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Elemental Analysis of Organic Species with Electron Ionization High-Resolution Mass Spectrometry

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Elemental Analysis of Organic Species with Electron Ionization High-Resolution Mass Spectrometry Anal. Chem. 2007, 79, 8350-8358 Elemental Analysis of Organic Species with Electron Ionization High-Resolution Mass Spectrometry Allison C. Aiken,²,³ Peter F. DeCarlo,³,§ and Jose L. Jimenez*,²,³ Department of Chemistry and Biochemistry (UCB 215), Cooperative Institute for Research in the Environmental Sciences (CIRES, 216 UCB), and Department of Atmospheric and Oceanic Sciences (311 UCB), University of Colorado, Boulder, Colorado 80309 We present a new elemental analysis (EA) technique for conductivity detection.1 Oxygen content must be measured organic species (CHNO) that allows fast on-line analysis separately using pyrolysis and reduction of the sample,1 adding (10 s) and reduces the required sample size to ∼1 ng, additional instrument reconfiguration and analysis time. For ∼6 orders of magnitude less than standard techniques. organic/inorganic mixtures, either the sample needs to be The composition of the analyzed samples is approximated subjected to some type of separation (which could introduce by the average elemental composition of the ions from biases), inorganic species need to be analyzed separately for every high-resolution electron ionization (EI) mass spectra. EA sample, or the results obtained will average both types of species. of organic species can be performed on organic/inorganic As these techniques were designed for off-line analysis, time mixtures. Elemental ratios for the total organic mass, such resolution was not a concern. as oxygen/carbon (O/C), hydrogen/carbon (H/C), and Due to the limitations of existing organic EA techniques, the nitrogen/carbon (N/C), in addition to the organic mass development of new techniques that require much smaller samples to organic carbon ratio (OM/OC), can be determined. As and provide much faster analysis is of great interest in, for deviations between the molecular and the ionic composi- example, environmental analyses. One potential method is to use tion can appear due to chemical influences on the ion the elemental composition of the ions formed from electron ionization (EI) high-resolution mass spectrometry (HRMS), which fragmentation processes, the method was evaluated and is very fast and sensitive, to approximate the composition of the calibrated using spectra from 20 compounds from the molecules analyzed. This potential technique has not been NIST database and from 35 laboratory standards sampled explored to our knowledge, which may be due to the lack of with the high-resolution time-of-flight aerosol mass spec- widespread availability of HRMS instruments until recently and trometer (HR-ToF-AMS). The analysis of AMS (NIST) also to the perception of widespread chemical biases in the spectra indicates that quantification of O/C is possible fragmentation process.2,3 After a molecule is ionized into a positive with an error (average absolute value of the relative error) ion, it may undergo one or several fragmentation reactions, each of 30% (17%) for individual species. Precision is much producing an ion and a neutral fragment. Various chemical ( better than accuracy at 5% in the absence of air for AMS properties such as ionization potential, ion and neutral stability, data. AMS OM/OC has an average error of 5%. Additional etc., as well as kinetic effects influence the pathways of fragmenta- calibration is recommended for types of species very tion reactions,2 which could lead to biases between the molecular different from those analyzed here. EA was applied to and the ion elemental compositions, e.g., if electronegative groups organic mixtures and ambient aerosols (sampled at 20 s preferentially become the neutral fragment rather than a positive from aircraft). The technique is also applicable to other ion during fragmentation. EI-HRMS measurements such as direct injection MS. Organic aerosols are one type of environmental sample that is currently receiving considerable attention from the scientific Elemental analysis (EA) of complex organic mixtures (C, H, community. Aerosols are responsible for important effects on N, O) is of interest in many fields such as environmental and food climate, health, and visibility,4 and organic species comprise a chemistry. Traditional EA generally requires a few milligrams of large fraction of the mass at most locations.5 They are an example sample and ∼10 min per sample analysis. Thermal methods are of a complex organic mixture found in the environment for which the most common type of EA for CHN analysis, which decompose the sample using high-temperature oxidation followed by thermal (1) Skoog, D. A.; Holler, F. J.; Nieman, T. A. Principles of instrumental analysis, 5th ed.; Saunders College Pub.: Orlando, FL, 1998. (2) McLafferty, F. W.; Turecek, F. Interpretation of mass spectra, 4th ed.; * Author to whom correspondence should be addressed. E-mail: University Science Books: Mill Valley, CA, 1993. [email protected]. (3) McLafferty, F. W. Personal communication, 2006. ² Department of Chemistry and Biochemistry. (4) Seinfeld, J. H.; Pandis, S. N. Atmospheric chemistry and physics : from air ³ Cooperative Institute for Research in the Environmental Sciences (CIRES). pollution to climate change, 2nd ed.; Wiley: Hoboken, NJ, 2006. § Department of Atmospheric and Oceanic Science. (5) Zhang, Q.; et al. Geophys. Res. Lett. 2007, 34, L13801. 