DILUTION MODULE continued a b
Figure 2 – a) Dependence of diluted solution on flow rate of microreactor. Split factor (3) and dilution flow rate (0.6 mL/min) were fixed. Only the micro- reactor flow rate (or sample flow rate) was varied from 0.6 mL/min to 6 mL/min. Test compounds: OS and 4-MOS in MeOH. b) Dilution with flow compen- sation approach with the microreactor flow rate from 0.6 mL/min to 6 mL/min.
to achieve the desired dilution ratio (1:100). Results for the modified dilution approach are presented in Figure 2b, which shows improved stability from flow rate fluctuation of the microreactor.
Experimental Materials and methods
Acetonitrile ROTISOLV and methanol (both HPLC gradient grade) were purchased from Roth (Karlsruhe, Germany); octanoic acid (OS) (>99%) was obtained from Merck-Schuchardt (Hohenbrunn, Germany) and 4-methyloctanoic acid (4-MOS) (>98%) was from Sigma-Aldrich (a part of
MilliporeSigma, Darmstadt, Germany). Concentrations of OS and 4-MOS in MeOH solvent were prepared as follows: stock solution 1 (low con- centration) contained 91 mg/L OS and 45.5 mg/L 4-MOS; stock solution 2 (medium concentration) 182 mg/L OS and 91 mg/L 4-MOS and stock solution 3 (high concentration) 273 mg/L OS and 136.5 mg/L 4-MOS.
For every concentration of the stock solution, four dilution ratios were performed in descending order—1:500, 1:300, 1:100 and 1:50. With every dilution ratio, different reaction stages of 3, 6 and 12 min were selected, which corresponded to microreactor flow rates of 4, 2 and 1 mL/min. These
Table 1 – Automatic and manual dilution of OS in MeOH (run 2; solution: 182 mg/L OS + 91 mg/L 4-MOS in MeOH) Dilution ratio
1:500
Automatic dilution Average peak area (4 mL/min) Average peak area (2 mL/min) Average peak area (1 mL/min) Average of all peaks STD
STD in %
Expected quotient Actual quotient
Manual dilution Average peak area (4 mL/min)
Average peak area (2 mL/min) Average peak area (1 mL/min) Average of all peaks STD
STD in %
572,316 558,824 517,558 549,566 78,059 14.20 1 1
833,103 823,043 830,081 828,592 83,829 10.12
AMERICAN LABORATORY 1:300 932,136
1,063,682 1,191,181 1,062,333 152,834 14.39 1.67 1.93
1,348,170 1,488,325 1,339,397 1,391,964 149,546 10.74
1:100
3,677,616 2,819,612 2,909,122 3,168,592 442,335 13.96 5
5.77
3,891,372 3,965,688 3,838,126 3,900,474 293,239 7.52
38 JANUARY/FEBRUARY 2017
1:50
5,962,748 5,963,894 6,054,573 5,993,738 257,865 4.30 10
10.91
6,926,684 6,886,017 6,876,310 6,895,291 596,194 8.65
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