HS-sample preparation problem.

[tinsang]

I should spike the residual working standard into a saline solution for accuracy validation. Saline solutions were prepared by diluting the stock standard solution to the required concentrations (5 levels over the range 1 ppm to 120 ppm of 1-propanol and MEK).
The specifications of the residual solvents in the saline: 1-propanol (NMT 100 ppm) and MEK (NMT 35 ppm).
Preparation:
Stock standard solution 1: 50, 0 mg of 1-propanol / 50 ml H2O
Stock standard solution 2: 50, 0 mg of MEK / 50 ml H2O
Range 1: required conc.: 1ppm 1-propanol and 1 ppm MEK
A: 1,0 ml stock std 1+ 1,0 ml stock std. 2/ 100,0 ml H2O.
Add 20µl of A + 180µl saline to HS-vial 1.
Range 2: required conc.: 10ppm 1-propanol and 5 ppm MEK
B: 10,0 ml stock std 1+ 5,0 ml stock std. 2/ 100,0 ml H2O.
Add 20µl of B + 180µl saline to HS-vial 2.
Range 3: required conc.: 50ppm 1-propanol and 10 ppm MEK
C: 25,0 ml stock std 1+ 5,0 ml stock std. 2/ 50,0 ml H2O.
Add 20µl of C+ 180µl saline to HS-vial 3.
Range 4: required conc.: 100ppm 1-propanol and 35 ppm MEK
D: How can I prepare the solution which I later use 20µl for vial 4 ?
Add 20µl of D + 180µl saline to HS-vial 4.
Range 5: required conc.: 120ppm 1-propanol and 42 ppm MEK
E: same question like in D.
Add 20µl of E + 180µl saline to HS-vial 5.
If you have another suggestion, can you please describe the step by step procedure of how to prepare samples and do the calculations? That would be great.
Parallel I want today to test the solution of saline, how many concentrations of 1-propanol and MEK they have. The RSDs for peak area of 1-propanol from 3 samples were 16% (around 2,2 ppm) and MEK wasn’t present. But the RSD values of the standard solution (two analytes) were below 2%. I think it’s critical for this analysis because saline plays a main role in recovery. Should I heat saline solution? If yes how much degree and time should I use?
Thanks.

[chromatographer1]

I have found for increased accuracy of measurement to prepare several spiking solutions at different concentrations. I would then spike each sample preparation with the same volume of solution at different concentrations.

For example, if I had a 10 microliter syringe I would add 10 microliters of solvent to different sizes of volumetric flasks to prepare my spiking solutions. And I would weigh the solvent additions to the water, not assuming an exact 10 microliters each time. Thus some of the concentrations might not be exactly at 1mg per mL or 0.1mg per mL, but 0.994 mg per mL and 0.115 mg per mL. Exact numbers aren't important as long as you know the actual value you created.

For example: 10 microliters might be 6.5 mg added to a 5mL flask containing 4mL of water. After adding and mixing the solvent fill to the mark with water (about 1 mL). Then add 6.6mg to 10mL flask containing 9mL of water. Then fill to the mark. Then add 6.55mg of solvent to a 25mL flask..... to a 100mL flask...... etc etc etc.

Record each time the actual amount of solvent added and the actual concentration of the solvent in water. In the three examples above, that would me 6.5mg/5mL, 6.6mg per 10mL, and 6.55 mg per 25mL. (You do the math)

Then add a std amount (the same volume) of the spiking solutions to each sample (perhaps 10microliters or 50microliters, you chose the amount) each sample or blank prepared the same way. (same solvent volume but possibly different weights of drug) Your preparations will be very consistent when this is done.

It is also helpful in the discussions about ppm if you specify which you mean, either:

1. the concentration in the dissolved drug sample volume

or

2. the concentration in the undissolved drug itself.

One comment that I have repeated over and over on this forum. When you use smaller samples (volumes) in the same vial your recoveries will INCREASE and salts will not be necessary. The lower ratio of headspace to liquid (more liquid in the same vial) will decrease the partition and extend the heating time required for the same recovery of the solvent. Using smaller vials will also increase recoveries. Smaller is better in headspace analysis. Except this one parameter: the larger amount of headspace gas phase you can inject the better.

