Isotope dilution studies: determination of carbon-13, nitrogen-15 and deuterium-enriched compounds using capillary gas chromatography-chemical reaction interface/mass spectrometry

Liquid chromatography/chemical reaction interface mass spectrometry as an alternative to radioisotopes for quantitative drug metabolism studies

Chemical reaction interface mass spectrometry (CRIMS) was coupled on-line with HPLC using a Vestec particle beam interface. A helium-assisted nebulizer provided added stability with no loss in accuracy or precision as compared to the thermospray nebulizer at flow rates of up to 1.0 mL/min using isocratic conditions. However, mass spectral response was found to be solvent-dependent for both the helium-assisted and thermospray nebulizers. Postcolumn solvent addition of methanol eliminated solvent-dependent decreases in mass spectral response. This allowed gradient HPLC elutions to be performed. Under these conditions, the flow of solvent into the particle beam interface was 2.5 mL/min, so a conventional thermospray nebulizer had to be used instead of the helium-assisted nebulizer. Experiments were conducted with the antianxiety agent buspirone in order to validate the methodology. Metabolites from in vitro incubations of [15N]/[14C]buspirone with rat liver slices were analyzed by gradient LC/CRIMS and by gradient LC/[14C] radioactivity counting. The response from LC/CRIMS analysis for individual metabolites was then compared with that obtained by LC/[14C] radioactivity counting. An excellent correlation was observed between the two methods for metabolites with quite different HPLC characteristics. Thus, gradient LC/CRIMS in combination with stable isotopes provides an alternative to using radioisotopes for carrying out drug metabolism studies.

Element- and isotope-specific detection for high-performance liquid chromatography using chemical reaction interface mass spectrometry

We have developed a combination of high-performance liquid chromatography (HPLC) and the chemical reaction interface mass spectrometry (CRIMS) method by using a Vestec Universal Interface (UI). This interface provides the extremely high degree of solvent removal that the CRIMS process requires. In doing so, we have produced an HPLC detector with the ability to carry out the element- and isotope-selective analyses with detection that is inherently: linear, structure-independent, sensitive, selective, comprehensive and flexible. The characteristics of the instrumentation and its performance are described.

An inductively coupled plasma-time-of-flight mass spectrometer for elemental analysis. Part III: Analytical performance

A time-of-flight mass spectrometer (TOFMS) was evaluated as a mass analyzer for inductively coupled plasma mass spectrometry (ICP-MS). The long-term drift of signals was in the range of 7-8% relative standard deviation, whereas the short-term precision was between 5 and 20%, somewhat worse than is typically reported for commercial ICP-MS instruments (5%). However, precision can be improved considerably in the TOFMS by ratioing isotopic peaks or through internal standardization, a consequence of its ability to extract all measured ions simultaneously from the inductively coupled plasma. This feature was demonstrated by monitoring the (206)Pb/(208)Pb ratio with boxcar averagers. In this ratioing mode, precision was improved to approximately 0. 5%. Detection limits were measured with two alternative signal processing systems: (1) discriminator-gated integration and (2) integration of digitized spectra. Both methods improved the signal-to-noise ratio by a factor of from 10 to 100, although detection limits were still 1-2 orders of magnitude poorer for most elements than from the best commercial ICP-MS instruments. The dynamic range of the discriminator-gated integration system is over 4 orders of magnitude, but can be extended to 10(6) with planned increases in primary ion-beam current, which is currently 10-100 times lower than is found in other instruments. Virtually simultaneous multielement and multiisotope analysis is possible for masses from (7)Li to (209)Bi with minimal mass bias and detection limits on the 0. 4-2-ppb level.

Tracing 15N with chemical reaction interface mass spectrometry: a demonstration using 15N-labeled glutamine and asparagine substrates in cell culture

This research demonstrates how the chemical reaction interface mass spectrometry (CRIMS) approach works for a study of amino acid metabolism in cell culture. 15N-selective chromatograms from both the culture medium and the cytosol of human hepatoma Hep G2 cells that were incubated in the presence of either 12 mM (alpha-15N)glutamine or (alpha-15N)asparagine have been produced. The time course of the distribution of 15N among different amino acids, as well as the enrichment for each amino acid, were observed over a 144 h period. Labeled glutamine was quickly converted into glutamate. After 144 h of incubation, the total amount of 15N was distributed primarily among alanine (50%), proline (28%) and glutamate (21%). The 15N enrichment of alanine and proline reached 44% and 41% respectively. Asparagine was only slowly metabolized by the cells. In addition to the 82% that was retained in asparagine, the remaining 15N in the media at 144 h was found primarily in alanine (8%), glutamate (6.8%) and proline (2.2%). Their enrichments were 20%, 36% and 19% respectively. The minimum detectable amount was 17 pg of 15N entering the CRI. CRIMS appears to be a powerful, facile approach for 15N-tracer experiments.

