Sampling small volumes of ambient ammonia using a miniaturized gas sampler

The development of a gas sampler for a miniaturized ambient ammonia detector is described. A micromachined channel system is realized in glass and silicon using powder blasting and anodic bonding. The analyte gas is directly mixed with purified water, dissolving the ammonia that will dissociate into ammonium ions. Carrier gas bubbles are subsequently removed from the liquid stream through a venting hole sealed with a microporous water repellent PTFE membrane. A flow restrictor is placed at the outlet of the sampler to create a small overpressure underneath the membrane, enabling the gas to leave through the membrane. Experiments with a gas flow of 1 ml min(-1), containing ammonia concentrations ranging from 9.4 ppm to 0.6 ppm in a nitrogen carrier flow have been carried out, at a water flow of 20 microl min(-1). The ammonium concentration in the sample solution is measured with an electrolyte conductivity detector. The measured values correspond with the concentration calculated from the initial ammonia concentration in the analyte gas, the fifty times concentration enhancement due to the gas-liquid volume difference and the theoretical dissociation equilibrium as a function of the resulting pH.     

Intermolecular interaction studies using small volumes

We present the use of 1-mm room-temperature probe technology to perform intermolecular interaction studies using chemical shift perturbation methods and saturation transfer difference (STD) spectroscopy using small sample volumes. The use of a small sample volume (5-10 μl) allows for an alternative titration protocol where individual samples are prepared for each titration point, rather than the usual protocol used for a 5-mm probe setup where the ligand is added consecutively to the solution containing the protein or host of interest. This allows for considerable economy in the consumption and cost of the protein and ligand amounts required for interaction studies. For titration experiments, the use of the 1-mm setup consumes less than 10% of the ligand amount required using a 5-mm setup. This is especially significant when complex ligands that are only available in limited quantities, typically because they are obtained from natural sources or through elaborate synthesis efforts, need to be investigated. While the use of smaller volumes does increase the measuring time, we demonstrate that the use of commercial small volume probes allows the study of interactions that would otherwise be impossible to address by NMR.     

Double transfer printing of small volumes of liquids

We report here a technique to print small volumes of liquid on a hydrophobic substrate. This process is based on the control of the critical parameters that govern a quasi-equilibrium liquid transfer process from one surface to another. We present a qualitative model that describes the physics of a transfer printing process between hydrophobic surfaces, and we use the parameters outlined in this model to manipulate the amount of liquid transferred between surfaces. We demonstrate the printing of discrete, small volumes (approximately 70 fL) of different liquid inks on a polymer substrate starting with volumes that are 8 orders of magnitude larger (a droplet of approximately 10 microL) in a simple two-step procedure.     

Fiber-optic pH detection in small volumes of biosamples

Determining the pH values of microscopic plant samples may help to explain complex processes in plants, so it is an area of interest to botanists. Fiber-optic probes with small dimensions can be used for this purpose. This paper deals with the fiber-optic detection of the pH values of droplets of plant xylem exudate based on ratiometric fluorescence intensity measurements with an internal reference. For this purpose, novel V-taper sensing probes with a minimum diameter of around 8 μm were prepared that enable the delivery of fluorescence signal from the detection site on the taper tip to the detector. The taper tips were coated with pH-sensitive transducer (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt; HPTS) and a reference [dichlorotris-(1,10-phenanthroline) ruthenium (II) hydrate (Ru-phen dichloride)] immobilized in a xerogel layer of propyltriethoxysilane and (3-glycidoxy)propyl trimethoxysilane. The prepared probes were sensitive to pH values mainly in the range from 6.0 to 9.0. In the pH range 6–9, the results were limited by measurement errors of about 0.2 pH units, and in the pH range 5–6 by measurement errors of about 0.5 pH units. Using the developed V-taper sensing probes, the pH values of in vivo and in vitro samples of small volumes (∼6 μl) of exudate were measured. The results were validated by comparison with conventional electrochemical pH measurements.     

