Gas Chromatography

Inertness

Inertness of Supelco GC Capillary Columns
 
The Proof: Inertness

SLBms columns achieve their high level of inertness by virtue of a Proprietary Surface Deactivation that we apply to the fused silica tubing.

Inertness, a measure of the absence of adsorptive or active sites in the column, is an important attribute in that it influences peak signal. Increased peak signal, in addition to decreased noise, is the best way to consistently achieve low detection limits. Additionally, a column that does not possess good inertness may end up reducing profits due to additional labor requirements to keep the system in working order.

Inertness = Sharp Peaks = Low Detection Limits

Most analytes have at least one distinguishing functional group in their molecular structure. The functionality may be subtle, such as a double bond, or pronounced, such as a chlorine substitution, and may adversely affect the chromatography. Polar functional groups, such as hydroxyls (-OH) or amines (-NH2), are problematic due to their tendency to adsorb to active sites on any surface (syringe needle, injection port liner, column, etc.) with which they come into contact. Adsorption results in broad, short peaks. Because more of the peak area is in the noise region, it tends to get lost by the software algorithm. Adsorption is essentially equivalent to the loss of area counts.

In Figure 1, the Extracted Ion Current Profile (EICP), also known as an Extracted Ion Chromatogram (EIC), for 2,4-dinitrophenol from a US EPA Method 8270D analysis on an SLB-5ms column is shown. This analyte contains a hydroxyl (-OH) and two nitro groups (-NO2) attached to an aromatic ring, and is historically a very troublesome analyte for this method. Figures 2 and 3 show EICPs of 4-nitrophenol and 4-nitroaniline, respectively, from the same analysis. 4-Nitrophenol contains a hydroxyl group and a nitro group attached to an aromatic ring, whereas 4-nitroaniline contains an amine group and a nitro group attached to an aromatic ring. These analytes are also known to be troublesome, although to a lesser degree than 2,4-dinitrophenol. Note the sharpness of the traces of the quantitation m/z, 184, 139, and 138, respectively, on the SLB-5ms column shown in these three Figures. It is important to keep in mind that sharper peaks result in greater peak area that is above the noise level. Since most analytes possess one or more groups of varying functionality, it is important to use a deactivated column, like SLB-5ms, to ensure good peak shape and sensitivity for all components of the sample.

Figure 1. EICP of 5 ng On-column of 2,4-Dinitrophenol.
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Figure 2. EICP of 5 ng On-column of 4-Nitrophenol.
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Figure 3. EICP of 5 ng On-column of 4-Nitroaniline.
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Inertness = Negligible Peak Tailing = Minimal Preventative Maintenance

As previously discussed, an active surface can lead to adsorption. Besides causing broad, short peaks, adsorption can cause peak tailing. Many environmental and pharmaceutical methods have a peak symmetry test that must be satisfied prior to the analysis of samples. Excessive peak tailing makes it difficult to pass the test. Failure to pass the test requires the analyst to perform maintenance (chemically treating the activity, clipping the column, replacing the column, etc.). These activities increase instrument downtime, which keeps the analyst from analyzing samples.

US EPA Method 625 requires that two analytes, one acidic and one basic, must pass a daily tailing factor test prior to the analysis of any sample extract. Figures 4 and 5 show EICPs on an SLB-5ms for pentachlorophenol and benzidine, respectively, at the method-required concentrations. Both tailing factors were extremely close to a value of 1, indicating excellent peak shape for both analytes in addition to falling well within the method-required criteria.

Figure 4. EICP of 50 ng On-column of Pentachlorophenol.
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Figure 5. EICP of 100 ng On-column of Benzidine.
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Inertness = Great RRFs = Minimal Preventative Maintenance

Many US EPA Methods that employ the use of a mass selective detector (MSD) require the use of internal standards, compounds that are structurally similar to the target analytes. The internal standards are added to the sample extract shortly before analysis and are used to compensate for injection discrepancies and drift in MSD response. The response for every target analyte relative to the response for a specific internal standard is calculated. This relative response factor (RRF) is plotted against the calibration curve to determine concentration.

These methods list minimum RRF criteria for poor responding target analytes, sometimes referred to as System Performance Check Compounds (SPCCs), as a measure of system performance. If an RRF is below the minimum criteria in either an initial or a continuing calibration, the analyst must take action to correct the problem prior to analysis of samples. This usually involves cutting off a section of column from the injector end to remove surface activity.

RRF values for these poor performing target analytes from US EPA Methods 8270D and OLM04.2 SVOA are shown in Table 1. Target analytes with polar functionality tend to have low RRFs due to adsorption. The observed RRFs clearly exceed the minimum RRF criteria specified for these analytes.

 
Table 1. RRFs at 50 ng On-column (Internals at 40 ng On-column) of Several Poor Performers.
Analyte Analyte
m/z
Internal
Standard *
8270D
Criteria
OLM04.2
Criteria
Observed
Bis(2-chloroisopropyl)ether 45 1 - 0.010 1.702
N-nitroso-di-n-propylamine 70 1 0.050 0.500 1.020
4-chloroaniline 127 2 - 0.010 0.441
Hexachlorobutadiene 225 2 - 0.010 0.153
Hexachlorocyclopentadiene 237 3 0.050 0.010 0.301
2-nitroaniline 65 3 - 0.010 0.365
Dimethyl phthalate 163 3 - 0.010 1.272
3-nitroaniline 138 3 - 0.010 0.348
2,4-dinitrophenol 184 3 0.050 0.010 0.160
4-nitrophenol 139 3 0.050 - 0.879
4-nitrophenol 109 3 - 0.010 0.152
Diethyl phthalate 149 3 - 0.010 1.290
4-nitroaniline 138 3 - 0.010 0.339
2-methyl-4,6-dinitrophenol 198 4 - 0.010 0.128
N-nitrosodiphenylamine 169 4 - 0.010 0.653
2,4,6-tribromophenol (surr.) [1] 330 3 - - 0.169
2,4,6-tribromophenol (surr.) [2] 330 4 - 0.010 0.106
4-bromophenyl phenyl ether 248 4 - 0.100 0.229
Pentachlorophenol 266 4 - 0.050 0.132
Carbazole 167 4 - 0.010 0.961
Di-n-butyl phthalate 149 4 - 0.010 1.088
Butylbenzyl phthalate 149 5 - 0.010 0.615
3,3'-dichlorobenzidine 252 5 - 0.010 0.401
Bis(2-ethylhexyl)phthalate 149 5 - 0.010 0.769
Di-n-octyl phthalate 149 6 - 0.010 1.258
[1] Using 8270D specified internal standard.
[2] Using OLM04.2 specified internal standard.
* Internal Standards
1 = 1,4-dichlorobenzene-d4, m/z 152
2 = Naphthalene-d8, m/z 136
3 = Acenaphthene-d10, m/z 164
4 = Phenanthrene-d10, m/z 188
5 = Chrysene-d12, m/z 240
6 = Perylene-d12, m/z 264