Methanol temperature calibration

The Evans method gives excellent results provided adequate care is taken. A most important requirement is that the solution temperature is measured reliably. One effective means of accomplishing this for H NMR is to insert into the NMR tube a capillary or additional coaxial sample of an NMR temperature calibrant solvent, usually methanol (158) or ethylene glycol (88). In this way the temperature measurement is made simultaneously with the susceptibility measurement. A second important factor is the variation of the solvent density with temperature (126). Because the density difference between the solvent and solution depends linearly on the concentration of the solute, it is only... 

NMR spectrometer, with variable temperature capability if possible several 10- or 25-mL volumetric flasks NMR tubes melting-point capillaries syringe torch for sealing capillaries ethylene glycol and methanol in NMR tubes for temperature calibrations. 

Table 2. Temperature calibrations for methanol and ethylene glycol...

Polymer Melting Points. The thermograms of the polymers were obtained under dry nitrogen on a Perkin-Elmer DSC-2 with aluminum pans containing 3-5 mg samples of polymers precipitated from methanol and dried as described. The instrument was temperature calibrated with indium and lead standards. For T determinations, separate samples were recorded at 10, 20, A(], 80, and 160°C/min heating rates. 

Many spectrometers are equipped with facilities to monitor and regulate the temperature within a probe head. Usually the sensor takes the form of a thermocouple whose tip is placed close to the sample in the gas flow used to provide temperature regulation. However, the readings provided by these systems may not reflect the true temperature of the sample unless they have been subject to appropriate calibration. One approach to such calibration is to measure a specific NMR parameter that has a known temperature dependence to provide a more direct reading of sample temperature. Whilst numerous possibilities have been proposed as reference materials [41], two have become accepted as the standard temperature calibration samples for solution spectroscopy. These are methanol for the range 175-310 K and 1,2-ethanediol (ethylene glycol) for 300-400 K. 

Figure 3.64. Temperature calibration chart for neat methanol. The shift difference A 5 (ppm) is measured between the CH3 and OH resonances.

It is evident that the procedure to be used with the Fischer reagent can be established only in terms of some standard reference method. Schroeder and Nair (31) adopted a calibration method which involved titration with the Fischer reagent after a prolonged extraction of water from the sample in methanol at room temperature. It was assumed that the extraction at low temperature, and the avoidance of an excess of the reagent, would minimize the extent of side reactions. Two procedures were used. 

Similarly, the m/z = 60 ion current signal was converted into the partial current for methanol oxidation to formic acid in a four-electron reaction (dash-dotted line in Fig. 13.3c for calibration, see Section 13.2). The resulting partial current of methanol oxidation to formic acid does not exceed about 10% of the methanol oxidation current. Obviously, the sum of both partial currents of methanol oxidation to CO2 and formic acid also does not reach the measured faradaic current. Their difference is plotted in Fig. 13.3c as a dotted line, after the PtO formation/reduction currents and pseudoca-pacitive contributions, as evident in the base CV of a Pt/Vulcan electrode (dotted line in Fig. 13.1a), were subtracted as well. Apparently, a signihcant fraction of the faradaic current is used for the formation of another methanol oxidation product, other than CO2 and formic acid. Since formaldehyde formation has been shown in methanol oxidation at ambient temperatures as well, parallel to CO2 and formic acid formation [Ota et al., 1984 Iwasita and Vielstich, 1986 Korzeniewski and ChUders, 1998 ChUders et al., 1999], we attribute this current difference to the partial current of methanol oxidation to formaldehyde. (Note that direct detection of formaldehyde by DBMS is not possible under these conditions, owing to its low volatility and interference with methanol-related mass peaks, as discussed previously [Jusys et al., 2003]). Assuming that formaldehyde is the only other methanol oxidation product in addition to CO2 and formic acid, we can quantitatively determine the partial currents of all three major products during methanol oxidation, which are otherwise not accessible. Similarly, subtraction of the partial current for formaldehyde oxidation to CO2 from the measured faradaic current for formaldehyde oxidation yields an additional current, which corresponds to the partial oxidation of formaldehyde to formic acid. The characteristics of the different Ci oxidation reactions are presented in more detail in the following sections. 

