Interception rates

The global interception rate for opiates continues to increase. .. 

Over the last decade, the annual growth in opiates-seizures averaged 6 per cent, which exceeded growth in global opium production and resulted in an increase in the global interception rate for opiates from 15 per cent in 1995 to 26 per cent in 2006. 

As a result, the global cocaine interception rate rose from 34 per cent in 2004 to 42 per cent in 20051, a significant increase from 20 per cent in 1995. However, yields and laboratory efficiency appear to have increased over the last few years and this may not yet be fully reflected in the current global cocaine production estimates. The result could be an overestimated global cocaine interception rate. While some adjustments in production figures may be expected in the future, there is little doubt that cocaine interception rates grew strongly in recent years. 

The global interception rate was calculated on the basis of a global cocaine production of 980 mt in 2005 and global seizures of 756 mt at street purity, which, given a global average cocaine purity of 55 per cent in 2005 (as reported by Member States to UNODC in the Annual Reports Questionnaires), would be equivalent to pure cocaine seizures of some 416 mt. 

For the fourth year in a row, Colombia topped the ranking of the world s largest cocaine seizures. It seized 217 mt of cocaine hydrochloride (HC1) and cocaine base in 2005, equivalent to 29 per cent of the world total and an increase from 19 per cent in 1995. Seizures in Colombia increased by 16 per cent from a year earlier, representing the highest cocaine seizures ever reported by a country. In 2005, the interception rate 2 of cocaine produced in Colombia rose to 29 per cent, up from 25 per cent in 2004 and 13 per cent in 2000, clearly reflecting continued enforcement efforts in the country. 

The interception rate of the cocaine seized in Colombia was calculated on the basis of domestic cocaine production and an average purity level of cocaine of around 85 per cent. 

There is no term in equation (21.3) for air flow. External air flow around organisms is sufficiently slow (subsonic) that it may usually be treated as incompressible (Vogel, 1994). This incompressibility means that the concentration of chemical stimulus molecules (n) will not be increased noticeably by the pressures that, develop adjacent to insect sensory hairs or antennae due to moving air (or moving antennae). The replacement of any captured molecules by the arrival of fresh odorant-laden air is the primary reason why air flow has such a dramatic effect on interception rate. One way of considering the influence of air flow is that at best the air flow could bring the interception rate closer to the limit predicted by equation (21.3). In order to discuss approaches more complex than that provided by equation (21.3), we have to consider the physical bases for molecular movements diffusion and convection. 

The apparent simplicity of equations (21.5)—(21.7) can be misleading the number of exact solutions is small, and while approximate solutions fill books (e.g. Crank, 1975), their application can be problematic. Seemingly small differences in the boundary conditions will completely change the character of the solution, as will be seen below, and identification of the appropriate boundary conditions is not easy. However, in practice one can make an educated guess about the approximate boundary conditions, or can estimate the interception rate for different kinds of possible boundary conditions. Below are a number of solutions that are the most useful for the geometry most relevant to insect antennae, the cylinder. Either sensory hairs or filiform antennae can be approximated as cylindrical in shape. 

Steady-state solutions for diffusion/convection When the air is moving, it becomes more difficult to calculate the interception rate. The mass transfer under these circumstances is generally expressed in dimensionless terms. Adam and Delbriick (1968) were able to generate a formula by making the simplifying assumption that the velocity of the air as it passed around the hair was everywhere constant (U) and very similar to the ambient air flow farther away (U0) ... 

Murray (1977) uses the same formula (equation 21.18) to predict interception rates of sensory hairs on insect antennae, but provides an additional insight in its application. As long as Pe < 1 (where L is hair diameter), the odorant molecules may be assumed to strike the hair anywhere. As Pe becomes larger (Pe >1), the interception rate will differ more with downstream/upstream location on the cylinder. The same logic may be used to estimate the potential for spatially dependent interception by a filiform antenna. 

A change in interception rate with air flow has been identified as a characteristic of flux detectors such as insect sensory hairs (Kaissling, 1998). However, in some instances (low Pe), an increase in flow will have only a negligible impact on the flux rate, even for a flux detector. This was shown by Berg and Purcell (1977) in the context of cells swimming through a liquid environment. That is, the flow rate doesn t matter if the molecules can walk themselves around just as quickly. For single cylindrical sensors (isolated hairs or filiform antennae), the slower the flow (the lower the Re and Pe), the less the interception rate is expected to increase with an increase in air speed (equations 21.18 and 21.20). 

Therefore, when compounds differ greatly in their molecular masses (which will be reflected in a smaller difference in the magnitudes of their diffusion coefficients), a modest difference in interception rates may be expected, particularly when the concentration of the compounds may be approximated as constant at some distance from the surface. 

---