Measuring PO4: Hanna ULR versus Triton ICP

[ScottB]

Wasn't sure where this post should go, but I will start here.

Just curious if anyone else is seeing a consistent discrepancy between their Hanna values and those from ICP for PO4. I use the Hanna for phosphorus and convert that value. According to Hanna, my PO4 stays around .13 +-. Each time I send out an ICP, I get .03 to .04.

I've now run 4 ICP on two systems and the same discrepancy appears each time with the Hanna showing an extra .1 above ICP.

I wear nitrile gloves and store cuvettes in RODI. I have replaced the cuvettes. My process is fairly meticulous and consistent and the values I get are fairly smooth. My reagents are always current. Should I just buy a new Hanna?

 

[Randy Holmes-Farley]

I don't know why there is a discrepancy. Might be something happened to the phosphate before it got to Triton (such as precipitation or bacterial uptake), and it might be one or both measurements are off.

 

[Dan_P]

Wasn't sure where this post should go, but I will start here.

Just curious if anyone else is seeing a consistent discrepancy between their Hanna values and those from ICP for PO4. I use the Hanna for phosphorus and convert that value. According to Hanna, my PO4 stays around .13 +-. Each time I send out an ICP, I get .03 to .04.

I've now run 4 ICP on two systems and the same discrepancy appears each time with the Hanna showing an extra .1 above ICP.

I wear nitrile gloves and store cuvettes in RODI. I have replaced the cuvettes. My process is fairly meticulous and consistent and the values I get are fairly smooth. My reagents are always current. Should I just buy a new Hanna?

@Rick Mathew posted a very nice expose on ICP phosphorous measurements. The PO4 in your sample becomes unavailable to ICP measurement during shipping. Rick’s data strongly suggest biological activity is the cause, probably it ends up in a biofilm on the sample container wall. I have confirmed his observations.

I do not trust the P measurement from any vendor because of this issue. Additionally, vendors do not state the accuracy or variability of their measurements on any element. We need to be cautious about interpreting the numbers we receive.

 

[Dr. Jim]

I have come to expect to see lower PO4 on ICP tests. I believe it is safe to say that every test I have sent out has come back that way. (I use Hanna P)

 

[ScottB]

Thank you all. We are often "taught" that our hobby grade kits are only moderately accurate versus (a well calibrated) ICP. This seems to be one exception.

I know the absolute value for PO4 is less important than just having some consistently available. Just wanted to understand which direction I should lean toward.

I'll do a little test. I will grab a couple samples then Hanna test one right away while letting the other rest for 5-6 days before testing. Let you know what I see.

 

[I’m Batman]

I'll do a little test. I will grab a couple samples then Hanna test one right away while letting the other rest for 5-6 days before testing. Let you know what I see.

Curious to see the results of this trial...

 

[schuby]

Thank you all. We are often "taught" that our hobby grade kits are only moderately accurate versus (a well calibrated) ICP. This seems to be one exception.

I know the absolute value for PO4 is less important than just having some consistently available. Just wanted to understand which direction I should lean toward.

I'll do a little test. I will grab a couple samples then Hanna test one right away while letting the other rest for 5-6 days before testing. Let you know what I see.

Any results?

 

[ScottB]

Any results?

Sorry. I've been slacking off.

Just pulled a couple samples. One is resting in a clean, plastic pee cup now. Will test that Jan 4th. My son will run the current sample this afternoon. Thanks for the reminder. Happy New Year.

Curious to see the results of this trial...

@chimbo84

 

[JMetaxas]

Sorry. I've been slacking off.

Just pulled a couple samples. One is resting in a clean, plastic pee cup now. Will test that Jan 4th. My son will run the current sample this afternoon. Thanks for the reminder. Happy New Year.

@chimbo84

Curious for the results.

 

[SaltyWaitress]

I have come to expect to see lower PO4 on ICP tests. I believe it is safe to say that every test I have sent out has come back that way. (I use Hanna P)

Have you noticed how big the difference is between ICP and your checker? OP noted .13 and .03, change of .1, big! Would you say the difference has been close to .1 or less or more? @Dan_P also

 

[Drew1900]

How are you filling your Hannah cuvettes? Are you filling too the line or filling with a 10ml syringe? I use a graduated pipette too fill my cuvettes and 10ml in my pipette fill’s each cuvette above the 10ml line. I’ve double checked the calibration on my pipette as well as cross checked by filling the cuvettes with other 10ml syringes. This could explain your findings being that if the 10ml line on your cuvette is not actually 10ml, it would give you a more concentrated solution and a higher test result.

