Dopamine Modulates Effective Connectivity in Frontal Cortex.

There is increasing evidence that the left lateral frontal cortex is hierarchically organized such that higher-order regions have an asymmetric top-down influence over lower order regions. However, questions remain about the underlying neuroarchitecture of this hierarchical control organization. Within the frontal cortex, dopamine plays an important role in cognitive control functions, and we hypothesized that dopamine may preferentially influence top-down connections within the lateral frontal hierarchy. Using a randomized, double-blind, within-subject design, we analyzed resting-state fMRI data of 66 healthy young participants who were scanned once each after administration of bromocriptine (a dopamine agonist with preferential affinity for D2 receptor), tolcapone (an inhibitor of catechol-O-methyltransferase), and placebo, to determine whether dopaminergic stimulation modulated effective functional connectivity between hierarchically organized frontal regions in the left hemisphere. We found that dopaminergic drugs modulated connections from the caudal middle frontal gyrus and the inferior frontal sulcus to both rostral and caudal frontal areas. In dorsal frontal regions, effectivity connectivity strength was increased, whereas in ventral frontal regions, effective connectivity strength was decreased. These findings suggest that connections within frontal cortex are differentially modulated by dopamine, which may bias the influence that frontal regions exert over each other.

Striatal dopamine synthesis and cognitive flexibility differ between hormonal contraceptive users and nonusers.

In rodents and nonhuman primates, sex hormones are powerful modulators of dopamine (DA) neurotransmission. Yet less is known about hormonal regulation of the DA system in the human brain. Using positron emission tomography (PET), we address this gap by comparing hormonal contraceptive users and nonusers across multiple aspects of DA function: DA synthesis capacity via the PET radioligand 6-[18F]fluoro-m-tyrosine ([18F]FMT), baseline D2/3 receptor binding potential using [11C]raclopride, and DA release using methylphenidate-paired [11C]raclopride. Participants consisted of 36 healthy women (n = 15 hormonal contraceptive users; n = 21 naturally cycling/non users of hormonal contraception), and men (n = 20) as a comparison group. A behavioral index of cognitive flexibility was assessed prior to PET imaging. Hormonal contraceptive users exhibited greater DA synthesis capacity than NC participants, particularly in dorsal caudate, and greater cognitive flexibility. Furthermore, across individuals, the magnitude of striatal DA synthesis capacity was associated with cognitive flexibility. No group differences were observed in D2/3 receptor binding or DA release. Analyses by sex alone may obscure underlying differences in DA synthesis tied to women's hormone status. Hormonal contraception (in the form of pill, shot, implant, ring, or intrauterine device) is used by ~400 million women worldwide, yet few studies have examined whether chronic hormonal manipulations impact basic properties of the DA system. Findings from this study begin to address this critical gap in women's health.

Enhancing dopamine tone modulates global and local cortical perfusion as a function of COMT val158met genotype.

The cognitive effects of pharmacologically enhancing cortical dopamine (DA) tone are variable across healthy human adults. It has been postulated that individual differences in drug responses are linked to baseline cortical DA activity according to an inverted-U-shaped function. To better understand the effect of divergent starting points along this curve on DA drug responses, researchers have leveraged a common polymorphism (rs4680) in the gene encoding the enzyme catechol-O-methyltransferase (COMT) that gives rise to greater (Met allele) or lesser (Val allele) extracellular levels of cortical DA. Here we examined the extent to which changes in resting cortical perfusion following the administration of two mechanistically-distinct dopaminergic drugs vary by COMT genotype, and thereby track predictions of the inverted-U model. Using arterial spin labeling (ASL) and a double-blind, within-subject design, perfusion was measured in 75 healthy, genotyped participants once each after administration of tolcapone (a COMT inhibitor), bromocriptine (a DA D2/3 agonist), and placebo. COMT genotype and drug interacted such that COMT Val homozygotes exhibited increased prefusion in response to both drugs, whereas Met homozygotes did not. Additionally, tolcapone-related perfusion changes in the right inferior frontal gyrus correlated with altered performance on a task of executive function. No comparable effects were found for a genetic polymorphism (rs1800497) affecting striatal DA system function. Together, these results indicate that both the directionality and magnitude of drug-induced perfusion change provide meaningful information about individual differences in response to enhanced cortical DA tone.

