NR 252.036(3)
(3) Procedure. (See Figure 1 for apparatus layout).
NR 252.036(3)(a)
(a) Place 50 ml. of 0.05M pH 7.0 potassium phosphate buffer in trap no.1.
NR 252.036(3)(b)
(b) Place 50 ml. of alkaline 3% hydrogen peroxide in trap no. 2.
NR 252.036(3)(c)
(c) Sample introduction and N
2 prepurge: Gently mix sample to be analyzed to resuspend settled material, taking care not to aerate the sample. Transfer 400 ml. of sample, or a suitable portion containing not more than 20 mg. sulfide diluted to 400 ml. with distilled water, to the distillation flask. Adjust the N
2 flow so that the impingers are frothing vigorously, but not overflowing. Vacuum may be applied at the outlet of trap No. 2 to assist in smooth purging. The N
2 inlet tube of the distillation flask shall be submerged deeply in the sample to ensure efficient agitation. Purge the sample for 30 minutes without applying heat. Test the apparatus for leaks during the prepurge cycle using snoop or soap water solution.
NR 252.036(3)(d)
(d) Volatilization of H
2 S: Interrupt the N
2 flow and vacuum and introduce 100 ml. of 6N HCl to the sample using the separatory funnel. Immediately resume the gas flow and vacuum. Apply maximum heat with the heating mantle until the sample begins to boil, then reduce heat and maintain gentle boiling and N
2 flow for 30 minutes. Terminate the distillation cycle by turning off the heating mantle and maintaining N
2 flow through the system for 5 to 10 minutes. Then turn off the N
2 flow, release vacuum, and cautiously vent the system by placing 50 to 100 ml. of distilled water in the separatory funnel and opening the stopcock carefully. When the bubbling stops and the system is equalized to atmospheric pressure, remove the separatory funnel. Extreme care shall be exercised in terminating the distillation cycle to avoid flash-over, draw-back, or violent steam release.
NR 252.036(3)(e)
(e) Analyze the contents of trap no. 2 for sulfate according to either EPA gravimetric test procedure 375.3 or EPA turbidimetric test procedure 375.4 (see
40 CFR 136.3, Table IB, parameter 65 (49 FR 43234, October 26, 1984, and correction notice at 50 FR 690, January 4, 1985)). Use the result to calculate mg/l. of sulfide in wastewater sample.
mg sulfide/l = (mg. BaSO
4 collected in Trap No. 2) x (137)
volume in ml. of waste sample distilled
mg sulfide/l = A x B x 333
C
where A=mg/l of sulfate in trap no. 2
B=liquid volume in liters in trap no. 2
and C=volume in ml of waste sample distilled
NR 252.036(4)(a)(a) Each laboratory that uses this method shall operate a formal quality control program. The minimum requirements of this program consist of an initial demonstration of laboratory capability and the analysis of replicate and spiked samples as a continuing check on performance. The laboratory shall maintain performance records to define the quality of data that is generated. Ongoing performance checks shall be compared with established performance criteria to determine if the results of analyses are within precision and accuracy limits expected of the method.
NR 252.036(4)(b)
(b) Before performing any analyses, the analyst shall demonstrate the ability to generate acceptable accuracy and precision by performing the following operations.
NR 252.036(4)(b)1.
1. Perform 4 replicate analyses of a 20 mg/l. sulfide standard prepared in distilled water. Refer to
sub. (2) (h) under “Reagents."
NR 252.036(4)(b)2.a.a. Calculate clean water precision and accuracy in accordance with standard statistical procedures. Clean water acceptance limits are presented in
subd. 2.b. These criteria shall be met or exceeded before sample analyses can be initiated. A clean water standard shall be analyzed with each sample set and the established criteria met for the analyses to be considered under control.
NR 252.036(4)(b)2.b.
b. Clean water precision and accuracy acceptance limits: for distilled water samples containing from 5 mg/l. to 50 mg/l. sulfide, the mean concentration from 4 replicate analyses shall be within the range of 72 to 114% of the true value.
