Direct detection of trimethylamine in meat food products using ion mobility spectrometry

Biogenic amines are degradation products generated by bacteria in meat products. These amines can indicate bacterial contamination or have a carcinogenic effect to humans consuming spoiled meats; therefore, their rapid detection is essential. Trimethylamine (TMA) is a good target for the detection of biogenic amines because its volatility. TMA was directly detected in meat food products using ion mobility spectrometry (IMS). TMA concentrations were measured in chicken meat juice for a quantitative evaluation of the meat decaying process. The lowest detected TMA concentration in chicken juice was 0.6 ± 0.2 ng and the lowest detected signal for TMA in a standard aqueous solution was 0.6 ng. IMS data were processed using partial least squares (PLS) and Fuzzy rule-building expert system (FuRES). Using these two chemometric methods, trimethylamine concentrations of different days of meat spoilage can be separated, indicating the decaying of meat products. Comparing the two methods, FuRES provided a better classification of different days of meat spoilage.     

Non-labeling multiplex surface enhanced Raman scattering (SERS) detection of volatile organic compounds (VOCs)

In this paper, we report multiplex SERS based VOCs detection with a leaning nano-pillar substrate. The VOCs analyte molecules adsorbed at the tips of the nano-pillars produced SERS signal due to the field enhancement occurring at the localized surface plasmon hot spots between adjacent leaning nano-pillars. In this experiment, detections of acetone and ethanol vapor at different concentrations were demonstrated. The detection limits were found to be 0.0017 ng and 0.0037 ng for ethanol and acetone vapor molecules respectively. Our approach is a non-labeling method such that it does not require the incorporation of any chemical sensing layer for the enrichment of gas molecules on sensor surface. The leaning nano-pillar substrate also showed highly reproducible SERS signal in cyclic VOCs detection, which can reduce the detection cost in practical applications. Further, multiplex SERS detection on different combination of acetone and ethanol vapor was also successfully demonstrated. The vibrational fingerprints of molecular structures provide specific Raman peaks for different VOCs contents. To the best of our knowledge, this is the first multiplex VOCs detection using SERS. We believe that this work may lead to a portable device for multiplex, specific and highly sensitive detection of complex VOCs samples that can find potential applications in exhaled breath analysis, hazardous gas analysis, homeland security and environmental monitoring.     

Closed tube sample introduction for gas chromatography-ion mobility spectrometry analysis of water contaminated with a chemical warfare agent surrogate compound

Ion mobility spectrometry (IMS) is a proven technology for detection of vapor phase chemical warfare agents. The technology is suitable for field portable instrumentation due to its small size, high sensitivity, speed of analysis, and low power consumption. However, it suffers from a limited dynamic range and potential difficulties in identifying compounds in complex matrices. The use of gas chromatography (GC) coupled to IMS can overcome the difficulty of chemical identification in mixtures by separating the sample into individual components before detection. Using this approach, IMS technology has previously been adapted to detect biological aerosols using an open tube pyrolyzer and a short GC column (Py-GC-IMS). The open sample introduction tube of a Py-GC-IMS instrument would be a convenient configuration to accept aerosol particulates, and while viewed as needed for aerosol trapping, is not optimal for liquid chemical analyses. To examine the usefulness of an existing Py-GC-IMS system for analysis of chemicals in water, an existing open-port sample interface was replaced with a septum-equipped closed tube injector to contain analyte vapors resulting from liquid injection. Tributylphosphate (TBP) was used as a surrogate chemical warfare agent, and aqueous injections into both closed and open tube assemblies were performed. Sample introduction into the closed tube inlet was also accomplished using solid phase microextraction (SPME) preconcentration. The limit of detection for TBP using an open tube, closed tube, and closed tube configuration with SPME sample introduction was 0.980, 0.196, and 0.0098 mg/L, respectively.     

