Design of magnetic photonic crystal microdroplet for sensitive detection of cationic organic pollutants by the coupled resonance effect.

Photonic crystals (PCs), periodically arranged nanoparticles, have emerged with extraordinary optical properties for light manipulation owing to their photonic band gaps (PBGs). Here, a novel strategy and method was developed for efficient enrichment and sensitive detection of cationic organic pollutants in water. Size-controlled Fe3O4@poly (4-styrenesulfonic acid-co-maleic acid) (Fe3O4@PSSMA) was prepared, and high surface charge were formed with the coating of PSSMA layer on the surface of Fe3O4, which could be used for adsorption and removal of cationic organic pollutants. The Fe3O4@PSSMA after adsorbing cationic organic pollutant were assembled to magnetic photonic crystal microdroplet (MPCM) structure in an external magnetic field, which was used as surface-enhanced Raman scattering (SERS) substrate. By coupling the magnetically tuned PBGs with Raman laser wavelength, the light utilization efficiency can be improved and the coupled resonance effect was greatly enhanced. The enhancement factor (EF) of MB was more than 800 attributing to the dual function of enrichment and coupled resonance effect of MPCM. The developed analytical strategy is the first time to use MPCM as a SERS substrate to realize the sensitive detection of 10 nmol L-1 MB in real water, which greatly improves the application of MPCM in the field of contaminant analysis and detection in water.

Developing a novel strategy for fabricating matrix film to assess the distribution of potassium perfluorooctanic sulfonate by matrix-assisted laser desorption/ionization mass spectrometry imaging.

Matrix deposition plays a critical role in image quality of matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). To improve the ionization efficiency and overcome the limitation of traditional matrix deposition methods in the face of difficult-to-sublimate or difficult-to-dissolve matrix, covalent organic frameworks (COFs) named COF-DhaTab was successfully synthesized and firstly used as matrix film. It was fabricated by imprinting of sieved COF-DhaTab powder on the surface of a double-sided adhesive tape. Outstanding reproducibility and uniformity of COF-DhaTab film were demonstrated by relative standard deviation (RSD) within 8.37% and 7.71% from dot-to-dot and plate-to-plate, respectively. With the introduction of double-sided adhesive tape, water contact angle (WCA) of COF-DhaTab film increased from 55° to 141°, resulting in significant suppression of analyte diffusion. Moreover, the intensity of potassium perfluorooctanic sulfonate (PFOS, C8F17SO3-, m/z 498.93) was 9.3 × 105, more than six hundred times higher than that using DHB matrix. This enhancement was attributed to the rough surface and multiple branches of the synthesized COF-DhaTab. To verify the ability of COF-DhaTab film as substrate, the spatial distribution of PFOS in zebrafish, rat liver and kidney tissues was explored. Superior imaging capability was displayed with high-spatial resolution and reliable location distribution. These results not only demonstrate the outstanding ability of COF-DhaTab as matrix for MALDI-MS and MALDI-MSI, but also provide a facile approach for fabrication of novel matrix films for MALDI-MSI.

Self-aspiration sampling design for rapid analyses of volatile organic compounds based on atmospheric pressure chemical ionization/photoionization combined ionization source mass spectrometry.

Development of combined mass spectrometry ionization sources has enabled expansion of the application and scope of mass spectrometry. A novel hybrid ionization system combining vacuum ultraviolet (VUV) and atmospheric pressure chemical ionization (APCI) was constructed. Gaseous samples were self-aspirated into an ionization zone through a capillary by negative pressure, generated by high-speed airflow based on the Venturi effect. Compared with APCI mode alone, the signal-to-noise ratio (S/N) in APCI/VUV mode was increased by about 276-times. To increase the ionization efficiency further, correlated experimental conditions were optimized. Four types of volatile organic compounds (VOCs) were tested to evaluate the performance of the APCI/VUV ion source. Excellent linearity and limit of detection were achieved for compounds in mixed solutions. Quantitative analyses of four VOCs (toluene, cyclohexanone, styrene and ethylbenzene) using APCI/VUV-MS were done, and the relative standard deviations (RSDs) were 1.57%, 6.30%, 4.49% and 8.21%, respectively, indicating that the APCI/VUV ionization source had excellent reproducibility. Our results demonstrated that the developed method was promising for analyzing VOCs as well as being rapid, simple, and easy to operate.

