Development of a corn flour certified reference material for the accurate determination of zearalenone

A certified reference material (CRM, KRISS 108-01-002) for zearalenone in corn flour was developed to assure reliable and accurate measurements in testing laboratories. Commercially available corn flour underwent freeze-drying, pulverization, sieving, and homogenization. The final product was packed in amber bottles, approximately 14 g per unit, and preserved at -70 °C. 13C18-Zearalenone was used as an internal standard (IS) for the certification of zearalenone by isotope-dilution liquid chromatography-tandem mass spectrometry (ID-LC‒MS/MS) and for the analysis of α-zearalenol, β-zearalenol, and zearalanone by LC‒MS/MS. The prepared CRM was sufficiently homogeneous, as the among-unit relative standard deviation for each mycotoxin ranged from 2.2 to 5.7 %. Additionally, the stability of the mycotoxins in the CRM was evaluated under different temperature conditions and scheduled test periods, including storage at -70°C, -20°C, and 4°C and room temperature for up to 12 months, 6 months, and 1 month, respectively. The content of each target mycotoxin in the CRM remained stable throughout the monitoring period at each temperature. Zearalenone content (153.6 ± 8.0 µg/kg) was assigned as the certified value. Meanwhile, the contents of α-zearalenol (1.30 ± 0.17 µg/kg), β-zearalenol (4.75 ± 0.33 µg/kg), and zearalanone (2.09 ± 0.16 µg/kg) were provided as informative values.

Development of a coffee bean certified reference material (KRISS CRM 108-10-023) for elemental analysis

The development of a novel coffee bean matrix certified reference material (CRM) for elemental analysis is described. The CRM was prepared by processing green coffee beans into a dry homogeneous powder. Mass fractions of elements in the CRM were measured using double isotope dilution inductively coupled plasma mass spectrometry (double ID-ICP-MS), and measurement results for eight elements (Mg, Ca, Fe, Cu, Zn, Cd, Hg, and Pb) of sufficient quality were certified. The mass fraction range was from 0.09476 mg/kg (Cd) to 1908 mg/kg (Mg), with relative expanded uncertainty range of 0.66% (Cd) to 12% (Pb). Measurement results of two elements (Cr and Ni) with insufficient quality were provided for information only. During characterization, an effective approach for the measurement of isotopic abundances and molar masses of elements with high natural isotopic variations for double ID-ICP-MS was developed and applied. The CRM developed in the present study is expected to be a useful measurement standard for assuring the quality of measurement procedures for coffee beans or related materials.

Accurate determination of 11 representative phthalates and di(2-ethylhexyl) terephthalate in polyvinyl chloride using isotope dilution-gas chromatography/mass spectrometry

Phthalates are mainly used as plasticizers in polyvinyl chloride (PVC). However, prolonged exposure to phthalates poses considerable risks to human health. Consequently, the utilization of phthalates in consumer products is subject to regulations, with a defined threshold of 0.1 %. In this study, we developed an accurate and simultaneous method for determination of 11 representative phthalates and a non-phthalate plasticizer (di(2-ethylhexyl) terephthalate, DEHT) in PVC as a higher-order reference method. Homogeneously prepared PVC samples, each containing approximately 0.1 % of the target plasticizer compounds, were analyzed using gas chromatography-mass spectrometry (GC-MS) with deuterium-labeled phthalates and DEHT. The developed method could effectively separate and quantify all target plasticizers without interference with each other and potential overlap between the isomeric forms of phthalates, di-isodecyl phthalate, and di-isononyl phthalate. The developed method has high-order metrological quality, exhibiting exceptional selectivity, accuracy, repeatability (≤ 2.17 %), reproducibility (≤ 2.16 %), and relative expanded uncertainty (≤ 5.6 %). This analytical method is thus suitable for accurately assessing the target plasticizer levels in PVC products for ensuring compliance with the established 0.1 % threshold. This method was successfully applied to quantify twelve distinct plasticizers in PVC products obtained from the Korean market, validating its effectiveness and reliability in real-world scenarios.

