As wlong01 said the causes can be many but if you at first suspect the ignition system it would be wise to know exactly how the ECM knows this. Some causes can be eliminated if this only occurs on one cylinder so you really know what the rest of them are doing. A scanner can reveal misfire counts on all cylinders in real time. The very first thing I would do is check the compression in all cylinders and compare them - perhaps cylinder 1 is quite low compared to the rest. High mileage, binding rings from carbon and poor valve operation (seats and springs) can cause low compression. If this seems pretty even albeit a bit low then what you attempted would be the next step.
The basic idea is that if a cylinder fires, it gives a kick to the crankshaft causing a slight variation in crank shaft rotation speed. The ECU has sensors that tell it the position of the crank shaft, from which it knows which cylinder should be firing next. When an ignition happens as it should, that's the power stoke and it gives a little kick to the crankshaft causing a slight increase in the RPM. The ECU sensors can tell it the crank position closely enough that it detects that slight increase. If the slight increase doesn't happen, the ECU knows that there was no ignition on that cylinder. Which is also how it knows which cylinder didn't fire, and how it reports which one it was through the code it throws. In this instance I would verify that the crank sensor was working properly and giving a signal for each and every cylinder.
If the misfires are occurring on other cylinders but not as frequent I might suspect the crank position sensor (probably not the issue). Unfortunately you'll have to clear all codes and run the car again to get accurate data. A suspected vacuum leak can be detected by spraying a bit of brake cleaner (with a straw) at any suspected leak spot - engine sound will change as well as rpm's.. As for injectors, suspect a carbon issue or failed/failing injector. If you don't have a bi-directional scanner you can go to a reputable repair shop but I'd definitely check cylinder compression first.
