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Is it OK to use THF (tetrahydrofuran) on my HPLC?

Well, it might be. In the early days of HPLC, THF was commonly used as a mobile phase solvent. It has eluting strength similar to acetonitrile, but just slightly stronger. Since it technically is an ether and is very miscible with water, it is sometimes useful with reverse phase HPLC. It also provides additional options for ternary mixes when methanol/water or acetonitrile/water mobile phases are not able to produce a fine tuned separation.

Like many organic solvents, there are some possible health hazards associated, which you can read about here. THF has a very low boiling point (66C) and emits fairly noxious fumes at room temperature, making it quite unpleasant to work with. You definitely need to use this in a hood.

A more concrete to reason to limit usage of THF for HPLC mobile phase is that it does have a tendency to swell PEEK (polyetheretherketone) material and may contribute to degradation over time. A side effect of the swelling could be increased system pressure, which may become an issue. The use of PEEK tubing and fittings has increased dramatically over the years, due to its ease of usage compared to stainless steel parts. However, THF should not be used with PEEK tubing, unless it is present only at low levels. I have read varying opinions on how much THF one should try using. Personally, I would prefer to stay less than 10% if using PEEK tubing. There is also the possibility that THF at higher levels can degrade other plastic-like materials, for example, pump seals. When using THF, it always best to check with the instrument manufacturer to ensure the proper seals are being used. Some pump seals are designed only for use with aqueous solutions and weaker organic solvents (these would be ones designated as “aqueous” or “reverse phase”).

As far as detection methods go, THF is OK to use for UV detection methods. Since its UV cutoff is around 212 nm, it usually does not produce interference. However, it is important to use HPLC-grade THF to avoid interference from stabilizers that often are used with other solvent grades. It is also important to make sure the THF is fresh, as the formation of peroxides over time will increase the UV background. Usage of THF for PDA and fluorescence detection is fairly similar and the same precautions exist. Some of the same concerns about purity, stabilizers and peroxides apply to most detection methods, including refractive index (RI) detectors.

Using THF with mass spec detectors presents some unique concerns. Agilent and Waters both suggest that its use for LC/MS should be very limited and special precautions should be taken:

Use of THF with LC/MS (Agilent)

Solvents and Caveats for LC/MS (Waters)

ThermoScientific mentions similar precautions for Charged Aerosol Detectors below:

Optimizing and Monitoring Solvent Quality for UV-Vis Absorption, Fluorescence and Charged Aerosol Detectors (ThermoScientific)

 

I hope you have found this information useful. Thank you for reading.

Analysis of N2O (nitrous oxide) via GC: an update

2011-jaap-pasfoto4-smallNitrous oxide (N2O), is commonly known as “laughing” gas, but is also used as a component in fuels in rockets and as an aerosol propellant. N2O is itself a stable gas and can be analyzed relative easy via gas chromatography. Often it is confused with “nitric oxide”, (NO). NO is a very reactive gas. When oxygen is preset, it will immediate oxidize into NO2. NO2 can be easily recognized as it has a dark brown color. Also NO2 shows reactivity, meaning that the analysis of NO and NO2 via gas chromatography is not commonly done, see for details: http://blog.restek.com/?p=4583

N2O elutes as a sharp peak from alumina.

Nitrous oxide on Alumina BOND / Na2SO4 Column: 30m x 0.53mm Rt Alumina BOND / Na2SO4; Helium, 4 mL/min; Oven: 40ºC; sample: 60µl; Detector: PlasmaDetek PED; sample: 5.4 ppm N2O.Chromatogram courtesy: L. Paradis, LDetec.

Recently a summary of N2O analysis was published by Separation science. Separations are shown on different adsorbents like, Porous polymer, Alumina, Molsieve 5A and ShinCarbon materials.

Full Article can be found here: http://www.sepscience.com/Information/Archive/All-Articles/4365-/Analysis-of-Gases-via-Gas-Chromatography-Part-1-Nitrous-Oxide

Especially the alumina PLOT is interesting (Fig.1) as often CO2 is present and can interfere with the N2O measurement. CO2 is adsorbed completely by Alumina, resulting is a single N2O peak. CO2 can be removed periodically by conditioning at 200C.

 

 

Preserving the First Dimension Separation in GCxGC

I mentioned in a recent blog post that to maximize peak capacity in GCxGC the first dimension separation needs to be preserved by having a very short second dimension separation (short modulation time), often on the order of 2 sec or less, even.  While maximizing peak capacity can be very important when trying to characterize a complex sample (e.g., for metabolomics or discovering emerging contaminants in the environment), maintaining the first dimension separation through short modulation times is even more critical when isomers are to be determined individually, including for mass spectrometry.  That’s because isomers that coelute in the first dimension for GCxGC are very rarely separated in the second dimension.  The second dimension column, no matter the alternate selectivity, is just too short.

