APR Nov/Dec 2022 - 68

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collected, raw data and signal-distribution views can be displayed
to accurately identify fractions, modify thresholds and interrogate
mass-distribution information. In turn, mean and median analyte
mass and number concentration figures can be calculated from this
data representation.
Measuring Selenium in
Selenized Yeast with scICP-MS
Selenium (Se) is a trace element micronutrient and antioxidant that
is thought to protect individuals from thyroid disease, cancer, cardiovascular
disease and even cognitive decline.1
Recent research delivering
Se via selenized yeast has shown reduced low-density lipoprotein
profiles in patients with atherosclerosis2
CRC gas flow
and reduced recurrence of
tumors in patients who initially had an advanced adenoma.3
The bioavailability and low cost of selenized yeast make this a
promising supplement and potential treatment for some diseases.
However, Se can be present in many different forms in yeast - organic
as selenomethionine,4
inorganic as selenite or selenate,5
and also in
nanoparticle form.6 To understand the full potential of how selenized
yeast might be used in disease prevention and treatment, plus how
reliable bioproduction can be maintained, full analyses of the mass
and distribution of Se in yeast are crucial.
In a recent experiment, the Thermo Scientific™ iCAP™ TQ ICP-MS
triple quadrupole system in single-cell acquisition mode was used to
evaluate the presence of Se in a certified reference material sample of
lyophilized yeast cells (SELM-1), a type of selenized yeast. Phosphorus
(P) was also evaluated as a cell marker, since it is a constituent element
in yeast cells, e.g., as a part of the DNA backbone. In this study, the
most common isotopes of both elements were measured in their ionic
forms: 31
P+ and 80
Se+.7
By comparing P and Se events as detected through the ICP-MS
instrument, the fraction of Se-containing cells could be calculated,
as well as the mean and median mass of the analytes within the
cell population.
Method
SELM-1 cells were resuspended in water, washed twice by
centrifugation and then diluted to a final concentration of 50,000 cells
per mL, as confirmed by flow cytometry. Specialized nebulizers and
spray chambers were then used to deliver a single stream of cells to
the scICP-MS while protecting their delicate structure. High transport
efficiencies of greater than 70% were achieved and confirmed by the
linked software.
The iCAP TQ ICP-MS was used in TQ-O2
mode to induce oxidation
and reduce polyatomic interferences. P and S were measured via the
product ions 31
P+ 16O+ and 80
Se+ 16
O+. The exact parameters established
for the separation and detection stages are shown in Table 1.
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| November/December 2022
The scQuant plug-in for Thermo Scientific™ Qtegra™ Intelligent
Scientific Data Solution™ Software was used to create the method and
provide data evaluation post-analysis. Data were acquired using the
time-resolved analysis mode at a dwell time of 5 ms and a detection
sensitivity of 0.2 µgL-1
was achieved for Se, equivalent to a minimum
detectable amount of 0.17 Se fg per cell.
Results
Quantitative assessment of the resulting ICP-MS signals allows the
determination of three key metrics: the number of cells that contain
a quantifiable amount of each element, the average mass of each
element within a cell and the distribution of each element across the
cell cohort.
The raw data indicates that the number of detected signals per unit
of time was slightly lower for Se than for P. This demonstrates that,
although each cell contains a significant amount of P (known to be
present in DNA), not all cells contain Se and, therefore, only a fraction
of the total cells showed detectable levels of Se (Figure 2). In fact, 57%
of cells had detectable levels of Se, in the range of 2.5 fg - 72.5 fg.
Further analysis showed a broad distribution of Se in the cell
population, a mean of 18.6 fg and a median of 16.8 fg were detected
with a standard deviation (SD) of ± 12.5 fg. The mean for P was 37.0 fg
and the median 30.9 fg, across the cell population, with an SD ± 23.1
fg (Figure 3). Although the SD was lower for the Se data, it was based
only on the detected cells and the overall score would have shown
greater inhomogeneity.
scICP-MS is an Effective Tool
for Analyzing Trace Elements in
Single Cells
This study into the presence of Se and P in selenized yeast demonstrates
that scICP-MS, used in single-cell mode, is an effective tool for the
Parameter
Nebulizer
Table 1. Instrument parameters set for the study of SELM-1 cells
Value
Spray chamber
Sample delivery
Sample flow
Forward power
Nebulizer gas flow
Sheath gas flow
Interface configuration
Analysis time
MicroMist™ HE U-Series nebulizer
Total consumption spray chamber
Chemyx™ Fusion 100-X syringe pump
10 µL/min
1,550 W
0.51 L-min-1
0.65 L-min-1
High sensitivity
60 s per element, 250 s total duration
(including uptake and wash)
CCT settings
0.35 mL-min-1
, 100% 02
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