In-stu chemical analysis requires sampling and ionization/excitation of sample in an open environment. The atmospheric pressure micro-plasma system is in a simpler structure and more advantageous for miniaturization than laser cutting or laser induced breakdown. Therefore, in-stu chemical analysis technology based on atmospheric pressure micro-plasma has attracted widespread attention in the industry. The micro-plasma sources produced by different excitation supplies have different parameter characteristics and sampling characteristics. In order to determine the appropriate micro-plasma source for in-stu element detecting, it is necessary to further understand the sampling and characteristics of the micro-plasma under each discharge mode and operating parameters. This paper focuses on the optical emission spectra of direct current(DC) glow and pulsed arc plasmas acting on iron samples in ambient atmosphere. The on-line detection of high melting point metal samples in an open environment was realized, and we also found that the arc discharge micro-plasma combined with the spectral analysis source has higher sampling efficiency. The high sampling efficiency of the arc discharge microplasma source provides a new method for the detection of metals and difficult to dissociate samples. At the same time, complex sample preparation and sample transfer processes were avoided compared to conventional sampling devices. The experimental device adopts a typical needle-plate discharge structure, which has the advantages of low price, convenient operation and quick analysis. Arc discharge and glow discharge are realized by using a high voltage pulse power source and a DC power source respectively. The sampling results of the two ways indicate that at the input of similar discharge power, the optical emission spectrum of the sample iron stimulated by the arc discharge micro-plasma, sample element lines occupies dominant position in the spectrum, accompanied by the line of nitrogen in the air. And the relative intensity of the iron ion (FeⅡ) line is higher than the relative intensity of the nitrogen molecule line. In the DC glow discharge, the relative intensity of the sample iron atom (FeⅠ) line is very weak. This shows that the micro-plasma generated by the arc discharge has higher sampling efficiency. The sputtering crater left on the surface of the sample can draw the same conclusion. The glow discharge current was increased to 25 mA, the spectral line of the sample elemental iron was still not significantly enhanced. At the same time, the effect of sampling spacing on the two sampling modes was also studied. Experimental results show that the spacing has no significant effect on the both sampling spectra of the two modes. In this paper, the above-mentioned comparative experiment was also carried out using the alloy aluminum foil whose main component is aluminum, and the same conclusion was obtained. It is concluded that the arc discharge micro-plasma is more suitable as a source of spectral analysis to achieve real-time rapid detection of metal samples.