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innovative petrochemical solutions
RESTEK REFINED
By Rick Morehead, Jan Pijpelink, and Jaap de Zeeuw
• Increased sample capacity results in improved peak shape and better accuracy.
• Optimized deactivation results in highest response for polar hydrocarbons.
• Rt®-Alumina BOND/MAPD columns give more flexibility in choice of sample size.
When using PLOT columns to analyze trace impurities in petroleum
gases, such as propylene, ethylene, or 1,3-butadiene, sample capacity
(loadability) is an important factor in obtaining accurate data. Phase
overload in adsorption chromatography results in peak tailing, which
can be problematic when trace-level impurities elute near the main
component where they may be obscured by the larger peak. Peak
tailing can be further exacerbated by residual activity on the adsor-
bent surface. Using a column with higher sample capacity and an
appropriate deactivation is a good strategy for reducing tailing and
improving quantification accuracy for low level polar hydrocarbon
impurities in volatile hydrocarbon streams.
MAPD-type alumina PLOT columns are commonly used for these
applications because the selectivity and degree of deactivation of
the alumina makes it very useful for separating the polar hydrocar-
bon analytes from the main C1-C5 components of the hydrocarbon
matrix. Although selectivity is very good for these compounds, sam-
ple capacity is often a challenge, which limits the amount of sample
that can be injected. Larger sample volumes can be desirable when
less sensitive detectors (e.g. TCDs) are used or when trace levels of
impurities, such as acetylene, propadiene, or methyl acetylene, must
be detected in main hydrocarbon streams in order to prevent dam-
age to polymerization catalysts.
Higher Retention With Good Peak Shape Yields
Higher Loadability
New Rt®-Alumina BOND/MAPD columns have an improved deacti-
vation and an increased sample capacity compared to other com-
mercially available MAPD PLOT columns. As shown in a comparison
of absolute retention times, the new MAPD column offers more than
twice the retention which results in greater resolution and increased
sample capacity (Figure 1). In this figure the absolute retention of
MAPD columns was compared using an isothermal oven tempera-
ture of 130 °C. Note that on the Rt®-Alumina BOND/MAPD column all
the C1-C5 hydrocarbons are well resolved and show perfect Gaussian
peak shape.
Greater Sample Capacity Improves Accuracy
To assess sample capacity, each column was tested at the temperature
shown on the manufacturer’s QA protocol in order to achieve compara-
ble retention. A range of sample volumes of a QA test mix were ana-
lyzed on each column using a 6-port sampling valve and 5 µL to 250 µL
sample loops. Peak tailing was measured for the analytes that were
most likely to exhibit tailing and to be sensitive to poor sample capacity
in actual impurity testing. As shown in Figure 2, much less peak tailing
was observed on the Rt®-Alumina BOND/MAPD column. Symmetrical
peaks were obtained across a wide sample volume range, indicating
that the column deactivation was highly effective and that sample
capacity was greater on the Rt®-Alumina BOND/MAPD column. Linearity
was also assessed, as shown in Figure 3, and excellent correlations were
achieved for all target impurities across the test range.
Summary
When analyzing impurities, such as acetylene, propadiene, and methyl
acetylene in petroleum gases, the sample handling capacity of the ana-
lytical column is an important consideration. Rt®-Alumina BOND/MAPD
columns offer higher sample capacity than other commercially available
MAPD columns and are recommended for analyzing polar impurities in
light hydrocarbon streams. Greater sample capacity improves data accu-
racy due to better peak symmetry and a wide linear range.
For more information on Rt®- and MXT®-Alumina BOND/MAPD PLOT
columns, visit
Improve Trace Analysis of Polar Impurities in PetroleumGases
Using Higher Sample Capacity Alumina MAPD Columns