Part 10. Preparing a groundwater sample
Sampling may seem simple, but it is one of the most critical steps in ensuring that analytical results are meaningful.
Sampling may seem simple, but it is one of the most critical steps in ensuring that analytical results are meaningful. Even the most precise measurements can be misleading if the sample was contaminated, altered, or poorly preserved.
10.1 Collecting a representative sample
- Obtain permission from the owner or manager of the well before collecting water.
- If sampling from a tap or well outlet, let the water run for several minutes before collection if the system has not been used recently.
- Avoid touching the inside of the bottle or cap.
- Use a clean plastic bottle, ideally new or acid-washed.
- Rinse bottle 2-3 times with sample water before filling.
- Label the bottle immediately with sample ID, date, time, location, collector, and whether it has been acidified.
10.2 Preserving the sample
To prevent chemical changes after sampling and iron/manganese precipitation, the water is normally acidified. Acidification helps keep arsenic in solution and reduces the chance of arsenic binding to container walls or precipitated material.
Acidify groundwater samples to pH <2 using hydrochloric or nitric acid. Do not assume a fixed acid concentration is sufficient for all groundwater samples; verify the final pH after acid addition, especially for alkaline or carbonate-rich waters.
10.3 Preparing the measurement cell
The standard OpenArsenic measurement cell uses a final volume of 40 mL. A common preparation is:
| Component | Typical volume | Purpose |
|---|---|---|
| Groundwater sample | Variable | Contains unknown arsenic concentration. |
| 1 M HCl | 4 mL | Produces 0.1 M HCl final concentration. |
| Deionized water | Variable | Used to reach 40 mL total volume and adjust dilution. |
The volume of groundwater sample used depends on the expected arsenic concentration. Selecting an appropriate dilution is important because the OpenArsenic method has a limited linear working range. Excessively concentrated samples may produce non-linear responses and unreliable standard addition calculations.
Choosing an appropriate dilution
The OpenArsenic method uses two standard additions:
- +2.5 ppb arsenic
- +5.0 ppb arsenic
As a rule of thumb, the combined standard additions should approximately double the analytical signal by the end of the measurement sequence.
For this reason, it is generally desirable for the diluted sample entering the electrochemical cell to contain approximately 5 ppb arsenic. After the two standard additions, the final concentration will then be approximately 10 ppb, which is typically within the validated linear working range of the method.
Suppose a groundwater sample contains approximately 20 ppb arsenic. If the sample were analyzed without dilution, the measurement sequence would be:
• Sample: 20 ppb arsenic
• Standard addition 1: 22.5 ppb arsenic
• Standard addition 2: 25 ppb arsenic
In this situation, the standard additions increase the concentration by only 25%, producing relatively small signal changes and increasing the risk of non-linear behaviour.
A better approach would be a four-fold dilution:
• Sample: 5 ppb arsenic
• Standard addition 1: 7.5 ppb arsenic
• Standard addition 2: 10 ppb arsenic
Recommended Starting Point
If the arsenic concentration of a sample is unknown, begin with a moderately diluted sample and adjust based on the observed peak size.
A common starting preparation is: 4 mL 1 M HCl + 26 mL DI water + 10 mL sample = 40 mL total volume
If the resulting peak is very small or not visible, increase the sample fraction. If the resulting peak is very large, prepare a more dilute mixture.
Remember: The objective is not to maximize peak height. The objective is to obtain a signal that responds clearly and approximately doubles over the course of the two standard additions.