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Superior Inertness Ensures Accurate Trace-Level Analysis

Inertness is one of the most difficult attributes to achieve in column manufacturing, but it is also one of the most critical as it affects detection through both peak shape and retention time stability. Rxi technology produces the most inert columns available, providing:

Increased Signal-to-Noise Ratios

Column inertness improves peak shape, which greatly determines the height of the signal and, therefore, analytical sensitivity. If capillaries are not sufficiently deactivated, peaks will become asymmetric and the signal will be reduced (Figure 1). Rxi columns are exceptionally inert, ensuring symmetric peak shape and high response for a wide range of analyte chemistries.

Figure 1  Strong interactions with residual active sites in the column increase tailing and reduce signal response.

Stabilized Retention Times

In addition to influencing signal-to-noise ratios, column inertness also affects retention time stability, which is an important factor for correct peak identification. Inertness is critical because peak tailing will increase as column activity increases, causing retention times to shift (Figure 2). A practical example is shown in Figure 3, where Rxi column technology is compared to a similar column from a different manufacturer using pyridine as a test probe. Pyridine peak shape and retention are significantly better on the Rxi column than the standard commercial column; pyridine elutes where it is supposed to elute—it is not retained by surface activity.

Figure 2  As column activity increases, signal decreases and retention time shifts.

GC_EX01116

Figure 3  Comparison of peak response for critical compounds: pyridine elutes faster and generates a higher signal on the Rxi column.

Another problem associated with poor inertness is that retention time becomes a function of the amount of analyte injected. As one does not know the exact level of target compound in samples, the peak may appear outside the retention time window in some samples, causing reporting errors or making additional analyses necessary. Figure 4A illustrates what happens if different amounts of pyridine are injected onto a typical commercial column. As the absolute amount decreases, the retention for pyridine increases. In contrast, Figure 4B shows the same injections made on an Rxi-5ms column. Due to the higher inertness of the Rxi column, the pyridine peak elutes at the same retention time, regardless of the amount injected.

Figure 4  Analyte levels in samples are unknown; only inert columns, which prevent concentration from affecting retention time, can assure accurate results.

GC_EX01118 GC_EX01119
extracted ion chromatogram, m/z 79

Improved Response for Polar, Acidic, and Basic Compounds

In addition to increased signal-to-noise ratios and stabilized retention times, the inertness of Rxi columns improves response for a wide range of challenging compound chemistries. Rxi columns are designed for enhanced inertness toward acidic, basic, and polar compounds. To demonstrate inertness, the response factors for 2ng injections of pyridine (basic) and 2,4-dinitrophenol (acidic) on an Rxi-5Sil MS column were compared with Agilent DB-5ms, Varian VF-5ms, and Phenomenex ZB-5ms columns. For both probes, the Rxi column outperformed the other columns, as demonstrated by the higher response factors for both compounds (Table I). The exceptional neutrality of Rxi columns allows a wide class of components to be analyzed on a single column (Figure 5).

Table I  Rxi-5Sil MS columns produce higher response factors for 2,4-dinitrophenol and pyridine than competitor columns.

 
2,4-dinitrophenol (average RF)
pyridine (average RF)
Rxi-Sil MS
0.24
0.74
Manufacturer A
0.20
0.63
Manufacturer B
0.22
0.64
Manufacturer C
0.24
0.65
Response factors are based on phenanthrene. (n=7)

Figure 5  Rxi technology provides good peak shapes for a wide range of compounds, including acids, alcohols, amines, aromatics, chlorinated hydrocarbons, ethers, hydrocarbons, nitro compounds, phenols, and polycyclic aromatic hydrocarbons.

