Next generation PLOT alumina technology for accurate measurement of trace polar hydrocarbons in hydrocarbon streams Jaap de Zeeuw, Tom Vezza, Bill Bromps, Rick Morehead and Gary Stidsen Restek Corporation, Bellefonte, USA 1 Next generation PLOT alumina technology for accurate measurement of trace polar hydrocarbons in hydrocarbon streams Jaap de Zeeuw, Tom Vezza, Bill Bromps, Rick More head and Gary Stidsen Restek Corporation Summary In light hydrocarbon analysis, the separation and detection of traces of polar hydrocarbons like acetylene, propadiene and methyl acetylene is very important. Using commercial alumina columns with KCl or Na2SO4 deactivation, often results in low response of polar hydrocarbons. Additionally, challenges are observed in response-in time stability. Solutions have been proposed to maximize response for components like methyl acetylene and propadiene, using alumina columns that were specially deactivated. Operating such columns showed still several challenges: Due to different deactivations, the retention and loadability of such alumina columns has been drastically reduced. A new Alumina deactivation technology was developed that combined the high response for polar hydrocarbon with maintaining the loadability. This allows the high response for components like methyl acetylene, acetylene and propadiene, also to be used for impurity analysis as well as TCD type applications. Such columns also showed excellent stability of response in time, which was superior then existing solutions. Additionally, it was observed that such alumina columns could be used up to 250 C, extending the Tmax by 50C. This allows higher hydrocarbon elution, faster stabilization and also widens application scope of any GC where multiple columns are used. In this poster the data is presented showing the improvements made in this important application field. Background In petrochemical arena analysis of volatile hydrocarbons and impurities has become a routine application. In the C1-C5 range many unsaturated hydrocarbons are present (alkenes, alkynes) and also these have to be separated and quantified. As hydrocarbons are generally believed to be inert they will elute from a wide variety of stationary phases. Liquid type phases like 100 % polydimethyl siloxanes will separate the saturated hydrocarbons, but selectivity is not present for separating the unsaturated hydrocarbons. Highly selective materials have been developed for the 2 separation of unsaturated hydrocarbons and especially the adsorption chromatography has proven to be very effective. Alumina is one of the most selective materials and is highly valued for this. Most widely used is the aluminum oxide. Aluminum oxides separate all C1-C5 hydrocarbons but has to be deactivated. Special alumina columns have been developed for trace polar hydrocarbon analysis but these columns still do not perform as customer prefers. Biggest challenges are stability of response in time for reactive hydrocarbons c.q. propadiene and methyl acetylene. Also loadability is a challenge which can be improved. Restek developed a new Al2O3 adsorbent called Rt-Alumina BOND/MAPD that shows to be very promising for this application. This new material is intensively deactivated and maximizes response for polar hydrocarbon, provide a constant response – in time -, and also offers higher retention/loadability and can also be used at temperatures higher then 200ºC. Test materials and conditions Restek Rt-Alumina BOND / MAPD column (50m x 0.53mm ID x 10um);Varian Select Al2O3/MAPD column (50m x 0.53mm ID x 10um df), Part number – CP7432, S/N 6505409; A Restek Alumina BOND Na2SO4 column (50m x 0.53mm ID x 10um df) (P/N 19756, S/N 971892. Carrier Gas : Helium Sample : DCG custom gas standard S/N 441868 Split Injection : 5ul gas injection Split Vent Flow Rate : 80ml/min Injection Port Temp. : 200°C Detector Temp. : 200°C Oven Temp : 60°C, hold 4 minutes, 10°C/minute to 200°C, hold 5 minutes Challenges with Present technology Alumina columns show best selectivity for separation of C1-C6 hydrocarbon isomers. Depending on deactivation, the surface will behave more or less polar. Using Na2SO4 deactivation, results in a polar surface. The selectivity of this surface is better then the KCl (less polar) deactivation. It was observed that standard alumina columns showed a different response for polar hydrocarbon. Components like propadiene, acetylene and methyl acetylene showed strong variation in response. As these components have to be measured at low ppm levels, the resulting data was not reliable and better solutions were requested. Besides non reproducible responses, the Na2SO4 deactivated alumina columns also showed drift of response in time. Fig. 1 shows the response for propadiene relative to n-butane when repeated injections are done. The adsorption of polar compounds is related to the activity of the alumina used, and by deactivation one can make alumina surfaces more inert for the target analytes. One of the commercial solutions developed was the Al2O3/MAPD column. 