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I can tell you that determining transitions (SRMs) for GC-MS/MS can be quite challenging with so many precursor ions to evaluate. I spent a considerable amount of time optimizing transitions for our GC Multiresidue Pesticide Kit which has 203 compounds. That is a lot of transitions to evaluate!
I also tested my chromatographic method for analyte to analyte interferences by comparing ion ratios. The ion ratio of qualifying Q3 ion to quantification Q3 ion for each compound tested in small subsets was compared to the ion ratio determined in the presence of all 203 compounds. When the two ratios fell outside of plus/minus thirty percent, this indicated analyte to analyte interference and alternative transitions were used.
Transitions for all compounds and notes about analyte to analyte interferences and shared transitions can be found in this downloadable spreadsheet. The data was collected and optimized on our Thermo Scientific™ TSQ™ 8000 Triple Quadrupole GC-MS System.
The spreadsheet provides the method conditions (last tab), retentions times and three SRMs per compound. CAS numbers and isomers information is also included.
I hope that this helps accelerate your method development!
Shopping list for this method:
- GC Multiresidue Pesticide Kit
- Rxi-5ms (cat# 13423)
- Sky liners (Single Taper with wool for Thermo TRACE™ 1310)
- Internal Standards and/or Surrogates
Passive Canister Sampler – Part I: How a flow controller works and how to select the proper critical orifice (featuring the all new Critical Orifice Calculator V1.03)
If you have been following my George Lucas-inspired, passive canister sampler trilogy, then you know how to calibrate your passive sampler and collect a sample; and assemble your passive sampler and leak check it. Today we will cover how the passive canister sampler (i.e., flow controller) works and how to select the proper critical orifice for your sampling scenario.
A passive canister sampler (see figure below) maintains a constant sample flow over an integrated time period, despite changes in canister vacuum. The critical orifice acts as a flow restrictor, upstream of a constant back pressure (i.e., vacuum in the canister). This constant back pressure is established by the balance between the mechanical spring rate of the diaphragm and the pressure differential across the diaphragm. The latter is established by the pressure difference between the atmospheric pressure (hence the atmospheric reference), the vacuum in the canister, and the flow through the critical orifice. The net result is a constant flow (up to ~-5” Hg and then the sampling rate drops off). The critical orifice determines the flow range. The adjustable piston is used to set a specific, fixed flow rate within the flow range. An adjustment to the position of the piston changes the back pressure, which changes the pressure differential across the critical orifice. If the piston is lowered away from the diaphragm, the flow rate will increase. If the piston is raised toward the diaphragm, the flow rate will decrease. And there you have your crash course in passive sampler theory of operation.
So the take away message for today is that the critical orifice is responsible for the flow range. Now how do we select the proper orifice? Let us start off with the scenario discussed in part III, in which we have a 6 L canister and we would like to sample for an 8 hour period.
We have the following:
- An evacuated canister starting at 29” Hg
- We are shooing for 29” Hg sampled down to 5” Hg
- This means 24” Hg (~83%) consumed
- 83% of 6 L = 5 L
- 5000 mL / (8 hr * 60 min/hr) = 10.3 mL/min
Now that we know we want a flow ~10.3 mL/min, we go to the chart below and see that we need a critical orifice of 0.0020”.
*Kit part #s are for the entire passive air sampling kit, which you may find here: http://www.restek.com/catalog/view/3361
*Orifice part #s are for the orifice only, which you may find here: http://www.restek.com/catalog/view/1177
OR… you could simply use the all new
Previously, I wrote about using a MS column for non-mass spec applications. There are occasions when you come across a method or journal article where a non-MS column is used in a mass spectrometer. People ponder if this is a good idea, and often contact Tech Support for guidance. So, can I use a non-MS column in my mass spectrometer?
The simple answer to that question is you absolutely can. There are times that based upon the target compounds, where the selectivity of one of these columns (ex: Rtx-1701, Rtx-20, etc.) may be required. With that said, there are a few things to consider when installing these columns in a GC/MS.
With a capillary column in a GC/MS, you want to minimize any potential for column bleed. Column bleed creates an elevated background, which decreases the signal to noise ratio. If the signal to noise ratio is decreased, detection limits and quantitation limits become elevated. Less bleed will also provide more reliable library matches and help minimize source maintenance.
To help minimize column bleed, start by using the thinnest film thickness that will provide the best separation for your assay. The less film you have, the less bleed you may have. Since you are using a MS, some coelution may be acceptable if quantitation ions are not common to both compounds. A film thickness under 0.50 µm is ideal, but sometimes a thicker film is the solution. Keep the transfer line temperature at least 20°C below the maximum temperature of the column. Exposing the column to constant high temperatures in the transfer line will cause bleed. Use the lowest possible oven temperature to elute/separate your compounds. Again, try to avoid the column’s maximum temperature. As a final note on bleed, remember that the potential for column bleed increases as polarity increases. A polyethylene glycol (wax) column will exhibit higher bleed than its less polar (dimethyl polysiloxane) cousin, such as the Rtx-1. But, always use the best column for your separation. It’s all about the chromatography, so you may need to clean your source a little more frequently.
