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Totally Reliable Column-to-Column Performance


Chromatographers today need assurances that every column they receive is going to perform in the same way as the column it replaces. The processes used by Restek to make Rxi columns lead the industry in manufacturing precision and quality control, resulting in the most consistently performing columns available. To demonstrate this, test data for one of the most popular columns (Rxi-5ms, 30m x 0.25mm, 0.25µm) are shown below. Statistics are based on data for more than 2,000 columns manufactured over a 2-year period.

Efficiency (plates/meter)

Rxi columns will separate analytes better than competitor columns because they have more plates/meter. Typical values for columns of the same dimensions from some vendors (Agilent, J&W, Varian) are listed at 3,900 plates/meter; other suppliers do not even specify a value, meaning there is no efficiency guarantee whatsoever. In contrast, the Rxi-5ms column has an average of 4,270 plates/meter with a very tight distribution (Figure 1). This means that the Rxi coating process is well-controlled and highly reproducible.

Figure 1  Rxi columns have higher efficiency and reliably produce better separations than non-Restek columns. (Average plates/meter: Rxi-5ms = 4,270; competitors = 3,900.)


In practical terms, this means peaks will be better resolved on a Restek column than on a competitor column, simply because the Rxi process consistently produces a more efficient column (Figure 2). This allows you to run analyses at higher than optimal linear velocity and reduce analysis time—easily by 10%—without sacrificing separation power.

Figure 2  More plates-per-meter mean better separations on an Rxi column.


Retention Factor (k)

Restek precisely determines the amount of stationary phase that is coated in each column. As a result, retention factor is tightly controlled with an average of 6.44 and standard deviation of 0.11 for pentadecane on the Rxi-5ms column (Figure 3). Narrow distribution ensures consistent polymer film thickness, which means retention times will be consistent and predictable, column-to-column.

Competitor columns typically have variations of ±7%. If we apply that standard to the Restek column, we calculate a window of ±0.45 around the average for pentadecane. As shown in Figure 3, this tolerance window is much wider than the entire distribution for the Rxi column, meaning that there will be much less variation in retention times on an Rxi column than on a competitor column. This is confirmed in Figure 4, which illustrates that retention times are much more consistent—and thus predictable—on the Rxi column.

Figure 3  Retention factor data for the Rxi-5ms, 30m x 0.25mm, 0.25µm column. Industry standard minimum and maximum for quality control is noted at the 7% lines.


Figure 4  A tightly controlled Rxi coating process ensures more consistent and predictable retention times on Restek columns than on columns produced by other suppliers.


Retention Indices (RI)

Most commercial columns have a tolerance on retention indices which is typically within ±0.5 units for acenaphthalene and undecanol. As shown in Figure 5 the variation in RI for the Rxi-5ms column is much less than the industry standard (1-undecanol: average = 1372.8; standard deviation ±0.15). This narrow distribution ensures consistent column selectivity, which means the components always elute in the same position relative to the other components. Resolution is, therefore, more consistent column-to-column on the Restek column than on columns from other manufacturers.

Figure 5  Relative retention is much more consistent on Restek columns than competitor columns because the variance is more tightly controlled. (Industry standard minimum and maximum for quality control is indicated by the ±0.5 lines.)



The industry standard for acceptable bleed is 4pA for the 30m x 0.25mm, 0.25µm column. The Rxi-5ms column consistently performs below this criterion. As shown in Figure 6, the average bleed is 1.4pA with a standard deviation of 0.5.

Figure 6  Bleed for the Rxi-5ms column (30m x 0.25mm, 0.25µm) is consistently less than the industry standard of 4pA.


All Rxi columns are guaranteed to be low bleed because Restek does not batch test for bleed to reduce testing costs. To assess bleed, first every Rxi column is conditioned for 16 hours at maximum isothermal temperature. After this, each column is individually tested for bleed using calibrated equipment. Bleed values obtained are listed on the test report for every Rxi column. 


Column inertness has been the biggest challenge in making fused silica columns until now—Rxi column technology has made it possible to make highly inert columns in a remarkably consistent fashion. Inertness is critical as it affects peak shape, response, retention time stability, integration, and data accuracy (a detailed discussion of inertness can be found here). Each Rxi column is evaluated with a rigorous test mixture designed to indicate different types of column activity. As shown in Table I and Figure 7, peaks for all test probes are highly symmetric and standard deviations are very low, indicating extremely consistent column-to-column inertness.

Table 1  Rxi columns are extremely inert and rigorously tested for different types of activity.

Test Probe   
Activity Indicated
Peak Asymmetry
Peak Asymmetry
(standard deviation)
silanol groups (hidden)
silanol groups
basic active sites
acidic active sites

Data shown are for Rxi-5ms columns (30m x 0.25mm, 0.25µm).

Figure 7  Tight distributions and excellent peak asymmetry values assure consistently inert columns.

A. 1,6-Hexanediol asymmetry plot


B. 1-Undecanol asymmetry plot


C. 4-Chlorophenol asymmetry plot


D. 1-Decylamine asymmetry plot



Rxi columns deliver more accurate, reliable trace-level results than any other fused silica column on the market. Take variation out of the equation by switching to highly reproducible Rxi columns.