Determination of mercury in AAS Its known that the only method to determine mercury in atomic absorbtion spectroscopy is the cold vapor method. 

What is the reason that mercury can't be determined with other methods, such as flame, electrothermal AAS or even emission spectroscopy ?
Mercury is an element with poor sensitivity when analyzed by traditional flame atomic absorption. This happens because the element is extremely volatile (remember mercury is a liquid at room temperatures and has significant vapor pressure at room temperature). Cold vapor is a widely used method, that's EPA approved, and shows a significant increase in sensitivity (nearly four fold). 
Source

There's many different devices out there that can be used to do elemental analysis, but from a personal experience, if you're working with dry solid material (ex. soil), then X-ray fluorescence (XRF) could be worth looking at. It's relatively cheap and does multi-element analysis. With mercury however, you're looking at a LOD of about 5 ppm, which is... okay. It depends on your experiment.