Storage and Use Information Guide of NMR Solvents
Storage and Handling
Please note that some packaging sizes of some solvents may require special handling not given below. The bottle or ampoule packaging information should be reviewed for further instructions.
Acetic Acid-d4 / Acetone-d6 / Benzene-d6 / Cyclohexane-d12 / Deuterium Oxide / N, N-Dimethylformamide-d7 / Dimethyl Sulfoxide-d7 / 1,4-Dioxane-d8 (p-Dioxane) / Ethanol-d6 / Methanol-d4 / Methylene Chloride-d2 / Pyridine-d5 / 1,1,2,2 Tetrachloroethane-d2 / Toluene-d8 / Trifluoroacetic Acid-d / 2,2,2-Trifluoroethanol-d3.
Store at room temperature away from light and moisture. The above products are stable if stored under recommended conditions.
Acetonitrile-d3
Store at room temperature away from light and moisture. This product is stable for one year after receipt of order if stored under above conditions (unopened). After one year, the solvent should be re-analyzed for chemical purity before use.
Chloroform-d / Tetrahydrofuran-d8
Store refrigerated (-5℃ to 5℃) away from light and moisture. These products are stable for six months after receipt of order if stored under above conditions (unopened). After six months the solvent should be re-analyzed for chemical purity before use.
Deuterium Exchange of Labile Protons in Deuterated Solvents Containing Residual D2O
Some deuterated solvents are prepared by catalytic exchange of protonated solvent with deuterium oxide and carefully purified by distillation. Residual water (H2O in equilibrium exchange with D2O) is kept to a minimum of 20-200 ppm, the higher value corresponds to the amount in the more hygroscopic solvents. The labile deuterons (and protons) of water are available to exchange with labile protons in the chemist’s sample and can result in inaccurate integration ratios. The example (Figure 1) below shows that just 100 ppm of D2O can cause problems when studying dilute solutions of analytes. A significant decrease in the integral of 1 labile proton may be observed in a sample containing 5 mg organic compound, MW~200, dissolved in 1 g DMSO-d6 containing 100 ppm D2O (Figure 2). The problem becomes worse as the molecular weight of the analyte increases.
(X for residual solvent; * for residual water)
Figure 2. 1H-NMR spectrum of 5.3 mg of 2,6-di-tert-butyl-4- methylphenol in DMSO-d6 with 100 ppm D2O added. Note the reduced ratio of the phenolic proton 18:3:2:0.47 (t-butyl: methyl: ring-H: -OH). Note that the HOH and HOD peaks are separated in the spectrum.
Solution
Water (as H2O, HOD or D2O) can be minimized by adding molecular sieves to the solvent, agitating the mixture and allowing it to stand for a few hours. The water content may be reduced to about 10-20 ppm in this manner. If exchange still causes a problem, it is recommended to use a less hygroscopic solvent, such as chloroform, methylene chloride or acetonitrile.
Use and Handling
ALD has implemented extensive quality control protocols for the evaluation of chemical and isotopic purities of our solvents. ALD understands that the increase in sensitivity and resolution of today’s high field NMR instruments requires solvents with the highest chemical purity as well as high isotopic enrichment. Each lot of our NMR solvents receives thorough quality control testing before being released for shipment. All ampoules and bottles are clearly marked with both a production and a packaging lot number for easy tracking in the unlikely event that a problem should occur.
Water Peaks
Water contamination is a common problem for deuterated NMR solvents. There are several things that can be done to minimize/eliminate water peaks.
• Consider using single-use ampoules. Many of our solvents are available in single-use breakseal ampoules ranging in size from 0.25 mL to 3 mL.
• Handle solvents in a dry atmosphere.
• Dry NMR tubes and pipettes used for sample preparation overnight in an oven and cool them in a dessicator prior to use.
• Precondition an NMR tube by rinsing it with D2O. Remove residual D2O by rinsing first with methanol-D4 or acetone-D6 and then with the solvent of choice. This process will not remove water, but it will exchange the protons for deuterium and minimize the water peak.
