The mixing cross

One liter of water/acetonitrile 65 35 is needed. There is a leftover of a 80 20 mixture. 

Mixmre 1 is 20% in acetonitrile. What needs to be added is pure acetonitrile, i.e. C2=100%. 

The volume fractions are 65 parts of mixture 1 and 15 parts of solvent 2, giving 

Be aware of the fact that old mixtures do not necessarily still have their original contents of solvents 1 and 2, therefore a new mixture may also differ from the ratio that is wanted. Due to the effect of volume contraction, it is not allowed to prepare water/ methanol mixtures in a measuring flask by filling the flask to the mark. 

The mixing cross does not work with buffer mixtures when a certain pH of the new buffer is needed  

Reproduced with permission from D. Patel, Liquid Chromatography Essential Data, Wiley, Chichester, 1997, p. 89. 

The volume fractions are 65 parts of mixture 1 and 15 parts of solvent 2, giving a total of 80 parts which must represent 1000 ml. Therefore 65 parts correspond to 812 ml and 15 parts to 188 ml. (The control of the result is simple 812 ml 0.2 = 162 ml, plus 188 ml = 350 ml of acetonitrile in 11 of new mixture.) The problem can also be solved on the base of the water content  

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Figure 7 Fluorescence intensity versus time for three runs of a step-gradient increase from 0.32 to 1.68 pM rhodamine B. The left axis fluorescence signal was collected 20 mm downstream from the mixing cross, and the right axis shows the concentration of rhodamine B corresponding to each signal plateau. (Reprinted with permission from Ref. 47.)...

When X impinges on the core of M at R R2 the angular velocity increases and this causes transitions between the components, thus leading finally to transitions between 2P1/2 and 2P3/2 atomic states (mechanism 2). This mechanism is operative at smaller distances compared to those involved in mechanism 1. The transition probability for mechanism 2 is proportional to ratio (< /w)2, which does not appear in mechanism 1, Both factors tend to decrease the importance of mechanism 2 compared to mechanism 1. Nevertheless, the difference in steepness of the repulsive interaction between X and M in B 22 and A 2II states can lead to preference of mechanism 2 for nearly adiabatic collisions when the mixing cross section is very small. 

Thus, the intramultiplet mixing in heavy alkalies can be accounted for by transitions in two nonadiabaticity regions that give competing contributions to the mixing cross sections. It seems clear now that an attempt [62] to interrelate experimental data resorting only to equation (57) cannot be justified. 

The mixing cross is a useful aid for the calculation of solvent mixtures. Figure 5.3 shows how to use it. The contents c (usually in vol%) with regard to a certain solvent of the three mixtures involved (or two mixtures and a pure solvent) are noted as well as the content differences between the initial mixtures and the needed one, thus representing a cross with the new mixmre in the middle. The negative sign that occurs in one of the two subtractions is ignored. The obtained differences represent the volume fractions to be mixed which finally must be converted into the needed volume parts. 

The mixing cross is a usefiil aid for the calculation of solvent mixtures. Figure 5.3 shows how to use it. The contents c (usually in volume %) with regard to... 

The updates and improvements of this new edition are mainly to be found in details such as new references and technical descriptions which match today s instrumentation. Four new sections have been written, namely on the shelf-life of mobile phases, the mixing cross, the phase systems in ion chromatography, and on measurement uncertainty. Some equations in the zeroth chapter . Important and Useful Equations for HPLC, have new numeric values because a porosity of 0.65 is more realistic than 0.8 for chemically bonded phases. 

The mixing cross part is attached to the engine. Subsequently the beaker is screwed on top of it. The constituents to mix are transferred into the beaker. The beaker may only be filled up to half its volume as otherwise insufficient mixing will result. 

The mixing cross also causes a tangential and a radial flow and turbulence. During mixing a lot of air is whipped into the mass and after long mixing the temperature will increase. 

It is interesting to note that overall, the mixed cross sections in Table 3 are small and further, unlike the unmixed cross sections, they are not strongly diagonally dominated (i.e., the j =j cross sections are not, in general, much larger than those for which j i) They appear to provide sensitive tests of dynamical approximations, and, of course, can give information on intermolecular anisotropies. 

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