Rapid detection of aflatoxin B1, zearalenone and ochratoxin A in grains by thermal desorption dielectric barrier discharge ionization mass spectrometry.

Dielectric barrier discharge ionization is increasingly used for rapid detection in ambient mass spectrometry, although more often for gaseous and highly volatile samples than for solids and liquids. In this project, we present a rapid and sensitive method for detecting mycotoxins and demonstrate its capability for the detection of aflatoxin B1, zearalenone, and ochratoxin A in food samples. Our method is based on thermal desorption coupled to dielectric barrier discharge ionization mass spectrometry (TD-DBDI-MS), which we show generates minimal interferences and produces almost exclusively molecular ions. We detected mycotoxins in various food samples, including corn, peanuts, millet, and rice. Our method has a linear dynamic range of 1 µg kg-1 to 100 µg kg-1 for all three mycotoxins and a limit of detection (LOD) of 0.31 µg kg-1, 0.28 µg kg-1 and 0.43 µg kg-1, respectively. It is simple, rapid, reduces the pretreatment steps and has significant potential for practical applications.

Label-free detection of biomolecules using inductively coupled plasma mass spectrometry (ICP-MS).

Bioassays using inductively coupled plasma mass spectrometry (ICP-MS) have gained increasing attention because of the high sensitivity of ICP-MS and the various strategies of labeling biomolecules with detectable metal tags. The classic strategy to tag the target biomolecules is through direct antibody-antigen interaction and DNA hybridization, and requires the separation of the bound from the unbound tags. Label-free ICP-MS techniques for biomolecular assays do not require direct labeling: they generate detectable metal ions indirectly from specific biomolecular reactions, such as enzymatic cleavage. Here, we highlight the development of three main strategies of label-free ICP-MS assays for biomolecules: (1) enzymatic cleavage of metal-labeled substrates, (2) release of immobilized metal ions from the DNA backbone, and (3) nucleic acid amplification-assisted aggregation and release of metal tags to achieve amplified detection. We briefly describe the fundamental basis of these label-free ICP-MS assays and discuss the benefits and drawbacks of various designs. Future research is needed to reduce non-specific adsorption and minimize background and interference. Analytical innovations are also required to confront challenges faced by in vivo applications.

Design of a dual Ir-Eu tag for fluorescent visualization and ICP-MS quantification of SIRPα and its host cells.

With the expansion of ICP-MS application into the field of bioanalysis, there is an urgent need for novel element tags today. Here, we report the design of a dual-element Ir-Eu tag, opening the door to simultaneous fluorescent imaging and ICP-MS quantification. The ratio of 153Eu/193Ir may serve as a precision control of the labeling process, allowing internal validation of the quantitative results obtained. As for SIRPα and its host cell analysis exemplified here, the Ir-Eu tag demonstrated superior figures of ICP-MS quantification with the LOD (3σ) down to 0.5 (153Eu) and 1.1 (193Ir) pM SIRPα and 220 (153Eu) and 830 (193Ir) RAW264.7 cells more than 130 times more sensitive compared with the LOD (3σ) of 65.2 pM SIRPα at 612 nm using fluorometry. Not limited to these demonstrations, we believe that the design ideas of the dual Ir-Eu tags should be applicable to various cases of bioanalysis when dual optical profiling and ICP-MS quantification are indispensable.

Spatio-Temporal Analysis of Anesthetics in Mice by Solid-Phase Microextraction: Dielectric Barrier Discharge Ionization Mass Spectrometry.

Local anesthetics, drugs that only affect a restricted area of the body, are widely used in daily clinical practice. Less studied but equally important is the distribution of local anesthetics inside organisms. Here, we present a rapid in situ testing method of drug distribution in various organs. The temporal and spatial distribution of anesthetics in mice was measured by solid-phase microextraction (SPME), thermal desorption (TD), and dielectric barrier discharge ionization (DBDI) atmospheric pressure mass spectrometry. A coated SPME probe using a tungsten wire as the support covered with a carbonaceous material was prepared by a simple, low-cost flame method. An in-line structure of the inlet allows TD and DBDI to share the same capillary tube, which greatly improves the transmission efficiency. Nine kinds of anesthetics, such as lidocaine and dyclonine, were detected, and the limit of detection was determined to be as low as 13 pg/mL. In addition, the time-dependent distribution of drugs in mice organs was studied. We also found that macromolecules in organisms do not noticeably interfere with the detection. This method is convenient and efficient because it does not require tissue homogenates and allows direct in situ detection. Compared with the conventional analytical methods, this method is simple and rapid, works in situ, and allows microscale analysis of trace analytes in biological organisms with high sensitivity.

