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Concentration Units for Air Folks

A couple of customers have recently asked about standard concentrations and converting from ppmto mg/m³. Honestly, I have no clue how I have never blogged about this subject, especially considering how fundamental and critical this topic is. So here we go:

Air concentrations at ppm are parts per million by volume and should therefore be expressed as ppmv. Although sometimes people forget the “v” at the end, it is implied. Unless of course they meant parts per million by mole, which is sometimes used for air as well; and just so happens to be conveniently identical for an ideal gas and practically identical for most compounds in air at standard temperature and pressure (STP).

But for the most part it should go as follows:


Which you may then conceptualize as:


Now… I have encountered some confusion as to the fact that ppmv may be directly interpreted as mg/m3. Please do not make this mistake! The water and soil folks have the luxury of jumping from ppm to mg/L. But that is because they are not dealing with the density of gases, which are temperature and pressure dependent.

So how do we go from ppmv to mg/m3 (this is exactly the same for ppbv to µg/m³)… here is how:


Where T = temperature in K = 273.15 + °C and 0.08205 is the Universal gas constant in L atm K-1 mol-1, which you may find on the good ole interweb.

Or you could stick to the short version of the aforementioned, which only requires you to remember an ideal gas will occupy 24.45 L/mol at 1 atm and 25 °C. Yes, not the 22.4 L/mol you see littered all over the internet. Unless of course your STP includes 0 °C. Therefore, the above equation simply reduces to the following:


Remember, use the exact same equation for ppbv to µg/m³.


Chemistry Nobel Prize 2016

The 2016 Nobel Prize in chemistry was jointly awarded today to Jean-Pierre Sauvage, Sir Fraser Stoddart, and Bernard Feringa for their research in supramolecular chemistry, specifically, for the development of molecular machines. These molecular machines use self-assembly, host-guest interactions, and mechanically interlocked molecular architectures to generate nanometer scale mechanical motion in response to an external chemical or physical stimulus.

The supramolecular mechanical structures produced through the efforts of the laureates as well as countless others in the area challenge our understanding of chemical bonding and shed light on nature’s own molecular machines, constantly at work within the cells of our bodies. Supramolecular chemistry is also critically important in chiral gas chromatography! The host-guest interaction between chiral molecules and the functionalized cyclodextrins in a chiral GC stationary phase allows for differential retention of enantiomers resulting in their chromatographic separation.


Cyclodextrins are macrocyclic polysaccharides with central cavities of various sizes. They are often used as "hosts" for smaller molecules which enter the cavity.

Cyclodextrins are macrocyclic polysaccharides with central cavities of various sizes. They are often used as “hosts” for smaller molecules which enter the cavity.

I first became fascinated with molecular machines when I presented this Science paper from David Leigh’s research group, himself a former student of Stoddart, at a graduate school journal club. In this groundbreaking work, the group developed a rotaxane based molecular machine that was able to perform synthesis of a small peptide. While it may not be as capable as our own ribosome, evolution has had a ~3 billion year head start! Other machines such a motors, ratchets, and elevators have been reported and the Chemistry Prize Science Background discusses some of these advancements in more depth.

Crystal structure of a rotaxane, a type of supramolecular architecture. The macrocycle is mechanically locked on the "shaft" by bulky end groups.

Crystal structure of a rotaxane, a type of supramolecular architecture. The macrocycle is mechanically locked on the “shaft” by bulky end groups.

Crystal structure of a catenane, a type of mechanically interlocked molecular architecture. The rings are locked together by their own covalent bonds.

Crystal structure of a catenane, a type of mechanically interlocked molecular architecture. The rings are locked together by their own covalent bonds.














Congratulations to the laureates and I’m excited to see where this rich area of research leads us. As scientists, it’s important to retain a sense of curiosity and creativity in our everyday work and Nobel season is a great opportunity to reflect on the fundamental research that has brought chemistry to the level of sophistication we have today.

What can I use as a cap for my mobile phase bottles?

