Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
corresponding compound if its chemical formula is known. Until<br />
now such methodology has been exclusively applied to the<br />
quantitative determination of compounds containing ICPMS<br />
detectable elements for trace-elemental speciation of metals 9 or<br />
semimetals. 10,11 Unfortunately, the use of the ICPMS for the<br />
detection of carbon, hydrogen, nitrogen, or oxygen is seriously<br />
hampered by the low ionization yields of these elements in the<br />
ICP and the high carbon background under normal ICP operating<br />
conditions (atmospheric pressure). In fact, there are only two<br />
reports so far describing the use of HPLC and the postcolumn<br />
addition of 13 C-labeled species for the quantification of organic<br />
compounds by ICP-MS. 12,13 In both cases, the observed detection<br />
limits were not satisfactory.<br />
For the MS detection of organic compounds, previously<br />
separated by GC, electron ionization (EI) is the most common<br />
ionization source employed. It provides both structural and<br />
quantitative information of any volatile organic compound injected<br />
in the gas chromatograph. Unfortunately, as pointed out before,<br />
it requires specific analytical standards as its response is structurespecific<br />
for each single molecule subjected to analysis. 14 Recently,<br />
we have introduced a new quantitative detection concept in gas<br />
chromatography, based on the postcolumn addition of 13 CO2 for<br />
carbon isotope dilution analysis using EI. 15 This concept<br />
constitutes a patented procedure in which organic compounds<br />
separated by liquid or gas chromatography are converted<br />
quantitatively into carbon dioxide, by an oxidation or combustion<br />
reaction, and then are mixed with a postcolumn flow of<br />
enriched 13 CO2. 16 This procedure should provide quantitative<br />
information of every single compound previously separated in<br />
the chromatograph without the need for individual standards.<br />
We selected electron ionization because it operates under high<br />
vacuum conditions providing much better sensitivity and lower<br />
background for carbon detection than the above-mentioned ICP<br />
atmospheric source. For the correct application of isotope<br />
dilution analysis conversion of all carbon containing compounds<br />
to a unique chemical species is required (isotope equilibration).<br />
The isotopic equilibrium between the 13 C-containing species<br />
continuously added postcolumn ( 13 CO2) and the separated<br />
organic compounds is reached by their quantitative conversion<br />
into CO2, after the chromatographic separation, in a combustion<br />
furnace. 17 As the only species to be finally measured by EI is<br />
CO2, compound independent response and isotopic equilibration<br />
is finally obtained. This approach can become a universal<br />
quantitative detector in GC-MS analysis, without the classical<br />
need for specific standards and, what is more, it is compatible<br />
with the exceptional structural identification capabilities provided<br />
by current GC-EI-MS instruments. Thus, in this work<br />
(9) Sariego-Muñiz, C.; Marchante-Gayón, J. M.; García-Alonso, J. I.; Sanz-Medel,<br />
A. J. Anal. At. Spectrom. 2001, 16, 587–592.<br />
(10) Giusti, P.; Schaumlöffel, D.; Ruiz-Encinar, J.; Szpunar, J. J. Anal. At. Spectrom.<br />
2005, 20, 1101–1107.<br />
(11) Heilmann, J.; Heumann, K. G. Anal. Chem. 2008, 80, 1952–1961.<br />
(12) Vogl, J.; Heumann, K. G. Anal. Chem. 1998, 70, 2038–2043.<br />
(13) Smith, C.; Jensen, B. P.; Wilson, I. D.; Abou-Shakra, F.; Crowther, D. Rapid<br />
Commun. Mass Spectrom. 2004, 18, 1487–1492.<br />
(14) Mark, T. D. Int. J. Mass Spectrom. Ion Phys. 1982, 45, 125–145.<br />
(15) Cueto-Díaz, S.; Ruiz-Encinar, J.; Sanz-Medel, A.; García-Alonso, J. I. Angew.<br />
Chem., Int. Ed. 2009, 48, 2561–2564.<br />
