TO-15 + PAMS + TO-11A = China’s HJ759 + PAMS + HJ683 part 2: Deans switching and TO-15/PAMS

In a previous blog (TO-15 + PAMS + TO-11A = China’s HJ759 + PAMS + HJ683) Jason Herrington mentioned a dual column MS/FID setup for China’s combined HJ759 + PAMS + HJ683 method. While this could be done with a simple Y splitter (such as https://www.restek.com/catalog/view/1983), a more elegant solution is to use a microfluidic switch, or Deans switch, to send some compounds to the secondary column and FID while maintaining the bulk of the analysis on the MS.

So how does it work? The Deans switch is composed of a pressure control module (PCM), a solenoid valve, and a 3 port switching plate. The primary column is connected to the switching plate with a short transfer line to the primary detector (MS), and a second column to the secondary detector (FID). The solenoid valve directs auxiliary carrier gas flow to the plate, with one of the outlet ports at higher pressure than the other, as shown in Fig. 1 below. The larger arrow on the MS output end of the Deans switch shows the higher pressure that directs the flow to the FID when the switch is on, and vice versa. A smaller pressure is applied to the other side to ensure that the flow from column 1 doesn’t backflow to the PCM.

Fig. 1 – Deans switch operation, with the flow from column 1 shown in red.

 

This has several advantages over simply splitting the flow. Since the entire sample isn’t passing through the secondary column it can be chosen without concern over it being robust enough to handle everything in the sample. No worries about trying to elute less volatile compounds off your thick film or plot columns. Also, by not splitting the sample sensitivity is maintained without having to decrease split ratios or increase injection volumes. It is important to note though that the Deans switch does increase carrier gas flow on the restrictor and column 2 due to the extra flow from the switching plate, so your MS may see a slight decrease in sensitivity. The extra flow is either 50% or at least 1mL/min more than column 1, so if your primary column flow is 2mL/min your final flow to the MS will be 3mL/min, so keep in mind the pumping efficiency of your MS.

What does it look like in the end? With no cryogenic cooling we have complete analysis of 112 VOCs in 35 minutes. The Deans switch sends the C2 and C3 hydrocarbons at the beginning of the run to the secondary column and FID for better separation and detection, then switches the rest of the run to the MS.

 

Fig.2 – FID chromatogram of C2 and C3 hydrocarbons at ~1ng on column.

Fig. 3 – MS chromatogram of PAMS compounds at ~1ng on column.

Fig. 4 –MS chromatogram of PAMS + HJ759 compounds at ~1ng on column.

