Using Microwave Digestion and FAST Flame Sample Automation to Analyze Micronutrients in Fresh and Dried Fruits

As the world is trending towards healthier living and consumption of healthy foods, knowing the nutritional quality of foods has become more critical. In the absence of fresh fruit, dried fruits are often used as a substitute. Therefore, manufacturers and customers are interested in knowing whether dried fruit loses certain nutritional value during processing.

Measurement of micronutrient concentrations in fresh and dried fruit is one way of monitoring the fruit’s quality. Micronutrients are trace elements with nutritional value and can be measured through various inorganic analytical methods.

Although inductively coupled plasma optical emission spectroscopy (ICP-OES) is usually employed to measure multiple elements, flame atomic absorption (AA) spectrometry has emerged as an alternative due to its operational speed, simplicity and cost-effectiveness. Flame AA can measure multiple elements by analyzing a sample once for each of the desired elements.

As a result, sample throughput for Flame AA is slower than for ICP-OES when measuring multiple elements. This issue can be addressed through the use of a rapid, high-throughput sample automation system.

Despite the need for multiple analyses of each sample, the analysis time per sample is considerably reduced, thereby increasing the sample throughput when compared to manual sample introduction. Furthermore, the use of an automated sample introduction system enhances the accuracy of the analysis by minimizing human involvement.

This article demonstrates the analysis of common nutritional elements present in different fresh and dried fruits using the PinAAcle™ 900 atomic absorption spectrometer operating in flame mode coupled to a FAST Flame sample automation accessory.

Experimental Procedure

All the micronutrient analyses were carried out using the combination of the PinAAcle 900T atomic absorption spectrometer and the FAST Flame 2 accessory. Table 1 shows the desired elements and instrument conditions. The sample introduction system includes a high-sensitivity nebulizer, standard spray chamber, and a 10 cm burner head.

All calibration curves were prepared automatically in-line by the FAST Flames system from a single intermediate calibration standard in 10% HNO3/deionized water. La2O3, at a concentration of 0.5% by weight, was added to the diluents, standards and solutions to control ionization during the analysis of calcium (Ca), sodium (Na), and potassium (K).

Table 1. PinAAcle 900 instrument and analytical conditions

Element Cu Fe Mg Mn Zn K Na Ca
Mode Absorption Absorption Absorption Absorption Absorption Emission Emission Absorption
Wavelength (nm) 324.75 248.33 285.21 279.48 213.86 766.49 589.00 422.67
Slit (nm) 0.7 0.2 0.7 0.2 0.7 0.2 0.2 0.7
Acetylene Flow (L/min) 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.7
Air Flow (L/min) 10 10 10 10 10 10 10 10
Burner Head Rotation 0 ° 0 ° 0 ° 0 ° 0 ° 45 ° 0 ° 0 °
Acquisition Time (sec) 1 1 1 1 1 1 1 1
Replicates 3 3 3 3 3 3 3 3
Sample Flow Rate (mL/min) 6 6 6 6 6 6 6 6
Intermediate Standard (mg/L) 1 5 1 1 5 200 10 10
Auto-Diluted Calibration Standards (mg/L) 0.05
0.1
0.2
0.5
1
0.25
0.5
1
2.5
5
0.05
0.1
0.2
0.5
1
0.05
0.1
0.2
0.5
1
0.25
0.5
1
2.5
5
10
20
40
100
200
0.25
0.5
1
8
10
0.5
1
2.5
5
10
Calibration Curve Type Non-Linear Through Zero Non-Linear Through Zero Non-Linear Through Zero Non-Linear Through Zero Non-Linear Through Zero Non-Linear Through Zero Non-Linear Through Zero Non-Linear Through Zero

The FAST Flame 2 accessory includes a high-speed autosampler, peristaltic pump and switching valve that ensure rapid sample turnaround with no sample-to-sample memory effect, short signal stabilization times, and fast rinse-out.

The FAST Flame 2 quickly fills a sample loop using vacuum before the valve switches to inject the sample as the autosampler moves to the subsequent sample. This process avoids any time delay related to the self-aspiration or peristaltic pumping, and the long rinse-in and rinse-out times caused by autosampler movement and flushing. As a result, complete sample-to-sample analytical times are reduced to 15 seconds.

The software can be set to determine QC over-range samples. The samples that are out of the calibration range can be automatically re-run at an increased dilution factor using the in-line dilution capability of the FAST Flame 2, thereby adjusting the signal to fall within the calibration range. This, in turn, provides accurate measurement in addition to successful QC check.

The desired elements need to be isolated from the fruit into an instrument-ready solution to ensure accurate analysis. Open-vessel digestion that involves nitric acid and a simple heating block can be an effective solution for extraction, but this process may result in undigested matter, which would require further filtration or centrifugation before introducing into the instrument. This could lead to poor recoveries and accuracy.

Closed-vessel microwave digestion, however, provides complete sample digestion, avoiding the need for any extra filtration procedures and enabling maximum element recovery. As a result, it is possible to achieve more accurate analyses.

Fresh and dried fruit samples were prepared with the PerkinElmer Titan MPS™ microwave sample preparation system, a sample digestion oven featuring a unique vessel and system design with a special focus on ease of use, throughput, and safety.