Accurate determination of four tetracycline residues in chicken meat by isotope dilution-liquid chromatography/tandem mass spectrometry

An analytical method based on isotope dilution-liquid chromatography-tandem mass spectrometry (ID-LC‒MS/MS) was developed to accurately determine four representative tetracyclines (tetracycline, chlortetracycline, doxycycline, and oxytetracycline) in chicken meat. Tetracyclines are known to have a great tendency for epimerization and keto-enol tautomerism, which often provoke major challenges in their determination. Since this isomerization was found to be unavoidable during the whole chain of the current analysis, the total content (µg kg^‒1) of individual tetracycline was quantified as a sum of each parent compound and its respective isomeric forms. Using this approach in combination with IDMS analysis, more consistent, accurate, and reproducible measurement results for the four tetracyclines in chicken meat were acquired. LC-MS/MS conditions and sample preparation processes were comprehensively optimized to minimize the chelating effect of tetracyclines and possible co-extracted interferences. Details of the sample preparation scheme, LC‒MS/MS detection, calculation equation, and method validation are described in this article. The method provided very good accuracy (97.7-102.6%) for all analytes across the concentration range of 10-200 µg kg^‒1, with relative standard deviations for intra-day and inter-day precision of less than 4%. The limits of quantification were below 0.2 µg kg^‒1, demonstrating the high sensitivity of the method. Furthermore, the measurement uncertainty was generally below 5.5%. Hence, the established method exhibits high-order metrological quality with superior performance over various existing methodologies. Moreover, this method can provide references for general food testing laboratories close to and far below the established maximum residue limits (100 µg kg^‒1) for animal muscle tissues.

Development of an isotope dilution liquid chromatography/tandem mass spectrometry method for the accurate determination of neonicotinoid pesticides, imidacloprid, clothianidin, and thiamethoxam in kimchi cabbage reference materials

A method based on isotope dilution liquid chromatography/tandem mass spectrometry (ID-LC/MS/MS) was established as a candidate reference method for accurate determination of neonicotinoid pesticides, imidacloprid, clothianidin, and thiamethoxam in kimchi cabbage. Their deuterated isotopes, imidacloprid-d4, chlothianidin-d3, and thiamethoxam-d4 were used as internal standards. The combination of HLB and Carb solid-phase extraction (SPE) cartridges was used to clean-up kimchi cabbage extracts. The ID-LC/MS/MS conditions were optimized with fortified kimchi cabbage samples for validation. Imidacloprid in the ERM-BC403 cucumber sample (0.627 ± 0.026) mg/kg was analyzed with the developed method, and the measured value (0.604 ± 0.028) mg/kg agreed within their uncertainties. The developed method was employed for the certification of kimchi cabbage reference materials prepared in this laboratory. The measured values of imidacloprid, clothianidin, and thiamethoxam are (0.860 ± 0.020) mg/kg, (0.524 ± 0.012) mg/kg, (0.787 ± 0.014) mg/kg, respectively. The standard deviation of the measured values for ten bottles was < 1%, and the measured values after one year agreed with their first measurements indicating reliable repeatability and reproducibility of the developed method.

Development of isotope dilution-liquid chromatography/tandem mass spectrometry for the accurate determination of zearalenone and its metabolites in corn

A method using isotope dilution-liquid chromatography-tandem mass spectrometry (ID-LC/MS/MS) was developed for the accurate determination of zearalenone (ZEN) and its five major metabolites in corn. ¹³C- or ²H-labeled analogues of the target mycotoxins were used as internal standards. As the immunoaffinity columns used demonstrated selectivity to a specific chiral isomer of a racemic mixture of zearalanone-d₆, a clean-up cartridge without stereoselectivity (Mycosep 226 column) was selected for the same recovery of the analyte and its internal standard with adequate elimination of matrix interferences. The method demonstrated sufficient selectivity, sensitivity, accuracy and precision over a concentration range of 20-400 µg/kg. The limit of detections and limit of quantifications were 0.14-0.33 µg/kg and 0.45-1.11 µg/kg, respectively. The accuracy values were 96.7%-103.6%, with intra and inter-day precisions of less than 3% and 4%, respectively. The expanded measurement uncertainty was less than 7% (with a 95% confidence level).

Candidate reference measurement procedure based on HPAEC-PAD for the development of certified reference materials for monosaccharides in serum

To achieve the measurement reliability of monosaccharides used as diagnostic markers in clinical fields, it is essential to establish certified reference materials (CRMs). The purpose of this study is to develop a serum CRM by adopting high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) as a new candidate reference measurement procedure for the measurement of glucose and galactose, common diagnostic markers of diabetes and galactosemia, respectively. Using various monosaccharides as internal standards, the accuracy of the HPAEC-PAD method was tested by measuring glucose CRM following treatment with three different deproteinization methods: ultrafiltration, protein precipitation by trichloroacetic acid (TCA), and protein precipitation by acetonitrile. Results showed that ultrafiltration and 5% TCA provided good accuracy with every tested monosaccharide as the internal standard. Accordingly, serum samples in this study were treated by ultrafiltration after adding 2-deoxy-D-glucose and arabinose, which were selected as internal standards for galactose and glucose, respectively. Both intra- and inter-day recovery tests showed good precision and accuracy within 2%. From the serum CRM batches prepared at two levels, 11 units were analyzed by exact-matched calibration methods, and the mass fractions of galactose and glucose were determined via HPAEC-PAD. The between-unit relative standard deviations were not more than 1.5%, showing homogeneity. The expanded uncertainties (%) of galactose and glucose for both levels were less than 3.6% and 2.3% at 95% confidence. The HPAEC-PAD method presented in this study can significantly improve the accuracy and precision of simultaneous monosaccharide analysis, allowing for the development of further serum CRMs for monosaccharides. Graphical abstract.

