Determining NVR: New Approaches
Barbara Kanegsberg & Mantosh Chawla, April 2002
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.
Barbara Kanegsberg at BFK Solutions, 16924 Livorno Dr., Pacific Palisades, CA
90272, 310-459-3614; barbara@bfksolutions.com;
contact Mantosh Chawla atPhotoemission Tech., (PET), 3255 Grande Vista Dr., Newbury
Park, CA, 91320; 805-499-7667
