ICP testing (again)

[JulesH]

Hello,

I work in an environment that requires precise and accurate measurements of certain variables. This is only possible if the instruments used to take those measurements are calibrated before the project commences and continuously verified during the project.

This brings me to the subject of the ICP test. They have always bothered me because of the above paragraph. I have done a fair amount of reading about these instruments, primarily OES versions. However I am aware of the methodology used for both the OES and MS ICP tests.

They are impressive but in saltwater the matrix is complex, which causes most of the problems with the test (OES).

These can include:

• Interference with the detection of trace metals such as vanadium, manganese or zinc.
• Suppression or enhancement of emission lines of target elements.
• Spectral overlaps, for example calcium lines overlapping with strontium or magnesium.
• Elements can read artificially high or low depending on the calibration and software correction.

Trace-level analysis means even minute contamination is significant:

• Metal from collection containers syringes or even human contact can skew results.
• Some mail-in kits use cheap plastic vials that are not pre-acid-washed and can leach elements.
• Cross-contamination in laboratories that run hundreds of samples a day is also possible.
• Certain ions (e.g. Fe, Al, Mn or even PO₄ bound to CaCO₃ particles) can adhere to particles or precipitate in the sample bottle before it reaches the laboratory.
• This means the true dissolved concentration may be lower than what the laboratory measures (if the particles are digested) or higher if some are lost.

If the sample is left for several days:

• Bacteria can alter redox states (Fe²⁺ → Fe³⁺).
• Precipitates can form with carbonate or phosphate.
• Some laboratories require acidification of samples, others do not – this affects stability.

Many reef ICP laboratories use automated correction algorithms to “normalise” results to typical seawater ion ratios.

If your salinity, Mg or Ca is slightly off, the corrections can misrepresent minor elements.

My conclusions are for what they are worth:

• You can have high confidence in Ca, Mg, Na, K, Sr, B, Br, Li measurements.
• Use to track for trends in Fe, Zn, Cu, Mo, Mn, I, Al, Si.
• Ignore absolute numbers for P, Cr, Ti, heavy metals (unless alarmingly high).
• Always compare changes over time, not single snapshots.

I am your average reef keeper and not a chemist and would be interested in other people’s opinions.

 

[Gumbies R Us]

@Randy Holmes-Farley

 

[JonasRoman]

Hello,

I work in an environment that requires precise and accurate measurements of certain variables. This is only possible if the instruments used to take those measurements are calibrated before the project commences and continuously verified during the project.

This brings me to the subject of the ICP test. They have always bothered me because of the above paragraph. I have done a fair amount of reading about these instruments, primarily OES versions. However I am aware of the methodology used for both the OES and MS ICP tests.

They are impressive but in saltwater the matrix is complex, which causes most of the problems with the test (OES).

These can include:

• Interference with the detection of trace metals such as vanadium, manganese or zinc.
• Suppression or enhancement of emission lines of target elements.
• Spectral overlaps, for example calcium lines overlapping with strontium or magnesium.
• Elements can read artificially high or low depending on the calibration and software correction.

Trace-level analysis means even minute contamination is significant:

• Metal from collection containers syringes or even human contact can skew results.
• Some mail-in kits use cheap plastic vials that are not pre-acid-washed and can leach elements.
• Cross-contamination in laboratories that run hundreds of samples a day is also possible.
• Certain ions (e.g. Fe, Al, Mn or even PO₄ bound to CaCO₃ particles) can adhere to particles or precipitate in the sample bottle before it reaches the laboratory.
• This means the true dissolved concentration may be lower than what the laboratory measures (if the particles are digested) or higher if some are lost.

If the sample is left for several days:

• Bacteria can alter redox states (Fe²⁺ → Fe³⁺).
• Precipitates can form with carbonate or phosphate.
• Some laboratories require acidification of samples, others do not – this affects stability.

Many reef ICP laboratories use automated correction algorithms to “normalise” results to typical seawater ion ratios.

If your salinity, Mg or Ca is slightly off, the corrections can misrepresent minor elements.

My conclusions are for what they are worth:

• You can have high confidence in Ca, Mg, Na, K, Sr, B, Br, Li measurements.
• Use to track for trends in Fe, Zn, Cu, Mo, Mn, I, Al, Si.
• Ignore absolute numbers for P, Cr, Ti, heavy metals (unless alarmingly high).
• Always compare changes over time, not single snapshots.

I am your average reef keeper and not a chemist and would be interested in other people’s opinions.

This is an excellent post. First we have the knowledge, that some lack, that ICP doesnt measure for instance PO4, but just calculate assuming all P is from PO4, which is never true. My opinion is that PO4 must be meausured by photometri and hence should me measured at home, and if you want to verify(or calibrate) your method, use a reference fluid. I did a post here recently where I showed the difference in ICP calculated and photometer measured. The diff varies with organics in the sampel. One more subject I like to emphasise that level of detection for ICP OES is in some traces higher than natural sea water level, meaning that a zero value (or non detectable) may still say you have too high levels. Some user make dose actions on these "zero" values. Here we need a ICP-MS to fill the gap, and many companies now provide. This is especially important if you are of that believe that every singel element must be spot on ( I do not believe in that).
For me I use ICP mainly to see that I at least doesnt have toxic levels of traces, so for instance happy with "0" or "n.d" in some traces, and also check mainly Sr, I, K and F, where we have no other good options at home and I believe those are imporant. The basic elements, like Ca, Mg, KH, PO4, NO3 I measure at home daily.

/Jonas

 

[CHSUB]

that ICP doesnt measure for instance PO4, but just calculate assuming all P is from PO4

FM uses some kind of photometer and gives both values of P, calculated and measured.

 

[CHSUB]

K

Also Salifert K home test kit is spot on with ICP MS and when I raised my value in my aquarium each 10 ppm adjusted registered perfectly.

 

[JonasRoman]

Also Salifert K home test kit is spot on with ICP MS and when I raised my value in my aquarium each 10 ppm adjusted registered perfectly.

i agree with that