Nonvolatile residue (NVR) is contamination that is not readily evaporated and therefore consists primarily of inorganics and particulates. It is used as a measure of solvent purity as well as an indication of residue or other contamination on surfaces.
NVR measurement is a widely used technique that can be time-consuming, cumbersome to perform, and prone to result variability. Classically, NVR determination involves evaporating large volumes of the appropriate organic solvent by boiling, then determining the residue gravimetrically using a sensitive balance and carefully-tared weighing vessels. NVR levels of 0.1 to 100 ppm are typical, so for accurately analyzing relatively pure solvents, as much as 1 liter of solvent may be required. Boiling large volumes of organic solvents involves safety, handling, and environmental issues. In addition, particles and other sources of contamination can be inadvertently introduced from the air and from glassware.
In response, modifications of NVR techniques and NVR solvents have been developed. For example, two “micro” or portable NVR processes have been introduced which show promise in taking NVR determination out of the analytical lab and into the realm of day to day process monitoring. Both require orders of magnitude less solvent than does classic NVR; one offers the potential for at least partial molecular identification. In that large volumes of extraction solvent may be inherently required for identification of contamination on surfaces, one would expect the technique to be particularly valuable for monitoring water or solvent quality. In addition, there have been a number of attempts to modify the solvent used for NVR.
One micro-gravimetric technique, which arose out of research in biological sensors, uses a very thin, vibrating silicon membrane, an application of microelectromechanical systems (MEMS). Developers realized that the silicon membrane without any biological coating could be used to detect mass as a decrease in resonant frequency. This frequency decrease is proportional to the deposited mass. Whereas classic methods may require 1 liter of solvent and hours of evaporation time, the silicon sensor requires 10 to 25 micrograms of solvent. Organic solvents or water can be used. The NVR readout is direct, without calibration curves.
Another approach uses an adaptation of Fourier Transform Infrared Spectroscopy (FTIR) to estimate NVR and to provide some degree of contaminant identification. This technique involves evaporating between 100 µl and approximately 4 mL of solvent in a cup that is the sample holder for FTIR determination. While the currently available evaporation device is designed for use with a specific portable FTIR, another version, which can be used with commercial FTIR equipment, is also being introduced. With FTIR of a given material, a spectrum with peaks at characteristic wavelengths is obtained. In this approach, NVR is determined by measuring peak-height relative to predetermined calibration curves. While additional initial calibration may be required for this technique, FTIR has the advantage of at least partial contaminant identification.
Steps are also being taken to eliminate the use of classic organic solvents in surface NVR determination. Where NVR is used to detect surface contamination, depending on the surface area or part configuration, appreciable organic solvent may be required for extraction. Typically, classic chlorinated organic solvents such as perchloroethylene have been used. Given the safety and environmental issues involved, some attempts have been made to do extractions using less aggressive solvents, or even using water. Such modifications require an understanding of the nature of expected contamination and of the substrate. The extractions for NVR determination on a solid surface are in fact cleaning or decontamination processes, and a non-aggressive solvent may not adequately remove the soils of interest. Water, however, is likely to be more effective in removing inorganic contamination than is perchloroethylene and less effective in removing organic contamination. Particularly if there are no issues with substrate compatibility or with corrosion, water can be an effective, environmentally preferred media for initial extraction; depending on the mixture of contaminants, however, results may change.
In all of these instances, new protocols will need to be developed. For micro-techniques, this could involve calibration or a re-defining of evaporation techniques, depending on the solvent used. Using FTIR allows greater definition of contamination, and, with this increased specificity, modifies the nature of NVR determination. In cases where new extracting solvents are used, the set of detected contaminants may change.
