Determination of functionalised carboxylic acids in atmospheric particles and cloud water using capillary electrophoresis/mass spectrometry

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Abstract

A capillary electrophoresis/electrospray ionisation mass spectrometry (CE/ESI-MS) method was developed for the determination of 38 organic acids in atmospheric particles and cloud water. The target analytes include many functionalised carboxylic acids, such as carboxylic acids with additional oxo-, hydroxy- or nitro-groups. These compounds are of large interest as their determination might give new insights into the atmospheric multiphase chemistry. OASIS HLB sorbent material (Waters) was used to extract and enrich polar carboxylic acids from aqueous solutions with recoveries greater than 80% for most analytes. Relative standard deviations in the range of 4–20% for peak areas (n = 5), including the SPE step, and 0.2–0.5% (n = 8) for migration times were found. The limits of detection (S/N = 3) ranged from 0.005 to 0.6 μmol l−1 for an ion-trap mass spectrometer and from 0.0004 to 0.08 μmol l−1 for a time-of-flight mass spectrometer. These detection limits translate into atmospheric concentrations in the low pg m−3 range based on the experimental conditions in this study. Severe matrix effects were observed for real samples, arising from complex co-extracted organic material. However, using the method of standard addition, most of the analytes could successfully be quantified in samples of ambient particles and cloud water with concentrations in the low ng m−3 to high pg m−3 range. These results demonstrate the suitability of the proposed method for the determination of a wide range of polar carboxylic acids at low concentrations in complex samples of different atmospheric phases.

Introduction

Atmospheric particles contain a significant amount of organic compounds, besides inorganic substances. In contrast to the well characterised inorganic fraction, the composition of the organic carbon (OC) is still largely unknown and only a small fraction of it could be identified so far [1], [2]. The determination of single organic species is therefore a major task in current atmospheric research [1], [3], [4].

Among the identified compounds of organic particle constituents, carboxylic acids usually represent a major fraction [5]. Most of the literature studies performed up to now were focused on particle phase straight-chain mono- and dicarboxylic acids, usually with 2–10 C-atoms (e.g. [6], [7], [8], [9], [10], [11]). Only very few measurements exist for functionalised carboxylic acids, i.e. mono- or dicarboxylic acids with additional functional groups, such as oxo-, hydroxy- or nitro-groups. However, a large fraction of particulate OC is known to consist of highly oxidised compounds [12]. Therefore, a determination of functionalised carboxylic acids can be expected to extend the knowledge of the chemical composition of particulate OC. Additionally, functionalised carboxylic acids are likely to be produced in oxidation reactions in the atmosphere. Potential sources could be radical reactions in the liquid phase, leading to hydroxylated or oxygenated acids [13]. A study of functionalised carboxylic acids could therefore give new insights into tropospheric multiphase chemistry.

For detailed studies of the OC composition, gas chromatography/mass spectrometry (GC/MS) is commonly used as an analytical technique. For GC/MS, polar compounds such as carboxylic acids need to be derivatised to more volatile products (e.g. esters), which makes the sample preparation time consuming. It also increases the risk of losses of the volatile esters during sample enrichment, which is usually done by volume reduction under vacuum [14], [15], [16], [17]. Therefore, modern liquid phase separation techniques such as high performance liquid chromatography (HPLC) or capillary electrophoresis (CE) coupled to mass spectrometry (MS) are attractive alternatives to GC/MS determinations for the determination of polar compounds [18], [19], [20], [21], [22], [23]. For ionisable compounds, CE is a very well suited tool due to the high separation efficiency, low sample consumption, short analytical times and low operation costs [24]. In combination with indirect UV detection it is a well established technique for the determination of short-chain organic acids in ambient particles (e.g. [2], [25], [26], [27]).

The aim of the present work is to develop and characterise a method for the determination of functionalised carboxylic acids in atmospheric particles and cloud water, using capillary electrophoresis/mass spectrometry (CE/MS).

Section snippets

Chemicals

The chemicals used in this work were obtained from the following suppliers: ammonium hydroxide solution (25%), acetic acid (>99.5%), and 2-propanol (>99.8%) from Fluka (Munich, Germany), methanol (Chromasolv, >99.0%), aqueous sodium hydroxide (NaOH, 1 mol l−1), and hydrochloric acid (HCl, 1 mol l−1) from Riedel-de Haën (Munich, Germany). All carboxylic acids were obtained from Sigma–Aldrich (Munich, Germany). Their purity was always better than 97%, except 5-oxoazelaic acid (96%),

Background electrolyte

In former studies of our group [19], a 20 mM ammonium acetate BGE with 10% (v/v) methanol and a pH of 9.1 resulted in good separation efficiency for acidic compounds. In the present work, slight variations of the ammonium acetate concentration (10–30 mM), the methanol fraction (0–15%), and the pH value (8 and 10) did not lead to significant improvements in the resolution of individual acids in a standard mixture. Therefore, this BGE was used for all further studies.

Electrospray ionisation

The optimisation of

Conclusions

A method has been developed to determine the concentrations of 38 carboxylic acids in aqueous extracts of atmospheric particles and cloud water using CE/MS and SPE for sample enrichment. The method shows excellent LODs, high separation efficiency, and good repeatability. A successful application of the method to complex real samples demonstrated that CE/MS in combination with SPE forms an attractive alternative to more established derivatisation-based GC/MS techniques. The proposed method

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