Wool in the liner improves chromatographic performance by increasing the evaporation of high molecular weight compounds, promoting sample mixing, reducing backflash and trapping non-volatile matrix. We have received reports where the use of wool in the liner has resulted in poor reproducibility and low response to semi-volatile compounds. This study will simulate these conditions and examine the effects of wool position in the liner using different split ratio and no split conditions.
A significant body of work detailed the optimization of gas chromatography (GC) inlets using splitless and split modes with and without wool in a variety of linear configurations. Linus Waklaski conducted a study to evaluate four splitless liners for the recovery of C8 to C40 hydrocarbons. The single-taper and straight liners showed reduced hydrocarbon (>C24) recovery when compared to the single-taper and double-taper cyclo-gooseneck liners with wool. A single taper with wool was found to be one of the best liners because wool enhanced the evaporation of heavier analytes (1). Another study compared split liners using a 50:1 split ratio, with the highest peak areas and lowest relative standard deviations (RSDs) involving wool (2).
worked using splitless injection techniques to confirm that wool content did not increase activity for the most active compounds, such as pentachlorophenol and benzidine. has gone A decrease in response to 2,4-dinitrophenol was observed only when three times the amount of specific wool was loaded, but other active analytes were not tested. Manufacturers passivate the wool after placing it in the liner to prevent the wool fibers from breaking. Passive wool fibers expose active silanols (3,4).
Over the years we have received calls from customers struggling with calibration curve linearity and reproducibility of standards when using split or splitless straight 4 mm liners with wool purchased from the manufacturer. The wool is placed about 25 mm into the liner, just below where a typical autosampler syringe needle would inject. Although the wool has not moved during shipping, a prevailing hypothesis is that the wool changes to the liner during installation in the injection port, thereby altering the evaporation rate. By changing the septum during injection port pressure, the wool can be pushed to the top of the liner, and if the needle pierces the wool, the sample can be injected under the wool. In this scenario, wool may not be effective in vaporizing the sample (Figure 1).
In this work, 50:1 split, 5:1 split, and splitless conditions will be used to evaluate liners with wool in different positions. Middle wool, bottom wool, top wool and no wool will be compared for responses and reproducibility as shown in Figure 1.
Experimental model
The column used for this work was a 100% polydimethylsiloxane (PDMS), “type 1 phase,” (Restek) 30 μm, 0.25 μm inner diameter with 0.25 μm film thickness in a GC–mass spectrometer (MS). was installed. Flow of 1 ml/min. The 5:1 and 50:1 split conditions used a GC oven program of 80 °C (1 min hold) ramped 20 °C/min to 300° (no hold), then 15 °C/min to 350 °C (hold 1 minute) hold 10 minutes). Under no-split conditions, an oven program of 50 °C (1.7 min hold) was used at 20 °C per min to 300 °C (no hold), then 15 °C per min to 350 °C (8 min hold). ) done. Splitless hold time was set for 1.7 min. The MS scan rate was 5.6 scans per second and scans were performed from m/z 45 to 550 with a solvent delay of 2.6 min. The selected compounds were alkanes with even numbers from C8 (n-octane) to C40 (n-tetracontane) in hexane. Concentrations were adjusted to maintain 10 ng on the column for both partition ratios, and conditions without partition were performed at 50 ng on the column. Splitting and no-splitting conditions were tested with wool in the center (as provided by the manufacturer), no wool, wool on the bottom, and wool on top of the autosampler syringe needle (Figure 1). Pierced by Tests were performed five times and wool position was inspected between liner changes. If migration of wool was detected, the wool was repositioned and the liner reanalyzed.
Results and discussion
Splitless injection provides the best sensitivity because most of the sample (and solvent) is directed down the split line to the column. Splitless injection techniques require careful optimization of hold time, onset temperature, stationary phase selection, and solvent. Slow flow rates in the liner can lead to solvent effects and degradation of active analytes. Since we were injecting hexane, our vapor cloud was calculated to be 223 μL with an effective liner volume of 493 μL (5), which means that most of our sample was trapped in the injection port during the hold time. and was transferred to the column. Konrad Grob designed and built a glass injection port immersed in hot oil, where he injected perylene and was able to observe the compound with ultraviolet light. The results of his work demonstrated the Leyden-frost effect, where the solvent is held slightly above the surface of the inlet cell by a vapor cloud (6). This revealed a source of discrimination as the column inlet is above the cell and less volatile sample is lost. Figure 2 is a comparison of wool in the center with similar peak area responses compared to wool. No wool was shown, which was almost identical to the top wool, and the bottom wool, which was almost identical to the middle. These findings are similar to other published work on this topic (7).
