HIGH PURITY ACIDS ACS REAGENT ACIDS REDISTILLED ACIDS Produced by taking ACS Reagent Grade acids and redistilling them in glass. This distallation yields high purity acids w i t h very low or absent trace elements.
DOUBLE-DISTILLED ACIDS Manufactured by taking the already very pure redistilled acids and passing them through vycor/ Quartz stills twice. This process removes virtually all volatile and non-volatile impurities. For details on the full line of CFS Chemicals High Purity Acids, circle the reader service number below.
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values are found for a given XL value. When t„s m = 0, (as in Equation 7), the value of CL is obtained. However, the reduced analytical sensitivity yields a value, CR, to the right of CL. If m » t a s m , then the CR will not be sta tistically different from CL- For the sit uation when t a s m is not sufficiently small as compared to m, a CR may be substantially larger than CL- There fore, only the larger value should be used in reporting a limit of detection. An important consideration in the use of this model is the choice of a t value. The recommended k value of 3 involves a 99.87% confidence level. Therefore, the t should reflect a simi lar level. An α = 0.0005 results in a confidence level of 99.9% for a twotailed t distribution. The degrees of freedom, v, are η — 2 for a linear re gression model. The η value used should be the number of points used to prepare the calibration curve, i.e., each point consists of a mean measure and a concentration value. Propagation of Errors Approach In the second approach used to evaluate CL, the error in the intercept term, i, as well as the error in m are considered. To include these errors, Equation 1 must be rearranged to
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c = ί—ΐ
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m Prom Equation 18, any value of χ can be related to a concentration using the m and i values obtained from the lin ear regression model. The i term is usually neglected in most cases and assumed to be 0 because the analytical measures are background subtracted. But in most linear regression analyses, i ^ 0. If a true reflection of the error in the determined concentration is sought, the error in m and i must be included in Equation 7. The contribution of each term to the total error may be found by taking the first derivative of c with respect to each term:
U7jt'/v weighing ct little weans a lot.
TARE Taking the designated derivatives and the square root gives
v — i\2
and finally combining like terms yields
Sc=l CIRCLE 33 ON READER SERVICE CARD 720 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
(21) L J ? ^ L J _ (21) m