ICP Analysis

The following discussion addresses practical aspects of ICP analysis as it pertains to the study and/or care of aquatic habitats and/or in the care of hydroponic / aquaponic systems. For assistance interpreting an ICP report, please email us the link or pdf and be sure to include net system volume and alkalinity value (at the time of drawing the sample) if the ICP provider does not include this parameter in their analyses.

The current gold standard for determining ionic content in an aqueous sample, particularly in the case of seawater, is inductively coupled plasma ("ICP") analysis, further divided into approaches that utilize atomic emission (ICP-AES), optical emission (ICP-OES), and mass spectrometry (ICP-MS).

Within very small margins of error, ICP analysis is capable of reporting the presence and concentration of various elements of interest (certainly, to aquaculture, horticulture, and research professionals studying aquatic organisms and/or habitats) when titrimetric and/or colorimetric analysis methods lack the necessary resolution or are generally unavailable. The choice of which ICP approach is most suitable for sample analysis on a consistent basis is a matter of the required degree of accuracy and resolution for ions of interest, as well as operating budget. Certain ions are very difficult to isolate using optical and/or atomic emission approaches; in these cases, mass spectrometry may be required. As of this writing, ICP-MS analysis is more expensive than either ICP-AES or -OES, and there are seemingly fewer providers of ICP-MS service, particularly within North America; resultantly, cost of analysis and turn around time frames are generally greater for ICP-MS analysis than for ICP-AES or -OES.

Requirements of resolution are unique to each individual system or research project, however it is generally the case that ICP-OES, performed properly, yields such resolution that values ≤1 ppb may be discernible for many elements of interest in recirculating aquatic ecosystems. Researchers and commercial growers, in particular, have use for such resolution because it enables them to monitor system chemistry and intelligently correlate ionic values with study outcomes and productivity, respectively. For these reasons, as well as relatively high availability of ICP-OES service providers within North America, analysis performed with ICP-OES accounts for >90% of the reports that we receive for interpretation. As accessibility to ICP-MS within North America increases, however, so too will the utilization of this approach, likely coinciding with a decrease in the cost of analysis.

We are often asked for recommendations for ICP service providers. Over time, as access to these providers has increased and standards within their community have changed, we have revised our initial response. In our experience as advisors to system administrators, the criteria that should be considered when selecting an ICP service provider are:

Reliability of reported values;
Resolution of reported values for critical ions;
Turn around time frame from date of sampling to date of report availability.

Reliability of reported values for a given ion is impacted by the capabilities of the ICP method and of the equipment utilized, as well as by the maintenance of that equipment by the service provider. These aspects also impact the resolution of reported values. It's implied that the service provider understands how to interpret data in the manner necessary to properly report values to the client; in years past, this aspect came into question from time to time, but has seemingly been less of an issue over the past couple of years as standards among providers have increased due to increasing competition for clientele.

Individuals seeking to identify a reliable, reputable ICP provider should inquire with multiple individuals (e.g. ≥10) who have been using a single provider to analyze at least four samples from one system under their care, and who have adequate experience to correlate reported values with observable changes within that system. In taking this approach, the "sample size" is adequate to make an intelligent decision and discard outliers that are not necessarily representative of an ICP provider's capabilities, whether the experience was negative or positive. If an ICP service is reporting "unusual" values for major ions which behave non-conservatively within the confines of the system being sampled, then this is a reason to make inquiries among other known users of that service to determine whether the reliability of the service has changed. In many cases, these ions can be tested for with traditional methods to verify or refute the values being reported; within recirculating aquatic ecosystems, these elements are Mg, Ca, K, Br, and Sr. This point is addressed more fully in the final paragraph of this piece.

With respect to the time frame between drawing the sample and receiving the analysis, "as quickly as possible" is recommended, for the simple reason that the conditions within the system at the time of report receipt will be more representative of those present when the sample was drawn assuming no acute changes in water chemistry during the interim than when the reporting period is relatively long, given the same ICP method of analysis. The time frame is a matter of the combination of proximity between the client and the ICP provider (assuming a standard mode of shipping the sample) and the speed with which the ICP service analyses the sample and provides the report to the client. From date of sampling to ICP analysis and provision of report to client, three days is as quickly as we have seen; this time frame represents a reasonable target, we feel.

