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Virtually Particle-Free Rt-Silica BOND Columns Provide Reliable PLOT Column Performance With Less Time Lost for Maintenance

By Amanda Rigdon, Bill Bromps, Tom Vezza, and Jaap de Zeeuw
  • Optimized manufacturing process practically eliminates particle release, reducing downtime due to system obstructions and damage from particles.

  • Bonded silica stationary phase minimizes impact of water, resulting in reproducible retention times for water-containing samples.

  • Versatile, highly retentive column ideal for analysis of light hydrocarbons, sulfur gases, halocarbons, and carbon dioxide at temperatures above ambient.

  • Individually QC tested with sensitive unsaturated C4 probes to ensure consistent selectivity.

Porous layer open tubular (PLOT) columns are very useful to GC analysts working on a wide variety of applications, and the unique selectivity of PLOT columns makes them particularly good for separating gaseous compounds without cryogenic cooling. However, the overall utility of traditional PLOT columns is hampered by the characteristic instability of the porous layer that coats the inside of the column. With most PLOT columns, particles that shed from the porous layer create significant problems because they can form obstructions inside the column that can alter flow, causing retention time instability. In addition, particle build-up makes frequent maintenance necessary as jets become obstructed and detectors become contaminated. In contrast, new Rt-Silica BOND columns from Restek are exceptionally robust due to optimized manufacturing and phase bonding steps that practically eliminate particle release. This exceptional stability—in combination with high loadability, inertness, and consistent selectivity—makes these new columns extremely reliable and ideal for the analysis of light hydrocarbons, sulfur gases, and halocarbons. In addition, carbon dioxide and other permanent gases can be retained at ambient temperature on this silica-based column. This article demonstrates the robustness of the Rt-Silica BOND column and its performance for many of the applications relevant to testing natural gas and light hydrocarbon streams.

Virtually Particle-Free and Water Resistant PLOT Performance

Restek’s proprietary manufacturing technique for the Rt-Silica BOND column results in an extremely stable porous layer with traditional PLOT column loadability and retention without loose particles that can damage valves and foul FID jets.

Figure 1 shows a magnified picture of three fused silica columns. The middle column is a traditionally-manufactured PLOT column, the bottom column is a wall-coated open tubular column, and the top column is an Rt-Silica BOND PLOT column. Note the uneven layer of particles on the middle column, as well as areas where the particles have completely detached from the column wall; this causes irregularities in the internal diameter of the column that can cause retention time instability. In comparison, the Rt-Silica BOND column looks identical to the wall-coated open tubular column, with no visible shedding of particles or peeling of the coating layer. While the Rt-Silica BOND column does contain a porous layer, the structure of this layer is extremely fine and well-adhered to the column wall, ensuring virtually particle-free operation over the lifetime of the column.

Figure 1:  Traditional PLOT columns (middle) have an uneven coating of particles that can shed, fouling instrument parts. Rt-Silica BOND columns (top) have a very fine porous layer with no visible particles and look very similar to wall-coated open tubular columns (bottom).

Rt®-Silica BOND columns

The manufacturing process used to make Rt-Silica BOND columns results in a PLOT column with high selectivity, retention, and capacity without the particle shedding associated with conventional PLOT columns. This provides improved column robustness and less downtime for maintenance. The particle-free nature of this column is evidenced by a particle-generation experiment in which a column was temperature and pressure ramped multiple times. Changes in temperature cause changes in pressure, which result in particle shedding in traditional PLOT columns. Free particles generate large spikes when they hit the flame ionization detector (FID), interfering with quantification. In addition, the particles themselves can obstruct FID jets and damage valves. Note that no large particle spikes were generated when this experiment was carried out on a brand new Rt-Silica BOND column (Figure 2).

Figure 2:  The Rt-Silica BOND PLOT column shows no large particle spikes, even with temperature and pressure variation.

Particle Generation Experiment on Rt-Silica BOND
GC_PC1276
ColumnRt-Silica BOND, 30 m, 0.32 mm ID (cat.# 19785)
Injectionsplit (split ratio 35:1)
Liner:Premium 2.0 mm ID straight inlet liner (cat.# 23313.1)
Inj. Temp.:250 °C
Oven
Oven Temp.:50 °C to 250 °C at 35 °C/min (hold 5 min) to 50 °C at 70 °C/min
Carrier GasHe, constant flow
Linear Velocity:114 cm/sec
DetectorFID @ 260 °C
Make-up Gas Flow Rate:50 mL/min
Make-up Gas Type:N2
Hydrogen flow:40 mL/min
Air flow:400 mL/min
Data Rate:10 Hz
InstrumentAgilent 7890A GC

