Fe isotope variations in natural materials measured using high mass resolution multiple collector ICPMS

We present the first measurements of Fe isotope variations in chemically purified natural samples using high mass resolution multiple-collector inductively coupled plasma source mass spectrometry (MC-ICPMS). High mass resolution allows polyatomic interferences at Fe masses to be resolved (especially, (40)Ar(14)N(+), (40)Ar(16)O(+), and (40)Ar(16)OH(+)). Simultaneous detection of Fe isotope ion beams using multiple Faraday collectors facilitates high-precision isotope ratio measurements. Fe in basalt and paleosol samples was extracted and purified using a simple, single-stage anion chemistry procedure. A Cu "element spike" was used as an internal standard to correct for variations in mass bias. Using this procedure, we obtained data with an external precision of 0.03-0.11 per thousand and 0.04-0.15 per thousand for delta(56/54)Fe and delta(57/54)Fe, respectively (2sigma). Use of Cu was necessary for such reproducibility, presumably because of subtle effects of residual sample matrix on mass bias. These findings demonstrate the utility of high-resolution MC-ICPMS for high-precision Fe isotope analysis in geologic and other natural materials. They also highlight the importance of internal monitoring of mass bias, particularly when using routine methods for Fe extraction and purification.     

Sulfur isotope analysis of sulfide and sulfate minerals by continuous flow-isotope ratio mass spectrometry

A continuous flow method (CF-IRMS) for the rapid determination of the sulfur isotope composition of sulfide and sulfate minerals has significant advantages over the classic extraction method in terms of the reduced sample quantity and a rapid analytical cycle of less than 8 min/ analysis. For optimum performance, the technique is sensitive to a number of operating parameters, including sample weight and the O2 saturation of the Cu-reduction reactor. Raw data are corrected using a calibration based on five international and internal standards ranging from -17.3 to +20.3 per thousand, which requires monitoring in order to correct the effect of changing delta18O of the sample gas on the measured mass 66 values. Measured sulfur contents are within 1-1.5% of expected values and the reproducibility of delta34S values is +/-0.1 per thousand (1sigma). The technique has been used successfully for more than 1000 analyses of geological samples with a wide range of delta34S from -20 to +20 per thousand.     

Determination of the S isotope composition of methanesulfonic acid

Sulfur (S) isotopes have been used to apportion the amount of biogenic and anthropogenic sulfate in remote environments, an important parameter that is used to model the global radiation budget. A key assumption in the apportionment calculations is that there is little isotope selectivity as reduced compounds such as dimethyl sulfide (DMS) are oxidized. This paper describes a method to determine, for the first time, the S isotope composition of methanesulfonic acid (MSA), the product of DMS oxidation. The isotope composition of MSA was measured directly by EA-IRMS and was used as an isotope reference for the method. Synthetic mixtures approximating the conditions expected for aerosol MSA samples were prepared to test this method. First, MSA solutions were measured alone and then in combination with MSA and SO4(2-). In synthetic mixtures, SO4(2-) was separated from MSA by precipitating it as BaSO4 prior to preparation of MSA for isotope analysis. The delta 34S value for MSA solutions was -2.6 per thousand (SD +/- 0.4 per thousand), which is not different from the delta 34S obtained from MSA filtrate after precipitating SO4(2-) from the mixture (-2.7 +/- 0.3 per thousand). However, these values are offset from direct EA-IRMS analysis of MSA used as the isotope reference by -1.1 +/- 0.2 per thousand, and this must be accounted for in reporting MSA measurements. The S isotope measurements using this method approach a limiting value above 300 microg of MSA. This is approximately equal to the MSA found in 20,000 m3 of air, assuming ambient concentrations of approximately 15 ng m(-3). Three samples of MSA from the Pacific Ocean measured using this technique have an average delta 34S value of +17.4 +/- 0.7 per thousand.     

