GC Inlet Liner Selection, Part I: Splitless Liner Selection

Splitless injections are used when detection of trace amounts of analytes is necessary and the goal is to recover close to 100% of all analytes that are injected into the instrument.  During a splitless injection, the split vent is closed for a predetermined amount of time, directing all inlet flow onto the column (with the exception of the septum purge).  Because of the slow flow rates, splitless injections can be tricky.  These slow flow rates can contribute to band broadening (wider peaks), as well as longer residence times in the liner, leading to increased interactions with any active sites.

I conducted an experiment to compare various liner configurations for use with splitless analyses of liquid injections.  I wanted to compare liners based on recovery across a wide molecular weight range, as well as reproducibility from injection to injection.  To do this, I injected a series of hydrocarbons ranging from C10 up to C40.  As these compounds are all in the test mix at equal mass, ideally, peak area responses for all compounds should be the same.  A common phenomenon, known as molecular weight discrimination, occurs in the GC inlet when there is incomplete vaporization and therefore incomplete transfer of heavier compounds, with the heaviest compounds showing much less recovery compared to lighter compounds.

The following liner configurations were compared using the splitless conditions listed in Table 1 below.

Table 1: Instrument conditions for liner comparisons.

Figure 1 shows how some common liner configurations compare for peak area response across the molecular weight range when performing splitless injections:

Figure 1: Comparison of peak area response across a wide molecular weight range for various liner configurations used in splitless mode.

As previously mentioned, an ideal liner will minimize molecular weight discrimination, leading to equal responses across all compounds.  You can see that the single taper liner with wool and the double taper cyclo liner achieve this.  Both the wool and the cyclo corkscrew provide extra surface area, which enhances vaporization by increasing the heat capacity of the liners.  The low pressure drop liner had similar performance for C20 and higher; however, there is some loss of more volatile compounds, perhaps from the wool being located higher up in the liner, leading to losses out of the septum purge vent. The presence of a taper helps to direct the sample to the column, as well as minimize interactions with the gold seal, which can otherwise be detrimental to performance with the slow carrier gas flows used in splitless injections.

When it comes to reproducibility, the liners with wool and the cyclo corkscrew also performed best (See Figure 2).  These features create a turbulent zone, allowing for reproducible mixing with the carrier gas upon injection.  They also serve to “catch” the sample, preventing analytes from hitting the bottom of the inlet where they can condense and get lost.

Figure 2: Liner reproducibility comparison across wide molecular weight range.

Conclusions

Overall, I would recommend the use of a single taper liner with wool or a cyclo liner for use with splitless injections of liquids.  The single taper liner with wool is the more cost effective solution; however, for those that do not want to use wool, the double taper cyclo is a viable second choice if you’re looking for the best splitless performance.  As you can see from the data above, if your analytes are on the more volatile side of the spectrum, the use of wool or a cyclo may not always be necessary for recovery and reproducibility.  Depending on matrix, though, these features can help to catch involatile material, as well as septa particles, preventing column contamination.

 

GC Inlet Liner Selection: An Introduction

GC inlet liners play an important role in GC sample introduction. The sample’s first contact is with the liner and from there it is transferred to the analytical column. In the case of liquid injections, the sample must be vaporized inside of the liner prior to transfer.  Choosing a proper inlet liner for your analysis is critical, as it can affect both analyte recovery, as well as injection to injection reproducibility.  Inertness must also be considered, especially when analyzing active compounds, such as pesticides, acids, or bases.

Liner selection can sometimes seem like a daunting task, because there are so many configurations available, including different shapes, sizes, packings, such as wool, etc.  The best liner is going to depend on your sample type, nature of analytes, and injection type.

In the following blog series, I would like to discuss selection of liners for split and splitless liquid injections, based on optimizing performance.  I will also blog about liners for use with gas analyses and PTV inlets, as well as direct injection uniliners.  I will post a final blog discussing the importance of liner inertness.  I hope this upcoming series will be useful to anyone who is confused by the myriad of liners available and wonders if they are actually using the best liner for their analysis.

