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 columnwith 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 quantifiedusing 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 heartof 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 theculprit 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 chromatographysystems, 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
NOTE: The diagram, “Fundamentals of Ion Chromatography” courtesy Dionex Corporation.
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and 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; [email protected]; www.bfksolutions.com.