Chromatographic purification for the determination of dissolved silicon isotopic compositions in natural waters by high-resolution multicollector inductively coupled plasma mass spectrometry

A procedure is described for accurate Si isotope ratio measurements by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Dissolved silicon was preconcentrated and separated from other elements present in natural surface waters using anion-exchange chromatography. The optimized procedure provides virtually complete elimination of major inorganic constituents while maintaining Si recovery in excess of 97%. High-resolution capabilities of MC-ICPMS used in this study allow interference-free measurements of 28Si and 29Si isotopes using conventional solution nebulization sample introduction without aerosol desolvation. Owing to the magnitude of polyatomic ion contributions in the region of mass 30, mostly from 14N16O+, measurements of the 30Si isotope can be affected by tailing of the interference signals, making exact matching of analyte and nitric acid concentrations in all measurement solutions mandatory. Isotope abundance ratio measurements were performed using the bracketing standards approach and on-line correction for mass-bias variations using an internal standard (Mg). Uncertainties, expressed as 95% confidence intervals, for replication of the entire procedure are better than +/-0.18/1000 for delta29Si and +/-0.5/1000 for delta30Si. For the first time with MC-ICPMS, the quality of Si isotope abundance ratio measurements could be verified using a three-isotope plot. All samples studied were isotopically heavier than the IRMM-018 Si isotopic reference material.     

Precise and traceable (13)C/(12)C isotope amount ratios by multicollector ICPMS

A new method for the measurement of SI traceable carbon isotope amount ratios using a multicollector inductively coupled mass spectrometer (MC-ICPMS) is reported for the first time. Carbon (13)C/(12)C isotope amount ratios have been measured for four reference materials with carbon isotope amount ratios ranging from 0.010659 (delta(13)C(VPDB) = -46.6 per thousand) to 0.011601 (delta(13)C(VPDB) = +37 per thousand). Internal normalization by measuring boron (11)B/(10)B isotope amount ratios has been used to correct for the effects of instrumental mass bias. Absolute (13)C/(12)C ratios have been measured and corrected for instrumental mass bias and full uncertainty budgets have been calculated using the Kragten approach. Corrected (13)C/(12)C ratios for NIST RM8545 (Lithium Carbonate LSVEC), NIST RM8573 (L-Glutamic Acid USGS40), NIST RM8542 (IAEA-CH6 Sucrose) and NIST RM8574 (L-Glutamic Acid USGS41) differed from reference values by 0.06-0.20%. Excellent linear correlation (R = 0.9997) was obtained between corrected carbon isotope amount ratios and expected carbon isotope amount ratios of the four chosen NIST RMs. The method has proved to be linear within this range (from (13)C/(12)C = 0.010659 to (13)C/(12)C =0.011601), and therefore, it is suitable for the measurement of carbon isotope amount ratios within the natural range of variation of organic carbon compounds, carbonates, elemental carbon, carbon monoxide, and carbon dioxide. In addition, a CO2 gas sample previously characterized in-house by conventional dual inlet isotope ratio mass spectrometry has been analyzed and excellent agreement has been found between the carbon isotope amount ratio value measured by MC-ICPMS and the IRMS measurements. Absolute values for carbon isotope amount ratios traceable to the SI are given for each NIST RM, and the combined uncertainty budget (including instrumental error and each parameter contributing to Russell expression for mass bias correction) has been found to be < 0.1% for the four materials. The advantage of the method versus conventional gas source isotope ratio mass spectrometry measurements is that carbon isotope amount ratios are measured as C(+) instead of CO2(+), and therefore, an oxygen (17)O correction due to the presence of (12)C(17)O(16)O(+) is not required. Organic compounds in solution can be measured without previous derivatization, combustion steps, or both, thus making the process simple. The novel methodology opens new avenues for the measurement of absolute carbon isotope amount ratios in a wide range of samples.     