Dopaminergic Mechanisms Underlying Normal Variation in Trait Anxiety.

Trait anxiety has been associated with altered activity within corticolimbic pathways connecting the amygdala and rostral anterior cingulate cortex (rACC), which receive rich dopaminergic input. Though the popular culture uses the term "chemical imbalance" to describe the pathophysiology of psychiatric conditions such as anxiety disorders, we know little about how individual differences in human dopamine neurochemistry are related to variation in anxiety and activity within corticolimbic circuits. We addressed this issue by examining interindividual variability in dopamine release at rest using [11C]raclopride positron emission tomography (PET), functional connectivity between amygdala and rACC using resting-state functional magnetic resonance imaging (fMRI), and trait anxiety measures in healthy adult male and female humans. To measure endogenous dopamine release, we collected two [11C]raclopride PET scans per participant. We contrasted baseline [11C]raclopride D2/3 receptor binding and D2/3 receptor binding following oral methylphenidate administration. Methylphenidate blocks the dopamine transporter, which increases extracellular dopamine and leads to reduced [11C]raclopride D2/3 receptor binding via competitive displacement. We found that individuals with higher dopamine release in the amygdala and rACC self-reported lower trait anxiety. Lower trait anxiety was also associated with reduced rACC-amygdala functional connectivity at baseline. Further, functional connectivity showed a modest negative relationship with dopamine release such that reduced rACC-amygdala functional connectivity was accompanied by higher levels of dopamine release in these regions. Together, these findings contribute to hypodopaminergic models of anxiety and support the utility of combining fMRI and PET measures of neurochemical function to advance our understanding of basic affective processes in humans.SIGNIFICANCE STATEMENT It is common wisdom that individuals vary in their baseline levels of anxiety. We all have a friend or colleague we know to be more "tightly wound" than others, or, perhaps, we are the ones marveling at others' ability to "just go with the flow." Although such observations about individual differences within nonclinical populations are commonplace, the neural mechanisms underlying normal variation in trait anxiety have not been established. Using multimodal brain imaging in humans, this study takes initial steps in linking intrinsic measures of neuromodulator release and functional connectivity within regions implicated in anxiety disorders. Our findings suggest that in healthy adults, higher levels of trait anxiety may arise, at least in part, from reduced dopamine neurotransmission.

Effects of Dopaminergic Drugs on Cognitive Control Processes Vary by Genotype.

Dopamine (DA) has been implicated in modulating multiple cognitive control processes, including the robust maintenance of task sets and memoranda in the face of distractors (cognitive stability) and, conversely, the ability to switch task sets or update the contents of working memory when it is advantageous to do so (cognitive flexibility). In humans, the limited specificity of available pharmacological probes has posed a challenge for understanding the mechanisms by which DA, acting on multiple receptor families across the PFC and striatum, differentially influences these cognitive processes. Using a within-subject, placebo-controlled design, we contrasted the impact of two mechanistically distinct DA drugs, tolcapone (an inhibitor of catechol-O-methyltransferase [COMT], a catecholamine inactivator) and bromocriptine (a DA agonist with preferential affinity for the D2 receptor), on the maintenance and switching of task rules. Given previous work demonstrating that drug effects on behavior are dependent on baseline DA tone, participants were stratified according to genetic polymorphisms associated with cortical (COMT Val158Met) and striatal (Taq1A) DA system function. Our results were partially consistent with an inverted-U-shaped relationship between tolcapone and robust rule maintenance (interaction with COMT genotype) and between bromocriptine and cued rule switching (interaction with Taq1A genotype). However, when task instructions were ambiguous, a third relationship emerged to explain drug effects on spontaneous task switching (interaction of COMT genotype and bromocriptine). Together, this pattern of results suggests that the effects of DA drugs vary not only as a function of the DA system component upon which they act but also on subtle differences in task demands and context.