“100%” D2O
To avoid loss of enrichment due to exchange with ambient moisture, “100%” D2O stored in a serum bottle should be sampled with a syringe that has been preflushed with dry nitrogen. Additionally, a volume of dry nitrogen equal to the amount of D2O being removed should be injected into the serum bottle prior to withdrawing D2O.
TMS Evaporation
When stored at room temperature (unless noted below) and properly capped, solvents containing TMS should not suffer from TMS evaporation. However, upon extended storage of these solutions, some loss of TMS may occur.
Storage
All serum bottles should be stored upright in the refrigerator; we do not recommend freezing. We also recommend that chloroform, diethyl ether, diglyme, tetrahydrofuran, and TMS be stored in the refrigerator.
Deuterated Chloroform as an Example
The deuterated chloroform produced at ALD is of the highest chemical purity. Over time, chloroform will decompose regardless of the storage container or conditions. Over many months of storage at room temperature (for example, in a stockroom), deuterated chloroform can become acidic. However, decomposition is minimized if bottles are stored refrigerated in the dark.
ALD takes several precautions during production and packaging of chloroform-d to further minimize decomposition. Exposure to oxygen is minimized by using an argon atmosphere during production and packaging. Amber bottles are used to protect the product from light. Finally, silver foil is added to the solvent to act as a radical scavenger, which helps to stabilize the material over time.
Quality Control of Deuterated Chloroform
To ensure the highest quality, ALD routinely tests each batch of solvent for chemical and isotopic purity. The chemical purity is monitored during production and packaged using 1H NMR, GC, Karl-Fischer titration for total water content, and other wet-chemical methods for acidity and various impurities.
Proper Storage and Use of Deuterated Chloroform
Unopened bottles of chloroform-d should be refrigerated (-5°C to 5°C) to maximize shelf life. Moisture and oxygen will be introduced to the solvent following initial use through air entering the bottle upon opening. Decomposition can follow which results in the deuterated chloroform becoming acidic. The acidity can be easily tested using the following method.
Testing Deuterated Chloroform Acidity
A 1 mL aliquot of the solvent is added to a test tube containing 1 mL of distilled water (pH 5.0-7.0) and 2 drops of bromothymol blue (0.04% w/v). The color is compared to a 2 mL blank of distilled water (pH 5.0-7.0). If the sample solution is discolored (yellow) relative to the blank (blue-green), the deuterated chloroform is acidic.
Samples of deuterated chloroform that have become acidic can be easily neutralized using the following procedure:
• Place 3-5 grams of a 5Å molecular sieves into a 50 or 100 g bottle of the solvent.
• Swirl slightly and allow to stand overnight. Excess water and traces of acidity will be removed. This is also the preferred way to store chloroform-d bottles once they have been opened, as it will keep the solvent dry and stable over time.
• Maintain an inert atmosphere (argon or nitrogen) in the bottle.
• Small dust or powder particles may break off from the molecular sieves. However, these particles can be removed simply by filtering the quantity of deuterated chloroform needed for an NMR sample through a small plug of glass wool or cotton in a glass pipette.
Special Applications that Require Ultra-Dry and Acid-Free Deuterated Chloroform
For applications involving highly acid-sensitive or moisture-sensitive compounds, deuterated chloroform can be purified further prior to use. Solvents treated in the following manner will be exceptionally dry and acid free.
• Place a glass wool plug into a disposable glass pipette (~7 mm diameter).
• Add dry alumina powder into the pipette to a height of 3-4 cm.
• Pass the solvent through the small alumina bed into the sample container of the product to be analyzed.
• Analyze the sample as soon as possible.
This procedure will ensure the deuterated chloroform is dry and free of trace amounts of acid prior to contact with the sample. Note that the chloroform will react with basic compounds, such as alkaloids or amines. If the product is to be recovered, this should take place as soon as possible to minimize possible reaction.