Our choices for mobile phase bottle tops are based primarily on two different products.

The first product is the Hub-Cap Bottle Top, catalog #26541. This is designed to fit a 4-Liter solvent bottle. :




The body (also called insert) of the Hub-Cap Top is cap is made of inert PTFE and has three threaded holes. The exterior ring is composed of high density polyethylene and does not usually come in contact with the mobile phase. Threads for all three holes in the body are for ¼-28 fittings, two of them for 1/8 inch OD fitting nuts and the other is for 1/16 inch nuts. However, the intention is really to run 1/8 inch OD tubing through the two larger holes without the use of a fitting nut. Using it this way without a nut creates a slight vent that can be beneficial, since it avoids the creation of a vacuum as solvent is being pumped from the bottle. The 1/16 inch OD hole comes with a PEEK plug that can be removed, if needed for ventilation, or can be replaced with a ¼-28 thread fitting nut if desired.  If needed, catalog numbers 25794 and 25975 can purchased as a nut and ferrule for the 1/8 inch OD holes.  However, at this time, we do not sell nuts and ferrules for 1/16 inch OD ¼-28 fittings, only the PEEK plug that is mentioned.


The other product is the Opti-Cap Bottle Top, catalog #25300. This is designed to fit a 1 or 2-Liter GL-45 bottle:














The body of the Opti-Cap is made of inert PTFE and the exterior ring is made of polypropylene. Similar to the Hub-cap, the body has three holes that are threaded for ¼-28 fittings. Two of them are for 1/8 inch OD fitting nuts and the other is for 1/16 inch nuts and comes with a PEEK plug installed.  Again, similar to the Hub-Cap Top, the intention is really to run 1/8 inch OD tubing through the two larger holes without the use of a fitting nut when used as for mobile phase. As is the case for the Hub-Cap, catalog numbers 25794 and 25975 can purchased as a nut and ferrule for the 1/8 inch OD holes if needed. Again, we do not sell nuts and ferrules for the 1/6 inch OD hole.

You may notice that we also sell the Eco-cap, catalog number 25395, which is a simplified version of the Opti-Cap. It has three holes like the Opti-cap, but none of the holes are threaded, so it cannot be used with fittings.  A luer-type polypropylene plug is provided for the 1/16 inch hole and the body of the Eco-Cap is composed of a translucent polymer called ETFE.




What to do you do when you have the wrong type of top for the bottle type that you are using?

Here is where it gets tricky.

If you have an Opti-Cap Bottle top already (see above), but you need to use it with a 4-Liter bottle, what you need is a Hub-Cap Adaptor, catalog #26538:26538

You can purchase the Hub-Cap Adapter with the Opti-Cap Bottle Top together as a kit as catalog # 26540.  I would suggest purchasing this kit if you expect to be using GL-45 bottles most of the time, but you might want to use a 4-Liter bottle occasionally and want to be prepared for either. You can also use the Hub-Cap Adapter with the Eco-Cap, to adapt it for use with 4-Liter bottles, if desired, although it will not produce an air-tight seal.


If you have a Hub-Cap Bottle Top already (see above), but you need to use it with a GL-45 bottle, what you need is an Opti-Cap Adaptor, catalog #27197:27197

You can purchase the Opti-Cap Adapter with the Hub-Cap Bottle Top together as a kit as catalog #26551.  I would suggest purchasing this kit if you expect to be using 4-Liter bottles most of the time, but you might want to use a GL-45 bottle occasionally and want to be prepared for either.

I hope you have found this informative.

Thank you for reading.


How Dirty Are You? Part 5…Micropipette Tips

Mouth Pipetting Source: http://collections.nlm.nih.gov/catalog/nlm:nlmuid-101446257-img

Mouth Pipetting
Source: http://collections.nlm.nih.gov/catalog/nlm:nlmuid-101446257-img

Pipettes are common tools for drawing and dispensing precise volumes in chemical, biological and medical labs. Thanks to the invention of micropipettes and other techniques, we don’t do mouth pipetting anymore.

