@article {935,
	title = {Water-induced errors in continuous-flow carbon isotope ratio mass spectrometry},
	journal = {Anal Chem},
	volume = {70},
	year = {1998},
	note = {Leckrone, K JHayes, J MengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.1998/07/17Anal Chem. 1998 Jul 1;70(13):2737-44.},
	month = {Jul 01},
	pages = {2737-44},
	abstract = {Formation of HCO2+ from CO2 and background H2O in isotope ratio mass spectrometers has been examined in detail. The process is troublesome because its product is not resolved from 13C16O2+. The resulting, artifactual enhancement of the mass 45 ion current (and analogous enhancement of the mass 46 ion current by transfer of hydrogen to mass 45 species) can cause systematic errors in analyses of 13C based on measurement of ion current ratios in the mass spectrum of CO2. Such errors are neutralized when isotopic analyses are based on differential comparisons in which ion currents and background water levels are precisely equal during admission and ionization of both sample and standard gases. In continuous-flow systems, however, that requirement is generally not met. The resulting systematic error is proportional to the 18/44 ion current ratio. When the widely used MAT252 mass spectrometer is tuned to yield maximum sensitivity, the constant of proportionality is 26 +/- 2/1000 (i.e., the error will be 0.26/1000 if the mass 18 ion current is 100 times smaller than that at mass 44). Errors can be reduced 5-fold when the ion-source residence time of CO2+ is decreased by use of stronger ion-extraction potential gradients. Under those same conditions, sensitivity is decreased by 60\%. For operation at highest sensitivity, carrier gas dew points on the order of -70 degrees C are required to obtain errors < or = 0.1/1000 for samples yielding mass 44 ion currents of 10 nA. Carrier gas dew points < or = -80 degrees C are conveniently reached by use of a Nafion dryer operated at approximately 0 degree C.},
	issn = {0003-2700 (Linking)},
	doi = {10.1021/ac9803434},
	author = {Leckrone, K. J. and Hayes, J. M.}
}
@article {2291,
	title = {Carbon isotopic fractionation associated with lipid biosynthesis by a cyanobacterium: relevance for interpretation of biomarker records},
	journal = {Geochim Cosmochim Acta},
	volume = {61},
	year = {1997},
	note = {Sakata, SHayes, J M
McTaggart, A R
Evans, R A
Leckrone, K J
Togasaki, R K
eng
Research Support, U.S. Gov{\textquoteright}t, Non-P.H.S.
1997/01/01 00:00
Geochim Cosmochim Acta. 1997;61(24):5379-89.
},
	month = {1997},
	pages = {5379-89},
	abstract = {For the cyanobacterium Synechocystis UTEX 2470, grown photoautotrophically to a logarithmic stage of growth, the total lipid extract is depleted in 13C by 4.8\% relative to average biomass. Depletions observed for acetogenic (straight-chain) lipids range from 7.6 (hexadecanoic acid) to 9.9\% (a C16 n-alkyl chain bound in a polar-lipid fraction), with a mass-weighted average of 9.1\%. Polyisoprenoid lipids fall into two isotopic groups, with phytol, diplopterol, and diploptene depleted by 6.4-6.9\% and bishomohopanol (produced from the extracts by the preparative degradation of bacteriohopanepolyol) depleted by 8.4\%. Analysis of the pattern of depletions indicates that two carbon positions in each C5 biomonomer leading to polyisoprenoid products are probably depleted in 13C relative to average biomass. The depletion of bacteriohopanepolyol relative to other polyisoprenoids can be ascribed to changes that occur over the life of each cell: (1) the 13C content of carbon flowing to lipid biosynthesis decreases as the cell size increases and (2) a greater proportion of the bacteriohopanepolyol which, unlike other polyisoprenoids, is present mainly in the cytoplasm rather than in membranes and is synthesized when cells are larger. Chlorophyll a is depleted relative to average biomass by O.7\%. Given the observed depletion of 13C in phytol, the heteroaromatic, chlorophyllide portion of chlorophyll must be enriched in 13C by 2.7\%. This enrichment is large relative to that in chlorophyllides produced by eukaryotes and may be related to a parallel enrichment of 13C in cyanobacterial glutamic acid. As in many previous investigations of cyanobacterial lipids, long-chain n-alkanes (C22-C29) are found in the extracts. They are, however, enriched in 13C relative to biomass and have isotopic compositions suggesting that they are contaminants of petrochemical origin. Available results indicate that cyanobacterial lipids will be depleted relative to dissolved CO2 that has served as a carbon source by 22-30\% and that a wider range of depletions will be characteristic of eukaryotic products. The absence of long-chain n-alkanes in cyanobacteria reduces the possibility that petroleum ever formed from pre-eukaryotic sedimentary debris.},
	keywords = {*Biomass, Acetic Acid/metabolism, Alkanes/*analysis/chemistry, Biomarkers/chemistry, carbon isotopes, Carbon/*chemistry, Chlorophyll/metabolism, Cyanobacteria/chemistry/*metabolism, Geologic Sediments/*chemistry, Lipids/*biosynthesis, NASA Discipline Exobiology, Non-NASA Center, Palmitic Acid/metabolism, Phytol/metabolism, Pigments, Biological/metabolism},
	isbn = {0016-7037 (Print)0016-7037 (Linking)
},
	url = {https://www.ncbi.nlm.nih.gov/pubmed/11540730},
	author = {Sakata, S. and Hayes, J. M. and McTaggart, A. R. and Evans, R. A. and Leckrone, K. J. and Togasaki, R. K.}
}
@article {936,
	title = {Efficiency and temperature dependence of water removal by membrane dryers},
	journal = {Anal Chem},
	volume = {69},
	year = {1997},
	note = {Leckrone, K JHayes, J MengComparative StudyResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.1997/03/01 00:00Anal Chem. 1997 Mar 1;69(5):911-8.},
	month = {Mar 01},
	pages = {911-8},
	abstract = {The vapor pressure of water in equilibrium with sorption sites within a Nafion membrane is given by log P(WN) = -3580/T + 10.01, where P(WN) is expressed in Torr and T is the membrane temperature, in kelvin. The efficiency of dryers based on selective permeation of water through Nafion can thus be enhanced by cooling the membrane. Residual water in effluents exceeds equilibrium levels if insufficient time is allowed for water to diffuse to the membrane surface as gas passes through the dryer. For tubular configurations, this limitation can be avoided if L > or = Fc(10(3.8)/120 pi D), where L is the length of the tubular membrane, in centimeters, Fc is the gas flow rate, in mL/ min, and D is the diffusion coefficient for water in the carrier gas at the operating temperature of the dryer, in cm2/s. An efficient dryer that at room temperature dries gas to a dew point of -61 degrees C is described; the same dryer maintained at 0 degrees C yields a dew point of -80 degrees C and removes water as effectively as Mg(ClO4)2 or a dry ice/acetone slush. The use of Nafion membranes to construct devices capable of delivering gas streams with low but precisely controlled humidities is discussed.},
	issn = {0003-2700 (Linking)},
	doi = {10.1021/ac9610220},
	author = {Leckrone, K. J. and Hayes, J. M.}
}
