The utility of adding ion mobility (IM) to liquid chromatography/mass spectrometry (LC/MS) for quantitation of 25hydroxyvitamin D 3 (25OHD3) is evaluated. Sodiated 25OHD3 ions adopt both closed and open conformations, whereas the stereoisomer 3-epi-25-hydroxyvitamin D (epi25OHD), when sodiated, adopts only a closed gas-phase conformation. The unique open conformation for sodiated 25OHD3 permits unambiguous quantitation. Nonetheless, the ratio of open versus closed gas-phase conformations for sodiated 25OHD3 can vary with instrumental conditions; conversion from the open to the closed conformer is attributed to radio frequency (rf) heating within the ion accumulation trap. Ion heating becomes significant when space charge in the trap forces ions to larger radii where the rf field is higher. To avoid biasing quantitation, an isotopically labeled internal standard must be used to account for changes in conformer ratio. Thirty-three serum extract samples were evaluated by LC/IM-MS and were found to not be biased by changes in ion conformer ratios, permitting reliable quantitation of 25OHD3 without interference from the epimer.
Ion mobility was
integrated with liquid chromatography/high resolution
mass spectrometry (LC/IM-HRMS) to quantify 25-hydroxyvitamin D (25OHD)
in human serum. It has previously been shown that 25OHD adopts two
gas-phase conformations which are resolved using ion mobility; in
contrast, the inactive epimer, 3-epi-25-hydroxyvitamin D (epi25OHD),
only adopts one. Interference from epi25OHD was eliminated by filtering
the chromatogram to retain the drift time that corresponds to the
unique gas-phase conformation of 25OHD. Although ion mobility separates
the epimers, some chromatography is required to separate compounds
which interfere with ionization or fall at the same nominal m/z. Standards were prepared in 4% albumin
solutions and compared against commercial serum quality controls.
Standards and quality controls were analyzed and validated using a
2 min LC/IM-MS method. 25-Hydroxyvitamin D3 and D2 were quantified
over the range between 2 and 500 ng/mL with bias and precision within
15%. When epi25OHD was spiked into quality control samples, no significant
bias was introduced, and analysis of 30 patient samples shows good
agreement between this LC/IM-MS and traditional LC/MS/MS methods.
This work shows that ion mobility can be incorporated with liquid
chromatography and mass spectrometry for rapid quantitation of 25OHD
in human serum.
<p>Ion mobility was integrated with liquid chromatography/high
resolution mass spectrometry (LC/IM-HRMS) to quantify 25-hydroxyvitamin D
(25OHD) in human serum. It has
previously been shown that 25OHD adopts two gas-phase conformations which are
resolved using ion mobility; in contrast, the inactive epimer,
3-epi-25-hydroxyvitamin D (epi25OHD) only adopts one. Interference from epi25OHD was eliminated by
filtering the chromatogram to retain the drift time that corresponds to the
unique gas-phase conformation of 25OHD.
Although ion mobility separates the epimers, some chromatography is
required to separate compounds which interfere with ionization or fall at the
same nominal <i>m/z</i>. Standards were
prepared in 4% albumin solutions and compared against commercial serum quality
controls. Standards and quality controls
were analyzed and validated using a two-minute LC/IM-MS method. 25-hydroxyvitamin D3 and D2 were quantified over
the range between 2 and 500 ng/mL with bias and precision within 15%. When epi25OHD was spiked into quality control
samples, no significant bias was introduced, and analysis of 30 patient samples
shows good agreement between this LC/IM-MS and traditional LC/MS/MS
methods. This work shows that ion
mobility can be incorporated with liquid chromatography and mass spectrometry
for rapid quantitation of 25OHD in human serum.</p>
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