Deactivation of Metal Surfaces: Applications in Gas Chromatography (GC) for the Past 15 Years

In 1987, Restek Corp. (Middelburg, The Netherlands) introduced a new way to deactivate metal columns, called SilcoSteel. The original surface was shielded in such a way that the traditional reactive metal surface no longer impacted the chromatography. Later, a secondary deactivation was applied‒Siltek‒wherein the surface of the silicon layer was further passivated. This deactivation allowed not only columns to be deactivated, but also liners, tubing, cylinders, connectors, etc. Surfaces were especially inert toward sulfur. Volatile sulfur compounds like H2S, methyl mercaptan, and even mercury are known to disappear when they are analyzed at trace levels or when they are stored for some time. The Siltek technology rectified this. In addition, deactivation also had a unique side effect that was not expected‒it increased the stability of high-temperature columns like simulated distillation and biodiesel applications such that column lifetime typically increased by a factor of 4.  

Figure 1 – High-temperature polyimide after 48 hr at 400 °C. The polyimide outside coating is pyrolyzed, forming bare fused-silica spots, which can cause column breakage.

Why a metal solution?

Fused silica is widely used in many GC applications, but has limitations; these are eliminated when metal is used as a capillary tubing. In particular, in applications in which the fused-silica column is “stressed,” a metal solution is preferred. Such applications are:

  1. High-temperature separations: When the components elute at temperatures >350 °C, the polyimide coating of fused silica is seriously compromised. There are so-called high-temperature polyimide coatings, but even these have limitations when operated at higher temperatures. Some manufacturers claim stability to 430 °C for polyimide coating, which is a risk that the user does not want to take. Figure 1 shows what happens when a high-temperature polyimide-type column is used for 48 hr at 400 ºC. The bare fused-silica surface is already visible, meaning there is danger of column breakage or even explosion. Aside from being fragile, sudden column breakage can lead to silica and pyrolyzed polyimide dust that will impact the lifetime of the oven heating elements. Typical applications are simulated distillation (simdist), biodiesel, triglycerides, waxes, oils, and surfactants.1,2
  2. Process industry-type separations: In process analyzers, the first priority is reliability. Fused silica is often considered to be too fragile, and if a metal solution is available that does the same separation it will be preferred. Also, many columns are often positioned in a relatively small place. Additionally, some process analyzers use the high circulation speed of air, which sometimes “sandblasts” fused silica.
  3. Small and portable instrumentation: Fused silica can easily be wound on small diameters, but the ring tension increases exponentially. In particular, if 0.53-mm columns have to be wound on a small coil diameter, the ring tension in 0.53 mm will increase the risk for breakage. There is no issue with metal columns being wound on very small diameters. Figure 2 shows examples of how metal columns are miniaturized.
  4. Direct heating applications: If mounted correctly, metal columns can also be used for direct heating.3,4

Figure 2 – Metal tubing is flexible and can be coiled in small diameters without risk of column breakage, which is especially useful for process analyzers and small oven designs.

Metal columns and hydrophobic surfaces

Stainless steel (SS) in itself is highly reactive. There are several ways to passivate the material so that it is not easily oxidizable, but for good deactivation quality a different approach was needed. By shielding the SS surface by depositing a layer of silicon on the surface, a new, well-defined surface was created. The first products were available under the name SilcoSteel. This material already showed high inertness toward sulfur compounds.

Later, a secondary deactivation (U.S. patent 6,511,760)5 was added to this process, where the surface was made more hydrophobic and showed an even higher inertness, also toward very polar analytes such as alcohols and acid/base compounds. These surfaces are called Siltek or Sulf-Inert.

Figure 3 shows a mixture containing a range of highly polar compounds run on a metal column with only a 0.1-μm film. Such thin layers will immediately reveal any unwanted surface activity, since the polar analytes will start to tail. Figure 3 shows a near-perfect chromatogram, proving the high inertness of the metal surface with the silicon-based Siltek deactivation.

Figure 3 – Test mixture containing polar analytes like 2,6 hexane-diol (very polar), chlorophenol (polar + acid), and propyl aniline (polar + base). Column: 10 m × 0.32 mm Siltek-treated capillary, coated with a 0.10-μm layer of 5% diphenyl/95% dimethyl polysiloxane.