In my published study, 1ng of solvent was recovered from 1mg of drug dissolved into 25 microliters of water. Hard to believe, huh? And people still want to take 10mL of water and dissolve 1g of drug and put it into a 20mL vial. And they are disappointed when they can't see 10 ppm (10 micrograms) of a solvent in the 1g of drug when I can see 1ng of solvent (that is 10,000 times less material) in a 6mL vial.

Good luck,

Rod

[tinsang]

Thanks very much, Rod, for your detailed explanation.
I think I didn’t make myself clear on my last post, because my English isn’t very good.

My work is 1-propanol and MEK in saline with HS method to validate. This means that saline solution is my sample matrix. The saline solution has a concentration of around 3ppm of 1 propanol. MEK is not present (below 1ppm).
The required concentrations of samples for the calibration curve as below:
1-Propanol: 1, 10, 50, 100, 120 ppm
MEK: 1, 5, 10, 35, 42 ppm.
My draft:
1 ppm of propanol +1 ppm of MEK:
10,0 mg propanol and 10,0 mg MEK/ 1000 ml Water .
Add 0,5 ml of solvent to a 5ml flask and fill to the mark with saline. Add 200 µl to Vial 1. Cap vial.
10 ppm of propanol +5 ppm of MEK:
10,0 mg propanol / 100 ml Water (solvent 1).10,0 mg MEK / 200 ml Water (solvent 2).
Add 0,5 ml solvent 1 and 0,5 ml solvent 2 to a 5ml flask and fill to the mark with saline. Add 200 µl to Vial 2. Cap vial.
50 ppm of propanol +10 ppm of MEK:
10,0 mg propanol / 20 ml Water (solvent 1).10,0 mg MEK / 100 ml Water (solvent 2).
Add 0,5 ml solvent 1 and 0,5 ml solvent 2 to a 5ml flask and fill to the mark with saline. Add 200 µl to Vial 3. Cap vial.
100 ppm of propanol +35 ppm of MEK:
10,0 mg propanol / 10 ml Water (solvent 1).7,0 mg MEK / 20 ml Water (solvent 2).
Add 0,5 ml solvent 1 and 0,5 ml solvent 2 to a 5ml flask and fill to the mark with saline. Add 200 µl to Vial 4. Cap vial.
120 ppm of propanol +42 ppm of MEK:
12,0 mg propanol / 10 ml Water (solvent 1). 8,4 mg MEK / 20 ml Water (solvent 2).
Add 0,5 ml solvent 1 and 0,5 ml solvent 2 to a 5ml flask and fill to the mark with saline. Add 200 µl to Vial 5. Cap vial.
I have only limited solution of saline and hope that it's enough for my analyses (2x). If not do you have any idea how I can reduce the saline?
I need the spreadsheet for the calculation. Do you know where I could download it?

By the way, to answer your question from the last post: The specifications of 1 propanol (NMT 100ppm) and MEK (NMT 35 ppm) are the concentrations in the dissolved saline sample volume.
I’d be glad to hear your opinion on that.

Thanks.

[chromatographer1]

Your draft:

1 ppm of propanol +1 ppm of MEK:
10,0 mg propanol and 10,0 mg MEK/ 1000 ml Water .
Add 0,5 ml of solvent to a 5ml flask and fill to the mark with saline. Add 200 µl to Vial 1. Cap vial.

What you did is described below:

Prepare solution of 10 micrograms per mL of IPA and MEK in water

Add to 4.5mL of saline 0.5 mL of 10mcg/mL solution.

Add 200 microliters to vial. (200 = 180 microliters of saline, 20 microliters of water, 5 mcg each of IPA and of MEK)

OK, STOP ! Why did you not just add 180 microliters of saline to the vial, add 10 microliters of 0.5 microgram per microliter of IPA solution and 10 microliters of 0.5 microgram per microliter of MEK solution? 0.5 mcg per mcL = 500 mg per liter

Your way uses 4.5 mL of saline. Mine uses 180 microliters.

Do you follow?

Do the same for the other solutions.

Best wishes,

Rod

[tinsang]

Thanks a lot, Rod

Regards.