With a PLOT column, one needs to keep any particles generated out of the source and the instrument’s turbo pump (if equipped). The best way to do that is with at least 5 meters of 0.18 mm or 0.25 mm internal diameter fused silica column. This section of capillary column can act as a particle trap and can handle the pressure drop of the MS vacuum. My colleague Jaap de Zeeuw wrote a blog post containing this information. I do want to emphasize that a good connection must be made. This can be problematic for leaks. Ideally, detectors, such as thermal conductivity (TCD), flame ionization (FID), or discharge ionization (DID) are better suited with PLOT columns due to the nature of the compounds being analyzed.
So, if you are contemplating a non-MS column for your GC/MS, review the considerations above and go for it. By following the above guidelines and knowing what you are getting into, you will be able to obtain reliable chromatography on your GC/MS. The tradeoffs can often times be worth the selectivity derived for one of these columns. Thank you for reading and good luck with your analyses!
I have talked to a few customers this week who were having some trouble identifying the sizes of the ends for our Swagelok® and Parker® Tube-End Reducers, so I thought I would write a post to clarify this information.
A general photo of the style of a Tube-End Reducer which we sell is shown below. The Red Arrow points to the “Tube” side, and the Green Arrow to the “Compression” side.
The table directly below lists our Swagelok® Tube-End Reducers. The first dimension (left-side) is the outside diameter (OD) of the “Tube” end (as shown by the Red Arrow above) and the second dimension (right-side) is the “Compression” end (as shown by the Green Arrow above). The “Compression” end is listed as the size of the OD tubing it is designed to connect to. For example, Restek 23127 has a 1/4″ OD tube on one end and the other side (compression end) will connect to 1/16″ OD tubing.
The table below lists our Parker® Tube-End Reducers.
I hope this helps clarify the end sizes of our Tube-End Reducers. If you still have questions, email firstname.lastname@example.org. Thanks.
Many of you may have read my response in the Restek Advantage, where I was questioned about the “MS” designation on some Restek GC columns. I explained how the MS suffix is for a mass spec grade column. These MS designated columns are tested and guaranteed for low bleed performance, which is ideal for sensitive detectors like a mass spec. Another question, we receive about MS columns, is can I use a MS column for non-mass spec applications? Can I use this type of column with an ECD, FID, FPD, or NPD?
The simple answer to that question is “You absolutely can.” Using a column engineered for low bleed specifications is also beneficial with any non-mass spec detector. Detectors such as an ECD or NPD are also very sensitive and selective. In addition to low bleed, MS columns exhibit excellent inertness for better signal to noise ratios, reproducibility, and robustness. Those are desired characteristics in all of your chromatographic assays. The tight manufacturing quality controls used on MS columns ensure better performance.
Restek’s applications lab has generated numerous chromatograms using an MS column in conjunction with a non-mass spec detector. Some quick examples are residual solvents, terpenes, drugs, and petroleum hydrocarbons.
The next time someone asks about the MS designation, in addition to mass spec, I plan to note Most Sensitive, More Stable, More Signal, Most Stringent (testing), etc. The MS column is a great column to use with any detector.
In my next post, I will answer the question of can I use a non-MS column for mass spec applications.
Electronic Cigarettes Part VIII: Vapor Analysis – Why we chose multi-sorbent thermal desorption for analyzing VOCs like acetaldehyde
In the last installment of this blog series we let you know that we were utilizing triple-sorbent thermal desorption (TD) tubes to collect electronic cigarette vapor. However, we did not tell you why, but obviously I am going to do so right now. As you may already know, Restek dabbles in the field of whole air canister sampling. So obviously we attempted to collect and analyze e-cigarette vapor with canisters. This did work fairly well and we were able to see many of the VOCs like formaldehyde, acetaldehyde, and acrolein; however, we were not able to see nicotine and glycerin. It was not clear if the nicotine and glycerin were getting stuck in the canisters and/or held up in our air preconcentrator. Regardless, we wanted to see these major constituents of e-cig vapor in our analyses. Not to mention it is very unsettling to know you are losing some compounds in your sampling/analysis train, but you do not know where.