A Hoppy Little Story

Now that we’re getting into the warmer months of the year, you’re probably starting to see pop-up beer gardens or people outside relaxing with a nice cold one. Since the beer industry has been evolving and people are enjoying craft beers more and more, there are tons of unique brews on the market. If you are not a beer connoisseur, then you may not know what gives your favorite India pale ale or summer lager its intriguing characteristics. The ingredient responsible for the bitterness and much of the aroma are the hops. There are an incredible number of different hop varieties and every single variety has its own personal profile! Some common U.S. hop varieties include Cascade, Centennial, Citra, and Nugget. But, you may be asking yourself, “what makes these hop varieties so different?” TERPENES! Terpenes are a class of organic compounds that are comprised of isoprene units and they are what give hops their unique aroma and flavor profiles. There are many different classes of terpenes, but the classes that we are interested in are the mono- and sesquiterpenes, which include limonene, humulene, pinene, myrcene, to name a few.

http://www.chemspider.com/Chemical-Structure.20939.html

Limonene – http://www.chemspider.com/Chemical-Structure.20939.html

Since I have an interest in quality craft beers, I wanted to dive into a variety and look for some terpenes. Luckily for me, a friend of mine in Restek’s reference standard department grows his own hops. Thanks, Joe! When these hops were harvested last year, Joe vacuum packed me his finest Hallertau variety.

Ground Hops

The hops were added to a Blixer processor with dry ice and then ground into a very fine powder. 0.5 g of the hop powder was measured into a 50 mL QuEChERS tube (cat# 25846), followed by 10 mL of isopropanol. The mixture was vortexed for 5 seconds, then sonicated for 5 minutes, repeated three times. 1 mL of the supernatant was filtered using a 13 mm, 0.22 µm, PTFE syringe filter (cat# 26142) and added to a 20 mL headspace (HS) vial (cat# 24685). 19 mL of RO water was then added to the 20 mL HS vial, which was then capped and ready for analysis. Further sample preparation included the CTC PAL RTC rail system, where samples were analyzed via solid phase microextraction (SPME), using direct immersion (DI), with gas chromatography-mass spectrometry. A divinylbenzene (DVB) SPME-Arrow (cat# 27486) was used for this SPME Arrow-DI-GC-MS method.

 

 

 

 

Total Ion Chromatogram of Terpenes in Hops

 

Using Restek’s Terpene Mix 1 & 2 (cat# 34095 & 34096), we were setup to identify 23 different terpenes. After normalizing the responses for terpenes to 100% we were able to gather a nice breakdown of the terpene profile for Joe’s Hallertau. Using the NIST mass spectral database, we were also able to identify several other terpenes (i.e. beta-Phellandrene), but these were not added to the pie chart.

 

 

So, the next time you’re enjoying a cold one on a hot summer day, give it a good sniff to get an idea of what terpenes may be in there. Every beer has its own unique hoppy little story to tell! Cheers!

 

Photo courtesy of Wil Stewart

Are You Interested in Ultrashort-Chain PFAS Analysis? Be Sure to Screen Your Solvents/Solutions for Contamination

Trifluoroacetic acid (TFA) is the perfluorinated analogue of acetic acid with the shortest possible chain length (C2) among per- and polyfluoroalkyl substances (PFAS). Together with perfluorinated C3 compounds, they are defined as ultrashort-chain PFAS. C2 and C3 PFAS are ubiquitous and very mobile in global aquatic environment including rain, snow, river, and even ocean. TFA, especially, can occur at very high concentration in both drinking and non-potable water sources. As we are developing a dilute-and-shoot LC-MS/MS method for ultrashort-chain PFAS analysis, we realized that the HPLC grade water and methanol could have TFA contamination!  By applying a HILIC/ion exchange column for ultrashort-chain PFAS analysis, a detectable TFA peak was observed upon blank diluent (50:50 LC/MS grade water:HPLC methanol) injection. Further analysis showed that the LC/MS grade water contains relatively higher trace level of TFA (~5 ppt) compared to the HPLC grade methanol we regularly used for LC-MS/MS analysis. In a search for TFA-clean reagent solvents, we tested a variety of water and methanol from different vendors. Using a 10 ppt standard solution (in 50:50 DI water:EMSURE® methanol) as the reference, it was shown that EMSURE® methanol (with plastic container) and JT Baker methanol are much cleaner compared to other brands with either detectable or significant higher level of TFA (Figure 1). As for water reagents, the reverse osmosis (RO) and deionized (DI) waters generated in our facility are much cleaner than the LC/MS grade water.

