Absolute uptake rate

Uptake rates can be presented as specific uptake (often designated as V), with units of per time, or absolute uptake (often designated as rho, p), with units of mass N per volume per time. Specific uptake rates are useful when comparing size fractions or systems that may differ substantially in biomass as they are more a measure of the physiological process of uptake. The absolute uptake rate is calculated by multiplying the specific uptake rate by the PN concentration (see Chapter 31 by Lipschultz, this volume). 

Subsequent to the ingestion of iodine in various forms, I is absorbed by the small intestine and enters the blood. Two competing pathways are involved in the clearance of I from the blood renal filtration into urine and thyroidal uptake. The renal clearance rate for I (30-50 mL/minute) varies only with the glomerular filtration rate. However, the thyroidal 1 clearance rate is autoregulated to maintain an absolute thyroidal I uptake rate of approximately 100 jig I each day. To accomphsh this, the thyroidal I clearance rate may vary (3 to 100 mL/minute) depending on the concentration of I in the blood. 

Nebulizer Solution uptake rate, mlmin 1 Sample volume, ml Absolute sensitivity, counts fg-1 Reference... 

Calculated or measured in situ specific uptake rates horn depth profiles were converted to absolute rates using particulate nitrogen values in tables and figures. 

Dickson, M. L., and Wheeler, P. A. (1995). Nitrate uptake rates in a coastal upwelhng regime A comparison of PN-specific, absolute, and Chi a-specific rates. Limnol. Oceanogr. 40, 533—543. 

Fe concentrations measured at the start of the bioassays, and based on the estimated half-saturation constant fQ of 0.09 nM Fe (by employing MichaeHs—Menten type kinetics), the availability of Fe during that time (ambient dissolved Fe concentration of 0.03—0.04 nM) would appear to limit NO uptake by phytoplankton. These studies aU demonstrate a clear enhancement of specific and absolute NO3 uptake rates by Fe during short-term incubations, and are supported by similar results conducted in other HNLC regions including the equatorial Pacific Ocean (Coale et al., 1996 Cochlan, 2001 Price et al., 1991, 1994) and the eastern (Boyd et al., 1996, 1998 Marchetti et al., 2006) and western (Kudo et al., 2005, in review) subarctic Pacific Ocean. 

Seawater is the most extreme environment on Earth in terms of its paucity of essential trace elements. Marine microorganisms have thus evolved some of the most efficient uptake systems possible and take up trace metals at rates near the maximum allowed by physics and chemistry. One can calculate absolute hmits to the cellular uptake rates of metals by considering the simple case of uptake via transmembrane proteins (Hudson and Morel, 1990). 

The diffusion coefficient is affected by temperature and pressure, but so too is the concentration. The mass collected by the sampler is therefore proportional to the square root of the absolute temperature and independent of pressure. The temperature effect should be less than 0.2%°C , and is normally ignored, since the variation from the measured uptake rate should be less than 5% from room temperature to 0°C and from room temperature to 40°C. 

Fig. 2.6 Effects of ferulic acid and initial nutrient solution pH on net phosphorous uptake (a 22 day old r for pH 5.5 = 0.71, and pH 6.5 = 0.45), absolute growth rates of leaf expansion (b 16-18 day old r for pH 5.5 = 0.90, pH 6.25 = 0.69, and pH 7.0 = 0.72), and net water utilization (c 16-18 day old r for pH 5.5 = 0.95, for pH 6.25 = 0.88, and for pH 7.0 = 0.69) of cucumber seedlings. Figures based on regressions and data from Lehman and Blum (1999b) (a) and regressions from Blum et al. (1985b) (b, c). Plenum Publishing Corporation, regressions and data used with permission of Springer Science and Business Media...

Another low-flow nebuliser used for coupling ICP-MS to chromatographic columns is the high-efficiency nebuliser (HEN). This device has a small capillary and is used with a spray chamber. Compared to pneumatic nebulisers, the HEN operates more efficiently at very low solution uptake rates. Micro-HPLC-HEN-ICP-MS coupling was applied to the speciation of five arsenic compounds. The HEN operated most efficiently at sample uptake rates of 40 pL/min and was shown to have excellent absolute detection limits. A possible drawback of the HEN, as for all low-flow nebulisers, is the poor tolerance in nebuhsing highly concentrated solutions. 

In this expression, U(t) is relative rate of uptake and Cx is relative to equilibrium, i.e. the sites available for ion exchange or adsorption for the specified ratio Vim. Thus, the absolute rate is a coupled result of kinetics and equilibrium. Note that in a solid diffusion-controlled process, U(t) is relative to the ease of movement of the incoming species in the solid phase (through Ds). 

A variety of experimental techniques have been used for the determination of uptake coefficients and especially Knudsen cells and flow tubes have found most application [42]. Knudsen cells are low-pressure reactors in which the rate of interaction with the surface (solid or liquid) is measured relative to the escape through an aperture, which can readily be calibrated, thus putting the gas-surface rate measurement on an absolute basis. Usually, a mass spectrometer detection system monitors the disappearance of reactant species, as well as the appearance of gas-phase products. The timescale of Knudsen cell experiments ranges from a few seconds to h lindens of seconds. A description of Knudsen cell applied to low temperature studies is given [66,67]. 

---