Phase coverage

The use of TFA as a mobile-phase additive in LC-MS can be problematical when using electrospray ionization. In negative ion detection, the high concentration of TFA anion can suppress analyte ionization. In positive ion detection, TFA forms such strong ion pairs with peptides that ejection of peptide pseudo-molecular ions into the gas phase is suppressed. This problem can be alleviated by postcolumn addition of a weaker, less volatile acid such as propionic acid.14 This TFA fix allows TFA to be used with electrospray sources interfaced with quadrupole MS systems. A more convenient solution to the TFA problem in LC-MS is to simply replace TFA with acetic or formic acid. Several reversed-phase columns are commercially available that have sufficient phase coverage and reduced levels of active silanols such that they provide satisfactory peptide peak shapes using the weaker organic acid additives.15... 

Improvement of the geometric structure of the working electrode by a well-controlled PEVD process benefits the performance of a CO sensor in many ways. To optimize kinetic behavior, the response and recovery times of CO potentiometric sensors were studied at various auxiliary phase coverages. This was realized by a unique experimental arrangement to deposit the Na COj auxiliary phase in-situ at the working electrode of type III potentiometric CO sensors by PEVD in a step-wise fashion. Since the current and flux of solid-state transported material in a series of PEVD processes can be easily moiutoredto control the amount of deposit... 

Figures 21a-e show SEM SE plan-view images of the first sample at five selected PEVD steps, indicated by a to e in Table 2. The auxiliary phase coverage at the working electrode of the sensor increased with PEVD processing time and PEVD flux from a to e. After 14 steps of auxiliary phase deposition and sensor response testing, the final thickness of the product was about 3 pm, which was estimated from an SEM SE image of a cleaved cross-section sample (Eigure 2If).

Fig. 21 SEM SE images of the working electrode (a) before PEVD, (b) first stable EMF response, (c) when the response time of the sensor just passed the minimum point, (d) when the recovery time of the sensor just passed the minimum point, (e) final auxiliary phase coverage (plan view), and (f) final auxiliary phase coverage (cross-section). Bars equal to 5 pm.

The sensor response and recovery behavior for various auxiliary phase coverages at the working electrode of a type III potentiometric sensor are revealed for the first time through a combination of Figure 23 and Figures 21a-e. 

Fig. 1 Dependence of k on adrenaline (squares) and L-tyrosine hydrazide (circles), on mobile-phase concentration of 1-hexane-sulfonate. Column Synergi Hydro-RP (Phenomenex) 150 x 4.6 mm ID, particle size 4 pm, and bonded phase coverage 4.05 pmol/m. Eluent phosphate buffer 37.10 mM KH2PO4 and 4.29 mM Na2HP04 calculated to provide a pH of 6.0. After addition of the desired amount of sodium 1-hexanesulfonate, NaCl was added so that the total sodium concentration was 50 mM (constant ionic strength). Experimental data were fitted by Eq. 8.

Bonded phase coverage depends on the number of silanols available and on the type of reaction. Low surface area silicas (e g. Zipax) require polymerisation but for high surface area silicas (e.g. Li Chrosorb) direct reaction without polymerisation is satisfactory. 

Packing material Phase Particle shape and size (pm) Pore size (A) Pore volume (ml g ) Surface area (m g- ) Carbon load (%) Bonded phase coverage (pmolm ) End capping... 

Surface area Carbon load (%) Bonded phase coverage (pmol m ) End capping... 

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