Figure 2–OptimizedRtx
®
-VGC columndimensions (30m x 0.45mm ID) allow for correct
desorb flow rates from thepurge and trap, faster analyses times, andbetter resolutionof
closely elutingpeaks compared to traditional 0.53mm ID columns.
30m, 0.25mm, 1.40µmRtx
®
-VGC (cat.# 19415); 1:10 split at injection port; 1mm ID liner; Compounds at 100ppb in 5mL of ROwater (unless otherwise noted);
Oven program:
35°C (hold 14min.) to 220°C@ 24°C/min. (hold 6min.);
Carrier gas:
He@ ~1mL/min. constant;
Concentrator:
Tekmar LSC-3100 Purge and Trap;
Trap:
Vocarb™ 3000;
Purge:
11min. @ 40mL/min. @ ambient temperature;
Dry Purge:
1min. @ 40mL/min. (MCS bypassed using Silcosteel
®
tubing);
Desorb Preheat:
245°C;
Desorb:
250°C for 2min., Flow 10mL/min.;
Bake:
260°C for 8min.;
Interface:
transfer line 0.32mm IDSiltek™ fused silica;
Detector:
HP 5973MS;
Scan range:
25 to 300 AMU.
26
22
18
14
10
6
2
min.
1
2
3
4
5
6,7
9
8
10
12
11
15
13
14
16
18
17
19
20, 21
23
22
24
25
26
27 28
29
30
1. methanol
100,000ppb
2. ethanol
10,000ppb
3. 2-methylpentane
4. 2-propanol
500ppb
5. 3-methylpentane
6. hexane
7. methyl tert-buyl ether
8. tert-butanol
500ppb
9. diisopropyl ether
10. ethyl-tert-butyl ether
11. isooctane
12. benzene
13. n-heptane
14. tert-amyl methyl ether
15. 1-butanol
500ppb
16.
a,a,a
-trifluorotoluene
17. toluene
18. 1-chloro-3-fluorobenzene
19. ethylbenzene
20. m-xylene
21. p-xylene
22. o-xylene
23. isopropylbenzene
24. decane
25. 1,3,5-trimethylbenzene
26. 1,2,4-trimethylbenzene
27. 4-bromochlorobenzene
28. naphthalene
29. 2-methylnaphthalene
150ppb
30. 1-methylnaphthalene
150ppb
GC_EV00401
the purge and trap, faster analyses times, and better resolution of
closely eluting peaks compared to traditional 0.53mm ID columns
(Figure 2).
Oxygenates also can be analyzed byGC/MS following the protocol
defined in US EPAMethod 8260B. GC/MS is a common way to
increase the level of confidence in chromatographicdataover theGC
methods. Using a 30m, 0.25mm ID, 1.4µmRtx
®
-VGC columnwith
a quadrapole MS can identify oxygenates and alcohols with a high
degree of certainty because the compounds that share ions are well
resolved using this column (Figure 2). TheMSwas used to positive-
ly identify oxygenates and pentanes from their spectra (Figure 3).
Peak shapes are symmetrical for all these compounds using theRtx
®
-
VGC column, regardless of detector.
Purgeand trapconditions suchaspurge timeand temperaturemust be
optimized to achieve good purging efficiency of oxygenates and
hydrocarbons. You can review optimized conditions in the figures
shown in thisApplications Note. The selection of the trap adsorbent
material also is critical to achieving accurate quantitative results. The
most demanding compounds to analyze are the alcohols because they
purge poorly and can interfere with other target analytes. A ‘J’ trap
(e.g., BTEXTRAP
™
trap) can handle a heavy sample loadwith per-
cent levels ofmethanol, but has a lowered ability to retain themore
polar analytes like ethers and alcohols. So, if the majority of your
samples are highly contaminated soils requiring methanol extract
and you are analyzing for MTBE, it is best to use a ‘J’ trap. For
cleaner samples where sensitivity is an issue, then youwill achieve
better results with a ‘K’ trap (e.g., VOCARB 3000
™
trap).
Additionally, GC analysis conditions such as temperature program
rate and flow rate are critical for achieving good separation of oxy-
genates from hydrocarbons.
To confirm the ability of the Rtx
®
-VGC to provide accurate quan-
titative results, a composite gasoline standard was analyzed using
both PID and GC/MS. The GC/MS analysis was used to confirm
the identity of the analytes thatmatched target compound retention
times on the PID. Any compound that was found within 0.10
minute of a target compound could be identified as a possible oxy-
genate. The GC/MS confirmed that only one target compound,
diisopropyl ether, gave a falsepositive retention timematchwith2-
methyl-1-pentene. Although 2-methyl-1-pentene is found at low
concentrations relative to the methylpentanes, it responds well on
the PID. Using the composite gasoline standard, no other oxy-
genates matched within the 0.10 minute retention time window,
thereby making positive identification for most of the oxygenates
possible using the PID (Figure 4). Because gasoline composition
can vary from state to state, the use of a confirmation column or
MS detection is strongly recommended because alkenes such as 2-
methyl-1-pentene can interfere with positive identification of oxy-
gen-containing compounds.
Chlorobenzene also is a common contaminant in drinking water
and is commonly analyzed in addition to gasoline using purge and
trap with PID detection. Because the boiling point and retention
time of chlorobenzene are similar to ethylbenzene,
m
-xylene, and