Positive displacement pipettes are particularly useful because the air gap in standard micropipettes are replaced by capillary pistons, which greatly improve the accuracy for handling aggressive liquids, such as organic solvents, and viscous solutions.

It sounds like the perfect way to transfer problematic solvents, however, the question of cleanliness has not be addressed. This reminds me of the blog series from my colleague Julie Kowalski, “How Dirty Are You? So maybe it is time to add another subject to it.

To mimic the worst case scenario, I soaked the positive displacement pipette tips (50-250 µL) into 10 mL of various organic solvents in glass centrifuge tubes and shook them for 30 min. The organic solvents are toluene, acetone, dichloromethane, hexane, and acetonitrile. The 0.5 ppm triphenylphosphate (TPP) was prepared as the internal standard. The solvents were then completely dried under N2 at 50 °C after extraction. Finally, 100 μL of the corresponding solvent was added for reconstitution using glass syringes.

For comparison, another set of samples were prepared by pipetting 250 μL of solvents 10 times. Blanks went through the entire preparation without soaking the micropipette tips.

Each sample was tested on a Rxi-5ms (30 m x 0.25 mm x 0.25 µm) using a Shimadzu GC-MS-QP2010 Plus instrument.

The 30 min soaking results are shown below. I was not surprised to see a contamination peak at higher intensity than the internal standard (TPP) for all solvents. What really surprised me was the absence of phthalate.

So, what is the contamination? It was identified as “Erucylamide” in the NIST MS search library with a relatively good match (>90%). Erucylamide is often employed as an antiblocking/smoothing agent for plastic products, such as PE, PP, and PVC. Therefore, it is a very common contaminant in plastic packages and products.


So, what is the worst solvent? Toluene produced the most intense signals. Acetonitrile and acetone generated relatively similar backgrounds which were cleaner relative to toluene. It pretty much followed the polarity trend except for hexane (see the picture below).

From the chromatograms below soaking for 30 min produced much higher signals than 10-time pipetting. For disposable pipette tips, many people only do one aspiration for each sample, sometimes two if doing sample priming. Therefore, you will get a cleaner background less times you use the micropipette.



The good news is the major ions of erucylamide (m/z 59/72/55) are relatively small, which may not interfere with your target in either full range scan or specific transitions. For anyone who may still be concerned about this contamination, I hope this blog will make you feel better.


RIP Philae

The exact resting place of the Philae lander on Comet 67P/Churyumov-Gerasimenko has finally been pinpointed, in new photographs by the Rosetta orbiter. Rosetta did a close pass of the comet on September 2nd and the survey caught the first clear glimpse of Philae’s position.

Philae's resting place on Comet Churyumov-Gerasimenko

Photograph shows Philae wedged into a deep crack on the comet’s surface.
Main image and lander inset: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; context: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0


I’m so proud that Restek was able to contribute columns to this astounding mission!

To find out more about the Rosetta mission and Restek’s contribution, read Chris English’s interview with Robert Sternberg, head of the GC space instrumentation group at Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA). There is also an in-depth paper on the COSAC instrument, COSAC, the Cometary Sampling and Composition Experiment on Philae, by Fred Goesmann, Helmut Rosenbauer, and Reinhard Roll of the Max Planck Institute for Solar System Research, if you’re interested in finding out more about the development of the instrument and the columns that accompany it.

Oh, and did you know that Restek has columns on three other space missions?! That’s right, four Restek columns can be found on the Curiosity rover (MSL 2011 mission) in its SAM instrumentthree columns are on the ExoMars 2018 mission in its MOMA instrument, and Restek’s MXT®-1701 column was the first capillary column that ever flew in space, on the Cassini/Huygens mission! And the venerable Pro ezGC application was crucial to the column selection for Curiosity, ExoMars, and Rosetta! (See the Sternberg interview, and this article by C. Szopa and R. Sternberg, for more details on Pro ezGC’s use.)


