Isoflurane as an Accurate Negative Mode Calibrant for Ion Mobility Spectrometry.

The search for suitable ion mobility spectrometry (IMS) calibrant compounds is ongoing and necessitates the use of highly accurate reduced ion mobility (K0) values across a range of instrumental conditions. Such values will be used in calibrating devices to shift the ion mobility scales and alarm windows for chemicals of interest to their proper locations based on the instrumental conditions present during calibration and sampling. Many positive ion mode calibrants have been investigated, whereas investigations for a negative ion detection mode calibrant have been more limited. Isoflurane (IsoF) is a strong candidate as a negative ion mode calibrant. This report documents the accurately measured K0 values for IsoF product ions as a function of multiple instrumental parameters. K0 values were measured in two instrumentation modes as a function of drift gas temperature, water concentration, and dopant concentration. This report culminates with our evaluation of the suitability of IsoF as a negative ion mode calibrant for IMS applications.

Accurate Evaluation of Potential Calibration Standards for Ion Mobility Spectrometry.

Ion mobility spectrometry (IMS)-based instruments have historically been accurate to, at best, ±2% of the reduced ion mobility (K0) value of the chemical of interest. Fielded IMS-based detectors that are in use for hazardous and illicit substance detection are subject to false-positive alarms because of this inaccuracy and the resulting wide alarm windows, which are required to maintain a high rate of true-positive alarms. To reduce false-positive alarm rates and improve the accuracy of any IMS-based instrument, accurate K0 values of an ion mobility reference standard need to be used for ion mobility scale calibration. However, a suitable calibrant has yet to be accurately analyzed and agreed upon by the IMS community. In this study, we have chosen five potential IMS calibrants on the basis of their rating against seven criteria for suitable standards and analyzed them as a function of drift gas temperature and humidity using an accurate ion mobility instrument. Recommendations are made herein for each potential calibrant's suitability as a standard for the wider IMS community.

High Accuracy Ion Mobility Spectrometry for Instrument Calibration.

Ion mobility spectrometry (IMS) is widely used to characterize compounds of interest (COIs) based on their reduced mobility ( K0) values. In an attempt to increase the accuracy and agreement of studies, the most recommended method has been to use a reference compound with a known K0 value to calibrate the instrument and calculate COI K0 values from normalized spectra. Researchers are limited by the accuracy of previous K0 value reference measurements on which to base their calibrations. Any inaccuracy in these reference K0 values, typically ±2%, will propagate through to the calculated K0 value of the COI. For this reason, there is a need to standardize reference K0 values with improved accuracy. Through improvement of the accuracy of reference measurements, a lower degree of error will propagate through new K0 value calculations. The K0 values of the ammonium reactant ion, the potential reference standard dimethyl methylphosphonate (DMMP), and three explosive COIs were characterized at multiple drift gas temperatures, drift gas water contents, and electric field strengths on an accurate ion mobility spectrometry instrument. K0 values reported here are known to ±0.1% as a result of reducing the error of all instrumental parameters.

Determination of E/N Influence on K0 Values within the Low Field Region of Ion Mobility Spectrometry.

The established theory of ion motion within weak electric fields predicts that reduced ion mobility (K0) remains constant as a function of the ratio of electric field strength to drift gas number density (E/N). However, upon increasing the accuracy and precision of K0 value measurements during a previous study, a new relationship was seen in which the K0 values of ions decreased as a function of increasing E/N at field strengths below 4 Td. Here the effect of E/N on the K0 value of an ion has been investigated in order to validate the reality of the phenomenon and determine its cause. The pertinent measurements of voltage and drift time were verified in order to ensure the authenticity of the trend and that it was not a result of a systematic error in parametric measurements. The trend was also replicated on a separate ion mobility spectrometer drift tube in order to further validate its authenticity. As a result, the theory of ion motion within weak electric fields should be revised to reflect the behavior seen here.

E/N effects on K0 values revealed by high precision measurements under low field conditions.

Ion mobility spectrometry (IMS) is used to detect chemical warfare agents, explosives, and narcotics. While IMS has a low rate of false positives, their occurrence causes the loss of time and money as the alarm is verified. Because numerous variables affect the reduced mobility (K0) of an ion, wide detection windows are required in order to ensure a low false negative response rate. Wide detection windows, however, reduce response selectivity, and interferents with similar K0 values may be mistaken for targeted compounds and trigger a false positive alarm. Detection windows could be narrowed if reference K0 values were accurately known for specific instrumental conditions. Unfortunately, there is a lack of confidence in the literature values due to discrepancies in the reported K0 values and their lack of reported error. This creates the need for the accurate control and measurement of each variable affecting ion mobility, as well as for a central accurate IMS database for reference and calibration. A new ion mobility spectrometer has been built that reduces the error of measurements affecting K0 by an order of magnitude less than ±0.2%. Precise measurements of ±0.002 cm(2) V(-1) s(-1) or better have been produced and, as a result, an unexpected relationship between K0 and the electric field to number density ratio (E/N) has been discovered in which the K0 values of ions decreased as a function of E/N along a second degree polynomial trend line towards an apparent asymptote at approximately 4 Td.

Detection of aqueous phase chemical warfare agent degradation products by negative mode ion mobility time-of-flight mass spectrometry [IM(tof)MS].

The use of negative ion monitoring mode with an atmospheric pressure ion mobility orthogonal reflector time-of-flight mass spectrometer [IM(tof)MS] to detect chemical warfare agent (CWA) degradation products from aqueous phase samples has been determined. Aqueous phase sampling used a traditional electrospray ionization (ESI) source for sample introduction and ionization. Certified reference materials (CRM) of CWA degradation products for the detection of Schedule 1, 2, or 3 toxic chemicals or their precursors as defined by the chemical warfare convention (CWC) treaty verification were used in this study. A mixture of six G-series nerve related CWA degradation products (EMPA, IMPA, EHEP, IHEP, CHMPA, and PMPA) and their related collision induced dissociation (CID) fragment ions (MPA and EPA) were found in each case to be clearly resolved and detected using the IM(tof)MS instrument in negative ion monitoring mode. Corresponding ions, masses, drift times, K(o) values, and signal intensities for each of the CWA degradation products are reported.

Nonlinear wavelet compression of ion mobility spectra from ion mobility spectrometers mounted in an unmanned aerial vehicle.

Linear and nonlinear wavelet compression of ion mobility spectrometry (IMS) data are compared and evaluated. IMS provides low detection limits and rapid response for many compounds. Nonlinear wavelet compression of ion mobility spectra reduced the data to 4-5% of its original size, while eliminating artifacts in the reconstructed spectra that occur with linear compression, and the root-mean-square reconstruction error was 0.17-0.20% of the maximum intensity of the uncompressed spectra. Furthermore, nonlinear wavelet compression precisely preserves the peak location (i.e., drift time). Small variations in peak location may occur in the reconstructed spectra that were linearly compressed. A method was developed and evaluated for optimizing the compression. The compression method was evaluated with in-flight data recorded from ion mobility spectrometers mounted in an unmanned aerial vehicle (UAV). Plumes of dimethyl methylphosphonate were disseminated for interrogation by the UAV-mounted IMS system. The daublet 8 wavelet filter exhibited the best performance for these evaluations.