Genome-wide identification and expression analysis of the trihelix transcription factor family in sesame (Sesamum indicum L.) under abiotic stress.

BACKGROUND: The plant trihelix gene family is among the earliest discovered transcription factor families, and it is vital in modulating light, plant growth, and stress responses. METHODS: The identification and characterization of trihelix family members in the sesame genome were analyzed by bioinformatics methods, and the expression patterns of sesame trihelix genes were assessed by quantitative real-time PCR. RESULTS: There were 34 trihelix genes discovered in the genome of sesame, which were irregularly distributed among 10 linkage groups. Also, the genome contained 5 duplicate gene pairs. The 34 trihelix genes were divided into six sub-families through a phylogenetic study. A tissue-specific expression revealed that SiTH genes exhibited spatial expression patterns distinct from other trihelix genes in the same subfamily. The cis-element showed that the SiTHs gene promoter contained various elements associated with responses to hormones and multiple abiotic stresses. Additionally, the expression patterns of 8 SiTH genes in leaves under abiotic stresses demonstrated that all selected genes were significantly upregulated or downregulated at least once in the stress period. Furthermore, the SiTH4 gene was significantly induced in response to drought and salt stress, showing that SiTH genes may be engaged in the stress response mechanisms of sesame. CONCLUSION: These findings establish a foundation for further investigation of the trihelix gene-mediated response to abiotic stress in sesame.

Zero-Interfacing µHPLC to ICPMS.

The chromatography-mass spectrometry hyphenated technique is the most widely adopted tool for quantifying trace analytes in a complex biosample. One issue we frequently encountered, however, is that the separated analyte-containing chromatographic peaks broaden and even remix prior to mass spectrometric quantification due to the inevitable molecular diffusion within the dead-volume introduced by hyphenation. We developed a zero-interfacing approach for coupling microbore (µ) HPLC with inductively coupled plasma mass spectrometry (ICPMS). Zero-interfacing µHPLC to ICPMS has been achieved by a column-nebulizer assembly (COL-NEB) of a self-designed glass framework with a tapered nozzle, in which a capillary chromatographic column can be harbored while an Ar gas flow is blown through the nozzle mouth. The COL-NEB can be positioned just before the base of the Ar-ICP serving as the central sampling channel of a conventional Ar-ICP torch for online nebulization and transportation of the analytes separated on µHPLC into ICPMS, maintaining the molecular resolution obtained on µHPLC and the limit of detection (LOD) of ICPMS. For example, the full width at half-maximum of a SLUGT peptide chromatographic peak was reduced to 1.71 ± 0.07 s (n = 5) with a 0.72 fg LOD (3σ) of 80Se. Moreover, at least 32 Se-containing peptides were determined in the trypsin lysate of the water-soluble fraction (≥3000 MW) from Se-enriched yeast CRM SELM-1 within a 10 min run, the highest record to date. We believe such an approach paves the way to determining accurate information on a heteroatom and its binding biomolecules that play key roles during life processes.

Quantification of active selenols in cells: a selenol-specific recognition europium-switched signal-amplification ICP-MS approach.

Selenium (Se) is a mysterious thus tempting element playing a dual bio-chemical function, mainly through selenol, during life processes. Quantification of the selenols is thus of great significance for understanding the biological roles of Se, but remains a big challenge. Herein we report a selenol-specific recognition-mediated and europium (Eu) signal-switched amplification inductively coupled plasma mass spectrometry (ICP-MS) approach for quantifying the free active selenols (act-SeH) in cells. A bifunctional molecule, 2,4-dinitrobenzenesulfonyl-piperidin-4-yl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic europium (DNBS-DOTA-Eu), was designed and synthesized for the specific recognition and highly sensitive quantification of act-SeH via switching Se to more sensitive Eu ICP-MS signals. The limit of detection (LOD, 3σ) of 3.41 pg/mL (22.43 pmol/L), corresponding to the absolute mass LOD of 6.82 ag act-SeH per cell, is almost 25 times lower than 83.76 pg/mL (1.06 nmol/L), 167.52 ag, when monitoring 80Se. The results indicate that act-SeH in the selenite-precultured cancerous HepG2 and paracancerous HL7702 cells are 0.090 ± 0.002 pg/cell (n = 7) and 0.021 ± 0.006 pg/cell (n = 7), more than 4.28 times higher in HepG2 than in HL7702. Preliminary application of this approach to the cells from real hepatic tissue samples suggested that act-SeH has a positive relationship with the degree of hepatic disease. act-SeH in cells appears to be a very promising relevant index for understanding the biochemical functions of Se, besides the total Se in cells and blood serum and/or plasma.

