Biomedical subjects

T W Burgoyne

Publications and source records attributed to T W Burgoyne.

3 recordsLinked to original sources

PubMed2001

Determination of the the H3 factor in hydrogen isotope ratio monitoring mass spectrometry.

The H3 factor, K, is a parameter required in high-precision, mass spectrometric analyses of hydrogen isotopic abundances. When H2 is used as the sample gas, R* = R - Ki2, where R* is the true HD/H2 ratio, R is the observed (mass 3)/(mass 2) ion-current ratio, and i2 is the ion current at mass 2. Four different methods for the determination of K were defined and tested under conditions characteristic of isotope ratio monitoring systems. Three of these were peak-based. The fourth employed steady flows of H2 from a conventional inlet system. Results obtained using the latter method were more precise (standard deviation of K = 0.1 versus approximately 0.6 ppm mV(-1) for the peak-based methods). However, use of the resulting values of K for correction of isotope ratio monitoring GC/MS results led to systematic errors as large as 9 per thousand, whereas use of the peak-based values led to no systematic errors. Values of K were only weakly dependent on the pressure of He, declining approximately 5% for each 10-fold increase in P(He). Small variations in partial pressures of H2O and CH4, potential contaminants under isotope ratio monitoring conditions, had no significant effect on values of K.

Journal Article↗

PubMed2001

Correction of H3+ contributions in hydrogen isotope ratio monitoring mass spectrometry.

Two fundamentally different approaches, termed "pointwise" and "peakwise," are currently used to correct hydrogen isotope ratio monitoring data for the presence of H3+ ion contributions. Consideration of the underlying assumptions shows that the peakwise approach is valid only for peaks with the same functional shape and only when background signals do not vary. The pointwise correction is much more versatile and can be used even when peak shapes and sizes, as well as background signals, vary significantly. It is not exact and is limited in accuracy by (1) the signal-broadening effects of electronic time constants, (2) the analog-to-digital conversion frequency, and (3) the highest frequency of the sample signal. To minimize errors for typical gas chromatographic signals, time constants of <500 ms and analog-to-digital sampling intervals of < or =250 ms are needed. Errors are further minimized by matching sample and standard peaks in both amplitude and D/H ratio. Using the pointwise algorithm, we demonstrate that a series of 14 homologous n-alkanes varying in concentration over a 5-fold range can be analyzed with a mean precision of 2.3 per thousand and no systematic errors.

Journal Article↗

PubMed1997

Space charge evaluation in a plasma-source mass spectrograph.

Space charge effects, and the matrix interferences they cause, are problems in inductively coupled plasma mass spectrometry (ICPMS). It has previously been observed that these deleterious space charge effects are not significantly present in sector-field instruments, a fact that has been attributed, but not demonstrated, to the high accelerating potentials they commonly employ. To examine the significance of space charge in our plasma-source mass spectrograph (which operates at only moderate accelerating potentials) and in other sector instruments, a graphite disk was placed approximately 7 cm behind the skimmer. An inductively coupled plasma was operated for 17 h while a 0.01 mM multielement solution was introduced. This disk was then analyzed by spatially resolved laser ablation ICP time-of-flight MS. Second vacuum-stage acceleration appears to be an important factor that governs the elemental distribution within the ion beam. The ion beam width at m/z 208 is one-third of its width at m/z 7 using an accelerating potential of 800 V; at an accelerating potential of 4000 V, the ion beam width does not vary with mass.

Mass Spectrometry↗