Detection of perfluorocarbons using ion mobility spectrometry

An ISAS custom-designed ion mobility spectrometer equipped with a ionization source is used for the sensitive detection of traces of perfluorocarbons (PFCs, C₅F₁₂ to C₉F₂₀) in air, a class of substances for which a growing interest for industrial and environmental applications arose within the last years. Mobility spectra of the PFCs are presented, compared and discussed with regard to the possibility of identifying these analytes; detection limits are determined to be in the upper ng l⁻¹ range. Using a specific PFC as an example, a way to prevent unwanted contributions of non-product ions, the difference mobility spectrum, is introduced and described. Advantages and possibilities of this technique are briefly discussed.     

Continuous on-line determination of methyl tert-butyl ether in water samples using ion mobility spectrometry

A rapid analytical procedure for the on-line determination of methyl tert-butyl ether (MTBE) in water samples was developed. A new membrane extraction unit was used to extract the MTBE from water samples. The concentration of MTBE was determined using ion mobility spectrometry with 63Ni ionization and corona discharge ionization without chromatographic separation. Both ionization methods permit the sensitive determination of MTBE. A detection limit of 100 microg/L was established for the on-line procedure. Neither the inorganic compounds, humic substances nor gasoline were found to exert a significant influence on the peak intensity of the MTBE. The screening procedure can be used for concentrations of monoaromatic compounds (benzene, toluene, xylene) up to 600 microg/L. No sample preparation is required and the analysis results are available within 5 min. In order to determine concentrations between 10 microg/L and 100 microg/L, a discontinuous procedure was developed on the basis of the same experimental set-up.     

Fingerprinting bacterial strains using ion mobility spectrometry

Ion mobility spectrometry (IMS) is currently in widespread use for the detection and identification of narcotic and explosive compounds without prior sample clean-up or concentration steps. IMS analysis is rapid, less than a minute, and sensitive, with detection limits in the nanogram to picogram range, depending on the target analyte. Our studies indicate that this technique has potential for detection of specific components of bacterial cells and for identification and differentiation of bacterial strains and species within a minute, and with no specialized test kits or reagents required. When microgram quantities of whole bacterial cells are thermally desorbed, complex positive or negative ion patterns (plasmagrams) are obtained. These plasmagrams differ reproducibly for different strains and species and for different conditions of growth, and can be used for the classification and differentiation of specific strains and species of bacteria, including pathogens. Methods for improved ion peak detection, most notably sequential sample desorption at stepped increases in temperature (programmed temperature ramping), are described.     

Chemical standards in ion mobility spectrometry

Positive ion mobility spectra for three compounds (2,4-dimethylpyridine (2,4-DMP, commonly called 2,4-lutidine), dimethyl methylphosphonate (DMMP) and 2,6-di-t-butyl pyridine (2,6-DtBP)) have been studied in air at ambient pressure over the temperature range 37-250 °C with (H₂O)_nH⁺ as the reactant ion. All three compounds yield a protonated molecule but only 2,4-dimethylpyridine and dimethyl methylphosphonate produced proton-bound dimers. The reduced mobilities (K₀) of protonated molecules for 2,4-dimethylpyridine and DMMP increase significantly with increasing temperature over the whole temperature range indicating changes in ion composition or interactions; however, K₀ for the protonated molecule of 2,6-di-t-butyl pyridine was almost invariant with temperature. The K₀ values for the proton-bound dimers of 2,4-dimethylpyridine and DMMP also showed little dependence on temperature, but could be obtained only over an experimentally smaller and lower temperature range and at elevated concentrations. Chemical standards will be helpful as mobility spectra from laboratories worldwide are compared with increased precision and 2,6-di-t-butyl pyridine may be a suitable compound for use in standardizing reduced mobilities. The effect of thermal expansion of the drift tube length on the calculation of reduced mobilities is emphasized.     