Novel fluorescent silica nanoparticle probe for ultrasensitive immunoassays

The preparation and utilization of a novel particulate label based on fluorescent hybrid silica (FHS) nanoparticles are reported in this article. These nanoparticles have shown several unique advantages over existing dye molecules, quantum dots, and latex-based fluorescent particles in easy preparation, good photostability and high sensitivity. A high molar ratio of the fluorescent molecules present in the core to biomolecules on the particle surface was achieved by using the well-developed silica surface immobilization chemistry for biomolecular linking. A fluoroimmunoassay method for detecting trace level Hepatitis B Surface Antigen (HBsAg) was developed. The calibration graph for HBsAg was linear over the range 0.5-220 ng/ml with a detection limit of 0.1 ng/ml. The sensitivity is greatly increased when compared with the corresponding immunoassay performed with direct fluorophore labeling. The present work shows that these FHS nanoparticles are high-quality markers for biochemical assays.     

Fast detection of methyl tert-butyl ether from water using solid phase microextraction and ion mobility spectrometry

Methyl tert-butyl ether (MTBE) is commonly used as chemical additive to increase oxygen content and octane rating of reformulated gasoline. Despite its impact on enhancing cleaner combustion of gasoline, MTBE poses a threat to surface and ground water when gasoline is released into the environment. Methods for onsite analysis of MTBE in water samples are also needed. A less common technique for MTBE detection from water is ion mobility spectrometry (IMS). We describe a method for fast sampling and screening of MTBE from water by solid phase microextraction (SPME) and IMS. MTBE is adsorbed from the head space of a sample to the coating of SPME fiber. The interface containing a heated sample chamber, which couples SPME and IMS, was constructed and the SPME fiber was introduced into the sample chamber for thermal desorption and IMS detection of MTBE vapors. The demonstrated SPME-IMS method proved to be a straightforward method for the detection of trace quantities of MTBE from waters including surface and ground water. We determined the relative standard deviation of 8.3% and detection limit of 5 mg L⁻¹ for MTBE. Because of short sampling, desorption, and detection times, the described configuration of combined SPME and IMS is a feasible method for the detection of hazardous substances from environmental matrices.     

Identification of volatile chemical signatures from plastic explosives by SPME-GC/MS and detection by ion mobility spectrometry

This study demonstrates the use of solid-phase microextraction (SPME) to extract and pre-concentrate volatile signatures from static air above plastic explosive samples followed by detection using ion mobility spectrometry (IMS) optimized to detect the volatile, non-energetic components rather than the energetic materials. Currently, sample collection for detection by commercial IMS analyzers is conducted through swiping of suspected surfaces for explosive particles and vapor sampling. The first method is not suitable for sampling inside large volume areas, and the latter method is not effective because the low vapor pressure of some explosives such as RDX and PETN make them not readily available in the air for headspace sampling under ambient conditions. For the first time, headspace sampling and detection of Detasheet, Semtex H, and C-4 is reported using SPME-IMS operating under one universal setting with limits of detection ranging from 1.5 to 2.5 ng for the target volatile signatures. The target signature compounds n-butyl acetate and the taggant DMNB are associated with untagged and tagged Detasheet explosives, respectively. Cyclohexanone and DMNB are associated with tagged C-4 explosives. DMNB is associated with tagged Semtex H explosives. Within 10 to 60 s of sampling, the headspace inside a glass vial containing 1 g of explosive, more than 20 ng of the target signatures can be extracted by the SPME fiber followed by IMS detection.     