A Biochemical Lanthanide-Encoding Approach Enables Quantitative Monitoring of the Bacterial Response to Vancomycin Treatment.

A pathogenic bacterium has its own mechanisms for not only pathogenic attack but also exogenous invasion defense, in which the bacterial cell wall is the front line of attack and defense. We developed a biochemical lanthanide-encoding approach to quantify the uncanonical d-amino acid (d-X) that was edited in a small proportion into the terminal acyl-d-Ala-d-X of nascent peptidoglycan UDP-MurNAc-pentapeptides in the bacterial cell wall. This approach overcomes the difficulties regarding quantification and accuracy issues encountered by the popular optical imaging and traditional high-performance liquid chromatography-based methods. Newly synthesized azide-d-Leu and ketone-d-Met were used together with alkynyl-d-Ala for their metabolic assembly and then bioorthogonally encoded by the correspondingly fabricated DBCO-DOTA-Gd, H2NO-DOTA-Eu, and azide-DOTA-Sm tags. This approach allows direct quantification of the d-X in situ in the cell wall using 158Gd, 153Eu, and 154Sm species-unspecific isotope dilution inductively coupled plasma mass spectrometry, avoiding any tedious and complex "cell-broken" pretreatment procedures that might induce racemization of the d-X. The obtained site-specific and accurate in situ information about the d-X enables quantitative monitoring of the bacterial response when Staphylococcus aureus meets vancomycin, showing that the amounts of azide-d-Leu and ketone-d-Met assembled are more important after determining the structure- and composition-dependent bacterial antibiotic resistance mechanisms. In addition, we found that the combined use of vancomycin and d-Ala restores the efficacy of vancomycin and might be a wise and simple way to combat vancomycin intermediate-resistant S. aureus.

Orderly MOF-Assembled Hybrid Monolithic Stationary Phases for Nano-Flow HPLC.

We report an approach that polymerizable handle-modified nanosized metal organic frameworks (MOFs) are used as independent monomers to be covalently organized by crosslinking molecules (CLMs) into an orderly MOF-assembled hybrid monolithic stationary phase, overcoming the respective problems of previously reported MOF-mixed or embedded stationary phases so far. It has a hierarchical micro-, meso-, and macropore structure throughout the monolithic matrix that is donated from MOF themselves, formed via CLM crosslinking in-between MOFs and expended by porogenic solvents, and a tunable surface chemistry derived inherently from MOFs, regulated by CLMs and initiated by the mobile phases as well. Such a pore structure and surface chemistry display multiplex interactions of sieving and electrostatic repulsion in addition to the polarity-based interactions that synergistically govern the partitioning way and degree of target molecules between the stationary and mobile phases, thus offering the ability to simultaneously separate small and large molecules during one chromatographic run on a nano-flow capillary high-performance liquid chromatography platform. A baseline mutual separation with the HETP and Rs of, for example, 9.2 µm butylbenzene and 4.56 (butylbenzene and pentylbenzene), 7.9 µm (phenylalanine) and 3.50 (tryptophan and phenylalanine), and 7.0 µm (myoglobin) and 1.91 (bovine serum albumin and myoglobin) was achieved when UiO-66/NH-methacrylate was exemplified as a model of MOFs and 1,6-hexanediol dimethacrylate and stearyl methacrylate together as CLMs. Not limited to the MOFs and CLMs demonstrated here, other available MOFs and CLMs or newly designed and synthesized ones are expected to be used for constructing one's own desired monolithic stationary phases toward her/his particular purposes.

Direct Infusion ICP-qMS of Lined-up Single-Cell Using an Oil-Free Passive Microfluidic System.

