6
Presented in Figure 6 is a typical example of mixing baseline noise for Case 1 appearing as a repeating sinusoidal pattern superimposed over baseline drift. Baseline drift is a slow increase or decrease of the background signal. It typically appears to be decreasing if the system was not allowed to equilibrate long enough but can appear as random drift even when the system is fully stabilised. The amount of this baseline drift tends to increase when the system is operating under steep gradient conditions or at higher back pressures. It is difficult to compare sample to sample results when this baseline drift is present, and this was overcome by applying a low-pass filter to the raw data to filter out these low frequency variations providing oscillation plots with flat baselines. Also shown in Figure 6 is a plot of the mixer baseline noise after the low-pass filter was applied.
Test Results
Upon completion of CFD modelling and initial experimental testing, three separate static mixers were subsequently developed utilising the internal structures noted above with three internal volumes, 30 µL, 60 µL, and 90 µL. This range covers the range in volumes and mixing performance needed for the majority of low level analyte HPLC testing where enhanced mixing with low dispersion is required to produce a low amplitude baseline. Presented in Figure 7 are the results of baseline sine wave measurements taken from the test system for Case 1 (Acetonitrile with ammonium acetate as a tracer) shown using the three volumes of static mixers along with no mixer installed. The experimental test conditions for the results shown in Figure 7 were held constant for all 4 tests following the procedure outlined in Table 1 with a solvent flow rate of 0.5 ml/min. Offset values were applied to the data set so they could be displayed next to each other without signal overlap. The offset does not affect the amplitude of the signal which is used to rate the mixer performance levels. The average amplitude of the sine wave with no mixer installed was 0.221 mAu with the amplitude dropping to 0.077, 0.017, and 0.004 mAu for the Mott 30 µL, 60 µL, and 90 µL static mixers, respectively.
static mixers achieve up to 98.1% reduction baseline sine wave, significantly outperforming commonly available mixers in use for the HPLC industry under these test conditions.
Figure 9. Plots showing offset HPLC UV detector signal versus time for Case 2 (Methanol with acetone as a tracer) showing solvent mixing with no static mixer (union), new line of Mott static mixers and a two commonly available mixers (actual data offset by 0.11 (No Mixer), 0.22, 0.3, 0.35 mAu respectively to best display results).
Seven commonly available mixers in the industry were also evaluated. These included three mixers of different internal volumes from each of Company A (labelled Mixer A1, A2 and A3) and Company B (labeled Mixer B1, B2 and B3). Only one size was evaluated from Company C.
Table 2. Static Mixer Mixing performance and Internal Volumes Static Mixer
Case 1
Sinusoid Reduction: Acetonitrile testing (Efficiency)
Figure 7. Plots showing offset HPLC UV detector signal versus time for Case 1 (Acetonitrile with ammonium acetate tracer) showing solvent mixing with no mixer, and Mott 30 µL, 60 µL, and 90 µL mixers installed showing improved mixing (smaller signal amplitudes) as the volume of the static mixer is increased (actual data offset by 0.13 (No Mixer), 0.32, 0.4, 0.45 mAu respectively to best display results).
Presented in Figure 8 is the same data displayed from Figure 7 but this time including the results for three commonly available HPLC static mixers, which have internal volumes of 50 µL, 150 µL, and 250 µL.
No Mixer Mott 30 Mott 60 Mott 90 Mixer A1 Mixer A2 Mixer A3 Mixer B1 Mixer B2 Mixer B3 Mixer C
- 65%
92.2% 98.1% 66.4% 89.8% 92.2% 44.8% 93.2% 96.9% 97.2%
Case 2
Sinusoid Reduction: Methanol Water test (Efficiency)
-
67.2% 91.3% 97.5% 73.7% 91.6% 94.5% 45.7% 84.5% 96.2% 97.4%
0
30 60 90 50
150 250 35
100 370 250
Examination of the results in Figure 8 and Table 2 show that the Mott 30 µL static mixer has a similar mixing efficiency to the Mixer A1, with 50 µL; however, Mott 30 µL has a 30% smaller internal volume. When the Mott 60 µL mixer was compared to the Mixer A2, with 150 µL internal volume, a slight improvement in mixing efficiency is observed - 92% versus 89%, but more importantly, this higher level of mixing is performed with 1/3 the volume of the comparable Mixer A2. The performance of the Mott 90 µL mixer compared to the Mixer A3, with 250 µL internal volume follows a similar trend. Improved mixing performance of 98% versus 92% is also observed along with an internal volume that is 3 times smaller. Similar results and comparisons can be observed with Mixers B and C. Thus, the new line of Mott PerfectPeakTM
static mixers achieves improved mixing efficiencies
over comparable competitors’ mixers, but with smaller internal volumes, thereby providing improved background noise, better signal to noise ratios, better analyte sensitivity, peak shapes, and peak resolution. Similar trends in the mixing efficiency were observed in both Case 1 and Case 2 studies.
For the Case 2 study using (Methanol with acetone as a tracer) testing was performed to compare the mixing efficiencies of the Mott 60 mL, the comparable Mixer A1 (with 50 µL internal volume) and comparable Mixer B1 (with 35 µL internal volume). As expected the performance when no mixer installed was poor but is used for a baseline of analysis. The Mott 60 mL mixer was the best performing mixer of the test group with a 90% increase in mixing efficiency. The comparable Mixer A1 mixer followed with a 75% increase in mixing efficiency followed by the comparable Mixer B1 with 45% improvement.
Figure 8. Plots showing offset HPLC UV detector signal versus time for Case 1 (Acetonitrile with ammonium acetate as a tracer) showing solvent mixing with no static mixer, new line of Mott static mixers and a three commonly available mixers (actual data offset by 0.1 (No Mixer), 0.32, 0.48, 0.6, 0.7, 0.8, 0.9 mAu respectively to best display results).
The percentage reduction in baseline sine wave was computed by taking the ratio of the sinusoid amplitudes to the amplitude with no mixer installed. Presented in Table 2 are the measured percentage sinusoid reduction, for Case 1 and 2, and internal volumes for the new static mixers along with seven standard mixers commonly used in the industry. The data in Figures 8 and 9, and the calculated results presented in Table 2, show that the Mott
Baseline sine wave reduction testing as a function of flowrate was conducted on the mixer series under the same conditions as the Case 1 sinusoid tests, changing only the flowrate. Over the flow rate range of 0.25 to 1 ml/min, the data shows that the baseline sine wave reduction remains relatively consistent for all three mixer volumes. For the two smaller volume mixers, there is a small rise in sinusoid reduction with decreasing flow rate, which is expected due to the increased residence time of the solvents within the mixer allowing for greater diffusional mixing. It is anticipated that the sinusoid deduction will further increase as the flow rates are further reduced. However, for the largest mixer volume, which had the highest baseline sine wave reduction, the baseline sine wave reduction was basically unchanged (within the limits of experimental uncertainty) with values ranging from 95 to 98%.
Internal Volume (µL)
INTERNATIONAL LABMATE - JANUARY/FEBRUARY 2017
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