Calibration Compounds Research Articles

Predicting ESI/MS Signal Change for Anions in Different Solvents

LC/ESI/MS is a technique widely used for qualitative and quantitative analysis in various fields. However, quantification is currently possible only for compounds for which the standard substances are available, as the ionization efficiency of different compounds in ESI source differs by orders of magnitude. In this paper we present an approach for quantitative LC/ESI/MS analysis without standard substances. This approach relies on accurately predicting the ionization efficiencies in ESI source based on a model, which uses physicochemical parameters of analytes. Furthermore, the model has been made transferable between different mobile phases and instrument setups by using a suitable set of calibration compounds. This approach has been validated both in flow injection and chromatographic mode with gradient elution.

Indirect quantitative structure-retention relationship for steroid identification: A chemometric challenge at “Chimiométrie 2016”

A chemometric challenge was proposed during the "Chimiométrie" congress 2016, held in Namur, Belgium, on 17–20 January. The aim of this contest was to challenge the ability of congress participants to build indirect Quantitative Structure-Retention Relationship models (QSRR) using the linear solvent strength (LSS) theory of reversed-phase liquid chromatography. QSRR is a very helpful method for the identification of unknown analytes, including the prediction of chromatographic retention time. Because of the potential presence of various isomeric compounds, accurate retention time prediction is particularly important in the context of steroid identification. In addition, the indirect prediction of retention time using the linear solvent strength (LSS) parameters S and log kW provides a great advantage for use in any gradient conditions. In the proposed dataset, the experimental values of S and log kW were estimated using Ultra High Pressure Liquid Chromatography separation with two linear gradients (5–95% ACN+0.1% FA) of 15 and 60min, respectively. The aim of the challenge was the accurate estimation of retention time for a 45min gradient by applying the LSS theory based on the predicted S and log kW values. Molecular descriptors were calculated from a series of reference steroid compounds using the VolSurf+ software. By these means, a collection of 128 variables related to molecular shape, volume, polarisability, polar surface area, hydrophobic surface area, lipophilicity, molecular diffusion, and solubility was generated automatically. The dataset (n=95) included 76 steroid compounds for calibration and 19 for validation. Experimental log kW, S and retention time values were provided for the calibration set only. The results were evaluated according to the smallest RMSEC obtained for the retention time predictions of the validation set with the 45min gradient using the LSS parameters. Moreover, each individual relative error should not exceed 5% of the experimental retention time for both the calibration and validation sets. This paper summarises the approaches proposed by the best three participants and the challenge organiser.

EXTH-36. SCREENING COMPOUNDS THAT TARGET THE DNA DAMAGE RESPONSE FOR BLOOD-BRAIN-BARRIER PERMEABILITY

For glioblastoma (GBM), development of strategies that stabilize disease progression and ultimately achieve a cure will require comprehensive approaches towards target selection and design of therapies. Activation of the DNA damage response (DDR) by temozolomide and irradiation induces signaling pathways with actionable therapeutic targets (Akt, Chk1, and Mdm2), providing fertile ground to develop novel anti-GBM combination therapies. Using pediatric and adult GBM cells with clinically relevant molecular profiles, isobologram analyses indicated that combinations of small molecule inhibitors to these DDR-induced targets can synergistically block in vitro cell growth and decrease resistance of GBM cells to temozolomide. Knowing the degree of blood-brain-barrier (BBB) permeability is particularly important in the selection of therapeutic compounds for the treatment of GBM. To this end, we adapted the Mouse Brain Uptake Assay (MBUA), which determines the BBB apparent permeability (Papp, nm/sec) for drugs across an intact BBB with the assumption that uptake rate is an important contributor to brain concentrations. When Papp is plotted as a function of cLogD and compared with calibration compounds, specific mechanistic interpretation is possible. Optimal BBB Papp values fall within the region of brain uptake limited only by cerebral blood flow, and not limited by low passive permeability or active efflux. Inhibitors of interest have been analyzed using the MBUA. BBB permeability, as measured by Papp, was in the order RG7388>RG7112>nutlin3a for MDM2 inhibitors and CHIR124>AZD7762 for Chk1 inhibitors. Survival in an intracranial GBM xenograft model indicated that an intermittent dosing regimen of temozolomide with Akt (GDC0068) and Mdm2 (RG7112) inhibitors exhibiting moderate BBB permeability significantly reduced GBM growth with minimal toxicity. Studies are in progress to determine if the next-generation Mdm2 inhibitor RG7388 with a more favorable BBB Papp can further improve efficacy of combination therapy and offer new therapeutic options for GBM.