Time-of-flight ion mobility spectrometry and differential mobility spectrometry: A comparative study of their efficiency in the analysis of halogenated compounds

The ion mobilities of halogenated aromatics which are of interest in environmental chemistry and process monitoring were characterized with field-deployable ion mobility spectrometers and differential mobility spectrometers. The dependence of mobility of gas-phase ions formed by atmospheric-pressure photoionization (APPI) on the electric field was determined for a number of structural isomers. The structure of the product ions formed was identified by investigations using the coupling of ion mobility spectrometry with mass spectrometry (APPI-IMS-MS) and APPI-MS. In contrast to conventional time-of-flight ion mobility spectrometry (IMS) with constant linear voltage gradients in drift tubes, differential mobility spectrometry (DMS) employs the field dependence of ion mobility. Depending on the position of substituents, differences in field dependence were established for the isomeric compounds in contrast to conventional IMS in which comparable reduced mobility values were detected for the isomers investigated. These findings permit the differentiation between most of the investigated isomeric aromatics with a different constitution using DMS.     

Determination of terpenes in humid ambient air using ultraviolet ion mobility spectrometry

Atmospheric humidity causes the major problem using ion mobility spectrometers (IMS) under ambient conditions. Significant changes of the spectra are decreasing sensitivity as well as selectivity. Therefore, the influence of humidity on the IMS signal was investigated in case of direct introduction of the analyte into the ionisation chamber and in case of pre-separation by help of a multi-capillary column (MCC). For direct analyte introduction, a significant decrease of the total number of ions in the range of 28-42% with increasing relative humidity was found. Simultaneously additional peaks in the spectra were formed, thus complicating the identification of the analytes. In case of pre-separation of the analyte, the spectra do not change with increasing relative humidity, due to the successive appearance of the analyte and the water molecules in the ionisation chamber. Detection limits were found in the range of 5 μg/m³ (about 1 ppbv) for selected terpenes and—with pre-separation—independent on relative humidity of the analyte. Without pre-separation, detection limits are in the same range for dry air as carrier gas but in the range of 200-600 μg/m³ when relative humidity reaches 100%. Thus, MCC-UV ion mobility spectrometry is optimally capable for the detection of trace substances in ambient air (e.g. indoor air quality control, process control, odour detection) without further elaborate treatment of the carrier gas containing the analyte and independent on relative humidity.     

Quantitative analysis of morphine and noscapine using corona discharge ion mobility spectrometry with ammonia reagent gas

Morphine and noscapine were determined using corona discharge ion mobility spectrometry. The detection limits were 5.6 × 10⁻¹¹ and 6.7 × 10⁻¹¹ g for morphine and noscapine, respectively. The linear dynamic ranges of the calibration plots for the compounds were about three orders of magnitude. The method has also been successfully applied for simultaneous determination of the compounds using the standard addition method.     

Hydrocarbon detection using laser ion mobility spectrometry

A Laser Ion Mobility Spectrometer has been set up and trace detection experiments have been performed. We find that laser ionization almost selectively ionizes aromatic hydrocarbons. Aliphatic hydrocarbons are only laser-ionized in case these contain conjugated double bonds. As, in contrast to radioactive ion mobility spectrometry, background air constituents and air contaminants cannot be ionized, drift spectra are inherently simple and easily interpretable. We show that a laser ion mobility spectrometer can be operated in two basically different modes, either using tunable or fixed-frequency laser sources. In the tunable laser mode, aromatic hydrocarbons can be detected in the positive mode and distinguished from each other on account of their different excitation wavelengths and ion drift times. In the fixed-frequency mode, specially chosen and intentionally admitted aromatic hydrocarbons are laser ionized and the primary ionization is transferred to non-aromatic species by means of atmospheric pressure chemical ionization. In this latter mode of operation nitroglycerin and triacetone triperoxide, two non-aromatic high explosives, could be detected.     

Analysis of biogenic amines using corona discharge ion mobility spectrometry

A new method based on corona discharge ion mobility spectrometry (CD-IMS) was developed for the analysis of biogenic amines including spermidine, spermine, putrescine, and cadaverine. The ion mobility spectra of the compounds were obtained with and without n-Nonylamine used as the reagent gas. The high proton affinity of n-Nonylamine prevented ion formation from compounds with a proton affinity lower than that of n-Nonylamine and, therefore, enhanced its selectivity. It was also realized that the ion mobility spectrum of n-Nonylamine varied with its concentration. A sample injection port of a gas chromatograph was modified and used as the sample introduction system into the CD-IMS. The detection limits, dynamic ranges, and analytical parameters of the compounds with and without using the reagent gas were obtained. The detection limits and dynamic ranges of the compounds were about 2 ng and 2 orders of magnitude, respectively. The wide dynamic range of CD-IMS originates from the high current of the corona discharge. The results revealed the high capability of the CD-IMS for the analysis of biogenic amines.     

