The Experts below are selected from a list of 135 Experts worldwide ranked by ideXlab platform
Alessandro Dulivo - One of the best experts on this subject based on the ideXlab platform.
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determination of thiocyanate in saliva by headspace gas chromatography mass spectrometry following a single step aqueous derivatization with Triethyloxonium Tetrafluoroborate
Journal of Chromatography A, 2015Co-Authors: Sara Ammazzini, Enea Pagliano, Massimo Onor, Emanuela Pitzalis, Zoltan Mester, Beatrice Campanella, Emilia Bramanti, Alessandro DulivoAbstract:Abstract A novel method for the determination of salivary thiocyanate is presented. Thiocyanate was converted into ethyl thiocyanate by single-step aqueous derivatization based on Triethyloxonium Tetrafluoroborate and measured by gas chromatography-mass spectrometry (15 min runtime). The ethyl thiocyanate derivative is volatile and can be sampled from the headspace. The derivatization chemistry proposed allows for separation of the analyte from saliva matrix whose introduction in the measurement system is avoided. Quantitation of the analyte was obtained by isotope dilution, employing a 13C-enriched thiocyanate as internal standard. Technical details and fundamental aspects of derivatization chemistry and calibration strategy are presented. The method was validated by comparison with a standard method based on ion chromatography. The two independent methodologies produced results in agreement within 3%. Also a three level spike recovery test was carried out for validation purpose and quantitative recoveries were attained. The method is fast, simple, safe, and sensitive. Measurement of a 1 mL volume 50 ng/g of thiocyanate standard produced a signal-to-noise ratio of 250 for the analytical peak. This method is therefore suitable for ultra-trace determination of thiocyanate (low part-per-billion range). For the application described the full detection potential of the method was not required and the sample preparation presented has been designed for quantitation of saliva samples containing 1–400 μg/g of thiocyanate with a combined standard uncertainty of 2% relative for saliva samples containing 25 μg/g of thiocyanate. This method was applied for the determination of thiocyanate in human saliva samples.
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negative chemical ionization gc ms determination of nitrite and nitrate in seawater using exact matching double spike isotope dilution and derivatization with Triethyloxonium Tetrafluoroborate
Analytical Chemistry, 2012Co-Authors: Enea Pagliano, Zoltan Mester, Ralph E Sturgeon, Juris Meija, Alessandro DulivoAbstract:The alkylation of nitrite and nitrate by Triethyloxonium Tetrafluoroborate allows determination of their ethyl esters by headspace gas chromatography/mass spectrometry (GC/MS). In the present study, significant improvement in analytical performance is achieved using negative chemical ionization providing detection limits of 150 ng/L for NO2– and 600 ng/L for NO3–, an order of magnitude better than those achieved using electron impact ionization. The derivatization procedure was optimized and alkaline conditions adopted to minimize conversion of nitrite to nitrate (determined to be 0.07% at 100 mg/L NO2–) and to avoid the exchange of oxygen between the analytes and the solvent (water). Quantitation entails use of isotopically enriched standards (N18O2– and 15NO3–), which also permits monitoring of potential conversion from nitrite to nitrate during the analysis (double spike isotope dilution).
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quantification of nitrite and nitrate in seawater by Triethyloxonium Tetrafluoroborate derivatization headspace spme gc ms
Talanta, 2011Co-Authors: Enea Pagliano, Massimo Onor, Emanuela Pitzalis, Zoltan Mester, Ralph E Sturgeon, Alessandro DulivoAbstract:Abstract Triethyloxonium Tetrafluoroborate derivatization combined with direct headspace (HS) or SPME-gas chromatography–mass spectrometry (GC–MS) is proposed here for the simultaneous determination of nitrite and nitrate in seawater at micromolar level after conversion to their corresponding volatile ethyl-esters (EtO-NO and EtO-NO2). Isotopically enriched nitrite [15N] and nitrate [15N] are employed as internal standards and for quantification purposes. HS-GC–MS provided instrumental detection limits of 0.07 μM NO2− and 2 μM NO3−. Validation of the methodology was achieved by determination of nitrite and nitrate in MOOS-1 (Seawater Certified Reference Material for Nutrients, NRC Canada), yielding results in excellent agreement with certified values. All critical aspects connected with the potential inter-conversion between nitrite and nitrate (less than 10%) were evaluated and corrected for by the use of the isotopically enriched internal standard.
