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Optimized Volatiles Analysis Ensures Fast VOC Separations

By Michelle Misselwitz, Innovations Chemist, Gary Stidsen, Product Manager, and Chris English, Innovations Manager
  • Optimized analysis allows for 36 runs per 12-hour shift, increasing instrument productivity.
  • Rxi-624Sil MS column inertness gives sharper peaks and more accurate data.
  • High temperature stability reduces bleed profile, resulting in lower detection limits.
Optimized conditions assure good resolution with minimal downtime.

Optimized methods for the analysis of volatile organic compounds (VOCs) can be time-consuming to develop because compound lists can be extensive and analytes vary significantly in chemical characteristics. For example, target compounds in EPA Method 8260 for solid waste matrices include volatiles that range from light gases (Freon) to larger aromatic compounds (trichlorobenzenes). These differences make column selectivity, thermal stability, and inertness critical to resolving volatiles. Often, “624” type columns are chosen for their selectivity, but thermal stability is usually poor, which can result in phase bleed that decreases detector sensitivity. New Rxi-624Sil MS columns offer reliable resolution of critical VOC pairs and also provide lower bleed and greater inertness than other columns. In order to provide optimized conditions for labs analyzing VOCs, we established parameters that ensure good resolution, while reducing downtime by syncing purge and trap cycles with instrument cycles. In addition, we present comparative data that demonstrate why Rxi-624Sil MS columns are the best choice for volatiles analysis.

Resolve Critical Pairs and Reduce Downtime

In order to achieve desired separations and minimize downtime between injections, several critical pairs were chosen for computational modeling using Pro ezGC software. The temperature program initially determined by the software was 35 °C (hold 5 min.) to 120 °C @ 11 °C/min. to 220°C @ 20 °C/min. (hold 2 min.). While this provided the best resolution of critical pairs, it also extended the analysis time to 19 min. Since the purge and trap cycle time was 16.5 min., we tested other conditions to see if adequate resolution could be maintained, while using a faster instrument cycle time that more closely matched the purge and trap cycle time, in order to maximize sample throughput. In other calculations, the software suggested changing temperature ramps at 60°C; therefore, a program of 35°C (hold 5 min.) to 60°C @ 11 °C/min. to 220°C @ 20 °C/min. (hold 2 min.) was tested. This final program reduced instrument downtime by better synchronizing injection and analysis cycles, and also provided excellent resolution of volatile compounds (Figure 1). Testing of faster conditions determined that the initial hold of 5 minutes at 35°C was critical for the best separation of early eluting compounds, such as the gases, as well as a favorable elution of methanol between gas compounds.

Figure 1  Rxi-624Sil MS columns resolve methyl ethyl ketone and ethyl acetate, a separation not obtained on other 624 columns.

