Improve Trace Analysis of Acetylene, Propadiene, and Methyl Acetylene Impurities With Higher Sample Capacity Alumina MAPD Columns

Content previously published in Petro Industry News

When using PLOT columns to analyze trace impurities in petroleum gases, such as propylene, ethylene, or 1,3-butadiene, sample handling capacity (loadability) is an important factor in obtaining accurate data. Phase overload 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 adsorbent surface. Using a higher sample capacity column with an appropriate deactivation is a good strategy for reducing tailing and improving accurate quantification of low level polar impurities in volatile petroleum streams.

MAPD alumina PLOT columns are commonly used for these applications because the selectivity of alumina makes it very useful for separating C1-C5 hydrocarbons. Although selectivity is very good for these compounds, sample capacity is often poor, 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 order to prevent damage to polymerization catalysts.

New Rt-Alumina BOND/MAPD columns have an improved deactivation, increased sample capacity, and greater absolute retention compared to other commercially available MAPD PLOT columns. As shown in a comparison of absolute retention times, all peaks are well resolved on the Rt-Alumina BOND/MAPD column and no coelutions are observed (Figure 1). Greater retention increases resolution, which reduces the likelihood of coelution and can lessen the impact of tailing. Absolute retention was compared using an isothermal oven temperature of 130 °C; however, several critical compounds were not resolved on the Select Al₂O₃ MAPD column at this temperature, so optimized conditions for each column were used for sample capacity evaluations.

To assess sample capacity, each column was tested at the temperature shown on the manufacturer’s QA protocol in order to achieve comparable retention and adequate resolution. A range of sample volumes of a QA test mix were analyzed 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 be sensitive to poor sample capacity in actual impurity testing. As shown in Table I, 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 also that sample capacity was greater on the Rt-Alumina BOND/MAPD column. Linearity was also assessed, as shown in Figure 2, and excellent correlations were achieved for all target impurities across the test range.

When analyzing impurities, such as acetylene, propadiene, and methyl acetylene in petroleum gases, the sample handling capacity of the analytical 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 accuracy due to better peak symmetry and a wide linear range.

Figure 1: Rt-Alumina BOND/MAPD columns have greater absolute retention than Select Al₂O₃ MAPD columns, resulting in greater sample handling capacity through increased resolution.

1A. Rt-Alumina BOND/MAPD

GC_PC1213

Peaks

	Peaks
1.	Methane
2.	Ethane
3.	Ethylene
4.	Propane
5.	Cyclopropane
6.	Propylene
7.	Acetylene
8.	Propadiene

	Peaks
9.	n-Butane
10.	trans-2-Butene
11.	1-Butene
12.	Isobutene
13.	cis-2-Butene
14.	Isopentane
15.	n-Pentane
16.	1,3-Butadiene
17.	Methyl acetylene

Conditions

Column	Rt-Alumina BOND/MAPD, 50 m, 0.53 mm ID, 10 µm (cat.# 19778)
Standard/Sample	DCG custom standard
Injection
Inj. Vol.:	5 µL split
Liner:	2 mm single taper
Inj. Temp.:	200 °C
Split Vent Flow Rate:	80 mL/min.
Oven
Oven Temp.:	130 °C (hold 15 min)
Carrier Gas	He, constant pressure (4.4 psi, 30.3 kPa)
Temp.:	130 °C

Detector	FID @ 200 °C
Instrument	HP5890 GC
Notes	Liner cat.# 20795 was used to produce this chromatogram, but it has since been discontinued. We recommend cat.# 20796 [or cat.# 23315] as an alternative. For assistance choosing a replacement for this application, contact Restek Technical Service or your local Restek representative.

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1B. Select Al₂O₃ MAPD

GC_PC1214

Peaks

	Peaks
1.	Methane
2.	Ethane
3.	Ethylene
4.	Propane
5.	Cyclopropane
6.	Propylene
7.	Acetylene
8.	Propadiene

	Peaks
9.	n-Butane
10.	trans-2-Butene
11.	1-Butene
12.	Isobutene
13.	cis-2-Butene
14.	Isopentane
15.	n-Pentane
16.	1,3-Butadiene
17.	Methyl acetylene

Conditions

Column	Varian Select Al2O3 MAPD (CP7432), 50 m, 0.53 mm ID, 10 µm
Standard/Sample	DCG Custom Standard
Injection
Inj. Vol.:	5 µL split
Liner:	2 mm single gooseneck
Inj. Temp.:	200 °C
Split Vent Flow Rate:	80 mL/min.
Oven
Oven Temp.:	130 °C (hold 8 min)
Carrier Gas	He, constant pressure (4.4 psi, 30.3 kPa)
Temp.:	130 °C

Detector	FID @ 200 °C
Instrument	HP5890 GC
Notes	GC liner cat.# 20795 was used to produce this chromatogram, but it has since been discontinued. For assistance choosing a liner for this application, contact Restek Technical Service or your local Restek representative.

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Table I: Higher sample capacity is also demonstrated by comparing peak symmetry. Rt-Alumina BOND/MAPD columns produce more symmetrical peaks, even when more material is injected.

Figure 2: Higher sample capacity results in a wide linear range and accurate quantification, even at levels that can produce tailing and incomplete separations on other MAPD columns. (yellow = methyl acetylene, pink = acetylene, blue = propadiene)

Authors

• 
  Rick Morehead

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• 
  Jan Pijpelink

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• 
  Jaap de Zeeuw

  Jaap is a world-renowned chromatographer with over 40 years of experience, including 15 years with Restek and 27 years with Varian/Chrompack prior to joining Restek. Throughout his career, Jaap has focused the chromatographic challenges of industrial analysis. For his 1979 graduation from The Institute for Higher Education, where he specialized in chemistry, Jaap authored a paper titled "The Challenge of Coating Flexible Fused Silica Capillary Columns" and has since distinguished himself as an authority on the subject. Jaap has been directly involved with the creation of numerous chemically bonded columns, including the first bonded PEG column and the stabilized PLOT columns widely used in the petrochemical arena. He has also helped develop new techniques, such as fast GC-MS using vacuum GC technology (low-pressure GC or LPGC). Over his innovative career, he has filed several patents for his work. Based out of the Netherlands, Jaap is extensively published and regularly travels internationally to share his knowledge. After retiring from Restek, Jaap founded CreaVisions, where he works as a GC Consultant and teaches master classes on key GC topics as well as on creativity in science.

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