Enea Pagliano - One of the best experts on this subject based on the ideXlab platform.
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A rapid and sensitive method for the determination of inorganic chloride in oil samples
Analytica Chimica Acta, 2019Co-Authors: Zuzana Gajdosechova, Zoltan Mester, Enea PaglianoAbstract:Abstract A novel isotope dilution method for the analysis of inorganic chloride in fuel oil matrix is presented. The samples were diluted by isopropyl alcohol:toluene, mixed with 37Cl− internal standard and reacted with Triethyloxonium Tetrafluoroborate at room temperature. This reagent promoted conversion of Cl− into stable ethyl chloride (EtCl) which was selectively detected by headspace GC–MS/MS with no matrix effects. A limit of detection of 0.2 mg kg−1 Cl− was obtained in fuel oil. The method was tested on the NIST SRM 1634c (trace elements in residual fuel oil) and validated through a series of robustness tests. Over 18 days, the variation in the signal response was less than 10% and the RSD for quantitative isotope dilution results was below 3%. Overall, the method is fast, simple and allows robust quantitation of inorganic chloride directly in the non-aqueous media.
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Determination of elevated levels of nitrate in vegetable powders by high-precision isotope dilution GC–MS
Food Chemistry, 2019Co-Authors: Enea Pagliano, Zoltan MesterAbstract:Abstract A high-precision isotope dilution GC–MS method was employed for the determination of nitrate in processed vegetables. The samples were extracted in water, derivatized with Triethyloxonium Tetrafluoroborate and analyzed by headspace GC–MS (15 samples/h). The method was applied to estimate the effect of drying on the content of nitrate in vegetables. The absolute amount of nitrate in a spinach sample before and after drying did not change even when the material was baked at 105 °C. Elevated levels of nitrate were found in commercial vegetable powders where the nitrate mass fraction exceed the percent level: 1.2–2.3% NO3− was found in spinach powders, 1.3–1.6% in kale powders, and 1.4% in a beetroot powder. The likely reduction of the antioxidant properties seen in vegetable powders along with their high nitrate content suggest the need to study the risk of endogenous N-nitrosation associated with these products.
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determination of thiocyanate in saliva by headspace gas chromatography mass spectrometry following a single step aqueous derivatization with Triethyloxonium Tetrafluoroborate
Journal of Chromatography A, 2015Co-Authors: Sara Ammazzini, Enea Pagliano, Massimo Onor, Emanuela Pitzalis, Zoltan Mester, Beatrice Campanella, Emilia Bramanti, Alessandro DulivoAbstract:Abstract A novel method for the determination of salivary thiocyanate is presented. Thiocyanate was converted into ethyl thiocyanate by single-step aqueous derivatization based on Triethyloxonium Tetrafluoroborate and measured by gas chromatography-mass spectrometry (15 min runtime). The ethyl thiocyanate derivative is volatile and can be sampled from the headspace. The derivatization chemistry proposed allows for separation of the analyte from saliva matrix whose introduction in the measurement system is avoided. Quantitation of the analyte was obtained by isotope dilution, employing a 13C-enriched thiocyanate as internal standard. Technical details and fundamental aspects of derivatization chemistry and calibration strategy are presented. The method was validated by comparison with a standard method based on ion chromatography. The two independent methodologies produced results in agreement within 3%. Also a three level spike recovery test was carried out for validation purpose and quantitative recoveries were attained. The method is fast, simple, safe, and sensitive. Measurement of a 1 mL volume 50 ng/g of thiocyanate standard produced a signal-to-noise ratio of 250 for the analytical peak. This method is therefore suitable for ultra-trace determination of thiocyanate (low part-per-billion range). For the application described the full detection potential of the method was not required and the sample preparation presented has been designed for quantitation of saliva samples containing 1–400 μg/g of thiocyanate with a combined standard uncertainty of 2% relative for saliva samples containing 25 μg/g of thiocyanate. This method was applied for the determination of thiocyanate in human saliva samples.