When coupled online with mass spectrometry (MS), widely applied water-in-oil droplet-based microfluidics for single cell analysis met problems. For example, the oil phase rumpled the stability, efficiency, and accuracy of MS, the conventional interface between MS and the microfluidic chip suffered the low sample introduction efficiency, and the transportation rates sometimes unmatched the readout dwell times for transient signal acquisition. Considering cells are already "droplets" with hydrophilic surface and elastic hydrophobic membrane, we developed an oil-free passive microfluidic system (OFPMS) that consists of alternating straight-curved-straight microchannels and a direct infusion (dI) micronebulizer for inductively coupled plasma quadrupole-based mass spectrometry (ICP-qMS) of lined-up single-cell. OFPMS guarantees exact single cell isolation one by one just using a thermo-decomposable NH4HCO3 buffer, eliminating the use of any oil and incompatible polymer carriers. It is more flexible and facile to adapt to the dwell time of ICP-qMS owing to the adjustable throughput of 400 to 25000 cells/min and the controllable interval time of at least 20 ms between the lined-up adjacent single cells. Quantitative single-cell transportation and high detection efficiency of more than 70% was realized using OFPMS-dI-ICP-qMS exemplified here. Thus, cell-to-cell heterogeneity can be simply uncovered via the determination of metals in the individual cells.

Silica Monolith Nested in Sponge (SiMNS): A Composite Monolith as a New Solid Phase Extraction Material for Environmental Analysis.

We report a new material of a composite silica monolith nested in sponge (SiMNS) and demonstrate an application in the trace analysis of environmental contaminants in water. SiMNS is prepared through sponge absorption of a hydrolyzed mixture of siloxanes and in situ gel formation within the pores. Images obtained using scanning electron microscopy show that the silica and sponge skeletons are mutually nested in SiMNS. This nested composite structure of SiMNS enhances the mechanical flexibility of the material, allowing for reproducible production of desirable sizes and shapes for solid phase extraction (SPE) cartridges without the need to use frits. Functionalization of SiMNS provides appropriate SPE options for selective and efficient extraction of specific contaminants. SPE cartridges packed with functionalized SiMNS-SO3Na have high extraction capacity, good stability in the pH range of 2 to 11, and efficient enrichment of dipeptides in water. Extraction of six dipeptides from water using these new SiMNS-SO3Na SPE cartridges followed by HPLC-MS/MS analysis results in improved method detection limits (MDLs) of 0.02-1.3 ng/L and method quantification limits (MQLs) of 0.05-4.3 ng/L. Successful identification and quantification of three dipeptides, Tyr-Gly, Phe-Gly, and Tyr-Ala, from raw water demonstrates a useful application of the new SPE materials for environmental analysis of trace contaminants. On the basis of this work, a range of functionalized SiMNS materials can be produced and tailored for various environmental and exposomic analyses.

Antibody-Bridged Beacon for Homogeneous Detection of Small Molecules.

In conventional competitive immunoassays for small molecules (SM), antibodies are either immobilized to solid phases or labeled with magnetic particles or probes. The former involves laborious blocking and washing steps, whereas the latter requires complicated labeling and purification steps. To circumvent these limitations, we describe here a new type of molecular beacon, termed antibody-bridged beacon (AbB), enabling homogeneous detection of SM without any immobilization or labeling of the antibody. The AbB is formed by the binding of an antibody to a pair of SM-labeled oligonucleotide probes that each comprise a stem sequence conjugated by either a fluorophore or a quencher. Competitive binding of the SM target to the antibody destructs the stem-loop structure of AbB, restoring the quenched fluorescence. A minimum binding energy of stem sequences is required for efficient formation of the desired stem-loop structure of AbB. A systematic study of the impact of stem sequences on the fluorescence background and quenching efficiency provided useful benchmarks, e.g., binding energy of -11 kcal/mol, for the construction of AbB. The optimized AbB showed fast signal responses, as demonstrated in the analyses of two small molecule targets, biotin and digoxin. Low nanomolar limits of detection were achieved. The novel AbB strategy, along with the guidelines established for the construction and application of AbB, offers a promising approach for homogeneous detection of small molecules, obviating immobilization or labeling of antibodies as required by other competitive immunoassays.

Methylated and thiolated arsenic species for environmental and health research - A review on synthesis and characterization.

