Quantity errors in HPTLC are often so large
that this excellent analytical tool is disqualified as applicable only for semi quantitative analyses.
On the other hand quantitative HPTLC - in its full instrumentalized version - is not replaceable for very many analytical tasks.
Well done it is as precise as the best HPLC method but more important: the danger of wrong identification is very much smaller than with HPLC. As HPTLC users repeat sampling and separation much more often than HPLC users, the danger of excessive quantitative errors is again much smaller with HPTLC than with HPLC. NOTE that structure identifying highly sensitive spectroscopic detectors are applicable on-line HPTLC even in their micro version. The first direct HPTLC/MS coupling was made by the author in the late sixties but there was not yet a real demand for at these early years.
The reasons, why HPTLC is in quite a good shape with respect to HPLC IF WELL DONE:
HPTLC users have for each sample a freshly unused “column”.
HPTLC users can do multiple simultaneous runs and critically compare data.
Well informed HPTLC users have thousand different and highly specific detection modes available and very many more mobile phases than HPLC users.
HPTLC users can easily multiply the separation power of a plate by multi dimensional separations - two dimensional runs are only a question of proper sampling and the use of proper plates and chambers.
HPTLC is highly sensitive and especially applicable to trace analyses.
As example: drinking water quality analysis MUST be done quantitatively, which is possible by HPTLC for more than hundred toxic traces simultaneously in the ppt-range of concentration (1 ppt = 1/1000 ppb; expressed as weight-concentration; 1 ppt equals 10 to the power of minus 10 weight %)
HPTLC users can easily and directly on-line combine chromatographic techniques with capillary GC, and HPLC/capillary HPLC. They can use HPLC as a quite helpful sample cleaning and sample preparation mode for the final HPTLC analysis.
HPTLC is unbeatable for environmental mass analysis of solid and liquid “dirt” which a HPLC user would never inject into his delicate instruments.
Although circular HPTLC is widely unknown, it is more powerful than linear HPTLC especially for micro preparative chromatography in its “rotation HPTLC mode (Sz. Nyiredy, see HPTLC literature)”.
HPTLC can be used under high pressure (easily up to 3000 kg mechanical forces on a 50 x 50 or 100 x 100 mm circular plate). There is also forced flow linear HPTLC available with promising possibilities for mass screening analyses not only in pharmaceutical research and development.
HPTLC is the only mode applicable in a technically underdeveloped area with missing electrical power and a minimum of space, especially to control product falsification in medicine.
HPTLC can be done in less than 30 seconds for up to 12 samples simultaneously by micro anti circular separation and in a few minutes for up to 80 samples simultaneously. The only problem is then that our to days computers are still too slow for a complete timely quantitation of such a high data stream.
To switch from classical HPTLC with large mobile phase troughs and large classical plates into high pressure circular micro planar chromatography = µ-PLC this is a question of seconds and quite a small investment, we could do this:
- take HPTLC plates
- replace grandfathers trough by a virtual trough in vertical position ( 1mm high, empty
(this can be seen after clicking a LINK in page µ-PLC of this site).
- use the gas phase with all its analytical power for improving drastically the plate clean up, the sampling,
the sample focusing to sharp sample bows
- use only one ml mobile phase in a one ml flask contacting the plate center through a wick.
- let separate and dry again completely with a 2 L/min gas flow from a micro pump (costs 10 Euro !)
- make a digital photo, crop it, enter it in µ-PLC integration software (it makes multiintegration and
reduces this way drastically the plate structure)
- and enjoy a data quality of easily a factor of fife to ten better than ever before. Soon are all details
in this site.
And why are much too often HPTLC data so badly reproducible, that this analytically most powerful technique has in many laboratories the level of a “semi quantitative” chromatography mode often even not anymore in use ?
* The main source of quantity errors in HPTLC standard as well as trace analyses is the plate structure. No existing HPTLC stationary phase is free from structure which kills the analytically possible data quality at any wave length or reduces it by a full factor of ten.
* A next source is the fundamentally non linear calibration function for each substance in any concentration range but the use of the “rule of three” evaluation mode or by the use of data transmission concepts.
* The third source is the quite low separation power of HPTLC resulting in substance overlapping which often causes quantitative side effects falsifying the correct quantity value.
* The fourth source is the still missing or widely underdeveloped planar quantitation techniques, starting with digital photography at any wavelength range but not yet ending with enough accurate and enough precise HPTLC quantitation software.
There are quite some further error sources which are in discussion in a series of publication in the only international PLC journal “JPC, Journal of Planar Chromatography - Modern TLC”;
Springer; ISSN 0933-4173.
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