PeakstR (min)
1.Dichlorodifluoromethane (CFC-12)2.198
2.Chloromethane2.459
3.Vinyl chloride2.659
4.Bromomethane3.226
5.Chloroethane3.434
6.Trichlorofluoromethane (CFC-11)3.876
7.Diethyl ether (ethyl ether)4.44
8.1,1-Dichloroethene4.909
9.1,1,2-Trichlorotrifluoroethane (CFC-113)4.998
10.Acetone5.029
11.Iodomethane5.195
12.Carbon disulfide5.323
13.Acetonitrile5.637
14.Allyl chloride5.715
15.Methyl acetate5.723
16.Methylene chloride5.981
17.tert-Butyl alcohol6.234
18.Acrylonitrile6.451
19.Methyl tert-butyl ether (MTBE)6.509
20.trans-1,2-Dichloroethene6.512
21.1,1-Dichloroethane7.315
22.Vinyl acetate7.359
23.Diisopropyl ether (DIPE)7.407
24.Chloroprene7.429
25.Ethyl tert-butyl ether (ETBE)7.97
26.2-Butanone (MEK)8.193
27.cis-1,2-Dichloroethene8.193
28.2,2-Dichloropropane8.193
29.Ethyl acetate8.265
30.Propionitrile8.276
31.Methyl acrylate8.318
32.Methacrylonitrile8.476
33.Bromochloromethane8.507
34.Tetrahydrofuran8.521
35.Chloroform8.651
36.1,1,1-Trichloroethane8.843
37.Dibromofluoromethane8.848
38.Carbon tetrachloride9.026
39.1,1-Dichloropropene9.037
40.1,2-Dichloroethane-d49.246
41.Benzene9.262
42.1,2-Dichloroethane9.334
43.Isopropyl acetate9.34
44.Isobutyl alcohol9.421
45.tert-Amyl methyl ether (TAME)9.421
46.Fluorobenzene9.598
47.Trichloroethene9.976
48.1,2-Dichloropropane10.243
49.Methyl methacrylate10.29
50.1,4-Dioxane (ND)10.299*
51.Dibromomethane10.326
52.Propyl acetate10.346
PeakstR (min)
53.2-Chloroethanol (ND)10.368*
54.Bromodichloromethane10.496
55.2-Nitropropane10.698
56.cis-1,3-Dichloropropene10.904
57.4-Methyl-2-pentanone (MIBK)11.026
58.Toluene-D811.148
59.Toluene11.21
60.trans-1,3-Dichloropropene11.407
61.Ethyl methacrylate11.435
62.1,1,2-Trichloroethane11.585
63.Tetrachloroethene11.662
64.1,3-Dichloropropane11.729
65.2-Hexanone11.749
66.Butyl acetate11.837
67.Dibromochloromethane11.921
68.1,2-Dibromoethane (EDB)12.035
69.Chlorobenzene-d512.412
70.Chlorobenzene12.44
71.Ethylbenzene12.507
72.1,1,1,2-Tetrachloroethane12.507
73.m-Xylene12.612
74.p-Xylene12.612
75.o-Xylene12.935
76.Styrene12.949
77.n-Amyl acetate13.018
78.Bromoform13.118
79.Isopropylbenzene (cumene)13.226
80.cis-1,4-Dichloro-2-butene13.268
81.4-Bromofluorobenzene13.385
82.1,1,2,2-Tetrachloroethane13.456
83.trans-1,4-Dichloro-2-butene13.496
84.Bromobenzene13.515
85.1,2,3-Trichloropropane13.526
86.n-Propylbenzene13.565
87.2-Chlorotoluene13.657
88.1,3,5-Trimethylbenzene13.699
89.4-Chlorotoluene13.751
90.tert-Butylbenzene13.965
91.Pentachloroethane14.007
92.1,2,4-Trimethylbenzene14.01
93.sec-Butylbenzene14.14
94.4-Isopropyltoluene (p-cymene)14.254
95.1,3-Dichlorobenzene14.263
96.1,4-Dichlorobenzene-D414.321
97.1,4-Dichlorobenzene14.34
98.n-Butylbenzene14.579
99.1,2-Dichlorobenzene14.635
100.1,2-Dibromo-3-chloropropane (DBCP)15.252
101.Nitrobenzene15.407
102.1,2,4-Trichlorobenzene15.935
103.Hexachloro-1,3-butadiene16.04
104.Naphthalene16.196
105.1,2,3-Trichlorobenzene16.396
* ND = not detected; retention time determined by wet needle injection
Volatiles by EPA Method 8260 on Rxi-624Sil MS (30m, 0.25mm ID, 1.40µm)
GC_EV1169
ColumnRxi-624Sil MS, 30 m, 0.25 mm ID, 1.40 µm (cat.# 13868)
Sample8260A surrogate mix (cat.# 30240)
8260A internal standard mix (cat.# 30241)
8260B MegaMix calibration mix (cat.# 30633)
VOA calibration mix #1 (ketones) (cat.# 30006)
8260B acetate mix (revised) (cat.# 30489)
California oxygenates mix (cat.# 30465)
502.2 calibration mix #1 (gases) (cat.# 30042)
Conc.: 25 ppb in RO water
Injectionpurge and trap split (split ratio 30:1)
Inj. Temp.:225 °C
Purge and Trap
Instrument:OI Analytical 4660
Trap Type:10 Trap
Purge: 11 min @ 20 °C
Desorb Preheat Temp.:180 °C
Desorb: 0.5 min @ 190 °C
Bake: 5 min @ 210 °C
Interface Connection:injection port
Oven
Oven Temp.:35 °C (hold 5 min) to 60 °C at 11 °C/min to 220 °C at 20 °C/min (hold 2 min)
Carrier GasHe, constant flow
Flow Rate:1.0 mL/min
DetectorMS
Mode:Scan
Transfer Line Temp.:230 °C
Analyzer Type:Quadrupole
Source Temp.:230 °C
Quad Temp.:150 °C
Electron Energy:70 eV
Solvent Delay Time:1.5 min
Tune Type:BFB
Ionization Mode:EI
Scan Range:36-260 amu
InstrumentAgilent 7890A GC & 5975C MSD
NotesOther Purge-and-Trap Conditions:
Sample Inlet: 40°C
Sample: 40°C
Water Management: Purge 110°C, Desorb 0°C, Bake, 240°C
AcknowledgementEclipse 4660 purge-and-trap courtesy of O.I. Analytical, College Station, TX.

Not all "624s" are Equivalent

While optimizing instrument conditions can improve sample throughput, obtaining adequate resolution depends largely on column selectivity, thermal stability, and inertness. Rxi-624Sil MS columns are optimized across these parameters, and therefore provide reliable separation of critical VOCs.

Lower Bleed Means Improved Sensitivity and Longer Column Lifetime

While 624 type columns generally provide good selectivity for most volatiles, they are limited by their low thermal stability. Poor thermal stability results in phase bleed that can reduce column lifetime, decrease detector sensitivity (especially ion trap mass spectrometers), and interfere with the quantification of later eluting compounds. Rxi-624Sil MS columns have the highest thermal stability and lowest bleed among 624 type columns due to the incorporation of phenyl rings in the polymer backbone (Table I, Figure 2). The conjugated ring system of this silarylene phase provides a more rigid structure that increases thermal stability compared to nonsilarylene phases.

