the kohlrausch regulation function ( krf ) is a law of conservation that describes the mechanism of electrophoretic migration of ions when they are subjected to a current of constant intensity , this law being expressed by relation ( 2 ): wx is a constant defined for each point of the electrophoretic system . this law defines a constant wx for each zone of electrolyte . in other words , the leading electrolyte m has a constant wx ( m ), the solution s has a constant wx ( s ) and the terminating electrolyte t has a constant wx ( t ). these values of constants remain unchanged during migration of the species . in other words , when a species migrates from a point x1 having a constant krf1 to a point x2 having a constant krf2 , the concentration of the species under consideration evolves so as to respect the new constant krf2 . thus , if krf1 is greater than krf2 , the species under consideration is diluted , extending the distance over which it can be detected . fig4 a - c show the progress of a conventional isotachophoresis analysis . these figures notably demonstrate the effect of the kohlrausch function described above on the electrophoretic migration of the species . fig4 a shows a capillary in the initial state as described in fig3 a . in the present case , the leading electrolyte m has a constant wx ( m ), the solution s has a constant wx ( s ) and the terminating electrolyte t has a constant wx ( t ). the values of the krf constants are shown in a diagram as a function of the coordinate x where we are located in the capillary 30 . fig4 b shows the capillary 30 in which the species a , b , and c are undergoing migration . for example , when the species c migrates towards the leading electrolyte m and the value of the krf constant of the solution wx ( s ) is greater than the value of the krf constant wx ( m ) of the leading electrolyte m , dilution of the species c takes place , so that the value of the concentration respects the value of the krf constant wx ( m ) of the leading electrolyte m . in other words , the concentration of species c decreases through increase in the distance over which species c is detected . in fig4 c , separation of the species a , b and c is completed . the species a , b , c are distributed in the form of contiguous single - element bands . moreover , in fig4 c , the terminating electrolyte t illustrates the kohlrausch law of conservation . the terminating electrolyte t is concentrated at the inlet of the zone corresponding initially to the zone of the solution s , whereas the terminating electrolyte t is then diluted at the inlet of the zone corresponding initially to the zone of the leading electrolyte m . thus , the concentration of the terminating electrolyte is not constant along the capillary 30 , which is reflected in the graph showing the values of constant krf as a function of x . isotachophoresis is a technique that differs from the other techniques for chemical separation by its capacity for autofocusing of the species in separate bands , whose length varies during separation and then remains constant once the quasi - stationary state is reached . it is this characteristic that is peculiar to the isotachophoresis technique that is exploited by the invention . quasi - stationary state means the state of the system once separation has been effected . this particular property makes it possible to stop and resume separation with limited degradation of the performance of the technique . thus , a principle of one aspect of the invention is based on a step of transient stoppage of the generator of voltage at the terminals of the capillary 30 , which has the result of stopping the migration of the species to be separated . fig5 a - e show the progress of an isotachophoresis analysis during which the voltage generator is stopped temporarily . fig5 a shows a capillary 30 in which the species a , b , c have been separated by isotachophoresis , as shown in fig4 c . the representation of the conductometric signal , obtained by a detector situated at the outlet end of the capillary as a function of time , has a first plateau corresponding to the leading electrolyte m and a second plateau corresponding to species a . fig5 b shows the capillary 30 when detection of species b begins . a second plateau corresponding to species b appears on the graph of the conductometric signal as a function of time . fig5 c shows the capillary when the voltage generator is stopped . the migration of the species is halted , species b remains opposite the detector , which is reflected by an increase in the length of the plateau corresponding to species b . fig5 d shows the capillary 30 when migration resumes , migration of species b comes to an end , and species c is then detected . a new plateau corresponding to species c then appears on the associated graph of the conductometric signal as a function of time . finally , fig5 e shows the capillary 30 during passage of the terminating electrolyte t at the level of the detector . the signal , as a function of time , has a fourth plateau corresponding to the terminating electrolyte t . the different steps of separation can be repeated if separation of species b and c has not been obtained . these figures clearly demonstrate that transient stoppage of the voltage generator leading to stopping of migration of the species does not cause the system to change . in other words , temporary stoppage of migration does not have an influence on separation by isotachophoresis . fig6 a to 6 e show the progress of an analysis by isotachophoresis during which the voltage generator is stopped after one cycle of migration and a counter - pressure is applied transiently . fig6 a shows the capillary in which the species a , b , and c have migrated to the level of the detector near the outlet of the capillary 30 , after a first cycle of separation . separation of the species a , b and c has started , according to the principle described in fig3 a - 3 c , but is not yet completed . in fact , one portion of species a has been isolated but the other portion of species a and species b and c are still mixed . fig6 b corresponds to a step of stopping the electric field and applying a counter - pressure δp in the direction opposite to the direction of migration