Determination of ochratoxin A in licorice root using inverse ion mobility spectrometry

Despite the recent, successful efforts to detect mycotoxins, new methods are still required to achieve higher sensitivity, more simplicity, higher speed, and higher accuracy at lower costs. This paper describes the determination of ochratoxin A (OTA) using corona discharge ion mobility spectrometry (IMS) in the licorice root. A quick screening and measuring method is proposed to be employed after cleaning up the extracted OTA by immunoaffinity columns. The ion mobility spectrometer is used in the inverse mode to better differentiate the OTA peak from the neighboring ones. After optimization of the experimental conditions such as corona voltage, injection port temperature, and IMS cell temperature, a limit of detection (LOD) of 0.010 ng is obtained. Furthermore, the calibration curve is found to be in the range of 0.01-1 ng with a correlation coefficient (R²) of 0.988. Licorice roots were analyzed for their OTA content to demonstrate the capability of the proposed method in the quantitative detection of OTA in real samples.     

Towards metals analysis using corona discharge ionization ion mobility spectrometry

For the first time, the capability of corona discharge ionization ion mobility spectrometry (CD-IMS) in the determination of metal complex was evaluated. The extreme simplicity of dispersive liquid–liquid microextraction (DLLME) coupled to the high sensitivity of CD-IMS measurement could make this combination really useful for simple, rapid, and sensitive determination of metals in different samples. In this regard, mercury, as a model metal, was complexed with diethyldithiocarbamate (DEDTC), and then extracted into the carbon tetrachloride using DLLME. Some parameters affecting the extraction efficiency, including the type and volume of the extraction solvent, the type and volume of the disperser solvent, the concentration of the chelating agent, salt addition and, pH were exhaustively investigated. Under the optimized condition, the enrichment factor was obtained to be 142. The linear range of 0.035–10.0 μg mL⁻¹ with r² = 0.997 and the detection limit of 0.010 μg mL⁻¹ were obtained. The relative standard deviation values were calculated to be lower than 4% and 8% for intra-day and inter-day, respectively. Finally, the developed method was successfully applied for the extraction and determination of mercury in various real samples. The satisfactory results revealed the capability of the proposed method in trace analysis without tedious derivatization or hydride generation.     

Temperature effects on resolution in ion mobility spectrometry

The separation efficiency of ion mobility spectrometry (IMS) may be measured in terms of either resolving power, based on a single-peak definition, or peak-to-peak resolution, based on the separation of pairs of adjacent peaks. Usually resolving power decreases with temperature. However, the experimental results show that the peak-to-peak resolution may be increased in some cases. Negative ion mobility spectra of halide ions are better resolved at elevated temperatures. In addition, the peaks corresponding to protonated monomer of amylacetate and the proton-bound dimer of ethylacetate are well separated at 100 °C while they fully overlap at 18 °C. This paper focuses on the effect of temperature on peak-to-peak resolution. It was also observed that in some cases peak-to-peak resolution decreases with temperature. Examples are the spectra of cyclohexanone and methyl-iso-butyl ketone (MIBK) as well as dimethyl methyl phosphonate (DMMP) and MIBK. The increase or decrease in resolution at elevated temperatures has been attributed to the changes in separation factor (α) which is governed by the different hydration and clustering tendency of ions.     

Determination and identification of malathion, ethion and dichlorovos using ion mobility spectrometry

Positive ion mobility spectra of different organophosphorus pesticides such as malathion (s-(1,2-dicarb-ethoxyethyl) o,o-dimethyl dithiophosphate), ethion (o,o,o′,o′-tetraethyl s,s′-methylene bis(phosphorodithioate)) and dichlorovos (2,2-dichlorovinyl dimethyl phosphate) have been studied in air at ambient pressure using ion mobility spectrometry method with ⁶³Ni ionization source. The limits of quantification (LOQs) were 1.0 × 10⁻⁹, 1.0 × 10⁻⁹ and 5.0 × 10⁻⁹ g for malathion, ethion and dichlorovos, respectively. The working range of these compounds was about three orders of magnitude and the relative standard deviation (R.S.D.) of repeatability at the 5 μg ml⁻¹ level were all below 15%. Furthermore, in this study, the influences of IMS cell temperature on the ion mobility spectra of these compounds were investigated.     