Stabilization effect on nonpolar phases

One of the most interesting characteristics of the Siltek deactivation is the impact on phase stability. Nonpolar siloxane phases are stabilized by the silicon surface. This translates into a significant lower degradation and high application temperatures. Figure 4 shows the difference between fused silica that is deactivated using silane reagents and Siltek deactivation on MXT. Column dimensions and film thickness were identical. The deactivated MXT showed almost four times lower phase degradation. This characteristic is very important since it not only allows the metal columns to be used at high temperature because of robustness, but also offers much lower bleed. Practically, this means that MXT columns do not lose their stationary phase coating, which results in much longer column lifetime and the need for less frequent calibration.

Figure 4 – Bleed behavior of same phases on different surfaces. Siltek deactivation stabilizes the stationary phase, resulting in lower phase degradation.

Simulated distillation

One application in which metal columns are beneficial is simulated distillation. In this application, the sample is separated on a nonpolar column and the resulting chromatogram can be converted in a boiling point range. As samples are analyzed that go up to C120, oven temperatures are required that exceed 430 °C. Metal columns are the only option.

Table 1 - Popular ASTM methods and column recommendations

Table 1 shows an overview of different ASTM methods that are used for simdist.1 For the D2887 for hydrocarbon ranges up to C44, typically a thick film simdist is used. This is because the injected sample is concentrated, and phase overload will result under the wrong elution profiles. For the extended-range hydrocarbons, the sample is diluted and MXT simdist columns are used with films of 0.09–0.16 μm. Such thin films allow elution of very high carbon numbers (see Figure 5). For this application, retention time reproducibility is very important because the accuracy of the boiling point distribution is directly impacted by the retention time. The low bleed provides the best retention time reproducibility.

Figure 5 – High-temperature application: SimDist 7139 method for crudes and resins. Column: 5 m × 0.53 mm, 0.1-μm film MXT-simdist. Oven: 40 °C, 30 °/min→430 °C, 1 min.

Biodiesels: glycerides analysis

Methods ASTM D-6584 and EN14105 describe the measurement of triglycerides and glycerol in biodiesel. The methods use fused-silica columns that are operated up to 380 °C. This brings the methods under stress because fused silica becomes fragile when used at these temperatures. Even more importantly, in order to follow the methods, the on-column technique is required. For on-column, a retention gap must be used (usually 1–2 m × 0.53 mm deactivated capillary) that has to be coupled. Coupling fused silica and high temperature operation is very challenging for the routine laboratory because it is not easy to make a coupling, and even the smallest leak may result in shortened column lifetime.

A metal capillary solution is thus preferred. Metal MXT-Biodiesel TG columns are factory-coupled with a 0.53-mm retention gap. An even better solution is an MXT column that is made with an integra-gap (see Figure 6); these columns can be prepared in 0.53-mm ID MXT tubing. Because there is no coupling present, there is no risk of leaks, and operation is very easy.

Figure 6 – Design of an integrated retention gap and stability test: 16 m × 0.53 mm MXT-Biodiesel TG, 0.16 μm with 2 m integra-gap after analysis 1 and analysis 100 of B-100 biodiesel according to ASTM 6584. Oven programming: up to 430 oC. Note the small change in retention times.

To achieve efficiency similar to the 10 m × 0.32 mm columns, listed in Methods ASTM D-6584 and EN14105, the 0.53-mm phase is manufactured as a 16-m column, with a 2-m section of integrated retention gap (only 14 m is coated). Figure 6 shows an overlay of analyses 1 and 100, using programs up to 430 °C. Retention times are almost identical, which again demonstrates the stabilization characteristics of the MXT surface.

Micropacked columns

In addition to its application as a capillary, the smaller-bore MXT tubing was used successfully for making micropacked columns with different types of packings. This column type is still preferred in some industries and is especially useful for the determination of trace sulfur compounds. For example, Ref. 6 refers to an application-specific sulfur column that is used for sulfur gas analysis.

Also, 0.53-mm metal columns can be packed with different materials. The columns require only 2–4 mL/min flow and can be mounted easily in any capillary GC since they have an o.d. of 0.76 mm. A significant advantage is the high loadability, which is beneficial for separations in which high concentrations are to be injected. Figure 7 shows the separation of CO and CO2 from air using a 0.53-mm-i.d. micropacked Shincarbon (Restek Corp., Bellefonte, PA) (for more details see Ref. 7). These 0.53-mm columns have been made successfully with molecular-sieve 5Å porous polymers, Shincarbon, and several liquid-phase-coated packing materials.