Points to consider for a validation study of iPS cell-derived cardiomyocytes using a multi-electrode array system

Human induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) provide a novel assay system to assess cardiac safety in drug development to overcome a problem of species difference in non-clinical testing during drug development. Using the multi-electrode array (MEA) platform, electrophysiological activities of iPS-CMs can be recorded easily to assess QT prolongation and proarrhythmic potential of drug candidates. Here we have established a standardized protocol to evaluate the possibility of iPS-CMs, and shared the protocol with an international consortium. To obtain reproducible and reliable experimental data from these cells, we determined the optimal experimental conditions, such as cell density, MEA coating, culture conditions, high-pass filter frequency, definition of early afterdepolarization or triggered activity, and calibration compounds. Based on the protocol, our validation study using 60 compounds is in progress. Thus, MEA-based experiments using iPS-CMs would be a standard testing method to evaluate QT prolongation and proarrhythmic potentials.

Crystallographic and Dynamic Aspects of Solid-State NMR Calibration Compounds: Towards ab Initio NMR Crystallography.

The excellent results of dispersion‐corrected density functional theory (DFT‐D) calculations for static systems have been well established over the past decade. The introduction of dynamics into DFT‐D calculations is a target, especially for the field of molecular NMR crystallography. Four 13C ss‐NMR calibration compounds are investigated by single‐crystal X‐ray diffraction, molecular dynamics and DFT‐D calculations. The crystal structure of 3‐methylglutaric acid is reported. The rotator phases of adamantane and hexamethylbenzene at room temperature are successfully reproduced in the molecular dynamics simulations. The calculated 13C chemical shifts of these compounds are in excellent agreement with experiment, with a root‐mean‐square deviation of 2.0 ppm. It is confirmed that a combination of classical molecular dynamics and DFT‐D chemical shift calculation improves the accuracy of calculated chemical shifts.

Development of a standard gas generating vial comprised of a silicon oil–polystyrene/divinylbenzene composite sorbent

In this work, a highly reproducible standard gas generating vial is proposed. The vial is comprised of a silicon diffusion pump oil spiked with an appropriate calibration compound, such as modified McReynolds probes (benzene, 2-pentanone, pyridine, 1-nitropropane, 1-pentanol, and n-octane), and then mixed with polystyrene/divinylbenzene (PS/DVB) particles. The concentrations of these compounds in gaseous headspace were found to substantially decrease in comparison to previously developed hydrocarbon pump oil based vials; hence, the amount of standard loaded onto SPME fibers was at most, half that of the previous vial design. Depletion for all compounds after 208 successive extractions was shown to be less than 3.5%. Smaller quantities of standards being used resulted in a vial that depleted slower while remaining statistically repeatable over a wider number of runs. Indeed, it was found that depletion could be largely predicted by using a mass balance theoretical model. This behavior allowed a further increase in the number of loadings that could be performed repeatedly. At a 95% level of confidence, the ANOVA test demonstrated that the prepared vials were statistically identical, with no significant intra- or inter-batch differences. In addition, it was found that vials stored under different conditions (e.g. under light exposure, room temperature, and within a refrigerator) were stable over 10 weeks. Silicon based vials proved to be ideal for performing instrument quality control and loading of internal standards onto fibers, both of which are of great importance when performing on-site analysis using portable GC–MS instrumentation and high throughput determinations in laboratory.

Chemical ionization mass spectrometry using carbon nanotube field emission electron sources.

