Mass signals

Figure 6.14 Oxidation of 0.01 M H COOH in 0.5 M H2SO4 on a polycrystalline electrode pre-coveted with CO. (a) Voltammetric currents, (b) Mass signals for C02 (m/e = 44) and C02 (m/e = 45). (Reproduced from Willsau and Heitbaum [1986].)...

After amplification the data can be directly recorded or digitalized and stored. Several mass signals can be detected during a given experiment. Depending on the type of detector used and on the sensitivity of the measurement, the mass spectrometer... 

Fig. 2.9. Current transient and mass signal responses during oxidation of methanol adsorbate in pure base electrolyte (flow cell procedure). Methanol was adsorbed from a 10 2 M CD3OH + 10 4 M HC104 + 0.1 M NaC104. ad = 356 mV tad = 400 s. Potential step to 975 mV vs. Pd-H for 0.5s to produce C02 (m/e = 44) and hydrogen ions, followed by a step to —574 mV vs. Pd-H to detect HD...

The use of conventional electrochemical methods to study the effect of metal adatoms on the electrochemical oxidation of an organic adsorbate may be in some cases of limited value. Very often, in the potential region of interest the current due to the oxidation of an organic residue is masked by faradaic or capacitive responses of the cocatalyst itself. The use of on-line mass spectroscopy overcomes this problem by allowing the observation of the mass signal-potential response for the C02 produced during the oxidation of the adsorbed organic residue. 

Fig. 4.1. Current and mass signal during an on-line mass spectroscopic experiment showing the effect of adsorbed tin on platinum upon methanol electrooxidation. 1 M CH3OH/0.5 M H2S04 sweep rate 10 mV/s, 24 °C.

Another advantage of the on-line MS method is the possibility to observe separately the catalyst effects on parallel reactions. This is illustrated in Fig. 4.1 for methanol electroxidation on Pt with 60% of tin. Current and mass signals for C02 and for HCOOCH3 (methylformate) during the oxidation of 1 M CH3OH on a porous platinum electrode were recorded. 

As we have pointed out in Section 1.1 the mass signal response for a given reaction is proportional to the respective current (see Eq. 1.1). The result in Fig. 4.1 shows that at 0.58 V an increase in current by a factor of 7.5 is obtained. At the same time, the mass signal for C02 increases by a factor of 13 and that for HCOOCH3 by a factor of 3. This result indicates that tin affects the current efficiency for both parallel reaction pathways to a different extent. 

Fig. 4.6a,b shows the effect of Sn(II) addition on current and C02-mass signal respectively. The enhancement of C02 production is clearly more pronounced than that produced by Sn(IV) addition. This effect can be observed at potentials as low as 0.25 V (lower than in the case of Sn(IV)). 

A voltammogram recorded after 15 min of interaction with tin is shown in Fig. 4.7a, together with the mass signal response for C02. The current signal shows... 

Temperature programmed desorption (TPD) of NH3 adsorbed on the samples was carried out on an Altamira TPD apparatus. NH3 adsorption was performed at 50°C on the sample that had been heat-treated at 120°C in a helium flow. After flushing with helium, the sample was subjected to TPD from 50 to 600°C (AT = 10°C min 1). The evolved NH3, H20 and N2 were monitored by mass spectroscopy by recording the mass signals of m/e = 16, 18 and 28, respectively using a VG Trio-1 mass spectrometer. 

Figure 3.29 Cyclic current (a) and mass signal (b,c,d) voltammograms for 0.1 M CH3OH in 1 M HCI04 at porous platinum, and room temperature. The sweep rate was 20mVs 1 and the surface roughness c. 50, After Iwasita and Vielstich (1990).

Comparative sequence analysis based on MALDI-TOF-MS analysis of nucleic acids cleaved at specific bases and reference sequences used to construct in silico cleavage patterns enable cross-correlation of theoretical and experimental mass signal patterns. Observed signal pattern differences are indicators of sequence variations and... 

On-line mass spectrometric meastirements were conducted using a platinum painted electrode. The potential was held at 50 mV for 20 s in 3 M sulfuric acid containing O.OIM Na2Mo04. Then, the electrolyte was replaced with 3 M sulfuric add with 1 M methanol. Then the potential was swept in the anodic direction at 10 mV/s while recording the electrochemical current and the mass signal (m/e = 44) corresponding to CO2. The residts are shown in Fig. 4-26 together with the results on pure platinum for comparison. 

For CE—MS, volatile buffers are common. To use MEKC systems combined with mass detection, volatile micelles have been tested.When using atmosferic pressure photo-inonization (APPI), non-volatile BGE constituents do not deteriorate the mass signal to... 

In Fig. 9.30 a broadening of the molecular weight distribution at increasing time of polymerization is observable which is typical for the polymerization of 2-oxazolines. Plotting the m/z value of the most intensive mass signal versus the reaction time, a strictly linear relationship is obtained (Fig. 9.30). These results are consistent with recent experimental and theoretical studies of surface-initiated... 

Fig. 9.30 a) Normalized MALDI TOF mass spectrum of all fractions (taken after 2 h to 72 h of polymerization time) of poly(2-phenyl-2-oxazoline) freed by dissolving the gold core with NaCN solution and collection of the polymer. The calculated mass of the monomer unit (147.17) is in good agreement with the spacing of the mass signals (AM = 146.93) of the most prominent peaks. Based on ear-... 

Table. ELEMENTAL COMPOSITION OF F.I. MASS SIGNALS DETECTED FOR A LOW RANK BITUMINOUS COAL,78%Cdaf...

As discussed above, the development of mild MS techniques has led to further progress in the determination of proanthocyanidin size distribution. In particular, ESI-MS studies have demonstrated that prodelphinidin and procyanidin units coexist within the polymers, where they seem distributed at random. A list of mass signals attributed to proanthocyanidins detected in grape or wine extracts is given in Table 5.2. 

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