Micro flow calorimeter for thermoelectrical detection of heat of reaction in small volumes

A micro chip element with an integrated flow channel and thin film thermopiles was developed in order to realize a micro calorimeter for small volumes in a flow arrangement. The flow channel consists of two inlets, a mixing region, a measurement region, and one outlet. Thermopiles of BiSb/Sb thermocouples were used as thermal transducers because of their extraordinary high thermopower. A thin film heater of a NiCr alloy was integrated in order to yield the possibility of internal electrical calibration. In addition, this heater can be used for thermal modulation of the whole measurement system. The calorimeter was prepared using vacuum evaporation, PECVD and magnetron sputtering for thin film deposition and photolithography as well as chemical wet etching for microstructuring. The function of the device was tested to neutralize 400 nmole, 100 nmole and 10 nmole NaOH by H₂SO₄.     

A novel device for the accurate dispensing of small volumes of liquid samples

A sample dispenser with electronic control of delivered volume has been designed and evaluated. With this device, liquid samples are dispensed in the form of uniform droplets less than 4 nl in volume; total volumes as small as 40 nl can be delivered with 1.5% precision. Important features of the microdroplet formation process have been studied and the effect of sample solution concentration and viscosity have been examined. The dispenser is expected to find application in several areas, including electrothermal atomization atomic absorption spectrometry, automated titrimetry, and high-precision, low-volume solution sampling.     

New valve switching modulator for comprehensive two-dimensional gas chromatography

This paper describes a new design for a valve switching modulation system developed for comprehensive two-dimensional gas chromatography (GC x GC). This system is based on the differential flow and is different from the earlier reported in the literature. It has several advantages and constructed of inexpensive, commercially available parts that are easily installed on most gas chromatographs. Operation is easy and robust and requires no external coolants. This new design allows GC x GC analysis to be conducted in any laboratory environment capable of supporting conventional GC and imposes no additional consumable costs. The system is flexible and permits the use of a wide range of stationary phase combinations. The system has been successfully tested for complete material transfer and is suitable for quantitative analysis. The flexibility of this system is demonstrated using several test mixtures that show its extended application in petroleum, biochemistry, and environmental studies.     

The dosage of small volumes for chromatographic quantifications using a drop-on-demand dispenser system

A commercially available piezo-driven drop-on-demand dispenser was tested for its suitability for the preparation of analytical calibration standards and in a standard addition approach prior to quantitative ultra performance liquid chromatography (UPLC) analysis of homoserines. The reproducibility of the drop-on-demand dosing system was tested and the verification of the droplet volume was performed by preparing a series of 1.0 mg/L caffeine standard solutions from a 1,000.0 mg/L stock solution and analysis of the concentrations obtained by UPLC. The reproducibility was better than 1% relative standard deviation from measurement to measurement and the highest was 1.6% from day to day. The results were compared with the conventional way of generating standard solutions (pipetting). A gravimetric method and a photography-based method for the determination of the average single droplet volume were compared and found to be in very good agreement. The system was employed for the quantification of N-decanoyl homoserine by standard addition in bacterial culture supernatants containing this analyte. The agreement with conventional quantification techniques was high. The paper shows the feasibility of the approach with advantages in low sample and solvent volume consumption and very good reproducibility and reliability combined with easy usage. Figure Ejected droplet, 60 μs after application of the pulse     

The use of SEM for multielement analysis in small volumes and low concentrations

Summary A preparation method is described for analysing size classified rain drops by SEM (scanning electron microscopy), equipped with an energy-dispersive detection system. The concentration is determined by addition of an internal standard. This allows the quantitative analysis of samples with a volume of 2 l and a concentration of 1 g/l. The time of measurement is 100 s.Dedicated to Professor Dr. V. Krivan on the occasion of his 60th birthday     

Nanoliter deposition unit for pipetting droplets of small volumes for Total Reflection X-ray Fluorescence applications

A nanoliter droplet deposition unit was developed and characterized for application of sample preparation in TXRF. The droplets produced on quartz reflectors as well as on wafers show a good reproducibility, also the accuracy of the pipetted volume could be proved by a quantitative TXRF analysis using an external standard. The samples were found to be independent of rotation of the sample carrier. Angle scans showed droplet residue behavior, and the fluorescence signal is relatively invariant of the angle of incidence below the critical angle, which is useful for producing standards for external calibration for semiconductor surface contamination measurements by TXRF. Further it could be demonstrated that the nanoliter deposition unit is perfectly able to produce patterns of samples for applications like the quantification of aerosols collected by impactors.     

