Is Metals Analysis of Water Samples considered "sampling"?

J

JodiB

#1
Trying to determine if we need a "sampling plan". I don't think so, but want to make sure...

One of our tests is metals analysis of water samples. The samples that we receive from our (internal) customers are of various quantities and TDS. We dilute most of the samples to overcome the TDS, and draw out the number of mls that we need for the analysis. Is this "sampling" ? Have we just created a sub-sample? Do we need some sort of sampling plan or specific "sampling" procedure for this? We describe the sample preparation (including dilution) in our method.

We also do solids analysis using XRF and XRD. We grind up the solid, etc. in our sample preparation. Obviously we don't use the entire amount of what they send us in the analysis, we take what should be a homogenous portion of what we've prepared.
 
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Ninja

Looking for Reality
Trusted Information Resource
#2
Re: Is this considered "sampling"?

...we take what should be a homogenous portion of what we've prepared.
Why "should" it be a homogenous portion?
Not because you want it to be, but because you sample it in a way to ensure that it is...right?

Assuming so, that way that you take that "should be homogeneous" sample is your sampling plan.
Sounds like you already have a sampling plan...so the question is whether everyone does it the same way, and if that way is documented anywhere...
 

Mikishots

Trusted Information Resource
#3
Trying to determine if we need a "sampling plan". I don't think so, but want to make sure...

One of our tests is metals analysis of water samples. The samples that we receive from our (internal) customers are of various quantities and TDS. We dilute most of the samples to overcome the TDS, and draw out the number of mls that we need for the analysis. Is this "sampling" ? Have we just created a sub-sample? Do we need some sort of sampling plan or specific "sampling" procedure for this? We describe the sample preparation (including dilution) in our method.

We also do solids analysis using XRF and XRD. We grind up the solid, etc. in our sample preparation. Obviously we don't use the entire amount of what they send us in the analysis, we take what should be a homogenous portion of what we've prepared.

What they send you for analysis is your LOT, irrespective of the fact that they call it a SAMPLE. If you are not testing the entire lot you have received, then you are using samples of that lot.

A sampling plan would define how many units you will look at out of the lot you have received. It should be based on some statistical method, rather than just some arbitrary amount; to be statistically valid, it needs to be consistent and representative.

Hope that helps.
 
J

JodiB

#4
We document what we do for sample preparation within our methods, does that meet ISO/IEC 17025's requirment for "sampling plans" ?

We obviously can't use the entire sample since they'll send us 8-16 oz of water and we only need a few ml max. Shake, filter, dilute, shake again, draw off what we need for the test. That's the plan :) !
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#5
your sample size is effectively 1. this is statistically valid IF:
The 'sample' sent to you is from a homogenous blend of the entire lot and the sample itself is homogenous at the time you take your sub-sample (things aggregate and precipitate) AND you have determined that the measurement error of your method is small compared to the specification.
 
J

JodiB

#6
Thanks Bev. What does that mean that my sample size is 1? Sorry, but this isn't something that I've looked into before. Somebody once told me that we don't do sampling, since we aren't picking and choosing what we test ; we test everything that is sent to us. Now I'm a bit confused because we do take just a portion of what they send to us since what they send is excessive of what we need. And that seems to be "sampling" to other people...
 

Ninja

Looking for Reality
Trusted Information Resource
#7
Thanks Bev. What does that mean that my sample size is 1? Sorry, but this isn't something that I've looked into before. Somebody once told me that we don't do sampling, since we aren't picking and choosing what we test ; we test everything that is sent to us. Now I'm a bit confused because we do take just a portion of what they send to us since what they send is excessive of what we need. And that seems to be "sampling" to other people...
I think we're into the definitions of words here...I turned some of this definition bold and underlined as it applies to the current thread:
++++++++++++++++++++++++++++++++++++
sam?ple

/ˈs?m
pəl, ˈsɑm-/ Show Spelled [sam-puh
l, sahm-] Show IPA noun, adjective, verb, sam?pled, sam?pling.
noun 1. a small part of anything or one of a number, intended to show the quality, style, or nature of the whole; specimen.
2. Statistics. a subset of a population: to study a sample of the total population.
3. a sound of short duration, as a musical tone or a drumbeat, digitally stored in a synthesizer for playback.

adjective 4. serving as a specimen: a sample piece of cloth.
verb (used with object) 5. to take a sample or samples of; test or judge by a sample.
+++++++++++++++++++++++++++++++++++++


They send you more than you need, so you take a small part of it...thus you take a sample. Said sample is intended to show the quality, style or nature of the whole specimen.

If you aren't using it all, you're taking a sample.
 

harry

Trusted Information Resource
#8
............ Somebody once told me that we don't do sampling, since we aren't picking and choosing what we test ; we test everything that is sent to us. Now I'm a bit confused because we do take just a portion of what they send to us since what they send is excessive of what we need. And that seems to be "sampling" to other people...
I think your job is to make sure that whatever portion that you use for your analysis is representative of the sample given to you - which could be as simple as shaking the container of the sample to make sure that is is well mixed and therefore homogeneous.

I frequently send samples of water to the lab for testing and the report I get always start with a caveat - One sample of water in a plastic container marked 'X' was received from the customer .............. I think this is important because the lab was not in a position to know how, when, where the sample was collected and thus cannot be responsible for the sampling part. The test result is only good for that particular sample received.
 

somashekar

Staff member
Admin
#9
Trying to determine if we need a "sampling plan". I don't think so, but want to make sure...

One of our tests is metals analysis of water samples. The samples that we receive from our (internal) customers are of various quantities and TDS. We dilute most of the samples to overcome the TDS, and draw out the number of mls that we need for the analysis. Is this "sampling" ? Have we just created a sub-sample? Do we need some sort of sampling plan or specific "sampling" procedure for this? We describe the sample preparation (including dilution) in our method.

We also do solids analysis using XRF and XRD. We grind up the solid, etc. in our sample preparation. Obviously we don't use the entire amount of what they send us in the analysis, we take what should be a homogenous portion of what we've prepared.
The water sample is your test input. The other things that you do is your assey procedure. You are simply not into any statistical sampling.
You may get 1000 ml of water which represent perhaps a tank of 1000 ltr of water, and you do things to take say 10 ml of this water for your assey. There is no statistical sampling of any sort here. We do the same for out STP water tests from a third party.
 
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J

JodiB

#10
Thanks for all the input. It seems like there's still two different opinions here on whether or not we are doing "sampling". So I'm still not clear on whether we have to address "sampling" in an ISO/IEC 17025 quality management system. Do we do "sampling"? If so, would a simple statement in the quality manual to the effect that our methods ensure that we test a representative portion of the samples that are sent to us?
 
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