Figure 7 – Separation of CO and CO2 on carbon adsorbent. Column: 1 m × 0.53 mm MXT-Shincarbon 80/100 mesh; oven: 30 oC; carrier: He, 140 kPa; split 1:30; detection: micro-TCD (thermoconductivity detector).

Inertness for sulfur compounds

Silcosteel and Sulf-Inert/Siltek deactivations are well-known for their performance in trace sulfur analysis. Volatile sulfur compounds, such as hydrogen sulfide, sulfur dioxide, and mercaptans, are extremely reactive and mostly present in low concentrations. Correct quantification is a big challenge. The Siltek deactivation contributed significantly here because all parts that are in contact with the sample can be passivated using this process, i.e., sample canisters, transfer lines, injection port liner, connectors, and fittings. Every part of the GC system should be passivated.

Additionally, samples should be stored in an inert environment. In case the buyer and seller find different values, there are ASTM methods in place that allow the samples to be stored for several months and then rerun. It is important that sample composition does not change. This is possible using gas storage devices (bombs) that are Siltek/Sulf-Inert deactivated. Figure 8 shows how the sulfur levels change after storage in deactivated and normal SS canisters. Here the Siltek deactivation is very beneficial.

Figure 8 – Sulfur-containing samples change minimally when stored in Siltek canisters. Column: 30 m × 0.53 mm Rtx-1, df = 7 μm; 1) H2S, 2) methyl mercaptan, 3) ethyl mercaptan, 4) dimethylsulfide, 5) 2-propanethiol, 6) 1-propanethiol, 7) ethyl methylsulfide, 8) 2-butanethiol, 9) diethyl sulfide.

Porous layer open tubular (PLOT) columns

In addition to the success of MXT with liquid stationary phases, several adsorbents have been successfully deposited on the MXT format. Alumina, molecular sieve 5Å, and porous polymers coat very efficiently on the Siltek-treated metal. Figure 9 shows a porous polymer coating on a metal MXT. Note the very good peak shapes for highly polar anaytes like methanol and ethanol. This is adsorption chromatography using metal surfaces, and it is very impressive. Also, the MXT PLOT column has the robustness of metal and can be used during difficult conditions. MXT columns demonstrate efficiency and inertness identical to that seen with fused-silica columns of the same diameter.8

Figure 9 – Inertness test for MXT Q-BOND. Column: 30 m × 0.53 mm; oven: 150 oC; carrier: H2, 40 cm/sec; injection: split, 200 mL/min; detection: FID (flame ionization detector). Note very good peak shape for alcohols.

Summary

As discussed in this article, capillary separation technology has benefitted greatly from the implementation of inert MXT tubing. In addition to high-temperature analysis applications such as simulated distillation and biodiesel, metal columns are used routinely in process analysis and in small, portable GC formats. Due to their inertness they can be used in applications in which fused silica is utilized.

Additionally, the Siltek/Sulf-Inert deactivation has demonstrated efficacy when accurate levels of sulfur need to be monitored. This technique is not only useful for transfer lines, but also for sample loops, liners, MXT columns, flame jets, etc.; thus the entire GC system can be optimized to produce the highest possible signal.

The latest development‒coating of adsorbents onto MXT‒will further improve analysis because smaller oven dimensions and direct heating options are possible without risk of column breakage. The technology used to deactivate metal columns was the foundation of a new company called SilcoTek. There are many more applications outside the field of chromatography that can benefit from the unique characteristics of silicon-type coatings.9

References

  1. Burger, B.; Pijpelink, J. PIN Jun/Jul 2009, 14.
  2. de Zeeuw, J.; Burger, B. Am. Lab. Jan 2008, 24–7.
  3. http://www.vernier.com/products/sensors/gc-mini/.
  4. http://patents.justia.com/2003/06511760.html.
  5. http://www.falconfast.net/.
  6. http://www.restek.com/pdfs/pres-2002-113.pdf.
  7. http://blog.restek.com/?p=1045.
  8. de Zeeuw, J.; Morehead, R. et al. PIN Jun/Jul 2011, 37.
  9. http://www.silcotek.com/.

Jaap de Zeeuw is an international specialist in gas chromatography, Restek Corp., Weerhaan 9, 4336 KT Middelburg, The Netherlands; tel.: +31 118 623 151; e-mail: Jaap.dezeeuw@restek.com.

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