Electrospray ionization-ion mobility spectrometry: a rapid analytical method for aqueous nitrate and nitrite analysis Prabha Dwivedi, ab Laura M. Matz, a David A. Atkinson† c and Herbert H. Hill, Jr* ab a Department of Chemistry, Washington State University, Pullman, WA 99164-4630, USA. E-mail: hhhill@wsu.edu; Fax: 509-335-8867; Tel: 509-335-5648 b Center for Multiphase Environmental Research, Washington State University, Pullman, WA 99164, USA c Chemistry Department, Idaho National Engineering and Environmental Laboratory, Idaho Falls, Idaho 83415, USA Received 11th September 2003, Accepted 15th December 2003 First published as an Advance Article on the web 12th January 2004 This paper reports the first example of electrospray ionization (ESI) for the separation and detection of anions in aqueous solutions by ion mobility spectrometry (IMS). Standard solutions of arsenate, phosphate, sulfate, nitrate, nitrite, chloride, formate, and acetate were analyzed using ESI-IMS and distinct peak patterns and reduced mobility constants (K 0 ) were observed for respective anions. Real world water samples were analyzed for nitrate and nitrite to determine the feasibility of using ESI-IMS as a rapid analytical method for monitoring nitrate and nitrite in water systems. The data showed satisfactory correlation between the measured value ( ~ 0.16 ppm) and the reported maximum nitrate-nitrogen concentration (0.2 ppm) found in a local drinking water system. For on-site measurement applications, direct sample introduction and air as an alternate drift gas to nitrogen were evaluated. The identities of the nitrite and nitrate mobility peaks were verified by comparison of reduced mobility constants with mass identified nitrate and nitrite ions reported in literature. In the mixing ratio, a linear dynamic range of 3 orders of magnitude and instrument detection limits of 10 ppb for nitrate and 40 ppb for nitrite were obtained. The calibration curves showed r 2 value of 0.98 and slope of 0.06 for nitrate and r 2 value of 0.99 and slope of 0.11 for nitrite. Introduction Ion mobility spectrometry (IMS) with electrospray ionization was first reported for ions such as peptides and other non-volatile compounds from aqueous solutions in the 1980’s by Shumate and Hill. 1 Later, in 1994, Wittmer et al. demonstrated that whole proteins could be electrosprayed under atmospheric conditions and their multi-charged states separated by IMS. 2 Other applications of ESI-IMS have included the determination of polar and non-volatile organic compounds such as pesticides in aqueous samples. Recently, Dion et al., have demonstrated that electrospray ioniza- tion with ion mobility spectrometry (ESI-IMS) could be used to detect inorganic cations such as metal ions from aqueous samples. 3 The potential for ESI/IMS to determine inorganic ionic species in aqueous samples opened a new and potentially huge application area for IMS. Because IMS has a resolving power significantly greater than that of liquid chromatography, it may be possible to perform similar analyses to those requiring ion chromatographs but in only a fraction of the time and with higher resolution and sensitivity. In this paper, selected inorganic anions were used to characterize the response of an ion mobility spectrometer. Nitrate and nitrite ions were selected as the primary test ions because their gas phase mobilities had previously been measured as product ions from explosive vapours. 4 Past studies have suggested the involvement of nitrate and nitrite in the pathogenesis of metheamoglomia, infertility, cancer, tumor, still birth in livestock, through a mechanism involving reduction of nitrate to nitrite and subsequent formation of potentially mutagenic nitroso-compounds. 5–10 Regular sampling of drinking water sys- tems is necessary to prevent health risks from these contaminants. Excessive use of fertilizers (artificial or natural), biodegradation of nitrogenous organic matter, and waste streams from industries (explosives, pharmaceuticals, food processing) are the main sources of nitrate–nitrite contamination in natural water re- sources. 11 For these reasons, a rapid and sensitive method capable of determining low levels of nitrate and nitrite directly from aqueous samples is desired. In routine water quality analysis, determination of nitrate and nitrite is achieved by traditional analytical methods, including spectrophotometry, cadmium reduction, and ion chromatogra- phy. 12 Traditional methods are limited for on-site field measure- ments due to their high cost and maintenance, interference from matrix effects, low sensitivity and/or extensive sample preparation required prior to analysis. 13 For today’s environmental analytical applications, methods that are low cost, fast, accurate, sensitive, and portable enough for field applications are desirable. In addition, these analytical methods should require minimum instrumental maintenance and sample preparation procedures. When compared to methods currently used for nitrate–nitrite analysis, ESI-IMS method has an analysis time advantage over traditional methods since separation by IMS is achieved in milliseconds. Furthermore, the ESI-IMS instrument is low cost, easy to handle and requires minimal maintenance. This manuscript reports the results and conclusions of using ESI-IMS for the direct detection of anions such as nitrate and nitrite ions in natural water samples. 1.0 Experimental 1.1 Chemicals, solvents and samples The chemicals used in this study were sodium nitrate (NaNO 3 ), sodium nitrite (NaNO 2 ), calcium nitrate (Ca(NO 3 ) 2 ), potassium nitrite (KNO 2 ), formic acid (HCOOH), acetic acid (CH 3 COOH), sodium chloride (NaCl), sodium arsenate (Na 2 HAsO 4 .7H 2 O), sodium phosphate monobasic (NaH 2 PO 4 .H 2 O), and magnesium sulfate 7-hydrate (MgSO 4 .7H 2 O) purchased from Aldrich Chem- ical Company (Milwaukee, WI). Solvents used were HPLC grade methanol (CH 3 OH), acetonitrile (CH 3 CN) and water (H 2 O), and were purchased from J.T. Baker (Phillipsburg, NJ). Aqueous environmental water samples were collected from a river, a creek and a household water tap located in Palouse region at the state of Washington, USA. † Current address: Pacific Northwest National Laboratory, (PNNL), 902 Battelle Blvd., Richland, WA 99352, USA. This journal is © The Royal Society of Chemistry 2004 DOI: 10.1039/b311098b 139 Analyst , 2004, 129 , 139–144