## 3 Experimental Results
### 1-kbp and \(\lambda\)-DNA Samples  
The character of the THz transmittance changed significantly with the measurement of the larger DNA samples: the 1-kbp ladder and \(\lambda\)-DNA. The results for the 1-kbp ladder sample are shown in Figure 5b, and the \(\lambda\)-DNA in Figure 6a over the frequency range 550 to 1000 GHz. Although not as strong as the prominent signatures in the smaller molecule samples, there are still three signatures in Figure 5b significantly deeper than the others, labelled (1), (2), and (3), and centered at frequencies of \(\
u_{0}\) = 750, 825, and 910 GHz, respectively. The Q\({}_{\text{S}}\) (As) values of these three are 23, 28, and 28 GHz (0.13, 0.12, and 0.12), respectively, as listed along with the other parameters in Table 2.  
Figure 5: (**a**) Transmittance spectrum for the 50-bp DNA sample between 700 and 1050 GHz. (**b**) Transmittance for the 1-kbp DNA sample between 550 and 1000 GHz.  
Figure 6: _Cont._For the \(\lambda\)-DNA of Figure 6a, there are no prominent signatures, but instead a quasi-sinusoidal behavior with a peak-to-trough transmittance difference of only \(0.1\). The increased noisiness in Figure 6a compared to the previous transmittance plots is mostly attributed to the magnified vertical scale. Because the peak-to-peak frequency separation is comparable to what is expected for longitudinal modes in the fused quartz substrate (etalon), the \(\lambda\)-DNA sample is likely just displaying broadband absorption modulated by an etalon transmittance. Because of this interpretation, there is no listing of signature parameters in Tables 1 and 2.  
However, since the \(\lambda\)-DNA sample approached the lower limits of the signature strength necessary for confident detection, it also provided an opportunity to study reproducibility. At the signal-to-noise ratios of our spectrometer, reproducibility would mostly be affected by systematic errors, like drift in the THz signal power, or by variation in the nanochannel fluidics. Figure 6b shows the results of two successive transmission scans  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \multicolumn{3}{c}{**1-kbp DNA**} \\ \hline Signature & (1) & (2) & (3) \\ \hline Center Freq (GHz), \(\
u_{0}\) & 750 & 825 & 910 \\ \hline FWHM (GHz), \(\Delta\
u\) & 32 & 30 & 32 \\ \hline Quality Factor, \(Q_{\text{S}}\) & 23 & 28 & 28 \\ \hline Tmin & 0.59 & 0.6 & 0.62 \\ \hline Tmax & 0.8 & 0.79 & 0.81 \\ \hline Signature Absorbance, As & 0.13 & 0.12 & 0.12 \\ \hline Molecule/cm\({}^{3}\) (diluted), \(\rho_{\text{m}}\) & \(6.2\times 10^{13}\) & \(6.2\times 10^{13}\) & \(6.2\times 10^{13}\) \\ \hline Molecular Cross Sec (nm\({}^{2}\)) & \(9.8\times 10^{3}\) & \(8.9\times 10^{3}\) & \(8.6\times 10^{3}\) \\ \hline Cross-Sec Diam, D (nm) & 112 & 106 & 105 \\ \hline D/Molecular Length & 0.086 & 0.081 & 0.080 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Spectroscopic and optical parameters for three prominent absorption signatures in 1-kbp DNA sample.  
Figure 6: (**a**) Transmittance spectrum for the \(\lambda\)-DNA sample between 500 and 1000 GHz. (**b**) Repeat measurement of same \(\lambda\)-DNA sample to check for reproducibility.  
with the \(\lambda\)-DNA sample. By averaging over frequency, we found that the variation in transmittance was \(\sim\)5% across this 550-1000 GHz range.