Nabil Saad | Picarro - Academia.edu (original) (raw)

Papers by Nabil Saad

We report here on the measurement of high precision delta13C from total inorganic carbon (TIC) an... more We report here on the measurement of high precision delta13C from total inorganic carbon (TIC) and dissolved organic carbon (DOC) using a sample preparation system coupled to a small footprint Wavelength- Scanned Cavity Ring-Down Spectrometer (WS-CRDS). This system is capable of applying a 5% H3PO4 solution or a sodium persulfate oxidation process to a water sample in an exetainer vial, thereby liberating gaseous CO2 and permitting stable carbon isotope measurement in TIC and DOC, respectively. The isotopic carbon signature determination can then be used to trace the origin of carbonates or organic carbon compounds. In a first phase, a manual process was employed in which TIC containing samples were acidified and the evolved CO2 was collected inside gas pillows. The gas pillows were then connected to the inlet of the isotopic WS-CRDS instrument for carbon ratio measurement. In a second phase, the CO2 liberation processes were automated in an integrated analyzer enabling software con...

Isotopes in environmental and health studies, Jan 17, 2015

This work examines the performance and limitations of a wet chemical oxidation carbon analyser in... more This work examines the performance and limitations of a wet chemical oxidation carbon analyser interfaced with a cavity ring-down spectrometer (WCO-CRDS) in a continuous flow (CF) configuration for measuring δ(13)C of dissolved organic carbon (δ(13)C-DOC) in natural water samples. Low-chloride matrix (<5 g Cl/L) DOC solutions were analysed with as little as 2.5 mg C/L in a 9 mL aliquot with a precision of 0.5 ‰. In high-chloride matrix (10-100 g Cl/L) DOC solutions, bias towards lighter δ(13)C-DOC was observed because of incomplete oxidation despite using high-concentration oxidant, extended reaction time, or post-wet chemical oxidation gas-phase combustion. However, through a combination of dilution, chloride removal, and increasing the oxidant:sample ratio, high-salinity samples with sufficient DOC (>22.5 µg C/aliquot) may be analysed. The WCO-CRDS approach requires more total carbon (µg C/aliquot) than conventional CF-isotope ratio mass spectrometer, but is nonetheless appl...

Journal of lipid research, 2001

Using [13C]-tracers and direct fetal doses, we show for the first time that the fetal primate con... more Using [13C]-tracers and direct fetal doses, we show for the first time that the fetal primate converts alpha-linolenic acid (18:3) to docosahexaenoic acid (22:6) in vivo, and we estimate the relative bioefficacy of the two substrates for brain 22:6 accretion. Pregnant female baboons consumed a diet free of long chain polyunsaturates (LCP), with n-6/n-3 ratio of 10/1. In the third trimester of pregnancy (normal gestation = 182 days), they were instrumented with chronic indwelling catheters in the maternal femoral artery and the fetal jugular artery. Doses of either [U-13C]-18:3 (18:3*, n = 3) or [U-13C]-22:6 (22:6*, n = 2) were administered directly to the fetus. Blood was collected from fetus and mother, and the fetus was taken by cesarean section when electromyographic activity indicated that parturition was imminent. Fetal liver, brain, retina, and retinal pigment epithelium (RPE) were collected, and (13)C fatty acids determined. In 18:3*- dosed animals, labeled n-3 LCP were detec...

Proceedings of the National Academy of Sciences, 2009

A continuous-flow cavity ring-down spectroscopy (CRDS) system integrating a chromatographic separ... more A continuous-flow cavity ring-down spectroscopy (CRDS) system integrating a chromatographic separation technique, a catalytic combustor, and an isotopic 13 C/ 12 C optical analyzer is described for the isotopic analysis of a mixture of organic compounds. A demonstration of its potential is made for the geochemically important class of short-chain hydrocarbons. The system proved to be linear over a 3-fold injection volume dynamic range with an average precision of 0.95‰ and 0.67‰ for ethane and propane, respectively. The calibrated accuracy for methane, ethane, and propane is within 3‰ of the values determined using isotope ratio mass spectrometry (IRMS), which is the current method of choice for compound-specific isotope analysis. With anticipated improvements, the low-cost, portable, and easy-to-use CRDS-based instrumental setup is poised to evolve into a credible challenge to the high-cost and complex IRMS-based technique.