Separation of benzodiazepines by electrospray ionization ion mobility spectrometry-mass spectrometry

Benzodiazepines are a commonly abused class of drugs; requiring analytical techniques that can separate and detect the drugs in a rapid time period. In this paper, the two-dimensional separation of five benzodiazepines was shown by electrospray ionization (ESI) ion mobility spectrometry (IMS)-mass spectrometry (MS). In this study, both the two dimensions of separation (m/z and mobility) and the high resolution of our IMS instrument enabled confident identification of each of the five benzodiazepines studied. This was a significant improvement over previous IMS studies that could not separate many of the analytes due to low instrumental resolution. The benzodiazepines that contain a hydroxyl group in their molecular structure (lorazepam and oxazepam) were found to form both the protonated molecular ion and dehydration product as predominant ions. Experiments to isolate the parametric reasons for the dehydration ion formation showed that it was not the result of corona discharge processes or the potential applied to the needle. However, the potential difference between the needle and first drift ring did influence both the relative intensity ratios of the two ions and the ion sensitivity.     

Detection of emissions from surfaces using ion mobility spectrometry

Emissions from surfaces (from furniture, wall paintings or floor coverings for instance) significantly influence indoor air quality and therefore the wellbeing or even the health of the occupants. Together with metabolites from mold they are responsible for the well-known “sick building syndrome”. Therefore, it is in the interest of the manufacturer as well as of the occupants to have a fast and accurate method for the detection of substances relevant to this syndrome in order to be able to monitor and control product quality and indoor air quality. The use of small and easy-to-transport ion mobility spectrometers that use UV light as the ionization source enables rapid in situ detection of such substances with high selectivity and sensitivity (detection limits in the lower ppb range). If a multicapillary column is used for preseparation as well, the selectivity is increased and the unwanted influence of humidity on the spectra can be eliminated, thus enabling the use of the instruments under normal ambient conditions. Furthermore, the use of air as carrier gas avoids the need for other sources of high-purity gas. An emission cell with a homogeneous and constant air flow over the surface to be investigated was developed in order to ensure reproducible results. Investigations of emissions from wooden surfaces with and without additional contamination as well as from complex mixtures are presented. The results demonstrate that relevant emissions can be identified and quantified with high sensitivity and selectivity in under five minutes. Therefore, the method is useful for indoor air quality monitoring, especially when miniaturized instruments are applied. Figure     

Spray nebulization for sample introduction in ion mobility spectrometry

A new sample introduction system based on spray nebulization has been successfully developed to perform direct analysis of liquid samples by IMS. The system comprises a concentric nebulizer that generates a spray plume which is introduced in the ionization region of the IMS instrument through a temperature controlled transfer line. This system avoids previous problems of direct injection of liquid samples and maintains the countercurrent flow of inert gas necessary for the operation of the IMS instrument. Evaluation of the qualitative and quantitative capabilities of the methodology has been performed after a carefully study of the main variables affecting the spray nebulization and the transport of the analyte molecules through the transfer line. To demonstrate the usefulness of the new sample introduction system, direct analysis of drugs and drug metabolites in saliva or urine samples have been performed, obtaining accurate, reliable and sensitive results. Moreover, analytes with physico-chemical properties that limited the capability of thermal desorption as sample introduction method such as amino acids can be analyzed by using the spray nebulization methodology.     

Kinetic study of proton-bound dimer formation using ion mobility spectrometry

A method to measure the rate constant for the formation of symmetrical proton-bound dimers at ambient pressure was proposed. The sample is continuously delivered to the drift region of an ion mobility spectrometer where it reacts with a swarm of monomer ions injected by the shutter grid. Dimer ions are formed in the drift tube and a tail appears in the ion mobility spectrum. The rate constant is derived from the mobility spectra. The proposed approach was typically examined for methyl isobutyl ketone (MIBK), 2,4-dimethyl pyridine (DMP), and dimethyl methyl phosphonate (DMMP). The rate constants measured in this study were: 0.25 × 10⁻⁹, 0.86 × 10⁻¹⁰, and 0.47 × 10⁻¹⁰ cm³ s⁻¹ for MIBK, DMP and DMMP, respectively. The logarithm of the measured rate constants were found to be almost independent of reciprocal temperature within 303 to 343 K, indicating that no activation energy is involved in the formation of proton-bound dimers.     