Figure 1. TFA in Water and Methanol Reagents

So watch out for what kinds of reagent solvents you are using if you want to include TFA in your PFAS analysis. For the trace level of TFA detection, it is necessary to use cleaner solvents for both sample preparation and LC analysis. And one more note, we also experienced that the use of glass HPLC vials could produce TFA contamination. We recommend to use polypropylene vials for ultrashort-chain PFAS or TFA LC-MS/MS analysis to avoid trace level contamination.

Are you analyzing TFA? We welcome you to share your experience in terms of what reagent solvents and labware you are using in the lab.

A Tale of Two Columns (CLPesticides and CLPesticides2)—Part IV: Fast 8081 Method Using GC Accelerator Kit

The moment has finally come to see how we can use the GC Accelerator to get the most horsepower for your 8081 analysis.  If you’ve been following this blog series, you will remember that in Part II, I talked about ways to make your runs faster.  I also showed you our previous fast “7 minute” method, using some of those tricks.  One of the limiting factors for that method was oven ramping ability; the ramps at the beginning and end were as high as a 120V Agilent GC would be able to maintain.  Fortunately, we have enough resolution at those points in the run to actually increase the ramp rate without causing any coelutions.

So how do we do this?  You’ve probably figured out by now that the answer is the GC Accelerator Oven Insert Kit.  With this kit, simply increasing the initial ramp rate and final ramp rate will allow you to elute all analytes in around 5 minutes, with a total oven cycle time close to 10 minutes.  In addition, flow rates can be increased to further capitalize on this benefit.  If, for example, your current total cycle time for 8081 is close to 20 minutes, this will effectively allow you to double the amount of samples you can process in a given amount of time.

The examples below demonstrate this fast method on both 0.32mm ID CLPesticides column as well as 0.25mm ID CLPesticides columns. Table 1 details the products you will need to try either of these methods.  Note that the columns are custom products and are not wound on cages.  This is necessary to fit both analytical columns in the oven with the entire GC Accelerator kit.  Refer to Part III of this series for detailed instructions on how to set this up.

 

Table 1: List of products needed to perform the methods below.  Both the 0.32mm ID columns and 0.25mm columns are listed for you to choose which pair works best for you.  All columns were custom products that are not on cages.

 

Figure 1: Fast GC Accelerator method on 0.32 mm ID Rtx-CLPesticides and Rtx-CLPesticides2 columns.  All analytes elute near 5 minutes with a total GC cycle time of around 10 minutes.  Link to chromatogram and conditions found here: https://www.restek.com/images/cgram/gc_ev1498.pdf

 

Figure 2: Fast GC Accelerator method on 0.25 mm ID Rtx-CLPesticides and Rtx-CLPesticides2 columns.  All analytes elute near 5 minutes with a total GC cycle time of around 10 minutes.  Link to chromatogram and conditions found here: https://www.restek.com/images/cgram/gc_ev1496.pdf

 

So there you have it!  The GC Accelerator kit can be used to increase sample throughput for dual column methods, in addition to its original intent to be used for GC-MS.  While the above example uses method 8081, similar changes could be made to other methods.  I encourage you to use Restek’s Pro EZGC Chromatogram Modeler to explore speeding up methods using increased ramp rates attainable with the GC Accelerator kit.  Any column can be ordered without a cage by attaching the suffix “-051” to the column catalog number.

