Detecting Everything: Extraction, Separation, Detection, Discrimination
Barbara Kanegsberg and Ed Kanegsberg, April 2005
Comprehensive, Non-specific Methods
Extractive gravimetric methods, for example, are comprehensive but very
non-specific techniques to determine contamination by weighing. Examples
include the newly proposed ASTM method for detecting contamination levels
in permanent implantable metal devices and most non-volatile residue
(NVR) methods. Weighing the residue detects almost everything that
can be extracted
from a component, including, potentially, some of the component substrate
as well.
These methods do not detect volatile contaminants (important where adsorption to the component is an issue). Like cleaning, extractive gravimetric techniques are dependent on the specifics of extraction including time, temperature, cleaning force, and the solvent used. Further, the technique of evaporating the mixture extracted into the solvent prior to analysis can influence the weight of the residue. Typically, heat is used for evaporation. With high-boiling solvents, high heat can result in a caramelized mixture of solvent residue and contaminant, resulting in a falsely elevated contamination level.
Gravimetric methods are comprehensive in that an array of organic and
inorganic contaminants, thin film, and particulate, are detected.
With appropriate
and consistent techniques they can be accurate and show high inter-test
precision. While gravimetric techniques do not elucidate the specific
contaminant, they are useful as a relatively rapid early warning
technique. Similarly,
OSEE, a non-extractive technique for overall surface residue can
detect essentially everything on a relatively flat surface, but there
is no
discrimination or identification. Contact angle determination can
also be a good early
warning signal of contamination. For all such methods, the technique
and limitations must be well understood [1].
Separation and Detection
The past three columns were devoted to an overview of chromatographic
methods. Current chromatography systems are “black boxes” consisting
of an injection port, a column, and a detection system. It is
important to keep in mind that the power of chromatography is
as a separation
technique. Early forms of chromatography were also detection
methods in that one could
visually see many of the separated bands on the column. While
some identification could be made based on elution time or position
on the
column, in order
to definitively identify chemical species, appropriate detection
techniques are required.
Detectors are quite varied; there is no “universal” detector.
A typical particle counting system discriminates by size, but
does not characterize the particle. For characterization, other
detection
methods
such as FTIR are needed. Then there is the issue of how specific
a detector is and how quantifiable the results are. For example,
FTIR
identifies contaminants
by circumstantial evidence. However, limited identification
and quantification can be achieved with FTIR with careful data analysis.
FTIR can help
track the usual suspects, without the cost and time associated
with more definitive
techniques.
Limited, Discriminating, or Standard Methods
We should also remember that some detectors are pre-set to
detect a subset of chemical species. This is particularly
true for field
testing techniques.
There is the temptation to use a detector used by the facilities
department to detect refrigerant leaks to test for other
volatile contaminants;
this may not provide accurate information.
Standard methods provide a common basis of understanding
and potentially lower undesirable inter-laboratory variability;
a test performed
in Cleveland can be readily compared with one obtained
in San
Francisco. The problem
is that there is a tendency to look at such standardized
methods as universally applicable, as absolute, as infallible.
However,
standard methods may
not be applicable to your particular set of contaminants.
Whether you are considering
an EPA method, an ASTM method, or any other recognized
test method, you have to understand the purpose and limitations
of the specific
standard
method.
Standard tests are often optimized to detect a particular
category of contaminants present on a particular matrix.
For example,
some methods
geared to environmental
contaminants are based on extraction with a single
solvent (sometimes the late, lamented CFC-113). If the standard
extraction solvent
is non-polar, like hexane, while the suspect contaminants
are relatively polar and
do
not readily dissolve in hexane, the method may be performed
correctly, but the contaminants will be missed. Standard
gas chromatography
methods may employ a pre-set injection temperature,
perhaps because there is
no concern with detection of materials above a certain
boiling point.
If you
have a mixture containing high boilers and want to
know what is actually present, the standard method will not
provide
complete results.
Reference:
1 B. Kanegsberg & M. Chawla, “Measuring Thin Film Surface Contamination”,
A2C2™ Magazine, September, 2001
Barbara Kanegsberg and Ed Kanegsberg
are independent consultants in critical cleaning, precision cleaning, surface
preparation, and contamination control. They are the editors of “Handbook
for Critical Cleaning,” CRC Press. Contact them at BFK Solutions
LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
