CharliesMom wrote:James_Ball wrote:
For over 20 years I have prepped mine the same way. Make a working standard in methanol and store that in a 5ml micro reaction flask with a Mininert cap so that I can use it for the whole month without it going bad. Store the standard in the freezer at -18C. I never allow it to warm more than a couple minutes simply because if you pull up the volume you need in a syringe, the syringe body will bring the methanol to room temp in a couple seconds, after a wait a couple seconds I remove the syringe from the flask and close the valve then inject into a volumetric flask of Organic free water. 50 or 100ml flask will fill the 40ml vial to a meniscus, add two drops of 1:1 HCL if it is a 524 standard, nothing if it is 8260, then cap and ready to go.
Over 100 analytes in our calibration standard and I can usually get 70% of them calibrated from 1ppb - 200ppb in 8 calibration standards with less than 10% RSD using average response factor. There are always a few that will need to be set to linear calibration (acetone, isobutyl alcohol, ect) and a couple quadratic (methyl iodide, 2-chloroethylvinyl ether).
Plus if using 100ml flasks I can make a duplicate standard for each level and calibrate two instruments at once and still have a little left over I can use to measure out 5ml aliquots into vials to do a Soil calibration following the water calibration. I have not had any problems doing it this way and I can make the entire eight point curve up in about 15 mintues.
I admire your experience Sir. Could you please comment further on this set up? We use 2000ug/L stock for 524.2
I feel volumetric is the best way but we use the 40 ml vial but change actual volume to 44mls and it changes the math on spiking amount. What is correct? Also, what volume syringe do you use to spike? Is it a Hamilton gas tight? Do you have needle full or empty?
Sorry I missed this one before.
If spiking into the vials we estimated 43mls, but for calibration standards we use the 50ml or 100ml class A volumetric flasks for improved accuracy. If you only invert them slowly three times to mix there isn't any noticeable loss of the gasses. Just dispense immediately into the 40ml vials and not leave the flask sitting at room temp for a long time.
Each analyst is a little different and use the syringe they are most comfortable with, but for me I use an extended handle Hamilton 50ul syringe(the one with the long blue aluminum handle with the glass syringe attached). The teflon washer in the handle controls the tension on the plunger so it doesn't slide down after you pull it to volume. I can use it from 1ul to 50ul with no problems, and for the larger volumes just pull it up to 4 times for 200ul. I found that using the same syringe eliminates any deviation from syringe to syringe. I had a co-worker several years ago who was not comfortable using it at such low volumes and would use three syringes to cover the volume range but I could always get better %RSD on my curves. He had good technique so I believe it was small differences in the accuracy of each syringe that was causing the problems.
Another thing to remember when using the syringes, the needle can have a large volume within it. One co-worker would pull up 5-10ul of air then bring up solution to mark using the meniscus. Problem is, when dispensing the air volume clears the needle volume, so you have the measured amount plus what ever the needle volume was. We found one 100ul syringe actually had 15ul of needle volume, so if you were measuring 50ul you could be off by as much at 30%. I always purge the air from the barrel, then measure to the marks with the plunger tip, that way you have accurate delivery and the needle volume stays in the needle. If switching standards, just pull air a few times to remove most of the liquid in the needle then wash a few times with solvent.
You can also be accurate if you leave the air in the needle, use the plunger tip to determine volume and despense, the air remains in the needle. If you don't aspirate any air and the plunger doesn't leak from above, you will still be accurate as the air acts like an extension of the plunger down to the needle tip. We tested it and found it to work, but I still like to purge the air out of habit.