Readers of the blog have been presented with a wealth of information on for their GC columns, LC columns, detectors, syringes, carrier gases, TO-Canisters, and the list go on. What’s missing? Analytical Reference Standards.

Although not all reference standards have special handling instructions, the following general guidelines should assist you with the proper handling of these critical resources.

Storage temperatures can have a big impact upon your reference standard solutions. Volatile compounds (ex: vinyl chloride through naphthalene) are normally recommended to be stored frozen, since they can readily evaporate at room temperature. Make sure you store these solutions with minimal headspace in a well-sealed vial. On the other hand, solutions containing less volatile compounds (ex: PAHs and Cannabinoids), are typically recommended to be stored refrigerated. Others, such as Aroclor PCBs, are to be stored at room temperature. Since some of our certificates of analysis (CoA) list “less than 10°C” as the storage condition, I do have some advice for anyone that stores these less volatile compounds in their freezer. Once the solution warms to room temperature, place the solution in a sonicator for 10-15 minutes to allow these compounds to dissolve back into solution. By doing so, you will not be “missing” these compounds from your assay.

Some compounds are sensitive to light and can easily degrade when exposed to it. These photosensitive compounds, such as organophosphorus pesticides, can be protected by storing them in amber vials. For this purpose, Restek supplies a deactivated amber secondary vial with all of its solutions. A good practice is to always store your reference standards, including those at room temperature, away from light. Some additional precautions to avoid light exposure are to work under yellow light and to use red-tinted glassware when preparing dilutions. If you are working with known photosensitive compounds, use amber autosampler vials for your solutions. Minimize exposure to light and to return all standards to their storage area as soon as possible after use.

Finally, some compounds are incompatible with acids, bases, other compound classes or specific compounds. Because of the number and variety of potential reactions, I’m not going to go into specific details. I will note that Restek’s standard solutions are evaluated for solution feasibility to ensure compatibility between compounds. Since there are numerous multi-component methods, analysts do end up mixing solutions to create calibration standards. If you experience the loss of analytes or see new analytes in these cocktails, try analyzing your purchased solutions individually. There’s likely an interaction occurring between compound(s).

If you’re ever in doubt, don’t forget the CoA. The CoA contains storage recommendations and any warnings be it photosensitivity or chemical incompatibility. The manufacturer should have data to support their recommendations. The ability to search Restek’s CoA documentation is one click at:

Documentation Search

Now, reference standards can have their 15 minutes of fame in ChromaBLOGraphy.

We (tech service) get calls all the time about fittings, especially the ones we sell which are manufactured by Swagelok®, Parker®, and Valco®. It seems like customers are either very familiar with fittings, or they know virtually nothing about them. I will try and provide some information that will hopefully take some of the confusion out of these different fittings.

First, the Swagelok and Parker fittings that Restek sells are designed for similar applications. Although each of these companies offer a huge variety of products, we only sell the items usually requested by our customers in the field of chromatography. These fittings are commonly used to plumb gases to a gas chromatograph, install packed columns, install gas filters (traps), etc…

A few things to keep in mind:

1. We sell both brass and stainless steel fittings. Brass is for use with copper tubing, and stainless steel is for use with stainless steel tubing. If you try and use a brass fitting with stainless steel tubing, brass isn’t hard enough for the ferrule to “bite” into the stainless steel tubing; therefore, you will not be able to obtain a proper seal. In addition, connecting two dissimilar metals may lead to galvanic corrosion . 

2. The fittings we sell may have compression or National Pipe Thread tapered ends (NPT). Compression ends are made for a tube which has a specific outside diameter (OD). For example, a ¼” compression fitting is designed to connect a section of tubing which has an OD of ¼”. When the nut (which contains the ferrules) is tightened, it compresses the ferrules onto the tubing to make the seal. Keep in mind that most metal tubing is designated by its OD while most plastic tubing is designated by its ID (and usually needs a plastic fitting to obtain a proper connection).

3. NPT (ends) look very different than compression (ends). NPT has no nuts or ferrules, but instead relies entirely on a thread connection. Like plastic tubing, the product designation for NPT (and pipe in general) is the internal diameter (ID), not tubing OD like compression. To help clarify the difference between compression and NPT, the photo below shows a fitting with male NPT end on the left, and compression end on the right.

4. The metal ferrules that are included in the compression end(s) can be replaced with ferrules of different materials. For example, if trying to connect a fitting to a ¼” OD glass tube, you would want to replace the metal ferrules with a softer material like graphite. Similarly, you would want to replace the metal ferrules with PTFE ferrules if using the fitting with PTFE tubing.

5. Finally, unlike compression, NPT fittings can be either male or female. Just remember that the threads on a male fitting are on the outside, and threads on a female fitting are on the inside.

We also sell Valco® fittings. These are considered low (or zero) dead volume fittings, meaning they are designed to eliminate any dead space (volume) within the fitting. Common uses for these fittings are to connect capillary columns and other small ID tubing where dead space may cause chromatography (or other) problems such as tailing peaks. These fittings usually include stainless steel ferrules. Just like with other compression fittings, the metal ferrules can be replaced with Valcon Polyimide ferrules when connecting tubing other than stainless steel. Note that these fittings, while containing nuts and ferrules like other compression fittings, have a very different style (see photo below), and are not compatible/interchangeable with the Swagelok® / Parker® fittings described above.

A few final thoughts on the topic of fittings:

For gas flow rates less than 5mL/min, consider Valco® connectors to minimize dead volume. For the most inert fittings, consider Siltek®/Sulfinert ® Treated products. When in doubt as to which fitting to use, always ask someone first, as using the incorrect fitting can be dangerous if a proper seal is not made. Remember, we are just a phone call or e-mail away.