The problem outlined above, first dimension coelution caused by a longer modulation time in GCxGC, and its solution, a faster modulation time to preserve the first dimension separation, are illustrated in the figure below.  See how the tetrachlorobenzenes coelute when the modulation time is 2 sec?  The peaks eluting from the first dimension column are “piling up” at the modulator, which might be OK if they were separated in the second dimension, but they are not.  Modulate faster = preservation of the first dimension separation, in GCxGC.  The general rule of thumb is modulate (slice) the first dimension peak at least 3 times.

Mod time

Restek at BFR2016 in Toronto – APGC of Brominated Flame Retardants Using Helium and Nitrogen Carrier Gases

In only a few short weeks, I will be giving a presentation on the analysis of brominated flame retardants at BFR2016 in Toronto.  My colleagues and I used an atmospheric pressure ionization mass spectrometer with gas chromatography on an Rtx-1614  (15m x 0.25mm x 0.10µm) column to look at polybrominated diphenyl ethers (PBDEs) in various samples.  First though, we explored optimizing the chromatography for speed, while using efficient helium carrier gas.  But we also looked at employing nitrogen carrier gas, since the APGC instrument we used can easily handle nitrogen carrier with no loss in sensitivity like would occur with the typical vacuum-pumped electron ionization MS system.  By employing a selective GC column like the Rtx-1614, you can get the same analysis times (and PBDE retention times!) for helium and nitrogen carrier gases and still meet necessary separation criteria (e.g., separation of Br4 PBDE congeners 49 and 71 as per EPA Method 1614).  Both helium and nitrogen approaches were facilitated by the EZGC Method Translator and Flow Calculator.

Check out the abstract and the preliminary program and I hope to see you at the conference.

Peak Capacity and Selectivity in Gas Chromatography

I recently had the pleasure of hearing Professor Taduesz Gorecki from the University of Waterloo lecture on comprehensive two-dimensional gas chromatography (GCxGC).   One of his chief areas of research is on the GCxGC modulator, which is essentially the “injector” for the second dimension column in this multidimensional chromatography approach.  A twist on the usual process from Taduesz involved “stop-flow” modulation, where the column flow in the first dimension is halted while the injection and separation occur for the second dimension.  Where the usual second dimension columns and separations in GCxGC are super short (sometimes L less than 1 m and time less than 2 sec) to preserve the first dimension separation, “stop-flow” modulation allows longer second dimension separations (and longer second dimension columns to improve them).  The first and second dimension separations are more independent of each other than in traditional GCxGC.

GCxGC in general, and especially stop-flow GCxGC, generate very large peak capacities versus one-dimensional GC, up to an order-of-magnitude higher.  But in his lecture Tadeusz reminded us that without good choices of stationary phase selectivity in both dimensions, that peak capacity won’t be fully realized.  I can easily demonstrate that with some recent work I did for an upcoming lecture at the 40th International Symposium on Capillary Chromatography and 13th GCxGC Symposium.  Note in the figure below how the Stabilwax column in the second dimension pulls apart the diesel, placing the aromatic analytes further away (higher) from the aliphatic compounds (line of peaks at the bottom of the contour plot).  However, the Rtx-200 (a trifluoropropylmethyl phase) does not have the necessary selectivity to allow full use of the 2D space.  Never fear; I exploited that space later by analyzing an environmental sample containing PAHs, pesticides, PCBs, explosives, and priority pollutants.  Sometimes it’s all about having selectivity choices in GCxGC, and that’s where Restek excels.

Diesel on Wax and 200

Split Injection Makes for Easier Polar Solvent GC Work versus Splitless Injection

As anyone who’s doing QuEChERS knows, analyzing acetonitrile extracts on nonpolar GC columns (like Rxi-5ms, e.g.) using splitless injection can be problematic because of the classic solvent – stationary phase mismatch.  To avoid split peaks we usually have an initial GC oven temperature slightly above the 82°C boiling point of acetonitrile (MeCN), but this causes tailing of early eluting peaks as they are not focused using the “solvent effect” or cold trapping.  Life is much easier if we can do split injection at, say, a ratio of 10:1, since instead of, e.g., 1 microliter, we now have approx. 0.1 microliter of polar solvent going onto our nonpolar GC column.  Of course you need to pay attention to the hit on your LODs and LOQs, but with more sensitive MS/MS instruments being developed every year, split injection for pesticide residue work is becoming practical.  Given that approx 10 times less extract goes onto the column and into the MS source, system uptime is greatly improved, too.  A Sky Precision split liner with wool and “shoot-and-dilute” GC (split injection GC) makes life easier!