Method development for accurate determination of eight polycyclic aromatic hydrocarbons in extruded high-impact polystyrene

An analytical method was developed for the accurate determination of eight polycyclic aromatic hydrocarbons (PAHs) in extruded high-impact polystyrene (HIPS) as a higher-order reference method. Uncleaned HIPS matrix rendered the accurate quantitation impossible and hampered the repeatability of the gas chromatography/mass spectrometry (GC/MS) measurement. This led to a bias in the results and increased the measurement uncertainty. Extracts were sufficiently purified through a two-step process: (i) dissolution and precipitation and (ii) molecularly imprinted polymers-solid phase extraction for clean-up. Co-elution of indeno(1,2,3-cd)pyrene (IP), dibenz(a,h)anthracene (DBahA), and their ¹³C₆-labeled isotopes resulted in a bias in the area ratios of IP/¹³C₆-IP and DBahA/¹³C₆-DBahA, thus increasing the measurement uncertainty. An optimized GC condition using an Rxi-PAH column improved the peak separation of IP and DBahA and their isotopes, thus improving the quality of the measurement. The optimized method was validated using PAH (0.36-0.45 mg/kg)-containing HIPS pellets. The optimized method had excellent repeatability (<2%) and reproducibility (<3%) for concentrations less than 0.5 mg/kg of the eight PAHs in HIPS. Using the optimized method, the relative expanded uncertainties for all the target PAHs were below 5% (with a 95% level of confidence).

Highly sensitive analytical method for the accurate determination of 5-methyltetrahydrofolic acid monoglutamate in various volumes of human plasma using isotope dilution ultra-high performance liquid chromatography-mass spectrometry

One predominant and bioactive folate vitamer circulating in the blood is 5-methyltetrahydrofolate (5-Me-THF). In this study, a method for the accurate determination of 5-Me-THF in human plasma samples of various volumes was established using isotope dilution ultra-high performance liquid chromatography-mass spectrometry (ID-UPLC-MS). For this purpose, 500 μL of homogeneous human plasma was initially employed, and the 5-Me-THF and the ¹³C₅-5-Me-THF standard solutions prepared using 1% ascorbic acid in water gave the calibration solution and spiking sample. The desired amount of ¹³C₅-5-Me-THF standard solution was spiked into the sample followed by sample pretreatment. The method was validated for its repeatability, reproducibility, recovery, and limits of detection and quantification. Subsequently, it was applied to smaller volumes of human plasma samples (i.e., 50 and 10 μL), the results of which corresponded well with those obtained using 500 μL. The feasibility of the method was further confirmed using 10 μL of a standard reference material, SRM 3949, which is a human serum sample containing three different levels of 5-Me-THF. The established ID-UPLC-MS method was successfully applied to various volumes of human plasma or serum ranging from 500 to 10 μL, which exhibited particularly good sensitivity in addition to reliable results for the quantification of 5-Me-THF. Our method therefore expands on the ability to obtain accurate quantitative results for 5-Me-THF using small volumes of blood.

Development of isotope dilution-liquid chromatography/tandem mass spectrometry for the accurate determination of type-A trichothecenes in grains

We report the development of a highly accurate method based on isotope dilution liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS) for the simultaneous determination of four major type-A trichothecenes in cereal grains. Uniformly labeled [¹³C] analogues of the target analytes were used as internal standards. An expedient sample preparation procedure was established. After extraction with acetonitrile/water (84:16; v/v), further clean-up was performed using MycoSep 227 solid-phase extraction cartridges. Unlike the commonly used immunoaffinity columns having strong selectivity for only certain target analytes, the cartridges allowed the simultaneous recovery of all four mycotoxins and efficient elimination of co-extracted matrix interferences. The ID-LC-MS/MS method exhibited very good analytical performance in the concentration range of 10-200 µg/kg; accuracy ranged from 97 to 103% with intra-day and inter-day relative standard deviations of less than 5% and 4%, respectively. Measurement uncertainties were generally below 5%. The applicability of the method was assessed by measuring the target mycotoxins in several samples at sub-µg/kg levels.

