of the measurement uncertainties for the quantification of benzoic acid in orange juice

Quantitative nuclear magnetic resonance measurements have become more popular over the last decade. The introduction of new methods and experimental parameters has been of fundamental importance in the development of new applications. Amongst these new developments is the introduction of electronic referencing for quantifications. The use of electronic referencing eliminates errors in the analyses as a result of weighting of internal standards as well as undesired problems as a result of the solubility of the standards in the analyte solution and chemical interactions between the analyte and the internal standard. In this work, we have studied the quantification of a very important analyte in a food matrix, benzoic acid in orange juice, as a model to the validation and measurement uncertainty estimation of electronic referencing using (1)H NMR in food analyses. The referencing method applied was the pulse length-based concentration measurement. Method was validated and showed good results for the precision and accuracy parameters evaluated. A certified reference material and a reference material candidate were analyzed, and extremely good results were obtained. Reported relative expanded uncertainties are in the 1.07-1.39% range that can be considered an extremely good performance for the analysis of a food complex matrix. Measurement uncertainty was evaluated by two different approaches, and the pulse calibrations for the samples and for the reference have been shown to account for approximately 80% of the total uncertainty of the measurement.

Method development in quantitative NMR towards metrologically traceable organic certified reference materials used as (31)P qNMR standards

Quantitative nuclear magnetic resonance (qNMR) spectroscopy is employed by an increasing number of analytical and industrial laboratories for the assignment of content and quantitative determination of impurities. Within the last few years, it was demonstrated that (1)H qNMR can be performed with high accuracy leading to measurement uncertainties below 1 % relative. It was even demonstrated that the combination of (1)H qNMR with metrological weighing can lead to measurement uncertainties below 0.1 % when highly pure substances are used. Although qNMR reference standards are already available as certified reference materials (CRM) providing traceability on the basis of (1)H qNMR experiments, there is an increasing demand for purity assays on phosphorylated organic compounds and metabolites requiring CRM for quantification by (31)P qNMR. Unfortunately, the number of available primary phosphorus standards is limited to a few inorganic CRM which only can be used for the analysis of water-soluble analytes but fail when organic solvents must be employed. This paper presents the concept of value assignment by (31)P qNMR measurements for the development of CRM and describes different approaches to establish traceability to primary Standard Reference Material from the National Institute of Standards and Technology (NIST SRM). Phosphonoacetic acid is analyzed as a water-soluble CRM candidate, whereas triphenyl phosphate is a good candidate for the use as qNMR reference material in organic solvents. These substances contain both nuclei, (1)H and (31)P, and the concept is to show that it is possible to indirectly quantify a potential phosphorus standard via its protons using (1)H qNMR. The same standard with its assigned purity can then be used for the quantification of an analyte via its phosphorus using (31)P qNMR. For the validation of the concept, triphenyl phosphate and phosphonoacetic acid have been used as (31)P qNMR standards to determine the purity of the analyte tris(2-chloroethyl) phosphate, and the resulting purity values perfectly overlap within their expanded measurement uncertainties.