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Rapid GC/MS Analysis of Semivolatile Organic Compounds
by Christopher English, Environmental Innovations Chemist, Restek Corp.
In a single gas chromatography/mass spectrometry analysis for semivolatile organic materials in environmental samples, analysts must monitor dozens of compounds exhibiting multiple chemical behaviors. Fast, reliable results depend on careful consideration of both column parameters and analytical conditions. Figure 1 shows an analysis of 90 US EPA Method 8270 target compounds, surrogates, and internal standards, achieved in 15 minutes. The silarylene based polymer Rtx®-5 Sil MS stationary phase provides excellent resolution, and exhibits low bleed at high temperatures. Responses and peak shape are excellent for active compounds such as 2,4-dinitrophenol (peak 50) and pentachlorophenol (66), difficult separations such as benzo(b) / benzo(k) fluoranthene (83 & 84) are achieved, and target compounds can be quantified to lower limits than with phenyl/methyl polymers. A short, narrow bore column with a thin phase film provides the needed results very rapidly.
click the image to enlarge
Figure 1. 15-minute separation of US EPA Method 8270D semivolatile organics on a 20 m 3 0.18 mm ID column.
Column: Rtx®-5 Sil MS, 20 m, 0.18 mm ID, 0.18 mm df (cat.# 42702); Sample: 8270 MegaMix™ 76-component calibration mix (cat.# 31686), benzoic acid (31415), benzidine (31441), 2,4-dinitrophenol (31291), Acid Surrogate Mix (31063), B/N Surrogate Mix (31062), SV Internal Standard Mix (31206), 0.5 mL, 5 ppm each compound / 2.5 ng on column (2,4-dinitrophenol: 10 ppm/5 ng; 3-, 4- methylphenol: 2.5 ppm/1.25 ng each, for calibration at lower levels and quantification at required limits); Injection: splitless, hold 0.15 min., pressure pulse 0.20 min. @ 30 psi, 2 mm ID cyclo double gooseneck inlet liner (cat.# 20907), 270 C; Instrument: Agilent 6890/5973 GC/MS; Carrier Gas: helium, 1.2 mL/min., constant flow; Oven Temp.: 40 C (hold 0.5 min.) to 90 C @ 14 C/min., to 330 C @ 22 C/min. (hold 1 min.); Transfer Line Temp.: 280 C; Scan Range: 35 to 550 amu; Solvent Delay: 1 min.; Tune: DFTPP; Ionization: EI. GC_EV00736 Peaks: 1) N-nitrosodimethylamine, 2) pyridine, 4) aniline, 6) phenol, 50) 2,4-dinitrophenol, 66) pentachlorophenol, 83) benzo(b)fluoranthene, 84) benzo(k)fluoranthene.
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To achieve this separation, analysis conditions also must be optimized. Splitless injection reduces solvent effects that could interfere with N-nitrosodimethylamine (1) and pyridine (2), but hold time is critical: a change of only several seconds can reduce sensitivity by 50%. A 0.20 min. pulse, 5 psi higher than the column flow backpressure, dramatically improves sample transfer onto the column; making the pulse 0.05 min. (3 seconds) longer than the splitless hold time allows excess solvent to be swept away quickly. A 2 mm ID injector liner with a spiraled bore allows the 0.5 μL sample to be completely vaporized in the injection port, ensuring more reproducible results than with a standard (straight bore) liner. The 270 C injection port temperature minimizes interactions between the injection port and reactive analytes. The temperature program is adjusted to resolve analytes that coelute and share ions: the initial temperature ramp resolves aniline and phenol (4 & 6), for example, and the benzo(b) / benzo(k) fluoranthene isomers (83 & 84) are resolved by eluting them during the second temperature ramp, rather than during the final hold time.
Finally, reference materials should be formulated carefully. Target compound N-nitrosodiphenylamine readily oxidizes to diphenylamine and nitric oxide, a highly reactive gas that can initiate other reactions in the calibration mix. For stability, diphenylamine, rather than N-nitrosodiphenylamine, should be included in the mix. Similarly, derivative azobenzene should be included in the mix, in place of unstable target compound diphenylhydrazine.
For identification of all peaks, request Restek Advantage newsletter 2004v2 (lit.# 59037) or see www.restek.com
Figure 1. 15-minute separation of US EPA Method 8270D semivolatile organics on a 20 m × 0.18 mm ID column.
Column: Rtx®-5 Sil MS, 20 m, 0.18 mm ID, 0.18 μm df (cat.# 42702); Sample: 8270 MegaMix™ 76-component calibration mix (cat.# 31686), benzoic acid (31415), benzidine (31441), 2,4-dinitrophenol (31291), Acid Surrogate Mix (31063), B/N Surrogate Mix (31062), SV Internal Standard Mix (31206), 0.5 μL, 5 ppm each compound / 2.5 ng on column (2,4-dinitrophenol: 10 ppm/5 ng; 3-, 4- methylphenol: 2.5 ppm/1.25 ng each, for calibration at lower levels and quantification at required limits); Injection: splitless, hold 0.15 min., pressure pulse 0.20 min. @ 30 psi, 2 mm ID cyclo double gooseneck inlet liner (cat.# 20907), 270 C; Instrument: Agilent 6890/5973 GC/MS; Carrier Gas: helium, 1.2 mL/min., constant flow; Oven Temp.: 40 C (hold 0.5 min.) to 90 C @ 14 C/min., to 330 C @ 22 C/min. (hold 1 min.); Transfer Line Temp.: 280 C; Scan Range: 35 to 550 amu; Solvent Delay: 1 min.; Tune: DFTPP; Ionization: EI. GC_EV00736 Peaks: 1) N-nitrosodimethylamine, 2) pyridine, 4) aniline, 6) phenol, 50) 2,4-dinitrophenol, 66) pentachlorophenol, 83) benzo(b)fluoranthene, 84) benzo(k)fluoranthene.
Restek Corp.
110 Benner Circle
Bellefonte, PA 16823-8812
Restek Corporation
110 Benner Circle
Bellefonte, PA, 16823
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