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high precision quadruple isotope dilution method for simultaneous determination of nitrite and nitrate in seawater by gcms after derivatization with Triethyloxonium Tetrafluoroborate
Analytica Chimica Acta, 2014Co-Authors: Enea Pagliano, Juris Meija, Zoltan MesterAbstract:Abstract Quadruple isotope dilution mass spectrometry (ID 4 MS) has been applied for simultaneous determination of nitrite and nitrate in seawater. ID 4 MS allows high-precision measurements and entails the use of isotopic internal standards ( 18 O-nitrite and 15 N-nitrate). We include a tutorial on ID 4 MS outlining optimal experimental design which generates results with low uncertainties and obviates the need for direct (separate) evaluation of the procedural blank. Nitrite and nitrate detection was achieved using a headspace GCMS procedure based on single-step aqueous derivatization with Triethyloxonium Tetrafluoroborate at room temperature. In this paper the sample preparation was revised and fundamental aspects of this chemistry are presented. The proposed method has detection limits in the low parts-per-billion for both analytes, is reliable, precise, and has been validated using a seawater certified reference material (MOOS-2). Simplicity of the experimental design, low detection limits, and the use of quadruple isotope dilution makes the present method superior to the state-of-the-art for determination of nitrite and nitrate, and an ideal candidate for reference measurements of these analytes in seawater.
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negative chemical ionization gc ms determination of nitrite and nitrate in seawater using exact matching double spike isotope dilution and derivatization with Triethyloxonium Tetrafluoroborate
Analytical Chemistry, 2012Co-Authors: Enea Pagliano, Zoltan Mester, Ralph E Sturgeon, Juris Meija, Alessandro DulivoAbstract:The alkylation of nitrite and nitrate by Triethyloxonium Tetrafluoroborate allows determination of their ethyl esters by headspace gas chromatography/mass spectrometry (GC/MS). In the present study, significant improvement in analytical performance is achieved using negative chemical ionization providing detection limits of 150 ng/L for NO2– and 600 ng/L for NO3–, an order of magnitude better than those achieved using electron impact ionization. The derivatization procedure was optimized and alkaline conditions adopted to minimize conversion of nitrite to nitrate (determined to be 0.07% at 100 mg/L NO2–) and to avoid the exchange of oxygen between the analytes and the solvent (water). Quantitation entails use of isotopically enriched standards (N18O2– and 15NO3–), which also permits monitoring of potential conversion from nitrite to nitrate during the analysis (double spike isotope dilution).
Zoltan Mester - One of the best experts on this subject based on the ideXlab platform.
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A rapid and sensitive method for the determination of inorganic chloride in oil samples
Analytica Chimica Acta, 2019Co-Authors: Zuzana Gajdosechova, Zoltan Mester, Enea PaglianoAbstract:Abstract A novel isotope dilution method for the analysis of inorganic chloride in fuel oil matrix is presented. The samples were diluted by isopropyl alcohol:toluene, mixed with 37Cl− internal standard and reacted with Triethyloxonium Tetrafluoroborate at room temperature. This reagent promoted conversion of Cl− into stable ethyl chloride (EtCl) which was selectively detected by headspace GC–MS/MS with no matrix effects. A limit of detection of 0.2 mg kg−1 Cl− was obtained in fuel oil. The method was tested on the NIST SRM 1634c (trace elements in residual fuel oil) and validated through a series of robustness tests. Over 18 days, the variation in the signal response was less than 10% and the RSD for quantitative isotope dilution results was below 3%. Overall, the method is fast, simple and allows robust quantitation of inorganic chloride directly in the non-aqueous media.
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Determination of elevated levels of nitrate in vegetable powders by high-precision isotope dilution GC–MS
Food Chemistry, 2019Co-Authors: Enea Pagliano, Zoltan MesterAbstract:Abstract A high-precision isotope dilution GC–MS method was employed for the determination of nitrate in processed vegetables. The samples were extracted in water, derivatized with Triethyloxonium Tetrafluoroborate and analyzed by headspace GC–MS (15 samples/h). The method was applied to estimate the effect of drying on the content of nitrate in vegetables. The absolute amount of nitrate in a spinach sample before and after drying did not change even when the material was baked at 105 °C. Elevated levels of nitrate were found in commercial vegetable powders where the nitrate mass fraction exceed the percent level: 1.2–2.3% NO3− was found in spinach powders, 1.3–1.6% in kale powders, and 1.4% in a beetroot powder. The likely reduction of the antioxidant properties seen in vegetable powders along with their high nitrate content suggest the need to study the risk of endogenous N-nitrosation associated with these products.
