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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, 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.
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.
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 1: 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 |
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).
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.