Little Change in Functional Brain Networks Following Acute Levodopa in Drug-Naïve Parkinson's Disease.

OBJECTIVE: The objective of this study was to investigate the effects of levodopa on functional brain networks in Parkinson's disease. METHODS: We acquired resting state functional magnetic resonance imaging in 30 drug-naïve participants with Parkinson's disease and 20 age-matched healthy controls. Each participant was studied following administration of a single oral dose of either levodopa or placebo in a randomized, double-blind, crossover design. RESULTS: The greatest observed differences in functional connectivity were between Parkinson's disease versus control participants, independent of pharmacologic intervention. By contrast, the effects of levodopa were much smaller and detectable only in the Parkinson's disease group. Moreover, although levodopa administration in the Parkinson's disease group measurably improved motor performance, it did not increase the similarity of functional connectivity in Parkinson's disease to the control group. CONCLUSIONS: We found that a single, small dose of levodopa did not normalize functional connectivity in drug-naïve Parkinson's disease. © 2019 International Parkinson and Movement Disorder Society.

Fragmentation pathways arising from protonation at different sites in aminoalkyl-substituted 3-hydroxy-1,2,5-oxadiazoles (3-hydroxyfurazans).

RATIONALE: The gas-phase fragmentation chemistry of multifunctional cations is highly influenced by the site of protonation. Possible relationships between protonation site and fragmentation processes were studied using 4-aminoalkyl-3-hydroxyfurazans. For these heterocyclic amines, the starting points for competing fragmentation pathways varied with protonation at multiple sites in two tautomers. METHODS: Mass spectra were acquired using electrospray ionization (positive mode) coupled to triple quadrupole and ion trap mass spectrometers; precursor-product ion relationships were studied by collision-induced dissociation. Quantum mechanical computations were performed at the MP2/6-311++G(2d,p)//ωB97X-D/6-311+G(d) level of theory. RESULTS: Prominent successive losses of NO and CO and competing losses of CH2 =NH or NH3 were observed as fragmentation processes. The lowest barrier computed for the initial step in a fragmentation pathway was associated with the [M + H]+ ion protonated at N5 in the heterocyclic ring, whereas an alternative ring cleavage leading to complementary product ions was initiated by protonation of the ring at N2. Side-chain protonation led to loss of NH3 without cleavage of the 3-hydroxyfurazan ring. CONCLUSIONS: The product ions obtained by the competing fragmentation processes varied with the site of protonation. Interestingly, the most abundant product ions observed at low collision energies were formed by cleavage of protonated molecules possessing more internal energy than other isomers.

Ghosts in the Machine II: Neural Correlates of Memory Interference from the Previous Trial.

Previous memoranda interfere with working memory. For example, spatial memories are biased toward locations memorized on the previous trial. We predicted, based on attractor network models of memory, that activity in the frontal eye fields (FEFs) encoding a previous target location can persist into the subsequent trial and that this ghost will then bias the readout of the current target. Contrary to this prediction, we find that FEF memory representations appear biased away from (not toward) the previous target location. The behavioral and neural data can be reconciled by a model in which receptive fields of memory neurons converge toward remembered locations, much as receptive fields converge toward attended locations. Convergence increases the resources available to encode the relevant memoranda and decreases overall error in the network, but the residual convergence from the previous trial can give rise to an attractive behavioral bias on the next trial.

Quantitative determination of the neurotoxin ß-N-methylamino-L-alanine (BMAA) by capillary electrophoresis-tandem mass spectrometry.

