ICP-MS can detect ppb concentrations. You wouldn't want to add the undiluted acid to the machine, but if you dilute it by 10 times or so (typical ICP-MS solution is 2wt% acid) you will still be able to detect sub-ppm concentrations. If you are not concerned about such low concentrations, you can also use ICP-OES (dilution also required) or AAS (check with specialist if dilution is necessary).

WDXRF is also able to detect elemental composition of liquids, and may be possible to detect the undiluted acid, as long as you use a support film that can withstand the strong acid. However this method is not sensitive enough for concentrations much below 1wt%.

Thanks Rafael Mattos Dos Santos and we will followup your valuable sugesstions

ICP-OES has proved to detect very low concentrations of metals in solution. Its detection limit is in sub-ppm too.