Simple equipment for titration,measurement and mixing of very small volumes of solution

Summary A simple ultramicrotitration equipment is described which permits measurement of from one to tens ofl of solutions, mixing of solutions, and certain titrations. The equipment can be used to prepare specimens for radiometric titrations, polarography, chromatography, and paper electrophoresis.

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Zusammenfassung Ein Ultramikrotitriergerät wurde beschrieben. Es dient zur Messung von einem bis zu mehreren zehn Mikroliter Lösung, zum Mischen von Lösungen und für gewisse Titrationen. Es kann zur Herstellung von Proben für radiometrische Titrationen, zur Polarographie, Chromatographie und Papierelektrophorese Verwendung finden.

     

Formation of crystal nuclei near critical supersaturation in small volumes

This work deals with the nucleation of crystals in confined systems in response to the recent high interest in research on crystallization in emulsion and microemulsion droplets. In these confined systems, crystallization often occurs at high supercooling; thus, nucleation determines the overall crystallization process. A decrease in the volume of the confined mother phase leads to the higher supercooling needed for the phase transition. We have numerically solved kinetic equations in order to determine the conditions under which the first crystal nuclei are formed by homogeneous and heterogeneous nucleation from supercooled melt and supersaturated solution, depending on the volume of the mother phase. Supersaturation (or supercooling) increases with decreasing volume of the mother phase. The nucleation barrier depends linearly on the logarithm of volume of the mother phase in all cases under consideration, as follows from the numerical solution of kinetic equations.     

HPLC‐fluorescence assay for measuring mosapride in small volumes of rat plasma

A simple and sensitive HPLC‐fluorescence assay was developed for the determination of a gastroprokinetic agent mosapride in small volumes of rat plasma. Samples (50 μL) were treated with 200 μL of the internal standard solution (cisapride, 0.1 μg/mL in acetonitrile). Chromatographic separation was achieved on a C₁₈ column by gradient elution with the mobile phase of acetonitrile‐water containing 20 mM potassium dihydrogen phosphate, at a flow rate of 1 mL/min. Fluorescence was measured with excitation and emission set at 315 and 354 nm, respectively. The retention time was about 16 min for cisapride and 20 min for mosapride. No endogenous substances were found to interfere. The calibration curve was linear from 0.015 to 10 μg/mL. The lower limit of quantification was 0.015 μg/mL. The intra‐ and inter‐day precision expressed as relative standard deviation did not exceed 7.7%, and the accuracy was within 4.7% deviation of the nominal concentration. The method was used successfully to investigate the disposition kinetics of mosapride in rats. Copyright © 2009 John Wiley & Sons, Ltd.     

Analyte preconcentration and separation from small volumes by electrodeposition for electrothermal atomic absorption spectroscopy

Electrodeposition of Pb from 50 μl volumes of 0.1M KCl solution was studied by electrolysis at uncontrolled potentials of 4.0–6.0 V on a pyrolytic graphite platform cathode. Deposition efficiency was evaluated as a function of time by ASV measurements on aliquots of the electrolysed solution. Quantitative separation of the analyte from the matrix was achieved in a relatively short time, aided by convective stirring of the sample achieved through gas evolution. Thus, the feasibility of rapid electrodeposition directly in a pyrolytic graphite-coated furnace has been demonstrated, allowing construction of an automated electrodeposition-electrothermal atomic absorption spectroscopic system.