Peaks TO-15 PAMS TR (min) Peaks TO-15 PAMS TR (min)
1 Ethane X 7.677 59 Carbon tetrachloride X 20.927
2 Ethylene X 8.68 60 3-Methylhexane X 21.011
3 Propane X 10.363 61 Benzene X X 21.401
4 Propylene X X 15.583 62 1,2-Dichloroethane X 21.513
5 Acetylene X 17.863 63 Isooctane X X 21.638
6 Dichlorodifluoromethane X 6.567 64 Heptane X X 22.01
7 1,2-Dichlorotetrafluoroethane X 7.148 65 1,4-Difluorobenzene X X 22.312
8 Isobutane X 7.204 66 Trichloroethylene X 22.841
9 Chloromethane X 7.348 67 Methylcyclohexane X 23.348
10 trans-2-Butene X 7.761 68 1,2-Dichloropropane X 23.399
11 n-Butane X X 7.859 69 Methyl methacrylate X 23.422
12 Vinyl chloride X 7.868 70 1,4-Dioxane X 23.506
13 1,3-Butadiene X 8.045 71 Bromodichloromethane X 23.915
14 cis-2-butene X 8.193 72 2,3,4-Trimethylpentane X 24.128
15 1-Butene X 8.621 73 2-Methylheptane X 24.472
16 Bromomethane X 9.322 74 3-Methylheptane X 24.751
17 Chloroethane X 9.796 75 cis-1,3-Dichloropropene X 24.755
18 Isopentane X 10.163 76 4-Methyl-2-2pentanone (MIBK) X 24.988
19 Vinyl bromide X 10.586 77 Toluene X X 25.415
20 Trichlorofluoromethane X 10.888 78 n-Octane X 25.555
21 1-Pentene X 11.013 79 trans-1,3-Dichloropropene X 25.801
22 n-Pentane X X 11.269 80 1,1,2-Trichloroethane X 26.186
23 Ethanol X 11.617 81 Tetrachloroethene X 26.381
24 trans-2-Pentene X 11.831 82 2-Hexanone (MBK) X 26.502
25 Isoprene X 12.24 83 Dibromochloromethane X 26.893
26 cis-2-Pentene X 12.296 84 1,2-Dibromoethane X 27.125
27 Acrolein X 12.63 85 Chlorobenzene-d5 X X 27.84
28 1,1-Dichloroethene X 13.025 86 Chlorobenzene X 27.891
29 1,1,2-Trichlorotrifluoroethane X 13.113 87 Ethylbenzene X X 27.994
30 Acetone X 13.192 88 n-Nonane X X 28.11
31 2,2-Dimethylbutane X 13.215 89 m- & p-Xylene X X 28.179
32 Isopropyl alcohol X 13.796 90 o-Xylene X X 28.769
33 Carbon disulfide X 13.875 91 Styrene X X 28.793
34 Allyl chloride X 14.53 92 Bromoform X 29.127
35 2,3-Dimethylbutane X 15.046 93 Cumene X X 29.285
36 Methylene chloride X 15.06 94 4-Bromofluorobenzene X X 29.573
37 2-Methylpentane X 15.176 95 1,1,2,2-Tetrachloroethane X 29.712
38 Cyclopentane X 15.292 96 n-Propyl benzene X X 29.87
39 Tertiary butanol X 15.469 97 1,2,3-Trimethylbenzene X 29.963
40 Methyl tert-butyl ether (MTBE) X 16.068 98 n-Decane X 30.01
41 trans-1,2-Dichloroethene X 16.096 99 p-Ethyltoluene X X 30.024
42 3-Methylpentane X 16.133 100 2-Chlorotoluene X 30.052
43 1-Hexene X 16.783 101 1,3,5-Trimethylbenzene X X 30.084
44 Hexane X X 17.071 102 m-Ethyltoluene X 30.377
45 1,1-Dichloroethane X 17.587 103 1,2,4-Trimethylbenzene X X 30.609
46 Vinyl acetate X 17.587 104 1,3-Dichlorobenzene X 31.06
47 2,4-Dimethylpentane X 18.711 105 o-Ethyltoluene X 31.171
48 Methylcyclopentane X 18.958 106 1,4-Dichlorobenzene X 31.185
49 2-Butanone (MEK) X 19.194 107 Benzyl chloride X 31.311
50 cis-1,2-Dichloroethene X 19.246 108 m-Diethylbenzene X 31.352
51 Ethyl acetate X 19.297 109 p-Diethylbenzene X 31.483
52 Bromochloromethane X X 19.873 110 n-Undecane, X 31.524
53 Tetrahydrofuran X 19.896 111 1,2-Dichlorobenzene X 31.673
54 Chloroform X 20.128 112 n-Dodecane X 32.853
55 1,1,1-Trichloroethane X 20.551 113 1,2,4-Trichlorobenzene X 33.745
56 2-methylhexane X 20.57 114 Hexachlorobutadiene X 33.866
57 Cyclohexane X X 20.723 115 Naphthalene X 34.172
58 2,3-Dimethylpentane X 20.797

 

 

That covers the PAMS and HJ759 methods, but what about HJ683? Don’t worry, there’s more to come on this application soon.

A Tale of Two Columns (CLPesticides and CLPesticides2)—Part III: Using the GC Accelerator Kit for Dual Column Analyses

In my previous two blogs (Part I and Part II), I mentioned the use of Restek’s GC Accelerator Oven Insert Kit (cat. #23849) for making your methods even faster.  The GC Accelerator kit was originally released with the intent of being used with an Agilent GC-MS system; however, this same kit can also be used for speeding up dual column / detector analyses, such as EPA 8081.  In this blog, I am going to show you how to install the GC Accelerator kit for use with dual columns.

Step 1: Obtain columns to be used in a tied format, without cages.  This is important, as it will allow for both columns to fit in the oven with the GC Accelerator kit installed.  When ordering columns, use the suffix “-051” after the column catalog number to request a specific column without a cage.

Step 2: Install the guard column in the back inlet and connect the two tied analytical columns to the guard using a connector, such as a “Y” press-tight.  Analytical columns can be installed in front and back detectors.  Ensure all columns are in the back position of the oven (Figure 1).  Be sure to pressurize and leak check before proceeding with installation of the GC Accelerator kit.

