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Changing from Helium to Nitrogen and Maintaining the Separation

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Changing from Helium to Nitrogen and Maintaining the Separation Cd The Chrom Doctor Changing from Helium to Nitrogen and Maintaining the Separation Efficiency in the Same Analysis Time Jaap de Zeeuw1 and Jack Cochran,2 1Restek Corporation, Middelburg, The Netherlands and 2Restek Corporation, Bellefonte, USA. For several reasons, there is interest in replacing helium with a different carrier gas in gas chromatography. Hydrogen is the obvious choice but there is a concern on safety and reactivity. It is also possible to use nitrogen but this is often not considered because it has a low optimal flow and velocity. Here we will show that it is possible to replace helium for nitrogen as the carrier gas while separation, peak elution order, analysis time, response and even oven temperature conditions can be kept the same. Summary exactly the same conditions for Carrier Gases is often avoided and helium is the By using method translation and oven programming. Even the inlet In gas chromatography there are preferred carrier gas. Also there are chromatogram modelling it became pressures are very close. mainly three carrier gases used: several detection systems that prefer clear that the loss of efficiency The only price that has to be paid nitrogen, helium and hydrogen. helium because of the detection using nitrogen could be perfectly is a loss in loadability, which means Figure 1 shows the van Deemter principle. Think here about the mass compensated by using a smaller that this concept will not work for plots, showing the column efficiency spectrometer and the pulse discharge ID capillary of a shorter length. By every application but for many it will. versus the linear gas velocity (one based detectors.(PDD, HID, BID). replacing a 30 m x 0.25 mm column Besides the guaranteed availability, can also use flow here). Nitrogen is for a 20 m x 0.15 mm column, it using nitrogen offers a big advantage considered a slow carrier gas with an Hydrogen is demonstrated that separations in the cost and consumption volume optimum of 11–13 cm/sec. Helium is Hydrogen is an interesting alternative under nitrogen are almost exactly of carrier gas, meaning cost per about 2 times faster and hydrogen is and has a number of advantages [1]: the same as obtained with helium, in analysis will also benefit. the fastest gas. Because of concerns • It has a high optimal linear the same analysis time, while using of safety and reactivity, hydrogen velocity, allowing analysis time to 2 Changing from Helium to Nitrogen separation science www.sepscience.com Figure 1 Figure 2 Figure 1: Van Deemter curves for nitrogen, helium and hydrogen. Figure 2: Van Deemter curves showing HETP loss when nitrogen is used under the optimal velocity of helium. be shortened by a factor of 1.7 concern. When combined with hot Because of the higher speed, there is analysis will increase. • It is widely available, cheap and inlets, components can hydrogenate. a loss of theoretical plates of about a • For the same reason the can be produced in the lab using Figure 3 shows the separation of factor 2. You can use the same oven maintenance intervals will be a generator pesticides using N2 at higher linear conditions as used for helium. The shorter. • Because of the speed, peaks are velocity. This was possible, because chromatogram will be the same but • Peaks are lower in intensity, so narrower and needs less sample the stationary phase used offered only peaks are a little broader and there is a sensitivity loss. for the same signal, benefitting very high selectivity and some loss of about 30% lower in intensity. Figure 3 shows the separation of the life time of liner and column. efficiency could be afforded. For separations that are good pesticides using N2 at higher linear You can even use hydrogen at the enough this can be an interesting velocity. This was possible, because same velocity as helium and get Using Nitrogen at The Speed of approach. The practical implication the stationary phase used offered the same chromatography: see [2]. Helium one has to realize, are: very high selectivity and some loss of Here also the same temperatare If nitrogen is operated under optimal • Because peaks are broader, the efficiency could be afforded. programme can be used. The flow conditions, analysis time will peaks will merge faster upon challenges using hydrogen mostly be 2–2.5 times longer and this will column ageing. That means that Using Nitrogen and Compensation concern safety. Despite all kinds not be appreciated. Nitrogen can the number of analyses that can for The Loss of Efficiency Using a of precautions that are already in also be used at the same velocity as be done under these conditions Different Diameter