Optimization of aptamer microarray technology for multiple protein targets

Aptamer-based microarrays for the quantitation of multiple protein analytes have been developed. A multiplex aptamer microarray was generated by printing two RNA aptamers (anti-lysozyme and anti-ricin) and two DNA aptamers (anti-IgE and anti-thrombin) on to either streptavidin (SA) or neutravidin (NA)-coated glass slides. However, substantial optimization was required in order to ensure the simultaneous function of the aptamer:analyte pairs. The effects of protein labeling, assay buffer, surface coating, and immobilization chemistry and orientation were investigated. A single buffer (PBS buffer containing 5 mM MgCl₂ and 0.1% Tween 20) was found to work well with all the aptamers, even though this was not the buffer originally used in their selection, while neutravidin-coated slides yielded a lower detection limit, wider detection range, and more uniform background than streptavidin-coated slides. Incubation with Cy3-labeled proteins yielded sensitive, target-specific, and dose-dependent responses to each protein. Target protein concentrations as low as 72 pg/mL (5 pM, lysozyme), 15 ng/mL (0.5 nM, ricin), 1.9 ng/mL (0.01 nM, IgE), and 170 ng/mL (5 nM, thrombin) could be detected. These results show that aptamer arrays can potentially be used with numerous proteins in parallel, furthering the notion that aptamer arrays may be useful in proteomics.     

Hydrophilic interaction liquid chromatography – charged aerosol detection as a straightforward solution for simultaneous analysis of ascorbic acid and dehydroascorbic acid

Ascorbic acid (AA) and dehydroascorbic acid (DHA) are small polar molecules difficult to be retained in conventional chromatographic RP systems. Hydrophilic interaction liquid chromatography (HILIC) using Obelisk R (100 × 3.2 mm, 5 μm, Sielc) analytical column and isocratic elution by ammonium acetate buffer pH 4.2 was found to be successful at this task, while other tested HILIC columns – Obelisk N (100 × 3.2 mm, 5 μm, Sielc) and Luna HILIC (100 × 3.0 mm, 3 μm, Phenomenex) were unsuccessful for the purposes of analysis. Charged aerosol detection (CAD) has recently become a new alternative universal detection system in HPLC, and was extremely convenient for the simultaneous analysis of AA and DHA without the need of subtraction approach and oxidation/reduction step. CAD response was found linear in defined range in spite of the fact that CAD is designated as non-linear detection method. A simple and fast HILIC-CAD method was applied for the analysis of pharmaceutical preparations containing AA. Method validation was performed including parameters of precision, accuracy, linearity, limit of detection and limit of quantitation (LOQ). The method was fast, accurate and precise for both detectors with LOQ_AA 5 μg/ml for UV detection and 10 μg/ml for CAD, respectively. DHA was detected only by CAD within tested concentration range with LOQ_DHA 1 μg/ml.     

Direct on-strip analysis of size- and time-resolved aerosol impactor samples using laser induced fluorescence spectra excited at 263 and 351 nm

We report a novel atmospheric aerosol characterization technique, in which dual wavelength UV laser induced fluorescence (LIF) spectrometry marries an eight-stage rotating drum impactor (RDI), namely UV-LIF-RDI, to achieve size- and time-resolved analysis of aerosol particles on-strip. The UV-LIF-RDI technique measured LIF spectra via direct laser beam illumination onto the particles that were impacted on a RDI strip with a spatial resolution of 1.2 mm, equivalent to an averaged time resolution in the aerosol sampling of 3.6 h. Excited by a 263 nm or 351 nm laser, more than 2000 LIF spectra within a 3-week aerosol collection time period were obtained from the eight individual RDI strips that collected particles in eight different sizes ranging from 0.09 to 10 μm in Djibouti. Based on the known fluorescence database from atmospheric aerosols in the US, the LIF spectra obtained from the Djibouti aerosol samples were found to be dominated by fluorescence clusters 2, 5, and 8 (peaked at 330, 370, and 475 nm) when excited at 263 nm and by fluorescence clusters 1, 2, 5, and 6 (peaked at 390 and 460 nm) when excited at 351 nm. Size- and time-dependent variations of the fluorescence spectra revealed some size and time evolution behavior of organic and biological aerosols from the atmosphere in Djibouti. Moreover, this analytical technique could locate the possible sources and chemical compositions contributing to these fluorescence clusters. Advantages, limitations, and future developments of this new aerosol analysis technique are also discussed.     