Journal of Chromatography A, 2007

Monolithic silica capillary columns for hydrophilic interaction liquid chromatography (HILIC) wer... more Monolithic silica capillary columns for hydrophilic interaction liquid chromatography (HILIC) were prepared by on-column polymerization of acrylic acid on monolithic silica in a fused silica capillary modified with anchor groups. The products maintained the high permeability (K = 5 × 10 −14 m 2 ) and provided a plate height (H) of less than 10 m at optimum linear velocity (u) and H below 20 m at u = 6 mm/s for polar solutes including nucleosides and carbohydrates. The HILIC mode monolithic silica capillary column was able to produce 10 000 theoretical plates (N) with column dead time (t 0 ) of 20 s at a pressure drop of 20 MPa or lower. The total performance was much higher than conventional particle-packed HILIC columns currently available. The gradient separations of peptides by a capillary LC-electrospray mass spectrometry system resulted in very different retention selectivity between reversed-phase mode separations and the HILIC mode separations with a peak capacity of ca. 100 in a 10 min gradient time in either mode. The high performance observed with the monolithic silica capillary column modified with poly(acrylic acid) suggests that the HILIC mode can be an alternative to the reversed-phase mode for a wide range of compounds, especially for those of high polarity in isocratic as well as gradient elution.

Journal of Analytical Atomic Spectrometry, 2013

Analytical Chemistry, 2002

We present a rapid and selective method to increase the volatility of tyrosine and phenylalanine ... more We present a rapid and selective method to increase the volatility of tyrosine and phenylalanine without adding derivative C for high-precision gas chromatography-continuous-flow isotope ratio mass spectrometry (GCC-IRMS) based on enzymatic decarboxylation to yield alkylamines and evaluated for 15N isotopic integrity. Purified tyrosine and phenylalanine were converted to tyramine and phenethylamine by tyrosine and phenylalanine decarboxylases, respectively. GC separation was achieved using a thick stationary phase (5-microm) capillary column. Recoveries were 95 +/- 2%. The reproducibility of delta15N of tyramine and phenethylamine measured by GCC-IRMS averaged SD(delta15N) = 0.33 per thousand. The absolute differences between delta15N of amino acids measured by elemental analyzer-IRMS and the alkylamines measured by GCC-IRMS was not significant. Phenethylamine and tyramine prepared from a mixture of 18 amino acids were extracted by ethanol with 95% recovery, and analysis yielded clean chromatograms and equivalent precision. These data indicate that enzymatic decarboxylation of phenylalanine and tyrosine is a convenient method to increase their volatility for continuous-flow isotopic analysis without introducing extraneous C or significant isotopic fractionation.