Peaks
1.1,4-Dioxane
2.N-Nitrosodimethylamine
3.Pyridine
4.2-Fluorophenol (SS)
5.Phenol-d6 (SS)
6.Phenol
7.Aniline
8.Bis(2-chloroethyl) ether
9.2-Chlorophenol
10.1,3-Dichlorobenzene
11.1,4-dichlorobenzene-d4 (IS)
12.1,4-Dichlorobenzene
13.Benzyl alcohol
14.1,2-Dichlorobenzene
15.2-Methylphenol
16.Bis(2-chloroisopropyl) ether
17.4-Methylphenol/3-Methylphenol
18.N-Nitrosodi-N-propylamine
19.Hexachloroethane
20.Nitrobenzene-d5 (SS)
21.Nitrobenzene
22.Isophorone
23.2-Nitrophenol
24.2,4-Dimethylphenol
25.Benzoic acid
26.Bis(2-chloroethoxy)methane
27.2,4-Dichlorophenol
28.1,2,4-Trichlorobenzene
29.Naphthalene-d8 (SS)
30.Naphthalene
31.4-Chloroaniline
32.Hexachlorobutadiene
33.4-Chloro-3-methylphenol
34.2-Methylnaphthalene
35.1-Methylnaphthalene
36.Hexachlorocyclopentadiene
37.2,4,6-Trichlorophenol
38.2,4,5-Trichlorophenol
39.2-Fluorobiphenyl (SS)
40.2-Chloronaphthalene
41.2-Nitroaniline
42.1,4-Dinitrobenzene
43.Dimethyl phthalate
44.1,3-Dinitrobenzene
45.2,6-Dinitrotoluene
46.1,2-Dinitrobenzene
Peaks
47.Acenaphthylene
48.3-Nitroaniline
49.Acenaphthene-d10 (IS)
50.Acenaphthene
51.2,4-Dinitrophenol
52.4-Nitrophenol
53.2,4-Dinitrotoluene
54.Dibenzofuran
55.2,3,5,6-Tetrachlorophenol
56.2,3,4,6-Tetrachlorophenol
57.Diethyl phthalate
58.4-Chlorophenyl phenyl ether
59.Fluorene
60.4-Nitroaniline
61.4,6-Dinitro-2-methylphenol
62.N-Nitrosodiphenylamine (Diphenylamine)
63.1,2-Diphenylhydrazine (as Azobenzene)
64.2,4,6-Tribromophenol (SS)
65.4-Bromophenyl phenyl ether
66.Hexachlorobenzene
67.Pentachlorophenol
68.Phenanthrene-d10 (IS)
69.Phenanthrene
70.Anthracene
71.Carbazole
72.di-n-Butyl phthalate
73.Fluoranthene
74.Benzidine
75.Pyrene-d10 (SS)
76.Pyrene
77.p-Terphenyl-d14 (SS)
78.3,3'-Dimethylbenzidine
79.Butyl benzyl phthalate
80.Bis(2-ethylhexyl) adipate
81.3,3'-Dichlorobenzidine
82.Benz[a]anthracene
83.Bis(2-ethylhexyl)phthalate
84.Chrysene-d12 (IS)
85.Chrysene
86.Di-n-octyl phthalate
87.Benzo[b]fluoranthene
88.Benzo[k]fluoranthene
89.Benzo[a]pyrene
90.Perylene-d12 (IS)
91.Dibenz[a,h]anthracene
92.Indeno[1,2,3-cd]pyrene
93.Benzo[ghi]perylene
C = Toluene
Semivolatile Organics by EPA Method 8270 on Rxi-5Sil MS (30m, 0.25mm ID, 0.25 µm) w/Single Taper Liner packed with Semivolatiles Wool
GC_EV01129
ColumnRxi-5Sil MS, 30 m, 0.25 mm ID, 0.25 µm (cat.# 13623)
Sample8270 MegaMix (cat.# 31850)
Benzoic acid (cat.# 31879)
8270 Benzidines Mix (cat.# 31852)
Acid Surrogate Mix (4/89 SOW) (cat.# 31025)
Revised B/N Surrogate Mix (cat.# 31887)
1,4-dioxane (cat.# 31853)
SV Internal Standard Mix (cat.# 31206)
Diluent:methylene chloride
Conc.:10 ng on-column
Injection
Inj. Vol.:1.0 µL pulsed splitless (hold 0.25 min)
Liner:Singel Taper Splitless (4mm) w/Application-Specific Wool (cat.# 20798-231.1)
Inj. Temp.:250 °C
Pulse Pressure:25 psi (172.4kPa)
Pulse Time:0.3 min
Purge Flow:60 mL/min
Oven
Oven Temp.:40 °C (hold 1 min) to 280 °C at 25 °C/min to 320 °C at 5 °C/min (hold 1 min)
Carrier GasHe, constant flow
Flow Rate:1.2 mL/min
DetectorMS
Mode:Scan
Transfer Line Temp.:280 °C
Analyzer Type:Quadrupole
Source Temp.:250 °C
Tune Type:DFTPP
Ionization Mode:EI
Scan Range:35-550 amu
InstrumentAgilent 7890A GC & 5975C MSD

Rigorous inertness testing was central to the development of Rxi technology. Evaluations were based on the Lautamo/Jennings test mix, as originally reported by Lautamo et al., which includes a challenging list of probes [1]. To test Rxi columns, Restek expanded the Lautamo/Jennings test mix to include probes for acidic and basic activity as shown in Table II.

Table II  An expanded compound list, based on the original Lautamo/Jennings test mix, provided a rigorous test of inertness.

Component
ng on Column    
Column Functionality Test
Propionic acid
11.5
Basicity
1-Octene
3.9
Polarity
Octane
3.9
Hydrocarbon marker
Nitrobutane
7.7
Acidity
4-Picoline
7.7
Acidity
Trimethyl phosphate
38.5
Acidity
1,2-Pentanediol
11.5
Silanol
n-Propylbenzene
7.7
Hydrocarbon marker
1-Heptanol
7.7
Silanol
3-Octanone
7.7
Polarity
Decane
7.7
Hydrocarbon marker

As shown in Figure 6, the Rxi-5Sil MS column shows excellent peak shape for all compounds, demonstrating a high degree of inertness. In contrast, when this mixture is analyzed on another manufacturer’s column that claims to be the most inert, results for several critical indicator compounds are very poor (Figure 7). This test mix clearly shows that standard commercial columns are not as inert as Rxi columns and cannot deliver the same level of performance.

Figure 6  Lautamo/Jennings test mix acquired on Rxi-5Sil MS, 30m x 0.25mm, 0.25µm film column.

 1.  propionic acid
 2.  1-octene
 3.  octane
 4.  nitrobutane
 5.  4-picoline
 6.  trimethylphosphate
 7.  1,2-pentanediol
 8.  N-propylbenzene
 9.  1-heptanol
10. 3-octanone
11. decane

Figure 7  Lautamo/Jennings mix acquired on a new DB-5ms columns, 30m x 0.25mm df = 0.25µm

 1.  propionic acid
 2.  1-octene
 3.  octane
 4.  nitrobutane
 5.  4-picoline
 6.  trimethylphosphate
 7.  1,2-pentanediol
 8.  N-propylbenzene
 9.  1-heptanol
10. 3-octanone
11. decane

Some suppliers take pride in hand-picking columns and market these columns as a selected “inert” series. Restek does not hand-pick columns as all columns are manufactured and tested under controlled conditions and are required to meet our stringent specifications. This means Rxi columns deliver more accurate, reliable trace-level results than any other fused silica column on the market.

REFERENCES

[1] R. Lautamo, M. Hastings, E. Kuhn, W. Jennings, Plenary Lecture, 27th International Symposium on Capillary Chromatography, Riva Del Garda, Italy, June 2004.



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