A novel chemical ionization (CI) source has been developed based on a carbon nanotube (CNT) field emission electron source. The CNT-based electron source was evaluated and compared with a standard filament thermionic electron source in a commercial explosives trace detection desktop mass spectrometer. This work demonstrates the first reported use of a CNT-based ion source capable of collecting CI mass spectra. Both positive and negative modes were investigated. Spectra were collected for a standard mass spectrometer calibration compound, perfluorotributylamine (PFTBA), as well as trace explosives including trinitrotoluene (TNT), Research Department explosive (RDX), and pentaerythritol tetranitrate (PETN). The electrical characteristics, lifetime at operating pressure, and power requirements of the CNT-based electron source are reported. The CNT field emission electron sources demonstrated an average lifetime of 320 h when operated in constant emission mode under elevated CI pressures. The ability of the CNT field emission source to cycle on and off can provide enhanced lifetime and reduced power consumption without sacrificing performance and detection capabilities. Graphical Abstract ᅟ.

Modification of a gas chromatography/atmospheric pressure chemical ionisation time-of-flight mass spectrometer as an alternative to automated atmospheric pressure solids analysis probe.

The atmospheric pressure solids analysis probe (ASAP) has been successfully introduced into laboratories as a routine analytical tool but the technique is labour-intensive and is not easily automated. This paper describes the modifications made to an existing atmospheric pressure gas chromatography (APGC) system to enable the automated analysis of samples that would have otherwise been analysed by ASAP. Liquid samples were injected into a 1 m piece of fused silica guard column using a standard split/splitless gas chromatography (GC) inlet, oven and autosampler. A novel timing delay circuit was developed and integrated into the system to control acquisition start times and a bespoke heating block/calibrant delivery system was used to transport either the calibration or reference compound into the ionisation source. The modified system has allowed the successful analysis of those samples requiring ASAP to be carried out in a fully automated manner using APGc. The analysis is rapid and can be carried out in less than 2 min. The results obtained are directly comparable with those obtained by ASAP. A typical mass accuracy of better than 5 mm/z units was achieved, allowing the molecules to be identified based on their elemental formulae. An existing APGC system has been successfully modified and is suitable for the automatic analysis of samples normally analysed by ASAP. The APGC instrument incorporates the use of a novel reference compound and delivery system which allows accurate mass measurements to be performed. This and the other modifications described have allowed the technique to be incorporated into an existing suite of mass spectrometry-based experiments for use in the characterisation of organic molecules.

Prediction of the air-water partition coefficient for perfluoro-2-methyl-3-pentanone using high-level Gaussian-4 composite theoretical methods

The air-water partition coefficient (Kaw) of perfluoro-2-methyl-3-pentanone (PFMP) was estimated using the G4MP2/G4 levels of theory and the SMD solvation model. A suite of 31 fluorinated compounds was employed to calibrate the theoretical method. Excellent agreement between experimental and directly calculated Kaw values was obtained for the calibration compounds. The PCM solvation model was found to yield unsatisfactory Kaw estimates for fluorinated compounds at both levels of theory. The HENRYWIN Kaw estimation program also exhibited poor Kaw prediction performance on the training set. Based on the resulting regression equation for the calibration compounds, the G4MP2-SMD method constrained the estimated Kaw of PFMP to the range 5–8 × 10−6 M atm−1. The magnitude of this Kaw range indicates almost all PFMP released into the atmosphere or near the land-atmosphere interface will reside in the gas phase, with only minor quantities dissolved in the aqueous phase as the parent compound and/or its hydrate/hydrate conjugate base. Following discharge into aqueous systems not at equilibrium with the atmosphere, significant quantities of PFMP will be present as the dissolved parent compound and/or its hydrate/hydrate conjugate base.

Quantitative analysis of molecular interaction potentials of ionic liquid anions using multi-functionalized stationary phases in HPLC.

The molecular interaction potentials, including S (dipolarity/polarizability), A (hydrogen bonding acidity), and B (hydrogen bonding basicity), of anions are experimentally determined using multi-functionalized stationary phases in high-performance liquid chromatography (HPLC) systems. We employ three different multi-functionalized stationary phase columns (Obelisc R, Obelisc N, and Acclaim Trinity-P1) combined with two ingredients, namely, acetonitrile (ACN) and methanol (MeOH). These conditions can cause neutral, cationic, and anionic compounds to be retained. By using the retention characteristics of calibration compounds, including cations, anions, and neutral compounds, system parameters including the ionic interaction terms (zc Zc , za Za ) are evaluated using multiple linear regression, resulting in a standard deviation (SD) of 0.090-0.158 log units. Based on the system parameters and retention characteristics of the anions of interest, their molecular interaction potentials are characterized on the same scale for neutral and cationic molecules. Furthermore, to verify the determined molecular interaction potentials, we predict anion hydrophobicity. The results show that the determined S, A, and B, together with the computable descriptors E (excess molar refraction) and V (McGowan volume), can predict anion hydrophobicity with R(2) =0.982 and SD=0.167 (dimensionless).