Determination of total sulfur at microgram per gram levels in geological materials by oxidation of sulfur into sulfate with in situ generation of bromine using isotope dilution high-resolution ICPMS

We have developed a new, simple, and accurate method for the determination of total sulfur at microgram per gram levels in milligram-sized silicate materials with isotope dilution high-resolution inductively coupled plasma mass spectrometry equipped with a flow injection system. In this method, sulfur can be quantitatively oxidized by bromine into sulfate with achievement of isotope equilibrium between the sample and spike. Detection limits for 32S+ and 34S+ in the ideal solution and silicate samples were 1 and 6 ng mL(-1) and 0.07 and 0.3 microg g(-1), respectively. The total blank was 46 ng, so that a 40-mg silicate sample containing 10 mirog g(-1) sulfur can be measured with a blank correction of < 10%. This total blank can be lowered to 8 ng if a low-blank air system is used for evaporations. To evaluate the applicability of this method, we analyzed not only silicate reference materials with sulfur content of 5.25-489 microg g(-1) and sample sizes of 13-40 mg but also the Allende meteorite with a sulfur content of 2%. The reproducibility for various rock types was < 9%, even though blank corrections in some samples of low sulfur content were up to 24%. This method is suitable for analyzing geological samples as well environmental samples such as soils, sediments, and water samples.     

Measurement of sulfur isotope compositions by tunable laser spectroscopy of SO2

Sulfur isotope measurements offer comprehensive information on the origin and history of natural materials. Tunable laser spectroscopy is a powerful analytical technique for isotope analysis that has proven itself readily adaptable for in situ terrestrial and planetary measurements. Measurements of delta(34)S in SO2 were made using tunable laser spectroscopy of combusted gas samples from six sulfur-bearing solids with delta(34)S ranging from -34 to +22 per thousand (also measured with mass spectrometry). Standard deviation between laser and mass spectrometer measurements was 3.7 per thousand for sample sizes of 200 +/- 75 nmol SO(2). Although SO(2)(g) decreased 9% over 15 min upon entrainment in the analysis cell from wall uptake, observed fractionation was insignificant (+0.2 +/- 0.6 per thousand). We also describe a strong, distinct (33)SO(2) rovibrational transition in the same spectral region, which may enable simultaneous delta(34)S and Delta(33)S measurements.     

Quantitative analysis of proteins via sulfur determination by HPLC coupled to isotope dilution ICPMS with a hexapole collision cell

Quantitative analysis of proteins is an essential part and also constitutes a major challenge in modern proteomics. Quantification of proteins by inductively coupled plasma mass spectrometry (ICPMS) offers an alternative method for quantitative proteomics. In this study, we developed a method of absolute quantification of proteins via sulfur by size exclusion chromatography (SEC) coupled to ICPMS with a collision cell (ICP-CC-MS) and postcolumn isotope dilution. Bovine serum albumin (BSA), superoxide dismutase (SOD), and metallothionein-II (MT-II) served as model proteins. Enriched 34S, 65Cu, and 67Zn isotopic solutions were continuously mixed with the eluate from the SEC. Oxygen was added as a reactive gas into the collision cell where sulfur reacts with oxygen to form sulfur-oxygen ion, the ratio of 32S16O(+)/34S16O(+) thus representing 32S(+)/34S(+). The absolute quantity of proteins could be calculated by the isotopic dilution equation and the content of sulfur in the proteins. The detection limits for BSA, SOD, and MT-II are 8, 31, and 15 pmol, respectively. The relative standard deviations for the proteins are less than 3%. The ratios of S/Cu and S/Zn in the proteins were also determined. The quantitative method was validated by comparing with gravimetric results.     