Figure 1: Guard column installed in back inlet, connected to two analytical columns using “Y” press-tight. Analytical columns installed into front and back detectors.

Step 3:  Install the first two blocks of the GC Accelerator kit as shown in Figure 2.  The column that is installed in the front detector should come through the opening to the right of the small block on top (Figure 3).

Figure 2: Blocks installed in front of columns.

Figure 3: Analytical column comes through the opening to the right of the small block to go into front detector.

Step 4: Lastly, install the insert of the GC Accelerator kit (Figure 4).  This piece will be pushed against the front inlet and front detector.  Close the oven door.  This should not take excessive force if GC Accelerator plate is installed correctly.

Figure 4: GC Accelerator insert installed.

That’s all there is to it!  In my final blog I will show you a method for accelerating run times for 8081 on the CLPesticides and CLPesticides2 columns, both in 30m x 0.32mm ID and 30m x 0.25mm ID formats.

A Tale of Two Columns (CLPesticides and CLPesticides2)—Part II: Gaining Speed

In my previous blog post, I gave you a little history of the CLPesticides columns.  You’ll remember that I pointed out the 24 minute run times, which were promoted as being fast at the time.  Fortunately, there are ways to attain faster runs on this column pair for standard 8081 pesticides, due to their awesome selectivity and high plate count.

So let’s talk a little about fast GC…what variables can we control to get faster runs?  Here are a few:

  1. Increase carrier gas flowrate: Higher linear velocities of carrier gas will reduce analysis time. The catch is that after a certain point, higher flows can sacrifice separation capacity.  If a column has a sufficient number of plates, however, this can be a powerful way to speed up a method and still maintain acceptable separations.
  2. Increase temperature ramp rate: Increasing the programming rate will allow for quicker elution of compounds. At the same time, a ramp rate that is too high can lead to co-elutions.  If enough plates are present and the column has good selectivity, the limiting factor may ultimately become the instrument itself and its maximum ramping capability.  In the next two blogs I will offer a solution to push an Agilent GC even further and achieve ramp rates that weren’t previously possible.  (Spoiler alert: It involves a product known as the GC Accelerator Oven Insert Kit, cat# 23849.)  As a final word of caution, increasing the temperature ramp rate can also result in changes in elution order of compounds, so care must be taken.
  3. Use hydrogen carrier gas: Because of the higher diffusivity of hydrogen compared to helium or nitrogen, it has a higher optimum linear velocity, providing more separation capacity at higher flow rates comparatively (see point #1 above). The Van Deemter Plot (Figure 1) shows how the optimum linear velocity of hydrogen compares to helium and nitrogen. If switching carrier gases, check out Restek’s EZGC Method Translator, which will allow you to transfer an existing method from one carrier gas to another, without changing your separation.  For additional information on using alternate carrier gases for organochlorine pesticides, check out this guide: https://www.restek.com/pdfs/EVAR1935-UNV.pdf.

    Figure 1: The Van Deemter plot shows the height equivalent of a theoretical plate vs the average linear velocity of different carrier gases. A lower plate height leads to more overall plates per length of column, thus allowing higher separation capacity. Hydrogen has the highest optimum linear velocity.

  4. Shorter columns and reduced I.D.: As intuition would suggest, it takes analytes less time to travel through a shorter column than a long column. If all other things are equal, however, you sacrifice plates and could lose resolving power if you don’t have much to spare in the first place.  The best approach for this to be effective is to actually “scale down” your column; that is, not only make it shorter, but also reduce the inner diameter to increase interactions with the stationary phase.  By doing this and also keeping the phase ratio the same, you can achieve the same separation at a faster speed.  In order to ensure your separation remains the same, use Restek’s EZGC Method Translator, which allows you to translate methods from one column dimension to another.  Note that decreasing length and I.D. will require a higher oven ramp rate to maintain the same separation, which may exceed the limits of your instrument.  (Spoiler Alert #2: This can also be solved with the use of Restek’s GC Accelerator Kit, cat# 23849.)

A few years ago, Restek set out to provide a faster run time on the CLPesticides and CLPesticides2 columns.  Innovations Chemist, Jason Thomas, utilized the first 3 above points to elute all 8081 compounds in under 7 minutes, with near baseline resolution.  Check out this method here: https://www.restek.com/chromatogram/view/GC_EV1325. This method uses hydrogen carrier gas at a high flow rate and a multi-step ramp rate with a fast beginning and ending ramp.  This was performed utilizing 30 meter columns with 0.32mm inner diameters, the most popular column format for this analysis.