Analytical Chemistry, 2001

Amino acids are routinely derivatized using carbon-containing groups prior to gas chromatography ... more Amino acids are routinely derivatized using carbon-containing groups prior to gas chromatography continuous-flow isotope ratio mass spectrometry (GCC-IRMS). Derivative C contaminates analyte C because the entire derivatized compound is combusted to CO2. Correction procedures are required to extract the analyte isotope ratio. We present a method for reduction of six nonpolar amino acids to their corresponding amino alcohols, demonstrate a GC strategy to produce acceptable peak shapes from the resulting strongly H-bonding analytes, and present isotopic analysis for amino acids and their corresponding amino alcohols to evaluate any possible isotopic fractionation. Alanine, valine, leucine, isoleucine, methionine, or phenylalanine was reduced using NaBH4 in THF with I2 as an electrophile. Reactions were performed with 2 g of analyte to permit isotopic analysis by conventional elemental analysis-IRMS. All reactions were quantitative as assessed by IR spectra, melting points, and GC. Recovery from the reaction mixture was 60-84%. GC separation of a mixture of the six amino alcohols was achieved using a thick stationary-phase (5 microm) capillary column to avoid tailing due to hydrogen bonding to the walls of the fused-silica capillary. The reproducibility of GCC-IRMS determinations of amino alcohols averaged SD(613C) = 0.25 +/- 0.19%. The absolute differences between delta13C of amino acids measured by an elemental analyzer coupled to IRMS and amino alcohols measured by GCC-IRMS was delta613C = 0.14% and showed no general trend. Reactions performed with 2 mg of analyte yielded equivalent chromatograms. These data indicate that the reduction method does not induce isotopic fractionation and can be used for continuous-flow isotopic analysis to avoid addition of contaminating carbon.

Analytical Chemistry, 2007

Theoretical calculations are presented to optimize modulation period for maximum total peak capac... more Theoretical calculations are presented to optimize modulation period for maximum total peak capacity in comprehensive two-dimensional HPLC (2D-HPLC) taking into account the effect of modulation on the apparent peak capacity of the first-dimension ( 1 D) separation. Results indicate that modulation periods are most favorable when they are adjusted to ∼2.2-4 times the standard deviation of a 1 D peak in order to avoid excessively short run times at the second dimension ( 2 D). Data are presented that effective peak capacities of several thousand in 60 min can be expected for practical 2D-HPLC conditions, utilizing 1 D gradient elution followed by 2 D isocratic elution, that remain at ∼50-70% of the theoretical maximum peak capacity. This work suggests that lower modulation frequencies and longer 2 D separation times than previously proposed are favorable under realistic chromatographic conditions, alleviating some practical problems associated with 2D-HPLC.

… Assembly 2010, held …, 2010

The regulation of Earth's climate and its ability to sustain life are critically linked to w... more The regulation of Earth's climate and its ability to sustain life are critically linked to water as it exists in all three of its phases (gas, liquid, and solid). Earth's water cycle, its movement between the hydrosphere, biosphere, and the atmosphere, and how it undergoes phase changes, ...

Analytical and Bioanalytical Chemistry, 2008

A polyacrylamide (PAAm)-modified monolithic silica capillary column of increased phase ratio, 200... more A polyacrylamide (PAAm)-modified monolithic silica capillary column of increased phase ratio, 200T-PAAm, for hydrophilic interaction liquid chromatography (HILIC) was prepared. The column showed high separation efficiency, with a theoretical plate height H=7-20 μm at a linear velocity, u=1-7 mm/s. From a kinetic plot analysis, it was expected that the monolithic column could provide three times faster separation than particle-packed HILIC columns under a pressure limit at 20 MPa. HILIC coupled with electrospray ionization (ESI)-mass spectrometry (HILIC-ESI-MS) using the 200T-PAAm column was employed for the analysis of underivatized carbohydrates to achieve fast and efficient separations of mixtures containing mono-, di-, and trisaccharides within 5 min. Under single MS full scan mode, 200 pg of oligosaccharides was detected by the system. The limit of detection (LOD) of the LC-ESI-MS/MS system was determined using selected reaction monitoring (SRM) to be as low as 3.2 ng/mL (attomol level) for nonreducing saccharides. The system was successfully applied to the detection of disaccharides in extracts of plant, such as corn, soybean, and Arabidopsis thaliana. Keywords Monolithic silica . HILIC . Polyacrylamide . Underivatized carbohydrates . HILIC-ESI-MS [2]. Aminopropyl-modified silica columns have been used for the separation of carbohydrates [3-5] using hydrophilic interaction chromatography (HILIC) mode [6, 7] although Anal Bioanal Chem

Journal of Geophysical Research: Biogeosciences, 2015