Results
Table 6 summarizes results.All oxygenateswere resolved from other target compounds and potentially interfering gasoline compo-
nents. TBA andMTBEwerewell resolved using the 35°C starting temperature. Figure 3 represents aTIC of 1ppm non-oxygenated
unleaded gasoline spikedwith 5ppb of oxygenates. The inset is an extracted ion chromatogram showing oxygenates are recovered
without interference from the gasolinematrix.
The internal standardmethyl-d3
tert
-butyl ether was added to compensate for variations in purging efficiency specific to the ethers.
After the datawere acquiredwe observed a small ion 73 component as part of the IS spectra (Figure 4).MTBE and the IS share
retention time andMTBE produces ion 73. Our initial thought was to discard the data, but we determined the distribution of the rela-
tive abundance of ion 73 to ion 76 for the ISwas between 0.3% and 0.5%. Thiswould affect calculated concentrations ofMTBE by
nomore than 2%.Analysts in environmental laboratories can decide if this is an acceptable degree of error.We are currently evaluat-
ing other potential internal standards that behave in amanner similar to the ethers. One such substitution used in some laboratories is
TBA-d9, which accounts for variations common to alcohols, but not to ethers.
Rtx
®
-VMS, 30m, 0.25mm ID, 1.4µm (cat.# 19915)
Sample:
calibrationmixes: cat.# 30475B, 30465, 30006, 30042;
unleaded gasoline (cat.# 30096); non-oxygenated unleaded gasoline (custom)
internal / surrogate standards: cat.# 30240, 30074
Purge and trap conditions
O.I. 4560 Purge andTrapConcentrator
Sample:
10mL
Sample temp.:
40°C
Trap:
#10 (Tenax
®
/silica gel/carbonmolecular sieve)
Trap temp.:
purge: 20°C; desorb: 190°C; bake: 210°C
Purge:
11min., 38mL/min.
Desorb:
1.0min., 32mL/min.
Bake:
10min.
Chromatography
Inj. temp.:
250°C, split 1:25
Carrier gas:
helium, constant flow
Flow rate:
1.3mL/min.
Dead time:
1.47min.@ 35°C
Oven temp.:
35°C (hold 7min.) to 90°C@ 4°C/min. (no hold)
to 220°C@ 45°C/min. (hold 1min.)
Det.:
Agilent 5971AGC/MS
Scan range:
35–260 amu
Tune:
PFTBA/BFB
GC_EV00679
Acknowledgement: purge& trap courtesy of O.I. Analytical.
5ppb oxygenates in 1ppm
non-oxygenated gasoline.
area of enlargement
Figure 3:
GC/MSwith anRtx-VMS
®
column allows reliable identification of 5ppb oxygenates added to 1ppm gasoline.
