Sample neutralization

In a MALDl experiment, the sample is mixed or dissolved in a matrix material that has an absorption spectrum matching the laser wavelength of energy, The sample may not have a matching absorption peak (a), but this is not important because the matrix material absorbs the radiation, some of which is passed on to the dissolved sample. Neutral molecules and ions from both sample and matrix material are desorbed (b). 

Figure 2. The high resolution spectra of species 5 and 6 show N(ls) peaks at 401 eV (nonprotonated nitrogen) and 402.7 eV (protonated nitrogen) while samples neutralized with base, T, do not show protonated nitrogen peaks. Also, the latter sample shows two species...

Another pathway is for NO2, formed in Eq. (4), to attach to excess sample neutrals to form an adduct ion, M-N02 as shown in Eq. (5). Although this can be a distinctive and pronounced pathway for some explosives, such as nitrogylcerin, the reaction comes at a cost — two molecules are consumed for each product ion, degrading quantitative response. One molecule provides N02, which clusters with the second molecule. 

The first step in response in an IMS analyzer is the conversion of a sample neutral to a gas-phase ion through coulombic associations resembling displacement reactions as shown in Eqs 1 and 2 for positive and negative polarity, respectively. 

Sample neutral Positive reactant ion Positive product ion ... 

The discussion about the possible presence of a small contribution of d-n overlap forces at the surface of NiO is of interest because it may occur with Ni2+ interacting with adsorbates with r-acceptor characteristics, such as CO, NO (Section IV.I.2), and O2. IR spectra of O2 adsorbed at 77 K on progressively sintered NiO sampels (274) follow a trend similar to that observed for CO. In particular, on high-surface-area samples, O2 species formed at edge, step, and corner sites are predominant, whereas on progressively more sintered samples neutral species adsorbed in side-on configuration on Ni2+ of the (001) faces become the only species detectable by IR spectroscopy. 

Determination of small concentrations of anions in samples containing 1 M sodium hydroxide and determination of trace cations in strong mineral acids are two examples where sample neutralization is needed. However, chemical neutralization with HCl or NaOH would introduce a high concentration of unwanted ions. A better way is to introduce the or OH needed for neutralization through an ion-exchange membrane. This process is called electrodialysis. 

In short, the system developed by Jackson et al. [1] features automatic on-line sample collection from a turntable, acidic-oxidizing digestion of the sample, neutralization, liquid-liquid extraction and Introduction of the extract Into a sampler vial. 

Neutrality.- The same chemist determines the neutrality of glycerine thus 50 c.c. of glycerine mixed with 100 c.c. of water and a few drops of alcoholic phenolphthalein[A] are titrated with hydrochloric acid or sodium hydroxide not more than 0.3 c.c. normal hydrochloric acid or normal soda solution should be required to render the sample neutral raw glycerines contain from. 5 to 1.0 per cent, of sodium carbonate. 

Should the vapor concentration of M in the reaction region increase further, a second product ion can be formed as another sample neutral attaches to the protonated monomer, displacing a water molecule and yielding a proton-bound dimer A/2H+-(H20) r... 

Furthermore, proton-bound trimer ions G3H" (H20) and higher clustered ions can form in the reaction region where ions and sample nentrals mix. When such ions are extracted from the reaction region into the drift region of the IMS, largely free of sample neutrals, decomposition of the trimer ions is rapid. Such ions exhibit lifetimes too short to be measnred in the drift tube at ambient temperature. 

Phosphonate acid sample neutralization (with solid ammonium carbonate, Fisher Catalog A-652, to cessation of carbon dioxide evolution, pH 8-9). 

Ion-exchange chromatography may be used for purification and group separation. As mentioned, Okishio et al. (24) purified a liver extract by applying it in 0.1 M NaOH to an Amberlyst A-26 column as described above for solid extraction. An extract can also be dissolved in chloroform-methanol, 1 1, saturated with water, and applied with a slow flow rate to an Amberlyst A-26 column in OH" form in the same solvent. A 10-ml column should be used for 250 mg of sample. Neutral lipid contaminants are removed by washing with a suitable organic solvent and bile acids are eluted with 0.2 M ammonium carbonate in 80% ethanol as described above (34). 

Sample Neutral lipids % Polar lipids % a-Tocopherol mg/lOOg Acid value mg KOH/g oil Color ... 

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