Recent reports of the widespread occurrence of the neurotoxin ß-N-methylamino-L-alanine (BMAA) in cyanobacteria and particularly seafood have raised concerns for public health. LC-MS/MS is currently the analytical method of choice for BMAA determinations but incomplete separation of isomeric and isobaric compounds, matrix suppression and conjugated forms are plausible limitations. In this study, capillary electrophoresis (CE) coupled with MS/MS has been developed as an alternative method for the quantitative determination of free BMAA. Using a bare fused silica capillary, a phosphate buffer (250 mM, pH 3.0) and UV detection, it was possible to separate BMAA from four isomers, but the limit of detection (LOD) of 0.25 µg mL-1 proved insufficient for analysis of typical samples. Coupling the CE to a triple quadrupole MS was accomplished using a custom sheath-flow interface. The best separation was achieved with a 5 M formic acid in water/acetonitrile (9:1) background electrolyte. Strong acid hydrolysis of lyophilized samples was used to release BMAA from conjugated forms. Field-amplified stacking after injection was achieved by lowering sample ionic strength with a cation-exchange cleanup procedure. Quantitation was accomplished using isotope dilution with deuterium-labelled BMAA as internal standard. An LOD for BMAA in solution of 0.8 ng mL-1 was attained, which was equivalent to 16 ng g-1 dry mass in samples using the specified extraction procedure. This was comparable with LC-MS/MS methods. The method displayed excellent resolution of amino acid isomers and had no interference from matrix components. The presence of BMAA in cycad, mussel and lobster samples was confirmed by CE-MS/MS, but not in an in-house cyanobacterial reference material, with quantitative results agreeing with those from LC-MS/MS. Graphical Abstract CE-MS separation and detection of BMAA, its isomers and the internal standard BMAA-d3.

Competing fragmentation processes of ß-substituted propanoate ions upon collision-induced dissociation.

RATIONALE: When subjected to collisional activation, gas-phase carboxylate ions typically undergo decarboxylation. However, alternative fragmentation processes dominate when the carboxylate group is located within certain structural motifs. In this work, the fragmentation processes of ß-substituted carboxylate ions are characterized to improve correlations between reactivity and structure. METHODS: Mass spectra were collected using both ion trap and triple quadrupole mass spectrometers operating in the negative ion mode; collision-induced dissociation (CID) of ions was used to study the relationship between product ions and the structures of their precursor ions. Quantum mechanical computations were performed on a full range of reaction geometries at the MP2/6-311++G(2d,p)//B3LYP/6-31++G(2d,p) level of theory. RESULTS: For a series of ß-substituted carboxylate ions, a product ion corresponding to the anion of the ß-substituent was obtained upon CID. Detailed computations indicated that decarboxylative elimination and at least one other fragmentation mechanism had feasible energetics for the formation of substituent anions differing in their gas-phase basicities. Predicted energetics for anti- and synperiplanar alignments in the transition structures for decarboxylative elimination correlated with the positions of crossover points in breakdown curves acquired for conformationally constrained ions. CONCLUSIONS: The feasibility of more than one mechanism was established for the fragmentation of ß-substituted propanoates. The contribution of each mechanistic pathway to the formation of the substituent anion was influenced by structural variations and conformational constraints, but mostly depended on the nature of the substituent. Copyright © 2016 John Wiley & Sons, Ltd.

Phenyl group participation in rearrangements during collision-induced dissociation of deprotonated phenoxyacetic acid.

RATIONALE: The identification of trace constituents in biological and environmental samples is frequently based on the fragmentation patterns resulting from the collision-induced dissociation (CID) of gas-phase ions. Credible mechanistic characterization of fragmentation processes, including rearrangements, is required to make reliable assignments for structures of precursor and product ions. METHODS: Mass spectra were collected using both ion trap and triple quadrupole mass spectrometers operating in the negative ion mode. Precursor ion scans and CID of ions generated in-source were used to establish precursor-product ion relationships. Density functional theory (DFT) computations were performed at the MP2/6-311++G(2d,p)//B3LYP/6-31++G(2d,p) level of theory. RESULTS: Product ions at m/z 93 and 107 obtained upon CID of phenoxyacetate were attributed to phenoxide and o-methylphenoxide, respectively. An isotopic labeling experiment and computations showed that the phenoxide ion was formed by intramolecular displacement with formation of an α-lactone and also by a Smiles rearrangement. Rearrangement of phenoxyacetate via the ion-neutral complex formed in the α-lactone displacement pathway gave the isomeric o-hydroxyphenylacetate ion which yielded o-methylphenoxide upon decarboxylation. Computations provided feasible energetics for these pathways. CONCLUSIONS: Previously unrecognized and energetically favorable rearrangements during the collision-induced fragmentation of phenoxyacetate have been characterized using isotopic labeling and DFT computations. Notably, the phenyl substituent plays an indispensable role in each rearrangement process resulting in multiple pathways for the fragmentation of phenoxyacetate.