Analysis of explosives using electrospray ionization/ion mobility spectrometry (ESI/IMS)

The analysis of explosives with ion mobility spectrometry (IMS) directly from aqueous solutions was shown for the first time using an electrospray ionization technique. The IMS was operated in the negative mode at 250°C and coupled with a quadrupole mass spectrometer to identify the observed IMS peaks. The IMS response characteristics of trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), 2-amino-4,6-dinitrotoluene (2-ADNT), 4-nitrotoluene (4-NT), trinitrobenzene (TNB), cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), cyclo-tetramethylene-tetranitramine (HMX), dinitro-ethyleneglycol (EGDN) and nitroglycerine (NG) were investigated. Several breakdown products, predominantly NO₂⁻ and NO₃⁻, were observed in the low-mass region. Nevertheless, all compounds with the exception of NG produced at least one ion related to the intact molecule and could therefore be selectively detected. For RDX and HMX the [M+Cl⁻]⁻ cluster ion was the main peak and the signal intensities could be greatly enhanced by the addition of small amounts of sodium chloride to the sprayed solutions. The reduced mobility constants (K₀) were in good agreement with literature data obtained from experiments where the explosives were introduced into the IMS from the vapor phase. The detection limits were in the range of 15–190 μg l⁻¹ and all calibration curves showed good linearity. A mixture of TNT, RDX and HMX was used to demonstrate the high separation potential of the IMS system. Baseline separation of the three compounds was attained within a total analysis time of 6.4 s.     

Solid phase micro-extraction coupled with ion mobility spectrometry for the analysis of ephedrine in urine

Quantitative solid phase micro-extraction (SPME) coupled with ion mobility spectrometry is demonstrated using the analysis of ephedrine in urine. Since its inception in the 1970's ion mobility spectrometry (IMS) has evolved into a useful technique for laboratories to detect explosives, chemical warfare agents, environment pollutants and, increasingly, for detecting drugs of abuse. Ephedrine is extracted directly from urine samples using SPME and the analyte on the fiber is heated by the IMS desorber unit and vaporized into the drift tube. The analytical procedure was optimized for fiber coating selection, extraction temperature, extraction time, sample pH, and analyte desorption temperature. The carryover effects, ion fragmentation characteristics, peak shapes, and drift times of ephedrine were also evaluated based on the direct interfacing of SPME to IMS. A limit of detection of 50 ng/mL of ephedrine in urine and a linear range of 3 orders of magnitude were obtained, showing that SPME-IMS compares well to other techniques for ephedrine and drug analysis presented in the literature.     

Improved design for high resolution electrospray ionization ion mobility spectrometry

An improved design for high resolution electrospray ionization ion mobility spectrometry (ESI-IMS) was developed by making some salient modifications to the IMS cell and its performance was investigated. To enhance desolvation of electrospray droplets at high sample flow rates in this new design, volume of the desolvation region was decreased by reducing its diameter and the entrance position of the desolvation gas was shifted to the end of the desolvation region (near the ion gate). In addition, the ESI source (both needle and counter electrode) was positioned outside of the heating oven of the IMS. This modification made it possible to use the instrument at higher temperatures, and preventing needle clogging in the electrospray process. The ion mobility spectra of different chemical compounds were obtained. The resolving power and resolution of the instrument were increased by about 15-30% relative to previous design. In this work, the baseline separation of the two adjacent ion peaks of morphine and those of codeine was achieved for the first time with resolutions of 1.5 and 1.3, respectively. These four ion peaks were well separated from each other using carbon dioxide (CO₂) rather than nitrogen as the drift gas. Finally, the analytical parameters obtained for ethion, metalaxyl, and tributylamine indicated the high performance of the instrument for quantitative analysis.