by
Hollow » Fri Feb 10, 2012 6:57 pm
chromeleon wrote:
One of my colleagues has been using it for quantitative work and found that he wasn't seeing any peaks at a certain injection volume.
is this "certain" volume on the high or low end?
before doing some tests, make sure your AS is purged properly with inital mobile phase. As for the "purge inj" make sure you have a column installed, cause it needs some backpressure for success.
If your detector is working fine, you could do a linearity check as well.
Inject different volumes of a solution of (eg.) caffein dissolved in mobile phase. Plot Area vs. Vinj
-> how does the curve looks like? Linear fit should give r2 >= 0.999.
Second check would be for reproducibility: do six replicates of a certain volume of the caffein solution and calculate %RSD of the peak areas. This should be <1%
As for the accuracy, the Waters OQ test is similar to the one you did, but they do six replicates of 50µl of water (in series). The average volume withrawed should be 50 +/- 0.5µL = mg (if I remeber correct). The balance should read at least 0.1 mg
The 717plus has the "needle in line", today also know as "FTN: Flow-through-needle" design, so it just takes up the volume specified. Afterwards the needle is just switched into the flow path and all of the sample goes to the column.
Possible sources of your errors:
- be aware that the point of inlet is on the side of the needle, about 4-5mm above the tip. Fill your vial with enough sample: if you make a lot of replicates of large volumes, maybe use several vials for not to fall below this needle inlet level.
- if you have limited sample volumes, use "total recovery vials" (insert style), but make sure the small part is filled with your sample and not with trapped air.
-
edit: valve #3 is defective (->call Waters) (not valve4)
-- does the "compression check" pass or fail?
- as Tom suggested: syringe is not sealing properly (plunger tip worn out?)
chromeleon wrote:
It also varies depending on the position of the vial.
very strange! No idea for this...
- were the vials filled with different levels?
- is the carousel working correct eg. does it move to the correct vial?
--> place several groups of 3 vials around the carousel, then specify injections from the middle vial of each group. Are all (and only) the correct vials punctured, none of the left or right ones?