Optimization and application of ICPMS with dynamic reaction cell for precise determination of 44Ca/40Ca isotope ratios

An inductively coupled plasma mass spectrometer with dynamic reaction cell (ICP-DRC-MS) was optimized for determining (44)Ca/(40)Ca isotope ratios in aqueous solutions with respect to (i) repeatability, (ii) robustness, and (iii) stability. Ammonia as reaction gas allowed both the removal of (40)Ar+ interference on (40)Ca+ and collisional damping of ion density fluctuations of an ion beam extracted from an ICP. The effect of laboratory conditions as well as ICP-DRC-MS parameters such a nebulizer gas flow rate, rf power, lens potential, dwell time, or DRC parameters on precision and mass bias was studied. Precision (calculated using the "unbiased" or "n - 1" method) of a single isotope ratio measurement of a 60 ng g(-1) calcium solution (analysis time of 6 min) is routinely achievable in the range of 0.03-0.05%, which corresponded to the standard error of the mean value (n = 6) of 0.012-0.020%. These experimentally observed RSDs were close to theoretical precision values given by counting statistics. Accuracy of measured isotope ratios was assessed by comparative measurements of the same samples by ICP-DRC-MS and thermal ionization mass spectrometry (TIMS) by using isotope dilution with a (43)Ca-(48)Ca double spike. The analysis time in both cases was 1 h per analysis (10 blocks, each 6 min). The delta(44)Ca values measured by TIMS and ICP-DRC-MS with double-spike calibration in two samples (Ca ICP standard solution and digested NIST 1486 bone meal) coincided within the obtained precision. Although the applied isotope dilution with (43)Ca-(48)Ca double-spike compensates for time-dependent deviations of mass bias and allows achieving accurate results, this approach makes it necessary to measure an additional isotope pair, reducing the overall analysis time per isotope or increasing the total analysis time. Further development of external calibration by using a bracketing method would allow a wider use of ICP-DRC-MS for routine calcium isotopic measurements, but it still requires particular software or hardware improvements aimed at reliable control of environmental effects, which might influence signal stability in ICP-DRC-MS and serve as potential uncertainty sources in isotope ratio measurements.     

Direct determination of tellurium in geological samples by inductively coupled plasma mass spectrometry using ethanol as a matrix modifier

Direct determination of tellurium in geological samples by inductively coupled plasma mass spectrometry (ICP-MS) is often complicated by its low abundance, poor analytical sensitivity, and the presence of xenon interferences. Therefore, a simplified and rapid method for direct determination of nanogram levels of tellurium in geological samples using ICP-MS by reduction of interferences and improvement of sensitivity was developed. It is impossible to resolve 126Te and 128Te from isotope interferences of Xe even by currently available high-resolution magnetic mass spectrometry due to the extremely small mass difference (0.001-0.002 amu). However, the addition of 4% ethanol was found to suppress the interferences of Xe by a factor of 6 and increases the sensitivity of Te determination in ICP-MS by a factor of 3 relative to the values obtained in conventional 3% (v/v) HNO3 solution at the corresponding optimum operating conditions, respectively. The detection limits of 126Te and 128Te were reduced by a factor of 7.2 and 8.8, respectively, and the limit of quantitation (LOQ) for 126Te in the presence of 4% ethanol was 1.5 ng g(-1) (the LOQ is expressed as the concentration in the solid samples, thereby taking into account the dilution factor of 1000). The agreement between the determined Te concentration values (r = 0.998) in various geological samples (n = 140) by using isotopes of 126Te and 128Te indicates negligible contributions of polyatomic interferences produced by the addition of ethanol at these m/z. The proposed method was successfully applied to the direct determination of nanogram levels of Te in a series of international geological reference materials.     