Photocatalytic degradation of perfluorooctanoic acid (PFOA) from water: A mini review.

Perfluorooctanoic acid (PFOA) has drawn increasing attention as a highly persistent organic pollutant. The inherent stability, rigidity and potential toxicities characteristics make it a challenge to develop efficient technologies to eliminate it from water. Photocatalytic technology, as one advanced method, has been widely used in the degradation of PFOA in water. In this review, recent progress in the design of photocatalysts including doping, defects engineering, heterojunction and surface modification to boost the photocatalytic performance toward PFOA is summarized. The relevant degradation mechanisms were also discussed in detail. Finally, future prospect and challenges are proposed. This review may provide new guidelines for researchers to design much more efficient photocatalysts applied in the elimination of PFOA.

Rapid and sensitive determination of free fatty acids based on in-source microdroplet-driven derivatization coupled with high-resolution mass spectrometry.

Accurate and sensitive measurements of free fatty acids (FFAs) in biological samples are valuable for diagnosing and prognosing diseases. In this study, an in-source microdroplet derivation strategy combined with high-resolution mass spectrometry was developed to analyze FFAs in lipid extracts of biological samples directly. FFAs were rapidly derivated with 2-picolylamine (PA) in the microdroplet which is derived by electrospray. With the proposed method, twelve typical FFAs were determined reliably with high sensitivity and acceptable linearities (R2 ≥ 0.94). The LODs and LOQs for the twelve FFAs were 9-76 pg mL-1 and 30-253 pg mL-1, respectively. The developed method was applied to analyze the alteration of FFAs in liver and kidney samples of rats induced by perfluorooctane sulfonate (PFOS) exposure. The good results demonstrate that the established analysis technique is dependable and has promising applications in detecting FFAs associated with complex biological samples.

Contamination profiles and potential health risks of environmentally persistent free radicals in PM2.5 over typical central Chinese megacity.

As one of the most important transportation hubs and industrial bases in China, Zhengzhou has suffered from serious PM2.5 pollution for a long time. However, the investigation of contamination status and possible exposure risks of environmentally persistent free radicals (EPFRs) in PM2.5 from Zhengzhou is rare. In this work, a comprehensive study of pollution levels, seasonal variations, sources, and potential health risks of PM2.5-bound EPFRs in Zhengzhou was conducted for the first time. The atmospheric concentrations of EPFRs in PM2.5 from Zhengzhou ranged from 1.732 × 1012 spin m-3 to 7.182 × 1014 spin m-3 between 2019 and 2020. Relatively serious contamination was noticed in winter and spring. Primary fossil fuel combustion and Fe-mediated secondary formation were apportioned as possible sources of PM2.5-bound EPFRs in Zhengzhou. Moreover, to avert the bias of the toxicity assessment induced by utilization of incompletely extracted EPFRs from sample filter, simulatively generated EPFRs were applied to toxicological evaluations (cell viability and reactive oxygen species assays). Corresponding experimental dosages were based on the estimated adults' annual exposure amounts of EPFRs in real PM2.5 samples. The results elucidated that EPFRs might cause growth inhibition and oxidative stress of human lung cells, suggesting the possible exposure-induced health concerns for local people in Zhengzhou. This study provides practical information of real contamination status of PM2.5-bound EPFRs in Zhengzhou, which is favorable to local air pollution control and reduction of exposure risks on public health in central China.

Sulfonic acid functionalized monolithic column for high selectivity capillary electrochromatography separation.

A new nano-scale spherical vinyl-functionalized covalent organic polymer (TAPT-DVA-COP) with uniform sizes around 300 nm was initially constructed using 2,5-divinyl-1,4-benzaldehyde (DVA) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) as monomers. Then, a sulfonic acid (-SO3H) modified COP termed COP-SO3H was developed based on post-sythesis method employing TAPT-DVA-COP as precursor. Capillary electrochromatography (CEC) monolithic columns were fabricated using the physical doping technique to exhibit the application potential of TAPT-DVA-COP and COP-SO3H. Compared to the TAPT-DVA-COP monolithic column, the COP-SO3H monolithic column achieved a highly selective separation between analytes with different properties, including monosubstituted benzenes, alkylbenzenes, hydroxybenzoates, nucleoside bases, and biogenic amines. Non-covalent interaction (NCI) analysis and experimental data show that the synergism of the sulfonic acid group and aromatic moieties on COP-SO3H endows the new stationary phase with diverse interactions, including ion exchange, hydrophobic, π-π and hydrogen bonding. In addition, the COP-SO3H monolithic column exhibited good reproducibility and excellent potential for the determination of hydroxybenzoates in compact powders and alkylbenzenes in effluent samples.