What is the pressure limit for my LC column?

This is a question we get in Tech Services fairly often.  Unfortunately, we are not able to provide a published limit for every column that we produce, but here are a few of them that we have specified:

  • For 3 and 5 um fully porous particle ROC LC columns, the maximum operating pressure is 400 Bar/ 5800 psi, which is also the limit for older or more conventional HPLC systems and close to the limit for column hardware. My personal preference is to operate below 200 Bar/ 2900 psi if possible, although the 3 um particle columns will tend be towards the upper range in terms of pressure.
  • For Raptor 2.7 um (superficially porous particle) columns, the maximum pressure rating for these columns and for many mid-pressure range HPLC instruments is 600 Bar/ 8700 psi. I would prefer to operate below 500 Bar/ 7250 psi as much as possible.
  • For Raptor 5.0 um (superficially porous particle) columns, the maximum pressure rating for these columns and for older and more traditional HPLC instruments is 400 Bar/ 5800 psi. I would prefer to operate below 300 Bar/ 4350 psi as much as possible.
  • EXP guard holder (catalog #25808) and EXP fittings that are used with Raptor columns and guard cartridges are rated up to 20,000 psi when tightened by wrench, 8,700 when hand-tightened.
  • UltraShield and Inline filters (catalog #24993), that are used with our Pinnacle DB UHPLC (1.9 µm) columns, are rated up to 1000 Bar/ 14700 psi.

The difficulty in giving an absolute pressure limit for various columns is that the expectations are different depending on the application and the column dimensions.  I usually refer customers to the table from my blog post Building up pressure on HPLC?, which was originally intended for fully porous particle columns.  Please see below for an updated version of this table. As you can see, the predicted pressure is affected by the column dimensions, flow rate and the specific solvents that are used.  Aqueous mobile phases usually produce a backpressure similar or slightly higher than what is shown here for methanol.  Please keep in mind that because backpressures typically increase over the lifetime of the column, flow conditions for a new column should be established towards the lower end of the operating range if possible.

predicted pressures LC

Although this table was intended for fully porous particle columns, the predicted pressure is not too far off from what it would be for a superficially porous column with the same particle size.  For example, the results above for 3.0 µm particle columns can be used also to give a rough estimate of pressures for a 2.7 µm Raptor SPP column.

In all cases, it is important to stress that for best chromatographic results, all LC and UHPLC columns should be used as close as possible to the optimal flow rate.  Operating close to the optimal value also helps to ensure that the pressure stays within a range appropriate for instrumentation and for the specific column hardware.

Optimal flow rates for all of our LC columns are shown here in this table from our catalog:

optimal flow rates

Here are some links to other related blog posts and articles you might read , if interested:

Should I use a 2.7 or 5.0 µ, Raptor™ column?

What is SPP and when should I use a Raptor™ column?

Technical Service “Red Flags” –LC

Resurrecting an old reverse phase LC column…

Frequently Asked Questions: HPLC and UHPLC


I hope this post has helped to clear up some questions you might have had.

Thank you for reading.

Impact of pH on Derivatization of Nitrophenol Compounds

Our goal of evaluating the Normalized French Environmental Method 12673 required testing the method’s ability to derivatize nitrophenols and alkylphenols in addition to the chlorophenols the method was designed for. Unfortunately, nitrophenols proved challenging to derivatize at low concentrations. In an effort to improve the derivatization of nitrophenols, we experimented with the pH of the reaction mixture.

The original method requires 5 mL of 1 molar potassium carbonate (0.7 g solid potassium carbonate) to be added to 50 mL of water sample, followed by the addition of 1 mL acetic anhydride, then extraction with hexanes. In an attempt to improve the conversion of nitrophenols to acetic acid esters, the amount of potassium carbonate being added to the solution was increased, or replaced with sodium hydroxide. Our hope was that increasing the basicity of the solution would create a better environment for the leaving group, promoting the nucleophilic attack process.