So that is where we decided to try TD tubes. Now to be completely fair, TD tubes are not the only game in town. There are various time-integrated and real-time instruments designed for the sampling and analysis of VOCs and SVOCs; however, only a limited number of these methodologies have been applicated to e-cigarette vapor testing. In fact, an exhaustive literature search produced the following peer-reviewed manuscripts, which have attempted to characterize electronic cigarette vapor: Goniewicz et al., Kosmider, McAuley et al., and Uchiyama et al. Goniewicz et al. utilized solid adsorbent tubes for fifteen carbonyl compounds (aldehydes and ketones) and twelve VOCs. Kosmider et al. and Uchiyama et al. utilized 2,4-dinitrophenylhyrdrazine (DNPH) coated silica cartridges to capture and analyze twelve and six carbonyls, respectively. And McAuley utilized solid adsorbent tubes for benzene, toluene, ethylbenzene, and m/p xylenes (BTEX).
We chose our multi-sorbent sampling and analytical approach, because it offered the following three distinct advantages over the aformentioned cited studies:
- The VOCs and SVOCs were not limited to a class of compounds (e.g., carbonyls). Therefore, a variety of alkanes, alkenes, aromatics, and halogenated compounds were evaluated.
- The multi-sorbent approach expanded our scope well beyond the previously limited BTEX lists to hydrocarbons in the C2 to C32 range.
- Derivatization and/or solvent extraction was not required. Like other studies, samples were immediately (i.e., <1 min) analyzed post- sampling, and therefore there was no need to form a “stable” carbonyl-hydrazone derivative, which then had to be solvent extracted.
Overall, the current method may be well suited for the easy and rapid screening of e-cigarette vapor for a large number of VOCs and SVOCs. So now we still need to address the compounds of interest we found and concentrations encountered. Stay tuned…
M.L. Goniewicz, J. Knysak, M. Gawron, L. Kosmider, A. Sobczak, J. Kurek, A. Prokopowicz, M. Jablonska-Czapla, C. Rosik-Dulewska, C. Havel, P. Jacob III, N. Benowitz, Levels of selected carcinogens and toxicants in vapour from electronic cigarettes, Tob Control 23 (2014) 133.
Kosmider, A. Sobczak, M. Fik, J. Knysak, M. Zaciera, J. Kurek, M.L., Goniewicz,Carbonyl compounds in electronic cigarette vapors: effects of nicotine solvent and battery output voltage,Nicotine Tob Res 16 (2014) 1319.
T.R. McAuley, P.K. Hopke, J. Zhao, and S. Babaian, Comparison of the effects of e-cigarette vapor and cigarette smoke on indoor air quality, Inhal Toxicol 24 (2012) 850.
Uchiyama, K. Ohta, Y. Inaba, and N. Kunugita, Determination of carbonyl compounds generated from the e-cigarette using coupled silica cartridges impregnated with hydroquinone and 2,4-dinitrophenylhydrazine, followed by high-performance liquid chromatography, Anal Sci 29 (2013) 1219.
You’re ready to install a column into your GC and realize you do not have the correct ferrules. Which ferrule do you choose? The parameters to consider when choosing the correct ferrule are instrument make and model, nut size and type, ferrule material, and column ID. A visit to our ferrules home page will provide a general overview of various ferrules offered by Restek. By navigating the page, you can narrow the selection by choosing these parameters. For my final post on ferrule selection I will cover Shimadzu GC options and tips.
Shimadzu 17A, 2010, and 2014 GCs use ferrules with two piece construction to install capillary columns. They also use specific nuts for column installation into Shimadzu inlets and detectors. Restek provides both these items for capillary column installation. The nut without the slot is a Restek design to increase its lifetime and durability.
Restek also offers an injector nut kit that will allow the use of standard compression ferrules in Shimadzu GCs.
Shimadzu GCs packed column hardware is commonly metric sizes- Restek does not supply metric size fittings or nuts. The only metric size packed column ferrules we sell are for Shimadzu model 17A. They have an ID of 5mm to accommodate a glass packed column (not manufactured by Restek). Restek can supply stainless steel packed columns for Shimadzu GCs but the end-user should check their existing fittings and adaptors.
This concludes my series on ferrule selection. I hope the information has been beneficial.
Old man winter is gone and the temperatures are rising… but why are my TO-15 air canister pressures rising too?
In case my colleagues have not gone on record with the following, I shall: “customer inquiries make for THE best blogs!”
So… as you can imagine I have been receiving some customer inquiries as of late. And it should come as no surprise that these questions are related to air sampling during the recent winter season. A couple of them have gone a little like this:
“They sampled ambient air into 6L cans back in February in Illinois at an outdoor ambient temperature of -3 F. The vacuum gauge readings for four of their cans read around 5 to 7 “Hg in the field. The lab report is showing the lab took canister vacuum/pressure readings upon receipt at the lab at ambient indoor temperature, most likely around 70 F and recorded gauge readings that were quite a bit different. Some were recorded at 1″Hg, some at 0, and one or two at 1psi.”
The rest of the inquiry goes on to ask if the aforementioned observations are reasonable or do we think there was a canister leak, etc… Short answer – YES, the aforementioned observation is reasonable. Long answer – continue reading.