Because the concentrations of ions in the system will have changed between the point of sampling and a short time later, however, the precise values reported are only an artifact relative to "now". In light of this, the more critical metric for individuals working with the observation and or care of aquatic ecosystems is likely to be the trend of ionic values reported over a period of time. When, for example, the value of a critical ion is deemed to be deficient, then the objective is to gradually increase the value towards the standardized value. For administrators of recirculating aquatic ecosystems, trending in the right direction is pragmatic and, due to the aforementioned time lag, the best that can be done. Given this, ICP-OES is likely still the most useful and cost-effective ICP approach currently available to the readers of this piece, because though the reported values of some ions may not have the resolution that ICP-MS is capable of providing, the values are likely adequately precise to illustrate trends, which can be acted upon. In the event that ions of interest, such as fluoride, cannot be analyzed with ICP-OES or -AES, then the choice must be made to use a service capable of reporting the value(s) accurately and with the necessary resolution (preferably within 5% - 10% of the standardized value).

Within the analysis of pollutants (defined in this instance as elements which have a deleterious impact on livestock in the system when the concentrations are within one order of magnitude of either the seawater standardized values or at values which are commonly present in the system and cannot be easily reduced), such as aluminum, cesium, lead, arsenic, and tin (among many others), it is vital to remain mindful of the potential resolution of the ICP equipment and to take reported values into the proper context. During the early part of 2024, tin has been consistently showing up in samples pulled from coral display and propagation systems at values that exceed, in many cases, 500,000% of the standardized value (largely, it seems, a result of corrosion from exposed magnets within the systems, but potentially from other sources). The problem in this particular instance is that tin is present in natural seawater (S=35) at a concentration of 4.748x10-7 ppm. In order for the ICP service to provide meaningful information to the client (with respect to tin, specifically, in this example), their equipment must be able to detect tin at that concentration and be capable of resolution of 0.0001 ppb. If the resolution for tin is only 0.001 ppb, then the reported values will seem far worse than they might actually be. In the systems in question, loss of small-polyp stony corals, in particular, has been high, which was the initial reason that many of the system administrators submitted samples to ICP providers. In these instances, identifying a "smoking gun" may be invaluable to remission of the situation. Think, for example, if the reason that acute SPS losses were occurring was unknown and that the system admin chose to address the issue through water changes, all the while being unaware that the source of the pollutant was still in the system and continuing to release harmful material. No amount of water changes would likely be sufficient to halt the issue before entire loss of the cohort occurred. In this, the ICP analysis would be invaluable.

When the choice of an ICP service has been made, then we recommend continuing to use that service to process samples as a matter of maintaining consistency for as long as the service is providing realistic and reliable data. Switching between ICP services (even those using the same approach) over a short term is likely to result in variances in reported values based upon a number of criteria, such as resolution capabilities of the equipment employed. To illustrate, there are now numerous instances recorded (several that we were consulted on) in which an individual drew four samples from the same system within seconds of one another, submitted the samples to four separate ICP services, and received different reported values for all ions analyzed. Further, there have been instances in which multiple samples from the same system were drawn and sent to the same provider, again with different values reported (albeit slight). For this reason, focusing on the specific values reported by the ICP service becomes secondary to understanding whether those values are deficient or excessive relative to their standards, and then taking action to modify the concentration as dictated by the requirements of the system inhabitants. To reiterate, choosing one ICP provider of repute and continuing to utilize them for sample analysis is recommended, for the sake of consistency of methods of analysis. Otherwise, the individual has a tendency to fall prey to analysis paralysis and focuses on the "on paper" performance capabilities of the various providers as opposed to using one as a benchmark and then proceeding with sound principles of husbandry, validating findings regularly as follows...

Lastly, there is a tendency, which is perhaps understandable to some extent given the increasing relegation of once tedious tasks to apps, services, and increasingly inexpensive electronic devices with rapid reporting capabilities, to believe values reported by electronic methods (i.e. digital meters) of analysis and ICP analysis unreservedly. In analytical science, it is critical to verify results, whether by repeating a test or by using another suitable means of testing to validate the original findings, before acting on conclusions; this is particularly true when an initial finding seems anomalous. At Captiv8 Aquaculture, we work with numerous commercial aquaculture and horticulture operations, as well as academic organizations and research institutions; none of them rely solely upon ICP analysis or digital meters to monitor chemical parameters in systems under their care. The reason is that independent testing of numerous critical parameters using titrimetric or colorimetric testing methods has yielded significantly different results to those reported by more modern methods; these verification tests have, in many cases, been undertaken out of necessity when a reported value seemed inaccurate or nonsensical, and had no action been taken the impact could have been loss of life within the system(s) in question. These points considered, we strongly recommend that system administrators possess and regularly employ traditional test kits to verify reported values of ions such as Mg, Ca, and K, as well as for alkalinity, nitrate, and phosphate. Our preference since 2000 has been for kits produced by Hach, though we have also used La Motte kits on occasion. There are a number of highly-rated kits produced and marketed for the marine aquarium hobby that we have heard positive feedback on, however we have no personal experience with these brands so cannot provide first-hand feedback.