* The water sample flows from a 1.5 ml micro flask through the wick accurately into the center of a top pre cleaned HPTLC plate of 100 x 100 mm size at room temperature.
* The PTFE tube does not directly touch the layer. A few hundred µm thick wick part looking out of the tube end assures the sample flow but avoids any mechanical plate layer damage. NOTE: the touch pressure is given only by the weight of the sample micro bottle - about 4.5 grams. The wick itself has only 33 mg weight corresponding to a 20 times 11 mm piece of viscose felt fabric area.
* The wick permeability is adjusted to a water sample transfer flow of about 50 µl / min. Water samples are so viscous that well permeable wicks are necessary. The permeability is measured by the back pressure of a 2 liter per min air flow from the µ-PLC gas pump. How this is done and for quantitative data see under “Making a µ-PLC instrument”.
* Air from a clean air room is blown by a strong hair dryer with removed heating wires at full speed onto the open plate surface with the blower tube symmetrically adjusted above the sample bottle in about 200 mm distance from top - see photo at the end of part 1. The heat generated by the blower motor warms up the blowing air from room temperature by about 2 to 3 degree centigrade.
* After about 20 minutes we see the one ml water sample now only as a wet circular area of about 20 mm diameter - this part of the sample water is much colder than the blowing air because of the evaporation energy taken. Organic water impurities remain sharp at the outer border of the tube holding ring. When the sample bottle is empty it is taken out of the tube holder ring and the wet sample area is dried to completeness by the still blowing dryer.
* Now the sample bottle is filled with one ml freshly distilled cleanest methanol. Under still flowing dryer air the bottle is reinserted into the center PTFE holder ring. Because of the lower viscosity the methanol flow is now larger than 50 µl/min, thus the residual sample parts and all remaining traces in the transfer wick are quicker transferred into the circular area of the adsorbed water impurity substances. The transmitting methanol dries off sharply outside the PTFE bottle holder ring, focussing possible substances from the water sample.
* Depending on the sample impurity level we can transfer instead of a 100% water sample mixtures of water with volatile solvents like methanol, ethanol, butanol, acetone, diethyl ether. This lowers of course the best possible detection / quantitation level but may avoid chemisorption of impurities on glass and/or the wick material.
* The next steps are standard µ-PLC procedures checking
- the sharpness and position of the focussed sample start circle,
- the detectability conditions,
- the separation procedure for very first raw group separations -
like highly non polar - highly polar,
- photo documentation,
- add-on of known quantities of external standards, locally brush sampled inside
selected positions of the first focussed sample circle and refocussed onto the final
water sample circle prior separation steps,
- final photo transfer to the multi-integration procedure
BUT if necessary or advisable only.
It is NOT advisable under the following conditions:
When checking the water quality as DRINKING water we may find on silica gel chemisorbed materials. They fluoresc and may not be movable by any solvent out of the about 5 mm large wick-to-layer touch area in the plate center. We may find under UV and under FLUORESCENCE conditions detectable impurities as a strong more or less thick substance circle at the former outer border of the bottle holder ring.
In this case further analytical effort is useless.
Because: if we see ANYTHING on the chromatogram area AND got before and after the water analysis the necessary critical BLIND run with zero signal results, the tested water is not of drinking quality.
We do NOT need any chemical identification, no MS/MS effort or what so ever.
This “No need for further effort” analysis is fast, economical and safe. It is based on the main condition of drinking water: it MUST be free from anything above the detectability limit given by µ-PLC with UV-signal control. This is the ppb/ppt-level for organic substances.
However we may use such “No Drinking Water” chromatogram data to support clean-up-techniques and installations. In this case it may be necessary to identify parts of the signals. They may correlate with repairable problems in the clean-up process and thus help to identify error sources like not yet correctly cleaned pumps or other hardware parts.
If the drinking water data found by the water-µ-PLC concept given here show ZERO substance signals we however need further effort, because we cannot state, the water is OK.
Because now we may be confronted with possible problems or systematic analytical errors:
> The water is possibly not of drinking quality because of bio impurities
(bacteria and much more). This type of impurities has to be checked always in addition
after high tech filtration and micro UV light treatment.
> Unknown substances may chemisorb on glass when transferred into the micro bottle or
later on the transfer wick material when flowing into the layer.
As the sample bottle and the wick however are flushed by volatile solvents after the
water transfer into the layer and the flushed materials are focussed inside the first
sampling area this error might not be of large impact. But who knows.
> The impurities cannot be detected - by NON of the up to hundred HPTLC detection
reactions including gas reactions after the final focussing step.
But still critical substances may pollute the drinking water.
> The detection limits are much larger for up to now not detected impurities than the
ppb/ppt values basically possible when one gram sample concentrates in a narrow
sharp layer area. Therefore other analytical techniques are still necessary.
Modern analytical development in the water analysis area is more and more directed to micro-enrichment techniques (as an example: W. DUENGES: 20 ml water sample based micro enrichment procedures) prior HPLC separation online MS and other detection systems.
If water has by chemical structure known detectable impurities - special organic traces - visible under UV by absorption or by fluorescence emission - and if the material amount of those traces is in the range of a few nano gram concentration per one gram of water sample, the quantition limit of these substances reaches 10 to the power of -7 weight-percent, which represents the ppb-level. However further effort depends on the special job to be done. If it is only the drinking water test, we do not need quantitation as discussed above. If the µ-PLC is taken to develop or improve clean up processes, quantitation is needed and easily possible by multi integration of the digital photo data. Quantitation problems however remain in case the impurities are multi substance mixtures of unknown structure like original medical residuals and metabolic substances.
See here .
For the simple drinking water test the use of an UV-lamp and light filtered photography is completely sufficient. If we see strong signals, the “Do not drink it” decision is mandatory. For special jobs like mentioned above we may need other detection modes easily applicable to the 100 x 100 mm plate with central circular signals. This includes MS. But as gas reactions may also lower the detectability limits it is a simple task with the µ-PLC concept. The plate - aluminium foil based - can be heated up on a hot area, is covered with the standard glass plate with central gas inlet open to all sides and the gas stream flowing from the aquarium pump can transfer the reaction chemicals. This is easily possible under inert gases due to the flatness of the “instrument”.
At the end of this “part 1” - µ-PLC trace analysis :
it may look like the concept is analytically quite weak. One however must realize, that the state of the art of water analysis by regulated standard techniques is critical with respect to the 100 % rate of knowledge: All enrichment techniques used up to now fail with respect to completeness besides low levels of yield. Highly polar traces with chemisorption character we have seen already at the very first experiments are not known by now in standard techniques. They are not extractable by solvents but only when using solid adsorption materials. And how to detect them when fixed on solids ? The standard water analyst may now know what to do. Taking the water sample as such for enrichment directly into the analytical system is promising but the first results were disappointing with respect to the drinking water quality. Especially shocking were some results found with water samples from bathes in connection with medical care institutions.