 

[ScottB]

How are you filling your Hannah cuvettes? Are you filling too the line or filling with a 10ml syringe? I use a graduated pipette too fill my cuvettes and 10ml in my pipette fill’s each cuvette above the 10ml line. I’ve double checked the calibration on my pipette as well as cross checked by filling the cuvettes with other 10ml syringes. This could explain your findings being that if the 10ml line on your cuvette is not actually 10ml, it would give you a more concentrated solution and a higher test result.

I fill to the point where the bottom of the concave shaped water level reaches the line.

It is such a large difference -- generally .07 to .1 -- with ICP always the smaller value. I have used ATI, ICP-analysis.com and now Triton.

On Monday, I will Hanna test the 5 day old sample to see if it reads lower than the "fresh" sample. The reference value is .07 on the fresh.

 

[Dr. Jim]

Have you noticed how big the difference is between ICP and your checker? OP noted .13 and .03, change of .1, big! Would you say the difference has been close to .1 or less or more? @Dan_P also

My results/ICP (company) ...since last May. (Only the first one showed a higher value by ICP)

0.13/0.27 (Triton)
0.02/0.19 (Triton)
0.02/0.01 (ATI)
0.055/0 (ATI)
0.03/0.022 (ATI)
0.071/0.011(OCEAMO)
0.064/0.03(ATI)
0.071/0.037(Triton)
0.067/0.04(ATI)

 

[Dr. Jim]

How are you filling your Hannah cuvettes? Are you filling too the line or filling with a 10ml syringe? I use a graduated pipette too fill my cuvettes and 10ml in my pipette fill’s each cuvette above the 10ml line. I’ve double checked the calibration on my pipette as well as cross checked by filling the cuvettes with other 10ml syringes. This could explain your findings being that if the 10ml line on your cuvette is not actually 10ml, it would give you a more concentrated solution and a higher test result.

I've been saying (and posting) this for a long time! Glad someone else has found this! I've written to Hanna 3 times asking if the test is designed to be accurate when filled to the line or when filled with an accurate graduated pipette. No answer! (I'm guessing the latter).

 

[SaltyWaitress]

My results/ICP (company) ...since last May. (Only the first one showed a higher value by ICP)

0.13/0.27 (Triton)
0.02/0.19 (Triton)
0.02/0.01 (ATI)
0.055/0 (ATI)
0.03/0.022 (ATI)
0.071/0.011(OCEAMO)
0.064/0.03(ATI)
0.071/0.037(Triton)
0.067/0.04(ATI)

First two were much higher, others lower though. Thanks for sharing

 

[Drew1900]

I fill to the point where the bottom of the concave shaped water level reaches the line.

It is such a large difference -- generally .07 to .1 -- with ICP always the smaller value. I have used ATI, ICP-analysis.com and now Triton.

On Monday, I will Hanna test the 5 day old sample to see if it reads lower than the "fresh" sample. The reference value is .07 on the fresh.

My point is, if you are filling the cuvette too the 10 ML line, but the line was printed incorrectly(which has been my experience), your sample size may be less than 10ml. This would give you a more concentrated sample and a higher phosphate reading.

Its an easy theory too test. Preform the test using a 10ml syringe too fill the cuvette instead of basing it off the 10ml line on the cuvette and see what your results are.

 

[SaltyWaitress]

How are you filling your Hannah cuvettes? Are you filling too the line or filling with a 10ml syringe? I use a graduated pipette too fill my cuvettes and 10ml in my pipette fill’s each cuvette above the 10ml line. I’ve double checked the calibration on my pipette as well as cross checked by filling the cuvettes with other 10ml syringes. This could explain your findings being that if the 10ml line on your cuvette is not actually 10ml, it would give you a more concentrated solution and a higher test result.

What would be more concentrated? The reagent? The phosphate concentration is the same just a difference in volume

 

[Drew1900]

What would be more concentrated? The reagent? The phosphate conc is the same just a difference in volume

Your sample would be more concentrated. If you’re sample size is say 9ml, and you add the reagent too that, it will create a more concentrated sample than a sample size of 10ml.
Hannah checkers work off of light readings. If you put the reagent into 5ml of water, it’s going to make a darker sample than if you put the reagent in 10ml of water. Kinda like when you mix Kool-Aid. The more water you add, the more the color of the Kool-aid is diluted.

 

[SaltyWaitress]

Your sample would be more concentrated. If you’re sample size is say 9ml, and you add the reagent too that, it will create a more concentrated sample than a sample size of 10ml.
Hannah checkers work off of light readings. If you put the reagent into 5ml of water, it’s going to make a darker sample than if you put the reagent in 10ml of water. Kinda like when you mix Kool-Aid. The more water you add, the more the color of the Kool-aid is diluted.

So the reagent is more concentrated, makes sense.

 

[Drew1900]

So the reagent is more concentrated, makes sense.

Correct