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determination of thiocyanate in saliva by headspace gas chromatography mass spectrometry following a single step aqueous derivatization with Triethyloxonium Tetrafluoroborate
Journal of Chromatography A, 2015Co-Authors: Sara Ammazzini, Enea Pagliano, Massimo Onor, Emanuela Pitzalis, Zoltan Mester, Beatrice Campanella, Emilia Bramanti, Alessandro DulivoAbstract:Abstract A novel method for the determination of salivary thiocyanate is presented. Thiocyanate was converted into ethyl thiocyanate by single-step aqueous derivatization based on Triethyloxonium Tetrafluoroborate and measured by gas chromatography-mass spectrometry (15 min runtime). The ethyl thiocyanate derivative is volatile and can be sampled from the headspace. The derivatization chemistry proposed allows for separation of the analyte from saliva matrix whose introduction in the measurement system is avoided. Quantitation of the analyte was obtained by isotope dilution, employing a 13C-enriched thiocyanate as internal standard. Technical details and fundamental aspects of derivatization chemistry and calibration strategy are presented. The method was validated by comparison with a standard method based on ion chromatography. The two independent methodologies produced results in agreement within 3%. Also a three level spike recovery test was carried out for validation purpose and quantitative recoveries were attained. The method is fast, simple, safe, and sensitive. Measurement of a 1 mL volume 50 ng/g of thiocyanate standard produced a signal-to-noise ratio of 250 for the analytical peak. This method is therefore suitable for ultra-trace determination of thiocyanate (low part-per-billion range). For the application described the full detection potential of the method was not required and the sample preparation presented has been designed for quantitation of saliva samples containing 1–400 μg/g of thiocyanate with a combined standard uncertainty of 2% relative for saliva samples containing 25 μg/g of thiocyanate. This method was applied for the determination of thiocyanate in human saliva samples.
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high precision quadruple isotope dilution method for simultaneous determination of nitrite and nitrate in seawater by gcms after derivatization with Triethyloxonium Tetrafluoroborate
Analytica Chimica Acta, 2014Co-Authors: Enea Pagliano, Juris Meija, Zoltan MesterAbstract:Abstract Quadruple isotope dilution mass spectrometry (ID 4 MS) has been applied for simultaneous determination of nitrite and nitrate in seawater. ID 4 MS allows high-precision measurements and entails the use of isotopic internal standards ( 18 O-nitrite and 15 N-nitrate). We include a tutorial on ID 4 MS outlining optimal experimental design which generates results with low uncertainties and obviates the need for direct (separate) evaluation of the procedural blank. Nitrite and nitrate detection was achieved using a headspace GCMS procedure based on single-step aqueous derivatization with Triethyloxonium Tetrafluoroborate at room temperature. In this paper the sample preparation was revised and fundamental aspects of this chemistry are presented. The proposed method has detection limits in the low parts-per-billion for both analytes, is reliable, precise, and has been validated using a seawater certified reference material (MOOS-2). Simplicity of the experimental design, low detection limits, and the use of quadruple isotope dilution makes the present method superior to the state-of-the-art for determination of nitrite and nitrate, and an ideal candidate for reference measurements of these analytes in seawater.
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negative chemical ionization gc ms determination of nitrite and nitrate in seawater using exact matching double spike isotope dilution and derivatization with Triethyloxonium Tetrafluoroborate
Analytical Chemistry, 2012Co-Authors: Enea Pagliano, Zoltan Mester, Ralph E Sturgeon, Juris Meija, Alessandro DulivoAbstract:The alkylation of nitrite and nitrate by Triethyloxonium Tetrafluoroborate allows determination of their ethyl esters by headspace gas chromatography/mass spectrometry (GC/MS). In the present study, significant improvement in analytical performance is achieved using negative chemical ionization providing detection limits of 150 ng/L for NO2– and 600 ng/L for NO3–, an order of magnitude better than those achieved using electron impact ionization. The derivatization procedure was optimized and alkaline conditions adopted to minimize conversion of nitrite to nitrate (determined to be 0.07% at 100 mg/L NO2–) and to avoid the exchange of oxygen between the analytes and the solvent (water). Quantitation entails use of isotopically enriched standards (N18O2– and 15NO3–), which also permits monitoring of potential conversion from nitrite to nitrate during the analysis (double spike isotope dilution).
Massimo Onor - One of the best experts on this subject based on the ideXlab platform.
