Cahn Instruments, Inc. - Analytical Chemistry (ACS Publications)


Cahn Instruments, Inc. - Analytical Chemistry (ACS Publications)https://pubs.acs.org/doi/10.1021/ac00258a730Cahn Instrum...

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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.

GFS CHEMICALS P.O. Box 23214

Columbus, Ohio 43223

(614) 881-5501

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

CIRCLE 99 ON READER SERVICE CARD

c = ί—ΐ

in small weighs. Sensitivity to 0.1 ug. Simple to operate. Auto calibration. HCI) output. Three models. Portable or remote weighing. The Cahn Series 20 Microbalances. Write for our free catalog today Cahn Instruments, Inc., 162(F South Carmenita Road, Cerritos, CAVXFOl.

CAHN

(18)

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

ll/2

-VM

(2o)

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