Due to instrument ramp rate limitations, this method is about as fast as possible using a standard Agilent 120V instrument.  In order to gain any more speed and maintain similar resolution, we must find a way to increase the maximum achievable oven ramp rates.  Enter the GC Accelerator Kit.  Stay tuned for parts 3 and 4 to see how to use this kit to shave a couple more minutes off of the analysis.

A Tale of Two Columns (CLPesticides & CLPesticides2)—Part I: A Little History

Chlorinated pesticides are persistent environmental contaminants commonly analyzed using a variety of GC methods, including US EPA 8081, 608 and 508.  Due to similarities in chemical properties of these pesticides, selectivity must be carefully considered when choosing GC columns.  Historically, columns with phenyl methyl (5% phenyl, 35% phenyl, 50% phenyl columns) and cyanopropyl (1701 column) were recommended; unfortunately, the phenyl methyl stationary phases do not offer the best selectivity for the commonly analyzed pesticides and some coelutions are present.  While the Rtx-1701 offers better selectivity over the phenyl methyl phases, one drawback is a lower maximum programmable temperature.  This allows build-up of less volatile matrix components within the column, both affecting performance and reducing lifetime.  The 1701 columns also may degrade active components, such as DDT and methoxychlor, due to interactions with the stationary phase.

In the late 90’s, as an answer to the aforementioned woes facing analysts, Restek released a pair of proprietary columns known as the CLPesticides and CLPesticides2 columns.  These columns feature excellent selectivity of chlorinated pesticides and complement each other well for dual column analyses.  With the release of the CLPesticides and CLPesticides2 columns, baseline resolution of the standard 8081 method pesticides was now possible and the columns could be operated at higher oven temperatures to ensure “baking out” of contaminants from the dirty extracts typically found in environmental samples.

Upon the initial release of the column pair, one of the marketing claims was the ability to baseline resolve the common method 8081 pesticides in under 24 minutes (Figure 1 and 2), an impressive feat at a time when 50-minute runtimes were common. Since then, fast GC has become a major focal point for many laboratories and a 20+ minute run would no longer be considered acceptable.  After all, time is money!

Figures taken from original Restek marketing literature.

Fortunately, since that time, Restek has optimized methods on these columns to obtain considerably faster run-times.  In the upcoming Part II of this blog series, I will discuss some ways in which faster run times can be achieved.  Parts III and IV of this series will push the pedal even further using Restek’s recently developed GC Accelerator Kit with Agilent GC’s to achieve elution of all compounds in close to 5 minutes with complete instrument cycle times around 10 minutes.  Stay tuned for details…

Pesticides are like Siblings – some get along well and some don’t – No. . .Really?

My colleague, Joe Konschnik and I have been asked by many food chemists out there about how much they can trust their pesticides mixes after they combine them into one single larger mixture for calibrating their instrument, or monitoring the accuracy and precision of their method – and rightfully so.  Well, like siblings, sometimes they get along well and sometimes they don’t.  As with many things we purchase there’s an expiration date on the label.  Through much testing and experience, companies can predict when our meat will spoil, when our canned goods are no longer safe and when our kid’s car seats need to be replaced.

If you’ve ever purchased a reference standard or certified reference material (CRM), you’ll notice there’s an expiration date on the label.  These dates tell you how long the components in that solution are stable and the date by when it should be used.  However, once you crack open the standard ampule, or other container, and especially when you mix its contents with any other CRM, chemical or matrix, that expiration date no longer applies.  All CRMs, which are those reference materials manufactured according to the strict ISO 17034 standard, are required to remain stable only until the container is opened for use.

To illustrate what happens when you combine the ten mixes contained in RESTEK’s multiresidue pesticides LC kit (Catalog #31971) let’s look at a recent study we did at Restek where over 200 pesticides were spiked into then extracted from celery matrix.  You can see the featured application here.

For this analysis we spiked all of the components of the LC Multiresidue Pesticide Kit  into the celery matrix.  The LC Multiresidue Pesticide Kit contains 204 pesticides that are grouped into 10 different vials for long-term stability.