Assessment of microcystin purity using charged aerosol detection

The increasing occurrence of toxic cyanobacterial blooms has led to a requirement for robust monitoring strategies and whilst several validated procedures have been developed these can be limited by the lack of high quality calibration standards. High quality standards must have confirmation of identity, purity and concentration by multiple methods. One aspect, purity, is rarely addressed but is essential. This is the first evaluation of the charged aerosol detector (CAD) to determine the benefits of incorporating a universal detector for more accurate purity determination of these peptides. Microcystins were detected at 5–10 ng on the column using the CAD, providing comparable quantification limits to those obtained using traditional UV detection. Purity determination of test compounds that had been partially purified, had showed that highest purity was at 238 nm > UV TIC > ESI TIC > CAD indicating that increased impurities could be detected using the CAD thus providing a more accurate indication of compound quality. Compounds purified by preparative HPLC were shown to have relative purities between 97% and 99%, however, when evaluated by CAD this dropped to 90–94% supporting the multi-detector strategy as essential for production of high quality compounds.     

Derivatization procedures and determination of levoglucosan and related monosaccharide anhydrides in atmospheric aerosols by gas chromatography-mass spectrometry

This study evaluated the derivatization procedures for detecting the three most commonly monosaccharide anhydrides (MAs) (levoglucosan, mannosan and galactosan) in atmospheric aerosols using gas chromatography-mass spectrometry (GC-MS). Various silylating agents, mainly trimethylsilylating agents (TMS), were compared and the effects of various contents of trimethylchlorosilane (TMCS, as a stimulator) were evaluated to optimize the conditions for detecting these compounds in aerosol samples. Differences among the abundances of the derivatives were caused by the sterical hindrance of three hydroxyl groups in the structures of monosaccharide anhydrides. The effects of the reaction time and temperature were also examined. The optimal reaction time and temperature were 60 min and 80 °C with 1% TMCS plus 0.2% 1,4-dithioerythritol (DTE). Under these conditions, the percentages of formation of bis-O-TMS derivatives (as by-products) were 23, 29 and 10% for galactosan, mannosan and levoglucosan, respectively. The concentrations of galactosan, mannosan and levoglucosan in particles of smoke samples ranged from 29 to 88, 23 to 69 and 77 to 380 ng/m³, respectively; and in particles of atmospheric aerosols ranged from 0.06 to 0.75, n.d. to 0.49 and 1.6 to 132 ng/m³, respectively. Levoglucosan was the dominant MAs detected in both type of samples. Less than 10% quantitation difference was obtained when bis-O-TMS derivatives were included in the calculation.     

Total-Reflection X-ray fluorescence analysis of elements in size-fractionated particulate matter sampled on polycarbonate filters — Composition and sources of aerosol particles in Göteborg,Sweden

This is the first study applying the technique of cold plasma ashing on polycarbonate filters as a preparative step for subsequent elemental analysis of aerosol particles by Total-Reflection X-ray fluorescence. The procedure has been validated by analyzing blanks of the filter material, chemicals used as additives as well as certified standard reference material. The results showed that cold plasma ashing is superior to conventional digestion methods with regard to the ease of sample preparation and contamination. A PIXE cascade impactor was used to collect size-fractionated aerosol particles in 9 size classes ranging from 16 to 0.06 µm aerodynamic diameter at an urban and a suburban site in Göteborg, Sweden. Filter segments loaded with the aerosol particles were cut out and fixed on Quartz carriers. After adding 10 ng of Ga as internal standard the samples were dried, digested by cold plasma ashing and analyzed by Total-Reflection X-ray fluorescence. The analysis of aerosol particles showed that elemental concentrations at both the urban and the suburban site in Göteborg were low compared to central Europe. More and concurrent sampling of size-fractionated particles is required to identify local sources of trace elements in the urban area of Göteborg.     