Triple quad ICPMS (ICPQQQ) as a new tool for absolute quantitative proteomics and phosphoproteomics

It is clear that sensitive and interference-free quantification of ICP-detectable elements naturally present in proteins will boost the role of ICPMS in proteomics. In this study, a completely new way of polyatomic interference removal in ICPMS for detection of sulfur (present in the majority of proteins as methionine or cysteine) and phosphorus (present in phosphorylated proteins) is presented. It is based on the concept of tandem mass spectrometry (QQQ) typically used in molecular MS. Briefly, the first quadrupole can be operated as 1 amu window band-pass mass filter to select target analyte ions ((31)P, (32)S, and their on-mass polyatomic interferences). In this way, only selected ions enter the cell and react with O(2), reducing the interferences produced by matrix ions as well as background noise. After optimization of the cell conditions, product ions formed for the targets, (47)PO(+) and (48)SO(+), could be detected with enhanced sensitivity and selectivity. The coupling to capillary HPLC allowed analysis of S- and P-containing species with the lowest detection limits ever published (11 and 6.6 fmol, respectively). The potential of the approach for proteomics studies was demonstrated for the highly sensitive simultaneous absolute quantification of different S-containing peptides and phosphopeptides.     

Determination of iron, cobalt, copper, zinc, rubidium, molybdenum, and cesium in human serum by inductively coupled plasma mass spectrometry

A method was developed for the determination of seven trace elements (Fe, Co, Cu, Zn, Rb, Mo, and Cs) in human serum by inductively coupled plasma mass spectrometry (ICP-MS). Sample preparation was kept as limited as possible. As the only sample pretreatment serum samples were diluted with nitric acid and indium was added as an internal standard. The results for iron, cobalt, copper, and zinc were corrected for interferences from polyatomic ions by using a blank solution containing the same concentration of sodium, sulfur, chlorine, and calcium as human serum. For copper and zinc the corrections are small, whereas for iron and cobalt they are important. No interferences occur for the considered isotopes of rubidium, molybdenum, and cesium. In order to test the accuracy and precision of the analytical technique, a "second-generation biological reference material (human serum)" was analyzed. The results obtained by ICP-MS for the seven elements considered showed good agreement with the certified values.     

Chlorine stable isotopes: a comparison of dual inlet and thermal ionization mass spectrometric measurements

Chlorine stable isotope ratios, 37Cl/35Cl, currently are measured using dual-inlet and thermal-ionization mass spectrometry. These two different analytical techniques, however, have never been cross calibrated. A set of samples with chlorine stable isotope delta values ranging from -4.4 to +0.3 % relative to standard mean ocean water chloride has been analyzed using both of these techniques. Our data show that both techniques can yield similar results within analytical uncertainty. CsCl thermal ionization data are extremely sensitive to the amount of chlorine being measured and cannot be used to determine absolute ratios without an independent means of correcting for machine-induced mass fractionation. As long as standards and samples are of equivalent size, however, the differences between samples measured by thermal ionization remain constant Dual inlet stable isotope mass spectrometry is suited best for samples of > 10 micromol Cl, yielding chlorine stable isotope data with < or =0.1% reproducibilities (2sigma). Thermal ionization mass spectrometry easily accommodates samples of approximately0.1-0.3 micromol Cl, with achievable uncertainties of < or =0.2% (2sigma).     

An approach for assessing total instrumental uncertainty in compound-specific carbon isotope analysis: implications for environmental remediation studies

Determination of compound-specific carbon isotope values by continuous flow isotope ratio mass spectrometry is impacted by variation in several routine operating parameters of which one of the most important is signal size, or linearity. Experiments were carried out to evaluate the implications of these operating parameters on both reproducibility and accuracy of delta13C measurements. A new method is described for assessing total instrumental uncertainty of routine compound-specific delta13C analysis, incorporating both accuracy and reproducibility. These findings have important implications for application of compound-specific isotope analysis in environmental geochemistry and in particular for the rapidly developing field of isotopic investigation of biodegradation and remediation of organic chemicals in contaminant hydrogeology.     