A review on recent advances in mass spectrometry analysis of harmful contaminants in food.

Food safety is a widespread global concern with the emergence of foodborne diseases. Thus, establishing accurate and sensitive detection methods of harmful contaminants in different food matrices is essential to address and prevent the associated health risks. Among various analytical tools, mass spectrometry (MS) can quantify multiple impurities simultaneously due to high resolution and accuracy and can achieve non-target profiling of unknown pollutants in food. Therefore, MS has been widely used for determination of hazardous contaminants [e.g., mycotoxin, pesticide and veterinary drug residues, polychlorinated biphenyls (PCBs), dioxins, acrylamide, perfluorinated compounds (PFCs) and p-Phenylenediamine compounds (PPDs) in food samples]. This work summarizes MS applications in detecting harmful contaminants in food matrices, discusses advantages of MS for food safety study, and provides a perspective on future directions of MS development in food research. With the persistent occurrence of novel contaminants, MS will play a more and more critical role in food analysis.

Recent advances in analysis of capsaicin and its effects on metabolic pathways by mass spectrometry.

Capsaicin is the main food active component in Capsicum that has gained considerable attention due to its broad biological activities, including antioxidation, anti-inflammation, anti-tumor, weight regulation, cardiac protection, anti-calculi, and diurnal-circadian regulation. The potent biological effects of capsaicin are intimately related to metabolic pathways such as lipid metabolism, energy metabolism, and antioxidant stress. Mass spectrometry (MS) has emerged as an effective tool for deciphering the mechanisms underlying capsaicin metabolism and its biological impacts. However, it remains challenging to accurately identify and quantify capsaicin and its self-metabolites in complex food and biological samples, and to integrate multi-omics data generated from MS. In this work, we summarized recent advances in the detection of capsaicin and its self-metabolites using MS and discussed the relevant MS-based studies of metabolic pathways. Furthermore, we discussed current issues and future directions in this field. In-depth studies of capsaicin metabolism and its physiological functions based on MS is anticipated to yield new insights and methods for preventing and treating a wide range of diseases.

Untargeted Metabolomics Using UHPLC-HRMS Reveals Metabolic Changes of Fresh-Cut Potato during Browning Process.

Surface browning plays a major role in the quality loss of fresh-cut potatoes. Untargeted metabolomics were used to understand the metabolic changes of fresh-cut potato during the browning process. Their metabolites were profiled by ultra-high performance liquid chromatography coupled with high resolution mass spectrometry (UHPLC-HRMS). Data processing and metabolite annotation were completed by Compound Discoverer 3.3 software. Statistical analysis was applied to screen the key metabolites correlating with browning process. Fifteen key metabolites responsible for the browning process were putatively identified. Moreover, after analysis of the metabolic causes of glutamic acid, linolenic acid, glutathione, adenine, 12-OPDA and AMP, we found that the browning process of fresh-cut potatoes was related to the structural dissociation of the membrane, oxidation and reduction reaction and energy shortage. This work provides a reference for further investigation into the mechanism of browning in fresh-cut products.

Silica microparticles modified with ionic liquid bonded chitosan as hydrophilic moieties for preparation of high-performance liquid chromatographic stationary phases.