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determination of thiocyanate in saliva by headspace gas chromatography mass spectrometry following a single step aqueous derivatization with Triethyloxonium Tetrafluoroborate
Journal of Chromatography A, 2015Co-Authors: Sara Ammazzini, Enea Pagliano, Massimo Onor, Emanuela Pitzalis, Zoltan Mester, Beatrice Campanella, Emilia Bramanti, Alessandro DulivoAbstract:Abstract A novel method for the determination of salivary thiocyanate is presented. Thiocyanate was converted into ethyl thiocyanate by single-step aqueous derivatization based on Triethyloxonium Tetrafluoroborate and measured by gas chromatography-mass spectrometry (15 min runtime). The ethyl thiocyanate derivative is volatile and can be sampled from the headspace. The derivatization chemistry proposed allows for separation of the analyte from saliva matrix whose introduction in the measurement system is avoided. Quantitation of the analyte was obtained by isotope dilution, employing a 13C-enriched thiocyanate as internal standard. Technical details and fundamental aspects of derivatization chemistry and calibration strategy are presented. The method was validated by comparison with a standard method based on ion chromatography. The two independent methodologies produced results in agreement within 3%. Also a three level spike recovery test was carried out for validation purpose and quantitative recoveries were attained. The method is fast, simple, safe, and sensitive. Measurement of a 1 mL volume 50 ng/g of thiocyanate standard produced a signal-to-noise ratio of 250 for the analytical peak. This method is therefore suitable for ultra-trace determination of thiocyanate (low part-per-billion range). For the application described the full detection potential of the method was not required and the sample preparation presented has been designed for quantitation of saliva samples containing 1–400 μg/g of thiocyanate with a combined standard uncertainty of 2% relative for saliva samples containing 25 μg/g of thiocyanate. This method was applied for the determination of thiocyanate in human saliva samples.
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quantification of nitrite and nitrate in seawater by Triethyloxonium Tetrafluoroborate derivatization headspace spme gc ms
Talanta, 2011Co-Authors: Enea Pagliano, Massimo Onor, Emanuela Pitzalis, Zoltan Mester, Ralph E Sturgeon, Alessandro DulivoAbstract:Abstract Triethyloxonium Tetrafluoroborate derivatization combined with direct headspace (HS) or SPME-gas chromatography–mass spectrometry (GC–MS) is proposed here for the simultaneous determination of nitrite and nitrate in seawater at micromolar level after conversion to their corresponding volatile ethyl-esters (EtO-NO and EtO-NO2). Isotopically enriched nitrite [15N] and nitrate [15N] are employed as internal standards and for quantification purposes. HS-GC–MS provided instrumental detection limits of 0.07 μM NO2− and 2 μM NO3−. Validation of the methodology was achieved by determination of nitrite and nitrate in MOOS-1 (Seawater Certified Reference Material for Nutrients, NRC Canada), yielding results in excellent agreement with certified values. All critical aspects connected with the potential inter-conversion between nitrite and nitrate (less than 10%) were evaluated and corrected for by the use of the isotopically enriched internal standard.
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Quantification of nitrite and nitrate in seawater by Triethyloxonium Tetrafluoroborate derivatization—Headspace SPME GC–MS
Talanta, 2011Co-Authors: Enea Pagliano, Massimo Onor, Emanuela Pitzalis, Zoltan Mester, Ralph E Sturgeon, Alessandro D'ulivoAbstract:Abstract Triethyloxonium Tetrafluoroborate derivatization combined with direct headspace (HS) or SPME-gas chromatography–mass spectrometry (GC–MS) is proposed here for the simultaneous determination of nitrite and nitrate in seawater at micromolar level after conversion to their corresponding volatile ethyl-esters (EtO-NO and EtO-NO2). Isotopically enriched nitrite [15N] and nitrate [15N] are employed as internal standards and for quantification purposes. HS-GC–MS provided instrumental detection limits of 0.07 μM NO2− and 2 μM NO3−. Validation of the methodology was achieved by determination of nitrite and nitrate in MOOS-1 (Seawater Certified Reference Material for Nutrients, NRC Canada), yielding results in excellent agreement with certified values. All critical aspects connected with the potential inter-conversion between nitrite and nitrate (less than 10%) were evaluated and corrected for by the use of the isotopically enriched internal standard.
Emanuela Pitzalis - One of the best experts on this subject based on the ideXlab platform.
