Metrohm describes a system that can be used for the automatic determination of fluoride in blood.
The system consists of an 855 Robotic Titrosampler with three 800 Dosinos for pipetting the blood samples and/or the NaF standards, adding TISAB or buffer solutions, titration with La(NO3)3 and a PC.
The Tiamo software package controls the system.
A right-side fitted, micro-titration vessel lid is equipped with an F ISE and reference electrode, magnetic stirrer and PFA titration vessel.
It also carries dosing tips, rinsing nozzles and an aspiration tip for aspirating spent sample solution to waste.
Rinsing and waste containers are fitted with 849 Level Control for continuous monitoring of rinsing and waste levels to prevent pumps from running dry and/or waste containers from overflowing.
Two different custom-built sample racks are used for the fluoride analysis.
Each of three different laboratories uses two automated systems with two different racks.
The first one is for glass vials (322 places) while the second one is for special McCartney bottles (107 places).
Two additional cap holding racks, specially developed for each of the two systems, store the caps during analysis as the samples are taken from either the opened glass vials or McCartney bottles.
The cap holding racks were required since each vial, once opened, must be closed with the original cap after analysis for ensuring sample integrity.
Glass sample vials are used for samples taken from alleged drunken driver offenders and McCartney sample bottles are used for post-mortem blood samples from various mortuaries for alcohol-related fatal accidents and for insurance purposes.
Incoming sample batches are logged on Lims and the sample data is wirelessly imported into Tiamo via sample tables.
The blood samples to be investigated are placed in the designated sample rack according to the relevant sample table.
The subsequent sample transfer, direct potentiometric measurement or titration, calculation of results as well as the rinsing and cleaning steps take place fully automatically.
Results are verified manually before exporting them to the Lims.
First, the transfer tube is completely filled with buffer solution (pH = 6).
Subsequently, a fixed sample volume (1.5mL aliquot for standards and samples alike), embedded between two air gaps, is aspirated from the sample vessel.
Afterwards, the sample is transferred into the external titration vessel using pipetting equipment that consists of the 800 Dosino with Dosing Unit (10mL) and a 10mL transfer tube with pipette tip.
Finally, the transfer tube is rinsed with 10mL buffer solution into the titration vessel and 10mL ultra-pure water is added.
Three certified NaF standards (0.8 per cent, 1.5 per cent, 3.0 per cent) are filled into PP vials and placed into three specially defined positions on the sample rack.
The corresponding true values are entered as common variable STD 1-3.
The sample series is started by recording a three-point calibration curve with certified NaF standards (beginning with 0.8 per cent).
The potential is measured under constant stirring and the mV value recorded.
Thorough cleaning of the sensors, magnetic stirring bar, tubing, buret tips and the vessel is then carried out.
This procedure is repeated with standards two and three (1.5 per cent and 3.0 per cent NaF, respectively).
A three-point calibration curve is automatically recorded.
The accuracy of the determination is verified after every 10 samples with a certified check standard (NaF content = 1.0 per cent).
The same sequence is followed for the determination of the fluoride content in blood samples.
With the specially designed sample rack for glass vials the automated system can analyse 317 samples in one continuous run; the time required for each blood sample is three minutes including sample transfer and rinsing.
In a similar way, the special rack for McCartney bottles allows 102 samples to be analysed in one run.
First, the transfer tube is completely filled with the buffer solution (pH = 6).
An air gap is created and a fixed volume (1.5mL aliquot) of the sample is aspirated from the sample vessel followed by another air gap.
The sample is transferred into the external titration vessel using pipetting equipment consisting of an 800 Dosino with Dosing Unit (10mL) and a 10mL transfer tube with pipette tip.
The transfer tube is rinsed with 10mL buffer solution into the titration vessel and 10mL deionised water is added.
The sample is titrated with c(La(NO3)3 = 0.1mol/L.
Endpoint evaluation is done using a Gran Plot.
The titration vessel is rinsed and emptied several times, after which the system is ready for the next analysis.
The same sequence is followed for the titer determination of the c(La(NO3)3 = 0.1mol/L, which is carried out using a certified standard (3.0 per cent NaF).
Most countries forbid operation of motor vehicles above prescribed levels of blood vehicle content (BAC).
BAC can be objectively measured and defines intoxication.
However, under the action of enzymes and bacteria the blood sugar can be fermented into ethanol, which increases the BAC.
The addition of a known amount of fluoride, which acts as an enzyme inhibitor, suppresses alcohol formation.
Therefore, in some countries the BAC value and the fluoride content of the sample has to be reported.
It has been shown that the fully automated determination of fluoride in blood samples can be carried out by precipitation titration with La(NO3)3 or, at lower concentration, with the standard addition method.
Thanks to constant pipetting and measurement procedures, the presented system offers more accurate results.
