Sample tubes, cleaning

As a general rule, APCI is less likely to demonstrate matrix effects and ESI is more likely to be affected by matrix effects. Sample clean-up is another important factor—protein precipitation is more likely to result in matrix effects than is solid phase extraction. Matrix effects may be caused by sample constituents that are not parts of the biological matrix. Mei et al.126 129 showed that certain brands of sample tube containers can produce matrix effects. They also demonstrated that Li-heparin, a common anticoagulant for plasma samples, can produce significant matrix effects... 

The sampling tube is made of heat-resistant glass and one end is drawn out to form a capillary injection needle (Figure 1). The other end of the tube is a glass cone (Quickfit 7/16) which allows a clean connection, without any gasket, to the vacuum pump or gas supply. The adsorbent, 0.3 g of TENAX TA, 35-60 mesh, is kept between plugs of glass wool. 

Procedure. Weigh 2 g of the oven dried sample ground to 1 mm into a porcelain or glass crucible and place in a cool muffle furnace. Increase the temperature to 550°C and ash for 2 h. Cool and dissolve in nitric acid solution, make up to 20 ml and store in a polythene capped tube. Allow to settle before transferring to another clean sample tube by decanting or using a polythene Pasteur pipette. Analyse the solution by AAS or ICP according to the manufacturer s recommendations. 

Clean sample tubes using very hot (almost boiling) 1% detergent solution to rinse out tubes, while brushing with a nylon brush, then boil 30 min in 1 % detergent solution. 

Transfer 2 mL of each neutralized (and filtered) sample into clean and labeled test tubes. Add 3 mL of a molybdate solution to each test tube and mix the contents. 

Standards were prepared by loading known amounts of vinyl acetate onto clean sampling tubes. Apparatus was assembled such... 

In the preferred procedure, samples of vinyl acetate on Chromosorb 107 were prepared. A glass U-tube was connected to the inlet end of a clean sampling tube, the outlet of which was connected to a small pump. As an aliquot of vinyl acetate in hexane was injected into the U-tube, the pump drew air through the system at 0.2 L/min such that the vapors were swept onto the sorbent bed. After 2-3 minutes, or approximately five volume changes, the pump was turned off and the apparatus was disassembled. Standard samples prepared in this manner were thermally desorbed and analyzed. A calibration curve thus... 

Caution All equipment must be scrupulously clean (for an acceptable cleaning procedure, see AOCS Official Method Cd 12-57). Do not use chromic acid or other acidic cleaning agents. All receptacles in the heater must be calibrated for temperature under the exact conditions of the test. During the test, the temperature must be monitored in a sample tube containing the recommended quantity of oil. 

Operation of the NMR instrument will be desoibed by the instructor. Particular care should be taken in inserting and removing the NMR sample tubes to prevent damage to the probe. All tubes should be wiped clean prior to insertion to avoid contamination of the probe. 

Place a clean, dry weighing boat or appropriately sized sample tube onto a simple two-decimal-place balance (see p. 22) and zero (tare) the balance and weigh about 9.80 g of the chemical. 

Remove the sample and container from the balance and tip the contents into a clean, dry beaker (400 mL), ensuring that there is no spillage outside the beaker. Do not attempt to wash out the sample tube with water. 

A break-seal attachment. This vacuum line attachment is used to recover samples sealed in relatively narrow tubes. The tubes are marked with a sharp glassknife so that, when they are inserted in the ground glass joint, passed through the stopcock key bore, and allowed to rest on the bottom of the envelope tube, the knife mark rests on the indentation in the envelope. After evacuation the crank is rotated slowly, as shown by the aiTow in Fig. A. 11. The sample tube will break cleanly at the knife mark. 

To check for possible contamination that might have occurred during sampling, samples were collected in the clean room, directly from the sampling tube after filtration. Tests were made on Cd, Co, Cu, Ni and Zn determined by ETAAS after APDC/DDDC complexation (Cd, Cu) or DPASV after UV-irradiation (Cd, Co, Cu, Ni and Zn). 

The quality of the tube (e.g., concentricity, wall thickness) is very important when very good resolution is required. Also, each type of probe has a lowest recommended quality sample tube and the use of lower quality tubes can lead to the damage of the probe. Each manufacturer specifies the quality parameters in terms of inner and outer diameters, waU thickness and concentricity. Each product has a specified tolerance for these parameters. Apart form the catalogue quality, the history of the tube is also very important (e.g., how it was cleaned, dried, stored, even what samples have been previously analyzed in it, as these operations might lower its quahty). 

Dissolve your compound CDCI3 (about 2mL) in a clean, dry sample tube. Use about 10 mg of sample for CW-NMR or 5 mg of sample for FT-NMR. Check to see if the solvent contains TMS if it does not, consult your instructor. 

Clean the sample tubes. You may leave the dry ice and acetone in the Dewar. 

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