Organophosphorous, Carbamate, Uron, and Organonitrogen pesticides are all represented in these mixtures.  However, once all ten vials are mixed, some pesticides in the mix begin to degrade AND matrix can further accelerate this degradation.

While CRM suppliers may have a lot of data regarding the stability of a single, or multi-component reference standard, very little data exists for what happens once they are mixed together.  Joe suggested we try an experiment to find out what happens and I agreed so here’s what we did.  For this experiment, the pesticides were solvated in 90:10 Water:ACN (containing celery matrix).  They were measured against a matrix matched calibration curve and a stable internal standard to determine precision, accuracy and concentration during the evaluation.  Over the testing period the samples were stored on a Peltier cooled autosampler (4 °C).

After a single day, 14 residues had lost 20% of their signal and after the entire eight day trial, more than 50 residues had lost more than 20% signal.  To compare the effect of matrix on the degradation, a matrix free study was performed with the same concentrations and solvents.  After eight days, only 19 residues lost 20% signal, showing that even a simple matrix like celery can hasten degradation.

This observation showed us our calibration curves and standards should be made daily.  The interactions with the pesticides themselves and the interactions of the pesticides with the matrix required it.  While this meant more work at the beginning of each day, we see this as a best practice for how to obtain data with the greatest integrity.

To clarify, this doesn’t mean having to crack open a new ampule each day, but simply using new aliquots from the vials provided with your ampules.  Upon opening each ampule, the contents should always be transferred to the vial provided and properly stored following the guidance in your CoA for future use.

We suggest, based on this information, it would seem appropriate for each lab to determine their own laboratory specific quality assurance procedures for handling and measuring the stability of their pesticide mixtures prior to and during analysis.  The decision whether or not to reuse a mix should be based upon a study which determines the stability of the compounds of interest in specified matrices.  This will ensure the best practices for handling reference materials for their intended purpose.

…by the way, May 2nd is National Brothers and Sisters Day according to National Today https://nationaltoday.com/national-brothers-sisters-day/, so don’t forget to reach out to thank, congratulate, tease, hug or otherwise irritate that special sibling of yours.  And. . .good luck getting along with your sibling(s)!!!

Aura Personal Air Samplers for NIOSH Canister Method 3900

The National Institute of Occupational Safety and Health (NIOSH) has published their “Volatile Organic Compounds, C1 to C10, Canister Method 3900 in the NIOSH Manual of Analytical Methods (NMAM), Fifth Edition. In short, the method is a variant of United States (U.S.) Environmental Protection Agency (EPA) Method TO-15; however, with a focus on a particular subset of VOCs often encountered in the occupational setting at low ppmv concentrations. So, why am I bothering to blog about this when we already have countless TO-15 blogs? Well, it just so happens that this long-awaited document is the first validated method for capillary flow-controlled sampling into evacuated canisters. Ta da!!! Queue the Aura Personal Air Sampler! Oh what, you do not know the Restek Aura? We can fix that…

A couple of years ago, we launched the Aura Personal Air Sampler kit (shown above) for the Environmental and Occupational Health/Industrial Hygiene market. The impetus was to provide a personal air sampling alternative, which overcomes some of the short-comings associated with active and passive (diffusive) solid sorbent sampling approaches. The heart of the Aura is a capillary flow controller, which will constantly sample at ~0.31 mL/min into a 400 mL canister over an 8-hour sampling duration. Think of the capillary flow controller like an elongated version of the critical orifice you may be familiar with in our soil gas and passive flow controllers. The biggest difference is that it achieves relatively constant flow rates (i.e., <10% change) well below anything the aforementioned devices can achieve. The other advantage is that we are talking about a sampling device light as a feather, especially when compared to standard flow controllers. This is canister-based sampling, so no need to know the atmosphere and select the appropriate sorbent, as required with other personal air sampling approaches. The Aura covers everything from C1 to C10. There is also no need for a mass-rich, power-dependent, noise-making, calibration-requiring sampling pump. You know, the same sampling pumps test-subjects despise wearing. See the photo below where I can freely move about with the confidence and comfort of full mobility, minimal weight addition, and no added noise. That’s it for now… Oh wait, the list of awesomeness associated with Aura continues. We ship all of the capillary units at pre-calibrated flow rates, so sampling with the Aura is about as easy as it gets. Okay, now I am done with the boasting.

