Trace Level Gases – Testing Procedures

Range: “Nominal minimum and maximum concentrations which a method is capable of measuring”[40 CFR Part §53.23 (a)]

  • Generate a calibration curve using 7 separate equally spaced, identifiable points from 0 to 90+/-5 percent of full scale. This plot is to be of recorder scale readings vs. pollutant concentrations or output units(millivolts, milliamps) vs. pollutant concentrations
  • Negative responses at the zero point are corrected using a zero offset on analyzer
  • The calibration curve should show the analyzer’s response over at least 95% of required range

Noise: "Spontaneous, short duration deviations in output, about the mean output, which are not caused by input concentration changes"[40 CFR Part §53.23 (b)] Noise = So

  • Determined at two concentrations, zero and 80+/-5% FS Range
  • Standard Deviation about the mean, expressed in concentration units
  • Setup and allow analyzer to stabilize
  • Run 60 minutes zero air, record 25 readings at 2-min intervals (instantaneous, not averaged readings)
  • Run 60 minutes at 80% FS, record 25 readings at 2-min intervals (instantaneous, not averaged)
  • Calculate standard deviation from both sets

Lower Detectable Limit: "Minimum concentration which produces a signal of twice the noise level"[40 CFR Part §53.23 (c)]

  • Run zero air through the analyze
  • Wait 10-15 minutes. At this point, the instrument and the associated output should be stable.
  • Record zero reading (in ppm or ppb, depending on the analyzer range). This reading is designated as Bz
  • Generate and measure a pollutant test atmosphere equal to the concentration of the vendor-specified lower detectable limit (LDL).
  • Again once reading is stable (approximately 10-15 minutes), record reading in ppm or ppb. This reading is designated as BL
  • Determine the LDL: Subtract BZ from BL (LDL=BL – BZ)
  • Compare the LDL value with the noise level, So [40 CFR Part §53.23 (b)] determined for a zero concentration test atmosphere.

Note: The LDL must be equal to or higher than 2So (LDL = ≥2 * So)

Method Detection Limit: “the minimum concentration of a substance that can be measured and reported with 99% confidence that the analyte concentration is greater than zero and is determined from analysis of a sample in a given matrix containing the analyte”[40 CFR Part §136, App. B].

  • Make an estimate of the detection limit using the following:
  • The concentration value that corresponds to 2.5 to 5 times the instrument noise.
  • Run zero gas through analyzer to determine analyzer will go to zero (or acceptable zero).
  • Dilute the gas to estimated concentration level and collect readings for a predetermined length of time: Suggested: 20-25 1 minute observations, repeated 7 times over the course of 5 -14 days. Average the concentration from these readings.
  • Calculate the standard deviation, s, of the replicate measurements.
  • Compute the MDL;

Where represents the 99th quantile of a Student’s t distribution with (n-1) degrees of freedom and n represents the number of replicate measurements. Refer to Table in 40 CFR §Part 136.

Zero Drift: "change in response to zero concentration over a 12- and 24-h period of continuous unadjusted operation"[40 CFR Part §53.23 (e)(i)]

Note: most vendors specify the line voltage and temperature to be held constant, some also specify longer time periods

  • Calculate: difference between maximum and minimum concentrations
  • While performing a zero air test, determine the minimum (Cmax) and maximum (Cmin) readings during a period of 12 consecutive hours.
  • Determine the 12 hour zero drift(12ZD) (12ZD= Cmax – Cmin)
  • Calculate the 24-hour zero drift (24ZD) for the nth test day as 24ZDn = Zn-Zn-1 or
  • 24ZDn= Zn – Z’n-1 if zero adjustments were made on the previous day where for L1 and L2 taken on the nth test day.
  • Compare 12ZD and 24ZD to the zero drift specification. Both 12ZD and 24ZD must be equal to or less than the specified value to pass the test for zero drift

Span Drift: "Percent change in response to a higher concentration over a 24-h period of continuous unadjusted operation"[40 CFR Part §53.23 (e)(ii)]

(Note: same as above, plus - Span Drift often expressed as percent of full scale)

  • Determine Span drift at 20 percent of Upper Range Limit (URL)
  • Choose a concentration at 20 percent of the URL

Run the concentration for 24-hour period

Calculate the drift:

: unadjusted

If adjustments are made, refer to equation in 40 CFR Part 53.23 ii (A)

  • Determine Span drift at 80 percent of Upper Range Limit (URL)
  • Choose a concentration at 80 percent of the URL
  • Run the concentration for a 24-hour period