8350 Analytical Chemistry, Vol. 79, No. 21, November 1, 2007 10.1021/ac071150w CCC: $37.00 © 2007 American Chemical Society Published on Web 10/04/2007 standard chemical analysis techniques such as GC/MS can only METHODS identify a small fraction (∼10%) of the total species present.6 The Elemental Analysis Procedure. It is a property of EI for aerosol mass spectrometer (AMS) has become a ªde factoº molecules containing small atoms such as C, H, N, O, and S that standard for studying ambient aerosol size-resolved composition the sum of the ion signal intensities from all fragments is due to its ability to quantitatively characterize submicrometer approximately proportional to the mass concentration of the 8,17,18 nonrefractory particles with high sensitivity and time and size original species. This dependence arises because of the resolution.7-9 AMS spectra of standards have been shown to be roughly linear increase of the electron ionization cross section with the number of electrons in the molecule, combined with the similar to those from EI databases, albeit with more fragmenta- proportionality between the number of electrons and molar mass tion.10,11 The most notable difference is the greatly enhanced 8 + (MM) for molecules with the small atoms mentioned above. The intensity of the CO2 ion for species such as carboxylic acids in proportionality constant can change for various groups of mol- the AMS, likely due to pyrolysis occurring on the vaporizer.10 With ecules such as sulfate or oxygenated organics, but these changes the recently developed high-resolution time-of-flight AMS (HR- are smaller than a factor of 2 and can be calibrated.8 Thus, species ToF-AMS),12 elemental composition of organic ions can be directly can be directly quantified from the measured ion currents, a result determined, especially for the lower m/z (<100) where most of that has been verified through multiple intercomparisons with the signal appears for the AMS. quantitative instruments operating based on other principles.8,9,19 EA of the total organic aerosol is of great interest but is difficult If the elemental composition of each fragment ion signal in a to perform with traditional CHNO instrumentation due to the large mass spectrum can be identified (as with HRMS), the average sample size needed, which limits time resolution to several hours. composition of the ions can be calculated. Since the ion current - 3 It would require filtering 1000 10 000 m of air per sample at produced by EI is approximately proportional to the total molec- typical ambient particle concentrations, in addition to the potential ular mass present, the same ion current at different m/z's for gas adsorption or particle evaporation artifacts when collecting represents the same original mass. For example, if there is 1-kHz samples on a filter for offline analysis using traditional tech- signal at both m/z 60 and 300, each being the molecular ion from niques.13 Some recent mass spectrometric instruments have one of two species, approximately the same mass of both species demonstrated EA of individual particles by ablating the particles was present in the ionization region. Therefore, 5 times more into atomic ions.14-16 molecules of the lighter molecule must have been present. The This paper describes the first application of electron ionization average elemental composition in that case would be the weighed high-resolution mass spectrometry (EI-HRMS) to EA of organic average of the composition of the two observed ions, with each molecules (rather than just individual ions) to our knowledge. ion carrying the same weight (since their ion currents are the Using EI mass spectra of small molecules from the NIST database, same). For a complex spectrum from an unknown molecule or we demonstrate that it is possible to determine atomic ratios for mixture, the best approximation for the elemental composition O/C, H/C, and N/C, as well as the ratio for total organic mass to of the original species that each ion represents is the composition organic carbon mass (OM/OC). The EA method is further of that ion. The best estimate of the molecular or mixture validated and characterized with pure-component aerosol stan- composition can then be found by summing the ion contributions dards and their mixtures sampled with the HR-ToF-AMS. We then across the whole mass spectrum. For an organic spectrum, the apply the method to ambient aerosols as sampled from aircraft to relative mass concentrations of C and O (MC and MO, respectively) highlight its advantages of high precision, high time resolution, can be estimated as and small sample size for EA. m/zmax ) MC ∑ IjFC (6) Rogge, W.
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