Another benefit of Restek’s proprietary manufacturing process for the Rt-Silica BOND column is that the stationary phase of the column is composed almost entirely of silica. While silica retains water, it does not adsorb it. Some PLOT materials adsorb water, which changes the retention and selectivity of the column. After analyzing samples containing water, these PLOT columns require extensive thermal conditioning (bakeout) to return their original retention and selectivity. Figure 3 shows a mixture of saturated and unsaturated hydrocarbons analyzed on the Rt-Silica BOND column both before exposure to water and then immediately after 10 large volume water injections. Even under these experimental conditions of extreme overwetting, the retention and selectivity of the column remain very similar and under normal use conditions would be effectively identical. This consistent water-resistant performance allows analysts to save time by minimizing maintenance and eliminating the extensive bakeout periods associated with other PLOT columns.

Figure 3:  Repeated water injections have minimal impact on Rt-Silica BOND column selectivity and retention, meaning, water-containing samples can be analyzed without requiring time-consuming thermal reconditioning.

PeakstR (min)
1.Methane1.514
2.Ethane1.653
3.Ethylene1.698
4.Acetylene1.854
5.Propane2.032
6.Cyclopropane2.345
7.Propylene2.438
8.Propadiene2.594
PeakstR (min)
9.n-Butane3.151
10.1-Butene4.485
11.Methyl acetylene4.625
12.trans-2-Butene5.096
13.1,3-Butadiene5.267
14.Isobutylene5.267
15.cis-2-Butene5.544
16.iso-Pentane5.811
17.n-Pentane6.352
Water Robustness Test on Rt-Silica BOND
GC_PC1275
ColumnRt-Silica BOND, 30 m, 0.32 mm ID (cat.# 19785)
SampleCustom hydrocarbon gas standard
Conc.: 1 mole percent in nitrogen
Injection
Inj. Vol.:15 µL split (split ratio 35:1)
Liner:Premium 2 mm ID straight inlet liner (cat.# 23313.1)
Inj. Temp.:250 °C
Oven
Oven Temp.:120 °C (hold 25 min)
Carrier GasHe, constant flow
Linear Velocity:40 cm/sec
DetectorFID @ 260 °C
Make-up Gas Flow Rate:50 mL/min
Make-up Gas Type:N2
Hydrogen flow:40 mL/min
Air flow:400 mL/min
Data Rate:10 Hz
InstrumentAgilent 7890A GC
NotesThe hydrocarbon mix was first analyzed to generate a baseline reference; then, ten separate 10 μL injections of water were made 1 minute apart using the same method. After the final water injection, the hydrocarbon mix was analyzed again to compare the chromatography before and after the water injections.

Versatile Column for Many Applications

The new Rt-Silica BOND column combines the retention, capacity, and selectivity of traditional PLOT columns with virtually particle-free, water-resistant performance. The bonded silica surface provides excellent retention for light hydrocarbons (Figure 4), permanent gases, and halocarbons, allowing for easy analysis of impurities in light hydrocarbon streams. In addition to light hydrocarbon analysis, the Rt-Silica BOND column is especially selective for sulfur compounds in hydrocarbon streams. Figures 5 and 6 illustrate good separation of sulfur compounds in propane and butane, respectively.

Figure 4:  Saturated and unsaturated hydrocarbons are resolved and retained well on the Rt-Silica BOND column.

Peaks
1.Methane
2.Ethane
3.Ethylene
4.Acetylene
5.Propane
6.Cyclopropane
7.Propylene
8.Propadiene
Peaks
9.n-Butane
10.1-Butene
11.Methyl acetylene
12.1,3-Butadiene
13.trans-2-Butene
14.Isobutylene
15.cis-2-Butene
16.Isopentane
17.n-Pentane
Saturated and Unsaturated Hydrocarbons on Rt-Silica BOND PLOT Column
GC_PC1266
ColumnRt-Silica BOND, 30 m, 0.32 mm ID (cat.# 19785)
SampleCustom DCG gas standard
Diluent:Nitrogen
Conc.:1 mole percent
Injection
Inj. Vol.:15 µL split (split ratio 35:1)
Liner:Premium 2 mm straight inlet liner (cat.# 23313.1)
Inj. Temp.:250 °C
Oven
Oven Temp.:60 °C (hold 2 min) to 175 °C at 2 °C/min
Carrier GasHe, constant flow
Flow Rate:3.3 mL/min
DetectorFID @ 260 °C
Make-up Gas Flow Rate:50 mL/min
Make-up Gas Type:N2
Hydrogen flow:40 mL/min
Air flow:400 mL/min
Data Rate:10 Hz
InstrumentAgilent 7890A GC

