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Topic: GC-MS Calibration Curve Help  (Read 1980 times)

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Offline MagicKitty96

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GC-MS Calibration Curve Help
« on: March 28, 2023, 02:23:05 PM »
 I will be making a calibration curve on a GC-MS and need to get a sample down to 500, 200, 100, 50, and 20 PPB.  The sample has a specific gravity of 1.145 and will be in Ethyl Acetate. Does anyone know how to go about doing the math for this?

I know that in water you can put .001 mL of sample into 1000 mL of water to get 1 PPM, but I don't know how to go about getting it in ethyl acetate.

The units at the end end up being ng/mL

Offline Borek

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Re: GC-MS Calibration Curve Help
« Reply #1 on: March 28, 2023, 06:26:38 PM »
Assuming ppb is intended to be w/w, convert volumes to masses using densities (or specific gravities) and calculate dilution factor using masses.
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Offline rjb

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Re: GC-MS Calibration Curve Help
« Reply #2 on: March 28, 2023, 06:43:13 PM »
I may have misunderstood your intentions, but based on your second sentence, the first thing I would do in your shoes would be to get my head around the idea of serial dilutions. Making a PPB solution in a single step (unless you're willing to make enormous quantities) is not the way to go and will result either in poor accuracy and repeatability or ridiculous volumes of analyte solution.

What am I on about? Let's imagine (like many labs) you have a standard 4dp analytical balance with 0.1mg readability, in that case, the minimum weight you can measure (and obtain less than 0.1% error) is 82mg and that's assuming absolutely ideal conditions in a ground floor weighing room with the balance set up perfectly on a weighing table! If we round this up to 100mg, then to make a 20PPB solution, you'd need to dissolve 100mg of your compound in nigh on 5000 litres of solvent - hardly practical!

Instead what we could do is make up a stock solution of say 100ug/ml (by dissolving 100mg of our analyte in 1L of solvent). By withdrawing 10ml of this and dissolving in 990ml of solvent we have a diluted stock solution which is at 1000ng/ml from which we can make up our calibration standards by further (1 in 2, 1 in 5 etc.) dilution. Doing it this way saves us 4998 litres of solvent! 2L of solvent is still a lot, so we could bring this down by making up a smaller volume of more concentrated initial stock solution (say 100ml) and adding additional dilution steps, remembering of course that each step contributes to the overall error.

What difference does using ethyl acetate (rather than water) as your solvent make to these calculations? Well that's up to you... Parts per notation is really annoying as it is often unclear as to what the scientist is on about as parts per notation can be weight:weight, volume:volume mole:mole and plenty of analytical chemists use weight:volume (in essence ignoring the difference in density between water and other solvents), so you're going to have to figure out which is appropriate for you and make the necessary mass to volume calculations where necessary! This is why I personally prefer to use weight volume (i.e. ng/ml) nomenclature in my work.

Out of curiosity what are you running and are you using SIM or Scan?
« Last Edit: March 28, 2023, 07:02:25 PM by rjb »

Offline MagicKitty96

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Re: GC-MS Calibration Curve Help
« Reply #3 on: March 28, 2023, 10:25:42 PM »
Thanks for the detailed response! I will be using SIM with a triple quadrupole. I have used GC-MS in the past, but my specialty has always been spectroscopy and I'm very anxious and second guessing everything I know. We need to be extremely precise. I figured serial dilution would be the way to go, I am still unsure if I use the density to get the required grams since my sample is a liquid.

Offline Borek

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Re: GC-MS Calibration Curve Help
« Reply #4 on: March 29, 2023, 03:17:59 AM »
I am still unsure if I use the density to get the required grams since my sample is a liquid.

[tex]density = \frac{mass}{volume}[/tex]

Seriously, this is a basic high school stuff.
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Offline MagicKitty96

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Re: GC-MS Calibration Curve Help
« Reply #5 on: March 29, 2023, 07:35:48 AM »
I know how to calculate density, I'm not an idiot. Making unnecessarily rude comments doesn't make you cool. That is not the question asked. I'm trying to figure out if PPB needs to be in by volume or by weight since I've never done this before. Way to push people down and make them not want to come back.
« Last Edit: March 29, 2023, 08:04:59 AM by MagicKitty96 »

Offline rjb

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Re: GC-MS Calibration Curve Help
« Reply #6 on: March 29, 2023, 09:55:14 AM »
Hi OP,

I personally think it would be wise to either clarify the type of 'parts per' calculation that you are expected to do, or alternatively to consider doing your work in absolute units like ng/ml

My reasons for this are manifold, but are best illustrated by the following:

Let's imagine that we take the 1000ng/ml diluted stock that I alluded to in my last post. From the fact that its a 1000ng/ml solution, its very clear how much solute and how much solvent is in there. By contrast the same solution expressed in parts per notation is a lot more ambiguous... 

In PPB w/v format this 1000ng/ml solution would be expressed as 1000 PPB

If we express this in PPB w/w, then this same solution would be expressed as 1108 PPB. This is calculated by dividing the mass of the compound by the mass of the solvent (which would be in this case 0.902 g as ethyl acetate has a density of 902g/L) and multiplying by 10^9 to get the concentration in PPB w/w.

If we express this PPB v/v, then this same solution would be 873 PPB. This is calculated by determining the volume of liquid compound weighing 1000ng which because of the density of your compound is 873nL...

If we express this in PPB mol:mol, the we need to know the molar mass of your substance and the solvent. If your substance has a molar mass of say 220 g/mol, then this same solution would equate to about 444 PPM mol/mol...

You can no doubt see the issue! Which PPB method you choose is up to you, but it is important to make sure that you are clear which convention is appropriate to your work before you start making up any solutions. Alternatively remove all this ambiguity by making up your solutions in unambiguous ng/ml units.

Kind Regards




Offline MagicKitty96

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Re: GC-MS Calibration Curve Help
« Reply #7 on: March 29, 2023, 10:03:44 AM »
Thank you uso much rjb, you really helped. I get it now!

Offline Borek

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Re: GC-MS Calibration Curve Help
« Reply #8 on: March 29, 2023, 02:43:32 PM »
I'm trying to figure out if PPB needs to be in by volume or by weight since I've never done this before.

Problem is, this is what you should know first, before starting to work, it is not something anyone can second guess for you. You know the method, you know the data, you know the context. ppb is ambiguous and can mean many things (rjb listed common meanings).

For w/w calculation you need to know mass of the solution.

I am still unsure if I use the density to get the required grams since my sample is a liquid.

Yes, you use the density. Just rearrange the density formula to

[tex]mass = volume \times density[/tex]

and the rest of the calculations should follow.

rjb suggested to divide by 0.902 g - but this is based on the assumption you deal with 1 mL sample. Good assumption for an example (especially as his starting point was ng/mL), but not an universal number.
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