The Titan MPS system incorporates non-contact temperature control for every vessel and pressure control using a reference vessel, thus ensuring accurate digestion method control and zero sample contamination irrespective of the sample type.

Table 2 shows the details of the microwave digestion method. Each vessel contained 10 mL of concentrated nitric acid and 0.5 g dried fruit or 1 g of fresh fruit. The spiking was carried out before sample digestion, while spike concentrations were chosen with respect to the expected sample concentrations.

Table 2. Titan MPS system digestion method

Method Step Target Temp (°C) Pressure Limit (bar) Ramp Time (min) Hold Time (min) Power Limit (%)
1 140 35 10 2 60
2 195 35 3 25 100
3 50 35 1 20 0

Results and Discussion

Using the in-line dilution capability of the FAST Flame 2, the calibration curves were built from a single intermediate standard. Table 3 shows the calibration results, which demonstrate the accuracy of the methodology through recoveries of the independent calibration verification (ICV) solution that are within 10% of the expected values.

Table 3. Calibration results

Element Correlation Coefficient ICV Concentration (mg/L) Measured ICV (mg/L) ICV (% Recovery)
Cu 0.99985 0.500 0.494 98.8
Fe 0.99999 2.00 1.98 99.0
Mg 0.99999 0.500 0.517 103
Mn 0.99995 0.500 0.495 99.0
Zn 0.99991 2.00 1.95 97.5
K 0.99860 100 96.7 96.7
Na 0.99865 5.0 4.55 91.0
Ca 0.99975 5.0 5.02 100

The results of the analyzed fruit samples are shown in Figure 1, with the fresh fruits displayed in orange and dried fruit shown in blue. It is evident from this plot that all the dried fruits contain higher nutrient concentrations than the fresh fruits.

Furthermore, the elemental concentrations among fruits vary, although potassium consistently has the highest concentration of all the elements. To obtain concentrations within the calibration range, the samples were automatically diluted by the factors given in Table 4, using the FAST Flame 2 accessory.

Results for dried (blue) and fresh (orange) fruit samples

Figure 1. Results for dried (blue) and fresh (orange) fruit samples

Table 4. In-line dilution factors

Fruit Cu Fe Mg Mn Zn K Na Ca
Dried Blueberry 1 1 20 1 2 2 1 5
Dried Strawberry 1 1 20 1 2 2 1 5
Dried Raspberry 1 1 20 1 2 2 1 5
Fresh Raspberry 1 1 20 1 2 2 1 5
Fresh Blueberry 1 1 20 1 2 2 1 5
Fresh Strawberry 1 1 20 1 2 2 1 5
Fresh Kiwi 1 1 20 1 2 2 1 5

All the samples were spiked at the levels shown in Table 5 to determine accuracy. The recoveries of all spikes were found to be within 10% of the calculated values for all elements as illustrated in Figure 2.

Per-sample matrix matching was not required in the spike recovery studies, corroborating the value and labor savings of employing the Titan MPS system to safely and completely digest the samples.

Table 5. Spike levels (all units in mg/kg)

Fruit Cu Fe Mg Mn Zn K Na Ca
Dried Blueberry 49.3 197 493 98.6 197 4880 195 488
Dried Strawberry 46.6 186 466 93.1 186 4930 197 493
Dried Raspberry 50.1 201 501 100 201 5236 209 524
Fresh Raspberry 19.6 78.6 196 39.3 78.6 2078 83.1 208
Fresh Blueberry 18.9 75.7 189 37.9 75.7 1850 74.0 185
Fresh Strawberry 21.0 83.9 210 42.0 83.9 1744 69.8 174
Fresh Kiwi 19.7 78.7 197 39.4 78.7 1991 79.6 199

Recovery of pre-digestion spikes for fresh and dried fruit samples

Figure 2. Recovery of pre-digestion spikes for fresh and dried fruit samples

The integration of the FAST Flame 2 accessory minimized the standard creation from one intermediate and five final standards to a single intermediate standard. Since the concentrations of a number of elements in various samples fell beyond the calibration curve, Syngistix™ for AA software flagged these elements and samples, which triggered the FAST Flame 2 to re-run the samples, but at a higher dilution.

The response of FAST Flame 2 to the over-range samples and the instrument’s ability to auto-dilute the samples in a precise and consistent manner without the interference of an analyst reduced the time consumption and avoided the need for additional sample handling and lengthy re-prep.

Conclusion

This article demonstrated that the PerkinElmer PinAAcle 900 AA spectrometer can be used for measuring Cu, Fe, Mg, Mn, Zn, K, Na, and Ca in fresh and dried fruit samples  in a reliable and effective manner.

Furthermore, the combination of the FAST Flame 2 accessory with the PinAAcle 900 eliminates human error while carrying out dilutions and making calibration standards, thereby improving throughput, aproviding excellent long-term stability and increasing productivity.

Moreover, the Titan MPS used for sample digestion avoids sample and matrix problems without specialized analytical parameters or matrix matching. Similar analyses can also be carried out for smaller sample batches in the absence of a FAST Flame 2 accessory.

This information has been sourced, reviewed and adapted from materials provided by PerkinElmer.

For more information on this source, please visit PerkinElmer.

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