Chromatography Forum

Can someone please give me some guidelines as to setting up this method? We have a Shimadzu gc with dual ecd's and I can get a passing calibration curve (0.998 or better for MCAA, MBAA, DCAA on linear and 0.995 for TCAA and DBAA and even better 0.998 or better for all analytes under quadratic fit). However, when I run CCV's everything passes except TCAA and DBAA. They are both coming out around twice the true concentration. Does anyone have any instrument parameters they are currently using that work really well for this method (ie oven program, flow rates, head pressure, septum purge flow, linear velocity, inj temp, det temp, etc)

Do you know that it is an instrument problem and not a derivatization problem? As in a reshoot of the standards from the calibration also have too high of recovery?
Back in the dark days when I ran this method I noticed that the methylation efficiency of TCAA and TBAA especially were often erratic. I did a temporary fix by moving on to 552.3 and using TAME. Then I fixed it completely by not running the method anymore!

Two things I remember from when I ran 552.2, not sure if they apply to your situation.

- I had some issues with the range of my calibration curve. I had to analyze some pretty high samples, and when I did a single curve it biased the low values and I couldn't get good recoveries even though the correlation coefficients were passing. So I broke it into a high and low curve, got tighter correlation coefficients, and my CCV's were much better.

-TCAA was definitely a challenge. One thing that helped was to make sure that the reactor temperature was as close to 50C as possible. Rather than trust the thermometer in the heat block, I rigged up another digital thermomenter with the thermocouple in a centrifuge tube with 7mL of water in it. This helped stabilize the TCAA recoveries some.

I too solved the problem by no longer running the method

If the heat block temp is too low or too high, how will this affect the methylation procedure? My cal curve ranges from
1-50 ppb. What volume of extract did you inject? Also, does the purity of the water affect this method drastically? We are using dI but it has high TOC levels so I boiled it for 40 min to remove the organics. Is this sufficient enough?

I suggest reading 552.3 even though you are running 552.2. That's where I got the temperature tip, and it has some good information in it. Some of the things you can apply to 552.2 without deviating from the method. My understanding is that too low a temperature and derivatization doesn't work properly, too high and MTBE hits its boiling point.

I can't remember what exactly what calibration range we used. It's pretty easy during development to experiment with adding and subtracting points to determine what range works. Your final curve will need at least 5 points. If your failing CCV is a low concentration, your calibration curve may be going too high. I think I tried to get better >0.999 on my curves in order for a low concentration standard check to pass.

Water purity can be a factor. In my lab our purified water was OK for HAAs, but for THMs it wasn't as we had detectable Chloroform . To address that we would fill a 1L glass bottle, loosely cover it with aluminum foil, and leave it in a 104C oven for 24 hours. Not sure if that would do the trick for HAA contamination or not, probably depends on the boiling point of the contaminants.

Make sure your sodium sulfate is muffled according to the method. Also, we had contamination from some brands and lots of sulfuric acid.

Per Steve's comment, most of the problems I had with this method were sample prep and/or calibration curve related and not instrument issues.

I don't have too many instrument parameter specifics, it's been a couple years.

I have been reading 552.3 to get some additional ideas to try...I have an analog temperature block so I tried putting an extra vial in with just water and monitored it's temperature and saw our block was about 45 degrees so I bumped that up to 50. I also tried to put the extracts in the fridge to see if any water crystals formed and they didn't this time but I did have them form in a previous extraction I did. Do you know what it does if you do have ice crystals in your extracts? I also boiled the water I used for the calibration for about 40 minutes to hopefully get rid of any organics or impurities in our dI. A few additional questions I had are do you need to use HPLC grade mtbe, pesticide grade methanol, pesticide grade sodium sulfate? Should I purchase the sodium sulfate and copper pentahydrate in glass containers? You said certain brands of sulfuric acid had contamination and others didn't. Do you have any suggestions on what brands of sulfuric worked for you?

Water (ice crystals) in the extract vials isn't good as this translates to water in your sample and onto the column. I had good luck with "saving" those samples by using a pasteur pipet to transfer the extract from the vial with ice crystals in it to a fresh vial (leaving the crystals behind).

I don't remember what grade MTBE I used. I did check it occasionally by derivatizing an MTBE blank - put the same amount of straight MTBE into a centrifuge tube as you would transfer from a sample (3mL?), and proceed with the sample prep (adding acidifed methanol, putting on the heat block, etc). We used EMD, but this was because we had chloroform interference in THM analysis and we used the same solvent bottle for both analyses. We had good luck with Fisher sulfuric acid. We used regular (ACS? maybe) grade sodium sulfate and cupric sulfate pentahydrate, but those should be considered as potential contamination sources. My auditor told me at one point that the sodium sulfate and the sulfuric acid were usual suspects for contamination.

Carryover in the gastight syringes was also a problem - had to rinse thoroughly between uses.

Water in your extracts means you could have acid in them too. That is very bad. I always checked for acid after I destroyed a pair of Restek columns.

Steve,

How did you check for acid?

I checked one of my standards with pH 0-6 range paper and it had a pH of 5. Would you say the solution isn't neutralized completely?