Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
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Figure 1. Schematic illustration of the setup showing the modification made in the GC-MS instrument. The two possible valve configurations<br />
are shown in the inset: Left, for structural identification by conventional GC-EI-MS analysis, and right, for absolute quantification using GC-<br />
Combustion-MS and postcolumn addition of 13 CO2. Adapted from reference. 15<br />
isotope ratio (12/13) in the tracer and Rn is the natural isotope<br />
ratio (13/12) in the sample, whereas Rb is the measured isotope<br />
ratio in the blend.<br />
Additionally, eq 1 can be used to quantify the mass flow of<br />
enriched carbon dioxide when an internal standard of known<br />
concentration is spiked to the sample.<br />
Head-Space SPME Procedure. Test solutions (∼10 ng/g) were<br />
prepared by diluting appropriately the BTEX standards in Milli<br />
Q water. Samples of ∼7 g were pipetted into 10 mL glass vials<br />
with open-top phenolic closures and PTFE/silicone septa (Supelco,<br />
Bellefonte, USA). Three fiber coatings, 100 µm polydimethylsiloxane<br />
(PDMS), 70/30 µm polydimethylsiloxane/divinylbenzene<br />
(PDMS/DVB), and 50/30 µm carboxen/polydimethylsiloxane/<br />
divinylbenzene (CAR/PDMS/DVB) were tested for comparative<br />
purposes. Samples were saturated in all cases with NaCl to<br />
improve extraction efficiency and stirred at 900 rpm at 30 °C<br />
during 30 min to allow equilibration between the liquid and gas<br />
phases. Then the fibers were inserted in the headspace and<br />
extracted during 15 min to ensure equilibrium conditions between<br />
the headspace and the fiber. After extraction, the fibers were<br />
removed and thermal desorption was carried out in the injection<br />
port of the GC at 250 °C.<br />
RESULTS AND DISCUSSION<br />
Online Continuous Measurement of the 12 C/ 13 C Isotope<br />
Ratios. The optimum conditions for the continuous measurement<br />
of 12 C/ 13 C isotope ratios were established by connecting directly<br />
the reservoir that contained the 13 CO2 spike to the mass<br />
spectrometer. 15 The mass spectrum obtained for the mass<br />
range 43-47 when such postcolumn flow was set at 1 mL/<br />
min is provided in the Supporting Information (Figure SI-1).<br />
The 13 C isotope enrichment in the postcolumn spike (∼97%)<br />
can be clearly seen from the spectrum. Resolution was<br />
optimized while still preserving a good sensitivity. The mass<br />
resolution finally obtained allowed baseline separation even for<br />
isotope ratios very far from 1. The experiments performed for<br />
the selection of the optimum integration time are given in the<br />
Supporting Information (Figure SI-2). An integration time of 70<br />
ms was finally selected considering both the precision in the ratio<br />
measurement and the number of points per chromatographic peak<br />
(∼15) to obtain well-defined peak profiles.<br />
Spike Enrichment and Stability of the 12 CO2/ 13 CO2<br />
Isotope Ratio. Again, to estimate the 13 C enrichment obtained<br />
after the preparation of the 13 CO2 spike, its reservoir was<br />
connected directly to the mass spectrometer. A constant flow<br />
of1mLmin -1 was set and the signals at m/z ratios 44 and 45<br />
were continuously monitored during 2 min for each independent<br />
isotope ratio measurement under the measurement<br />
conditions described in the procedures. The isotope ratio was<br />
calculated obtaining an average value of 0.034 ± 0.0003, which<br />
corresponds to a 13 C abundance of 96.70 ± 0.02% (n ) 5) (see<br />
the mass spectrum shown in the Figure SI-1). The slight<br />
difference between the experimentally obtained enrichment and<br />
that provided for the precursor Na2 13 CO3 by the supplier (99%),<br />
can be ascribed to small contaminations by natural abundance<br />
CO2 from the ambient air during the generation of the tracer<br />
<strong>Analytical</strong> <strong>Chemistry</strong>, Vol. 82, No. 16, August 15, 2010<br />
6865