Beverage-Air NC49 Manual descarregar pdf (Página 53)

Document No. 031481-12 AAA-Direct, Dionex Amino Analyzer and AminoPac PA10 Page 52 of 66

10.4.4Incorrect or Variable Retention Times

A. Check your eluent preparation procedure for possible errors.

B. Prime the pump if necessary.

C. Measure the flow rate by weighing out the eluent collected during exactly five minutes. Recalibrate the pump if

necessary.

D. Your sodium hydroxide eluent contains too much carbonate and/or the re-equilibration period at the end of the gradient

method is too short.

E. Set the eluent composition for 100% for each eluent and draw out at least 40 mL of eluent from each eluent line.

F. Samples containing high salt content (> 50 mM) will decrease the retention times.

G. Check and/or service the pump’s proportioning valve. With the pumping turned off, the flow through the pump outlet

tubing (disconnected from the injector) should be zero in all three eluent positions. Check this separately for each

eluent line at the 100% setting.

10.4.5 Unidentified Peaks Appear Alongside the Expected Analyte Peaks

During the acetate gradient a number of small peaks may appear (See Figure 2). These peaks are usually due to trace contaminants

in the water supply. The contaminants accumulate on the column during the isocratic section of the chromatogram and are

released, frequently as irregular baseline deformations or sharp spikes, with the increasing eluent strength.

Some trace contaminants can co-elute with glutamate and aspartate compromising accuracy of quantitation of these amino acids

at lower concentrations. If extraneous peaks are observed even after the water supply is excluded as a possible cause, clean the

autosampler lines and sample loop. The autosampler should be cleaned using the following protocol:

A. Disconnect column and detector cell from the autosampler.

B. Set the pump to 100% deionized water.

C. Place the following solutions in the autosampler in autosampler vials and inject in sequence. Use 25 µL full loop

injections:

1. 1 M NaOH

2. Deionized water

3. IPA

4. Deionized water

5. 1 M HCl

6. Deionized water

10.5 Sodium Hydroxide Cleanup

The sodium hydroxide (2 M) rinse used to decrease column or system-related elevated background is essentially identical with

the rinse performed during an installation of a new system, Section 6.1.3. Following the rinse, check the background again while

pumping the 60 mM sodium hydroxide and repeat the rinse at least once if necessary. Leave the old gold working electrode in

place during the first and the second checking of the detection background. Use a new or reconditioned electrode only if the

background remains high even after the second rinse. Should the new electrode also produce a reading of > 80 nC, remove it from

the system within 30 minutes, rinse it with water and reinstall the old electrode. In case the repeated rinse does not lower the

background, perform the nitric acid cleanup described in Section 10.6. Then try the background with old electrode first and if

necessary only briefly with the new electrode again. In case the new electrode delivers < 80 nC, leave it in the system and recondition

the old electrode using chemical cleanup described in Section 10.8.2.

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Beverage-Air NC49 Manual Página 53