NR 252.036(4)(c)
(c) The method detection limit or MDL may be determined periodically by each participating laboratory in accordance with the procedures specified in "Methods for Chemical Analysis of Municipal and Industrial Wastewater," EPA- 600/4-82-057, July 1982, EMSL, Cincinnati, OH 45268. For the convenience of the user, these procedures are contained in
s. NR 252.0365.
NR 252.036(4)(d)
(d) A minimum of one spiked and one duplicate sample shall be run for each analytical event, or 5% spikes and 5% duplicates when the number of samples per event exceeds 20. Spike levels are to be at the MDL and at x when x is the concentration found if in excess of the MDL. See
par. (c) for MDL samples. Spike recovery shall be 60 to 120% for the analysis of a particular matrix type to be considered valid.
NR 252.036(4)(e)
(e) Report all results in mg/liter. When duplicate and spiked samples are analyzed, report all data with the sample results.
NR 252.036 History
History: Cr. Register, May, 2001, No. 545, eff. 6-1-01. NR 252.0365(1)(1)
Definition. “Method detection limit" or "MDL" means the minimum concentration of a substance that can be identified, measured and reported with 99% confidence that the analyte concentration is greater than zero and determined from analysis of a sample in a given matrix containing analyte.
NR 252.0365(2)
(2) Scope and application. This procedure is designed for applicability to a wide variety of sample types ranging from reagent or blank water containing analyte to wastewater containing analyte. The MDL for an analytical procedure may vary as a function of sample type. All sample processing steps of the analytical method shall be included in the determination of the MDL. The MDL obtained by this procedure is used to judge the significance of a single measurement of a future sample. The MDL procedure was designed for applicability to a broad variety of physical and chemical methods. To accomplish this, the procedure was made device or instrument independent.
NR 252.0365(3)(a)(a) Make an estimate of the detection limit using one of the following:
NR 252.0365(3)(a)1.
1. The concentration value that corresponds to an instrument signal or noise ratio in the range of 2.5 to 5. If the criteria for qualitative identification of the analyte is based upon pattern recognition techniques, the least abundant signal necessary to achieve identification shall be considered in making the estimate.
NR 252.0365(3)(a)2.
2. The concentration value that corresponds to 3 times the standard deviation of replicate instrumental measurements for the analyte in reagent water.
NR 252.0365(3)(a)3.
3. The concentration value that corresponds to the region of the standard curve where there is a significant change in sensitivity at low analyte concentrations, such as a break in the slope of the standard curve.
NR 252.0365(3)(a)4.
4. The concentration value that corresponds to known instrumental limitations.
NR 252.0365(3)(b)
(b) Prepare reagent or blank water that is as free of analyte as possible. Reagent or interference free water is defined as a water sample in which analyte and interferent concentrations are not detected at the method detection limit of each analyte of interest. Interferences are defined as systematic errors in the measured analytical signal of an established procedure caused by the presence of interfering species or interferent. The interferent concentration is presupposed to be normally distributed in representative samples of a given matrix.
NR 252.0365(3)(c)1.1. If the MDL is to be determined in reagent or blank water, prepare a laboratory standard such as an analyte in reagent water at a concentration which is at least equal to or in the same concentration range as the estimated MDL. It is recommended to be between one and 5 times the estimated MDL. Proceed to
par. (d).
NR 252.0365(3)(c)2.
2. If the MDL is to be determined in another sample matrix, analyze the sample. If the measured level of the analyte is in the recommended range of one to 5 times the estimated MDL, proceed to
par. (d).
NR 252.0365(3)(c)3.
3. If the measured concentration of analyte is less than the estimated MDL, add a known amount of analyte to bring the concentration of analyte to between one and 5 times the MDL. In the case where an interference is coanalyzed with the analyte and the measured level of analyte is greater than 5 times the estimated MDL, there are 2 options:
NR 252.0365(3)(c)3.b.
b. The sample may be used as is for determining the MDL if the analyte level does not exceed 10 times the MDL of the analyte in reagent water. The variance of the analytical method changes as the analyte concentration increases from the MDL, hence the MDL determined under these circumstances may not truly reflect method variance at lower analyte concentrations.
