This basic primer on Chromatography Glassware will aid in understanding the different types of glass used in the industry.
As most people know, glass has been around not just for centuries but probably several millennia. The earliest form of glass is what’s commonly known as soda-lime glass. Like any glass, the primary component of soda-lime glass is silicon dioxide (sand, or SiO2) which is combined with oxide additives (sodium oxide, Na2O, soda ash; or calcium oxide, CaO, lime—hence the name soda-lime glass). While the percentages of each additive differ based on the manufacturer and the use of the glass itself—windows or bottles—soda-lime glass is generally composed of 74 % sand, 14 % Na2O, 10 % CaO with the rest being oxides of several other common metals. Iron oxides give rise to the typical colors we see in glass such as the green of 7Up, Beck’s or Heineken beer or the brown associated with virtually every other beer on the planet. This type of glass is relatively inexpensive to prepare, readily recycled and repurposed, worked rather easily (with a liquid-glass transition temperature below 600 °C) and for the most part, chemically resistant. However, soda-lime, or ‘soft-glass’ has one flaw that makes it virtually useless in a laboratory setting—it cannot handle extreme changes in temperature without shattering. A new type of glass was needed. Enter PYREX.
The name PYREX is synonymous with glassware—be it kitchen cookware or the ubiquitous laboratory beaker. Pyrex glassware, or borosilicate glass, has been around for over a century and has been sold under a variety of trade names (such as Refmex, Kimble and Duran just to name a few), with Pyrex being the most well-known. Borosilicate glass differs from soda-lime glass in the composition and identity of the added oxides. In contrast to soda-lime glass which has sodium and calcium oxides as the primary additives, borosilicate glass has a higher composition of sand (around 80 %) and its primary additive is boron oxide (B2O3—approximately 13 %) with the rest of the mixture composed of other oxide additives. Altering the percentages and the additives imparts several changes to the glass produced. Borosilicate glass has a very low coefficient of thermal expansion (3.3 x 10–6 /K for ‘typical’ borosilicate glass while PYREX has a specific coefficient of 3.2 x 10–6 /K). The “3.3” in the coefficient of thermal expansion for typical borosilicate glass is an important moniker that’s used industrially to denote the type and quality of glass (except that instead of 3.3, it’s just ‘33’).
Borosilicate glass can be further subdivided into several categories based on the amount of boron oxides and the other additives used. While the “3.3” type borosilicate glass is quite common, there are a couple of other types of borosilicate glasses manufactured. Glasses with lower percentages of sand and boron oxides (~75 and 10 % respectively) contain a relatively high amount of alkaline oxides and aluminum oxides (up to 10 %). This type of borosilicate glass has a slightly higher coefficient of thermal expansion (4.0–5.0 x 10–6 /K) and is slightly softer than “3.3” glass, making this type of glass slightly easier to work relative to typical borosilicate glass. Glass that contains a relatively high concentration of boron oxides (~20 %) and lower amounts of sand (70%) finds several uses in chemical industry. This type of glass is ideal for making glass-to-metal connections, possesses a low expansion coefficient, a very low softening point, is easily worked, has a high chemical resistance and a wide UV transmission window (<200 nm).
When choosing glassware for chromatographic applications, the specifications can become a little confusing. Is there a difference between the two typical types of expansion glassware (33 expansion and 51 expansion)? For most routine chromatographic applications, the answer is probably no. The designations of “33” and “51” most likely come from the coefficients of linear expansion for the type of glass manufactured. Both types of glass are high quality and the ’33’ glass is often marketed as “Type 1, Class A” while the ‘51’ glass is marketed as “Type 1, Class B”. Both types of glass possess low thermal expansion coefficients, both are chemically resistant and are quite suitable to produce autosampler vials. The differences between the two types of glass are quite minor—the 33 expansion glass has a lower coefficient of thermal expansion than 51 expansion glass as well as a lower combined total of arsenic and antimony (less than 0.005 %). In effect, there is very little difference between the two types of glass for most chromatography applications and either can be utilized with full confidence.