Split MeCN

Don’t Forget to Change Your GC Inlet Bottom Seal and Trim Your GC Column as Part of Your Maintenance Routine!

While doing PAH analysis for dirty samples with splitless GC I saw increased peak tailing and loss of response, so I did what anybody would do: changed my GC inlet liner.  In this case I used a Sky single taper inlet liner with wool.  Unfortunately when I did my next standard analysis, not much was improved.  While sometimes an inlet liner change is indeed enough to restore performance, in this case it wasn’t, and so I changed the gold dual vespel ring inlet seal at the inlet bottom and trimmed the column slightly.  Problem solved!

PAH 01PAH 02

Restek at the 40th International Symposium on Capillary Chromatography and 13th GCxGC Symposium

Restek is proud to have numerous presentations in the orals scientific program for the 4oth ISCC and 13th GCxGC Symposium, which will take place in Riva del Garda, Italy in May.

Exploding a Peak Capacity Increase Record by Using Hydrogen Carrier Gas for Comprehensive Two-Dimensional Gas Chromatography – Time-of-Flight Mass Spectrometry (Jack Cochran, Julie Kowalski, Christopher Rattray, Amanda Rigdon, Jaap de Zeeuw; Restek and Mark Merrick; LECO)

Achieving a Near-Theoretical Maximum in Peak Capacity Gain for the Analysis of Ignitable Liquids Using GCxGC-TOFMS (Katie Nizio, Shari Forbes; University of Technology Sydney, and Jack Cochran; Restek)

Time Interval Deconvolution of Polychlorinated Biphenyls in Aroclor Mixtures Using Gas Chromatography – Vacuum Ultraviolet Spectroscopy (Kevin Schug, Changling Qiu; University of Texas at Arlington and Jonathan Smuts, Phillip Walsh; VUV Analytics and Jack Cochran; Restek)

3D Printed Capillary Columns (Roy Lautamo, Bill Bromps; Restek)

Finally, A Significant Improved Chromatographic Separation of Argon and Oxygen Using New Plot Column Phase and Deposition Technologies (Jaap de Zeeuw, Bill Bromps, Ashlee Reese, Roy Lautamo; Restek)

Snag the programs below to see what other scientists will present…

Preliminary Program 13th GCxGC

Preliminary Program 40th ISCC

Fast Analysis of PAHs using Split Injection and a Short GC Column

Click on the this link:  Fast Analysis of PAHs  to see how I analyzed the EPA 16 polycyclic aromatic hydrocarbons (PAHs) in under 10 min using GC-MS by employing split injection with a Sky Precision split liner with wool and a 15m x 0.25mm x 0.25µm Rxi-5Sil MS GC column.  Split injection allows a higher GC oven temperature start with good focusing for naphthalene, the most volatile PAH.  And the Rxi-5Sil MS stationary phase (5% phenyl type) is silphenylene, a backbone modification that provides a faster separation for benzo[b]fluoranthene and benzo[k]fluoranthene versus a typical 5% diphenyl column like the Rxi-5ms.

In addition to the speedy separation of the benzofluoranthenes, note the almost baseline separation for indeno[1,2,3-cd]pyrene and dibenz[a,h]anthracene.  Not bad!

Although you’ll see literature that encourages faster GC analyses through employment of smaller inside diameter columns, I prefer the 15m x 0.25mm x 0.25µm due to the better sample loading capacity for the 0.25mm x 0.25µm over something like 0.18mm x 0.18µm or 0.15mm x 0.15µm columns.  In addition, the larger ID column has better ruggedness than the smaller bore columns.

One more note on the use of 15m x 0.25mm x 0.25µm versus 30m x 0.25mm x 0.25µm columns.  Under vacuum-outlet conditions, i.e., using MS, the analysis time is about 3 times shorter for the 15m column.  You can prove that to yourself with the EZGC Method Translator and Flow Calculator.

Shoot-and-Dilute GC – Practical GC – The Column

Check out my recently published articles in LCGC – The Column on Shoot-and-Dilute GC (split injection GC), where I outline the benefits of this technique through recently collected experimental data in my lab at Restek.  I’m two articles into my Practical GC series, with another one soon to be published.  This is a continuation of other split injection work I’ve done and posted on in ChromaBLOGraphy.  Interested in keeping your GC systems up longer and avoiding problems with compound degradation and adsorption in your GC inlet liner?  Read on…

Split Injection GC: The Benefits of Shoot-and-Dilute GC

Split Injection GC: GC Inlet Liner Choice for Shoot-and-Dilute GC