Without making this blog too long, we can use the combined gas law in the following fashion:
Pi = 5” Hg*
Vi = 5 L
Ti = -3 F = 252 K
Vf = 5 L
Tf = 70 F = 294 K
This means we are solving for Pf. If we do so with the aforementioned information we get Pf = ~ 0.8” Hg. So everything checks out and life is good. *I find it much easier to work with all of the aforementioned in psia, so consider that if you find yourself getting twisted up with ” Hg. Hopefully the Illinois field technician has thawed out by now! Just remember… if the temperature goes up, so does the pressure.
In my last blog, I showed how FID is a more suitable detection method for cannabis residual solvent analysis than MS. But what about terpene analysis? Can our old friend the FID hold its ground against the mighty mass spectrometer for this application? Actually, it can!
Terpenes are much larger molecules than residual solvents, so terpene analysis via MS doesn’t suffer from low molecular weight interferences like we see for residual solvents. However, terpenes fall into another weakness of MS: mass spectrometers are wonderful at separating and quantifying ions of different masses, but if a MS encounters two co-eluting compounds with the same mass and/or mass fragments, there is no way for the MS to differentiate between those compounds, which means that they can’t be quantified/identified separately. Let’s take a look at a table of the molecular weights and major ions for a bunch of terpenes of potential interest to the cannabis industry:
In this list of 37 terpenes, we actually only have 11 unique molecular weights and 12 unique major fragment ions. If you look through the list, you can see that these compounds share a lot of ions. To show a graphical example, we can look at the mass spectra for alpha-phellandrene and delta-3-carene, which elute very close to each other on Restek’s recommended column (by the way, you’ll always have groups of closely-eluting terpenes on any column you use – there are just too many of them to separate them all to baseline):
Mass Spectrum for Alpha-Phellandrene from NIST Database*
Mass Spectrum for 3-Carene from NIST Database*
These spectra are almost identical, and will be very difficult to deconvolute for quantitative purposes. There are many more cases like this amongst all the terpenes since they’re all part of the same structural class. So basically, you can definitely analyze terpenes using MS, but you lose much of the advantage MS gives you from a spectral deconvolution standpoint, since the MS can’t really tell the difference between many of these terpenes anyway. You might achieve a slight increase in sensitivity using MS versus FID, but since we’re interested in terpenes in the percent level (even fractions of a percent), FID has plenty of sensitivity at a fraction of the cost.
So once again, the humble (and affordable) GC-FID is the best choice, even over MS. I hope this helps explain why. Please feel free to comment or contact me with any questions or comments, and if you’d like to read more on terpene analysis, check out our application note.
Over the past few months, I’ve gotten numerous questions about the best detection method for terpenes and residual solvents in cannabis. It seems that a lot of people are purchasing GC-MS instruments for both of these analyses. While GC-MS is indeed a powerful tool, it’s not really necessary for either analysis. In fact, the use of MS for residual solvent analysis can be problematic enough to make its use prohibitive. So what’s the best analysis and detection method for both terpenes and residual solvents? The humble GC-FID. If you’re interested in the reasons why, read on! I’ll be splitting this blog into two parts (residual solvents and terpenes), so make sure to stay tuned for the next portion on terpenes to be posted in a few days.
When most people think about doing trace analyses, their first thought is to go with the most sensitive piece of equipment they can easily get their hands on, which is MS, which can be operated in selected ion monitoring mode. Most of the time, this is a really good approach, but for residual solvents – especially cannabis residual solvents – this can backfire on you due to the air that is injected along with your headspace sample. Remember headspace sampling involves injection of a large volume of gas from your headspace vial. This is completely different from a liquid injection in which a very small amount of liquid is injected and no air is introduced into the system.
Let’s pretend that we have a sample of headspace containing 500ppm of butane. That’s a lot of butane, but if we think about it another way, our sample contains 99.95% room air. Since butane often elutes under the tail end of our air peak, when butane elutes from our column, it’s also eluting with a much larger amount of air. When both butane and air are introduced into a mass spectrometer, the much larger amount of air will interfere with butane in the MS source. This interference may cause signal suppression, resulting in loss of sensitivity and linearity.
In addition to direct interference of air for butane, most of the rest of our analytes (through pentane) all share very small mass fragments. One of the drawbacks for MS is that in general, MS has low detectability for low molecular weight fragments due to background interferences from leaks, column phase, and carrier gas impurities. So most of the sensitivity you gain by purchasing a MS is lost due to interferences. In reality, FID is generally at least as sensitive as MS for analysis of low molecular weight volatiles, if not more, and FIDs are blind to air, which is important with headspace analyses of early-eluting compounds.
Remember to stay tuned for part two of this blog, which will discuss terpene analysis via GC-MS versus GC-FID!