Intramolecular interactions and the neutral loss of ammonia from collisionally activated, protonated ω-aminoalkyl-3-hydroxyfurazans.

Gas phase fragmentation reactions of monoprotonated 4-(3-aminopropyl)- and 4-(4-aminobutyl)-3-hydroxyfurazan were investigated to examine potential interactions between functional groups. The two heterocyclic alkyl amines were ionized by electrospray ionization (ESI, positive mode) and fragmented using tandem mass spectrometry (MS/MS). The fragmentation pathways were characterized using pseudo MS3 experiments, precursor-ion scans, and density functional computations. For both heterocyclic ions, loss of ammonia was the only fragmentation process observed at low collision energies. Computational analysis indicated that the most feasible mechanism was intramolecular nucleophilic displacement of ammonia from the protonated ω-aminoalkyl side chain by N5 of the furazan ring. The alkylated nitrogen in the resulting bicyclic product ion facilitated N-O bond cleavage; subsequent neutral losses of nitric oxide (NO) and carbon monoxide (CO) occurred by homolytic bond cleavages. Next in the multistep sequence, neutral loss of ethylene from a radical cation was observed. A less favorable, competing fragmentation pathway of protonated 4-(3-aminopropyl)-3-hydroxyfurazan was consistent with cleavage of the 3-hydroxyfurazan ring and losses of NO and CO. Overall, the similar fragmentation behavior found for protonated 4-(3-aminopropyl)- and 4-(4-aminobutyl)-3-hydroxyfurazan differed from that previously characterized for furazan analogs with shorter alkyl chains. These observations demonstrate that a small change in the structure of multifunctional, heterocyclic alkyl amines may significantly influence interactions between distinct functional groups and the nature of the fragmentation process.

Tandem mass spectrometry of homologous 3-hydroxyfurazan and nitrile amino acids: Analysis of cooperative interactions and fragmentation processes.

The assignment of structure by tandem mass spectrometry (MS/MS) relies on the interpretation of the fragmentation behavior of gas-phase ions. Mass spectra were acquired for a series of heterocyclic mimetics of acidic amino acids and a related series of nitrile amino acids. All amino acids were readily protonated or deprotonated by electrospray ionization (ESI), and distinctive fragmentation processes were observed when the ions were subjected to collision-induced dissociation (CID). The deprotonated heterocycles showed bond cleavages of the 3-hydroxyfurazan ring with formation of oxoisocyanate and the complementary deprotonated nitrile amino acid. Further fragmentation of the deprotonated nitrile amino acids was greatly dependent on the length of the alkyl nitrile side chain. Competing losses of CO2 versus HCN occurred from α-cyanoglycinate (shortest chain), whereas water was lost from 2-amino-5-cyanopentanoate (longest chain). Interestingly, loss of acrylonitrile by a McLafferty-type fragmentation process was detected for 2-amino-4-cyanobutanoate, and several competing processes were observed for ß-cyanoalanate. In one process, cyanide ion was formed either by consecutive losses of ammonia, carbon dioxide, and acetylene or by a one-step decarboxylative elimination. In another, complementary ions were obtained from ß-cyanoalanate by loss of acetonitrile or HN=CHCO2H. Fragmentation of the protonated 3-hydroxyfurazan and nitrile amino acids resulted in the cumulative loss (H2O + CO), a loss that is commonly observed for protonated aliphatic α-amino acids. Overall, the distinct fragmentation behavior of the multifunctional 3-hydroxyfurazan amino acids correlated with the charged site, whereas fragmentations of the deprotonated nitrile amino acids showed cooperative interactions between the nitrile and the carboxylate groups.

Thermal analysis by variable temperature infrared spectroscopy with a button sample holder and Peltier heating/cooling.

