Amine Blush Testing: Elusive Mystery or Good Old-Fashioned Organic Chemistry?

Author: Melissa Tuchscherer

Another method developed and being utilized for detecting amine blush is based on the presence of the carbamate compounds and not on physical phenomena such as pH. This method has field and lab applications, is quick and easy to use, and does not require expensive pieces of laboratory equipment. The medical field and Homeland Security have several different quick methods to test for carbamates in case of emergency, such as pesticide poisoning or a terror threat involving chemical agents. In these instances, there is not an abundance of time to allow for the use of the GC/MS and FTIR equipment; lost time equals lost lives. Instead, these agencies use testing methods that involve a biological response or inhibition of the enzyme cholinesterase to the carbamate class of compounds. The inhibition of cholinesterase takes place as a result of the phosphorylation by organophosphates or carbamylation by carbamates of the serine residue in the site of the enzyme.

The Elcometer 139 Amine Blush Screen Test Kit(s) testing method was developed for the coatings industry by adapting these test methods based on the biological response of the enzyme to carbamates. Cholinesterase normally reacts with acetylthiocholine (ATC), resulting in the cleavage of acetylthiocholine and yielding acetic acid and thiocholine (Figure 2).

This reaction may easily be observed using Ellman’s reagent. Thiocholine reacts with the chromogen (Ellman’s reagent, also called coloring agent) and turns yellow. If the cholinesterase is exposed to a sample containing a carbamate compound, the carbamate aligns itself with the cholinesterase due to the different polarities at specific active sites within the cholinesterase attracting the oppositely charged sites of the carbamate salt (Figure 3a). The spatial arrangement assists in the reaction of the serine OH from the cholinesterase active site with the carbamate blocking the cholinesterase activity (Figure 3b), thereby preventing the enzyme from reacting with acetylcholine. This phenomenon is referred to as cholinesterase inhibition.

When ATC is added to the sample test solution, the amount of cholinesterase available to cleave the ATC is reduced by the carbamate compound. This results in less thiocholine being available to react with the chromogen, thus yielding a less intense yellow color. As the amount of carbamate compound present in a sample increases, the amount of cholinesterase available decreases and the yellow color decreases.

Field sampling the amine blush using the testing method used in the Elcometer test kit is as simple as getting the amine blush into a solution through either wiping the suspect surface with a swab moistened with IPA or by placing a chip into the test tube supplied. A color control standard is prepared in the same way as the test sample but with no carbamate compound introduced into it, and this control is used for color comparison. The sample solution, prepared with the carbamate compound present or suspected, is tested and the color is compared to the control standard. If all of the available cholinesterase is depleted (inhibited), a colorless solution is observed in the test sample vial, while the control sample vial results in a yellow solution. An alternate method of conducting this test (generally in a lab environment) uses a field-portable spectrophotometer as a quantitative means to evaluate color differences.

Several benefits of this testing kit are that it requires only a few chemicals, it can be performed in a test tube or small vial, it can be conducted onsite in a short period of time by most inspectors or applicators, and the cost of conducting this test is relatively inexpensive compared to the other methods discussed. The testing method of the Elcometer 139 Amine Blush Screen Test Kit(s) is specific to, and qualitatively identifies the presence of, carbamate compounds, thus resulting in the elimination of the prospect of obtaining false positive or false negative results associated with the flawed process of measuring for pH values.

In actuality, one of the few false positives that this test kit could potentially be subject to would be sources of carbamate compounds deposited through means other than the amine blush. Carbamate-based pesticides will result in a carbamate compound being detected, and if the user was not aware of the potential presence of these carbamates, a false positive for amine blush presence would be indicated. It is therefore suggested that the user know their surroundings and identify potential variables or sources of contamination before assigning the origin of the carbamate. However, chemistry is chemistry and contamination is contamination, and even if the source of the carbamate was not the amine, good painting practice would dictate that the carbamate be remediated or the coating may experience the same recoating or performance issues as if it had come from the amine.

As alluded to earlier, a challenge for the coatings industry for this type of test method as compared to the medical, agricultural and Homeland Security personnel is that these entities also detect for organophosphates, while the coatings industry does not. This poses a challenge for the coatings industry, as the potential exists for a false positive result being returned due to organophosphates that may be present on coatings from other outside sources such as agricultural overspray when treating crops. The Elcometer 139 Amine Blush Screen Test Kit(s) addresses this issue by not oxidizing the organophosphates, which results in this kit not detecting organophosphates and thereby lowering the risk of a false positive result being reported.