NR 252.0365(3)(d)1.1. Take a minimum of 7 aliquots of the sample to be used to calculate the MDL and process each through the entire analytical method. Make all computations according to the defined method with final results in the method reporting units. If blank measurements are required to calculate the measured level of analyte, obtain separate blank measurements for each sample aliquot analyzed. The average blank measurement is subtracted from the respective sample measurements.
NR 252.0365(3)(d)2.
2. It may be economically and technically desirable to evaluate the estimated MDL before proceeding with
subd.1. This will prevent repeating this entire procedure when the costs of analyses are high and insure that the procedure is being conducted at the correct concentration. It is quite possible that an incorrect MDL can be calculated from data obtained at many times the real MDL even though the background concentration of analyte is less than 5 times the calculated MDL. To insure that the estimate of the MDL is a good estimate, it is necessary to determine that a lower concentration of analyte will not result in a significantly lower MDL. Take 2 aliquots of the sample to be used to calculate the MDL and process each through the entire method, including blank measurements as described in
subd.1. Evaluate these data:
NR 252.0365(3)(d)2.a.
a. If these measurements indicate the sample is in the desirable range for determining the MDL, take 5 additional aliquots and proceed. Use all 7 measurements to calculate the MDL.
NR 252.0365(3)(d)2.b.
b. If these measurements indicate the sample is not in the correct range, reestimate the MDL, obtain new sample as in
par. (c) and repeat either
subd.1. or
2.
NR 252.0365(3)(e)
(e) Calculate the variance (S
2) and standard deviation (S) of the replicate measurements, as follows:
n n
S2 = 1 [S xi2 - (
S xi)2/n
n-1 i=1 i=1
s+(s2) 0.5
where: the xi, i = 1 to n are the analytical results in the final method reporting units obtained from the n sample aliquots and
n
S xi2
i=1
refers to the sum of the X values from i = 1 to n.
MDL = t(n-1, 1-a = .99) (S)
where:
MDL=the method detection
t (n-1, 1-a=.99)=the students' t value appropriate for a 99% confidence level and a standard deviation estimate with n-1 degrees of freedom as given in subd. 2.
S=standard deviation of the replicate analyses.
NR 252.0365(3)(f)2.
2. The 95% confidence limits for the MDL derived in
subd. 1. are computed according to the following equations derived from percentiles of the chi square over degrees of freedom distribution (X2/df) and calculated as follows:
MDLLCL=0.69 MDL
MDLUCL=1.92 MDL where MDLLCL and MDLUCL are the lower and upper 95% confidence limits respectively based on 7 aliquots.
NR 252.0365(3)(g)1.1. Optional iterative procedure to verify the reasonableness of the estimated MDL and calculated MDL of subsequent MDL determinations.
NR 252.0365(3)(g)2.
2. If this is the initial attempt to compute MDL based on the estimated MDL in
par. (a), take the MDL as calculated in
par. (f), spike in the matrix at the calculated MDL and proceed through the procedure starting with
par. (d)1.
NR 252.0365(3)(g)3.
3. If the current MDL determination is an iteration of the MDL procedure for which the spiking level does not permit qualitative identification, report the MDL as that concentration between the current spike level and the previous spike level which allows qualitative identification.
NR 252.0365(3)(g)4.
4. If the current MDL determination is an iteration of the MDL procedure and the spiking level allows qualitative identification, use S
2 from the current MDL calculation and S
2 from the previous MDL calculation to compute the F ratio.
if S2
A < 3.05
S2B
then compute the spooled standard deviation by the following equation:
spooled = [ 6SA2+ 6SB
2 ]0.5
____________
12
if S2
A > 3.05
S2B
respike at the last calculated MDL and process the samples through the procedure starting with par. (d).
NR 252.0365(3)(g)5.
5. Use the
Spooled as calculated in
subd. 3. to compute the final MDL according to the following equation:
MDL=2.681 (Spooled)
where 2.681 is equal to t (12, 1-a=.99)
NR 252.0365(3)(g)6.
6. The 95% confidence limits for MDL derived in
subd. 4. are computed according to the following equations derived from percentiles of the chi squared over degrees of freedom distribution.
MDLLCL=0.72 MDL