Implementation of acoustic, radiofrequency and microwave rotating fields in analytical plasma sources

Four helium plasma sources operating at atmospheric pressure have been developed for analytical emission spectrometry by applying a synchronically rotating field with three or more phases operating at 1 kHz, 27 MHz or 2.45 GHz. The plasma takes the form of a disk and has minimum field strength at the axis. Thus, a channel is formed at the center through which the sample in the form of wet aerosol or a chemically generated vapor of halogen may be introduced. A dual-flow concentric ceramic injector was used to supply helium plasma gas and the sample to the plasma. The helium plasma operated at low power levels (40-300 W) and low gas flow rates of below 3 L min^− 1 and was self-igniting. The acoustic, radio-frequency (rf) and microwave-driven plasmas can withstand wet aerosol loadings of 5, 30 and 100 mg min^− 1, respectively, generated by an ultrasonic nebulizer without a desolvation unit. The plasma physical characteristics were compared at these three frequencies under otherwise similar operating conditions. The helium excitation temperature, OH rotational temperature and electron number density increased with increasing frequency in ranges of 2800-4000 K, 1100-3200 K and 0.1-7 × 10¹⁴ cm^− 3, respectively. To demonstrate the effect of frequency on the plasma excitation efficiency the emission intensity from halogen ions was evaluated using chemical vapor generation with continuous sampling without desiccation. Using 3-phase microwave, 6-phase microwave, 4-phase rf and 1 kHz helium plasma sources the detection limits (3σ) for chlorine at 479.40 nm were 26, 60, 230 and 1200 ng mL^− 1, respectively. The microwave-driven plasma was the densest and had the highest excitation potential toward chlorine and bromine ions.     

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.     

The use of silver nanoparticles as an effective modifier for the determination of arsenic and antimony by electrothermal atomic absorption spectrometry

Silver nanoparticles (AgNPs) were proposed as a new chemical modifier for the elimination of interferences when determining arsenic and antimony in aqueous NaCl or Na₂SO₄ solutions and in sea-water by electrothermal atomic absorption spectrometry. For this purpose, the AgNPs were prepared simply by reducing silver nitrate with sodium citrate. The effects of pyrolysis and atomization temperatures and the amounts of interferents and modifiers on the sensitivities of these elements were investigated. In the presence of the proposed modifier, a pyrolysis temperature of at least 1100 °C for arsenic and 900 °C for antimony could be applied without the loss of analytes, and the interferences were greatly reduced to allow for interference-free determination. The detection limits (N = 10, 3σ) for arsenic and antimony were 0.022 ng and 0.046 ng, respectively. AgNPs are cheaper and more available compared to many other modifiers. No background was detected, and the blank values were negligible.     

Highly selective determination of methylmercury with methylmercury-imprinted polymers

In this paper a new prototype of solid phase microextraction-ion mobility spectrometry (SPME-IMS) system was developed to effectively couple the extraction efficiency of SPME with the detection capability of IMS. The main component of this system was the transfer line/desorber, which was a low-thermal-mass (ca. 0.15 g) silicosteel coatings stainless steel tube. The transfer line/desorber was designed to rapidly desorb and transfer the analytes extracted by SPME to IMS. A custom-made temperature controller with a proportional-integral-differential (PID) was used to maintain the temperature of the transfer line/desorber stable and avoid overheating or oscillating. The low thermal mass of this interface allowed it to be rapidly heated and cooled with much less electrical power and could substantially reduce the demand for high capacity batteries. The operational characteristics of this system were demonstrated through the analysis of camphor vapour. The precision of reduced mobility and the peak amplitude of camphor were good (R.S.D. 0.62%, n = 10; R.S.D. 2.5%, n = 10, respectively). SPME-IMS system was also shown to be capable of on-site measurement by monitoring the biogenic volatile organic compounds (BVOCs) emitted from eucalyptus citriodora leaves. In addition, this system was applied to quantitation of diazepam and cocaine in aqueous solution. Limits of detection were 10 ng/mL for diazepam and 50 ng/mL for cocaine with the reported experimental conditions. This SPME-IMS system exhibits considerable promise as a robust, simple, rapid, energy-saving fieldable approach for on-site analysis of analytes in various matrix.     