Two novel stationary phases, 1-(4-bromobutyl)-3-methylimidazolium bromide bonded chitosan modified silica and 1-(4-bromobutyl)-3-methylimidazolium bromide bonded chitosan derivatized calix[4]arene modified silica stationary phase, were synthesized using 1-(4-bromobutyl)-3-methylimidazolium bromide bonding chitosan as a polarity regulator solving the limitation of the strong hydrophobicity of calixarene in the application of hydrophilic field. The resulting materials were characterized by solid-state nuclear magnetic resonance, Fourier-transform infrared spectra, scanning electron microscopy, elemental analysis, and thermogravimetric analysis. Based on the hydrophilicity endowed by 1-(4-bromobutyl)-3-methylimidazolium bromide bonded chitosan, the retention mode of ILC-Sil and ILCC4-Sil could be effectively switched from the hydrophilic mode to a hydrophilic/hydrophobic mixed mode and could simultaneously provide various interactions with solutes, including hydrophilic, π-π, ion-exchange, inclusion, hydrophobic, and electrostatic interactions. On the basis of these interactions, successful separation and higher shape selectivity were achieved among compounds that vary in polarity under both reverse-phase and hydrophilic interactive liquid chromatography conditions. Moreover, the ILCC4-Sil was successfully applied to the determination of morphine in actual samples using solid-phase extraction and mass spectrometry. The LOD and LOQ were 15 pg/mL and 54 pg/mL, respectively. This work presents an exceptionally flexible adjustment strategy for the retention and selectivity of a silica stationary phase by tuning the modification group.

An ultrastable 2D covalent organic framework coating for headspace solid-phase microextraction of organochlorine pesticides in environmental water.

Herein, a quinoline-linked ultrastable 2D covalent organic framework (COF-CN) coated fiber was successfully prepared and used for highly-sensitive headspace solid-phase microextraction (HS-SPME) of organochlorine pesticides (OCPs) in environmental water. The extraction efficiency of the COF-CN coating for all 14 OCPs was higher than that of four commercial SPME fiber coatings and most of the published works, with enrichment factors ranging from 540 to 5065. In combination with gas chromatography-tandem mass spectrometry (GC-MS/MS), a wide linear range (0.05-200 ng/L), low detection limits (LODs, 0.0010-13.54 ng/L) and satisfactory reproducibility and repeatability were obtained under optimal conditions. Compared with the published works, the LODs of the developed technique were improved 2-5.9 times, and the enrichment factors (EFs) of the developed method were enhanced at least 2 times. The COF-CN coated fiber can be easily recycled and reused at least 70 times without any washing step. The adsorption mechanism was first characterized by density functional theory calculations and X-ray photoelectron spectroscopy analysis. Besides, the established method was successfully applied to the analysis of the distribution of trace OCPs in real water samples from Henan Province. All these results proved the promising application of the developed HS-SPME-GC-MS/MS method for organic pollutants analysis in water samples.

Rapid identification and relative quantification of disaccharide isomers by three fragment ion pairs using ESI-MS/MS and its application in yellow rice wine.

Small structural differences bring great difficulties on carbohydrates identification, especially in terms of their quantification. Herein, a novel ESI-MS/MS based strategy was established to discriminate and relatively quantified protonated PMP-disaccharides with different composition and glycosidic bond. Interestingly, protonated PMP labeled-disaccharides provided abundant fragment ions arising from cross-ring cleavage and glycosidic bond cleavage, which could afford diagnostic fragment patterns for isomers differentiation in combination of statistical analysis. It was worth to note that the relative intensity ratios (RIR) of three ion pairs could completely discriminate 16 disaccharides, and subsequently used to relatively quantified isomers in a binary mixture. Ultimately, this method was applied for the discrimination of yellow rice wine, and then the relative content of maltose and isomaltose were confirmed as well. In general, this method was easy to operation and effective for rapid differentiation and quantification of isomeric disaccharides in complex matrices.

Characterization of Traditional Chinese Sesame Oil by Using Headspace Solid-Phase Microextraction/Gas Chromatography-Mass Spectrometry, Electronic Nose, Sensory Evaluation, and RapidOxy.