A small experiment was designed in order to evaluate the effect of pH on derivatization of nitrophenol compounds. This experiment included both increasing and decreasing the pH of the derivatization reaction in order to observe the relative response from compounds, which were analyzed on the Rxi-5Sil MS column.

blog 2 - ph 9 11.66 13.5Figure 1: Three overlaid chromatogram showing peaks for (1) 2-nitrophenyl acetate, (2) 3-nitrophenyl acetate, and (3) 4-nitrophenyl acetate. Each overlaid chromatogram represents a derivatization in a different pH solution.

Only nitro-containing phenols were determined in this experiment. Nitrophenyl acetates performed significantly better than dinitrophenyl acetates, and were chosen to reflect to effect of pH. The pH 11.66 derivatization performed best for two of the three nitrophenyl acetates, followed by pH 9, and pH 13.5. The increasing pH, therefore, did not enhance the derivatization process. There appears to be a point at which the pH can impede the derivatization process; the increased pH was also observed to inhibit the derivatization of the alkyl and chlorophenol compounds.

blog 2 - tableTable 1: Responses of chloro- and alkyl phenyl acetate compounds after derivatization in pH 11.6 and pH 13.44 conditions.

The increased pH had a negative effect on the derivatization of these compounds. Despite being a higher concentration, the responses of the compounds derivatized at pH 13.44 are significantly lower than the responses of those derivatized at pH 11.66.

Improved Peak Symmetry as a Result of Derivatization of Phenols

Peak symmetry is important for determination and quantification of compounds. In order to create a symmetric peak, the compound must be able to elute all at more or less the same time. If a compound interacts with the column itself (ex: by temporarily adsorbing to the fused silica surface), it delays travel and effects peak morphology, causing “tailing”.

When comparing EPA method 525.2 to NF EN 12673 for the extraction of phenols, peak symmetry was notably improved using NF EN 12673.

The hydroxyl group on phenol compounds is highly active and able to hydrogen bond with the silanol groups present to some extent in all fused silica GC columns. This is one possible causes of phenol peak tailing. Phenyl acetates, however, have an acetic acid ester group rather than a hydroxyl group, which greatly reduces compound activity,  resulting in cleaner, symmetric peaks.

Relative response factors (RF) were calculated for each of 10 calibration points for pentachlorophenol and 12 calibration points for pentachlorophenyl acetate, to illustrate the effect of peak tailing. Ideally, relative response factors should be equal at each calibration point because they are normalized by concentration. Peak tailing, however, complicates the peak integration, which pushes the RF further from the “ideal”. The relative standard deviation (RSD) shows how close the RFs are to one another. The higher RSD in Figure 1 shows that the RFs deviated more from one another in the pentachlorophenol calibration, than they did in the pentachlorophenyl acetate calibration.

blog 3 - pentaCP RF graphs - 2

Figure 1: A graphical representation of RRF’s for pentachlorophenol (right) and pentachlorophenyl acetate (left) peaks used for calibration purposes. Concentrations used for calibration represent the concentration of phenols in the original water sample.

Accordingly, the peaks in the pentachlorophenol calibration exhibited more tailing than the peaks in the pentachlorophenyl acetate calibration.

blog 3 - pentaCP chromatogramFigure 2: A chromatogram showing peak shapes for pentachlorophenol (left) and pentachlorophenyl acetate (right).

Nobel Laureate Talks and Restek Champagne Reception

From July 19th – 22nd 2016 Restek Corporation with the collaboration of the German Restek Office (Restek GmbH) attended the 39th ISEAC conference, organized from the “International Association of Environmental Analytical Chemistry” (IAEAC) at the Hamburg School of Food Science, lead by  Prof. Dr. Markus Fisher. The central subject of the ISEAC-39 conference is the innovative use of analytical methods for the investigation of environmentally and food relevant questions.

