Search

In situ measurements of sea ice thickness (I), snow depth (S), and snow freeboard (Fsn) from drilling profile lines from 15 cruises into the Southern Ocean, Antarctica, were analyzed. Iwas calculated from in situ Fsnand Susing an isostatic approach. Iwas also directly estimated from Fsnas can be obtained from laser altimetry. The root‐mean‐square difference (RMSD) between observed and calculated Ireduces, and the correlation between Fsnand Iincreases substantially, when (1) using values averaged over the survey lines (∼50 m) instead of single drill hole measurements (∼1 m) and (2) treating positive and negative sea ice freeboard (Fi) separately. For small Fi, however, Sapproximates Fsnpointing toward an isostatic balance also between Sand I. Our linear regression analysis between the in situ measurements suggests a direct conversion of Fsninto Iusing a region‐specific set of equations. RMSD values are similar to those obtained employing isostatic balance models and treating positive and negative Fiseparately. However, more data would have been needed to obtain significant differences between most of the various models suggested. Still our new approach gives a viable alternative for Antarctic Iretrieval from altimetric measurements of Fsnalone. Correlation between in situ observations of Fsnand Sis high. RMSD between observed and calculated Sis small. This suggests estimation of Sfrom altimetric Fsnmeasurements. Such Shas an estimated precision of ∼5 cm, and is neither affected by snow wetness or grain size nor limited to S< 50 cm. In situ observations from drilling profiles into Antarctica were analyzed.Statistical relationships between sea ice and snow properties were developed.

Reactive nitrogen species such as peroxynitrite can nitrate specific amino acids, whether free or protein bound, and 3-nitrotyrosine is believed to be one marker of this reaction. To examine the significance of this pathway in biological systems we have developed an accurate, sensitive, and specific assay for 3-nitrotyrosine based on combined liquid chromatography tandem mass spectrometry. Our approach allowed simultaneous analysis of both tyrosine and 3-nitrotyrosine and employs isotopomer standards (i.e., [15N1, 13C9]-tyrosine and [13C6]-3-nitrotyrosine). Calibration curves were linear (r2= 0.999) across the range 0.5–100 pg/µL (i.e., 2.2–442 fmol/µL), and the detection limit for standard samples was 0.5 pg/µL (2.2 fmol/µL, or 10 fmol on column; S/N = 5) or 1 pg/µL (4.4 fmol/µL) for extracted (biological) samples. As a component of this study we have undertaken an extensive investigation of artifactual formation of 3-nitrotyrosine under conditions that exist during sample extraction and derivatization. Our studies show that under appropriate conditions (low pH, elevated temperatures, and in the presence of a vast excess of the two substrates, tyrosine and the nitrate anion), 3-nitrotyrosine can readily be formed as an artifact.

Abstract: Reactive nitrogen species such as peroxynitrite can nitrate specific amino acids, whether free or protein bound, and 3-nitrotyrosine is believed to be one marker of this reaction. To examine the significance of this pathway in biological systems we have developed an accurate, sensitive, and specific assay for 3-nitrotyrosine based on combined liquid chromatography tandem mass spectrometry. Our approach allowed simultaneous analysis of both tyrosine and 3-nitrotyrosine and employs isotopomer standards (i.e., [15N1, 13C9]-tyrosine and [13C6]-3-nitrotyrosine). Calibration curves were linear (r 2 = 0.999) across the range 0.5–100 pg/μL (i.e., 2.2–442 fmol/μL), and the detection limit for standard samples was 0.5 pg/μL (2.2 fmol/μL, or 10 fmol on column; S/N = 5) or 1 pg/μL (4.4 fmol/μL) for extracted (biological) samples. As a component of this study we have undertaken an extensive investigation of artifactual formation of 3-nitrotyrosine under conditions that exist during sample extraction and derivatization. Our studies show that under appropriate conditions (low pH, elevated temperatures, and in the presence of a vast excess of the two substrates, tyrosine and the nitrate anion), 3-nitrotyrosine can readily be formed as an artifact.

The nitration of aromatic compounds by electrophilic substitution is often utilized in analyses of nitrate concentrations in physiological samples by gas chromatographic methods. Problems associated with the use of concentrated sulfuric acid, which is normally used to catalyze this reaction, led us to investigate an alternative method. We describe here a facile GC/MS assay for nitrate in plasma or urine samples which takes advantage of the ability of trifluoroacetic anhydride (TFAA) to catalyze the nitration of aromatics. Toluene, utilized as both reaction solvent and electrophile, was shown to react with nitrate in the presence of TFAA to quantitatively produce the three nitrotoluene isomers (ratioo-:m:p-, approx 57:3:40). Following the incorporation of15N-labeled nitrate as internal standard, nitrotoluene was quantified using GC/MS by analysis of the selected the ion pairsm/z120 and 121 (M+-OH) for theo-isomer orm/z137 and 138 (molecular ion, M+) for thep-isomer. The limit of detection for nitrate after TFAA-catalyzed conversion to nitrotoluene was less than 100 fmol on column (s/n; 40:1). The TFAA-based GC/MS assay was compared with that utilizing the usual catalyst, concentrated sulfuric acid. With the exception of samples containing nitroarginine analogues, good correlation was found for urine or plasma samples analyzed using either a standard sulfuric acid-catalyzed method or the TFAA-catalyzed procedure. Nitroarginine analogues, which can be present in samples following their use as nitric oxide synthase inhibitors, did not decompose under the conditions of the TFAA-catalyzed assay and, hence, do not give rise to significant interference with nitrate analysis in this procedure. In contrast, catalytic sulfuric acid caused nitroarginine analogues to decompose (essentially quantitatively) and cause spuriously high nitrate levels in samples. The use of TFAA as a catalyst for the nitration of toluene enables a facile and sensitive GC/MS analysis for nitrate which offers improved safety and sample integrity.

Peroxynitrite (ONOO−) can react with a wide range of biomolecules resulting in peroxidation, oxidation, and/or nitration and as a consequence cause their inactivation. In this study mass spectrometry (MS) combined with both liquid (LC) and gas chromatography (GC) has been employed to identify the products formed following ONOO−treatment of three peptides at physiological pH: leucine–enkephalin (YGGFL), V3 loop (GPGRAF), and LVV-hemorphin7 (LVVYPWTQRF). LC–MS analysis of leucine–enkephalin following ONOO−treatment indicated the formation of products corresponding in mass to mono- and dinitrated forms of the starting material. LC–MS–MS and GC–MS analyses revealed no evidence for the formation of nitrophenylalanine; however, both 3-nitrotyrosine and 3,5-dinitrotyrosine were observed and characterized. GC–MS analysis of hydrolyzed peptides following ONOO−treatment confirmed the presence of nitrated and dinitrated tyrosine. However, when a 20-fold molar excess of ONOO−was reacted with leucine–enkephalin, only about half of the tyrosine originally present in the peptide could be accounted for in the acid hydrolysate. The main product was 3-nitrotyrosine which represented ca. 50% of the original tyrosine; traces of 3,5-dinitrotyrosine (ca. 3% of the original tyrosine) were also present.