Determination of active pharmaceutical ingredients by heteroatom selective detection using inductively coupled plasma mass spectrometry with ultrasonic nebuilization and membrane desolvation sample introduction

The combination of ultrasonic nebulization with membrane desolvation (USN-MD) is utilized to determine active pharmaceutical ingredients (API) by heteroatom inductively coupled mass spectroscopy (ICP-MS) detection. Ultrasonic nebulization provides efficient sampling while use of the membrane desolvator acts to reduce solvent-based interferences. This approach reduces interferences sufficiently so that a standard argon ICP-quadrupole MS can be utilized. Examined APIs and associated heteroatoms included: phosphomycin (P), amoxicillin (S), chlorpropamide (Cl), and ofloxacin (F). The optimum plasma r.f. powers for P, S, and Cl were in the 1000 to 1200 watts range. The high ionization energy of F required that the plasma be operated at 1500 W. The 16O2+ interference at mass 32 precluded determinations using the sulfur-32. The sulfur-34 (4.2% natural isotopic abundance), however, was relatively free of isobaric interferences. Interferences were relatively small at the mass 35 isotope of Cl, but increased with higher ICP r.f. powers. Overlaps were significant at the masses of monoisotopic species, fluorine-19 and phosphorus-31. Detection limits for P, S, Cl, and F of 2, 3, 90, and 3000 ng/mL, respectively, were generally lower than those produced with other quadrupole systems and comparable to or better than values published utilizing high-resolution instruments.     

Precolumn isotope dilution analysis in nanoHPLC-ICPMS for absolute quantification of sulfur-containing peptides

A novel generic approach based on precolumn isotope dilution nanoHPLC-ICPMS analysis was developed for the accurate absolute quantification of sulfur-containing peptides. A 34S-labeled, species-unspecific sulfur spike (sulfate), noninteracting with analyte peptides under the optimized HPLC condition, was added directly to the chromatographic eluents. Thus a generic sulfur standard permanently present during analysis was used for peptide quantification. Interference-free detection of the 32S and 34S isotopes in ICPMS was achieved by eliminating O2+ ions in a collision cell using Xe gas at 130 microL min-1. The detection limit for sulfur was 45 microg L-1 which corresponded to 1-2 pmol of individual peptides. The method was validated by the analysis of a standard peptide solution showing high accuracy (recovery 103%) and good precision (RSD 2.1%). The combination of nanoHPLC-ICP IDMS with nanoHPLC-ESI MS/MS allowed the precise quantification and identification of sulfur-containing peptides in tryptic digests of human serum albumin and salt-induced yeast protein (SIP18) at the picomole level.     

Evaluation of gas chromatographic isotope fractionation and process contamination by carbon in compound-specific radiocarbon analysis

The relevance of both modern and fossil carbon contamination as well as isotope fractionation during preparative gas chromatography for compound-specific radiocarbon analysis (CSRA) was evaluated. Two independent laboratories investigated the influence of modern carbon contamination in the sample cleanup procedure and preparative capillary gas chromatography (pcGC) of a radiocarbon-dead 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169) reference. The isolated samples were analyzed for their 14C/12C ratio by accelerator mass spectrometry. Sample Delta14C values of -996 +/- 20 and -985 +/- 20 per thousand agreed with a Delta14C of -995 +/- 20 per thousand for the unprocessed PCB 169, suggesting that no significant contamination by nonfossil carbon was introduced during the sample preparation process at either laboratory. A reference compound containing a modern 14C/12C ratio (vanillin) was employed to evaluate process contamination from fossil C. No negative bias due to fossil C was observed (sample Delta14C value of 165 +/- 20 per thousand agreed with Delta14C of 155 +/- 12 per thousand for the unprocessed vanillin). The extent of isotopic fractionation that can be induced during pcGC was evaluated by partially collecting the vanillin model compound of modern 14C/12C abundance. A significant change in the delta13C and delta14C values was observed when only parts of the eluting peak were collected (delta13C values ranged from -15.75 to -49.91 per thousand and delta14C values from -82.4 to +4.71 per thousand). Delta14C values, which are normalized to a delta13C of -25 per thousand, did not deviate significantly (-58.9 to -5.8 per thousand, considering the uncertainty of approximately +/-20 per thousand). This means that normalization of radiocarbon results to a delta13C of -25 per thousand, normally performed to remove effects of environmental isotope fractionation on 14C-based age determinations, also cor-rects sufficiently for putative isotopic fractionation that may occur during pcGC isolation of individual compounds for CSRA.     

