Hold-back carriers

The irradiated sample, diluted with Alundum in a porcelain boat containing mercuric nitrate carrier, is combusted very slowly in a slow oxygen stream in a 96% silica combustion tube. The volatile products are collected in two consecutive traps, both containing a solution of acetic acid-sodium acetate buffer, bromine, and mercuric nitrate hold-back carrier. The collection solutions in 2N HC1 are loaded onto Dowex 2, and radioactive interferences are eluted with aliquots of water and 2N HC1. The resin, in a small vial, is counted for the 0.077 MeV photopeak from 197Hg. 

Iodine vapor was added as a hold-back carrier to the air stream after It left the reactor by passing about 100 of the gas over Iodine crystals. This side stream then rejoined the main stream which was subsequently scrubbed of Its Iodine by an acid solution of KBrO In trap A. Trap B was used only for auxiliary tests. In some tests trap A was removed also. 

After addition of 1 mg Ga hold-back carrier, the Fe(QS)a Is ppted. 

The sainple la dissolved in H SO containing a few pg of I" hold-back carrier. 

The Au Is dissolved In 1-2 ml of warm aqua regia (2 N 4-10 min HN0a 4 N HCl) and hold-back carriers for Hg, Pt, Ir, and Os ( 1 mg each) added. The Au Is extracted Into ethyl acetate and the organic phase 1b washed with 6 H HCl. Held = lOOjt. 

If coprecipitation of radionuclides present in low concentrations is to be avoided, hold-back carriers are added. Isotopic carriers are most effective foi this purpose, but they lead to low specific activity. 

Traces of elements or compounds, respectively, of other elements with similar properties may serve as non-isotopic carriers for radioisotopes of stable elements as well as for isotopes of radioelements. The influence of non-isotopic carriers depends on the nature of the compounds and the chemical operation. For example, in precipitation reactions, non-isotopic carriers or hold-back carriers, respectively, may play a major role. 

If macroamounts of other elements are to be separated from microamounts of radioactive substances by precipitation, isotopic or non-isotopic hold-back carriers may be added to suppress coprecipitation of the radioactive substances. 

Radiochemical purification of the induced activity in the presence of carrier involves chemical operations such as precipitation, distillation, solvent extraction, chromatography, and ion exchange. While the chemistry performed on the carrier and sample should be designed to isolate the material in a pure state, a useful operation frequently carried out is scavenging. Strongly adsorptive precipitates such as ferric hydroxide, lanthanum fluoride, and antimony sulfide may be formed in the solution. These precipitates, by coprecipitation, occlusion, and surface adsorption can be used to remove unwanted traces of activity. A scavenging agent should be chosen that wdll not carry down appreciable amounts of the carrier from solution. An alternative method is to add hold-back carriers for the unwanted traces of activity and precipitate the required element in their presence. 

Non-isotopic carriers have their uses as may be judged by the following example. Sr is made from by an (n, p) reaction. The addition of a lead salt followed by fuming nitric acid precipitates lead and strontium together as nitrates and leaves the yttrium in solution. The lead, having thus acted as a non-isotopic carrier, may be precipitated as sulphide from a solution of the lead and strontium salts. Here again a hold-back carrier of inactive strontium ions is necessary to prevent adsorption of most of the Sr on the PbS. 

An additional complication can arise in solution if the radioactive species in trace amounts react with trace concentrations of impurities. For example, in an investigation of the properties of pentavalent protactinium, Pa(V), it was foimd that the protactinium was extracted into pure xylene from 1 M HCIO4 solutions. Further experimentation showed that this extraction was due to the presence in the xylene of organic impurities at concentrations below the detectable limit of 0.01 %. Support for this interpretation was provided when the solution was made 10" M in thorium, which was expected to form complexes with the probable impurity, thereby preventing the reaction with the protactinium. In fact, no protactinium was extracted into xylene from this solution. The thorium in this case acts as a hold-back carrier. 

Studies of this kind are often easy to do. However, the Lu-acetylacetone system was chosen as it was not as simple, yet good results were obtained by careful use of the tracer technique Because the Lu concentration was 10" M, 10" M Nd " was added as hold-back carrier to avoid sorption losses ( 9.2.3). At the pH s that had to be investigated Lu (and... 

Precipitation with isotopic carrier Precipitation with non-isotopic carrier Precipitation with hold-back carrier Separation of the precipitations by filtration or centrifugation... 

Now for the carrier. (Fig. 8) It consists first of a 4 3/4 inch long tube to hold the Fang and another 31/8 inches long for the cover. However you want to measure the carrier tube, make sure the barb projects a bit over a half inch. If delivered by hand youll feel a sense of assurance to know that the fish line will be cut and the back of the tube will start the plunger on its way just in case you neglected to cut off the last ring on the plunger s tip. 

Spunbond processes and fabrics have been commercial for over 40 years. The processes, fabrics, and markets are quite varied. The best known spunbond fabrics and markets are cover stock for diapers and feminine hygiene products. However, spunbond fabrics are also found in carpet backing, geotextiles, furniture dust covers, carriers for fabric softeners, and a host of other industrial and consumer applications. The key to most of these markets has been a low cost covering material with sufficient tensile and opacity properties to fit the application. The latest market trend is towards pre-wetted wipes for cleaning and similar uses. The product needs for this fabric include short term abrasion resistance and ability to hold solvents. 

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