Capillary place, many labs do not want to use helium. Figure 2 shows what we can will be smaller. You will have to It has been known for a long time hydrogen. Reactivity also remains a expect using the van Deemter curve. use more columns and cost per that analysis time can be shortened 3 Changing from Helium to Nitrogen separation science www.sepscience.com Figure 3 Figure 4 Figure 3: Example of using nitrogen at higher velocity. Pesticides on Rtx-Cl-Pesticides. Because this Figure 4: Relation of column diameter and plate number. phase is very selective, the loss of efficiency due to operation under nitrogen will not be a problem. using a smaller diameter capillary [3]. this is what we will use. The figure 1. Shows the custom option for nitrogen are just a little higher The same separation can be obtained shows helium as carrier gas but with 2. Shows the carrier gas selection then the pressure required for helium. using a shorter length column. nitrogen the increase of optimum will 3. Column dimensions, original 0.25 The big question is: how much Figure 4 shows a series combination also be comparable. m and the new 0.15 mm resolution is sacrificed when this of column diameters and length In this experiment a 30 m x 0.25 mm 4. Hold up times are set at a SIMILAR exercise is done? Running nitrogen at required for 120.000 plates. It is clear capillary with 0.25 um film operated value ( gas velocity for nitrogen a higher velocity will cost efficiency. that shorter columns will provide under helium was substituted for will be higher then optimal) Also what is the benefit of using the faster separations. a 20 m x 0.15 mm capillary with a 5. To check the analysis time, these SAME temperature programme? Practically 0.15 mm columns have 0.15 um film, which was operated values should be similar: exact proven to be very effective and can be under nitrogen. The stationary phase same run time Using the Same Temperature coated with different phases with phase and phase ratio was kept the same. 6. Shows the temperature Programme ratios to match the capillaries that are The method translator[4], that was programme, which will also be Normally when changing column available in 0.25 and 0.32 mm columns. recently made available by Restek, will exactly the same. dimension or linear gas velocity, What makes this exercise especially be used to calculate conditions. We see that when the hold-up the temperature programme has interesting is that the optimum flow/ Figure 6 shows a snapshot of times are matched, the exact SAME to be adjusted to obtain the same velocity also increases when internal the data obtained using the EZ- temperature programme can also be separations and peak sequence. diameters are reduced (see Figure 5). GC method translation. As this is a used and the result is that the analysis If the temperature programme is 0.15mm columns can be operated at custom calculation the translation is time will also be exactly the same. NOT adjusted the components will relatively higher linear velocity and done in a “custom” mode. Also note that the pressures required elute at different temperatures and 4 Changing from Helium to Nitrogen separation science www.sepscience.com Figure 5 Figure 6 Figure 5: Van Deemter plots for helium and different diameter capillary columns. Optimum Figure 6: Screen shot of method translation showing the impact on parameters when changing velocities move to the higher values with decreasing ID. column dimensions from 30m/0.25/0.25 and helium under efficiency optimized flow/velocity, to 20m/0.15/0.15 and using nitrogen. Operating nitrogen at 0.36mL/min shows the same analysis time and the same programming conditions. Numbers are explained in the text. the separations can become worse elution temperatures are similar, the and even peak swapping can occur. chromatogram will be similar. Practical Experiment required. As expected, the translator Figure 7 shows an example of a In our exercise here, when using In order to proof the concept, a 30 m calculated the same programming complex mixture using the similar nitrogen, the void time was adjusted x 0.25 mm Stabilwax column with conditions and also the same column under different conditions, using a smaller ID column and this a film of 0.25 µm and a 20 m x 0.15 retention time. where elution temperatures are way it is possible to use the SAME mm with a film of 0.15 µm were Figure 8 shows the chromatograms changed. As can be seen, just by temperature programme. This compared using helium and nitrogen that were obtained. Separations are having around 15 degrees different automatically means that peaks elute as the carrier gas. The sample chosen exactly the same and are achieved in elution temperatures, the elution at the same temperature and the was a perfume containing many exactly the same analysis time.
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