Part II, Complex Chromatography Systems, Ion Chromatography
Barbara Kanegsberg and Ed Kanegsberg, February 2005
The Basics
In the previous column, we introduced chromatography, a powerful separation
technique that typically involves partitioning of mixtures between an adsorbent
stationary phase and a liquid or gas mobile phase. Separation of mixtures
of contaminates, without contaminant modification, is important for accurate
quantification and identification. In the context of contamination control,
we concentrate on chromatography for separation and identification; chromatography
can also be used to purify product.
In liquid-solid column chromatography, the mixture is applied to a column
packed with sorbent. The mixture is separated by washing or eluting the
column with one or more solvents (in this context solvent could include
water).
Complex Chromatography Systems
Current chromatography systems employ the same basic principles as did
the nearly century-old systems but they can be extremely sophisticated
and
therefore extremely daunting. In current analytical systems, the humble
chromatography column is hidden in one or more attractive but uninformative “black
boxes” (or grey or blue boxes) that are typical of what you might
have seen on a visit to an analytical laboratory. The column is augmented
by a number of assistive devices. A system typically contains a sample
injection device, eluting material(s) with a pump, perhaps a gradient maker,
and a chromatography column packed with sorbent. The eluent may be a single
chemical, a mixture, a sequence of chemicals, or a gradient. In a gradient,
the relative concentrations of elution chemicals are gradually changed.
The sorbent (the stationary phase packed into the column) and the length
of column are specific to the application in question. Rather than visually
observing the bands, the separated materials are analytically “visualized” and
quantified using an appropriate detector along with data analysis software.
Between the column and the detector there may be a device for post-column
modification of the eluted bands. This modification can include ion exchange
or a more complex chemical reaction to enhance the level of detection.
In addition, there is usually a small “guard” column to protect
the main analytical column from gross (and hopefully irrelevant) residue;
and if many samples are to be separated and detected at once there can
be a turntable with an auto-sampling device.
There is no need to be intimidated by all the bells and whistles; at the heart of it all, is the basic chromatography column, separating complex mixtures.
Ion Chromatography
Ion chromatography is a specific chromatographic technique. An anion
exchange column might be used to resolve (or separate) a complex
mixture containing
fluorine, chloride, nitrate, and organic acids such as oxalate.
On the principle of “like dissolves like,” the appropriate eluting medium for
ion chromatography systems is typically aqueous and the elution medium contains
a counter ion, perhaps sodium hydroxide. If sodium hydroxide is used for
elution, one obtains eluted “bands” of sodium salts.
If a conductivity detector is employed, the sodium ions (the counter
ions)
limit sensitivity
of detection of the anions. Therefore, in a post-column modification,
an anion exchange column, termed a suppressor, is used to replace
Na+ with
H+, converting the sodium salts to the corresponding acids.
Applications
In the world of electronics, ion chromatography is considered a more
specific, more informative version of the classic ROSE (Resitivity
of Solvent Extract)
test for ionic contamination of electronics. Ionic contaminants
from components of fluxes, cleaning chemistries, and plating chemistries
can interfere with
functionality and longevity of the electronics device. Both tests
are extractive, so the component has to be extracted with an
effective
chemistry.
Whereas
the ROSE test provides a change in reactivity as a measure of
contamination control, ion chromatography can identify specific components.
This
is useful in quality control and in the detective work involved
in tracking
down the
culprit in failure analysis.
In a very timely example, hexavalent chromium (Cr6+) can be specifically and sensitively detected by ion chromatography. There are EPA methods for wastes and water testing; and there is an OSHA method (215) for industrial hygiene samples. Measurement of hexavalent chromium is subject to interferences and requires very careful sample handling. Ion chromatography is more sensitive and specific than are traditional colorimetric methods. The OSHA method involves air sampling, aqueous extraction, ion chromatography, post-column modification, and detection of the reacted eluent at a light wavelength of 540 nm. In October 2004, OSHA proposed that the permissible exposure level of hexavalent chromium be lowered from 52 to one microgram per meter3 in air, 8 hour time weighted average; the action level is 0.5 microgram. In California, OEHHA sets even lower levels. If the OSHA proposal is enacted, the impact will not be limited to plating operations. An array of applications such as welding, production of some sealants, and chemical mixing may be impacted. Use of ion chromatography systems, with specific, accurate separation and detection will be critical.
Next month: More of the wonders of chromatography!
References:
We appreciate the comments of Beverly Newton, Dionex Corporation
and of Jim Unmack, CIH, Unmack Corporation.
Some web resources:
* EMPF cleanliness testing.
http: //www.empf.org/empfasis/oct03/603clean.htm
* Hexavalent Chromium
http://www.wcaslab.com/tech/hexchrom2.htm
* OSHA Method ID-215. Hexavalent Chromium in Workplace
Atmospheres.
http: //www.osha-slc.gov/dts/sltc/methods/inorganic/id215/id215.html
* Navigate to: “OSHA Proposes Revised Rule on Hexavalent Chromium”
www.osha.gov
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.