3 Figure 1. Development of response factor for propadiene / butane ratio Na2SO4 deactivated alumina during 50 repeated injections With this technology a more intense deactivation was applied [ 1 ], making polar hydrocarbons elute with higher response factor. Practically the variation in response factor was improved, but still a linear decrease in time was observed, when repeated injections of hydrocarbon mixture was done, see figure 2. By changing deactivation, another challenge was developing: retention (and loadability) of the alumina columns was greatly reduced, which would directly impact the chromatography. Figure 2: Development of response factor for propadiene / butane ratio of Agilent CP-Al2O3/MAPD during 50 repeated injections 4 This resulted in lower sample capacity and more difficult method optimization as peaks were more affected by concentrations. Figure 3 shows a comparison of 50m x 0.53m Alumina - MAPD columns with similar dimensions. Columns were tested in same GC same temperature and same linear gas velocity. There is a big difference in retention/loadability which is in favor of the Rt-Alumina BOND / MAPD. New technology Restek developed a new alumina based column, with a new deactivation technology that maintains the absolute retention characteristics of alumina columns, but shows a clear improved stability for polar hydrocarbon response, compared with present commercial solutions. The new column is called Rt-Alumina BOND / MAPD and it has comparable retention/loadability as is obtained with current alumina columns, but it will show maximal responses for polar hydrocarbons. With the deactivation applied, we tried to create a polar surface as these surfaces are preferred. The new Rt-Alumina BOND / MAPD has a slightly lower polarity, but can practically be used for all separation challenges. The main differences are: - Constant response for polar hydrocarbons in time ( propadiene, acetylene, methyl-acetylene); - High loadability (peaks stay more symmetrical when level increases); - Extended temperature stability: column is useable up to 250 ºC Loadability In Gas-solid chromatography, phase overload results in strong tailing peaks. Adsorption surfaces have an additional challenge. It’s not only the amount of active sites that are available for distribution, but also the level of “activity distribution” of the active sites. Ideally all active sites show similar activity, but in reality, there always is a distribution of activity. This is defined by the nature of the alumina, its specific area, impurities, and form how it is used in the PLOT column. That is also why we find differences in behavior between alumina columns. 5 Figure 3: Comparison of absolute retention of 50m x 0.53mm Alumina – MAPD columns; Columns tested under identical conditions. Fig. 3 shows a comparison of 50m x 0.53m Alumina - MAPD columns with similar dimensions. Columns were tested in same GC, same temperature and same linear gas velocity. There is a big difference in retention which is in favor of the Rt-AluminaBOND / MAPD, having almost 2.5 times higher retention. This translates immediately in a higher loadability, which especially affects the polar hydrocarbons. Impact of loadability on peak shape was tested under 2 types of conditions: Series A: When the components elute with a comparable retention factor. The Rt-Alumina BOND / MAPD columns was tested at 130 ºC and the CP-Al2O3/MAPD was tested at 100ºC; These conditions also simulate what will practically happen when both columns are used under temperature programmed analysis. Series B: Comparing the loadability of both columns at the same temperature. This is a different comparison as here we do not have the extra tailing caused by the extra surface activity that is developed at lower test temperature. 6 Series A: Figure 4 shows the peak shape for acetylene, propadiene and butane for both columns. The CP-Al2O3/MAPD elutes the propadiene and acetylene after butane because of the lower oven temperature; As expected there is a big difference in peak symmetry in favor for the new alumina technology. Figure 4: Overlay of peak shape between MAPD columns. Zoom into peak shape of butane, acetylene and propadiene. To get comparable retention factors, the Rt-Alumina BOND / MAPD was tested at 130ºC; The CP-Al2O3/MAPD was tested at 100ºC. Data see table 1; The differences are even bigger when testing is done for 1,3 butadiene and methyl acetylene, see figure 5. Also note that the retention times are strongly