Suppressed modes

It is appropriate to refer here to the development of non-suppressed ion chromatography. A simple chromatographic system for anions which uses a conductivity detector but requires no suppressor column has been described by Fritz and co-workers.28 The anions are separated on a column of macroporous anion exchange resin which has a very low capacity, so that only a very dilute solution (ca 10 4M) of an aromatic organic acid salt (e.g. sodium phthalate) is required as the eluant. The low conductance of the eluant eliminates the need for a suppressor column and the separated anions can be detected by electrical conductance. In general, however, non-suppressed ion chromatography is an order of magnitude less sensitive than the suppressed mode. 

The pH value also affects the ionization of acidic and basic analytes and their electromigration. Since this migration can be opposite to that of the electroos-motic flow, it may both improve and impair the separation. This effect is particularly important in the separation of peptides and proteins that bear a number of ionizable functionalities. Hjerten and Ericson used monolithic columns with two different levels of sulfonic acid functionalities to control the proportion of EOF and electromigration. Under each specific set of conditions, the injection and detection points had to be adjusted to achieve and monitor the separation [117]. Another option consists of total suppression of the ionization. For example, an excellent separation of acidic drugs has been achieved in the ion-suppressed mode at a pH value of 1.5 [150]. 

Figure 4 Transverse scan of axillary and subscapular lymph nodes in a rabbit 5 min postinjection of Gd-containing liposomes. Liposomes (egg lecithin cholesterol Gd-poly-NGPE = 70 25 5, 20 mg total lipid) were injected subcutaneously into the forepaw of anesthesized rabbit in 0.5 mL of HEPES-buffered saUne. Images were acquired by using a 1.5 Tesla GE Signa MRl scanner operated at fat suppression mode and Tj-weighted pulse sequence [16].

Figure 6 Transverse MR images of axillary-subscapular lymph node area in the rabbit 4 min after s.c. administration of PEG (5 kDa) -phosphatidyl ethanolamine micelles containing coreincorporated Gd-loaded amphiphilic chelate DTPA-phosphatidyl ethanolamine. The dose was 0.5 pmol Gd per injection site. Fast and clear visualization of both lymph vessel (1) and lymph node (2) was achieved. Images were acquired by using a 1.5 Tesla GE Signa MRI scanner operated at fat suppression mode and Ti-weighted pulse sequence [20].

Since all ions are electrically conducting, conductivity detection can be considered universal in response. Conductivity detection comprises non-suppressed and suppressed modes. 

Eluent Flow rate Detection 0.05 MNaOH in 40% methanol 1 mL/min Suppressed conductivity, chemical suppression mode... 

When negatively charged, the velocity of acidic compounds is smaller than that of EOF, since their electrophoretic mobility opposes the electroosmotic mobility. In such cases, electrokinetic injection must be avoided since very little or no sample is introduced into the column. Moreover, in instances when the magnitude of electrophoretic mobility is larger than that of the electroosmotic mobility, the analyte is not transported along the column. The obvious solution is to operate in ion-suppressed mode, i.e., at low pH, where acidic compounds are not ionized. The drawback is extensive analysis time, since the majority of silanol groups are also not ionized and EOF is significantly smaller. 

Octadecyl (octyl) bonded phase with low percentage of free silanol groups ION-SUPPRESSION MODE methanol (acetonitrile) -water containing ca. 0.01 - 0.1 M phosphate buffer, ammonium carbonate or sodium acetate (pH 4-7). 

Alkaline-earth metals, which elute in the order Mg24 < Ca2+ < Sr2+ < Ba2+, can also be determined using both conductivity detection modes. While an eluent mixture of hydrochloric acid and 2,3-diaminopropionic acid is used in the suppressed mode, ethylenediammonium ions are suitable for the direct conductivity detection mode. Fig. 3-143 shows a separation of alkaline-earth metals on Shimpack IC-C1 obtained with this eluent. Because of the high elution power of the mobile phase, all monovalent... 

Most decay modes (other than the semileptonic modes) that involve a neutral K meson are now given as modes, not as modes. Nearly always it is a tliat is measured, and interference between Cabibbo-allowed and doubly Cabibbo-suppressed modes can invalidate the assumption that 2r(Xj) = r K ). 

Cb, Ci8 or phenyl- Ion-suppression mode methanol bonded phase with low (acetonitrile)-water containing percentage of free 0.01-0.1 mol 1 phosphate... 

Reversed-phase chromatography in the ion-pair or ion-suppression mode on... 

In the suppressed mode, the solutes appear as bands of sodium and potassium hydroxides in essentially deionized water. The positive deflection in conductivity caused by the sodiimi band, for example, is calculated according to Eq. (8.32) ... 

In a hypothetical suppressed mode, one could in principle assume the extremely low conductivity of pure water, but a real system is more fairly represented by choosing a value of about 2 pS/cm, because inevitable impurities in the eluent and any slight leakage from the suppressor system have to be taken into account... 

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