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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.


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.


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).
Regression of the PF technique against other measurement methods Same as Figure 1 but showing low SO2 mixing ratio scale
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.

Rejection ratio versus 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.


  • 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
1315 East West Hwy, Rm 3439
Silver Spring, MD 20910
Telephone: (301) 713-0971
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