Figure 5:  Sulfur Compounds in Propane

Peaks
1.Carbonyl sulfide
2.Propane
3.Hydrogen sulfide
4.Carbon disulfide
5.Methyl mercaptan
Sulfur Compounds in Propane on Rt-Silica BOND
GC_PC1273
ColumnRt-Silica BOND, 30 m, 0.32 mm ID (cat.# 19785)
Sample
Conc.:6 ppm in 100% propane
Injectionsample valve
Sample Loop Vol.:250 µL
Inj. Temp.:250 °C
Oven
Oven Temp.:35 °C (hold 8 min) to 200 °C at 10 °C/min (hold 5 min)
Carrier GasHe, constant flow
Flow Rate:2 mL/min
DetectorPFPD from OI Analytical @ 250 °C
Element Mode:sulfur
InstrumentThermo Trace GC
NotesThis valve/loop injection employed a split injection technique. Split flow was set to 40 mL/min.
AcknowledgementChromatogram courtesy of Jean-Louis Brix and Joeri Vercammen (Global Analyser Solutions, Belgium)

Figure 6:  Sulfur Compounds in Butane

Peaks
1.Carbonyl sulfide
2.Hydrogen sulfide
3.Butane
4.Carbon disulfide
5.Methyl mercaptan
6.Ethyl mercaptan
7.1-Propanethiol
Sulfur Compounds in Butane on Rt-Silica BOND
GC_PC1274
ColumnRt-Silica BOND, 30 m, 0.32 mm ID (cat.# 19785)
Sample
Conc.:6 ppm in 100% butane
Injectionsample valve
Sample Loop Vol.:250 µL
Inj. Temp.:250 °C
Oven
Oven Temp.:40 °C (hold 5 min) to 200 °C at 10 °C/min (hold 8 min)
Carrier GasHe, constant flow
Flow Rate:2 mL/min
DetectorPFPD from OI Analytical @ 250 °C
Element Mode:sulfur
InstrumentThermo Trace GC
NotesThis valve/loop injection employed a split injection technique. Split flow was set to 40 mL/min.
AcknowledgementChromatogram courtesy of Jean-Louis Brix and Joeri Vercammen (Global Analyser Solutions, Belgium)

Rigorous QC Testing Ensures Ultimate Column-to-Column Reproducibility

While column-to-column reproducibility is a must for all analysts, it is especially important in valve, backflushing, or column-switching applications. With this in mind, a special QC test was designed for the Rt-Silica BOND column. Performance parameters, including efficiency, selectivity (RI), retention (k), and inertness are evaluated for each and every column. While the QC tests from some manufacturers include some of these parameters, the compounds used to measure RI are not well-retained and not sensitive to changes in column selectivity. The RI compounds used in the QC test for the Rt-Silica BOND are 1,3-butadiene and methyl acetylene, which are not only very sensitive probes for selectivity, but are of high interest to many analysts. Additionally, while some commercially available PLOT columns are not evaluated for inertness, Rt-Silica BOND column inertness is measured with propylene, which is a more active, unsaturated hydrocarbon. This QC testing ensures the highest level of column-to-column reproducibility available in the industry for PLOT columns. Figure 7 shows QC results from three separate lots of Rt-Silica BOND columns.

Figure 7:  Rigorous QC testing ensures column-to-column reproducibility.

Peaks
1.Methane
2.Propylene
3.n-Butane
Peaks
4.1,2-Dichlorotetrafluoroethane (CFC-114)
5.Methyl acetylene
6.1,3-Butadiene
7.n-Pentane
3-Lot Comparison for Rt-Silica BOND
GC_PC1277
ColumnRt-Silica BOND, 30 m, 0.32 mm ID (cat.# 19785)
SampleCustom gas standard
Diluent:Nitrogen
Conc.:1 mole percent each component
Injection
Inj. Vol.:15 µL split (split ratio 35:1)
Liner:Premium 2.0 mm ID straight inlet liner (cat.# 23313.1)
Inj. Temp.:250 °C
Oven
Oven Temp.:90 °C (hold 20 min)
Carrier GasH2, constant flow
Linear Velocity:38 cm/sec
DetectorFID @ 260 °C
InstrumentAgilent/HP6890 GC

Conclusion

The Rt-Silica BOND column gives you the retention and capacity you need from PLOT columns, along with virtually particle-free and water-resistant operation. The combination of rugged manufacturing and rigorous QC testing ensures every Rt-Silica BOND column will provide optimal performance and reliable results for every analysis, while minimizing downtime due to maintenance from particle shedding or time-consuming bakeouts due to water contamination. The column’s unique selectivity makes it ideal for analysis of hydrocarbons, halogenated compounds, and sulfur gases.

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