Here are the full Aura instructions, but consider the following your Aura crash course:

  1. Attach the capillary flow controller to a quick connect.
  2. Attach the quick connect to the 400 mL canister.
    1. Sampling thereby commences.
  3. Place the Aura into the Aura purse.
  4. Affix to the sampling subject.
  5. Wait 8 hours and then reverse the process.

Easy peasy…  As for post sampling, go ahead and analyze the Aura just like you would any other canister sample. And NOW… you can do all of the aforementioned with the confidence the approach is backed by NIOSH Method 3900. If you want to know more about Aura and just happen to be out in Minneapolis, Minnesota on May 20-22, be sure to stop by at the American Industrial Hygiene Conference and Exposition (AIHce). Aura will be featured at our vendor booth and the Learning Pavilion (Expo Hall) on May 21, 1130 – 1155 am.

 

TO-15 + PAMS + TO-11A = China’s HJ759 + PAMS + HJ683

It is very fitting that I write this blog while I am in Shanghai, China. The impetus for this blog, and the blogs to follow, is that the Chinese Ministry of Ecology and Environment (formerly the Ministry of Environmental Protection) published their “VOCs Monitoring Scheme of Environmental Air Quality.” In short, this standard outlines the sampling and analysis of 117 VOCs via the following three methods: HJ759, PAMS, and HJ683. These methods are very similar to U.S. EPA Methods TO-15, PAMS, and TO-11A, respectively. They have done so, because PM2.5 is not solely responsible for hazy skies and VOCs play a critical role in atmospheric reactions, which generate ozone, smog, etc. The following picture illustrates why the People’s Republic of China (PRC) is interested in VOCs:

In the photo, those are large shipping boats on the Yangtze river. I took this photo on final approach at 4 pm, and no, it was not rainy or overcast. This was smog. And it gets better… I could actually see the sun today, which I did not see during my 13 day visit in November of 2018. So, this was a good day! Okay, you get the point.

Before we break down HJ759 and PAMS, let us quickly review up to present day: I have spent the last several years presenting at conferences, publishing blogs, and writing application notes on TO-15. However, there is also the Photochemical Assessment Monitoring Stations (PAMS) network, which utilizes a lesser-known and -discussed canister-based method; since this method is executed by government monitoring sites and laboratories. We have countless blogs, chromatograms, etc. for TO-15, but we have never blogged about PAMS and confess that we only have one PAMS chromatogram. Despite our lack of focus, PAMS is interesting. Long story short, the PAMS network was put in place for the monitoring of ozone. Therefore, this network focuses in on aliphatic, aromatic, and carbonyl VOCs. Whereas, TO-15 has some overlap, but places more of an emphasis on halogenated VOCs. Overall, the sampling and analysis is very similar between the two methods. The divergence is in the target analyte lists. For more on PAMS, be sure to check out the EPA’s website.

Now for present day: The PRC has used TO-15 as the model for HJ759. Most of the method is very similar, which makes sense. And PAMS is simply PAMS. Why reinvent the wheel, when the U.S. already tackled the same problems over 3 decades ago. However, there appears to be a new trend coming out of the PRC, which is a push for one sampling and analytical approach for all the VOCs found in To-15 (HJ759 in China), PAMS (PAMS in China), and TO-11A (HJ683 in China). I have the following mixed feelings about this movement:

  1. I love the fact that manufacturers/providers like Restek are being pushed to develop innovative approaches for meeting the PRC’s desire for one analytical method.
  2. There is a reason TO-11A requires the sampling of carbonyls by derivatization on cartridges and not by collection into canisters in the U.S. We already know this does not work well in canisters. However, despite the PRC document specifying the use of cartridges for carbonyls, some of the air sampling community in the PRC seems to think otherwise. For the record, today’s blog will only focus on combing TO-15 and PAMS, and not TO-11A. More blogs to follow on this specific topic later.

The concept of combining the TO-15 and PAMS target analyte lists into one analytical run is not new. In fact, I have been presenting a lot of my canister cleaning work at the National Environmental Monitoring Conference (NEMC), Air and Waste Management Association (A&WMA), and to the U.S. EPA since 2014; for all of which I used the following single analysis combining TO-15 and PAMS target analyte lists:

As you may see in the above chromatogram, there were 116 compounds (i.e., 111 target analytes, 3 internal standards, and BFB). There were only 2 critical coelutions (2-Methylpentane and vinyl acetate; and n-hexane and ethyl acetate), which I never bothered to work out, because this method was only used internally. Not sure why I never published this method until now.  Wait, I take that back! Here are the following reasons:

  1. I believe most laboratories do not have interest in the above, as they are conducting either TO-15 or PAMS, not both; and/or not in one analysis. This is evident since we have never received a request.
  2. This method requires on-column cooling, which labs tend to avoid.