Thermodynamic properties of chloramine formation and related reactions during water treatment: A G4MP2, G4, and W1BD theoretical study

A high-level gas and aqueous phase theoretical thermodynamic study was conducted on the primary and related chemical reactions which occur during chloramination for water treatment using the G4MP2, G4, and W1BD composite methods with the SMD, PCM, and CPCM solvation models. The standard state (298.15 K, 1 atm or 1M) formation of mono-, di-, and tri-chloramines from their precursors via hypochlorous acid chlorination is substantially exothermic and exergonic in both the gas and aqueous phases. The excellent agreement between experimental and theoretical values for a range of structural and thermodynamic calculations on a suite of calibration compounds suggests that the G4MP2, G4, and W1BD calculations meet or exceed criteria for thermochemical accuracy. The temperature influence on the thermodynamics of chloramine formation is projected to be negligible regardless of phase between 0 and 100°C. Additional thermodynamic calculations were undertaken on associated chloramination reactions involving the disproportionation of monochloramine, the decomposition of di- and tri-chloramine, and the reactions of trichloramine with ammonia and dichloramine. The results from these investigations not only provide a better understanding of the reaction thermodynamics, they also allow for a more rigorous interpretation of proposed chloramination mechanisms.

Undisturbed and disturbed above canopy ponderosa pine emissions: PTR-TOF-MS measurements and MEGAN 2.1 model results

Abstract. We present the first eddy covariance flux measurements of volatile organic compounds (VOCs) using a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) above a ponderosa pine forest in Colorado, USA. The high mass resolution of the PTR-TOF-MS enabled the identification of chemical sum formulas. During a 30 day measurement period in August and September 2010, 649 different ion mass peaks were detected in the ambient air mass spectrum (including primary ions and mass calibration compounds). Eddy covariance with the vertical wind speed was calculated for all ion mass peaks. On a typical day, 17 ion mass peaks, including protonated parent compounds, their fragments and isotopes as well as VOC-H+-water clusters, showed a significant flux with daytime average emissions above a reliable flux threshold of 0.1 mg compound m−2 h−1. These ion mass peaks could be assigned to seven compound classes. The main flux contributions during daytime (10:00–18:00 LT) are attributed to the sum of 2-methyl-3-buten-2-ol (MBO) and isoprene (50%), methanol (12%), the sum of acetic acid and glycolaldehyde (10%) and the sum of monoterpenes (10%). The total MBO + isoprene flux was composed of 10% isoprene and 90% MBO. There was good agreement between the light- and temperature dependency of the sum of MBO and isoprene observed for this work and those of earlier studies. The above canopy flux measurements of the sum of MBO and isoprene and the sum of monoterpenes were compared to emissions calculated using the Model of Emissions of Gases and Aerosols from Nature (MEGAN 2.1). The best agreement between MEGAN 2.1 and measurements was reached using emission factors determined from site-specific leaf cuvette measurements. While the modeled and measured MBO + isoprene fluxes agree well, the emissions of the sum of monoterpenes is underestimated by MEGAN 2.1. This is expected as some factors impacting monoterpene emissions, such as physical damage of needles and branches due to storms, are not included in MEGAN 2.1. After a severe hailstorm event, 22 ion mass peaks (attributed to six compound classes plus some unknown compounds) showed an elevated flux for the two following days. The sum of monoterpene emissions was 4–23 times higher compared to emissions prior to the hailstorm while MBO emissions remained unchanged. The monoterpene emission (in mg compound m−2) during this measurement period is underestimated by 40% if the effect of this disturbance source is not considered.