Links to other blogs in this series:

Part I: A Little History

Part II: Gaining Speed

Part III: Using the GC Accelerator Kit for Dual Column Analyses

ASTM D 3606 17

Keep your benzene in check with ASTM D3606

 

While benzene historically has been used as an additive in gasoline and aviation fuel, it is a known toxic air pollutant and is regulated by both the EPA and European Union. While the ASTM has published both capillary and packed column methods this blog will focus purely on the more robust packed approach. With the addition of ethanol to gasoline modifications have been made to the method to prevent the coelution of benzene / ethanol which allows for the quantification of benzene at concentrations ranging from 0.1% to 5% by volume. Toluene, while less toxic, can be quantified at between 2% and 20% by volume.

 

Since benzene and ethanol are poorly resolved on some column configurations, we have found a solution using a two column set. Column #1 is a 6’ x 1/8” OD (1.8m x 2mm id) nonpolar Rtx-1 polymer, which separates components in boiling point order. After the elution of n-octane (C8), column #1 is back flushed to prevent the heavier compounds from entering column #2, the main analytical column.  Column #2 a 16’ x 1/8” OD (4.9m x 2mm id) column packed with a proprietary polymer that allows complete resolution of ethanol/benzene, R = > 1.50. One thing to remember; if the column set is housed in an auxiliary oven, manually check the oven temperature to be sure it is actually at the method temperature of 135°C.  An incorrect oven temperature can result in poor resolution. If you suspect your oven is not at the correct temperature check the retention time of toluene. One of the most important, yet commonly overlooked problems is oxygen and moisture in the chromatographic system. Regardless of what GC system you’re using always have oxygen and moisture filters installed on the carrier gas line as close to the GC as possible to assure a trouble-free analysis.

 

Stay tuned for more blogs on this topic

Take Care of Your LC System Investment and Minimize Downtime with Routine Maintenance!

For cannabis QA laboratories and producers, developing methods for HPLC analysis of cannabinoids can be time consuming and resource heavy.  While a lot of focus has been given to sample preparation and optimization of method conditions to maximize sample throughput, one VERY critical factor is often  overlooked: routine LC system maintenance.

Keeping a log book to document LC system maintenance and replacing consumable parts on a regular basis can help minimize system downtime.  It also provides a “known good” system baseline to reference for troubleshooting if problems occur.  It’s worth the small investment in record-keeping and routine maintenance time to ensure your expensive system stays up and running to maximize productivity.

We often use “the car example” to help people relate: would you purchase a vehicle for tens of thousands of dollars and not take it in every 5,000 miles for a $30 oil change as recommended by the manufacturer?  Probably not unless you want to risk catastrophic engine damage and a much bigger price tag than $30 somewhere down the road.  Not to mention your customer’s frustrations with longer turnaround times while your system is down.  Regular preventative maintenance is critical for optimum LC performance and prolonging the lifetime of your investment.  Restek offers replacement parts and kits for a variety of LC systems.  Start here to search by instrument manufacturer or part type, or input the catalog number of the vendor part you need to replace in the search box at the top of the page.

If you have an Agilent 1100, you know those are not supported anymore by the manufacturer, but we’ve got you covered with these critical parts:

Autosampler PM Kit

Pump PM kit

Lamps

In future posts, we’ll go into more details about what parts should be changed routinely and what issues they might help prevent for your workflow.  So stay tuned for a closer look at replacement parts that you should keep on hand and be ready to routinely change to minimize the risk of leaks, flow rate and gradient percent inconsistencies, peak area/height inconsistencies, and carryover.

If you can’t wait and need to do some maintenance right away, check out our video library for help with changing a lamp, choosing the proper tubing and fittings, and more!

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.

Links to other blogs in this series:

Part I: A Little History

Part II: Gaining Speed

Part IV: Fast 8081 Method Using GC Accelerator Kit

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.

Links to other blogs in this series:

Part I: A Little History

Part III: Using the GC Accelerator Kit for Dual Column Analyses

Part IV: Fast 8081 Method Using GC Accelerator Kit