Increasing Sample Throughput When Analyzing Major Elements in Drinking Water

Knowing the quality of our drinking water is a serious concern to most people around the world. In addition the continuous expansion of related regulations has made the capability of quantification and monitoring of contaminants in drinking water highly significant. Flame atomic absorption (AA) is a valuable technique for monitoring major elements in drinking water in areas where system cost, simplicity, and accessibility are concerns.

Complying with regulatory standards is an essential requirement that must be met at all times for municipal and bottled water. Productivity, precision, and accuracy are the essential qualities for laboratories to process samples successfully without additional sample handling or excessive re-prep.

There can be significant variation in elements of interest between regions and regulatory bodies. This article discusses the analysis of some commonly regulated high-level elements in various drinking water samples, including bottled and municipal water. The elements of interest with their regulatory limits are listed in Table 1.

Table 1. Select elements in drinking water

Element Limit (mg/L)
Copper (Cu) 1.3
Iron (Fe) 0.3
Magnesium (Mg) 50
Zinc (Zn) 5
Sodium (Na) 200
Calcium (Ca) 200

Experimental Procedure

A PerkinElmer PinAAcle™ 900T atomic absorption spectrometer running in flame mode was used to perform all analyses, although PinAAcle 900H, 900F, and 500 models could also be used to obtain equivalent results. Table 2 outlines the instrumental conditions used in the analyses. Important instrumental components used include a high-sensitivity nebulizer, a 10 cm burner head, and a standard spray chamber.

Table 2. PinAAcle instrument and analytical conditions

Parameter Cu Fe Mg Zn Na Ca
Wavelength (nm) 324.75 248.33 285.21 213.86 589.00 422.67
Slit (nm) 0.7 0.2 0.7 0.7 0.2 0.7
Air Flow (L/min) 2.5 2.5 2.5 2.5 2.5 2.5
Acetylene Flow (L/min) 10 10 10 10 10 10
Acquisition Time (sec) 3 3 3 3 3 3
Replicates 3 3 3 3 3 3
Sample Flow Rate (mL/min) 6 6 6 6 6 6
Intermediate Standard (mg/L) 1 2 1 2 20 20
Auto-Diluted Calibration Standards (mg/L) [0.05]
[0.1]
[0.25]
[0.5]
[1]
[0.1]
[0.2]
[0.5]
[1]
[2]
[0.05]
[0.1]
[0.25]
[0.5]
[1]
[0.1]
[0.2]
[0.5]
[1]
[2]
[0.5]
[1.0]
[2.0]
[5.0]
[1.0]
[2.0]
[5.0]
10.0]
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

A single intermediate standard prepared in 2% HNO3 and deionized water was used to perform external calibrations. This standard was then diluted using the in-line dilution capability of the PerkinElmer FAST Flame 2 sample automation accessory. After acidifying the water samples with nitric acid, sample analyses were performed directly with no further preparation except spiking.

The FAST Flame 2 accessory consists of a switching valve, peristaltic pump, and high-speed autosampler. Besides providing rapid sample turnaround with quick rinse-out, the FAST Flame 2 accessory eliminates memory effect between samples and ensures short signal stabilization times.

The sample loop is rapidly filled with a vacuum and then sent to the instrument for analysis while the autosampler moves to the next sample.  In this manner the wait time for self-aspiration or peristaltic pumping and the prolonged rinse-in and rinse-out times associated with an autosampler were eliminated, thus shortening complete sample-to-sample analytical times to 15 seconds.

The in-line dilution capability of the FAST Flame 2 accessory enables automated generation of all required calibration standards in-line from a single intermediate standard, minimizing the amount of time the analyst spends preparing standards.

It is also possible to set the FAST Flame 2 accessory to determine QC over-range samples. With this capability, samples whose concentrations are higher than the highest calibration standard are automatically diluted further and re-run to bring the signal within the calibration range. The accuracy of this approach was demonstrated at lower-than-regulated levels by spiking the water samples at levels much higher than the regulatory guidelines.

Experimental Results

The in-line dilution capabilities of the FAST Flame 2 accessory were used to create the calibration curves from a single intermediate standard. The corresponding calibration results given in Table 3 show the excellent correlation for the calibration standards, demonstrating the advantage of the automatic in-line sample and standard dilution capabilities of the FAST Flame 2 accessory. The independent calibration verification recoveries corroborate the validity of the calibration and accuracy of the standards created through the dilution system.

Table 3. Calibration results

Element Correlation Coefficient ICV Concentration (mg/L) Measured ICV (mg/L) ICV (% Recovery)
Copper (Cu) 0.99994 0.500 0.494 98.8
Iron (Fe) 0.99989 1.00 0.979 97.9
Magnesium (Mg) 0.99999 0.500 0.508 102
Zinc (Zn) 0.99999 1.00 1.03 103
Sodium (Na) 0.99997 5.00 5.05 101
Calcium (Ca) 0.99954 5.00 5.34 107

The results from the water sample analyses are presented in Tables 4 to 9. There was a significant variation in the concentration of magnesium, sodium and calcium in the waters.  For those samples where certain elements read beyond the calibration range, the FAST Flame 2 automatically diluted the samples so that they would fall within the calibration range, as indicated in the “In-line Dilution Factor” column in Tables 4-9.