High-precision position-specific isotope analysis of 13C/12C in leucine and methionine analogues

We report an automated method for high-precision position-specific isotope analysis (PSIA) of carbon in amino acid analogues. Carbon isotope ratios are measured for gas-phase pyrolysis fragments from multiple sources of 3-methylthiopropylamine (3MTP) and isoamylamine (IAA), the decarboxylated analogues of methionine and leucine, using a home-built gas chromatography (GC)-pyrolysis-GC preparation system coupled to a combustion-isotope ratio mass spectrometry system. Over a temperature range of 620-900 degrees C, the characteristic pyrolysis products for 3MTP were CH4, C2H6, HCN, and CH3CN and for IAA products were propylene, isobutylene, HCN, and CH3CN. Fragment origin was confirmed by 13C-labeling, and fragments used for isotope analysis were generated from unique moieties with > 95% structural fidelity. Isotope ratios for the fragments were determined with an average precision of SD(delta13C) < 0.3% per thousand, and relative isotope ratios of fragments from different sources were determined with an average precision of SD(delta(delta)13C) < 0.5% per thousand. Delta(delta)13C values of fragments were invariant over a range of pyrolysis temperatures. The delta(delta)13C of complementary fragments in IAA was within 0.8% per thousand of the delta(delta)13C of the parent compounds, indicating that pyrolysis-induced isotopic fractionation is effectively taken into account with this calibration procedure. Using delta(delta)13C values of fragments, delta(delta)13C values were determined for all four carbon positions of 3MTP and for C1, C2, and the propyl moiety of IAA, either directly or indirectly by mass balance. Large variations in position-specific isotope ratios were observed in samples from different commercial sources. Most dramatically, two 3MTP sources differed by 16.30% per thousand at C1, 48.33% per thousand at C2, 0.37% per thousand at C3, and 5.36% per thousand at C(methyl). These PSIA techniques are suitable for studying subtle changes in intramolecular isotope ratios due to natural processes.     

Development of a compound-specific carbon isotope analysis method for atmospheric formaldehyde via NaHSO3 and cysteamine derivatization

A novel method has been developed for the compound-specific carbon isotope analysis of atmospheric formaldehyde using gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). The method allows the determination of the delta13C value for atmospheric formaldehyde at nanogram levels with higher precision and lower detection limit. In the present work, atmospheric formaldehyde was collected using NaHSO3-coated Sep-Pak silica gel cartridges, washed out by water, then derivatized by cysteamine of known delta13C value, and the delta13C value of its derivative (thiazolidine) determined by GC/C/IRMS. Finally, the delta13C value of atmospheric formaldehyde could be calculated by a simple mass balance equation between formaldehyde, cysteamine, and thiazolidine. Using three formaldehydes with different delta13C values, calibration experiments were carried out over large ranges of formaldehyde concentrations. The carbon isotope analysis method achieved excellent reproducibility and high accuracy. There was no carbon isotopic fractionation throughout the derivatization processes. The differences in the carbon isotopic compositions of thiazolidine between the measured and predicted values were always <0.5 per thousand, within the specifications of the GC/C/IRMS system. The present method was also compared with the previous 2,4-dinitrophenylhydrazine derivatization method, and this method could be performed with lower analytical error and detection limit. Using this method, four 6-h ambient atmospheric formaldehyde samples were consecutively collected from 8 to 9 March 2005. The results showed that the delta13C values of atmospheric formaldehyde were different during the daytime and nighttime. This method proved suitable for the routine operation and may provide additional insight on sources and sinks of atmospheric formaldehyde.