Quantitative analyses of essential fatty acids in cereals and green vegetables by isotope dilution-gas chromatography/mass spectrometry

Abstract

An analytical method for the accurate determination of essential fatty acids in cereals and green vegetables is established based on isotope dilution-gas chromatography/mass spectrometry (ID-GC/MS). In this approach, 13C-labeled fatty acids were added as internal standards to the samples prior to the sample preparation. Repeatability and reproducibility for the analyses of the essential fatty acids extracted from homogenized cereals and green vegetables were examined to validate the ID-GC/MS method. The repeatability and reproducibility were within 2% relative standard deviation. The developed method provides higher-order measurement results with the relative expanded uncertainties of 1–2% and 2–4% for cereals and green vegetables, respectively. Based on the fatty acid measurements, the developed ID-GC/MS method can be used as a candidate reference method for the quantitation of essential fatty acids in cereals and green vegetables. The developed method was also applied to analyze several commercially available products including cereal and vegetable powders.

Bias reduction in the quantitative analysis of a target analyte present in a limited quantity in human plasma using dual-mode heart-cutting two-dimensional liquid chromatography coupled with isotope dilution mass spectrometry

Dual-mode heart-cutting two-dimensional liquid chromatography (DMHC 2D-LC) was applied to isotope dilution mass spectrometry (IDMS) to reduce the bias in the quantitative analysis of a target analyte present in a limited quantity in human plasma. Based on a Waters I-Class LC system, the DMHC 2D-LC system was operated in one- and two-dimensional modes to facilitate the determination of heart-cutting time and the efficient trapping of the target LC eluate. Experiments to determine the feasibility of coupling with IDMS were performed with triple quadrupole mass spectrometry using folic acid standards and/or ¹³ C₅ -folic acid. To validate the performance of the DMHC 2D-LC/IDMS system on a complex sample, human plasma was analyzed for folic acid and the result was compared with that obtained using conventional single-column LC. The total run time of the DMHC 2D-LC system was 20 min, the same as that of the single-column LC system. The peak profile of the spiked ¹³ C₅ -folic acid obtained with single-column LC/MS was affected by matrix effects, but resolved with DMHC 2D-LC/MS, thus improving the accuracy of the analysis. The DMHC 2D-LC/IDMS system showed reliable performance in analyzing the target analyte in human plasma, eliminating matrix effects and saving analysis time.

Development of an infant formula certified reference material for the analysis of organic nutrients

Infant formula certified reference material (CRM, KRISS CRM 108-02-003) were developed for the analysis of organic nutrients. The CRM is a milk-based infant formula powder, packaged at 14 g per unit. Ten thousand units were prepared and stored at -70 °C. For the certification of each nutrient, ten units were analyzed for simultaneous value-assignment and homogeneity test. Analytical methods used were isotope dilution mass spectrometry (IDMS) based on liquid chromatography mass spectrometer (LC/MS) or gas chromatography mass spectrometer (GC/MS) as higher-order reference methods.13 vitamins, 3 fatty acids, and total cholesterol were certified. The between-unit relative standard deviation of measurement results for each nutrient ranged 0.2% to 2.5%, showing very good homogeneity. The expanded relative uncertainties of the certified values ranged from 1% to 8%, indicating that they have higher-order metrological quality. The values of proximates (proteins, lipids, carbohydrates, water, and ash) were assigned through inter-laboratory comparisons.

Accurate determination of hexabromocyclododecane diastereomers in extruded high-impact polystyrene: Development of an analytical method as a candidate reference method

Herein, an isotope dilution-liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS) method was developed as a candidate reference method for the accurate quantitation of hexabromocyclododecane (HBCDD) diastereomers in plastics. The repeatability, reproducibility, and uncertainty results showed that developed ID-LC/MS/MS method is reliable and reproducible. As homogeneous samples, HBCDD-containing extruded high-impact polystyrene (HIPS) pellets were fabricated via an extrusion process. Notably, we detected α-, β-, γ-, δ-, ε-, η-, and θ-HBCDDs in the extruded HIPS pellets, and then determined their exact mass fractions using the ID-LC-MS/MS method. The relative contents (average ± standard deviation) of HBCDD diastereomers in HIPS obtained using the phenyl-hexyl column were 67.08 ± 0.41% (α-HBCDD), 19.73 ± 0.37% (β-HBCDD), 11.59 ± 0.16% (γ-HBCDD), and 1.6 ± 0.07% (sum of δ-, ε-, η-, and θ-HBCDDs). These values differed significantly from the ones determined for the technical HBCDD mixtures (10.42% α-HBCDD, 5.30% β-HBCDD, 82.13% γ-HBCDD, 2.15% minor HBCDDs) used to fabricate the HIPS pellets and thus demonstrating the HBCDD isomerization during the extrusion. The proportion of minor HBCDDs was smaller than the uncertainty of the total HBCDD and that the sum of α-, β-, and γ-HBCDDs was comparable to the total HBCDD in the investigated samples. Notably, a real-life sample (expanded polystyrene board obtained from a local construction site) also showed a similar HBCDD profile, being rich in α-HBCDD.