The assay was carried out using a Varian gas chromatograph (model 5000 LC) under the following experimental condition. The oven injector and flame ionization detector temperatures were 125°C and 225°C respectively. A Porapak column was used, the eluent was N2 at a flow rate of 30 ml/min and the injected volume 2 pi. Various concentrations of purified methylene chloride in purified methanol were injected (both solvents were distilled to discard any impurity which might interfere with the sensitive assay). Calibration curves were linear in the range 50-500 ppm (the limit of detection was 10 ppm). Methylene chloride detection in the microspheres was performed by dissolving various amounts (20-200 mg) of microspheres in 220 ml of purified methanol prior to the injection. 

Sultana et al. [88] developed a reversed-phase HPLC method for the simultaneous determination of omeprazole in Risek capsules. Omeprazole and the internal standard, diazepam, were separated by Shim-pack CLC-ODS (0.4 x 25 cm, 5 m) column. The mobile phase was methanol-water (80 20), pumped isocratically at ambient temperature. Analysis was run at a flow-rate of 1 ml/min at a detection wavelength of 302 nm. The method was specific and sensitive with a detection limit of 3.5 ng/ml at a signal-to-noise ratio of 4 1. The limit of quantification was set at 6.25 ng/ml. The calibration curve was linear over a concentration range of 6.25—1280 ng/ml. Precision and accuracy, demonstrated by within-day, between-day assay, and interoperator assays were lower than 10%. 

Hollander and co-workers [303—305] dealt with the problem in detail and developed a method for the isolation of hormones from blood, using Bio-Rad AG 50W-X2 (100—120 mesh) ion-exchange resin. Acylation with pivalic anhydride—methanol—triethylamine (20 1 1) was performed at 70°C for 10 min. The derivatives were purified with the aid of Amberlite IR-45 resin and benzene as a solvent. The dry residue was dissolved in 100 jul of benzene and 5 /il were injected directly on to a 60 cm X 4 mm I.D. column packed with 5% OV-1 on Chromosorb W HP after an isothermal period at 220°C for 12 min, the temperature was increased at 3°C/min up to 300°C. Calibration standards were injected immediately after the sample. Almost identical results were obtained for T3 by GC and radioimmunoassay [304], Other workers [306] applied the same procedure to the seeds and analysed pivalyl methyl esters of T3 and T4 on an 81 cm column packed with 3% of Dexsil on Chromosorb W HP at 305°C. 

Etofenprox calibration solution. Weigh in duplicate (to the nearest 0.1 mg) about 0.06 g of etofenprox standard (M and Mg) into two 50-ml stoppered volumetric flasks. Add 10ml of di-cyclohexyl phthalate internal standard solution, shake to dissolve the etofenprox and dilute to 50 ml with methanol/tetrahydrofuran (50 50, v/v) (solutions and Cg). Keep the solutions in a thermostat bath if room temperature varies by more than 2°C. 

Plpet 5 mL of the hexane solution containing 4-dodecylbenzenesulfonyl azides into a 100-mL volumetric flask and dilute to the mark with methanol. Pipet 5 mL of this solution Into a small stoppered flask. Add 2 mL of aqueous t N potassium hydroxide solution and heat at 75°C for 20 min. Allow to cool to room temperature, add 2 drops of 0.1% phenolphthalein solution, and 10 mL of 1.5% Na2S04. Shake, then transfer quantitatively to a 60-mL separatory funnel. Add 10 mL of butanol (or isoamyl alcohol) to the sample flask, shake, then transfer to the separatory funnel. Shake the funnel, let the layers separate, then remove the bottom (H2O) layer into a 100-mL volumetric flask, Add an additional 10 mL of 1.5% Na2SC>4 to the alcohol layer in the separatory funnel, shake, let the layers separate, then transfer the water layer to the volumetric flask. Neutralize the lined water layers to the phenolphthalein endpoint with 1 N hydrochloric acid, then immediately add 25 mL of Fe(NH4)(S04>2. Dilute to the mark with 1.5% Na2S04 solution, let stand 10 min, then read absorbance at 458 nm. Read azide concentration against the NaN3 calibration curve. 

