diff --git a/docs/tutorials/preprocessing.ipynb b/docs/tutorials/preprocessing.ipynb
index d3d27b5..28731c4 100644
--- a/docs/tutorials/preprocessing.ipynb
+++ b/docs/tutorials/preprocessing.ipynb
@@ -82,15 +82,6 @@
     "We split the data matrix into the marker intensity part and the FSC/SSC part. Moreover, we move all height related features to the `.obs` part of the anndata file. Notably. the function `split_signal` checks if a feature name is either FSC/SSC or whether a name endswith `-A` for area related features and `-H` for height related features.   "
    ]
   },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "pm.pp.split_signal(adata)"
-   ]
-  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -111,33 +102,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Let us modify the feature column `signal_type` manually."
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "adata.var[\"signal_type\"] = adata.var[\"signal_type\"].cat.add_categories([\"area\"])\n",
-    "adata.var[\"signal_type\"][3:] = \"area\""
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "adata.var"
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "Repeat to split the data matrix."
+    "We use the `channel` column of the `adata.var` data frame to split the matrix."
    ]
   },
   {
@@ -146,7 +111,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "pm.pp.split_signal(adata)"
+    "pm.pp.split_signal(adata, var_key=\"channel\")"
    ]
   },
   {
@@ -162,7 +127,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "This time, we did not get the warning that all features are returned. Indeed, the data matrix was reduced by three features (`FSC-A`, `FSC-H` and `SSC-A`). "
+    "The data matrix was reduced by three features (`FSC-A`, `FSC-H` and `SSC-A`). "
    ]
   },
   {
diff --git a/pytometry/preprocessing/_process_data.py b/pytometry/preprocessing/_process_data.py
index 2f630de..df454ac 100644
--- a/pytometry/preprocessing/_process_data.py
+++ b/pytometry/preprocessing/_process_data.py
@@ -37,7 +37,7 @@ def create_comp_mat(spillmat: pd.DataFrame, relevant_data: str = "") -> pd.DataF
 
 def find_indexes(
     adata: AnnData,
-    var_key=None,
+    var_key: str = None,
     key_added="signal_type",
     data_type="facs",
     inplace: bool = True,
@@ -94,7 +94,7 @@ def find_indexes(
             f"{data_type} not recognized. Must be either 'facs' or               "
             " 'cytof'"
         )
-    adata.var["signal_type"] = pd.Categorical(index_array)
+    adata.var[key_added] = pd.Categorical(index_array)
     return None if inplace else adata
 
 
@@ -241,6 +241,8 @@ def split_signal(
     # merge non-idx entries in data matrix with obs
     non_cols = adata.var_names[non_idx].values
     for idx, colname in enumerate(non_cols):
+        if colname == "":
+            colname = adata.var["channel"][non_idx[idx]]
         adata.obs[colname] = adata.X[:, non_idx[idx]].copy()
 
     # subset the anndata object