Can anyone assist with sample preparation procedures for analysing VMA in urine using a GBC EC detector.
Any assistance greatly appreciated

I used ICI EC detector an HPLC (now is GBC) with wall-jet cell. Try this VMA screening with a C18 125 x 4 mm (I had a Lichrocart RP18 5 um, Merck)+ precolumn. Mobile phase: NaH2PO4 0.05 M pH=2.7 (H3PO4)+ 2-3% Acetonitrile at 1 ml/min. EC +0.85 V vs Ag/AgCl, Range 50 nA.
Inject 20 ul reference sample 0.3 ug/ml. We diluted urine (filtered through 0.45 um) 1/10 with mobile phase and this solution was injected directly onto HPLC column. VMA peak had a rt of 5 min approx. and after that we injected 100 ul of pure acetonitrile for elueting later peaks (I now it is not conventional but it worked). We saw the detector out of scale but after 3 minutes the sytem was ready again. With 7% acetonitrile instead of 2% in mobile phase you can also quantify 5-HIAA and HVA. Amines like cathecolamines and 5-HT should not interfere.
SPE C18 doesn't work because VMA is very polar. We also used liquid-liquid extraction of 200 ul acidified (20 ul HCl 2N) urine + 1 ml Ethyl acetate) in an eppendorf, vortex-mixing 5 min, centrifuging 5 min. Transfer 500 ul aliquot to another eppendorf tube containing 500 ul of NaH2PO4 20 mM pH=7.5, vortex again and centrifuge.
transfer 400 ul to a sample vial and dilute with 400 ul H3PO4 40 mM. Inject 20 ul.
Internal standard for VMA: With direct injection of diluted urine you don't need it but for liq-liq extraction you should use it.
Good luck and I'm sorry my English!

Indeed, sample preparation with a C18 is not likely to work, but you can use an anion exchanger such as Oasis MAX or similar things.