ESMS was perfonned with a Fisons VG Quattro outfitted with a Hsons Electrospray Source. Samples were dissolved in 1.0 mL of 50% methanol-1% acetic acid, then diluted 1 10 with 50% acetonitrile-1.0 mM anmumium acetate to give 25 pmol/pL. A 10 pL aliquot of each sample was injected into a 10 pL/min stream of 50% acetonitrile-1.0 mM ammonium acetate. Data was processed using Fisons MassLynx Software. MALDI-MS was performed with a Vestec Benchtop lit linear dme-of-flight mass spectrometer, opmted in the linear mode with an N2 laser (337 nm). Samples were dissolved in 1.0 mL of 25% acetonitrile-0.1% TFA, then diluted 3 100 to give 5-10 pmol/pL. A 0.5 pL aliquot of each sample solution was added to 0.5 pL of matrix [a-cyano-4-hydroxycinnamic aci saturated solution in 50% acetonitrile-2% TFA]. Samples were dried at ambient temperature and pressure. Each spectrum was the sum of ion intensity from 10-50 larer pulses. Tlie mass axis was calibrated externally. 

To test the solubility of a solid, transfer an amount roughly estimated to be about 10 mg (the amount that forms a symmetrical mound on the end of a stainless steel spatula) into a 10 x 75-mm test tube and add about 0.25 mL of solvent from a calibrated dropper or pipette. Stir with a fire-polished stirring rod (4-mm), break up any lumps, and determine if the solid is readily soluble at room temperature. If the substance is readily soluble in methanol, ethanol, acetone, or acetic acid at room temperature, add a few drops of water from a wash bottle to see if a solid precipitates. If it does, heat the mixture, adjust the composition of the solvent pair to produce a hot solution saturated at the boiling point, let the solution stand undisturbed, and note the character of the crystals that form. If the substance fails to dissolve in a given solvent at room temperature, heat the suspension and see if solution occurs. If the solvent is flammable, heat the test tube on the steam bath or in a small beaker of water kept warm on the steam bath or a hot plate. If the solid completely dissolves, it can be declared readily soluble in the hot solvent if some but not all dissolves, it is said to be moderately soluble, and further small amounts of solvent should then be added until solution is complete. When a substance has been dissolved in hot solvent, cool the solution by holding the flask under the tap and, if necessary, induce crystallization by rubbing the walls of the tube with a stirring rod to make sure that the concentration permits crystallization. Then reheat to dissolve the solid, let the solution stand undisturbed, and inspect the character of the ultimate crystals. 

For materials applications, the chemical shifts of methanol and ethylene glycol can be monitored in the liquid state to follow temperature [Hawl]. The most sensitive ehemical shift is the Co resonance of aqueous Co(CN)e with a sensitivity of 0.05 K at 7 T and 0.2 K at 2T [Dorl]. Furthermore, dibromomethane dissolved in a liquid crystal is a temperature sensitive NMR compound [Hed 1 ], and known phase-transition temperatures can be exploited to calibrate the temperature control unit [Hawl J. In temperature imaging of fluids, temperature can be determined from the temperature dependence of the selfdiffusion coefficient but convective motion may arise in temperature gradients [Hedl]. In the solid state, the longitudinal relaxation time of quadrupolar nuclei like Br is a temperature sensitive parameter [Suil, Sui2]. In elastomers, both T2 and Ti depend on temperature (Fig. 7.1.13). In filled SBR, T2 is the more sensitive parameter with a temperature coefficient of about 30 xs/K [Haul]. 

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