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determination of thiocyanate in saliva by headspace gas chromatography mass spectrometry following a single step aqueous derivatization with Triethyloxonium Tetrafluoroborate
Journal of Chromatography A, 2015Co-Authors: Sara Ammazzini, Enea Pagliano, Massimo Onor, Emanuela Pitzalis, Zoltan Mester, Beatrice Campanella, Emilia Bramanti, Alessandro DulivoAbstract:Abstract A novel method for the determination of salivary thiocyanate is presented. Thiocyanate was converted into ethyl thiocyanate by single-step aqueous derivatization based on Triethyloxonium Tetrafluoroborate and measured by gas chromatography-mass spectrometry (15 min runtime). The ethyl thiocyanate derivative is volatile and can be sampled from the headspace. The derivatization chemistry proposed allows for separation of the analyte from saliva matrix whose introduction in the measurement system is avoided. Quantitation of the analyte was obtained by isotope dilution, employing a 13C-enriched thiocyanate as internal standard. Technical details and fundamental aspects of derivatization chemistry and calibration strategy are presented. The method was validated by comparison with a standard method based on ion chromatography. The two independent methodologies produced results in agreement within 3%. Also a three level spike recovery test was carried out for validation purpose and quantitative recoveries were attained. The method is fast, simple, safe, and sensitive. Measurement of a 1 mL volume 50 ng/g of thiocyanate standard produced a signal-to-noise ratio of 250 for the analytical peak. This method is therefore suitable for ultra-trace determination of thiocyanate (low part-per-billion range). For the application described the full detection potential of the method was not required and the sample preparation presented has been designed for quantitation of saliva samples containing 1–400 μg/g of thiocyanate with a combined standard uncertainty of 2% relative for saliva samples containing 25 μg/g of thiocyanate. This method was applied for the determination of thiocyanate in human saliva samples.
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quantification of nitrite and nitrate in seawater by Triethyloxonium Tetrafluoroborate derivatization headspace spme gc ms
Talanta, 2011Co-Authors: Enea Pagliano, Massimo Onor, Emanuela Pitzalis, Zoltan Mester, Ralph E Sturgeon, Alessandro DulivoAbstract:Abstract Triethyloxonium Tetrafluoroborate derivatization combined with direct headspace (HS) or SPME-gas chromatography–mass spectrometry (GC–MS) is proposed here for the simultaneous determination of nitrite and nitrate in seawater at micromolar level after conversion to their corresponding volatile ethyl-esters (EtO-NO and EtO-NO2). Isotopically enriched nitrite [15N] and nitrate [15N] are employed as internal standards and for quantification purposes. HS-GC–MS provided instrumental detection limits of 0.07 μM NO2− and 2 μM NO3−. Validation of the methodology was achieved by determination of nitrite and nitrate in MOOS-1 (Seawater Certified Reference Material for Nutrients, NRC Canada), yielding results in excellent agreement with certified values. All critical aspects connected with the potential inter-conversion between nitrite and nitrate (less than 10%) were evaluated and corrected for by the use of the isotopically enriched internal standard.
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Quantification of nitrite and nitrate in seawater by Triethyloxonium Tetrafluoroborate derivatization—Headspace SPME GC–MS
Talanta, 2011Co-Authors: Enea Pagliano, Massimo Onor, Emanuela Pitzalis, Zoltan Mester, Ralph E Sturgeon, Alessandro D'ulivoAbstract:Abstract Triethyloxonium Tetrafluoroborate derivatization combined with direct headspace (HS) or SPME-gas chromatography–mass spectrometry (GC–MS) is proposed here for the simultaneous determination of nitrite and nitrate in seawater at micromolar level after conversion to their corresponding volatile ethyl-esters (EtO-NO and EtO-NO2). Isotopically enriched nitrite [15N] and nitrate [15N] are employed as internal standards and for quantification purposes. HS-GC–MS provided instrumental detection limits of 0.07 μM NO2− and 2 μM NO3−. Validation of the methodology was achieved by determination of nitrite and nitrate in MOOS-1 (Seawater Certified Reference Material for Nutrients, NRC Canada), yielding results in excellent agreement with certified values. All critical aspects connected with the potential inter-conversion between nitrite and nitrate (less than 10%) were evaluated and corrected for by the use of the isotopically enriched internal standard.