That is of course until now! The PRC is pushing the envelope and asking for just that (i.e., the single analytical method). It would be nice to know that we just have to work out those 2 critical coelutions and have a solution. But it is not quite that simple. We need to develop further because:

  1. Preconcentrator instrument manufacturers have moved away from using liquid nitrogen to cool their traps. These makes on-column cooling that much less attractive.
  2. The C2 compounds in PAMS (ethylene, acetylene, ethane) do not respond well on a mass spec, so the desired detection limits (200 pptv) may be hard to achieve.

Okay, I have reached my personal blog size limit and jet lag remains present. So, stay tuned for the next blog where we show you a dual column GC-FID-MS analysis with all 116 TO-15 (HJ759) and PAMS compounds resolved. Oh yeah, if you happen to be in Shanghai this week, be sure to stop by our booth at the IEexpo.

Choosing Your Citral Column

Image credit: Wikipedia

The name ‘citral’ or 3,7-dimethyl-2,6-octadienal, suggests the scent of lemons so it’s not surprising that another name for this compound is lemonal. There are two isomers of this compound with the same chemical formula (C10H16O); geranial (citral A) and neral (citral B). As seen in the image to the right, the difference between these compounds is subtle1.

Geranial and neral both have a lemon scent, however, neral has a milder, and sweeter lemon odor. These compounds are used individually or together depending upon the desired scent or flavor since they are used in perfumes, candy and even soft drinks. They can also be added to enhance other flavors for artificial grapefruit, orange and lime. The most common uses are cleaning products, laundry and dishwashing detergents. Chances are good you will use citral today. Interestingly lemongrass has significantly higher amounts of citral compared to lemons1.

As you will see from the chromatograms below column choice is important. Two different columns were evaluated; the Stabilwax and the Rtx-Wax, for peak shape, resolution and bleed. The obvious column of choice is the Rtx-Wax as shown in the following chromatograms.

Shown below are chromatograms of a custom citral mix on both columns with the Rtx-Wax column showing excellent peak shapes and separation. The Stabilwax column shows a fronting effect leading up to the citrals, which can cause problems with integration/quantitation.

 

Figure 1: Image of Custom Citral Mix on an Rtx-Wax

 

Figure 2: Image of Custom Citral Mix on a Stabilwax, notice the fronting peak shape indicating interaction with the analyte and stationary phase.

 

Figure 3: Zoomed image of the Custom Citral Mix on the Rtx-Wax and the Stabilwax. The red trace represents Rtx-Wax whereas the blue trace represents the Stabilwax.

 

Figure 4: Chromatogram of lemon Oil using the Rtx-Wax column. Lemon oil contains percent levels of citrals.

Reference/link: https://www.chemistryworld.com/podcasts/citral/3008193.article

 

Paul’s excellent questions on “Liner Selection for HS VOCs”

I had a feeling the blog I posted yesterday was sure to prompt some thought-provoking questions, as some of my peers had already been doing so. So, it came as no surprise that Paul posted the following excellent questions (in black) to which I have responded to in blue. Normally, I would just address all this in the comments section, but in my opinion the comments section tends to get lost in the weeds. In addition, the questions and my responses were all so long it justified the following blog, so I will respond to the following questions from Paul below.

Paul says:

First thanks for this blog series. I like fundamentals. I read the linked article by Jason S. Herrington and got a few questions about it.

Thank you for the thoughtful questions.

1. Why is there a difference in peak area between the 0,75 mm and 2 mm liner? The Peak width should be wider for the 2 mm liner but the Peak area should be the same. (Degradation or lost in the split?)

My alternative theory is that the 0.75 mm liner fits the fiber like a glove. So, thermal transfer to the fiber needle, fused silica, and most importantly phase is more efficient. Add in the increased velocities of the 0.75 mm liner and now we have further pushed the partitioning equilibrium in favor of desorption. This all translates into faster and more complete partitioning of VOCs out of the fiber. Honestly, although never explicitly stated, I thought this was all the “logic” behind other vendors pushing the 0.75 mm liners for SPME. But then again, I say “logic” because I certainly do not see the data to support any of the above, so it appears to be based on what I shall call logical theory. Of course, maybe it was all just in my head in the first place. Bottom line, the lack of data surrounding the subject (i.e., SPME liner dimensions) was the impetus for me to collect this data in the first place.