Automatic Analysis of Quantitative NMR Data of Pharmaceutical Compound Libraries

In drug discovery, chemical library compounds are usually dissolved in DMSO at a certain concentration and then distributed to biologists for target screening. Quantitative (1)H NMR (qNMR) is the preferred method for the determination of the actual concentrations of compounds because the relative single proton peak areas of two chemical species represent the relative molar concentrations of the two compounds, that is, the compound of interest and a calibrant. Thus, an analyte concentration can be determined using a calibration compound at a known concentration. One particularly time-consuming step in the qNMR analysis of compound libraries is the manual integration of peaks. In this report is presented an automated method for performing this task without prior knowledge of compound structures and by using an external calibration spectrum. The script for automated integration is fast and adaptable to large-scale data sets, eliminating the need for manual integration in ~80% of the cases.

Determination of LFER Descriptors of 30 Cations of Ionic Liquids—Progress in Understanding Their Molecular Interaction Potentials

In order to understand molecular interaction potentials of 30 cations of ionic liquids (ILs), the well-known linear free energy relationship concept (LFER) was applied. The LFER descriptors for the excess molar refractivity and the molar volume were calculated in silico and for hydrogen-bonding acidity and basicity, and the polarizability/dipolarity of IL cations were experimentally determined through high performance liquid chromatography (HPLC) measurements. For the study, three different columns (RP-select B, Cyan, and Diol) and buffered mobile phases, based on two organic solvents acetonitrile (ACN) and methanol (MeOH), were selectively combined to the HPLC separation systems RP-select B-ACN, RP-select B-MeOH, Cyan-MeOH, Diol-ACN, and Diol-MeOH. By measuring the retention factors of 45 neutral calibration compounds and calculating LFER descriptors of three cations in the HPLC systems, the system parameters, including an ionic z coefficient, were determined. Conversely, the LFER descriptors of 30 ionic liquid cations were determined, based on the parameters of five systems and their retention factors in the HPLC systems. The results showed that the type of head group, alkyl chain length and further substituents of the cation have a significant influence on the dipolarity/polarizability and the hydrogen-bonding acidity, and functionalized groups (hydroxyl, ether, and dimethylamino) lead to hydrogen-bonding basicity of the cation. The characterization of cationic LFER descriptors opens up the chance for a more quantitative understanding of molecular interaction potentials and physicochemical properties of ILs.

Comparison of multiple linear regression, partial least squares and artificial neural network for quantitative structure retention relationships of some polycyclic aromatic hydrocarbons

Quantitative structural-retention relationships (QSRR) of retention phenomena of polycyclic aromatic hydrocarbons (PAHs) facilitate resolving complex mixtures of them by gas chromatography (GC). The structural descriptors of 38 PAH compounds were calculated. Stepwise variable selection was applied for selection of meaningful descriptors. MLR, PLS and ANN models were built with calibration compounds. The predictive ability of the models was evaluated on 6 PAHs, which were not used in training steps and also by leave-6-out cross-validation. The best prediction results were obtained by the MLR model. The difference in predictive ability of the ANN and PLS model was trivial.

Which Drinking Water Disinfection Process Generates the Least Toxic DBP Mixture: Chlorination, Chloramination, Ozonation, or Chlorine Dioxide Treatment?

NOM in natural waters mainly originates from the decomposition of plants. It consists of a wide variety of complex organic compounds with molecular weights ranging from hundreds to hundreds of thousands of Da [1]. The complexity of NOM determines a higher magnitude of complexity of the reaction products from disinfectants and NOM with/without bromide and iodide. Since Rook [2] first found trihalomethanes as DBPs in finished drinking water in 1974, numerous DBPs have been identified in real or simulated drinking waters. Commonly known DBPs include trihalomethanes, haloacetic acids, haloacetonitriles, haloketones, cyanogen halides, chloropicrin, chloral hydrate, N-nitrosodimethylamine, bromate, and chlorite. Richardson [3] compiled a review on drinking water DBPs formed from using four major disinfectants, in which she listed about 600 DBPs that were identified by gas chromatography/mass spectrometry (GC/MS). Of the 600 DBPs, some are commonly known DBPs and occur at low-μg/L levels, some occur at sub-μg/L or ng/L levels, and some may still not be quantifiable [4]. The non-quantifiable DBPs are because of no corresponding standard compounds for calibration or their concentrations below detection limits. In addition, there are a countless number of unidentified/unknown DBPs.

