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Bulk carbon-13 for commercial use, e.g. in chemical synthesis, is enriched from its natural 1% abundance. Although carbon-13 can be separated from the major carbon-12 isotope via techniques such as thermal diffusion, chemical exchange, gas diffusion, and laser and cryogenic distillation, currently only cryogenic distillation of methane (boiling point −161.5°C) or carbon monoxide (b.p. − ...
Carbon (6 C) has 14 known isotopes, from 8 C to 20 C as well as 22 C, of which 12 C and 13 C are stable.The longest-lived radioisotope is 14 C, with a half-life of 5.70(3) × 10 3 years. . This is also the only carbon radioisotope found in nature, as trace quantities are formed cosmogenically by the reactio
This material had an anomalously high 13 C: 12 C ratio (0.0112372 [4]), and was established as δ 13 C value of zero. Since the original PDB specimen is no longer available, its 13 C: 12 C ratio can be back-calculated from a widely measured carbonate standard NBS-19, which has a δ 13 C value of +1.95‰. [ 5 ]
The δ 13 C of C3 plants depends on the relationship between stomatal conductance and photosynthetic rate, which is a good proxy of water use efficiency in the leaf. [19] C3 plants with high water-use efficiency tend to be less fractionated in 13 C (i.e., δ 13 C is relatively less negative) compared to C3 plants with low water-use efficiency. [19]
Twelve dozen (12 2 = 144) are known as a gross; and twelve gross (12 3 = 1,728, the duodecimal 1,000) are called a great gross, a term most often used when shipping or buying items in bulk. A great hundred, also known as a small gross, is 120 or ten dozen. Dozen may also be used to express a moderately large quantity as in "several dozen" (e.g ...
For females, it is menarche, the onset of menstruation, which occurs, on average, between ages 12 and 13. [12] [23] [24] [25] The age of menarche is influenced by heredity, but a girl's diet and lifestyle contribute as well. [12] Regardless of genes, a girl must have a certain proportion of body fat to attain menarche. [12]
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Tween is an American neologism and marketing term [10] for preteen, which is a blend of between and teen. [9] [11] People within this age range are variously described as tweens, preadolescents, tweenies, preteens, pubescents, middle schoolers, [12] or tweenagers. [13] [14] A junior high school can sometimes be confused with a middle school.