Xiao Mo Xiang You (XMXY) is a traditional Chinese sesame oil variety that is obtained through a hot water flotation process. This unique process gives the oil a unique aroma, health benefits, and excellent product stability. Although XMXY is always the most expensive among all the sesame oil varieties, it is usually used as a flavoring in many traditional Chinese daily food products and is increasingly popular. In order to reveal the characteristics of the oil, the volatile components, sensory evaluation, and oxidation stability of five XMXY samples were, respectively, analyzed by using headspace solid-phase microextraction/gas chromatography−mass spectrometry, an electronic nose, sensory evaluation, and RapidOxy. Comparisons and multidimensional statistical analysis were also carried out to distinguish XMXY from roasted sesame oil (RSO) and cold-pressed sesame oil (CSO) samples. In total, 69 volatiles were identified from XMXY, RSO, and CSO samples. Some compounds possessed high odor activity value (OAV > 1) in XMXY, including heterocyclic compounds, phenols, and sulfur-containing compounds. Additionally, they were also the main volatile components that distinguish XMXY from RSO and CSO. Roasted and nutty aromas were the dominant aroma attributes of XMXY. XMXY had better flavor intensity and oxidation stability than the other two sesame oil samples. These results are very valuable for the quality control and product identification of traditional Chinese sesame oil.

Developing a Noncontact Heating Matrix Spraying Apparatus with Controllable Matrix Film Formation for MALDI Mass Spectrometry Imaging.

Matrix deposition plays an important role in obtaining high-quality and reliable molecular spatial location information for matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). To control the matrix film formation, an automatic matrix spraying apparatus was developed with the introduction of a noncontact heating lamp. Compared with the unheated condition, the noncontact heating lamp suppressed the coffee-ring effect and the diffusion phenomenon of the analyte effectively by controllable matrix film formation. Meanwhile, the signal intensity was increased by 2-5 fold. To prove the ability of the matrix deposition apparatus, the apparatus combined with metabolomics analysis was used to show the spatial distribution of the substance in sprouted potato tubers. The potential biomarkers at m/z 868.5049 and m/z 852.5101 were identified as α-solanine and α-chaconine, and the synthesis pathways were further searched. To further demonstrate the quality of MALDI images including localization and spatial resolution, lipid distribution in rat brain tissue was investigated by the developed noncontact heating matrix spraying apparatus. An excellent match with distinguishable compartments of lipids in the rat brain was obtained between the H&E-stained sections and MALDI-MSI images. These results indicate that the developed noncontact heating matrix spraying apparatus is reliable and provides a low-cost, high-quality, rapid approach for MALDI-MSI.

Selective Detection of Nucleotides in Infant Formula Using an N-Rich Covalent Triazine Porous Polymer.

The aromatic structure and the rich nitrogen content of polymers based on covalent triazine-based frameworks (CTF) and their unique hydrophilic-lipophilic-balanced adsorption properties make them promising candidates for an adsorbent that can be used for sample pretreatment. Herein, a new covalent triazine-based framework (CTF-DBF) synthesized by a Friedel−Crafts reaction was used for the determination of the content of nucleotides in commercial infant formula. It was shown that the synthetic materials had an amorphous microporous structure, a BET surface area of up to 595.59 m2/g, and 0.39 nm and 0.54 nm micropores. The versatile adsorption properties of this material were evaluated by quantum chemistry theory calculations and batch adsorption experiments using five nucleotides as probes. The quantum chemistry results demonstrated that CTF-DBF can participate in multiple interactions with nucleotides. All the analyses performed present good linearity with R2 > 0.9993. The detection limits of targets ranged from 0.3 to 0.5 mg/kg, the spiked recoveries were between 85.8 and 105.3% and the relative standard deviations (RSD, n = 6) were between 1.1 and 4.5%. All these results suggest that this versatile CTF-DBF has great potential for sample pretreatment.

Carboxyl-Based CPMP Tag for Ultrasensitive Analysis of Disaccharides by Negative Tandem Mass Spectrometry.

Here, we develop a sensitive method for glucose-containing disaccharide analysis by 1-(4-carboxyphenyl)-3-methyl-5-pyrazolone (CPMP) derivatization using mass spectrometry. The intense anion of [M - H]- (m/z 759) was observed for CPMP-labeled disaccharides in a negative mode. After derivatization, its sensitivity was significantly increased with the limits of detection (LODs) and limits of quantification (LOQ) ranging from 3.90 to 8.67 ng L-1 and 12.99 to 28.92 ng L-1, respectively. During CID-MS/MS analysis, the fragment patterns of CPMP derivatized disaccharides in the negative mode were simpler and clearer than their counterparts in a positive mode, which further could be applied to distinct and relatively quantitative isomeric disaccharides with ultrahigh sensitivity and good reproducibility. The great linear relationships could be achieved under wider concentration ratios from 0.01 to 20 compared to the previous report. Eventually, the developed methodology was applicable to identify isomeric disaccharides in beers. No sucrose was discovered. All beers contain 1,4- and 1,6-linked disaccharides. Some of them also have a mixture of 1,2- and 1,3-linked disaccharides. Through the integration of statistical analysis, beers with different production processes were finally discriminated, and the relative quantification of isomaltose and maltose was realized. In general, this method is sensitive, fast, and reliable for the discrimination and relative quantification of isomeric disaccharides in complex matrices. This study provides a new idea for the structural analysis of oligosaccharides in food, plants, and animals and an important theoretical basis for the exploration of new functions of oligosaccharides.