Table 4. Magnesium in drinking water

Sample Measured Concentration (mg/L) In-line Dilution Factor Measured Sample + Spike Concentration (0.250 mg/L Spike) % Spike Recovery
Municipal Water A 0.062 1 0.311 99.6
Municipal Water B 1.09 5 1.34 99.6
Bottled Water A 0.010 1 0.262 101
Bottled Water B 0.232 1 0.487 102
Bottled Water C 1.15 4 1.40 101
Bottled “Mineral” Water A 0.354 2 0.611 103
Bottled “Mineral” Water B 6.52 10 6.75 93.2

Table 5. Iron in drinking water

Sample Measured Concentration (mg/L) In-line Dilution Factor Measured Sample + Spike Concentration (0.500 mg/L Spike) % Spike Recovery
Municipal Water A 0.136 1 0.654 104
Municipal Water B 0.008 1 0.538 106
Bottled Water A 0.018 1 0.522 101
Bottled Water B 0.000 1 0.509 102
Bottled Water C 0.037 1 0.510 94.6
Bottled “Mineral” Water A 0.057 1 0.547 98.0
Bottled “Mineral” Water B 0.048 1 0.524 95.2

Table 6. Copper in drinking water

Sample Measured Concentration (mg/L) In-line Dilution Factor Measured Sample + Spike Concentration (0.500 mg/L Spike) % Spike Recovery
Municipal Water A 0.100 1 0.600 100
Municipal Water B 0.187 1 0.683 99.2
Bottled Water A 0.004 1 0.499 99.0
Bottled Water B 0.006 1 0.476 94.0
Bottled Water C ND 1 0.494 102
Bottled “Mineral” Water A 0.002 1 0.463 92.2
Bottled “Mineral” Water B 0.000 1 0.463 92.6

Table 7. Zinc in drinking water

Sample Measured Concentration (mg/L) In-line Dilution Factor Measured Sample + Spike Concentration (0.500 mg/L Spike) % Spike Recovery
Municipal Water A 0.080 1 0.589 102
Municipal Water B 0.002 1 0.500 99.6
Bottled Water A ND 1 0.492 99.0
Bottled Water B ND 1 0.500 101
Bottled Water C ND 1 0.502 101
Bottled “Mineral” Water A 0.003 1 0.520 103
Bottled “Mineral” Water B ND 1 0.498 100

Table 8. Sodium in drinking water

Sample Measured Concentration (mg/L) In-line Dilution Factor Measured Sample + Spike Concentration (10.00 mg/L Spike) % Spike Recovery
Municipal Water A 9.55 10 19.0 94.6
Municipal Water B 20.3 15 30.0 97.1
Bottled Water A 2.84 10 12.3 94.1
Bottled Water B 0.996 15 10.9 98.8
Bottled Water C 15.6 10 25.2 96.2
Bottled “Mineral” Water A 9.81 10 19.3 94.9
Bottled “Mineral” Water B 37.4 25 47.1 97.7

Table 9. Calcium in drinking water

Sample Measured Concentration (mg/L) In-line Dilution Factor Measured Sample + Spike Concentration (10.00 mg/L Spike) % Spike Recovery
Municipal Water A 7.18 5 17.2 100
Municipal Water B 17.9 10 27.8 99.3
Bottled Water A 2.08 5 12.5 105
Bottled Water B 0.027 5 10.0 100
Bottled Water C 5.24 5 15.2 99.1
Bottled “Mineral” Water A 146 40 * *
Bottled “Mineral” Water B 10.9 10 21.2 103

*For Bottled “Mineral” Water A, the Ca concentration was too high to obtain a meaningful spike recovery.

The calcium concentration in the Bottled "Mineral" Water A sample was high enough that the spike must have been greater than 10 mg/L for ideal recovery. In all other cases, the water samples met the regulatory limits of elements analyzed.

Other than the calcium in Mineral Water A, the spike recoveries were all within 10% of the spiked values, even for those spiked at much below the regulatory limits, demonstrating the accuracy of the methodology. Moreover, the FAST Flame 2 accessory minimized human error by reducing standard preparation from one intermediate and five final standards to a single intermediate standard.

Furthermore, the FAST Flame 2 accessory was also able to respond to the over-range samples and accurately and precisely dilute the samples automatically with no operator interference, thus shortening time and removing additional sample handling and re-prep.

The long-term stability of the methodology is shown in Figure 1 for magnesium spiked into Bottled Water B, where the results are mostly within 1% of the mean value and the 10 minute running mean does not vary by more the +/- 1%. The absence of a rising or falling trend demonstrates the ability of the PinAAcle 900 AA spectrometer to perform continuous sample analysis and successful QC checks well beyond 4h without recalibrations and analyst interaction.

Four-hour stability of Mg spiked at 0.600 mg/L in Bottled Water B.

Figure 1. Four-hour stability of Mg spiked at 0.600 mg/L in Bottled Water B.

Conclusion

The results clearly demonstrate the capability of the PinAAcle 900 AA spectrometer to perform reliable and effective analysis of drinking water samples for elements of interest over a wide range of concentrations. The combination of the PinAAcle and the FAST Flame 2 sample automation accessory provides the following advantages:

  • Automated dilutions
  • Creation of calibration standards from a single intermediate standard
  • Increased throughput
  • Outstanding long-term stability
  • Optimized productivity for laboratories

For smaller sample batches, the same analyses can be performed without using the FAST Flame 2 sample automation 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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