In this work, the feasibility of infrared thermal imaging (ITI) is demonstrated to show its potential application in analytical chemistry. A system of ITI was combined with disposable microplates to perform enthalpimetric analysis, which was selected as an example in order to show the reliability of this method. In this way, the novel thermal infrared enthalpimetry (TIE) method was evaluated in neutralization, precipitation, redox, and complexation reactions, with a multichannel pipet for adding the reagent and an infrared camera to monitor the temperature of multiple reactions (up to 24 simultaneous reactions) in a contactless way. Analytical signals were obtained in only 10 s, and the difference in temperature (ΔT) before and after the reaction was used for the construction of calibration curves by use of reference solutions. More than 10,000 values were considered for the temperature determination for each reaction. The proposed method was applied for determination of the total acidity of vinegar as well as the chloride, iron, and calcium content of pharmaceuticals. The results were compared with those from conventional techniques (titration), and agreement between 96% and 101% was obtained. Sample throughput could even reach thousands of samples analyzed in 1 h. These preliminary results demonstrate the important features of TIE and possible application for other matrices and analytical parameters. The proposed TIE could be spread to cover other enthalpimetric techniques, different reactors (e.g., microfluidic and paper analytical devices), and portable devices, thus reaching other fields of chemistry.
Sample preparation and determination steps were performed in microplates using infrared thermography for the residual KOH determination (heat of neutralization).
In
this work, the in-tip thermal infrared enthalpimetry (in-tip
TIE) method is proposed for fast enthalpimetric analysis. In this
method, the reactions inside the tips of a multichannel pipette were
combined with temperature monitoring by an infrared camera. The filter
paper was used inside the tips to retain reagents as solutions (wetted
paper mode) or as solids (dried paper mode) to perform neutralization,
redox, or precipitation reactions. The dried reagents inside the tips
were obtained by oven drying a solution retained in the filter paper.
The determination of the total acidity of the vinegar, ascorbic acid
in vitamin C tablets, and chloride in soy sauces and saline inhalation
solutions was performed as examples of the application of the proposed
method. The agreement with reference methods ranged from 98 to 107%.
The use of reagents dried inside the tip was feasible, leading to
a simple aspiration of sample solution within the 12 tips of the pipette
to perform a rapid analysis (1 min). Therefore, up to 720 measurements
in 1 h were feasible for in-tip TIE over up to 12 measurements for
the reference methods. Moreover, miniaturization reduced reagent consumption
and residue generation. For example, for in-tip TIE, only 3.6 mL of
residues was generated (n = 12) over 60–240
mL in reference methods (n = 3). Contrarily to other
TIE methods, no microplates or stirring was required, opening possibilities
for field analysis since the multichannel pipette and the infrared
camera are both operated with batteries.
Content uniformity (CU) is a parameter used to ensure the uniformity of the amount of active substances in pharmaceutical preparations. It is often evaluated in the quality control of pharmaceutical dosage forms, and a novel method was proposed for CU determination using thermal infrared enthalpimetry (TIE). Captopril tablets were dissolved in 24-well microplates and the heat released from the reaction of captopril and I 2 was monitored with an infrared camera. Some experimental parameters were adjusted, as captopril/I 2 ratio, stirring speed and dispensing rate, and the most suitable conditions were 1.00:1.00 (v/v), 250 rpm and 0.57 mL s-1 , respectively. Interferences from tablet adjuvants (e.g., starch, cellulose and lactose) were not observed. The results obtained by TIE did not differ statistically (Student's t-test, p > 0.05) from those from official method (UV spectrophotometry) described in pharmacopeia. The TIE method required 31 and 12 times less reagent and time, respectively. Therefore, the proposed method was suitable for routine analysis and agrees with the requirements of green analytical chemistry.
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