2. With a split of 1:5 the velocity in the inlet is much higher than with splitless injection. I can imagine with the higher velocity in the liner because of the split, the liner diameter is not that important any more.

My one colleague has been hounding me about this very point. Obviously, you are both correct about the split minimizing the significance of liner dimensions. Confession: in my previous blog, I made a serious mistake by not justifying the 5:1 split due to the chromatography looking so horrible without the split. TEASER: I ran the same type of experiments in splitless, and initial review of the data continues to say that liner dimensions do not make a Tinkers Dam for HS-SPME VOCs. Future blog to follow…

3. Why is the % RSD of the 2 mm liner seams to be better than for the 0,75 mm liner?

Ah, you saw this too!? I am not entirely sure this is a consistent trend. Perhaps we will know further in the future with a more substantial data set. With that said, I will pose the following theory to explain this observation: as stated above, the 0.75 mm liner fits around the SPME fiber like a glove. Well this means that the SPME fiber has occupied a lot of the available real estate in the liner. This could translate into one or both of the following phenomenon taking place, which may explain the observation in question:

  1. Flow has been constricted/obstructed in the 0.75 mm fiber and this impacts the split, thereby causing some inconsistencies. For the record, this may be a contributing theory to help explain the discrepancy we addressed in the first question.
  2. The SPME fiber “clogging” up the 0.75 mm liner results in turbulent flow, which also causes some inconsistencies.

Everything here is theoretical and will be hard to isolate. Is it turbulent flow, higher velocities, better thermal transfer, inaccurate splits, residence time, etc… the list goes on ad infinitum. I say it is probably the culmination of all of the above, but doubt we will truly ever know. However, it is certainly fun to hypothesize and debate some of the theories.

4. Which role does the distance of the column to the liner Play in this case? (keyword tapered liner)

I have only been running straight-walled liners and have not evaluated column distance.

I hope the community got some ideas.

I hope so too, as the data surrounding SPME liners appears to be hidden. Thanks again for your questions.

SPME Fundamentals: Liner Selection for HS VOCs

My colleague Linx Waclaski is usually the one doling out excellent liner advice, so bear with me as I take a crack at this liner stuff. Ever since we came out with traditional SPME and the SPME Arrow, a lot of customers have had concerns/questions regarding SPME liner dimensions. The short answer to most of these questions is that none of this “makes a tinker’s dam” (an old expression my dad used to say to me, so as to let me know I was focusing in on inconsequential minutia) for head space (HS) volatile organic compounds (VOCs). Look up tinker’s dam for a fun fact. If you want the long answer (with appropriate caveats), turn to page 36 of the following:

The Column (17 December 2018 Volume 14 Issue 12)

I really wrote the current blog and the above article for the new traditional SPME and/or SPME Arrow end user. In particular, to let them know they need not get wrapped around the axle when it comes to liner selection. Oh, and to challenge the unsupported claims I see some vendors make that narrow bore inlet liners are more efficient for SPME.

If you are interested in moving the needle significantly, there a far more important details surrounding SPME extraction and desorption that deserve your precious time and consideration. In fact, the SPME end-user has over a dozen extraction and desorption conditions (e.g., extraction temperature, desorption duration, etc…) they can manipulate. For example, look at Colton’s recent blog on incubation/extraction temperatures for cannabis residual solvents using SPME. In particular, look at what happens when you incubate/extract at 30 vs 80°C for o-Xylene. Yes, I know it may be hard to see with the log scale. Luckily, I happen to share an office with Colton and can tell you that we are looking at average peak area responses of 1.66 x 107 vs 5.97 x 106, respectively. That is an 89% difference in response, without having to purchase different liners. Mind you, in the LCGC article I wrote the largest statistically significant difference we saw was only 57%. I hope you see the point.

I am not saying to ignore liner selection. What, I am saying is that there is only so much time in a day and I would encourage you to invest your limited time into optimizing the extraction/desorption conditions, as illustrated in Colton’s blog. So, stay tuned for some up-coming blogs where we continue to demonstrate which SPME parameters actually make a tinker’s dam.