Implementation of the chemical reaction interface mass spectrometry technique on a Hewlett-Packard mass-selective detector

A microwave-powered chemical reaction interface has been installed in a Hewlett-Packard gas chromatograph-mass spectrometer (GC-MS) system (5890 IT gas chromatograph-S971 mass-selective detector). The technical details and optimization strategies are discussed. The evaluation of this new setup is presented, showing detection limits of 1 ng of (13)C-, (15)N-, and Cl-containing compounds with signal-to-noise ratios greater than or egual to 3. Selective detection was evaluated with a urine sample from a dog dosed with (15)N3-midazolam that had been previously analyzed by using a differentially pumped research-level quadrupole mass spectrometer. The results show that the detection of (15)N and Cl remains highly selective and the mass-selective detector gives comparable sensitivity to the larger instrument when the latter is operating over a conventional mass range. The capability for chemical reaction interface mass spectrometry can be easily accomplished with an inexpensive GC-MS system.

Development of an isotope-selective high-performance liquid chromatography detector using chemical-reaction-interface mass spectrometry: application to deuterated cortisol metabolites in urine

An isotope-selective detector for HPLC based on the particle-beam-interface and the chemical-reaction-interface mass spectrometer (CRIMS) principle is described. This paper focuses on the selective detection of deuterium-labeled analytes. The CRIMS product HD is detected as has been previously described for GC-CRIMS. The analytical performance was not affected by analyte structure or solvent composition. Deuterium detection was linear from 20 ng to 490 ng using 2H3-labeled cortisol. Based on this method, the fractional abundance of three labeled metabolites of cortisol were determined in the urine of a patient infused with tracer amounts of deuterated cortisol.

Improved measurement of stable isotope ratios in gas chromatography/mass spectrometry using the microwave-powered chemical reaction interface for mass spectrometry

The microwave-powered chemical reaction interface for mass spectrometry (CRIMS) has been successfully used for selective detection of analytes labeled with 13C, 15N and D following capillary gas chromatography separation with good analytical characteristics in biological applications. In this study we evaluated how an advanced data system coupled to a quadrupole mass analyzer could improve precision and sensitivity of stable isotope ratio measurements for 13C and 15N. The enrichments of 13C and 15N are determined by monitoring CO2 (m/z 44 and 45) and NO (m/z 30 and 31). These small molecules are produced from the analyte in the chemical reaction interface in the presence of SO2 as a reactant gas. Using caffeine and its 13C1, 15N2-labeled analog for these quantitative studies, we have found that the Vector 2 system improves overall precision (RSD = 0.6% for both carbon and nitrogen) and sensitivity of stable isotope measurements by at least a factor of two compared to the Vector 1 system, and by more than an order of magnitude compared to our older results. With the optimum system we are now able to measure an atom% enrichment of 0.0044 for 15N and 0.015 for 13C in caffeine in the presence of 300 ng of unlabeled material. This is more than half way between isotopic detection limits of conventional gas chromatography/mass spectrometry and the state-of-the-art, which is a gas chromatograph coupled to a chemical combustor and a dual-collector isotope ratio mass spectrometer.

A moving belt device to couple high-performance liquid chromatography and chemical reaction interface mass spectrometry

A device is described to enable selective detection of 13C- and 15N-labeled compounds following separation by high-performance liquid chromatography (HPLC). A thermospray vaporizer deposits the materials eluting from the HPLC column onto a continuously moving endless belt. The belt carries these compounds into the chemical reaction interface, where a microwave-induced helium plasma converts complex organic molecules in the presence of a reactant gas into small stable molecules that are detected by the mass spectrometer. Chromatograms showing only compounds enriched with 13C and 15N can be obtained by subtracting the abundance of naturally occurring isotopes from the observed M + 1 signal. This work demonstrates the feasibility of this approach and encourages its further development.

Selective detection of sulfur-containing compounds by gas chromatography/chemical reaction interface mass spectrometry

Addition of a reactant gas to a low pressure microwave-induced plasma creates a reaction interface in which complex molecules are converted into small polyatomic neutral species. For a given reactant gas the array of these small molecules reflect s the elemental composition of the original analyte. In this study HCI has been found highly effective as a reactant gas for selective detection of sulfur-eontaining compounds using capillary gas chromatography/ chernical reaction interface mass spectrometry. Detection limits as low as 30 pg of a sulfur-containing compound and a dynamic range of two orders of magnitude were achieved.