For critical products, particularly products assembled in a cleanroom or other controlled environment, the purity and cleanliness of the liquids used for cleaning and assembly are every bit as important as the quality of the air. Many critical products require the use of high purity solvents. But just as for the quality of water used in aqueous processes,¹ there are many grades of high purity solvents.

ACS REAGENT GRADE
The generally accepted standard for high purity laboratory chemicals is ACS Reagent grade. ACS Reagents meet minimum impurity specifications determined by the American Chemical Society. These specifications vary among chemicals. For example, allowable maximum levels of impurities specified for acetone has a limit for isopropyl alcohol (IPA) and methanol, and the IPA spechas a limit for acetone.

Solvent manufacturers may offer additional grades developed to address requirements of specific applications. These grades generally meet the ACS specifications, and they also have additional constraints or specifications. Examples are “Electronic Grade” for electronic assemblies, “HPLC Grade” for use with liquid chromatography applications, and “CMOS (Complementary Metal Oxide Semiconductor) Grade” or “Semiconductor Grade” for wafer preparation uses. One supplier’s specification for “Electronic Grade” acetone additionally lists a chloride limit that is unspecified in the “ACS Reagent Grade”. The “CMOS Grade” acetone specification from the same supplier lists trace limits for more than 30 elements in addition to those with limits for the“ ACS Reagent Grade.”

READ THE FINE PRINT
With the exception of semiconductor grades specified by Semiconductor Equipment and Materials International (SEMI), we know of no industry-wide independent or consensus standards for specialized grades that exceed ACS Reagent Grade specifications. The specifications for specialized grades are generally set by the supplier. As a result, differences in specifications exist even for what is nominally the same reagent grade from different suppliers. A comparison of the specifications for “Electronic Reagent Grade” acetone from two major chemical suppliers shows small but potentially important variations. Each supplier lists a limit for trace elements that are not listed by the other. One lists the limit for Iron at half the specification of the other. Do these differences in specification mean that the chemicals are different or that one supplier is better than the other? Maybe, but not necessarily. The specifications indicate what is tested for, not necessarily what is present. Suppose your application is sensitive to the presence of a particular substance; for example, nickel. If nickel is not listed on the specification, we suggest that you contact the supplier directly to determine if the level of nickel is determined. If not, then your choices are to either find a supplier that does test for nickel or arrange to have a test run specifically for you, either by the supplier, yourself, or a third party.

“HPLC Grade” is not necessarily purer than other grades. The main reason for using “HPLC grade” solvents is that other grades may contain impurities that absorb UV light and that can thus interfere with peak detection during chromatography. For example, if hexane contains traces of benzene, the benzene might absorb enough 254 nm light to cause interference with HPLC detectors.