of the species so as to return all the species a , b , c to the level of the inlet of the capillary 30 . application of the counter - pressure δp mixes species a , b and c again . thus , it is necessary to configure the counter - pressure δp so that agglomeration or mixing of the species does not completely cancel the start of separation obtained by the first cycle of separation . more precisely , the speed of return of species a , b , c to the inlet of the capillary must be lower than the speed of migration of species a , b , c during the first cycle of separation . advantageously , a second detector is positioned at the level of the inlet of the capillary so as to stop the application of the counter - pressure when species a , b and c arrive at the level of the inlet of the capillary 30 . fig6 c shows the second cycle of migration of the species when migration resumes . species a , b , and c resume their migration with a speed depending on their charge / size ratio , forming bands . fig6 d corresponds to a stage of separation of the more advanced species ; the single - element bands begin to appear . in fig6 e , separation is completed ; species a , b and c are distributed in the form of single - element bands . the detector situated near the outlet of the capillary 30 then detects the different species , as in fig5 e . fig7 a to 7 f show the evolution of the separation of a mixture of four lanthanides nd , sm , eu and gd introduced in identical amount , namely 177 ng , into a capillary with a length of 1 m and a diameter of 75 microns . separation by isotachophoresis is effected by applying a current of 3μa . fig7 a shows that after one cycle of migration , it is not possible to separate these four lanthanides with a separation system used conventionally . effectively , after cycle 0 , one portion of the neodymium has been isolated , and the other portion of the neodymium is still mixed with the samarium , europium and gadolinium . thus , the graphical representation of the conductometric signal as a function of time has only two plateaux . separation of the four lanthanides is obtained after five repetitions of the various steps . multi - cycle isotachophoresis makes it possible to increase the length of the capillary artificially without needing to increase the voltage applied to the terminals of the capillary 30 , thus making it possible to separate species that have similar speeds of electrophoretic migration . the succession of cycles shown in fig7 b to 7 f shows a gradual evolution of the separation profile , leading to the formation of four bands corresponding to the four lanthanides . optionally , if one species has a speed of migration well above that of the other species , it is possible to extract it before continuing separation with the other species . fig8 a to 8 d show the steps of the method of multi - element isotopic measurement coupling a method of multi - cycle isotachophoresis as described above and a method of mass spectrometry using a mass spectrometer of the multi - detector icp - ms type . fig8 a shows a capillary after separation of species a , b and c , species a , b , and c being situated at the level of the outlet of the detector in the form of single - element bands , as shown in fig6 e . species a is transferred to the mc - icpms mass spectrometer . in the present case , three isotopes a 1 , a 2 , and a 3 of element a are detected at the detectors . the graph showing the level of the signals by mc - icpms has three peaks corresponding to the three isotopes . fig8 b shows the start of analysis of element b by mc - icpms ; two isotopes b 1 and b 2 of element b are demonstrated . however , only one of the two isotopes b 1 is detected at the level of the detectors of the mc - icpms spectrometer , the detectors not being positioned so as to detect the other isotope . fig8 c corresponds to the step of stopping separation and applying counter - pressure so as to return the remaining species b and c to the inlet of the capillary . during this stoppage , the detectors of the mc - icpms spectrometer are repositioned so as to be able to detect the second isotope b 2 of element b . fig8 d corresponds to the step of isotopic measurement of species b after a new cycle of migration of the species . species b is transferred to the mc - icpms spectrometer ; the two isotopes b 1 and b 2 of species b are detected , which corresponds to the two peaks on the graph showing signal intensity as a function of time . fig9 shows separation of a mixture of four natural lanthanide elements nd , sm , eu and gd having 29 isotopes . this figure is able to confirm the validity of the method for multi - element isotopic measurement with delayed sequential elution . in fact , acquisition of the conductometric signals of the 29 isotopes was performed in a single separation . therefore 29 rectangular signals are obtained with similar widths , distributed in four groups each corresponding to one lanthanide element of the mixture . the graph shows measurement of the signals of the isotopes obtained by an mc - icpms spectrometer after separation by isotachophoresis according to the principle of the invention . in the present case the detectors are repositioned during the stoppages of separation so as to detect all the isotopes . the graph of the signal as a function of time reveals four groups of curves p 1 , p 2 , p 3 and p 4 . separation of the mixture of lanthanides is stopped after appearance of each of the groups and a counter - pressure is applied . during application of the counter - pressure , the detectors are repositioned . the first group of curves has a peak p 1 at 1800s corresponding to the element neodymium , the second group of curves corresponding to the element samarium has a peak p 2 at 2200 s , the third group of curves corresponding to the element europium has a peak p 3 at 2700 s and the fourth group of curves corresponding to the element gadolinium has a peak p 4 at 3000 s . the first group comprises the plateaux corresponding to the isotopes of neodymium ; mc - icpms notably detects the isotopes 142 nd , 143 nd , 144 nd 145 nd 146 nd , 148 nd , 150 nd . in the same way , the other groups comprise the curves corresponding to the isotopes of samarium , europium and gadolinium .