Temperature ascorbic acid addition

Marti, N. et al., Influence of storage temperature and ascorbic acid addition on pomegranate juice, J. Sci. Food Agric., 82, 217, 2001. 

In similar work, Sturgeon et al. [125] compared direct furnace methods with extraction methods for cadmium in coastal seawater samples. They could measure cadmium down to 0.1 pg/1. They used 10 pg/1 ascorbic acid as a matrix modifier. Various organic matrix modifiers were studied by Guevremont [116] for this analysis. He found citric acid to be somewhat preferable to EDTA, aspartic acid, lactic acid, and histidine. The method of standard additions was required. The standard deviation was better than 0.01 pg/1 in a seawater sample containing 0.07 pg/1. Generally, he charred at 300 °C and atomised at 1500 °C. This method required compromise between char and atomisation temperatures, sensitivity, heating rates, and so on, but the analytical results seemed precise and accurate. Nitrate added as sodium nitrate delayed the cadmium peak and suppressed the cadmium signal. 

Figure 19. ESR spectrum (B) produced by the addition of ascorbic acid to the reaction mixture of glucose with X-butylamine (each 1 MJ in ethanol refluxed for 15 min. A reaction mixture. B reaction mixture (0.5 mL) kept at room temperature for 15 min after the addition of ascorbic acid (0.1 g/mL, 0.05 mL). (Reproduced with permission from Ref. 5.)...

The recommended procedure for the determination of arsenic and antimony involves the addition of 1 g of potassium iodide and 1 g of ascorbic acid to a sample of 20 ml of concentrated hydrochloric acid. This solution should be kept at room temperature for at least five hours before initiation of the programmed MH 5-1 hydride generation system, i.e., before addition of ice-cold 10% sodium borohydride and 5% sodium hydroxide. In the hydride generation technique the evolved metal hydrides are decomposed in a heated quartz cell prior to determination by atomic absorption spectrometry. The hydride method offers improved sensitivity and lower detection limits compared to graphite furnace atomic absorption spectrometry. However, the most important advantage of hydride-generating techniques is the prevention of matrix interference, which is usually very important in the 200 nm area. 

Modifiers are, however, used in a rather indiscriminate way in many laboratories. If used carelessly they can contaminate the sample solution with the element that is being determined and they can actually add to the background interference which one intends to reduce. By carefully optimizing the ashing and atomization temperatures for specific food matrices, as described above, the use of matrix modifiers can be reduced to the cases when they are really necessary. An additional benefit of matrix modification is that the sample and standard matrix are made very similar, this often making the standard addition method unnecessary. How this is carried out is described in detail in most instrument manuals and in specific textbooks. Commonly used modifiers are ammonium nitrate, ammonium phosphate, Mg nitrate, Pd nitrate, and ascorbic acid. 

Peptide Modification lodination was carried out on a stainless steel probe target by adding 0.1 % aq. I2 (1 pi) to the dried peptide (ca. 1 pmol). The reaction was stopped after 1 minute by addition of ascorbic acid and the MALDI matrix, a-cyano cinnamic acid in excess. Esterification with ethanol was carried out using the method of Hunt et al. (15), where an acetylchloride and ethanol solution (1 6, v v) was added (5 pi) to the peptide dried in a microcentrifuge tube (ca. 1 pmol). After incubation for 15 minutes at room temperature a 2 mM p-mercaptoethanol (in ethanol) solution was added (1 pi) and the mixture was dried. The matrix, a-cyano-4-hydroxycinnamic acid (2 pi), was added to the micro tube and after 5 minutes 1 pi of this matrix was removed and applied to a target. 

---