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Gordon J Nelson

Publications and source records attributed to Gordon J Nelson.

6 recordsLinked to original sources
PubMed2005

Assessing analytical specificity in quantitative analysis using tandem mass spectrometry.

OBJECTIVES: The necessity of confirmation of compound identity in quantitative analysis is well recognized for methods utilizing single mass spectrometry detection but is not commonly addressed for applications utilizing multiple-stage mass spectrometry (MSn). For MSn detection, no commonly accepted rules for assessment of analytical specificity in quantitative analyses have been established to date. METHODS: To assure compound identity, we evaluated approaches based on monitoring multiple mass transitions of a target compound followed by comparison of the branching ratios of the mass transitions. RESULTS: Monitoring multiple mass transitions along with evaluation of the ratio of their relative intensities allows the analyst to distinguish the target analyte from interferences in quantitative analysis. The strategy and the acceptance criteria are compound and method specific and should be established during the method development and validation. CONCLUSIONS: The certainty of analyte identity is very important in quantitative analysis using MSn detection; methods to verify analyte identity should be used in all critical applications.

Cortisone↗
PubMed2005

Simultaneous analysis of the Delta9-THC metabolites 11-nor-9-carboxy-Delta9-THC and 11-hydroxy-Delta9-THC in meconium by GC-MS.

Neonates that are exposed to cannabinoids in utero may have characteristic physical and mental developmental problems throughout their lives. The early identification of exposed neonates allows early intervention and anticipation of potential problems. Testing meconium detects maternal marijuana use over the last four months of gestation, providing a better drug exposure marker than urine. However, the distribution of metabolites in meconium is not identical to urine and analytical methods must be adapted. Both the major urine metabolite, 11-nor-9-carboxy-Delta9-tetrahydrocannabinol (9-carboxy-THC), and a minor urine metabolite, 11-hydroxy-Delta9-tetrahydrocannabinol (11-hydroxy-THC), are common in meconium. Currently published methods to extract these two metabolites for instrumental analysis are time-consuming and laborious, often involving the preparation of two fractions. This study describes a simple solid-phase extraction method and an optimized hydrolysis method that allow the preparation and analysis of both metabolites in a single extract. The limit of detection by this extraction method was 5 ng/g for both metabolites with an analytical measurement range from 10 to 500 ng/g. The recovery at 100 ng/g was greater than 62% for both analytes. The analysis of 246 cannabinoid screen positive specimens illustrated the importance of including the 11-hydroxy-THC in a meconium marijuana confirmation: 16 specimens confirmed positive for 11-hydroxy-THC only, resulting in a 6.5% increase in the positivity rate compared to 9-carboxy-THC alone.

Dronabinol↗
PubMed2004

Simultaneous quantitative analysis of isobars by tandem mass spectrometry from unresolved chromatographic peaks.

A method was developed for the simultaneous quantitation of isobars from unresolved chromatographic peaks. The method is based on differences in branching ratios of ion abundances in their tandem mass spectra and an assumption that the product ion mass spectra of a mixture can be considered as a linear combination of the spectra of individual constituents. We present analytical equations and a matrix-based approach for deconvoluting the concentration of individual components from the total peak intensity for two and three isobars and also a matrix-based generalization to any number of compounds. The feasibility of the simultaneous analysis of mixtures containing two compounds was assessed. The approach was evaluated for the analysis of structural isomers of methylmalonic and succinic acids in human plasma and urine samples for a group of 270 samples. The linear regression equation, standard error and correlation coefficient for the agreement with a traditional method utilizing chromatographic separation of the isomers were y = 0.999x - 0.005, 0.024 micro mol l(-1), and 0.985, respectively. The utility of a spectral contrast angle as a predictor of analysis feasibility was evaluated.

Algorithms↗
PubMed2003

Dissociation of individual isotopic peaks: predicting isotopic distributions of product ions in MSn.

Traditional practice in tandem mass spectrometry is to select the mono-isotopic ion for dissociation. However, high molecular weight compounds often have weak mono-isotopic peaks, which limit that approach. Furthermore, the traditional approach does not take advantage of the very rich store of information available in the isotopic patterns from the dissociation of individual non-mono-isotopic peaks. Interpretation of these isotopic patterns requires a theory capable of predicting the patterns. However, a general theory for the prediction of these patterns has been lacking. This paper shows that the patterns can be obtained from a certain vector product, the outer product, of the full isotopic distribution of the product ion with the full isotopic distribution of the complementary product. Unlike previous approaches, the method is applicable to systems of arbitrary isotopic complexity. The patterns are potentially useful for elucidation of dissociation pathways, elemental composition, and chemical structure. The paper presents several applications of the theory.

Journal Article↗
PubMed2003

Pseudo-Cushing syndrome caused by fenofibrate interference with urinary cortisol assayed by high-performance liquid chromatography.

Urinary free cortisol (UFC) excretion over 24 h reflects the production rate of cortisol and is used commonly in the diagnosis of Cushing syndrome. We report on two patients evaluated for Cushing syndrome who had elevated UFC when analyzed by HPLC but normal values for the analysis performed by RIA and HPLC-mass spectrometry/mass spectrometry (HPLC-MS/MS). Other laboratory testing was inconsistent with the diagnosis of Cushing syndrome and raised doubts about the diagnosis. We identified a probable cause of analytical interference as coming from fenofibrate (Tricor), medication taken by the patients. Fenofibrate peak overlapped with the HPLC peak of cortisol and produced an MS/MS transition overlapping the major transition of cortisol. A second MS/MS transition was free from interference. In summary, fenofibrate administration may cause false elevation of UFC values determined by HPLC or HPLC-MS/MS in patients evaluated for Cushing syndrome. An HPLC-MS/MS method using multiple mass transitions, rather than a single transition, allows accurate quantitation of urinary cortisol in patients taking fenofibrate.

Adult↗