Calculate the drift

: unadjusted

If adjustments made, refer to equation in 40 CFR Part 53.23 ii (B)

Lag or Response Time: "Time interval between a step change in input concentration and the first observable corresponding change in response"[40 CFR Part §53.23 (e)(iii)]

  • Switch analyzer to read specific concentration (in CFR states this test is to begin when measuring test atmospheres) R1
  • Mark chart or note time on data logger
  • Note first observable response (two times the noise level) R2
  • Determine the elapsed time in minutes: R2 – R1 This time must be equal to or less than time specified in table B-1 (from CFR for reference and equivalency methods)

Precision: "Variation about the mean of repeated measurements of the same pollutant concentration, expressed as one standard deviation about the mean"[40 CFR Part §53.23 (e)(vi)]

  • Determine concentration of the precision check for each analyzer. Range for precision check can be determined using analyzer sensitivity, linear range, and expected ambient concentrations.
  • Run precision check every day.Record the actual concentration of the precision check and the response of the instrument
  • Calculate relative percent difference (di) using the following:

where Yi is the indicated value and Xi is the actual value

  • Additionally, the precision estimate can be calculated from the equation below, found in the proposed revisions to 40 CFR Part 58 Appendix A Section 4.1.2. The precision estimator is the coefficient of variation upper bound and is calculated using equation 2 as follows:

where is the 5th percentile of a chi-squared distribution with n-1 degrees of freedom.

Below is a table of the proposed performance evaluation levels taken from 40 CFR Part 58 Appendix A Section 3.2.2

Audit Level / ConcentrationRange, PPM
O3 / SO2, / NO2 / CO
1......
2......
3......
4......
5...... / 0.02-0.05
0.06-0.10
0.11-0.20
0.21-0.30
0.31-0.90 / 0.0003-0.005
0.006-0.01
0.02-0.10
0.11-0.40
0.41-0.90 / 0.0002-0.002
0.003-0.005
0.006-0.10
0.11-0.30
0.31-0.60 / 0.08-0.10
0.50-1.00
1.50-4.00
5-15
20-50

Rise Time: “Time interval between initial response and 95% of final response after a step increase in input concentration"[40 CFR Part §53.23 (e)(iv)]

  • Calculate 95 percent of the 90 ± five percent reading (R5) of the upper range limit
  • Determine the elapsed time between the first observable(two times noise level) response and a response equal to ninety-five percent of the R5 reading

Fall Time: "Time interval between initial response and 95% of final response after a step decrease in input concentration"[40 CFR Part §53.23 (e)(v)]

  • Subtract the stable zero air reading (Z1) from the 80 ± five percent reading of the upper range limit (D1); (D1- Z1)
  • Calculate five percent of this number
  • Determine the elapsed time in minutes between the first observed decrease in response following the eighty percent ± five percent reading and a response equal to five percent of the difference of (D1- Z1)

Bias: “the systematic or persistent distortion of a measurement process which causes error in one direction” "[40 CFR Part §58 Appendix A, Section 3.01(c)]

Note: Procedure has been revised (11/17/04)

  • Bias will be calculated using the equation(s) in the proposed revisions to 40 CFR Part 58 Appendix A, Section 4.1.3. These equations use the precision checks generated daily.

4.1.3 Bias Estimate. The bias estimate is calculated using the one point QC checks for SO2, NO2, O3, or CO described in section 3.2.1. The bias estimator is an upper bound on the mean absolute value of the relative percent differences as described in equation 3 as follows:

(Eq.3)

where n is the number of single point checks being aggregated; t0.95,n-1 is the 95th quantile of a t-distribution with n-1 degrees of freedom; the quantity AB is the mean of the absolute values of the di’s and is calculated using equation 4 as follows:

(Eq.4)

and the quantity AS is the standard deviation of the absolute value of the di’s and is calculated using equation 5 as follows:

(Eq.5)

  • Since absolute values are used to calculate bias, a positive or negative sign will be determined as defined in Section 4.1.3.1 in the proposed revisions to 40 CFR Part 58, Appendix A.
  • The absolute bias calculated will be verified as per Section 4.1.4 of the proposed revisions to 40 CFR Part 58, Appendix A. This section calls for an annual performance evaluation or audit per section 3.2.2 of the proposed revisions to 40 CFR Part 58 Appendix A.

LinearityFrom the study plan: “linearity will be checked to determine if it’s within +/-1% of full scale of best-fit straight line

Revised 04/25/06 jrice

1