Hundreds of millions of people around the world are exposed to elevated concentrations of inorganic and organic arsenic compounds, increasing the risk of a wide range of health effects. Studies of the environmental fate and human health effects of arsenic require authentic arsenic compounds. We summarize here the synthesis and characterization of more than a dozen methylated and thiolated arsenic compounds that are not commercially available. We discuss the methods of synthesis for the following 14 trivalent (III) and pentavalent (V) arsenic compounds: monomethylarsonous acid (MMAIII), dicysteinylmethyldithioarsenite (MMAIII(Cys)2), monomethylarsonic acid (MMAV), monomethylmonothioarsonic acid (MMMTAV) or monothio-MMAV, monomethyldithioarsonic acid (MMDTAV) or dithio-MMAV, monomethyltrithioarsonate (MMTTAV) or trithio-MMAV, dimethylarsinous acid (DMAIII), dimethylarsino-glutathione (DMAIII(SG)), dimethylarsinic acid (DMAV), dimethylmonothioarsinic acid (DMMTAV) or monothio-DMAV, dimethyldithioarsinic acid (DMDTAV) or dithio-DMAV, trimethylarsine oxide (TMAOV), arsenobetaine (AsB), and an arsenicin-A model compound. We have reviewed and compared the available methods, synthesized the arsenic compounds in our laboratories, and provided characterization information. On the basis of reaction yield, ease of synthesis and purification of product, safety considerations, and our experience, we recommend a method for the synthesis of each of these arsenic compounds.

p-Azidophenylarsenoxide: An Arsenical "Bait" for the In Situ Capture and Identification of Cellular Arsenic-Binding Proteins.

Identification of arsenic-binding proteins is important for understanding arsenic health effects and for developing arsenic-based therapeutics. We report here a strategy for the capture and identification of arsenic-binding proteins in living cells. We designed an azide-labeled arsenical, p-azidophenylarsenoxide (PAzPAO), to serve bio-orthogonal functions: the trivalent arsenical group binds to cellular proteins in situ, and the azide group facilitates click chemistry with dibenzylcyclooctyne. The selective and efficient capture of arsenic-binding proteins enables subsequent enrichment and identification by shotgun proteomics. Applications of the technique are demonstrated using the A549 human lung carcinoma cells and two in vitro model systems. The technique enables the capture and identification of 48 arsenic-binding proteins in A549 cells incubated with PAzPAO. Among the identified proteins are a series of antioxidant proteins (e.g., thioredoxin, peroxiredoxin, peroxide reductase, glutathione reductase, and protein disulfide isomerase) and glyceraldehyde-3-phosphate dehydrogenase. Identification of these functional proteins, along with studies of arsenic binding and enzymatic inhibition, points to these proteins as potential molecular targets that play important roles in arsenic-induced health effects and in cancer treatment.

Click chemistry mediated Eu-tagging: activity-based specific quantification and simultaneous activity evaluation of CYP3A4 using 153Eu species-unspecific isotope dilution inductively coupled plasma mass spectrometry.

P450 3A4 (CYP3A4) is one of the most important isoforms in the human cytochrome P450 superfamily. It was used as an example in this proof-of-concept study in order to demonstrate an activity-based labeling and then click chemistry (CC) mediated element-tagging strategy for simultaneously specific quantification and activity measurement of an enzyme using species-unspecific isotope dilution inductively coupled plasma mass spectrometry (SUID ICPMS). A dual functional hexynylated 17α-ethynylestradiol activity-based probe was synthesized for specifically labeling CYP3A4 and then CC-mediated Eu-tagging with an azido-DOTA-Eu complex for CYP3A4 quantification and activity measurement in human liver microsome and serum samples using (153)Eu SUID ICPMS. The LOD (3σ) of CYP3A4 reached 20.3 fmol when monitoring (151/153)Eu ICPMS signals, in addition to the merits of specificity and simultaneous activity measurement achieved. We believe that this activity-based CC-mediated element-tagging strategy will liberate more potential advantages of ICPMS in bioanalysis.

Single cell glycan-linkages profiling for hepatocellular carcinoma early diagnosis using lanthanide encoded bacteriophage MS2 based ICP-MS.

Early diagnosis is paramount for enhancing survival rates and prognosis in the context of malignant diseases. Hepatocellular carcinoma (HCC), the second leading cause of cancer-related deaths worldwide, poses significant challenges for its early detection. In this study, we present an innovative approach which contributed to the early diagnosis of HCC. By lanthanide encoding signal amplification to map glycan-linkages at the single-cell level, the minute quantities of "soft" glycan-linkages on single cell surface were converted into "hard" elemental tags through the use of an MS2 signal amplifier. Harnessing the power of lanthanides encoded within MS2, we achieve nearly three orders of magnitude signal amplification. These encoded tags are subsequently quantified using single-cell inductively coupled plasma mass spectrometry (SC-ICP-MS). Linear discriminant analysis (LDA) identifies seven specific glycan-linkages (α-2,3-Sia, α-Gal, α-1,2-Fuc, α-1,6-Fuc, α-2,6-Sia, α-GalNAc, and Gal-ß-1,3-GalNAc) as biomarkers. Our methodology is initially validated at the cellular level with 100% accuracy in discriminating between hepatic carcinoma HepG2 cells and their normal HL7702 cells. We apply this approach to quantify and classify glycan-linkages on the surfaces of 55 clinical surgical HCC specimens. Leveraging these seven glycan-linkages as biomarkers, we achieve precise differentiation between 8 normal hepatic specimens, 40 early HCC specimens, and 7 colorectal metastasis HCC specimens. This pioneering work represents the first instance of employing single-cell glycan-linkages as biomarkers promising for the early diagnosis of HCC with a remarkable 100% predictive accuracy rate, which holds immense potential for enhancing the feasibility and precision of HCC diagnosis in clinical practice.