Split injections use a higher flow rate and reduce the time the sample spends in the injection port. A wool or other method is required to thoroughly vaporize the sample, which is then transferred to a column producing narrow peaks. The initial oven temperature is possible because very little solvent is transferred to the column (8). Figure 3 is a comparison of different wool positions in a straight 4 mm inner diameter liner using 5:1 and 50:1 split ratios. Not surprisingly, wool in the center provides the best overall reproducibility because the heater block and heat sensor are located in the middle of the ejection port. With our set point of 250 °C, the top and bottom of the inlet can be 100 °C cooler (9). This causes higher RSDs, lower reactivity and greater discrimination for high molecular weight analytes with no wool, bottom wool and top wool. Wool that moved to the top of the liner and was pierced by the needle produced the most variation in the five simulated analyzes (Figure 3). A high partition ratio reduces the time analytes spend in the injection port and therefore is not completely volatilized.
Customer problems can be simulated in split mode as long as two conditions are met: she is on top of the wool liner and the needle of the autosampler penetrates the wool. In more than half of the cases where the wool was placed on top using an autosampler syringe with a cone-style tip, the wool was pushed back into place and the analysis duplicated the result of the middle wool. . Differences were more subtle for analysis without partitioning and may be related to other variables.
Results
For partition analysis, the wool in the center of the liner provides the best evaporation of heavy analytes, resulting in the largest peak areas and lowest standard deviations. Correct fit liners are recommended for distribution analysis as the wool plug is held in place. For those using straight liners with wool, always press the inlet before maintenance and confirm that the wool has not moved. For analysis without partitioning, bottom wool and middle wool had similar recoveries. A single taper liner with wool at the bottom has been shown to provide excellent sample evaporation with less analyte breakdown (2).
We have found that wool in the liner can go straight to the top of the liner after septum replacement when the device is not pressurized. If the needle pierces the wool, the sample can be injected under the wool, resulting in the highest RSDs measured for this study.
references
(1) Waclaski, L. Optimizing splitless GC injection. Column 2018, 14 (8), 11-15.
(2) Waklaski, LGC inlet liner selection. American Laboratory. CompareNetworks, Inc., 2016. https://www.americanlaboratory.com/914-Application-Notes/189474-Selecting-a-GC-Inlet-Liner/ (accessed 2024-05-14).
(3) Waclaski, L. Does wool content matter in prepackaged liners?—Part I: Experimental setup. Chromablography. Restek Corporation, 2020. https://www.restek.com/chromablography/does-the-amount-of-wool-in-prepacked-liners-matter-part-i-experimental-setup (accessed 23-04-2023 ).
(4) Waclaski, L. Does wool content matter in prepackaged liners?—Part II: Results. Chromablography.. Restek Corporation, 2020. https://www.restek.com/chromablography/does-the-amount-of-wool-in-prepacked-liners-matter-part-ii-results (accessed 2023-04-23).
(5) Solvent Expansion Calculator. Restac. Restek Corporation, 2024. https://www.restek.com/solvent-expansion-calculator (accessed 2024-05-14).
(6) Grove, K. Split and splitless injection in capillary gas chromatography: with some remarks on PTV injection, Third Edition. Hüthig Buch Verlag Heidelberg, 1993. https://pdfs.semanticscholar.org/193a/e59f1d1e78a792bd383f8f45e5097f605892.pdf (accessed 2024-05-14).
(7) Waklaski, LGC Inlet Linear Selection, Part I: Splitless Linear Selection. Chromablography.. Restek Corporation, 2019. https://www.restek.com/chromablography/gc-inlet-liner-selection-part-i-splitless-liner-selection (accessed 2023-04-23).
(8) Cochrane, J. Rattray, C. Semivolatiles Analysis Using Split Injection. LCGC Europe – Application Notebook 2016, 29 (9), 533–534.
(9) Grossman, S. It's a matter of degrees, but do degrees really matter? Technical Literature Library. Restek Corporation, 2024. https://www.restek.com/articles/its-a-matter-of-degrees-but-do-degrees-really-matter (accessed 2024-05-14).
Chris English Managing a team of chemists at Restek's innovation laboratory since 2004. Before taking the reins of the laboratory, he spent seven years as an environmental chemist and was instrumental in the development of Restek's current line of GC columns. Chris holds a BS in Environmental Science from St. Michael's College, USA. E-mail: chris.english@restek.com