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.     

Analysis of the volatile chemical markers of explosives using novel solid phase microextraction coupled to ion mobility spectrometry

Ion mobility spectrometry (IMS) is routinely used in screening checkpoints for the detection of explosives and illicit drugs but it mainly relies on the capture of particles on a swab surface for the detection. Solid phase microextraction (SPME) has been coupled to IMS for the preconcentration of explosives and their volatile chemical markers and, although it has improved the LODs over a standalone IMS, it is limited to sampling in small vessels by the fiber geometry. Novel planar geometry SPME devices coated with PDMS and sol-gel PDMS that do not require an additional interface to IMS are now reported for the first time. The explosive, 2,4,6-trinitrotoluene (TNT), is sampled with the planar SPME reaching extraction equilibrium faster than with fiber SPME, concentrating detectable levels of TNT in a matter of minutes. The surface area, capacity, extraction efficiency, and LODs are also improved over fiber SPME allowing for sampling in larger volumes. The volatile chemical markers, 2,4-dinitrotoluene, cyclohexanone, and the taggant 4-nitrotoluene have also been successfully extracted by planar SPME and detected by IMS at mass loadings below 1 ng of extracted analyte on the planar device for TNT, for example.     

Simultaneous determination of tetracyclines and their degradation products in environmental waters by liquid chromatography–electrospray tandem mass spectrometry

A sensitive method was developed for the trace determination of six tetracyclines and ten of their degradation products in influent, effluent, and river waters using liquid chromatography–electrospray tandem mass spectrometry detection, combined with Oasis hydrophilic–lipophilic balance (HLB) cartridge extraction and Oasis mixed-mode strong anion exchange (MAX) cartridge cleanup. Tetracyclines and their products were separated by liquid chromatography in 9.5 min, and the instrument detection limits were generally between 0.03 and 0.1 μg/L except for minocycline (0.5 μg/L). The chromatograms were improved through the MAX cleanup and no apparent matrix effect was found. The recoveries of all the target compounds except for 4-epianhydrochlortetracycline and anhydrochlortetracycline (34–52%) were 75–120% for influent, 61–103% for effluent, and 64–113% for river waters. The method detection limits (MDLs) of the analytes varied in the range of 0.8–17.5 ng/L in all studied matrices. The method was applied for the determination of tetracyclines and their products in a sewage treatment plant (STP) and surface waters in Beijing, China. Oxytetracycline (3.8–72.5 ng/L), tetracycline (1.9–16.5 ng/L), and five products including 4-epitetracycline, 4-epioxytetracycline, isochlortetracycline, anhydrotetracycline, and 4-epianhydrochlortetracycline (5.7–25.3 ng/L) were detected in wastewater, while only oxytetracycline and tetracycline (2.2 and 2.1 ng/L) were detected in surface water samples.     

Rapid preseparation of interferences for ion mobility spectrometry

Two new approaches to reduce false positive interferences commonly observed with explosives and drugs detection in the field were reported for ion mobility spectrometry (IMS). One of the approaches involved the rapid preseparation of potential interferences prior to detection by IMS. Firstly, it was found that the introduction of a short column packed with adsorption packing material before an IMS could help to reduce the false positive rates. Secondly, the retention time at which the most intense response occurred over the analysis time period could be utilized to separate false positive responses from target analytes with the same drift times. Rapid preseparation of potential interferences provided a greater degree of confidence for the detection (in less than 30 s) of drugs, explosives and chemical warfare agents (CWAs). Detection limits as low as 10 pg of TNT with a sensitivity of 12 A g⁻¹ were reported. Successful development of this technique may lead to the construction of a simple interface fitted with a short column of adsorption packing material to enhance either initial separation or to hold-back interferences mixed with explosive and drug responses in the field.