MALDI-TOF High Mass Calibration up to 200 kDa Using Human Recombinant 16 kDa Protein Histidine Phosphatase Aggregates

BackgroundProtein histidine phosphatase (PHP) is an enzyme which removes phosphate groups from histidine residues. It was described for vertebrates in the year 2002. The recombinant human 16 kDa protein forms multimeric complexes in physiological buffer and in the gas phase. High-mass calibration in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has remained a problem due to the lack of suitable standards. Large proteins can hardly be freed of their substructural microheterogeneity by classical purification procedures so that their use as calibrants is limited. A small adduct-forming protein of validated quality is a valuable alternative for that purpose.Methodology/Principal FindingsThree major PHP clusters of ∼113, 209 and >600 kDa were observed in gel filtration analysis. Re-chromatography of the monomer peak showed the same cluster distribution. The tendency to associate was detected also in MALDI-TOF MS measuring regular adducts up to 200 kDa.Conclusions/SignificancePHP forms multimers consisting of up to more than 35 protein molecules. In MALDI-TOF MS it generates adduct ions every 16 kDa. The protein can be produced with high quality so that its use as calibration compound for high mass ranges above 100 kDa, where standards are difficult to obtain, is feasible.

High-throughput Total Cupric Ion Reducing Antioxidant Capacity of Biological Samples Determined Using Flow Injection Analysis and Microplate-based Methods

High-throughput cupric ion reducing antioxidant capacity (CUPRAC) methods were developed for assessment of total antioxidant capacity (TAC) in urine and serum, based on reduction of Cu(II)-neocuproine complex to highly colored Cu(I)-neocuproine complex, measured spectrophotometrically at 450 nm. The reaction time was significantly reduced from 30 to 4 min by application of a calibration compound (uric acid) with kinetic behavior similar to that shown by urine samples. The method was implemented in a microformat (96 well plates) and also in an automatic fashion (flow injection analysis, FIA). A determination throughput value of 288 h(-1) (microplate method) or of 15 h(-1) (automatic FIA) was attained. Application of both methods to human serum (SRM 909b, level I) and urines (n = 9) provided TAC values in agreement with those of the end-point batch method.

Extension of a prediction model to estimate vapor pressures of perfluorinated compounds (PFCs)

Perfluorinated compounds (PFCs) are persistent and have been found globally as environmental contaminants. Release into the environment can occur from manufacturing, industrial and consumer uses. The vapor pressure is an important physical property influencing both the release and the environmental partitioning, but few reliable experimental determinations are available. Here we update a previous PLS regression model to cover also this compound class, using only a few calibration compounds. The recalibration is accomplished by applying a leverage-based weighting scheme that is generally applicable in updating structure–property relationships. The predictive performance is validated with an external validation set and is considerably better than for other standard estimation software, both with regard to accuracy and precision. The model can be given a chemical interpretation and the prediction error for the liquid vapor pressure is within 0.2 log units of Pa. Finally, the model is applied and vapor pressure estimates are reported for more than 200 PFCs where no reliable experimental data are available.

Temperature-Dependent Vapor Pressure of Selected Cyclic and Linear Polydimethylsiloxane Oligomers

The isothermal retention times of seven cyclic (D4 to D10) and nine linear (L5 to L13) dimethylsiloxane oligomers on a nonpolar gas chromatographic column were determined at temperatures between (308.15 and 438.15) K. Using normal alkanes as standard reference and calibration compounds, these retention times were used to derive liquid state vapor pressures at 298.15 K as well as enthalpies of vaporization that allow for the estimation of vapor pressures over a wide range of temperatures. The data agree within 0.25 log units with vapor pressure values previously reported by Flaningam (1986) but extend those values to lower temperatures and larger oligomers. Cyclic dimethylsiloxanes are more volatile than linear dimethylsiloxanes of a similar size. Oligomers with 10 and more dimethylsiloxane units are expected to sorb appreciably to aerosol at environmentally relevant temperatures.