(16) Ruiz-Encinar, J.; García-Alonso, J. I World Intellectual Property Organization,<br />
Spain; International Patent WO 2007/042597 A1, 2007.<br />
(17) Merritt, D. A.; Freeman, K. H.; Ricci, M. P.; Studley, S. A.; Hayes, J. M.<br />
Anal. Chem. 1995, 67, 2461–2473.<br />
we describe the instrumental developments and its analytical<br />
features for integral characterization and determination of<br />
organic compounds in detail.<br />
Furthermore, the compound-independent quantification provided<br />
by the developed approach can be an inestimable tool in<br />
the optimization and quantitative assessment of sample extraction<br />
and preconcentration procedures applied prior to GC-MS analysis.<br />
For instance, solid-phase microextraction (SPME) is today a<br />
widely used preconcentration technique in Gas Chromatography. 18<br />
Fiber absorption and recovery are common parameters used when<br />
validating a given SPME procedure. However, those two parameters<br />
are almost impossible to compute using conventional<br />
approaches. 19 In fact, it is necessary to inject specific liquid<br />
standards and to assume that the transfer efficiency in the GC<br />
injector for every compound under analysis is the same when<br />
using conventional injection and the thermal desorption. Thus,<br />
the important topic of absolute absorption yields will be evaluated<br />
in this work, as exemplified for the HS-SPME analysis of BTEX<br />
(Benzene, Toluene, Ethylbenzene, and o,m,p-Xylenes) in different<br />
water samples, using the proposed IDA-EI-MS quantification<br />
procedure.<br />
EXPERIMENTAL SECTION<br />
Reagents and Materials. Solid enriched Na2CO3 (99% 13 C<br />
enrichment) was obtained as a highly pure chemical reagent<br />
(purity >98%) from Cambridge Isotopes Laboratories (Andover,<br />
Massachusetts, USA). A standard mixture of n-alkanes (40 µg/<br />
mL each in n-hexane) was purchased from Fluka (Seelze,<br />
Germany). Phosphoric acid (99% puriss.) was purchased from<br />
Sigma-Aldrich (St. Louis, USA). Individual compounds (undecane,<br />
dodecane, tridecane, pentadecane, butyl butyrate, and<br />
hexyl butanoate) with certified purities ranging from 99 to 99,7%<br />
were used as analytical standards (Fluka, Seelze, Germany or<br />
Sigma-Aldrich, St. Louis, USA). A solution of 1,2,4-trimethylbenzene<br />
in methanol (5000 µg/mL) and a BTEX standard<br />
mixture (2000 µg/mL each in methanol) were both from<br />
Supelco (Bellefonte, USA). Ultrapure water (18.2 MΩcm) was<br />
obtained with a Milli-Q system (Millipore, Bedford, MA).<br />
n-Hexane for organic trace analysis grade was purchased from<br />
Merck (Darmstadt, Germany). SPME holder and fibers were<br />
purchased from Supelco (Bellefonte, USA).<br />
Instrumentation. GC-MS. A Konik-Tech (Sant Cugat del<br />
Vallés, Spain) 4000-B Gas Chromatograph coupled to a Konik-<br />
Tech MS-Q12 quadrupole mass spectrometer with an electron<br />
ionization source was used. This instrument uses a specially<br />
designed injector which keeps the septum at relatively low<br />
temperature, does not require a septum purge flow, and minimizes<br />
losses of organic compounds in the injector port. The analytical<br />
column was a 30 m long (0.32 mm internal diameter, 0.25 µm<br />
stationary phase) DB-XLB column (Agilent J&W Scientific, Santa<br />
Clara, USA). The sample volume injected in the GC was always 1<br />
µL using manual injection. Sample injection was performed in the<br />
splitless (1 min)/split mode. Carrier gas flow was set at 1 mL/<br />
min for all samples and conditions.<br />
Six-Way Valve. A manually actuated 0.25 mm bore stainlesssteel<br />
six-way two-position valve (VICI AG International, Schenkon,<br />
(18) Vas, G.; Vekey, K. J. Mass Spectrom. 2004, 39, 233–254.<br />
(19) Langelfeld, J. J.; Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1996, 68,<br />
144–155.<br />
<strong>Analytical</strong> <strong>Chemistry</strong>, Vol. 82, No. 16, August 15, 2010<br />
6863