Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
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indicators of atmospheric processing of volatile organic compounds;<br />
9 for example, carbon isotope ratio measurements allow<br />
the calculation of the extent of photochemical processing of<br />
isoprene in the atmosphere. 10 Furthermore, Rudolph et al. 11<br />
developed a gas chromatography/combustion/isotope ratio mass<br />
spectrometry (GC/C/IRMS) technique to measure the stable<br />
carbon isotope ratios of isoprene and its gas-phase oxidation<br />
products, MACR and MVK, and to gain insight into the atmospheric<br />
oxidation of isoprene. Very recently, this group has<br />
reported studies on the stable carbon kinetic isotope effects (KIE)<br />
of the reactions of isoprene, MACR, and MVK with OH radicals,<br />
as well as with ozone in the gas phase. 12-14 These data are<br />
valuable for obtaining insight into the role of loss processes in<br />
determining the atmospheric mixing ratios. However, there are<br />
no isotopic studies of isoprene SOA products in the aerosol phase.<br />
Methylboronic acid (MBA) has been reported to determine<br />
natural 13 C abundances of monosaccharides by derivatizing<br />
adjacent hydroxyl groups of monosaccharides followed by N,Obis(trimethylsilyl)trifluoroacetamide<br />
(BSTFA) derivatization of<br />
the remaining single OH groups. 15,16 Recently, Boschker et al. 17<br />
reported a versatile method for stable carbon isotope analysis of<br />
carbohydrates by high-performance liquid chromatography/<br />
isotope ratio mass spectrometry. To develop a compound-specific<br />
isotope analysis method for 2-methyltetrols, marker compounds<br />
of photooxidation products of isoprene, a technique from our<br />
previous work was adapted. 18 MBA was used as the derivatization<br />
reagent prior to gas chromatography/combustion/isotope ratio<br />
mass spectrometry (GC/C/IRMS). The derivatizing C from the<br />
reagent accounts for 29% of the analyte in the boronates, but 70%<br />
in the trimethylsilyl (TMS) derivatives. Therefore, the sensitivity<br />
of the MBA method should be high. The oxidation reaction of<br />
isoprene, atmospheric sampling, accuracy, and reproducibility of<br />
the method will be discussed in detail, and the stable carbon<br />
isotope effects during the procedure will be evaluated. δ 13 C data<br />
for atmospheric 2-methyltetrols will also be presented to<br />
demonstrate the practical utility of this method.<br />
EXPERIMENTAL SECTION<br />
Materials. Isoprene was obtained from three suppliers: Fluka,<br />
Sigma-Aldrich (>98% pure, M1); Alfa-Aesar (Lancaster, England)<br />
(99% pure, M2); and Toyo Kasei Kogyo Co. (Osaka, Japan) (99%<br />
pure, M3). Hydrogen peroxide (30% in water) was purchased from<br />
Guoyao (Shanghai, China). Methylboronic acid (MBA) was<br />
(9) Rudolph, J.; Czuba, E. Geophys. Res. Lett. 2000, 27, 3865–3868.<br />
(10) Rudolph, J.; Anderson, R. S.; Czapiewski, K. V.; Czuba, E.; Ernst, D.;<br />
Gillespie, T.; Huang, L.; Rigby, C.; Thompson, A. E. J. Atmos. Chem. 2003,<br />
44, 39–55.<br />
(11) Iannone, R.; Koppmann, R.; Rudolph, J. J. Atmos. Chem. 2007, 58, 181–<br />
202.<br />
(12) Rudolph, J.; Czuba, E.; Huang, L. J. Geophys. Res. 2000, 105, 29323–29346.<br />
(13) Iannone, R.; Koppmann, R.; Rudolph, J. Atmos. Environ. 2008, 38, 4093–<br />
4098.<br />
(14) Iannone, R.; Koppmann, R.; Rudolph, J. Atmos. Environ. 2009, 43, 3103–<br />
3110.<br />
(15) van Dongen, B.; Schouten, S.; Damste, J. Rapid Commun. Mass Spectrom.<br />
2001, 15, 496–500.<br />
(16) Gross, S.; Glaser, B. Rapid Commun. Mass Spectrom. 2004, 18, 2753–2764.<br />
(17) Boschker, H. T. S.; Moerdijk-Poortvliet, T. C. W.; van Breugel, P.;<br />
Houtekamer, M.; Middelburg, J. J. Rapid Commun. Mass Spectrom. 2008,<br />
22, 3902–3908.<br />
(18) Wang, W.; Li, L.; Li, H.; Zhang, D.; Wen, S.; Jia, W.; Wang, B.; Sheng, G.;<br />