An apparatus and methodology for variable temperature infrared spectroscopy measurements of neat samples contained in a button sample holder are described. Sample heating and cooling are achieved by applying voltage to stacked Peltier thermoelectric devices. Between 0 and 150 °C, samples can be heated and cooled at 2 °C s-1 rates, facilitating temperature step heating/cooling profiles with minimal delay between isothermal infrared spectrum measurements. Examples of correlating temperature-dependent spectral variations with specific sample changes are provided for α-quartz heating/cooling, ibuprofen melting, and acetylsalicylic acid thermal decomposition. Trends in α-quartz infrared spectra obtained with a step heating/cooling temperature profile are used to evaluate spectrum measurement reproducibility. Detection of vibration band intensity variations of less than 1% resulting from a 10 °C sample temperature increment illustrates the measurement sensitivity. By comparing infrared spectra obtained at different temperatures, reversible and irreversible sample changes are identified. Infrared spectra acquired during linear ramp heating are employed to determine the ibuprofen melting point, which confirms the temperature measurement accuracy of the apparatus. Selective analysis is demonstrated by determining isoconversion effective activation energies for processes involved in the thermal decomposition of the acetylsalicylic acid component of a commercial pharmaceutical tablet.

Precise temperature control and rapid heating/cooling of infrared spectroscopy samples with a two-stage thermoelectric device.

The design and performance of an apparatus for heating and cooling samples during variable temperature infrared spectroscopy studies are described. The apparatus incorporates two thermoelectric device modules in a stacked configuration. The cascaded devices are powered in parallel and contained within a metal enclosure that maintains their alignment and applies clamping pressure between them to maximize thermal conductivity. By using this apparatus, sample temperatures can be increased or decreased at 2 °C s-1 rates and isothermal temperatures can be maintained precisely (±0.1 °C). The rapid heating and cooling capabilities of the apparatus facilitate programmed temperature step heating/cooling profiles with isothermal infrared spectrum measurements at pre-selected temperatures. Using linear heating and cooling temperature ramps, subtle temperature-dependent poly(styrene) infrared spectrum changes are elucidated and correlated with sample temperature. Results obtained by using a temperature step sample heating profile are compared with those obtained by using linear temperature ramp heating and cooling to characterize silica gel dehydration and re-hydration processes. By comparing infrared spectra acquired at different temperatures while heating and cooling the sample, silica gel spectrum changes associated with water desorption/adsorption and the thermal expansion/contraction of the Si-O-Si network are differentiated.

Neuroinflammation and white matter alterations in occupational manganese exposure assessed by diffusion basis spectrum imaging.

OBJECTIVE: To evaluate in-vivo neuroinflammation and white matter (WM) microstructural integrity in occupational manganese (Mn) exposure. METHODS: We assessed brain inflammation using Diffusion Basis Spectrum Imaging (DBSI) in 26 Mn-exposed welders, 17 Mn-exposed workers, and 26 non-exposed participants. Cumulative Mn exposure was estimated from work histories and the Unified Parkinson's Disease Rating Scale motor subsection 3 (UPDRS3) scores were completed by a movement specialist. Tract-based Spatial Statistics allowed for whole-brain voxel-wise WM analyses to compare WM DBSI-derived measures between the Mn-exposed and non-exposed groups. Exploratory grey matter region of interest (ROI) analyses examined the presence of similar alterations in the basal ganglia. We used voxelwise general linear modeling and linear regression to evaluate the association between cumulative Mn exposure, WM or basal ganglia DBSI metrics, and UPDRS3 scores, while adjusting for age. RESULTS: Mn-exposed welders had higher DBSI-derived restricted fraction (DBSI-RF), higher DBSI-derived nonrestricted fraction (DBSI-NRF), and lower DBSI-derived fiber fraction (DBSI-FF) in multiple WM tracts (all p < 0.05) in comparison to less-exposed workers and non-exposed participants. Basal ganglia ROI analyses revealed higher average caudate DBSI-NRF and DBSI-derived radial diffusion (DBSI-RD) values in Mn-exposed welders relative to non-exposed participants (p < 0.05). Caudate DBSI-NRF was also associated with greater cumulative Mn exposure and higher UPRDS3 scores. CONCLUSIONS: Mn-exposed welders demonstrate greater DBSI-derived indicators of neuroinflammation-related cellularity (DBSI-RF), greater extracellular edema (DBSI-NRF), and lower apparent axonal density (DBSI-FF) in multiple WM tracts suggesting a neuroinflammatory component in the pathophysiology of Mn neurotoxicity. Caudate DBSI-NRF was positively associated with both cumulative Mn exposure and clinical parkinsonism, indicating a possible dose-dependent effect on extracellular edema with associated motor effects.

