Precautions Against Blocking

Besides the causes of increased pressure and blockages described in the previous section, there are two other causes. Firstly, solvent impurities, pump abrasion, etc., and secondly the sample itself. 

Classical sample preparation procedures such as filtration centrifugation, liquid-liquid extraction, and SPE are effective against blocking by the sample. The most important technology, however, is the guard column switching. The different variants are described in Section 2.7.1.3.5. 

The use of in-line filters can be very helpfid. Some stainless steel sieves and frits with a diameter of 1/16 are available. These can be placed directly into a 1 /16 screw coimection. Likewise, a set of micro couplings with filters inserted is available. Both the guard column and filter can worsen a separation after prolonged use, even if the pressure is only slightly raised. 

With ever smaller flow rates, recognizing leakages becomes increasingly difficult. Down to the lower pL range (2 pL min ), leakages can still usually be recognized visually, in particular if tissue paper is used to make the liquid visible. 

The most important indication of a leakage is the injecting peak. With constant flow rate and identical capillaries, it must always come at an identical time. If the time increases without a visible leakage, an increase in the flow rate is recommended, until about 80-90% of the maximum pressure of the column is reached. In this way, the leakage should become larger and more visible. 

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Lead is a toxic metal and rigid precautions against lead poisoning are essential. Sprayed lead is, like other sprayed metals, porous, and the sprayed layers will not as a rule withstand attack by strong acids. On the other hand, lead from 0-13 to 0-25 mm (0-005 to 0-01 in) in thickness has proved extremely useful in atmospheres containing sulphuric acid. In this case the pores in the lead become blocked with lead sulphate, with the result that complete protection is assured. In the event of mechanical breakdown, lead does not exhibit any sacrificial action and therefore corrosion may lift the lead layer. 

Verticillate epithelial keratopathy due to amiodarone, in which there is whorl-shaped pigmentation of the cornea, has been proposed to be worsened by soft contact lenses (SEDA-15, 171). Two patients with hard contact lenses and amiodarone-associated keratopathy both complained of increased sensitivity to sunlight and were fitted with ultraviolet light-blocking lenses instead, as a precaution against further corneal damage however, the authors did not think that the contact lenses had contributed to the damage (102). 

The collar sensors showed her she had picked up a considerable tumble since losing her grip on the rope ladder. Her neural nanonics had automatically blocked the impulses from her inner ears as a precaution against nausea. There were also a number of analgesic blocks erected across the nerve paths from her forearms. A physiological status... 

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