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, increased 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.

Optimised methods for the analysis of volatile organic compounds (VOCs) can be time-consuming to develop. Time-consuming because not only can compound lists be extensive, but 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 optimised 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 exactly 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 minutes. Since the purge and trap cycle time was 16.5 minutes, 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 minutes) to 60°C @ 11 °C/min. to 220°C @ 20 °C/min. (hold 2 minutes) was tested. This final program reduced instrument downtime by better synchronizing injection and analysis cycles, and also provided excellent resolution of volatile compounds (see 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.

Not all "624s" are Equivalent

While optimising 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 (see 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.

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 GC_GN1147 Column Rxi®-624Sil MS (see notes), 30 m, 0.25 mm ID, 1.4 µm (cat.# 13868) Sample Fluorobenzene (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 Gas He, constant flow Linear Velocity: 40 cm/sec. Detector FID @ 250 °C Instrument Agilent/HP6890 GC Notes Columns 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. Minimising 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 (see Figure 4).

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

Peaks Conc. (...) 1. Isopropylamine 100 2. Diethylamine 100 3. Triethylamine 100 GC_PH1162 Column Rxi®-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 Gas He, constant flow Linear Velocity: 37 cm/sec. Detector FID @ 250 °C Instrument Agilent/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 GC_EV1175 Column Rxi®-624Sil MS, 30 m, 0.25 mm ID, 1.40 µm (cat.# 13868) Sample Conc.: 25 ppb in RO water Injection purge 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 Gas He, constant flow Flow Rate: 1.0 mL/min. Detector MS 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 Instrument Agilent 7890A GC & 5975C MSD Notes Other 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 optimising resolution and sample throughput can adopt the conditions established here on Rxi^®-624Sil MS columns to maximise productivity and assure accurate, reliable results.