The interesting concept of this conference series is to show the close connection between Food Safety Analysis and Environmental Analysis, as was mentioned by the vice-president of the Federal Institute for Risk Assessment (Bundesinstitut für Risikobewertung, BfR), Prof. Dr. Reiner Wittkowski, talking about “What goes in must come out – environmental impact on food safety”. In fact, both communities are often facing the same challenges, as shown by the discussion about the Glyphosate issue or about MOSH/MOAH, both of them related to food safety as well as to environmental aspects. The same co-incidence can be recognized in observing polyfluorinated surfactants like Perfluorooctane Sulfonate ((PFOS) or Perfluorooctane Acid (PFOA), widely used to impregnate breathable functional clothing. Prof. Wittkowski used these examples to explain the need and the challenges of proactive consumer protection, a concept which is followed strongly by the European Community.

One of the purposes of the Organizers of this conference series is the idea of “Thinking out of the Box”, an inbuilt series of talks with related topics out of different scientific fields not directly connected to analytical challenges. This interesting part of the conference showed up with talks from Nobel Laureate Prof. Dr. Robert Huber, giving a summary about his work about the “Beauty and  Fitness for Purpose of Proteins” as well as Prof. Dr. Christoph Kutter, Director of Fraunhofer Research Institution for Microsystems and Solid State Technologies, talking about “Sensors for the Internet of Things”. As a high light for the Chromatography Community the talk of Prof. Dr. Alexander Makarov gave a short cut about the development of the Orbitrap Mass Spectrometer Technology.

During the conference, Restek could present its innovations in  Separation Science. To honor the scientific work of those researchers who showed up with Posters, Restek opened the first poster session with a Champagne Reception to welcome the interested audience.

Restek Corp. especially wants to thank the group of Prof. Markus Fischer, the organizing young scientists are dedicated to make things possible, and the secretary of the IAEAC.


Champagne Reception

Preparing the Champagne Reception at the first Poster Session

Poster Session

Interesting Discussions during the Champagne Reception and Poster Session


Last Sunday, I was sitting in the garden, recapping the experiences of attending the scientific conferences I had the pleasure to visit during this year. One of the honoring sessions has left a quite inspiring impact on my mind. In the beginning of the year, I had the opportunity to listen to one of the most experienced and successful scientists in our Chromatography Community, Dr. Hernan Cortes, who as one of the first researchers was recognizing the power of hyphenated techniques in Chromatography (he even experienced the LC/GC coupling as a helpful technique in solving practical challenges – Sorry, Dr. Cortes that I couldn’t believe that the evening before). He was honored with the  LCGC Europe’s Lifetime Achievement Award during HTC in Ghent/Belgium in January.

Hernan Cortes  always has an open heart and an open mind for young Scientists. Therefore his laudation talk had a clear focus on how to help Young Scientists to develop their skills, as they are the future not only for our field of Science. As my daughter is studying Biology at the University of Cologne in the moment, I was electrified by one of his slides, showing some bullet points he called his “Hernanisms”, which he claimed not be developed only by himself, but were taken over from him as some of the keys of his success. So I asked him to provide this slide to me, because I liked to share these ideas with my young daughter.

Re-reading these “Hernanisms”, I thought it would be valuable to share them with you as well, hoping that you may transfer these ideas to your young colleagues as well.

I have copied these  Bullet Points below (my explaining comments in Grey, for I cannot recap his complete talk).

  • Never stop learning and teaching.
  • Normal is a setting on a washing machine
  • No set roadmap.
    • There is no “normal” path
  • Intelligence, hard work, luck.
    • –Two out of three don’t make it.
  • Continuous improvement.
  • Karma, help others and be true to yourself.
  • Happiness percentage (in everything you are doing, in the end happiness should win over frustration)
  • Time management.
    • The only commodity you can never get back (….is wasted time ).

Restek is very aligned with these Short Cuts for our company also is dedicated to the idea of supporting young scientists, as our RASP (Restek Academic Support Program) may show.*

Thank you, Dr. Cortes for your inspiring work and thoughts.


*Please ask for our RASP program

Flyer RASP_GNOT2495-UNV_RASP_001