Table I  The Rxi-624Sil MS column has the highest thermal stability of any 624 column.

Column Manufacturer Highest Temperature Limit (Isothermal)
Rxi-624Sil MS Restek 320 ºC
VF-624ms Varian 300 ºC
DB-624 Agilent J&W 260 ºC
ZB-624 Phenomenex 260 ºC

Figure 2  The Rxi-624Sil MS column has the lowest bleed of any column in its class and provides true GC/MS capability.

Peaks
1.Fluorobenzene
Bleed Comparison of Rxi-624Sil MS and VF-624ms
GC_GN1147
ColumnRxi-624Sil MS (see notes), 30 m, 0.25 mm ID, 1.4 µm (cat.# 13868)
SampleFluorobenzene (cat.# 30030)
Diluent:methanol
Conc.:200 µg/mL
Injection
Inj. Vol.:1 µL split (split ratio 20:1)
Liner:4mm Split Liner with Wool (cat.# 20781)
Inj. Temp.:220 °C
Oven
Oven Temp.:40 °C (hold 5 min) to 60 °C at 20 °C/min (hold 5 min) to 120 °C at 20 °C/min (hold 5 min) to 200 °C at 20 °C/min (hold 10 min) to 260 °C at 20 °C/min (hold 10 min) to 300 °C at 20 °C/min (hold 20 min)
Carrier GasHe, constant flow
Linear Velocity:40 cm/sec
DetectorFID @ 250 °C
InstrumentAgilent/HP6890 GC
NotesColumns are of equivalent dimensions and were tested after equivalent conditioning.

Better Peak Shape Means More Accurate Results

Rxi-624Sil MS columns are the most inert 624 column available. Figure 3 shows the differences between vendor columns using primary amines, which are good indicators of column activity. The unique Rxi deactivation results in symmetric peaks with minimal tailing, which improves quantitative accuracy. Minimizing tailing is especially important with concentration techniques, such as purge and trap, since the act of desorbing analytes off of the packing material results in some tailing. If a column is not inert, additional tailing due to column activity can magnify this problem. The sharp, symmetric peaks seen on Rxi-624Sil MS columns allow greater resolution, higher signal-to-noise ratios, and more accurate results for active volatiles such as alcohols (Figure 4).

Figure 3  Highly inert Rxi-624Sil MS columns provide better peak shape and more accurate results for active compounds.

PeaksConc.
(µg/mL)
1.Isopropylamine100
2.Diethylamine100
3.Triethylamine100
Inertness Comparison (Basic Compounds): Primary, Secondary, and Tertiary Amines on Rxi-624Sil MS
GC_PH1162
ColumnRxi-624Sil MS, 30 m, 0.32 mm ID, 1.8 µm (cat.# 13870)
Sample
Diluent:DMSO
Conc.:100 µg/mL
Injection
Inj. Vol.:1 µL split (split ratio 20:1)
Liner:5mm Single Gooseneck with Wool (cat.# 22973-200.1)
Inj. Temp.:250 °C
Oven
Oven Temp.:50 °C (hold 1 min) to 200 °C at 20 °C/min (hold 5 min)
Carrier GasHe, constant flow
Linear Velocity:37 cm/sec
DetectorFID @ 250 °C
InstrumentAgilent/HP6890 GC

Figure 4  Obtain more accurate results for active volatiles, such as alcohols, by using highly inert Rxi-624Sil MS columns.

Peaks
1.tert-Butyl Alcohol
tert-Butyl Alcohol Peak Shape on Rxi-624Sil MS (30m, 0.25mm ID, 1.40µm)
GC_EV1175
ColumnRxi-624Sil MS, 30 m, 0.25 mm ID, 1.40 µm (cat.# 13868)
Sample
Conc.: 25 ppb in RO water
Injectionpurge and trap split (split ratio 30:1)
Inj. Temp.:225 °C
Purge and Trap
Instrument:OI Analytical 4660
Trap Type:10 Trap
Purge: 11 min @ 20 °C
Desorb Preheat Temp.:180 °C
Desorb: 0.5 min @ 190 °C
Bake: 5 min @ 210 °C
Interface Connection:injection port
Oven
Oven Temp.:35 °C (hold 5 min) to 60 °C at 11 °C/min to 220 °C at 20 °C/min (hold 2 min)
Carrier GasHe, constant flow
Flow Rate:1.0 mL/min
DetectorMS
Mode:Scan
Transfer Line Temp.:230 °C
Analyzer Type:Quadrupole
Source Temp.:230 °C
Quad Temp.:150 °C
Electron Energy:70 eV
Solvent Delay Time:1.5 min
Tune Type:BFB
Ionization Mode:EI
Scan Range:36-260 amu
InstrumentAgilent 7890A GC & 5975C MSD
NotesOther Purge and Trap Conditions:
Sample Inlet: 40°C
Sample: 40°C
Water Management: Purge 110°C, Desorb 0°C, Bake, 240°C

Conclusion

Labs interested in optimizing resolution and sample throughput can adopt the conditions established here on Rxi-624Sil MS columns to maximize productivity and assure accurate, reliable results.

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