Fu, J. Rapid Commun. Mass Spectrom. 2009, 23, 2675–2678.<br />
purchased from ABCR GmbH and Co. KG (Karlsruhe, Germany)<br />
(97% pure) and recrystallized three times from a benzene/acetone<br />
mixture (3:1). MBA of the same lot number was used for all<br />
derivatizations. Anhydrous pyridine (99% pure) was supplied by<br />
Acros Organics (Geel, Belgium). BSTFA [N,O-bis(trimethylsilyl)<br />
trifluoroacetamide] was purchased from Pierce (Rockford, IL). All<br />
solvents employed were HPLC grade.<br />
Preparation of Standard 2-Methyltetrols. 2-Methyltetrols<br />
were made by photooxidation of isoprene, which we conducted<br />
by exposing a 30 mL flask with a mixture of 5 mL of 30% H2O2<br />
and 5 mL of isoprene (0.05 mol, 3.4 g) to sunlight. A few drops<br />
of sulfuric acid (0.1 M) was added until the pH of reaction<br />
mixture was between 1 and 2. The resulting mixture was then<br />
maintained with continuous and vigorous stirring in sunlight<br />
for4h; 19 15 mg of barium carbonate was added to 1 mL of the<br />
reacted solution for neutralization. After centrifugation, the<br />
supernatant was dried, and a slightly yellow oil (2.5 g, 36% yield)<br />
was obtained. It worth noting that exposure to sunlight is<br />
crucial for the production of 2-methyltetrols.<br />
The purification of crude 2-methyltetrols was performed<br />
according to a procedure reported by Wang et al. 20 The purity of<br />
2-methyltetrols was verified by gas chromatography/mass spectrometry<br />
(GC/MS) after they had been derivatized with BSTFA,<br />
and the δ 13 C value was determined by elemental analyzer/<br />
isotope ratio mass spectrometry (EA/IRMS).<br />
Derivatization of 2-Methyltetrols. The derivatization technique<br />
of 2-methyltetrols with methylboronic acid was adapted from<br />
Wang et al.; 18 100 µL of a solution of 2-methyltetrols (approximately<br />
1 mg/mL in methanol) was dried under a gentle<br />
nitrogen flow, and then 5 mL of a solution of 1 mg of methylboronic<br />
acid in 10 mL of anhydrous pyridine was added. The molar<br />
ratio of MBA to 2-methyltetrols was ∼10:1. The mixture was<br />
allowed to react at 60 °C for 60 min. It should be noted that the<br />
pretreatment of pyridine with 4 Å molecular sieves in excess is<br />
crucial for a successful MBA derivatization. The δ 13 C value of<br />
methylboronic derivatives was determined by gas chromatography/combustion/isotopic<br />
ratio mass spectrometry (GC/C/<br />
IRMS).<br />
Measurements of the δ 13 C Value of Standard Isoprene.<br />
The method for determining the δ 13 C value of isoprene was as<br />
follows. 21 Isoprene (1 mL) was sealed in a2mLglass vial with<br />
an open screw cap containing a Teflon-lined silica septum. After<br />
∼1 h for equilibrium, 15 µL gas samples from the glass bottle<br />
were injected into the split/splitless injection port of the gas<br />
chromatograph using a Hamilton gastight locking syringe.<br />
Aerosol Sampling. Samples were collected in boreal-temperate<br />
Changbai Mountain Forest Ecosystem Research Station in Jilin<br />
Province and subtropical Dinghu Mountain Nature Reserve in<br />
Guangdong Province. The details for sampling sites have been<br />
described previously. 6 The sampling occurred during the summer<br />
when the meteorological conditions and the maximum solar<br />
radiation, as well as high temperatures, were favorable for the<br />
photooxidation of isoprene. A high-volume PM2.5 air sampler<br />
(19) Santos, L.; Dalmazio, L.; Eberlin, M.; Claeys, M.; Augusti, R. Rapid Commun.<br />
Mass Spectrom. 2006, 20, 2104–2108.<br />
(20) Wang, W.; Vas, G.; Dommisse, R.; Loones, K.; Claeys, M. Rapid Commun.<br />
Mass Spectrom. 2004, 18, 1787–1797.<br />
(21) Yu, Y.; Wen, S.; Feng, Y.; Bi, X.; Wang, X.; Peng, P.; Sheng, G.; Fu, J. Anal.<br />
Chem. 2006, 78, 1206–1211.<br />
<strong>Analytical</strong> <strong>Chemistry</strong>, Vol. 82, No. 16, August 15, 2010<br />
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