Chromatography Forum

I have always struggled with conversion factors when making standards for GC, HPLC (mg/l, ul/ml, ppm, ppb, % etc.) . I just cant wrap my head around the differences, to make matters worse, I searched the internet but found conflicting conversion factors. I generally use pipetters to make up a standards because the densities of my purchased standard is close to water. I am hoping someone can explain to me a way to keep the concept straight in my head. Thanks Jeni

1. v1c1=v2c2

2. Make sure your units cancel.

3. Have someone review your standard preparations.

Hi Jen,

You aren't alone, after 40 years I still have to stop and take out a pen and paper.
Part of it is just practice. Don't be afraid to write out some tables with conversions and print them off. For example, put a range of % down 1 column then across the top have what that is in, for example ppm,ug/ml, mg/10ml, g/100ml

I'd start off by memorising

0.1% = 1000ppm
1000ppm = 1000ug/ml (or 1mg/ml or 1mg/g)
so 1ppm = 1ug/ml

1g = 1000mg
1mg=1000ug
1ug= 1000ng

for water 1ul=1mg

I find with those you can work out most of what you need, although sometimes I have to just convert everything to 10E0x(g) to get them in the same units.

Is that the sort of thing you wanted?

Regards,

Ralph

2. Make sure your units cancel.

This is, for me, the most valuable tool in doing any calculation. I have figured out how to do many calculations just by making the units work out.

In this case, write down your conversion factors as another item in the calculation, e.g., " 1000 mg/g". It seems tedious at first but it makes it a lot easier to either get the calculation right the first time, or go back through the calculation to find errors.

I remember back when I started doing my PhD and was first introduced to ICPMS, I was stuck with some data in wt% and wanted to compare it with another set of data in ppm.

Seems so embarrasingly stupid now , but I was stuck for a whole afternoon banging my head against a brick wall going through pages and pages of paper and rough notes doing molar calculations involving volumes, concentrations, molecular weights etc etc etc until it finally hit me that 1 wt% was the same as one part per hundred, or 10000 ppm. doh!

From then on I started at 1 Kg in 1L = 100% (w/v), and just back calculated from there until I got the hang of it.

Thank you kindly for your help. I have been @ this for over 10 years but when it comes to making up new standards or training a new technician, I start to sweat just thinking about standard prep and concentrations. I will definateley make a chart to keep instead of the throw away scrap papers that I usually work on for 1/2 the day while I & for not documenting the calculations last time. Thanks again Jeni

Blame the government (it's almost always safe to do so, and a favorite pastime for many people...)

EU MRLS tend to be written in ISO terms: mg kg^-1
U.S. tolerances are in the Code of Federal Regulations mostly as "parts per million" or ppm.

1 ppm = 1 mg kg^-1

Most of the time density is 1, so 1 kg = 1 L (note, this is a capital L not a lower case l)

Most of us work with g and mL, not kg and L. Also, most of my standards come in a purity something less than 100%, and a lot of them come in a salt form that has to be corrected for.

Some folks (biochemists most noteably) think it is nice to use molarity. It's always fun to convert a manuscript from nM steroid to ppb steroid


I've also been doing this awhile, and the first time I do a calculation, I write it all down, with the units, making sure they all cancel out nicely. Of course, working in a regulated environment, I would never ever do this on a scrap of paper.

We recognize that this is both a difficult undertaking and one where a mistake could affect results for weeks or months to come. Therefore, weighing, dissolving and calculating the stock concentration of a standard is the one activity that is ALWAYS witnessed and checked by a colleague, even for our non-GLP studies. Any subsequent mistakes in dilution would be caught in an audit, since all volumes used are always written down. Sometimes it is even possible to salvage something in this case.

Finally, fairly early on in method development/validation, a method SOP is prepared. These always have fairly detailed descriptions (weights, volumes of stocks and diluents used, exact calculations, etc.) of how to prepare stock solutions, fortification solutions, standard curve solutions, QCs, etc. One of my coworkers with a background in pesticide residue analysis actually takes this a step further and prepares worksheets to be filled in for each standard solution made. You spend a lot of time up front making sure all your calculations are right, but after that you just "follow the recipe."

Technically speaking, it's incorrect to say 1 ppm = 1 ug/mL. If water is the solvent, and concentration is very low, mathmatically 1 ppm is equal to 1 ug/mL. But most of the time the stock standards we buy are made in organic solvents, the specific weights of some the solvents are not even close to 1 g/ mL. Even in water, if concentration is high, 1 ppm is not 1 ug/mL. For example, if we have a 20% w/w acetone solution in water,

20% w/w = 200,000 ppm

But because the density of 20% w/w acetone solution in water is less than 1 g/mL, 200,000 ppm < 200,000 ug/mL.

Although I use them all the time, ppm and ppb, like %, are not concentration units, they are just units for quantity. In air analysis, ppbv or ppmv sometimes are used as concentration units for air samples, and the conversion factor from ppbv to ug/m^3 is

ppbv=ug/m^3 * 24.45/MW

this formula holds true only under certain conditions.

Doesn't that depend on the exact definition of a "part" in the term ppm/ppb (is there an exact definition?). i.e. is a "part" an atom/molecule (wouldn't that be Molarity?), a mass unit (ug/mg), or a volume unit (ul/ml)?

i.e. if I make up a 10% acetone solution in DIW, I would report it as 10% Acetone (v/v), since I simply dilute 1mL acetone to 10mL with DIW. Whereas if I make up a 10% NaCl solution from a solid standard, I would label it 10% NaCL (w/v), since I dissolve 1g NaCl in 10mL DIW.

So for ppm/ppb solutions, wouldn't the same principle apply, since one is just parts per hundred and the other is parts per million? i.e. shouldn't we be using (w/v) or (v/v) when mentioning ppb/ppm values?

A quick look at Wikipedia reveals this...

"According to the U.S. National Institute of Standards and Technology (NIST) Guide for the Use of the International System of Units (SI), "the language-dependent terms part per million, part per billion, and part per trillion ... are not acceptable for use with the SI to express the values of quantities." NIST's Guide for the Use of the International System of Units(SI) has examples of alternative expressions.

Acceptable SI units are:
1 millimole/mole = 1 part per thousand
1 micromole/mole = 1 part per million
1 nanomole/mole = 1 part per billion
1 picomole/mole = 1 part per trillion"

"Examples:
a sensitivity of 2 ng/kg but not: a sensitivity of 2 ppt"

i.e. stick to the same units...
1ppm = 1ug/g or 1ul/L, but not 1ug/L

Yes! It´s about time to make live easier and get rid of ppm......, feet, inches, barrels, Fahrenheit, etc, etc.

This will probably only make sense to Americans and older Canadians, Brits, et al.:

I have an aversion
to metric conversion
though it's great from the scientists' view.

But describing a dame
will not be the same
when she's ninety-six, sixty-four, ninety-two.

yes, and how can centimetres be metric when there are 2.54 of them to the inch?

Ralph

Just thought of another one -

nph - needles per haystack!

Sorry

Ralph

Hi Ralph

That is truly brilliant

Peter

Thanks Peter,

Regards,

Ralph