|
Reagents & Solutions 800-356-9526 608-274-4330 enotes@promega.com
|  |
FAQspeakWhat is an absorbance unit?Some of Promega's products are given in quantities that are expressed as absorbance units. For example, Ribo m7G Cap Analog (5´-7-methyl guanosine nucleotide; Cat.# P1711, P1712) is sold in quantities of A254 units, and Random Hexamers(a), of the Prime-a-Gene® Labeling Buffer(a), are defined in A260 units. The Absorbance Unit is unusual in that, although it appears to be a measure of concentration, it is in fact a measure of quantity. One absorbance unit is defined as the amount of that compound which, when dissolved in 1 milliliter of solution, possesses an absorbance of 1.0 at a specified wavelength. Thus, 1.0 A254 unit of Cap Analog is the amount with an optical density (O.D.) of 1.0 when dissolved in 1ml of water. Cap Analog has a molar extinction coefficient (e) of 18,200 at 254nm (1); that is, a one-molar (1mol) solution of Cap Analog has an absorbance of 18,200 at 254nm. An absorbance of 1.0 at 254nm will be generated by a 1/18,200 molar solution of Cap Analog (or a 55µmol solution, which corresponds to 55nmol per milliliter). Therefore, 1.0 A254 (in 1ml of water) will give an absorbance of 1.0 at 254nm, which corresponds to 55nmol (5.5 x 10-8mol) of Cap Analog. The 25 A254 unit size of Cap Analog contains 25 x 5.5 x 10-8mol = 1.37 x 10-6mol (1.37µmol) of Cap Analog. Similarly, the 10 A254 unit size corresponds to 10 x 5.5 x 10-8mol = 0.55 x 10-6mol (0.55µmol) of Cap Analog. Cap Analog has a molecular weight of 882g/mol. Therefore, 25 A254 units is 1.37 x 10-6mol x 882g/mol = 1.2 x 10-3g or 1.2mg of the compound. Likewise, 10 A254 units is 0.55 x 10-6mol x 882g/mol = 0.49 X 10-3g or 0.49mg. Both sizes of Cap Analog offered by Promega are supplied at a final concentration of 40mM. This corresponds to 34.3ml for the 25 A254 unit size and 13.7ml for the 10 A254 unit size. Similar calculations can be performed for other compounds that are expressed in absorbance units. Reference
(a)For Laboratory Use.*Products may be covered by pending or issued patents. Please visit our patent and trademark web page for more information.
|
||||
|
||||||