Chemical constituents from stem bark and roots of Clausena anisata.

Phytochemical investigations on the stem bark and roots of the tropical shrub Clausena anisata led to the isolation and characterization three carbazole alkaloids: girinimbine, murrayamine-A and ekeberginine; two peptide derivatives: aurantiamide acetate and N-benzoyl-L-phenylalaninyl-N-benzoyl-L-phenylalaninate; and a mixture of two phytosterols: sitosterol and stigmasterol. The structures of these compounds were established by nuclear magnetic resonance (1H-NMR, 13C-NMR, COSY, HSQC, HMQC, HMBC and NOESY) spectroscopy and electrospray ionization mass spectrometry (MS).

Discrimination of commercial ibuprofen tablets by using a button sample holder and mid-infrared spectroscopy.

Infrared spectra obtained from commercial ibuprofen tablets are categorized by using principal component analysis. A stainless steel abrasive button is used to sample the coatings and interiors of ibuprofen tablets with three different formulations. Rubbing the button wire mesh surface across tablet surfaces removes material for analysis. Small fragments are retained within mesh void spaces and larger fragments are swept away prior to analyses. Infrared spectra for tablet coatings exhibit significant differences and can be used for identification. Tablet interior compositions consisting primarily of ibuprofen from different manufacturers are more distinguishable from pre-processed spectra than from spectrum second derivatives. The speed and sensitivity afforded by this methodology suggests that rapid detection of counterfeit pharmaceuticals based on mid-infrared spectroscopy measurements of microgram quantities of material removed with a button sample holder is feasible.

Variable temperature infrared spectroscopy with a button sample holder and thermoelectric heating/cooling.

An apparatus and methodology are described that can be used for variable temperature infrared spectroscopy measurements of liquids and powders. Neat samples placed in a stainless steel button sample holder are heated and cooled by applying voltage to a Peltier thermoelectric device. Temperatures between -50 and 100 °C can be attained with the apparatus. By moderately cooling volatile liquids to slow evaporation, the time available for spectrum measurements is substantially increased. Reproducible volatile liquid infrared spectra measured at 10 °C resemble thin film transmittance spectra. Various sample temperature profiles can be created by linking heating and cooling segments to form customized sequences. Measurements made during linearly varying temperature ramps reveal temperature-dependent variations that can be correlated with specific sample structure changes. Difference spectra derived from isothermal measurements made during temperature steps permit discrimination between reversible and irreversible sample changes. Results obtained by using different programmed heating/cooling profiles to monitor the temperature-dependent changes in pure kaolinite and montmorillonite containing 15% (w/w) acetylsalicylic acid are presented to demonstrate the operation and sensitivity of the apparatus and methodology.

Multiple path length mid-infrared spectra of liquids obtained by using a modified button sample holder.

A modified mesh button sample holder is characterized by examining the effects of multiple absorption path lengths on infrared spectra. Unlike transmission cells, which are designed to provide a single radiation penetrating distance, reflections by the button stainless-steel mesh result in a wide distribution of absorption path lengths. Consequently, detector signals represent radiation traversing different distances through the sample. Higher absorptivity bands are measured with shorter effective path lengths than lower absorptivity bands. Therefore, band intensity ratios in measured infrared spectra differ from their relative absorptivities. In addition, overlapping band shapes are retained in spectral regions that would be opaque in transmission cell measurements. Wavenumber dependent effective path lengths can be systematically and reproducibly varied by changing the volume of liquid added to the button reservoir. Although Beer's law is not applicable, absorbance is proportional to concentration when sample volumes are constant, facilitating quantitative analyses.