The concentrations of the solutions to be prepared to reach the content cited previously were determined through the carbon percentage calculated theoretically (from the empirical formulae of the different compounds) for each of the solutions:

In order to be free of a possible influence of deposit size on the final quantity of residue after evaporation, we chose to standardize the volume deposited for each type of solution and for each concentration studied. Consequently, it was necessary to prepare 3 deposit solutions for each type of alcoholic solution studied (one solution per concentration level).

The theoretical concentrations desired for the deposit solutions were as follows:

In order to avoid any partial evaporation, we chose to select a volume of alcoholic solution rather than carrying out a weighing operation. Volume transfer allows the solution to be exposed for a much shorter time than during a weighing operation. The deposit solutions were thus prepared as described below:

Sample preparation consisted in depositing 100μL of each deposit solution prepared beforehand, then in adding ultrapure water until the final volume of 40mL was reached (immediately after the deposit or after evaporation of the deposit), the corresponding concentrations for the solutions prepared in this way were, consequently, the following:

These preparations had a dual aim: on the one hand to check that the TOC meter was capable of analyzing and supplying the results projected by the theoretical calculations; and the other hand, to also obtain a reference value for the interpretation of evaporated samples. The quantity of carbon so determined by the TOC meter corresponds to the quantity of carbon present before evaporation. It was used to calculate the efficiency of the evaporation of carbon-containing products.

The reference samples were prepared in several steps:

Step 1: Deposition of 100 μL of ethanol or isopropyl alcohol deposit solution prepared beforehand at a concentration that allowed the desired levels to be achieved, directly in a vial for TOC analyses.

Step 2: Immediate addition of 39.9 mL ultrapure water to reach a final volume of 40mL. As the reference samples were used to determine the quantity of carbon present in the quantity of alcoholic solution deposited, water was added to the TOC vials which were then closed, immediately after depositing the product in order to avoid any partial evaporation. The immediate closure of the vials also avoided any partial inter-sample contamination. For the same reason, each sample of each type of alcohol was prepared and processed independently from the others, so as to never have several TOC vials open simultaneously.

The analytical technique used for this study was non-specific (TOC meter). It was therefore also necessary to prepare sample blanks to determine the quantity of carbon contributed by the equipment and the ultrapure water and thus to be able to deduct this value from the different samples.

Step 3: For the preparation of solvent blanks, 40mL of the ultrapure water used in step 2 of the preparation of reference samples was introduced directly into a TOC vial. For each test series, 3 vials of blanks were prepared in order to obtain a more robust TOC content value. The mean of 3 blanks was used to deduct TOC content from the reference samples.

The nature and quantity of each reference sample prepared, per series, are summarized below:

3.3 Preparation of evaporated samples

The evaporated samples were the subject of the study. These were samples of alcoholic solutions left deliberately to evaporate in order to study and detect potential carbon-containing residues, once the surface was dry. They were prepared in a manner strictly identical to reference samples and from the same deposit solutions for each test series.

The preparations for each of the evaporated samples were as follows:

Step 1: Deposition of 100 μL of ethanol or isopropyl alcohol deposit solution prepared beforehand at a concentration that allowed the desired levels to be achieved, directly in a vial for TOC analyses.

Analysis of samples in the TOC meter is sensitive to contamination (non-specific method), this is why evaporation of the 100μL samples was conducted naturally, without heating or airflow. The TOC vials were placed open under an extractor hood to assist evaporation without inputting external contamination.

Despite all these precautions, and in order to take account of potential carbon contamination appearing during sample evaporation, sample blanks were also prepared and left to evaporate beside the test samples.

Step 2: Deposition of 100 μL of ultrapure water used to prepare the alcoholic deposit solutions, used for the preparation of samples.

Step 3: The vials prepared in steps 1 and 2 were left until the deposits evaporated completely

Step 4: After total evaporation, 40mL of water was added to the dry vials to obtain a volume equivalent to that of the reference samples and so allow the results obtained to be compared. The same quantity of the same ultrapure water was added to the evaporated blanks to determine the TOC content of the ultrapure water added to the evaporated samples, as well as the potential quantity of carbon contributed by the evaporation step. For each test series, 3 vials of evaporated blanks were prepared to obtain a more robust TOC content value. The mean of the 3 results was used to deduct TOC content from the evaporated samples.