Gas Chromatography

Consistency

Consistency of Supelco GC Capillary Columns
The Proof: Consistency

The very consistent nature of SLBms columns can be traced to the Innovative Manufacturing Processes that we have established for these columns.

Today’s GC analysts rightly expect to receive high quality columns on which fundamental performance parameters have been rigorously tested. However, beyond the importance of the absolute value of a column performance parameter is the importance of consistent performance, which goes a long way in ensuring the reliability of the method and the satisfaction with the column.

 

The Column as a Source of Variation

Anyone who has worked in an analytical environment or relied on analytical data knows how variation can cause data to be questioned, analyses to be repeated, and conclusions to be rejected. No analysis is valuable if it cannot be repeated; tomorrow, next year, or by other operators in other laboratories. In chromatographic separations, variation can arise from many sources. The column is one of these sources.

As column manufacturers, it is our responsibility to ensure minimal contribution from the column to overall variation. For chromatographic separations, variation in retention time can lead to compound misidentification. Variation in inertness can reduce the reliability of quantification, sensitivity, and limit of detection. Variation in column lifetime or durability reduces the method’s ease of automation, adds to the analysis costs, and causes instrument downtime. For GC separations in particular, column bleed, both the overall level of bleed and its variability between columns, is a common problem. High bleed columns reduce reliability of identification and sensitivity, and should be avoided.


SLB = Consistent Resolution and Retention

Figure 1 shows the separation of an 8-component QA test mix on three columns from three different phase lots. The first two components represent a critical pair, 2-octanone/n-decane. Note that each phase lot provided baseline resolution and consistent retention of these closely eluting compounds. A second critical QA parameter is retention factor (indicative of film thickness), k’, which is measured using the last eluting peak, n-tridecane. As shown, k’ values are consistent across these columns.

Figure 1. QA Column Test Data From Three SLB-5ms Columns
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Conditions
column: SLB-5ms, 30 m x 0.25 mm I.D., 0.25 µm (28471-U)
oven: 110 °C (14 min.), 15 °C/min. to 325 °C (15 min.)
inj.: 250 °C
det.: FID, 360 °C
carrier gas: helium, 30 cm/sec @ 110 °C
injection: 1.0 µL, 100:1 split
liner: 4 mm I.D. split, cup design
sample: Nonpolar Column Test Mix, each analyte at 500 µg/mL in methylene chloride

Peak IDs
1. 2-Octanone
2. n-Decane
3. 1-Octanol
4. n-Undecane
5. 2,6-Dimethylphenol
6. 2,6-Dimethylaniline
7. n-Dodecane
8. n-Tridecane


SLB = Consistently Inert

Some compounds may exhibit poor response or poor peak shape due to interaction with active sites on the column fused silica surface. The inertness of the SLB-5ms column, attributed to its exceptional surface deactivation, is demonstrated as follows. The relative response factors (RRFs) of an acidic, a basic, and a neutral compound, from three columns made with different lots of stationary phase were compared. As shown in Table 1, the %RSD values were <10%. Figure 2 shows the excellent peak shapes of extracted ion current profiles (EICPs) from 5 ng on-column amounts of pentachlorophenol, a notorious poor performing analyte, on these same three columns.

Table 1. RRFs of Poor Performers From Three SLB-5ms Columns
Column # Phase Lot 4-Chloroaniline Acenaphthylene 2,4-Dinitrophenol
1 A 0.413 1.833 0.105
2 B 0.415 1.921 0.109
3 C 0.479 1.981 0.108
         
Avg. RRF   0.436 1.912 0.107
Std. Dev.   0.038 0.074 0.002
% RSD   8.6 3.9 1.9

Conditions
column: SLB-5ms, 30 m x 0.25 mm I.D., 0.25 µm (28471-U)
oven: 40 °C (2 min.), 22 °C/min. to 240 °C, 10 °C/min. to 330 °C (1 min.)
inj.: 250 °C
MSD interface.: 330 °C
scan range.: m/z 40-450
carrier gas: helium, 1.0 mL/min. constant (11 min.), programmed rapidly to 1.5 mL/min. (hold remainder of run)
injection: 0.5 µL, pulsed (15 psi until 0.10 min.) splitless (0.75 min.)
liner: 4 mm I.D., single taper
sample: 10 ppm 8270 standards in methylene chloride, with internal standards at 40 ppm


Figure 2. EICPs of Pentachlorophenol (5 ng on-column) From Three SLB-5ms Columns
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SLB = Consistently Exhibit Low Bleed

The consistently low bleed performance of the SLB-5ms is demonstrated in Figure 3 on three columns made from different phase lots by the mass spectra of benzo(g,h,i)perylene. Each spectrum was taken from a 5 ng on-column analyses, and at an elution temperature of 330 °C. Note that these columns all exhibit a minimal amount of the primary column bleed ion m/z 207 present in the spectra. Another prominent column bleed ion, m/z 281, is barely noticeable in any of these spectra. Low bleed levels will result in higher quality MS spectra, and better library search matches.

Figure 3. Mass Spectra of Benzo(g,h,i)perylene From Three SLB-5ms Columns
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