Reinvent Aliphatic Arsenicals as Reversible Covalent Warheads toward Targeted Kinase Inhibition and Non-acute Promyelocytic Leukemia Cancer Treatment.

The success of arsenic in acute promyelocytic leukemia (APL) treatment is hardly transferred to non-APL cancers, mainly due to the low selectivity and weak binding affinity of traditional arsenicals to oncoproteins critical for cancer survival. We present herein the reinvention of aliphatic trivalent arsenicals (As) as reversible covalent warheads of As-based targeting inhibitors toward Bruton's tyrosine kinase (BTK). The effects of As warheads' valency, thiol protection, methylation, spacer length, and size on inhibitors' activity were studied. We found that, in contrast to the bulky and rigid aromatic As warhead, the flexible aliphatic As warheads were well compatible with the well-optimized guiding group to achieve nanomolar inhibition against BTK. The optimized As inhibitors effectively blocked the BTK-mediated oncogenic signaling pathway, leading to elevated antiproliferative activities toward lymphoma cells and xenograft tumor. Our study provides a promising strategy enabling rational design of new aliphatic arsenic-based reversible covalent inhibitors toward non-APL cancer treatment.

ICP-MS-based multiplex and ultrasensitive assay of viruses with lanthanide-coded biospecific tagging and amplification strategies.

Highly sensitive and a multiplex assay of viruses and viral DNAs in complex biological samples is extremely important for clinical diagnosis and prognosis of pathogenic diseases as well as virology studies. We present an effective ICP-MS-based multiplex and ultrasensitive assay of viral DNAs with lanthanide-coded oligonucleotide hybridization and rolling circle amplification (RCA) strategies on biofunctional magnetic nanoparticles (MNPs), in which single-stranded capture DNA (ss-Cap-DNA)-functionalized MNPs (up to 1.65 × 10(4) ss-Cap-DNA per MNP) were used to recognize and enrich target DNAs, and single-stranded report DNA (ss-Rep-DNA-DOTA-Ln) coded by the lanthanide-DOTA complex hybridized with the targeted DNA for highly sensitive readout of HIV (28 amol), HAV (48 amol), and HBV (19 amol). When utilizing the RCA technique in association with the design and synthesis of a "bridge" DNA and a corresponding ss-Rep-DNA-DOTA-Ho, as low as 90 zmol HBV could be detected. Preliminary applications to the determination of the viral DNAs in 4T1 cell lysates and in serum confirmed the feasibility of this ICP-MS-based multiplex DNA assay for clinical use. One can expect that this element-coded ICP-MS-based multiplex and ultrasensitive DNA assay will play an ever more important role in the fields of bioanalysis and virology and in medical studies after further sophisticated modifications.

Detection and quantification of proteins and cells by use of elemental mass spectrometry: progress and challenges.

Much progress has been made in identification of the proteins in proteomes, and quantification of these proteins has attracted much interest. In addition to popular tandem mass spectrometric methods based on soft ionization, inductively coupled plasma mass spectrometry (ICPMS), a typical example of mass spectrometry based on hard ionization, usually used for analysis of elements, has unique advantages in absolute quantification of proteins by determination of an element with a definite stoichiometry in a protein or attached to the protein. In this Trends article, we briefly describe state-of-the-art ICPMS-based methods for quantification of proteins, emphasizing protein-labeling and element-tagging strategies developed on the basis of chemically selective reactions and/or biospecific interactions. Recent progress from protein to cell quantification by use of ICPMS is also discussed, and the possibilities and challenges of ICPMS-based protein quantification for universal, selective, or targeted quantification of proteins and cells in a biological sample are also discussed critically. We believe ICPMS-based protein quantification will become ever more important in targeted quantitative proteomics and bioanalysis in the near future.