Inter-Laboratory Validation of Method to Determine Residual Enrofloxacin in Chicken Meat

An inter-laboratory study was performed to evaluate the performance of a method developed for the quantification of enrofloxacin in chicken meat. Liquid-liquid extraction combined with a clean-up procedure based on solid-phase extraction followed by a liquid chromatography-tandem mass spectrometric method was used by three individual laboratories. All the investigated results of calibration curves and limits of quantification were within the acceptable range for regulatory testing of enrofloxacin. The three laboratories received blind a certified reference material to analyze in triplicate and assess using statistical analysis. From the results, no statistical differences were found between the laboratories in the precision of the method. Additionally, all the results of the z-score, which is an indication of fixed interval bias criteria for accuracy from the laboratories, fell within the allowable limits (±2σ). Based on this proficiency testing by inter-laboratory comparisons, the analytical method including the sample preparation step was proven to be applicable.

Development of isotope dilution-liquid chromatography/tandem mass spectrometry for the accurate determination of trans- and cis-vitamin K1 isomers in infant formula

A method based on isotope dilution-liquid chromatography/tandem mass spectrometry (ID-LC/MS/MS) using a C30 column has been developed for the separate and accurate determination of trans- and cis-vitamin K₁ in infant formula. Vitamin K₁ and the deuterium-labeled internal standard eluted at slightly different retention times experiencing different matrix effects, and this possibly resulted in biased measurement. The matrix effect profiles obtained from post-column infusion experiments showed that atmospheric pressure chemical ionization (APCI) was less susceptible to matrix effects near the retention time than electrospray ionization (ESI); therefore, APCI was used in this study. The developed method was validated by measuring fortified samples, and the results agreed with the gravimetric values. Its repeatability and reproducibly were within 2% relative standard deviation. The relative expanded uncertainty was approximately 5%, indicating that the method was of higher-order metrological quality as a reference method.

Accurate determination of ochratoxin A in Korean fermented soybean paste by isotope dilution-liquid chromatography tandem mass spectrometry

Ochratoxin A (OTA), a naturally occurring mycotoxin, has been frequently detected in doenjang, a traditional fermented soybean paste, when it is fermented under improper conditions. Reliable screening of OTA in traditional fermented soybean paste (doenjang) is a special food-safety issue in Korea. Our laboratory, the National Metrology Institute of Korea, established an isotope dilution-liquid chromatography tandem mass spectrometry (ID-LC/MS/MS) method as a higher-order reference method to be used for SI-traceable value-assignment of OTA in certified reference materials (CRMs). (13)C20-OTA was used as an internal standard. Sample preparation conditions and LC/MS measurement parameters were optimised for this purpose. The analytical method was validated by measuring samples fortified with OTA at various levels. Repeatability and reproducibility studies showed that the ID-LC/MS/MS method is reliable and reproducible within 2% relative standard deviation. The analytical method was applied to determine OTA in various commercial doenjang products and home-made doenjang products.

Investigation of isotope dilution mass spectrometric (ID-MS) method to determine niacin in infant formula, breakfast cereals and multivitamins

An isotope dilution LC/mass spectrometric (ID-LC/MS) method was developed as a candidate reference method for the accurate determination of niacin in infant formula, breakfast cereals and multivitamin. After spiking nicotinamide-d(4) as an internal standard, infant formula and breakfast cereal samples were hydrolysed under alkaline condition. Samples were then analysed in SRM mode to detect nicotinic acid and nicotinic acid-d(4) at m/z 124→80 and 127→84, respectively. In the case of multivitamin sample that contains mainly free nicotinamide, LC/MS monitored nicotinamide and nicotinamide-d(4) at their SRM channels of m/z 123→80 and m/z 127→84, respectively, after simple extraction. The repeatability and reproducibility were tested for the validation of the developed ID/LC-MS method. Additionally, the developed analytical method was applied to determine total niacin contents in homogenised infant formula, homogenised multivitamin, and commercially available products including different types of infant formula, breakfast cereals, and multivitamin tablets.