GASIE
GASIE was a blind comparison of seven analytical techniques, at SO2 concentrations ranging from
0-500 pptv, and was conducted at the University of Delaware, Lewes, in September-October, 1994.
Briefly, test mixtures were delivered to the seven GASIE investigators through a common Teflon
manifold for fixed, 90-minute sampling intervals, during which time [SO2] remained constant.
Almost half of the controlled were characterized by [SO2] less than 50 pptv. GASIE consisted of a
total of 115 ninety-minute measurement periods, divided into four phases.
Experiment
The Air Resources Laboratory (ARL) PF detector deployed for GASIE was a Thermo
Environmental Instruments (Franklin, MA) Model 43s detector. SO2 is electronically excited at 190-230 nm
with the output of a xenon flash lamp pulsed at 10 Hz. An orthogonally oriented
photomultiplier tube (PMT) detects the emitted fluorescent radiation at 240-420 nm.
Modifications
Factory modifications:
- High energy lamp trigger pack
- Higher applied lamp voltage for greater photon flux
- Tuning of the excitation wavelengths for maximum sensitivity
- Additional optics
Additional modifications made at ARL:
- Addition of a second hydrocarbon "kicker" membrane and charcoal canister to maximize
hydrocarbon removal
- Carbonate filter for chemical zeroing
- Addition of a mass flow controller
Operation During GASIE
- Exclusive use of Teflon or stainless steel on all wetted surfaces.
- Calibration by zero air dilution of a compressed gas standard; calibrated several times daily at
concentrations of 280-4040 pptv SO2.
- Baseline monitoring via selective removal of sulfur dioxide using a carbonate-impregnated paper
filter. SO2 determined by difference.
- Use of Nafion dryer to remove water vapor interference, with no apparent loss of SO2.
Results from GASIE
- Regression of the PF technique against other measurement methods (Fig. 1) shows excellent
overall agreement and a high degree of correlation, and suggest a possible ARL calibration error
of approximately 10%. The technique agreed well with other methods at low SO2 mixing ratios
(Fig. 2).
 |
 |
| Figure 1 |
Figure 2 |
- The average PF response from Runs 51-105 is presented below, as a function of test mixture set
point. Included is the mean, standard deviation (1) and coefficient of variation (1/mean) of
the measured values. Each data point included in n corresponds to an entire 90-minute
measurement period.
| Set Point (pptv) |
PF Response (pptv)
Mean Response |
|
n |
Coefficient of Variation |
Total Uncertainty |
| 0 |
-0.04 |
12.1 |
6 |
N/A |
N/A |
| 18 |
29.0 |
11.4 |
9 |
39.5% |
92.4% |
| 41 |
64.7 |
5.71 |
10 |
8.83% |
25.2% |
| 127 |
170 |
11.24 |
12 |
6.55% |
21.2% |
| 260 |
328 |
8.98 |
6 |
2.74% |
16.3% |
| 501 |
621 |
17.2 |
11 |
2.77% |
16.3% |
- Results of low-level SO2 determination during GASIE indicate that the PF method was always
able to distinguish between set points of 41 and 0 pptv, and 41 and 18 pptv, and was occasionally
able to distinguish between a set point of 0 and 18 pptv. A detection limit of approximately 20-40 pptv may be inferred.
Post -GASIE Interference Testing
The PF detector will respond to any compound which absorbs radiation at 190-230 nm and
fluoresces at 240-420 nm. Detailed interference studies were conducted after GASIE, and results are
expressed below and in Figure 3 as a rejection ratio, defined as the ratio of the concentrations of
interferant to SO2 required to produce an equivalent instrumental response. For example, nitric
oxide (NO) exhibits a rejection ratio of approximately 35, so an NO concentration of 35 ppbv is
required to produce an instrumental response equivalent to 1 ppbv SO2. Carbon disulfide (CS2)
exhibits a rejection ratio of approximately 20. Results of interference tests of several aromatic
hydrocarbons are presented below, as a function of sample flow rate.
None of the tested interferants were retained on the carbonate zeroing filter. Thus, although the PF
detector will respond to a number of interfering compounds, these compounds will pass into the
detector in both the zero and measurement modes, and thus should induce no net response, as sulfur
dioxide concentrations are determined by the difference between zero and measure modes. Reducing
the sample flow rate will dramatically decrease or eliminate hydrocarbon interferences.
Conclusions
- The modified commercial pulsed fluorescence detector is a low-cost, easy to use instrument
which compares favorably to more sophisticated, labor-intensive, and expensive techniques.
- The PF detector can reliably and precisely measure SO2 at concentrations down to 40 pptv.
During GASIE, the detection limit was estimated at 20 pptv, and the limit of quantitation at 40
pptv.
- A variety of compounds, notably NO, CS2, and aromatic hydrocarbons, will interfere with the
measurement of SO2. The effects of these interferences may be subtracted out by using a
carbonate-impregnated filter to selectively scrub SO2 while allowing interferants to pass. The
interference posed by hydrocarbons may be further reduced by installing additional hydrocarbon
"kickers" and by reducing sample flow rate.
- Variations in interferant concentrations on time scales shorter than the zero/ measurement cycle
will cause errors.
- Periodicity in the noise spectrum of the instrument (likely arising from variations in lamp
stability and flash intensity) necessitates the use of 30 minute signal averaging periods for
greatest accuracy and precision at low mixing ratios..
For more information contact:
Winston Luke
NOAA/ARL (R/E/AR)
1315 East West Hwy, Rm 3439
Silver Spring, MD 20910
Telephone: (301) 713-0971
e-mail: Winston.Luke@noaa.gov
|
