Download The lowest analyte response which can be detected above the

S-10-1
Statistical Principles
DETECTION LIMIT (Spectrophotometric)
PRINCIPLE
The lowest analyte response which can be detected above the reagent blank
response is computed at a defined confidence level based on the standard
deviation of the reagent blank response. The analyte level that will produce the
least detectable analyte response is obtained from the appropriate standard
calibration curve. The detection limit is computed from the least detectable
analyte level and the appropriate sample weight (Note 1).
SCOPE
The method is applicable to spectrophotometric procedures where the 100 percent
transmittance can be established with a substance other than the reagent blank.
PROCEDURE
According to the analytical method, prepare for analysis a minimum of ten
independent reagent blank solutions. Prepare for analysis the standard solutions
that are specified in the analytical method. Set the spectrophotometer to the
wavelength specified in the analytical method. Set the zero percent transmittance.
Establish the 100 percent transmittance with a substance other than the reagent
blank (Note 2). Measure and record the percent transmittance of each reagent
blank solution and each standard solution. Convert the transmittance values to
absorbance values.
CALCULATION
Compute the mean reagent blank absorbance according to the equation:
Xb =
Σ
i
1
X b /n
where X b is the mean reagent blank absorbance, Xb is a given reagent blank absorbance and n is the
number of reagent blank solutions analyzed.
Analytical Methods of the Member Companies of the
Corn Refiners Association, Inc.
Accepted 4-14-78
Revised 4-9-98
S-10-2
Statistical Principles
DETECTION LIMIT (Spectrophotometric) ⎯ continued
Subtract the mean reagent blank absorbance from the absorbance obtained for each standard solution.. Plot
the resultant net absorbance for each standard solution against the respective analyte level. Compute the n1 standard deviation for the blank absorbance according to the equation:
Sb =
Σ
i
1
(X b - X b ) 2
n -1
where Sb is the standard deviation of the reagent blank absorbance. Compute the lowest analyte net
absorbance, which can be detected above the mean reagent blank absorbance according to the equation:
R L = X - X b = K 2 • Sb
where RL is the lowest net analyte absorbance which can be detected above the mean reagent blank
absorbance.
X is the average analyte value
X b is the mean blank value
Sb is the standard deviation of the blanks
K is the degree of confidence required (Note 3)
2 is the quadratic sum of sample signal noise and blank signal noise
To compute RL at the 95% confidence level, set K equal to 1.96. Obtain from the standard calibration
curve the analyte level (in μg) which corresponds to RL. Compute the analytical detection limit according
to the equation:
Detection Limit (μ g/g analyte) =
(G L )(Dilution Factor)
Sample Wt., g
where GL represents the lowest detectable analyte level in μg. The dilution factor is equal to the original
sample weight divided by the weight of sample employed in the analysis.
NOTES AND PRECAUTIONS
1.
The procedure is adapted from that described by H. Kaiser and H. Specker,
Z. Anal. Chem., 149, 46 (1956). The procedure is further described in
Trace Characterization: Chemical and Physical, National Bureau of
Standards Monograph 100, 1967, pp. 154-156 and Trace Analysis Physical
Methods, Interscience Publishers, 1965, pp. 2-5.
Statistical Principles
S-10-3
DETECTION LIMIT (Spectrophotometric) ⎯ continued
2.
The material which is used to establish the 100 percent transmittance
should have an absolute transmittance which is greater than the reagent
blank at the analytical wavelength. Purified water may be used for most
applications.
3.
The values of K may be 1.00, 1.96 or 3.00 which correspond to confidence
levels of 68.3%, 95%, and 99.7%, respectively.