Pd-Pd/PdO as active sites on intercalated graphene oxide modified by diaminobenzene: fabrication, catalysis properties, synergistic effects, and catalytic mechanism.

Pd-Pd/PdO nanoclusters well dispersed on intercalated graphene oxide (GO) (denoted as GO@PPD-Pd) were prepared and characterized. GO@PPD-Pd exhibited high catalytic activity (a TOF value of 60 705 h-1) during the Suzuki coupling reaction, and it could be reused at least 6 times. The real active centre was Pd(200)-Pd(200)/PdO(110, 102). A change in the Pd facets on the surface of PdO was a key factor leading to deactivation, and the aggregation and loss of active centres was also another important reason. The catalytic mechanism involved heterogeneous catalysis, showing that the catalytic processes occurred at the interface, including substrate adsorption, intermediate formation, and product desorption. The real active centres showed enhanced negative charge due to the transfer of electrons from the carrier and ligands, which could effectively promote the oxidative addition reaction, and Pd(200) and the heteroconjugated Pd/PdO interface generated in situ also participated in the coupling process, synergistically boosting activity. Developed GO@PPD-Pd was a viable heterogeneous catalyst that may have practical applications owing to its easy synthesis and stability, and this synergistic approach can be utilized to develop other transition-metal catalysts.

A novel methodology and strategy to detect low molecular aldehydes in beer based on charged microdroplet driving online derivatization and high resolution mass spectrometry.

The concentration of aldehydes is one of the important indicators in the food quality and safety. To efficiently analyze the four aldehydes (methanal, ethanal, propanal and n-butanal) in beer, charged microdroplet driving online derivatization apparatus coupled with high resolution mass spectrometry was firstly developed. Utilizing the high-speed reaction accelerated by microdroplets, the offline derivative of aldehydes with 2,4-dinitrophenylhydrazine in bulk was transferred into online derivatization. The developed method featured acceptable linearities (R2 ≥ 0.95), high sensitivities (LODs at ng mL-1 level) and qualified precisions (RSDs ≤ 8.4 %) for target compounds. Four aldehydes with trace amount were successfully determined in beer. The results indicated that the novel online analytical strategy did not require complex sample preparation and could conduct simple, rapid, sensitive detection of small molecule aldehydes with high throughput in beer or even other food samples.

Covalent Triazine Framework Sorbent for Solid Phase Extraction of Fipronil and its Metabolite in Eggs with Ultra-High Performance Liquid Chromatography-Tandem Mass Spectrometry.

A solid-phase extraction (SPE) method was established for fipronil and its metabolite residues (fipronil desulfinyl, fipronil sulphone and fipronil sulphide) in eggs with a covalent triazine framework (CTF) porous material as the adsorbent followed by ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) detection. Multiple probes and quantum chemistry theory calculations were conducted to describe the versatile adsorption property directly and quantifiably. The conjugated structure of CTF and N-containing triazine generated π-π interactions and hydrogen bonds between the CTF and the targets, which led to high extraction efficiency and recoveries. The solid-phase extraction parameters, including amount of the adsorbent, type of eluent, amount of eluent and loading rate were investigated. Under the optimal experimental conditions, the recoveries of the analytes were between 85.5% and 103.2%, and the RSD (n = 5) was between 1.8% and 3.6%. The LODs and LOQs were 0.13-0.2 ng g-1 and 0.5-0.8 ng g-1, respectively. The sorbent can effectively reduce the interference of the matrix and meet the detection requirements of fipronil and its metabolites in eggs. These results imply that the CTF as adsorbents have great potential in the analysis of trace targets in samples with complex matrices.